PLC—Programmable Logic Controllers

6.5

PLC—Programmable Logic Controllers W. N. CLARE, G. T. KAPLAN, D. R. SADLON, A. C. WIKTOROWICZ n ^

R. A. GILBERT o*»)

Reviewed by B. G. Liptak (1994)

Types of Input/Output (I/O):

Discrete I/O: 120 V AC, 220 V AC, 0 to 5 V DC, 0 to 24 V DC, transistor-transistor logic (TTL) Analog I/O: 4 to 20 mA, 1 to 5 V DC, 0 to 10 V DC, -2.5 to +2.5 V DC Special I/O: Thermocouple, RTD, stepper motor (pulses), strain gauge, high-speed counters, PID

Typical Specifications:

Scan Time per WOO Words (IK) of Logic: 1 msec to 50 msec, depending on manufacturer and enhanced software features Word Size: 4 bit, 8 bit, or 16 bit (typical) Amount of I/O: Small PC—20 to 256 I/O; medium PC—256 to 1024 I/O; large PC—1024 and greater I/O Size of Memory: Small PC—256 to 2K words (K = 1024 bits, bytes, or words of digital data); medium PC—2K to 12K words; large PC—12K and larger Type of Memory: CMOS (complementary metal oxide semiconductor); RAM (random access memory); EPROM (erasable programmable read-only memory); CORE (ferrite cores) Environmental Conditions: 0 to 60°C (32 to 140°F), relative humidity, to 95% noncondensing, 115 V AC ±15% and 230 V AC ±15%

Costs:

Small PC Hardware: Basic PC with CPU, 2K to 3K RAM memory, 10 to 20'I/O points costs $600 to $2000 (extra discrete I/O when available costs $25/point) Medium PC Hardware: Basic system with 256 I/O points costs $5000 to $8000; with 512 I/O points costs $10,000 to $20,000; and with 1024 points costs $14,000 to $28,000 (RAM quantity adjusted to handle designated I/O count) Large PC Hardware: $15,000 to $70,000 (cost driven by network and information display requirements)

Software and Engineering:

Software costs range from 50 to 100% of the hardware cost. System engineering costs and documentation costs each range from 25 to 50% of hardware cost. Therefore, the total cost (without installation labor) is about twice (if not more) the hardware cost

Support Equipment Cost:

Hand-held programmers cost $100 to $500; PC-supported programmers cost $2500 to $5000; CRTdedicated programmers cost $1100 to $1800

Partial List of Suppliers:

U.S. Suppliers: Allen-Bradley; ASC Systems; Bailey Controls; Cincinnati Milacron Co.; Control Techniques Process Instruments; Eagle Signal Control; Eaton/Cuttler-Hammer; Foxboro Co.; G.E. Fanuc Automation; Giddings & Lewis; HMW Enterprises Inc.; Honeywell; Icon Corp.; International Parallel Machines Inc.; Jumo Process Control Inc.; Klockner-Moeller Corp.; Leeds & Northrup; Minarik Electric Co.; Modicon; Nolatron; Omega Engineering; Omron Electronics Inc.; Pro-log Corp.; Reliance Electric Co.; RTWare Inc.; SKH Systems Inc.; Square D Automation; Tektronix Inc.; Texas Instruments Inc.; Uticor Technology Inc.; Wizdom Systems Inc. Foreign Suppliers: Fuji Electric Corp.; Mitsubishi Electric; Siemens; Telemechanique; Toshiba Inc.; Westinghouse Canada Inc. (Note: The most popular sources are Allen-Bradley, Modicon, and Siemens.)

PLC VERSUS DCS

During the past decade, the capabilities of programmable logic controllers (PLCs) and distributed control systems (DCS) have changed to the extent that today many applica­ tions that used to be the exclusive province of one or the other can be handled by both. PLCs were developed by manufacturers who had been making relays for logic and

interlock applications, while DCS systems were developed by process control manufacturers, having substantial experi­ ence in PID-type analog control. Therefore, in the past it made good sense to use each type of controller in its area of superior experience. If the bulk of the I/O was digital (discrete), the logical choice was to use a PLC, whereas if the I/O was mostly analog, a DCS system was selected. This logic, while still valid to some extent, is no longer univer721

722

PLCs and Other Logic Devices

sally true, and personal preference and end-user familiarity has become a decisive factor in system selection. In terms of pros and cons between the two designs, PLC I/O is likely to be more rugged and PLCs are likely to handle discrete logic faster than DCS systems. PLCs are also likely to be more desirable because their languages, such as ladder logic, are usually more familiar to plant personnel and there­ fore there is less resistance to using them. On the other hand, ladder logic type languages can be undesirable in some situations because they are not well suited to analog process control. Some users have overcome the limitations of PLCs by coupling them to personal computers (PCs) using cus­ tom-coded programming. The disadvantage of this approach is that such a nonstandardized system is usually understood fully only by its designer, and when that person leaves the company, the system can be ruined. When it comes to communication redundancy and data security, the DCS sys­ tems are superior. The DCS systems are also superior in their programming library, in advanced or optimizing con­ trol, in self-tuning algorithms, and, particularly, in their total plant architecture and information management capabilities.

INTRODUCTION

A programmable logic controller (PLC) is an industrially hardened computer-based unit that performs discrete or con­ tinuous control functions in a variety of processing plant and factory environments. Originally intended as relay replace­ ment equipment for the automotive industry, the PLC can now be found in some part of virtually every type of industry imaginable. The programmable controller is produced and sold worldwide as stand-alone equipment by at least 10 major control equipment manufacturers. Several more com­ panies are in the business of producing PLCs for original equipment manufacture (OEM) applications. (Note that "programmable logic controller" is sometimes referred to as a PC, rather than a PLC. Since this term is also used to designate personal computers, use of it can cause confu­ sion.)

TABLE 6.5a

History of Programmable Logic Controllers (PLCs) 1968

Design of PLCs developed for General Motors Corporation to eliminate costly scrapping or assembly-line relays during model changeovers.

1969

First PLCs manufactured for automotive industry as electronic equivalents of relays. First application of PLCs outside the automotive industry. Introduction of "smart" PLCs for arithmetic operations, printer control, data move, matrix operations, CRT interface, etc.

1971 1973

1975

Introduction of analog PID (proportional, integral, derivative) control, which made possible the accessing of thermocouples, pressure sensors, etc.

1976

First use of PLCs in hierarchical configurations as part of an integrated manufacturing system.

1977

Introduction of very small PLCs based on microprocessor technology.

1978

PLCs gain wide acceptance, sales approach $80 million.

1979

Integration of plant operation through a PLC communication system.

1980

Introduction of intelligent input and output modules to provide high-speed, accurate control in positioning applications.

1981

Data highways enable users to interconnect many PLCs up to 15,000 feet from each other. More 16-bit PLCs become available. Color graphic CRTs are available from several suppliers.

1982

Larger PLCs with up to 8192 I/O become available.

1983

"Third party" peripherals, including graphic CRTs, operators' interfaces, "smart" I/O networks, panel displays, and documentation packages, become available from many sources.

HISTORY

Programmable controllers were originally designed for Gen­ eral Motors Corporation in 1968 to eliminate costly scrapping of assembly-line relays during model changeovers. Table 6.5a lists some of the milestones in the develop­ ment of PLCs. (For a discussion of more recent advances, refer to Section 6.6.) The automotive industry fostered the development of the PLC primarily because of the massive rewiring that had to be done every time a model change occurred. Solid-state logic is much easier to change than relay panels, and this advantage was reflected in the cost of installing and operating the PLC instead of traditional relay systems. Table 6.5b provides a summary of the differences among various technologies that perform logic control.

Although impressive, cost reductions alone are not the only reason that programmable controllers are the major replacement candidate for traditional relay logic. Compared with electromechanical relay systems, PLCs offer the fol­ lowing additional advantages: • Ease of programming and reprogramming in the plant • A programming language that is based on relay wiring symbols familiar to most plant electrical personnel • High reliability and minimal maintenance • Small physical size • Ability to communicate with computer systems in the plant • Moderate to low initial investment cost

6.5 PLC—Programmable Logic Controllers

723

TABLE 6.5b Cost Advantages Over Relay

Relays

SolidState Controls

Micro­ processor

Mini­ computer

Hardware cost

Low

Equal

Low

High

High to Low, Depending on Number of Controls

Versatility

Low

Low

Yes

Yes

Yes

Usability

Yes

Yes

No

No

Yes

Maintain-Ability Troubleshoot

Yes

No

No

No

Yes

Computer-compatible

No

No

Yes

Yes

Yes

Arithmetic capability

No

No

Yes

Yes

Yes

Information gathering

No

No

Yes

Yes

Yes

Industrial environment

Yes

No

No

No

Yes

Programming cost

(Wiring) High

(Wiring) High

Very High

High

Low

Reusable

No

No

Yes

Yes

Yes

Space required

Largest

Large

Small

OK

Small

PLCs

• Rugged construction • Modular design

ply, I/O, central processor, memory, and programming and peripheral device subsections. Each is discussed below.

Today's programmable controller is at best a distant rela­ tive of the first- and second-generation PLCs built through­ out the 1970s and 1980s. Today there are at least two recognized PLC sizes to select from. The small programma­ ble controller is primarily a relay replacement unit that provides a few additional functions. This is an extremely basic controller that is amazingly inexpensive. This small dedicated controller is enclosed in a single-mounted hard­ ened case. It is intended to be a relay replacement unit and provide reliable control to a stand-alone section of a process. The modern medium-sized programmable controller per­ forms all the relay replacement functions expected of it but also adds many other functions—including counting, timing, and complex mathematical applications—to its rep­ ertoire. Most medium-sized PLCs can perform PID, feedforward, and other control functions as well (see Chap­ ter 1). In addition, medium-sized and large-scale PLCs now have data highway capabilities and can function well in DCS environments.

Power Supply

The power supply may be integral or separately mounted. It always provides the isolation necessary to protect solid-state

I

T POWER SUPPLY

^H

MEMORY

CPU

 I / O BUS I / O SYSTEM MODULES

\I OUT PUT DEV ICES

FUNCTIONAL DESCRIPTION

A programmable controller manufactured by any company has several common functional parts. Figure 6.5c illustrates a generic PLC architecture. The diagram shows power sup­

1 PROGRAMMING , DEVICE

SOLENOIDS MOTOR STARTERS ETC.

FIG. 6.5c

PLC architecture.

1\ irJPUT DE VICES

SWITCHES PUSHBUTTONS ETC.

724

PLCs and Other Logic Devices

components from most high-voltage line spikes. The power supply converts power line voltages to those required by the solid-state components. All PLC manufacturers provide the option to specify line voltage conditions. In addition, the power supply is rated for heat dissipation requirements for plant floor operation. This dissipation capability allows PLCs to have high-ambient-temperature specifications and represents an important difference between programmable controllers and personal computers for industrial applica­ tions. The power supply drives the I/O logic signals, the central processor unit, the memory unit, and some peripheral de­ vices. As I/O is expanded, some PLCs may require addi­ tional power supplies in order to maintain proper power levels. The additional power supplies may also be separate or part of the I/O structure. Input Systems

Inputs are defined as real-world signals giving the controller real-time status of process variables. These signals can be analog or digital, low or high frequency, maintained or momentary. Typically they are presented to the programma­ ble controller as a varying voltage, current, or resistance value.1'2 Signals from thermocouples (TCs) and resistance temperature detectors (RTDs) are common examples of ana­ log signals. Some flowmeters and strain gauges provide variable frequency signals, while pushbuttons, limit switches, or even electromechanical relay contacts are fa­ miliar examples of digital, contact closure type signals. One additional type of input signal, the register input, reflects the computer nature of the programmable controller. The register input is particularly useful when the process condition is represented by a collection of digital signals delivered to the PLC at the same time. A binary coded decimal (BCD) thumbwheel is a good example of an input device that is compatible with a register input port. If the thumbwheel represented three and one-half digits of process data, then all fourteen data output wires from the thumb­ wheel would provide their digital signal directly to the programmable controller register input unit, which would, in turn, signal condition and transfer the data to the central processor unit.

central processor unit. Higher point densities are possible, but their selection may involve a trade-off in wire size used, as well as in ease of wire harness installation to the module. One of the most important functions of the I/O is its ability to isolate real-world signals (0 to 120 V AC, 0 to 24 V DC, 4 to 20 mA, 0 to 10 V, and thermocouples) from

ELECTRICAL OPTICAL ISOLATION

FILTER

ELECTRICAL OPTICAL ISOLATION

FILTER

ELECTRICAL STOP —O 1 O-> OPTICAL ISOLATION

— FILTER

LS-I

LS-2

-> ->

START Λ

'Λ

- >

u 1

1

1 T

J

J

ELECTRICAL OPTICAL ISOLATION

ADDRESS L

1 t

J

1

FILTER

f\

J

BUS

L> L>

1

1

'

T1

'11

/

L

TO I/O BUS

COMMON HOT EXTERNAL 115 V AC SOURCE FIG. 6.5d

Typical input area; typical AC input unit.4 LIGHT SENSITIVE TRANSISTOR

LED

/

Outputs

There are three common categories of outputs: discrete, register, and analog. Discrete outputs can be pilot lights, solenoid valves, or annunciator windows (lamp box). Regis­ ter outputs can drive panel meters or displays; analog out­ puts can drive signals to variable speed drives or to I/P (current to air) converters and thus to control valves. Most I/O systems are modular in nature; that is, a system can be arranged by use of modules that contain multiples of I/O points. These modules can be composed of 1, 4, 8, or 16 points and plug into the existing bus structure. The bus structure is really a high-speed multiplexer that carries infor­ mation back and forth between the I/O modules and the

TO THE

\

PROCESSOR ϊ_^_ INPUT ^

SIGNAL OUTPUT

SIGNAL INPUT

LIGHT PATH

OPTICAL

0NOFF

INPUT

ISOLATOR FILTERING

FIG. 6.5e

Electrical optical isolator.4

ON OFF

'J—L OUTPUT

6.5 PLC—Programmable Logic Controllers ELECTRICAL OPTICAL ISOLATION

STORAGE

STORAGE

H

ELECTRICAL OPTICAL ISOLATION

POWER l / J £ \ DRIVES | \ j y

POWER I /JisN DRIVES

STORAGE

ELECTRICAL OPTICAL ISOLATION

POWER DRIVES

STORAGE

ELECTRICAL OPTICAL ISOLATION

POWER DRIVES

\ \ £ )

'ADDRESS LINES FROM MEMORY OUTPUT BUS

 FROM THE PROCESSOR

BLOWN FUSE INDICATOR |

CIRCUIT STATUS INDICATOR

VΔRάfOR *

n

725

time it takes for the programmable controller to interrogate the input devices, execute the application program, and provide updated signals to the output devices. Scan times can vary from 0.1 milliseconds per IK (1024) words of logic to more than 50 milliseconds per IK of logic. Although scan times are often given as performance measures, there are factors that make them misleading. Word size varies from 4 bits to 32 bits depending on the PLC model and manufac­ turer. Special features, which vary from preprogrammed drum times to full floating point mathematics, have different processing times and may generate longer scan times. Therefore, when selecting a programmable controller other performance factors must be considered. The user should take into account the application as well as the speed of the controller. Generally speaking, process applications need to take advantage of microprocessor power, whereas machine control applications are usually more concerned with pro­ gram execution speed. Memory Unit

COMMON HOT EXTERNAL 115 V AC SOURCE

FIG. 6.5f

Typical output area; typical AC output unit.' the low signal levels (typically 0 to 5 V DC MAX) in the I/O bus. This is accomplished by use of optical isolators, which trigger a process switch to transfer data in (input module) or out (output module) to a solenoid valve without violating bus integrity. Typical discrete I/O schematics are shown in Figures 6.5d, 6.5e, and 6.5f. Central Processor Unit (Real Time) The central processor unit (CPU), or central control unit (CCU), performs the tasks necessary to fulfill the PLC func­ tion. Among these tasks are scanning, I/O bus traffic control, program execution, peripheral and external device commu­ nications, special function or data handling execution (en­ hancements), and self-diagnostics. Central processing units can use TTL (transistor-transis­ tor logic), CORE (ferrite cores), or CMOS (complementary metal oxide silicon) technology or can be microprocessorbased (VLSI). Although TTL is faster (faster scan times), CORE requires no battery backup, CMOS is more compact and requires lower power levels, and microprocessor-based systems are both more powerful and more flexible. Gener­ ally, trade-offs must be made between speed and special features. It should be noted that the CPU and memory units are considered separate functions. One common way of rating how a PLC performs these tasks is its scan time. Scan time is roughly defined as the

The memory unit7 of the PLC serves several functions. It is the library where the application program is stored. It is also where the PLC's executive program is stored. An executive program functions as the operating system of the PLC. It is the program that interprets, manages, and executes the user's application program. Finally, the memory unit is the part of the programmable controller where process data from the input modules and control data for the output modules are temporarily stored as data tables. Typically, an image of these data tables is used by the CPU and, when appropriate, sent to the output modules. Memory can be volatile or nonvolatile. Volatile memory is erased if power is removed. Obviously, this is undesirable, and most units with volatile memory provide battery backup to ensure that there will be no loss of program in the event of a power outage. Nonvolatile memory does not change state on loss of power and is used in cases in which extended power outages or long transportation times to job site (after program entry) are anticipated. The basic programmable controller memory element is the word. A word is a collection of 4, 8, 16, or 32 bits that is used to transfer data about the programmable controller. As word length increases, more information can be stored in a memory location. Even with the ambiguity associated with word length, programmable controllers that provide the equivalent of 32K of 8-bit memory locations can execute application programs that are moderately complicated and interact with 50 to 100 discrete I/O points. More details about programmable controller memory and computer memory in general are available in Chapter 7. Programmer Units The programmer unit provides an interface between the PLC and the user during program development, start-up, and troubleshooting. The instructions to be performed during

726

PLCs and Other Logic Devices

each scan are coded and inserted into memory with the programmer. Programmers vary from small hand-held units the size of a large calculator to desktop stand-alone intelligent CRTbased units. These units come complete with documenta­ tion, reproduction, I/O status, and on-line and off-line pro­ gramming ability.3'4 Many PLC manufacturers now offer controller models that can use a personal computer as the programming tool. Under these circumstances, the manufac­ turer will sell a program for the personal computer that usually allows the computer to interface with a serial input module installed in the programmable controller. Programming units are the liaison between what the PLC understands (words) and what the engineer desires to occur during the control sequence. Some programmers have the ability to store programs on other media, including cassette tapes and floppy disks. Another desirable feature is auto­ matic documentation of the existing program. This is ac­ complished by a printer attached to the programmer. With off-line programming, the user can write a control program on the programming unit, then take the unit to the PLC in the field and load the memory with the new program, all without removing the PLC. Selection of these features depends on user requirements and budget. On-line programming allows cautious modification of the program while the PLC is controlling the process or the machine. Peripheral Devices

Peripheral devices are grouped into several categories: pro­ gramming aids, operational aids, I/O enhancements, and computer interface devices are the most common.5 Each category is described below. Programming aids provide documentation and program recording capabilities. Although some devices can program many models of different manufacturers' PLCs, most are dedicated to single suppliers and specific models. The defi­ nite trend in programming aids is PC-compatible software that allows the programmable controller to be emulated by the personal computer. This software is sold by the PLC manufacturer or a licensee and is often model-specific. If the software also offers on-line programming and troubleshoot­ ing characteristics it may in fact be used only on a single specific programmable controller. This isolation is achieved by means of software or hardware keys that come with each copy of the software purchased. Operational aids include a variety of resources that range from color graphics CRTs to equipment or support programs that can give the operator specific access to processor pa­ rameters. In this situation the operator is usually allowed to read and modify timer, counter, and loop parameters but not have access to the program itself. Some aids facilitate the interaction between the programmable controller and dumb terminals, such as printers, to deliver process information in a desired format. Some devices have the ability to set up an entire panel and plug into the PLC through external RS 232C ports, thereby saving enormous panel and wiring costs.

The I/O enhancement group is a large category of PLC peripheral equipment. It includes all types of modules, from dry contact modules to intelligent I/O to remote I/O capabili­ ties. Some I/O simulators used to develop and debug pro­ grams can be categorized in the I/O enhancement group. These specific devices are typically hardware modules that can be plugged into the PLC. The computer interface device group is a rapidly expand­ ing section of programmable controller peripheral devices. These devices allow peer-to-peer communications (i.e., one programmable controller connected directly to another), as well as network interaction with various computer systems. In fact, this group of devices will certainly expand in number as communication standards become commonly accepted and more and more products are provided to facilitate such network interactions.6 DIFFERENCES BETWEEN COMPUTERS AND PLCS

PLCs are often thought of as computers. To a certain extend this is true; however, there are four important differences between PLCs and computers. Real-Time Operation/Orientation

The PLC is designed to operate in a real-time control envi­ ronment. Most PLCs have internal clocks and "watch-dog timers" built into their operations to ensure that some func­ tional operation does not send the central processor into the ' 'weeds." The first priority of the CPU is to scan the I/O for status, to make sequential control decisions (as defined by the program), to implement those decisions, and to repeat this procedure all within the allotted scan time. Environmental Considerations

PLCs are designed to operate near the equipment they are meant to control. This means that they function in hot, humid, dirty, noisy, and dusty industrial environments. Typ­ ical PLCs can operate in temperatures as high as 140°F (60°C) and as low as 32°F (0°C), with tolerable relative humidities ranging from 0% to 95% noncondensing. In addition, they have electrical noise immunities comparable with those required in military specifications. Manufactur­ ers' experience shows mean time between failures (MTBF) ranging from 20,000 to 50,000 hours. Programming Languages and Techniques

PLC languages are designed to emulate the popular relay ladder diagram format. This format is read and understood worldwide by maintenance technicians as well as by engi­ neers. Unlike computer programming, PLC programming does not require extensive special training. Applications know-how is much more important. Although certain spe­ cial techniques are important to programming efficiency, they are easily learned. The major goal is the control pro­ gram performance. Another difference between computers and PLCs is the sequential operation of the PLC. Program

6.5 PLC—Programmable Logic Controllers ( START)

( START )

TASK 1

TASK 2

TASK 3

TASK 4

(A)

T

TASK 1

TASK 2

ΪI

^*

"T" TASK 3

TASK 4

Y

T

(B)

F/fc &5£ PLC versus computer. Program structure for a programmable controller (A) requires sequential execution with a scan, starting with task 1 and proceeding through task 4. The program structure for a general-purpose computer (B) permits task execution in any order.

727

The danger involved in using the desired results and the budget as the primary selection criteria is that at the begin­ ning of a project the understanding of the problem is limited. If the user's focus on the problem is too narrow, the solution might solve only the perceived symptoms of the problem while the problem itself may merely reappear in an alternate form at some other point in the process. If the budget assigned to solve a problem is based only on the initial investment required for the programmable con­ troller hardware, this will underestimate the total cost by neglecting costs associated with labor, maintenance, and downtime. The effect of this oversight usually fosters a "penny wise but pound foolish" philosophy and results in the selection of a controller that initially costs less but does not quite get the job done. Purchasing a larger and therefore more expensive pro­ grammable controller might in some situations be costeffective if labor, maintenance, and downtime costs are also considered. Similarly, while programmable controllers are more expensive than individual solid-state control units, if the indirect costs are also considered then the PLC becomes a cost-competitive alternative. Versatility

operations are performed by the PLC in the order they were programmed (Figure 6.5g). This is an extremely useful fea­ ture that allows easy programming of shift registers, ring counters, drum timers, and other useful indexing techniques for real-time control applications. Maintenance and Troubleshooting

As a plant floor controller, the PLC must be maintainable by the plant electrician or the instrument technician. It would be highly impractical to require computer-type maintenance service. To this end, PLC manufacturers build in self-diag­ nostics to allow for easy troubleshooting and repair of prob­ lems. Most PLC components are modular and simple to isolate; remove-and-replace (system modules) diagnostic techniques are usually implemented. JUSTIFICATION FOR THE USE OF PLCS

The only consideration of importance in any control system is to get the process variable under control effectively and reliably. When control options are available, several factors can be taken into consideration in making an implementa­ tion decision. Some of these factors include the controller's cost, versatility, flexibility, and expandability. Cost

The ideas associated with cost are discussed first because this is usually the first issue which users consider when selecting a control technology. In most cases, the two things initially known about a problem are the results desired and the budget available for fixing the problem.

The multifunction capability of a PLC allows control logic decision making, a versatility rarely possible with other systems. The ability to combine discrete and analog logic is a powerful tool for the controls engineer. This is especially evident in the control of batch processes. Entire start-up and shutdown sequences can be performed by the sequencer logic, and analog logic can be brought in during the batch run. Control of critical start-up parameters, such as tempera­ ture and pressure, can be precisely preprogrammed for each start-up step. Temperature stepping is easily programmed, as are the feedforward calculations that are used in some polymer reactors. All of these types of PLC applications are currently in use today and are well documented.8-11 Flexibility

As a process goes on-line and is refined, the control equip­ ment should be easily reconfigured to accommodate such modifications. The multifunction use of the PLC has already been discussed. In addition, digital blending applications, boiler control of either carbon monoxide or excess oxygen, and some other forms of optimizing control are also within the capabilities of PLCs. Because one common device per­ forms multiple functions in a plant, fewer spare parts are needed, and the programming language is technicianfriendly. In addition, the digital nature and self-diagnostic capabilities are strong additional justification for the PLC. Expandability

As a process matures, it is inevitable that enhancements will be added. These usually require more inputs and outputs. For hand-wired relay systems this usually necessitates ex-

728

PLCs and Other Logic Devices Summary

tensive panel changes, which generally are problematic. A PLC easily accommodates the additional I/O without re­ quiring changes in the existing wiring: the new points are merely placed in the system. If a PID loop or two is being added, no panel rework is necessary; only the wiring of the new points and some reprogramming to incorporate them is required. Of course, if the initially selected PLC is "tight," additional I/O bases might be necessary. For this reason, most manufacturers recommend sizing the system to allow for 10 to 20% expansion. Another advantage of the PLC is that it allows piecemeal implementation of projects. Systems can be brought up on­ line quickly and can be gradually converted over to the PLC while on-line. The ability of the PLC to be reprogrammed while operating permits automation of processes that are too expensive to shut down. This technique is valuable to new as well as to retrofit projects (revamps).1213

Table 6.5h together with Figures 6.5i and 6.5j provide good illustrative summaries of the characteristics of programma­ ble controllers as discussed thus far. Table 6.5h illustrates an I/O list for a milling application, while Figure 6.5i shows in some detail the programmable controller and all its key parts. The CPU is shown in the center of the figure as the PLC processor. Power for this unit is delivered by the power supply shown to its right. The programming unit connected directly to it on the left is the way a ladder logic program line, like the one shown just above the CPU, is entered into the controller. Representations of two I/O modules are shown in Figure 6.5j. The input module is on the left of the drawing and indicates a variety of contacts directly attached to seven of the eight points on the module. The point at address zero is shown as a spare for use as a replacement or future enhance-

TABLE 6.5h

Example of a Detailed I/O List for a Milling Machine Input

Used in Rung(s)*

Definition

XO

MOA

Automatic

/ -46

40 -47

3 3 7

-3 -3 -22

9 9

3 6 1 11 12

8 -52

-39

3 3 6 5 16

15 15

-19 -21

3 2 48 -48 -40 -48 49 -52 -52 -52

XI X2 X3

Rt. Shot Left Shot Machine

Pin In Pin In Slide Adv

L.S. 1 L.S. 2 L.S. 3

X4 X5 X6 X7 X8

Machine Shot Pins Pump Rt. Bore Left Bore

Slide Ret Out #1 Slide Ret Slide Ret

L.S. L.S. 3M L.S. L.S.

X9 X10 Xll X12 X13

Rt. Bore Left Bore Swing Pull Swing

Slide Adv Slide Adv Clamp On Clamp On Clamp Off

L.S. 9 L.S. 10 P.S. 1 P.S. 2 P.S. 3

X14

Cycle

Start

X15 X16 X17 X18

Cycle Auto Lube Complete Hi Lube

Stop On/Off Pin Level

2 -45 1 40 -40 37

Lo Lube Manual Clamp Shot Pins Clamp

Level Light On/Off In/Out On/Off

-40 -1 49 50 51

X19 X20 X21 X22 X23

Lube

Pull Swing

4 5&6 7 8

*Rung is a grouping of PLC instructions which controls one output or storage diagram.

40 48

43

44

45

-17

-36

-36

-37

50

51

52

46 47

This is represented as one section of a logic ladder

6.5 PLC—Programmable Logic Controllers

729

IF LS15 AND PB4 ARE BOTH CLOSED, SOL 3 IS ENERGIZED THROUGH THE PLC INSTRUCTIONS ADDRESSED TO: -LS15 PB4 SOL 3«

|

T

12102

+-] [

T

02304

] C

()—+

v

N PLC PROGRAM PANEL

1

12105

I

'

PLC (MAIN) POWER SUPPLY

PLC PROCESSOR

L2[T]

SOL 3

Q ] USER-SUPPLIED I/O POWER

FIG. 6.51

Configuration drawing. (Courtesy of Allen-Bradley)

ment site. The output module shown has all eight of its output addresses (from address 140 through 147) in use. Figure 6.5i shows an example of a remote I/O rack. Although the rack shown (rack No. 2) is powered by the main power supply, that is not a requirement. A remote I/O rack functions the same way as its direct I/O counterpart

does. Various modules can be inserted in the rack to match the application's control needs. Often manufacturers provide a remote I/O distribution panel or module to serve the efficient multiplexing of the modules on the remote I/O rack back to the CPU. Most medium-sized PLCs can support several remote racks,

730

PLCs and Other Logic Devices OUTPUTS

INPUTS

10

2 2LT AUTO CYCLE

O 2SS

1 ° 1 SPARE

140

75

- &

CYCLE

1LT MANUAL CYCLE

MANUAL OFF AUTO

OA CH

8PB CYCLE START

ΟΛ

5PS CLOSED WHEN CHUCK CLAMPED O

f

CH

141

&

/ 142

77

I T

4 PS CLOSED WHEN CHUCK ADVANCED

10

CHUCK CLAMPED 10 LT

Φ

143

-Hr

78

21 SOL CLAMP CHUCK 144

79

-o-^r-oCLAMP AND JACK RETRACTED ,_ ,

145

80

T

- & / \

13 LT JACK ADVANCED 19

I



2 0 SOL UNCLAMP CHUCK

CHUCK CLAMP , UNCLAMP

CM

w

76

—"4^-

20

CH CH

Φ-

1 146

11 LT CLAMP ADVANCED 147

82

—U— /"-"\

FIG. 6.5J

Point-to-point wiring diagram.' which in turn contain 4, 8, or 16 I/O modules. It may or may not be possible to mix various modules in a remote rack. Specific information about module compatibility and remote I/O multiplexing is available from the manufacturer. This information is required to facilitate PLC selection and sizing for specific applications. PROJECT EXECUTION

The PLC project must take into account the important con­ siderations of schedule and budget as is true of any major undertaking. The PLC can facilitate the transition, however, by simultaneously pursuing several activities, thereby con­ densing the overall project schedule. A review of each major activity is presented in the following paragraphs. Systems Analysis

The control system should be analyzed as a whole to deter­ mine plant control requirements. The PLC plays an integral part in these analyses, and its capabilities should be thor­ oughly understood by the controls engineer. Vital to systems analysis are the process and instrument diagram (P&ID), the descriptive operational sequence, and a logic diagram or electrical schematic. Part of this evaluation will be system sizing and selection. Once the appropriate PLC is selected and purchase orders are placed, two activities should begin immediately: engineering design and software development.

Engineering Design

The first step is development of the I/O list (Table 6.5h). This detailed document will be used extensively and should be developed with great care. (Once I/O numbers are as­ signed, it becomes very difficult to change all references to these numbers.) The I/O list is followed by the configuration drawing. The configuration drawing (Figure 6.5i) shows the ar­ rangement of the I/O and support hardware. The point-topoint wiring diagram (Figure 6.5j) will be used by the panel shop and the installation contractor to make the I/O device interconnect. Panel, or enclosure, design should now be coordinated with the addition of panel instrumentation, such as light switches, meters, and recorders. Once these steps are com­ pleted, panel fabrication and assembly can begin. Software (Program) Development

The I/O list mentioned previously will be used to begin the program development. Basic control philosophy decisions need to be made at this point. Should valves fail open or closed? What fail-safe provisions are necessary for analog control? These philosophical decisions should be docu­ mented and included with the process operational descrip­ tions.15 Quite often this document will be referred to as the software functional specification. Its purpose is to define, as

6.5 PLC—Programmable Logic Controllers precisely as possible, the operation of the controls.16 It also has several other functions: 1. It communicates the functional requirements of the con­ trol system to those writing the PLC code. 2. It records the thought process (regarding control) of the system designer to be used in the event of a personnel change. Such information can be invaluable.17 3. It provides a review document for personnel working in other capacities (mechanical, process, and project man­ agement) to ensure that they understand the operation of the controls. 4. It provides a guide for developing the operational de­ scription for the operator's manual. After the functional specification has been reviewed and approved, a detailed operational sequence chart, timing dia­

731

gram, logic diagram, flow chart, or electrical schematic is developed from it. This schematic is translated or coded into the appropriate PLC language, cross-referencing I/O with PLC designator tags. The piping and instrument diagram is also cross-referenced with PLC designators. In this way, future cross-referencing of system drawings and PLC codes is facilitated. As the code is entered, a memory map or register index is kept by the programmer (Table 6.5k). This map is useful for organizing program data in logical arrangements and will prove invaluable during start-up, when the programmer may need to locate available blocks of memory quickly for program revisions. Once the program is entered, a simulation is recom­ mended, and the program checkout process is begun "on the bench." This process uses the functional specification to

TABLE 6.5k Example • of Memory Map for Milling Machine (Source Xcel) Used in Rung(s):

Definition Coil

CRO CR1 CR2 CR3 CR4 CR5

Cycle Shot Machine

R.H. Head L.H. Head Clamp Part Pins Slide Adv

Retract Retract Part Unclamp Out Milling

CR6 CR7 CR8

Machine Milling Start

Slide Spindles Boring

Retract Off Spindles

CR9 CR11

Unclamp

R.H. Bore Swing

Complete Clamps

Pull Hold Complete Pump # 1 Pull Retract Swing L.H. Bore Pump Time Off Auto Lube

Clamps Light On S.Pins In Pressure Clamp Shot Pins Clamps Complete Start Pump #1 Cycle

CR12 CR13 CR15 CR17 CR18 CR19 CR20 CR29 CR31 CR32 CR101

Unclamp Milling Unclamp Unclamp Press. Off Pressure

Lube Off

CR102 CR103 CR104 CR105 CR106

60 Min. Contin.

Lube Start/ Lube

Lube On Restart Disable Restart

13 14 4 2 5 6 30 7 9 10 31 11 15 -30 -3 3 8 25 26 27 28 12 17 18 36 -43 41 -48 -41 43 -40 40

15 15 5 2 5 6 32 7 9 10 33 11 15 -31 16 3 8 26 49 50 51 12 17 -52 36 48 42

19 21 -34 25 6 7 35 8 10 11 38 13 16 -31 16 4 9 28

-20 -33

-33 -38

-46 -47

25 27 22

-28 24

29

-22 -24 12

39 -24 20

-32 23

13 -23

46 28

-29

17 4 -27 52

26 -26 -39

14 18

14 18

47

40

40

41

41

42

-43

-44

45

-41 44 45 44

42 45

34

732

PLCs and Other Logic Devices

prove that the software is acceptable. A large percentage of the program can be proved in this manner. Program de­ bugging can be completed before field installation. Field corrections will therefore be minimized, and high-salaried electrical and installation personnel will not be standing around waiting. The savings that can be realized are quite large.18 There is no substitute for a bench-simulated program check. The software simulation proves the program and allows acceptance by the customer. The program should be reproduced and documented after it has been checked. Software/Hardware Integration

Using the PLC and programming aids, the panel wiring can be "rung out" (that is, checked point by point) in through the PLC. Each I/O point should be activated separately to the terminal block or from the panel controls (buttons, lights, switches). In this manner, the electrical integrity of the panel from the terminal blocks inward is ensured. If any continuity problems exist hereafter, they will be located in the field wiring. Some organizations prefer to perform a simulated opera­ tion checkout at this time. This is a highly useful approach and can be implemented if the simulators and the I/O point arrangement are organized to simulate the process outside the panel. Occasionally, operations personnel are brought in to demonstrate this process. This is an effective way for the operations personnel to become familiar with the new sys­ tem, especially in new automation projects, in which there may be some "fear of the unknown" to overcome. Occa­ sionally, operations personnel find operational flaws or make suggestions for improvements that can be easily im­ plemented in the panel shop but would be difficult or impos­ sible in the field. At the conclusion of this exercise the panel is accepted by the customer and is shipped to the job site.

After Start-up

Once again, it is imperative for future successful plant oper­ ation that complete, current documentation be available. This documentation should include the items discussed pre­ viously. Especially useful for future changes or additions are the start-up notes and notes pertaining to future modifica­ tions. Training of operations, maintenance, and engineering personnel should be timely and "hands-on." It is useful to videotape these training sessions for future reference (e.g., for training of new personnel). Suggested programs for PLC training can be obtained from the PLC vendor. This is an important function provided locally at the job site, at a nearby metropolitan area, or at the PLC vendor factory. PROGRAMMING FUNDAMENTALS

The use and understanding of PLC programming depends on knowledge of the process to be controlled, an understanding of electrical schematics, and an appreciation for logic opera­ tions and for various types of logic and relay devices. Other sections of Chapter 6 provide information on each of these topics. A review of those topics might be useful at this point. One popular programming technique involves defining the sequential logic in electrical schematic format, using actual tag numbers, and then translating this diagram into the appropriate programming language. Figure 6.51 shows the translation of some examples of typical circuits to ladder diagrams, Boolean algebra, and mnemonics. Because this translation is relatively simple, maintenance and engineer­ ing personnel have accepted programmable controllers, al-

RELAY LADDER DIAGRAM 1PB I

System Checkout and Start-up

After electrical interconnections are made and point-to-point wiring is completed (mechanical completion), the system is ready for start-up. The ability of the PLC to operate step by step through the start-up becomes very useful at this stage. Experienced PLC personnel may provide temporary STOP, CONTINUE, and STEP switches in the back of the panel in order to facilitate the start-up procedures. These switches can be key-locked, software-locked, or discon­ nected for normal operation. They are also very useful as future maintenance and troubleshooting tools to diagnose future problems as being either hardware- or softwarebased. Unanticipated circumstances always are a factor during start-up. For this reason it is not uncommon to have pro­ gramming personnel available at this time for implementa­ tion of any program changes that might be necessary. Quite often, these changes can be accomplished over long-distance telephone lines using modems. These changes are not easily implemented without adequate documentation. After suc­ cessful start-up, the plant is signed off.

2CR

4CR

5CR

HI

^

S0 L

A

O

3LS

^ ^ FREE FORMAT EQUIVALENT PLC DIAGRAM 1PB

II

2CR

4CR

II—|—II

5CR

S 0 L_ A

ά—O-

3LS

BOOLEAN STATEMENT ( ( 1 P B » 2 C R ) + 3 L S ) · 4 C R · 5CR = SOL A CODE OR MNEMONIC LANGUAGE LOAD 1PB

AND OR AND

NAND STORE

2CR 3LS 4CR 5CR

SOL A

FIG. 6.51

Ladder translation. Here is a comparison of programming languages that are used with various programmable controllers. The most popular is still the relay ladder diagram because plant personnel are more familiar with it.19

6.5 PLC—Programmable Logic Controllers though they have not accepted computers as readily. The unknown has been replaced with the familiar. Although the programming style and language used is, to some extent, dictated by the size of the PLC used, there are fundamental programming elements besides logic opera­ tions—e.g., timers, counters, and arithmetic capabilities— that are provided in all models. Some of the characteristics of these important elements are discussed below.

733

within the PLC. Obviously, the scan rate of the PLC (scans per second) must be twice the pulse rate of the turbine (pulses per second). Arithmetic Capabilities Figure 6.5o shows an arithmetic program that permits the rapid addition of pulse counts from two counters hooked to two electric meters. The resulting sum is displayed through a

Timing and Counting Figure 6.5m is a schematic representation of a timer and a counter. Although their formats differ, the principles are the same. The legs of the timer represent start/stop and reset. Timers can be on-delay or off-delay and can be cascaded (that is, linked together in series). Counters can be up or down and have a count leg (in which the number of switch closures is the count) and an up/down leg (in which the position of the switch determines up count or down count). A PLC with arithmetic capability can use a combination timer and counter as an integrator. Figure 6.5n shows a turbine meter pulse counter turned into a low-cost integrator

RUNG I0 (0)52

LEFT MILL SPINDLE DV5B Y6:24

-HI

RT. MILL SPINDLE DVI5A YI5:32

MILLING SPINDLES OFF CR7;9

—II—

START BORING SPINDLES CR8

<> I0 12 23 33

START BORING SPINDLES CR8:I0

RT. BORE SLIDE RET L.S. 7 X7

RUNG 11 (9)57

START BORING SPINDLES CR8;I0

R.H. BORE COMPLETE CR9

HI

<>

11 13

R.H. BORE COMPLETE CR9:11 HORIZONTAL PRESET CURRENT h-OUTPUT TIME

START/, PRESET STOP

■OUTPUT 1

R E S E T - | C UT ! M E E N T| - 0 U T2 P UT

RESET

LEFT BORE SLIDE RET L.S. 8 X8

RUNG 12 (0)61

lh

START/STOP

CURRENT

PRESET COUNT

^

RESET COUNT SIGNAL

13 46

VERTICAL

TIMER:

COUNTER:

11 20 31 38

OUTPUT

Γ

RUNG

Timer/counter schematic. A key part of any PLC programming is its capability to do timer/counter functions. The instructions are entered either horizontally or vertically, depending on the make. Horizontal programming, however, is more commonly used.19

METER COUNT METER 1 PULSES

COUNT METER 2 PULSES

CR1

13 (q)65

-O 12 14

RUNG 14 (0)71

-o-

<>

TMR

15 -20 -46

2

19 -33

2

L.H. HEAD RETRACT CR1

L.H. BORE COMPLETE CR29-. 12

<>

TMR L.H. BORE COMPLETE C R 2 9 : 12

14 47

R.H. HEAD RETRACT CRO

R.H. BORE COMPLETE CR9:11

R.H. BORE COMPLETE CR9 :11

15 -33 -47

2

21 -38

2

CR2

RUNG 15 (0)77 TOTALIZE PULSES

-HI

L.H. BORE COMPLETE CR29

L.H. BORE COMPLETE CR29:I2

COUNT -J PRESET LOUTPUT SIGNAL" 1 CURRENTl RESET-J COUNT LOUTPUT 2

FIG. 6.5m

START BORING SPINDLES CR8:10

CR3

L.H. HEAD RETRACT CR1:I4

UNCLAMP SWING CLAMPS CR11: 15

-o-

RT. BORE SLIDE ADV L.S. 9 X9

LEFT BORE SLIDE ADV R.H. HEAD L.S. I0 RETRACT XIO CR0:I3

UNCLAMP SWING CLAMPS CR11

M—(> 15 -23 -29 -3I

FIG. 6.5n

FIG. 6.50

Pulse counter totalizer.

Sample ladder diagram—annotated. (Courtesy of Xoel)

16 28 -30 -3I

734

PLCs and Other Logic Devices

panel meter. This logical addition is performed using integer mathematics (that is, no decimal calculations can be per­ formed). Most PLCs use an approximation technique called "double-precision integer mathematics" to do calculations of more complexity (such as PID). Some PLCs have true floating point mathematics capability. Floating point mathematics is a powerful tool for process applications. For example, in the integrator example in Figure 6.5p the division of pulses by elapsed time can be expressed as a decimal number rather than as a truncated integer. Feedforward calculations and PID can be performed in double-precision integer mathematics but are more mem­ ory-intense than in floating point mathematics. Floating point mathematics may require the use of a separate micro­ processor within the CPU and usually involves two adjacent memory locations to store the mantissa and the abscissa in a form of scientific notation. The programmer automatically translates when the memory location is followed by a period (.), indicating a floating point number. Programming Documentation The following technique can be used for PLC programming applications. We have found many advantages to this ap­ proach. 1.

2.

3. 4.

5. 6. 7.

8.

Develop detailed I/O lists. Table 6.5h shows an I/O cross-reference relating tag numbers to I/O points. This list should be used extensively; starting without it will cause confusion and errors resulting from inevitable changes. Develop a detailed descriptive operational sequence of events. Figure 6.5q shows a sample sequence using a process batch application. Develop electrical schematics or ladder diagrams for sequential control. Develop piping and instrumentation drawings (or a logic diagram) for process control. Figure 6.5r shows a diagram that will allow maintenance personnel to find important activities quickly. Note the I/O matrix index, showing what happens inside the PLC software, and the cross-referenced I/O memory locations and tag num­ bers. Translate the drawings in steps 3 and 4 to the program­ mable controller language. Enter the program code using a memory map (see Table 6.5k). Debug the program at the programmer's facility. Use a simulator to debug the program. Run through the opera­ tional sequence defined in step 2. If it has changed, be sure to ascertain how that change has affected other parts of the program. Rewrite the sequence description to reflect current operations, if necessary. Save and document the program. Reproduce the pro­ gram on transportable media, such as cassettes or floppy disks. (Do this daily.) Document programming changes

COUNT TURBINE METER PULSES

TIME PULSE INTEGRATING PERIOD

LATCH ON CALCULATION REQUEST

CRIO

YIO

-O CR11

<>

CALCULATE GALLONS/MIN

SPECIAL FUNCTION USER MATH START ADDRESS : V 4 0 0 NEXT ADDRESS : V 4 0 6 ERROR OUTPUT ? ERROR OUTPUT DESIGNATOR :

N

ENTRY FORMAT: (MATH T E R M : OPERATOR, MATH TERM : OPERATOR, . . . = RESULT)

V20I

V202

C200

NO. PULSES

TIME PERIOD

CONST.

=V205 GAL/MIN

FIG. 6.5p

Floating point integrator.

using peripherals, such as CRT programmers or tape loaders. These devices can be purchased or rented. 9. Enter and debug the program in the field. It is essential to note all changes made on the documentation. One of the biggest problems with relay systems is undocu­ mented field modifications. 10. Redocument and reproduce the final program.14'21 The final documentation package should include the follow­ ing: I/O list and cross-reference, descriptive operational sequence, electrical schematics, process schematics, pro­ gram listing (see Figure 6.5o for annotated final document), memory maps (showing the memory areas that have been used and those that are available), and notes for future program changes or additions. One important word about the documentation package: A major advantage of the programmable controller is its ability to be reprogrammed as plant requirements change. Without proper documentation, previous programming efforts will have to be reproduced in order to make the changes that are required. Poor documentation results in wasteful efforts at reconstruction. The programmable controller, like all engi­ neering tools, requires good control systems engineering practices in order for its full potential to be realized. Good documentation is an essential element of any PLC project.22

6.5 PLC—Programmable Logic Controllers

735

OUTPUT TABLE Outputs

Step

Description

Home

Start

Auto select and start

1

Fill

Tank full (Level switch 1)

2

Heat

Temperature setpoint or timer

3

Cool

Timer

4

Agitate

Timer

5

Empty

Tank empty (Level switch 2)

6

End

Automatic reset

LEVEL SWITCH 1 LEVEL SWITCH 3 LEVEL SWITCH 4 M

TEMPERATURE SENSOR CO) LEVEL SWITCH 2

Input conditions to advance

AUTO

START MANUAL

AGITATOR

φ Q

CM φ t-

CO

«CO

(0 CO

«J CO

>o

>

X

g-co

>

001 002 003 004 005 006 007 008 009

Description

Auto select switch Start pushbutton Level switch 1 Temperature sensor Level switch 2 Valve 1 open limit switch Valve 2 open limit switch Valve 3 open limit switch Valve 4 open limit switch

Input no.

010 011 012 013 014 015

X

X

is

CO CO

X

X

X

X

X

I T3

Conditional interlocks:

C

Vo CM yφ φ

(0 CO

X = Output energized

£8

X

Description

Recipe 1 select Recipe 2 select Recipe 3 select Level switch 3 Level switch 4 Manual step pushbutton

Φ

>

INPUT TABLE Input no.

is

>>

I

5*

o Έ. c E Φ

Φ

25 CO

EB

C

Φ w. O

1-

?

S'CM

&« O V)

fid

B)

C)

FIG. 6.5q

Batch sequence. A. Graphic representation of a simple batch input interlocks fbottomj, which affect certain outputs. C. The process reactor vessel, with inputs and outputs. B. Outputs for ' 'Input Table'' correlates the connection terminal number with each step in the process are described in the ' Output Table.''the word description of each input.9 The advance conditions for each step are shown along with

HARDWARE AND SYSTEM SIZING AND SELECTION

Despite the variety of available PLC models, system sizing is relatively simple. Hardware and system size can be deter­ mined by an analysis of the following system characteristics: 1. 2. 3. 4. 5. 6.

I/O quantity and type I/O remoting requirements Memory quantity and type Programming requirements Programmers Peripheral requirements

Although sizing is generally straightforward, selection of the right PLC requires considerable judgment regarding trade-offs between future requirements and present cost. 1/0 Quantity and Type

In most programmable controllers, plug-in modules are used to convert the I/O signal level to one that is compatible with the bus architecture. These modules can be composed of 1, 4, 8, or 16 points, depending on the manufacturer's standard design. For small projects (20 to 256 I/O), I/O requirements are usually easy to define and group. A systematic approach

is required for medium-sized projects (256 to 1024 I/O), however, in order to avoid confusion of I/O allocation. Obviously, the organization of I/O for large systems (1024 I/O and above) requires careful planning. The I/O base (rack or housing) is used to hold the I/O module in place and to provide a termination point for the wiring. The bases may be mounted anywhere in the control enclosure; however, there are cable length requirements that must be met. The majority of bases mount horizontally to allow proper module cooling. A terminal strip is built into the mounting base for field connections so that no wiring need be disturbed in order to remove or replace a module. These bases typically hold various quantities of I/O— anywhere from 1 to 128 I/O points. Whereas in most sys­ tems the module has the intelligence to communicate with the CPU, some systems require the use of serial interface modules. In any case, some provision is made to accept register input data from the input modules and to send these data (on or off status of field device) in serial format to the PLC processor. Serial data are also converted into register data to be sent to the output module. Input modules are typically transistor-triggered and have built-in time delays to protect against contact bounce. The

736

PLCs and Other Logic Devices HS-IOI-2 IN STOP I POSITION v

HS-IOI-2 ' ΊΓ IN RUN I L POSITION

s

LEVEL ΓΤΟΟΤ GREATER ά 2 2 1 THAN 6ft.l· (1.8 m) LSL-103-2

d

N

HS-IOI-2 IN AUTO POSITION

E>

LEVEL [1002] GREATER THAN 3 ft.1 ~~~

(0.9m)

CD

v

LSLL-I03-I

V

^

HS-IOI-I N0.1 PUMP IN MANUAL POSITION [JΦΦe]

HS-IOI-I

h

r=

J

LEVEL GREATER Π003" THANI2ft.|lόό3 (3.6m) ' ALARM LSHH-I03-4

J__ CL)/UAHH\

Discrete I/O modules come equipped with an LED indi­ cator light to indicate the status of the module (on or off) for troubleshooting. The input module LED indicates field side status of the pushbutton, and the output module LED indi­ cates logic side status. Built-in fusing on output modules is becoming standard in the industry and provides good protection for overload conditions. The type of fuse depends on the module, and the way in which fuses are accessed varies from one PLC manufacturer to another. Field connection is made to the I/O base by way of a built-in terminal block. Some PLC vendors provide wireway as part of the rack structure to "clean up" the panel design. Each new generation of I/O becomes more dense, so these newer systems become progressively harder to wire. Interposing terminal blocks between the field connections and the rack may become standard in the future. Standard cabling is provided for connection from the base to the PLC processor. These cables are generally multi-bit data I/O con­ nector/cable assemblies to simplify installation.

ioo-iy

Analog I/O Systems The analog I/O system is designed to interface with analog field devices such as flowmeters, pres­ sure transmitters, and valve positioners. This system accepts SYMBOLS DEFINED FOR PROGRAMMABLE CONTROLLER LOGIC DIAGRAM inputs of 0 to 5 V DC or 4 to 20 mA. The output analog | I 0 0 4 | , |ΐΟΟβ | ETC. INDICATES PC INPUT module can output a signal of 4 to 20 mA current loop up to CD» C D ' CD» C D ETC · INDICATES PC OUTPUT 750 ohms as well as 0 to 10 V DC. Some of the newer I/O modules include direct thermocouple wiring and RTDs. On­ INDICATES FIELD WIRED OR GATE board cold junction compensation is often included with thermocouple input modules, and many different types of 4 0 | — I N D I C A T E S "AND" GATE IN PC LINE 4 0 thermocouples can be accommodated. These modules are sometimes referred to as A/D (analog THIS LOGIC DIAGRAM IS FOR SUMP PUMP to digital) and D/A (digital to analog) modules. They pro­ vide optical isolation for electrical noise protection and are FIG. 6.5r Process and logic diagrams. This logic diagram is for a sump typically arranged in a quad module or an eight-point mod­ 20 ule. pump. L

Γ—· L ^ R ^ P >

? \ y

C

input signal from a field device (limit switch) has to be energized for some amount of time in order for the module to notify the processor of a true "on" condition. The discrete output module uses a solid-state switch (triac) to power a field device, such as a motor starter, a valve, or lights. Outputs are available for voltage ranges of 5 to 240 V at currents up to 5 A, with typical 120 V outputs operating at 2 A maximum. Solid-state drivers of this type are not intended to drive large loads directly (e.g., a large motor starter). Highly inductive loads or those with a high surge current may also require an interposing dry contact relay in order to power the field device. Manufacturers rate their equipment output current at dif­ ferent temperatures. All current ratings for solid-state out­ puts will vary with ambient temperatures. Therefore, one should be sure to check the PLC output rating for each application and manufacturer.

Register 1/0 Systems The register I/O system provides direct interface to multi-bit data field devices, such as thumbwheel switches, position encoders, and digital read­ outs to the PLC. These devices are typically TTL level, which allows interface to other types of electronic hardware as well. Intelligent I/O and other special-purpose I/O re­ quirements are becoming increasingly common. The user should arrange any special I/O types as well as the commonly available modules according to an I/O matrix by logical area, as shown in Table 6.5s. In this table, I/O types are listed across the first row and plant areas are listed down the first column. In this way, one can accurately reconstruct the decision-making process concerning I/O quantity and type. It is important to include at least 10 to 20% spare rack space in all I/O considerations. For example, consider a typical process application. As­ sume a total I/O count of 764, broken down into 436 inputs and 328 outputs. This application falls into the medium PLC

6.5 PLC—Programmable Logic Controllers

12Π

TABLE 6.5s I/O

Matrix Model No.2

Plant Area

Process area1

Analog

In

24

4-20 mA

Tank farm #1 3

0

Tank farm #2 3

0

Qty

Model No.2

Discrete In

Model No.2

Qty

Voltage

Qty

Analog Out

230 24VDC 62 120 VAC 52 120 VAC 68 120 VAC

Loading station

Discrete Out Voltage

Model No.

Qty

136

16

10

24VDC 36 120 VAC 34 120 VAC 24 120 VAC 98 240 VAC

10

14

Notes: (1) Discrete in 16 pts/card Discrete out 16 pts/card Analog in 16 pts/card Analog out (2) Add model numbers after award of contract. (3) DI and DO are 8 points per card

category. Since the majority of the field devices are located a good distance from the CPU, a PLC with remote I/O is desirable. The I/O requirements by locations are as follows: Process Area: Total of 390 in and outputs (I/O), of which 230 are 24 Volts (discrete DC inputs), 24 are 4 to 20 mA analog inputs, and 136 are 24 V DC discrete outputs Tank Farm # 1 : Total of 98 I/O, of which 62 are 120 V AC discrete inputs and 36 are 120 V AC discrete outputs Tank Farm #2: Total of 86 I/O, of which 52 are 120 V AC discrete inputs and 34 are 120 V AC discrete outputs Loading Station: Total of 190 I/O, of which 68 are 120 V AC discrete inputs and 122 are discrete outputs (of which 24 are 120 V AC and 98 are 240 V AC) Let us assume that the PLC system being considered has the following features: (1) no constraints on input and output mixture; (2) the I/O modules are available in two formats, 16 points per module and 8 points per module; and (3) 10% spare I/O is required. For the sake of illustration, we will use the 16 point per module I/O structure in the process area and the 8 point per module structure in the tank farms and loading station. The I/O distribution (including spares) per location would now be as follows: Process Area: Total of 390 I/O points, which require the following modules: • 230 + 10% = 253 24 V DC inputs points with 16 points/ module = 15.8 modules; use 16 modules • 24 4- 10% = 26.4 4 to 20 mA analog inputs at 16 points per module = 1.6; use 2 modules

•

136 + 10% = 149.6 24 V DC discrete outputs at 16 points per module = 9.35; use 10 modules

Tank Farm # 1 : Total of 98 I/O points, which require the following modules: • 62 + 10% = 68.2 are 120 V AC discrete inputs at 8 points per module = 8.5; use 9 modules • 36 + 10% = 39.6 are 120 V AC discrete outputs at 8 points per module = 4.9; use 5 modules Tank Farm #2: Total of 86 I/O points, which require the following modules: • 52 + 10% = 57.2 are 120 V AC discrete inputs at 8 points per module = 7.2; use 8 modules • 34 + 10% - 37.4 are 120 V AC discrete outputs at 8 points per module = 4.7; use 5 modules Loading Station: Total of 190 I/O points, which require the following modules: • 68 + 10% = 74.8 are 120 V AC discrete inputs at 8 points per module = 9.4; use 10 modules • 24 + 10% = 26.4 are 120 V AC discrete outputs at 8 points per module = 3.3; use 4 modules • 98 + 10% = 107.8 are 240 V AC discrete outputs at 8 points per module = 13.5; use 14 modules If we assume that one remote communication channel can service up to 128 I/O in groups of sixteen 8-point modules or eight 16-point modules, the system becomes: • Process Area—390 I/O; 28 16-point modules; 4 remote channels

738

PLCs and Other Logic Devices

• Tank Farm #1—98 I/O; 14 8-point modules; 1 remote channel • Tank Farm #2—86 I/O; 13 8-point modules; 1 remote channel • Loading Station—190 I/O; 28 8-point modules; 2 remote channels I/O demoting Requirements

A unique feature of the PLC is the multiplexed nature of the I/O bus. This can be used to great advantage to reduce overall wiring cost. If I/O racks are centralized in logical clusters, plant wiring requirements can be greatly reduced. Wiring between racks and the CPU can be reduced to a few twisted pairs of wires or a single cable. The tremendous cost savings that result can be realized without a compromise of control accuracy or capability. A system configuration diagram (such as that shown in Figure 6.5i), when used in conjunction with the I/O matrix in Table 6.5s, aids in keeping track of the overall system configuration. Remote I/O is broken down into two distinct types: the integral type, which allows a limited transmission distance (up to 15,000 feet, or 4500 meters); and the transmitter/ receiver type, which allows virtually unlimited transmission capability. Most PLC manufacturers and third-party periph­ erals manufacturers can provide some form of either type. Technology is advancing greatly in this area as systems change from fiber optics to microwave and radio transmis­ sion. It is important to remember the major weakness of remote I/O systems. If the bus is cut or interrupted, the effects of I/O failure will be relatively unpredictable. One must consider the effect of a possible system failure on each step in the sequence. For this reason, duplication of smaller CPUs at each remote location is often considered preferable to a large central CPU. This is actually an extension of distributed control within the network of the PLC itself. This approach can be very cost-effective, since requirements for the central unit size can be reduced. Serious consideration should be given to distributed versus centralized architecture in remote I/O systems in which control system integrity is important. Memory Quantity and Type

The type and quantity of PLC memory used depends on the controller's size and the company that manufactured it. Most small PLCs come with a fixed quantity of RAM. Although this is usually 2K to 4K of memory, the actual number of memory locations is not as important as the average size application program the PLC can be expected to handle. (In this case, size refers to the number of I/O points that are to be controlled and the average number of logic, timer, counter, and mathematics operations that are to be performed.) Some manufacturers may provide an extra expense option of PROM or ROM memory with their small PLCs. Midsize and large PLCs provide users an option for

almost any type of memory desired. This includes various types of nonvolatile (i.e., CORE) memory. Quantity limits imposed by the PLC will exceed most application demands. When this is not the case, it usually suggests that a more efficient control scheme is in order or that the application really does not belong on a PLC in the first place. Total memory, as stated in manufacturers' literature, does not necessarily mean that the entire content is available to the user. Some manufacturers reserve large blocks for the PLC executive. A system with 4K of 16-bit words of user memory may comfortably accommodate a program, whereas another system with 8K of 8-bit words may have too small a memory for the same program. Special programming language features are an important aspect of memory sizing, especially in process control. The PID algorithm is a perfect example: One manufacturer re­ quires 33 words of user-available memory, whereas another may need in excess of 1000 words. Obviously, the memory sizing for a loop control program would vary in these two systems. Another example is the use of special functions, such as shift registers. An alternative way of developing a shift register in ladder logic is to use a special function shift register or handling data to require less user memory. Word (or register) moves are also powerful in terms of memory efficiency. Programming languages, which can be binary- or octal-based or alphanumeric Boolean, affect memory use. The closer the language is to machine code (binary-based), the more user memory is required to perform the more complex functions. The closer the language to alphanumeric Boolean, the less memory will be required for complex functions. The best way to determine program memory prerequi­ sites is to write a representative sample program reflecting some actual project requirements and to request information about user memory size from the various manufacturers. If the manufacturer's suggestions are followed, the user can be reasonably assured that the memory will not be undersized. The final area of caution about memory size concerns the consideration of data storage. Data tables, scratch pads, and historical data retrieval requirements can inflate the size of the PLC memory. It should be remembered that the primary task of a PLC is control of the process. If data requirements are large, connection to auxiliary devices, such as mini- and microcomputers, should be given serious consideration. Many of these devices are currently available and are of an industrial grade; furthermore, the price of these systems is coming down rapidly. It is not good engineering practice to degrade control capabilities by burdening the PLC with excessive data acquisition functions. As a plant goes on-line, operational requirements for data generally increase astro­ nomically. These will be easily accommodated by a mini- or microcomputer but not by the PLC memory. Programmers

Three basic programming tools are provided by manufactur­ ers of midsize PLCs. These include hand-held programmers,

6.5 PLC—Programmable Logic Controllers CRT programmers, and CRT programmer simulators that run on personal computers. The CRT programmer may not be an option if a small programmable controller is pur­ chased. The hand-held programmer enables the operator to enter a program one contact at a time. These units are widely used because they are rugged, portable, and easy to operate. They are very cost-effective and give an engineer the capability to enter a program and to diagnose trouble in logic and field devices. The CRT programmer provides the engineer with a visual picture of the program in the PLC. Ladder diagrams are drawn on the screen, just as they would be drawn on paper. Design and troubleshooting time is reduced with the use of the CRT. With menu-driven software, programmer training time is decreased. The CRT is designed for desktop or factory floor orientation. The exterior is made of impactresistant foam or metal with a spill-proof tactile keyboard. The screen size varies from 4 to 9 in. (100 to 225 mm), with keylock protection to prevent unauthorized program alter­ ation. These units can be ordered with memory storage capabil­ ities. Information is stored on either floppy disks or magnetic tape. With floppy disk memory, programs can be copied from one disk to another and then verified without need for loading the PLC's memory. Stand-alone programming is a feature of disk memory—an engineer can develop a pro­ gram on the disk and then load into the PLC. Some CRT programmers provide complete documenta­ tion capability, including ladder diagrams, cross-reference listing, and I/O listing. More sophisticated CRT program­ mers supply "user"-defined contacts and coil names along with commentary above each network or rung. The CRT programmer also includes an external RS232C port for con­ nection to a printer. The CRT screen typically shows 8 rungs of ladder logic by 11 contacts across. The ladder diagrams can be placed into the real-time mode, which allows visual contact status. A whole screen of contacts and coils can be updated in as fast as 40 msec. These programmers are designed for porta­ bility but do weigh 45 to 60 lbs (20 to 27 kg). With a modem connection these CRTs can be used at remote locations for programming and troubleshooting. Although the CRT programmer simulator that operates on a personal computer is a popular option as a program­ ming tool, there are some factors that need to be noted. Cost is certainly a factor. The initial impression is that the pro­ gram that behaves like a CRT programmer does not cost as much as the real thing (a CRT programmer). This is cer­ tainly true, but the program version may not be as versatile as the CRT. This versatility mismatch is in the functions it provides and the number of programmable controllers that can use it. Since the CRT was initially the primary program­ ming tool sold by the PLC manufacturer and the CRT pro­ grammer simulator may have been developed by a software house licensed by the PLC manufacturer, there is no guaran­

739

tee that the program will have all of the functions or operate the same way that the CRT programmer does. A CRT simulator might be restricted to use on only one personal computer or on one programmable controller. Se­ curity keys are one way to obtain this isolation. Security keys are devices that plug into the back of the computer or programmable controller or both. Without the key there is no communication connection between the PLC and the per­ sonal computer. Some manufacturers use alternate security schemes such as providing extra keys at group prices or issuing site license agreements. In any case, it is unlikely that a PLC manufacturer would allow use of a CRT simula­ tion program on an unlimited number of personal computers or programmable controllers.

PLC INSTALLATION

Installation of programmable controller systems is not a difficult or mysterious procedure, but the following general rules will save time and trouble for the systems designer or installer. The basic principles of PLC installation are the same as those for installation of relay or other control sys­ tems. Safety rules and practices governing proper use of electrical control equipment in general should be observed. These include correct grounding techniques, placement of disconnect devices, proper selection of wire gauge, fusing, and logical layout of the device. PLCs can often be re­ trofitted into existing hardwired relay enclosures because they are designed to withstand the typical plant environ­ ment. PLC vendors provide installation manuals upon re­ quest. Safety Considerations

Perhaps the most important safety feature, which is often neglected in PLC system design, is emergency stop and master control relays. This feature must be included when­ ever a hardwired device is used in order to ensure operator protection against the unwanted application of power. Emer­ gency stop functions should be completely hardwired (Figure 6.5t). In no way should any software functions be relied upon to shut off the process or the machine. Discon­ nect switches and master control relays should be hardwired to cut off power to the output supply of the PLC. This is necessary because most PLC manufacturers use triacs for their output switching devices, and triacs are just as likely to

Implementation

Planning ahead is every bit as important in designing a complete PLC system as in laying out a relay logic panel. Care in counting I/O points in the beginning—and leaving a safety factor—will save headache in the panel fabrication stage. Panels should always have plenty of expansion room left over, since I/O is invariably added as the job progresses

740

PLCs and Other Logic Devices

2FUI

oV-Q

3FU 460 VAC -pH2

H10

H3Q-

OH4

AC GND AC COM LINE REMOVE FOR 220 V MODEL 530 CPU 202

X2

ni

FIG. 6.5u

in

ΗΘ1ΝΝΚ 202

X2

Typical enclosure layout. A. This 9" X 12" (225 mm X 300 mm) area reserved for disconnecting means. B. Area available for fuses, control relay, transformers, or other user devices. C. AC input line. Routed separately from I/O wiring. D. Wiring duct for I/O wiring within enclosure. E. Unobstructed minimum vertical space required. Above the controller: typical = 6" (150 mm); worst case = 12" (300 mm). Below the controller: typical = 6" (150 mm); worst case = 10" (250 mm). Between two controllers: typical = 6" (150 mm); worst case = 12" (300 mm). F. Unobstructed horizontal space required: 4" (100 mm) minimum. G. Minimum 2" (50 mm) between I/O wiring duct or terminal strip and I/O chassis. H. 8-gauge wire or 1" (25 mm) braid for bonding purposes. 6" = 150 mm; 36" = 900 mm; 25" = 625 mm. (Courtesy of Allen-Bradley)

FIG. 6.5t

Master stop switch installation. (Courtesy of Texas Instruments)

and the operators see the advantages of PLCs. The designer should refer to the layout considerations provided by the manufacturer. Extra space should be left to provide access to the boards and connectors of the PLC. The diagnostic and status indicators should all be visible. The designer should leave room between I/O racks for wire ways and large hands. One good technique for ensuring efficient panel layouts is to involve maintenance personnel in the design procedure. This not only optimizes the layout but also introduces the staff to the hardware (Figure 6.5u). In general, the best defense against creating a tangled mess when designing a PLC system is to follow proper documentation techniques. A little more time spent docu­ menting panel layout, I/O counts, and wiring diagrams re­ sults in a lot less time spent starting up the system. PLCs can handle large amounts of I/O points with varying electrical characteristics, so things can get pretty confusing in a hurry.

Cable requirements between hardware boxes vary from one type of PLC to another, so this is an important consideration in panel layout. Enclosure Enclosures should nearly always be provided for the PLCs themselves. This protects the electronics from moisture, oil, dust particles, and unwanted tampering. Most manufacturers recommend a NEMA 12 enclosure for the standard indus­ trial environment. This type of enclosure is readily available in a variety of sizes and, in fact, may be already included with a new system. Programmable controllers are designed to be located close to the machine or the process under control. This keeps the wiring runs short and aids in the troubleshooting proce­ dure. At times, however, mounting the PLC directly on the machine or too close to the process is not advisable, such as in cases of vibration inherent in the machine, electrical noise interference, or excessive heat problems. In these situations,

6.5 PLC—Programmable Logic Controllers the PLC must be either moved away or successfully protec­ ted against these environmental conditions. Temperature Considerations

Installing any solid-state device requires paying attention to ambient temperatures, radiant heat bombardment, and the heat generated by the device itself. PLCs are typically de­ signed for operation over a broad range of temperatures, usually from 0 to 60°C. When analyzing the proposed PLC environment, however, one should remember that enclosure temperatures usually run a few degrees higher than ambient temperatures. Radiant heat on an enclosure from surround­ ing tanks can raise the internal temperature beyond that specified by the manufacturer. Heat generated by the PLC is a key issue when the device is placed in ambient temperatures close to the extreme men­ tioned in the specifications. The temperature rise caused by the power consumption of the PLC itself is not hard to estimate. In addition, most manufacturers will provide a notation of the power consumption of the triacs driving field loads. When designing the hardware layout within the panel, one should adhere to the manufacturer's suggestions regard­ ing ways to minimize heating problems. Most PLCs use convection over fins to take heat away from particular areas within the hardware. Care must be taken to ensure that no obstruction to air flow over these fins is introduced by placement of the PLC in the enclosure. Wire ways are typi­ cally provided with holes to allow air to pass through. Generally, one can avoid problems with PLC enclosures by simply leaving plenty of air space around the heat producers. Should all of these factors combine to cause a tempera­ ture problem, the panel can be vented, air conditioned, or moved to another location. Usually, simply blowing filtered air through the enclosure will resolve minor difficulties. If air conditioning is required, small units that are designed for cooling electronic enclosures are readily available. Noise

Noise or unwanted electrical signals can generate problems for all solid-state circuits, particularly microprocessors. Each PLC manufacturer suggests methods for designing a noise-immune system. These guidelines should be strictly followed in the design and installation phases, since noise problems can be very difficult to isolate after the system is

L1 O

Cf^-i

O

·

INPUT

|R

N O

1

COMMON

FIG. 6.5v

Dummy load for leakage. If a ' 'leaky'' triac, such as a proximity switch, is used to switch the hot side of the line, then a pull-down resistory (R) is required to counteract the leakage.

741

up and running. I/O systems are isolated from the field, but voltage spikes can still appear within the low-voltage envi­ ronment of the PLC if proper grounding practices are not followed. A well-grounded enclosure can provide a barrier to noise bombardment from outside. Metal-to-metal contact between the PLC and the panel is a must, as is a good connection from the panel to the ground. Noise producers within the panel should be noted during the panel design phase, and the PLC must not be located too close to these devices. Wiring within the panel should also be diverted around noise produ­ cers so as not to pick up any stray signals. Often, it is necessary to keep AC and DC wiring bundles apart, particu­ larly when high-voltage AC is used at the same time that low-level analog signals are present. Line voltage variations can cause hard-to-trace problems in the operation of any computer-based system. PLCs are no exception, even though they are designed to operate over a much larger variation in supply voltage. Large spikes or brownout conditions can cause errors in program execution. Most manufacturers protect against this, enabling the con­ troller to come up running after a brownout, but these measures may not be acceptable in all applications. The designer may wish to add an isolation transformer to a proposed PLC system, sized for twice the anticipated load. This is cheap insurance, and PLC manufacturers will help determine the required load. Hookup

PLC panels can be very neat and orderly if all the terminals are arranged in a logical fashion. The actual result is a direct function of the time spent during the design process. Inter­ posing terminal blocks between the PLC I/O structure and the field is suggested, since the terminations provided by PLC manufacturers are shrinking in the race to provide higher-density I/O. This also gives the panel designer the ability to place the field termination points where they are easily accessed. Wiring ducts keep the panel neat and protect the wire from mishap. Many noise problems can be averted by following good wiring practices. Low-voltage signal wiring should be kept away from noise sources. Analog signals should be shielded, with the shield terminated at an isolated ground in the panel only (to prevent shield ground loops). Again, these analog signals should be separated from power wiring. Triac outputs require some special attention that will be new to relay users. Triacs used for AC loads typically leak a small amount of current. In the case of triac outputs from a PLC, this leakage may be enough to keep panel lamps glowing or small relays energized. When a triac is used to switch the input on a PLC, the leakage may be enough to make the PLCs "think" the input is on. A dummy load (shown in Figure 6.5v) can be used to drain this leakage when the input should be off. Whenever a mechanical con­ tact is used in series with a load energized by a triac (as

742

PLCs and Other Logic Devices COMMON

L1 O R I—y/VW-

N O-

OUTPUT (TRIAC)

FIG. 6.5w

Mechanical contact in series, RC suppressor.

L10-

COMMON MOV

Q} N o-

O

. OUTPUT (TRIAC)

FIG. 6.5x

Mechanical contact in parallel, MOV suppressor.

shown in Figure 6.5w), a resistance-capacitance (RC) net­ work should be used as shown to protect the triac from inductive kickback. A varistor should be provided in parallel with a load whenever the load can be "hot-wired" around the triac (Figure 6.5x). The user should check with the PLC manufacturer for the suggested RC and MOV (metal oxide varistor) types for the particular application. Triacs cannot directly drive large motor starters and similar devices. PLC manufacturers provide surge specifications for the various I/O cards. Sometimes an interposing relay or dry contacts will be required for large loads. Programmable controllers are basically similar to most other electrical control systems. To be sure, solid-state de­ vices, microprocessors, and triacs require some special con­ siderations during the design, installation, and start-up phases of a project, but these concepts are not too unreason­ able or difficult to assimilate. As always, good design habits in the beginning will ensure a safe and reliable control system. APPLICATIONS

Applications for PLCs can fill a whole book. Many sources documenting various PLC applications are listed in the bibli­ ography of this section. PLC vendors can supply numerous application notes for the products that they offer. Most major PLC vendors also publish detailed articles about applications in technical jour­ nals and prepare papers for engineering societies and indus­ trial symposia on control, automation, and so forth. Each manufacturer's software package usually has its own appli­ cation programming techniques. Vendors also are a valuable source of "how-to" information, providing training courses in their local office or at the factory and actual hands-on experience to help users gain familiarity with the PLC. Most vendors offer an applications or programming manual that gives insight on how to use available programming features.

Of course, familiarity with one brand of PLC will help the engineer learn to use another brand quickly. PLC PERIPHERALS

The popularity of programmable controllers has lead to the creation of a strong third-party peripheral manufacturing industry. These companies are always developing new prod­ ucts that assist the PLC user with interfacing a specific application to a PLC. Three categories of these products— operator stations, I/O enhancements, and programming and documentation tools—are presented below. The operator stations facilitate operator interface with the PLC-controlled process to monitor process variables, to alter program pa­ rameters, to conduct on-line program alterations, and to conduct troubleshooting procedures. I/O enhancements in­ clude all capabilities not ordinarily supplied with PLCs or those items that a particular manufacturer may not choose to support. Programming and documentation tools include products supplied by the manufacturers or made available by third-party vendors.5'25 Operator Stations

Operator stations include those provided by manufacturers and intended to be used with their particular PLC and those offered by third parties for use with either a particular brand or anyone's PLC. These stations may include devices such as timer/counter access modules (TCAMs), loop access modules (LAMs), data terminals, color graphics consoles, computers, printers, and manual backup stations. Most PLC manufacturers provide an operator interface unit (OIU) designed specifically for their PLC. These are either part of the standard system or offered as an option. They are usually mounted directly on the PLC but may be designed to be panel-mounted and cabled back to the con­ troller. Functions include access to read/write register data, simple programming, and diagnostics. Some specialized de­ vices, such as TCAMs, LAMs, and OIUs, provide operator interaction with PLC internal registers and loop tables. This gives the systems designer the ability to provide real-time changing of variables, loop tuning and inspection, manual control of analog outputs, and the ability to provide batch- or menu-type information at low cost. Communications with the PLC are multi-dropped over an RS422 or a similar differential line. Unauthorized data entry is prevented with software locks, keylock protection, or both. Some PLCs can support communications directly with dumb data terminals. Operators enter the data by issuing special control characters to the PLC communications port. Data terminals can be provided in industrial versions in­ tended for the plant floor or in office machines for entry of data by a supervisor. This operator interface approach is not very user-friendly and can be intimidating. Color graphics consoles offer process graphics and com­ munications facilities to many brands of PLCs simulta-

6.5 PLC—Programmable Logic Controllers neously. These systems range from those that can simply be purchased and put on-line with a minimum of engineering effort to those that require some programming. The basic differences are in flexibility. Those that do not require programming may not be able to provide the custom menus and graphics that are required. The ease of communications with different types of PLCs also varies according to manu­ facturer. Finally, the method of generating the graphics pages differs greatly. Most color graphics consoles offer multiple graphics pages that are animated by reading data tables in the PLCs. Operators enter the data by means of standard keyboards, user-configurable industrial keyboards, light pens, touch screens, and the like. Different graphics pages may be selected with preformatted menus or custom menus programmed by the user or the systems house. Devel­ opment stations are often required to give the final user the ability to change graphics menus or key commands after the initial project is completed. Computer systems can be made to perform man-machine interface functions. Indeed, the color graphics consoles de­ scribed in the previous paragraph are simply computers with standard graphics and communications software packages. Most PLC manufacturers provide board-level additions or modules that give the PLC the ability to converse via the RS232 protocol to most any computer. Of course, both the communications software and the particular applications software must be generated to provide an interface. Many vendors and systems houses are providing communications packages for various PLCs to run on microcomputers and personal computers. These small systems offer low-cost operator interfaces to PLCs, providing data handling capa­ bilities and the ability to be networked into a true distributed architecture. In this way, PLC purchasers can be assured that their investment will be protected from factory automation. Microcomputers that have the ability to multi-task and ac­ cess large amounts of both RAM and nonvolatile memory, have proper software support, and are able to be networked will provide a good investment in terms of operator interface functions as well as total system capability. Printers have always been an important part of the PLC system both as a development tool and for handling some of the operator interface functions. Many PLCs are able to provide communications directly to dumb printers. A stand­ alone PLC system then can often provide performance re­ ports, alarm logging, and the like without ever involving a computer. This feature is usually somewhat limited, since PLCs were designed primarily to control the process ma­ chine. Large amounts of data, sophisticated print logs, and multiple alarms are not really within the realm of a stand­ alone PLC system. This type of data manipulation is too cumbersome and requires too much memory for most PLCs. Manual control stations are important as backups in case of failure of the PLC controlling PID loops. Loop access modules provide manual control capabilities but still rely on the integrity of the PLC, so they are not truly manual in the

743

hardwired sense. A manual control station is an important part of the distributed control system because it gives true manual control of the loops locally or in the control room, even when the local controllers are down. I/O Enhancements PLC manufacturers are providing more and more types of input and output capabilities for their products. There are, however, many third-party peripherals that aid the PLC in interfacing to the field devices. New I/O capabilities that are being offered include faster response, new analog capabili­ ties, intelligence, high-speed pulse counters, dry contact, and specialty modules. Fast-response I/O is currently offered in both discrete and analog versions. Discrete rapid response modules are facili­ tated by the PLC logic, but the output does not rely on ladder logic scan times to get updated. For analog modules provide quicker analog-to-digital (A/D) and digital-to-analog (D/A) conversions. This gives PLCs the ability to control faster PID loops and to make analog measurements of assembly line parts (weight, for example). Analog I/O capabilities for PLCs are being expanded from the conventional 4 to 20 mA, 0 to 5 V, 0 to 10 V versions to include direct thermocouple and RTD inputs. These modules typically accept eight to ten points each, and different types of T/Cs and RTDs are accommodated. Intelligent I/O modules include all modules that are able to perform processing functions. Because the tasks per­ formed by the PLC are further distributed, greater speed and reliability for the overall system can be realized. Intelligent I/O modules give the PLC multiple additional capabilities, which may include memory storage and retrieval, comput­ ing tasks, and communications. Memory modules provide additional room to store data points, alarm messages, lookup tables, and the like. This approach leaves the main operating memory free for the control tasks. Computing modules give PLCs the ability to perform true computer functions using a language like BASIC. Again, the real-time tasks are left in the main memory, but tasks such as setpoint calculation, formation of data, and some operator interface tasks may be placed in the computer module. Communications modules can provide the PLC with a range of capabilities, from simple ASCII output strings to communication networking. The storage of ASCII messages for a printer or display can be contained outside the main memory of the PLC, and the data can be output when required. Full-system communication networking capabili­ ties are provided with network modules, giving the designer the ability to multi-drop PLCs off a single operator interface device or a supervisory computer. High-speed pulse counter modules provide the ability to interface with turbine meters, stepper motors, and optical encoders. High-speed pulses cannot normally be interfaced to PLC inputs because of the scan time of the ladder logic. These modules provide an interface that does not rely on the

744

PLCs and Other Logic Devices

scan time, so that the PLC is able to monitor pulses that indicate position or flow. Dry contact modules are offered by both manufacturers and third-party vendors. These modules solve the problems normally associated with triacs, low power, and uncertainty of failure state. Specialty modules are designed to solve a single interface problem. X/Y positioner modules can be included in this category, as well as servo axis controllers, stepper motor outputs, and even maintenance access modules. These mod­ ules are a further extension of the distributed technology. Clock modules that fit into the I/O bus may be considered to be part of this group. These modules provide real-time and day/date functions upon interrogation from the PLC. Most are backed up by a battery to ensure timekeeping during power outages.

configuration drawings, point-to-point wiring diagrams, and I/O layout. (One system even prints out the wire labels.) Some of these devices offer still other computer services— word processing, BASIC programming, and even computeraided design (CAD) facilities. COMMUNICATIONS NETWORKING

Many PLC manufacturers have responded to the increasing industry demands for equipment that allows communica-

HIERARCHICAL CONTROL

Programming and Documentation Tools

Both PLC and aftermarket parties offer programming and documentation tools for the system designer or user. These tools include programmers, CRT documentors, and com­ plete microcomputer-based systems. Programmers are typically provided by the PLC manu­ facturer and are designed to program a specific machine or family of machines. Some third parties are offering univer­ sal programmers and documentors. These microcomputer devices vary greatly in price and capabilities but offer onand off-line programming to many different types of PLCs, real-time status, and some very sophisticated annotation. Communications to different PLCs are usually supported with different software packages. Each vendor's product offers different types and amounts of ladder and contact comments. Again, many types of cross-references are avail­ able to be printed out. Often other PLC design documenta­ tion problems may be solved, such as the generation of panel

FIG. 6.5z

Hierarchical control. (Courtesy of Allen-Bradley)

PROCESS

MACHINE

PROCESS

PLC|

|PLC|

MANAGEMENT INPUT Γ

O

|PLC|

r—i

IPLC

MANAGEMENT DATA

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CONVEYORS

PACKAGING

FIG. 6.5y

With master control, one PLC controls a number of related machines and processes. This system is simple but may require long runs of multiple-wire cables, and the entire system is vulnerable to the failure of the one PLC. (Courtesy of AllenBradley)

MACHINE

TEST AND INSPECTION

FIG. 6.5aa

Distributed control. (Courtesy of Allen-Bradley)

6.5 PLC—Programmable Logic Controllers tions among multiple process areas. 26-29 The original master control layout shown in Figure 6.5y is adequate for PLC control of moderate-size applications of perhaps 100 to 500 I/O points, but as the application grows the PLC becomes overburdened under this arrangement. The hierarchical plan illustrated in Figure 6.5z provides a supervisory PLC con­ trolling network that reduces this burden. The plan allows for a "master" PLC that oversees the process and controls a set of "slave" PLCs that control the actual process activities in the plant. An alternate approach is illustrated in Figure 6.5aa. This distributed control system allows dedicated PLCs to control sections of the process with an interface to management. As indicated in the figure, this interface could be to a human operator. This person would monitor selected data from the collection of PLCs. Any management decisions generated from this data are passed back to the PLCs by an input terminal. Under normal conditions it is expected that each dedicated PLC can monitor and control its section of the process.

745

Most major PLC manufacturers offer network communi­ cations for their own products. Various network protocols exist and efforts to generate standards are gaining momen­ tum. Instrument and control organizations such as the Instru­ ment Society of America constantly exert pressure on the PLC industry to agree on one or at least a few communica­ tion standards. In any event, there are several communica­ tions philosophies of interest. Some of these are outlined below. Universal Communications Networking

Some PLC manufacturers and third parties are offering universal communications networking. This is sure to be the way of the future, because the need for networking different brands of PLCs together will increase. Several exciting new developments in the networking arena are showing up. Included are peer-to-peer communi­ cations, "hot" redundancy, on-line engineering, and costeffective networking of intelligent man-machine interfaces.

TO OTHER Tl NETWORKS

TO OTHER PUBLIC INDUSTRIAL NETWORKS

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5TI

TI5IO

FIG. 6.5bb

Peer-to-peer communications. (Courtesy of Texas Instruments)

5TI

TI520

TI520

TIWAY

TI500

T 5TI

y

PM550

OPERATOR INTERFACE

746

PLCs and Other Logic Devices

FIG. 6.5CC

Hot backup PLC networking. (Courtesy of Allen-Bradley)

Peer-to-Peer Communications Many of the PLC manufac­ turers have already addressed the need for peer-to-peer com­ munication among the PLCs on a distributed network. Without this feature, every time one PLC needs to know the status of another part of the machine or plant, it must interrupt the activities of the supervisory computer in order to get the information (Figure 6.5bb). The ability of one PLC to "talk" to another along the data highway greatly speeds the control activities of each machine and allows the super­ visory computer to "concentrate" on its tasks.

Hot Backup PLCs The new breed of data highways can provide a process with a "hot" backup PLC to take over in the event of a failure. With the supervisory computer in­ volved, sufficient intelligence exists to determine whether or

not a particular PLC is performing properly. Figure 6.5cc depicts a distributed system with redundant PLCs. On-Line Engineering On-line engineering of PLC systems can now be offered from a remote location with the combi­ nation of the programming/documentation tools and the distributed network. Systematic start-up and debugging of processes are available with this technique. Figure 6.5dd depicts on-line engineering as part of a distributed system. Intelligent Man-Machine Interfaces Intelligent man-ma­ chine interfaces (MMI), or enhanced computer-based oper­ ator interfaces, can be networked into a total distributed system to give a redundant or local interface to the system (Figure 6.5ee). This technique can be used to provide the

6.5 PLC—Programmable Logic Controllers

Ί4Π

Γ DEVELOPMENT

TERMINAL

SUPERVISORY

MMI

TERMINAL

COMPUTER

1

I

IU

ENGINEERING

Ί REPORT PRINTER

OPERATORS MMI REPORT PRINTER

ALARM PRINTER

OPERATOR S MMI

ID

h

STAGE 3

I

OPERATOR S MMI

INTELLIGENT MULTIPLEXER

SUPERVISORY COMPUTER

DATA HIWAY

| DATA HIWAY

SUPERVISORS TERMINAL

PLC

|

|PLC

PLC

STAGE 1

PLC

PLC

STAGE 2

FIG. 6.5ff PLC

PLC

PLC

PLC

Entry-level networking grows with user's

PLC

needs.

FIG. 6.5dd

Networking with on-line

engineering.

INTELLIGENT MMI

SUPERVISORY COMPUTER

TERMINALS

^

INTELLIGENT MULTIPLEXER

DATA HIWAY

PLC

PLC

PLC

PLC

FIG. 6.5ee

Local or redundant MMI in network.

control room interface while the supervisory computer sup­ ports its own interfaces. Another powerful technique is to allow the networking of a few PLCs together solely for the purpose of providing a single man-machine interface to all parts of the system (Figure 6.5ff). This solution is ideal for small PLC users because it is very economical yet allows expansion as the plant grows.

References 1. "The PC User Buyer's Guide," PC User. 2. "Fast Growing PC Market Encourages Wide Range of Product Of­ fering," Control Engineering, January 1983. 3. "PC Users Guide," Instruments and Control Systems. 4. Londerville, S., "Programmable Controllers in Boiler Control Applica­ tions," Instrument Society of America, Northern California Section, ISA Days, June 2, 1982. 5. Laduminsky, A.J., "Peripherals Enhance PC Performance," Control Engineering, January 1983.

6. Whitehouse, R.A., "The Future of Programmable Controllers," ISA 1976 Annual Conference, Advances in Instrumentation, Vol. 33, Part I, Instrument Society of America, October 15-19, 1976. 7. Korarek, L.A., "Sizing Criteria for the Evaluation of Programmable Controllers," Instruments and Control Systems Seminar Workshop (available from Allen-Bradley). 8. Bennett, L.E., and Lockert, C.F., "Evaluating Controls for Batch Processing," InTech, July 1976. 9. Dietz, R.O., "Programmable Controllers in Batch Processing," InTech, May 1975. 10. Larsen, G.R., "A Distributed Programmable Controller System for Batch Control," InTech, March 1983. 11. Melville, H.J., "PLC Applications in Batch Digester Control," ISA 1980 Annual Conference, Advances in Instrumentation, Vol. 35, Part II, Instrument Society of America, October 20-23, 1980. 12. Hawkinson, G.M., "Selecting a Programmable Controller for a Retrofi," 9th Annual Programmable Controllers Conference Proceedings, ESD, March 1980. 13. King, J„ and Ptak, J., "Toothpaste Processing Using a PC Mini Microcomputer Network," 12th Annual Conference and Equipment Display Proceedings, ESD, March 1983. 14. Conrad, W., "Document Your PC Design," 9th Annual Programmable Controllers Conference Proceedings, ESD, March 1980. 15. Bryant, J.A., "Is Your Plant's Control System Safe," Power, August 1979. 16. Penz, D.A., "Organizing PC Software Development," Instruments and Control Systems, February 1982 and March 1982. 17. Heider, R.L., "The Programmable Controller as a Process Link to a Distributed Computer Network," ISA 1980 Annual Conference, Ad­ vances in Instrumentation, Vol. 35, Part II, Instrument Society of America, October 20-23, 1980. 18. Kraemer, W.P., "Testing and Start-up of Programmable Controller Systems," Paper PC 179-14, IEEE Transactions on Industry Applica­ tions, Vol. 1A-16, No. 5, September/October 1980. 19. Programmable Controller Course, Instruments and Control Systems, Radnor, Pennsylvania: Chilton Co., 1981. 20. Klostermeyer, W.H., and Thurston, C.W., "PC's Prove Reliable in Chemicals and Plastics," Control Engineering, April 1976. 21. Stockweather, J.W., "Proper PC Documentation for the User," 11th Annual Conference and Equipment Display Proceedings, ESD, March 1982.

748

PLCs and Other Logic Devices

22. Langhans, J.D., "The Programmable Controller in a Continuous Pro­ cess," Control Engineering, July 1972. 23. Hoffman, E.L., "Redundant Control Systems," 10th Annual Confer­ ence and Equipment Display Proceedings, ESD, March 1981. 24. Sykora, M.R., "The Design and Application of Redundant Program­ mable Controllers," Control Engineering, July 1982. 25. Lutz-Nagy, R., "The Dawn of Intelligent Motion," Power Transmis­ sion Design, February 1980. 26. Eckard, M., "Tackling the Interconnect Dilemma," Instruments and Control Systems, May 1982. 27. ESD Vendor Workshop, "Control Networks versus Data Acquisition (A Case for Dividing the Tasks)," 12th Annual Conference and Equip­ ment Display, ESD, March 1983 (available from ISSC). 28. Hoag, D.S., "Programmable Controllers in Distributed Process Con­ trol Systems," International Conference on Power Transmissions, Houston, June 1982 (sponsored by Power Magazine). 29. Jannotta, K.L., "Factory Communications: How to Talk to the Ma­ chines," International Conference on Power Transmissions, Houston, June 1982 (sponsored by Power Magazine).

Bibliography Asfahl, C.R., Robots and Manufacturing Automation, 2d edition, New York: John Wiley, 1992. Bailie, N.B., "The Application of Programmable Controllers to Compres­ sor Systems for Offshore Applications," Measurement and Control: Journal of the Institute of Measurement and Control, England, Vol. 14, No. 9, September 1981. Baker, A.T., and Rahoi, R.J., "Programmable Controller Applications for Power Plant Waste Water Treatment Facility," ISA Power Instru­ mentation Symposium, Chicago, May 14-16, 1980. Barrett, L.E., and Lemmon, R.H., "Use of Programmable Controllers to Control Series Slurry Pumping Systems," 10th Annual Programmable Controller Conference and Equipment Display Proceedings, ESD, March 10-12, 1981. Bartlett, P.G., Jr., "Installing a PC," Instruments and Control Systems, August 1980. Bauman, D.E., and Meilott, M.T., "Automatic Small Refinery Blending Operations," Hydrocarbon Processing, November 1981. Bedworth, D.D., Henderson, M.R., and Wolfp, P.M., Computer Integrated Design and Manufacturing, New York: McGraw-Hill, 1991. Boehem, B.W., Software Risk Management, IEEE (Computer Science), 1990. Booty, D.J., "So You're Buying a Programmable Controller," InTech, September 1980. Burr, J., "Smart Programmable Controller: Key to Pipeline Pump Station Control," (available from Gould Modicon). Carlisle, B.H., "More Power for Programmable Controllers," Machine Design, March 6, 1980. Cherba, D.M., "Redundancy in Programmable Control Systems," Instru­ ments and Control Systems, April 1983. Clare, W., and Wilson, G., "Programmable Controllers Application to Water Treating Plants: What, Why, When, Where, How," 7th Annual Joint Instrumentation Conference and Workshop ISA and AWWA, Santa Ana College, California, August 20, 1980. Cleaveland, P., "Programmable Controllers: A Technology Update," In­ struments and Control Systems, May 1983. Coelho, W.A., "Putting Programmable Controllers to Work on Material Handling," Instrumentation Technology, March 1974. Cooke, E.F., and Halajko, J.S., et al., "Supervisory Programmable and Setpoint Control for Automating a Batch Plant," InTech, July 1980. Daiker, J., "Operator Interface: Link-up of a Color-Graphic CRT and PLC," 9th Annual Conference and Equipment Display Proceedings, ESD, March 1980. Dechont, G.R., and Ware, W.E., "Performing PID with a Programmable Controller on an Extrusion Line," 11th Annual Conference and Equip­ ment Display Proceedings, ESD, March 1982.

Deltano, D., "Programming Your PC," Instruments and Control Systems, July 1980. Ellis, R., "Color Graphics CRTs Provide 'Window' into Factory Opera­ tion," Instruments and Control Systems, February 1983. Farrar, L., "Discrete Control Products for Reliable Automation," Instru­ ments and Control Systems, April 1982. Farrar, L., "Programmable Controllers in the Factory," Instruments and Control Systems, July 1983. Fischer, G.I., "Use of PC's for Energy Management," 9th Annual Pro­ grammable Controllers Conference Proceedings, ESD, March 1980. Franck, D.T., "Programmable Controllers Today and Tomorrow," Electri­ cal Consultant, 1974. Frost, H.C., "New Processes Improve Food Quality, Yields Help Energy Efficiency, Labor Utilization," Food Product Development, April 1980. Gilbert, R.A., and Llewellyn, J.A., "Programmable Controllers: Practices and Concepts," Industrial Training Co., 1985. Girard, B., Programmable Logic Controllers: Architecture and Applica­ tion, New York: Wiley & Sons, 1990. Harold, R.A., "Designing Automated Systems for High Reliability and Maintainability," 12th Annual Conference and Equipment Display Proceedings, ESD, March 1983. Hasenfuss, F.C., "Programmable Controllers Control Natural Gas Under­ ground Storage," 10th Annual Programmable Controller Conference and Equipment Display Proceedings, ESD, March 10-12, 1981. Henry, D., "PC Input/Output Considerations," Instruments and Control Systems, June 1980. Herndor, W., "Advantages of Digital Control in Small Process Loop Applications," ISA 1980 Annual Conference, Advances in Instru­ mentation, Vol. 35, Part I, Instrument Society of America, October 20-23, 1980. Hickey, J., "Programmable Controller Update," Instruments and Control Systems, July 1979. Hill, C , "Microprocessors Simplify an Adaptive Gain Problem," ISA 1980 Annual Conference, Advances in Instrumentation, Vol. 35, Part I, October 20-23, 1980. Hollopeter, W.C., "Man/Machine Interfaces: Yesterday, Today, and To­ morrow," ISA Regional Show 1979, Los Angeles, Gould Modicon, Andover, Massachusetts. Huber, R.F., "Programmable Controls: Where the Action Is," Production, September 1971. Hyde, J.F., "Sophisticated PC's: The Right Direction But Don't Stop Now," 11th Annual Conference and Equipment Display Proceedings, ESD, March 1982. Kalpakjian, S., Manufacturing Engineering and Technology, 2d edition, Reading, Massachusetts: Addison-Wesley, 1992. Larson, K., "The Great DCS versus PLC Debate," Control, January 1993. Nelson, J.E., and Menefee, W.F., "Application of Programmable Control­ lers in the Loop Supervisory Control and Monitoring System" (detailed paper available from Gould Modicon). Obert, P., "The 12 Most Often Asked Questions About PLCs," Instru­ ments and Control Systems, August 1976. Owens, D.R., "A Case for Custom Software," Instruments and Control Systems, November 1981. Palko, E., ed., "The Solid State Industrial Programmable Controller," Plant Engineering, April 1973. Parkin, T.O., "How Cost-Effective Are PC's?" Instruments and Control Systems, April 1980. Persson, N.C., "Programmable Controllers," Design News, April 1978. PLC-5 Programming Software, Allen-Bradley, 1991. Poisson, N.A., and Anderson, R., "Batch Process Control with Programma­ ble Controllers," (available from Gould Modicon). Programmable Controller Handbook, Cincinnati Milacron Inc., Electronics Systems Division, Lebanon, Ohio, 1972. "Programmable Controllers," Measurements and Control, September 1993. Quatse, J.T., "Programmable Controllers of the Future," Control Engi­ neering, Vol. 33, No. 1, January 1986.

6.5 PLC—Programmable Logic Controllers Saitta, G.N., "PC Manages Vapor Recovery System at Alaska Pipeline Terminal," InTech, May 1978. Sampson, W.H., "Controlling Electric Demand with a Programmable Controller," 11th Annual Conference and Equipment Display Proceed­ ings, ESD, March 1982. Savelyev, M.K., "How to Use PCs for Energy Management Systems," Control Engineering, February 1979. Stockweather, J.L., "Suppliers to Users: What is Their Role?" PC User, March 1982. Standard ICS3-1978, Industrial Systems, National Electrical Manufacturers Association, 1978. SYMAX 650 Programming Manual, Square D, 1991. Taebel, T.A., "An Introduction to Programmable Controllers," presented at the ESD PC Conference, May 1977. Thumann, A.,' 'For Greater Control Sequence Flexibility, Use the Program­ mable Controller," Electrical Consultant, 1974. Urmie, M., "Programmable Controllers for Batch Processing," Measure­ ments and Control, December 1982.

749

Waite, R., "PC Study Reveals User Wants and Needs," Instruments and Control Systems, February 1983. Walker, F.H., "Use of Programmable Controllers for Industrial Power Distribution System Monitoring, Control and Protection," IEEE Pro­ ceedings Conference Record 79CH1423-3 1 A, Paper Number PCI-7925, 1979. Ward, P.T., and Mellor, S.J., Structured Development for Real-Time Sys­ tems, Yourdon Press, 1991. Whitehouse, R.A., "Programmable Controllers: Cost and Maintenance," 1979 Plant Engineering and Maintenance Conference, Chicago, May 8, 1979. Wiktorowicz, A.C., and Jeffreys, J., "Boiler Combustion Control with Programmable Controllers," International Conference on Power Trans­ mission, Houston, June 1982 (sponsored by Power Magazine). Williams, R.L, "Fundamentals of Pipeline Instrumentation, Automation and Supervisory Control," ISA 1981 Annual Conference, Advances in Instrumentation, Vol. 36, Part I, Instrument Society of America, March 23-26, 1981.