Microprocessor-based controls for drilling — retrofitting aspects

Microprocessor-basedcontrols for driUing- retrofitting aspects Numerical control systems have been around for some time. R C Yadav and R H Weston* describe a method whereby computer control can be incorporated into an existing drilling machine This paper considers the updating of NC (numerical control) machine tools with control systems demonstrating modern lectures. To illustrote such machine tool retrofitting, the paper describes the structure of microprocessor-bosed hardwore end software which was developed to achieve improved control of o first generotion NC drilling machine. microprocessors numericalcontrol

retrofit

Since their introduction over 20 years ago, NC machines have found numerous applications in the metal working industry. NC applications have ranged from simple automatic positioning machines to sophisticated machine forms integrated within flexible manufacturing systems1-3 . Such developments have been possible as improved digital data processing facilities have become available. The cost of such facilities has also fallen with the development of time-sharing data processing systems, minicomputers and, more recently, LSI technology 4. Microprocessors and family elements provide a cost effective base for developing sophisticated controls for present day machine tools. Additionally, low cost LSl devices can be used to produce control systems which can be used for the updating of obsolete machine tools therby providing advanced control features such as manual data input (MDI) and interactive processor communication s'6. As part of an R & D project, the updating of a first generation NC drilling machine (a Herbert coordatrol drilling machine) was carried out at Loughborough University of Technology, UK. This updating was achieved by 1"etrofitting new drives and feedback mechanisms to the machine and developing a microprocessor-based NC system to provide the machine operator with enhanced operational facilities. Texas TMS 9900 microprocessor family elements were used to form the basis of the controller. RETROFITTING

ASPECTS

As a direct consequence of the present rate of technological change, many of first and second generation NC machines have been discarded or at least are being under used. By retrofitting new controls, obsolescence can be forestalled in some casess-7 . The state-of-the-art of retrofitting is quite well developed. There are many reported examples where applications of retrofitting are being illustrated and a number of machine tool control system builders are now Department of Mechanical Engineering, MNR EngineeringCollege, Allahabad, India *Department of Engineering Production, Loughborough University of Technology, Loughborough, Leics, UK

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providing control systems which are particularly suitable for retrofitting applications 8 . (Control systems such as COMMANDO-DCD and COMMANDO-DVL of Anllam Electronics Corp., Series 7100 of Allen-Bradley Co. and the M-1700 of Autotech Corp., et al, are typical examples). RETROFITTING

OF A DRILLING

MACHINE

A Herbert drilling machine with autotop column was updated with modern controls, this machine being a prototype single spindle drilling machine, developed in 1963. The original specification for this first generation NC drilling machine included the following: • • • • •

working surface of table longitudinal travel traverse travel maximum table positioning speed overall positioning accuracy (noncumulative) • air-bearing working pressure • free air consumption

600 x 355 m m 450 m m 300 m m 3050 m m / m i n -+0.05 m m

4.2 kN/m 2 0.10 m3/min

The table and saddle slides are supported on a cushion of air when traversing from one location to another. For stability during the drilling operation, the air supply to the slides is cut off and the slides are positively clamped. "['he origina~l hardwired controller of the machine was proving to be extremely unreliable and severe maintenance problems existed as many of the electrical components used could no longer be purchased. However, the mechanical condition of the machine was quite sound, hence there was a justified case for retrofitting a new controller to the machine. At the outset of the retrofitting work it was necessary to formulate a control specification. Some of the major factors considered in achieving this specification can be summarized as in the following. Comprehensive microprocessor development facilities were available at Loughborough and a decision made to use LSl family devices to form the base of the control system. Furthermore, all associated hardware and software was to be designed to demonstrate modularity to facilitate extreme flexibility when retrofitting and when providing future control features. Where necessary suitable machine drive and feedback elements would be fitted to provide modern features. As the drilling machine was itself a prototype design, it had a number of design and operational shortcomings. Included within these were: • manual selection of spindle speed and feed • manual adjustment of limit switches for implementing a change in depth of drilling

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To overcome the limitation imposed by the manual selection, considerable mechanical modification would be required with regard to change in spindle head design and change in spindle drive actuator and control circuitry which was beyond the project budget. However, the disadvantages associated with the manual adjustment could be overcome with a microprocessor-based CNC (computer numerical control) system by providing a suitable software interrogable position feedback mechanism and software driven contactors to facilitate programmed control of drilling depth. A particular requirement of the completed system was to provide ease of operator use and to achieve this, manual data input facilities were incorporated.

DRIVES AND FEEDBACK RETROFITTING The choice of drive and position feedback elements were made with reference to desired machine performance and the required interface structure between the developed controller and the machine. The major factors considered in affecting a choice of drive systems and feedback elements for this particular application could be summarized as follows 9 : • The original table and saddle position arrangement of the machine was found to be unsatisfactory in operation and had limited capabilities when compared with equivalent computer controlled systems. Thus it was necessary to action a repair to the control system or develop (or purchase) a new one. • As a state-of-the-art digital controller was to be developed, it was appropriate to use drives and feedback elements which would allow simplicity of interfacing with digital controlling elements. • Similarly, it was necessary to provide suitable switching and interface circuitry for the spindle to achieve programmed control of drilling depth, thereby adapting the machine to suit a wider range of drilling tasks. • It was also necessary to study the mechanical construction and characteristics of the drilling machine slides so that retrofit elements could be correctly specified. The salient features of this study are summarized below.

Table and saddle mechanical characteristics The drilling machine table and saddle slides are air-cushion supported and drive is through a lead screw with recirculating ball nut assembly. An air-cushion supported drive system has been a characteristic feature of the Herbert drilling machines and provides a very low friction drive. Low power actuators can thus be used with air bearing slides; typically a motor of 1/50 HP output being adequate. The suitability of the mechanical construction of the drive system was studied in a series of tests which were devised and conducted to check for backlash error and to evaluate drive friction torque 9. These tests revealed that the drive system is free of backlash error and that the slides maintain a satisfactory air-float condition at the recommended working air pressure.

Choice of servodrive actuators Commonly used electromechanical actuators for NC machine tools have been DC motors and stepping motors. DC servo-

288

Romped

Fast speed

acceleration

i

Ramped

~

deceleration

speed

Figure 1. Slide velocity characteristics for a stepping motor actuated point at point control system

drives are suitable for contouring machines which require high power servo systems with better performance than equivalent point-to-point servo control systems I°'11 . However, the use of high performance DC servo systems is necessarily associated with high cost and high complexity motor drive systems. In this application, stepping motors were chosen as electromechanical actuators to realize advantages of associated low cost and ease of interfacing 12'13 . However, it was necessary to choose stepping motors with suitable torque and speed characteristics. In a point-to-point control system, any stepping motor used as slide actuator should provide the features listed below 12'14(see Figure I ). • • • • •

instantaneous start and stop drive operation at preselected slow speed adequate acceleration of friction and inertial loads to an adequately high fast speed which is preselected ability to maintain slide drive at the fast speed resonance-free operation at selected speeds drive step resolution suitable to the particular application

'Sigma' Series-20 stepping motors (20-3437. D200/F075), which satisfy the above stated requirements, were selected and used as drive actuators for both the table and saddle of the Herbert drilling machine.

Choice of feedback mechanisms Open-loop controls employing stepping motors have been very popular as low complexity drive systems. However, in applications where the reliability of a control system is a prime consideration, a closed-loop control would be advisable. The inherent features of greater accuracy and reliability were considered to be sufficiently important to employ closed-loop control in this application. Thus, it was necessary to choose and use a suitable feedback element. Position feedback elements, used in machine tool applications, were considered within three categories9,i s, 16. • rotary position transducers • linear or planar transducers • proximity sensors Rotary digital incremental encoders (Ferranti type 24ST) were chosen as position measuring devices for both the table and saddle of the drilling machine, as they provided: • convenience in mounting them on the machine • simplicity of interfacing with the digital control system to be developed • favourable cost compared to alternatives

microprocessors and microsystems

Choice of processingelement and microcomputer development system Recent technological advances dictate that any new generation machine tool control system should be based on LSI microprocessor family elements 1'4'17-~9. However, an extremely wide range of such LSI devices are available on the market, demonstrating a variety of levels of sophistica-

tion with regard to performance characteristics and with regard to hardware and software development support available to the system builder during the development engineering phases of a project. It is often extremely difficult to ensure that a choice of microprocessor family is optimal, especially as new products are fast becoming available. However, when selecting a family of microprocessor elements, the following factors

Terminal

0 Controller

Microcomputer

EPROM expansion module I00 CPU module

RAM expansion module

I I I I

! I I 1

Input / output expansion modules

Translator drives

Signal conditioning isolation direction sensing

Table and saddle step motors

Table and saddle encoders

U

12- bit

Contactors and drive amplifiers

A/D converter and

signal conditioning

Miscellaneous function

switching

0

Spindle feed and speed AC motor drives

Feed

LVDT

Eli Table and saddle

SAir bearing clamp

Clutch coolant

Spindle

Hardware limit switches

Figure 2. Hardware modules of the HDM microprocessor-based control system

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289

should be considered. • the hardware characteristics of the microprocessor family, • the power of its instruction set4,11,2°, • the aids and support available for developing microprocessor systems 19. In this application, the third criterion, i.e. the aids and support available for developing the microprocessor system dictated largely the choice of microprocessor family. The TMS family of microprocessor elements was well supported within the Department of Engineering Production at Loughborough with the availability of an FS 990/4 development system with in-circuit emulation facilities. Nevertheless, the other two criteria (as listed above) were also well satisfied with such z choice. Having chosen a processing element it was necessary to develop a microcomputer system with sufficient processing capability to facilitate control of the drilling machine. A low cost TM 990/100-1 CPU module was chosen to provide a base processing capability. This CPU module was upgraded to provide a microcomputer system appropriate for control of the drilling machine by designing and constructing memory and I/O expansion modules.

To allow programmed control of spindle feed, contactor drive and interface circuitry was included and used in conjunction with an LVDT which was fitted to the machine to establish drilling depth. A 12-bit A/D converter and sample and hold circuit was included to achieve z-axis position feedback. The air bearing and clamping pneumatic circuits were simplified and solenoid drive interface circuitry included. Simplification of the pneumatics was possible as switching and interlock functions could be included within the control software. Table 1. Software module functions MODULE

FUNCTIONS

Manual data input

Controls interactive communication between the operator and the processor

Table position control

Controls the drive of the machine table and saddle as commanded

Spindle drive control

Controls the spindle drive as commanded

Decimal number read

Read signed decimal number, decimal to binary conversion and data storage

Read and write ASC11 messages

Message handling to/from terminal

Parameter handling

] nterrupt vector, axis parameter and software and hardware limit switch handling

CONTROL HARDWARE AND SOFTWARE Control hardware and software was developed in modular form and used the microcomputer system described above. Figure 2 illustrates the hardware structure adopted. Interfacing of the retrofitted motor and feedback devices, for both the table and saddle, was achieved using drive and feedback electronics recommended by the device manufacturers. Additional interfacing was incorporated to allow the position of both table and saddle to be established on an interrupt basis.

Control initialization

t Terminal

Read and write message handling

ASCII

k

Parameter

handling

)

module

Table/saddle

MEI mad

Jl(

position control module

kl

Table and saddle

I I I I I I

t

R e a d decimal numbers module

\

Spindle drive control module

I:l

Spindle

1 1 e 3. Software modules of the HDM microprocessor-based control system

290

microprocessors and microsystems

A number of software modules were developed to facilitate real-time control of the machine axis and to provide operator communications. Figure 3 illustrates these modules and Table 1 describes the function of each module. The manual data input module was constructed as a master module and controls the transfer of data from an input terminal to the axis drive modules and provides a limited set of prompts to aid the operator in describing the drilling functions to be performed. Figure 4 illustrates the interactive format used during a typical terminal part programming session.

TABLE IS AT ZERO DATUM:

START (CR)

SETTABLE LEFTTRAVEL LIMIT ? = 450.00 SET SADDLE OUT TRAVEL LIMIT ? = 300.00 TABLE TRAVEL COMMAND: LEFT/RIGHT (+/-) ? = 200.00 OUT/IN (+/-) ? = 150.00 SPINDLE TRAVEL COMMAND: SPINDLE DOWN ? = 100.00 RAPID TRAVEL ~ = 50.00 OPERATION (MC/ED/ZP) ? MC

CONCLUSIONS Significant retrofitting of a first generation single spindle drilling machine was required to provide a reliable machine which could be controlled effectively using a modern NC system. Major modifications were made to the table, saddle and spindle drive systems and further modifications required to achieve efficient and reliable operation of the table and saddle air bearing pneumatics. A microprocessorbased NC system was designed and constructed for this machine to provide enhanced metal cutting facilities and to provide part programming aids for the operator. The decision to base the control hardware on a commercially available CPU module rather than to design and develop all controller hardware from the chip level proved to be effective. Considerable development time was saved and the reliability was improved throughout the testing phase. Among the range of available TMS 9900based CPU boards, the TMS 990/100M was chosen for its low cost, flexible hardware architecture, and extremely flexible I/O capability. This CPU module was complemented by static RAM expansion, EPROM expansion and programmable I/O expansion modules which were developed in-house to provide a customized microcomputer system. This microcomputer system is used in conjunction with specially developed interface modules to provide control hardware, which has proved to be satisfactory in both performance and flexibility. Although the hardware and control software of the developed control system was configured to suit a particular drilling machine, both the hardware and software were configured in modular form. Thus the control system can be used to control one of a range of single spindle drilling machines. Modification to modules can be actioned to account for variation in machine form and/or machine operational requirements. For some machines the required modifications could be achieved by making minor software changes while for other machines major changes to a number of modules, both hardware and software, would be required. However, for all machines a kerriel of hardware and software could be used to minimize the engineering development required. Similarly the use of a modular software structure will allow the inclusion or deletion of many advanced functions such as part-program editing, storing, and retrieval, pattern repeat, tool off-setting, diagnostic aids and comprehensive MDI facilities. In a current research project one such module is being developed to provide MDI facilities to augment those reported here. This module includes cutting technology to allow optimum drilling conditions to be established by an unskilled operator for a wide range of engineering materials.

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DRILLING OVER TABLE TRAVEL COMMAND: LEFT/RIGHT (+/-) ? = 200.00 OUT/IN (+/-) ? = 100.00 SPINDLE TRAVEL COMMAND: SPINDLE DOWN ? = = RAPID TRAVEL ? OPERATION (MC/ED/ZP) ? ED EDIT DATA: TABLE TRAVEL COMMAND: LEFT/RIGHT (+/-) ? = 200.00 OUT/IN (+/-) ? SPINDLE TRAVEL COMMAND: SPINDLE DOWN ? RAPID TRAVEL ? - OPERATION (MC/ED/ZP) ? MC DRILLING OVER TABLE TRAVEL COMMAND: LEFT/RIGHT (+/-) ? - OUT/IN (+/-) ? - SPINDLE TRAVEL COMMAND: SPINDLE DOWN ? - RAPID TRAVEL ? = = OPERATION (MC/ED/ZP) ? ZP TABLE AT ZERO DATUM TABLE TRAVEL COMMAND: LEFT/RIGHT (+/-) ? =

Figure 4. Terminal listing illustrating the base level of the MDI interactive format

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2

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microprocessors and microsystems