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Automation System Structure
Automation System Structure
2
Chapter Outline 2.1 Introduction 15 2.2 Subsystems 15
2.2.1 Instrumentation 15 2.2.2 Human Interface 16 2.2.3 Control 17
2.3 Input Instrumentation Subsystem 18
2.3.1 Measurement of Data 18
2.4 Output Instrumentation Subsystem 19
2.4.1 Transfer of Control Command 19
2.5 Human Interface Subsystem 20
2.5.1 Direct Monitoring 20 2.5.2 Direct Control 21
2.6 Control Subsystem 21
2.6.1 Data Acquisition 21 2.6.2 Data Analysis and Decision Making 21 2.6.3 Control Execution 21 2.6.4 Communication 23
2.7 Summary 23
2.1 Introduction In Chapter 1, we briefly discussed the automation functionalities, cycle, and steps. In this chapter, those concepts are further elaborated to define the physical structure of the overall automation system, the functions of its independent subsystems, and their interconnections.
2.2 Subsystems The automation system is broadly divided into three subsystems: instrumentation, control, and human interface. They are interconnected, as illustrated in Fig. 2.1.
2.2.1 Instrumentation The instrumentation1 is the branch of engineering that deals with the measurement and control of process parameters. An instrumentation device measures the physical variable 1
www.wikipedia.org.
Overview of Industrial Process Automation. http://dx.doi.org/10.1016/B978-0-12-805354-6.00002-5 Copyright © 2017 Elsevier Inc. All rights reserved.
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Overview of Industrial Process Automation
Figure 2.1 Basic structure of automation system.
and/or manipulates it to produce an output in an acceptable form for further processing in the next stage device. Instrumentation devices are not normally intelligent and are not capable of making decisions (exceptions are discussed later in the book). The instrumentation subsystem primarily works in two directions: one on the input side of the control subsystem and the other on the output side. Hence the instrumentation subsystem can be further divided into input and output subsystems to explain the flow of data in the order they are received from a process, processed in the control subsystem, and sent to a process for actions: 1. The input instrumentation subsystem interfaces with the process to acquire data on the behavior of the process (measurement of process parameters) and sends them to the control subsystem in an acceptable form. 2. The output instrumentation subsystem interfaces with the process to send the command received from control subsystem to the process in an acceptable form, to change the behavior of the process (control of process parameters).
2.2.2 Human Interface The human interface subsystem2 presents information to the operator or user on the state of the process and facilitates implementing the operator’s control instructions to the process. The human interface subsystem is also called the human–machine interface, man–machine interface, human–system interface, and so forth. The human interface subsystem is a facility for the user or operator to interact directly with the process, via the control subsystem, for: 1. Direct monitoring of process parameters to know what is happening inside the process (monitoring of process behavior), and 2. Direct control of process parameters by forcing a change, if required, by issuing manual commands (controlling of process behavior). 2
www.wikipedia.org.
Automation System Structure
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2.2.3 Control The control subsystem3 is a mechanism or device for automatically manipulating the output of a process and for managing, commanding, directing, or regulating the behavior of the process to achieve the desired result. The control subsystem is the heart of the automation system. It is an intelligent device capable of making decisions. The control subsystem manages data flow to/from the instrumentation subsystems for process monitoring and control, and to/from the human interface subsystem for direct interaction with the process; it can exchange data bidirectionally with other external compatible systems if required. Apart from data analysis and decision making, the control subsystem performs the following functions with respect to other subsystems: 1. Acquires data on process parameters via the input instrumentation subsystem to monitor the behavior of the process continuously, 2. Issues commands to process via the output instrumentation subsystem to correct or change the behavior of the process, 3. Routes process data to the human interface subsystem displays for direct monitoring, and 4. Acquires direct commands from the human interface subsystem and routes them to process for the control of process parameters.
The following sections further explain the implementation of functions associated with the subsystems with specific reference to automation of the water-heating process.
Figure 2.2 Basic water-heating process.
Fig. 2.2 illustrates the basic water-heating process with a manual control facility for letting cold water into the tank and switching power on to the heater. Here, the breaker controls the flow of power to the heating element while the valve controls the flow of water to the tank. To implement automatic control of the water level, water temperature, and measurement of water and power consumption, the following functions need to be implemented: 1. Check the actual water temperature to know whether it is higher than, equal to, or less than the desired value. 2. Keep the breaker closed if the temperature is less than the desired value. Otherwise open the breaker. 3. Check the actual water level to know whether it is higher than, equal to, or less than the desired level. 3
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Overview of Industrial Process Automation
4. Keep the valve open if the level is lower than the desired value. Otherwise close the valve. 5. Measure the water and power consumptions.
The following sections illustrate the application of automation subsystems to implement these steps.
2.3 Input Instrumentation Subsystem 2.3.1 Measurement of Data Because they are electronics-based, modern automation systems understand and process signals only in electronic form. Hence in measuring data from the process, instrumentation devices need to convert physical process signals (electrical, mechanical, chemical, thermal, etc.) into their electronic equivalents so that the control subsystem can accept and process them. Their conversion into electronic form is carried out with no loss of information. Required input instrumentation devices are: 1. Temperature and level sensors to measure the water level and temperature, 2. Flow switch and supervision relay to measure the valve and breaker status, and 3. Water and energy meters to measure water and power consumption.
Because they are physical signals, level and temperature are measured, converted into their electronic equivalents, and sent to the control subsystem as analog inputs by temperature and level sensors. Fig. 2.3 illustrates the measurement of water temperature and level.
Figure 2.3 Measurement of water level and temperature.
Valve and breaker status are measured indirectly. The presence or absence of water flow in the pipe indicates valve status, open or closed, whereas the presence or absence of power flow in the line indicates breaker status, on or off. These status measurements are converted into their electronic equivalents and sent to the control subsystem as digital inputs by a flow switch and supervision relay. The flow switch detects the presence or absence of water flow in the pipe, whereas the supervision relay detects the presence or absence of power in the heater.
Automation System Structure
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Fig. 2.4 illustrates the measurement of valve and breaker status.
Figure 2.4 Measurement of status of valve and breaker.
In their physical form, water and power consumption are measured as a series of pulse inputs converted into their electronic equivalents and sent to the control subsystem by water and energy meters. The number of pulses over a period indicates consumption. The water meter measures the consumption of water, whereas the energy meter measures the consumption of power. Fig. 2.5 illustrates the measurement of water and power consumption.
Figure 2.5 Measurement of water and power consumption.
2.4 Output Instrumentation Subsystem 2.4.1 Transfer of Control Command Because the process understands only physical signals, electronic control signals generated by the control subsystem are converted into their physical equivalents (electrical, mechanical, etc.) in an acceptable form for the process to receive control
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Overview of Industrial Process Automation
commands from the control subsystem. Here also, the conversion or transformation is carried out with no loss of information. Control commands (to open or close the valve and open or close the breaker) generated by the control subsystem in electronic form are converted into their physical equivalents (electronic to mechanical) and sent to the process by a solenoid control and control relay (digital outputs). The solenoid opens or closes the valve to allow or disallow water to the tank, while the control relay opens or closes the breaker to allow or disallow power to the heater. Required output instrumentation devices are: 1. An on–off solenoid control to open and close the valve, and 2. An on–off control relay to open and close the breaker.
Figure 2.6 Control of valve and breaker.
Fig. 2.6 illustrates control of the valve and breaker.
2.5 Human Interface Subsystem The human interface subsystem is the means by which the users or operators directly interact with the process for direct monitoring and control of the process. The human interface subsystem is the interface between physical processes and the operator, via the control subsystem, for direct monitoring of the process parameters and to effect direct control of the process parameters. There is no need for any interface devices between the control subsystem and the human interface subsystem (unlike the need for an instrumentation subsystem between the process and the control subsystem). Signals between the control and human interface subsystems are electronic in both directions and therefore are compatible.
2.5.1 Direct Monitoring Direct monitoring provision is made for observing process variables of interest on the displays whenever there is a need. In other words, operators can observe exactly what is happening inside the process through visual display of parameters of interest.
Automation System Structure
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2.5.2 Direct Control Direct control provision is made for manipulating or controlling process parameters of interest whenever there is a need. This action overrides the functions performed automatically by the control subsystem. The provision for setting reference values for limit checking is also part of the human interface subsystem. The operator panel allows operators to perform the following manually: 1. Observe actual values of temperature and level, 2. Observe the actual status of the valve and breaker, 3. Observe the actual consumption of water and power, 4. Set reference values for temperature and level, and 5. Control (close or open) the valve and breaker.
Fig. 2.7 illustrates a typical hardware-based human interface or operator panel.
Figure 2.7 Operator panels for water-heating system.
2.6 Control Subsystem The control subsystem performs the following steps continuously.
2.6.1 Data Acquisition In this step, the control subsystem acquires process data in an electronic form from the input instrumentation subsystem, which in turn receives them from the process in a physical form.
2.6.2 Data Analysis and Decision Making In this step, the control subsystem analyzes the acquired data by comparing them with preset reference values for any deviations and decides whether to effect changes in the behavior of the process or not.
2.6.3 Control Execution In this step, the control subsystem issues commands to process in electronic form to output instrumentation devices, which in turn transfer them in a physical form to the process.
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Overview of Industrial Process Automation
Figure 2.8 Typical automation cycle.
Figure 2.9 Inputs and outputs in control subsystems.
Figure 2.10 Automation system for water-heating process.
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These steps are illustrated in Fig. 2.8 as a cyclic operation or automation cycle. Fig. 2.9 illustrates the input–output to–from control subsystems to the instrumentation and human interface subsystems for level and temperature control in the water-heating process. Fig. 2.10 illustrates the fully integrated automation system with all of its subsystems interconnected to monitor and control the water-heating process.
2.6.4 Communication Because they are intelligent and communicable, modern control subsystems can communicate externally with other compatible systems. This will be discussed in detail in Chapter 7.
2.7 Summary In this chapter, we discussed the overall structure of automation systems as well as their functional subsystems and interconnections. These functional subsystems are discussed further individually in greater detail in subsequent chapters.
2
Chapter Outline 2.1 Introduction 15 2.2 Subsystems 15
2.2.1 Instrumentation 15 2.2.2 Human Interface 16 2.2.3 Control 17
2.3 Input Instrumentation Subsystem 18
2.3.1 Measurement of Data 18
2.4 Output Instrumentation Subsystem 19
2.4.1 Transfer of Control Command 19
2.5 Human Interface Subsystem 20
2.5.1 Direct Monitoring 20 2.5.2 Direct Control 21
2.6 Control Subsystem 21
2.6.1 Data Acquisition 21 2.6.2 Data Analysis and Decision Making 21 2.6.3 Control Execution 21 2.6.4 Communication 23
2.7 Summary 23
2.1 Introduction In Chapter 1, we briefly discussed the automation functionalities, cycle, and steps. In this chapter, those concepts are further elaborated to define the physical structure of the overall automation system, the functions of its independent subsystems, and their interconnections.
2.2 Subsystems The automation system is broadly divided into three subsystems: instrumentation, control, and human interface. They are interconnected, as illustrated in Fig. 2.1.
2.2.1 Instrumentation The instrumentation1 is the branch of engineering that deals with the measurement and control of process parameters. An instrumentation device measures the physical variable 1
www.wikipedia.org.
Overview of Industrial Process Automation. http://dx.doi.org/10.1016/B978-0-12-805354-6.00002-5 Copyright © 2017 Elsevier Inc. All rights reserved.
16
Overview of Industrial Process Automation
Figure 2.1 Basic structure of automation system.
and/or manipulates it to produce an output in an acceptable form for further processing in the next stage device. Instrumentation devices are not normally intelligent and are not capable of making decisions (exceptions are discussed later in the book). The instrumentation subsystem primarily works in two directions: one on the input side of the control subsystem and the other on the output side. Hence the instrumentation subsystem can be further divided into input and output subsystems to explain the flow of data in the order they are received from a process, processed in the control subsystem, and sent to a process for actions: 1. The input instrumentation subsystem interfaces with the process to acquire data on the behavior of the process (measurement of process parameters) and sends them to the control subsystem in an acceptable form. 2. The output instrumentation subsystem interfaces with the process to send the command received from control subsystem to the process in an acceptable form, to change the behavior of the process (control of process parameters).
2.2.2 Human Interface The human interface subsystem2 presents information to the operator or user on the state of the process and facilitates implementing the operator’s control instructions to the process. The human interface subsystem is also called the human–machine interface, man–machine interface, human–system interface, and so forth. The human interface subsystem is a facility for the user or operator to interact directly with the process, via the control subsystem, for: 1. Direct monitoring of process parameters to know what is happening inside the process (monitoring of process behavior), and 2. Direct control of process parameters by forcing a change, if required, by issuing manual commands (controlling of process behavior). 2
www.wikipedia.org.
Automation System Structure
17
2.2.3 Control The control subsystem3 is a mechanism or device for automatically manipulating the output of a process and for managing, commanding, directing, or regulating the behavior of the process to achieve the desired result. The control subsystem is the heart of the automation system. It is an intelligent device capable of making decisions. The control subsystem manages data flow to/from the instrumentation subsystems for process monitoring and control, and to/from the human interface subsystem for direct interaction with the process; it can exchange data bidirectionally with other external compatible systems if required. Apart from data analysis and decision making, the control subsystem performs the following functions with respect to other subsystems: 1. Acquires data on process parameters via the input instrumentation subsystem to monitor the behavior of the process continuously, 2. Issues commands to process via the output instrumentation subsystem to correct or change the behavior of the process, 3. Routes process data to the human interface subsystem displays for direct monitoring, and 4. Acquires direct commands from the human interface subsystem and routes them to process for the control of process parameters.
The following sections further explain the implementation of functions associated with the subsystems with specific reference to automation of the water-heating process.
Figure 2.2 Basic water-heating process.
Fig. 2.2 illustrates the basic water-heating process with a manual control facility for letting cold water into the tank and switching power on to the heater. Here, the breaker controls the flow of power to the heating element while the valve controls the flow of water to the tank. To implement automatic control of the water level, water temperature, and measurement of water and power consumption, the following functions need to be implemented: 1. Check the actual water temperature to know whether it is higher than, equal to, or less than the desired value. 2. Keep the breaker closed if the temperature is less than the desired value. Otherwise open the breaker. 3. Check the actual water level to know whether it is higher than, equal to, or less than the desired level. 3
www.wikipedia.org.
18
Overview of Industrial Process Automation
4. Keep the valve open if the level is lower than the desired value. Otherwise close the valve. 5. Measure the water and power consumptions.
The following sections illustrate the application of automation subsystems to implement these steps.
2.3 Input Instrumentation Subsystem 2.3.1 Measurement of Data Because they are electronics-based, modern automation systems understand and process signals only in electronic form. Hence in measuring data from the process, instrumentation devices need to convert physical process signals (electrical, mechanical, chemical, thermal, etc.) into their electronic equivalents so that the control subsystem can accept and process them. Their conversion into electronic form is carried out with no loss of information. Required input instrumentation devices are: 1. Temperature and level sensors to measure the water level and temperature, 2. Flow switch and supervision relay to measure the valve and breaker status, and 3. Water and energy meters to measure water and power consumption.
Because they are physical signals, level and temperature are measured, converted into their electronic equivalents, and sent to the control subsystem as analog inputs by temperature and level sensors. Fig. 2.3 illustrates the measurement of water temperature and level.
Figure 2.3 Measurement of water level and temperature.
Valve and breaker status are measured indirectly. The presence or absence of water flow in the pipe indicates valve status, open or closed, whereas the presence or absence of power flow in the line indicates breaker status, on or off. These status measurements are converted into their electronic equivalents and sent to the control subsystem as digital inputs by a flow switch and supervision relay. The flow switch detects the presence or absence of water flow in the pipe, whereas the supervision relay detects the presence or absence of power in the heater.
Automation System Structure
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Fig. 2.4 illustrates the measurement of valve and breaker status.
Figure 2.4 Measurement of status of valve and breaker.
In their physical form, water and power consumption are measured as a series of pulse inputs converted into their electronic equivalents and sent to the control subsystem by water and energy meters. The number of pulses over a period indicates consumption. The water meter measures the consumption of water, whereas the energy meter measures the consumption of power. Fig. 2.5 illustrates the measurement of water and power consumption.
Figure 2.5 Measurement of water and power consumption.
2.4 Output Instrumentation Subsystem 2.4.1 Transfer of Control Command Because the process understands only physical signals, electronic control signals generated by the control subsystem are converted into their physical equivalents (electrical, mechanical, etc.) in an acceptable form for the process to receive control
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Overview of Industrial Process Automation
commands from the control subsystem. Here also, the conversion or transformation is carried out with no loss of information. Control commands (to open or close the valve and open or close the breaker) generated by the control subsystem in electronic form are converted into their physical equivalents (electronic to mechanical) and sent to the process by a solenoid control and control relay (digital outputs). The solenoid opens or closes the valve to allow or disallow water to the tank, while the control relay opens or closes the breaker to allow or disallow power to the heater. Required output instrumentation devices are: 1. An on–off solenoid control to open and close the valve, and 2. An on–off control relay to open and close the breaker.
Figure 2.6 Control of valve and breaker.
Fig. 2.6 illustrates control of the valve and breaker.
2.5 Human Interface Subsystem The human interface subsystem is the means by which the users or operators directly interact with the process for direct monitoring and control of the process. The human interface subsystem is the interface between physical processes and the operator, via the control subsystem, for direct monitoring of the process parameters and to effect direct control of the process parameters. There is no need for any interface devices between the control subsystem and the human interface subsystem (unlike the need for an instrumentation subsystem between the process and the control subsystem). Signals between the control and human interface subsystems are electronic in both directions and therefore are compatible.
2.5.1 Direct Monitoring Direct monitoring provision is made for observing process variables of interest on the displays whenever there is a need. In other words, operators can observe exactly what is happening inside the process through visual display of parameters of interest.
Automation System Structure
21
2.5.2 Direct Control Direct control provision is made for manipulating or controlling process parameters of interest whenever there is a need. This action overrides the functions performed automatically by the control subsystem. The provision for setting reference values for limit checking is also part of the human interface subsystem. The operator panel allows operators to perform the following manually: 1. Observe actual values of temperature and level, 2. Observe the actual status of the valve and breaker, 3. Observe the actual consumption of water and power, 4. Set reference values for temperature and level, and 5. Control (close or open) the valve and breaker.
Fig. 2.7 illustrates a typical hardware-based human interface or operator panel.
Figure 2.7 Operator panels for water-heating system.
2.6 Control Subsystem The control subsystem performs the following steps continuously.
2.6.1 Data Acquisition In this step, the control subsystem acquires process data in an electronic form from the input instrumentation subsystem, which in turn receives them from the process in a physical form.
2.6.2 Data Analysis and Decision Making In this step, the control subsystem analyzes the acquired data by comparing them with preset reference values for any deviations and decides whether to effect changes in the behavior of the process or not.
2.6.3 Control Execution In this step, the control subsystem issues commands to process in electronic form to output instrumentation devices, which in turn transfer them in a physical form to the process.
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Overview of Industrial Process Automation
Figure 2.8 Typical automation cycle.
Figure 2.9 Inputs and outputs in control subsystems.
Figure 2.10 Automation system for water-heating process.
Automation System Structure
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These steps are illustrated in Fig. 2.8 as a cyclic operation or automation cycle. Fig. 2.9 illustrates the input–output to–from control subsystems to the instrumentation and human interface subsystems for level and temperature control in the water-heating process. Fig. 2.10 illustrates the fully integrated automation system with all of its subsystems interconnected to monitor and control the water-heating process.
2.6.4 Communication Because they are intelligent and communicable, modern control subsystems can communicate externally with other compatible systems. This will be discussed in detail in Chapter 7.
2.7 Summary In this chapter, we discussed the overall structure of automation systems as well as their functional subsystems and interconnections. These functional subsystems are discussed further individually in greater detail in subsequent chapters.