-1 P ROCESS C ONTROL B ASICS

1 4 -1 P ROCESS C ONTROL B ASICS

Process control is the regulation of designated process parameters to within

a specified target range or to a set target value called the set point. Process control is most often used in product manufacturing, because many factors, such as color, composition, and density, must be accurate for a product to be well made. Therefore, to implement a quality product, process control is used to monitor and correct process parameters by analyzing the state of dynamic variables. Dynamic variables are process characteristics, such as tempera- ture, flow, and pressure, that vary with time. Through its I/O interfaces, a PLC can regulate these dynamic variables to a desired set point, thus implementing process control.

Figure 14-1 illustrates the basic concept of process control using a reactor tank in which steam controls the temperature in the tank. In this case, the temperature must be maintained at a target value, or set point, of 125 ° C. Because it varies with time, the temperature is the dynamic variable, which is also called the process variable (PV). The steam level, which regulates the process variable (i.e., raises and lowers the temperature), is called the control variable (CV). The valve that controls the amount of steam entering the reactor tank’s jacket is called the control element, or final output field device , because the more the controller opens the valve, the more the steam increases the temperature.

Figure 14-2 shows the block diagram of the process control system illus- trated in Figure 14-1. The PLC reads the process variable from the system (i.e., obtains feedback) and compares it with the set point to determine how well the temperature is being regulated. This configuration is known as a closed-loop system, because the controller uses feedback to monitor the system. An open-loop system does not use feedback, so the controller does not receive process variable data. Figure 14-3 illustrates the configuration of an open-loop system.

If the temperature reading in the process in Figure 14-2 is low, the controller will adjust the control variable by opening the valve to allow steam to enter the tank, thereby raising the temperature. The controller will then recheck

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S ECTION PLC Process Process Responses C HAPTER 4 Applications

and Transfer Functions 14

Material 1 Material 2

Temperature Sensor

Product Discharge Figure 14-1. Reactor tank control system.

SP

CV Process Σ PV

E = SP – PV Controller

Control Feedback

Figure 14-2. Block diagram of the closed-loop reactor tank system.

Figure 14-3. Open-loop process control system.

the process variable. If the temperature is still low, it will again open the steam valve to increase the temperature. The controller will repeat this process until the actual temperature (process variable) is as close as possible to the target temperature (set point value). The difference between the process variable and the set point is called the error. The error can be either positive or negative, depending on whether the process variable is too high or too low. However, regardless of the sign of the error, the controller still performs the same basic function—adjusting the process variable until it equals the set point (i.e., making the error equal to zero). Once equality is achieved, the

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S ECTION PLC Process Process Responses C HAPTER 4 Applications

and Transfer Functions 14

process is said to be regulated. As we will discuss later, many factors can disturb the system, thus altering the process variable. Therefore, the controller must adjust the control variable to correct for errors created by these factors, as well as to correct for errors due to a change in the set point.

In a PLC-based system, a control system block diagram like the one shown in Figure 14-2 can be expanded to include interfaces that control the field output devices, as well as those that read process variable input data (see Figure 14-4). Figure 14-5 shows a control system that uses a PID interface (discussed in Chapter 8) to implement process control independent of the

PLC. The next chapter will further explain PID control.

Analog Output

Controller CV 1C Material 1 Material 2

Analog Input

Product Discharge

Figure 14-4. Control system block diagram including I/O interfaces.

The adjustment of the control variable according to data obtained by reading the process variable and analyzing the error between it and the set point is referred to as the control loop. Most control loops are affected by distur- bances , which influence the process and alter the process variable (see Figure 14-6). To understand disturbances, let’s examine a simple control loop example—a car’s cruise control mechanism. As shown in Figure 14-7, once the cruise control has been set at a target speed (set point), the system will maintain that speed by keeping the accelerator (control variable) at a constant level. However, if the system experiences a disturbance, such as pavement with higher friction or an uphill climb, the system will increase the control variable (i.e., increase acceleration) to maintain the set point speed. This is demonstrated by the fact that the accelerator pedal of a car

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S ECTION PLC Process Process Responses C HAPTER 4 Applications

and Transfer Functions 14

Processor

PID Module

Block Transfer

Material 1 Material 2

Temperature Sensor

Transmitter

Product Discharge Figure 14-5. Control system using a PID interface.

Disturbance

SP

Σ E = SP – PV Controller

CV Process PV

– PV

Controller must adjust its output to correct for the error created by a disturbance

Figure 14-6.

A control loop with a disturbance.

Accelerator

Actual

Set Point +

Car’s Engine Σ Speed

E Cruise Speed

& Drive Train

Cruise Speed)

– PV

(Actual Speed)

Figure 14-7. Cruise control process loop.

under cruise control is depressed further when the car is going uphill than when it is going downhill. Figure 14-8 illustrates how a cruise control system compensates for disturbances. The components that form this simple system respond to keep the process variable at the set point by adjusting the control variable to maintain the error at zero during the disturbance. This, in fact, is the main function of process control—monitoring the error signal generated by the system and adjusting the outputs accordingly.

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S ECTION PLC Process Process Responses C HAPTER 4 Applications

and Transfer Functions 14

Hill Hill

Speed Begins Ends

PV

Begins Ends

Figure 14-8. Cruise control compensation graphs showing (a) the set point speed, (b) the reaction of the process variable to the disturbance, (c) the reaction of the control variable to the disturbance, and (d) the error.