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The block diagrams shown in Figures 2-12 and 2-13 indicated symbolically that the error was computed as being the difference between the set point and the process variable, specifically

e = SP – PV. (2-9)

However, we could have computed error in the following way:

e = PV – SP. (2-10)

Either method is correct under some circumstances, but for any given loop only one method is correct.

A controller, whether it is implemented in hardware or software, has an attribute that is either direct-acting or reverse-acting. These terms refer to the relative direction of movement of the process variable and controller output. If on an increase in the process variable the controller’s response is to increase its output, then the controller is said to be direct-acting because the con-troller output directly follows the measurement. If on an increase in the process variable the controller’s response is to decrease its output, the controller is said to be reverse-acting. Thus, computing the error as SP – PV implies that the controller is reverse-acting, whereas PV – SP implies that the controller is direct-acting.

This attribute, direct- or reverse-acting, must be set properly by an instrumentation or control system engineer in order for the control loop to function correctly. The proper setting depends upon the process as well as the failure mode final actuator. Within the loop, there must be an attribute called negative feedback, so named because if a measurement moves away from set point, the control action will be in the direction that returns it to set point. The opposite of neg-ative feedback is obviously positive feedback. As an example of positive feedback, suppose we have a temperature control loop in which the controller adjusts the position of a steam valve.

If, on a rise in temperature, the control action is such that the steam valve is opened, this will cause a further rise in temperature, which will cause a further opening of the steam valve, which will cause ... and so on. This is positive feedback; it must be avoided.

Engineers may use two thought processes to determine the proper setting. One way is to con-sider the control problem by thinking of the process action; the other is to concon-sider the control-ler action. Either way will arrive at the same results.

To consider the process (plant) itself, as shown in Figure 2-12, suppose there is an increase in

due to the action of a manual regulator installed in lieu of a controller.) Does this cause the sig-nal that represents the process variable to increase or decrease? To answer this question, we must know the failure mode of the valve (fail-closed, often called “air-to-open,” or fail-open, often called “air-to-close”). We also need to know the effect of the manipulated variable on the process variable (is it steam or cooling water?). We need to know whether the valve positioner is direct- or reverse-acting; we may also encounter a transmitter whose output signal increases when the physical variable decreases.

If an increasing signal to the process (valve signal) causes the measurement signal to increase, we say that the process is direct-acting. If an increasing signal causes the measurement signal to decrease, the process is said to be reverse-acting. Then, to have negative feedback, the con-troller must be the opposite of the process. A majority of processes are direct-acting (due to the fail-closed action of the valve); hence a majority of controllers are set reverse-acting. Figures 2-12 and 2-13, which show error computed as SP – PV, depict reverse-acting controllers.

The other thought process is to consider the required controller action. We ask ourselves the question, “If the measurement signal increases, do we want the controller output to increase or decrease?” If the desired action of the controller output is to increase, then set the controller direct-acting. Otherwise, set the controller reverse-acting. This thought process requires us to implicitly consider all of the factors mentioned above: Is the valve fail-open or fail-closed? Is the manipulated variable steam or cooling water? And so on. In contrast, the first thought pro-cess explicitly considers all of these factors.

Some manufacturers of digital control systems separate the consideration of the controller’s direct or reverse action from the failure mode of the valve. The controller output signal, rang-ing from 0 to 100 percent, always represents the “percent open” of the valve. Therefore, the direct or reverse action of the controller represents the relative direction of the process variable and valve movement, regardless of whether the valve is fail-open or fail-closed. Then, a sepa-rate configuration question, which is applicable to the analog output function block, asks whether the signal should be reversed or not. If the signal is not reversed, 0 to 100 percent of the signal from the controller is converted into a 4–20 mA signal to the valve. This would nor-mally be the choice for fail-closed valves. If the signal is reversed, 0 to 100 percent of the sig-nal from the controller is converted into a 20–4 mA, typically for fail-open valves. This application is depicted in Figure 2-14.

™ REFERENCES

2-1. Bob Connell. Basic Math for Process Control. ISA – The Instrumentation, Systems, and Automation Society, 2003.

2-2. ISA-5.1-1984 (R 1992), Instrumentation Symbols and Identification. ISA – The Instrumentation, Systems, and Automation Society, 1992.

2-3. SAMA Standard PMC 22.1-1981, Functional Diagramming of Instrument and Control Systems. Scientific Apparatus Makers Association, 1981 (MCAA web site: http://www.measure.org).

2-4. The Automation, Systems, and Instrumentation Dictionary, 4^th Edition. ISA – The Instrumentation, Systems, and Automation Society, 2003.

Figure 2-14. Direct- and Reverse-Acting Configuration Used in Some Digital Systems 3,'

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PROCESS AND CONTROL LOOP