Appendix A: Pid Fundamentals; Pid Control Fundamentals; Proportional Control; Integral Control - FACTS Engineering Direct Logic 405 Manual

Four loop

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APPENDIX A: PID FUNDAMENTALS

This section describes the proportional, integral and derivative control actions (PID) of the Temperature

Controller. The Four Loop Temperature Controller maintains the temperature of a process. This

temperature is called the Process Variable (PV). The controller action is called the Output (sometimes

referred to as the manipulated variable).

Proportional Control action produces an output signal that is proportional to the difference between the

SetPoint and the PV (sometimes referred to as the deviation or the error). A large error will generate a

large output. The output signal is normally represented as a percent of output for the PID Control Period

(V5020). Thus a 25% output with a 2 second control period will turn the solid state relays on for ½

second and off for 1½ seconds.

Proportional Control action includes a gain adjustment. The amount of gain determines the amount of

output produced for a given error. For example, a gain setting of 10 will produce a 50% change in output

for a 5% change in temperature (here temperature is expressed as a percent of range such that 100% is

3276.7 degrees).

The highest gain settings approach on-off control. Too much gain may result in an unstable or oscillating

temperature. Too little gain results in a sustained error called offset.

Integral Control action is dependent upon time. It responds over time to the offset inherent in the

proportional only controller. It will accommodate SetPoint and load changes by continuing to adjust the

output until the error is eliminated. The actual response of the integral action depends on the setting of

the reset adjustment.

The integral and proportional actions combine to force the temperature to the SetPoint. This is

sometimes referred to as automatic reset.

Even though proportional plus integral action eliminates offset, it may cause temperature overshoot.

Derivative Control action is used to overcome excessive temperature overshoot. It is used only in

combination with the proportional and integral control actions. Derivative Control action responds to the

rate of change and direction of the error. The actual response of the integral action depends on the

setting of the rate adjustment.

Traditional higher performance PID controllers include some form of derivative gain limiting. This is

used when instabilities in the PV due to noise or other sources cause the output to become unstable.

The FACTS Engineering Temperature Controller uses a proprietary Derivative Control action filter which

de-glitches the PV while at the same time retaining the full effect of the rate setting. For correctly tuned

loops, this method results in a lower damping ratio than a comparable controller with derivative gain

limiting.

5.1

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