Working of PID control

PID control is closed loop control system used widely in industrial automation. PID stands for Proportional-Integral-Derivative control. PID control is the combination of these three controllers.

Conventional ON/OFF controllers are used only where we needed two-stage control operation, which always produces offset in the output. Oscillating nature of ON/OFF system between On and Off limits its usage and hence, it is replaced by PID controller.

Proportional control:

Proportional control never reaches a steady state condition, so the controller requires a manual reset. A fundamental limitation of proportional control has to do with its response to changes in setpoint and changes in process load.

A “load” in a controlled process is any variable not controlled by the loop controller which nevertheless affects the process variable the controller is trying to regulate. In other words, a “load” is any factor the loop controller must compensate for while maintaining the process variable at setpoint.

Process response for PID is shown in above figure. Every process has a fixed setpoint, every controller tries to maintain the PV ( process variable) at the SP( setpoint). The PID controller takes a control action when the PV reaches the setpoint. The incoming PV will definitely have an impact on the process output. This will affect the PV in the output to produce an offset below or above the setpoint.

Integral / Reset control :

If we wish to restore the process to the setpoint after a disturbance then proportional action alone will be insufficient. Integral control system integrates the errors in the process until the error becomes zero.

Process response and reset action for an integral controller is shown above.

If the integration happens at too fast a rate, the controller’s output will “saturate” either high or low before the process variable can make it back to setpoint.

If we wish to restore the process to the setpoint we must increase the input over and above that required to restore a balance. The additional input must replace the lost process parameter and then revert to a mass balance situation to maintain the PV at the setpoint.

The additional action for the integral signal is reset action, it reset the process to a setpoint, reset action is the integral of an error signal to the integral controller action that it is possible to operate the control loop on integral action alone, without proportional. Typically, though, process controllers implement some form of proportional plus integral (“PI”) control.

Derivative/ Rate control :

The derivative is the rate of change of the variable with respect to the other variable. A controller with derivative action looks at how fast a variable change per unit of time, and take control action proportion to the rate of change variable.

Derivate control doesn’t look into the present PV value to control the output, but track the rate of change of variable and control the output with respect to it. It can be seen that the derivative action gives a large, immediate, control signal, which will limit the deviation. The proportional action is then superimposed upon this step. When the error stops changing derivative action ceases.

The rate response gives an immediate control signal, which will be equal to what the proportional response would be after some time. Derivative units are given in minutes.

If the process variable starts to change at a high rate of speed, the job of the derivative action is to move the final control element in such a direction as to counteract this rapid change, and thereby moderate the speed at which the process variable changes. In simple terms, derivative action works to limit how fast the error can change.

PID Control advantages:

  • Increased productivity

  • Decreased equipment strain.

  • Increased process safety