PID Basics

Direct action and Reverse action

Direct action: It will lead the MV to increase when the PV is larger than the SV.
Reverse action: It will lead the MV to decrease when the PV is larger than the SV.

Proportional (P) control action

Proportional control (P) generates the MV in proportion to the error (E: difference between SV and PV).
In the P control action, the MV is calculated as follows.
MV=KP + E
KP : Proportional gain
If the KP value is too large, the control process is getting fast but the system will be in danger to oscillate. If the value is too small, the control process is getting slow to make it stable.

Under proportional control, the offset (residual error) will remain until the operator manually changes the bias on the controller’s output to remove the offset.

Integral (I) control action

Integral (I) control will generate the MV in proportion to the time-integral of the error (E). Integral action eliminates offset. If the integral time is set too long, the controller will be sluggish, if it is too short, the control lop will oscillate and become unstable.

The Integral (I) action is used with PI control or PID control. It is not used by itself.

Derivative (D) control action

Derivative (D) control will generate the MV in proportion to the rate of change in the error (E). By adding the D control, quick corrective action can be obtained at the beginning of upset condition.
If the derivative time is set too long, oscillations will occur and the control loop will run unstable. If the derivative time is set 0, the derivative control does not function.

The Derivative control is used with PI and PID control. It is never used alone.