PID control loop diagram (courtesy of Wikipedia) |
Many types of PID controllers exist on the market and are used for controlling temperature, pressure, flow, and just about every other process variable. Here is a brief explanation of the three actions that make up PID.
Proportional Control Action (P): The controller output responds in proportion to error signal. The characteristic equation for this action is:
Proportional Control Action (P): The controller output responds in proportion to error signal. The characteristic equation for this action is:
- Where, Kp is called proportional gain, e is the error magnitude and B is the output from controller when there is no error. It is also called bias.
- In a proportional controller, the value of gain is set as required by the process and can be varied from 0 to ∞.
- Where, e= error, Ti= Time interval of integral action.
- Purpose of integral action is to provide adequate control action on varying demands of process. In this type of action, output varies as per the time integral of error. This action does not exist independently and always associated with proportional control.
Today's loop controllers are much easier to set the PID, thanks to auto-tuning algorithms. What used to be a very time consuming and tedious job can now be done with the push of a button and allowing the controller to "learn" the process dynamics. PID controllers minimize error and optimize the accuracy of any process.