Interview question - Proportional Integral Derivative controller Selection Advantages Disadvantages


#1

Proportional Controller

Proportional controller means that the percent change in the output of the controller is some multiple of the controller of the percent change in the measurement . The multiple is called gain of the controller.

Disadvantage of Proportional controller - Offset
Offset means that controller will maintain the measurement at a value different from the set point.

Integral Controller -
Integral controller is used to avoid offset.
As long as the measurement remains at set point there is no change in the output due to the integral mode in the controller. However ,when any error exists between measurement and set point, the integral action will cause the output to begin to change and continue to change as long as the error exists.
The more integral action in the there is in the controller ,the more quickly the output changes due to the integral response.

Disadvantages of Integral controller - It oscillate around the set point .
Integral wind-up

Derivative action

The derivative mode responds to how quickly the error is responding.
The derivative output is proportional to rate of change of error.The greater the rate of change the greater the output due to derivative response .
Derivative action is unnecessary on processes that respond fairly quickly to valve motion and cannot be used at all on processes with noise in the measurement signal such as flow since the derivative controller will respond to the rapid changes in measurement it sees in the noise.