Question

PID controllers consist of Proportional, Integral, and Derivative blocks that manipulate the error of the Process Variable signal depending on their own characteristics. Some controllers use these three blocks while some other controllers use the P block or the P block in conjunction with another one. In any case, the P block seems to be always present.

Discuss if we can have controllers that employ only an I block or a D block alone.

Describe why or why not using these blocks alone is possible and the possible consequences of this type of use.

Answer 1

Firstly the output of controller changes smoothly in response to an error.

There are three types of controllers:

1.P-controller

2.I-controller

3.D-controller

P-controller: The proportional controller produces an output proportional to error signal.

I-controller: integral controller produces an output which is integral of error signal.

D-controller: Derivative controller produces an output which is derivative of error signal.

The Integral controller is not used alone because it responds slowly to an error signal and it will allow large deviation at the instant the error is produced. This may lead to system instability and cyclic operation.Due to this integral controller is not used alone.

The derivative controller is not used alone because if the error is constant and non zero then the output of derivative controller is zero so it won't work satisfactorily.

Due to these reasons the I and D controllers are not used alone and they are combined with P controller.