Rockwell Automation T6200 Compressor Anti-Surge and Capacity Controller - Manual (Page 110)

Control Primer
7-8
Integral (I) Action
The integral action, also called "reset" action, has a dynamic response. The controller output
response may not have the same shape as the controller error response, as it does in proportional
action. Refer to Figure 7-4. The controller error is integrated over time. The integral time, also
called the "reset" time, is the time constant of the controller.
The advantage of the integral action is that it does not have the offset problem that the proportional
action has. The disadvantage is that it has a lag, causing it to have a slower response time than the
proportional action.
Derivative (D) Action
The derivative action, also called "rate control", also has a dynamic response, but is the inverse of
integral action. This action takes the derivative of the controller error with respect to time, refer to
Figure 7-5. Derivative action is not used by itself. It is normally used with proportional action or
with proportional-plus-integral action. This action is normally in the controllers output stage rather
than in the calculator.
Integral (I) Control
Action
Figure 7-4