Rockwell Automation T6200 Compressor Anti-Surge and Capacity Controller - Manual (Page 106)

Control Primer
7-4
The input block represents the input signal conditioning circuits. The setpoint is the controller's
reference. The setpoint is usually set by an operator, but sometimes the setpoint is the output of
another controller. The comparator compares the input and setpoint and the difference is called the
error. This error is passed on to the calculator. The expression for the error is:
e = r - c
(7-1)
where: c = controller input
e = comparator error
r = controller setpoint
In an analog controller, the calculation is done continuously. In a digital controller, the calculations
are done on a predetermined interval called the scan time. This scan time is fast enough that the
result is the same.
The calculator performs the proportional-plus-integral-plus-derivative (PID) algorithm. The P-I-D is
called the mode of the controller. The controller may use one or more of these functions, ie., P, I, PI,
PD, or PID (the D and DI are not normally used). A process controller that can perform all three
modes is called a PID controller even if it is not using all three modes.
Figure 7-1.
A Simplified
Feedback Controller
in a Process Loop