Rockwell Automation T6200 Compressor Anti-Surge and Capacity Controller - Manual (Page 105)

Control Primer
7-3
Control Primer
Controllers are used by the process industries as well as other industries. They are usually used, among
other things, to improve accuracy, efficiency, safety, environmental, and to automate, ie., when the
controller takes action it has a way to measure the result and then modify the action. Most of these
controllers are used in a closed-loop with negative feedback. Sometime they are used in an open-loop,
normally referred to as feedforward control.
With feedback control the output of the controller could be used to regulate a process, usually by
controlling a valve, and the input to the controller monitors the same process and compares it to a
reference, normally called the setpoint. If an error exists the output of the controller will change the
process to correct the error. In a typical feedforward control, the input to the controller would be from a
point ahead of the process being controlled. The controller, by measuring this disturbance, would try to
predict the required changes to the process and make them.
Feedback is either positive or negative. Positive feedback tries to produce an unbalance condition
causing instability. In an application where a pump is used to fill a tank and positive feedback was used in
the controller, the controller is intended to control the level of a tank by turning the pump on and off. In a
condition where the pump was on and the level was above the desired level, the pump would continue to
run, overfilling the tank. With negative feedback the controller works to restore balance. The pump would
stop when the level in the tank reaches the desired level.
The controller is made up of several elements. Figure 7-1 represents a simplified digital controller
showing the most common elements.