Controllers and Control Modes

As use of the personal computer (PC) became widespread, a number of applications were found for controller use. Distributed control systems (DCSs) began to replace the older pneumatic and 

Controllers can be operated in manual, automatic, or cascade control. During plant start-up, the controller is typically placed in the manual position and left there until the process has lined out. This process initiates a setpoint on the final control element however, it does not utilize a controlling function. It only opens the valve 50% or 25% and keeps it there until the technician changes the mode. This keeps the process from swinging up and down during start-up. After the process is stable, the operator places the controller on automatic and allows it to supervise the control loop function. At this point, the controller will open and close the control valve to maintain the setpoint. Cascade control describes how one control loop controls or overrides the instructions of another control loop in order to achieve a desired setpoint. In this case, a control loop s controller may use all five elements of another control loop as its final control element. 

The proportional band on a controller describes the scaling factor used to take a controller from 0% to 100% output. For example, if the proportional band is set at 50% and the amount of lift the final control element (in this case a giobe vaive) has off the seat is 4 inches, the control valve will open 2 inches. Range is the portion of the process controiied by the controller. For example, the temperature range for a controller may be limited to 80°F to 140°F. Span is the difference (A) between the upper and lower range limits. This value is always recorded as a single number. For example, the difference between 80 and 140 is 60, so the span is 60. 

The reset (or integral) mode is designed to reduce the difference between the setpoint and the process variable by adjusting the controller output continuously until the offset is eliminated. The reset mode responds proportionally to the size of the error, the length of time that it lasts, and the integral gain setting. 

A graph of the process should be a straight line when the process is in control. 

---