The term RSET RPM means reset in repeats per minute. The value you set as RSET RPM governs how frequently proportional action is repeated within each minute. Be careful not to confuse that term with RSET MIN, which means reset in minutes per repeat. The latter means there is one or more minutes between each repeat of proportional action. In either case, the reset is for integral time.
This correction is needed because of an inherent weakness of proportional control. Proportional control requires a significant error condition to create an output signal. Accordingly, proportional control alone can never actually achieve the desired condition. Some small amount of error, known as system offset will always be present. Fig. 9 shows the type of control response typical of proportional control alone. Note the offset from set point.
The integral action is designed to eliminate offset. Because the offset’s magnitude is relatively small, it cannot change the control signal significantly by itself. An integrating term is used to observe how long the error condition has existed, summing the error over time. The summation value becomes the basis for an additional control signal, which is added to the signal produced by the proportional term. The control loop then continues to produce a control action over time, allowing the elimination of offset.
Adding integral action to the controller output can:
- Respond to the presence of error in the control loop.
- Relate the magnitude of the control signal to that of the error.
- Respond to offset over time to achieve zero error-set point.
Fig. 10 shows the control response typically produced with proportional-integral control. The significant difference is the elimination of offset once the system has stabilized.
The term RATE MIN refers to rate per minute, which can range from 0.00 to 10.00 (0.08 or less = OFF). The value you set as RATE MIN governs how much braking action is applied to the output of the controller as it corrects for error. This correction is applied only when the error is changing and increases when error changes faster.
This correction is needed because proportional control has a tendency to overshoot. Overshoot refers to a control loop’s tendency to overcompensate for an error condition, causing a new error in the opposite direction. Overshoot can cause unnecessary overheating.
Overshooting is corrected by a derivative action that provides an anticipatory function to exert a “braking” action on the control loop. The derivative term is based on the error’s rate of change. It observes how fast the PV approaches SP and produces a control action based on this rate of change. This additional action anticipates the convergence of PV and SP, in effect counteracting the control signal produced by the proportional and integral terms. The result is a significant reduction in overshoot.
Fig. 11 shows the effect of both integral and derivative actions to reduce overshoot and eliminate offset in proportional control.