7.1.8 High and Low Cutback Cb.Hi and Cb.Lo

Cutback is a unique feature of the algorithm which is used to avoid overshoot while allowing highly responsive control.

 

Cutback High and Cutback Low are values that modify the amount of overshoot, or undershoot, that occurs during large step changes in PV (for example, under start-up conditions). They are independent of the PID terms, which means that the PID terms can be set for optimal steady state response and the cutback parameters used to modify any overshoot which may be present.

 

Cutback involves moving the proportional band towards the cutback point nearest the measured value whenever the latter is outside the proportional band and the power is saturated (at 0 or 100% for a heat only controller). The proportional band moves downscale to the lower cutback point and waits for the measured value to enter it. It then ‘escorts’ the measured value with full PID control to the setpoint. In some cases it can cause a ‘dip’ in the measured value as it approaches setpoint, as shown in the diagram below, but generally decreases the time needed to bring the process into operation.

 

The action described above is reversed for falling temperature.

 

If cutback is set to Auto the cutback values are automatically configured to 3*PB.