The lag block reduces overshoot when the position demand changes faster than the actuator can respond. Ideally, the lag exactly matches the response of the actuator and therefore no change is required out of the Ki/s block. An SPC adjusted to an under-damped condition will be more responsive with less gain margin. It is more susceptible to instability due to changes in the system. An SPC adjusted to an over-damped condition will have slower response and better stability. It is less susceptible to instability due to changes in the system. The ideal adjustment is a critically damped condition. This produces the best stability and response combination. Ref. Manual 83402 PID Control for additional information on tuning the control.
Configuring and Adjusting a P Control
P—The P Controller provides the best stability. This control is the simplest controller, is very robust and works well for systems that aren’t very sensitive to position errors. Steady-state errors will exist if the Actuator Null Current value does not equal the actual null current of the actuator. Forward/Reverse Acting Control Selects the sense of the error signal in the controller. For example, when the demanded position exceeds the actual position (positive error), a forward-acting controller drives the output more positive, while a reverse- acting controller drives the output more negative.
Actuator Minimum Current
Determines the most negative actuator current that can be output, and sets the scaling for the -100 % controller output point. Allowed Range: –250 mA to +250 mA Actuator Maximum Current Determines the most positive actuator current that can be output, and sets the scaling for the 100 % controller output point. Allowed Range: –250 mA to +250 mA Actuator Null Current Should be set equal to the actual null current of the actuator. Allowed Range: Actuator Minimum Current to Actuator Maximum Current
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