The quarter decay method is a closed loop controller tuning method. This means that the controller remains in automatic while tuning adjustments are made.
The quarter decay method defines the ultimate limit for tight controller tuning. Often, the tuning constants it produces are too tight (too sensitive) in processes that have sticky valves and noisy measurements.
To prevent controllers from going unstable unexpectedly, tuning constants should be set to values one-half as sensitive as those obtained with the quarter decay method. After these less sensitive tunings are exposed to actual upsets and irregularities, and the operators gain confidence in the controller tuning, it may be appropriate to make the tunings more sensitive.
The general tuning sequence is as follows:
1. With the controller in automatic, adjust all tuning constants to their least sensitive (least effective) setting. Proportional band should be at its highest value (proportional gain should be at its lowest value). Integral time should be at its highest value (most minutes per repeat or least repeats per minute). Derivative time should be at its highest value.
2. Make a small step change in controller setpoint and record the controller measurement until it settles out.
3. Change the setpoint back to its original value. Record the measurement as before.
4. Increase the proportional gain (reduce the proportional band) in small steps and repeat steps 1-3 until the recording of the output resembles Figure 300-20, curve B; that is, until the amplitude of the first positive excursion of curve B is approximately four times that of the second (thus the name, quarter decay method).
5. Measure the period of oscillation. Set the reset and derivative:
TR = P/1.5 minutes
(Eq. 300-7)
TD = P/6 minutes
(Eq. 300-8)
6. With TR and TD set at above values, reestablish controller gain for quarter decay.
Figures 300-21, 300-22, and 300-23 show how the three tuning parameters affect the response of a controller. With proportional-only control, settling time is fairly long and there is a permanent offset from the setpoint. Adding integral control reduces settling time and eliminates offset. Adding derivative control to proportional control reduces settling time but not offset. Only integral control eliminates the offset.
