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Control Loop Tuning
How to Tune a Loop the Old Way
SET UP
Place the control loop in manual mode when the process value and set point are close. Adjust the output so the process and set point are together, turn off integral and derivative responses and
switch to automatic mode.
STEPS
1. Create a disturbance in the system by adjusting the set point a small amount either up or down. The resulting disturbance must be large enough to observe, but not so large that it causes signal saturation at any point in the control loop. This means that the controller output should be within its working range of 0 to 100% (4 to 20mA) at all times during the recovery and the process variable change during recovery should be small enough to be considered linear about the set point.
2. Hold the set point at this value until the process variable begins to move, then return the set point to its original value.
3. Compare the recorded oscillation with the graphic shown. Cycle B should be approximately 1 /4 of cycle A as shown in the figure. If it is not, change the proportional band setting and repeat Steps 1 and 2.
4. Decreasing the proportional band setting (increasing gain) increases cycle B relative to cycle A. If the controller has proportional response (P) only, no further adjustment is required. If the controller has two or three responses, proceed to Step 5.
5.. Measure the period (P) of oscillation in minutes. The period is the time required for one complete cycle.
8) Calculate I (integral) and D (derivative) values as follows:
a. Controller with P and I responses: I = P (Minutes)
b. Controller with P and D responses: D = P (Minutes) / 8
c. Controller with P, I, and D responses: I = 1.5 / P (Minutes) D = P (Minutes) / 6. 9)
