American Power Conversion Central Air Conditioning System room load 1.Select the PID loop to tune

Operation: Control the Environment

How to tune the PIDs

A qualified service technician must tune the PID loops to optimize the performance of each mode. The units shown are for temperature. In the dehumidify loop, the units are in percent relative humidity (%RH).

The PID loops must be tuned after the equipment in the room is installed and running. The loops should be tuned periodically to account for changes in the

room load.

1.Select the PID loop to tune.

2.Adjust the integral and derivative constants to zero, and the proportional constant to twenty.

3.Set the temperature (cool or reheat) setpoint value and start the System.

4.When the temperature reaches a stable value, increase the proportional constant by 10. If the temperature does not respond to this change, adjust the setpoint.

5.Repeat step 4 until the temperature value starts to oscillate and the peak amplitude of the oscillations is constant.

6.Note the time, in minutes, between the peaks of the oscillations. This is the Ultimate Period, PU.

7.Note the value of the proportional constant. This is the Ultimate Gain, GU.

8.Compute the PID constants using the table below. Select the type of control from the rows of the table, and use the equations in the columns to compute the values for each constant.

Control Method	Proportional	Integral	Derivative
P	0.5*GU	–	–
P + I	0.45*GU	0.54*GU/PU	–
P + I + D	0.6*GU	1.2 * GU/PU	0.075*GU*PU

9. Set each constant according to these calculations.

You may need to further tune the PID loops in the following ways to account for changes in the room load:

Loop Behavior	PID Tuning Correction
Slow response to temperature/humidity	Decrease the proportional multiplier or increase the
changes	derivative multiplier.
Over-cooling/Under-cooling after changes	Increase the derivative or the proportional multipliers.
in the room load or changes in setpoint
Environmental temperature/humidity never	Increase the integral setting.
reaches the setpoint
Over-cooling/Under-cooling on constant	Decrease the integral multiplier.
room load

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