Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 87)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 87
Drive Configuration Chapter 1
PID Gains
Parameters P1086 [PID Prop Gain], P1087 [PID Int Time], and P1088 [PID
Deriv Time] determine the response of the PID.
Proportional control (P) adjusts output based on size of the error (larger error =
proportionally larger correction). If the error is doubled, then the output of the
proportional control is doubled. Conversely, if the error is cut in half then the
output of the proportional output is cut in half. With only proportional control
there is always an error, so the feedback and the reference are never equal. [PID
Prop Gain] is unit less and defaults to 1.00 for unit gain. With [PID Prop Gain]
set to 1.00 and PID Error at 1.00% the PID output is 1.00% of maximum
frequency.
Integral control (I) adjusts the output based on the duration of the error. (The
longer the error is present, the harder it tries to correct). The integral control by
itself is a ramp output correction. This type of control gives a smoothing effect to
the output and continues to integrate until zero error is achieved. By itself,
integral control is slower than many applications require and therefore is
combined with proportional control (PI). [PID Int Time] is entered in seconds.
If [PID Int Time] is set to 2.0 seconds and PI Error is 100.00% the PI output
integrates from 0 to 100.00% in 2.0 seconds.
Derivative Control (D) adjusts the output based on the rate of change of the error
and, by itself, tends to be unstable. The faster that the error is changing, the larger
change to the output. Derivative control is usually used in torque trim mode and
is usually not needed in speed mode.
For example, winders using torque control rely on PD control not PI control.
Also, P1084 [PID LP Filter BW] is useful in filtering out unwanted signal
response in the PID loop. The filter is a Radians/Second low pass filter.
PID Lower and Upper Limits/Output Scaling
The output value produced by the PID is displayed as ±100% in P1093 [PID
Output Meter].
P1082 [PID Lower Limit] and P1081 [PID Upper Limit] are set as a percentage.
In exclusive or speed trim mode, they scale the PID Output to a percentage of
P37 [Maximum Freq]. In torque trim mode, they scale the PID Output as a
percentage of rated motor torque.