Allen-Bradley 22C-B012N103, User Manual

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Rockwell Automation Publication 22C-UM001J-EN-E - January 2017 163
Application Notes Appendix D
• A154 [PID Prop Gain]
The proportional gain (unitless) affects how the regulator reacts to the
magnitude of the error. The proportional component of the PID
regulator outputs a speed command proportional to the PID error. For
example, a proportional gain of 1 would output 100% of max frequency
when the PID error is 100% of the analog input range. A larger value for
[PID Prop Gain] makes the proportional component more responsive,
and a smaller value makes it less responsive. Setting [PID Prop Gain] to
0.00 disables the proportional component of the PID loop.
• A155 [PID Integ Time]
The integral gain (units of seconds) affects how the regulator reacts to
error over time and is used to get rid of steady state error. For example,
with an integral gain of 2 seconds, the output of the integral gain
component would integrate up to 100% of max frequency when the
PID error is 100% for 2 seconds. A larger value for [PID Integ Time]
makes the integral component less responsive, and a smaller value makes
it more responsive. Setting [PID Integ Time] to 0 disables the integral
component of the PID loop.
• A156 [PID Diff Rate]
The Differential gain (units of 1/seconds) affects the rate of change of
the PID output. The differential gain is multiplied by the difference
between the previous error and current error. Thus, with a large error the
D has a large effect and with a small error the D has less of an effect. This
parameter is scaled so that when it is set to 1.00, the process response is
0.1% of [Maximum Freq] when the process error is changing at 1% /
second. A larger value for [PID Diff Rate] makes the differential term
have more of an effect and a small value makes it have less of an effect. In
many applications, the D gain is not needed. Setting [PID Diff Rate] to
0.00 (factory default) disables the differential component of the PID
loop.
Guidelines for Adjusting the PID Gains
1. Adjust the proportional gain. During this step it may be desirable to
disable the integral gain and differential gain by setting them to 0.
After a step change in the PID Feedback:
– If the response is too slow increase
A154 [PID Prop Gain].
– If the response is too quick and/or unstable (see Figure 37 ), decrease
A154 [PID Prop Gain].
– Typically, A154 [PID Prop Gain] is set to some value below the point
where the PID begins to go unstable.
2. Adjust the integral gain (leave the proportional gain set as in Step 1).
After a step change in the PID Feedback:
– If the response is too slow (see
Figure 38 ), or the PID Feedback does
not become equal to the PID Reference, decrease A155 [PID Integ
Time].