Rockwell Automation Publication 750-AT006D-EN-P - January 2022
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Chapter 7 Applications
Features
PID Preload
– Steps the PID output to a preload value for better dynamic response when the PID output is enabled. If PID is not enabled, the
PID integrator can be initialized to the PID preload value or the current value of the commanded speed. The operation of preload is selected
in the PID configuration parameter. By default, preload command is off and the PID load value is zero, which causes a zero to be loaded into
the integrator when the PID is disabled. When the PID is enabled, the PID output starts from zero and regulates to the required level. When
the PID is enabled with the PID load set to a nonzero value, the output begins with a step. This step can result in the PID reaching steady state
sooner, however the drive can go into current limit and extend the acceleration if the step is too large.
Figure 90 - Effect of PID Preload
Preload command can be used when the PID has exclusive control of the commanded speed. With the integrator preset to the commanded
speed, there is no disturbance in commanded speed when the PID is enabled. After the PID is enabled, the PID output is regulated to the
required level.
Figure 91 - Effect of PID Preload to Commanded Speed
When the PID is configured to have exclusive control of the commanded speed, and the drive is in the current limit or voltage limit, the
integrator is preset to the commanded speed so that it knows where to resume when it is no longer in the limit.
PID Ramp Reference
– This feature is used to provide a smooth transition when the PID is enabled and the PID output is used as a speed
trim (not exclusive control). When the PID ramp reference is selected in the PID configuration parameter, and the PID is disabled, the value
that is used for the PID reference is the PID feedback. This value causes the PID error to be 0. Then when the PID is enabled, the value used
for the PID reference ramps to the selected value for the PID reference at the selected acceleration or deceleration rate. After the PID
reference reaches the selected value, the ramp is bypassed until the PID is disabled and enabled again. S-curve is not available as part of the
PID linear ramp.