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Rockwell Automation Publication 750-AT006D-EN-P - January 2022
Chapter 7 Applications
Zero Clamp
– This feature limits the possible drive action to one direction only. Output from the drive is from 0 to maximum forward speed,
or from 0 to maximum reverse speed. This limitation removes the chance of doing a plugging type operation in an attempt to bring the error
to 0. This bit is active only in trim mode.
The PID has options to limit operation so that the output speed always has the same sign as the master speed reference. The zero clamp
option is selected in the PID configuration parameter. Zero clamp is disabled when the PID has exclusive control of the speed command.
For example, if the master speed reference is +10 Hz and the output of the PID results in a speed adder of –15 Hz, zero clamp limits the output
frequency not to become < 0. Likewise, if master speed reference is –10 Hz and the output of the PID results in a speed adder of +15 Hz, zero
clamp limits the output frequency not to become > 0.
Feedback Square Root
– This feature uses the square root of the feedback signal as the PID feedback. This feature is useful in processes
that control pressure, because centrifugal fans and pumps control pressure as a function of the square root of speed.
The PID has options to take the square root of the selected feedback signal.
This function is used to linearize the feedback when the transducer produces the process variable squared. The result of the square root is
normalized back to full scale to provide a consistent range of operation. The option to take the square root is selected in the PID
configuration parameter.
Figure 92 - Normalized Square Root of Feedback
:
Stop Mode
– When 10:110 [Mtr Stop Mode A] or 10:111 [Mtr Stop Mode B] is set to Ramp (1) and a stop command is issued to the drive, the PID
controller continues to operate during the deceleration ramp until the PID output becomes more than the master reference. When 10:110
[Stop Mode A] or 10:111 [Stop Mode B] is set to Coast (0), the drive disables PID and performs a normal stop. This bit is active in Trim mode
only.
Anti-Wind Up
– When 9:1 [PID Cfg] Bit 5 Anti-Windup is set to Enabled (1), the PID controller automatically stops the integrator from creating
an excessive error that can cause instability. The integrator is automatically controlled without the need for PID reset or PID hold inputs.
Percent Speed Reference
– When using process PID control, the output can be selected as percent of the speed reference. This function
works in speed trim mode only, not in torque trim, or exclusive mode.
An example is when percent reference selected:
•
Speed Reference = 43 Hz
•
PID Output = 10%
•
Maximum Frequency = 130 Hz
As a result, 4.3 Hz is added to the final speed reference. Another example is when percent reference is not selected. As a result, 13.0 Hz is
added to the final speed reference.
PID Enable
– The PID controller can be enabled or disabled. The enabled status of the PID controller determines when the PID controller
output is part or all of the commanded speed. The logic that is evaluated for the PID enabled status is shown in
.
