Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 81)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 81
Drive Configuration Chapter 1
Zero Clamp
This feature limits the possible drive action to one direction only. Output from
the drive is from zero to maximum frequency forward or zero to maximum
frequency reverse. This removes the chance of doing a “plugging” type operation
as an attempt to bring the error to zero. This bit is active only in trim mode.
The PID has the option to limit operation so that the output frequency 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 PID has
exclusive control of speed command.
For example, 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 to not
become less than zero. 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 to not become greater than zero.
Feedback Square Root
This feature uses the square root of the feedback signal as the PID feedback. This
is useful in processes that control pressure, because centrifugal fans and pumps
vary pressure with the square of speed.
The PID has the option to take the square root of the selected feedback signal.
This 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.
-100.0 -75.0 -50.0 -25.0 0.0 25.0 50.0 75.0 100.0
-100.0
-75.0
-50.0
-25.0
0.0
25.0
50.0
75.0
100.0
Normalized Feedback
N
orm
aliz
ed
SQR
T (F
eedback)