Rockwell Automation 20B PowerFlex 70, PowerFlex 700 Reference Manual - Manual (Page 142)

Process PI Loop
142 Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
When a digital input is configured as “PI Enable,” the PI Enable bit of [PI
Control] must be turned on for the PI loop to become enabled.
If a digital input is not configured as “PI Enable” and the PI Enable bit in
[PI Control] is turned on, then the PI loop may become enabled. If the PI
Enable bit of [PI Control] is left continuously, then the PI may become
enabled as soon as the drive goes into run. If analog input signal loss is
detected, the PI loop is disabled.
•
PI Hold - The Process PI Controller has the option to hold the integrator
at the current value so if some part of the process is in limit the integrator
will maintain the present value to avoid windup in the integrator.
The logic to hold the integrator at the current value is shown in the
following ladder diagram. There are three conditions under which hold
will turn on.
– If a digital input is configured to provide PI Hold and that digital input is
turned on then the PI integrator will stop changing. Note that when a
digital input is configured to provide PI Hold that takes precedence over
the PI Control parameter.
– If a digital input is not configured to provide PI Hold and the PI Hold bit
in the PI Control parameter is turned on then the PI integrator will stop
changing.
– If the current limit or voltage limit is active then the PI is put into hold.
•
PI Reset – This feature holds the output of the integral function at zero.
The term “anti windup” is often applied to similar features. It may be used
for integrator preloading during transfer and can be used to hold the
integrator at zero during “manual mode”. Take the example of a process
whose feedback signal is below the reference point, creating error. The
drive will increase its output frequency in an attempt to bring the process
into control. If, however, the increase in drive output does not zero the
error, additional increases in output will be commanded. When the drive
reaches programmed Maximum Frequency, it is possible that a significant
amount of integral value has been “built up” (windup). This may cause
Stopping
PI_Status
.Enabled
DigInCfg
.PI_Enable
DigInCfg
.PI_Enable
DigIn
.PI_Enable
PI_Control
.PI_Enable
PI_Control
.PI_Enable Signal Loss Running
PI_Status
.Hold
DigInCfg
.PI_Hold
DigInCfg
.PI_Hold
DigIn
.PI_Hold
PI_Control
.PI_Hold
Current Lmt
or Volt Lmt