Process Pid; Pi Enable - Allen-Bradley powerflex 700 User Manual

Appendix C Application Notes

Process PID

134
The internal PI function of the PowerFlex 700 provides closed loop process
control with proportional and integral control action. The function is designed
for use in applications that require simple control of a process without external
control devices. The PI function allows the microprocessor of the drive to follow
a single process control loop.
The PI function reads a process variable input to the drive and compares it to a
desired setpoint stored in the drive. The algorithm will then adjust the output of
the PI regulator, changing drive output frequency to try and make the process
variable equal the setpoint.
It can operate as trim mode by summing the PI loop output with a master speed
reference.
Slip Adder
Spd Ref
PI Ref
Process PI
Controller
PI Fbk PI Enabled
Or, it can operate as control mode by supplying the entire speed reference. This
method is identified as "exclusive mode."
Slip Adder
Spd Ref
PI Ref
Process PI
Controller
PI Fbk PI Enabled

PI Enable

The output of the PI loop can be turned on (enabled) or turned off (disabled).
This control allows the user to determine when the PI loop is providing part or
all of the commanded speed. The logic for enabling the PI loop is shown below.
Drive
Drive Ramping Drive
Running to Stop Jogging
Bit 0 Bit 6
[PI Configuration]
The drive must be running for the PI loop to be enabled. The loop will be
disabled when the drive is ramping to a stop (unless "Stop Mode" is configured in
[PI Configuration]), jogging or the signal loss protection for the analog input(s)
is sensing a loss of signal.
If a digital input has been configured to "PI Enable, " two events are required to
enable the loop: the digital input must be closed AND bit 0 of the PI Control
parameter must be = 1.
Rockwell Automation Publication 20B-UM002G-EN-P - July 2014
Slip
Comp
+
+
Open
Loop
Linear Ramp
& S-Curve
Process
+
PI
+
Speed Control
Slip
Comp
+
+
Open
Loop
Linear Ramp
& S-Curve
Process
PI
Speed Control
Bit 0 of
[PI Control] = 1
(enabled) Signal Loss
A Digital Input The Configured
is Configured Digital Input
to PI Enable is Closed
Spd Cmd
Spd Cmd
The PI Loop
is Enabled
"Enabled" Status
Digital Input
is Reflected
in [PI Status]
Bit 0 = 1