Rockwell Automation Publication 20A-UM001N-EN-P - July 2013 119
Application Notes Appendix C
Process PI for Standard
Control
The internal PI function of the PowerFlex 70 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 enables 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 then adjusts 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.
Or, it can operate as control mode by supplying the entire speed reference. This
method is identified as Exclusive mode.
PI Enable
The output of the PI loop can be turned on (enabled) or turned off (disabled).
This control enables 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 in
below.
The drive must be running for the PI loop to be enabled. The loop is disabled
when the drive is ramping to a stop, jogging, or the signal loss protection for the
analog input or inputs is sensing a loss of signal.
+
Spd Cmd
Process PI
Controller
Linear Ramp
& S-Curve
+
+
+
PI Enabled
Speed Control
Spd Ref
PI Ref
PI Fbk
Slip Adder
Open
Loop
Slip
Comp
Process
PI
Spd Cmd
Process PI
Controller
Linear Ramp
& S-Curve
+
+
PI Enabled
Speed Control
Spd Ref
PI Ref
PI Fbk
Slip Adder
Open
Loop
Slip
Comp
Process
PI
Drive
Running
Drive
Ramping
to Stop
Drive
Jogging
Bit 0 of
[PI Control] = 1
(enabled) Signal Loss
The Configured
Digital Input
is Closed
A Digital Input
is Configured
to PI Enable
"Enabled" Status
Digital Input
is Reflected
in [PI Status]
Bit 0 = 1
The PI Loop
is Enabled
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