Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 76)

76 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 1 Drive Configuration
Process PID Loop The internal PID function provides closed loop process control with
proportional and integral control action. The function is designed to be used in
applications that require simple control of a process without the use of a separate
stand-alone loop controller.
The PID 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
PID regulator, changing drive output frequency to attempt zero error between
the process variable and the setpoint.
The Process PID can be used to modify the commanded speed or can be used to
trim torque. There are two ways the PID Controller can be configured to modify
the commanded speed.
• Speed Trim - The PID Output can be added to the master speed reference.
• Exclusive Control - PID can have exclusive control of the commanded
speed.
The mode of operation between speed trim, exclusive control, and torque trim is
selected in P1079 [PID Output Sel].
Speed Trim Mode
In this mode, the output of the PID regulator is summed with a master speed
reference to control the process. This mode is appropriate when the process needs
to be controlled tightly and in a stable manner by adding or subtracting small
amounts directly to the output frequency (speed). In the following example, the
master speed reference sets the wind/unwind speed and the dancer pot signal is
used as a PID Feedback to control the tension in the system. An equilibrium
point is programmed as PID Setpoint, and as the tension increases or decreases
during winding, the master speed is trimmed to compensate and maintain
tension near the equilibrium point.
0 Volts
Dancer Pot
P1072 [PID Fdbk Sel]
Master Speed Reference
Equilibrium Point
P1067 [PID Ref Sel]
10 Volts