Rockwell Automation 1398-PDM-xxx IQ Master Version 3.2.4 for IA-2000 and IQ-5000 Positioning Drive Modules, IQ-55 - Manual (Page 184)

162 Closed Loop Control • Introduction
Publication 1398-PM601A-EN-P — October 2000
TUTORIAL
Closed Loop Control
Introduction
Closed loop control systems operate by using feedback to measure the actual output of the system being
controlled and comparing the measured output to the desired or commanded output. The comparison of
actual to commanded output yields the error of the system, and the closed loop regulator acts on this
error to cause the controlled variable to follow the command. The regulator acts on the error based on
various gains, which may produce an output proportional to the error (proportional gain, an output pro-
portional to the error accumulated over time (integral gain), or some combination of these. The output
of the regulator is the input to the system being controlled, and as the system error changes, the regula-
tor output changes to cause the system to follow the command.
A typical motion control system has several closed control loops operating together to control motion,
with a velocity control loop closed inside of a position control loop. The motion controller generates a
position command for the system to follow, and the position regulator compares this command to the
measured position from the feedback device. The resulting position error is acted on by the loop gains to
produce an output which is the input to the inner velocity control loop. This velocity command is then
compared to measured velocity, and the velocity loop gains act on the velocity error to produce a torque
command. The amplifier applies a current to the motor to produce the desired torque in the motor.
The ULTRA Plus or IQ controller architecture uses a velocity loop inside a position control loop as
described above. The feedback device for both loops is an optical incremental encoder, which is used to
measure motor position and calculate motor speed. The encoder mounted on the motor is always used
for the velocity feedback device, and is usually also used as the position feedback device. Optionally, an
external encoder can be used for the position feedback.
Following is a block diagram of the internal workings of the ULTRA Plus or IQ.
• Items within the dashed box represent functions that are performed within the ULTRA Plus or IQ.
• Boxes that are cross hatched are read only system variables that can be read in an application pro-
gram or read with a Host Language command.
• Clear boxes represent system variables that can be read and written.