Rockwell Automation 1398-PDM-xxx IQ Master Version 3.2.4 for IA-2000 and IQ-5000 Positioning Drive Modules, IQ-55 - Manual (Page 186)

164 Closed Loop Control • Closed Loop Position Control
Publication 1398-PM601A-EN-P — October 2000
TUTORIAL
Closed Loop Position Control
Position control applications generally fall into two broad categories: contouring and point-to-point
positioning. Contouring applications require that the actual position follows the commanded position in
a very predictable manner and that the system has high stiffness to reject external torque disturbances.
Note that the actual position must follow the commanded position in a predictable manner, not necessar-
ily with zero following error at all times. Point-to-point positioning systems are typically not concerned
about following a particular path, but move time, settling time, and velocity profile are important.
The position control loop in the ULTRA Plus or IQ-Series implements a Proportional plus Integral (PI)
type loop with velocity and acceleration feedforward. The position command input to the position loop
is the sum of the motion profile generator output, the jog commands, the external position command
generated by the electronic gear, and the cam profile generator output. This total position command is
used to generate the velocity and acceleration feedforward terms, and is compared to the position feed-
back from the encoder. The position feedback is typically from the encoder mounted on the back of the
motor, or optionally from an encoder mounted externally on the machine. The selection of the position
feedback source is made in the Feedback Config menu, as shown by the switches in the block diagram.
The position feedback is subtracted from the position command, resulting in the Following Error (FE).
The proportional and integral gains act on the following error.
The proportional gain, KP, is the most important term and generates a velocity command proportional to
position error. This means that if only KP gain is used, motion is only possible if a position error exists,
and higher velocities require proportionally higher position errors. Increasing the KP gain will reduce
the following error. However, KP gains that result in a position loop bandwidth above about a third of
the velocity loop bandwidth tend to cause the actual position to overshoot the commanded position,
which is usually undesirable.
The velocity feedforward gain, KFF, generates a velocity command signal proportional to the derivative
of the position command. Therefore, if there is no change in the position command, the feedforward
command is always zero. Ideally, 100% velocity feedforward would provide the exact velocity com-
mand without the need for any position error. In practice, actual systems include loads which are not
ideal, so a more conservative approach to setting the velocity feedforward gain (100% or less) is often
used. Because the feedforward command is generated open-loop, there is no effect on the position loop
stability. The function of the velocity feedforward command is to significantly reduce the constant
velocity following error when the KP gain is maintained at the proper value for stability.
The acceleration feedforward term, FGAIN, scales the second derivative of the position command. This
term allows position following error to be reduced when the velocity is changing. The acceleration feed-
forward term is useful in combination with the velocity feedforward to maintain a low following error at
all times, such as in a tracking application or when a fast settling time is required.
The last term is the integral gain, KI, which can provide a velocity command signal to reduce static
position error to zero. Integral gain provides stiffness against torque disturbances and friction torques
which are usually handled by the velocity controller. Integral gain in the position controller is normally
avoided except for special situations. Integral gain in the position controller tends to cause overshoot, so
the ULTRA Plus or IQ-Series controller allows the integrator to be active only when the actual position
is close to the commanded position. This zone around the commanded position where KI is active is set
by the IZONE parameter.