Position Pi Gains; Velocity Feedforward - Allen-Bradley ControlLogix Reference Manual
Integrated motion on the ethernet/ip network
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- User manual (203 pages) ,
- Installation instructions manual (16 pages) ,
- Installation instructions manual (16 pages)
Table of Contents
Chapter 2
Behavior models used in CIP Motion
Position PI Gains
Velocity Feedforward
60
units prior to applying the output to the velocity loop summing junction. This is
done by scaling the position loop output using the Feedback Unit Ratio.
See also
Position Control Behavior
The Proportional Gain of the classic proportional-integral (PI) controller sets the
unity gain bandwidth of the position loop in radians/second, while the Integral
Gain is used to devise the Position Error signal to zero to compensate for the effect
of any static and quasi-static torque or forces applied to the load.
See also
Position Control Behavior
The inner velocity loop requires a non-zero command input to generate
steady-state axis motor velocity. To provide the non-zero output from the device
to the motor, a non-zero position loop output is required, which translates to a
non-zero position error.
This dynamic error between command position and actual position while moving
is often called "following error". Most closed loop motion control applications
require zero following error all of the time. This could be achieved to some extent
through use of the position integral gain control, but typically the response time of
the integrator action is too slow to be effective in high-performance motion
control applications. An alternative approach that has superior dynamic response
is to use Velocity Feedforward.
The Velocity Feedforward feature is used in Position Control mode to provide the
bulk of the Velocity Reference input necessary to generate the desired motor
velocity. It does this by scaling the Fine Velocity Command signal output of the
Command Generator by the Velocity Feedforward Gain and adding the resultant
Velocity Feedforward Command signal to the Position Loop Output generated by
the position loop to form the Velocity Reference signal. With this feature, the
position loop does not need to generate much effort to produce the required
velocity command level, hence the Position Error value is significantly reduced.
The Velocity Feedforward Command signal allows the following error of the
position control loop to be reduced to nearly zero when running at a constant
velocity. This is important in applications such as electronic gearing and
synchronization applications where it is necessary that the actual axis position not
significantly lag behind the commanded position at any time.
Theoretically, the optimal value for Velocity Feedforward Gain is 100%. In reality,
however, the value may need to be adjusted to accommodate velocity loops with
finite loop gain. One thing that may force a smaller Velocity Feedforward value is
that increasing amounts of feedforward tend to exacerbate axis overshoot. For this
Rockwell Automation Publication MOTION-RM003I-EN-P - February 2018
on
page 58
on
page 58
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