Rockwell Automation 999 IMC S Class Compact Motion Controller (Cat. No. 4100-999-122) - Manual (Page 38)

3-8 Technical Overview
Publication 999-122 - January 1997
Electronic gearing is accomplished by first reading the master axis’
actual position and computing the distance increment from the previous
reading. This increment is then multiplied by a programmable gear ratio
and added to the slave axis’ command position. In this way the slave
axis is forced to track the master axis according to the specified gear
ratio. The slave axis may be programmed to move in the same or the
opposite direction from the master axis.
Electronic gearing ratios may be specified as a number between
0.00001:1 and 9.99999:1. Alternately, the gear ratio may be specified
as a pair of integer values–a numerator and denominator– representing
the exact ratio of slave axis feedback counts to master axis feedback
counts. The ability to specify numerator/denominator gear ratios makes
it possible to perform electronic gearing using an irrational gear ratio
such as 1/3 with no accumulated error.
By combining the jog and electronic gearing capabilities of the
IMC-S/23x, the slave axis may be smoothly accelerated and decelerated
into and out of electronic gearing motion. This merged motion
capability is equivalent to a software clutch.
Interpolation
Two independent interpolators for all axes allow any two or three axes
to be moved as a group along a linear, circular, or helical path. Motions
from the two interpolators may be combined with each other or with
other types of motion. Motion segments may be blended to one another
to accomplish continuous path motion as long as they are tangent at
their intersection.
Velocity Feedforward The IMC-S/23x is capable of providing velocity feedforward to reduce
following error. Following error is the servo position error that is
present when the axis is moving at a commanded speed. Without
velocity feedforward, a following error necessary to produce sufficient
output to drive the motor at the commanded speed will always exist.
Many applications require that the following error be near zero over
the entire speed range of the motor. Velocity feedforward may be used
to satisfy this requirement.
Velocity feedforward is provided by pre-computing the command
velocity as the rate of change with respect to time of the command
position. The command velocity is then scaled by the F Gain
(Feedforward Gain) and added to the velocity command. By adjusting
the F Gain it is possible to produce, from the feedforward term alone,
the required velocity command to drive the motor at the desired speed.
Thus, only a little position error is needed to "fine tune" the motor speed
and position.