Chapter 2
Behavior models used in CIP Motion
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Rockwell Automation Publication MOTION-RM003I-EN-P - February 2018
If the Velocity Control loop is tuned for peak performance with the load applied,
the axis will be, at best, under-damped and, at worst, unstable in the condition
where the gear teeth are not engaged. In the worst case scenario, the motor axis
and the input gear oscillates wildly between the limits imposed by the output gear
teeth. The net effect is a loud buzzing sound when the axis is at rest, commonly
referred to as 'gearbox chatter'. If this situation persists, the gearbox will wear out
prematurely. To prevent this condition, the conventional approach is to de-tune
the velocity loop so that the axis is stable without the gearbox load applied.
Unfortunately, system performance suffers.
With a Backlash Stabilization Window value commensurate with the amount of
backlash in the mechanical system, the backlash stabilization algorithm is very
effective in eliminating backlash-induced instability while still maintaining full
system bandwidth. The key to this algorithm is a tapered Kj profile that is a
function of the position error of the position loop, illustrated in the following
diagram.
The reason for the tapered profile, as opposed to a step profile, is that when the
position error exceeds the backlash distance a step profile would create a very large
discontinuity in the torque output. This repulsing torque tends to slam the axis
back against the opposite gear tooth and perpetuate the buzzing effect. The
tapered profile can be qualified to run only when the acceleration command or the
velocity command to the control loop structure is zero, for example, when not
commanding motion that would engage the teeth of the gearbox.
Properly configured with a suitable value for the Backlash Compensation
Window, this algorithm entirely eliminates the gearbox buzz without sacrificing
any servo performance. The Backlash Compensation Window parameter
determines the width of the window over which backlash stabilization is applied.
In general, this value is set equal to or greater than the measured backlash distance.
A Backlash Stabilization Window value of zero effectively disables the feature.