Rockwell Automation 1756-HYD02 ControlLogix Hydraulic Servo Module - Manual (Page 73)

Publication 1756-UM525A-EN-P - June 2003 73
Using the 1756-HYD02 Module Features Chapter 4
Backlash Compensation:
Reversal Offset Use this feature to compensate for positional inaccuracy introduced by mechanical backlash. For example,
power-train type applications require a high level of accuracy and repeatability during machining operations.
Lost motion is often generated by a number of mechanical components that may introduce inaccuracies and
that are subject to wear over their lifetime. Hence, when an axis is commanded to reverse direction,
mechanical lost motion in the machine may result in a small amount of actuator motion without axis motion.
As a result, the feedback device may indicate movement even though the axis has not physically moved.
The Reversal Offset specifies a directional offset that is added to the motion planner’s command position as
it is applied to the associated axis loop to compensate for mechanical lost motion. When the commanded
velocity changes sign (a reversal), the ControlLogix controller adds, or subtracts, the Reversal Offset value
from the current commanded position; the axis immediately moves the motor to the other side of the lost
motion window and engages the load. The application of this directional offset is completely transparent
and does not affect the value of the Command Position attribute.
If a value of zero is applied to the Reversal Offset, the feature is effectively disabled. Once enabled by a
non-zero value, and the load is engaged by a reversal of the commanded motion, changing the Reversal
Offset can cause the axis to shift as the offset correction is applied to the command position.
Backlash Compensation:
Stabilization Window Eliminates backlash-induced instability while maintaining full system bandwidth. This algorithm value
should be commensurate with the amount of backlash in the mechanical system.
Mechanical backlash is a common problem in applications that use mechanical gearboxes. Until the input
gear is turned to the point where its proximal tooth contacts an adjacent tooth of the output gear, the
reflected inertia of the output is not felt at the actuator (i.e. when the gear teeth are not engaged, the
system inertia is reduced to the motor inertia).
If the axis 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. If this situation persists the gearbox will wear out
prematurely. To prevent this condition, the conventional approach is to de-tune the loop so that the axis is
stable without the gearbox load applied. Unfortunately, system performance suffers.
The key to this algorithm is a tapered Torque Scaling profile, that is a function of the position error of the
axis loop. 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 is only run when the acceleration command to the axis loop is zero, i.e. when we
are not commanding any acceleration or deceleration that would engage the teeth of the gearbox.
Properly configured with a suitable value for the Backlash Stabilization Window, this algorithm entirely
eliminates the gearbox buzz without sacrificing any axis performance. The Backlash Stabilization parameter
determines the width of the window over which backlash stabilization is applied. In general, this value
should be set to the measured backlash distance. A Backlash Stabilization Window value of zero effectively
disables the feature. (Patent Pending)
Velocity Offset Provides a dynamic velocity correction to the output of the position loop, in position units per second.
Because the position loop output value is updated synchronously every Coarse Update Period, the Velocity
Offset can be tied into custom outer control loop algorithms using Function Block programming. Position
loop is still closed and compensates for velocity offset values by changing position error. Position error
faults are possible.
Feature: Definition: