212 Rockwell Automation Publication MOTION-RM003I-EN-P - February 2018
Torque or force scaling gain value that converts commanded acceleration into
equivalent rated torque/force. Properly set, this value represents the total system
inertia or mass.
Backlash Reversal Offset
Usage Access T Data
Type
Default Min Max Semantics of Values
Required - P Set/SSV
REAL 0 0
Position Units
The Backlash Reversal Offset attribute value is used to compensate for positional
inaccuracy introduced by mechanical backlash. Backlash manifests itself when an
axis is commanded to reverse direction. During such a reversal there is a small
amount of displacement of the motor that does not translate to displacement of
the load due to mechanical play in the machine, for example, through the gearing
or ball-screw. As a result, there is an error in the control system's indication of the
actual position for the axis versus the true position of the mechanical load, that
error being equal to the lost displacement due to the mechanical backlash.
Compensation for this positioning error due to mechanical backlash can be
achieved by adding a directional offset, specified by the Backlash Reversal Offset
attribute, to the motion planner's command position before sending to the drive.
Whenever the commanded velocity changes sign (a reversal), the Logix controller
will add, or subtract, the offset value from the current commanded position. This
causes the servo to immediately move the motor to the other side of the backlash
window and engage the load. It is important to note that the application of this
directional offset is completely transparent to the user; the offset does not have
any impact on the value of the Command Position attribute. If a value of zero is
applied to the Backlash 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 Backlash Reversal Offset can cause the axis to
shift as the offset correction is applied to the command position.
Backlash Compensation Window
Usage Access T Data
Type
Default Min Max Semantics of Values
Optional - P Set/SSV
REAL 0 0
Position Units
Defines a window around the command position. When the actual position is
within this window, the effective System Inertia gain is reduced by a factor of the
ratio of the Position Error and the Backlash Compensation Window. When the
actual position is outside the window, the configured System Inertia gain is
applied.
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