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TB265 Rev1 Lash/Screw Compensation Acceleration Coefficients
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Purpose
To describe the use of the Lash/Screw Compensation Axis Acceleration Coefficient parameters 208 thru 215 which were introduced
and are used in CNC11 v3 02 and higher  This document assumes the reader is using a Renishaw ballbar and software to analyze the
results of lash or screw compensation Â
Background
When screw compensation or lash compensation is enabled and setup correctly a ballbar plot will show that backlash has been
minimized  However the lash/screw compensation may have produced anomalies seen on the ballbar plot as reversal spikes These
reversal spikes can be caused by having the speed of the lash/screw compensation either too fast or too slow This document will
serve as a guide on how to minimize reversal spikes caused by lash/screw compensation by adjusting parameters 208 thru 215
Reversal spikes can be caused by other machine issues as well such as excess friction or even servo drive issues  Parameters 208
thru 215 are not necessarily intended to solve these issues Â
Technical
The values of parameters 208 thru 215 are used to compute the acceleration rate which is used for the lash or screw compensation Â
Adjusting these parameters therefore effects the speed and time of the lash/screw corrections made by the motion processing unit for
each individual axis  The acceleration rate for the axis also effects the speed of the lash/laser compensation  This acceleration rate is
computed based on the acceleration time from the PID menu settings and the maximum velocity of the axis from the Jog menu Â
Prerequisites for adjusting parameters 208 thru 215
Parameters 216 and 217 should both be set to the recommended values of zero
Centroid PID values/3rd party drives have been properly tuned  PID menu see also third party drive documentation
All mechanical issues have been resolved
The correct maximum feedrates have been set for all axes Jog menu "Max Rate"
The correct acceleration time has been set for all axes PID menu "Accel"
Lash or screw compensation has been successfully setup so that backlash is minimized
Procedure
By default the values for parameters 208 thru 215 are set to 0 1250 Unfortunately there isn't an easy way to calculate the best values
to use other than trial and error  The amount that the parameter will need to be changed will vary depending on the machine the
motors the drive the ballscrew tuning and other factors   The value of the parameter must be greater than zero for lash or laser screw
compensation to work
Negative reversal spike
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Negative reversal spikes are an indicator that the lash/screw compensation corrections are performed too quickly  To minimize this
effect lower the value of the Acceleration Coefficient parameter for the axis with the reversal spikes  For example if the parameter was
originally set to 0 1250 try setting the parameter to 0 0125  Rerun the ballbar and see what effect this change made  The goal is to get
the reversal spikes as close to zero as possible  Repeat this procedure of lowering the value until the goal is reached  If the reversal
spikes have become positive the parameter has been changed too far Â
Positive reversal spike
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The procedure for positive reversal spikes is the same as negative reversal spikes except that the parameter value should be
increased instead of decreased  For example if the parameter was originally set to 0 1250 and the ballbar plot reported positive
reversal spikes try changing the parameter to 0 25 and rerun the ballbar Â
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Example barbar plots with negative reversal spikes
The following barbar plots show the results of screw compensation on the X and Y axes Â
In the plot below the amount of backlash for X and Y have been minimized  However the plot shows reversal spikes on the X axis Â
The value of parameter 208 was 0 1250 default
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In the plot below parameter 208 was changed to 0 00125 and the barball test was run again  The reversal spikes have improved
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 In the plot below the value of parameter 208 was changed to 0 00013 and the barball test was run again  The parameter has been
lowered too far causing the reversal spikes to swing to a positive value
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Document History
Rev1 Created on 2012 06 15
by #270