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TB050 Rev1 Long/Short Step in Incremental Movements
When jogging small increments typically 0001" to 001" jog steps may produce an alternating series of long
and short errors in the measured position which is caused by mechanical imperfections In these cases the
Digital Read Out DRO of the control properly indicates the requested movement and the motor shaft
movement as indicated by absolute position in the PID Configuration table verifies it but the physical
movement of the table does not agree Obviously there is lost motion
The lost motion can be found in "springy lash" Springy lash is a combination of high drag starting friction and
windup between the motor shaft and the table Springy lash is different than true mechanical lash because
true mechanical lash will never spring back Some possible mechanical components of the "spring" in springy
lash are
1 Ball screw bearing supports are flexing Check bearing supports with a dial indicator
2 Yoke is flexing
3 Ball screw itself is winding up
4 Timing belt creep Note that the belts themselves aren't stretching but rather the belt teeth are actually
deflecting against the pulley teeth *Incorrect square tooth timing belts make it worse See TB#003
The mechanical problems listed above are only some of the possible parts of the observed long step short
step movement The other factor in this situation is the high force required to initially move the table or high
drag forces An indication of this contributing factor can be found by observing Kv1 values in the PID
Configuration screen after a successful Autotune is performed See TB# 045 Kv1 is proportional to the
amount of torque force required to move the table at a slow jog rate and thus an indicator of mechanical
drag Values for Kv1 that are above 20 are considered high drag with Kv1 values exceeding 35 considered
very high drag Note that undersized motors and drives on a larger machine may give high Kv1 values but
this does not necessarily add to springy lash Some possible causes of high drag or high force requirements
are
1 Lack of lubrication reaching the ways and/or ballscrews
2 Incorrect gib adjustment
3 General machined surface and ball screw mounting misalignments
4 Oversized balls in the ball screw nut
5 Too much tension in double nut adjustable ball screws
The above mechanical problems combine to cause the short step long step movement Suppose at first there
is no significant windup or rotational tension in the pulley belt and ballscrew system The high drag required
to initially move the table results in a small jog step being partially lost by the springy lash of the machine A
movement of the table occurs that is less than the jog increment requested The belts and other mechanical
parts are being twisted like a spring On a subsequent jog increment the high drag required to initially move
the table results in a measured table distance greater than the requested jog increment due to unwinding of
the springy forces followed by windup in the opposite direction when slight over travel occurs
Modifications introduced in software release 5 27 attempt to counteract these mechanical imperfections to
achieve more consistent table movement when jogging incrementally Software however cannot perfectly
remedy all mechanical imperfections The only sure remedy for this problem is mechanical improvements to
reduce drag and/or springy lash
Document History
Rev1 Created on 1999 05 05