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Backlash in RapidPath

There are two parameters that affect backlash compensation behavior, the amount and the rate. The backlash amount is a measurable quantity. The rate governs how quickly all the backlash is applied. This value is probably application specific based on the inertia involved in moving the axis and the speeds that are commonly employed during direction-changing motion (e.g. any sort of circular motion). (citation needed)

Backlash Parameters

Versions of RapidPath™ prior to (FIXED_REVISION) incorrectly expect you to provide values for these parameters in motor counts rather than in setup units (e.g. inches).

Backlash Amount

The backlash amount (called backlash width in RMP parlance) is how much motion can be commanded (when reversing direction) in the motor without any appreciable movement or force being applied to the next mechanical component.

Backlash_gear.png (source: Wikipedia)Backlash_ballscrew.gif (

Backlash Rate

The backlash rate is how much additional backlash compensation is applied to the commanded positions every millisecond.

This parameter determines how long it will take to apply the backlash amount.

Choosing Parameters

The backlash amount can be measured by moving a meaningful distance in one direction (to position X0), commanding motion in the opposite direction, then commanding the motor back to X0, and measuring how far away from the actual position it is.

The backlash rate is determined by how quickly you want compensation to be applied. If the geometries you need are small, a smaller time for applied compensation (or a larger rate) will be necessary.

Recommendations: Applying backlash within 50ms might be good. (citation needed)

Mach 4 and RapidPath

The backlash amount is set in the Mach 4 configuration for a motor.

With the control disabled, pull down the Configure menu and select Control


The backlash rate is set in the RapidPath configuration for a motor.

Pull down the Configure menu, select Plugins... then RapidPath. Select the Drives tab at the top, and the correct motor from the list on the left side. 

For the backlash value to take effect you will need to set a Backlash Rate, and we normally start with a value of 20.


The default backlash rate is zero, which means that backlash compensation will not be applied.

Other Resources