# 4.2.5. Backlash compensation¶

Backlash occurs in any mechanical device, e.g., in reduction gear or in worm-gear. Backlash results in differences in physical stage position when approaching the same point from different directions, whereas the motor shaft is exactly in the required position.

Backlash compensation mode is used in order to eliminate such ambiquity. Its activation allows user to determine the direction from where the stage would approach the destination point. Further on, the stage will approach the stop point from the defined direction only, eliminating the mechanical backlash. If the natural approaching direction doesn’t match the selected one, then the controller drives the motor for some user-defined distance beyond the destination point and after that turns the motor around and completes the approach from the required direction.

While a loaded mechanical system is moving, its dynamic characteristics in the backlash zone do differ from the regular motion mode. Therefore, the rotation in the backlash zone should be performed with user-defined speed.

The following parameters of backlash-compensating system are available for adjustment by user:

• Backlash compensation on/off flag.
• Rotation speed while performing the compensating motion.
• Backlash compensation distance. The plus or minus sign for that parameter is used to determine the approach direction. The plus means the approach from the left side whereas the minus means the approach from the right side.

The controller indicates if the backlash compensation is active using MOVE_STATE_ANTIPLAY flag in the state structure which is also displayed in XILab main window.

A forced backlash compensation by using the LOFT command may be performed if there is no confidence that the actual position is backlash-free. While carrying out this command, a displacement for backlash compensation distance is performed with subsequent return. Calling this command while driving will lead to a smooth stop of the engine. This command makes sense only when the backlash system is active.

Note

The backlash compensation mode presumes no axis position correction, providing the user with just the choice of the direction from where the stage should approach the destination point, sticking to this selected direction.

The backlash compensation adjustment using XILab software is described in Settings of kinematics (stepper motor) section. Switching on and backlash compensation parameters detection commands are described in Programming guide.

The minimum backlash is reached if the approach to the setpoint is performed with the same movement parameters, so the optimal values of the backlash parameters are: the play speed must be equal to the nominal speed, the backlash compensation distance must be such that the device could reach the nominal speed.

Backlash compensation distance can be calculated from formula:

$S = \frac{U^2}{2} \Big[ \frac{1}{Ac} + \frac{1}{Dc} \Big] + 0.2 U$

S - backlash compensation, Ac, Dc - acceleration and deceleration, U - nominal speed, 0.2 - even motion time.