8.13. The problem of compensation for backlash (example from technical support)

Note

In this Chapter, we will clearly show the problem of backlash and its solution. The description and solution is taken from the actual user request to the technical support service


Dear Service Engineer,
We are using one of your motorised mirrors (8MKVDOM-1) with the 8SMC5-USB controller in a very sensitive spot to align our laser.
To align our system we observe the power at the output of our laser system. The problem we a re facing is that we get different results when scanning in one direction and go backwards again, i.e. the ‘optimal’ position changes. We observe this when using our homemade Labview program as well as the XILab software.
Using the ‘Go home’ command in the XILab software does not make the motor go to position 0, but results in an oscillating motion, stopping somewhere near Zero. Is this intentional and does it have anything to do with our problem? The stop setting is using the limit switch, we have not tried to use ‘revolution sensor’ or ’synchronization pulse’. Is that an option? Also do you have any suggestions how to make the motor as precise as possible?

Video showing the problem can be found at this link

Best regards,
Marvin


Hello dear Marvin,
First of all, set up a profile for your stage. Open XILab -> Settings -> “Restore from file…” button -> open “Standa” folder -> chose 8MKVDOM-1.cfg. Don’t forget to click “Apply” and “Save to flash” buttons.
After that click the “Go home” button in the xilab main window and click “Zero” button. Revolution sensor or encoder will help you better control position

In your mechanical system, the moving to the desired position on the left and right is not the same and there is a backlash, but it is possible to eliminate this problem. To do this, check the box next to “Backlash compensation” in the “Stepper motor” tab and specify a value that exceeds the amount of backlash (in your case, this is 8-10 steps). The sign of this setting determines the direction of moving to the position. Positive sign means move from the left while negative - from the right. In the “Backlash compensation speed” field, set the speed at which the compensation movement will be performed. This value should be low (50 s/sec is enough) in order to avoid “drifting” during play compensation.

Important: The backlash compensation mode presumes no axis position correction, providing only a choice of the direction from where the stage should approach the destination point, sticking to this selected direction.

Best regards,
Vlad


Hey Vlad,
Awesome! We did some more tinkering and it kind of works now

Thanks!
Marvin