Jump to content
Cura Connect | Survey Read more... ×
Ultimaker Community of 3D Printing Experts
Sign in to follow this  
mayoff

unexpected motion pattern while measuring backlash

Recommended Posts

I clamped a dial indicator to my bed to measure my Ultimaker's backlash. While working on that, I noticed something interesting about the behavior of the needle on the indicator. Please watch this video to see it:

 

I told the head to move 2.54 mm (0.1 inches) slowly along the Y axis, to make the needle revolve once. The interesting thing (to me) is that the needle didn't move smoothly all the way around. It moved smoothly for about 0.008 inches, then paused, then moved smoothly another ~0.008 inches, then paused, and so on.

I can think of several things that could cause this pattern. One is that the indicator itself is causing it. If so, that's a benign problem in the sense that it doesn't affect prints. Here is the certificate of inspection that came with my indicator:

8409872160_d16f24e871.jpg

I don't know if there's enough resolution in the certificate's error graph to decide whether the pattern is caused by the indicator.

Another potential reason is interference between the firmware's clock frequency and the wave frequency required to drive the stepper at the desired speed. I don't think this is the problem because if I run the test at a higher (but still fairly slow) speed, I still see the pattern, with the same distances (but shorter time intervals) between pauses. If it were a clock interference problem, I would expect the time interval between pauses to remain constant.

Another possible reason is that the stepper itself has irregularities.

The last reason that occurs to me is that the timing belt/pulley interface is irregular. Perhaps due to the way the teeth mesh, the pulley's response to timing belt motion is nonlinear.

Share this post


Link to post
Share on other sites

I decided that it's definitely not the indicator, because if I put my finger on the print head and run a move command, I can feel the pauses even when the head's not touching the indicator.

It also occurred to me that it could be some flaw in the linear bearings cause friction periodically, but I get the pattern on both axes so that seems unlikely.

Share this post


Link to post
Share on other sites

I would point the finger towards the stepper, because your measured "error" coincides with the micro stepping distance:

the UM has 78.7402 micro steps/mm (1/16th steps), or 0.0127mm/step, or (if you insist on dirty units) 0.0005in/step.

0.008in/0.0005in=16

I think what you are seeing is the stepper "nudging" to a full step position, before it continues with the micro stepping, which can possibly explained with the energizing of the coils.

Share this post


Link to post
Share on other sites
Thank you for the explanation. I only used inches because that is how my indicator is marked.

Should I try to adjust the motor power, as described here: http://wiki.ultimaker.com/Electronics_build_guide#Tuning_the_stepper_motor_drivers I do have a multimeter but I'm not sure how I would use it for this.

How are your prints looking? If they are looking good, you are probably getting the measurement associated with the slight tightening and loosening of the belts as the eccentric point of the pulley on the axis comes around. If that's the case, then I wouldn't touch the drivers. They can be a bit finicky and are easy to fry. Snowygrouch looked into some high tech pulleys that would maintain concentrically correctness but found they cost far too much.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Our picks

    • Architect Design Contest | People
      The goal of this contest is to design a set of people figurines that could be used in such a project to make an area, office or mall seem populated. 
      Think of different types of people in different environments, like walking people, people standing still, working people, and both men and women.
       
      • 31 replies
    • Taking Advantage of DfAM
      This is a statement that’s often made about AM/3DP. I'll focus on the way DfAM can take advantage of some of the unique capabilities that AM and 3DP have to offer. I personally think that the use of AM/3DP for light-weighting is one of it’s most exciting possibilities and one that could play a key part in the sustainability of design and manufacturing in the future.
        • Like
      • 3 replies
×

Important Information

Welcome to the Ultimaker Community of 3D printing experts. Visit the following links to read more about our Terms of Use or our Privacy Policy. Thank you!