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IndustryArena Forum > CNC Electronics > Stepper Motors / Drives > motor skipping steps on random - usual rookie problem
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  1. #1
    Join Date
    Feb 2015
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    9

    motor skipping steps on random - usual rookie problem

    Hi all!

    I do realize that this question pops out from time to time, but after 3 days of calibration and tests I need help from someone more experienced.

    The said problem is a behaviour I can only describe as stepping motor skipping steps on random. The motor is Nema23 4Nm paired with driver CW5045 (I'm not sure about their quality, can someone confirm that these are not some variations of generic chinese products?)

    Sometimes the motor does not return exactly to the starting point and is even 3mm off, sometimes it returns to the starting point with sub 100um accuracy (all in calibration mode over the material - the software I use is UCCNC).
    The problem is that a shift in the starting point can occur even for relatively small movements (50mm forward and 50mm back movement can result in shift of the starting position of 0.5mm, sometimes more, sometimes less, all in calibration mode).
    The linear speed for the axis I have tested thoroughly was set to 2m/minute (and decreased from 5m/minute after some testing).
    One of the most annoying things is that even though the calibration was more or less repeatable over large distances, when drilling small shapes they were always at least 1mm smaller than they should be (what should be a 10mm wide hole was only 9mm wide - soft wood, 2mm deep).

    If you know by any chance what might be the reason for such behaviour please share this information with me Even if your experience does not exactly relate to my problems it might help me with finding the solution. If you need some additional information I will gladly provide it.

  2. #2
    Join Date
    Jan 2005
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    1695

    Re: motor skipping steps on random - usual rookie problem

    Do you have a photo of your machine? Troubleshoot one step at a time so you can isolate the problem.

    - test one axis
    - step slowly. Try .5m/min and move 10cm or more. Is it still off?

  3. #3
    Join Date
    Feb 2015
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    9

    Re: motor skipping steps on random - usual rookie problem

    I'm pretty sure changing the speed to .5m/min did not help much, but tommorow I will have chance to test it again (I only did thorough tests for speeds above 1m/min.).
    Naturally I only tested one axis at a time (except for cutting in wood, when I tested both axes).

    Below are the photos of the system (I do not have anything better at the time, one of them are from early days of the system, when it still wasn't complete).
    Attachment 267950
    Attachment 267948

    I will test the CNC at very low speeds tommorow and give you some more information.

  4. #4
    Join Date
    Apr 2004
    Posts
    5950

    Re: motor skipping steps on random - usual rookie problem

    When a stepper motor loses steps it's usually not an issue with the speeds per se, but the acceleration value. Try lowering that until your machine works more reliably. Generally the acceleration value will be about 1/10 as large a number as the maximum reliable velocity figure.
    Andrew Werby
    Website

  5. #5
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    Feb 2015
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    9

    Re: motor skipping steps on random - usual rookie problem

    So this is how the situation looks like. I have checked different speeds and accelerations (I went as low as 200mm/min with max speed and 10mm^2/s with acceleration) and it didn't help. Maybe this is not a problem with missing steps after all.
    Below I attach a simple graph with today's results. The Delta denotes a shift from the starting point to the endpoint (starting point should be exactly the same as the endpoint, that is, Delta should be equal 0). The shifts where usually 10cm long (example directions and results are shown in the graph).

    I will post exact results in about an hour (I did at least 100 repetitions with different parameters).

    Attachment 268070

  6. #6
    Join Date
    Jan 2005
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    1695

    Re: motor skipping steps on random - usual rookie problem

    The machine has quite a large span. I suspect that the mechanism might be too loose, or not rigid enough. Also, I cannot tell from the pictures how the screw ends are held. Are there bearings? Note that stepper motors cannot be relied on to take the axial loading. The rotor will move if it is pushed hard enough.

  7. #7
    Join Date
    Feb 2015
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    9

    Re: motor skipping steps on random - usual rookie problem

    The system is basically identical to this one: https://www.robocutters.co.uk/prodde...CutterX4CNCKit (its 2.5 by 1.5 meters).
    It does not use screws for X and Y moves only belts (which are screwed in at the side of the frame). Just tell me if you need a photo of some specific part of the cnc and I will provide it.

  8. #8
    Join Date
    Feb 2015
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    9

    Re: motor skipping steps on random - usual rookie problem

    Here is some additional info regarding previous test. In the test the starting point was in the middle of Y axis. First I went 100 units (circa 10cm) in one direction (let's denote it by ↑ character), then 100 units in the other direction (↓ direction). Then I measured the number of the endpoint was off from the correct position. The speed was set to 1m/min., the acceleration was 10mm/s^2.
    ↑↓ (up/down) - no error
    ↓↑ (d/u) - 2,6 units off
    ↓↑ (d/u) - no error
    ↓↑ (d/u) - no error
    ↑↓ (u/d) - 2,4 units off
    ↑↓ (u/d) - no error
    ↓↑ (d/u) - 2,6 units off
    ↓↑ (d/u) - no error
    ↓↑ (d/u) - no error
    ↓↑ (d/u) - no error
    ↓↑ (d/u) - no error
    ↑↓ (u/d) - 2,6 units off

    I have also performed the same tests with slower and faster speeds (from 0,2m/s to 8m/s), with accelerations as low as 1mm/s^2 and with microstep division number from 5 to 50. Exactly the same pattern has emerged. Whenever I change the direction of the test (that is, when i change from 'first step up then step down' to 'first step down then step up') some error appears. This error usually vanishes when i repeat the same type of movement.

    I have also tried loosening up the belts by circa 5mm (some vids on youtube were showing that the belt does not need extreme tension). There was no measurable change in average error.

    Thank you for all the replies!

  9. #9
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    Jan 2005
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    1695

    Re: motor skipping steps on random - usual rookie problem

    Can you post a close up of the drive mechanism? The measurements are in mm, is that correct?

    Here's a test. Power up the motors. Push and pull against the slides in various directions with your gauge in place. If you are easily able to cause "permanent" displacement errors, it means that the slide mechanism has too much slack.

  10. #10
    Join Date
    Feb 2015
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    9

    Re: motor skipping steps on random - usual rookie problem

    Sorry for late response, I was offline for a few days. Yes, the results are in mm (for presented data 1 unit = 1 mm).

    If I apply some force to the mechanism it moves slightly (less that one mm) and then it moves back when I remove the force. Is it ok or should there be no movement possible at all?

    Below are some additional photos.
    Attachment 269040
    Attachment 269042

  11. #11
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    Jan 2005
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    1695

    Re: motor skipping steps on random - usual rookie problem

    I think the problem is mechanical. There should be no flexing when subjected to cutting forces.

    -The flex might explain why why it's off during actual machining.
    -The backlash and belt stretch could also be a contributor.
    -The stepper rotor will have some amount of give under load.

  12. #12
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    Feb 2015
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    9

    Re: motor skipping steps on random - usual rookie problem

    Indeed there is some flexing during machining, but only for high speeds (slowing the system down should help).

    I guess that the main problem might be with the belt stretch (or is it rubber band effect?). Is it even possible to apply proper tension to the timing belt without additional tools?

  13. #13
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    Re: motor skipping steps on random - usual rookie problem

    I wonder if it might be backlash in the belt pulleys might be a bigger contributor than stretch.

  14. #14
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    Feb 2015
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    9

    Re: motor skipping steps on random - usual rookie problem

    Do you mean backlash between the motor shaft and belt pulley or between belt pulley and the belt?

    It cannot possibly be the former since the shaft is glued to the pulley (it wasn't at the beginning).

  15. #15
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    Re: motor skipping steps on random - usual rookie problem

    Belt and pulley.

  16. #16
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    Feb 2015
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    Re: motor skipping steps on random - usual rookie problem

    Well, now simple question arises, what can be done about that? Is there any way to counteract backlash? (different belts, pulleys?)

  17. #17
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    Jan 2005
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    1695

    Re: motor skipping steps on random - usual rookie problem

    The GT pulleys and belts are suppose to be low backlash. You could also machine your own pulley and adjust the shape slightly to eliminate the backlash, although this might reduce the service life of the belt.

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