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IndustryArena Forum > Machine Controllers Software and Solutions > PlanetCNC > Lost position with closed loop stepper motors! Nightmare! Please help!
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  1. #1
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    Lost position with closed loop stepper motors! Nightmare! Please help!

    In a CNC router I have I use the old MK2/4 with CNC USB Controller software [I don't plan to upgrade it's an old machine]


    This machine has closed loop nema23 steppers, specifically 2HSS57 on both 3 axis.

    While in a pocket, the unexpected happen, position was lost.

    While in that pocket, I saw the machine doing spatial moves, instead of helixing it was doing something strange, like moving in straight lines.

    Then position was lost (about 1 mm in both X and Y axis). The machine is fine, no backlash, no ballscrew problems no coupling problems.

    The only thing that comes to mind, is maybe the motor settings in PlanetCNC are too aggresive and the closed loop motor drivers never got the pulses.

    This is huge problem, today I lost 3 hours of work and the material.

    Anybody with advice on what settings should I use with 2HSS57?

    Currently all axis use same settings (metric):

    Step Delay: 15?sec
    Axeleration: 160
    Steps/Unit: 160
    Inital Speed: 0
    Maximum Speed: 4000

    Any ideas on what to look to understand what's causing the issue ?


    - Petros

  2. #2

    Join Date
    Nov 2015
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    166

    Re: Lost position with closed loop stepper motors! Nightmare! Please help!

    Has the reticle in the encoder loosened?

  3. #3
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    Re: Lost position with closed loop stepper motors! Nightmare! Please help!

    Quote Originally Posted by tantemay View Post
    Has the reticle in the encoder loosened?
    I can't tell, but I don't think it's the first place to look, inside the encoder of a enclosed stepper..... After all , it's working since then, it only happened once, a loose encoder would create a permanent issue I guess

  4. #4

    Re: Lost position with closed loop stepper motors! Nightmare! Please help!

    Seems unlikely the stepper system misbehaved. More likely it didn't see the correct number of step pulses. Reducing the max frequency and increasing the pulse width would seem like possible solutions.

    I don't know what "stepper delay" refers to - is this the deadtime between pulses? That would need to have a minimum value to avoid them merging but it sounds rather long / slow.

    Sounds as if the maximum step frequency would be 4000 x 160, in which case that would be 640kHz?? .

  5. #5
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    Re: Lost position with closed loop stepper motors! Nightmare! Please help!

    Hi,

    Sounds as if the maximum step frequency would be 4000 x 160, in which case that would be 640kHz?? .
    No, it would be 640,000 pulses per minute so 640,000 / 60=10,600 pulses per second or 10.6 kHz

    Craig

  6. #6

    Re: Lost position with closed loop stepper motors! Nightmare! Please help!

    Ah - there had to be an obvious explanation! However, the max axis speed and the pulse frequency are two different parameters. If you had the pulse frequency set to 400kHz, the pulse width would be little more than 1us, whereas at 100kHz it would be closer to 5us. Even at low axis speeds, that fixed pulse width could be an issue.

    I had problems with a Leadshine stepper driver that turned out to be incorrect setting of the pulse frequency in the controller (my fault - RTFM!). I dropped from 200kHz to 100kHz as recommended and it stopped dropping steps.

  7. #7
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    Re: Lost position with closed loop stepper motors! Nightmare! Please help!

    Hello.

    Here we go again.

    Excuse me for asking but, are you 100% sure you don´t have any mechanical mismatch?

    From what you say your machine had been working properly and then started loosing steps. Am I correct?

    If such is the case i´d skip the electronics and go for the mechanics.

    You have to remember to always check the mechanics under load. If you do not do it that way mechanical slippage can easily conceal itself.

    One simple way to find out is drawing a line with a felt marker going along the motor shafts across the couplings and over the transmission shafts. Then do some part, in special one that changes the direction of motors extensively. In any case you can write a program that can do precisely that. Then you can check the drawn lines and check for misaligments. Again you have to do it under actual load conditions. Cutting air might not get you anywhere.

    Another test requires that you remove the couplings and then check the shafts for scratches. If there are any you can be sure there is slippage.

    If the mechanics result ok after the aforementioned tests you can do the following. Write a program to move all the axes in the equivalent of say 10 cms. stop and then return to the initial position. Make sure you mark the initial position of the motors. No load is needed in this case. After repeating for no less than 50 cycles (200 is recommended) check the position of the motors. If they appear as displaced from the original markings suspect some encoder error.

    Note: while encoder errors are more common than drivers at this point interchanging any two if possible can help detect where the problem is.

    Please let us know what you find.

    Regards.

  8. #8
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    Re: Lost position with closed loop stepper motors! Nightmare! Please help!

    I just remembered.

    Is it possible to override the feedback in your drives? If such thing is possible you can try doing your part without it and check how the motors behave.

    Good luck.

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