Motor won't move in single microsteps - problem, or normal? - Page 2


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Thread: Motor won't move in single microsteps - problem, or normal?

  1. #13
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    Default Re: Motor won't move in single microsteps - problem, or normal?

    @109jb - you're right - a large RC servo could potentially work. I've been a little reluctant to go down that path, as the output shafts on even the large servos are surprisingly small, so I worry a bit about strength and stability. In order to work with the cable arrangement, I'd need to add a spool/spindle to it, and given the small/short output shaft, I suspect I'd need to add a bearing and support above the spool/spindle to adequately support side loading on that small output shaft joint - its just seemed like more trouble than I wanted to get into, but if the stepper approach continues to be problematic, I'm going to have to broaden my horizons, and a big RC servo will definitely be near the top on the list of options. (The good news is, something like the Hitec HS-5765MH can be had for under $100 - that is a beast of a servo, and should definitely have the needed strength - just a question of mating it up to the cable in a strong enough manner.)



  2. #14
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    Default Re: Motor won't move in single microsteps - problem, or normal?

    Sigh... Its with much deflation and disappointment that I report that while the stepper is definitely running better/stronger on 48v, I still can't get it to work in full-step mode, and its still missing occasional steps when making full-step moves in 1/2 step mode. (I also found the same thing in 1/4 step mode.) It will make full steps in 1/2 step mode, by moving 2 steps at a time. (And via 4 steps in 1/4 step mode.) At this point, I'd be happy to just have a reliable full-step - if I have to do that via 2 half-step moves, I'm Ok w/ that. There is still also a very narrow range of speeds that it will move reliably in 1/2 step mode.

    The missing steps, ironically, seem to be occurring mostly when I'm letting cable out (ie less load on the stepper), than when pulling cable in (more load on the stepper). I need to perhaps try experimenting with different speeds for pulling cable vs releasing cable, I dunno. The missing steps also seem to be associated with changing direction a bunch of times. (So, if I home it, move it out a bit, jog it back and forth a bit, then return to what should be the step zero position, and its either not quite all the way there, or it has bottomed out a few steps before reaching the home position.)

    I'm thinking the giant-honking RC servo might be the next logical step. I just can't help but feel that I must be doing something wrong w/ the stepper - steppers are used all the time w/o these problems. I don't know if its my driver, my wiring, the stepper, itself. (I have another stepper I will pull from something tomorrow, and try it out, just to be sure these problems are not specific to the motor I'm using.)

    Much confusion and dejection here...



  3. #15
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    Default Re: Motor won't move in single microsteps - problem, or normal?

    It seems like step one is to get the motor and driver to reliably make full steps. It might take gearing to hit the resolution you are after. (which frankly I don't like but it appears to be reality)

    I am sure that you are tired of throwing money at it, but perhaps try a gecko stepper motor driver and a bigger motor?

    Gekco doesn't sell it any more, but at one time they offered a size 34 motor with something like 1200 oz-in. For fun I tried to hold the shaft still when it was turning about 5 - 10 rpm - not a chance. It does what it wants to do.

    It just occurred to me that some drivers apply full current to "hold" the position of a motor and some drivers reduce the current to prevent over heating. The assumptions are:
    - in a cnc, the threaded drive shaft is providing most of the holding torque to be stationary
    - if the cutting head is stationary, it doesn't need as much power as when moving and cutting
    - sometimes a fan is attached to the shaft to provide cooling, and if the motor is "idled" it will over heat under full current.

    It looks like a fun project - don't give up.



  4. #16
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    Default Re: Motor won't move in single microsteps - problem, or normal?

    I looked at some of your older posts and see that you have a gecko 540 setup with 48 volts on your mill. Maybe you can temporarily try to run the test using your already proven setup and see if that helps any.



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    Default Re: Motor won't move in single microsteps - problem, or normal?

    Use a mechanical gear like a worm drive.
    Or a real AC brushless servo around 100W or so. Vastly better.

    Steppers will position fine to 1/4 step with good drives. No load or low load.
    But as load increases steppers lag on microsteps, and lag more on more microsteps.
    At 1/10 step steppers have iirc about 1/10 of max torque.
    Mariss / geckodrives wrote about this.

    I suspect;
    your motion-control train has 3 errors, each of various size.
    - Hysteresis
    The minimum movement is not controlled and jumps around erratically.
    -Sticktion
    The minimum movement has a major component of stick-slide vs the desired movement
    -Uniformity
    The required torque or force probably varies quite a lot.
    Pulleys and wires often indicate this

    Stretch/compliance leading to jumps is likely. Cables and pulleys.

    It is quite hard to make a stable, steady, accurate, high-resolution motion-control system using anything other than industrial components - like linear rails and ballscrews and timing belts and ac bearings.
    It is extremely hard to do so without industrial tooling and experience in a suitable field like cnc, astronomy, metrology, etc.
    It is very extremely hard to make a cheap motion control system, small, from non-industrial components - unless you are a cnc/automation specialist // similar.



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Motor won't move in single microsteps - problem, or normal?

Motor won't move in single microsteps - problem, or normal?