1. Got a question or need help troubleshooting? Post to the troubleshooting forum or Search the forums!

Stepper motor question

Discussion in 'General Questions' started by Lance Weston, Dec 6, 2020.

  1. Lance Weston

    Lance Weston Active Member

    Joined:
    Jun 2, 2018
    Messages:
    515
    Likes Received:
    171
    I have attached a PDF that explains what the steppers need in current. The R2 stepper (17HS4402) rating is 1.3 amps. The Robo board uses an A4988 driver with a 0.1 ohm sense resistor. The Robo R2 board Imax current equals the reference, 0.9v, divided by (8 x 0.1 sense resistor) yielding 1.125 amps Imax.

    From the PDF I can see that microstepping requires 1.414 x 1.3 amp rating of the motor yielding 1.84 amps Imax. There are two windings so I think that number should be cut in half to 0.92 amps. This is less than the 1.125 amps the stepper is being driven with. 1.84 amps would burn up the stepper.

    Is there anyone more facile with steppers that can help me resolve the current. I am using the Partsbuilt system board an can set the Imax current up to 1.5 amps Imax with an A4988 plug in driver.

    I thought that I had read that for microstepping the current must be exactly right for accuracy, but I can not find that article right now. Does anyone have any info on over/under driving the steppers in microstepping mode?
     

    Attached Files:

  2. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
    Staff Member

    Joined:
    Feb 21, 2013
    Messages:
    23,526
    Likes Received:
    7,305
    Yes.
    Agreed.
    Possibly, but certainly not good long-term.

    There is a generic, but decent, paper here: https://www.faulhaber.com/en/suppor...r-tutorial-microstepping-myths-and-realities/

    That generically covers some details on steppers/microstepping.

    I have never had issues with adjusting the current on the Robo regardless of where I set it -- the stepper drivers are not going to let you overdrive the stepper (if they are working correctly) to the point of failure, but if the temperature of the motor gets too high then you have a long-term wear issue and should bring it back down. I usually keep an eye on RMS current as well, rather than peak.
     
  3. Lance Weston

    Lance Weston Active Member

    Joined:
    Jun 2, 2018
    Messages:
    515
    Likes Received:
    171

    Ther paper talks about torque, but it does not refer to max/min current. I have put heatsinks and fans on the steppers and they run cold regardless of the current I pump through them. It seems "reasonable" that I can pump any amount of current and increase torque and accuracy as long as I keep the motors cold. That just means I do not know enough and I do not know where to go to get the info.

    On the two boards that I have from Robo the DAC that controls the reference voltage was removed (Ver 1.15) and a fixed 0.9v reference was used. A resistor divider off the 5v. You may not be adjusting anything even though the software allows it.

    I just for the reason for not going over current. If you accelerate to slippage and the current is too high you will start to demagnetize the magnets. I am not sure why over current matters though, it would seem any time you accelerate to slippage you would start to demagnetize.
     
    #3 Lance Weston, Dec 7, 2020
    Last edited: Dec 7, 2020
  4. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
    Staff Member

    Joined:
    Feb 21, 2013
    Messages:
    23,526
    Likes Received:
    7,305
    The demagnetization is certainly a concern, but to get it to the point of concern you would need to run them at or near max current for extended periods -- basically the heat will do it if it stays too hot for too long. They are pretty tough little motors (even the generic ones)... If you stay within the current specs and add heat sinks if they get warm... you are not going to have much trouble. The issues I have seen to date with the steppers in 3D printers have largely been more general failures (a winding fails or the motor mechanically self-destructs)
     
  5. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
    Staff Member

    Joined:
    Feb 21, 2013
    Messages:
    23,526
    Likes Received:
    7,305
    In the sense of full disclosure -- I have had a couple that would slip... those might well have been weak magnets, but they were certainly not "young" motors
     
  6. Lance Weston

    Lance Weston Active Member

    Joined:
    Jun 2, 2018
    Messages:
    515
    Likes Received:
    171
    I think undercurrent is of more concern. Micro stepping is interesting. The individual steps have to reach enough torque to step, if enough torque is not reached for the step the motor will not move. When enough steps are reached to overcome all resistance then the motor will move the sum of the steps. So the error is not cumulative. I doubt it is visible except for odd moire patterns that could occur. I think it is probable that Robo did overdrive the motors (1.125 amp drive vs 0.92 amp rating) just to the point they did not overheat.

    Since I have fans on the motors to keep them cool I am going to way overdrive on one machine and see what the results are and the long term effect on the motors.
     
    mark tomlinson likes this.
  7. Lance Weston

    Lance Weston Active Member

    Joined:
    Jun 2, 2018
    Messages:
    515
    Likes Received:
    171

    I was wrong, the Robo motors are specified as 1.13 amps per phase DC. The current limit on the system board is 1.125 amps. The problem is that microstepping alters the RMS (DC equivalent). The peak current now needs to be 1.414 times 1.13 amps per phase or 1.6 amps to get the specified torque, because a sine wave delivers less power than a square wave. I am guessing this was a serendipitous design error. I think the motors would have run way too hot, to the point the plastic mounts would soften. I am hoping, but still can not verify that when the steppers are run under current they are fine but the torque is proportionally less. Empirically this appears to be true.
     

Share This Page