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Robot design - stepper motor heat. — Parallax Forums

Robot design - stepper motor heat.

BitsBits Posts: 414
edited 2012-07-27 15:38 in General Discussion
I am designing a robot PCB and while testing the A3967 stepper motor driver I have discovered the the motor gets very hot when I hold it after stepping out x amount of steps.

I am considering pulsing the sleeping pin, in my mind it would creating a duty cycle if you will. That said would this even help remove some heat? How hot can a stepper motor safely run. At this point I cant touch the motor chassis without burning my finger I am guessing its over 40C.

Would hole stepping help vs 1/8 stepping etc?

Any ideas....?

Comments

  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-26 13:28
    Is the stepper holding a mechanical load in place when the load isn't moving? Does the stepper need to maintain the torque all the time?
  • BitsBits Posts: 414
    edited 2012-07-26 13:41
    For testing I have a large wheel that the motor turns about 13" in diameter. So to answer your question there is a load (overcome inertia & moving the wheel) but once it stops the load presumably decreases greatly. I am ramping the motor.

    I have two modes I am interested in.

    1. Move stepper motor a desired number of steps then put the ic in sleep mode. There are no issues except a lower precision when I stop the wheel
    ± a step or 2.

    2. Move stepper motor a desired number of steps then hold the wheel. This is where I am getting fantastic precision
    ±0.05° but the heat is significant.
  • ercoerco Posts: 20,259
    edited 2012-07-26 13:49
    Steppers can & will get warm & even burn up if you apply full power torque at stop for long periods of time. Ideally, your stepper is geared low enough so you can shut it off once in position and it will stay there.

    You want to ramp the motor up to speed, move it toward position, then ramp it down to stop exactly on target, possibly holding it stopped briefly until all your inertia dies down, then shut it off entirely. Can you live with that?

    Some applications apply reduced voltage & current to hold a stepper in a fixed position. That's burning power to stay still, makes less sense. :)
  • BitsBits Posts: 414
    edited 2012-07-26 14:06
    erco wrote: »
    You want to ramp the motor up to speed, move it toward position, then ramp it down to stop exactly on target, possibly holding it stopped briefly until all your inertia dies down, then shut it off entirely. Can you live with that?

    I will give this a shot.
    erco wrote: »
    Some applications apply reduced voltage & current to hold a stepper in a fixed position. That's burning power to stay still, makes less sense. :)

    This is what I was thinking when I mentioned pulsing the sleep pin. So that the current would be cut in half.

    I just PWM the sleep pin and it looked bad. Ill test it out Erco. Thank you
  • BitsBits Posts: 414
    edited 2012-07-26 15:08
    Ahh looks like I had to turn down the voltage on Pin1 "VRef". Now the current is manageable and the heat problem is looking promising.
  • idbruceidbruce Posts: 6,197
    edited 2012-07-26 15:16
    Bits

    I have had the opportunity to work with steppers quite a bit. Steppers can get quite warm, but it sounds like yours are getting too much current. There are two main ways to get around this, either with current limiting resistors or with PWM and current sensing technology. When I started messing with steppers, I built my drivers from TIP120s, and by not have the proper currenting limiting, my motors would get very hot, just like you describe. Now that I purchased some Gecko drives, they run pretty darn fast, with decent torque, and remain fairly cool.

    Bruce
  • kwinnkwinn Posts: 8,697
    edited 2012-07-27 13:39
    If possible stop on a step rather than a micro step so you can maintain that position without providing power to the motor.
  • BitsBits Posts: 414
    edited 2012-07-27 15:34
    No I needed to ramp to a stop and hold the position of the stepper motor. This issues is now solved, just cant figure out how to make it say solved in the OP.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-27 15:38
    Bits wrote: »
    ... This issues is now solved, just cant figure out how to make it say solved in the OP.

    Go to your post, click on EDIT POST, and in the new window click on GO ADVANCED. From there, you can use the Prefix drop down menu to mark it as "solved".
  • idbruceidbruce Posts: 6,197
    edited 2012-07-27 15:38
    just cant figure out how to make it say solved in the OP

    Edit the prefix in the original post
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