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Schlepper Motors — Parallax Forums

Schlepper Motors

ercoerco Posts: 20,255
edited 2014-06-23 17:26 in Robotics
Man, it's a lot of work coming up with something that makes those $2 Chinese stepper motors look useful. They're sloppy, lots of backlash, slow, low torque, just not a lot of good things to say. Here's a quickie demo that without any load, they can repeat 360-degree turns for a while until they get hot. Why hot? They are 5V motors driven at ~12V here just to make them faster and repeatable. I was considering making a drawing bot with these, but I will have to find other motors more suitable after seeing how frustrating this simple thing was to do.

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-06-22 18:40
    erco,

    Those motors look pretty nice, really, for only two bucks. How are you driving them? If you're driving them directly with 12V, it's no wonder that they're getting hot. You could obtain the same performance with less heat using a current-limited PWM drive or a simple series-resistor drive that provides the initial oomph but limits steady-state current to the motor's specs. In fact, you could go up to 24V with such arrangements for even better performance that what you're getting now.

    -Phil
  • WhitWhit Posts: 4,191
    edited 2014-06-22 19:02
    Cool demo video erco!
  • ercoerco Posts: 20,255
    edited 2014-06-23 09:27
    Excellent calls, PhiPi. Yes, I'm using a straight connection to 12V worth of Li-Ion batteries. My hope was to achieve higher RPM and limit current by switching the coils on for a few microseconds, but these motors only go so fast before they start skipping pulses. I do like your series resistor drive idea, but IMO it would be wasted on these cheesy motors. When I find some decent, low backlash steppers, I'll be chatting with you again, my learned proponent!
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2014-06-23 10:21
    Ack, there was something about those turning discs and your camera work that made me dizzy.

    I wouldn't think over-volting steppers would make them faster. It does increase torque, though, so you can run them at higher PPS rates without their skipping steps.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-06-23 10:41
    GordonMcComb wrote:
    I wouldn't think over-volting steppers would make them faster.

    Using a higher voltage has the advantage of getting current to flow in the motor coils more quickly. This translates into a higher possible step rate. With a series resistor, the coil will "see" the higher voltage until current starts to flow, then settle to the rated voltage when the current reaches its rated value. This so-called RL drive is very inefficient, though, since the resistors dissipate so much energy as heat. PWMing a high voltage with current feedback is much better, but more complicated.

    -Phil
  • ercoerco Posts: 20,255
    edited 2014-06-23 13:58
    GordonMcComb wrote: »
    Ack, there was something about those turning discs and your camera work that made me dizzy.

    It might also be Youtube's autostabilization effect. I checked the "Shall we fix the shakiness for you" box on this one.

    GordonMcComb wrote: »
    I wouldn't think over-volting steppers would make them faster.

    C'mon Gordon. You're one of us good old boys who know how stuff really works. If some'um don't work, double the voltage! More power's always gooder-er.
  • Mark_TMark_T Posts: 1,981
    edited 2014-06-23 14:32
    I reckon there's a gap in the market for an affordable NEMA11 sized 0.9 degree 6-wire stepper with 5V 0.25A windings.
    Would be unipolar at 5V or H-bridge drivable at 12V, or chopper-drivable from higher voltages and useful for a lot
    of small belt-driven movement control...
  • ercoerco Posts: 20,255
    edited 2014-06-23 17:26
    Barely related: this DRV8825 driver chip looks too good to be true for bipolar motors (sadly, the $2 is unipolar). Under $6, 8-45V, 1.5+ amps, too many features to list here. Select the the DRV8825 from the dropdown.

    http://www.ebay.com/itm/StepStick-Pololu-stepper-driver-A4983-A4988-DRV8825-RepRap-Sanguinololu-RAMPS1-4-/310763158057#ht_3337wt_1166
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