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I'm having an issue with my stepper motor, while I am aware they should make a noise that is dependent on the current and speed of the stepping, the issue I'm having is the noise is changing. For example, if I tell it to take 5000 micro steps (1/16), 1 every 700 micro seconds, perhaps after 500 steps, the frequency of the sound changes for a brief period (perhaps 1/4 second) then returns to what it was.

The typical sound is a fairly high pitched whine, the "different" sound is a slightly lower pitched sound.

The reason I ask is that I am having accuracy issues with this stepper, it is attached to an axis and if I move it 5000 steps forward, then 5000 steps backward, in some cases (not all) it is not in precisely the same position.

The power supply used is just a regular computer PSU (I had one spare) and the connector is providing 12V at upto 40 amps, the manual on the PSU states that the current is regulated to within 5% and the volts may fluctuate by upto 120mv, could this fluctuation be causing the issue? If so, how can I rectify this?

I'm working with this stepper motor: https://www.sparkfun.com/products/13656 and this driver: http://www.robotshop.com/ca/en/pololu-single-bipolar-stepper-motor-driver.html

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  • \$\begingroup\$ Sounds like it is skipping steps. You are probably going faster than the motor can keep up with (it loses torque as it goes faster because of inductance) - especially if you are using microstepping, maximum torque is gained by doing half-stepping or possibly full-stepping depending on the motor. Try running at a lower speed and see if your accuracy improves. \$\endgroup\$ – Tom Carpenter May 21 '16 at 3:55
  • \$\begingroup\$ @TomCarpenter, That was my initial thought, but it does it at different speeds too. \$\endgroup\$ – Zack Newsham May 21 '16 at 4:19
  • \$\begingroup\$ Definitely you miss steps. just going slower is not enough. You must use position profile- accelerate and decelerate instead just jumping between 0 and max speed. Adjust voltage to 24V, it will allow higher speed and better current regulation. You have a potentiometer there- make sure you allow maximum current. remove the load... \$\endgroup\$ – Gregory Kornblum May 21 '16 at 4:49
  • \$\begingroup\$ You don't mention WHAT you are programming that is creating those steps? Are you using a computer or microcontroller which may be intermittently going off handling interrupts or doing other business? \$\endgroup\$ – Richard Crowley May 21 '16 at 5:16
  • \$\begingroup\$ @RichardCrowley, its a Raspberry Pi (one of the older models, I forget which). The Pi isn't doing anything else, other than idle OS tasks. \$\endgroup\$ – Zack Newsham May 21 '16 at 5:18
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Because "5000" steps one way and then "5000" the other doesn't result in the original position, it's indisputable that you're missing steps. A could of ideas to narrow it down:

  • Try without micro-stepping. Repeat the 5000 step exercise with full steps.
  • Change the speed. You might be right on the edge of a resonant speed, which can cause the change in pitch you describe.
  • And yes, definitely use a trapezoidal speed profile as another suggested already. Going from stopped to 1428 steps/sec is an abrupt start for sure.
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    \$\begingroup\$ what? Motor are first-order system, they can't change speed instantly therefore putting u(t) command will not result in any problems.... \$\endgroup\$ – MathieuL May 21 '16 at 13:14
  • \$\begingroup\$ @MathieuL Perhaps true on paper but practical experience with some of the low-cost drivers suggests differently. We're talking about a $5 driver board here. Microstepping (particularly smaller like 1/4 or higher) is not perfectly linear and I've seen it cause exactly the step loss the OP is describing, especially under load. \$\endgroup\$ – TomServo May 21 '16 at 13:28

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