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DRV8825 stepping indexer settings halving the total travel distance

HPB
Expert 1350 points
Other Parts Discussed in Thread: DRV8825

I have a custom stepper controller board based on grbl and the DRV8825. I use NEMA 17 motors with 1.7A maximum current per phase and the machine operates at 12V (essentially the voltage at VMA/B of DRV8825 is 12V). Attached here is an image of how the microswitches are connected on my board. I used this site to calculate the steps/mm value that I need to specify in grbl. If I set M2 at 5V and leave M1 and M0 open, I should see 1/16 steps/mm resolution and based on my pulley tooth count (20) and belt pitch (2mm) the calculated value for grbl is 80. However, this does not seem to be working. With a value of 80 I seem to be halving the total travel distance in any axis. What am I missing? Is the way the microswitches are connected wrong?

  • 0
    Expert 1350 points

    Sorry missed the attachment.

  • 0 in reply to HPB
    TI__Guru** 114905 points
    Hi HPB,

    There are additional factors that could be causing this. Some of them are starting speed, acceleration/deceleration, chopping current, and decay mode.

    What is your starting speed and acceleration/deceleration when you move?
    What is VREF and the shunt resistor?
    What decay mode are you using?

    Can you provide a scope capture of the current through four full steps? This will help determine if you are running at 1/16 step.
  • 0 in reply to Rick Duncan
    Expert 1350 points
    The machine is starting from a stand still. Decay mode is mixed mode. Shunt resistors are 0.1R and Vref is 0.75 setting the current limit to 1.5A. I don't have a scope so wouldn't be able to provide any screen captures. Let me check the micro-switches and their placement again. I have a feeling its connected the wrong way.
  • 0 in reply to HPB
    Expert 1350 points
    I figured out the problem. I had connected the micro-switches that set the stepping resolution in the reverse way which was setting the resolution to half of what I had intended. Put the switches back in the right way and everything works great now. Thank you for helping me troubleshoot this.