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DRV8825 in 32 micro step mode

richard markham
Prodigy 110 points
Other Parts Discussed in Thread: DRV8825, DRV8834

I have been testing the DRV8825 with a 2A stepper motor running at full power on 24V.

I have been trying to get smooth low speed rotation, by using 32 step mode.

I seem to be getting some 'cogging' where the motor progresses smoothly through the 32 steps until the end when it suddenly jumps.

This makes movement a bit jerky.

Is this a function of the drive or the motor? I am guessing that the motor is jumping to the next full step position?

The effect is less obvious at 1/16th step and dissapears at 1/8th, but at these settings there is too much vibration in the drive so I really want to use 1/32 mode.

  • TI__Expert 7455 points

    Hi Richard,

    What decay mode are you using? If you are on slow decay, can you try mixed or fast?

    Best regards,

    Jose Quinones

  • in reply to Jose Quinones
    Prodigy 110 points

    Hi Jose,

    I have tried both fast and slow decay. I cannot see any difference.

    It's almost like the motor jumps 2 steps at the end of the 32 step sequence.

    You can actually hear the steps running. You get a 31 regular steps then the 32 step is louder.

    The motor is driving a CCTV camera and the jump on every 32nd step is quite obvious when viewing the camera video.

  • in reply to richard markham
    TI__Expert 7455 points

    Hi Richard,

    I would say then you are facing a problem with the zero crossing distortion. If you have a current probe you should be able to see it.

    The problem with microstepping is that as current becomes smaller and smaller, it gets harder to read it, so there is a higher current regulation error percentage. Also, the MIN TIME ON will cause a small deformation around the zero crossing point.

    A suggestion would be to add some series resistance, or decrease the input voltage to limit the current charge rate and diminish the zero crossover distortion.

    What voltage is your application? If you can operate at less than 11V, then I recommend trying the DRV8834 which has dynamic TBLANK designed in place especifically to decrease the zero crossover distortion.

    Hope this info helps. Best regards,

    Jose Quinones 

  • in reply to Jose Quinones
    Prodigy 110 points

    Hi Jose,

    I'll try running at lower voltage to see if there is any difference. Currently 24v.

    I don't have a current probe but I'll manually step the drive to identify the position where the motor jumps and see what I can measure with my scope.

    We need quite a bit of power, so i don't think the 8834 will cut it.

  • in reply to richard markham
    Prodigy 110 points

    I don't see much difference when lowering the input voltage.

    In 32 step mode, step 18 seems to be a big step and the following 4 or 5 steps don't create any movement.

    Same in 16 step mode. 9th step is a big step. 10th and 11th steps don't create much movment of the rotor.

  • in reply to richard markham
    Prodigy 110 points

    Hi Jose,

    Still not getting smooth movement in 16 and 32 step modes.

    Anything else i can check?

  • in reply to richard markham
    TI__Expert 7455 points

    Hi Richard,

    One thing that I forgt to ask is what kind of motor you are trying to drive? can you share its part number or related information? You can send to my personal email if you don't want to make it public, at jquinones@ti.com.

    Best regards,

    Jose Quinones

  • in reply to Jose Quinones
    Prodigy 110 points

    Hi Jose,

    The motor is a KA50 from NIDEC Servo Corp.

    The exact model is KA50 KM2-552

    Spec is:

    KA50KM2-552

    degree/step 1.8
    V/Ø  5.1
    A/Ø  1
    Ω/Ø  5.10
    mH/Ø  10

    holding torque
    mN·m 520  
    oz·in  74

    Detent torque
    mN·m  20
    oz·in  2.8

    Rotor interia
    g·cm2  100
    oz·in2 0.5

  • in reply to richard markham
    Prodigy 110 points

    Problem solved!

    DRV8825 decay pin wasn't getting set correclty.

    Once set to FAST mode, drive runs nice and smooth.

    I found this by monitoring the Sense resistor and looking at the voltage across it. In mixed mode I was getting a missing quadrant. In Fast mode I see a complete wave shape.