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DRV8825_EVM

Timothy Luca
Intellectual 790 points
Other Parts Discussed in Thread: DRV8825

I am running a stepper using the DRV8825 EVM and its GUI, and I am looking at the current at the ISENA testpoint. I am also looking at the average current which my power supply is providing.

If I grab the motor shaft to prevent it from turning, I observe that the current increases - based on both the ISENA testpoint as well as my power supply's current reading.

I thought that steppers driven by a bipolar chopping circuit should show no current change if they stall. Can you explain what I am seeing? 

  • 0
    TI__Guru** 114905 points
    Hi Tim,

    You are seeing the effect of the the motor back emf. As the motor speed increases, the torque (current) decreases. When the motor stalls, there is no back emf, and the current increases. In some high speed cases, the current cannot reach the maximum programmed chopping levels.

    You should also be able to observe this current difference between slow speed and fast speed.
  • 0 in reply to Rick Duncan
    Intellectual 790 points

    Hi Rick,

         Thanks for your response. I am puzzled by another aspect of the current draw. I have the motor enabled but idle (speed set to zero). I am monitoring the current from the power supply (24V). I see that the current is limited since it is far less than V/R (and the supply is not limiting it).

    So I thought the chopping threshold that is controlled by the Vref value is limiting it. So I changed that value (and checked with a voltmeter to make sure it was changing). But it had no effect at all on the idle current from the power supply. I changed from slow decay to fast decay - and then the power supply current was immediately affected and in concert with my changes in the reference voltage.

    Yet when I clicked the Decay mode back to slow, the power supply current did not return to its previous value.

    What am I missing?

    My motivation is simply to run the stepper with less power when it is idle. I could completely turn it off using teh enabloe or sleep pins, but I'd prefer to have some current flowing so that it better holds its position.

    Thanks,
    Tim

  • 0 in reply to Timothy Luca
    TI__Guru** 114905 points
    Hi Tim,

    I am not sure why changing the decay mode back to slow had no impact. The best way to determine why is to observe the two output currents with a current probe.

    Slow decay can cause loss of regulation in the windings. If using a current probe, the lower steps when microstepping are not maintained. In some severe cases, the current will increase beyond the desired 100% setting. This could be the reason that the current did not change when adjusting VREF.

    Fast decay avoids this but has more current ripple. You noticed that the voltage reference controlled the current in fast decay.

    One method of adjusting current when holding is to create an adjustable VREF. A simple method is use a GPIO as an open drain output to turn on/off a parallel resistor at the center point of the VREF voltage. When running, turn off the parallel resistor. When holding, turn on the parallel resistor to reduce VREF.
  • 0 in reply to Rick Duncan
    Intellectual 790 points
    Hi Rick,
    Thanks again for your prompt response. Will adjusting the reference to reduce the idle power only work if I am in fast decay mode?
    Tim
  • 0 in reply to Timothy Luca
    TI__Guru** 114905 points
    Hi Tim,

    Fast decay will definitely work. Without knowing why slow decay acted as described, I cannot say only. With the output scope captures as the motor is running slowly, it would be easier to definitively say.
  • 0 in reply to Rick Duncan
    Intellectual 790 points
    Hi Rick,
    How do I send you a file? I'm ready to attach a scope photo, but haven't figured out how to do so.
    Thanks,
    Tim
  • 0 in reply to Timothy Luca
    TI__Guru** 114905 points

    Hi Tim,

    Select "Use rich formatting" below and right of the reply text window.

    More options will pop up. Select either the "Insert Media" or "Insert File" icon.

  • 0 in reply to Mary West
    TI__Guru** 114905 points
    Hi Mary,

    Can you have clarify? What are the two signals in the scope capture?

    Is it possible to capture the current in both windings at the minimum step rate? I am interested in seeing a complete sine and cosine wave of both windings.

    Also, please let us know what the step rate (pulses per second) and the microstep mode used.

    Thank you.
  • 0 in reply to Rick Duncan
    Intellectual 790 points
    Hi Rick,
    I'm sorry, I must have confused you. For some unknown reason, even using the rich format mode, I was not able to get my computer to post the scope picture that you had requested, so Mary West was kind enough to do so for me. The scope picture is a comparison of the fast decay mode and the slow decay modes when the motor is idle, and we were trying to determine why the slow mode appeared to be unaffected by changes in the chopping current (voltage reference). The slow decay mode has the tall, rectangular waveforms.

    Do you also need to see the same comparison pictures with the motor running at a very slow rpm?
    Thanks,
    Tim