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DRV8711: Optimizing DRV8711

Part Number: DRV8711

We are using the DRV8711-EVM to drive a motor that is fairly inductive (200 step/rev, 6.5mH, 2.1 ohms). For accuracy, we need to use microstepping of 64, and for speed, we need pps =100kHz. Our power supply is 28V, though we could use a 48V supply. We set the torque register to get 1A through the motor. As we try to increase the speed, our current waveform deviates from  a sinusoid and becomes triangular. We are using the forced fast decay mode. Increasing the supply from 28 to 48V helps, but doesn't quite get us to where we need to be. TBLANK = 1.5us, TOFF = 5us, ISGAIN=40.

Any insights as to how we could improve things?

5 Replies

  • Hi Tim,

    This is a common problem. There is not enough time to inject the current into the windings at 28V. At 100kHz, the current level is changed every 10us.

    As you noted, raising the voltage helps.

    There are a few things you can try in addition to raising the voltage. Some are more difficult than others.
    1) Change to lower microstepping --
    a) use 1/8 or 1/4 instead of 1/64 or
    b) count the number of step pulses issued and dynamically switch step rates. Switching from 1/64 microsteps to full step and increasing the torque DAC to 140% of the full step should help.
    2) Locate a motor with less inductance

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

      Hi Rick,

           Thanks. We are still experimenting with your suggestions - so far, it still seems like we fall short (we don't have the option of changing motors). We have observed something that we can't explain - as we increase the step frequency from what is a "good" frequency ("good" meaning that our current waveform looks undistorted), we find that we the current gets more distorted and then starts looking more reasonable again. Can you explain what's going on?

  • In reply to Timothy Luca:

    Hi Timothy,

    As the motor speed increases, the back EMF prevents the current from being injected into the winding as efficiently as desired.

    The current becomes more and more distorted until either the motor stalls, or the back EMF moves to a point where it now assists.

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi Rick,

           Could you elaborate a bit more on how "the back EMF moves to a point where it new assists"? I don't understand how it can do that.

    Thanks,
    Tim

  • In reply to Timothy Luca:

    Hi Rick,

         Could you explain how the back EMF can assist? I don't understand.

    Thanks,
    Tim

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