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DRV8711 stall not detected

Mark81
Expert 2610 points
Other Parts Discussed in Thread: BOOST-DRV8711, DRV8711

I've got a BOOST-DRV8711 dev board (http://www.ti.com/tool/boost-drv8711) to have some fun with the DRV8711 IC.

I'm interested in stall detection, to detect when the motor looses steps. Using a general purpose MCU I wired the SPI and the STEP/DIR pins. I'm able to read/write the internal registers and to move the motor.

I read a lot of other threads and the relevant docs. I'm aware I have to find the correct values by experiments, they depend upon motors, voltage, speed, I cannot use full-step, etc....

To try this feature I set these parameters:

  • torque = 128, smplth = 200 us
  • microstepping = 1/16
  • sdthr = 0xFF, sdcnt = 1 step, vdiv = / 32
  • motor voltage: 12 V

All other settings are left on their default value. This is the motor I'm using (with no load):

http://www.applied-motion.com/sites/default/files/HT17-268_RevC.pdf

I set the motor to run @ 5000 step/s and I clamped the axis, but nothing happened (I was expecting the stall led turns on). Because the stall condition is met when the BEMF drops below the sdthr value I reduced both torque and speed settings - with no luck.

I'm using the most sensitive settings to detect a stall condition (maximum value for sdthr and lowest value for vdiv).
Anyway I tried to set different values to that register with no luck.

Perhaps I miserunderstood the datasheet?

  • 0
    TI__Guru** 114905 points
    Hi femtotech,

    Have you tried reducing the smplth to 50 or 100 us? At 5000 step/s the next step arrives prior to the sample completing.

    If you have a current probe, please determine the amount of time required for the current to reach zero. This will help you decide whether 50 or 100us is the better choice.
  • 0 in reply to Rick Duncan
    Expert 2610 points
    You're right, at 5000 step/s the smplth is too high, but the meaning of my sentence should be "I tried 'up to' 5 kstep/s". Lower the speed doesn't change the behavior. Anyway I've just tried reducing the smplth to 50 or 100 us with no luck.
    Unfortunately I don't have a current probe. Is there something I could do with a normal oscilloscope?
  • 0 in reply to Mark81
    TI__Guru** 114905 points
    Yes, there is are a few things that can be done with a normal scope.

    You can set the EXSTALL bit to see the BEMF on the BEMF pin. The STALLn/BEMFVn becomes an indicator of when the sample is valid.

    This can be confirmed by measuring the voltage across the sense resistors.

    At each zero current step for the windings, the BEMFVn signal should assert to allow you to see the value. This should help you determine how to set the stall bits.