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DRV8711: Overcurrent shutdown while running: current regulation doesn't work anymore after a while

Christoph Schwarz
Prodigy 40 points
Part Number: DRV8711

Hello,

we are using the DRV8711 driver IC in an application with this stepper motor: Nanotec ST8918M6708 (https://de.nanotec.com/produkte/566-st8918-schrittmotor-nema-34/).

The schematic is nearly the same than on the eval board, only other FETs were used. We use CSD19532QB5 (http://www.ti.com/lit/ds/symlink/csd19532q5b.pdf). The supply is 48V

The motor is specified to run until 2100/min, but when we reach about 1200/min the current regulation cannot hold the current on the desired value and then the overcurrent protection stops the driver. Here you can see when this happens:

In detail: The desired current is 2.6A (the orange line) but neither slow nor fast decay can decrease the current.

To config the driver I do the following:

    Stepper.Driver.Ctrl |= STEPPER_CTRL_ENBL;
    Stepper.Driver.Ctrl |= STEPPER_CTRL_EXSTALL;
    Stepper.Driver.Ctrl |= STEPPER_CTRL_MODE_HALF_STEP;
    Stepper.Driver.Ctrl |= STEPPER_CTRL_ISGAIN_20;
    Stepper.Driver.Ctrl |= STEPPER_CTRL_DTIME_450;

    Stepper.Driver.Torque |= TORQUE;
    Stepper.Driver.Torque |= STEPPER_TORQUE_SMPLTH_50;

    Stepper.Driver.Off |= 20;

    Stepper.Driver.Blank |= 50;

    Stepper.Driver.Decay |= 100;
    Stepper.Driver.Decay |= STEPPER_DECAY_DECMOD_ALL_AUTO;

    Stepper.Driver.Stall |= 60;
    Stepper.Driver.Stall |= STEPPER_STALL_SDCNT_4;
    Stepper.Driver.Stall |= STEPPER_STALL_VDIF_8;

    Stepper.Driver.Drive |= STEPPER_DRIVE_OCPTH_500;
    Stepper.Driver.Drive |= STEPPER_DRIVE_OCPDEG_2;
    Stepper.Driver.Drive |= STEPPER_DRIVE_TDRIVEN_250;
    Stepper.Driver.Drive |= STEPPER_DRIVE_TDRIVEP_250;
    Stepper.Driver.Drive |= STEPPER_DRIVE_IDRIVEN_300;
    Stepper.Driver.Drive |= STEPPER_DRIVE_IDRIVEP_200;

I tested all decay modes and modified all other settings, but it always is the same result. When reaching a specific rpm value (maximum around 1200/min) this behavior happens.

Does anyone has an idea what I am doing wrong?

Cheers,

Christoph
  • 0
    TI__Guru** 114905 points
    Hi Christoph,

    Thank you for providing such detailed information.

    Are you using the motor in series mode? The graph appears to show that the motor may stop running around 1400 rpm in series mode.

    If you have not done so, may I suggest you contact the manufacturer to understand what the difference between the series and parallel graphs.

    On the DRV8711, you can try using full step with the step input decreased by half and the current increased by 1.4x
  • 0 in reply to Rick Duncan
    Prodigy 40 points
    Hi,

    thank you for your fast response.

    I use parallel mode.

    What do you mean with step input decreasing and current increasing?
    I already tried several microstepping mode (from full step to 1/32 step).

    I also tried different current settings. The best result (seen above) is with around 2.6A. When I set this value much higher (5A or more) I get a overcurrent shutdown with the first pulse.
    Between the supply of the H bridges and the supply of the driver is a shottky diode for reverse polarity protection so the supply for the driver is always a bit below the supply of the H bridges. Does this create the overcurrent problem?

    Cheers,
    Christoph
  • 0 in reply to Christoph Schwarz
    TI__Guru** 114905 points
    Hi Christoph,

    You probably already did this, but step input decreasing meant halfing the step input frequency for full step mode.

    In the images above, it appears the current approaches 5A in the unzoomed image and 4A in the zoomed image.

    Separating the supply of the H bridge and the driver can result in overcurrent events. If the H bridge supply droops and the driver supply does not, the OCP monitor (DRV8711 VM - XOUTn) could interpret that as an overcurrent.
  • 0 in reply to Rick Duncan
    Prodigy 40 points

    Hello,

    now I have a direct junction between the bridge supply and the driver supply. The overcurrent errors nearly disappeared and I also can use a lot more current. In the image I attached you can see current and voltage of one winding of the motor with roughly 9.5A current (max. motor current).

    When accelerating with a ramp first a sine current is seen but after some steps the current decreases and at the ruler X2 the motor stops.

    I currently use microstepping with 32 steps. The behavior of the motor doesn't change with a small mechanical load. I already changed the starting speed and the ramp but only the time until the current breaks changes.

    Do you have any advice what problem I am facing here?

    Cheers,

    Christoph

  • 0 in reply to Christoph Schwarz
    TI__Guru** 114905 points
    Hi Christoph,

    Sorry for the delay. After searching the stepper motor Frequently Asked Questions, the torque curves were recorded in 1/4 step with current compensation (not sure what current compensation means). This was found under the question "Does the stepper motor have less torque in half step mode than in full step mode?"

    Please try operating the motor at 1/4 step.

    Also, please consult with the motor manufacturer for details on current compensation.