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DRV 8834 behavior on nENBL pin high and low state

Anuj Tanksali
Prodigy 20 points
Other Parts Discussed in Thread: DRV8834

Hi,

 

I have a query regarding the DRV 8834 driver. We are using DRV8834 to drive stepper motors.

The motor interface document is attached for your reference.

 

The application is driving 6 motors through 6 DRV8834 drivers one at a time.

The STEP, Dir are common which go to all 6 DRV8834 drivers.

Only the enable pin of DRV8834 is selected by a MUX to enable the specific DRV8834 to drive respective motor.

 

The following configuration is used

 

Controller used to drive DRV 8834 driver      :              LPC1788

STEP size                                                              :               1/16 set through the controller

Direction                                                              :               Forward/Reverse set through the controller

PWM frequency for STEP                                  :               250 Hz set through the controller                                                                                             

 

Flowing are the steps to drive the stepper motor through DRV8834 driver

 

  1. Set Backward Direction

  2. Enable the DRV 8834 output by driving the nENBL pin low

  3. Wait for 500 ms

  4. Start PWM at 250 Hz frequency

  5. Stop PWM pulses and then motor by driving the DRV 8834 nENBL pin high

  6. Set Forward Direction

  7. Enable the DRV 8834 output by driving the nENBL pin low

  8. Wait for 500 ms

  9. Start PWM at 250 Hz frequency

  10. Wait for fixed no of pulses to complete

  11. Stop PWM pulses and then motor by driving DRV 8834 nENBL pin high

  12. Go to step 1 and then step 2 to enable next DRV 8834

 

Observations

 

  1. When DRV 8834 nENBL pin is driven high to stop the motor and again enabled when switching to next motor a small jerk is observed on the output mechanical assembly which is connected to next motor. This jerk is observed on all motors connected to respective DRV 8834 drivers. This jerk is observed exactly when DRV8834 nENBL pin becomes low and no PWM pulses are started at this point.

     

Query

 

  1. In our application we have to control 6 motors one by one and due to this jerk we cannot control the motor position accurately.

How to eliminate this initial jerk?

Thanks and Regards,

Anuj Tanksali

  • 0
    TI__Mastermind 26065 points
    Hi Anuj,

    What you find is true in your driving scheme.

    If a motor stopped by nENABLE high, the phase current is totally removed. So the motor has no hold torque for its loading state, or even for the micro stepping position maintaining. The motor will departure from its position when disabled. Then, when it is enabled again, it will jerk and resume to the previous indexer position.

    Normally we don't use nENABLE to stop and hold a stepper. We have to keep the current on the phase to keep the holding torque and its angular position. If the current make the motor too hot in holding state, we can reduce the two phase currents with the same rate together, such 50% of each. We can done this by change Vref.

    Best regards,
  • 0 in reply to Wilson Zuo
    Prodigy 20 points
    Hi Wilson,

    Thanks a lot for your timely response.

    I did some tests by keeping the nENABLE pin continuously low and I do not make the pin high after that.
    No jerk was observed in this condition.

    In my application I will be running a test for maximum 3 to 6 hrs. So the DRV8834 nENABLE pin will be low for that much time.
    My question is what are the effects of keeping the DRV8834 nENABLE continuously low.

    Regards,

    Anuj Tanksali
  • 0 in reply to Anuj Tanksali
    TI__Mastermind 26065 points
    Hi Anuj,

    If the nENABLE continuously low, the driver is always turning on the phase current even if the stepper is not running. That is true how a stepper keeps its holding torque although not so effeciently. Also the motor could be very hot with the current always there. That is why we need half/ or lower current keeping state.

    Best regards,