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DRV8811 Stepper Motor Holding Current

Fiona Campbell
Prodigy 50 points
Other Parts Discussed in Thread: DRV8811

Hi,

 I am using a DRV8811 to drive a stepper motor and am looking for some application assistance:

The application uses full step control to move the motor through a fixed number of steps.  In the past once the motor reached its required position the H-bridge was de-activated using the enable function and the motor windings are therefore un-powered and is hence he motor free to move.

My requirements haveI now changed so that once the motor has transitioned the demanded steps the mechanism is held in this fixed position and will not move under externally applied load.

The data sheet implies that with the enablen pin tied low the H-Bridge is always active and hence a drive currnt should be present on the output but I am not seeing a drive current on either of motor windings.

Is tthis correct ?  If so then is there a way to hold the motor in its fixed position ?

Regards

Fiona

  • 0
    TI__Guru** 114905 points

    Hi Fiona,

    ENABLEn low will keep the DRV8811 outputs active, unless there is another signal overriding it. For example, SLEEPn being low will place the device in a low power state.

    Another signal that can affect the amount of current is VREF. If it is placed at a low voltage, the drive current will be reduced.

    Please check these signals first, and then look for other signals changing values from driving to holding. That should provide a clue why you are not seeing drive current.

  • 0 in reply to Rick Duncan
    Prodigy 50 points

    Hi Rick,

    Control signals are :

    SLEEPn = high (3.3V)

    RESETn = high

    SRn = low (0V)

    USM 0/1 = low

    DECAY = 1.5V

    VREF = 1.18V to give an Ichop=147mA

    Motor current per phase = 75mA @ 12V

    RC a/B 100K//1n

    Its moving the demanded steps OK its just not holding the motor once it completes the steps.

    Fiona

  • 0 in reply to Fiona Campbell
    TI__Guru** 114905 points

    Hi Fiona,

    When you say it is not holding the motor once it completes, what is it doing? Does it continue to advance, jitter, or do something else?

    Can you provide a scope capture of the currents when running and stopped? If not the currents, please provide a scope shot of either AOUT1 and AOUT2 or BOUT1 and BOUT2 when stopped. I would prefer to see the current if possible.

    Also, as a quick experiment can you set SRn=1 when you have stopped?

    Thanks.

  • 0 in reply to Rick Duncan
    Prodigy 50 points

    Hi,

    Once demanded nubmer of steps completed it stops moving and remaiins in requested position but if an external force is applied it moves off with no resistance as terhe is no winding current on wither winding to maintian a holding torque.

    See attached plots for example of drive demand and  winding current.

    8510.TI_Plots.pdf

    I have a new firmware build avaialble today to try SRn=1.

    Fiona

     

  • 0 in reply to Fiona Campbell
    Prodigy 50 points

    Updated plot file to show what happens when mechanism pussed off from static position.

    0488.TI_Plots.pdf

    Fiona

  • 0 in reply to Rick Duncan
    Prodigy 50 points

    Hi,

    Not sure why but making the SRn=1 seems to have solved problem !

    Now getting a DC steady state current of approx 50mA on each winding and mecahanism being held in position when external fource applied.

    Fiona

  • 0 in reply to Fiona Campbell
    TI__Guru** 114905 points

    Hi Fiona,

    Glad to hear it is working. Now let's try to determine why this solved your problem.

    An overcurrent event (OCP) could cause the loss of drive current. If an OCP occurs, the outputs will stop driving, the device will be reset to the HOMEn state, and ENABLEn has no effect until it is pulsed high and back low.

    With SRn=0, please watch the HOMEn pin when stopping the motor. If it is not set low (there is a three in 4 chance it is not), please watch it and see if it goes low unexpectedly. This could be an indication of an overcurrent event.

    Another method to confirm this is to pulse ENABLEn high and low once the current is no longer driving.

    Page 13 of the datasheet explains the OCP circuit and the action taken, including the need to pulse ENABLEn to re-activate the outputs.

    What is the inductance and resistance of your motor? This may also provide a clue.