• State
  • Locked Locked
  • Replies 7 replies
  • Subscribers 30 subscribers
  • Views 2049 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DRV8412 always in brake

Alastair Barrow
Prodigy 20 points
Other Parts Discussed in Thread: DRV8412

Hi, hopefully this is a simple question.

We are evaluating the DRV8412 using the evaluation board (DRV8412EVM) for a haptic/robotic application. Currently setting up a torque controller to test the closed loop position gain of a single axis.

However, we can't seem to achieve a "free to spin" mode. We can drive in either direction and create a good response or if we drop the duty cycle low or to zero we are in brake mode.

Can someone quickly explain what we're doing wrong here?

Thanks

Ally

  • 0
    TI__Guru** 114905 points
    Hi Ally,

    What are the RESET_AB and RESET_CD inputs? To disable the outputs, the RESET_xx inputs should be a logic low.
  • 0 in reply to Rick Duncan
    Prodigy 20 points
    Hi, thanks for the quick reply.

    Because we are using the EVM RESET_AB is a mechanical so to run it is always logic high. So do we then need to PWM the RESET_AB line to drive it low during the dead time in the drive PWM in order to coast?

    Thanks!
    Ally
  • 0 in reply to Alastair Barrow
    Prodigy 30 points
    To add some more information to this thread and share new developments, by using a PWM matched to the motor direction PWM (Ie PWM A or B) on the RESET_AB input pin a coast mode is achieved. However if hold RESET_AB in a high state the motor continually turns.

    I wonder is this caused by applying a pwm to the RESET_AB pin?

    Many thanks,
    Chris
  • 0 in reply to Chris Shackleton
    Prodigy 30 points
    This problem can be solved by only driving one of the PWM inputs at any given time, IE not using complimentary PWM for each half of the gates.


    Chris
  • 0 in reply to Chris Shackleton
    TI__Guru** 114905 points
    Hi Chris,

    It is good to hear you have solved your problem. If using CBC mode, please note there could be a problem with the outputs disabling and remaining off. The second to last paragraph on page 15 of the datasheet (Rev G) describes this scenario.
  • 0 in reply to Rick Duncan
    Prodigy 30 points
    Hi Duncan,
     
    It seems as though I may have been overly excited, and I have not solved this problem. I will try to be more clear.
     
    We are still unsure if this chip is intended to do what we need. The data sheet lists “haptics” as a use case so we think it must but we’re scratching our heads at the moment.
     
    In our use case we need to be able to dynamically control whether the motor is in coast (terminals open) or damped (terminals shorted) during the off portion of the PWM.
     
    Here’s a simple example very similar in principle to the actual use case which recreates the necessary result and the difficulty we are having.
     
    1)We connect a lever arm to a motor and apply a constant, small force with the motor by closing the control loop around measured current.
    2)A person then holds the lever and moves it back and forth.
    3)We want to be able to have the motor in one of two modes: either coasting during the PWM off time so the lever feels free to move (with a small bias force) or in damped mode so the lever feels viscous/frictional to move.
     
    This is basically our simplest test case before moving on to a multi-axis version of the robot.
     
    In both modes we command a fixed duty PWM to the A input channel and hold B at ground. In damped mode we leave the resetAB pin high and in coast mode (from your explanation above) we expected to set the resetAB pin high during the ‘on’ portion of the PWM and low during the ‘off’ portion.
     
    If we follow this process, the damped mode works exactly as expected. However, the coast mode has an artefact in that when the bridge is commanded to be in reset (resetAB=low) the bridge is actually allowing current to flow in the opposite direction across the motor. The effect of this is that the average force is significantly less than the damped mode.
     
    If we hold the resetAB line low for long enough, the bridge opens and we achieve a coast mode. So we can create ‘zones’ of coasting as long as there is no commanded force, but as soon as we need to command a force we hit problems with cycling the resetAB line.
     
    Hopefully the above explains the use case clearly. Can you confirm that, using this chip we should be able to achieve a coasting response during the PWM dead time? If so, can you shed any light as to what we are doing wrong?
     
    Many thanks
  • 0 in reply to Chris Shackleton
    TI__Guru** 114905 points

    Hi Chris,

    Chris Shackleton said:
    If we follow this process, the damped mode works exactly as expected. However, the coast mode has an artefact in that when the bridge is commanded to be in reset (resetAB=low) the bridge is actually allowing current to flow in the opposite direction across the motor. The effect of this is that the average force is significantly less than the damped mode.

    When entering coast mode (disabling the outputs), the current continues to flow through the body diodes of the FETs until the current reaches zero or the outputs are re-enabled. The current should not flow in the opposite direction.

    Can you provide a scope capture of the current flowing in the opposite direction? Are you measuring the current in the motor windings, or somewhere else?