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DRV8834 Pulse Period

Al Bencomo
Intellectual 370 points
Other Parts Discussed in Thread: DRV8834

I'm using the DRV8834 to control a hybrid stepping motor (1.8° step angle @ 200 steps/revolution).

The driver is configured to a microstep resolution of 1/8 step.

Is there an equation or way to calculate the pulse period [TON + TOFF] required to hold a specific torque value?

In other words, if I need to hold a torque of 6.032 oz·in (0.0426 N·m), then how can I translate that to a pulse period?

Here some additional information:

My microcontroller clock rate is 62.5 KHz (@ 16us time-base) and I need to supply the DRV8834 with a pulse period value between 2 - 65536 to control that torque.


Thanks in advance!

-Al

  • 0
    TI__Guru** 114905 points

    Hi Al,

    The DRV8834 handles this for you in indexer mode. Once enabled, each pulse on the STEP input moves to the next value in the step table (Table 5).

    There are a few items that need to be addressed.

    First is the chopping current, which should be set at the rated current of the motor. The indexer table will set the current as described in the datasheet under "Current Control". Assuming normal operation, once the required current for the designated step is achieved, current regulation begins until the next STEP input.

    Second is setting the decay mode. This usually requires experimenting as VM and motor parameters can affect which decay mode works best.

     

  • 0 in reply to Rick Duncan
    Intellectual 370 points

    Hi Rick,


    In my case, the indexer mode is the default.   However, is there a way to calculate the pulse period (TON-TOFF) between each pulse sent to the STEP input, based on the known required torque?


    Thanks, and best regards!

    -Al

  • 0 in reply to Al Bencomo
    TI__Guru** 114905 points

    Hi Al,

    I think I understand your question now. Let me repeat it back in a different way. Are you asking:

    What speed should I issue STEP commands to produce a known torque?

    If you are, I have seen this information typically in the motor manufacturer specifications.

    I have seen that value referred to as Pull-out torque. The specifications usually show the maximum speed for a given torque value. Exceeding that speed may result in lower torque or loss of synchronization.

    If that is not your question, can you please explain in more detail?

  • 0 in reply to Rick Duncan
    Intellectual 370 points

    Hi Rick,

    Yes, that was my question.  Unfortunately, most of the motor manufacturers don't provide too much details or information at the microstepping level.  Only charts that map Torque vs PPS.

    I was looking for some kind of equation, but I think figured out an empirical extracted factor to map the torque to the required pulse period.  Here is the preliminary results:

    http://youtu.be/Rifz1suig60

    Thanks Rick, and best regards!

    Cheers,

    -Al