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Original question:

DRV8234: Question on DRV8234

DRV8234: Delay time from ripple detection to RC_OUT output

Daisuke Higa
Genius 4465 points
Part Number: DRV8234


Tool/software:

Hi support team all.

Is it possible to specify the delay time from when current ripple is detected internally until it is output to RC_OUT?

The internal threshold for detecting current ripple is unclear, so the detection timing is unclear. When we actually measure RC_OUT, there is very large jitter.

We are concerned that this will affect position detection.
(We are verifying this using an EVM.)

Best regards,

DH

  • 0
    TI__Mastermind 20270 points

    Hi DH,

    A specified delay time would need to be implemented in the firmware to account for any missed ripples on startup. This is a known behavior in these applications and is seen due to the slow back-EMF build during startup. There is an amount of inrush current observed before the motor begins to spin that causes seen ripples during inrush current conditions. 

    Under same operating conditions and settings, the device consistently misses the same number of ripples each time so this can be offset in firmware. There isn't a way to evaluate this with the GUI alone, additional firmware would need to be implemented. This can easily be accounted for simply by subtracting the missed ripples from the target ripple count. Programming the microcontroller to offset for x-ripples enables position tracking with better accuracy.

    Best,

    David

  • 0 in reply to David Medis
    Genius 4465 points

    Hi David-san

    Thank you for your prompt reply.

    However your answering isn't that I am intended.

    I would like to know that the delay time (Δtdelay) from current ripple detected to RC_OUT pulse be outputted. 

    (Please refer to the following figure.)

    What could be the cause of this variation in delay time? 

    Best regards,

    DH

  • 0 in reply to Daisuke Higa
    TI__Mastermind 20270 points

    Hi DH,

    This time is dependent on the motor construction and specific commutation speed. It's not possible for us to define these delay times for all motors. The delay time will be consistent when tuned properly.

    Why do you need to know this time?

    As long as each ripple has a corresponding RC_OUT pulse then the motor position will be known.

    Best,

    David

  • 0 in reply to David Medis
    Genius 4465 points

    Hi David-san

    Understood.
    The reason I would like to confirm this is as follows:
    My customer's application has rapid changes in rotation speed and the rotation speed changes are frequent, so they are concerned that fluctuations in latency will have an impact.

    I think that delay time fluctuations are mainly due to uneven rotation of the motor, but what do you think?

    Best regards,

    DH

  • 0 in reply to Daisuke Higa
    TI__Mastermind 20270 points

    Hi DH,

    It would likely depend on the load torque during speed changes. 

    What's the end application?

    How frequent are the speed changes?

    Best,

    David

  • 0 in reply to David Medis
    Genius 4465 points

    Hi David-san

    Application is paper feeder for thermal printer.

    The rotation speed must be changed in sync with the printing timing.

    Best regards,

    DH

  • 0 in reply to Daisuke Higa
    TI__Mastermind 20270 points

    Hi DH,

    This should be no concern. 

    Has the customer seen any variation in speed or position control or is this just an initial design concern?

    Best,

    David

  • 0 in reply to David Medis
    Genius 4465 points

    Hi David-san

    Yes, as you guessed, it's an early design concern.
    As a distributor, I need to respond to my customer's concerns.
    If you have any information that could reassure them, please share it.
    My customer is about to decide whether to evaluate.

    Best regards,

    DH

  • 0 in reply to Daisuke Higa
    TI__Mastermind 45865 points

    Hi, 

    Thank you for your post. Monday September 1st is national holiday in US. Please expect delay or please wait for our expert assignment on Tuesday US time

  • +1 in reply to Daisuke Higa
    TI__Mastermind 20270 points

    Hi DH,

    Understood. 

    The delay from current ripple to RC_OUT pulse shouldn't be of concern as long as each RC_OUT pulse corresponds to a current ripple. This ensures accurate position tracking. If the customer sees missing or additional RC_OUT pulses that don't match current ripple, then they can be addressed through proper motor tuning. 

    The following app note discusses concepts, examples, and common challenges and workarounds of ripple counting that can be referenced.

    Solving Sensorless Brushed DC Motor Speed and Position Control Using Ripple Counting (Rev. A) 

    Best,

    David

  • 0 in reply to David Medis
    Genius 4465 points

    Hi David-san

    Thank you for your kind support.

    I will propose your advice and the application note information to my customer.

    Thanks!

    Best regards,

    DH