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DRV8829: DRV8829 with linear actuator

Part Number: DRV8829

Hello Good day,

I have a DRV8829 setup like the typical application page in the datasheet, i am trying to drive a 12V, 0.5A typical linear actuator (it becomes open when it reaches one end). 

I notice that the fault pin goes LOW when the linear actuator reaches one end indicating that the protection was activated. It can't be over-temperature because nothing goes hot,

it can't be under voltage because voltage supplied to VM is constant (12V), so it must be overcurrent, but i dont understand why since my linear actuator only reaches up to 0.5 A. 

  • Marion,

    When the linear actuator reaches one end, does it stop? If it stops, the motor winding cannot generate EMF to reduce the winding current going up quickly. If the winding current is driven up to 6A OCP threshold before the current sense blank time (tblank=3.75us), it could trigger the OCP.

    To fix this issue, we can try to limit the current goes up to 6A in 3.75us without EMF. A series inductor can reduce the current ramp up speed. If the input is 12V, the inductor value should be L= V/(di/dt)=12V /[(6A-0.5A)/3.75us]= 8.2uH. Give some margin for input voltage; inductor value; blank time and OCP threshold variations, would you put a 15uH or 22uH inductor in series with the linear actuator to avoid the OCP issue.

    Best Regards,

    Wang Li

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  • In reply to Wang5577:

    Hello Wang Li

    Thank you for answering. Yes the linear actuator stops when it reaches one end and it cuts its connection (open circuit) using internal limit switches. Will the motor winding current still go up in an open circuit condition in that scenario?

    I will try the series inductor solution tomorrow and let you know how it goes.

    Thanks Again,
    Marion
  • In reply to Marion Ramos:

    Marion,

    For an inductor, if a voltage is applied on it, its current will be driven up. I would think the input capacitance could hold the input voltage much longer than 3.75us blanking time.

    Best Regards,

    Wang Li

    Motor Drive Solutions

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  • In reply to Wang5577:

    Hello Wang Li,

    I tried putting a series inductor of 25uH with the linear actuator and i still had the same problem, the fault pin gets low when the linear actuator goes to one end.

    Do you think it's the same issue with this one? except there he's using a drv8825
    e2e.ti.com/.../1392704

    Thanks,

    Marion

  • In reply to Marion Ramos:

    Marion,

    1. Would you share the schematic to me through the e2e private message? I want to check I[4:0] pull-up resistor value and any other possible reason to cause this issue.

    2. Would you check the output current to see the OCP triggering event when the motor moved to the end?

    Best Regards,

    Wang Li

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  • In reply to Wang5577:

    Hello Wang Li,

    1. I've sent you the current schematic im using through e2e private message. I did not use pull-up resistors on I[4:0] pins.
    2. Im sorry i do not have access to an oscilloscope, the reading on my multimeter does not go pass 0.6 A but the spike might too fast for the multimeter to detect it.

    I apologize for the late reply.
    Thanks again,
    Marion
  • In reply to Marion Ramos:

    Marion,

    1. I[4:0] pins are connected to 5V source directly. It is Ok to set 100% percentage of full-scale.
    2. In your schematic, the ISEN pins are connected to ground directly. So, you don't do any winding current regulation. The current regulation (check Equation 1 in datasheet) is Vref/(5*Risense). If Risense is zero, you don't have a current control. So, OCP is not surprise. If you want to regulate at 0.5A, please add the sense resistor: Risense= 3.3V/(5*0.5A)= 1.32ohm. If 1.32ohm is too big for your application, you can play I[4:0] setting to get lower Risense value.
    On datasheet page 10th: "If the current regulation feature is not needed, it can be disabled by connecting the ISENSE pins directly to ground and the VREF pins to V3P3."

    Best Regards,

    Wang Li

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  • In reply to Wang5577:

    Hello Wang Li,

    1. I tried using the current regulation, i now used 0.75 ohm resistor for Risense, so that gives me Ichop = (3.3V/(5*0.75)) = 0.88A.

    then I used 0x0Dh  on I[4:0] which sets to 60 % of full scale which will make about 0.88*0.6 = 0.53A but im still getting a LOW on the fault pin when the actuator reaches one end

    2. At the bottom of page 9 of the datasheet it says "The overcurrent protection does not use the current sense

    circuitry used for PWM current control, and is independent of the ISENSE resistor value or VREF voltage. " 

    3. I think this has something to do with disconnecting the motor while the drv8829 is driving it.

    Thanks again,

    Marion

  • In reply to Marion Ramos:

    Marion,

    Do you mean the OCP is still triggered after adding the current sense resistor and 8.2uH series inductor?

    Best Regards,

    Wang Li

    Motor Drive Solutions

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  • In reply to Wang5577:

    Wang Li,

    Yes. i added a Rsense resistor and an inductor value of 25uH and it's still the same. Ill try to find an 8.2uH and check it out.