DRV10974: DRV10974 can't switch from open to close loop

Part Number: DRV10974

Dear Expert

Our  motor is a rated motor voltage 5V, maximum voltage 5.5V;Rated speed 12000rpm, maximum speed 135000rpm;Rated current 400mA, maximum current 550mA;Phase resistance 6.5± 10% ω;Three-phase dc brushless motor .

DRV10974 set:Rcs=7.32KΩ,Radv=59KΩ,Rrmp=17.8KΩ

Below is phase current:

Could u give some advice about how to enter open loop?

  • Hello Gabriel,

    How many pole pairs does this motor have? Can you also share the motor back-EMF constant?

    Thanks,

    Matt

  • Dear Expert Matt

    The torque constant Kt is 2.569mNmV/A;The back emf constant BEMF is 0.272V/ KRMP.The polar pairs is 1 .

  • Hi Gabriel,

    From the phase current plot that you shared, it looks like the motor is spinning at approximately 40 Hz electrical speed which is equivalent to 2400 rpm. I didn't quite understand your question on how to enter open loop. Did you mean closed loop? Do you see the motor spinning constantly at this speed without triggering any fault? If so, then the device is already in closed loop. If you are using analog mode as speed input, make sure you apply 1.8 V (Analog full-speed voltage) on the speed pin to spin the motor at maximum speed. If the motor speed is controlled using PWM signal, make sure the duty cycle is 100%. 

    Regards,

    Vishnu

  • Dear Vishnu

    The problem is that switching from open-loop acceleration to closed-loop operation always triggers the chip's locking logic, then reboot-locking, and so on;Motor running state is: alignment - open loop acceleration - stop, cycle all the time, there is no way to transition to closed loop running state.

  • Gabriel,

    Device fails to transition to closed loop either due to the "Kt measure" fault or "BEMF abnormal" fault. Let us look at both faults in detail. 

    1. "Kt measure" fault

    Device measures the actual Kt of the motor when transitioning from open-loop acceleration to closed-loop acceleration. If the measured Kt is less than 200 mV, the device indicates that the handoff Kt level was not properly reached and the lock is triggered. In your case, the open loop frequency is 40 Hz and the Kt of your motor is around 16 mV/Hz so the BEMF generated during handoff is 40*16 = 645 mV which is well above 200 mV, so the device might not be triggering this fault. 

    2. "BEMF abnormal" fault

    In closed loop, device constantly estimates Kt (let us call this as Ktc) and checks if the estimated Kt is above 0.5*measured Kt and below 2*measured Kt as shown in below image. If the estimated Kt does not fall within this range, device triggers "BEMF abnormal" fault. Ktc might be incorrect if the lead time is not tuned properly. I would highly recommend you to check the Tuning guide for step by step procedure to tune lead time. 

    Also, I noticed that the current limit is chosen as 200 mA. In the phase current plot the current is close to 500 mA. Can you increase the current limit to a higher value which is close to the rated current of the motor? 

    Regards,

    Vishnu

  • Dear Vishnu

    Thanks for your reply! We changed the phase current plot the current is close to 1000 mA though slecting 54.9kΩ CS resistor . The RMP Resistor is 17.8 kΩ.

    1.But we tested below all of ADV Resistor ,the motor still could not opration in  close loop. The motor runs for about 6.7 seconds, then stops, restarts after 5 seconds, stops after 6.7 seconds, and repeats. We tested the phase crrent ,peak 1.15A ,we think this is not OCP.

    Pls give us more suggestion ,many thank!

    BR

    Gabriel

  • Gabriel,

    Can you try lowering the acceleration rates by choosing RMP resistor to 7.32 kOhms and re-tune the ADV resistor? Please follow the procedure explained in section 3.3 in the tuning guide to tune ADV resistor.

    Regards,

    Vishnu

  • Gabriel,

    Do you need any additional help on this thread?

    Regards,

    Vishnu