Unable to enter closed loop mode while driving BLDC. This is the A-phase current waveform of the alignment and go stage. I found that the current ripple is much larger than the current ripple in Tuning Guide.Is it because the LR constant is too small that the commutation in the closed loop goes wrong?
Hi,
Thanks for posting your question in MD forum. Are you using the DRV10975EVM or a custom board? There can be several reasons for the device to not able to enter closed-loop. Primary reason for this failure could be the wrong estimation of BEMF constant in open loop. If the device cannot estimate the correct BEMF constant value in open-loop, then while entering the closed loop, the estimated commutation point will be wrong. If the commutation point is wrong then the device might trigger several faults such as "Abnormal BEMF", "Closed loop stuck", "Current limit" etc. Please follow the below steps.
1. Check if DRV10975 can drive the motor
Firstly, let us check if the motor you are working on falls within the recommended application range of DRV10975. Can you provide the motor datasheet to compare the actual motor parameters with the below table?
2. Tune Open-loop settings
If the motor parameters fall within the operating range, next step will be to tune the open-loop settings. Disable closed-loop operation and keep increasing the open to closed loop threshold frequency up to a point where the estimated BEMF constant Kt matches the actual value in the datasheet. Set that frequency as the open to closed loop threshold frequency and now enable closed loop operation. Make sure the abnormal Kt lock detect threshold is set between 2Kt (upper limit) and 0.5Kt (lower limit). Also, set the AdjMode to "Half cycle".
If this doesn't seem to work, let me know what type of fault gets triggered in the device. Based on the fault type we can predict next steps.
Regards,
Vishnu
Hi,
As you said, motor constant is too low for your motor. This can be compensated to some extent by operating the device at higher PWM switching frequency. Looking at the phase resistance, DRV10975 may not be a good option. We have another device DRV10987 which has a wider range of phase resistance and more features compared to DRV10975. Can you try operating DRV10987 at 50 kHz PWM driver switching frequency? This is not a default setting so you will have to set CONFIG6 Register (address = 0x95) bit 14 to 1. If you are using DRV10987EVm GUI, you can simply go to PWM output options in the Advanced settings tab in the GUI and enable "Double the output PWM frequency" as shown below. This can minimize the ripple to some extent.
Note: 50 kHz is the maximum PWM frequency this device can support.
Regards,
Vishnu
