Electronic – Dealing with coupling of PWM noise into Hall-Sensors as spikes in BLDC driver circuit

analogbrushless-dc-motornoisepcbpwm

I have designed the first revision of a BLDC driver board (based on STM32 and DRV8323 chips). Right out of the bath (after writing the firmware of course) I noticed that the coupling of PWM into the hall signals is very terrible!

The motors that I have designed this circuit for are rated at ~20Watts. Their hall sensor only works at 5V or higher. I use the XOR mode of the STM32 timer to capture and generate commutation event…the STM32 is 3.3V…but I assume it can tolerate 5V input but I do not know what is the hysteresis for detecting a low or high signal when the GPIO pins are configured in XOR mode:

At below 100% PWM, I get a lot of noise. The yellow trace is one of the hall sensors (the blue is commutation event). I only have 2 probes so I could not capture all 3 sensors and the commutation event (pulling an gpio high and low before and after updating timer output). As you can see, the noise over the hall sensor is quite bad:

I blame 2 things:

1.The layout: As I did not have prior experience, and because there are 2 motor connectors, I routed the hall sensors very close to the phase signals (greens are phases and yellow are halls).

2. Motor Cable:
The motor cables are just 8 wires next to eachother (no twisting) for almost 50cm…so even if I did the best layout in the PCB the cable itself would cause the coupling issue.

I have tested the driver with 100% PWM (meaning no switching) and everything looks much better (the noise is very small and limited to when the half bridges switch at hall sensor update). So I really need to take care of this noise. I know that I can mitigate this effect in the software (e.g. by inserting timeout and other filtering methods…) but nothing can beat having a nice clean signal anyways.

So to make it short here is my questions:

1) What can I improve in the layout? separation of hall sensors and phase signals into different connectors?**

2) I will tear apart the motor cable and separate halls from phases and make them twisted pair…before I commit this, do you think it makes sense?

3) Should/Can I use a level-shifter and perhaps a Schmidt trigger to remove the noise and convert 5V to 3.3V before feeding it into the STM32 controller? If so, what chips do you suggest?

UPDATE:

Here is the RC filter for hall sensors, placed near MCU input.

Best Answer

Here is basically what I have (I also have a ferrite bead in series right at the connector, but that is for EMI, not part of the low-pass filter):

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Here is what I would suggest for you, since you also want to reduce the voltage from 5V to 3.3V. The RC time constant (ignoring the 1k) is the product of R2 and R3 in parallel and 56pF. R2 and R3 in parallel is 10k, so, it is pretty close to the same time constant as my circuit. You can experiment with different values of capacitor to change the time constant, but I suspect you will see a large benefit from this. The rise time is around 500 or 600 ns, which should be OK.

schematic

^{simulate this circuit}

Put the cap close to the processor. Resistor can be close, also, but that is less critical than making sure the capacitor GND connection is close to processor GND connection.

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Electronic – Speed measurement of brushless dc motor with hall sensors with STM32

I read the code, and I have a few comments or things that could help (or not).

Your code: I'm still figuring that out. According to the picture you added the state will never be 0b101? Are you trying to measure the time between each XORed pulse? I imagine that void XOR_ISR() will be called on each pulse's upwards edge. That should be rather straightforward, for example:

void XOR_ISR(){
    long time = TIM4->CCR1;
    int rpm = 10/time; // 60 * 1/(time*6)
}

I would add a moving average calculator to smooth out the measurements a little as well.

Upper Limit: Running at an maximum of 3000RPM the wheel will turn 3000/60=50 times per second. Since there are in total 6 poles on the wheel, it means that every 6th time the ISR is called a full rotation occured. This means there is a pulse 50*6=300 times per second. Or one pulse every 1.6 milliseconds. Seeing as your timer clock runs at 80MHz, it will count to 66640 for every pulse. This will give you very accurate readings at high speeds.

Lower limit: However, it could make the lower speed readings a bit more complex. Say the wheel is spinning at 10 RPM. That results in 10/60*6=1 pulse every second. This means the timer will count to 80 000 000 with every pulse. As @Harry said, this will only fit in a long variable with 5 bits to spare.

My thoughts: Running your timer at 80 MHz is a little overkill. I would lower the clock frequency slightly to make the measurement slightly more robust at very low RPM. But it all depends on what the smallest RPM is you want to measure. 10RPM? 1RPM? 0.1RPM? You will need to ensure that your counter does not overflow.

The accuracy: The biggest measurement error you could will be off by a maximum of one half of the timer period. This will be 1/80MHz/2 = 6.25ns. At an maximum RPM of 3000RPM this will result in a 6.25ns/1.6ms=0.00036% error. This is an error of 0.11RPM RPM. This is very good for most applications.

I hope this helps! And if someone finds a problem with my math, please tell me :)

Edit: Thank you @Harry for correcting my late night math.

Electrical – Pull-up scheme for Hall effect sensors on a BLDC

The spec says...

4.5V min for Hall sensor so you can use 5V and open collector may pull-up to your 3.3V. I don’t understand open collector and 6.3V floating. Unless that is simply stray leakage voltage from 24V Motor supply with 10M probe. Using 3.3mA to 3.3V with 1K will absorb any stray leakage and possibly 10K as well.

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Electronic – Speed measurement of brushless dc motor with hall sensors with STM32

Electrical – Pull-up scheme for Hall effect sensors on a BLDC

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