BEMF Oversampling

[manishsangram]

"The main idea is to get more samplesof the BEMF envelope within a PWM pulse instead of one sample.The captured samples are then used for modeling the relationship between BEMF voltage and sampling instances using a least squares (LSQ) method. LSQ method is a standard approach in linear regression analyses to approximate the oversampled (overdetermined) system."

The above technique is proposed using S12Z by MAREK STULRAJTERSYSTEM APPLICATION ENGINEER in FTF-AUT-N1806 - MAY 18, 2016

We would like to explore the same for low speed commutation. Is there any sample code which implements this which we can use to evaluate this for our project ?

[RadekS]

Hi Manish,

The BEMF Oversampling method will not help you with sensorless driving BLDC motor at low speed.

This method is used for high speeds.

The sensorless software for S12ZVM uses zero crossing technique for commutation calculation.

The problem is that BEMF voltage amplitude depends on motor speed – zero speed = zero induced voltage.

So, an ideal solution for low speeds is using the motor with sensors (e.g. hall).

For low-speed systems, we may use integration technique, however, this solution was not published for S12ZVM systems yet.

The integration techniques are basically described in DRM144 Three-phase BLDC sensorless motor control application document:

https://www.nxp.com/webapp/Download?colCode=DRM144&Parent_nodeId=1444433485590698280924&Parent_pageT...

And this solution was used for low speed driving mode with Kinetis devices:

http://www.nxp.com/products/software-and-tools/hardware-development-tools/freedom-development-boards...

Another system, where this solution was used is the Kinetis quadcopter drone reference design:

http://www.nxp.com/products/automotive-products/power-management/engine-and-dc-motor-control/h-bridg...

Even the integration techniques cannot help you at low motor speeds. According to our tests, the reasonable minimum speed was typically at 1%~3% of nominal motor speed and it strongly depends on motor characteristics (inductances).

I hope it helps you.

Have a great day,
Radek

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