Back EMF Integration technique

[manishsangram]

Hello,

We are using the S12ZVM128 chip to control BLDC motors. At present we are using the conventional back-emf + calculated time approach to commutation and it is working fine except for low rpm. When we need very low mechanical RPM this technique fails.

We are looking to use the BACK-EMF Integration technique to improve the Low RPM performance but we cannot find any proper reference code or documentation which explains how this can be achieved in general or specifically for S12Z. 

Essentially please bring out documents that explain in maximum detail how to achieve back-emf integration technique for commutation especially with S12Z MCU and if not then with any other MCU with code samples.

[RadekS]

Hi Manish,

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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