Friday, September 7, 2012

Excellent Video Presentation on Field Oriented Control

Here's a really great video presentation on Field Oriented Control (FOC) by Dave Wilson of Texas Instruments:


I was lucky enough to see this presentation live at a TI workshop in the spring. It covers all the basics of (sensorless) FOC, which is what most of my motor controllers are using now. By itself, it's probably not enough to write an FOC controller from scratch, but combined with the other information on TI's Motor Blog, most of the important techniques are covered. Of course, for any gaps there is still the ultimate guide to brushless motors, James Mevey's 2009 Masters Thesis.

In case you're wondering how the back EMF observer in this presentation compares to the dirt-simple flux observer I've been using in all of my FOC code, here's roughly one page of math converting between them:


The conclusion is that my dirt-simple flux observer differs from the back EMF observer in the presentation by only a second-order low-pass filter (2DLPF). This is pretty amazing since the structure of the two observers, and even the quantity that they are observing, seem so different. 

The properties of the extra 2DLPF are determined by the PI controller gains in the current error feedback of the back EMF observer. Higher gains give a lower time constant and faster tracking, but also more sensitivity to current sensor noise. My flux observer doesn't have this degree of freedom; it trusts the current sensor all the time and is therefore more susceptible to noise on that signal. (It's the extreme case of the feedback gains being very high.)

Speaking of flux observers...I promise I will write up my Gen. 1 sensorless algorithm soon...

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