I know typically it is not possible to sense BEMF voltage when using FOC but while playing around with some other controllers and reading data sheets I found they were using a techniques to extract phase voltage data in addition to current sense. The first one I found was on Ti instaspin, the sensing layout is identical to the VESC with one small change, phase voltage sense must have a filter capacitor (0.1uf on reference designs) and the software must know the frequency of the filter, as to what it does with the information and phase voltage, though manual loosely implies its to reject PWM signals and some sort of clarke transformation is being applied, what it is being used for is unknown as their FAST estimator is closed and secret. I have found instaspin relies heavily on whatever it is using it for, removing those vsense filter caps cause it to fail, interestingly when a VESC has them it becomes possible to see voltage sine waves under load in VESC tool.
Another I found in a dedicated motor control IC, when using sinusoidal mode it was inserting a small dead time where the estimator believed BEMF zero crossing should theoretically occur and compared the results from the inserted deadtime. This one was mainly to compensate for a very basic current sense system and may not offer any possible enhancement over multiphase current sensing.
There's very limited FOC motor control information online, I was interested to know if additional sensorless sensing enhancement were considered but not found useful in testing or if only current sense data was considered in the initial design and perhaps performance could be further increased.
Hi lizardmech,
Lebowski (Endless Sphere) seems to have a lot of knowledge about FOC.
Have you looked at those posts, I think it worth the pain.
Have a Nice Day.
Thierry
Using a low pass filter directly on the phase voltages and feeding that to the ADCs can help against dead time distortion and some other effects. The reason that the VESC does not have them is that BLDC mode heavily relies on sampling the voltage during a very specific time during the modulation to get the best low speed performance. FOC would benefit from adding the filter capacitors, and adapting the software is straight forward. This would have a larger impact when there is more dead time though, or when using IGBTs though.
The 75V/300A VESC that I'm working on (parts will arrive next week) has low pass filters on the phase voltages that can be switched on or off with small MOSFETs, to get the best performance both in BLDC and FOC mode. I made some LTSPICE simulations with the switchable filters, and so far it looks promising.
I have seen those, his firmware is completely closed isn't it? I have seen him with hardware related threads but not much in the way of open software.
Yes, you're right. He sells programmed chips.
Have a Nice Day.
Thierry
Very interesting. How would you make use of the filtered phase vsense data? Would you transform it to two coordinates and then compare it against the estimator? Aside from improving slower switching inverters would you expect to see it track better at low RPM in general?