I never understood the FOC, really. I have the bad habit that before I implement something, I want to understand it. I spent about a year reading on AC induction motor FOC (which, to be fair, is an order of magnitude more complex as a whole than BLDC FOC, including an additional "rotor flux estimator" black box no one was ever able to describe in any terms to me), trying to make sense, and the more I delved into maths, the more apparent it became that people writing all these manuals, appnotes, academic papers, etc., didn't have a clue either, just copypasting the same fancy math terms they had read about previously. And, you can't teach something you don't really understand! What became apparent was that I'd need to find someone who really understands it, and then have they personally teach it to me, so I can ask all the stupid questions I need to. I never found anyone.
This hasn't prevented me from creating great and usable motor drives - including torque control (yes, you can only control it within limits, just like in constant current lab supply there are limits as well) - for both AC induction motors, and BLDC. They are very efficient, as well.
FOC is typically presented as one huge black box with a massive "curriculum vitae" for "benefits" such as "high efficiency" over "competing" (what exactly?) control schemes.
OTOH, "crap in, crap out" applies. Books have been written on tuning the FOC to work at all.
And, for each person, the acronym "FOC" may mean completely different things.
In BLDC, the FOC seems to sometimes mean sensorless operation where the phase synchronization happens based on current direct measurement of two phases.
For me, the FOC seems an equivalent of compensating a voltage controlled switch mode supply - a great academic discussion of 1970's, difficult to achieve stability with, requiring careful tuning of PID loops. Other simpler schemes would be equivalent to current mode controlled converter, but I don't see much discussion on anything but FOC. "DTC" or "direct torque control" was a term I saw a few times in the induction motor world, and it made more sense to me, although I never understood that completely, either, just that it's closer to my "inherently stable, simpler current mode converter" analogy.
In the end, I don't know. All BLDC controllers I have designed use low-side current sensing on two phases, just to support any control scheme requiring this. OTOH, every current limit I have actually ended up implementing would work from the DC bus current measurement as well, as long as it doesn't include too much bulk capacitance after the shunt. OTOH, I use hall sensing to sense the rotor angle for phase synchronization. I can imagine that a well tuned current measurement could get me more resolution than the limited 60 electrical degrees I get from the halls, especially during load changes at high speeds.
TL;DR:
FOC can really mean anything. The presentation is often unnecessary complex and involved in math. FOC, in any form, may not be necessary for you, or it may be the best thing ever for you. If you find it overwhelming, implement something of your own first, which is exactly what I have always done, and never ended up with FOC so far.
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