[Animats]

3) is interesting. Sensorless operation is common for running motors, but usually doesn't sense position when the motor is stopped, or being moved by external forces.

So if you put a carrier on the power signals, you can potentially sense position, even at zero speed? No need for even Hall sensors? There's a paper from 2001 on that.[1] But it doesn't seem to be a mainstream technique. Are there problems with that approach? If it worked well, you'd expect to see it built into common motor control ICs by now. This paper [2] indicates some of the problems. Detecting the sensing signals in the presence of noise is reported to be hard, especially for small motors. It's a PhD thesis topic.[3] Looking for info about this, I'm finding academic papers, but not IC data sheets.

It would sure be nice to have motors with full positional feedback and only 3 wires, instead of 16.

[1] http://ieeexplore.ieee.org/document/955987/ [2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3231115/ [3] http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=10...

[stevendhansen]

It is not easy per-se, and yes it is used in many dissertations, but most of the limitations up to now have been more related to the performance required to implement a demodulator that runs at high enough frequencies in a low cost microcontroller or FPGA. This is becoming less of a problem with more powerful chips. I don't know if I'd call it a mainstream technique, but it is widely used in high performance IPM motor drives for servo applications.

I think this technique isn't commonly used in lower performance applications because it is an additional cost both in performance (need a more powerful processor) and complexity.

The big problem with high freq injection is that it requires some kind of saliency on the rotor, which means this only works for interior permanent magnet machines and not induction or surface PM machines unless they are specially constructed. I guess I don't know enough about hobby brushless DC machines to know if they are IPM or SPM, but my guess was IPM because they can be cheaper to build for high speed designs and I know hobby BLDCs can spin at 10s of thousands of rpm. Perhaps someone with more hobbyist know-how can chime in.

For this application (robotics) I think the other typical problems with high freq injection are negligible. For example you need a well-defined switching frequency that stays out of the way of your high frequency carrier. Many inverters pull back on switching frequency at high loads in order to lower switching losses, but if you lower the switching frequency too much you won't have enough voltage bandwidth to synthesize the high frequency carrier required for the self-sensing algorithm. I don't think this would be a big deal for robotics applications.

[Animats]

How high a frequency do you have to inject for sensing? Much higher than the power chopping frequency, presumably. Megahertz? Harmonics from the PWM drive are going to interfere. I can see why separating signal from noise is a problem, and why mating a motor and a controller isn't going to be plug and play.

Larger drone prop motors seem to be mostly permanent magnet brushless. Often the stator is on the inside, and the rotor is a rotating shell carrying permanent magnets that fits over the stator. 12 coils and 14 permanent magnets is a common configuration.

Other drone motors look more like standard motors, with an outside stator and an interior rotor. Some of those are definitely permanent magnet.

Here's a good overview.[1] If you wanted to repurpose such motors for industrial control, the main problem would be cooling. They're intended for use with the prop blowing air through them, so they can dissipate much more heat than most motor configurations. Slow speed, high-torque operation would probably overheat them.

[1] http://www.barnardmicrosystems.com/UAV/engines/electric_moto...

[stevendhansen]

The high freq signal has to be lower than the switching frequency - otherwise you end up with harmonic interference from the PWM like you mentioned. For high performance servo control in the 5-200kW range with switching frequencies up to 20kHz I typically see high frequency injection in the 1-4kHz range. This is high enough to produce negligible torque ripple and minimal estimation error, but low enough to be synthesizable without an ultra high PWM frequency. Of course for smaller systems switching at 100s of kHz or even MHz a higher high frequency signal could be injected.

I suspect that this type of sensorless strategy will become more popular with the rise of GaN and SiC devices, but it is currently implemented with good results today using only standard Si IGBTs and MOSFETs.

[madcowswe]

The hobby outrunners come in both varieties: the cheaper ones are surface magnet ones, like this one: https://hobbyking.com/media/file/1010923149X745660X9.jpg and the slightly more expensive ones have the magnets recessed into a thicker back-iron, and hence have saliency: https://www.electric-skateboard.builders/uploads/db1493/orig...

[madcowswe]

Ignore the red circle/arrow, this is just a picture I found online.