I know, aww I love them so much! I really want one too but I have not gotten around to doing such a thing. Since the electrical steel laminations or even more specific other core materials are kind of the most odd and difficult element of an electric machine to manufacture for a custom prototype motor, making a coreless machine is relatively easy if you have some patience and like that sort of work. Alex Borg has a site called amazingdiyprojects.com and he has made a little ebook about this, it is called build your own electric motor. I really liked his very practical approach that I found very refreshing to take as a base idea to develop from. You can also see his motor on youtube, it is quite cool for such a low-tech approach. Also there is lots of more academic content on the topic of ironless axial flux BLDC machines.
In my bookmarks, I found http://scolton.blogspot.com/search/label/axial%20motor
and http://build-its-inprogress.blogspot.com/2015/02/coreless-axial-flux-motors.html for more personal blog examples of such things.
I think this is a question for madcowswe to answer. I don’t think this has been explored before.
Speaking of this, does anybody know of a fitting 21+bit absolute encoder that is obtainable from accessible sources?
Not unless you have hundreds per encoder to pay for it. You also need SSI to use them.
Broadcom has some new magnetics but I think they only sell to OEMs.
Thanks Roiki. I had a feeling. So SSI is preferred in these ultra high resolution encoders because of the differential signal integrity, making it a logical choice for top of the line servo systems with utmost reliability? I have seen some info about a 20-bit encoder that has an SPI option, but I am not too familiar with the absolute encoder landscape. What is the highest resolution SPI magnetic absolute you are aware of that can be purchased? If they cost a few hundreds, that may be acceptable for what it is.
SSI(and Biss) and rs485 are the standards in the industry for digital connections. Canopen and ethercat are the latest preferred ones. Also SPI is not really meant for outside-board communication, it’s meant for interboard communication between ICs where traces are short and EMI is low. This is why it’s not used on industry grade products.
I’m not aware of any that go that high, especially with SPI. Magnetics just aren’t advanced enough yet. Broadcom aeat-6000 is 16 bits for slow rotation speeds but you need an extra expensive programmer to program the chip for use. AS33-M50M, also from broadcom, is 18 bit single turn but is available only to OEMs on bulk order. None of these have SPI, only SSI, Biss-c and rs485.
If you need that high of a resolution you need to look at optics but they’re pricey and bulky.
Right, that makes perfect sense and I figured so. Well, it would be a nice future addition to the ODrive to include SSI.
Are you talking about units such as these large Heidenhain ring-shaped large through-bore encoders? Yes, I guess I am following what you say about magnetics. I suppose for a turntable of 1 million counts or more, the natural choice would always be a large-diameter true material pattern brute-force precision device that would most of the time be optical.
It would be but it’s some work to get it done. I’ve been looking at putting an ethercat board on it but rtos programming is too bothersome. I’ll just use a teensy in the middle.
There are many kinds. The cheapest I’ve found were broadcom AS37 and AS38, they’re about 200.
You can get magnetic rings and encoders from rls. Both incremental and absolute if ring form is what you need. The cheapest is about 120 and the absolute one I think is about 400. You can get different sized rings(with different pole count) and the read head specifies the resolution(you can get 8192 per pole with 72 poles = 589k counts.) but they their speed is limited by the speed of the counter.
In my earlier employment, we used 22+14 bit absolute multi-turn encoders from Kűbler with SSI or BiSS option. They also had a SIL 3 rated sin/cos output for safe velocity monitoring.
The BiSS/SSI output was absolute over 10^14 (16384) turns.
They cost about 3 grand each. 
That’s industrial for you. 