A friend showed me this today, not powered up yet:

https://www.kickstarter.com/projects/tropicallabs/mechaduino-powerful-open-source-industrial-servo-m

At first I was confused, it says it's a stepper, but with an encoder, 
and lowers or disables drive current when not needed.  Or you can 
freewheel it and it'll maintain the coordinate system.  It cannot stall 
without the system knowing, and a stall won't corrupt the coordinate system.

But there's no mechanical connection to the motor, it's this new AS5047D 
high speed, high resolution magnetic rotary position sensor:

http://ams.com/eng/Products/Magnetic-Position-Sensors/Angle-Position-On-Axis/AS5047D

You glue a magnet to the rear of the motor shaft, and keep the sensor 
like 1/4" away.  Note it not only counts delta, it knows absolute rotor 
position.

Gives 2000 counts/rev.  That's enough that it could effectively show 
rotor phase through a single fullstep, which would allow for more stable 
electrical control of stepping.  There's an initial calibration phase 
that drives a phase so it knows where the TDC of Coil A is in absolute 
position.

They set it up so it powers down and freewheels, except it will hold its 
position by powering coils when needed.  It can also reduce current to 
product only the torque needed to follow the step commands, instead of 
always operating at full current.

Well, my mind is blown!

Danny



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