FOC controlled Stepper Motors are better suited for our application
Stepper motors have more torque than BLDC motors, but are they less efficient?
The way stepper motors are used today they are lossy and have problems like “loosing steps”, since they are commutated completely blindly - no hall sensors, no zero voltage crossing detection or current analysis. No feedback at all. Surprisingly, they still work fine in most cases where huge torque is demanded, even in “blind mode”. The reason is, they were designed to work this way: They were designed for most torque and most simple driving. The torque density they archive still is unbeaten, but the way we drive them can be done way better today – even without any encoders or sensors – the keyword is FOC for hybrid steppers, which works without sensors besides the current sensors in the controller.
Hybrid steppers are a bit like BLDC motors with very high pole count that use reluctance and permanent magnetic forces at the same time – the reason they are called “hybrid” steppers. Almost all stepper motors used today are Hybrid Steppers. They are no true reluctance motors but have permanent magnets, just like BLDC motors or brushed DC motors do, thus they have a back EMF, a voltage that is generated if you turn the shaft. This voltage can be detected, unlike true reluctance motors that are turned by their “soft magnetic” properties, without permanent magnets, without any voltage generated if you turn the shaft.
ODrive can drive BLDC motors with or without any encoder. It assumes the actual rotor angle by measuring the motor currents and thus uses the motor itself as a sensor [citation], a sensorless operation. Hybrid stepper motors can be treated almost the same, with two phases instead of three:
The picture was taken from http://www.diegm.uniud.it/petrella/Azionamenti%20Elettrici%20I/Presentazione%20HSM.pdf
It would be nice if ODrive could not only drive BLDC but also Stepper Motors with FOC. This technique was developed for at least 15 years now, but commercially available controllers are rare. I did not find one yet. Long cables may disturb the sensorless measurement of rotor position, as stated from page 50 http://oa.upm.es/32464/1/tesis-Ricardo-Picatoste.pdf