I am actually developing an application using instaspin from texas that uses that concept.
I use the motor encoder to get 200% torque at zero speed and switch to a completely sensorless application when the motor spins fast enough. So far the drawback I've found is that I cannot have a true "coast" state using the sensorless and I am having trouble combining both signals, once the motor is spinning. I also have not been able to test on the vehicle how their sensorless operates at zero speed. At least on bench it seems to work fine.
Right now, sensorless apart I use both the vehicle speed sensor and the motor encoder. I simply ignore the motor encoder below a certain speed and use open loop. Then (generally 100RPM+) I switch to closed loop. This is to avoid encoder noise at low speeds.
Lots of options really... I wonder how low can Paul sensorless operate. Texas is claimed to track down to 1Hz. Idd be queen to test that on a vehicle. Apparently some guys did that on a bus and they actually seem quite happy...
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