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Sensorless control

I am curious about the definition of "sensorless control".  When you talk about sensorless control, are you in fact meaning a lack of physical position sensor such as e.g. a magnet plus vane plus hall effect? i.e. not having a unit whose sole objective is position detection.
Is the sensorless control based around alternative methods of measurement or detection to predict position using components that have to exist for the machine to function (such as measuring or detecting voltages or currents in the windings)?

I had long ago wondered about designing a motor, fully measuring its voltage and current profiles and phase firing timings for normal operation (from stationary to full speed full load) using a position sensor for getting the motor to work and to determine the best required phase firing sequences and associated voltage/current profiles then program a microprocessor to replicate the entire required profile such that I would attempt to eliminate the need for any sensing or measurement at all (but I concluded it would come very unstuck for any fault conditions or restarting while it was still turning). So in my mind don't all such machines require a form of measurement (i.e. some form of "sensing") to work properly so could never be truly sensorless?

A completely sensor-less control would be completely open-loop, which isn't reliable with some motors like PMSMs. Even if you knew the switching instants for one ideal case, too many "random" variables could influence the system (just think of the initial position), so that those firing instants could be inappropriate for other situations.

Actually, induction machines, thanks to their inherent stability properties, can be run really sensor-less (i.e. just connected to the grid or in V/f). To be honest, even in the simple grid-connection case there is an overcurrent detection somewhere in the grid, which requires some sensing.

But there can also be said the term sensorless relates to el. motor itself. In another words, it means there are not any sensors “attached” to the el. motor (which does not mean sensors cannot be in the inverter, in such a case). In our company we are using the second meaning, since it indicates no sensor connections are needed between the el. motor and the ECU (inverter).

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