Performance and design of an electromagnetic sensor for brushless DC motors

The electromagnetic position sensing device described operates both at standstill and running speeds by detecting the phase change of the second harmonic component of induced voltage in pick-up coils around a stator of magnetically nonlinear material. The magnetic circuit associated with each pick-up coil is excited by a high frequency sinusoidal current and has a magnetic bias that is switched by the position of a simple magnet attached to the rotor of the machine. Results are presented from a prototype device and signal processing is considered to the point where three logic position signals are available, as in Hall effect units. An analytical treatment of the device is given such that geometrical optimisation is possible