The electromagnetic position sensing device described in the paper operates both at standstill and running speeds by detecting the phase change of the second harmonic component of induced voltage in pickup coils around a stator of magnetically nonlinear material. The magnetic circuit associated with each pickup 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. List of principal symbols b0 = bias flux density in active zones b S = bias flux density averaged over unit breadth X B,, B , = instantaneous flux density in outer and inner D = breadth of winding slot e, = instantaneous standardised output of one unit e s 2 , E,, = instantaneous and peak values of second harmonic standardised EMF induced in a pickup coil, respectively H , , H , =instantaneous magnetic field in outer and inner zones, respectively i, I = instantaneous and peak excitation currents, respectively J, J^ = instantaneous and peak drive current densities averaged over winding slot, respectively 57. = maximum current density in winding slot area L = axial length of unit L = length of winding slot MI, M, = gradients of line segments on B / H characterNd NO t = time X Y a @ 0 zones, respectively istic operative at time t = number of turns in exciting coil of a unit = number of turns in a pickup coil = total breadth of a unit = breadth of outer zone = fraction of winding slot allotted to pickup coil = total magnet flux passing through a unit = angular frequency of excitation current Paper 7198B (Pl), first received 25th September 1989 The authors are with the Department of Electrical Engineering, University of Manchester, Manchester M13 9PL, United Kingdom 174 1 introductionThe range of applications for brushless DC motors continues to increase and with it the need for these motors to operate in more hostile environments. The usual rotor position sensing device incorporated in these motors is a Hall effect ring [l] where three suitably positioned semiconductor devices detect rotor position via the influence either of a special switching magnet on the rotor or of the main rotor poles. The motor volume that must be allotted to the Hall effect ring is often brought into question, particularly for very small motors. However, sometimes of greater significance is the suitability of these devices in terms of reliability in environments involving high temperatures, high vibration levels and ionising radiation. Whether or not the Hall effect device is criticised on a volume or a reliability basis, the search for an alternative unit continues. If a unit independent of the main magnetic circuit of the motor is considered, there are several alternative systems possible [ 1, 21 involving ...
