Giant magnetoresistive sensors for rotational speed control

This article demonstrates spin valve (sv) sensor applications as rotational speed control devices, in a Wheatstone bridge configuration. The sensor architecture uses two active self-biased sv elements (dimensions 400 x 6 mu m(2)) in a CoZrNb flux guide. The other two spin valves are inactivated for external fields inferior to 150 Oe. Inactivation is achieved by increasing the ferromagnetic coupling of the inactive spin valves above 200 Oe, by roughening the surface of the underlying oxide layer (12 Angstrom average roughness). When excited by a magnetized wheel with 60 N-S poles rotating at 360 rpm, the sensor yields a 400 mV pp square wave output with rise/fall times corresponding to an angular resolution of 0.17 degrees. Amplitude is independent of speed (0-3000 rpm), and of sensor to wheel separation (0.5 less than or equal to d(mm) less than or equal to 2.0). Bridge output decreases 26% when temperature is increased to 150 degrees C. Bridge offset of 350 mV is caused by the resistance differences between the active and inactive sv elements. (C) 1999 American Institute of Physics. [S0021-8979(99)31208-1].