Colpitts Oscillation Type Sensitve Micro Magnetic Sensor Using Amorphous Wire Magneto-Impedance (MI) Head

Abstract

A new sensitive micro-sized single head field sensor using Colpitts oscillator with a negative feedback loop is constructed based on the Magneto-Impedance (MI) effect using a slightly negative magnetostrictive amorphous wire (Fe_4.35 Co_68.15 Si_12.5B₁₅, 30μm dia., 1-3mm length, 2kg/mm² tension annealed), The MI element is used as an inductive element in the Colpitts oscillator, and is magnetized with a resonant circuit ac current (70MHz) biased with a DC current. A DC bias field Hb and a feedback field Hf are generated by the coil current around the MI element for realizing an asymmetrical MI characteristic regarding the sign of an external field Hex. An accurate linear relation of the sensor output voltage (E_out) versus Hex characteristics was obtained.
This paper reports the principles and basic properties of the sensor. Applications to a rotary encoder head for a ring magnet of 19mm dia. having 1, 000 poles, and a direction sensor utilizing the terrestrial magnetic field are also shown.