Design and characterization of a linear position transducer based on Giant Magneto Resistance (GMR) effect

In this report we describe a new linear position transducer based on the giant magneto resistance (GMR) effect. The GMR effect is a phenomenon observed in multilayer structures consisting of ferromagnetic, antiferromagnetic and paramagnetic metal films. The transducer consists of four GMR elements configured in a Wheatstone bridge with 380 Ohms nominal resistance of each leg. GMR elements were subjected to a high-gradient magnetic field created by specially designed Ne-Fe-B permanent magnets. A movable magnetic assembly was attached to a probe, the linear position of which was varied. This design represents an absolute, analog, linear position sensor. The dimensions of each sensitive element were 35 x 3 x 2 mm and the whole transducer was packaged in a 12 x 2 x 2 cm housing. The sensitivity of the sensor was approximately 1 Ohm/mm. The best achieved deviation from linearity over a range of 23 mm was + 10 m (0.05% of full scale), with hysteresis of 5 m. Noise density of the transducer was 60 nVrms/Hz, which allows 0.2 m resolution with a 4 Hz update rate. The transducer possesses some temperature dependence. Each element of the bridge has temperature dependence around 2200 ppm/C which scales to 34 m/C at the ends of the range of the full bridge. Non-uniformity of the thickness of layers in the GMR structures, defects and inhomogeneity in metal films are discussed as possible causes of non-linearity, hysteresis, relaxation and noise observed in some samples.