An Ultra Low-Power MR*1) Sensor for a Smart Water Meter or a Smart Gas Meter

Abstract

A new ultra low power MR sensor comprising MR elements integrated with Bi-CMOS IC chip, which is optimal for the use in smart water meters and gas meters playing important role in global smart gridization, has been developed. MR elements are configured as a Wheatstone bridge circuit. MR element patterns are placed in the perpendicular configurations as resistive components of the circuit. Those detect the magnetic field applied to X and Y axis respectively, and then outputs either high or low-level electric signal. Optimization of the MR element pattern aspect ratio (ratio of the element length to the element width) and configuration of the two-row parallel MR patterns in a longitudinal direction enable the equivalent current flow in the direction. This provides mutual magnetostatic coupling and cancellation of each demagnetizing field, and led to achieve minimizing the hysteresis of the resistance change rate affected by up and down of the applied magnetic field. Furthermore, this led to the improvement of mutual symmetry for both S to N and N to S direction of the magnetic field. This resulted in enhanced magnetic sensitivity and successfully obtained magnetic sensitivity higher than 0.8kA/m. For the power supply of the MR sensor, external intermittent control was adopted. Furthermore, active (on) time of the MR element was considerably shortened by utilizing the internal circuit of the IC. As a result, the sampling time was shortened to 1μs and the drive current of MR sensor was reduced to 0.3 μA. *1)MR: Magneto-resistance