Signal conditioning for GMR magnetic sensors: Applied to traffic speed monitoring GMR sensors

Abstract

The Earth's magnetic field provides a uniform magnetic field over a wide area of several kilometers, among 30 and 60 μT depending on the location. Thus, cars, which have a high ferrous content mainly under car parts, provoke local disturbances around 1 μT over the constant Earth's magnetic field whether it is moving or still standing. Giant Magneto-Resistance (GMR) gradient sensors are able to convert this Earth's magnetic field variations into a voltage signature, due to the “Giant Magnetoresistive Effect”. Nevertheless, the output of the GMR sensors is too weak to operate with, and it must be amplified and filtered enough to be digitized and then processed. In this paper, we show the design and implementation of a practical conditioner circuit for GMR sensors and a simple speed calculation algorithm used in our embedded speed monitoring system for vehicles. The signal conditioner has been adjusted for car applications, where a high gain in a certain bandwidth is required due to the relatively large distance between the sensors on the pavement and the under car parts. Our conditioning circuit amplifies the sensor's output voltage by 36.360 in a bandwidth among 0.19 and 48.23 Hz. We present, compare and analyze two real car signals captured with our embedded system. With the small and cheap magnetic sensors and only a few additional components, our embedded speed monitoring system is suitable in automotive applications, where it would drastically reduce the cost of the expensive installation procedures of current car speed monitoring systems or even car detecting systems.