Magnetic position sensors are widely used, especially in automotive applications. This is due to their low cost, absence of friction and insensitivity to dirt. Modern magnetic position sensors use measurement of magnetic field direction. These sensors have a diametrically magnetized magnet which is attached to the extremity of a rotating shaft, and an ASIC measures the magnetic field angle in the plane below the shaft. The rotating part of the sensor is an assembly of two parts: the rotating shaft itself and the magnet. These materials, in general case, have different thermal expansion coefficients. In order to improve the mechanical robustness of the sensor and to simplify the production process, we propose to use directly the shaft (made of steel) already in the application (to guide the rotating part) as the source of magnetic field or a low coercivity magnetized steel press fitted at the end of the shaft. A particular geometry of the magnetized part is proposed in order to obtain sufficient induction in the sensor. In this paper, the properties and the choice of the Iow coercivity materials are briefly exposed, the practical realization of prototypes and measurement results are given. Measurements of induction after temperature cycles and presence of demagnetizing field are presented. A simple and robust sensor for up to 360 deg strokes is presented. The low coercivity materials such as magnetic steels represent a trade-off between magnetic and mechanical properties. With an adaptation of sensor geometry, the magnetic field in the range of 150 G - 250 G can be obtained, which is sufficient for the probes which are normally used in magnetic sensors (examples: MLX90316, HMC1512. The field amplitude that can be obtained is limited by sensor size. It was shown, however, that this type of sensor can fit in the sensor housings for the automotive applications, still producing enough magnetic fields for the probe. Stability of magnetization in presence of external field is examined and tests over wide temperature range are done. Very good stability in temperature is shown. Moderate external demagnetizing field (40 G, for example) can decrease the sensor field once, but the magnetization then remains stable if the same demagnetizing field is applied. Strong external fields (permanent magnets) can demagnetize steel part or remagnetize it in a wrong direction. Long term stability of the magnetization in such sensors should be carefully examined.
Magnetic position sensor with low coercivity material
Magnetischer Positionssensor mit Werkstoffen geringer Koerzitivfeldstärke
Sensor, International Conference, 13 ; 237-242
2007
6 Seiten, 13 Bilder, 6 Quellen
Aufsatz (Konferenz)
Englisch
Kraftfahrwesen | 1990
|SAE Technical Papers | 1990
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