Magnetic position and speed sensor with hall probe in an air gap

Details Magnetic position and speed sensor with hall probe in an air gap Magnetic position and speed sensor with hall probe in an air gap

1992-10-01

1996-06-18

Snow, Walter E.

Electricity: measuring and testing

Magnetic

Displacement

32420725, G01B 714, G01B 730, G01R 3306, H01L 4306

Patent

active

055281397

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a magnetic position sensor, and, according to a variant, a magnetic position and speed sensor.
2. Discussion of the Background
Position sensors using Hall probes detecting the magnetic flux generated by a magnet by relative movement relative to the Hall probe are known in the prior art. In particular, French patent 2624966 describes a coder for a print wheel comprising a Hall-effect linear detector and a permanent magnet mounted in a nonferrous metal shaft exhibiting a ring forming a conductive flux spiral molded in the wheel and encircling the shaft. The angular position of the print wheel is determined in absolute value by the amplitude of the signal relative to the transition point of the spiral. In another embodiment, a second Hall-effect detector is placed on the shaft, being opposite to the first detector to provide a reinforced signal.
The sensors thus produced do not exhibit a signal which is actually linear, and, in the prior art, this drawback has been remedied by digitizing the signal delivered by the Hall probe and by processing the signal by data-processing means.
It has also been proposed in the prior art to remedy the defect of linearity of magnetic position sensors by complex geometries. During the European colloquium on the "modern magnets and new machines with magnets" which was held in Grenoble on Jun. 13 to 15, 1990, a position sensor delivering an output signal approximately proportional to the angular position was exhibited. This sensor comprised a magnetized ring of elliptical shape according to a first embodiment, or an original qualified geometry consisting of two arcs of a circle. The linearity error is thus cancelled by the eccentricity. If, on the theoretical plane, this solution is advantageous, it is technically difficult to achieve for sensors produced industrially in large scale. The production cost is thereby excessive for many applications.


SUMMARY OF THE INVENTION

The object of this invention is to remedy these drawbacks by proposing an angular or linear position sensor of low production cost, exhibiting a linearity error less than one percent and whose extent of measurement is slightly less than the length of a magnetic pole.
The position sensor according to this invention comprises a stator defining an air gap inside of which a mobile magnet integral with a coupling means moves. The sensor further comprises two secondary air gaps approximately perpendicular to the main air gap, a Hall probe measuring the variation of the induction in at least one of the secondary air gaps. The stator consists of a first stationary part and a second part which can be either stationary or mobile. The two parts define between them said air gap in which the magnetized part of the mobile element moves. The mobile element exhibits at least two thin parts magnetized crosswise in alternate direction made of a material exhibiting in the entire working area a practically linear demagnetization characteristic and a reversible permeability close to that of the air. The stationary stator part exhibits two secondary air gaps perpendicular to the air gap in which the mobile element moves, a Hall probe being housed in said secondary air gap. The L/E ratio is advantageously greater than 6, and preferably greater than 10, where L designates the linear width of the magnetic pole in the case of a linear sensor, or the length of the arc corresponding to the average radius of the pole, in the case of a rotary sensor and where E designates the width of the air gap. According to a particular embodiment, the mobile element comprises only a single thin part magnetized crosswise, exhibiting the shape of a tile extending on 120.degree. for a sensor with a rotary cylindrical rotor or mobile in translation, or the shape of a half-disk for a flat rotary sensor. The output signal is, however, of weaker amplitude and poorer quality than in the embodiment using at least two magnetized parts.
The Hall probe produces an electric signal pro

REFERENCES:
patent: 3194990 (1965-07-01), Kendall
patent: 5072181 (1991-12-01), Burger

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