Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2000-03-01
2003-02-11
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207250, C338S03200R
Reexamination Certificate
active
06518749
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
It relates more specifically to the field of sensors whose operating principle is based on detection of the variation of the magnetic flux produced by a permanent magnet in a ferromagnetic structure containing an air gap in which there is housed a Hall probe or an equivalent magnetic-field-sensing detection means.
2. Discussion of the Background
The general principle of such sensors is known. They are described in, for example, European Patents EP514530, EP665416 or EO596068.
There is also known a rotary sensor described in European Patent EP611951, made from a first stator part integral with a magnet which has semi-discoid shape and which is movable relative to two fixed stator parts defining a secondary air gap, inside which there is placed a Hall probe.
Another patent, German Patent DE29520111, describes a sensor containing an annular outer stator part defining an annular air gap with an inner stator part. The annular permanent magnet is placed in the main air gap. One of the annular stator parts has a secondary air gap, inside which there is placed a Hall probe.
In both cases, the magnet is plunged permanently and completely into the main air gap, and the signal detected by the Hall probe is a combination of two magnetic fields, making it impossible to guarantee perfect linearity of the output signal.
SUMMARY OF THE INVENTION
The objective of the invention is to simplify the construction of such sensors and to improve the response linearity, or in other words the amplitude of the electrical signal delivered as a function of the position of the mobile portion. The invention differs from sensors of the prior art mainly by the fact that the magnet progressively penetrates into the air gap, and in that it is completely plunged into the air gap only for an extreme position. In the other positions, the magnet is plunged only partly into the air gap, and the signal delivered by the Hall probe is a function of the degree of penetration of the permanent magnet into the air gap. This embodiment also makes it possible to modify the response function by choosing a particular geometry of the magnet. It is possible to generate a response signal corresponding to any desired function of the degree of penetration, by choosing a shape of the magnet or a shape of the stator parts defining the main air gap such that the variation of width, for example, corresponds to the sought function.
The invention is applicable just as well to linear sensors as to rotary sensors.
The invention relates to a magnetic position sensor for delivery of an electrical signal proportional to position, comprising a mobile part having at least one thin permanent magnet transversely magnetized in the direction of least thickness, which magnet is movable in a main air gap. The main air gap is defined by at least two ferromagnetic portions. These two ferromagnetic portions define between them a secondary air gap containing a. magnetic-field sensor. The mobile portion is designed such that the penetration of the magnetized portion into the main air gap is partial over a portion of the useful travel.
Advantageously, the length of the magnetized portion is at least equal to C+E, where C denotes the useful travel of the mobile portion and E denotes the thickness of the main air gap.
Preferably, the length of the air gap is at least equal to C+E, where C denotes the useful travel of the mobile portion and E denotes the thickness of the main air gap.
The main air gap comprises the space between the two stator portions, inside which space the magnet can be displaced during its useful travel, or in other words the travel over which it is desired that a position signal be obtained.
The secondary air gap is the zone in which the flux of the main air gap is collected. It is a space inside which the permanent magnet does not penetrate during its useful travel.
According to a preferred embodiment, the length of the magnetized portion is greater than the length of the air gap.
According to a preferred variant, the magnetic position sensor is characterized in that the mobile portion comprises two magnets magnetized in opposite directions parallel to the least thickness, the two magnets being disposed side-by-side in the direction perpendicular to the displacement.
According to a first embodiment, the mobile portion comprises two magnetized magnets of semicylindrical shape and is movable in a main air gap defined between a cylindrical yoke and semicylindrical stator parts, these semicylindrical parts defining between them a secondary air gap perpendicular to the axis of rotation.
According to a second embodiment, the mobile portion comprises at least one thin permanent magnet of rectangular shape and is movable in an air gap perpendicular to the magnetization direction, defined by a substantially U-shaped stator structure, the stator structure having a secondary air gap containing an axis parallel to the direction of displacement and perpendicular to the magnetic field lines generated by the permanent magnet.
According to a third embodiment, the mobile portion comprises two magnets in the form of discoid sectors magnetized in opposite directions, the fixed portion comprising two ferromagnetic parts in the form of discoid sectors.
According to another variant, the magnet has a geometric deformation specifically designed to permit compensation for a linearity defect due to a magnetic leak or to a perturbing field. As an example, this deformation will be a ridge inclined relative to the displacement direction in the case of a linear sensor, or a variable length of the magnet in the case of a rotary sensor.
REFERENCES:
patent: 5861745 (1999-01-01), Herden
patent: 6232771 (2001-05-01), Herden et al.
Gandel Pierre
Oudet Claude
Prudhamm Daniel
MMT (S. A.)
Snow Walter E.
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