Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1999-04-01
2001-09-11
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207120, C324S207250
Reexamination Certificate
active
06288533
ABSTRACT:
FIELD OF THE INVENTION
The invention is in the field of contactless angle measurement. It concerns a method and an apparatus for detecting the rotation position of a rotor with the aid of a magnetic source connected to the rotor and of stationary sensor means for measuring the magnetic field of the magnetic source. The method and arrangement according to the invention are applicable e.g. for rotary switches or for rotary position detection.
BACKGROUND OF THE INVENTION
The measurement of rotation angle is required in various applications, such as manual electrical switches or position detection of a motor. Depending on cost and accuracy constraints, this task can be accomplished by various methods, such as mechanical contacts, optical encoders, or magnetic encoders. Modern integrated circuit technology offers the possibility to integrate magnetic sensors and their readout and angle calculation electronics on one die. This allows embodiments of detectors of mechanical rotation which consist of a permanent magnet attached to the rotor and monolithically integrated sensor means attached to a stator, at competitive cost, yet good performance. The absence of mechanical contact between the rotor with the magnet and the stator with the sensor means allows for hermetic encapsulation of the sensor means. This permits wear free angle measurements under harsh environmental conditions.
The robustness of the angle measurement against mechanical tolerances, against device variation, and against external electromagnetic fields, while keeping fabrication cost low, is a major performance criterion.
SUMMARY OF THE INVENTION
An object of the invention is to show a method and to create an arrangement for detecting the rotation position of a rotor using a magnetic source connected to the rotor and stationary magnetic field sensor means for measuring the magnetic field of the magnetic source, which method and arrangement allow, compared to known such methods and arrangements, increased robustness against sensitivity and offset variations of the sensor means, against external magnetic fields and furthermore against mechanical tolerances regarding the relative positions of the sensor means and the magnetic source.
The inventive arrangement for contactless angle measurement comprises a magnetic source mounted to the rotating part (rotor) rotatable around a rotation axis and an array of magnetic sensors mounted on the non-rotating part (stator). The magnetic source is arranged such that the magnetic field has no rotational symmetry relative to the rotation axis. The sensor means consists of at least three sensors arranged in at least two sensor pairs, whereby each sensor may be replaced by a cluster of sensors (plurality of sensors arranged very close to each other). The sensors, e.g. Hall sensors, are arranged in such a way that at least the two sensors of each sensor pair are sensitive to parallel components of the magnetic field. Furthermore, the sensors are such arranged that connecting lines each connecting two sensors of one sensor pair have projections in a plane perpendicular to the rotation axis which are angled relative to each other. Advantageously, the sensors of each pair are positioned in one plane perpendicular to the rotation axis, e.g. all pairs in the same plane.
According to the inventive method, the mechanical angle (rotation position of the rotor) is determined by calculating at least one ratio of two differential signals of one sensor pair each (the differential signal of a sensor pair being the difference between the signals of the two sensors of the pair) and by comparing the calculated ratio with a predetermined (calculated or experimentally determined) function of said ratio versus the rotation angle. If instead of single sensors clusters of sensors are used it is the mean value of the sensor signals of the cluster which is used for calculating the differential signals.
This method yields an angle determination which is insensitive to variations common to the two sensors of each pair (e.g. offset) and to sensitivity variations common to all sensors contributing to one ratio, as well as to external magnetic fields.
For reducing or even suppressing the influence of mechanical misalignment of the rotating magnetic source and the sensor array with respect to each other and to the rotation axis, the magnetic source and the sensor array are such designed and such arranged that the magnetic field component to be measured at any possible sensor location is described by the product of a first order polynomial within a planar surface substantially perpendicular to the rotation axis (magnetic field changing linearly within such a planar surface, i.e. having no curvature), and a function perpendicular to said planar surface, which function is essentially the same for all sensor locations and is advantageously well approximated by a linear function.
In addition to providing information about the angle of mechanical rotation, the field of the magnetic source can be used to hold the rotor in place by adding a ferromagnetic yoke to the stator. Said yoke, shaping the magnetic field of the rotor, further enhances the insensitivity of the device against mechanical misalignment.
REFERENCES:
patent: 4283679 (1981-08-01), Ito et al.
patent: 4490674 (1984-12-01), Ito
patent: 5055781 (1991-10-01), Sakakibara et al.
patent: 5148106 (1992-09-01), Ozawa
patent: 5861747 (1999-01-01), Kubinski
patent: 38 26 408 (1989-02-01), None
patent: 2 143 328 (1985-02-01), None
Haeberli Andreas
Maier Christoph
Metz Matthias
Oppedahl & Larson LLP
Physical Electronics Laboratory
Strecker Gerard R.
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