Magnetic Sensor Assembly for Stray Field Compensation
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Solution Overview
Problem
Magnetic stray fields interfere with the accuracy of magnetoresistive sensors detecting rotational angles, requiring robustness against disturbing fields up to 3 kA/m, which existing solutions often achieve at increased system costs or with large, expensive magnets.
Innovation Solution
An assembly comprising a permanent magnet on a rotary member generating a parallel magnetic field, with two magnetic sensing elements and an electromagnetic coil to determine the magnetic stray field influence by comparing field strengths and directions at different distances, using three voltage dividers for compact design and enhanced measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If shielding external magnetic fields or using field gradients is implemented, then robustness against disturbing fields is improved, but system costs increase significantly
Solution Approach 1:
The patent introduces an intermediary approach by using a second magnetic sensing element positioned at a different distance from the permanent magnet. This intermediary sensor measures the total magnetic field (permanent magnet field + stray field) at its location, while the first sensor measures the same at a different location. By comparing the two measurements, the stray field influence is extracted without requiring physical shielding or complex field gradient structures, thus reducing system costs while maintaining robustness.
Solution Approach 2:
The patent replaces mechanical shielding structures with a field-based measurement and computation approach. Instead of physically blocking external magnetic fields using shielding materials or complex mechanical field gradient structures, the system uses two magnetic sensing elements to detect field strengths at different distances and computationally determines the stray field influence. This substitution of mechanical systems with sensor-based detection and processing reduces device complexity and cost.
2Reliability
If large and strong disc or block magnets based on expensive materials are used, then impact of stray fields is minimized, but system size and cost increase
Solution Approach 1:
The patent uses a second magnetic sensing element as an intermediary to measure the magnetic field strength at a different distance from the permanent magnet. This intermediary measurement allows the system to characterize the stray field influence without requiring larger or more expensive magnets. The sensor at the second position provides the data needed to compute and compensate for stray field effects, enabling the use of smaller, less expensive permanent magnets while maintaining measurement accuracy.
Solution Approach 2:
The patent changes the measurement parameters by taking magnetic field measurements at multiple different distances from the permanent magnet using two sensing elements. This multi-point parameter measurement allows the system to mathematically determine the stray field influence and compensate for it, eliminating the need to increase magnet size or strength to resist stray fields. The solution shifts from changing physical magnet parameters to changing measurement parameters.
3Measurement precision
If two magnetic sensing elements at different distances are used, then magnetic stray field influence can be determined, but device complexity increases
Solution Approach 1:
The patent makes the magnetic sensing elements multi-functional by using them for both primary angular measurement and secondary stray field detection. The same two sensing elements that measure the angular position of the permanent magnet are also used to measure the magnetic field strength at different distances for determining stray field influence. This multi-functionality allows the system to achieve accurate stray field determination without adding separate dedicated sensors, thus minimizing the increase in device complexity.
Solution Approach 2:
The patent applies self-service by using the magnetic sensing elements to serve dual purposes: their primary function of measuring angular position and their secondary function of characterizing the magnetic field for stray field compensation. The sensing elements essentially measure themselves at different positions to provide the data needed for stray field determination, eliminating the need for separate measurement systems and reducing overall device complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The assembly effectively compensates for magnetic stray field interference, maintaining measurement accuracy while reducing system size and cost, and relaxing volume requirements for the magnet, thus meeting the ISO11452-8 standard for automotive applications.
Implementation Method 1
a permanent magnet for generating a magnetic field, the permanent magnet being arranged on the rotary member so that at the sensor position a parallel field, perpendicular to the rotation axis is created
Implementation Method 2
an induction channel comprising an electromagnetic coil wound around the axis of rotation, the induction channel being adapted to pass an electric current
Implementation Method 3
The sensors make use of the magnetoresistive effect, which is the tendency of a material (e.g. ferromagnetic) to change the value of its electrical resistance in an externally applied magnetic field
Data Source
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AI summary
The present invention relates to an assembly 100 for determining the influence of an external disturbing magnetic field HD in a magnetic senor 200 detecting a rotational angle ω of a rotary member 300. The assembly 100 comprises a permanent magnet 310 for generating a magnetic field H0, the permanent magnet 310 being arranged on the rotary member 300 so that the magnetic poles 312, 314 generate a magnetic field perpendicular to the axis at the magnetic sensor about which the rotary member 300 rotates. The assembly further comprises a first channel comprising a first magnetic sensing element 210 positioned on the axis of rotation, the first channel configured to provide first angular data, and a second channel comprising a second magnetic sensing element 220 positioned on the axis of rotation, the second channel configured to provide second angular data, wherein the second magnetic sensing element 220 is spaced in the direction of the axis of rotation by a predetermined value z from the first magnetic sensing element 210. The assembly further comprises an induction channel comprising an electromagnetic coil 260 wound around the axis of rotation, the induction channel being adapted to pass an electric current Icoil+, Icoil-. The assembly further comprises a processor configured to compute a first field strength H0 based on a plurality of first angular data received by the first channel and a value Icoil+, Icoil- of the electric current passed through the induction channel and compute a second field strength fH0 based on a plurality of second angular data received by the second channel and the value Icoil+, Icoil- of the electric current passed through the induction channel; and compute the magnetic stray field component orthogonal to the magnetic field H0 by comparing the first field strength with the second field strength. The assembly being further characterized that each of the first magnetic sensing element 210 and the second magnetic sensing element 220 comprises three voltage divider 252, 254, 256.