Magnetic Sensor Bridge Circuit Stress Cancellation

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Solution Overview

Problem

Magnetic sensors using magnetoresistive effect elements are sensitive to stress, which affects their output when no external magnetic field is applied, making it difficult to ensure accuracy due to unpredictable and uncontrolled stress.

Innovation Solution

The magnetic sensor design incorporates a bridge circuit with MR elements arranged in groups where the magnetization direction of free layers rotates differently under stress, canceling out the variations in electric resistance and reducing output sensitivity to stress, using a combination of end and side bias magnets to stabilize the magnetization direction.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stability of the object's composition

If bias magnets are provided on both sides of the free layer to stabilize magnetization direction, then magnetization stability is improved, but stress sensitivity increases due to inverse magnetostrictive effect

Engineering Contradiction:

Improvemagnetization direction stabilityVSAvoidstress sensitivity

Core Design Contradiction:

Stability of the object's compositionVSObject-affected harmful factors

Solution Approach 1:

The bridge circuit is divided into four arms with MR elements having different initial magnetization directions (first and second directions). This segmentation allows different stress responses in different arms, enabling stress effect cancellation through differential measurement while maintaining stable operation points through bias magnets.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Each MR element in the bridge circuit is designed with specific local properties - different initial magnetization directions (parallel or perpendicular to reference layer) and different bias magnet configurations. This local differentiation enables the system to respond differently to stress in different arms, allowing stress compensation while maintaining overall stability.

Inventive Principle:

Principle #3Local quality

2Measurement precision

If MR elements are arranged in a bridge circuit to detect magnetic fields, then measurement sensitivity is improved, but output sensitivity to stress increases

Engineering Contradiction:

Improvemagnetic field detection sensitivityVSAvoidoutput sensitivity to stress

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The bridge circuit uses asymmetric configuration of MR elements with different initial magnetization directions relative to the reference layer. Some elements have magnetization parallel to reference layer while others are perpendicular, creating asymmetric stress responses that cancel out in differential measurement, reducing output sensitivity to stress while maintaining magnetic field detection capability.

Inventive Principle:

Principle #4Asymmetry

Solution Approach 2:

The bridge circuit configuration creates counterbalancing effects where stress-induced resistance changes in some arms are compensated by opposite changes in other arms. The differential measurement arrangement ensures that stress effects act as counterweights that cancel each other, while magnetic field effects add constructively for enhanced detection sensitivity.

Inventive Principle:

Principle #8Anti-weight (Counterweight)

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

This design minimizes the offset of output voltages under stress, maintaining accuracy and reducing sensitivity to external stress, thereby enhancing the reliability of magnetic field measurements.

Implementation Method 1

A magnetic sensor having a magnetoresistive effect element detects an external magnetic field based on the change of electric resistance that is caused by a magnetoresistive effect

Methodology Applied

Scientific EffectMagnetoresistive effect: Magnetoresistance

Implementation Method 2

a magnetically free layer (hereinafter, referred to as a free layer) whose magnetization direction changes depending on an external magnetic field

Methodology Applied

Scientific EffectMagnetization: Magnetism

Implementation Method 3

permanent magnetic layers that apply a bias magnetic field (hereinafter referred to as bias magnets) are provided on both sides of the free layer

Methodology Applied

Scientific EffectBias magnetic field: Magnetic Field

Implementation Method 4

when it is subjected to stress, the magnetization direction varies due to an inverse magnetostrictive effect

Methodology Applied

Scientific EffectInverse magnetostrictive effect: Magnetostriction

Data Source

PatentUS11156478B2Magnetic sensor

Publication Date: 2021.10.26 TDK CORP

AI summary

Magnetic sensor 1 has MR elements 11A to 14A that are connected to each other. MR elements 11A to 14A belongs either to group G1 in which electric resistance increases when the magnetization direction of each free layer 26 is rotated a predetermined angle in a same direction, or to group G2 in which the electric resistance decreases when the magnetization direction of each free layer 26 is rotated the predetermined angle in the same direction. A variation of an output of magnetic sensor 1 due to an increase of the electric resistance of the electric resistance elements of one group and a variation of the output of magnetic sensor 1 due to a decrease of the electric resistance of the electric resistance elements of another group are cancelled out.