Position Sensor Non-Uniform Magnetisable Material External Field Immunity
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Solution Overview
Problem
Existing inductive sensors using magnetisable material are prone to saturation by external magnetic fields, leading to inaccuracy or failure in position readings, especially when the length of the magnetisable material is much larger than its width, and shielding is not always feasible due to space or cost constraints.
Innovation Solution
A non-uniform pattern of magnetisable material, such as Mu-metal, is used to increase the effective reluctance in specific directions, reducing external magnetic field flux, and a conductive layer is optionally added to enhance signal levels by allowing eddy currents, while maintaining the magnetic flux generated by the permanent magnet.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a continuous film of magnetisable material is used, then signal levels are maintained, but external magnetic fields cause saturation and inaccuracy
Solution Approach 1:
The continuous film of magnetisable material is divided into discrete segments or patterns, creating a non-uniform structure that reduces the effective area exposed to external magnetic fields while maintaining sufficient magnetic flux paths for accurate position sensing. This segmentation prevents saturation by distributing the magnetic flux more evenly across the sensor structure.
Solution Approach 2:
Different regions of the magnetisable material are designed with varying properties - some areas have higher permeability or thickness to guide magnetic flux, while other areas are reduced or patterned to limit exposure to external fields. This local variation in material properties optimizes both signal strength and immunity to external saturation.
2Power
If the length of magnetisable material is increased to improve signal, then signal levels increase, but susceptibility to external magnetic field saturation increases
Solution Approach 1:
The magnetisable material is segmented into discrete regions rather than forming a continuous long film. This segmentation maintains sufficient magnetic flux paths for strong signals while limiting the continuous exposure length to external magnetic fields, thereby reducing saturation susceptibility.
Solution Approach 2:
The magnetisable material is designed with asymmetric patterns or non-uniform thickness distributions that optimize magnetic flux concentration in specific directions. This asymmetry allows strong signal generation in the measurement direction while minimizing exposure to external fields from other directions.
3Reliability
If shielding is added to protect against external magnetic fields, then immunity improves, but device complexity and cost increase
Solution Approach 1:
The shielding function is merged with the magnetisable material structure itself. By designing the magnetisable material with specific patterns, non-uniform thickness, or segmented configurations, the structure simultaneously performs both magnetic flux guidance and external field rejection functions, eliminating the need for separate shielding components.
Solution Approach 2:
The permeability, thickness, or geometric parameters of the magnetisable material are optimized to achieve high immunity to external magnetic fields without adding separate shielding structures. By carefully controlling these parameters, the material itself provides the shielding effect needed for reliability.
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 approach significantly improves the immunity of the position sensor to external magnetic fields, allowing accurate position readings even in stronger external fields, with some configurations achieving twice the immunity of sensors with uniform magnetisable material, and maintaining signal levels comparable to continuous films.
Implementation Method 1
a permanent magnet generates a DC magnetic field that can pass through a metallic wall and saturate a localised region of the magnetisable film
Implementation Method 2
saturate a localised region of the magnetisable film
Implementation Method 3
a magnetic field generated by alternating current flowing through the transmit aerial induces an electromotive force in the receive aerial
Implementation Method 4
a conductive layer is optionally added to enhance signal levels by allowing eddy currents
Data Source
Figure 1
Figure 2~3
Figure 4A~4D
AI summary
There is described a position sensor for outputting a signal indicative of the relative position between a first member and a second member, the position sensor having at least one conductive winding fixed relative to the first member and a magnetic field generator fixed relative to the second member. An extent of magnetisable material is positioned relative to the at least one conductive winding and the magnetic field generator such that when a magnetic field generated by the magnetic field generator saturates a localised region of the extent of magnetisable material, a component is introduced to a signal in the at least one conductive winding that varies in accordance with the position of the localised saturated region. The extent of magnetisable material has a property that is non-uniform in such a manner that the effective reluctance of the extent of magnetisable material is increased in at least one direction. In this way, less flux from external magnetic fields is concentrated in the film of magnetisable material.