Magnetic Encoder Pole Layout for Inter-Track Harmonic Cancellation
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Solution Overview
Problem
In small format magnetic encoders, the close proximity and small size of tracks lead to significant magnetic field interference between adjacent tracks, resulting in harmonic noise and distortion in sensor outputs due to manufacturing tolerances and differing pole numbers and designs.
Innovation Solution
The use of alternating pole designs and shapes in one track to partially cancel out the periodic variations in the magnetic field from the other track, achieved by varying the spacing, shape, area, and orientation of poles, and incorporating different magnetic materials or features such as holes and slots to reduce interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the encoder is made small to meet packaging constraints, then the encoder size is reduced, but magnetic field interference between tracks increases
Solution Approach 1:
The patent applies local quality by varying the pole arc width at different angular positions around the encoder disk. Poles closer to the inner track have smaller arc widths than poles closer to the outer track, creating a gradient that compensates for the stronger magnetic field coupling at smaller radii. This localized adjustment of pole geometry reduces harmonic distortion caused by inter-track interference while maintaining compact encoder dimensions.
Solution Approach 2:
The patent changes the geometric parameters of the magnetic poles, specifically the pole arc width, as a function of angular position. By varying this parameter around the circumference of the encoder disk, the magnetic field distribution is optimized to reduce interference between inner and outer tracks, enabling small format encoders to achieve acceptable signal quality despite close track spacing.
2Volume of moving object
If tracks are placed close together to reduce encoder size, then the encoder diameter is reduced, but harmonic noise in sensor outputs increases
Solution Approach 1:
The patent implements local quality by making the pole arc width a function of angular position, with narrower arcs positioned to reduce magnetic field overlap between closely spaced tracks. This localized geometric variation compensates for the increased field coupling that occurs when tracks are placed close together, thereby reducing harmonic noise while maintaining small encoder diameter.
Solution Approach 2:
The patent introduces asymmetry in the pole geometry by using different pole arc widths at different angular positions rather than uniform poles throughout. This asymmetric design creates a non-uniform magnetic field distribution that actively counteracts the symmetric interference pattern that would otherwise occur between closely spaced tracks, reducing harmonic distortion in the sensor outputs.
3Adaptability or versatility
If different numbers of poles are used in inner and outer tracks to provide high and low resolution measurements, then measurement versatility is improved, but magnetic field interference between tracks increases
Solution Approach 1:
The patent applies local quality by varying the pole arc width as a function of angular position, with the variation pattern specifically designed to reduce magnetic field interference between tracks having different pole counts. This localized geometric adjustment allows the encoder to maintain versatile measurement capabilities across multiple resolutions while minimizing the harmful magnetic coupling between the inner and outer tracks.
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 reduces unwanted harmonic noise, particularly 4th order harmonics, in the sensor outputs, improving the accuracy and reliability of position measurement in small diameter encoders.
Implementation Method 1
The encoder typically comprises three main components. The first is an encoder element that has a plurality of encoding regions arranged along at least one elongate track... The magnetic poles can be formed by locally magnetising regions of a material which can be permanently magnetized
Implementation Method 2
A set of sensors are located adjacent the encoder that detects the passing of the encoder regions as the encoder translates or rotates around the axis... The sensor detects the changing magnetic field and this produces a corresponding change in the output of the sensor
Data Source
AI summary
A magnetic encoder includes one or more sensors and an encoder element having at least two tracks of encoder regions. Each region comprises a magnetic pole. Each sensor has one or more sensing elements associated with a respective track and generates an output that is indicative of the magnetic field associated with that track. At least one track has a differing number of poles to at least one of the other tracks. The properties of the poles of a first one of the tracks differ along the track such that there is a periodic variation along the first track of the magnetic field emitted by the first track that is detected by the sensing elements associated with the first track which at least partially cancels out a corresponding periodic variation in field from the other tracks that is also detected by the sensing elements associated with the first track.