Linear Motor Position Sensing Using Magnetic Leakage Fields
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Solution Overview
Problem
Existing linear permanent magnet motors with position sensors face issues such as the need for a reference portion with different magnetic properties, sensor saturation due to high magnetic fields, and heat generation from coils, which affect accuracy and reliability.
Innovation Solution
Positioning the sensing element within the magnetic leakage field of the permanent magnets, rather than the main field, to avoid saturation and heat, allowing for accurate position sensing without expensive encoders and enabling closer coil arrangement for higher force density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Hall sensors are positioned within the main field of permanent magnets to detect position, then position detection capability is achieved, but sensor saturation occurs leading to measurement errors
Solution Approach 1:
A non-magnetic spacer element is introduced as an intermediary between the permanent magnets and the Hall sensors. This spacer mediates the magnetic field interaction by preventing direct exposure of sensors to the strong main field while still allowing detection of position through the leakage field that passes through or around the spacer structure.
Solution Approach 2:
The sensing region is differentiated from the main field region by positioning Hall sensors to detect only the leakage field portion. The spacer creates a local field distribution where the main field is confined to the motor's active region while the leakage field extends to the sensor location, enabling position detection without saturation.
2Measurement precision
If Hall sensors are mounted adjacent to coils for position sensing, then position detection is enabled, but coil heat generation increases sensor temperature and reduces sensitivity
Solution Approach 1:
The sensor positioning is moved from the lateral dimension (adjacent to coils) to a different spatial arrangement where sensors detect the leakage field extending from the magnet-coil interaction zone. This dimensional relocation places sensors in a cooler region while maintaining position detection capability through the extended leakage field.
3Measurement precision
If a reference portion with different magnetic properties is added to the mover for position detection, then absolute position measurement is achieved, but motor performance deteriorates
Solution Approach 1:
The permanent magnets themselves serve the dual function of both motor operation and position indication. The leakage field naturally generated by the permanent magnets during motor operation contains sufficient position information, eliminating the need for separate reference portions and allowing the motor components to serve multiple functions.
4Measurement precision
If multiple Hall sensors are used to achieve higher position resolution, then measurement precision improves, but device complexity and cost increase
Solution Approach 1:
The magnetic field detection parameter is changed from sensing the strong main field to sensing the leakage field. This parameter change allows a single Hall sensor to achieve high-resolution position measurement by detecting the spatial variation of the leakage field, eliminating the need for multiple sensors and associated 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
This approach provides reliable, cost-effective, and accurate positioning over long strokes with reduced risk of sensor saturation and heat, eliminating the need for linear encoders and enhancing motor performance.
Implementation Method 1
The position sensing device comprises a sensing element operable to sense a magnetic field of the mover (i.e. of the array of permanent magnets)
Implementation Method 2
Electromagnetic fields of the permanent magnets of the array and electromagnetic fields generated by the at least one coil interact in the air gap to create a traction force on the mover relative to the stator unit
Data Source
AI summary
A linear permanent magnet motor may include a stator unit having a coil, a mover including an array of permanent magnets, and a position sensing device operable to determine a position of the mover. The mover is arranged to move along a motion direction. The position sensing device has a sensing element operable to sense a magnetic field of the array of permanent magnets The sensing element is fixed to the stator unit. The array of permanent magnets is spaced apart from the stator unit by an air gap in which electromagnetic fields created by the array of permanent magnets and by coil are configured to interact thereby generating traction. The sensing element is positioned such that it is within a magnetic leakage field of the array of permanent magnets when the array of permanent magnets is positioned in correspondence with the sensing element.