Crankset Force Sensor on Bearing Outer Ring for Torque Accuracy
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Solution Overview
Problem
Existing force measurement systems for rotating shafts, such as bicycle crankset spindles, face challenges including complexity, cost, reliability issues, and inaccuracies due to friction, interference, and sensitivity to bending moments, especially when measuring torque and chain tension, which affect motor control in electric-assisted bicycles.
Innovation Solution
A crankset assembly with a force sensor positioned around the outer ring of the bearing, featuring a top frame inclined at an angle of 10° to 30° relative to the vertical axis, utilizing a single measurement channel and a parallelogram structure with strain gauges to minimize signal differences between pedaling phases and reduce sensitivity to moments, ensuring accurate torque measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If extensiometry gauges are glued onto the rotating shaft, then force measurement is achieved, but the system requires complex transmission devices (revolving slip ring or wireless transmission) that increase complexity, cost, and reduce reliability
Solution Approach 1:
Instead of measuring force directly on the rotating shaft and transmitting the signal outward, the invention inverts the approach by measuring the reaction force on the stationary outer bearing ring. This eliminates the need for revolving slip rings or wireless transmission systems, as the measurement point is now stationary and directly connected to the external processing unit through fixed wiring.
2Measurement precision
If gauges are mounted on the outer wall of the small arm, then force measurement is possible, but the gauges become highly sensitive to bending and twisting moments, reducing measurement accuracy
Solution Approach 1:
The invention extracts the measurement from the problematic small arm structure where bending and twisting moments affect accuracy. By relocating the measurement to the outer bearing ring, the system separates the force measurement function from the components subjected to complex stress states, measuring only the desired force component without interference from bending and twisting moments.
3Measurement precision
If multiple pairs of gauges are positioned on opposing faces of the arm, then sensitivity to moments is reduced, but the gluing process becomes complex and cost increases significantly
Solution Approach 1:
Instead of complicating the gauge installation on the small arm by adding multiple pairs of gauges on opposing faces, the invention inverts the measurement location to the outer bearing ring. This single-location measurement achieves moment insensitivity without requiring complex multi-face gauge installation, significantly simplifying manufacturing while maintaining measurement precision.
4Measurement precision
If the deformation gradient zone is used for gauge gluing, then measurement is achieved, but inaccuracies in gluing have great influence on sensitivity, reducing measurement reliability
Solution Approach 1:
The invention extracts the measurement from the high deformation gradient zone on the small arm where gluing inaccuracies significantly affect sensitivity. By measuring on the outer bearing ring, the system relocates the gauge installation to a zone with more favorable deformation characteristics, reducing the impact of gluing inaccuracies and improving measurement reliability.
5Ease of manufacture
If wide apertures are provided in the bottom bracket shell, then sensor installation is enabled, but sealing and protection of gauges against damp and chemical attacks becomes complex
Solution Approach 1:
The invention extracts the measurement function from the internal small arm structure that requires wide apertures and complex sealing. By measuring on the outer bearing ring, the system enables sensor installation with simpler aperture requirements and reduces the complexity of sealing and protecting gauges against environmental attacks.
6Object-affected harmful factors
If protective lacquer or gels are applied to gauges, then protection against environmental attacks is achieved, but polymerization time increases and the products deteriorate over time
Solution Approach 1:
The invention extracts the gauges from the harsh internal environment that requires protective lacquer or gel coatings. By installing sensors on the outer bearing ring with improved sealing, the system reduces the need for time-consuming protective coatings and their associated polymerization times, while also improving long-term reliability by removing gauges from deteriorating environments.
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 solution provides a reliable, efficient, and cost-effective method to measure torque transmitted to the bicycle's drive wheel, optimizing motor control by equalizing contributions from both legs and minimizing signal variations, thus enhancing the accuracy and stability of electric-assist systems.
Implementation Method 1
the deformation gauges which measure μ-deformations reflected by μ-resistance variations
Data Source
AI summary
A crankset assembly to be mounted on a bicycle includes at least one right-hand bearing for mounting a crankset shaft and a force sensor positioned around the outer ring of the bearing. The force sensor includes a peripheral portion, a central portion, and a top frame connecting a top section of the peripheral portion to a top portion of the central portion. The frame is positioned at a diameter oriented along an axis (Y′). The axis (Y′) makes an angle (φ) of between 10° and 30° with the vertical axis (Y) and the top frame is positioned behind the vertical axis (Y).