Crank Power Meter Auto-Zero Using Y-Axis Gravity Compensation
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Solution Overview
Problem
Conventional power meters using strain-gauges on bicycle cranks face inaccuracies due to the mass of the pedal and crank applying torque that varies with rotation, introducing a force offset that existing angle-based correction methods fail to address efficiently.
Innovation Solution
A power meter that auto-zeroes strain-gauge outputs using a Y-axis accelerometer to determine a correction factor without calculating the crank angle, by subtracting a correction factor derived from the Y-axis accelerometer readings to correct for the effect of gravity on the crank.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If angle-based correction methods are used to compensate for gravity effects, then power measurement accuracy is improved, but device complexity and calculation complexity increase due to requiring angle determination
Solution Approach 1:
The patent extracts only the Y-axis acceleration component needed for gravity compensation, eliminating the need to calculate crank angle. By taking out only the essential correction data from the accelerometer, the system achieves accuracy improvement without the complexity of full angle determination
Solution Approach 2:
The patent replaces the mechanical/geometric angle calculation system with a direct accelerometer-based correction system. Instead of using complex angle determination mechanisms, it substitutes a simpler acceleration-based correction approach that directly compensates for gravity effects
2Measurement precision
If angle-based correction methods are used to compensate for gravity effects, then power measurement accuracy is improved, but calculation complexity increases due to angular error calculations
Solution Approach 1:
The patent extracts only the Y-axis acceleration component needed for gravity compensation, eliminating the need to calculate crank angle. By taking out only the essential correction data from the accelerometer, the system achieves accuracy improvement without the complexity of full angle determination
Solution Approach 2:
Instead of calculating angle from acceleration data and then using that angle for correction, the patent inverts the approach by directly using acceleration data to calculate the correction factor without intermediate angle computation, thereby eliminating angular error propagation
3Power
If strain-gauges are used to measure crank torque, then power measurement capability is provided, but measurement accuracy deteriorates due to gravity-induced force offsets
Solution Approach 1:
The patent uses accelerometer data as feedback to continuously determine gravity-induced force offsets and apply real-time correction to the strain-gauge measurements. This feedback loop compensates for the harmful gravitational effects on the torque measurements
Solution Approach 2:
The patent introduces accelerometer-based correction factors as an intermediary element between the strain-gauges and the final power calculation. This intermediary compensates for gravity effects by providing correction data that adjusts the raw strain-gauge measurements
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
Improves power measurement accuracy by eliminating angular errors and simplifying calculations, providing more precise power readings without the need for angle determination.
Implementation Method 1
a Y-axis accelerometer for sensing a Y-axis acceleration relative to the crank
Implementation Method 2
a strain-gauge for sensing a bend force applied to a crank of the pedaled vehicle
Data Source
AI summary
A power meter has a strain-gauge attached to a crank of a pedaled vehicle to measure a bend force applied to the crank. The power meter also includes an accelerometer positioned on the crank to sense a Y-axis acceleration. The power meter also includes a controller having a processor and memory storing machine-readable instructions that when executed by the processor cause the controller to: read a bend force value from the strain-gauge, read an accelerometer value from the Y-axis accelerometer, calculate a correction factor based on the accelerometer value and a maximum error force value, and subtract the correction factor from the bend force value to determine an auto-zero bend force value that is corrected for an effect of gravity on the crank.