Manual Torque Estimation in Power Assist Wheelchairs
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Solution Overview
Problem
Existing power assist wheelchairs face challenges in accurately estimating manual torque, particularly when a power assist unit is retrofitted, due to varying output values from acceleration sensors depending on their position.
Innovation Solution
A power assist wheelchair system that calculates total torque based on an encoder detection signal, subtracts assist torque to estimate manual torque, and determines target currents for the electric motor, allowing for accurate manual torque estimation with a simple hardware configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an acceleration sensor is used to estimate manual torque, then torque estimation is possible, but the output value changes depending on sensor position making accurate estimation difficult
Solution Approach 1:
The invention extracts the torque estimation function from the acceleration sensor and relocates it to the encoder by calculating total torque from encoder detection signals. This removes the dependency on acceleration sensor position and orientation, eliminating the measurement accuracy problem while maintaining device simplicity.
Solution Approach 2:
The invention introduces an intermediary calculation process that uses encoder data to compute total torque, which then serves as the basis for manual torque estimation. This intermediary approach bypasses the need for direct acceleration sensor measurements and their associated positioning issues.
2Reliability
If acceleration sensors are used for torque estimation, then torque data can be obtained, but hardware configuration becomes complex especially for retrofit applications
Solution Approach 1:
The invention extracts the reliable torque estimation function from the complex acceleration sensor system and implements it using the existing encoder. This maintains reliability by using a proven sensor (the encoder already present in powered wheelchairs) while eliminating the need for additional acceleration sensors and their complex configuration.
Solution Approach 2:
The invention enables the encoder to serve dual purposes: its original function of detecting motor rotation and the additional function of estimating manual torque. This self-service approach eliminates the need for separate torque sensing hardware, simplifying the system especially for retrofit applications.
3Device complexity
If total torque is calculated using only encoder signals, then hardware configuration is simplified, but accurate separation of manual and assist torque is challenging
Solution Approach 1:
The invention uses feedback from the motor current detector to measure assist torque, which is then subtracted from the total torque (calculated from encoder signals) to isolate manual torque. This feedback loop enables accurate separation of torque components using simple hardware.
Solution Approach 2:
The invention segments the total torque into two distinct components: assist torque (measured from motor current) and manual torque (calculated as the difference). This segmentation allows each component to be measured and controlled independently, achieving precision without hardware complexity.
Data Source
AI summary
A power assist wheelchair includes a wheel, an electric motor that drives the wheel, an encoder that detects rotation of the electric motor, and a control device that controls the electric motor. The control device includes a total torque value calculator that calculates a total torque value based upon only a detection signal of the encoder, an assist torque value calculator that calculates an assist torque value based upon an output current of the electric motor, a manual torque value calculator that calculates a manual torque value based upon a difference obtained by subtracting the assist torque value from the total torque value, and a target current determiner that determines a target current of the electric motor based upon the manual torque value.