Steering Transmission Belt Ratios for Accurate EPAS Positioning
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Solution Overview
Problem
Worm drive column assist Electric Power Assist Steering (EPAS) systems are limited by low motor speed and limited ratio, leading to inefficiencies and challenges in implementing these systems on heavy vehicles, with typical efficiency ranging from 65% to 85% during back-driving conditions, and require high torque motors which are not effectively utilized.
Innovation Solution
A vehicle steering system transmission comprising a driver connected to a driver shaft via a flexible link, with sensors measuring the angular positions of the input, intermediate, and output shafts, allowing for precise control and positioning of the output shaft, even when power is lost, by using a configuration of sprockets and belts with non-whole number drive ratios to maintain positional accuracy and prevent repeated shaft positions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a worm drive mechanism is used in column assist EPAS systems, then the steering assist function is achieved, but the motor speed is limited and the system efficiency drops to 65-85% during back-driving conditions
Solution Approach 1:
The transmission system is divided into multiple independent belt drive stages (first belt drive, second belt drive, third belt drive) with different drive ratios. Each stage uses separate sprockets and belts, allowing the system to segment the total reduction ratio into manageable portions while maintaining flexibility in configuration and optimizing efficiency at each stage.
Solution Approach 2:
The system dynamically selects and adjusts drive ratios through the combination of multiple belt drives with different ratios. The flexible linkages and modular belt drive configuration allow the system to adapt to different operating conditions, optimizing the drive ratio selection based on steering requirements and power availability.
2Force
If a high ratio worm drive is used to increase steering assist, then the assist force is improved, but the motor speed becomes too low for effective operation
Solution Approach 1:
The total reduction ratio is segmented across multiple belt drive stages rather than using a single high-ratio worm drive. This segmentation allows each stage to operate at more favorable speed and torque conditions, preventing excessive speed reduction in a single stage while still achieving the required overall force multiplication.
Solution Approach 2:
The system changes the drive ratio parameter across different belt drive stages. By using multiple stages with different reduction ratios (first ratio, second ratio, third ratio), the system optimizes the speed-torque conversion at each stage, maintaining better motor speed characteristics while achieving the required steering assist force.
3Measurement precision
If multiple belt drives with different ratios are used, then the positional accuracy is improved, but the device complexity increases
Solution Approach 1:
The transmission system is segmented into multiple independent belt drive stages, each contributing to the overall reduction ratio. This segmentation allows for more precise control and measurement at each stage, improving the ability to track and control output shaft position accurately while distributing the complexity across modular components.
Solution Approach 2:
The multiple belt drives serve multiple functions: they provide the reduction ratio, enable positional tracking through sensors, and offer flexibility in configuration. The sensors on input, intermediate, and output shafts work together with the multi-stage transmission to achieve precise positional control, making the system multi-functional despite increased component count.
4Measurement precision
If sensors are added to measure shaft positions, then the control precision is improved, but the manufacturing cost increases
Solution Approach 1:
Sensors are installed on the input shaft, intermediate shaft, and output shaft to provide feedback on angular positions. This feedback enables precise control of the multi-stage transmission system, allowing the control module to accurately determine output shaft position and adjust the belt drives accordingly, improving measurement precision through active sensing and control.
Data Source
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AI summary
A vehicle steering system transmission comprising a driver connected to a driver shaft (4), the driver shaft (4) connected to an output shaft (10) by a flexible link (6, 7, 8), a first sensor (1) sensing the driver shaft (4), a control unit (500) receiving a signal from the first sensor (1), and the control unit (500) transmitting a signal to the driver to control an output shaft (10) movement.