Friction Compensation in Electric Power Steering
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Solution Overview
Problem
Existing electric power steering systems face challenges in accurately compensating for internal friction, which affects steering effort and directional stability due to varying friction forces caused by temperature and wear, leading to potential delays in steering response.
Innovation Solution
A method that dynamically calculates and compensates for friction forces by continuously monitoring the rack load and motor speed using sensors and state observers, adjusting the motor torque to account for friction, and applying compensation values that adapt based on steering angle and vehicle speed to ensure precise torque control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If friction force compensation is implemented using conventional methods (monitoring steering wheel torque and integrating torque over time), then friction compensation around the centre position is improved, but compensation accuracy at off-centre positions deteriorates
Solution Approach 1:
The patent implements dynamic friction compensation by continuously adapting the compensation torque based on real-time monitoring of rack load and motor speed. The friction compensation is no longer static or centered around the centre position, but dynamically adjusted according to the actual operating conditions at any steering angle, thereby resolving the contradiction between centre position accuracy and off-centre position adaptability
Solution Approach 2:
The patent employs feedback mechanisms by monitoring rack load and motor speed signals, and using these signals to continuously adjust the friction compensation torque. This closed-loop feedback approach allows the system to adapt to varying friction conditions across the entire steering angle range, not just around the centre position, thus improving both measurement precision and adaptability
2Device complexity
If friction forces are not correctly compensated, then steering system simplicity is maintained, but directional stability and steering response time deteriorate
Solution Approach 1:
The patent implements self-service friction compensation by utilizing signals already present in the steering system (rack load and motor speed from existing sensors and state observers). The system compensates for its own friction forces using its own operational data, without requiring additional dedicated friction sensors or complex external measurement systems, thus maintaining relative simplicity while improving directional stability
Solution Approach 2:
The patent changes the parameters used for friction compensation from static or centre-position-based values to dynamic parameters derived from rack load and motor speed. By using these varying parameters to calculate compensation torque, the system achieves better directional stability and steering response without significantly increasing device complexity
Data Source
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AI summary
A method of operating an electric power steering system, wherein the steering system comprises a steering wheel (1) coupled to a steering column (2), a pinion engaging a toothed rack (4) to operate steerable wheels (5) of a vehicle, at least one sensor (6) for measuring a torque applied to the steering column (2) or a force applied to the rack (4), and an servo motor (14) supplying steering assist to support a steering effort of a driver, the torque sensor (6) and the servo motor (14) being connected to a control unit (10,11), the method comprising the following steps: a) continuously monitoring the rack load (17) or the pinion torque using the signal from at least one sensor by direct measurement or estimation in a state observer, b) continuously monitoring the motor speed (23), c) when the motor speed (23) is near zero or equal to zero: deriving the rack load change from the rack load (17) or the pinion torque, d) calculate a reference motor movement which would be expected for the rack load change if no friction were present, e) if the motor movement is smaller than the reference motor movement, then request first additional steering assist torque, and f) proceed to step c).