Electric Power Steering Torque Damping via High Pass Filtering
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Solution Overview
Problem
Existing electric power assisted steering systems experience oscillatory and unstable behavior during high-speed driving maneuvers due to lightly damped yaw responses, leading to 'fish-tailing' and overshoot, particularly when driver actions are unsupported or involve rapid handwheel changes.
Innovation Solution
The system incorporates a signal processing unit that calculates a torque demand signal by differentiating the torque signal and applying a high pass filter, reducing low-frequency gain to prevent destabilization and enhance damping, while also adding damping torque to the initial torque demand signal to improve stability and reduce oscillations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a damping component is added to the torque demand signal to reduce oscillations, then vehicle stability is improved, but the system complexity increases
Solution Approach 1:
The patent changes the parameters of the damping component by making it a function of both torque and angular velocity. The damping component is reduced at low torques compared to high torques, and varies with angular velocity from zero at zero speed to maximum at arbitrary maximum speed. This parameter adaptation resolves the contradiction by providing stability only when needed (high torque/velocity conditions) rather than constantly, thus improving reliability without excessive complexity
Solution Approach 2:
The damping component is made dynamic rather than static. It adjusts in real-time based on operating conditions (torque and angular velocity), being higher during hands-free maneuvers and lower during hands-on maneuvers. This dynamic adaptation allows the system to maintain stability when required while avoiding unnecessary complexity in normal operating conditions
2Ease of operation
If the damping component is reduced at low torques, then the on-centre feel is improved, but the damping effect at high speeds is reduced
Solution Approach 1:
The patent applies local quality by differentiating the damping behavior based on the operating region. At low torques (on-centre region), the damping component is reduced to improve feel. At high torques (off-centre region), the damping component is increased to provide stability. This spatial differentiation of damping quality resolves the contradiction by optimizing for feel in one region and stability in another
3Reliability
If the damping component increases with angular velocity, then high-speed stability is improved, but the torque output is reduced
Solution Approach 1:
The damping component is applied partially rather than fully across all operating conditions. It increases with angular velocity from zero at zero speed to maximum at arbitrary maximum speed, providing just enough damping to achieve stability without excessively reducing torque output. This partial application resolves the contradiction by achieving the minimum necessary damping for stability while preserving useful torque output
Data Source
AI summary
An electric power assisted steering system for a vehicle, comprising: a steering mechanism which operatively connects a steering wheel to road wheels of the vehicle, a torque sensor arranged to, in use, produce a torque signal indicative of the torque carried by a portion of the steering mechanism, an electric motor operatively connected to the steering mechanism, a signal processing unit arranged to, in use, produce from the torque signal a torque demand signal representative of a torque to be applied to the steering mechanism by the motor, the signal processing unit comprising a first calculating unit arranged to calculate an initial torque demand signal dependent on the torque signal, and a second calculating unit arranged to, in use, calculate a torque damping signal indicative of an amount the initial torque signal is to be damped in order to generate the torque demand signal, and a motor drive stage arranged to provide, in use, a drive current to the motor responsive to the torque demand signal, in which the second calculating unit comprises an input for a signal dependent upon the torque demand signal, a differentiator, arranged to, in use, differentiate the input signal dependent upon the torque signal and a high pass filter arranged to filter the input signal to reduce low frequency components thereof.