Electric Power Steering Friction Compensation via Plant Observer
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Solution Overview
Problem
Existing electric power steering systems face challenges in compensating for friction in the power column, leading to oscillatory and non-responsive steering systems, which affect the overall performance and accuracy of steering input.
Innovation Solution
An electric power steering system that includes a power column with an electric motor and speed reduction mechanism, an assist torque command value calculation unit, a plant observer, a frictional force estimating unit, and a motor torque command value calculation unit, which calculates and compensates for frictional forces using observer and reference models to ensure non-oscillatory and responsive control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If friction compensation is implemented using conventional methods (adding frictional force to assist torque command), then friction compensation is achieved, but the control system becomes oscillatory and non-responsive
Solution Approach 1:
The patent implements a feedback mechanism where the plant observer continuously monitors the actual system state (rotation angle, frictional force) and compares it with the model predictions. The frictional force estimated by the observer is fed back to correct the assist torque command value, creating a closed-loop control system that adapts to actual friction conditions without causing oscillations
Solution Approach 2:
The patent replaces conventional friction compensation methods with an observer-based estimation approach. Instead of using fixed friction models or simple sign-based friction addition, the system uses a plant observer that combines system models with actual measurements to dynamically estimate frictional force, substituting mechanical intuition with computational estimation
2Force
If the speed reduction mechanism is used to amplify motor torque, then torque amplification is achieved, but large friction is generated in the mechanism
Solution Approach 1:
The patent converts the harmful friction effect into a measurable and compensatable parameter. By using the plant observer to estimate frictional force based on system state and model comparisons, the previously harmful friction becomes a quantifiable value that can be compensated for in the assist torque command, turning a system weakness into a controllable variable
3Reliability
If frictional force is added to assist torque command based on steering speed sign, then friction compensation is achieved, but the response to steering input is delayed and oscillatory
Solution Approach 1:
The patent transitions from static friction compensation (based on steering speed sign) to dynamic friction compensation. The plant observer continuously updates frictional force estimation based on real-time system state and model comparisons, allowing the friction compensation to adapt dynamically to changing steering conditions without causing oscillations or delays
Data Source
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AI summary
In an electric power steering system, an ECU 12 includes a rotation angle calculation unit 41, an assist torque command value calculation unit 42, a reference model 43, an angle deviation calculation unit 44, a PD control unit 45, an addition unit 47, and a plant observer 48. A proportional gain and a derivative gain which are used in the PD control unit 45 and a first gain and a second gain which are used in a first gain multiplication unit and a second gain multiplication unit of the plant observer 48 are set so that a characteristic equation corresponding to an equation of motion representing behavior of the electric power steering system has a double root.