Electric Power Steering Road Wheel Angle Control
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Solution Overview
Problem
Existing electric power steering systems lack precise control over the road wheel angle, which is crucial for autonomous driving, as they can only accept steering torque requests and not direct angle requests, leading to inefficiencies in trajectory control and lane changes.
Innovation Solution
A method involving dynamic inversion and feedback loops to determine the assist torque, using vehicle velocity and lateral acceleration to calculate a smooth reference assist torque, and iteratively adjusting the torque to match the desired steering angle, allowing for precise control of the road wheel angle without requiring an inverse torque map.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a dual torque interface with boost curve amplification is used in electric power steering system, then steering torque can be amplified to assist driver input, but precise control of road wheel angle becomes unachievable due to inability to inversely model the high frequency part of boost curve
Solution Approach 1:
The control approach is segmented into two independent interfaces: Type 1 for steering assist with boost curve amplification, and Type 2 for precise road wheel angle control without boost amplification. This segmentation allows each interface to optimize for its specific function without the limitations of the other.
Solution Approach 2:
A road wheel angle sensor is introduced as an intermediary measurement device to directly measure the actual road wheel angle. This sensor feedback enables precise angle control by providing direct measurement data to the controller, bypassing the need to inversely model the complex boost curve dynamics.
2Ease of operation
If static inverse boost curve model is used, then partial torque compensation can be achieved, but high frequency dynamics cannot be inverted leading to control errors
Solution Approach 1:
A feedback loop using road wheel angle sensor measurement is implemented to continuously monitor the actual road wheel angle and compare it with the reference angle. The controller adjusts the assist torque based on this feedback to eliminate angle errors, achieving precise control without requiring an accurate inverse model of the boost curve dynamics.
3Manufacturing precision
If direct road wheel angle request is implemented, then precise trajectory control can be achieved, but electric power steering system cannot accept direct angle requests as it only accepts torque requests
Solution Approach 1:
The road wheel angle sensor acts as an intermediary that enables the controller to directly measure and control road wheel angle, bridging the gap between the controller's ability to generate angle requests and the steering system's ability to execute torque commands. This intermediary measurement capability allows precise angle control through torque actuation.
Solution Approach 2:
The control system replaces the need for mechanical direct angle actuation with an electrical control approach. The controller calculates the required assist torque based on the angle error and feeds it to the electric power steering system, substituting mechanical angle direct control with an electrical torque-based control system that achieves the same precision.
Data Source
Figure 1~2-3
AI summary
The present invention relates to a method to control the steering of a vehicle steering system having at least an electric power steering system comprising at least the steps of Determining of a smooth reference assist torque, Tuning a desired angle gain, Measuring assist torque, Tuning a desired assist gain, Generating a desired steering wheel torque, and Input said desired steering wheel torque to said electric power steering system.