Electric Power Steering Virtual Rack End Control
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Solution Overview
Problem
Conventional electric power steering devices face challenges in reducing assist torque near the maximum steering angle, leading to potential impact and abnormal noise, and fail to consider steering velocity and physical models in control adjustments, resulting in inconsistent performance across different road conditions.
Innovation Solution
An electric power steering device that calculates a control rotational displacement based on steering torque and self-aligning torque, adjusts the rotational displacement input to the rack end control system, and uses feedforward control to output a current command value, reducing reaction force near the virtual rack end while maintaining assist force, thereby enabling smooth steering to the physical rack end without affecting the vehicle's turning radius.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If assist torque is reduced near the maximum steering angle to suppress impact and abnormal noise, then rack end impact noise is reduced, but steering force becomes insufficient and driver cannot turn steering wheel to rack end smoothly
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the virtual rack end position based on steering velocity and steering torque. When steering velocity is high or steering torque is large, the virtual rack end position is shifted toward the physical rack end, maintaining assist force to enable smooth steering. When steering velocity is low or steering torque is small, the virtual rack end position is shifted away from the physical rack end, reducing assist force to suppress impact and abnormal noise. This dynamic parameter adjustment resolves the contradiction between suppressing rack end impact noise and maintaining sufficient steering assist force.
2Object-affected harmful factors
If virtual rack end position is fixed before physical rack end, then impact and abnormal noise are suppressed, but steering discomfort occurs and turning radius is influenced
Solution Approach 1:
The patent applies dynamics by making the virtual rack end position dynamic rather than fixed. The virtual rack end position is continuously adjusted based on real-time steering velocity and steering torque conditions. This dynamic adjustment allows the system to adapt to different steering scenarios: maintaining a fixed offset from the physical rack end under normal conditions to suppress impact and abnormal noise, while shifting closer to the physical rack end when high steering velocity or large steering torque indicates the driver intends to turn to the rack end, thereby preventing steering discomfort and maintaining natural steering feel.
3Device complexity
If conventional feedback control is used without considering steering velocity, then control simplicity is maintained, but control performance becomes inconsistent across different road conditions
Solution Approach 1:
The patent applies parameter changes by introducing steering velocity as an additional control parameter alongside steering torque. The control system calculates the virtual rack end position based on both steering velocity and steering torque, allowing the control characteristics to adapt to different road conditions and steering scenarios. This multi-parameter approach improves control consistency and reliability across varying conditions while maintaining relatively simple control logic through map-based or calculated determination of the virtual rack end position.
Data Source
AI summary
An electric power steering device including: a control rotational displacement calculation unit configured to calculate a control rotational displacement when a steering angle of the steering system is in an angular range from a maximum allowable steering angle for the steering system to a predetermined threshold steering angle; a control steering angle shifting unit configured to calculate the control rotational displacement corrected by a correction amount based on one of the steering torque and rack axial force and a sign of one of the control rotational displacement and the steering angle as a shift control steering angle; and a feedforward control unit configured to output a second current command value based on the shift control steering angle and steering velocity. An assist control is controlled with a third current command value calculated by adding the second current command value to the first current command value.