Steer-by-Wire Steering Reaction Force Under Motor Current Limits
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Solution Overview
Problem
In steer-by-wire type steering devices, the separation of power transmission between the steering wheel and turning wheels makes it difficult for drivers to feel road surface conditions, and overheat protection functions for the turning motor can limit the steering reaction force, preventing the originally demanded force from being secured.
Innovation Solution
A steering control device that calculates a steering reaction force based on both the current value of the turning motor and other vehicle state variables, adjusting the distribution ratios of these forces to maintain the steering reaction force even when the turning motor's current is limited.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the turning motor's current is limited for overheat protection, then the turning motor is protected from overheating, but the steering reaction force decreases and cannot meet the driver's information demand
Solution Approach 1:
The control device introduces a distribution ratio as an intermediary parameter to balance between the ideal axial force (from target turning angle) and road surface axial force (from turning motor current). By adjusting this distribution ratio, the system mediates between protecting the turning motor and maintaining adequate steering reaction force for driver information
Solution Approach 2:
The system dynamically changes the distribution ratio parameter based on the turning motor's operating state. When the turning motor approaches overheat conditions, the distribution ratio is adjusted to reduce reliance on road surface axial force, thereby maintaining steering reaction force while protecting the motor
2Loss of information
If the road surface axial force is calculated by multiplying the turning motor current by a coefficient, then the road surface condition is reflected in the steering reaction force, but the steering reaction force becomes limited when the turning motor current is limited
Solution Approach 1:
The distribution ratio is made dynamic rather than fixed, allowing the system to adapt between reflecting road surface conditions and maintaining steering reaction force. The distribution ratio changes based on turning motor current limits, enabling the system to prioritize information feedback when possible and force maintenance when necessary
Data Source
AI summary
A steering control device includes a first calculator configured to calculate a first axial force, a second calculator, and a third calculator configured to calculate a third axial force. The third calculator is configured to, when a specific situation occurs in which a current of a turning motor has to be limited, decrease a reflection degree of the first axial force in the third axial force and increase a reflection degree of the second axial force in the third axial force.