Electric Steering Drag Force Threshold Control
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Solution Overview
Problem
In electric power steering systems, drivers are often unaware when the steering angle approaches its maximum limit, leading to potential excessive force application and contact between the rack shaft and rack housing, which can result in reduced controllability and increased risk of collision due to uncontrolled torque input.
Innovation Solution
A turning system with a control device that calculates and applies a drag force to the steering wheel when the turning angle exceeds a threshold, using a combination of elastic and sticky forces to resist excessive angle increases, thereby limiting the force required to turn the wheels and maintaining controllability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If feedback control based on steering torque difference is used, then steering accuracy is improved, but driver awareness of maximum steering angle is lost leading to potential rack shaft contact
Solution Approach 1:
The system applies a drag force to the steering wheel before the rack shaft actually contacts the rack housing, when the turning angle reaches a predetermined threshold. This preliminary counteracting force prevents the driver from applying excessive torque that would cause harmful contact, while maintaining accurate feedback control for normal steering operations.
Solution Approach 2:
A drag force mechanism acts as an intermediary between the driver's steering input and the rack shaft mechanism. This intermediary applies a resistive force on the steering wheel when the turning angle approaches the maximum limit, providing a tactile warning to the driver without directly interfering with the feedback control system's ability to maintain steering accuracy.
2Strength
If current command value is slowly decreased when rack shaft approaches housing, then mechanical damage is prevented, but driver cannot sense the limit and excessive torque may still be applied
Solution Approach 1:
Instead of waiting for the rack shaft to approach the housing before taking action, the system proactively applies a drag force when the turning angle reaches a predetermined threshold. This preliminary action prevents excessive torque application by the driver while providing tactile feedback about the approaching limit.
Solution Approach 2:
The drag force mechanism provides tactile feedback to the driver about the steering angle limit. When the turning angle reaches the threshold, the drag force creates a noticeable resistance that informs the driver they are approaching the maximum steering angle, enabling the driver to self-correct without mechanical contact occurring.
3Reliability
If drag force is applied to limit steering angle, then rack shaft contact is prevented, but feedback control of turning angle may deteriorate
Solution Approach 1:
The drag force is applied locally and selectively only when the turning angle reaches the predetermined threshold, rather than continuously. This localized application prevents rack shaft contact during critical moments while minimizing interference with feedback control during normal steering operations where the threshold is not reached.
Solution Approach 2:
The drag force is applied partially - only when necessary to prevent excessive steering angles. The force magnitude and application duration are controlled to be sufficient for protection but minimal enough to avoid significantly degrading the feedback control performance during normal operation.
Data Source
AI summary
A turning system includes a steering wheel, a turning actuator that has an electric motor incorporated thereinto and turns turning wheels, and a control device. The control device is configured to perform: a steering operation amount calculating process of calculating a steering operation amount; an angle command value calculating process of calculating an angle command value; an angle operation amount calculating process of calculating an angle operation amount; an operation process of operating a drive circuit of the electric motor; and a correction process of correcting a parameter upstream from the angle operation amount calculating process such that a drag force resisting an operation of the steering wheel for increasing magnitude of the turning angle is applied when the magnitude of the turning angle is equal to or greater than a turning angle threshold value.