Motor-Assisted Steering Ball-Screw with Variable Gear Ratio
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Solution Overview
Problem
Large vehicles require significant driver effort to steer, and existing power-assisted steering systems face limitations in gear ratio and driving feel, especially at high speeds, while mechanisms like worm gears fail to transmit torque in both directions, causing issues in returning to neutral.
Innovation Solution
A power-assisted steering system incorporating a ball-screw, rack, and sector gear with dual inputs from an electric motor and steering wheel, utilizing underdrive and overdrive gear ratios to optimize torque transmission and reduce friction through slide bearings, allowing for efficient steering angle changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a high gear ratio is used to reduce steering effort in large vehicles, then the driver effort is reduced, but the number of steering wheel turns required to accomplish necessary steering angle changes increases excessively
Solution Approach 1:
The system dynamically adjusts the gear ratio between the steering wheel and ball screw based on operating conditions. The variable gear ratio mechanism allows the system to provide high reduction ratio when low steering effort is needed, and low reduction ratio when rapid steering angle changes are required, thereby resolving the contradiction between reducing driver effort and maintaining steering responsiveness.
2Force
If a high gear ratio is used to reduce steering effort, then the driver effort is reduced, but the driving feel is adversely impacted especially at high vehicle speeds
Solution Approach 1:
The variable gear ratio mechanism dynamically changes the transmission ratio based on vehicle speed and steering conditions. At high speeds, the system provides lower gear reduction to maintain direct steering feel and responsiveness, while at low speeds or during maneuvering, it provides higher reduction to assist the driver. This dynamic adaptation resolves the contradiction between force assistance and speed-responsive driving feel.
3Productivity
If a worm gear mechanism is used to provide high gear reduction, then the gear ratio is achieved, but the mechanism acts as a lock and cannot transmit torque in the opposite direction, preventing return to neutral
Solution Approach 1:
The system replaces the static, self-locking worm gear mechanism with a dynamic variable gear ratio mechanism that allows bidirectional torque transmission. This enables the steering system to be driven from either the steering wheel side or the ball screw side, facilitating automatic return to neutral position when steering forces are removed, while still providing high gear reduction when needed.
Solution Approach 2:
The patent substitutes the traditional worm gear mechanical locking mechanism with an electric motor-assisted variable gear ratio system. This replacement eliminates the self-locking characteristic that prevents reverse torque transmission, allowing the steering column to rotate freely in both directions and return to neutral position under the influence of steering wheel forces or electric motor assistance.
4Power
If a smaller electric motor is used to assist steering, then the system size and power requirements are reduced, but the torque transmission capability and driving feel may be compromised
Solution Approach 1:
The variable gear ratio mechanism amplifies the torque capability of a smaller electric motor by dynamically adjusting the gear reduction ratio. When maximum torque assistance is needed, the system provides high gear reduction, allowing a smaller motor to deliver sufficient torque to the ball screw. This resolves the contradiction between using a smaller, more compact motor and maintaining adequate torque transmission capability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system reduces driver effort and maintains driving feel by enabling precise steering angle changes with a smaller electric motor, minimizing friction and ensuring proper system return to neutral, suitable for both manual and autonomous vehicles.
Implementation Method 1
A ball-screw includes a spindle supported for rotation about an axis and a ball-nut supported for axial movement along the axis and constrained against rotation about the axis
Implementation Method 2
Two slide bearings may be fixed to a second side and a third side of the block respectively to decrease friction as the block slides against a housing
Implementation Method 3
The sector gear meshes with the rack and is configured to change the axis of rotation of vehicle wheels
Implementation Method 4
The first helical gear may be fixed to the spindle. The second helical gear may be fixed to the rotor and may mesh with the first helical gear
Implementation Method 5
The first bevel gear may be fixed to the fourth helical gear. The second bevel gear may mesh with the first bevel gear and be fixed to the steering wheel
Data Source
AI summary
A power assisted steering system provides a gear reduction between a steering wheel and a steering mechanism and a much larger gear reduction between an electric motor and a steering mechanism. The system is back-driveable from the steering mechanism. The system utilizes a ball-screw, several helical gears, and two bevel gears. A rack fixed to the ball-screw nut drives a sector gear.