Asymmetric Roller Screw Spindle Drive for Self-Locking Steering

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Solution Overview

Problem

Existing spindle drives for steer-by-wire steering systems face challenges in efficiently managing lateral forces and unexpected loads, requiring additional locking mechanisms to prevent automatic return to neutral position, which increases complexity and space requirements.

Innovation Solution

A roller screw drive with different efficiencies in driving and reverse directions is used, offering low friction and torque in the driving direction while being self-locking in the reverse direction, reducing the need for additional locking units and minimizing space requirements, utilizing a belt drive or coaxial motor to achieve axial movement with minimal lubrication and noise.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Power

If a non-self-locking threaded spindle with roller screw drive is used, then the drive torque and fitting space are reduced, but the system requires additional locking units to prevent automatic return under lateral forces

Engineering Contradiction:

Improvedrive torqueVSAvoidlocking unit

Core Design Contradiction:

PowerVSDevice complexity

Solution Approach 1:

The roller screw drive is designed with asymmetric efficiency characteristics: high efficiency in the driving direction (low friction) and low efficiency in the reverse direction (self-locking). This local differentiation of friction properties eliminates the need for separate locking units while maintaining the benefits of low drive torque and compact size.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

Instead of using a self-locking screw drive (which would require high reverse efficiency) and adding a brake to enable movement, the invention inverts the approach by using a non-self-locking drive with high efficiency in both directions, then selectively applying friction only when needed through the asymmetric design. The system naturally locks in the reverse direction without additional components.

Inventive Principle:

Principle #13The other way round (Inversion)

2Reliability

If a conventional ball screw drive or trapezoidal screw drive is used, then self-locking is achieved, but the fitting space and drive torque requirements increase

Engineering Contradiction:

Improveself-locking capabilityVSAvoidfitting space

Core Design Contradiction:

ReliabilityVSArea of stationary object

Solution Approach 1:

The invention changes the friction parameter of the roller screw drive by designing asymmetric roller contact geometry and lubrication characteristics. This results in highly different efficiency values for forward and reverse directions, enabling self-locking behavior without the space and torque penalties of conventional screw drives.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The invention replaces the mechanical self-locking mechanism of conventional screw drives (which rely on high friction angles) with a roller screw drive that uses controlled asymmetric friction. This substitution maintains self-locking capability while reducing the mechanical complexity and space requirements.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Loss of energy

If a roller screw drive with high efficiency in both directions is used, then low friction and low drive torque are achieved, but the system becomes non-self-locking and requires locking units

Engineering Contradiction:

Improvefriction lossVSAvoidself-locking capability

Core Design Contradiction:

Loss of energyVSReliability

Solution Approach 1:

Inventive Principle:

Principle #3Local quality

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

This design allows for efficient steering with low drive power, compact integration, and low-noise operation, ensuring wheels remain on track under lateral forces without automatic return, using the motor's stall torque to maintain self-locking and prevent axial displacement.

Implementation Method 1

A roller screw drive is distinguished by exceptionally low friction in the driving direction... the roller screw drive has different efficiencies in its driving direction and in its reverse-driving direction

Methodology Applied

Scientific EffectFriction: Friction

Implementation Method 2

The rotational movement of the spindle nut can be produced by a motor with its axis parallel... The displacement or movement of the threaded spindle produced by the spindle drive in the axial direction

Methodology Applied

Scientific EffectElectromagnetic conversion: Electromagnetic Induction

Implementation Method 3

using the motor's stall torque to maintain self-locking and prevent axial displacement

Methodology Applied

Scientific EffectTorque: Torque

Data Source

PatentUS11623682B2Steer-by-wire steering system having a spindle drive

Publication Date: 2023.04.11 ZF FRIEDRICHSHAFEN AG

AI summary

A spindle drive (20) for an actuator of a steer-by-wire steering system (10). The spindle drive has a linearly displaceable threaded spindle (27). The spindle drive (20) is in the form of a roller screw drive. The roller screw drive has a first efficiency in the driving direction and a second different efficiency in the reverse-driving direction.