Steering Assist Torque Limiting for Ball Screw Protection
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Solution Overview
Problem
Existing electromechanical steering systems for vehicles with variable chassis height do not adequately protect the re-circulating ball screw drive from damage due to high radial forces, which can lead to noise and reduced service life, as they fail to account for the geometry of the steering system and varying chassis height influenced by spring deflection or adjustable settings.
Innovation Solution
A method for operating an electromechanical steering system that limits steering assistance power based on chassis height, vehicle speed, and the angle between the toothed rack and track rods, using a control unit to adjust the assist torque and prevent overload on the re-circulating ball screw drive, by calculating a weighted mean value of motor current that balances steering torque, chassis height, and vehicle speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the steering assistance power is increased to improve steering performance, then the steering assistance is improved, but the re-circulating ball screw drive is subjected to high radial forces causing damage, noise and reduced service life
Solution Approach 1:
The control unit dynamically adjusts the assistance torque based on real-time chassis height data from height sensors. The assistance torque is varied according to the detected chassis height, allowing the system to provide high assistance when safe (normal height) and reduce assistance when radial forces become dangerous (reduced height), thus resolving the contradiction between steering performance and component reliability
Solution Approach 2:
The system changes the operational parameters of the steering assistance by adjusting the assistance torque as a function of chassis height. When the chassis height decreases below a threshold value, the control unit reduces the maximum permitted assistance torque, thereby changing the parameter of assistance force to prevent damage to the re-circulating ball screw drive while maintaining normal operation above the threshold
2Reliability
If the assistance power is reduced to protect the re-circulating ball screw drive, then the service life is extended, but the steering assistance capacity is insufficient when needed
Solution Approach 1:
The system dynamically changes the assistance torque parameter based on chassis height conditions. When the chassis height is above the threshold, full assistance torque is permitted. When the chassis height drops below the threshold, the maximum permitted assistance torque is reduced. This parameter adaptation ensures protection of the ball screw drive while maintaining full assistance capacity when operating conditions are safe
Solution Approach 2:
The control unit implements dynamic adjustment of assistance torque based on real-time height sensor feedback. The system transitions between different assistance levels (full assistance above threshold, reduced assistance below threshold) based on the detected chassis height, thereby extending service life without permanently sacrificing steering assistance capacity
3Adaptability or versatility
If the chassis height is adjusted for different driving conditions, then the adaptability is improved, but the angle between track rods and toothed rack changes causing variable radial forces on the ball screw drive
Solution Approach 1:
The control unit continuously receives feedback from height sensors about the actual chassis height and adjusts the maximum permitted assistance torque accordingly. This feedback mechanism ensures that when chassis height changes (whether due to suspension deflection or active adjustment), the system responds by appropriate torque limitation to maintain reliable operation of the ball screw drive
Solution Approach 2:
The system dynamically adapts the assistance torque based on the actual chassis height resulting from suspension deflection or active adjustment. The control unit calculates the appropriate maximum assistance torque as a function of the detected height, allowing the system to accommodate variable chassis positions while maintaining protection of the ball screw drive from excessive radial forces
Data Source
Figure 1
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Figure 3~4
AI summary
A method for operating an electromechanical steering system for a motor vehicle with variable chassis height, - with a steering rack (4), which is driven by a servo motor (18) through a recirculating ball screw drive, - with a control unit (10), which receives input signals and generates output signals for driving the servo motor (18), the output signals being representative of an assist torque (T) to be provided by the steering system, is characterised in that - in operation the control unit (10) is, receiving or generating a chassis height signal (X) indicative of the current chassis height of the vehicle, and - limiting the assist torque (T) on the basis of the chassis height signal (X) to an upper limit, wherein the upper limit is decreased with an increase of the difference between the current chassis height and a mean chassis height.