Steering Rack Force Estimation With Friction Compensation
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Solution Overview
Problem
Existing methods for determining rack force in steering systems, particularly in steer-by-wire systems, are imprecise due to the neglect and/or imprecise representation of frictional influences, leading to inaccurate feedback torques and steering feel.
Innovation Solution
The method involves determining the target and actual actuator torque and position parameters to estimate frictional influences within the steering system, using a cascade control system and Kalman filter to calculate the rack force, incorporating frictional forces into a force balance or equilibrium.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If rack force is determined using sensors, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces direct mechanical sensing of rack force with a control-theoretic approach. By measuring actuator torque and using a mathematical model of the steering system (including friction compensation), the rack force is calculated indirectly rather than measured directly with sensors. This substitution of mechanical sensing with control-based estimation resolves the contradiction by achieving accurate rack force determination without adding complex sensor systems.
2Device complexity
If frictional influences are neglected in rack force determination, then device complexity is reduced, but measurement precision deteriorates
Solution Approach 1:
The patent implements friction compensation by continuously determining frictional forces based on actuator torque measurements and system state, then feeding this information back into the rack force calculation. The control unit dynamically adjusts the rack force determination by adding the determined frictional forces to the actuator-derived force component. This feedback mechanism ensures high measurement precision without requiring complex additional hardware.
3Measurement precision
If actuator torque is precisely measured to determine rack force, then measurement precision is improved, but use of energy increases
Solution Approach 1:
The patent leverages the actuator's own torque measurements, which are already taken for normal steering control operations, to simultaneously determine rack force. The actuator effectively 'services' dual functions: executing steering commands and providing data for rack force calculation. This self-service approach allows precise rack force determination without additional energy-consuming measurement systems.
Data Source
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AI summary
The invention relates to a method for determining the magnitude of a rack force (Fext) for a steering system (1) having a steering rack (9) that can be displaced by an actuator, the method comprising: - obtaining a target actuator torque (MS) for implementing a steering specification (L); - obtaining an actual actuator torque (MI) which is applied for implementing the steering specification (L); - determining the magnitude of a rack force (Fext) on the basis of the target actuator torque (MS) and the actual actuator torque (MI). The invention also relates to a control device (6) for carrying out a method of this kind.