Steer-By-Wire Rack Force Estimation Without Torque Sensors
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Solution Overview
Problem
The existing methods for estimating rack force in Steer-By-Wire (SBW) systems face accuracy issues, especially at low vehicle speeds, and are not compatible with the lack of physical connections in SBW systems, leading to high manufacturing costs and estimation errors.
Innovation Solution
A method that calculates the restoring force applied to a reducer in the SBW system using position differences and a spring constant, then estimates rack force by incorporating inertia and friction forces into a motion equation, eliminating the need for expensive torque sensors and leveraging the strength of system members.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If automotive kinetic model is used for rack force estimation, then the system can estimate rack force, but the accuracy is particularly low at low vehicle speeds
Solution Approach 1:
The patent replaces the automotive kinetic model with a steering system model that uses a motion equation specifically designed for the SBW system. This new model incorporates the actual mechanical components (reducer, rack bar, tie rod) and their relationships, substituting the generic kinetic model with a tailored mechanical model that accurately represents the SBW system's behavior at all speeds.
Solution Approach 2:
The patent changes the parameters used in the estimation model from general kinetic parameters to specific steering system parameters including reducer efficiency, rack bar mass, tie rod length, and spring constant. By adjusting these parameters to match the actual SBW system configuration, the model achieves accurate rack force estimation across all vehicle speeds, particularly improving low-speed accuracy.
2Measurement precision
If steering system model is used with torque sensor on column, then rack force can be estimated, but it is not compatible with SBW system due to lack of physical connection
Solution Approach 1:
The patent extracts the torque sensor requirement from the traditional steering system model and removes it entirely from the SBW system implementation. Instead of measuring torque at the column (which doesn't exist in SBW), the system uses position sensors on the rack bar and calculates rack force through a motion equation, completely eliminating the need for torque sensors and making the model compatible with SBW's mechanical disconnection.
Solution Approach 2:
The patent introduces a motion equation as an intermediary mathematical model that bridges the gap between the available position measurements and the desired rack force estimation. This intermediary model uses the relationship between rack bar position, velocity, and acceleration along with system parameters to calculate rack force without requiring direct torque measurement, enabling compatibility with SBW system architecture.
3Measurement precision
If rod cell is used to directly measure force on tie rod, then rack force can be measured, but the manufacturing cost increases
Solution Approach 1:
The patent replaces the expensive rod cell force sensor with cheaper position sensors (rack position sensor and motor position sensor) that have lower manufacturing costs. While position sensors are less direct than force sensors, they provide sufficient measurement capability when combined with the motion equation, achieving acceptable accuracy at reduced cost, effectively using cheaper components to replace expensive ones.
Solution Approach 2:
The patent substitutes the mechanical force measurement approach (rod cell directly measuring force on tie rod) with a computational approach using position measurements and mathematical modeling. Instead of mechanically sensing force, the system uses position sensors and calculates force through the motion equation, replacing a mechanical sensing system with a computational one that is more cost-effective.
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 approach allows for accurate estimation of rack force without torque sensors, reducing manufacturing costs and minimizing estimation errors by measuring force transmission between the steering motor and rack bar based on system member strengths.
Implementation Method 1
calculating the restoring force by multiplying the difference by a spring constant of the reducer
Data Source
AI summary
The present disclosure relates to a technology of estimating rack force using force that is transmitted from a steering motor of a Steer-By-Wire (SBW) system to a rack bar. The present disclosure provides a method for estimating rack force of an SBW system, the method calculating restoring force that is applied to a reducer on the basis of the difference between position values of the rack bar that are obtained at the front end and the rear end of the reducer when the position of the rack bar is changed by operation of a steering motor; and calculating rack force by reflecting the restoring force to a motion equation including inertia force of the SBW system.