Wheel Grip Factor Estimation Using Rack Force and Slip Angle
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Solution Overview
Problem
Existing technologies struggle to accurately estimate the grip factor of a vehicle wheel due to limitations in detecting the slip angle of the front wheel, reliance on unreliable sensor data, and punctual estimation methods that require reaching the grip limit, leading to inaccurate and unsafe driving conditions.
Innovation Solution
A system with independently operating electric motors for each wheel, electronically controlled shock absorbers, and an electronic control unit that processes steering parameters and state variables to estimate grip factor using real-time data and reliability filtering, allowing for pre-emptive and precise grip estimation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If punctual estimation methods are used to define grip as maximum acceleration, then the estimation can be considered reliable, but the acceleration must be reached which limits real-time applicability
Solution Approach 1:
The system performs preliminary estimation of the grip factor using rack force and slip angle data before the vehicle reaches maximum acceleration. The control unit continuously monitors these parameters and predicts grip conditions in advance, allowing the estimation to be available prior to reaching the grip limit rather than waiting for it to be reached.
Solution Approach 2:
The invention introduces rack force as an intermediary parameter that correlates with grip factor without requiring the vehicle to reach maximum acceleration. By using rack force measurements combined with slip angle calculations, the system mediates between the need for reliable grip estimation and the time constraint of reaching grip limit conditions.
2Measurement precision
If yaw angle sensor data is used for grip factor estimation, then differential values can be obtained, but sensor interferences cause difficulties in precisely estimating the grip factor
Solution Approach 1:
The system extracts the essential information needed for grip factor estimation (rack force and slip angle) while eliminating the problematic yaw angle sensor from the calculation chain. By deriving slip angle from rack force measurements and wheel speeds rather than relying on yaw rate integration, the harmful sensor interferences are removed while preserving the measurement precision.
Solution Approach 2:
The invention replaces the sensor-based mechanical measurement system (yaw angle sensor) with a computational approach using rack force transducers and wheel speed sensors. This substitution eliminates the integration drift and interference issues associated with yaw rate sensors while maintaining the ability to calculate slip angle and grip factor with high precision.
3Measurement precision
If slip angle of the front wheel is detected directly, then accurate grip factor estimation can be achieved, but the slip angle cannot be detected and can hardly be estimated by means of a steering angle
Solution Approach 1:
The rack force serves as an intermediary measurement that directly reflects the lateral forces at the wheel contact patch. By measuring rack force and combining it with wheel speed and steering angle data, the system indirectly determines slip angle with high precision without requiring direct mechanical slip angle sensors, thus avoiding excessive device complexity.
Solution Approach 2:
The rack force transducer performs multiple functions: it measures both the lateral force magnitude and, when combined with steering angle and wheel speed, provides slip angle information. This multi-functionality eliminates the need for separate slip angle sensors while maintaining measurement precision across different driving conditions.
Data Source
AI summary
Apparatus for estimating a grip factor of at least one wheel comprising: a control unit configured to process a current slip angle of said wheel; a storage unit, within which, in a consultation table, a plurality of grip curves correlating a plurality of values of a steering parameter comprising the rack force with a plurality of values of the slip angle are recorded; along a same curve, the value of the grip factor remains unchanged; wherein the control unit is configured to cyclically estimate at least one raw value of the grip factor of said at least one wheel based on the position of a current condition within the consultation table, as a function of the current slip angle and of the current rack force.