Vehicle Friction Estimation via Pitch Rate and Axle Torque
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Solution Overview
Problem
Current friction estimation technologies face challenges in accurately determining the friction coefficient, especially when all wheels experience longitudinal forces during braking or all-wheel drive, and struggle to estimate friction during low force excitement or on low friction surfaces, requiring improved methods for tire-to-road friction determination in vehicles.
Innovation Solution
A method and system that acquire front and rear wheel axle torque, vehicle longitudinal acceleration, pitch rate, and wheel rotational velocities to determine the vehicle's longitudinal velocity and pitch angle, using a state dynamics model to estimate the friction coefficient, which includes using a pitch-rate sensor to cancel out accelerometer contamination and estimate vehicle speed over ground, and employing a brush model to select the best fit data for improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If slip-force based estimation is used, then friction can be estimated during aggressive maneuvers, but estimation becomes very difficult when all wheels experience longitudinal forces during braking or all-wheel drive
Solution Approach 1:
The patent replaces traditional mechanical slip measurement methods with a state dynamics model approach that uses readily available sensor data (wheel speeds, vehicle acceleration) to calculate friction coefficients. This substitution eliminates the need for direct slip measurement while providing continuous friction estimates even during braking and all-wheel drive conditions where traditional slip-based methods fail.
2Measurement precision
If vehicle speed information is used for friction coefficient estimation, then accuracy can be improved, but vehicle speed becomes very hard to estimate when all wheels are braked or propelled due to high wheel slip
Solution Approach 1:
The patent introduces a state dynamics model as an intermediary that processes readily available sensor measurements (wheel rotational velocities and vehicle longitudinal acceleration) to indirectly determine vehicle longitudinal velocity and pitch angle. This intermediary approach allows accurate friction coefficient estimation without requiring direct vehicle speed measurement, even during braking or propulsion when traditional speed sensing fails.
3Measurement precision
If dedicated sensors are used for friction measurement, then measurement accuracy can be improved, but system cost and complexity increase
Solution Approach 1:
The patent makes existing vehicle sensors serve multiple functions: wheel speed sensors provide data for both ABS control and friction estimation, while accelerometers provide both vehicle dynamics control data and friction calculation inputs. This multi-functionality approach enables accurate friction measurement without adding dedicated sensors, reducing system complexity and cost while maintaining measurement precision.
Data Source
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AI summary
There is provided a method for estimating friction between a tire of a vehicle and a road surface, the method comprising acquiring, a front wheel axle torque, a rear wheel axle torque, a vehicle longitudinal acceleration, a vehicle pitch rate and wheel rotational velocities. The method further comprises determining a front wheel normal force and a rear wheel normal force, based on a center of gravity of the vehicle and the longitudinal acceleration; determining a longitudinal tire stiffness, jointly determining a vehicle longitudinal velocity, based on the wheel rotational velocities and vehicle longitudinal acceleration, and a vehicle pitch angle relative to the horizontal plane based on the vehicle pitch rate; and determining a friction coefficient between tires and ground based on the front and rear wheel axle torque, the front wheel normal force and the joint estimation of pitch angle and vehicle longitudinal velocity. There is also provided a system for performing the described method.