Vehicle Road Friction Estimation via Tire Force Gradient
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Solution Overview
Problem
Current methods for estimating road surface friction between a tire and a vehicle are inadequate, particularly in low friction conditions, as they often result in uncontrollable sliding and instability, and are not independent of tire and road conditions.
Innovation Solution
A method involving the application of controllable torque to vehicle wheels to create a controlled slip, determining a gradient threshold to avoid sliding while maintaining accurate friction estimation, using a system with a torque applying unit, inertial measurement unit, and control unit to compute friction estimates based on tire force and slip gradients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If opposing torques are applied to wheels to generate slip for friction estimation, then friction estimate can be obtained without disturbing the driver, but uncontrollable sliding and instability occur in low friction conditions
Solution Approach 1:
The system continuously monitors the gradient of tire force with respect to slip and uses this feedback to determine when to compute friction estimates. By establishing a feedback loop that tracks tire behavior in real-time, the system can identify optimal moments when the gradient falls within safe thresholds, ensuring accurate measurement without triggering sliding conditions.
Solution Approach 2:
The patent changes the parameter being monitored from raw tire force or slip magnitude to the gradient of tire force with respect to slip. This parameter transformation allows the system to distinguish between beneficial slip (which generates measurable friction data) and harmful sliding (which causes instability), thereby resolving the contradiction between measurement accuracy and tire stability.
2Measurement precision
If larger slip angle is used to maintain same lateral force in low friction conditions, then friction estimation can be performed, but tire sliding and loss of control occur
Solution Approach 1:
The patent replaces direct mechanical observation of slip angle with a mathematical derivative approach - computing the gradient of tire force with respect to slip. This substitution allows the system to infer friction characteristics through rate-of-change analysis rather than relying on absolute slip magnitude, enabling accurate measurement while avoiding the mechanical threshold that triggers sliding.
3Loss of information
If controllable torque is applied to create slip for friction estimation, then friction data can be collected, but the driver experiences disturbance and vehicle instability
Solution Approach 1:
The system applies only the minimal necessary torque to generate sufficient slip for accurate gradient calculation, rather than applying fixed or excessive torque. By using partial action - just enough excitation to obtain reliable friction data - the system minimizes driver disturbance while still acquiring the necessary information for friction estimation.
Data Source
AI summary
The present disclosure relates to a method for computing a friction estimate between a road surface and a tire of a vehicle. The method comprises applying a controllable torque to at least one of wheel of the vehicle and determining a vehicle velocity when the controllable torque is applied. Estimate a present tire force acting on the first tire when the torque is applied. Based on a vehicle velocity, estimate the present slip of the tire. Determining a present gradient of the tire force with respect to the slip based on the present tire force and the value indicative of the present slip. When the present gradient exceeds a predetermined first gradient threshold value and is below predetermined second gradient threshold value, compute the friction estimate.