Electric Drive Friction Estimation Using Periodic Wheel Torque

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Solution Overview

Problem

Existing systems for estimating the coefficient of friction for electric vehicles are inefficient, relying on additional sensors that increase costs and cannot differentiate between all road surfaces, and methods like test braking cause wear and are not energy-neutral.

Innovation Solution

A method using an electric drive to estimate the coefficient of friction by operating a wheel with a torque, applying a periodic excitation torque, and measuring the change in slip to determine the coefficient of friction without additional sensors.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If additional sensors (camera systems, moisture sensors, noise sensors) are used to estimate the coefficient of friction, then measurement capability is improved, but device complexity and cost increase

Engineering Contradiction:

Improvecoefficient of friction estimationVSAvoidsensor system

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The electric drive system performs self-diagnosis by using its own operational data (torque, speed, slip) to estimate the coefficient of friction. The system serves its own measurement needs without requiring external sensors, thereby reducing device complexity and cost while maintaining measurement capability.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The patent replaces mechanical/optical/acoustic sensor systems with an electrical control system that uses the existing electric drive infrastructure. By substituting physical sensors with electrical signal processing of drive parameters, the system achieves coefficient of friction estimation without adding mechanical complexity.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Measurement precision

If test braking is performed to ascertain the coefficient of friction, then measurement accuracy is improved, but harmful factors (brake and tire wear, energy consumption) increase

Engineering Contradiction:

Improvecoefficient of friction measurementVSAvoidbrake and tire wear

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The system uses the normal operational slip data from the electric drive during regular vehicle operation to estimate the coefficient of friction. No separate test braking maneuver is required, as the drive system continuously monitors its own performance parameters under normal driving conditions.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

The patent converts the normally harmful effect of wheel slip (which causes energy loss and wear) into a useful measurement signal. By analyzing the slip that occurs during regular operation, the system extracts coefficient of friction information without requiring additional harmful test maneuvers.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Ease of operation

If camera-based systems are used to estimate the coefficient of friction, then non-contact measurement is achieved, but reliability decreases due to inability to differentiate between all road surfaces

Engineering Contradiction:

Improvenon-contact measurementVSAvoidroad surface differentiation

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The patent replaces optical camera-based measurement with electrical signal analysis from the drive system. By using electrical parameters (torque, speed, slip) rather than optical images, the system achieves more reliable differentiation between road surfaces based on their actual friction characteristics during vehicle operation.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The system uses feedback from the drive system's actual performance (torque application, speed response, slip occurrence) to continuously estimate the coefficient of friction. This closed-loop approach provides reliable real-time information about road surface conditions based on actual vehicle-response data rather than visual estimation.

Inventive Principle:

Principle #23Feedback

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 method allows for reliable and cost-effective estimation of the road surface coefficient of friction, improving the performance of automated driving functions and energy efficiency in electric vehicles.

Implementation Method 1

A method using an electric drive to estimate the coefficient of friction by operating a wheel with a torque, applying a periodic excitation torque

Methodology Applied

Scientific EffectTorque: Torque

Data Source

PatentUS20250065884A1Method for estimating coefficients of friction, computer program, controller, vehicle, in particular utility vehicle

Publication Date: 2025.02.27 ZF FRIEDRICHSHAFEN AG

AI summary

A method is for estimating coefficients of friction for a vehicle, in particular utility vehicle, which can be driven by an electric drive. The method includes the steps of: operating, with a torque, a wheel of the vehicle, in particular utility vehicle, that is arranged on an underlying surface; ascertaining the slip of the wheel; applying a temporally predetermined excitation torque to the wheel, wherein the excitation torque is applied to the wheel periodically with a frequency; ascertaining a change in slip depending on the excitation torque, wherein the change in slip is ascertained taking into account the frequency; and ascertaining a coefficient of friction on the basis of the change in slip.