Electric Differential Torque Control for Split-Friction Wheel Slip
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Solution Overview
Problem
Existing differential drive arrangements for electric vehicles struggle to manage wheel slip effectively in varying friction conditions, leading to potential wheel lock and reduced maneuverability, especially in split friction scenarios.
Innovation Solution
A control unit with wheel slip control modules for each driven wheel determines obtainable torque based on current wheel state, including speed and friction conditions, and adjusts torque requests from the electric machine to prevent excessive slip, while service brakes are used to maintain wheel slip within configured limits, allowing for differential locking avoidance and improved traction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a differential allows the outer drive wheel to rotate faster than the inner drive wheel during a turn, then the vehicle's turning capability is improved, but the wheel shafts split wheel longitudinal forces equally which limits the force available from each wheel
Solution Approach 1:
The patent applies local quality by allowing each wheel to have different torque characteristics based on its individual friction conditions. The control system independently manages torque distribution to each wheel, enabling the high-friction wheel to deliver maximum force while the low-friction wheel operates within its limits, rather than forcing equal force distribution on both wheels.
2Productivity
If one wheel experiences lower friction than the other wheel, then it takes less torque to turn the low friction wheel, but the low friction wheel decides the longitudinal force possible to generate by both wheels which may not be ideal
Solution Approach 1:
The control system dynamically changes the torque parameter distributed to each wheel based on real-time friction condition detection. When split friction is detected, the system adjusts torque distribution to match each wheel's friction characteristics, preventing the low-friction wheel from limiting the overall force generation capability of the drive system.
3Power
If electric machines generate significant torque already at low speeds, then propulsion capability is improved, but severe wheel slip occurs if the requested torque is not carefully controlled
Solution Approach 1:
The patent implements feedback control by continuously monitoring wheel speed, vehicle speed, and friction conditions, then adjusting the torque requests to the electric machines accordingly. This closed-loop control prevents severe wheel slip by detecting slip conditions and reducing torque requests in real-time, while still allowing the electric machines to deliver their high torque capability when conditions permit.
Solution Approach 2:
The system dynamically adjusts torque distribution based on changing friction conditions detected during operation. The wheel slip control modules continuously update torque requests to electric machines based on current wheel state, allowing the propulsion system to adapt its power delivery characteristics to match instantaneous road conditions.
4Reliability
If service brakes are used to maintain wheel slip within configured limits, then wheel slip control is improved, but the system complexity increases
Solution Approach 1:
The control system achieves multi-functionality by using the service brakes not only for their primary braking function but also as an active component in wheel slip prevention during propulsion. This unified approach allows the same brake system to serve multiple purposes: normal braking, wheel slip control during acceleration, and torque vectoring assistance, thereby managing complexity through functional integration rather than adding separate dedicated slip control actuators.
Data Source
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AI summary
A control unit (110) for a heavy duty vehicle (100), the vehicle comprising an electric machine connected to first and second driven wheels via an differential, the control unit (110) comprising a first wheel slip control module associated with the first driven wheel, and a second wheel slip control module associated with the second driven wheel, where each wheel slip control module is arranged to determine an obtainable torque by the respective wheel based on a current wheel state, wherein the control unit (110) is arranged to determine a required torque to satisfy a requested acceleration profile by the vehicle (100), and to request a torque from the electrical machine corresponding to the smallest torque out of the obtainable torques for each driven wheel and the required torque.