Electric Power Steering Control for Torque Ripple Mitigation
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Solution Overview
Problem
Existing electric power steering devices face challenges in maintaining a consistent steering feel for drivers during component failures, such as short-circuit failures in switching elements, due to limited failure coping mechanisms in complex systems.
Innovation Solution
An electric power steering device with a control unit that includes inverter circuits with multiple switching elements, capable of independent control of motor windings, detects failures and employs field-weakening control with d-axis current restriction to maintain steering stability when one motor winding set fails, using a d-q-axis coordinate system for current command calculations and drive signal output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If field-weakening control is applied during failure operation, then motor speed range is extended, but torque ripple increases causing poor steering feeling
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the d-axis current command value based on the motor speed. Specifically, when motor speed exceeds a predetermined threshold during failure operation, the controller restricts the d-axis current command value to reduce torque ripple. This speed-dependent parameter adjustment allows the system to maintain both extended speed range and acceptable steering feeling by adapting control parameters to operating conditions.
2Ease of operation
If d-axis current is restricted to reduce torque ripple, then steering feeling is improved, but field-weakening control effectiveness is reduced
Solution Approach 1:
The patent implements dynamics by making the d-axis current restriction conditional rather than fixed. The controller dynamically determines whether to restrict d-axis current based on real-time motor speed feedback. When speed is below the threshold, full field-weakening control is applied for maximum speed range. When speed exceeds the threshold, restriction is applied to improve steering feeling. This dynamic adaptation resolves the contradiction by optimizing both parameters across different operating regions.
3Reliability
If multiple independent motor winding sets are provided for failure continuation, then system reliability is improved, but device complexity increases
Solution Approach 1:
The patent applies segmentation by dividing the motor into multiple independent winding sets (e.g., two three-phase winding sets: first and second windings). Each winding set can be independently controlled through separate inverter circuits. This segmentation enables failure continuation capability - when one winding set fails, the other can continue to provide steering assistance. The independent segmentation of windings and their associated control circuits directly implements the redundancy strategy while managing complexity through modular architecture.
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
Enables continued control with a satisfactory steering feel by detecting failures and restricting d-axis current based on torque ripples, ensuring stable operation even when one motor winding set is compromised.
Implementation Method 1
a motor configured to assist in controlling a driver's steering torque of a vehicle
Data Source
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AI summary
Provided is an electric power steering device capable of continuing control while ensuring a satisfactory level of a steering feeling of a driver at a time of occurrence of failure of one motor winding set. A control part is configured to detect a failure of each component of the electric power steering device, and when control is continued with field-weakening control by a normal motor winding and an inverter circuit depending on the detected failure, restrict a d-axis current based on a torque ripple that occurs in a steering wheel of a vehicle with respect to the field-weakening control of a state in which the failure has not occurred.