Two-Motor Electric Power Steering Torque Control
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Solution Overview
Problem
Electric power steering apparatuses experience a point of inflection in steering torque due to the transition from static to dynamic friction in the worm drive, leading to nonlinear steering force and deteriorated steering feel at the early stage of steering.
Innovation Solution
A two-motor configuration is employed, where the first motor generates torque in the direction of the steering operation, and the second motor, activated after detecting the rotation of the first motor, generates a smaller torque in the opposite direction to counteract the friction torque variation during the transition from static to dynamic friction, thereby reducing the point of inflection in steering torque.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a worm drive is used to provide large assist torque, then the assist torque capability is improved, but the steering feel deteriorates due to friction torque variation during transition from static to dynamic friction
Solution Approach 1:
The assist torque generation is segmented into two phases: a first period where the first motor generates assist torque alone, and a second period where both the first and second motors generate assist torque. This segmentation allows the system to address the friction torque variation issue by introducing a second motor that compensates for the friction changes, while maintaining the high assist torque capability provided by the worm drive mechanism.
Solution Approach 2:
The second motor acts as an intermediary that compensates for the friction torque variation in the worm drive. By generating additional assist torque in the second period, the second motor mediates the transition from static to dynamic friction, smoothing out the torque characteristics and improving steering feel without sacrificing the overall assist torque capability.
2Ease of operation
If control is used to change steering force characteristics model, then the steering feel at early stage is improved, but the friction torque transition effects cannot be eliminated
Solution Approach 1:
The second motor serves as an intermediary that directly compensates for the friction torque transition effects. By detecting the rotation of the first motor and activating the second motor accordingly, the system can counteract the friction changes in the worm drive, ensuring consistent steering torque characteristics throughout the transition from static to dynamic friction.
Solution Approach 2:
The control system uses feedback from the first motor's rotation detection to determine when to activate the second motor. This feedback mechanism ensures that the second motor is engaged at the appropriate moment to compensate for friction torque variation, maintaining reliable and consistent steering torque characteristics during the transition phase.
3Device complexity
If a single motor is used for assist torque generation, then the device complexity is reduced, but the ability to compensate for friction torque variation is lost
Solution Approach 1:
The assist torque generation is segmented into two phases: a first period where the first motor generates assist torque alone, and a second period where both the first and second motors generate assist torque. This segmentation allows the system to address the friction torque variation issue by introducing a second motor that compensates for the friction changes, while maintaining the high assist torque capability provided by the worm drive mechanism.
Solution Approach 2:
The system dynamically adjusts the motor configuration based on the steering phase. During early-stage steering, both motors are activated to compensate for friction torque variation. During later stages, only the first motor is needed. This dynamic adjustment optimizes the balance between device complexity and steering torque smoothness.
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 configuration improves the steering feel at the early stage of steering by minimizing the resistance and stepped reduction in steering force, enhancing the overall quality of steering experience.
Implementation Method 1
a first motor and a second motor that give an assist torque to the steering mechanism according to a steering operation of the steering wheel
Implementation Method 2
The torque of the second motor is smaller than the torque of the first motor and opposite from the direction of the steering operation
Implementation Method 3
The friction torque is generated in a transmission mechanism that transmits the torque of the first motor to the steering mechanism
Implementation Method 4
a point of inflection in the steering torque before and after the assist torque is generated by the electric motor, resulting in a nonlinear steering force... caused by a reduction in a coefficient of friction at the engagement sections (sliding parts) of the worm drive when the assist torque is generated by the motor because the engagement sections transits from a static friction state to a dynamic friction state
Data Source
AI summary
An electric power steering apparatus includes steering mechanisms that direct left and right front wheels of a vehicle equipped with the electric power steering apparatus according to rotation of a steering shaft to which a steering wheel is coupled, a first motor and a second motor that give an assist torque to the steering mechanisms, and a controller that controls the assist torques generated by the first motor and the second motor. The controller causes the first motor to generate the torque in a direction of the steering operation when a generation of the assist torque is started. The controller causes the second motor to generate the torque after a rotation of the first motor is detected. The torque of the second motor is smaller than the torque of the first motor and opposite from the direction of the steering operation.