Electric Power Steering Wheel Return Control
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Solution Overview
Problem
Conventional electric power steering systems face challenges in smoothly returning the steering wheel to a neutral position, especially at low vehicle speeds due to high friction and at high speeds due to inertia, leading to unstable vehicle characteristics and driver discomfort.
Innovation Solution
An electric power steering apparatus that calculates a target returning torque based on steering angle and vehicle speed, corrects it with steering torque and assist torque, and performs P, I, and D control calculations to generate a steering wheel returning control current, effectively managing the steering system's virtual inertia and viscosity coefficients to achieve smooth wheel return without driver discomfort.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional feedback control is used to generate assist torque, then the steering system can provide basic steering assistance, but the steering wheel cannot smoothly return to neutral position at low vehicle speeds due to high friction
Solution Approach 1:
The control device changes control parameters dynamically based on vehicle speed. At low speeds, it applies a steering wheel returning control current to overcome friction and enable smooth return to neutral. At high speeds, it switches to conventional feedback control to maintain stability. This parameter adaptation resolves the contradiction between return smoothness and system stability.
Solution Approach 2:
The system transitions from static feedback control to dynamic control that adapts to operating conditions. The control device actively calculates target steering angle speed and generates returning control current when needed, making the system responsive to real-time conditions rather than relying on fixed friction compensation characteristics.
2Reliability
If conventional feedback control is used to generate assist torque, then the steering system can provide basic steering assistance, but the steering wheel return becomes unstable at high vehicle speeds due to inertia
Solution Approach 1:
The system dynamically adjusts control strategy based on vehicle speed. At high speeds where inertia dominates, it maintains conventional feedback control to ensure stability while avoiding excessive returning force that would cause oscillation. The dynamic switching between control modes resolves the contradiction between stability and return smoothness.
Solution Approach 2:
The control parameters are changed according to vehicle speed conditions. The control device determines whether to apply steering wheel returning control current based on speed thresholds, changing the system behavior from active returning assistance at low speeds to passive stability maintenance at high speeds.
3Ease of operation
If a steering wheel returning control current is applied at all speeds, then the steering wheel can return smoothly at low speeds, but the steering system becomes overly complex and energy-consuming
Solution Approach 1:
The control system is segmented into different operational modes based on vehicle speed. The control device divides the operating range into low-speed and high-speed zones, applying steering wheel returning control current only in the low-speed zone where it is needed to overcome friction. This segmentation avoids unnecessary complexity in the high-speed zone.
Solution Approach 2:
Instead of applying returning control current at all speeds (excessive action), the system applies it partially only when vehicle speed indicates friction is the dominant factor. This partial action provides the necessary assistance without adding unnecessary complexity or energy consumption.
4Ease of operation
If friction compensation characteristic is increased to improve steering wheel return, then the steering wheel can return more smoothly, but the steering system becomes unstable due to inertia at high speeds
Solution Approach 1:
The friction compensation characteristic is made dynamic rather than fixed. The control device adjusts the level of returning control current based on vehicle speed, providing high compensation at low speeds for smooth return while reducing or eliminating it at high speeds to maintain stability. This dynamic adjustment resolves the contradiction between return smoothness and stability.
Solution Approach 2:
The control parameter for friction compensation is changed according to operating conditions. The system uses vehicle speed as a parameter to determine the appropriate level of returning control current, transitioning from high compensation at low speeds to low or zero compensation at high speeds, thereby maintaining both return smoothness and stability across the operating range.
Data Source
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AI summary
[Problem] An object of the present invention is to provide an electric power steering apparatus that is capable of realizing a smooth steering wheel returning control by the correction based on a steering torque in which vehicle characteristics are considered and a vehicle speed even in the case of steering intervention by a driver in a straight-advancing running state. [Means for solving the problem] An electric power steering apparatus comprising a steering wheel returning control section for driving a motor by means of a compensation current command value that comprises a target returning torque calculating section which calculates a target returning torque based on a steering angle and a vehicle speed, a correcting section which corrects the target returning torque with a steering torque and an assist torque, a steering system characteristic section which multiplies by a transmission characteristic corresponding to a virtual steering system characteristic based on a correction torque and a viscosity coefficient, a steering wheel returning control gain calculating section which multiplies a deviation between a target steering angle speed and an actual steering angle speed from the steering system characteristic section by a vehicle speed gain and a steering torque gain to obtain a steering wheel returning control gain, and a steering wheel returning control current calculating section which performs control calculation with respect to the steering wheel returning control gain, performs an output limit in response to the vehicle speed gain and the steering torque gain, and obtain a steering wheel returning control current.