Electric Power Steering Motor Neutral Point Correction
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Solution Overview
Problem
In electric power steering systems with a variable gear ratio, the conventional method for determining the motor neutral point can result in deviations from the actual steering angle, leading to inaccurate detection of the steering angle based on motor angle detection values.
Innovation Solution
An electric power steering system with a rack and pinion mechanism and a control device that computes the motor neutral point using a motor angle detection value and a conversion value derived from the steering angle detection value, taking into account the variable steering gear ratio, ensuring accurate absolute steering angle detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a variable gear ratio is employed in the EPS with a conversion mechanism other than rack and pinion, then steering feel is improved, but the motor neutral point calculation becomes inaccurate due to steering angle deviations
Solution Approach 1:
The system performs preliminary detection of the actual steering angle using the steering sensor before calculating the motor neutral point. This preliminary action ensures that the conversion value is computed based on the true steering angle rather than an assumed neutral position, thereby resolving the inaccuracy caused by variable gear ratio effects.
Solution Approach 2:
The steering sensor provides feedback on the actual steering angle to the control device. This feedback loop enables the system to continuously update the conversion value based on real steering angle information, compensating for the variable gear ratio and maintaining accurate motor neutral point calculation throughout the steering range.
2Device complexity
If the motor neutral point is obtained by conventional method using constant coefficient, then calculation is simplified, but steering angle detection accuracy deteriorates at large steering angles
Solution Approach 1:
The system transitions from a static constant coefficient to a dynamic conversion value that varies with steering angle. The control device adjusts the conversion value based on the detected steering angle, allowing the calculation to adapt to changing steering conditions while maintaining accuracy across the full steering range.
Solution Approach 2:
The conversion value is changed as a parameter based on the steering angle detection. Instead of using a fixed constant, the system modifies the conversion parameter dynamically according to the actual steering angle, ensuring accurate motor neutral point calculation regardless of the steering position or variable gear ratio effects.
Data Source
Figure 1
Figure 2A~2B
AI summary
An EPS (1) includes a first rack and pinion mechanism (11) that converts rotation of a pinion shaft (9) due to a steering operation, to reciprocating motion of a rack shaft (5); and a steering force assist device (21) that applies a motor torque of a motor (23) as an assist force by using a second rack and pinion mechanism (27); and an ECU (22) that controls an operation of the steering force assist device (21). Rack teeth (5a) of the rack shaft (5) constituting the first rack and pinion mechanism (11) are formed so that a steering gear ratio is changed depending on a steering angle. The ECU (22) computes a motor neutral point by subtracting a conversion value from a motor angle detection value. The conversion value is computed based on a steering angle detection value by referring to a conversion map that is set taking into account the steering gear ratio corresponding to the steering angle.