EPAS Steering Angle Recovery After Power Loss
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Solution Overview
Problem
Existing electric power assisted steering (EPAS) systems face challenges in maintaining accurate absolute steering angles without a reference position sensor, particularly when power is lost, leading to ambiguity and increased system costs due to the need for high-resolution sensors and index markers.
Innovation Solution
Combining a low-resolution Hall-effect sensor with an inductive sensor within the electric motor, where the Hall-state transitions and inductive sensor pulses are counted relative to a factory-calibrated center position, allowing for accurate steering angle maintenance without an index marker, and dynamically recovering the center steering position using vehicle stability control systems after power restoration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high-resolution angular position sensor is used to accurately determine steering angle, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent divides the position sensing function into two segments: a high-resolution sensor (e.g., Hall effect sensor) mounted on the steering shaft for accurate absolute position detection, and a motor position sensor for tracking motor rotor position. This segmentation allows each sensor to operate at optimal resolution for its specific function, achieving overall high measurement precision without requiring a single high-resolution sensor for the entire system.
Solution Approach 2:
The patent combines information from multiple sensors (steering shaft position sensor and motor position sensor) to determine the absolute steering angle. By merging the high-resolution absolute position data from the steering shaft sensor with the motor position data, the system achieves accurate steering angle measurement while avoiding the need for a single complex high-resolution encoder on the motor.
2Reliability
If an index marker and reference position sensor are used to restore absolute position after power loss, then reliability is improved, but device complexity and manufacturing cost increase
Solution Approach 1:
The patent implements a self-service mechanism where the microcontroller automatically detects power restoration and initiates the absolute position recovery process without external intervention. The system uses the stored calibration data and current sensor readings to automatically recalculate the steering angle offset, eliminating the need for manual calibration procedures or physical index markers.
Solution Approach 2:
The patent performs preliminary calibration during system initialization or manufacturing, storing the relationship between motor position and steering angle in non-volatile memory. This preliminary action allows the system to quickly restore absolute position accuracy after power loss by retrieving stored calibration data and applying it to the current sensor readings, avoiding the need for physical reference markers or complex recovery procedures.
3Measurement precision
If the motor position sensor operates continuously to track steering angle, then measurement precision is maintained, but energy consumption increases
Solution Approach 1:
The patent implements periodic sampling of the motor position sensor at controlled intervals during motor operation, rather than continuous monitoring. The microcontroller reads the motor position sensor at specific moments when the motor is actively moving the steering system, which is sufficient to track steering angle changes while significantly reducing energy consumption compared to continuous operation.
Solution Approach 2:
The patent maintains continuous useful action by using the motor position sensor data throughout the entire motor operation cycle to update the steering angle calculation. Even though the sensor is not continuously powered, the useful information is captured continuously during motor operation, ensuring no loss of steering angle tracking accuracy while minimizing energy consumption during idle periods.
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 solution enables accurate and cost-effective maintenance of absolute steering angles without the need for high-resolution sensors or index sensors, ensuring reliable steering angle determination even after power loss, by using a combination of Hall-effect and inductive sensors with nonvolatile memory to store and recover the calibrated center position.
Implementation Method 1
The motor position sensor typically comprises an electromagnetic type switch or switches which change state whenever a magnet provided on the rotor passes the sensor. Alternatively, a magnetized disc can be mounted on the rotor shaft and the sensor may detect movement of the magnets on the disc.
Implementation Method 2
In a 3-phase brushless permanent magnet motor, for example, three Hall effect sensors can be located around the rotor in such a manner that a rough measurement of rotor electrical position can be obtained.
Data Source
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AI summary
The steering angle of a vehicle is monitored using position sensors of an electric motor of an electric power assisted steering (EPAS) system. A position of the electric motor corresponding to the straight-ahead, center position of the steering system is stored in non-volatile memory during a steering calibration procedure, such as an end-of-line calibration in a vehicle assembly plant. Following power loss due to a dead battery, a steering angle zeroing procedure performed in a vehicle stability control (VSC) system generates a center position with enough accuracy to be within one electrical cycle of the motor. The pre-stored electric motor position is then used to determine the electrical cycle where the center position was located, and accurate monitoring of steering angle is resumed.