EPAS Torque Limiter for Safe Restart
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Electric power assisted steering (EPAS) systems face challenges in safely restarting after an unexpected shutdown, particularly when the vehicle is moving, as sudden application of high assistance torque can alarm the driver, and existing speed-based reinstatement strategies may be slow due to reliance on speed sensors that are slow to initialize.
Innovation Solution
The EPAS system incorporates a torque limiter and controlled torque slew-up rate determined by the state of the system prior to restart, using flags and stored information to differentiate between normal and abnormal shutdowns, applying a zero or gradual torque limit and blending default speed with actual speed measurements to ensure safe and gradual torque application.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the motor applies high assistance torque immediately after restart, then the steering assistance is restored quickly, but the driver may become alarmed due to sudden torque application
Solution Approach 1:
The system performs preliminary classification of the shutdown state (normal vs abnormal) using stored information before restart, and pre-determines the appropriate torque reinstatement strategy. This allows the system to prepare the correct torque limit and slew-up rate in advance, enabling rapid appropriate response without causing driver alarm.
Solution Approach 2:
The torque limit and maximum rate of increase are made dynamic rather than fixed, adjusting based on the classified shutdown state. For abnormal shutdowns, a gradual torque increase is applied, while for normal shutdowns, faster torque restoration is permitted. This dynamic adaptation resolves the contradiction between speed and driver comfort.
2Measurement precision
If the system waits for speed sensor re-initialization before determining restart strategy, then accurate speed information is available, but the restart response is delayed
Solution Approach 1:
The system performs the shutdown state classification using stored information (flags, engine speed, vehicle speed, torque data) before restart occurs, rather than waiting for speed sensor re-initialization. This preliminary action enables immediate determination of the appropriate restart strategy, eliminating the time delay while ensuring accurate classification based on pre-shutdown conditions.
Solution Approach 2:
The system uses stored information (flags and parameters) as an intermediary to represent the shutdown state, allowing the controller to make restart decisions without directly relying on the slow-to-initialize speed sensor. This intermediary data enables rapid decision-making while maintaining accuracy.
3Device complexity
If a fixed torque limit is applied after restart, then the control logic is simple, but it cannot differentiate between normal and abnormal shutdown scenarios
Solution Approach 1:
Different torque limit and slew-up rate parameters are applied locally based on the specific shutdown scenario. Rather than a single fixed limit, the system has distinct parameter sets for normal shutdowns (faster restoration) and abnormal shutdowns (gradual restoration), allowing optimized control for each local condition without excessive overall complexity.
Solution Approach 2:
The torque limit and maximum rate of increase parameters are changed based on the shutdown state classification. The system transitions between different parameter values depending on whether the shutdown was normal or abnormal, enabling adaptability through parameter variation rather than complex control logic.
Data Source
AI summary
An electric power assisted steering apparatus for a vehicle, comprising:a steering mechanism, which operatively connects a steering wheel to road wheels of the vehicle;an electric motor operatively connected to the steering mechanism;a torque sensing means adapted to produce a first output signal indicative of the torque carried by a portion of the steering mechanism;a vehicle speed sensing means for producing a second output signal indicative of the speed of the vehicle;a signal processing means adapted to receive the first and second signals and to produce a torque demand signal representative of the torque to be applied to the steering mechanism by the motor; anda motor drive stage adapted to provide a drive current to the motor responsive to the torque demand signal;wherein the apparatus include at least one of the torque limiter arranged to limit the torque and the maximum rate of increase of the torque following a restart of the apparatus, the limit and rate being determined at least in part by information indicative of a state of the system prior to the restart.