Electric Power Steering Current Limiting During Engine Cranking
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Solution Overview
Problem
Existing electric power steering systems face challenges in maintaining optimal power supply voltage during engine cranking, leading to excessive current limitation and poor steering operation feelings due to the temporary decrease in power supply voltage, which is not adequately addressed by conventional current limitation techniques.
Innovation Solution
The system incorporates cranking state detection to selectively apply a cranking upper limit characteristic that relaxes current limitation compared to non-cranking conditions, allowing for appropriate current limitation during engine cranking and non-cranking scenarios, thereby improving steering operation feelings and maintaining power supply voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional current limitation technique is applied during engine cranking, then power supply voltage can be maintained, but steering assist is excessively limited and steering operation feeling deteriorates
Solution Approach 1:
The patent applies dynamics by making the current upper limit value adjustable based on operating conditions. Specifically, the system dynamically switches between a first current upper limit value during engine cranking and a second current upper limit value during normal operation. This dynamic adjustment allows the system to maintain appropriate steering assist characteristics for each operating state, resolving the contradiction between voltage maintenance and steering feel.
Solution Approach 2:
The patent changes the parameter of current upper limit value based on the engine operating state. During engine cranking, a first current upper limit value is applied that allows greater current flow than the second current upper limit value used during normal operation. This parameter change enables the system to tolerate temporary voltage fluctuations during cranking while maintaining strict voltage control during normal operation, thereby improving steering operation feeling without compromising power supply stability.
2Reliability
If current upper limit value is decreased to maintain power supply voltage, then voltage stability is improved, but steering assist torque is reduced
Solution Approach 1:
The system dynamically adjusts the current upper limit value based on engine operating state. During engine cranking, the first current upper limit value is applied which permits higher current and thus higher steering assist torque. During normal operation, the second current upper limit value is applied to maintain voltage stability. This dynamic switching resolves the contradiction by applying appropriate current limits for each operational context.
Solution Approach 2:
The patent changes the current upper limit parameter according to engine state. The control unit stores and switches between different current upper limit values: a first value during cranking that allows sufficient power for steering assist, and a second value during normal operation that prioritizes voltage stability. This parameter adaptation resolves the trade-off between power output and voltage stability.
3Reliability
If hysteresis is set in upper limit characteristic to prevent hunting phenomenon, then power supply voltage stability is improved, but steering assist response becomes delayed
Solution Approach 1:
The patent segments the control strategy by creating separate current upper limit characteristics for different engine operating states. Instead of using a single hysteresis-based characteristic that delays response, the system implements distinct characteristics: one for engine cranking and another for normal operation. This segmentation eliminates the unnecessary response delay during cranking while maintaining voltage stability during normal operation through the hysteresis mechanism.
Solution Approach 2:
The patent changes the current upper limit characteristic parameters based on engine state. During engine cranking, a first characteristic without restrictive hysteresis effects is applied, enabling fast steering assist response. During normal operation, a second characteristic with hysteresis is applied to prevent hunting phenomenon and maintain voltage stability. This parameter adaptation resolves the contradiction between response speed and voltage stability.
Data Source
AI summary
A characteristic selection/current upper limit value calculation section 64 selects a cranking upper limit characteristic map MP2 in the case where a cranking state is detected by a cranking state supposition section 63, and selects a non-cranking upper limit characteristic map MP1 in the case where the cranking state is not detected. Upper limit limitation on a current caused to pass through a motor 20 is relaxed in a cranking upper limit characteristic as compared with a non-cranking upper limit characteristic. Consequently, it is possible to prevent excessive current limitation during cranking.