Electric Power Steering Assist Control Restriction
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Solution Overview
Problem
The existing electric power steering systems (EPS) face challenges in applying appropriate assist forces when steering torque is in a small range, leading to potential self-assist issues due to incorrect computation of assist current command values.
Innovation Solution
An EPS with a control device that computes assist control amounts based on various state amounts, applies restriction values to these amounts, and transitions between primary and secondary assist control amounts based on elapsed time and internal states to ensure accurate assist force application.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the assist current command value is restricted to zero when steering torque is small, then safety is improved, but steering behavior adjustment capability is lost
Solution Approach 1:
The patent applies different restriction strategies based on the magnitude of steering torque. When steering torque is large, the assist current command value is restricted to zero. When steering torque is small, the assist current command value is restricted within a margin that allows compensation amount to be effective. This local differentiation resolves the contradiction by adapting the restriction level to the specific operating condition.
2Ease of operation
If the assist current command value is restricted within a margin when steering torque is small, then steering behavior adjustment is maintained, but unintentional assist force may occur
Solution Approach 1:
The patent uses feedback control to monitor the assist current command value and steering torque relationship. When the assist current command value deviates abnormally from the expected range based on steering torque, the system detects this through feedback and applies appropriate restriction. This feedback mechanism maintains steering behavior adjustment capability while preventing unintentional assist force.
3Ease of operation
If the assist current command value is not restricted when steering torque is small, then steering behavior adjustment is maintained, but self-assist phenomenon occurs
Solution Approach 1:
The patent applies preliminary anti-action by proactively restricting the assist current command value within an appropriate margin before abnormal assist force can occur. This preventive restriction is applied based on the relationship between steering torque and compensation amount, preventing the self-assist phenomenon while maintaining necessary steering behavior adjustment capability.
Data Source
AI summary
Restriction values (upper limit and lower limit) for an assist control amount are set individually for each of state amounts including steering torque τ used to compute the assist control amount. A value obtained by summing such restriction values is set as a final restriction value for the assist control amount. Even though the assist control amount with an abnormal value is computed, the assist control amount is restricted to an appropriate value by the final restriction value. In addition, a transition is made from a primary assist control amount which is restricted to a secondary assist control amount computed separately from the primary assist control amount at the timing when a certain time elapses since the assist control amount is restricted. As the steering torque τ is increased, the speed of transition from the primary assist control amount to the secondary assist control amount is increased.