Electric Power Steering Assist Current Limiting
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Solution Overview
Problem
Existing electric power steering systems fail to accurately limit assist current command values when steering torque is within small values, leading to potential self-assist issues due to incorrect abnormality detection, resulting in unexpected assist forces.
Innovation Solution
An electric power steering system with a control unit that sets individual limiting values for assist control amounts based on various state quantities, allowing for precise limiting and preventing unexpected assist forces by accounting for command values and compensation gains.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the assist current command value is strictly limited to zero when signs differ, then safety is improved, but steering behavior adjustment capability deteriorates when steering torque is small
Solution Approach 1:
The limiting value of the assist current command value is made dynamic by setting it to zero when steering torque exceeds the threshold (abnormality detected) and to a non-zero value when steering torque is within the threshold (no abnormality). This dynamic adjustment resolves the contradiction by adapting the safety limit to the actual operating conditions.
Solution Approach 2:
The invention changes the parameter of the assist current command value limit based on the steering torque magnitude. When steering torque is large, the limit is set to zero for safety; when steering torque is small, the limit is relaxed to preserve steering behavior adjustment capability through compensation amounts.
2Adaptability or versatility
If the assist current command value is not limited when steering torque is small, then steering behavior adjustment is maintained, but unexpected assist force may occur due to erroneous abnormal assist current computation
Solution Approach 1:
The invention dynamically changes the limiting parameter based on steering torque magnitude. When steering torque is within the threshold, the limit is set to a non-zero value to maintain steering behavior adjustment. When steering torque exceeds the threshold, the limit is set to zero to prevent unexpected assist forces from erroneous computations.
Solution Approach 2:
The invention introduces steering torque as an intermediary parameter to mediate between safety requirements and steering behavior adjustment needs. The steering torque magnitude determines whether the assist current command value should be strictly limited or allowed to vary for compensation purposes.
3Device complexity
If a fixed limiting value is applied to assist current command value, then abnormality detection is simplified, but precision in limiting deteriorates when compensation amounts vary
Solution Approach 1:
The invention changes the limiting parameter from a fixed value to a dynamic value based on steering torque. This allows the limit to adapt to varying compensation amounts while maintaining a clear abnormality detection criterion (sign coincidence check), thus preserving simplicity while improving precision.
Data Source
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AI summary
An electric power steering system includes a control unit (40) configured to: compute an assist control amount (current amount), based on multiple kinds of state quantities indicating a steering state; control a motor (31) based on the assist control amount, the motor (31) being a source of assist force applied to a steering system of a vehicle; individually set, for the respective state quantities, limiting values that limit a variation range of the assist control amount, based on the respective state quantities used to compute the assist control amount; limit a value of the assist control amount using the limiting values; acquire a command value generated by a host control unit (55) to change the assist control amount; and take the command value into account in setting of at least one of the limiting values, when executing control of the motor based on the assist control amount that reflects the command value.