Electric Power Steering Controller Limiting Assist Current
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The existing electric power steering system (EPS) fails to limit the assist current command value to zero when steering torque is within a range of small values, leading to potential unintended assist forces and self-steering issues due to incorrect computation of assist current command values.
Innovation Solution
An EPS with a controller that individually sets limits for state quantities to control the assist controlled variable, allowing precise limiting of the assist force and preventing unintended assist forces by computing and limiting the assist controlled variable based on multiple state quantities, including steering torque, torque derivative, steering angle, angular velocity, and angular acceleration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the assist current command value is strictly limited to zero when the steering torque is within a small value range, then safety is improved, but the steering behavior adjustment capability deteriorates
Solution Approach 1:
The patent applies different limiting strategies to different ranges of steering torque. When steering torque is large, the assist current command value is strictly limited to zero to ensure safety. When steering torque is small, the assist current command value is limited within a margin range that allows compensation amount to prevent self-steer while maintaining steering behavior adjustment capability. This local differentiation resolves the contradiction between safety and steering behavior adjustment.
2Ease of operation
If the assist current command value is not strictly limited when the steering torque is within a small value range, then steering behavior adjustment is maintained, but unintended assist force and self-steer may occur
Solution Approach 1:
The patent changes the limiting parameter (assist current command value) based on the steering torque range. For small steering torque values, instead of strict zero limitation, a margin-based limitation is applied that allows the compensation amount to function properly while still preventing excessive assist current that could cause self-steer. This parameter adjustment resolves the contradiction between maintaining steering behavior adjustment and preventing unintended assist force.
3Device complexity
If a single limit value is applied to the assist current command value, then control simplicity is maintained, but control precision deteriorates
Solution Approach 1:
The patent implements dynamic limiting where the limit value for the assist current command value is not fixed but varies based on the steering torque magnitude. When steering torque is large, the limit is set to zero. When steering torque is small, the limit is set to a margin value that allows compensation. This dynamic adjustment of the limit parameter improves control precision while maintaining reasonable control simplicity through a clear decision rule based on steering torque threshold.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
There is provided an electric power steering system that is able to generate a more appropriate current command value. Limits of an assist controlled variable, formed of multiple upper limits and multiple lower limits (upper limits IUL1* to IUL5* and lower limits ILL1* to ILL5*) are individually set for multiple signals (τ, dτ, θs, ωs, αs) used to compute the assist controlled variable. The values obtained by adding the limits together are set as final limits of the assist controlled variable, that is, an upper limit IUL* and a lower limit ILL*. Thus, even if the assist controlled variable exhibiting an abnormal value is computed for some reason, the abnormal value of the assist controlled variable is limited to an appropriate value corresponding to each signal value by the final limits. By supplying the appropriate assist controlled variable to a motor control signal generating unit as a final current command value, an appropriate assist force is applied to a steering system.