Electric Power Steering D-Q Current Limiting for High Speed Followability
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Solution Overview
Problem
Conventional electric power steering control apparatuses experience a drop in assist torque and followability at high motor rotation speeds due to current limitations, where both d-axis and q-axis current command values are limited at the same rate, leading to insufficient d-axis current command values and reduced steering wheel followability.
Innovation Solution
An electric power steering control apparatus and method that prioritize d-axis current limiting over q-axis current limiting by calculating a d-axis current limiting value based on a dq-converted current limiting value, allowing the d-axis current command value to be clipped to this value or less, and subsequently calculating a q-axis current limiting value based on the limited d-axis current command value, ensuring the d-axis current command value is maintained at a higher level than the q-axis current command value.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If both d-axis and q-axis current command values are limited at the same rate, then the maximum assist torque is maintained at high rotation speeds, but the steering wheel followability deteriorates due to insufficient d-axis current command values
Solution Approach 1:
The current limiting process is segmented into two distinct stages: first limiting the d-axis current command value based on the dq-converted current limiting value, then limiting the q-axis current command value based on the remaining current capacity. This segmentation allows differential treatment of d-axis and q-axis currents, prioritizing d-axis current for followability while still managing overall power output.
Solution Approach 2:
The patent maintains continuous current command output to the inverter controller by implementing a sequential limiting process rather than a binary on/off approach. The d-axis current limiter and q-axis current limiter work in continuous sequence to provide smooth, uninterrupted current commands that maintain steering assist functionality across the full rotation speed range.
2Reliability
If the dq-converted current limiting value is applied equally to both axes, then the motor current is protected from overload, but the d-axis current command value becomes insufficient at high rotation speeds
Solution Approach 1:
The patent applies different quality characteristics to d-axis and q-axis current limiting. The d-axis current limiter uses the full dq-converted current limiting value as its basis, giving d-axis current higher priority. The q-axis current limiter then uses the remaining current capacity (square root of the difference between squared dq-limiting value and squared limited d-axis current). This local differentiation ensures motor protection while optimizing high-speed followability.
Data Source
AI summary
A main controller includes: a d-axis current limiter configured to calculate a d-axis current limiting value with priority, based on a dq-converted current limiting value, and calculate a limited d-axis current command value by clipping the d-axis current command value to be the d-axis current limiting value or less; a q-axis current limiter configured to calculate a q-axis current limiting value, based on the dq-converted current limiting value and the limited d-axis current command value, and calculate a limited q-axis current command value by clipping the q-axis current command value to be the q-axis current limiting value or less; and an inverter controller configured to control motor current, based on the limited d-axis current command value and the limited q-axis current command value.