Electric Power Steering Inverter Dead Time Compensation
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Solution Overview
Problem
Conventional electric power steering apparatuses face issues with current waveform distortion and torque ripple due to inverter dead time, leading to poor steering feel and increased noise and vibration, especially in low and middle speed regions, where conventional dead time compensation methods require tuning operations and are not effective for non-rectangular waveforms.
Innovation Solution
The electric power steering apparatus calculates dead time compensation values based on the motor rotational angle and feeds them forward to dq-axes or 3-phase voltage command values, eliminating the need for tuning operations and improving current waveform distortion and control responsibility, while adjusting compensation magnitude and direction with steering-assist command values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional dead time compensation methods are used, then the inverter dead time is compensated, but the current waveform distortion and torque ripple are not effectively suppressed, especially in low and middle speed regions
Solution Approach 1:
The patent applies preliminary action by calculating dead time compensation values in advance based on the motor rotational angle and storing them in a reference table. This pre-calculated compensation data is then directly applied to the voltage command values, allowing the system to proactively compensate for dead time effects before they cause waveform distortion and torque ripple, particularly in low and middle speed regions where conventional methods fail.
2Reliability
If conventional dead time compensation is applied, then the compensation is performed, but tuning operations are required and the method is not effective for non-rectangular waveforms
Solution Approach 1:
The patent changes the parameter basis for dead time compensation from fixed or waveform-type dependent values to values dynamically calculated based on the motor rotational angle. By creating a reference table that maps rotational angles to compensation values, the system adapts the compensation magnitude and direction according to the actual operating conditions, eliminating the need for manual tuning operations and making the method effective for non-rectangular waveforms.
3Reliability
If the inverter dead time is increased for fail-safe operation, then the safety is improved, but the current waveform distortion increases and steering sound is generated
Solution Approach 1:
The patent converts the harmful effect of increased dead time (which causes waveform distortion and steering sound) into a benefit by calculating compensation values that account for the actual dead time duration. The compensation reference table is specifically designed to counteract the effects of the increased dead time, transforming the safety-critical parameter into a controllable variable that can be compensated for, thereby maintaining both fail-safe operation and low noise performance.
4Measurement precision
If feed-back control of motor current is performed, then the steering assist torque is accurately generated, but the steering sound and vibration occur in low and middle speed regions
Solution Approach 1:
The patent applies preliminary action by incorporating dead time compensation into the voltage command values before they are applied to the motor. This pre-compensation approach addresses the root cause of steering sound and vibration (inverter dead time effects) before they manifest in the current waveform, allowing feed-back control to maintain accurate steering assist torque generation without the harmful side effects in low and middle speed regions.
Data Source
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AI summary
[Problem] An object of the present invention is to provide an electric power steering apparatus of a vector control system that compensates a dead time of an inverter without tuning operation, improves a distortion of a current waveform and a responsibility of a current control, and suppresses a sound, a vibration and a ripple. [Means for solving the problem] The present invention is the electric power steering apparatus of the vector control system that calculates dq-axes steering-assist command values based on at least a steering torque, calculates dq-axes current command values from the dq-axes steering-assist command values, converts the dq-axes current command values into 3-phase duty command values, driving-controls a 3-phase brushless motor by an inverter of a PWM control, and applies an assist torque to a steering system of a vehicle, wherein dead time reference compensation values are calculated based on a motor rotational angle, wherein dead time compensation of the inverter is performed by adding dead time compensation values in which the dead time reference compensation values are processed by using a gain, a sign and the like, to dq-axes voltage command values or to 3-phase voltage command values.