Motor Control Device Reducing Torque Ripple via Current Correction
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Solution Overview
Problem
Conventional electric power steering systems using one-shunt type current detectors face challenges in accurately measuring motor currents due to current transient responsibilities, leading to errors in average current detection, which results in operating sound degradation and torque ripples, especially at low speeds.
Innovation Solution
A motor control apparatus that employs a current detection correcting section to calculate and apply correction values based on power-supply voltage, back-EMF, PWM duty command values, and electric characteristic formulas to align detected currents with motor average currents, thereby reducing errors and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a one-shunt type current detector is used to reduce cost and weight, then manufacturing cost and device weight are reduced, but current detection precision deteriorates due to current transient responsibilities
Solution Approach 1:
The system performs preliminary actions by detecting current at specific timings (when all switching elements are OFF or when at least one upper switching element is ON) before the actual control calculation, and pre-calculates the average current value using the accumulated detection values. This preliminary detection and calculation approach compensates for the limitations of the one-shunt type detector, maintaining measurement precision while using the cost-effective detector type.
Solution Approach 2:
The patent introduces an intermediary mechanism - the microcomputer-based control system that accumulates current detection values at specific timings and calculates average current values through software processing. This intermediary calculation layer bridges the gap between the simplified one-shunt detection hardware and the requirement for precise current measurement, enabling accurate control despite hardware limitations.
2Adaptability or versatility
If current detection is performed at arbitrary timings in PWM control, then control flexibility is improved, but detection accuracy deteriorates due to current transient variations
Solution Approach 1:
The system applies local quality by selecting specific local timings (when all switching elements are OFF or when at least one upper switching element is ON) for current detection within the PWM cycle. Rather than using arbitrary timings, the detection is localized to these specific moments when the current state is more stable and representative, improving measurement precision while maintaining control flexibility through the microcomputer's timing control.
Solution Approach 2:
The system implements feedback by accumulating current detection values over multiple PWM cycles and calculating average current values. The detected current information is fed back to the control system, which uses this averaged data to adjust PWM duty ratios and switching patterns. This feedback mechanism compensates for transient variations and improves overall detection accuracy while maintaining control flexibility.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces motor operating noise and torque ripples while maintaining steering performance, enabling cost-cutting and weight savings through accurate current detection and correction.
Implementation Method 1
a one-shunt type current detector that detects motor phase currents
Implementation Method 2
a back-EMF of the motor
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
[Problem] An object of the present invention is to provide a motor control apparatus that performs current detection of a motor by a one-shunt type current detector, has less operating sound and reduces torque ripple and an electric power steering apparatus mounting the same. [Means for solving the problem] A motor control apparatus that drive-controls a motor by an inverter based on each-phase duty command values of PWM and detects each-phase motor currents of the motor by a one-shunt type current detector, comprises a current detection correcting section that calculates a current detection correction value based on a power-supply voltage of the motor, the each-phase duty command values, a back-EMF information of the motor, the each-phase motor currents detected by the current detector, an arrangement information of the PWM, and an electric characteristic formula of the motor, thereby to correct the each-phase motor currents detected by the current detector to a motor average current by the current detection correction value and drive-controls the motor.