Electric Motor Control Device Reducing Current Detection Aliasing
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Solution Overview
Problem
Existing control devices for electric motors struggle to effectively reduce the influence of aliasing in current detection, particularly when the detection period matches harmonic components of the fundamental frequency, limiting the applicability of current feedback control.
Innovation Solution
A control device that includes a drive control unit, rotation angle detection sensor, and current detection sensor, utilizing a current processing unit to perform three-phase/two-phase conversion, average value calculation, difference calculation, low-pass filtering, and correction calculations to reduce aliasing effects, with the option to adjust the carrier period division number N based on the rotation speed or number of carrier periods per fundamental wave period.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If current detection is performed at a carrier period matching harmonic components, then detection timing is simplified, but aliasing occurs in the detected current value
Solution Approach 1:
The patent applies periodic action by performing current detection multiple times at different phases within the carrier period. Specifically, it detects current at N different timing points distributed throughout the carrier period, then averages these N detected values to obtain the final current value. This periodic multi-point detection approach eliminates aliasing while maintaining systematic detection timing.
Solution Approach 2:
The patent uses feedback by calculating the average of N periodically detected current values and using this averaged value for control purposes. The control device feeds back this averaged current value to the PWM control unit, which adjusts the inverter output based on the corrected current information, thereby eliminating the aliasing effect while maintaining closed-loop control.
2Measurement precision
If current detection is performed multiple times to reduce aliasing, then current value accuracy improves, but control complexity increases
Solution Approach 1:
The patent merges N individual current detection results by calculating their average value. Instead of treating each detection separately, the control device combines all N detected current values through averaging to produce a single corrected current value that eliminates aliasing. This merging approach reduces complexity compared to more sophisticated signal processing methods.
Solution Approach 2:
The patent changes the detection parameter by varying the detection timing points across N different phases within the carrier period. By systematically varying the detection timing parameter and then averaging the results, the system eliminates aliasing without requiring complex filtering or transformation algorithms, thus maintaining relatively simple control processing.
3Measurement precision
If average value of currents is used for control, then aliasing influence is reduced, but control responsiveness deteriorates
Solution Approach 1:
The patent applies partial action by performing current detection at N timing points within the carrier period, where N is typically 2 or 3. This partial multi-point detection provides sufficient aliasing reduction without requiring excessive detection points that would significantly increase processing time and reduce responsiveness. The averaged value from these limited detection points maintains both accuracy and responsiveness.
Data Source
AI summary
In a control device for an electric motor (101), a three-phase/two-phase conversion section (203) outputs N d-axis current values and N q-axis current values in each measurement period that is 1/N of a carrier period. An average value calculation section (204) calculates average values of those values. A difference calculation section (205) calculates a difference between a k-th d-axis current value and the average value of the d-axis current values as a d-axis current difference, and calculates a difference between a k-th q-axis current value and the average value of the q-axis current values as a q-axis current difference. A filtering section (206) performs low-pass filtering on each difference, and outputs d-axis and q-axis current correction values. A correction calculation section (207) performs a linear operation of each k-th current value and the corresponding current correction value, and outputs the corrected current values.