Shunt-Based Metrology Circuit Using High-Frequency Feedback

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Solution Overview

Problem

Existing electricity meters face challenges in accurately measuring small differential voltages amidst large common mode values in shunt-based current measurement circuits, particularly due to calibration issues and environmental changes affecting resistive voltage dividers, which require precise matching that is difficult to maintain.

Innovation Solution

The implementation of a feedback loop system that uses a high-frequency signal to adjust current sources and match voltage drops across shunt-based circuits, ensuring accurate measurements by tuning the voltage drop to acceptable levels, and utilizing a single chip with its own power supply to measure all shunt voltages in polyphase meters without the need for isolation circuitry.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If resistive voltage dividers are used for voltage measurement, then the circuit is simple and convenient, but calibration accuracy deteriorates due to component matching issues and environmental changes

Engineering Contradiction:

Improvecircuit simplicityVSAvoidvoltage division matching accuracy

Core Design Contradiction:

Ease of manufactureVSMeasurement precision

Solution Approach 1:

The patent implements a feedback mechanism where a test signal is applied to the voltage dividers, the output is measured, and the results are used to adjust the dividers through a digital-to-analog converter and control voltage application. This closed-loop feedback system compensates for mismatches between voltage dividers and maintains accurate voltage division ratios despite environmental changes or component variations.

Inventive Principle:

Principle #23Feedback

2Measurement precision

If manually adjustable potentiometer type devices are used for voltage division adjustment, then initial matching can be achieved, but maintaining matched division ratio over time deteriorates due to environmental changes and component aging

Engineering Contradiction:

Improveinitial voltage matchingVSAvoidvoltage division ratio stability

Core Design Contradiction:

Measurement precisionVSStability of the object's composition

Solution Approach 1:

The system performs self-adjustment by automatically measuring its own voltage divider outputs using an integrated ADC, comparing the measurements against expected values, and generating correction voltages through a DAC to adjust the dividers. This self-service mechanism eliminates the need for manual recalibration and maintains accurate voltage division ratios over time without external intervention.

Inventive Principle:

Principle #25Self-service

Solution Approach 2:

Inventive Principle:

Principle #23Feedback

3Device complexity

If a single chip with its own power supply is used to measure shunt voltages, then cost and device complexity are reduced, but the ability to handle high common mode voltages deteriorates without isolation circuitry

Engineering Contradiction:

Improveisolation circuitry requirementVSAvoidhigh common mode voltage handling

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent extracts and measures only the differential voltage component between the two voltage dividers while rejecting the common mode voltage. By using matched voltage dividers and measuring the difference between their outputs, the system isolates the small differential signal from the large common mode voltage, allowing direct coupling to the ADC without isolation circuitry.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces matched voltage dividers as intermediary components that scale down and differentialize the high common mode voltages. These dividers act as mediators that transform the high voltage differential measurement problem into a low voltage differential measurement that can be handled by standard ADCs without isolation.

Inventive Principle:

Principle #24Intermediary (Mediator)

4Measurement precision

If high-frequency test signals are used for voltage divider matching, then matching accuracy at line frequency is improved, but susceptibility to external signal interference increases

Engineering Contradiction:

Improvevoltage divider matching accuracyVSAvoidexternal signal interference

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The patent converts the potential harm of external signal interference into a benefit by using frequency discrimination. The test signal is applied at a specific high frequency that is distinct from the line frequency and other interference signals. By measuring only at this specific test frequency, the system benefits from the frequency separation that naturally filters out external interference while maintaining accurate matching measurements.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Data Source

PatentUS8598885B2Instrumentation circuit for shunt-based metrology measurement

Publication Date: 2013.12.03 ITRON GLOBAL SARL

AI summary

Current flow at a line frequency may be measured from a source using matched voltage drops in a pair of voltage drop circuits. The voltage drop circuits may each includes a fixed value component, such as a resistor, and an adjustable value component, such as an adjustable current source, coupled in series. The adjustable valued components may be controlled based on differences in voltage drops produced by the voltage drop circuits based on a high-frequency signal, higher in frequency than the line frequency, applied to a control input for each of the adjustable value components.