Capacitive ECG Sensing With Motion Artifact Compensation
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Solution Overview
Problem
Bio-signals, such as electrocardiograms, are often contaminated by motion artifacts due to the movement of electrodes relative to the skin, which complicates accurate heart function assessment.
Innovation Solution
A displacement current sensor and associated circuitry are used to measure and compensate for motion-induced noise by detecting variations in capacitance between electrodes and the skin, employing a reference signal to separate motion artifacts from the bio-signal, utilizing a demodulation circuit and adaptive filtering to refine the electrocardiogram signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If capacitive electrodes are used for ECG measurements, then ease of operation is improved, but motion artifacts increase
Solution Approach 1:
A guard electrode is introduced as an intermediary element between the sensing electrode and the body. The guard electrode is driven at a higher frequency (e.g., 100 kHz) to create a virtual ground that shields the sensing electrode from motion-induced capacitance changes. This intermediary structure allows capacitive electrodes to maintain ease of operation while significantly reducing motion artifacts through electromagnetic shielding.
Solution Approach 2:
The system changes the operating frequency parameter by injecting a high-frequency signal (e.g., 100 kHz) onto the guard electrode. This frequency transformation allows the system to operate in a regime where motion artifacts are minimized while still extracting the low-frequency ECG signal through demodulation. The parameter change from DC/low-frequency to high-frequency operation resolves the contradiction between ease of use and artifact reduction.
2Object-affected harmful factors
If adhesive electrodes are used for ECG measurements, then motion artifacts are reduced, but ease of operation deteriorates
Solution Approach 1:
The mechanical attachment system (adhesive electrodes requiring skin preparation and placement) is replaced with a capacitive coupling system using guard electrodes. Instead of relying on mechanical adhesion to reduce motion artifacts, the system uses electromagnetic field shielding through the guard electrode driven at high frequency. This substitution maintains artifact reduction while dramatically improving ease of operation.
3Measurement precision
If high frequency regime is used for impedance measurement, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The guard electrode serves multiple functions: it provides shielding against motion artifacts during ECG measurement, enables impedance measurement through high-frequency operation, and acts as a reference for demodulation. This multi-functionality allows the system to achieve high measurement precision for both ECG and impedance without proportionally increasing device complexity, as the same hardware structure serves multiple measurement purposes.
Solution Approach 2:
The system uses periodic high-frequency signaling on the guard electrode to enable both impedance measurement and motion artifact reduction. By modulating the guard electrode at a specific frequency and using synchronous demodulation, the system extracts precise measurements while filtering out motion artifacts. The periodic action at high frequency improves measurement precision without requiring complex continuous monitoring systems.
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 noise in electrocardiogram signals, enhancing the accuracy of heart function monitoring and assessment in patient, personal health, and fitness applications.
Implementation Method 1
measuring a sensed signal, from a displacement current sensor (20), dependent upon electrical activity of a subject's heart
Implementation Method 2
A displacement current sensor and associated circuitry are used to measure and compensate for motion-induced noise by detecting variations in capacitance between electrodes and the skin
Implementation Method 3
employing a reference signal to separate motion artifacts from the bio-signal, utilizing a demodulation circuit and adaptive filtering to refine the electrocardiogram signal
Data Source
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Figure 4A~6C
Figure 7A~9
AI summary
an apparatus comprising: a displacement current sensor configured to measure a sensed signal dependent upon electrical activity of a subject's heart; and circuitry configured to process the sensed signal to compensate for artefacts arising from motion of the subject.