A Novel Sensor Design for Amplitude Modulated Measurement of Capacitive ECG

Abstract

Capacitive electrocardiogram (cECG) has been proposed for various applications to extend cardiac monitoring to everyday life. While offering unobtrusive measurement of an electrocardiogram, cECG measurements experience severe motion artifacts with higher amplitudes and longer durations, which results in gaps in the signal availability and deterioration in the signal quality. Time-variant coupling impedance of the cECG results from the variations in the contact pressure of through-clothing cECG measurements and is often used to model the nature of motion artifacts. This article offers a new perspective to utilize the sensitive nature of the coupling impedance by introducing high-frequency sinusoidal variations in the contact pressure. The multiplicative interaction between the source signal and the coupling impedance modulates the amplitude of this sinusoidal with the ECG signal. This new technique is named modulated ECG (mECG), as the ECG signal is merged to the envelope of the resulting carrier wave. The proposed idea of a contact pressure modulated with a high-frequency sinusoidal is investigated analytically in a cECG electrode model. The numerical simulations based on the electrode model are supported by the physical simulations conducted in a test bench using an electrode prototype proposed for mECG measurements. Among the factors influencing mECG measurements, the type of textile, contact pressure, and modulation frequency are investigated. Moreover, the electrode model with a parallel capacitor and resistor as the coupling impedance is evaluated by impedance measurements of textiles. mECG amplitudes are found lower than those of cECG with a factor greater than 398, yet cECG measurements are not affected by the parallel mECG measurements in the tests. Therefore, the mECG technique can supplement the cECG by being measured simultaneously using the same electrode.