Dry-Electrode EDA Monitoring with AC Excitation and DFT
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Solution Overview
Problem
Conventional wearable electronic devices for monitoring electrodermal activity (EDA) using dry electrodes and direct current (DC) excitation signals face issues such as electrolysis, corrosion, skin irritation, and noise interference, leading to increased complexity and cost.
Innovation Solution
Employing dry electrodes with alternating current (AC) excitation signals and discrete Fourier transform (DFT) processing to generate complex frequency domain representations, reducing electrolysis, computational complexity, and noise interference, while obtaining accurate measures of skin impedance or conductance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If DC excitation signals are used with dry electrodes, then the system structure is simpler, but electrolysis, corrosion, and skin irritation occur leading to reduced reliability
Solution Approach 1:
The patent changes the excitation signal parameter from DC to AC, specifically using a sinusoidal waveform at frequencies between 50-200 Hz. This parameter change eliminates electrolysis and corrosion issues associated with DC signals while maintaining system simplicity, directly resolving the contradiction between device complexity and reliability
Solution Approach 2:
The patent employs periodic AC excitation signals instead of continuous DC signals. The periodic nature of the AC signal at specified frequencies allows for reduced computational complexity through synchronized sampling while preventing the harmful effects of DC, thus improving reliability without significantly increasing system complexity
2Measurement precision
If AC excitation signals with DFT processing are used, then signal-to-noise ratio is improved, but computational complexity increases
Solution Approach 1:
The patent performs preliminary action by sampling the AC signal at frequencies synchronized with the excitation signal frequency before DFT processing. This pre-synchronization reduces the number of samples needed and optimizes the DFT computation, thereby improving signal-to-noise ratio while minimizing computational complexity
Solution Approach 2:
The patent optimizes the DFT processing by changing parameters such as using reduced FFT sizes (e.g., 8-point or 16-point transforms) and selecting specific frequency bins corresponding to the excitation frequency. These parameter changes maintain high measurement precision while significantly reducing computational burden compared to full-spectrum analysis
3Device complexity
If conventional DC-based EDA monitoring is used, then device structure is simpler, but noise interference increases reducing measurement accuracy
Solution Approach 1:
The patent changes the measurement approach by using AC excitation signals with specific frequencies (50-200 Hz) and processing the response through DFT at corresponding frequency bins. This parameter change enables frequency-selective measurement that rejects DC offsets and low-frequency noise, improving measurement accuracy while maintaining relatively simple device structure
Solution Approach 2:
The patent substitutes physical filtering mechanisms with digital signal processing methods. Instead of using complex analog filters to remove noise, the system uses DFT processing to selectively extract the AC response at the excitation frequency, replacing mechanical/filtering complexity with computational simplicity that achieves better noise rejection
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 system achieves reduced structural complexity, cost, and improved signal-to-noise ratio (SNR) by using AC excitation and DFT processing, providing accurate EDA monitoring in wearable devices.
Implementation Method 1
The AC excitation signal can cause an AC current to flow through the stratum corneum (i.e., the outer layer of the user's skin) from the first dry electrode contacting the first area of the user's skin to the second dry electrode contacting the second area of the user's skin
Implementation Method 2
a trans-impedance amplifier for converting the AC current received at the second dry electrode to an AC voltage
Implementation Method 3
an analog-to-digital (A-to-D) converter for sampling the AC voltage at a predetermined sampling frequency, such as about four times (4×) the predetermined excitation frequency, in order to obtain a voltage level sequence
Implementation Method 4
a discrete Fourier transform (DFT) processor for generating a complex frequency domain representation of the sampled voltage level sequence
Data Source
AI summary
Systems and methods of monitoring electrodermal activity (EDA) in human subjects suitable for use in wearable electronic devices. An EDA monitoring system can include first and second dry electrodes, an alternating current (AC) excitation signal source, a trans-impedance amplifier, an analog-to-digital (A-to-D) converter, a discrete Fourier transform (DFT) processor, and a microprocessor. The AC excitation signal source can produce an AC excitation signal having a predetermined excitation frequency, such as about 100 or 120 Hertz (Hz). The analog-to-digital (A-to-D) converter can include a sample-and-hold circuit that operates at a predetermined sampling frequency, such as about four times (4×) the predetermined excitation frequency of 100 or 120 Hz. The DFT processor can generate complex frequency domain representations of digitized, sampled voltage level sequences provided by the A-to-D converter for use in obtaining measures of a user's skin impedance or skin conductance.