Wearable PPG Sensing With Synchronous Demodulation Against Motion Noise
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Solution Overview
Problem
Wearable computing devices face challenges in accurately capturing photoplethysmography (PPG) data due to various aggressors such as motion, ambient light fluctuations, skin color, and sweat, which affect the accuracy of biometric measurements.
Innovation Solution
The device employs a control circuit that modulates light signals with a carrier frequency to distinguish between reference and PPG data, synchronously demodulates these signals to detect phase shifts, and adjusts the light signal amplitude to mitigate the impact of aggressors, using machine-learned models to enhance accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If PPG data is collected using conventional methods without modulation, then the device complexity is low, but the measurement precision deteriorates due to interference from motion and ambient light
Solution Approach 1:
The patent applies periodic action by modulating the light source with a carrier signal at a specific frequency (e.g., 100-200 Hz). This periodic modulation allows the PPG signal to be distinguished from noise through frequency domain separation. The demodulation process extracts the modulated PPG signal while rejecting non-modulated interference from motion and ambient light, thereby improving measurement precision without significantly increasing device complexity
Solution Approach 2:
The patent introduces an intermediary carrier signal as a mediator between the light source and the PPG measurement. By modulating the light source with this carrier signal, the system creates a frequency-tagged reference signal that enables selective extraction of PPG data through synchronous demodulation. This intermediary approach allows the system to separate desired signals from noise without requiring complex hardware modifications
2Measurement precision
If complex backend models are used to process PPG data, then the measurement precision improves, but the use of energy increases
Solution Approach 1:
The patent applies preliminary action by performing signal conditioning and noise rejection through synchronous demodulation before the data reaches the processing stage. The modulated reference signal and demodulation process pre-process the PPG signal to remove motion artifacts and ambient light interference, delivering cleaner data to the backend model. This preliminary processing reduces the computational burden on energy-consuming processors while maintaining measurement precision
3Measurement precision
If the amplitude of light signal is increased to overcome motion interference, then the measurement precision improves, but the object-affected harmful factors increase due to skin saturation and discomfort
Solution Approach 1:
The patent replaces the mechanical approach of increasing light amplitude with an electrical/signaling approach using frequency modulation. Instead of boosting the light signal strength to overcome motion interference, the system modulates the light source with a carrier signal and uses synchronous demodulation to extract the PPG signal. This substitution allows high signal-to-noise ratio achievement without increasing light amplitude, thereby avoiding skin saturation and user discomfort
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
This approach improves the accuracy of biometric data by isolating PPG components from noise, reduces computational complexity, and conserves power by minimizing the need for complex backend models.
Implementation Method 1
an emitter configured to output a light signal that is modulated with a carrier signal to generate a modulated light signal
Implementation Method 2
one or more detectors configured to receive a first reflected light signal that is a reference signal of the modulated light signal
Implementation Method 3
The control circuit is configured to synchronously demodulate the first reflected light signal and the second reflected light signal to obtain a first demodulated signal and a second demodulated signal
Data Source
AI summary
A wearable computing device includes a PPG sensor that includes an emitter configured to output a light signal that is modulated with a carrier signal to generate a modulated light signal. The PPG sensor further includes one or more detectors configured to receive a first reflected light signal that is a reference signal of the modulated light signal without PPG data in the carrier signal and a second reflected light signal that is a reference signal of the modulated light signal with PPG data in the carrier signal. The wearable computing device further includes a control circuit configured to synchronously demodulate the first and second instances of the modulated light signal to obtain a first demodulated signal and a second demodulated signal, respectively. The control circuit is further configured to generate data indicative of one or more aggressors affecting the PPG data based on the first and second demodulated signals.