PPG Motion Artifact Mitigation via Capacitive Intermediary
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Solution Overview
Problem
Photoplethysmography (PPG) and pulse oximetry measurements are susceptible to motion artifacts due to minute movements of the light emitter and/or sensor relative to the tissue, which corrupt the calculation of oxygen saturation values.
Innovation Solution
The use of two circuits, one for illuminating the tissue and another for sensing artifacts, where the second circuit processes the PPG signal to filter out noise caused by surface motion, utilizing impedance-based sensors or capacitance circuitry to measure and suppress motion-induced errors, thereby producing an accurate determination of local blood volume and blood composition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If optical detection is used to measure PPG signals, then heart rate and blood oxygen saturation can be determined, but motion artifacts corrupt the measurement accuracy
Solution Approach 1:
A capacitive sensor is introduced as an intermediary element that detects motion artifacts separately from the PPG signal. The capacitive sensor measures changes in capacitance caused by motion between the sensor and tissue, providing a reference signal that is then used to filter the PPG signal and remove motion-related noise.
Solution Approach 2:
The system uses the capacitive sensor output as feedback to dynamically adjust and filter the PPG signal. The processed capacitive signal serves as a reference input to a noise cancellation algorithm that continuously removes motion artifacts from the PPG measurement in real-time.
2Ease of operation
If minute movements of the light emitter and sensor occur, then the device can be portable and comfortable, but artifacts are introduced that render signal extraction difficult
Solution Approach 1:
The capacitive sensor acts as an intermediary that specifically detects motion-related changes without interfering with the optical PPG measurement. It provides a separate measurement channel that captures only the motion artifacts, enabling their removal while preserving the underlying physiological signal.
Solution Approach 2:
The motion artifact component is extracted from the combined signal using the capacitive sensor reference. By separating the motion-related capacitance changes from the optical PPG signal through signal processing, the system removes only the harmful motion components while retaining the useful physiological information.
3Device complexity
If a single circuit is used for both illumination and sensing, then device complexity is reduced, but motion artifacts affecting the light path cannot be distinguished from physiological signals
Solution Approach 1:
The sensing function is segmented into two separate circuits: one for optical PPG detection and another for capacitive motion sensing. This segmentation allows each circuit to specialize in detecting its specific signal type, enabling the system to distinguish between physiological changes and motion artifacts through differential measurement.
Solution Approach 2:
The sensor housing serves multiple functions: it acts as both the mounting structure for the optical components and as one element of the capacitive sensor. This multi-functionality reduces the need for additional separate motion sensing components while still providing motion artifact detection capability.
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 effectively mitigates motion-induced errors in PPG signals, providing a more accurate measurement of heart rate and blood oxygen saturation by filtering out noise due to surface motion, ensuring reliable physiologic monitoring.
Implementation Method 1
measuring the transmission (transmission PPG) or scattering (reflectance PPG) with a photodetector
Implementation Method 2
the second circuit includes an impedance-based sensor that measures the surface motion artifacts
Implementation Method 3
or capacitance circuitry to measure and suppress motion-induced errors
Data Source
AI summary
Aspects of the present disclosure are directed toward devices, apparatus, and methods for interfacing a PPG apparatus with the skin surface of a patient and sensing artifacts due to surface motion attributable to contact-based surface motion at or near where the apparatus in contact with the skin surface of the patient. The devices, apparatus, and methods include circuitry that contacts the skin surface of the patient, illuminates tissue at the surface, and senses a pulse photoplethysmography (PPG) signal of the patient in response thereto. Further, the circuitry senses artifacts due to surface motion, and responds to the sensed PPG signal by processing the sensed PPG signal relative to the sensed artifacts to produce a version of the sensed PPG signal that is indicative local blood volume and composition of the patient, and filtered to suppress noise therein due to the contact-based surface motion.