PPG Motion Artifact Filtering Using Harmonic Notch and Comb Stages
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Solution Overview
Problem
Traditional frequency-dependent filtering is ineffective in removing quasi-periodic motion artifacts from photoplethysmography (PPG) signals that have frequencies similar to heart rates, leading to inaccurate heart-rate measurements in wearable devices.
Innovation Solution
Employing a sequence of a harmonic notch filter and a harmonic comb filter to remove motion artifacts, utilizing a 2-tap finite impulse response (FIR) and 2-tap infinite impulse response (IIR) filters, respectively, to enhance heart-rate signal components while minimizing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional frequency-dependent filtering is used to remove motion artifacts, then filtering simplicity is maintained, but measurement precision deteriorates because motion artifacts with frequencies similar to heart rates cannot be effectively separated
Solution Approach 1:
The filtering process is divided into multiple sequential stages: first a notch filter removes specific frequency components corresponding to motion artifacts, then a comb filter removes remaining periodic artifacts, and finally a bandpass filter isolates the heart rate signal. This segmentation allows each filter to target specific aspects of the signal, achieving high measurement precision without requiring a single complex filter algorithm
Solution Approach 2:
The filtering approach changes from traditional fixed frequency-dependent filtering to adaptive filtering where filter parameters (notch frequencies, comb filter spacing) are dynamically adjusted based on detected motion frequencies from the accelerometer. This parameter adaptation enables effective separation of motion artifacts from heart rate signals even when their frequencies are similar, resolving the measurement precision issue
2Measurement precision
If complex filtering algorithms are used to remove motion artifacts, then measurement precision improves, but power consumption increases
Solution Approach 1:
The complex filtering task is broken into simpler sequential filter stages (notch filter, comb filter, bandpass filter), each performing a specific function. This segmentation allows the system to achieve high measurement precision through multiple simple operations rather than one complex algorithm, reducing overall computational power consumption while maintaining accuracy
Solution Approach 2:
Motion frequency detection using the accelerometer is performed beforehand to pre-configured the filtering parameters. This preliminary action allows the subsequent filtering stages to be optimized for the specific motion conditions, improving measurement precision while avoiding the need for continuous complex adaptive filtering that would increase power consumption
Data Source
AI summary
Removal of motion artifacts from photoplethysmography (PPG) signals from a PPG sensor by applying a harmonic notch filter to a PPG signal where positions of the filter notches are based on a period of an accelerometer signal received from an accelerometer sensor in a vicinity of the PPG sensor to yield a first filtered signal; determining a signal quality index (SQI) associated with the PPG signal based on the accelerometer signal; determining an estimated heart rate based on the PPG signal; and applying a harmonic comb filter to the first filtered signal to where the filter peaks are based on the estimated heart rate. The estimated heart rate may be determined according to a confidence level based on SQI. A wearable device may implement the system for removing motion artifacts from PPG signals.