Apparatus and method for measuring photoplethysmogram
Inventors
Laakkonen, Harri Matti-Heikki • Kinnunen, Hannu Olavi • Koskela, Markku Olavi
Assignees
Publication Number
US-12042258-B2
Publication Date
2024-07-23
Expiration Date
2035-01-27
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Abstract
Disclosed is an apparatus for measuring photoplethysmogram. The apparatus includes a ring structure with at least one photon source and at least one photon detector positioned on an inner surface of the ring structure. The apparatus further includes a controller configured to measure a preliminary photoplethysmogram during a first time period by taking a first number of samples, determine a form factor from said preliminary photoplethysmogram, determine an inter beat interval from said preliminary photoplethysmogram, and use the form factor and the inter beat interval to determine a second number of samples to be taken during a second time period of measurement of the photoplethysmogram and the distribution of the samples to be taken in function of time.
Core Innovation
The invention disclosed is an apparatus and a method for measuring a photoplethysmogram (PPG), which incorporates a ring structure with at least one photon source and one photon detector positioned on its inner surface. The apparatus further includes a controller that first measures a preliminary photoplethysmogram during a first time period by taking a first number of samples, determines a form factor and an inter beat interval (IBI) from this preliminary measurement, and then uses these parameters to adaptively determine a second number of samples and their temporal distribution for subsequent measurement periods.
The problem being solved arises from the challenges in wearable PPG devices related to power efficiency, artifact interference, and signal quality distortion. Wearable photoplethysmogram devices typically require continuous measurement, demanding substantial battery power due to the continuous operation of components such as LEDs and photodiodes. Moreover, user movement may introduce ambient light artifacts into the measurement, commonly addressed by applying a direct current (DC) offset which, if not tactfully applied, can distort the signal quality. Thus, there is a need for a more power-efficient operation and improved signal quality management without distortion during PPG measurement.
The present invention addresses these issues by implementing an adaptive sampling rate approach based on the preliminary PPG waveform characteristics. The controller uses the form factor and inter beat interval determined from the initial measurement to compute a probability function that guides the distribution and number of samples taken during subsequent measurements. This enables the apparatus to intensify sampling around peaks of the PPG waveform, where relevant physiological information is concentrated, and reduce sampling in less informative areas, thereby conserving battery power while maintaining measurement accuracy. Additionally, dynamic DC offset biasing is applied during off-peak time periods to reduce artifacts without distorting the important signal peaks.
Claims Coverage
The claims define one independent method claim involving an adaptive sampling approach for measuring photoplethysmogram using a wearable device, involving novel features related to sample distribution based on photoplethysmogram characteristics.
Adaptive measurement using binary probability function
The method measures photoplethysmogram from a user during a first time period by taking a first number of samples, then determines an inter beat interval from the photoplethysmogram, and uses this inter beat interval to determine a binary probability function that governs a distribution of a second number of samples for subsequent measurement periods.
Use of form factor in defining sample distribution
The method further involves using a form factor derived from the photoplethysmogram to determine the binary probability function that controls sampling distribution.
Biased sampling towards photoplethysmogram peaks
The binary probability function is specifically constructed to result in more samples being taken from the peaks of the photoplethysmogram compared to other parts, enhancing measurement focus on physiologically relevant data.
Dynamic biasing of direct current offset during off-peak periods
The method includes dynamically biasing the direct current offset during off-peak time periods of the photoplethysmogram to reduce artifacts without distorting signal quality during peaks.
Adjustment of sampling based on inter beat interval changes
The method adjusts the second number of samples and their distribution over time during the second time period if the inter beat interval changes, thus adapting to dynamic physiological variations.
Filtering measurement results using the probability function
Measurement results obtained during the second time period are filtered using the binary probability function to reduce noise and artifacts in the photoplethysmogram signal.
The claims collectively cover an adaptive photoplethysmogram measurement method in a wearable device that utilizes a binary probability function based on inter beat interval and form factor to distribute sampling effort, dynamically adjusts sampling based on physiological signals, applies signal conditioning through dynamic DC offset, and filters results using the probability function, thereby providing power-efficient and artifact-minimized PPG measurements.
Stated Advantages
Enhanced power efficiency through adaptive sampling concentrating measurements around peaks of the photoplethysmogram waveform.
Reduction in signal artifacts by dynamically biasing DC offset during off-peak time periods without distorting the signal quality at peaks.
Capability to use smaller range analog-to-digital converters due to DC offset control, thereby saving battery power.
Improved measurement precision by ensuring sufficient samples are taken at physiologically relevant points while reducing redundancy elsewhere.
Dynamic adjustment of sampling according to changes in inter beat interval, providing adaptability to user’s physiological state.
Documented Applications
Wearable devices configured to be worn on a user’s finger or wrist to continuously monitor biological signals such as blood volume pulse for deriving heart rate.
Personal health monitoring devices using photoplethysmogram technology to derive information including respiration, pulse, oxygen saturation, and user movement.
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