BIOMETRIC SENSOR FOR DETERMINING HEART RATE USING PHOTOPLETHYSMOGRAPH
A wearable computing device includes a display device for displaying augmented reality (AR) images and a biometric sensor for obtaining a heartrate of a user wearing the wearable computing device. The biometric sensor includes a photosensor that emits a light into the surface of a body part of the user. Using photoplethysmography, the photosensor measures voltages from the light reflected from or transmitted through the user's body part at a sampling rate of at least 100 Hz. The measured voltages are then filtered and normalized. Slopes for the resulting set of voltages are then determined on a sliding window basis of approximately 90 milliseconds (ms). Interpulse intervals are then determined for consecutive local minima within each set of sliding windows. The biometric sensor then computes a real-time heartrate for the user from the interpulse intervals, which may then be displayed on the display device of the wearable computing device.
1 . A biometric sensor for measuring a heart rate through photoplethysmography, the biometric sensor comprising:
a machine-readable memory storing computer-executable instructions; and
at least one hardware processor in communication with the machine-readable memory that, when the computer-executable instructions are executed, configures the biometric sensor to:
obtain a plurality of voltages in response to a photosensor emitting light into a surface of a human body;
filter at least one predetermined frequency from the plurality of voltages to obtain a plurality of filtered voltages;
normalize the plurality of filtered voltages to obtain a plurality of normalized voltages;
determine a plurality of slopes based on the plurality of normalized voltages;
determine a plurality of local minima based on the determined plurality of slopes;
determine a plurality of interpulse intervals based on the plurality of local maxima, wherein at least one interpulse interval represents a time between a first local minima selected from the plurality of local minima and a consecutive, second local minima selected from the plurality of local minima;
determine at least one heartrate from the determined plurality of interpulse intervals; and
communicate the determined at least one heartrate to a display.
2 . The biometric sensor of claim 1 , wherein the filter applied to the plurality of voltages comprises a bandpass infinite impulse response filter.
3 . The biometric sensor of claim 2 , wherein the at least one predetermined frequency of the bandpass infinite impulse response filter comprises a range of frequencies from approximately 1 Hz to approximately 50 Hz.
4 . The biometric sensor of claim 1 , wherein the biometric sensor is further configured to:
determine a median voltage from the plurality of filtered voltages; and
adjust each voltage of the plurality of filtered voltages by the determined median voltage.
5 . The biometric sensor of claim 1 , wherein:
the plurality of slopes occur within a preconfigured time duration; and
the preconfigured time duration is changed by a predetermined amount in response to a determination that the number of the plurality of slope minima occurring within the preconfigured time duration is less than a minimum threshold limit or greater than a maximum threshold limit.
6 . The biometric sensor of claim 1 , wherein the plurality of filtered voltages are decimated by a preconfigured amount.
7 . The biometric sensor of claim 1 , wherein the plurality of voltages are obtained by the photosensor at a sampling rate of approximately 100 Hz.
8 . A method for measuring a heart rate through photoplethysmography, the method comprising:
obtaining, by a photosensor, a plurality of voltages in response to emitting light into a surface of a human body;
filtering, by at least one hardware processor, at least one predetermined frequency from the plurality of voltages to obtain a plurality of filtered voltages;
normalizing, by at least one hardware processor, the plurality of filtered voltages to obtain a plurality of normalized voltages;
determining, by at least one hardware processor, a plurality of slopes based on the plurality of normalized voltages;
determining, by at least one hardware processor, a plurality of local minima based on the determined plurality of slopes;
determining, by at least one hardware processor, a plurality of interpulse intervals based on the plurality of local minima, wherein at least one interpulse interval represents a time between a first local minima selected from the plurality of local minima and a consecutive, second local minima selected from the plurality of local minima;
determining, by at least one hardware processor, at least one heartrate from the determined plurality of interpulse intervals; and
communicating, using at least one communication interface, the determined at least one heartrate to a display.
9 . The method of claim 8 , wherein the at least one predetermined frequency is filtered from the plurality of voltages by at least one bandpass infinite impulse response filter.
10 . The method of claim 9 , wherein the at least one predetermined frequency comprises a range of frequencies from approximately 1 Hz to approximate 50 Hz.
11 . The method of claim 8 , further comprising:
determining a median voltage from the plurality of filtered voltages; and
adjusting each voltage of the plurality of filtered voltages by the determined median voltage.
12 . The method of claim 8 , wherein:
the plurality of slopes occur within a preconfigured time duration; and
the preconfigured time duration is changed by a predetermined amount in response to a determination that the number of the plurality of slopes occurring within the preconfigured time duration is less than a minimum threshold amount or greater than a maximum threshold amount.
13 . The method of claim 8 , wherein the plurality of filtered voltages are decimated by a preconfigured amount.
14 . The method of claim 8 , wherein the plurality of voltages are obtained by the photosensor at a sampling rate of approximately 100 Hz.
15 . A machine-readable medium having computer-executable instructions stored thereon that, when executed by at least one hardware processor, causes a biometric sensor to perform a plurality of operations, the plurality of operations comprising:
obtaining a plurality of voltages in response to emitting light into a surface of a human body;
filtering at least one predetermined frequency from the plurality of voltages to obtain a plurality of filtered voltages;
normalizing the plurality of filtered voltages to obtain a plurality of normalized voltages;
determining a plurality of slopes based on the plurality of normalized voltages;
determining a plurality of local minima based on the determined plurality of slopes;
determining a plurality of interpulse intervals based on the plurality of local minima, wherein at least one interpulse interval represents a time between a first local minima selected from the plurality of local minima and a consecutive, second local minima selected from the plurality of local minima;
determining at least one heartrate from the determined plurality of interpulse intervals; and
communicating the determined at least one heartrate to a display.
16 . The machine-readable medium of claim 15 , wherein the at least one predetermined frequency is filtered from the plurality of voltages by at least one bandpass infinite impulse response filter.
17 . The machine-readable medium of claim 16 , wherein the predetermined frequency comprises a range of frequencies from approximately 1 Hz to approximate 50 Hz.
18 . The machine-readable medium of claim 15 , wherein the plurality of operations further comprise:
determining a median voltage from the plurality of filtered voltages; and
adjusting each voltage of the plurality of filtered voltages by the determined median voltage.
19 . The machine-readable medium of claim 15 , wherein:
the plurality of slopes occur within a preconfigured time duration; and
the preconfigured time duration is increased by a predetermined amount in response to a determination that the number of the plurality of slopes occurring within preconfigured time duration is less than a threshold amount.
20 . The machine-readable medium of claim 15 , wherein the plurality of voltages are obtained by the photosensor at a sampling rate of approximately 100 Hz.