Photoplethysmography (PPG) device and the method thereof
View Patent ↗A method of measuring Photoplethysmography (PPG) signals may comprise detecting dual wavelength illumination from a light detector so as to provide detected signals, converting the detected signals into digital signals so as to provide digital signals, reducing noises from the digital signals and increasing independency between the digital signals so as to provide preprocessed signals, subtracting the average value from the preprocessed signals so as to provide adjusted signals, obtaining whitening matrix based on covariance, eigen value, and eigen matrix obtained from the adjusted signals, and restoring data using the whitening matrix.
1. A method for measuring arterial oxygen saturation (SpO 2 ), the method comprising:
in a light detecting step, detecting red and infrared lights from a light detector;
in an A/D conversion step, converting the detected signals of the light detecting step into digital signals;
in a preprocessing step, removing noises from the digital signals of the A/D conversion step and then performing an innovation process, to increase independence between a measured Photoplethysmography (PPG) signal and motion artifact noise;
in a centering process step, subtracting an average value from output signals from the preprocessing step;
in a whitening process step, obtaining a whitening matrix by calculating covariance from output signals from the centering process, and by calculating eigen value and eigen matrix of the covariance;
in a W matrix calculating step, calculating a demixing matrix W by generating a new vector V using the whitening matrix and the output signals from the centering process, by generating an arbitrary 2x2 matrix W, and by applying the V and W to a non-linear function w + , where w + =w−E{vg(w T v)}−E{g′(w T v)}w, where g is a Gaussian function;
in a A matrix calculating step, calculating a mixing matrix A by calculating an inverse matrix of output signals from the W matrix calculating step; and
in a data restoring step, restoring data using output signals from the A matrix calculating step and output signals from the centering process step.
2. The method as claimed in claim 1 , wherein the preprocessing step comprises:
a period measuring step for obtaining a period of the output signals from the A/D conversion step, using autocorrelation;
an interleaving process step for interleaving the output signals from the A/D conversion step, data by relocating the output signals from the A/D conversion step in a predetermined sequence of amplitude according to the period obtained in the period measuring step;
a time low-pass filtering step for transforming, for each sampling point of output signals from the interleaving process step, a current value of the sampling point into an average value of the sampling point, a previous sampling point and a next sampling point; and
an innovation process step for subtracting, from a current output value from the time low-pass filtering step, an average value of previous output values from the time low-pass filtering step.
3. The method as claimed in claim 1 , wherein the data restoring step comprises:
a step for deciding Photoplethysmography(PPG) signal between separated Photoplethysmography(PPG) signal and noise signal using the demixing matrix W, and then restoring sign and gain of the measured Photoplethysmography(PPG) signal using the mixing matrix A;
a step for calculating a ratio R of normalized transmitted amount of the red and infrared lights from the mixing matrix A; and
a step for calculating the arterial oxygen saturation (SpO 2 ), using the ratio R, based on below formula
Arterial oxygen saturation (SpO 2 )(%)= a−bR,
where 107 ≦a≦ 110, 25≦ b 32.