Reducing Motion Induced Artifacts in Photoplethysmography (PPG) Signals

Abstract

A method for heart rate measurement in a photoplethysmograph (PPG) heart rate monitor device is provided that includes performing motion compensation on a PPG signal wherein a motion compensated PPG signal PPG accX is generated with reference to an X-axis acceleration signal, a motion compensated PPG signal PPG accY is generated with reference to a Y-axis acceleration signal, and a motion compensated PPG signal PPG accZ is generated with reference to a Z-axis acceleration signal, combining PPG accX , PPG accY , and PPG accZ to generate a final motion compensated PPG signal, wherein a first weight is applied PPG accX , a second weight is applied to PPG accY , and a third weight is applied to PPG accZ , performing a single Fourier Transform (FT) on the final motion compensated PPG signal to generate a frequency domain PPG signal; and estimating a heart rate based on the frequency domain PPG signal.

What is claimed is: 
     
         1 . A method for heart rate measurement in a photoplethysmograph (PPG) heart rate monitor device, the method comprising:
 receiving a PPG signal from a PPG sensor of the heart rate monitor device;   receiving an X-axis acceleration signal, a Y-axis acceleration signal, and a Z-axis acceleration signal from an accelerometer of the heart rate monitor device;   performing motion compensation on the PPG signal, wherein a first motion compensated PPG signal is generated with reference to the X-axis acceleration signal, a second motion compensated PPG signal is generated with reference to the Y-axis acceleration signal, and a third motion compensated PPG signal is generated with reference to the Z-axis acceleration signal;   combining the first motion compensated PPG signal, the second motion compensated PPG signal, and the third motion compensated PPG signal to generate a fourth motion compensated PPG signal, wherein a first weight is applied to the first motion compensated PPG, a second weight is applied to the second motion compensated PPG signal, and a third weight is applied to the third motion compensated PPG signal;   performing a single Fourier Transform (FT) on the fourth motion compensated PPG signal to generate a frequency domain PPG signal; and   estimating a heart rate based on the frequency domain PPG signal.   
     
     
         2 . The method of  claim 1 , wherein the first motion compensated PPG signal is generated by applying a first adaptive filter to the PPG signal with the X-axis acceleration signal as a reference signal, the second motion compensated PPG signal is generated by applying a second adaptive filter to the PPG signal with the Y-axis acceleration signal as a reference signal, and the third motion compensated PPG signal is generated by applying a third adaptive filter to the PPG signal with the Z-axis acceleration signal as a reference signal. 
     
     
         3 . The method of  claim 2 , wherein the first adaptive filter, the second adaptive filter, and the third adaptive filter are a same filter type, the filter type selected from one of a normalized least mean squares (NLMS) filter and a recursive least squares (RLS) filter. 
     
     
         4 . The method of  claim 2 , wherein weights of the first adaptive filter, the second adaptive filter, and the third adaptive filter are updated only when a user of the heart rate monitor device is in motion. 
     
     
         5 . The method of  claim 2 , wherein weights of the first adaptive filter, the second adaptive filter, and the third adaptive filter are reset to initial values when a user of the heart rate monitor device is stationary. 
     
     
         6 . The method of  claim 1 , further comprising determining whether or not a user of the heart rate monitor device is stationary. 
     
     
         7 . The method of  claim 6 , wherein determining whether or not a user of the heart rate monitor device is stationary comprises:
 computing a variance of the sum of the squares of the X-axis acceleration signal, the Y-axis acceleration signal, and the Z-acceleration signal over a window of time; and   comparing the variance to a threshold.   
     
     
         8 . The method of  claim 1 , wherein the first weight, the second weight, and the third weight are equal. 
     
     
         9 . The method of  claim 1 , wherein a value of the first weight, a value of the second weight, and a value of the third weight are based on an amount of correlation between the PPG signal and the X-axis acceleration signal, an amount of correlation between the PPG signal and Y-axis acceleration signal, and an amount of correlation between the PPG signal and the Z-axis acceleration signal. 
     
     
         10 . The method of  claim 2 , wherein a value of the first weight, a value of the second weight, and a value of the third weight are based on predicted interference energy removed from the PPG signal by the first adaptive filter, predicted interference energy removed from the PPG signal by the second adaptive filter, and predicted interference energy removed from the PPG signal by the third adaptive filter. 
     
     
         11 . A photoplethysmograph (PPG) heart rate monitor device comprising:
 a PPG sensor component configured to generate an analog PPG signal;   an accelerometer configured to generate an X-axis acceleration signal, a Y-axis acceleration signal, and a Z-axis acceleration signal;   a motion compensation component coupled to the PPG sensor to receive the PPG signal and coupled to the accelerometer to receive the X-axis acceleration signal, the Y-axis acceleration signal, and the Z-axis acceleration signal, wherein the motion compensation component is configured to generate a first motion compensated PPG signal with reference to the X-axis acceleration signal, a second motion compensated PPG signal with reference to the Y-axis acceleration signal, and a third motion compensated PPG signal with reference to the Z-axis acceleration signal;   a combiner configured to combine the first motion compensated PPG signal, the second motion compensated PPG signal, and the third motion compensated PPG signal to generate a fourth motion compensated PPG signal, wherein a first weight is applied to the first motion compensated PPG, a second weight is applied to the second motion compensated PPG signal, and a third weight is applied to the third motion compensated PPG signal;   a Fourier Transform (FT) component configured to perform a single FT on the fourth motion compensated PPG signal to generate a frequency domain PPG signal; and   a heart rate tracker component configured to estimate a heart based on the frequency domain PPG signal.   
     
     
         12 . The PPG heart rate monitor device of  claim 11 , wherein the motion compensation component comprises a first adaptive filter configured to generate the first motion compensated PPG signal using the X-axis acceleration signal as a reference signal, a second adaptive filter configured to generate the second motion compensated PPG signal using the Y-axis acceleration signal as a reference signal, and a third adaptive filter configured to generate the third motion compensated PPG signal using the Z-axis acceleration signal as a reference signal. 
     
     
         13 . The PPG heart rate monitor device of  claim 12 , wherein the first adaptive filter, the second adaptive filter, and the third adaptive filter are a same filter type, the filter type selected from one of a normalized least mean squares (NLMS) filter and a recursive least squares (RLS) filter. 
     
     
         14 . The PPG heart rate monitor device of  claim 12 , wherein weights of the first adaptive filter, the second adaptive filter, and the third adaptive filter are updated only when a user of the heart rate monitor device is in motion. 
     
     
         15 . The PPG heart rate monitor device of  claim 12 , wherein weights of the first adaptive filter, the second adaptive filter, and the third adaptive filter are reset to initial values when a user of the heart rate monitor device is stationary. 
     
     
         16 . The PPG heart rate monitor device of  claim 11 , further comprising a stationary detector component configured to determine whether or not a user of the heart rate monitor device is stationary. 
     
     
         17 . The PPG heart rate monitor device of  claim 16 , wherein the stationary detector component is coupled to the accelerometer to receive the X-axis acceleration signal, the Y-axis acceleration signal, and the Z-axis acceleration signal and is configured to determine whether or not a user of the heart rate monitor device is stationary by computing a variance of the sum of the squares of the X-axis acceleration signal, the Y-axis acceleration signal, and the Z-acceleration signal over a window of time and comparing the variance to a threshold. 
     
     
         18 . The PPG heart rate monitor device of  claim 11 , wherein the first weight, the second weight, and the third weight are equal. 
     
     
         19 . The PPG heart rate monitor device of  claim 11 , wherein a value of the first weight, a value of the second weight, and a value of the third weight are based on an amount of correlation between the PPG signal and the X-axis acceleration signal, an amount of correlation between the PPG signal and Y-axis acceleration signal, and an amount of correlation between the PPG signal and the Z-axis acceleration signal. 
     
     
         20 . The PPG heart rate monitor device of  claim 12 , wherein a value of the first weight, a value of the second weight, and a value of the third weight are based on predicted interference energy removed from the PPG signal by the first adaptive filter, predicted interference energy removed from the PPG signal by the second adaptive filter, and predicted interference energy removed from the PPG signal by the third adaptive filter.