Low-complexity sensor displacement tolerant pulse oximetry based heart rate measurement
Methods for heart rate measurement based on pulse oximetry are provided that can tolerate some degree of relative displacement of a photoplethysmograph (PPG) heart rate monitor device. In some methods, artifact compensation based on a reference signal is performed on the PPG signal data to remove artifacts in the signal that may be caused, for example, by changes in ambient light and/or motion of a person wearing the monitor device. The reference signal used for artifact compensation may be generated using an LED of a complementary wavelength to that of the LED used to generate the PPG signal, or by driving an LED at a lower current than the current applied to generate the PPG signal.
1. A method for heart rate measurement using a photoplethysmograph (PPG) heart rate monitor device, the method comprising:
capturing a PPG signal generated by using a first light emitting diode (LED) of the PPG heart rate monitor device;
capturing a reference signal generated by using a second LED of the PPG heart rate monitor device, wherein a current used to drive the second LED is lower than a current used to drive the first LED;
using the reference signal to remove motion noise from the PPG signal, wherein a motion noise compensated PPG signal is generated; and
estimating a heart rate using the motion noise compensated PPG signal, the estimating including:
estimating a start point of a pulse in a portion of the motion compensated PPG signal;
estimating a summit point of the pulse in the portion; and
estimating an end point of the pulse in the portion, wherein estimating the end point comprises:
computing a score for each of two candidate end points, wherein the score includes a difference between a first length of a first direct path from the estimated start point to the estimated summit point and a second length of a second direct path from the estimated summit point to the candidate end point; and
selecting one of the two candidate end points as the estimated end point based on the scores of the two candidate end points.
2. The method of claim 1 , wherein the first LED and the second LED are a same LED.
3. The method of claim 1 , further comprising:
determining, prior to the using the reference signal to remove motion noise, whether or not a portion of the PPG signal includes displacement noise; and
discarding the portion of the PPG signal when the portion includes displacement noise.
4. The method of claim 3 , wherein determining whether or not a portion of the PPG signal includes displacement noise comprises:
analyzing the portion to identify rapid transitions in position of the PPG heart rate monitor device; and
determining whether or not the portion includes displacement noise based on the identified rapid transitions.
5. The method of claim 4 , wherein analyzing the portion comprises:
determining a first number of zero crossings in a first threshold interval in the portion of the PPG signal; and
determining a second number of zero crossings in a second threshold interval in the portion of the PPG signal, and
wherein a ratio of the first number of zero crossings and the second number of zero crossings is used to determine if the portion includes displacement noise.
6. A method for heart rate measurement using a photoplethysmograph (PPG) heart rate monitor device, the method comprising:
capturing a PPG signal generated by using a light emitting diode (LED) of the PPG heart rate monitor device;
analyzing a first portion of the PPG signal to identify rapid transitions in position of the PPG heart rate monitor device, the analyzing comprising:
determining a first number of zero crossings in a first threshold interval in the first portion of the PPG signal; and
determining a second number of zero crossings in a second threshold interval in the first portion of the PPG signal;
determining whether or not the first portion includes displacement noise based on a ratio of the first number of zero crossings and the second number of zero crossings;
discarding the first portion of the PPG signal when the first portion includes displacement noise; and
using the first portion of the PPG signal for heart rate measurement when the first portion does not include displacement noise.
7. The method of claim 6 , wherein the LED is a first LED, and the using the first portion of the PPG signal comprises:
using a reference signal to remove motion noise from the first portion of the PPG signal to generate a motion noise compensated first portion of the PPG signal, wherein the reference signal is captured using a second LED of the PPG heart rate monitor device, wherein a current used to drive the second LED is lower than a current used to drive the first LED; and
estimating a heart rate using the motion noise compensated first portion of the PPG signal.
8. The method of claim 7 , wherein the first LED and the second LED are a same LED.
9. The method of claim 6 , wherein using the first portion the PPG signal for heart rate measurement comprises:
estimating a start point of a pulse in a second portion of the PPG signal comprising the first portion of the PPG signal;
estimating a summit point of the pulse in the second portion; and
estimating an end point of the pulse in the second portion, wherein estimating the end point is based on the estimated summit point.
10. The method of claim 9 , wherein estimating an end point comprises:
computing a score for each of two candidate end points, wherein the score for a candidate end point is a difference between a length of a direct path from the estimated start point to the estimated summit point and a length of a direct path from the estimated summit point to the candidate end point; and
using the computed scores to determined which of the two candidate end points is the better candidate end point.
11. The method of claim 6 , wherein the PPG heart rate monitor device is housed in a watch-like form factor.