According to the author's knowledge, the smart bracelet/watches with heart rate monitoring function on the market include Microsoft Band, microsoft band2, Fitbit Surge, Jawbone UP 3, Apple Watch, and Xiaomi Bracelet Light Edition. What specific "black" technologies are applied to them, and what is the difference between them and conventional monitoring equipment? Let us all come together today. (This article is only for technical discussion, not for product evaluation.)
At present, mature heart rate non-invasive measurement methods include blood oxygenation method, photoelectric volume method, ECG signal method, and arterial pressure method.
Blood Oxygen Method: If you have done a step test (a heart-forging test for heart and lung function), you must be familiar with this heart rate test method. The heart rate tester that is attached to the tip of the index finger uses this approach. In general, a complete oximeter often has two kinds of light-emitting diodes, one wavelength of 660 nm, which is red light of visible light, and one wavelength of more than 900 nm, which is infrared light. The oxygen-carrying hemoglobin in the blood vessels and the hemoglobin not carrying oxygen have different absorption rates for the two kinds of light. As shown below. At the same time, the oxygen content in the blood vessels is consumed, and the heart pumping blood is increased, and the cycle process is consumed. This cycle is just the same as the heart rate.
figure 1
The advantage of the oximetry method is to provide both heart rate and oxygen saturation signals. The disadvantage is that because the transmitted light signal needs to be received at the other end, then this part of the human body must be thin enough, and the proper position of the whole body is only the fingertip and the earlobe. The wrist is too thick, the visible light cannot penetrate at all, and the range of use is limited. None of the above sports watches have adopted this solution.
Photoelectric volume method: This scheme is to track the reflection of visible light (green light) in human tissue. Usually there are two green LEDs that emit visible light to the wrist, and then a photosensor in the middle to sense the reflected light. as follows:
figure 2
image 3
The skin's skin, bones, meat, fat, etc. reflect a fixed value of light, while the capillaries and arterial veins become fluctuating because they become larger and smaller as the pulse volume increases. The frequency of this fluctuation is the pulse, which is generally consistent with the heart rate.
This method can only get the heart rate signal, but it is relatively resistant to the noise caused by the movement, which is very suitable for the current sports watch. This method is currently used in sports watches on the market. As for the accuracy. . . In addition to appleWatch (Figure 3), several other (Figure 2) are using the same Philips patented sensor... The only thing that can be done is that the accelerometer is different from the algorithm for compensating for motion noise, from me. From the experience, it should be similar.
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