The smart bracelet should be the most intelligent product in addition to VR glasses at this stage, because the price is low, the penetration rate is far greater than smart watches and other expensive intelligent hardware. The smart bracelet can be roughly divided into a wristband with a heart rate monitoring function and a common wristband. Today, I will talk about the technical content of this toy with the accuracy of the heart rate sensor of the smart bracelet!
From a technical point of view, there are roughly two types:
The first one: principle of PPG photoelectric volume pulse wave tracing
Simply put, it is just reflection, using the pulsating change of the light transmittance in the blood, converted into an electrical signal, and converted by a software algorithm, the corresponding heart rate.
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When a beam of a certain wavelength is irradiated onto the surface of the skin of the fingertip, the beam will be transmitted to the photoreceiver by transmission or reflection. In the process, the light detected by the detector is attenuated by the absorption of the skin muscle and blood of the end. The intensity will be weakened. The absorption of light by skin, muscle tissue, etc. is kept constant throughout the blood circulation, and the blood volume in the skin changes pulsatingly under the action of the heart. When the heart contracts, the peripheral blood volume has the largest amount of light absorption, and the detected light intensity is the smallest. In the case of diastole, the opposite is true, the intensity of the detected light is the greatest, and the intensity of the light received by the light receiver changes pulsatingly.
Then there must be someone who wants to ask why the common green LEDs?
Because in the blood of this red liquid, the absorption rate of green light is the largest, and the data is relatively accurate.
When the user's heart beats, more blood will flow through the user's wrist, and the amount of green light will be absorbed. In the heartbeat gap, blood flow is reduced, resulting in a decrease in green light absorption.
For example, suppose the luminescence value of the bracelet is 100, the skin muscle tissue absorbs a constant 10, the total blood absorption is 15, and the reflection is 100-10*2-15=65, then the arterial blood comes over, and the red blood cells have increased oxygen. The total blood absorption becomes 2, and the reflection is 50.
After that, you will always be at 65-50-65-50-65-50-65-50-65-50..., by calculating how many pulse changes per second, you will get your heart rate.
The principle of this method is also its shortcomings. If the bracelet and skin directly have a lot of sweat? That value will be inaccurate.
If your value changes like this, 65-50-65-50-65-50-66-51-62-50-65, etc., this will also make the machine obscured.
In general, this method of measuring resting pulse and normal regular movement (running, etc.) is relatively accurate, but for irregular movements, such as football badminton and other irregular movements, the assumed value of the example will be chaotic. Will be slightly less accurate. But it is also a few different values. For non-professionals, I feel that Xiaobian should be enough.
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