A variety of non-invasive heart rate measurement methods have been commercialized through smart watches, including photoplethysmography (PPG), electrocardiogram (ECG), arterial blood pressure, and video. In the future, the lack of measurement methods for production-level applications will gradually increase.
What is Photoplethysmography (PPG) used for?
An application of infrared non-destructive testing technology in biomedicine is the use of photoplethysmography (PPG) to measure human heart rate. With photoelectric sensors, it detects differences in light intensity reflected by blood and tissue after absorption, which can be used to track blood vessels. This can also be used for calculating the heart rate and the volume change during the cycle.
PPG's basic principle is that an emitter first emits light of a certain wavelength (for instance, green light is used to measure the heart rate), then the light is reflected and transmitted, and then the intensity of the light is measured by a photodiode. Because the reflection/transmission of light through the skin, the volume of the veins and veins of the capillaries does not change with each pulse, the reflection intensity of the above light is relatively fixed. On the other hand, only the arteries part of the blood volume changes with the pulse, and this part of the blood reflects the fluctuations in light. The pulse, which fluctuates in frequency with the heart rate, is usually the signal we are most interested in, so this fluctuation is the one we are tracking.
PPG, or photoelectric volumetric measurement, is the most widely used technique at present. In comparison with other methods, the biggest advantage of PPG is that it involves very little contact between the sensor and skin, even if the method is loose for the sensor to work. If there is an air gap in the middle of the sensor, the sensor remains able to calibrate itself perfectly, even in direct contact. This is extremely important in the real world use environment, as the feeling of discomfort when the back of the watch applying pressure continuously on the skin. Often we find that users are looking for wearables that are easy to wear.
Noise and Ambient Light Compensation
The watch panel and skin typically generate a great deal of noise in relative motion when the user is moving. To compensate for this noise, an accelerometer is usually placed next to the PPG sensor. There are different types of sensor vendors, but the solutions are basically the same. There are also multiple statistical principles for smart watches. For now, only the two most common ones will be presented.
Compensating for ambient light signals and motion noise is usually one of the primary purposes of signal processing and compensation circuits,As the human living environment also contains a strong light signal, there are some ambient light signals which are even stronger than the light generated by a LED itself, such as sunlight. Especially if the user wears the smartwatch loosely, ambient light leaks will drown out the LED's signal light, so the sensor must automatically compensate/filter out the ambient light for a more accurate result. As of now, there seems to be a certain gap between different sensor suppliers in terms of the solution, but it is not a significant one.
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