Before smartwatches emerged, early smartbands didn't even have a screen – some only had a single button and required connection to a mobile app to view synced data. The period from 2015 to 2018 can be considered the market expansion phase for smartbands. Essentially, smartwatches are an extension of smartbands; both belong to the same broad category. It's not necessarily true that a watch is always worth buying over a band, because while watches indeed have larger screens and more features, their battery life typically doesn't last as long as bands.
Therefore, when discussing the sensor technology in smartbands, it logically should include the smartwatch category. Furthermore, with technological advancements, the sensor technology in modern smartbands is increasing year by year. Limited by space, I'll briefly introduce a few common and several newer sensor technologies here.
1. Dedicated Step Counting: Accelerometer
Simply put, the accelerometer detects acceleration, converts it into electrical signals, and uses this to estimate steps, distance traveled, and calories burned. This generally involves technologies like Hall effect, GMR (Giant Magnetoresistance), TMR (Tunnel Magnetoresistance), and specific algorithms.
2. Heart Rate Monitoring: Optical Heart Rate Sensor & Bioelectrical Impedance Sensor
The most common method for heart rate monitoring is the optical heart rate sensor, a traditional sensor placed on the back of the band/watch. It works by emitting green LED light onto the skin and blood vessels pressed against the sensor. By calculating fluctuations in light absorption, it determines heart rate status, assists in activity detection, and can also detect heart abnormalities for timely alerts.
Another type is the bioelectrical impedance sensor, which utilizes the body's own impedance to monitor blood flow, converting this data into specific metrics like heart rate, respiration rate, and galvanic skin response. As it synthesizes diverse data, its detection accuracy is enhanced, making it more meaningful for reference.
3. Sleep Monitoring: Three Different Approaches
Basic sleep monitoring also relies on the accelerometer to determine if you are asleep. The principle is simple: during sleep, body movement is minimal and infrequent. If no movement is detected, it assumes you are asleep. This has a certain degree of accuracy but is prone to misjudgment. For example, if you lie still in bed continuously looking at your phone, this method might also register it as sleep.
The second method combines heart rate to determine sleep state, utilizing the heart rate sensor. It employs PPG (Photoplethysmography) to perform HRV (Heart Rate Variability) detection. This is more accurate than relying solely on the accelerometer.
The third method uses CPC analysis to detect sleep. The principle involves utilizing the coupling relationship between ECG (electrocardiogram) and respiration during sleep to comprehensively determine states of wakefulness, light sleep, and deep sleep. Currently, this method offers higher accuracy and can reduce misjudgment rates, such as when the user is ill or remains still but awake (as mentioned before). However, this approach is typically found in higher-end smart wearable products and is more expensive; it's generally not used in hundred-yuan bands or sub-thousand-yuan watches.
4. Blood Oxygen Saturation (SpO2) Monitoring: Optical Sensor
As mentioned before, the principle is consistent with heart rate monitoring: the module on the back, pressed against the skin, emits light, and a photoresistor detects fluctuations in the light partially absorbed by the blood to analyze blood oxygen status. The difference is that this process often uses infrared light and is susceptible to various interfering factors. Therefore, the accuracy of SpO2 monitoring on bands/watches is limited and should only be used as a reference.
5. Screen Brightness Adjustment: Ambient Light Sensor
Similar to the auto-brightness adjustment on smartphones, this function uses an ambient light sensor to detect surrounding light levels, ensuring the screen brightness is automatically adjusted to be clearly visible to the user.
6. Wrist-raise Wake Screen: Accelerometer and Gyroscope
This function detects the band/watch status using the accelerometer and gyroscope, often involving complex algorithms. Determining a "wrist-raise" involves assessing the device's position, changes in screen orientation, and other factors to ensure it's a genuine wake gesture, thereby avoiding unnecessary battery drain from accidental screen activations.
7. Global Positioning and Activity Route Recording: GPS Sensor
Like GPS in phones, smartwatches equipped with the same module can independently achieve positioning and track activity routes. However, this functional module is relatively expensive and less common in budget smartbands; it's more frequently found in professional sports watches.
8. Body Temperature Detection: Temperature Sensor
The detection principle is straightforward: using thermistors and high-precision temperature sensors to achieve single or continuous body temperature monitoring. This is commonly found in health monitoring smartwatches (e.g., those for blood pressure/blood sugar) launched in recent years, serving as an auxiliary function.
9. Blood Pressure & Blood Glucose Monitoring: Diverse and Complex Corresponding Sensor Modules, Possibly Relying Only on Algorithms
These two major monitoring functions are starting to lean towards the medical check-up domain, targeting middle-aged and elderly users and those with specific health monitoring needs. However, the accuracy of these functions varies significantly.
Taking smartwatches supporting blood pressure monitoring as an example, currently, conventional smartwatches only rely on optical sensors to estimate a range based on detected heart rate status. Those adopting the true oscillometric measurement principle are much more reliable. Such watches must incorporate a micro airbag in the wrist strap during measurement. This approach can at least produce values with some reference significance.
Regarding non-invasive blood glucose monitoring watches, this is currently a very non-standardized feature. Most rely on optical sensors + algorithms to provide estimates, which should only be considered as a reference value for daily body monitoring.
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