Modern wearables have become an integral part of the lives of millions of people, and the Xiaomi Mi Band is leading the segment due to its accessibility and functionality. However, few people think about the complex physical processes and software algorithms hidden behind a simple plastic case. Understanding how the tracker reads your movements and biometric data helps to use its capabilities more effectively and correctly interpret the data obtained.
The device works by integrating data from sensors that are processed by the microcontroller, the accelerometer and the optical sensor work in tandem, feeding raw data to the software layer for analysis, and it's the software shell that turns a set of electrical signals into user-readable metrics: kilometers traveled, calories burned, and sleep phases.
In this article, we will take a closer look at the architecture of a smart bracelet, explain the physics of each sensor, and explain the nuances of synchronization with a smartphone, explain why the device sometimes makes mistakes when counting steps, and how heart rate monitoring occurs without using electrodes.
Hardware architecture and basic components
The internal device of the tracker is a compact printed circuit board, which houses all the key elements. the centerpiece is a microcontroller that controls the periphery, processes incoming signals and provides communication with the outside world. Depending on the generation of the device, for example, Mi Band 7 or Mi Band 8, the power of the processor and energy efficiency can vary significantly.
A key element of the power system is a small-capacity lithium polymer battery, which, despite its modest size, can keep the device running for one to two weeks, and its energy consumption is strictly controlled by software: when the sensors are not active, the controller puts them into sleep mode to save charge.
The Bluetooth Low Energy (BLE) module is responsible for wireless communication, and this protocol is not chosen by chance, since it allows you to maintain a constant connection to your smartphone with a minimum amount of energy, data transfer is carried out in short packets, which ensures the stability of the connection even at a distance of 10 meters.
⚠️ Warning: Opening the bracelet body breaks factory tightness, after self-repair, the device may lose protection from moisture and cease to be waterproof.
An important component is the vibrator motor, which provides tactile returns at notifications and alarms. The latest models use a linear vibrator motor, which creates clearer and more pleasant vibrations compared to the eccentric counterparts in the older versions.
Mechanics of step counting and accelerometer
The main tool for tracking activity is a three-axis accelerometer, which is a miniature sensor that responds to the change in position of the device in space and measures the acceleration along three axes: X, Y and Z. When you take a step, the hand makes a characteristic movement, which is recorded by the sensor.
The resulting acceleration data is converted into a digital signal and transmitted to algorithms for processing. The system analyzes the amplitude and frequency of the oscillations to distinguish walking from other movements, such as waving your hands or typing on a keyboard. Noise filtering is critical here, because without it, the number of steps would be greatly inflated.
The algorithm takes into account individual gait characteristics if the user has precalibrated the device or entered his anthropometric data. Step length is calculated based on the growth of the user, which allows more accurate determination of the distance traveled.
And it's worth noting that the accelerometer is not perfect, and it can be wrong if you're driving a wheelchair or pushing a cart in the supermarket, because the hand moves differently than when you're walking freely, and understanding this limitation helps you not to be too fanatical about numbers.
The principle of operation of the optical pulse meter
Xiaomi's heart rate monitoring is implemented with PPG technology, and the back of the capsule is green LEDs and photo sensors, based on the change in blood volume in the wrist capillaries during the heart cycle.
Green light is not chosen by chance: the hemoglobin in the blood actively absorbs light in this spectrum. When the heart contracts and ejects blood, the absorption of light increases, and when it relaxes, it decreases. The sensor detects these fluctuations in the intensity of reflected light.
The frequency of these fluctuations is your pulse in beats per minute. Modern models like the Xiaomi Smart Band 8 use an array of multiple LEDs and sensors to improve measurement accuracy, which allows you to compensate for signal loss when you're driving actively.
- 🟢 Green LEDs are used to continuously monitor the pulse at rest and with light activity.
- 🔴 Red and infrared sensors (in some models) can be used to measure blood oxygen levels (SpO2).
- 📉 Motion compensation algorithm tries to filter out noises caused by shaking your hand while running.
⚠️ Warning: Tattoos, thick wrist hair, or too weak fit of the bracelet can block the light of the sensor, leading to a lack of readings or serious errors.
It's important to understand that the optical method is inferior to the chest girdle, especially at interval loads, when the pulse changes very quickly, but for everyday use and amateur sports, the accuracy of the Mi Band is quite sufficient.
Sleep phase analysis and night monitoring
Night tracking is one of the most popular features, which works on the basis of complex data analysis. The bracelet uses a combination of readings from an accelerometer and a pulsemeter to determine the state of the user, and when you fall asleep, your motor activity decreases and your heart rate becomes more even.
The algorithm distinguishes several phases: superficial sleep, deep sleep and sleep. REM-Deep sleep is characterized by minimal mobility and a reduced, stable pulse, during which the body undergoes major recovery.
REM, or paradoxical sleep, is often accompanied by small movements and changes in heart rhythm, which sensors successfully detect. Based on the duration of each phase, a final rest quality report is formed, which is synchronized with a smartphone in the morning.
Why can’t the bracelet see a day’s sleep?
The accuracy of sleep phases is not medical, but it gives a good idea of rest dynamics, and regular wear of the device reveals patterns between lifestyle and sleep quality.
Data synchronization and application operation
The bracelet itself is just a data collector, all analytics are done in the cloud or on a smartphone. The Mi Fitness app (formerly Mi Fit) is used to transmit information, and the connection is automatically set up when a smartphone approaches, and the data is uploaded in the background.
The synchronization process involves transferring the data accumulated over the day or week about activity, sleep and heart rate. The app visualizes this data in the form of graphs and diagrams, allowing you to track progress. Without an Internet connection, the graphs will be based on the phone data, but the full story will be stored in the cloud after you connect.
In the app settings, you can set the pulse rate, from economy mode to measurement every minute. The more often you take the measurement, the faster the battery goes down, but the more detailed the statistics, and you can set up notifications from applications that are duplicated by vibration on the wrist.
| Parameter | Description of function | Impact on the battery |
|---|---|---|
| Pulse monitoring | Background heart rate measurement | High (with frequent measurements) |
| Notifications | Vibration in calls and messages | Low. |
| Wrist lifting | Activation of the screen when moving the hand | Average. |
| NFC module module | Payment and access (in NFC versions) | Short-term high |
It is important to remember that for proper work GPS-Tracking while jogging the bracelet often uses GPS-In this case, the phone must be close by, and the application must have permission to use geolocation.
Accuracy of measurements and influence factors
No wearable device gives you absolute data, and the Mi Band is no exception, and there are many external and internal factors that influence accuracy, and understanding these nuances will help you avoid disappointment and misguided conclusions about your health.
One of the main factors is the density of the strap, and if the bracelet dangles, the pulse sensor loses contact with the skin or catches light from outside light, and if the strap is too tight, it can clamping the bloodstream, which will also distort the readings.
- 🌡️ Ambient temperature: in severe frost, the vessels narrow, which can make it difficult to read the pulse with an optical sensor.
- 🏃 Intensity of movements: with sharp swings of hands (for example, in tennis), the accelerometer can count extra steps.
- 🧬 Individual features: skin color and tattoos affect the passage of light through tissues.
💡
For maximum accuracy of pulse measurements during exercise, try to keep your hand still when the bracelet is measuring, or look at the screen to initiate a forced measurement.
It is also important to update the firmware of the device periodically, and Xiaomi engineers are constantly improving the data processing algorithms, correcting errors and improving the accuracy of sensors in new versions of the software.