Application of sensors in smart wearable devices

Wearable devices are computing devices that can be installed on people, animals and objects and can sense, transmit and process information. Sensors are the core components of wearable devices. The sensors in wearable devices are an extension of human senses and enhance the human "sixth sense" function.

With the development of biotechnology and the miniaturization and intelligence of sensors, wearable devices may evolve into smart devices implanted in the human body.

In all kinds of refreshing wearable devices, there is a key device - sensor. It can sense changes in external conditions, such as cold or warm, fast or slow, and respond accordingly, just like our skin.

What sensors are in wearable devices?

Generally speaking, sensors in wearable devices can be divided into the following categories.

1. Motion Sensor

Motion sensor Motion control sensor is a component that converts changes in non-electrical quantities (such as speed and pressure) into changes in electrical quantities. It includes: acceleration sensor, gyroscope, geomagnetic sensor or electronic compass sensor, atmospheric pressure sensor (the altitude can be calculated by measuring atmospheric pressure), etc.

The main functions of these sensors include motion detection, navigation, entertainment, human-computer interaction, etc. Among them, electronic compass sensors can be used to measure direction and realize or assist navigation. Life lies in movement, and movement is an indispensable and important part of life. Therefore, it is of great value to measure, record and analyze human activities anytime and anywhere through motion sensors. Users can know the number of running steps, swimming laps, cycling distance, energy consumption and sleep time, and even analyze sleep quality, etc.

Functions and implementation principles of motion sensors

By measuring, recording and analyzing human activities anytime and anywhere through motion sensors, users can know the number of running steps, swimming laps, cycling distance, energy consumption and sleep time, etc., while also being able to navigate, entertain, and interact with people.

Taking the function of a wearable human motion capture system as an example, in order to capture human motion in real time, a wearable human motion capture system needs to be designed. It obtains real-time human posture information through inertial measurement units distributed on the human body. Each inertial measurement unit is composed of a micro MEMS 3-axis gyroscope, a MEMS 3-axis accelerometer, a MEMS 3-axis magnetometer and an MCU. The MCU obtains the data of each sensor and uses the extended Kalman filter based on quaternions to solve the posture angle of the corresponding part. The data is uploaded to the computer in real time through the CAN bus and Bluetooth module. The computer drives the virtual human motion through VC++ and OpenGL programs to realize real-time human motion reproduction.

The functions that motion sensors need to achieve include the collection of sensor data from three different types of inertial devices, attitude angle calculation and wired communication. The motion sensor mainly consists of five parts: MCU, accelerometer, gyroscope sensor, magnetometer and CAN interface. MCU includes modules such as synchronous serial communication bus (I2C), asynchronous receiver transmitter (UART), and buttons, which control a series of operations of the node. The node is powered by wires, and the MCU controls the data collection, attitude angle calculation and CAN transmission and reception of the three sensors.

The hardware system block diagram is shown below:

2. Biosensors

Biosensors are instruments that are sensitive to biological substances and convert their concentrations into electrical signals for detection. They are analytical tools or systems composed of immobilized biological sensitive materials as identification elements (including enzymes, antibodies, antigens, microorganisms, cells, tissues, nucleic acids and other biologically active substances) and appropriate physical and chemical transducers (such as oxygen electrodes, photosensitive tubes, field effect tubes, piezoelectric crystals, etc.) and signal amplification devices. Biosensors have the functions of receivers and converters.

Biosensors include blood glucose sensors, blood pressure sensors, electrocardiogram sensors, electromyography sensors, body temperature sensors, brain wave sensors, etc. The main functions realized by these sensors include health and medical monitoring, entertainment, etc.

Functions and implementation principles of biosensors

Health warning and disease monitoring: with the help of wearable technology, doctors can improve their diagnosis level and family members can communicate better with patients.

For example, wearable medical devices composed of blood pressure sensors can track and monitor users' body data, analyze and extract medical diagnostic models, predict and shape users' health development status, provide users with personalized cardiovascular medical care and health management plans, and also help family members care about the health of their loved ones.

The blood pressure monitor uses a sensor to detect the tiny changes in the cuff pressure caused by the vibration of the human arterial wall. The most commonly used method is the oscillation method. Its basic principle is to use the cuff tied to the arm and inflate the cuff through an air pump to block the propagation of pulsation in the blood vessels. After reaching a certain pressure (generally 124~316kPa), it begins to deflate. When the air pressure reaches a certain level, blood can flow through the blood vessels, and there is a certain amount of oscillation wave. It gradually deflates, and the oscillation wave becomes larger and larger. When it is deflated again, the contact between the cuff and the arm becomes looser and looser. Therefore, the pressure and fluctuation detected by the pressure sensor become smaller and smaller, and the pressure sensor can detect the pressure and fluctuation in the cuff in real time. The oscillation wave is transmitted to the pressure sensor in the machine through the trachea. After the corresponding amplification, filtering circuit, analog/digital signal conversion, central processor control and other processing links, the pulsation signal and pressure signal transmitted to the gas path through the cuff are converted into digital signals, and then further processed to obtain the systolic pressure, diastolic pressure, mean pressure and other data of blood pressure. This dynamic blood pressure monitor can be connected to mobile devices via Bluetooth and USB, and upload data to medical staff. It is usually worn outside by the user to provide 24-hour blood pressure monitoring.

In addition, wearable devices that sense changes in human emotions through sensors such as EEG and ECG can achieve entertainment interaction. For example, the Mind Cat Ears uses NeuroSky's advanced TGAM EEG chip, which can read human brain waves. Different patterns of brain waves represent different emotions and states of people. The chip converts the EEG signals representing people's emotional states into digital signals that can be recognized by the cat ears, thereby executing corresponding instructions and completing different actions. For example, when a person is in a state of concentration, it will stand up high, and when relaxed, it will droop down.

3. Environmental Sensors

Environmental sensors include: soil temperature sensors, air temperature and humidity sensors, evaporation sensors, rainfall sensors, light sensors, wind speed and direction sensors, etc. They can not only accurately measure relevant environmental information, but also connect to the host computer to achieve networking, which can meet the user's testing, recording and storage of measured data to the maximum extent. They are the first choice of high-quality instruments for scientific research, teaching, laboratories, and agricultural soil and fertilizer stations, agricultural science institutes and related agricultural environmental monitoring departments.

Functions and implementation principles of environmental sensors

In today's world, people often live in environments that threaten their health, such as air/water pollution, noise/light pollution, electromagnetic radiation, extreme climate, etc. What's more frightening is that we often don't even know that we are in such an environment, such as PM2.5 pollution, which can cause various chronic diseases. The air quality detection device composed of particle sensors - PM2.5 portable detector, can be worn on the human body, and can be displayed alone or used in conjunction with a mobile phone and shared with friends.

The portable PM2.5 detection uses an air pump dust monitor to suck the aerosol to be tested into the detection chamber. The aerosol to be tested is split into two parts at the dust monitor splitter. One part is filtered into clean air after passing through a high-efficiency filter, which serves as a protective sheath gas to protect the components of the sensor room from being contaminated by the gas to be tested. The other part of the aerosol, as the sample to be tested, directly enters the sensor room.

Particles and molecules will scatter light when exposed to light, and at the same time, absorb part of the energy of the irradiated light. When a beam of parallel monochromatic light is incident on the particle field to be measured, it will be affected by the scattering and absorption around the particles, and the light intensity will be attenuated. In this way, the relative attenuation rate of the incident light passing through the concentration field to be measured can be obtained. The magnitude of the relative attenuation rate can basically linearly reflect the relative concentration of dust in the field to be measured. The magnitude of the light intensity is proportional to the strength of the electrical signal converted by photoelectric conversion, and the relative attenuation rate can be obtained by measuring the electrical signal.