With the arrival of the intelligent era, the research and application of various smart sensors have been increasingly emphasized. Smart sensors also have rich information processing capabilities on the basis of traditional sensors and can provide more comprehensive functions.
This paper introduces the current research status of commonly used temperature, pressure, inertial, biochemical and RFID sensors and their key roles in industrial upgrading and innovative applications such as the Internet of Things (IoT), Virtual Reality (VR), robotics, healthcare, etc., and looks forward to the future development trend of smart sensors.
Smart sensor (smart sensor) refers to the sensor with information detection, information processing, information memory, logical thinking and judgment functions.
Relative to the traditional sensors that only provide analog voltage signals that characterize the physical quantity to be measured, smart sensors make full use of integration technology and microprocessor technology, set perception, information processing, communication in one, and can provide a certain level of knowledge of the information disseminated in digital form.
Since NASA (NASA) proposed the concept of smart sensors in the 1980s, after decades of development, smart sensors have become a major development direction of sensor technology, representing a country's industrial and technological research capabilities.
Driven by the current smart era, the importance of sensors has become more prominent, not only in the "Made in China 2025", "Germany 2020 High-Tech Strategy" and the European Union, the United States, South Korea, Singapore, etc. to promote the smart city and other strategies to play an important supporting role, but also in the Internet of Things (IoT), Virtual Reality (VR), robotics, smart homes, self-driving cars and other industrial development plays a Key Role.
High-performance, highly reliable multifunctional complex automatic, measurement and control systems and the rise and development of the Internet of Things based on radio frequency identification technology, more and more highlights the importance of smart sensors with perception, cognitive ability and its vigorous and rapid development of the urgency.
With the development of CMOS-compatible MEMS technology, the development of miniature smart sensors has received strong technical support, smart sensor industry is facing a very important historical development opportunity.
This paper summarizes the development status of different kinds of smart sensor technologies and applications, and makes an outlook on the future development trend.
1 Rich and diverse intelligent sensors that
In order to meet the needs of various intelligent applications, the sensor categories are very diverse, such as: environmental sensors, inertial sensors, analog sensors, magnetic sensors, biosensors, infrared sensors, vibration sensors, pressure sensors, ultrasonic sensors and so on.
Among them, the following sensors are more commonly used.
Environmental sensors, mainly gas sensors, barometric pressure sensors, temperature sensors, humidity sensors and so on. Gas sensors can be applied to air purifiers, DUI monitors, detectors of formaldehyde and other toxic gases in home furnishings, as well as detection devices for industrial exhaust gases. As people pay attention to environmental issues, the importance of environmental sensors is becoming more and more prominent, and there is a lot of room for development in the future.
Inertial sensors, mainly used in wearable products, such as smart bracelets, smart watches, VR helmets and so on. The inertial sensor is used to detect the tracking and identification of movement, informing the wearer of the amount of exercise, calories consumed and the effect of exercise on that day.
Magnetic sensors, which are mainly used in household appliances, such as coffee makers, water heaters, air conditioners, etc., are used to detect how much the angle has been turned or how much travel has been done, and are usually displayed on the dashboard. In addition, magnetic sensors are also used in door magnets and window magnets, etc. The intelligence and accuracy of robots also need magnetic sensors for support.
Analog sensors, which are mainly used in smart medical devices, can be used as inputs for signals such as heartbeat and electrocardiograms, and visualize the output of health data, so that users can understand their first-hand health and exercise data.
Infrared sensors are often used in infrared cameras, sweeping robots and other smart home.
2 Progress of technical research on smart sensors
A truly intelligent sensor should have the following functions:
1) self-calibration, self-calibration and automatic compensation functions;
2) automatic data collection, logical judgment and data processing functions;
3) self-adjustment and self-adaptation functions;
(4) A certain degree of storage, identification and information processing functions;
5) two-way communication, standard digital output or symbol output function;
(6) algorithmic judgment, decision-making and processing functions.
The following is an example of commonly used temperature, pressure, inertial, biochemical and RFID sensors to introduce the research progress of intelligent sensing technology.
2.1 Smart Temperature Sensors
The development of temperature sensors has gone through the following three stages: traditional discrete temperature sensors, analog integrated temperature sensors and intelligent temperature sensors as a whole.
Into the 21st century, intelligent temperature sensors are moving towards high precision, multi-functional, bus standardization, high reliability and safety, the development of virtual sensors and network sensors, the development of monolithic temperature measurement system and other directions of rapid development.
The current smart temperature sensor contains a temperature sensor, A/D converter, signal processor, memory and interface circuits, some products also have a multiplexer, central controller, random access storage and read-only memory.
Intelligent temperature sensors are characterized by the ability to output temperature data and the related temperature control volume, adapted to a variety of micro-controllers, and is based on hardware through the software to achieve the test function, the degree of intelligence depends on the level of software development.
(1) Improve the measurement accuracy and resolution
The earliest intelligent temperature sensors began in the mid-1990s, using 8-bit A/D converter, its temperature measurement accuracy is low, the resolution can only reach 1 ℃.
At present, foreign countries have launched a variety of high-precision, high-resolution intelligent temperature sensors, using 9 ~ 12-bit A/D converter, the resolution can reach 0.5 ~ 0.625 ℃. By the U.S. Dallas Semiconductor Corporation's newly developed DS1624-type high-resolution intelligent temperature sensor that can output 13-bit binary data, resolution as high as 0.03 ℃, temperature measurement accuracy of ± 0.2 ℃.
In order to improve the conversion rate of multi-channel intelligent temperature sensor, there are also chips using high-speed successive approximation A/D converter. AD7817 type 5-channel intelligent temperature sensor, for example, it is the local sensor, each remote sensor conversion time of only 27 ms, 9 ms, respectively.
In terms of high-precision temperature measurement, some scholars have designed a high-performance digital temperature sensor, which consists of a quartz tuning fork resonator, a digital interface circuit and a sensor reset control algorithm based on the field-programmable gate array, and the sensitivity of the sensor can reach the number of 10 -6 ℃, that is, the temperature measurement resolution of 0.001 ℃, the response time of 1 s, and the accuracy of the measurement is 0.01 ℃.
(2) Enhanced test functions
The new intelligent temperature sensor test function is constantly enhanced. Intelligent temperature sensors have a variety of operating modes to choose from, including single conversion mode, continuous conversion mode, standby mode, and some also added a low-temperature limit expansion mode.
For some intelligent temperature sensors, the host (external microprocessor or microcontroller) can also be set through the corresponding registers of its A/D conversion rate, resolution and maximum conversion time.
In addition, intelligent temperature sensors are developing from single channel to multi-channel direction, which creates good conditions for the development of multi-channel temperature measurement and control system.
(3) Standardization and standardization of bus technology
At present, the intelligent temperature sensor bus technology has also been standardized and normalized, the bus used are mainly single-wire (-Wire) bus, I 2 C bus, SMBus bus and SPI bus.
(4) Reliability and safety design
In order to avoid false action when the temperature control system is subjected to noise interference, in the internal of some intelligent temperature sensors, a programmable fault queuing counter is set up, dedicated to setting the number of times the measured temperature value is allowed to exceed the upper and lower limits. The interrupt port can be triggered only when the number of times the measured temperature continuously exceeds the upper limit or falls below the lower limit reaches the set number of times, which avoids the influence of occasional noise interference on the temperature control system.
In order to prevent damage to the chip due to human electrostatic discharge, some intelligent temperature sensors have also increased the electrostatic protection circuit, generally can withstand 1 ~ 4 kV of electrostatic discharge voltage.
For example, the TCN75 intelligent temperature sensor serial interface, interrupt/comparison signal outputs and address inputs can withstand 1 kV of electrostatic discharge voltage. LM83 intelligent temperature sensor can withstand 4 kV of electrostatic discharge voltage.
2.2 Smart Pressure Sensors
Intelligent pressure sensors are a combination of microprocessors and pressure sensors, so their implementation paths can be divided into: non-integrated intelligent pressure sensors, integrated intelligent pressure sensors and hybrid intelligent pressure sensors.
Non-integrated smart pressure sensors are smart pressure sensor systems that combine traditional pressure sensors, signal conditioning circuits, and microprocessors with digital bus interfaces into a single system.
This non-integrated pressure sensor is actually a conventional pressure sensor system with an added microprocessor connection. Therefore, it is one of the fastest ways and means to realize a smart pressure sensor system.
The integrated intelligent pressure sensor is a monolithic integration of pressure sensitive elements with signal processing, calibration, compensation, microcontroller, etc. It mainly adopts microelectromechanical system (MEMS) technology and large-scale integrated circuit process technology, and utilizes silicon as the base material to make the sensitive elements, signal conditioning circuits, and microprocessor units, and integrates them on a single chip.
With the rapid development of microelectronics technology and the application of micro-nanotechnology, the resulting smart pressure sensor has the characteristics of miniaturization, structural integration, high precision, multi-function, array, full digitalization, easy to use, easy to operate and so on.
Hybrid intelligent pressure sensor is according to the needs and possibilities of the system of various integrated links, such as sensitive units, signal conditioning circuits, microprocessor units, digital bus interfaces, in different combinations integrated in 2 ~ 3 chips, and packaged in a shell.
Hybrid integration to achieve intelligence is an intelligent pathway that is well suited to the current technological development. In the intelligent pressure sensor system, the microprocessor can realize software control of the sensor in accordance with the given program, changing the sensor from a single function to a multi-function. Intelligent pressure sensors generally have the following basic functions .
(1) data processing functions. Intelligent pressure sensors not only measure the measured parameters, but also according to the known measured parameters, can be automatically zeroed, automatically balanced, automatically compensated and so on.
(2) Automatic diagnostic function. This is the main function of intelligent pressure sensors, intelligent pressure sensors through its fault diagnosis software and self-testing software, automatic sensors and system status for regular and irregular detection, testing, timely detection of faults, to assist in the diagnosis of the cause of the failure, the location, and to give operational tips.
(3) Software configuration function. Intelligent pressure sensors due to the use of microprocessors, so there are not only the necessary hardware components, such as detection, amplification, A/D, D / A, communication interfaces, etc., but also software resources for the control and processing of data. In the intelligent pressure sensor, there is a multi-modular hardware and software set up so that the user can send commands through the microprocessor to accomplish different functions, which increases the flexibility and reliability of the sensor.
2.3 Intelligent Inertial Sensors
Inertial sensors, the most widely used type of MEMS sensors, include accelerometers, gyroscopes, and orientation sensors.MEMS technology has the unique advantage of miniaturizing inertial sensors and reducing costs.
Today's inertial measurement modules (IMUs) can integrate a triaxial accelerometer, triaxial gyroscope, and triaxial magnetometer in a size of 10 mm x 10 mm x 4 mm at a cost of less than a dollar. This inertial measurement module can be applied to smartphones and wearable devices to realize sports and health functions including gait monitoring, step counting, fall detection, sleep monitoring, indoor navigation, etc., as well as entertainment functions such as gesture recognition and direction sensing.