Sensor Technology in the Information Age

Information from the real world is obtained through sensors , which are closely related to people's lives. Sensors have entered the fields of industrial automation , automotive industry, aerospace, biology, and medicine in a big way, and there is also a wide range of development space in the fields of wireless communication and consumer products. There are many types of sensors, involving physics, chemistry, electronics, mechanics, biology, medicine and other disciplines. Among the many sensor technologies, microsystem technology plays a key role. Various types of reliable sensors have been developed using this technology. In addition, the wireless networking of sensors is also a hot topic at present, which has opened up another way for sensor applications. This article will introduce the development in this area in detail. Microsystem technology MST (microsystem technology) refers to a technology that uses two types of microstructure components, active and passive, in the system. MST implies MEMS (microelectromechanical system), and the two terms are often used interchangeably. Microstructure or microcomponent refers to the process and technology with a length unit of micrometer level, with a minimum of 100nm. The rise of this technology has also led to the emergence of NEMW (nanoelectromechanical system), which can be regarded as an extension of MEMS. MNT (micro and nano technology) is a fusion of the two, and is a more advanced technology that combines the characteristics of both processes and technologies. The advantages of sensor miniaturization are self-evident. Since it is processed using the same materials and processes as semiconductor devices, embedded sensors, array sensors, multi-function sensors, and smart sensors can be made. Due to the small size and low power consumption of the sensor, it is convenient for distributed measurement, and further can realize wireless sensor networks. The automotive industry is a large market for MEMS sensors. In addition to the current temperature, pressure, airbag accelerometers and wheel speed sensors, it will also be widely used in tire pressure monitoring, vehicle dynamic control and gyroscope/speed sensing, brake pressure sensing, engine ejection pressure sensing, and fuel vaporization pressure sensing. Accelerometer is a more representative product. Early MEMS adopted a multi-chip solution, with the sensor element (MEMS structure) on one chip and the signal conditioning circuit on another chip. From the perspective of process processing, this method is relatively simple, but there are many disadvantages: * Increased total silicon wafer area. * Multi-chip modules require additional assembly steps. * Requires a large sensor output signal to overcome the stray capacitance formed by the chip-to-chip interconnection and the inherent stray field effects of the larger sensor structure. * Low yield rate The latest generation of accelerometer sensors are all single-chip, such as AD's ADXL 202E integrated MEMS accelerometer, which is a small, inexpensive, low-gravity integrated dual-axis accelerometer that is currently in mass production. Figure 1 is a schematic diagram of the mechanical structure of the ADXL202E. The MEMS structure is suspended on the substrate with polysilicon springs, and the sensor body can move along the X-axis and Y-axis. There are 32 sets of radial finger-shaped crossbars around the square sensor body. These finger-shaped crossbars are located between two plates fixed to the substrate. Each finger-shaped crossbar and a pair of fixed plates form a differential capacitor. Any deviation of the sensor body from the sub-center position is determined by measuring the differential capacitance. This sensor can measure both dynamic acceleration (such as shock or vibration) and static acceleration (such as inclination or gravity).

Figure 1 Schematic diagram of the mechanical structure of ADXL202E

Figure 2 is the general block diagram of the ADXL202E. The differential capacitance is measured using a synchronous modulation/demodulation technique. The amplified X-axis and Y-axis acceleration signals are added to the CX and CY output pins and the PWM modulator through 32KΩ resistors, respectively. Adding capacitors at the CX and CY pins limits the bandwidth, thereby reducing the noise level. The PWM is proportional to acceleration, and its output can be directly sent to the digital input of a microprocessor, where a counter is used to demodulate the PWM signal.

Figure 2 ADXL202E block diagram

There are many challenges in MEMS design. The first is the mechanical problem. Even for the simplest system, the mechanical properties of each component must be understood. Most MEMS systems use polysilicon to build micromechanical structures, but its mechanical properties are not very ideal. In addition, what changes will occur in the mechanical properties of materials in the microscopic world? The second is the electrical measurement of the sensor. The sensor uses differential capacitance to achieve measurement, and then amplifies and linearizes the signal, and in some cases, temperature compensation is also required. The signal output by the differential capacitor is very small and has reached the limit of measurement. For integrated devices, there is still the problem of mutual influence between mechanical properties and electrical properties during the processing. Improving the mechanical structure affects the electrical performance, and vice versa. For example, annealing can increase mechanical strength, but it deteriorates the performance of BiMOS devices. Therefore, it is necessary to improve the process technology that takes into account both mechanical processing and electrical performance. At present, the design of devices is not done manually, and CAD tools and simulation software are widely used. However, there is no standard design software in the MEMS field. It is necessary to further strengthen the development of MEMS CAD tools, simulation software, and diagnostic software.

Biomedical sensors are a very active research field. The advantage of MST in biomedical diagnosis lies in the miniaturization of the measurement system, which can greatly reduce the number of samples and reactants. MST biosensors are composed of two parts: microstructured elements produce measurable outputs caused by changes in physical or biological properties; the biological part is usually an enzyme, antibody, nucleic acid, microtissue or cell, which uses the selectivity of antibody-antibody to provide a sensor response to a specific substance. IVD (clinical diagnosis) and biochips are the main MST products. IVD typical products are fusible blood test strips for measuring blood sugar. Biochips refer to DNA chips, protein chips, gene chips, microarrays, etc. DNA and protein microarray immunoassays use silicon or glass to process microarray structures and use fluorescent markers for detection. The thickness of the protein or DNA layer is about 10nm and is deposited on the microarray by printing or sputtering.

Microspectrometers are used for portable spectrometer-based applications. LIGA uses deep X-ray lithography and electroplating technology to make metal micromolds, and then uses this micromold to cast plastic gratings. LIGA can process a variety of materials, with the advantages of small structure (micrometer level), large aspect ratio (up to 100) and good surface finish (less than 50nm). MSC mass spectrometer also has MST version, mainly involving magnet part, WEIN filter, traveling wave tube, etc. The reduction of MS volume means that the mean free path of ions can be reduced, that is, it can work at a higher pressure (10-2mbar). The combination of small volume and high pressure creates conditions for omitting large and expensive vacuum pump systems. Research topics include silicon-based hot filament source MSTQMS (quadrupole mass spectrometer), new micro-machined electron spectrometer and ion capture MS.

LOC (chip-scale laboratory) integrates a variety of chemical processing and analysis technologies, and is processed and manufactured with silicon, glass and polysilicon materials. Since the performance of glass is relatively stable, glass substrates are generally preferred. The technologies to be solved are cheap light sources, detection arrays and integrated electronic circuits. Microfluidics refers to the technology of controlling liquid on a chip.

Wireless sensor technology

Replacing wired with wireless is the general trend of electronic technology, and sensor technology is no exception. In most new industrial systems, wireless links are indeed much more economical and convenient than wiring. Early products were point-to-point links, such as wireless weighing devices, which had an electronic scale on one end and a standard digital reader or display on the other. Motorola combines wireless sensors with communication technology to develop a wireless home monitoring and control system with wireless temperature sensors, wireless water leakage sensors, wireless door/window sensors, and wireless cameras. This type of sensor can preset upper and lower limits. Once an abnormal measurement occurs, it will alert the user by phone or email, and promptly notify the user of the fault and troubleshoot the fault. RFID (radio frequency identification) sensor tags are a very distinctive technology. RFID tags are integrated circuits. The chip contains the product's ID data, and the reader can read the data stored in the chip from a long distance. RFID was originally used for logistics tracking and supervision systems. If RFID is combined with sensing technology to make a sensor tag, it will not only contain the electronic barcode information of the goods, but also carry real-time information such as temperature, pressure, and location. MicroStrail Inc's Embed sense wireless sensor is such a product. It is equipped with sensor and data acquisition functions and can be embedded in products to create smart materials, smart structures, and smart machines. The product uses an inductor to obtain energy from an external coil, and provides 3V, 200μA power after rectification of the external magnetic field, and the reader obtains digital stress, temperature, and ID information. Crossbow is a company that can provide large-scale commercial smart particle wireless sensors. The hardware platform consists of a processor/radio board (MPR) commonly known as MOTES. The wireless network processor node can be integrated with the RFID reader component to form a cheap, mobile, networked RFID tag reader. This battery-powered device uses the Tiny OS operating system and supports dual-channel wireless networks. The sensor and data acquisition card (MTS and MDA) can have built-in sensors or external sensors, and the card is fully matched with the processor and radio board. Gateways and interface products (MIB) allow developers to interface MOTES to PCs, PDAs and other existing networks and protocols. The Tiny OS operating system is open source, expandable and scalable. ZigBee is the first choice for low-speed application wireless networks. The IEEE standard name for ZigBee is 802.15.4. The 802.15.4 standard defines the PHY layer and MAC layer, while the ZigBee Alliance sets standards for the network, security and application layers. ZigBee has three versions. The 868MHz European version has a data rate of 20kb/s; the 10-channel 915MHz version has a data rate of 40 kb/s; the real speed supreme is the 2.4GHz version, 16 channels, and a data rate of 250 kb/s. All three versions use DSSS (direct sequence spread spectrum) technology. ZigBee is also a PAN (personal area network) technology that can be configured in star, tree, and network topologies, with very ideal results. Its most important advantages are ultra-low power consumption and simplicity. In some cases, low power can keep the battery working for several years. Motorola claims to have developed a variety of standard-based pressure, accelerometer and other wireless sensor integrated circuits. The company takes full advantage of its MEMS-based sensors and advanced IEEE802.15.4RF technology to create economical and practical terminal products. The product uses standard hardware interfaces and optional OEM and embedded processors. The RF transceiver operates in the global 2.4GHz frequency band, and the communication transmission uses a symmetric key code of the 128-bit AES (Advanced Key Standard) algorithm to enhance the confidentiality of the communication. Ember's EM2420 wireless solution is an ideal communication product. It operates in the 2.4GHz global ISM band, 16 channels, and a channel spacing of 0.5MHz. 250Kbps QQOSK DSSS fully complies with the IEEE802.15.4 specification, 0dBm output power, and a working distance of 75m; it also has a +10 dBm output amplifier for longer distance transmission. It has good security and has hardware-based CRC and AES-128 keys. The device has an integrated CMOS transceiver and on-chip TX/RX switch, and the chip is packaged in a 7mm×7mm, 48QLP, requiring only a few external components. The EM2420 is equipped with a variety of high-speed synchronous serial hardware interfaces to communicate with the host processor. To facilitate user development, the Ember Net API software library provides a simple and consistent interface for routing, discovery, and management functions in the Ember Net protocol stack.











Millennial Net is a wireless sensor network product that provides OEMs and system integrators with a comprehensive platform to deploy efficient, scalable, self-organizing wireless sensor network systems. It is available in a variety of small-size modules that can be easily embedded and integrated into user devices. The IB-5324 is IEEE802.15.4 compatible and uses a 2.4GHz DSSS wireless solution to resist RF interference and provide data confidentiality. The IB-5324 Endpoint streamlined functional component is a small microcomputer with digital and module interfaces and wireless communication links. It uses a dual MCU design to facilitate the development of user applications. An application MCU merges and pre-processes sensor data. In addition, the system also provides routers, USB routers and gateways to complete the ground connection between the network and the host. Millennial Net's Windows XP or 2000-based network monitoring software serves as a graphical user interface to control and monitor wireless networks. The complete API software library allows users to use Microsoft Visual C/C++ to develop various applications and programs.

Conclusion

In summary, sensor technology is widely used and closely related to many disciplines. Sensor technology is a comprehensive high-tech, which integrates optics, mechanics, electricity, and biomedicine. It is no exaggeration to say that the level of sensor technology reflects the level of microelectronics, MEMS, nanotechnology, optoelectronics, biotechnology and other high-tech technologies. Therefore, in order to further promote the development of sensor technology, it is crucial to cultivate comprehensive talents, coordinate efforts among various disciplines, and establish new experimental bases and production bases.