The electronic nose is an electronic system that uses the response patterns of the gas sensor array to identify odors. It can continuously and real-time monitor the odor status of a specific location in a few hours, days, or even months. The electronic nose is mainly composed of three functional devices, an odor sampling manipulator, a gas sensor array, and a signal processing system. The main mechanism for the electronic nose to recognize odors is that each sensor in the array has a different sensitivity to the measured gas. For example, the No. 1 gas can produce a high response on a certain sensor, but it has a low response to other sensors. The sensor with high response of No.2 gas is not sensitive to No.1 gas. In the final analysis, the response pattern of the entire sensor array to different gases is different. It is this difference that enables the system to identify odors based on the response pattern of the sensor. . There are many types of electronic noses, and the typical working procedure is: First, a vacuum pump is used to suck air sampling into a small container chamber equipped with an electronic sensor array. Then, the sampling operation unit exposes the initialized sensor array to the odorant, and when the volatile compound (VOC) comes into contact with the surface of the sensor's active material, it produces a transient response. This response is recorded and transmitted to the signal processing unit for analysis, compared with a large number of VOC patterns stored in the database, and identified to determine the type of odor. Finally, use alcohol vapor to "wash" the surface of the sensor's active material to remove the measured odor mixture. Before entering the next round of new measurement, the sensors still have to be initialized again (that is, each sensor needs to be cleaned with dry gas or some other reference gas between work to reach the reference state). The time of the measured odor effect is called the "response time" of the sensor array, and the time spent in the cleaning process and the initialization process of the reference gas action is called the "recovery time". In the electronic nose system, the gas sensor array is the key factor. In addition to the basic gas chromatography (GC) analysis method, the main types of electronic nose sensors include conductive sensors, piezoelectric sensors, field effect sensors, and fiber optic sensors. The basic feature of a conductivity sensor is that its response form when exposed to a volatile compound (VOC) is a change in resistance value. Conductivity sensors are divided into two categories: metal oxide sensors and polymer sensors. Metal oxide sensors are more widely used in electronic nose systems, and their structure is shown in Figure 1. The active material in contact with the VOC in this type of sensor is oxides of tin, zinc, titanium, tungsten or iridium, and the substrate material is generally silicon, glass, plastic, and the temperature condition of 200-400℃ is required for contact reaction. A heater is installed at the bottom. The oxide material is doped with precious metals such as platinum and palladium to form two metal contact electrodes. The interaction with the VOC changes the conductivity of the active material and changes the resistance between the two electrodes. This resistance change can be measured with a single-arm bridge or other circuits. In fact, the active material of a sensor is always designed to be most sensitive to certain specific odors. The sensitivity range of the sensor is 5-50ppm. The disadvantages of metal oxide sensors are: (1) The operating temperature is relatively high; (2) After a long time of work, the response reference value is prone to drift, which needs to be overcome by signal processing calculations; (3) The sulfurization in the gas mixture The substance showed a "poisoning" reaction. However, it has a wide range of application and relatively low cost, so it is still a widely used gas sensor today. In the conductive polymer sensor, the active material in contact with the VOC is generally a conductive polymer composed of thiophene, indole, furan and other components. When gas molecules contact the above-mentioned polymer materials, ionization or covalent interaction occurs. The effect affects the transmission of electrons along the polymer chain, that is, changes the conductivity. In polymer materials, using microfabrication technology to form two electrodes with an interval of 10-20μm, the polymer is electropolymerized by applying an alternating voltage between the two electrodes, the voltage scanning rate is changed, and a series of polymerization is applied The precursor can produce a variety of active materials, so that different materials have specific responses to different gases. Conductive polymer sensors work at general ambient temperature without heating, so they are easier to manufacture, and their electronic interface is more direct, which has greater advantages in portable instrument applications. The sensitivity of this kind of sensor to detect smell can reach 0.1ppm, which is higher than that of metal oxide sensor, but it is generally in the range of 10-100ppm. The main disadvantages of current conductive polymer sensors are: (1) The electropolymerization process of active materials is difficult and time-consuming; (2) The contact response with VOC drifts over time; (3) It is extremely sensitive to humidity. It is easy to cover up and interfere with the normal response to VOC. In addition, certain gases will penetrate the entire polymer material, thereby slowing down the process of removing VOC from the polymer, that is, delaying the recovery time of the sensor. The basic feature of piezoelectric sensors is that the contact response form with VOC is reflected in the change of frequency. It is divided into quartz crystal microbalance (QCM) sensor and surface acoustic wave (SAW) sensor. Piezoelectric sensors can measure changes in temperature and mass, as well as parameters such as pressure, force and acceleration, but in electronic nose systems, they are generally used only as mass variable sensing detectors. The QCM sensor is a resonant disk with a diameter of several millimeters. The surface of the disk is covered with a polymer material, and each side has a metal electrode connected with a wire. The structure is shown in Figure 2. When the sensor is excited by the oscillating signal, it resonates at the characteristic frequency (10Hz～30MHz), and once the gas molecules are absorbed to the surface of the polymer coating, the mass of the disc is increased, thus reducing the resonant frequency. The height is inversely proportional to the mass of the gas molecules absorbed. The response and selectivity of the QCM sensor to different gases can be changed by adjusting the polymer coating of the resonant disk, and reducing the size and mass of the quartz crystal, and reducing the thickness of the polymer coating, can further shorten the response time of the sensor And recovery time. The main difference between a surface acoustic wave (SAW) sensor and a QCM sensor is: (1) Rayleigh waves run through the surface of SAW, not through the body like QCM; (2) SAW sensors work at a higher frequency, so they can produce more Large frequency changes. The typical operating frequency of QCM is only 10MHz, while SAW devices are several hundred MHz; (4) As SAW is a planar device, it can be manufactured by photolithography technology commonly used in the microelectronics industry, instead of requiring microelectronic machinery like QCM The system (MEMS) performs three-dimensional processing, so the cost of mass production is lower. However, the signal-to-noise ratio of the SAW sensor is inferior to that of the QCM sensor, so in many cases, the sensitivity of the former is lower than that of the latter. The third category of electronic nose sensors is the metal oxide silicon field effect tube sensor (MOSFET). Its working principle is: the reaction product (such as hydrogen) generated by the contact between VOC and the catalytic metal material will diffuse through the control electrode of the MOSFET to change the electrical conductivity of the device. As shown in Figure 3, a typical MOSFET structure has a P-type substrate and two N-type regions with high doping concentration diffused on the substrate. The metal contacts of the two N-regions are called source and drain respectively. pole. The sensitivity and selectivity of the device can be changed by changing the type and thickness of the metal contact agent and changing the operating temperature. One of the advantages of MOSFET is that it can rely on the IC manufacturing process, mass production, and stable quality. The main problem is that the contact reaction product (such as hydrogen) must penetrate the catalytic metal coating to affect the charge in the channel, which proposes a closed package method for the chip More demanding requirements. The MOSFET, like the conductivity sensor, also has the problem of reference value drift. The fourth type of practical odor sensor is an optical fiber sensor. Its response to gas compounds is a change in the color of the spectrum. As shown in Figure 4, the main part of this sensor is glass fiber, and a thin coating of chemically active material is applied on each surface of the glass fiber. The chemically active material coating is a fluorescent dye fixed in an organic polymer matrix. When in contact with a VOC, a single-frequency or narrow-band light pulse from an external light source propagates along the optical fiber and excites the active material to interact with the VOC. This reaction changes the polarity of the dye, thereby changing the fluorescence emission spectrum. As long as the spectral changes produced by a sensor array composed of many fiber optic devices coated with different dye mixtures are detected and analyzed, the corresponding gas compound composition can be determined. The optical fiber sensor has strong anti-noise ability and extremely high sensitivity. Its sensitivity unit is measured in ppb (parts per billion), which is far less than other electronic nose sensor types. The main shortcomings of the current optical fiber sensor are: (1) the equipment control system is more complicated and the cost is higher; (2) the fluorescent dye is affected by the white photochemical effect and has a limited service life. Ggd Low Voltage Switchgear,Lv Switchgear,Lv Switchgear Panel,Low Tension Switchgear TRANCHART Electrical and Machinery Co.,LTD , https://www.tranchart-electrical.com
