Electronic Nose mainly by smell sampling operation, the three functional devices composed of the gas sensor array and the signal processing system. Recognition odor of the electronic nose is the main mechanism for each sensor in the array has a different sensitivity of the measured gas, e.g., the 1st gas may be generated on a sensor with high response, while the other sensor is low response, the same , the 2nd gas generating high response of the sensor is not sensitive on the 1st gas, ultimately, the entire sensor array is different response pattern of the different gases, it is this difference, to enable the system to be identified in accordance with the response of the sensor pattern odor .
Many types of electronic nose, which is typical of the work program are: First, the use of a vacuum pump air sampling lessons to small container chamber equipped with an electronic sensor array. Next, the sampling operation unit to the initialization of the sensor array is exposed to the odor body, when the volatile compounds (VOC) and the sensor surface of the active material in contact with, to produce the transient response. Such a response is recorded and transmitted to the signal processing unit performs the analysis the VOC pattern and stored in the database for comparison, identification, to determine the type of odor.
Finally, use of alcohol vapor "wash" the sensor surface of the active material removal measured Bi smell mixture. Before entering a new measurement of the next round, the sensors still have to again implement the initialization (i.e., between the work, each sensor required drying gas or some other reference gas for cleaning, in order to achieve the reference state). The time spent by the measured odor effect of time is called the "response time" and the purge process and the reference gas role of the sensor array initialization process known as "recovery time".
The
gas sensor array is a key factor in the electronic nose system. In addition to the basic gas chromatography (GC) analysis method, the main types of the electronic nose sensors conductivity type sensor, a piezo sensor, a field effect sensor, fiber optic sensor and the like.
The basic characteristics of the conductive sensor is response when placed in the volatile compounds (VOC), form the resistance value change. The conductivity sensor is divided into two categories of metal oxide sensors and polymer sensor. Metal oxide sensor in the electronic nose system more widely used, its structure is shown in Figure 1. Such sensors in contact with the active material and VOC is a tin, zinc, titanium, tungsten, or iridium oxide, the substrate material is typically silicon, glass, plastic, and contact reaction occurs must satisfy a temperature condition of 200 ~ 400 ℃, so set at the bottom of the heater.
Oxide materials using a platinum, palladium and other precious metals adulterated form two metal contact electrode. Interaction with VOC change the conductivity of the active material, so that the change in resistance between the two electrodes, this change in resistance can be used to measure a Wheatstone bridge or other circuit. In fact, the active material is always one sensor designed to be the most sensitive response to certain odors.
The sensitivity of the sensor range 5 ~ 50ppm. The shortcomings of the metal oxide sensor is:
(1) Operating temperature;
(2) after working for a long time, in response to the reference value prone to drift, the need to overcome the use of signal processing operations;
(3) the sulfides in the gas mixture is the reaction of the "poisoning".
However, it is applicable to a very wide range and a relatively low cost, it is still to become a gas sensor that is widely applied.
Sensor of the conductive polymer, the active material in contact with VOC generally composed of a conductive polymer, thiophene, indole, furan and other ingredients will occur when the gas molecules in contact with the polymer material ionized or covalent interactions, this mutual The role affect the transport of electrons along the polymer chain, i.e. to change the conductive. In the polymeric material, the use of the micro fabric technology to form two interval 10 ~ 20μm of the electrode, by applying an alternating voltage between the two electrodes to make the polymer electrolyte polymerized, changing the voltage scan rate, and apply a series of polymerization the precursor active material, can produce a variety of different materials respectively showed a specific response to different gases.
Conductive polymer sensors work in a normal ambient temperature without heating, and therefore easier to manufacture, more direct electronic interface, and thus have a greater advantage in portable instrumentation applications. The sensitivity of this sensor to detect the odor can be achieved 0.1ppm, higher than the metal oxide sensor, but generally within the range of 10 ~ 100ppm. The main defects of the conductive polymer sensors are: (1) active material electro-polymerization process is more difficult and time consuming; (2) and VOC contacting drift phenomenon occurs in response to the present time; (3) is extremely sensitive to humidity, this sensitivity easy to conceal and interfere with the normal response to VOC. In addition, some gas will penetrate the polymeric material as a whole, thus slowing down the process of the VOC is removed from the polymer, i.e. delayed the recovery time of the sensor.
The basic characteristics of the piezo sensor response form of contact with the VOC is reflected in the change of frequency. It is divided into two kinds of quartz crystal microbalance (QCM) sensors and surface acoustic wave (SAW) sensor. Piezo sensor can measure changes in temperature and quality, but also the measurement of pressure, force and acceleration parameters, but the electronic nose system, they generally only as the qualitative variables sensing detectors use. QCM sensor is a diameter of a few millimeters of the resonator disk, the disk is coated with a polymeric material, each side has a wire connected to a metal electrode.
When the sensor by the excitation of the oscillation signal, it is resonant at the characteristic frequency (10Hz to 30MHz), and once the gas molecules are absorbed into the surface of the polymer coating, this increases the quality of the disc, thereby reducing the resonant frequency, the resonant frequency inversely proportional to the level with the quality of the gas molecules absorbed by. QCM sensor response to different gases, the selectivity can be changed by adjusting the resonant disc polymer coating, and to reduce the size and mass of the quartz crystal, and reduces the thickness of the polymer coating can further shorten the response time of the sensor and recovery time.
The main difference of the surface acoustic wave (SAW) sensor QCM sensor:
(1) the Rayleigh wave is the surface of the operation of the SAW not like QCM through its body;
(2) SAW sensor for higher frequencies, and therefore can generate a larger frequency change.
(3) the QCM of only typical operating frequency is 10MHz, and the SAW device is in the hundreds of MHz;
(4) Since the SAW is a planar device, so it can be commonly used photolithographic techniques of microelectronics industry to manufacture, rather than the QCM as the need for microelectronic mechanical systems (MEMS) for three-dimensional processing, therefore the lower the cost of mass production.
However, the SAW
co sensor signal-to-noise ratio is inferior to the QCM sensor, therefore, in many cases, the sensitivity of the former is lower than the latter.
The third category of the electronic nose sensors are metal oxide silicon FET sensor (MOSFET). Its working principle is: VOC with a catalytic metal material in contact with the generated reaction product (such as hydrogen) will be spread through the MOSFET control electrode to change the device's conductive material. As shown in Figure 3, a typical MOSFET structure, a P-type substrate and on the substrate diffusion two high doping concentration N-type region, two N-area metal contacts referred to as 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 working temperature.
One of the advantages of the MOSFET is relying on the IC manufacturing process, mass production, stable quality, the main problem is the contact with the reaction products (such as hydrogen) must infiltrate the the catalytic metal coating to affect the charge in the channel, which the chip sealed package the more stringent requirements. MOSFET and the conductive sensor is the same, there are also the drift of the reference value.
The fourth practical odor sensor is a fiber optic sensor. Its gaseous compounds form the response is a the spectral color change. The trunk portion of Figure 4, such a sensor is a glass fiber, in the respective surfaces of the glass fiber coated with a thin chemically active material coating. Chemically active material coating is fixed in the organic polymer matrix, a fluorescent dye, and when contact with VOC from the external light source is a single frequency or narrow-band light pulses along the fiber propagation and motivate the active material, with a VOC interaction reaction. This reaction changes the polarity of the dye, thus changing the fluorescence emission spectra.
Many cladding different dye mixtures of fiber optic device comprising an array of sensors generated spectral changes were detected and analyzed, it can determine the corresponding gas compound component. The fiber optic
sensor manufacturer has a strong anti-noise capability and high sensitivity, sensitivity unit ppb (parts per billion), far less than other type of electronic nose sensors. The main drawback of the optical fiber sensor is: (1) the equipment control system is more complex, high cost, (2) a fluorescent dye is influenced by the white effect, limited service life.
novaking - 26. Nov, 07:26
