Pressure sensors are the most commonly used sensors in industrial practice. They are widely used in various industrial automatic control environments, including water conservancy and hydropower, railway transportation, intelligent buildings, production automatic control, aerospace, military, petrochemical, oil wells, power, ships, and machine tools , Pipelines and many other industries, the following briefly introduces some common sensor principles and their applications 1. Principle and application of strain gauge pressure sensor There are many types of mechanical sensors, such as resistance strain gauge pressure sensors, semiconductor strain gauge pressure sensors, piezoresistive pressure sensors, inductive pressure sensors, capacitive pressure sensors, resonant pressure sensors, and capacitive acceleration sensors. But the most widely used is the piezoresistive pressure sensor, which has a very low price and higher accuracy and better linear characteristics. Below we mainly introduce such sensors. In understanding the piezoresistive force sensor, we first understand the element such as resistance strain gauge. The resistance strain gauge is a sensitive device that converts the strain change on the test piece into an electrical signal. It is one of the main components of piezoresistive strain sensor. The most widely used resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. There are two types of metal resistance strain gauges: filament strain gauges and metal foil strain gauges. Usually, the strain gauges are tightly bonded to the substrate that generates mechanical strain through a special adhesive. When the stress of the substrate changes, the resistance strain gauge also deforms together, causing the resistance of the strain gauge to change, so that The voltage applied to the resistor changes. The change in resistance of such strain gauges when stressed is usually small. Generally, these strain gauges form a strain bridge, which is amplified by subsequent instrumentation amplifiers, and then transmitted to the processing circuit (usually A / D conversion) And CPU) display or actuator. The internal structure of the metal resistance strain gauge is shown in Figure 1, which is a schematic diagram of the structure of the resistance strain gauge. It is composed of a base material, a metal strain wire or a strain foil, an insulating protection sheet, and a lead wire. According to different uses, the resistance value of the resistance strain gauge can be designed by the designer, but the value range of the resistance should be noted: the resistance value is too small, the required driving current is too large, and the heat of the strain gauge causes the temperature itself to be too high When used in different environments, the resistance value of the strain gauge changes too much, the output zero drift is obvious, and the zero adjustment circuit is too complicated. The resistance is too high, the impedance is too high, and the ability to resist external electromagnetic interference is poor. Generally it is around tens of ohms to tens of thousands of ohms. Working principle of resistance strain gauge The working principle of metal resistance strain gauge is the phenomenon that the resistance of the strain resistance adsorbed on the base material changes with the mechanical deformation, which is commonly known as the resistance strain effect. The resistance value of a metal conductor can be expressed by the following formula: where: Ï—the resistivity of the metal conductor (Ω? Cm2 / m) S—the cross-sectional area of ​​the conductor (cm2) L—the length of the conductor (m) Take wire strain resistance as an example. When the metal wire is subjected to external force, its length and cross-sectional area will change. It can be easily seen from the above formula that its resistance value will change. If the metal wire is elongated by external force When the length increases and the cross-sectional area decreases, the resistance value will increase. When the metal wire is compressed by external force, the length decreases and the cross section increases, and the resistance value decreases. As long as the change in the resistance is measured (usually the voltage across the resistance), the strain of the strained wire can be obtained 2. Principle and application of ceramic pressure sensor The corrosion-resistant ceramic pressure sensor has no liquid transmission. The pressure directly acts on the front surface of the ceramic diaphragm, causing a slight deformation of the diaphragm. The thick film resistor is printed on the back of the ceramic diaphragm and connected into a Wheatstone bridge (closed Bridge), due to the piezoresistive effect of the varistor, the bridge generates a highly linear voltage signal proportional to the pressure and proportional to the excitation voltage. The standard signal is calibrated to 2.0 / 3.0 / 3.3 according to the different pressure ranges mV / V, etc., compatible with strain sensors. Through laser calibration, the sensor has high temperature stability and time stability. The sensor comes with temperature compensation of 0 ~ 70 ℃, and can be in direct contact with most media. Ceramic is a recognized material with high elasticity, corrosion resistance, wear resistance, impact resistance and vibration. The thermal stability of ceramics and its thick film resistance can make its operating temperature range up to -40 ~ 135 ℃, and has high accuracy and high stability of measurement. The degree of electrical insulation is> 2kV, the output signal is strong, and the long-term stability is good. Ceramic sensors with high characteristics and low prices will be the development direction of pressure sensors. There is a trend to replace other types of sensors in Europe and the United States. In China, more and more users are using ceramic sensors to replace diffused silicon pressure sensors. 3. The principle and application of diffused silicon pressure sensor The pressure of the measured medium directly acts on the diaphragm of the sensor (stainless steel or ceramic), causing the diaphragm to generate a micro-displacement proportional to the pressure of the medium, causing the resistance value of the sensor to change, and detecting this change with electronic circuits The conversion outputs a standard measurement signal corresponding to this pressure.
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