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.
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 the metal conductor can be expressed by the following formula:
In the formula: Ï——Resistivity of metal conductor (Ω.cm2 / m)
S——Cross-sectional area of ​​conductor (cm2)
L——Length of conductor (m)
We take the wire strain resistance as an example. When the 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 wire is subjected to external force When elongating, its 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. Principle and application of diffused silicon pressure sensor
working principle
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.
4. Principle and application of sapphire pressure sensor
Using the strain-resistance working principle, silicon-sapphire is used as the semiconductor sensitive element, which has unmatched measurement characteristics.
Sapphire is composed of single crystal insulator elements, which will not cause hysteresis, fatigue and creep phenomena; sapphire is stronger than silicon, has higher hardness and is not afraid of deformation; sapphire has very good elasticity and insulation properties (within 1000 OC), therefore, use The semiconductor sensitive element made of silicon-sapphire is not sensitive to temperature changes, and it has very good operating characteristics even under high temperature conditions; the radiation resistance of sapphire is extremely strong; in addition, the silicon-sapphire semiconductor sensitive element has no pn drift, Therefore, the manufacturing process is fundamentally simplified, the repeatability is improved, and a high yield is ensured.
Pressure sensors and transmitters made of silicon-sapphire semiconductor sensitive components can work normally under the harshest working conditions, and have high reliability, good accuracy, minimal temperature error, and high cost performance.
The gauge pressure sensor and transmitter are composed of double diaphragms: a titanium alloy measuring diaphragm and a titanium alloy receiving diaphragm. A sapphire sheet printed with a heteroepitaxial strain sensitive bridge circuit is welded to a titanium alloy measuring diaphragm. The measured pressure is transmitted to the receiving diaphragm (the receiving diaphragm and the measuring diaphragm are firmly connected together with a tie rod). Under the effect of pressure, the deformation of the titanium alloy receiving diaphragm occurs. After the deformation is sensed by the silicon-sapphire sensitive element, its bridge output will change, and the magnitude of the change is proportional to the measured pressure.
The sensor circuit can ensure the power supply of the strain bridge circuit, and convert the unbalanced signal of the strain bridge into a unified electrical signal output (0-5, 4-20mA or 0-5V). In the absolute pressure sensor and transmitter, the sapphire sheet is connected with the ceramic base glass solder, which acts as an elastic element, which converts the measured pressure into strain gauge deformation, so as to achieve the purpose of pressure measurement.
5. Principle and application of piezoelectric pressure sensor
The piezoelectric materials mainly used in piezoelectric sensors include quartz, potassium sodium tartrate and dihydrogen phosphate. Among them, quartz (silica) is a natural crystal. The piezoelectric effect is found in this crystal. Within a certain temperature range, the piezoelectric property always exists, but after the temperature exceeds this range, the piezoelectric property is completely Disappear (this high temperature is called the "Curie point"). Since the electric field changes little with the change of stress (that is, the piezoelectric coefficient is relatively low), quartz is gradually replaced by other piezoelectric crystals. Potassium sodium tartrate has a great piezoelectric sensitivity and piezoelectric coefficient, but it can only be used in environments with relatively low room temperature and low humidity. Dihydrogen phosphate is an artificial crystal, which can withstand high temperature and quite high humidity, so it has been widely used.
Now the piezoelectric effect is also applied to polycrystals, such as current piezoelectric ceramics, including barium titanate piezoelectric ceramics, PZT, niobate-based piezoelectric ceramics, lead niobate magnesium piezoelectric ceramics, etc.
The piezoelectric effect is the main working principle of the piezoelectric sensor. The piezoelectric sensor cannot be used for static measurement, because the electric charge after the external force is only saved when the loop has an infinite input impedance. The actual situation is not the case, so this determines that the piezoelectric sensor can only measure dynamic stress.
Piezoelectric sensors are mainly used in the measurement of acceleration, pressure and force. Piezoelectric acceleration sensor is a commonly used accelerometer. It has the characteristics of simple structure, small size, light weight and long service life. Piezoelectric acceleration sensors have been widely used in the measurement of vibration and shock in airplanes, automobiles, ships, bridges and buildings, especially in the aviation and aerospace fields. Piezoelectric sensors can also be used to measure the combustion pressure inside the engine and the vacuum. It can also be used in the military industry. For example, it can be used to measure the changes in the pressure of the barrel and the shock wave pressure of the muzzle at the moment when the bullet of the gun fires in the bore. It can be used to measure both large and small pressures.
Piezoelectric sensors are also widely used in biomedical measurement. For example, ventricular catheter microphones are made of piezoelectric sensors. Because the measurement of dynamic pressure is so common, the application of piezoelectric sensors is very extensive.
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