Pressure sensor is the most commonly used sensor in industrial practice. It is widely used in various industrial automation environments, involving water conservancy and hydropower, railway transportation, intelligent buildings, production automation, aerospace, military, petrochemical, oil well, electric power, ships, machine tools , Pipelines and many other industries.
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, high accuracy and good linearity. Below we mainly introduce this type of sensor.
When understanding the piezoresistive force sensor, we first understand the element of 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 sensors. The most widely used resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. There are two kinds of metal resistance strain gauges: wire strain gauge and metal foil strain gauge. Usually the strain gauge is tightly bonded to the mechanical strain matrix through a special adhesive. When the matrix is subjected to a stress change, the resistance strain gauge also deforms, so that the resistance value of the strain gauge changes, so that The voltage applied to the resistor changes. The resistance change of this strain gauge is usually small when it is stressed. Generally, this strain gauge is composed of a strain bridge, which is amplified by a subsequent instrument amplifier, and then transmitted to the processing circuit (usually A/D conversion And CPU) display or actuator.
Internal structure of metal resistance strain gauge
The resistance strain gauge is composed of base material, metal strain wire or strain foil, insulation protection sheet and lead wire. According to different purposes, 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 heating of the strain gauge causes its own temperature to be too high , 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. But the resistance is too high, the impedance is too high, and the ability to resist external electromagnetic interference is poor. Generally, it is about tens of ohms to tens of kiloohms.
Working principle of resistance strain gauge
The working principle of the metal resistance strain gauge is the phenomenon that the resistance value of the strain resistance 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:
In the formula: ρ——the resistivity of the metal conductor (Ω·cm2/m)
S——cross-sectional area of conductor (cm2)
L——The length of the conductor (m)
Let’s take a wire strain resistor 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 it is stretched, its length increases, while its cross-sectional area decreases, and the resistance value increases. When the metal wire is compressed by an 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 is measured), 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 to form 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 pressure range. mV/V, etc., compatible with strain gauge sensors. Through laser calibration, the sensor has high temperature stability and time stability. The sensor comes with a temperature compensation of 0～70℃ and can directly contact most media.
Ceramic is a recognized material with high elasticity, corrosion resistance, wear resistance, shock and vibration resistance. The thermal stability of ceramics and its thick film resistors can make its operating temperature range as high as -40～135℃, and it has high accuracy and high stability of measurement. Electrical insulation degree> 2kV, strong output signal, good long-term stability. 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 European and American countries. In China, more and more users use ceramic sensors to replace diffused silicon pressure sensors.
3. 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 produce a micro-displacement proportional to the pressure of the medium, changing the resistance value of the sensor, and detecting this change with an electronic circuit, and Convert and output a standard measurement signal corresponding to this pressure.
4. Principle and application of sapphire pressure sensor
Utilizing the working principle of strain resistance, using silicon-sapphire as the semiconductor sensitive element, it has unparalleled measurement characteristics.
Sapphire is composed of single crystal insulator elements, which will not cause hysteresis, fatigue and creep; 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), so use The semiconductor sensor made of silicon-sapphire is not sensitive to temperature changes, and has good working characteristics even under high temperature conditions; sapphire has extremely strong radiation resistance; in addition, the silicon-sapphire semiconductor sensor 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 worst 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 by a tie rod). Under the action of pressure, the titanium alloy receiving diaphragm produces deformation. After the deformation is sensed by the silicon-sapphire sensor, 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 absolute pressure sensors and transmitters, the sapphire sheet, connected with the ceramic base glass solder, functions as an elastic element, converting the measured pressure into strain gauge deformation, thereby achieving the purpose of pressure measurement.
5. Principle and application of piezoelectric pressure sensor
The piezoelectric materials mainly used in piezoelectric sensors include quartz, sodium potassium 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, piezoelectric properties always exist, but after the temperature exceeds this range, the piezoelectric properties are completely Disappear (this high temperature is the so-called "Curie point"). As the electric field changes slightly 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 great piezoelectric sensitivity and piezoelectric coefficient, but it can only be used in environments where room temperature and humidity are relatively low. Dihydrogen phosphate is an artificial crystal that can withstand high temperatures and relatively high humidity, so it has been widely used.
The piezoelectric effect is now also applied to polycrystals, such as current piezoelectric ceramics, including barium titanate piezoelectric ceramics, PZT, niobate piezoelectric ceramics, lead magnesium niobate piezoelectric ceramics, and so on.
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 action of the external force can only be saved when the circuit has an infinite input impedance. The actual situation is not like this, 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. The piezoelectric accelerometer is a commonly used accelerometer. It has the advantages of simple structure, small size, light weight and long service life. Piezoelectric acceleration sensors have been widely used in vibration and shock measurement of airplanes, automobiles, ships, bridges and buildings, especially in the fields of aviation and aerospace. Piezoelectric sensors can also be used to measure internal combustion pressure and vacuum in the engine. It can also be used in the military industry. For example, it can be used to measure the change in chamber pressure at the moment when a bullet is fired in the chamber and the shock wave pressure of the muzzle. It can be used to measure large pressures as well as small pressures.