The working principle and application of some commonly used pressure sensors
Pressure sensor is the most commonly used sensor in industrial practice. It is widely used in various industrial self-control environments, involving water conservancy and hydropower, railway transportation, intelligent building, production automation, aerospace, military, petrochemical, oil well, electric power, ship, machine tool. , 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 a piezoresistive pressure sensor, which has a very low price, high precision and good linearity. Below we mainly introduce such sensors.
When we understand the piezoresistive force sensor, we first understand the components of the resistance strain gauge. A strain gage is a sensitive device that converts the strain change on the device under test into an electrical signal. It is one of the main components of a piezoresistive strain sensor. The most widely used resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. The metal resistance strain gauge has two kinds of filament strain gauges and metal foil strain gauges. Usually, the strain gauge is tightly bonded to the mechanical strain matrix by a special adhesive. When the stress changes due to the force of the substrate, the strain gauges are also deformed together, so that the resistance of the strain gauge is changed, thereby The voltage applied to the resistor changes. The strain gauges typically have a small change in resistance when stressed. Typically, such strain gauges form a strain bridge and are amplified by a subsequent instrumentation amplifier and transmitted to the processing circuit (usually A/D conversion). And CPU) display or actuator.
Internal structure of metal resistance strain gauge
The strain gauge is composed of a base material, a metal strain wire or a strained foil, an insulating protective sheet, and a lead wire. Depending on the application, the resistance of the strain gauge can be designed by the designer, but the range of resistance should be noted: the resistance is too small, the required drive current is too large, and the heat of the strain gauge causes the temperature to be too high. Used in different environments, the resistance value of the strain gauge is changed too much, the output zero drift is obvious, and the zero adjustment circuit is too complicated. The resistance is too large, the impedance is too high, and the ability to resist external electromagnetic interference is poor. Generally, it is about tens of euros to several tens of kiloohms.
The working principle of the resistance strain gauge
The working principle of the metal resistance strain gauge is a phenomenon in which the strain resistance adsorbed on the base material changes with the mechanical deformation, which is commonly called the resistance strain effect. The resistance value of the metal conductor can be expressed by the following formula:
Where: ρ——resistivity of metal conductor (Ω·cm2/m)
S——cross-sectional area of ​​the conductor (cm2)
L——the length of the conductor (m)
Take 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 the resistance value will change if the wire is subjected to external force. When it is stretched, its length increases, and the cross-sectional area decreases, and the resistance value increases. When the wire is compressed by an external force, the length is decreased and the section is increased, and the resistance value is decreased. As long as the change in resistance is measured (usually the voltage across the measured resistance), the strain of the strained wire is obtained.
2. Principle and application of ceramic pressure sensor
The corrosion-resistant ceramic pressure sensor has no liquid transfer, the pressure acts directly 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. Bridge), due to the piezoresistive effect of the varistor, the bridge produces a highly linear voltage signal proportional to the pressure, which is proportional to the excitation voltage. The standard signal is calibrated to 2.0 / 3.0 / 3.3 depending on the pressure range. mV/V, etc., compatible with strain gauge sensors. With laser calibration, the sensor has high temperature stability and time stability. The sensor comes with temperature compensation of 0 to 70 ° C and can be in direct contact with most media.
Ceramic is a recognized material that is highly elastic, resistant to corrosion, abrasion, shock and vibration. The thermal stability of ceramics and its thick film resistance allow it to operate over a temperature range of -40 to 135 ° C, with high precision and high stability. The degree of electrical insulation is 2kV, the output signal is strong, and the long-term stability is good. High-performance, low-priced ceramic sensors will be the development direction of pressure sensors. There is a trend to replace other types of sensors in Europe and the United States. More and more users in China use ceramic sensors instead of diffused silicon pressure sensors.
3. Principle and application of diffused silicon pressure sensor
The pressure of the measured medium acts directly 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, causing the resistance value of the sensor to change, and detecting the change by electronic circuitry, and The conversion outputs a standard measurement signal corresponding to this pressure.
4. Principle and application of sapphire pressure sensor
Using strain-resistive operation, silicon-sapphire is used as a semiconductor sensor with unparalleled metrology.
The sapphire is composed of a single crystal insulator element, which does 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 Semiconductor sensitive components made of silicon-sapphire are insensitive to temperature changes and have excellent operating characteristics even under high temperature conditions; sapphire has excellent radiation resistance; in addition, silicon-sapphire semiconductor sensitive components have no pn drift. Therefore, the manufacturing process is fundamentally simplified, the repeatability is improved, and high yield is ensured.
Pressure sensors and transmitters made of silicon-sapphire semiconductor sensitive components operate under the toughest operating conditions with high reliability, high accuracy, minimal temperature error and cost-effectiveness.
The gauge pressure sensor and transmitter consist of a double diaphragm: a titanium alloy measuring diaphragm and a titanium alloy receiving diaphragm. A sapphire sheet printed with a heterogeneous epitaxial strain sensitive bridge circuit is soldered to the titanium alloy measuring diaphragm. The pressure to be measured 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 is deformed. After the deformation is sensed by the silicon-sapphire sensing element, the bridge output changes, and the magnitude of the change is proportional to the measured pressure.
The sensor's circuitry ensures power to the strained bridge circuit and converts the strain bridge's unbalanced signal into a uniform 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 to act as an elastic element to convert the measured pressure into strain gauge deformation for pressure measurement.
5. Principle and application of piezoelectric pressure sensor
Piezoelectric materials mainly used in piezoelectric sensors include quartz, sodium potassium tartrate, and dihydrogen phosphate. Among them, quartz (silicon dioxide) is a kind of natural crystal. The piezoelectric effect is found in this crystal. In a certain temperature range, the piezoelectric property always exists, but after the temperature exceeds this range, the piezoelectric property is completely Disappeared (this high temperature is the so-called "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 large piezoelectric sensitivity and piezoelectric coefficient, but it can only be applied in a low room temperature and humidity environment. 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 also applied to polycrystals, such as the current piezoelectric ceramics, including barium titanate piezoelectric ceramics, PZT, tantalate-based piezoelectric ceramics, lead magnesium niobate piezoelectric ceramics, and the like.
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 saved only when the loop has an infinite input impedance. This is not the case, so this determines that the piezoelectric sensor can only measure dynamic stresses.
Piezoelectric sensors are mainly used in the measurement of acceleration, pressure and force. A piezoelectric accelerometer is a commonly used accelerometer. It has the characteristics of simple structure, small size, light weight and long service life. Piezoelectric accelerometers have found wide application in vibration and shock measurement of aircraft, automobiles, ships, bridges and buildings, especially in the aerospace and aerospace fields. Piezoelectric sensors can also be used to measure the measurement of combustion pressure inside the engine and the measurement of vacuum. It can also be used in the military industry, for example, to measure the change in the pressure of the gun bullet in the moment of firing and the shock wave pressure of the muzzle. It can be used to measure large pressures as well as to measure small pressures.