The internal structure of the pressure sensor_the working principle of the pressure sensor
1. Principle and application of strain gauge pressure sensor
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 wells, electricity, ships, machine tools , pipeline and many other industries, the following briefly introduces some common sensor principles and their applications.
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 extremely low price, high precision and good linearity. Below we mainly introduce such sensors.
When understanding the piezoresistive force sensor, we first understand the element of the resistance strain gauge. A resistance strain gauge is a sensitive device that converts the strain change on the measured object into an electrical signal.
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. (Electrical Technology Home www.dgjs123.com) According to different uses, the resistance value of the resistance strain gauge can be designed by the designer, but the range of the resistance value should be noted: the resistance value is too small, the required driving current is too large, At the same time, the heating of the strain gauge makes itself too high. When used in different environments, the resistance 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 large, 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 thousands of ohms.
How resistance strain gauges work
Crystals are anisotropic and amorphous are isotropic. When some crystalline medium is deformed by mechanical force in a certain direction, polarization effect occurs; when the mechanical force is removed, it will return to an uncharged state, that is, when subjected to pressure, a certain These crystals may produce an electrical effect, which is called the polarization effect. Scientists have developed a pressure sensor based on this effect.
The piezoelectric materials mainly used in piezoelectric sensors include quartz, potassium sodium tartrate and ammonium dihydrogen phosphate. Among them, quartz (silicon dioxide) is a natural crystal, and the piezoelectric effect is found in this crystal. Within a certain temperature range, the 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").
Since the electric field changes slightly with the change of stress (that is to say, the piezoelectric coefficient is relatively low), quartz is gradually replaced by other piezoelectric crystals. And potassium sodium tartrate has great piezoelectric sensitivity and piezoelectric coefficient, but it can only be applied in the environment of room temperature and low humidity. Dihydrogen phosphate is an artificial crystal that can withstand high temperature and relatively high humidity, so it has been widely used.
The working principle of the metal resistance strain gauge is the phenomenon that the resistance value changes with the mechanical deformation of the strain resistance adsorbed on the base material, commonly known as the resistance strain effect. The resistance value of the metal conductor can be expressed by the following formula:
In the formula: ρ - 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 )
Taking 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. When it is elongated, its length increases, while its cross-sectional area decreases, and its resistance value increases. When the wire is compressed by external force, the length decreases and the cross-section increases, and the resistance value decreases. As long as the change applied to the resistance (usually the voltage across the resistance) is measured, the strain condition 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, and the pressure acts directly on the front surface of the ceramic diaphragm, causing the diaphragm to deform slightly. The thick film resistor is printed on the back of the ceramic diaphragm and connected to form a Wheatstone bridge (closed). 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 according to the different pressure ranges. .0 / 3.3 mV/V, etc., compatible with strain gauge sensors.
Ceramic is recognized as a highly elastic, corrosion-resistant, wear-resistant, shock- and vibration-resistant material.
3. Principle and application of diffused silicon pressure sensor
Working principle The pressure of the measured medium acts directly on the diaphragm of the sensor (stainless steel or ceramics), 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
Using the working principle of strain resistance, using silicon-sapphire as a semiconductor sensitive element, it has unparalleled metrology characteristics.
Sapphire is composed of single crystal insulator elements without 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), therefore, using Silicon-sapphire semiconductor sensitive components are not sensitive to temperature changes, and have good working characteristics even under high temperature conditions; sapphire has extremely strong radiation resistance; in addition, silicon-sapphire semiconductor sensitive components have no p-n Drift, is welded to the titanium measuring diaphragm. The measured pressure is transmitted to the receiving diaphragm (the receiving diaphragm and the measuring diaphragm are firmly connected by a pull rod). Under the action of pressure, the titanium alloy receiving diaphragm is deformed. After the deformation is sensed by the silicon-sapphire sensitive element, the output of the bridge will change, and the magnitude of the change is proportional to the measured pressure.
The circuit of the sensor 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, acts as an elastic element, converting 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 ammonium dihydrogen phosphate. Among them, quartz (silicon dioxide) is a natural crystal, and the piezoelectric effect is found in this crystal. Within a certain temperature range, the 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"). And potassium sodium tartrate has great piezoelectric sensitivity and piezoelectric coefficient, but it can only be applied in the environment of room temperature and low humidity. Dihydrogen phosphate can withstand high temperature and relatively high humidity, so it has been widely used.
Now the piezoelectric effect is also applied to polycrystals, such as the 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, and the piezoelectric sensor cannot be used for static measurement, because the charge after the external force is only saved when the loop has an infinite input impedance. This determines that piezoelectric sensors can only measure dynamic stress.
Piezoelectric sensors are mainly used in the measurement of acceleration, pressure and force. Piezoelectric accelerometer is a commonly used accelerometer. It has the advantages of simple structure, small size, light weight and long service life. Piezoelectric accelerometers have been widely used in the vibration and shock measurement of aircraft, automobiles, ships, bridges and buildings, especially in the fields of aviation and aerospace. It can be used to measure both large and small pressures.
Piezoelectric sensors are also widely used in biomedical measurements. For example, ventricular catheter microphones are made of piezoelectric sensors. Because measuring dynamic pressure is so common, piezoelectric sensors are widely used.