Working principle and application of Saisiwei pressure sensor

Pressure sensor is the most commonly used sensor in industrial practice. It is widely used in various industrial automatic control environments, involving many industries such as water conservancy and hydropower, railway transportation, intelligent buildings, production automatic control, aerospace, military industry, petrochemical, oil wells, electricity, ships, machine tools, pipelines, etc. The following is a brief introduction to the principles and applications of some commonly used sensors.

1. Principle and application of strain gauge pressure sensor

There are many types of mechanical sensors, such as resistance strain gauge pressure sensor, semiconductor strain gauge pressure sensor, piezoresistive pressure sensor, inductive pressure sensor, capacitive pressure sensor, resonant pressure sensor and capacitive acceleration sensor, etc. However, the most widely used is the piezoresistive pressure sensor, which has extremely low price, high accuracy and good linearity. Below we mainly introduce this type of sensor.

When learning about piezoresistive force sensors, we first need to get to know the resistance strain gauge. The resistance strain gauge is a sensitive device that converts the strain change on the measured object into an electrical signal. It is one of the main components of the piezoresistive strain sensor. The most commonly used resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. There are two types of metal resistance strain gauges: wire strain gauges and metal foil strain gauges. Usually, the strain gauge is tightly bonded to the mechanical strain matrix through a special adhesive. When the matrix is ​​subjected to stress and the stress changes, the resistance strain gauge also deforms, causing the resistance of the strain gauge to change, thereby changing the voltage applied to the resistor. The resistance change of this strain gauge when subjected to force is usually small. Generally, this strain gauge forms a strain bridge, and 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 uses, the resistance value of the resistance strain gauge can be designed by the designer, but the range of resistance should be noted: if the resistance value is too small, the required driving current is too large, and the heat of the strain gauge causes the temperature of the strain gauge 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. If 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 kiloohms.

Working principle of resistance strain gauge

The working principle of metal resistance strain gauge is that the resistance value of the strain gauge adsorbed on the base material changes with mechanical deformation, which is commonly known as 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 ​​conductor (cm2)

L——conductor length (m)

Let's take the metal wire strain resistor 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 stretched by external force, its length increases, while the cross-sectional area decreases, and the resistance value will increase. When the metal wire is compressed by external force, the length decreases and the cross-sectional area increases, and the resistance value decreases. As long as the change in the resistor is measured (usually the voltage across the resistor is measured), the strain of the strained metal wire can be obtained.

2. Principle and application of ceramic pressure sensor

The corrosion-resistant ceramic pressure sensor has no liquid transmission. 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 into a Wheatstone bridge (closed bridge). Due to the piezoresistance effect of the varistor, the bridge generates a highly linear voltage signal that is proportional to the pressure and proportional to the excitation voltage. The standard signal is calibrated to 2.0 / 3.0 / 3.3 mV/V according to the different pressure ranges, and is compatible with strain sensors. Through laser calibration, the sensor has high temperature stability and time stability. The sensor has its own temperature compensation of 0 ~ 70 ° C and can be in direct contact with most media.

Ceramic is a material that is recognized as highly elastic, corrosion-resistant, wear-resistant, shock-resistant and vibration-resistant. The thermal stability of ceramics and its thick film resistor can make its operating temperature range as high as -40 to 135°C, and it has high precision and high stability in measurement. The electrical insulation level 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. In Europe and the United States, there is a trend of fully replacing other types of sensors. 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

How it works

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 medium pressure, causing the resistance value of the sensor to change, and the electronic circuit detects this change and converts and outputs a standard measurement signal corresponding to this pressure.

4. Principle and application of sapphire pressure sensor

It uses the working principle of strain resistance and adopts silicon-sapphire as semiconductor sensitive element, which has unparalleled measurement characteristics.

Sapphire is composed of single crystal insulator elements and will not experience hysteresis, fatigue and creep. Sapphire is stronger and harder than silicon and is not afraid of deformation. Sapphire has very good elasticity and insulation properties (within 1000 OC). Therefore, semiconductor sensitive elements made of silicon-sapphire 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 elements have no pn drift, which fundamentally simplifies the manufacturing process, improves repeatability and ensures a high yield rate.

Pressure sensors and transmitters made of silicon-sapphire semiconductor sensitive elements can work normally under the worst working conditions, and have high reliability, good accuracy, extremely small temperature error and high cost performance.

The gauge pressure sensor and transmitter are composed of two 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 the 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 is deformed. After the deformation is sensed by the silicon-sapphire sensitive element, its bridge output will change, and the amplitude 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 imbalance 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, connected with the ceramic base glass solder, plays the role of 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 main piezoelectric materials used in piezoelectric sensors include quartz, potassium sodium tartrate and diammonium phosphate. Quartz (silicon dioxide) is a natural crystal in which the piezoelectric effect is found. Within a certain temperature range, the piezoelectric property always exists, but when the temperature exceeds this range, the piezoelectric property completely disappears (this high temperature is the so-called "Curie point"). Since 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 a large piezoelectric sensitivity and piezoelectric coefficient, but it can only be used at room temperature and in an environment with relatively low humidity. Diammonium 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 piezoelectric sensors. Piezoelectric sensors cannot be used for static measurements because the charge after the external force is applied can only be saved when the circuit has infinite input impedance. The actual situation is not like this, so 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 excellent characteristics such as simple structure, small size, light weight and long service life. Piezoelectric accelerometer has been widely used in the vibration and impact measurement of aircraft, automobiles, ships, bridges and buildings, especially in the fields of aviation and aerospace. Piezoelectric sensors can also be used to measure the combustion pressure and vacuum degree inside the engine. It can also be used in the military industry, for example, to measure the change in chamber pressure and the shock wave pressure at the muzzle of a gun when a bullet is fired in the chamber. It can be used to measure both large and small pressures.