What are the principles for pressure sensors ?

[maryzhu]

I read an electronic magazine in which there is a report on what are the principles for pressure sensors. After reading, I became interested in pressure sensors, http://www.kynix.com/Product/Cate/180.html. So I searched some related materials and information about this topic on net, but found that it is not specific at all. Today I come to this forum to turn to you to discuss my understandings are appropriate.

Here are my personal understandings:

Pressure sensor is one of the most commonly used sensors in industrial practice. It is widely used in various industrial environment, which involves water conservancy and hydropower, railway transportation, intelligent building, production control, aerospace, military, petrochemical, oil well, electric power, ship, machine , Pipelines and many other industries, the following brief introduction to some commonly used sensor principle and its application.

Principle and Application of Strain Gauge Pressure Sensor

A wide range 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 high accuracy and better linearity. Here we mainly introduce this type of sensor.

In understanding the piezoresistive force sensor, we first understand the resistance strain gauge this component. A strain gauge is a sensitive device that converts a strain change on a DUT into an electrical signal. It is one of the main components of a piezoresistive strain sensor. Resistance strain gauges are the most used metal resistance strain gauge and semiconductor strain gauge two. Metal resistance strain gauge and wire-like strain gauge and metal foil strain gauge two. Usually the strain gauges are tightly adhered to the mechanical strain matrix by means of a special adhesive. When the stress of the substrate is changed, the strain gauges are also deformed together to change the resistance of the strain gages so that The voltage applied to the resistor changes. This strain gauge in the force generated when the resistance change is usually small, the general strain gauges are composed of strain bridge, and through the subsequent instrumentation amplifier to amplify, and then transmitted to the processing circuit (usually A / D conversion And CPU) display or actuator.

The internal structure of metal resistance strain gauge

As shown in Fig. 1, it is a structural diagram of a strain gauge which consists of a base material, a metal strain wire or a strain foil, an insulating protective sheet and a lead wire. Depending on the purpose, the resistance of the strain gauge can be designed by the designer, but the range of resistance should be noted that the resistance is too small and the required drive current is too large, and the heat of the strain gauge causes the temperature to be too high , The use of different environments, so that the resistance of the strain gauge changes too much, the output zero drift significantly, zero circuit is too complicated. And the resistance is too large, the impedance is too high, resistance to external electromagnetic interference is poor. Generally tens of euros to tens of thousands of euros or so.

The working principle of resistance strain gauge

Metal resistance strain gauge working principle is adsorbed on the matrix material strain resistance with mechanical deformation and resistance to change the phenomenon, commonly known as resistance strain effect. The resistance value of the metal conductor can be expressed as follows:
Where: ρ - metal conductor resistivity (Ω.cm2 / m)
S - conductor cross-sectional area (cm2)
L - length of conductor (m)

Our wire strain resistor, for example, when the wire by external force, its length and cross-sectional area will change from the above formula can be easily seen, the resistance value will change, if the wire by external action And elongation, its length increases, while the cross-sectional area decreases, the resistance value will increase. When the wire is compressed by external force, the length decreases and the cross section increases and the resistance decreases. As long as the measurement of resistance changes in the resistance (usually measuring the voltage across the resistor), you can get strain of strain wire.

2, ceramic pressure sensor principle and application

Corrosion-resistant ceramic pressure sensor without the transmission of liquid, the pressure directly on the front surface of the ceramic diaphragm, the diaphragm has a small deformation, thick film resistance printed on the back of the ceramic diaphragm, connected into a Wheatstone bridge Bridge), due to the varistor piezoresistive effect, so that the bridge produces a linearity proportional to the pressure, and the voltage is proportional to the voltage signal, the standard signal according to the pressure range of different calibration for the 2.0 / 3.0 / 3.3 MV / V, etc., and strain sensor can be compatible. Through the laser calibration, the sensor has a high temperature stability and time stability, the sensor comes with temperature compensation 0 ~ 70 ℃, and can be in direct contact with the vast majority of media.

Ceramics are a recognized high elasticity, corrosion resistance, abrasion resistance, impact and vibration of the material. The thermal stability of the ceramic and its thick film resistance can make its operating temperature range up to -40 ~ 135 ℃, and has a high precision measurement, high stability. Electrical insulation degree> 2kV, the output signal is strong, long-term stability is good. High-performance, low-cost ceramic sensor will be the direction of development of pressure sensors in Europe and the United States have a comprehensive alternative to other types of sensor trends in China, more and more users to use ceramic sensors instead of diffusion silicon pressure sensor.

3, diffusion silicon pressure sensor principle and application

Working principle The pressure of the measured medium acts directly on the diaphragm of the sensor (stainless steel or ceramic) so that the diaphragm produces a slight displacement proportional to the medium pressure, changes the resistance of the sensor, and detects the change with the electronic circuit , And converts and outputs a standard measurement signal corresponding to this pressure.
Schematic diagram.

4, sapphire pressure sensor principle and application

The use of strain resistors working principle, the use of silicon - sapphire as a semiconductor sensitive components, with unparalleled measurement characteristics.

Sapphire is composed of single crystal insulator elements without lagging, fatigue and creep; sapphire is stronger than silicon, hardness is higher, not afraid of deformation; sapphire has a very good flexibility and insulation properties (1000 OC or less), therefore, the use of Silicon-sapphire semiconductor-sensitive components, the temperature changes are not sensitive, even in high temperature conditions, but also has a good working characteristics; sapphire anti-radiation characteristics of strong; In addition, silicon - sapphire semiconductor sensor, no pn drift, Therefore, the manufacturing process is fundamentally simplified, the repeatability is improved, and the high yield is ensured.

Pressure sensors and transmitters made of silicon-sapphire semiconductor sensor can work in the harshest working conditions, with high reliability, good accuracy, minimal temperature and high cost.

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 heteroepitaxial strain sensitive bridge circuit was welded on a titanium alloy measuring diaphragm. The measured pressure is transmitted to the receiving diaphragm (the receiving diaphragm is firmly connected with the measuring diaphragm with a tie rod). Under the action of pressure, the titanium alloy receives the deformation of the diaphragm, which is sensed by the silicon-sapphire sensor, the bridge output will change, the amplitude of the change is proportional to the measured pressure.

The sensor circuit can guarantee 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, sapphire flakes, and ceramic base glass solder together, played a role in the elastic components, the measured pressure into strain gauge deformation, so as to achieve the purpose of pressure measurement.

Principle and Application of Piezoelectric Pressure Sensor

Piezoelectric materials mainly used in piezoelectric materials include quartz, potassium tartrate and dihydrogen phosphate. The quartz (silica) is a natural crystal, the piezoelectric effect is found in this crystal, within a certain temperature range, the piezoelectric properties have been there, but the temperature exceeds this range, the piezoelectric properties of the complete Disappear (the high temperature is the so-called "Curie point"). As the change in stress with the electric field changes slightly (also said that the piezoelectric coefficient is relatively low), so the quartz was gradually replaced by other piezoelectric crystals. Potassium tartrate has a large piezoelectric sensitivity and piezoelectric coefficient, but it can only be in the room temperature and humidity is relatively low environment can be applied. Dihydrogen phosphate is an artificial crystal, can withstand high temperature and very high humidity, it has been widely used.

Piezoelectric effect is also used in polycrystalline, such as the current piezoelectric ceramics, including barium titanate piezoelectric ceramics, PZT, niobate piezoelectric ceramics, lead magnesium niobate piezoelectric ceramics and so on.

Piezoelectric effect is the main working principle of piezoelectric sensors, piezoelectric sensors can not be used for static measurement, because after the external force of the charge, only when the loop has an infinite input impedance to be saved. 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 a simple structure, small size, light weight, long life and other excellent features. Piezoelectric accelerometers have been widely used in vibration and shock measurements of aircraft, automobiles, ships, bridges and buildings, particularly in the aerospace and aerospace fields. Piezoelectric sensors can also be used to measure the internal combustion pressure measurement and vacuum measurement. Can also be used in the military industry, for example, to use it to measure the gun guns in the chamber of the moment of the changes in the chamber pressure and muzzle shock wave pressure. It can be used to measure large pressure, can also be used to measure the small pressure.

Piezoelectric sensors are also widely used in biomedical measurements, such as ventricular catheter microphones are made by the piezoelectric sensor, because the measurement of dynamic pressure is so common, so the application of piezoelectric sensors is very extensive.

Ps: Excuse me if I was wrong in words or expressions as I am a green hand in the field of pressure sensors. I need continual learnings.

What is your idea ? Do you agree with my ideas ? Any of your ideas would be highly appreciated.

May someone would like to help ?

thanks in advance.