Pressure sensor is the most commonly used sensor in industrial practice and instrumentation control, and is widely used in various industrial self-control environments, involving water conservancy and hydropower, railway transportation, production automation, aerospace, military, petrochemical, oil well, electric power, ships, In many industries such as machine tools and pipelines, the comrades of the Skills Department of Shenzhen Liqin Sensing Skills Co., Ltd. briefly introduced 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 a piezoresistive pressure sensor, which has a very low price, high accuracy and good linearity. Below we will introduce these sensors first.

1. Principle and application of strain gauge pressure sensor:

When understanding the piezoresistive force sensor, let us first understand the components of the resistance strain gauge. A strain gage is a sensitive device that converts strain changes on the device under test into an electrical signal. It is one of the primary 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 mechanically strained substrate by a special adhesive. When the stress is changed by 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 is changed. The strain gauges that are subjected to force are usually modified to be small. Usually, the strain gauges form a strain bridge and are amplified by a subsequent instrumentation amplifier and then transmitted to the processing circuit (usually A/D conversion). And CPU) display or actuator.

1. Internal structure of metal resistance strain gauge: it consists of matrix data, metal strained wire or strained foil, insulating protective sheet and lead wire. Depending on the application, the resistance of the strain gauge can be designed by the designer, but the scale of the resistor 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. In different environments, the resistance of the strain gauge is changed too much, the output zero drift is significant, and the zero adjustment circuit is too messy. The resistance is too large, the impedance is too high, and the electromagnetic interference resistance against the outside is poor. Usually it is tens of ohms to tens of thousands of euros.

1.2, the working principle of the resistance strain gauge: the working principle of the metal resistance strain gauge is the phenomenon that the resistance of the strain resistance changes with the mechanical deformation on the substrate data, which is commonly known as 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 be changed. From the above formula, it can be easily seen that the resistance value will be changed, assuming that 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 effect, the length is decreased and the section is increased, and the resistance value is decreased. The strain pressure of the strained wire can be obtained by simply measuring the change in the resistance (usually the voltage across the resistance).

2. Principle and application of ceramic pressure sensor: The corrosion-resistant ceramic pressure sensor has no liquid transfer. The direct effect of pressure on the front surface of the ceramic diaphragm causes the diaphragm to undergo small deformation. The thick film resistor is printed on the back of the ceramic diaphragm. Connected into a Wheatstone circuit closed bridge, because of the piezoresistive effect of the varistor, the bridge generates a high linearity proportional to the pressure, and the voltage signal is proportional to the excitation voltage. The standard signal is different according to the pressure range. It is calibrated to 2.0 / 3.0 / 3.3 mV / V, etc., and is compatible with strain gauge sensors. Through laser calibration, the sensor has high temperature stability and time stability. The sensor comes with temperature compensation of 0 ~ 70 ° C, and direct touch with most media.

Ceramic is a recognized material of high elasticity, corrosion resistance, abrasion resistance, impact resistance and oscillation. The thermal stability of ceramics and its thick film resistance enable it to operate at temperatures ranging from -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-featured, low-priced ceramic sensors will be the development direction of pressure sensors. In Europe and the United States, there is a tendency to replace other types of sensors. In China, the increasing number of users use ceramic sensors instead of dispersed silicon pressure sensors.

3, the principle and application of the dispersed silicon pressure sensor: the pressure of the measured medium directly affects the diaphragm of the sensor (stainless steel or ceramic), so that the diaphragm generates a micro-displacement proportional to the pressure of the medium, so that the resistance value of the sensor is changed. And check the change with an electronic circuit and convert the output to a specification measurement signal corresponding to this pressure.

4. Principle and application of piezoelectric pressure sensor

The primary piezoelectric materials 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. The piezoelectric property always exists within a certain temperature scale, but the piezoelectric property after the temperature exceeds this scale. Completely disappeared (this high temperature is the so-called "Curie point"). Because 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 in a low room temperature and humidity environment. Dihydrogen phosphate is an artificial crystal that can withstand high temperatures and moderately high humidity, so together). Under the effect of pressure, the titanium alloy receiving diaphragm is deformed. After the deformation is sensed by the silicon-sapphire sensitive component, the bridge output will be changed, and the magnitude of the change is proportional to the measured pressure.

The piezoelectric effect is now also applied to polycrystals, such as piezoelectric ceramics, including barium titanate piezoelectric ceramics, PZT, tantalate-based piezoelectric ceramics, lead magnesium niobate piezoelectric ceramics, and the like.

Piezoelectric effect is the primary working principle of piezoelectric sensors. Piezoelectric sensors cannot be used for static measurement because the charge after external force effect is retained as long as the loop has an infinite input impedance. The situation in practice is not the case, so this resolution of the piezoelectric sensor can only measure dynamic stress.

Piezoelectric sensors are primarily 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 been used extensively in the measurement of oscillations and shocks in aircraft, automobiles, boats, bridges and buildings, especially in the aerospace and aerospace fields. Piezoelectric sensors can also be used to measure the measurement of the internal combustion pressure of the engine and the measurement of the vacuum. It can also be used in the military industry, for example, to measure the momentary pressure changes of the guns and bullets in the smash and the shock wave pressure of the muzzle. It can be used to measure large pressures as well as to measure small pressures.

Piezoelectric sensors are also widely used in biomedical measurement. For example, ventricular catheter microphones are made of piezoelectric sensors. Because measuring dynamic pressure is so common, piezoelectric sensors are widely used.

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