The working principle of pressure sensor and its application

- - Reproduced

Pressure sensors are the most commonly used sensors in industrial practice. They are widely used in various industrial automation environments, involving water conservancy and hydropower, railway transportation, intelligent buildings, production automation, aerospace, military, petrochemical, oil wells, electric power, ships, and machine tools. , pipelines and many other industries, the following briefly introduces some common sensor principles and their applications.

1. The 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 the piezoresistive pressure sensor, which has a very low price, high accuracy and good linearity. Below we mainly introduce such sensors.

In understanding the piezoresistive force sensor, we first understand the element resistance strain gauge. The resistance strain gauge 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. Metal strain gauges are also available in wire strain gages and metal foil gages. Usually, the strain gauges are tightly bonded to a mechanical strain matrix through a special adhesive agent. When the stress of the base body changes, the strain gauges also deform together, so that the resistance of the strain gauges is changed, and thus The voltage applied to the resistor changes. These strain gauges usually have a small change in resistance when they are subjected to stress. In general, these strain gauges form a strain bridge and are amplified by subsequent instrumentation amplifiers and transmitted to a processing circuit (usually A/D conversion). And CPU) display or actuator.

Internal structure of metal resistance strain gauge

Resistance strain gauges consist of a base material, a strained metal wire or a strain foil, an insulating protection sheet, and lead wires. According to different uses, the resistance strain gauge resistance can be designed by the designer, but the value of the resistance range should be noted: the resistance is too small, the required drive current is too large, while the heat of the strain gauge caused its own temperature is too high , The use of different environments, so that the resistance of the strain gauge changes too much, the output zero drift is obvious, the zero adjustment circuit is too complicated. The resistance is too high, the impedance is too high, and the resistance to external electromagnetic interference is poor. Generally, they are several tens of euros to several tens of thousands of euros.

How does the strain gage work?

The working principle of the metal resistance strain gauge is the phenomenon that the strain resistance changes with mechanical deformation due to adsorption on the base material, which is commonly known as resistance strain effect. The resistance of a metal conductor can be expressed by the following formula:

In the formula: ρ——Resistivity of metal conductor (Ω·cm2/m)
S - cross-sectional area of ​​the conductor (cm2)
L——length of conductor (m)

We take wire strain resistance as an example. When the wire is subjected to an 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 affected by external force. When extended, 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 decreases and the cross section increases, and the resistance value decreases. As long as the change in the resistance is measured (usually the voltage across the measuring resistor), the strain of the strained wire can be obtained.

2. Principle and Application of Ceramic Pressure Sensor

Corrosion-resistant ceramic pressure sensors do not transmit liquid. The pressure acts directly on the front surface of the ceramic diaphragm, causing a slight deformation of the diaphragm. The thick film resistors are printed on the back of the ceramic diaphragm and are connected to a Wheatstone bridge (closed). Bridge), due to the piezoresistive effect of the piezoresistors, causes the bridge to produce a highly linear voltage proportional to the pressure and a voltage signal 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. Through laser calibration, the sensor has high temperature stability and time stability, the sensor comes with temperature compensation 0 ~ 70 °C, and can be in direct contact with most of the media.

Ceramics is a well-known material with high elasticity, corrosion resistance, wear resistance, impact resistance and vibration. The thermal stability of ceramics and its thick film resistance can make its operating temperature range up to -40 ~ 135 °C, but also has high precision and high stability. Electrical insulation >2kV, strong output signal, good long-term stability. High-performance, low-cost ceramic sensors will be the development direction of pressure sensors. In Europe and the United States, there is a trend to replace other types of sensors. In China, more and more users use ceramic sensors instead of diffusion silicon pressure sensors.

3, diffusion silicon pressure sensor principle and application

working principle

The pressure of the measured medium directly acts on the diaphragm (stainless steel or ceramic) of the sensor, so that the diaphragm generates a micro-displacement proportional to the pressure of the medium, so that the resistance value of the sensor changes, and the change is detected by the electronic circuit, and The conversion outputs a standard measurement signal that corresponds to this pressure.

4. Principle and Application of Sapphire Pressure Sensor

Using strain-resistance operating principles, silicon-sapphire is used as a semiconductor sensing element and has unparalleled metering characteristics.

Sapphire is composed of single-crystal insulator elements that do not suffer from 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), and therefore, is utilized. Silicon-sapphire semiconductor sensing devices are insensitive to temperature changes and have good operating characteristics even at high temperatures; sapphire has extremely strong radiation resistance; in addition, silicon-sapphire semiconductor sensitive devices have no pn drift. Therefore, the manufacturing process is fundamentally simplified, the repeatability is improved, and a high yield is ensured.

Pressure sensors and transmitters manufactured with silicon-sapphire semiconductor sensing elements can operate normally in the harshest operating conditions, with high reliability, accuracy, minimal temperature error, and cost-effectiveness.

Gauge pressure sensors and transmitters consist of two diaphragms: a titanium alloy diaphragm and a titanium alloy diaphragm. The sapphire sheet printed with the heteroepitaxial strain-sensitive bridge circuit was welded on the titanium alloy measurement diaphragm. The measured pressure is transmitted to the receiving diaphragm (the receiving diaphragm and the measuring diaphragm are firmly connected together by a pull rod). Under the effect of pressure, the titanium alloy receives the deformation of the diaphragm. After the deformation is perceived by the silicon-sapphire sensor, the output of the bridge will change, and the amplitude 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 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 solder of the ceramic base glass and functions as an elastic element. The measured pressure is converted into strain gauge deformation to achieve the purpose of pressure measurement.

5, piezoelectric pressure sensor principle and application

The piezoelectric materials mainly used in piezoelectric sensors include quartz, potassium sodium tartrate, and diammonium phosphate. Among them, quartz (silicon dioxide) is a kind of natural crystal. The piezoelectric effect is found in this kind of crystal. Within a certain temperature range, the piezoelectricity is always there, but after the temperature exceeds this range, the piezoelectricity is completely Disappear (this high temperature is the so-called "curie point"). Because 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 environments with low room temperature and low humidity. Dihydrogen phosphate is an artificial crystal that can withstand high temperatures and high humidity, so it has been widely used.

Now the piezoelectric effect is also applied to polycrystals, such as current piezoelectric ceramics, including barium titanate piezoelectric ceramics, PZT, niobate piezoelectric ceramics, lead magnesium niobate piezoelectric ceramics, and the like.

Piezoelectric effect is the main working principle of piezoelectric sensor. Piezoelectric sensor cannot be used for static measurement because the charge after external force is only saved when the loop has infinite input impedance. The actual situation is not like this, so this determines that the piezoelectric sensor can only measure the 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 the advantages of simple structure, small size, light weight and long service life. Piezoelectric accelerometers have been widely used in the measurement of vibration and shock in aircrafts, automobiles, ships, bridges, and buildings, particularly in the aviation and aerospace fields. Piezoelectric sensors can also be used to measure the internal combustion pressure measurement and vacuum measurement. It can also be used in the military industry, for example, to measure the momentary pressure change of a gun dan fired in a cannon and the shock wave pressure of the muzzle. It can be used to measure large pressures, but it can also be used to measure tiny pressures.

The aluminum die casting part production flow chart

Commonly used materials of aluminum die casting: ADC12, ZL101, ZL102,A380, AlSi9Cu3, AlSi12(Fe);

The precision of the aluminum die casting: ±0.1mm;

Smoothness of surface of rough part: Ra1.6~Ra3.2;

Shrinkage rate of aluminum die casting products: 0.5%;

Material wastage rate: 5%;

General aluminum die-casting machining allowance: 0.5mm~0.7mm;

Surface preparation methods: Sand blasted, Shot blasting, Powder coating, Painting, Plating, Electrophoresis, Polishing

The specific pressure parameters of aluminum die casting part

Injection speed of die cast process

High speed computing formula

V -------------Cavity volume (CM³);

N ------------ Cavity No;

D ------------ Die-casting drift (CM);

T ------------- Proper filling time.

Holding pressure time

Aluminum die casting mold temperature

T_m --------------- Aluminum Die Casting Mould required working temperature (℃);

Th -------------- Aluminum Alloy pouring temperature (℃);

△ t---------------The temperature control of tolerance (The general is 25℃).

The Aluminum Alloy Die Casting production and QC control

Aluminum Die Casting Parts

Aluminum Alloy Die Casting, Aluminum Pressure Die Casting, High Pressure Aluminum Die Casting, Aluminum Injection Die Casting

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