Principle of pressure transmitter

Pressure transmitter 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 building, production automatic control, aerospace, military industry, petrochemical, oil well, electric power, ship, machine tool, pipeline and so on. The principles and applications of some common pressure transmitters are briefly introduced below

Principle and application of strain gauge pressure transmitter

There are many kinds of mechanical sensors, such as resistance strain gauge pressure transmitter, semiconductor strain gauge pressure transmitter, piezoresistive pressure transmitter, inductive pressure transmitter, capacitive pressure transmitter, resonant pressure transmitter and capacitive acceleration sensor. But the most widely used is piezoresistive pressure transmitter, which has very low price, high precision and good linear characteristics. Next, we mainly introduce this kind of sensor.

When designing the decompressing resistance pressure transmitter, we first understand the element of resistance strain gauge. Resistance strain gauge is a kind of sensitive device that converts the strain change on the tested part into an electrical signal. It is one of the main components of piezoresistive strain transmitter. The most widely used resistance strain gages are metal resistance strain gages and semiconductor strain gages. There are two kinds of metal resistance strain gauges: wire strain gauge and metal foil strain gauge. Usually, the strain gauge is tightly bonded to the matrix producing mechanical strain through a special adhesive. When the stress of the matrix changes, the resistance strain gauge also deforms together to change the resistance value of the strain gauge, so as to change the voltage applied to the resistance. The resistance change of this strain gauge is usually small when it is stressed. Generally, this strain gauge forms a strain bridge, which is amplified by subsequent instrument amplifiers, and then transmitted to the processing circuit (usually a / D conversion and CPU) display or actuator.

Internal structure of metal strain gauge

The resistance strain gauge is composed of base material, metal strain wire or strain foil, insulating protective sheet and outgoing line. According to different uses, the resistance value of the resistance strain gauge can be designed by the designer, but the value range of the resistance should pay attention to: the resistance value is too small, the required driving current is too large, and the heating of the strain gauge causes its own temperature 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 complex. 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 euros to tens of thousands of euros.

Working principle of resistance strain gauge

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

Where: ρ—— Resistivity of metal conductor (Ω· cm2 / M)

S -- sectional area of conductor (cm2)

L -- length of conductor (m)

We take the wire strain resistance as an example. When the wire is subjected to external force, its length and cross-sectional area will change. From the above formula, it can be easily seen that its resistance value will change. If the wire is extended by external force, its length will increase and the cross-sectional area will decrease, the resistance value will increase. When the wire is compressed by external force, the length decreases, the section increases and the resistance decreases. As long as the change applied to the resistance (usually the voltage at both ends of the resistance) is measured, the strain condition of the strained wire can be obtained.