Pressure sensors are one of the most commonly used sensors in industrial practice and are widely used in a variety of industrial self-control environments involving water conservancy and hydropower, railway transportation, intelligent construction, production self-control, aerospace, military, petrochemical, oil well, power, ship, machine tool, pipeline and many other industries. The following is a brief introduction to some common sensor principles and their applications.

There are many types of force 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. However, the most widely used is the piezoresistive pressure sensor, which has a very low price, high accuracy and good linearity. In the following we focus on this type of sensor.

When learning about piezoresistive force sensors, we first meet the element of resistance strain gauges. A resistive strain gauge is a sensitive device that converts the strain change in the measured part into an electrical signal. It is one of the main components of a piezoresistive strain transducer. The two most used types of resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. There are two types of metal strain gauges: wire strain gauges and metal foil strain gauges. The strain gauges are usually bonded tightly to a mechanical strain-generating substrate by means of a special adhesive. When the substrate is subjected to stress changes, the strain gauges are deformed together, resulting in a change in the resistance of the strain gauges and thus in the voltage applied to the resistance. These strain gauges usually produce a small change in resistance when subjected to stress. Generally these 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) for display or actuation.

I. Strain gauge pressure sensors

1、Strain tube type

Also known as strain gauge. Its elastic sensitive element is a thin-walled cylinder closed at one end, the other end of which is flanged to the measured system. Two or four strain gauges are attached to the cylinder wall, half of which are attached to the solid part as temperature compensation gauges and the other half as measuring strain gauges. When there is no pressure, the four strain gauges form a balanced full bridge circuit; when pressure is applied to the inner cavity, the cylinder becomes "drum-shaped" and the bridge is unbalanced, resulting in a voltage output in relation to the pressure. This sensor can also use a piston to convert the measured pressure into a force to be transmitted to the strain relief cylinder or through a pendant chain shaped diaphragm to transmit the measured pressure. The strain barrel type pressure sensor is simple, easy to manufacture and highly applicable, and is widely used in dynamic pressure measurement of rockets, artillery shells and artillery.

2、Diaphragm type

Its elastic sensitive element is a round metal flat diaphragm fixed around the perimeter. When the diaphragm is deformed by pressure, the radial and tangential strains at the centre reach a positive maximum, while the radial strain at the edge reaches a negative maximum and the tangential strain is zero. For this reason, two strain gauges are often attached to the positive and negative maximum strains and connected into a half-bridge circuit with adjacent bridge arms to obtain greater sensitivity and temperature compensation. The use of circular foil strain gauges (see resistance strain gauges) makes maximum use of the strain effect of the diaphragm. The non-linearity of these sensors is more significant. The latest product of the diaphragm type pressure sensor is the role of the elastic sensitive element and the strain gauge set in a single crystal silicon diaphragm, that is, the use of integrated circuit process on the single crystal silicon diaphragm diffusion production resistance strip, and the use of the surrounding fixed structure made of solid-state pressure sensor (see piezoresistive sensor).

3、Strain beam type

When measuring smaller pressures, a fixed beam or a structure of equal strength beam can be used. One method is to use a diaphragm to convert the pressure into a force which is then transmitted to the strain beam via a force transmission rod. The maximum strain in the fixed beam at both ends in Figure 3 is at the ends and midpoints of the beam, where the strain gauges are attached. There are other forms of this structure, for example a suspended beam with a diaphragm or bellows can be used.

4. Combination type

In the combined strain pressure sensor, the elastic sensitive element can be divided into a sensing element and an elastic strain element. The receptor element converts the pressure into force to the most strain-sensitive part of the elastic strain element, while the strain gauge is attached to the maximum strain of the elastic strain element. In practice the more complex strain tube and strain beam types are of this type. The sensing elements are diaphragms, membrane boxes, bellows, borden tubes, etc. The elastic strain elements are cantilevered beams, fixed beams, Π-shaped beams, ring beams, thin-walled cylinders, etc. They can be combined into various types according to different needs. Strain gauge pressure sensors are mainly used to measure the dynamic or static pressure of flowing media, such as the pressure of inlet and outlet gases or liquids in power pipelines and equipment, the pressure of internal combustion engine pipelines, etc.

The principle and mechanism of strain gauge pressure sensors

1、The internal structure of metal resistance strain gauges

Resistance strain gauges consist of a base material, a metal strain wire or strain foil, an insulating protective sheet and a lead wire. Depending on the application, the resistance value of the strain gauge can be designed by the designer, but the range of resistance values should be noted: a resistance value that is too small will require too much driving current, while the heating of the strain gauge causes the temperature itself to be too high for use in different environments, making the resistance value of the strain gauge change too much, the output zero point drift is obvious and the zero adjustment circuit is too complex. The resistance is too large and the impedance is too high, making it less resistant to external electromagnetic interference. Generally, the resistance is a few tens to tens of kilo-ohms.

The above diagram shows the structure of a resistance strain gauge, which consists of a base material, a metal strain gauge or strain gage, an insulator and a lead wire. Depending on the use' the resistance of the resistance strain gauge can be designed designer' but the range of resistance values should be noted: the resistance value is too small' requiring too much driving current' while the strain gauge' gives off heat to make itself highly heated 'use in different environments' makes the strain gauge resistance vary too much 'output zero drift is noticeable' the zero adjustment circuit is too complex. However, the resistance is too high' the impedance is too high' and the resistance to external electromagnetic interference is poor. Generally a few tens of ohms to tens of kiloohms approximately.

2、The working principle of resistance strain gauge

The working principle of the metal resistance strain gauge is the phenomenon of change in the resistance value of the adsorption strain resistor' in the base material 'with mechanical deformation' named the resistance strain effect. The metal conductor resistance value can be indicated using the type '

where: lu - metal conductor resistivity (omega square centimetres per metre)

-Conductor cross-sectional area (cm2)

I conductor length (m)

We use the metal foil strain resistance as an example' when the buoyant exogenous process' its length and cross-sectional area can be changed' can easily be seen in the formula' its resistance value that can have changes' such as the exogenous process of the slender metal wire time' its length increases 'but the cross-sectional area decreases' the resistance value can increase. When a liquid compresses an exogenous process 'its length decreases while the increase in section' the resistance value decreases. As soon as the (usually investigative start and end of the resistance change in voltage) 'and thus the strain wire strain mood is determined

3、The working principle of resistance strain gauges

The working principle of metal resistance strain gauges is the phenomenon of strain resistance adsorbed on the base material to produce a change in resistance value with mechanical deformation, commonly known as the resistance-strain effect. The resistance value of a metal conductor can be expressed by the following formula

Where: ρ - the resistivity of the metal conductor (Ω-cm2/m)

S - the cross-sectional area of the conductor (cm2)

L - the length of the conductor (m)

When a metal wire is subjected to an external force, its length and cross-sectional area will change, as can be easily seen from the above formula, its resistance value will change, if the wire is stretched by an external force, its length will increase, while the cross-sectional area will decrease, the resistance value will increase. When the wire is compressed by an external force, the length decreases and the cross-sectional area increases, the resistance value decreases. The strain on the wire can be obtained by measuring the change in resistance (usually by measuring the voltage across the resistance).

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