Strain gauge pressure sensors convert changes in pressure into changes in resistance values for measurement. Strain gauges are resistors made of metal conductors or semiconductors, and their resistance values change with the strain generated by pressure.

1. Introduction to Classification of Strain Gauge Pressure Sensors

The most commonly used strain gauge is the adhesive strain gauge (i.e. strain gauge). Its main disadvantages are small output signal, narrow linear range, and poor dynamic response (see resistance strain gauges, semiconductor strain gauges). However, due to the small size of strain gauges, there are multiple specifications to choose from for commercial strain gauges, and the flexible design of elastic sensitive components can be adapted to various application scenarios. Therefore, strain gauge pressure sensors manufactured with strain gauges still have a wide range of applications. According to the different structures of elastic sensitive components, strain gauges can be roughly divided into four types: strain tube type, diaphragm type, strain beam type, and combination type.

① Strain tube type, also known as strain tube type. Its elastic sensitive element is a thin-walled cylinder with one end closed, and the other end is connected to the tested system with a flange (Figure 1). Attach 2 or 4 strain gauges on the cylinder wall, with half attached to the solid part as temperature compensation gauges and the other half as measuring strain gauges. When there is no pressure, 4 strain gauges form a balanced full bridge circuit; When pressure is applied to the inner cavity, the cylinder transforms into a "waist drum shape", causing the bridge to lose balance and output a voltage that is proportional to the pressure. This type of sensor can also use a piston to convert the measured pressure into force and transmit it to the strain tube, or transmit the measured pressure through a diaphragm in the shape of a vertical chain. The strain tube pressure sensor has a simple structure, easy manufacturing, and strong applicability, and is widely used in dynamic pressure measurement of rockets, shells, and artillery.

② The elastic sensitive element of the membrane type is a circular metal flat membrane fixed around the periphery. When the diaphragm deforms under pressure, the radial strain and tangential strain at the center reach their maximum positive values, while the radial strain at the edge reaches its maximum negative value, and the tangential strain is zero. Therefore, two strain gauges are often attached to the positive and negative maximum strain points respectively, and connected to form a half bridge circuit with adjacent bridge arms to achieve greater sensitivity and temperature compensation. The use of circular foil strain gauges (see resistance strain gauges) can maximize the strain effect of the membrane (Figure 2). The nonlinearity of this sensor is significant. The latest product of the diaphragm pressure sensor is a solid-state pressure sensor that integrates the functions of elastic sensing elements and strain gauges into a single crystal silicon diaphragm. It uses integrated circuit technology to diffuse and produce resistance strips on the single crystal silicon diaphragm, and adopts a peripheral fixed structure to make it (see piezoresistive sensor).

③ When measuring small pressures with strain beam, a fixed beam or an equal strength beam structure can be used. One method is to use a diaphragm to convert pressure into force, which is then transmitted to the strain beam through a force transfer rod. The maximum strain of the fixed beam at both ends in Figure 3 is located at the ends and midpoint of the beam, where strain gauges are attached. There are other forms of this structure, such as using suspended beams and membranes or corrugated pipes.

④ In a combined strain pressure sensor, the elastic sensing element can be divided into a sensing element and an elastic strain element. The sensing element converts pressure into force and transmits it to the most strain sensitive part of the elastic strain element, while the strain gauge is attached to the maximum strain point of the elastic strain element. In fact, both the more complex strain tube and strain beam types belong to this type. Sensing elements include membranes, membrane boxes, corrugated tubes, Bourdon tubes, etc. Elastic strain elements include cantilever beams, fixed beams, π - shaped beams, annular beams, thin-walled tubes, etc. They can be combined into various forms according to different needs.

2. Method of using strain gauges

Usually, strain gauges are tightly adhered to a mechanically strained substrate using a special adhesive. When the substrate undergoes stress changes, the resistance strain gauges also deform, causing a change in the resistance value of the strain gauges and a change in the voltage applied to the resistance. The resistance change of this strain gauge when subjected to force is usually small. Generally, this strain gauge forms a strain bridge and is amplified by subsequent instrumentation amplifiers before being transmitted to processing circuits (usually A/D converters and CPUs) for display or execution.

The commonly used force measurement method is to measure the surface strain of the component using strain gauges and strain gauges, and determine the stress state of the component based on the relationship between strain, stress, and force.

When the strain gauge uses an AC bridge, its output characteristics are similar to those of a DC bridge. The arrangement of strain gauges and the connection of bridge groups (referred to as patch group bridges) should be determined based on the force distribution of the measured object. Appropriate fabric bridging methods should also be used to eliminate the effects of temperature changes and composite loads.

When measuring tensile (compressive) strain, an appropriate cloth bridge assembly method should be used to achieve temperature compensation (temperature compensation during axial tension (compression) measurement), eliminate bending moment effects (using double working pieces to eliminate temperature effects), and improve measurement sensitivity (using four working pieces to improve measurement sensitivity).The commonly used methods for stress measurement of cloth and bridge assembly: When the specimen is subjected to bending moment, tensile strain or compressive strain will occur on its upper and lower surfaces, respectively. The bending moment can be obtained through strain measurement. When connecting the patch bridge, attention should be paid to utilizing the characteristics of the bridge, retaining the influence of bending strain in the output, and eliminating the strain components caused by axial tension and pressure.