Analysis of strain measurement principle based on virtual instrument

0 Virtual instrument technology and LabVIEW
Virtual instrument technology uses high-performance modular hardware combined with efficient and flexible software to complete various test, measurement and automation applications. Virtual instrument technology has the advantages of high performance, strong scalability, less development time and excellent integration capabilities. Based on virtual instrument technology, virtual instrument test solutions that can adapt to different application scenarios can be developed to better build test systems with a high degree of automation and strong data processing and analysis capabilities. Lab-VIEW is a revolutionary graphical development environment with built-in signal acquisition, measurement analysis and data display functions. It abandons the complexity of traditional development tools and ensures the flexibility of the system while providing powerful functions. LabVIEW concentrates a wide range of data acquisition, analysis and display functions in the same environment, and can seamlessly integrate a complete set of application solutions on its own platform. Because of such huge advantages, it has become the best choice for establishing test and measurement and control systems.

1 Strain measurement principle
Semiconductor strain gauges can be used for stress measurement and stress analysis, as well as as force-to-electric conversion elements of various sensors. They are based on the physical property that metal wires undergo elastic deformation after being pulled or compressed, and their resistance values ​​also change accordingly.
Within the elastic range of metal wire deformation, the relative change in resistance △R/R is proportional to the strain △L/L, so △R/R=K△L/L. Among them, K is the sensitivity coefficient of the resistance strain gauge. When using a strain gauge to measure strain or stress, the strain gauge is pasted on the object to be measured. Under the action of external force, the surface of the object to be measured produces a small mechanical deformation, and the strain gauge pasted on its surface also undergoes the same change, so the resistance of the strain gauge also changes accordingly. When the pressure F is within a certain range, the strain εx is proportional to F as a constant.

Where: L is 50% of the center distance between the two holes; E is the elastic modulus of the beam; b is the width of the beam; h is the thickness of the thinnest part of the hole.
As shown in Figure 1, resistance strain gauges are attached to the top of the holes at the upper and lower ends of the double-hole beam, and they are combined into a full-bridge measurement circuit as shown in Figure 1 (b). This bridge measurement circuit can sensitively measure extremely small resistance changes. When the elastic body is acted upon by an object, the elastic body produces elastic deformation, and the resistance strain gauges attached to its surface deform synchronously with it, thereby changing their resistance values. Since the bridge circuit composed of resistance strain gauges is balanced, the resistance change of the resistance strain gauges will cause the bridge to be unbalanced, thus outputting a voltage signal, which is proportional to the force F at the end of the beam.

When the force F causes the resistance of a single strain gauge to change by △R, the output voltage of the full-bridge measurement circuit is U0 = U△R/R. The strain test process is shown in Figure 2.

Among them, the signal conditioning circuit includes signal amplification and filtering, and its function is to perform necessary conditioning on the signal. If you choose a data acquisition card with a signal conditioning module from NI, you don’t have to design a signal conditioning circuit separately. This article uses LabJack’s U12 data acquisition card as the data acquisition device and designs the signal conditioning circuit.

2 Selection of strain gauges - BP type semiconductor strain gauges
BP type semiconductor strain gauges can be used for stress measurement and stress analysis, and as force-to-electricity conversion elements for various sensors. They have the advantages of high sensitivity (about 50 to 100 times greater than metal strain gauges), small mechanical hysteresis, small size, and low power consumption. Its high sensitivity increases the output signal level by dozens of times, so it is not necessary to use an amplifier and can directly record the measurement results with simple instruments such as voltmeters or oscilloscopes, thereby greatly simplifying the measuring instrument; coupled with its small mechanical hysteresis, it can measure static strain, low-frequency strain, etc. In many new technologies, such as rockets, missiles, aircraft and other manufacturing industries and telemetry systems, semiconductor strain gauges have unique application value.
Technical parameters of BP-6-120 semiconductor strain gauge:
silicon wire size is 6mm×0.4mm×0.05mm; substrate size is 10mm x 6mm; sensitivity coefficient is 120; resistance temperature coefficient is less than 0.2%; sensitivity temperature coefficient is less than 0.16%; allowable working current is 25mA; allowable maximum strain is 2000με; maximum working temperature is 100℃.

3 Signal filtering circuit
The voltage signal coming out of the bridge circuit composed of strain gauges is usually accompanied by noise, vibration and other disturbances. In order to obtain a more accurate low-frequency strain signal, the collected strain signal needs to be filtered before it is sent to the computer. The dual second-order loop filter circuit uses an operational amplifier circuit composed of more than two adders, integrators, etc., and introduces appropriate feedback according to the required transfer function to form a filter circuit. Its outstanding features are low circuit sensitivity, very stable characteristics, and can realize a variety of filtering functions. Low-pass filtering is used here. The specific circuit is shown in Figure 3.

When forming a low-frequency filter, the circuit's natural frequency and passband gain are as follows:

4.1 Temperature effect of resistors
Resistors are made of metal materials, and their resistance will change with temperature. In addition, due to the different linear expansion coefficients of the test piece and the resistor material, the resistance of the resistor will change. The resistance change caused by temperature change is objective, but it is not expected to be reflected in the measurement results. [page]

4.2 Temperature compensation
As the four arms of the measuring bridge, when subjected to the load F during weighing, the resistance of R1 and R2 increases due to stretching, and the resistance of R3 and R4 decreases due to compression. The four strain gauges form a differential bridge, which has good linearity of output characteristics and also has temperature compensation. The output strain signal voltage is:

Where: ε1, ε2, ε3, ε4 are the strains felt by the strain gauges of each bridge arm respectively; εT is the strain produced by each strain gauge with temperature change.
4.3 Pre-adjustment of bridge zero point
For the convenience of measurement, before the specimen is deformed, the initial output voltage of the bridge is required to be zero, that is, U0=0. It is very difficult to select exactly the same bridge resistance. Zeroing the bridge can be taken to achieve the purpose of balancing it. The parallel resistance method is used here, as shown in Figure 4. Adjust the potentiometer Ra to adjust the parallel resistance on R3 and R4 to achieve the effect of zeroing.

5 Measurement system hardware composition and software design
The system hardware mainly includes double-hole beam, BP-6-120 semiconductor strain gauge, full-bridge circuit, general PC, data acquisition card, and signal conditioning circuit. NI's LabVIEW 8.0 software is used as the program design and compilation environment and tool.
This system uses a relatively simple measurement program flow, as shown in Figure 5.

Start the application, input the relevant test parameters and the storage path of the sampled data, click "Collect Data" to start the collection of strain, click Process Data, and the program will calculate and process the sampled data to obtain the strain value under the action of force F. Figure 6 shows the front panel of the double-hole beam strain measurement experiment.

The graphical programming method based on LabVIEW is direct and simple, making the program more concise, reducing the difficulty of program development and reducing the development time of the program. This software design mainly completes the collection, management, processing and analysis of strain signal data and the output of results. The program design block diagram is shown in Figure 7.

6 Conclusion
This paper introduces the method of designing a strain measurement system under the LabVIEW platform. LabVIEW has outstanding advantages in the field of test and measurement and is a new development direction in the field of instrument development. This paper gives a simple application of it, realizing real-time data acquisition and detection of stress and strain signals.