Design of vibration test system based on virtual instrument technology

0 Introduction

Vibration is the most common phenomenon in nature. Traditional vibration test systems mostly use electronic measuring instruments, which are characterized by single functions, special use, poor flexibility, and greatly restrict the scope of vibration testing. Nowadays, a technology that introduces virtual instrument technology into the field of vibration testing is popular, combining computer technology and vibration testing technology to form a virtual vibration test system. Practice shows that virtual vibration test instruments are not only powerful and versatile, but also have a friendly user interface and a simplified graphical programming method. They are generally welcomed and highly valued by the majority of users and have become a new development direction for vibration testing.
1 Hardware Design of Virtual Vibration Test System

The hardware of the virtual vibration test system mainly includes accelerometers, force sensors, signal amplifiers, data acquisition cards, and general PCs.

1.1 Accelerometer

Accelerometers mainly test the vibration acceleration of vibrating bodies. Piezoelectric accelerometers are commonly used in mechanical vibration measurement. They are sensors based on the piezoelectric effect that generates charges on the surface of certain materials after being subjected to force. The amount of charge output by the piezoelectric accelerometer is proportional to the vibration acceleration of the object. The amount of charge detected by an appropriate test system can achieve the measurement of vibration acceleration. It has the advantages of small size, light weight, high sensitivity and wide frequency range, and is most commonly used in vibration testing. The design of this system uses a piezoelectric sensor made of parallel piezoelectric materials, which is suitable for the use of a charge amplifier. The circuit feature is that the amplifier output voltage is only related to the charge input generated by the sensor and the amplifier feedback capacitance, and has nothing to do with the distributed capacitance formed by the cable constituting the circuit and the signal frequency. This feature makes the distributed capacitance of the transmission line of the charge amplifier insensitive, and the transmission distance can reach hundreds of meters.

1.2 Charge amplifier

The charge signal output by the piezoelectric sensor is relatively weak and cannot be directly collected by the data acquisition card. A signal amplifier is required to convert the weak charge signal into a stronger voltage signal that can be collected by the data acquisition card. The charge amplifier is an amplifier whose output voltage is proportional to the input charge. Its core is an operational amplifier with capacitive negative feedback, input impedance and high gain. This system uses the BC97 charge amplifier as an example. The charge amplifier also has a low-pass filter and an adaptive amplifier that can adjust the amplification factor according to the sensor sensitivity.

1.3 Data Acquisition Card

The data acquisition card used in this system is PCI-6024, which is a multi-function interface card produced by NI Corporation of the United States. This card is designed based on the PCI bus, with high data transmission rate and large throughput. It is the mainstream of data acquisition card design. It is a product with good cost performance, supports DMA mode and double buffer mode, and ensures uninterrupted acquisition and storage of real-time signals. It supports unipolar and bipolar analog signal input, and the signal input range is -5v~+5v and 0~10v respectively. It provides 16 single-ended/8 differential analog input channels, 2 independent D/A output channels, 24-line TTL digital I/O, 3 16-bit timing counters and other functions. The actual measurement is that the input signal enters the data acquisition card from the input terminal through the BNC connector for data acquisition. At the same time, the communication between the system software and the data acquisition card can be completed by simple settings using the Measurement Automation software provided by NI Corporation of the United States.
2 Software Design of Virtual Vibration Test System

The software design of virtual vibration test system uses graphical programming language LabVIEW as software development platform. In the process of program development, modular design concept is used. According to the needs of different functions, various functional modules are respectively formed. This system includes data acquisition module, data storage and reading module, data processing module, and result display module. In order to integrate various modules together, a main interface is also designed to realize the call of each module. Finally, the system is integrated and debugged.

2.1 Design of system main interface

In the design of the main interface of the system, the Edit Menu menu provided in LabVIEW is used to first use the function to be implemented as the content of the menu option so that it can be called at run time. Then, the call of each menu in the block diagram is selected through the Case loop, so that each menu corresponds to each subVI. In the Eexecution Options of VI Setup in each subVI, the Show Front Panel When Called option is selected. In this way, when a certain content in the menu is selected during operation, the subVI is selected and called.

2.2 Data acquisition module

The data acquisition module uses the AI ​​Waveform Scan module in the Analog Iuput function block in LabVIEW for acquisition control. According to different needs, you can choose continuous signal acquisition or single signal acquisition, and can control the acquisition channel, sampling rate, number of sampling points, windowing method, average number of times, and display spectrum type. You can also observe the time domain value and frequency domain value of the signal by moving the cursor. In terms of triggering mode, you can also choose signal triggering or free acquisition. When the signal is triggered, you can select parameters such as trigger level, trigger edge, and number of points reserved before triggering. [page]

2.3 Data storage and reading module

The main function of the data storage module is to store the time domain data corresponding to the image displayed on the monitor into a binary file; and store the parameters related to the acquired data: average times, data length, analysis bandwidth, trigger point sampling point, acquisition time, etc. into a text file with the same name as the data file, which is convenient for the data reading module and users to use.

The data reading module can easily read and write files. The process of LabVIEW reading and writing files is: open a file - read and write content according to a certain format - and finally close the file. The main functions used in the data reading module are: open file function, read file function, and close file function.

2.4 Data processing module

The design of the data processing module program is the key part of the software design of this system. It needs to complete many functions such as digital filtering, windowing, spectrum analysis, power spectrum analysis, correlation analysis, and cepstrum analysis. Time domain analysis includes autocorrelation and cross-correlation analysis. Amplitude domain analysis can perform mean, variance, probability density and probability distribution statistics. Data preprocessing can be performed by inputting calibration coefficients for each channel and filtering with digital filters. Low-pass, high-pass, band-pass and band-stop filtering can be performed. Frequency analysis based on FFT includes auto-power spectrum, cross-power spectrum, amplitude inverse spectrum and frequency response function. The frequency response function can use different estimation formulas as needed, and can choose to display the real part and imaginary part, amplitude frequency and phase frequency and coherence function. In addition, in frequency domain analysis, each signal can be windowed to reduce leakage. There are mainly rectangular windows, Hanning windows, Hamming windows, exponential windows, etc., which make full use of computer resources. The number of points for calculating FFT can range from 512 to 16384 points. Multiple averaging can be performed to reduce errors. Two channels can be selected to calculate frequency response functions and cross-power spectra. SubVI modules such as auto power spectrum, spectrum unit conversion and power frequency estimate are called in signal analysis.

2.5 Data display module

The data display module displays the collected data and the analyzed data on the display. It also contains many auxiliary display items, including coordinate unit display, maximum value and its corresponding position display, time limit display, and data acquisition file index display, so that users can observe the system test results.
3 Actual test of virtual vibration test system

This paper introduces the whole process of the development of virtual vibration test system. In order to verify the correctness of the whole system program, the hammer method is used to test the test piece. The test structure is hammered with a force sensor, and the acceleration sensor is used to pick up vibration. The two signals are amplified by the charge amplifier and sent to the data acquisition card. The acquisition conditions are set by software to control the acquisition, and the collected data is stored and various analyses are performed. When collecting, the sampling channels are set to 2, the sampling frequency is 1000Hz, the average number of times is 5, the number of points collected each time is set to 1024 points, the trigger channel is 0 channel, the trigger level is 100mv, the trigger edge is the default rising edge, and the number of reserved points is 20. From the test results, the excitation signal and the response signal reflect the typical shape of the excitation and response signals when the hammer method is used for excitation. It can be seen that the system program runs well and the system reliability is high.
4 Conclusion

This paper introduces the design of the virtual vibration test system. Practice has proved that it is feasible to construct a virtual vibration test system using LabVIEW and a PC-based data acquisition card. The test system is powerful and its modular programming makes program expansion very convenient. It can be foreseen that virtual instrument technology will have a broader application space in the entire test field.
5 Innovation of the author of this paper

The design of the vibration test system adopts advanced virtual instrument technology, which improves the test accuracy of the system, saves development time, and reduces development costs.