Laser Doppler Signal Processing System Based on LabVIEW

introduction

Laser Doppler displacement measurement technology has the characteristics of high precision, high signal-to-noise ratio, fast dynamic response, good linearity, strong anti-interference ability, large measurement range and non-contact. It has obvious advantages in the detection of dynamic parameters such as vibration displacement and deformation of long-distance targets, and can be widely used in machinery, aviation, construction and military fields. The measurement signal needs to be collected and processed to obtain the necessary parameters to achieve the test purpose and requirements. At the same time, virtual instrument is a powerful instrument system, and data acquisition and processing is the core technology of LabVIEW. To this end, a test system based on the Lab-VIEW platform was developed, combined with the measurement hardware part, to collect and process signals in a graphical and humanized way.

2 System Hardware Design

The hardware design of the test system consists of three parts: optical system, mechanical system and circuit system. The light of a certain frequency emitted by the optical element is divided into reference light and measurement light by a spectroscope. After being reflected by the measured object, the reference light has a certain frequency difference with the measurement light, which is then converted into an electrical signal by a photoelectric converter, filtered, amplified, and collected to the computer through a data acquisition card. The hardware system block diagram is shown in Figure 1.

The optical system uses the laser Doppler effect to measure moving objects and generates a signal with a certain frequency shift. Due to the relatively weak signal, low signal-to-noise ratio and wide bandwidth, high requirements are placed on the hardware system. In order to process this type of signal, a photoelectric converter is required to convert it into an electrical signal. According to the characteristics of the signal, the photoelectric converter is required to have high sensitivity and fast dynamic response. According to the system design performance requirements, the three components of photomultiplier tube, photodiode and avalanche photodiode meet the design requirements. At the same time, according to the comprehensive comparison of quantum efficiency, current amplification factor, frequency response, detector noise and cost performance, combined with the characteristics of the system, avalanche photodiode is selected here. Since the signal contains low-frequency components with more frequency components and high-frequency components with single frequency components, a bandpass filter is used to filter out the noise and improve the signal-to-noise ratio. The system uses an active high-pass filter and a passive low-pass filter to synthesize a bandpass filter. The active filter has low cost, reliable quality and small parasitic effects, simple design and adjustment process, and faster stopband attenuation than the passive filter. Therefore, the active high-pass filter is more suitable for low-frequency filtering of the system. The noise in the high-frequency part of the signal is caused by the acousto-optic modulator drive. Since the stopband attenuation speed is not required to be high, a passive low-pass filter is used for high-frequency filtering. The output signal needs to be amplified. Considering the parameters, ADI's ADL5531 fully calibrated logarithmic-limiting IF amplifier is selected, with a total gain of 80dB and a bandwidth of 480MHz, thus meeting the technical requirements of the signal acquisition card. Figure 2 shows the circuit design of the bandpass filter.

The data acquisition card is the entrance of VI. It collects the conditioned signal at a certain sampling frequency and stores it in the data acquisition card. It uses UAD08S acquisition card, PCI bus connection, and uses an independent 8-bit resolution A/D converter to maintain synchronization, realize accurate and synchronous acquisition of measurement data, and eliminate phase errors. The sampling rate is adjusted from 2MS/s to 400MS/s.

3. System software design

LabVIEW software is the most representative graphical programming software in the field of virtual instruments. Its intuitive front panel is combined with a flow-based programming method, and it simplifies and makes it easier to use the development environment based on the graphical programming language G. It has flexible program debugging methods, powerful function library functions, and can support multiple system platforms.

Program development establishes the engineering file of LV and develops it step by step. The main functions include hardware basic information report, interface design, signal acquisition design, signal processing, data printing, storage, etc. After all functional designs are completed, a laser.exe program based on LabVIEW engine that can run independently is generated. The test system program flow chart is shown in Figure 3.

The interface is designed based on the principles of consistency, usability, functionality, etc., including settings, signal processing, tools, operations, help and other functional menus, and message response boxes. At the same time, according to the requirements of human-computer interaction interface design, the interface is beautified and a welcome message is set. [page]

3.1 Signal Acquisition

The signal acquisition part includes the function initialization program, the acquisition card parameter initialization program, the acquisition card parameter setting program and the main program. NI-DAQ is used to allocate channels. There are two methods: selecting virtual channels and using channel strings. The former has the advantages of strain acquisition label definition, flexible surface, zero drift compensation, and channel setting without additional code when configuring the strain virtual channel, so the virtual channel is selected. The main program adopts a loop structure. When the program is running, the .vi is initialized using the function menu, and then the acquisition card parameters are read: acquisition card serial number, storage depth, maximum sampling rate and initialization error, set sampling rate, number of sampling points and channel range. The loop structure adopts a while loop. When the execution condition is true, the program is repeatedly executed to display the signal in real time; when the execution condition is false, the program execution stops and the display signal is saved.

3.2 Signal Processing

Figure 4 is a flowchart of the signal processing program. The filter subVI is called through the signal collected by the data. According to the signal requirements, the lower limit frequency of the filter is 1MHz and the upper limit frequency of the filter is 100MHz. The signal of 1 to 100MHz is processed, and the other signals are cleared to ensure the integrity and accuracy of the collected data. In the signal processing design part, according to the signal processing requirements, the variable step size algorithm is selected, the singular point elimination subVI is called, and the time-frequency analysis method is used to convert the time-voltage relationship into a time-frequency relationship. The velocity and displacement are further accurately obtained using the relationship and stored separately.

4 Experimental verification

Connect the laser, vibration table, signal conditioning circuit, and acquisition card correctly, turn on the laser power supply, run the laser Doppler signal processing system based on LabVIEW, set the relevant parameters of the acquisition card, click Start Measurement, and the signal begins to be collected. After calculation by the signal processing function module, the displacement curve is shown in Figure 5.

5 Conclusion

As a graphical programming software, LabVIEW is a powerful, convenient and fast programming tool for developing virtual test systems. Using the LabVIEW development environment for instrument system design, testing and implementation can reduce system development time and have obvious use effects. At the same time, the application of virtual instruments in laser Doppler signal measurement technology is also the first case. Practice has proved that the laser Doppler signal processing system developed using LabVIEW can better complete functions such as signal display, automatic time storage, and data processing and storage, thereby providing strong technical support for the application prospects of laser Doppler measurement technology.