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Hydropower unit vibration monitoring system based on virtual instrument

Abstract: This paper expounds the basic composition, implementation method and function of the vibration monitoring system based on virtual instrument, adopts PXI bus instrument and lab view visual virtual instrument system development platform, integrates all the functional modules of traditional instruments into a computer, and users can change its function and scale by modifying the software of virtual instrument. The system realizes the automatic acquisition of vibration signals of hydropower units, and can process and analyze vibration signals by computer

key words: virtual instrument vibration monitoring of hydropower units

1 introduction

with the development of hydropower industry in China, the production of large units, the number of units of various capacities is increasing. How to ensure the stable operation of hydropower units is an important issue of common concern. At this stage, the maintenance of hydropower units in China generally implements the planned maintenance system. No matter what the state of the equipment, it is required when it expires, resulting in a lot of waste of resources. This traditional expected maintenance system can no longer meet the requirements of modern maintenance, operation and management. Hydropower units have huge equipment, complex structure and various causes of failure. Although some failures are not necessarily manifested in the form of vibration, statistical data show that about 80% of the failures or accidents of hydropower units are reflected in the vibration signal. For example, the vibration parameters of the lower frame of hydropower units represent the balance of the rotating part, and the extreme frequency component of its vibration indicates the electromagnetic vibration of the generator, etc. Therefore, vibration monitoring is the most widely used and effective method at present. Through the analysis of these vibration signals, it is of great practical significance to fully explore the fault information contained in them for the safe production and decision-making of hydropower units

the vibration monitoring of hydropower units can be composed of traditional instrument systems, as shown in Figure 1. The system functions are completed by traditional instruments with fixed and unchangeable functions defined in advance by the manufacturer. Due to the lack of flexibility in the functions of traditional instruments, it is sometimes difficult to meet the diverse needs generated by the changing tasks, even though the capital investment is large. Virtual instrument technology has changed this situation, and it has created a new era in which instrument users can become instrument designers. Virtual instrument is a virtual panel that is similar to the physical panel of real instrument, which will rapidly produce aging at 60 ℃ through software platform. Hardware is no longer the main body of the system, it only realizes the input and output of signals, and powerful software completes the signal acquisition, analysis and processing and result display, realizing the concept of "software is instrument". Virtual instruments use computer software to replace some hardware functions of traditional instruments. Users can define the functions of instruments according to their needs. The improvement and function expansion of virtual instruments only need to update the design of relevant software, without adding new instruments. Therefore, virtual instrument technology has the characteristics of short development cycle, low cost, convenient maintenance, flexibility, powerful functions, user-defined and so on

2 system hardware structure

the hardware of this system is composed of sensor, scb-68 terminal box, pxi-1010 combined chassis, scxi-1125 programmable isolation and amplification module, scxi-1141 programmable low-pass filter module, scxi-1140 sampling/holding module, pxi-6052e data acquisition card, pxi-pci833x computer control PXI module, mxi-3 optical fiber communication module, dfe-530txi network adapter card, etc. its hardware structure is shown in Figure 2

2.1 selection and installation of vibration sensors

compared with thermal power units, the rated speed of hydropower units is lower, especially the vibration frequency caused by hydraulic factors is lower, and the vibration signal of hydropower units is a low-frequency signal. Due to the harsh environment at the installation site of the vibration sensor, large electromagnetic interference, large temperature changes, and the long-term vibration of the sensor support itself will increase the measurement error, it is necessary to choose a vibration sensor with high reliability, strong anti-interference ability, high precision and stable performance. In order to avoid the distortion of the measured value caused by the additional error caused by the installation of the vibration sensor, the vibration sensor should be installed reasonably. When measuring the shaft swing, the system selects the eddy current sensor, which uses the eddy current effect to measure the position, and has the advantages of non-contact measurement and strong anti-interference ability. Install the eddy current sensor on the bearing shell for relative measurement. The measuring points can be selected at the upper guide, lower guide, hydraulic guide and thrust, and install two eddy current sensors with 90 ° each other. When measuring the vibration of the frame and top cover, the seismic sensor is selected, which measures the absolute vibration of the object connected to the base, and has the characteristics of earthquake resistance and high stability. The seismic sensor can be directly fixed on the casing, and the installation should be as close to the rotating shaft as possible, and avoid the positions with strong electromagnetic fields such as bus outgoing line as far as possible. The measuring points can be selected to install two seismic sensors at the upper and lower frames and thrust frames respectively to monitor the horizontal and vertical vibration of the frames. The photoelectric proximity switch is used to obtain the Keyphasor signal and determine the reference point of the whole period sampling. In addition, in order to analyze the relationship between vibration and pressure, working head and upstream and downstream water levels, sensors corresponding to active power, pressure, upstream and downstream water levels should also be installed

2.2 data acquisition module

in the field of data acquisition, there are PC-DAQ data acquisition cards based on a variety of PC buses, as well as various data acquisition modules based on VXI bus. But in the three virtual instrument systems of GPIB, PC-DAQ and VXI, GPIB is essentially the expansion and extension of the functions of traditional instruments through computers; PC-DAQ directly uses the standard industrial computer bus, which has no bus performance required by the instrument; The construction of VXI system at one time requires large capital investment. PXI is a new open and modular instrument bus specification introduced by Ni company in 1997. It combines the integrated trigger function of COM pactpci with Windows operating system. On the basis of retaining the structural functions of PCI bus and CompactPCI module, the system reference clock and trigger bus are added, and the familiar Windows environment makes PXI system more suitable for the construction of industrial automatic measurement and control system. The system based on PXI bus specification combines the cost performance advantage of PC with the expansion of PCI bus in the field of instruments, and becomes a new virtual instrument system. PXI not only has the same performance as VXI, but also has the characteristics of short development cycle, low price and easy to build portable automatic test system

in this system, the data acquisition is to send the signals of each measurement point through the field sensor to the signal conditioning modules scxi-1125 (programmable isolation amplifier), scxi-1141 (programmable low-pass filter) and scxi-1140 (sample/hold amplifier) through the scb-68 terminal box for signal conditioning, Finally, the conditioned signal is sent to the data acquisition card pxi-6052e (16 single ended/8 differential analog inputs, sampling frequency 333 thousand times/s, 2 analog outputs, 8 digital I/O lines, 2 24 bit counting/locator) for data acquisition. The ordinary protection of the PXI film tensile testing machine of the data acquisition card is mainly divided into daily protection and weekly protection. For the first time, if we review and sort out the cleanness and ordinary function of the machine, the anti-interference ability of the 24 bit counting/timer on the 6052e is not strong. In order to make up for this deficiency, we can use ls7084 chip, resistance and capacitance to form a filter circuit to eliminate the interference caused by noise and vibration

2.3 computer control module

the system adopts the combined chassis pxi-1010 (8 PXI/CompactPCI and 4 scxi slots) of Ni company, and the zero slot control module adopts pxi-pci833x. Pxi-pci833x adopts mxi-3 technology, which is a connection technology between PCI buses. It adopts standard pci-pci bridge technology and 1.5gb/s high-speed serial port connection, which introduces a faster and more convenient expansion mode for PXI control. Mxi-3 technology can not only connect PXI/compact PCI chassis, but also directly control the PXI system through the master computer. In this system, the data collected by pxi-6052e data acquisition card is transmitted to the field computer through pxi-pci833x module and mxi-3 optical fiber communication module with transmission rate up to 132mb/s. Mxi-3 includes a pcimmxi-3 card inserted in the field computer and a pximxi-3 module inserted in the pxi-1010 chassis control slot. The two cards are connected through optical cables to realize the communication between each module in the pxi-1010 chassis and the field computer. Mxi-3 technology can realize signal transmission within 200m distance, and solve the problem of signal long-distance transmission between field computer and data acquisition module. The field computer is connected with the network remote monitoring terminal through dfe-530txi network adapter card to realize remote monitoring

3 system software structure

this system selects Labview6i of Ni company as the development tool, which adopts graphical programming scheme, also known as G language. LabVIEW provides a wealth of functions and subroutine libraries, from basic mathematical functions to advanced analysis libraries (including signal processing, window functions, filter design, linear algebra, probability theory and mathematical statistics, curve fitting, Fourier transform, wavelet analysis, etc.). Through these functions and subroutine libraries, we can realize the software of hardware system and design a vibration monitoring system that meets the technical requirements. Since the data acquisition card used in this vibration monitoring system is a product of Ni company, a large number of data acquisition subroutines provided by LabVIEW can be used, and there is no need to compile a driver for the data acquisition card. In addition, the network tool kit attached to lab view also facilitates the design of remote monitoring

labview development environment consists of front panel and flow chart. The front panel is a graphical user interface of human-computer interaction, which integrates a variety of commonly used control objects (such as switches, buttons, oscilloscopes, indicators, timers, etc.), and it is equivalent to the operation panel of actual instruments. When designing, you only need to select the required controls from the control library, and design reasonable attributes (such as size and range) and specific positions for them. These attributes and positions can be easily adjusted through the program. The reasonable design of the front panel is conducive to the realization of the function of the vibration monitoring system and is convenient for operation. Therefore, the front panel should be equipped with multi section switches to realize different data processing methods. The main part of the front panel is to display graphics and data. Multiple windows can be used to complete the simultaneous output of different signals. The front panel should also have control windows and switches to realize the operation of the vibration monitoring system. The flow chart is the graphical source code of the program, which is mainly composed of control icons, function icons and wiring on the front panel. When designing, select the required function icons from the function library, arrange them and the position of the control icons in accordance with the order of data transmission in the program, and then connect the icons with the wiring tool. The system software adopts modular design, and its system software function module is shown in Figure 3. Adopting modular design is conducive to software design and future improvement and upgrading

3.1 elimination of "spectrum leakage" phenomenon

in the process of transforming the sampled signal by using Fourier operation, it will cause "spectrum leakage" phenomenon. In order to eliminate "spectrum leakage" phenomenon and improve the accuracy of spectrum analysis, realize full period truncation during Fourier transform and implement full period sampling for vibration signal. Whole period sampling refers to the dynamic change of the sampling frequency of the system