| As an atomic emission spectroscopy, the laser-induced breakdown spectroscopy(LIBS) has numerous advantages, such as simultaneous multi-element analysis, speed,and in situ analysis. Therefore, this method has been widely applied in mineralexploration, detection of heavy metal, environmental monitoring, and many otherfields. In LIBS, a high power laser radiation source is focused on a sample’s surfaceto generate a plasma. A portion of the emission spectrum from atoms, ions, ormolecules is recorded and processed to facilitate the digital display of qualitative andquantitative information related to the elemental composition of the target.Commercial LIBS instruments have achieved good performance in theirrespective fields of applications. However each instrument of those always is focusedon one certain field. When the applied field is changed, the instrument cannot becompatible with new components. Now, development of LIBS instrument is still atthe beginning stage in China. As an important part of LIBS instrument, measurementand control system which is responsible for the coordinated work of the componentsis a key to develop this instrument. The main research contents are as follows:(1) Design and development of the measurement and control system usingplug-in mode.Being in charge of controlling all the components, measurement and controlsystem is the fundamental for achieving the aim of testing using LIBS instruments.Commercial LIBS instruments in the market are integrated by fixed models ofcomponents. When one of the components cannot satisfy some analysis needs, thoseinstruments do not support the replacement of single components leading to poorcompatibility for components. For this kind of problem, a universal measurement and control system was proposed in this paper. The master equipment was selectedaccording to application requirements for measurement and control system. And inorder to ensure all the components with different interfaces can access toinstrumentation system, interface adapters were designed. Control blocks for all thecomponents were developed independently in module. For the purpose of integratingthe whole control software, characteristic parameters of each control block wereextracted and packaged as independent DLL using plug-in application framework.The measurement and control system just need to replace relative plug-in when a newcomponent accesses to this system. The proposed system improves the compatibilityof LIBS instrument, and provides a multipurpose testing platform for differentrequirements of the user.(2) Design and development of a miniature digital delay generator forlaser-induced breakdown spectroscopy.To accelerate the development of LIBS instrument in our country, thedevelopment of the core components has become an urgent demand. Delay generatorwhich is used to make laser and spectrometer work synchronously is a corecomponent for LIBS instrument, as well as an important part for measurement andcontrol system. Nowadays, the most common delay generators used for LIBSexperiment (DG535/645) are large in size. Therefore, they are not suitable forinstrument integration. A miniature digital delay generator for laser-inducedbreakdown spectroscopy was designed based on FPGA in this paper. The digital delayused a100MHz crystal oscillator as the counter for the clock signal, therebygenerating a10ns resolution. The analog delay was produced by a slope circuit with a0.1ns resolution. These two kinds of delay modes could be combined to eliminate thejitter which was ranging from0to10ns caused by external signal and clock signal.FPGA chip’s flash storage was used to calibrate errors caused by the differencesbetween the analog electronic devices, the linearity of the capacitance’s chargingvoltage in the slope circuits, and the transient behavior of the capacitance’s chargingprocess during start-up and shut-off. Experiment results showed the rise time, fallingtime, inherent delay and RMS for output signal were5ns, less than30ns,38.06ns, and328ps respectively. The delay accuracy was less than1ns which was satisfied withLIBS requirement. The digital delay generator has advantages in size, weight andprice which could have important applications in portable LIBS systems. (3) Integration and testing of the measurement and control system.The measurement and control system was integrated on the fundamental ofhardware and software platform. It was tested from three aspects: compatibility,functionality and stability. The results showed that this system could be compatible of15components of different types. And the compatibility could be improved bychanging plug-ins for other components. The system realized all the designedfunctions including connecting components, controlling, acquiring data and savingdata. In about1700tests, measurement and control system was working stably incontrolling the operation of each component without appearing any hardware andsoftware failures.(4) Research on optimization of measurement and control process.Improving the signal quality obtained from LIBS instrument throughoptimization of measurement and control process is always a hotspot in LIBS researchand is the key to promote the development of LIBS instrument. The laser inducedprocess is complicated so that many factors could directly affect the ionizationproperties of the material and the final spectrum signal. According to this problem,some optimization was proposed in this paper. Firstly, the evolution of plasma wasresearched by focusing on the time delay parameter. It was obtained that the S/N ofplasma spectrum dropped quickly after the first rise. Secondly, a new laser inducedbreakdown spectroscopy based on single-beam-splitting (SBS-LIBS) has beeninitially explored in this work for effective signal enhancement. The laser beam wasdivided into two streams with adjustable angles to ablate/excite the target sample indifferent directions instead of the conventional one beam excitation in single pulseLIBS (SP-LIBS). The results showed that with the optimized angles of60°and70°for Al and Cu atomic emission lines at396.15nm and324.75nm respectively, about5.6and4.8times signal enhancements were achieved for aluminum and coppersamples compared to SP-LIBS, respectively. And under this condition, the S/N waspromoted by2.16times and2.23times respectively. The plasma of Al alloy lasted forroughly30μs in SBSLIBS which was6μs in SP-LIBS. In addition, a multilayeriterative testing method is designed to improve the efficiency of optimization ofmeasurement and control process. The experimental results had shown that theautomatic optimization of the delay time compared to the manual testing providedsignificant gain in testing efficiency. The range of delay time in the experiments is 1.28μs to10.28μs and the step value is1μs,0.5μs,0.2μs and0.1μs, respectively. Thegain in testing efficiency has been found73.76%,75.93%,78.81%, and80.42%,respectively.(5) Application research of the measurement and control system.Firstly, the measurement and control system was used to analysis Zn alloyqualitatively in an experiment platform. Combined with some data process methods,the elements Zn, Al, Cu, Fe and Re were identify correctly. Secondly, this system wasintegrated in desktop LIBS instrument, portable LIBS instrument and desktopLIBRAS instrument in project ‘development and application of innovativemultifunction laser spectroscopy instruments’ supported by national major scientificinstruments and equipment development special funds. |
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