The Research Of Nano-particle Size Measurement Method Using Dynamic Light Scattering Based On FPGA

Measurement of the size of nano-particle is of great significance to thedevelopment and application of nano science and technology.In order to develop a kindof advanced nano-measuring system that can be used widely, the author has conductedthe system research to the principle and techniques of dynamic light scatteringnanoparticles measurement based on FPGA in this paper.This paper focuses on the digital correlator and particle inversion algorithm,carries out the following research work:(1) Describe several representative methods of nano particle sizing.On theirapplications and measuring ranges, compare their advantages anddisadvantages.Focuse on the dynamic light scattering technique. Dynamic lightscattering (DLS) is an effective method of measuring the size of nanoparticles, alsoknown as photon correlation spectroscopy (PCS). Light scattering method has widerange of particle size measurement, measurement accuracy, less need to know thephysical parameters of the measured particle and dispersion medium, so it gets rapiddevelopment and more widely used. This method can give better results formonodisperse particle or particle with small distribution, but for the particle sizedistribution is quite broad particle system, the resolution and the reliability of themeasurement results is relatively poor.(2) Based on theory of PCS, study the structure and the advantages anddisadvantages of the linear correlator and nonlinear index correlator. The channel delaytime of linear correlator grows according to a linear law. But the actual hardwareresources are limited, the increasing number of channels can not be unlimited. Bychanging the laws that the delay time between path grows by linear, index correlatormake it grow by a certain percentage.But this will lead rapid growth in the dynamicrange,the curve resolution decreases, resulting in the loss of some information.(3) Use the FPGA to design a double pulse counter and a multi-delay andadjustable digital correlator. Dual photon pulse counter count the pulse signalsalternately, which is a seamless pulse counting. The digital correlator makes thecorrelation calculation in real time to photon pulses. By doing this, it ensures theaccuracy of particle inversion. Between the computer and the FPGA, they can transferdata via the serial port. The host computer via serial control correlator, to adjust the delay time of the channel.(4) Analyze the working principle of multi-delay and adjustable digital correlatorworks. Use the FPGA to design a high-speed multi-delay and adjustable digitalcorrelator, the core module includes reset control module, clock control module,photon counting module, correlation calculation module, FIFO module and serialcommunication module. Each module is designed by the programming of Verilog.(5) Study systematically the cumulant method and Tikhonov regularizationinversion algorithm. The cumulant method is used to achieve an average particle sizeof the inversion. Tikhonov regularization achieves the inversion of particle sizedistribution. In order to simulate the actual situation, add the random noise. Whennoise figure is0.001,0.01and0.1, the case was simulated respectively.Based on the theoretical study of photon correlation spectroscopy and digitalcorrelator structure, build a physical simulation platform, which mainly includes a lightsource, the incident light path, scattering light path, photoelectric detector, multi-delayand adjustable digital correlator and signal processing system. Green laser works as thelight source. Dynamic light scattering method is used as the method of particle sizemeasurement. The incident light path includes polarizer and focusing lenses. Thescattering light path includes filters and two porous. Photoelectric detector is mainlyrealized by the photomultiplier tube with good anti interference ability, low noise, highdetection efficiency. Multi-delay and adjustable digital correlator use XC5VLX50Tthat is in Virtex-5family of Xilinx FPGA to achieve the photon pulses to be collected,and complete light intensity autocorrelation function operation. Signal processingsystem is mainly realized by software with computer, mainly complete the inversionalgorithm of particle size. Multi-delay and adjustable digital correlator can achieve128channels. The dynamic range can be achieved to105. The lower limit of samplingtime is25ns. Use Visual C++to achieve the Levenberg-Marquardt algorithm, whichintroduces damping factor in the foundation of Gauss-Newton method and absorb theadvantage of one fell swoop optimization in Gauss-Newton method. And the selectionof the initial value has also been relaxed. It can achieve fast and accurate measurementof nano-particle size with a global convergence. And for Tikhonov regularizationinversion algorithm, realize simulation calculation in the Matlab. Obtain better stabilityand noise immunity.