The Design And Research Of Laser Scattering Wetness Sensor Circuit

The thermal efficiency of coal-fired power plants has been concerned and researched hot spots for many years, and in order to improve the thermal efficiency of coal-fired power plants, the problem that how to effectively recycle the steam at the end of turbine should be pay more attention to. In this paper, the prime task is to design and research the circuits which were used in the laser scattering wetness sensor. The sensor could be used to measure and monitor the steam wetness at the end of turbine in real time, so as to enhance the thermal efficiency and increase economic benefits for the coal-fired power plants.In this thesis, the basic theory-Mie scattering theory was introduced at first. According to this theory, the spectral distribution of the light scattered by particles related to the particle radius. In other words, when the particle radius is different, the scattering phase function of scattered light is various. Using the theory and the VC++ program which has been completed, the scattering function of single particle and particles were calculated. Then, through comparing the phase function of the spectral distribution between theoretical calculation and actual measurement, the inversion fitting methods of look-up table and recursive were raised. In this paper, the method of look–up table was applied. The most probable radius of liquid water droplets at the end-turbine was obtained by this method, and assuming that the size distribution of the steam condensate particle meets the Generalized Gamma Distribution, the content of liquid water could be calculated with the most probable radius of liquid water droplets and some related spectrum parameters. And then, according to the ideal gas equation and using the pressure and temperature in the steam turbine, the dry steam content was calculated. The values of temperature and pressure were measured by temperature sensor and pressure sensor in the turbine. At last, using the values of the content of liquid water and dry steam and wetness formula, the wetness could be obtained.In this thesis, the actual design is divided into two major parts: the design of laser diode driver power circuit and data acquisition hardware circuit.In the process of designing laser diode driver power circuit: firstly, the the working principle of semiconductor lasers was introduced briefly, and then Basic operating characteristics-threshold characteristics and dynamic characteristics. The threshold characteristics include the characteristics of P-I and V-I, and getting the ac small-signal equivalent circuit of semiconductor laser from the dynamic characteristics. Secondly, according to the specific requirements of semiconductors for the driving power, proposing the design plan which based on negative feedback theory. The negative feedback circuit has an advantage of stabilizing the system output voltage. There were four modules in this design plan. They are: voltage reference circuit, slow-start delay circuit, over-current protection circuit and stable power control circuit. The stable power control circuit is composed of photoelectric detection circuit, negative feedback network, Subtraction operation circuits, error amplifier circuit and voltage - current converter circuit. The module has the purpose of stabilizing the output power. It directly changes the output optical power into a current signal which is a certain proportion of the output optical power and the current signal could be used as a feedback signal to control the drive current source. Owe to this method, the phenomenon of the output power instability which is as a result of changes in temperature and some factors such as power surges can be eliminated. After completing the design of the four modules, the modules were stimulated with using spice models of the device in each module and the stimulation results is in line with expectations. In the stimulation of the stable power control circuit, because the coupled model of photoelectric conversion device is used in the actual stimulation, photoelectric coupled model-3N262 in the spice model library is applied to instead the photoelectric conversion device to complete the stimulation. Finally, complete PCB layout of the driven power circuit by using of EDA tools-protel software and make a real object.In the process of designing data acquisition circuit: summarizing the data acquisition system at first, and according to the requirements, select the amplifier chip and the analog-digital conversion chip used in the data acquisition circuit. Through the comparison and analysis, high-precision logarithmic amplifier-LOG114 and high-speed, high-precision 24-bit,Σ-△ADC-ADS1278 produced by TI company were chosen. Then do the specific design for the hardware circuit. The hardware circuit has two parties: analog part and digital part. The analog part includes: Signal amplification circuit, differential input interface circuit, and reference voltage circuit. The digital part includes: analog-digital conversion, ADS1278 with FPGA interface circuit, DC-DC converter circuit. And then, complete PCB layout with the Protel software for the designed circuit. Lastly, design ADS1278 data acquisition control procedures based on hardware description language-Verilog HDL and run the procedures under the integrated simulation environment of QuartusⅡ. the simulation results meet our requirements .In this thesis, the design of laser diode driver power circuit and data acquisition circuit laid a solid foundation for the further studying and the designing of the intelligent integrated laser light scattering wetness sensor. At the same time, it made realize the goal of measuring and monitoring the steam wetness at the end of turbine in real time more easy.