Research And Development Of Automatic Refractometer Based On Linear CCD

The measurement in refractive index is of great importance for studying optical properties,distinguishing material composition,and determining the concentration or purity and judging material quality,so the measurement of refractive index is widely used in many fields and industries such as petroleum,oil,pharmaceutical,chemical,medical and clinical inspection,wine,sugar food,jewelry,glasses.Some traditional instruments can measure the refractive index of liquid and solid in China,such as traditional Abbe refractometer.However,the instrument in the measurement process needs to visually observe the terminator and brings some problems such as complex operation,high error and low efficiency,so the instrument can not satisfy demands from market and industrial measurement.The automatic refractometer has been widely used in developed countries,but the instrument has a higher price than the traditional Abbe refractometer.With the rapid development of industrial automation technology,the traditional measurement techniques has been gradually replaced by automatic measurement technology,so an automatic refractometer,which can measure the refractive index of liquid and solid by a automation and high precision way,and has some advantages such as simple structure,low price,higher efficiency,is developed based on the linear CCD image sensor in this paper.The automatic refractometer,which is based on the total reflection phenomenon of light,adopts the measuring method of critical angle in view of the characteristics and measurement methods about common measurement instrument in refractive index.The image which contains terminator corresponding to refractive index is acquired by using linear CCD.What really matters is that terminator position of the light and shade is extracted by the edge detection algorithm to realize the goal that refractive index of liquid or solid is measured automatically.The mechanical structure of automatic refractometer,surface light source with strip,linear CCD drive circuit and signal pre-processing circuit are designed in this paper.The signal pre-processing circuit mainly includes the differential circuit and low pass filter which are used to process the output video signal about linear CCD.The optical image signal is exchanged into digital signal by using A/D conversion module in C8051F060 MCU.The collected data are denoised by three point neighborhood filtering method which are weighted.Three point linear fitting are proposed to find terminator position and position the turning point.Besides,the driver procedure of line CCD and software design of touch screen are designed.The feasibility and rationality of the design for the automatic refractometer is verified by the set of standard sample test.The analysis of experimental results and errors show that the measurement effect is better when the algorithm in edge detection of straight line fitting by three points is applied.Finally,a new method is proposed to solve the distortion phenomenon and measurement system for instrument is modified.The measurement accuracy of automatic refractometer is ±0.0002 and the instrument meets the requirements of design.