| On-line measurement for the surface temperature of moving billets is a yet unsolved dif-ficult problem in metallurgical detections for three reasons. Firstly, the temperature measuring result fluctuates substantially with up to around 70 K due to the effect of loose oxide coatings, which cannot be eliminated by a spot IR thermometer to achieve a stable value. Secondly, the temperature span of billets is as large as 400 K for different billets, and still reachs up to 200 K even for a specific billet. A commercial image camera cannot meet the requirements with equal-accuracy measurements for the various temperature ranges of such large gradient field due to CCD sensor’s narrow dynamic range. In addition, the exact temperature is difficult to be measured due to the uncertainty of billet emissivity.The surface temperature of billets is an important parameter in a continuous casting pro-cess. It is a key criterion to optimize casting speed, determine secondary cooling intensity, and judge the depth of liquid cores, so the on-line measurements for the surface temperature of billets are very significant to optimize secondary cooling system and improve billets quality.In order to solve the problem of billet surface temperature measurements, a single-color and a differential multi-color image pyrometers based on a planar CCD and DSP are devel-oped in this dissertation. The main works include:creation and analysis of image temperature measuring model and equal-accuracy compensation model, extension of CCD dynamic range, design of beam-spliting and 8-layer PCB of DSP module, optimization and scheduling of 6-stage pipeline structure, formulation of video capture sequence and driving scheme, im-provement of accuracy and real-time, reasonable extraction of temperature field, instrument test and field experiment.In summary, the contents and innovations are as follows:(1) A multi-color image temperature measuring modle based on CCD is established.To guide the design and analysis of the pyrometer, three image temperature measuring models respectively about single-color, bi-color and tri-color are established through profound analysis about the therories of radiation and CCD photoelectric conversion. The models pro-vide scientific basis for the temperature measuring method with self-adaptive light-integration time proposed in this dissertation, the wavelength selection of filters, and a single-point cali-bration method. According to the models, the measuring error caused by the factor of emmisivity deviating from ideal gray-body or preset values is quantified. The simulation re-sults show that the tri-color can obtain smaller error and higher sensitiviy compared with the bi-color at the three carefully chosen wavelengths of 0.71 μm,0.82μm and 0.93μm. In addi-tion, the models indicate that the measuring accuracy with the single-color method is related to object-system distance and CCD pixel’s spatial position, but has nothing to do with the above two factors for the multi-color methods.(2) Equal-accuracy measurements of the CCD image pyrometer is realizedIn order to eliminate the influences from distance variations and pixel dissimilarities on the accuracy, a distance error compensation model and a flatness-accuracy nonuniformity compensation model are established for the single-color image pyrometer. The former quanti-fies the temperature measuring error due to the deviation of working distance from the py-rometer’s calibration distance, and the latter describes the differences of measuring results between the central calibrated pixel and other pixels of a CCD. To extend the dynamic range of a CCD and improve its accuracy in low-temperature range, a temperature measuring method by means of self-adaptive adjustment for light-integration time is put forward. In this method, the intelligent pyrometer can automatically choose an optimal light-integration time according to the temperature distributions of different billets, throught which the dynamic range of 400 K is obtained, linearity is increased by 3.5 times and nosise sensitivity is de-creased by 4.5 times in low temperature range. As a result, the accuracy of the pyrometer tends to be equal distribution across entire ranges, which effectively solves the problem of the temperature field measurements with large gradient.(3) The protoype of differential multi-color CCD image pyrometer is researchedIn order to reduce the influence from the uncertainties of emmisivity and CCD dark current on the accuracy, a novel temperature measuring method with differential tri-color image is proposed. The selection principle of wavelength and its effect on the pyrometer are deeply analyzed; a photochopper beam-splitting and the circuit of video capture with a DSP as the core are designed; the control sequence of RAW video capture mode and its driving scheme are set; the quick-sync control for chopper disk is realized by means of PID algorithm with a constant offset. Finally, a prototype of the differential multi-color image pyrometer is devel- oped. Through the elaborate photochopper device and DSP, the prototype sequentially collects a dark image and 3 thermal images with specified wavelengths on a monochrome CCD. The thermal images provide the basis for 3-color thermometry, while the dark image is used to on-line reduce dark voltage of 3 thermal images by 4 times through a differential method.(4) The methods of improving accuracy and real-time are researchedAccuracy and real-time are the most important parameters for a pyrometer. This disserta-tion firstly analyzes the main factors affecting the accuracy of the pyrometer, which include dark current, random noise and reset noise, and then use pixel or image difference method, spatial digital filtering, and CDS circuit to effectively restrain them. For the great amount of data treatment and complicated algorithm, a 6-stage pipeline composed by a pryorneter to-gether with PC, an improved mechanism in Ethernet, and a multi-thread technology are adopted in this system, so the data-handling capability is greatly enhanced. The experiment results show that image acquisition frequence can reach up to 18 frames per second, the effec-tive measuring cycle for the single-color and tri-color pyrometers are respectively 0.3 and 0.8 seconds, which can meet the real-time requirements of temperature field measurements.(5) The real information of surface temperature field of billets is extractedIn order to eliminate the disturbance of oxide coatings to temperature field measuring results, a spatial digital filtering method is proposed. In this method, firstly the temperature extremums of each row of pixels are extracted from the raw measurements by the image py-rometer, so the surface temperature distribution with noises along the width direction of billets can be obtained after columns reorganization. Then, the distribution values are considered as the information from a single pixel sensor in time domain. By using DFT and LFT with cut-off frequency of 8Hz for reducing noises to restrain the fluctuations caused by oxide coatings and CCD random noises, the real information extraction of billet surface temperature field is finally realized.Based on the above research works, by means of a black body and an integrating sphere, the coefficient of the pyrometer is calibrated by the least square method, and its performance along with the mechanism models are evaluated. The results show that, the accuracy in low temperature range is increased by 7 times compared with traditional method, so the accuracy tends to be equal distribution within the range of 1073-1473K; the dissimilarities of accuracy among pixels are only 2.7K, and the error caused by distance variations is 1.9K. The surface temperature field of billet GGrl5 has been measured in field experiments. The influence of oxide coatings is effectively eliminated through the spatial digital filtering method, so the ac-tual distribution of the temperature field is obtained.The research results indicate that the pyrometer developed in this dissertation can meet the requirements of equal-accuary measurements for the surface temperature field of casting billets with large gradient. In addition, it can also be adopted at other fields of high tempera-ture measurements, and has wide application prospects. |
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