CONSIDERATIONS AND PERSPECTIVES IN ELECTRON BEAM PROCESSING

Transcription

1 CONSIDERATIONS AND PERSPECTIVES IN ELECTRON BEAM PROCESSING STELIAN-EMILIAN OLTEAN 1, LASZLO DAVID 2, MIRCEA DULĂU 1 Key words: Electron beam processing, Material processing, Electron beam control. The material processing with the aid of electron beams solves great topical problems, wherever conventional techniques failed or proved to be inefficient. The electron beams have many special properties, which make them particularly well suited for use in materials handling. This paper presents the electron beam system principle and the perspectives for improvement of the material processing. High quality electron beam control can be made using digital equipments. Two control problems will be presented in the following paragraphs: focus distance and target position point. 1. INTRODUCTION Electron beam material processing is an important non-conventional technique used industrial manufacturing. Nuclear technologies, aeronautics, microelectronics are some examples where this equipment is used. Industrial applications of electron beam techniques begin around The special electron beams properties like high resolution, long depth of field attainable, high power density energy sources make them very useful in material handling [1]. An electron beam system can be used in melting, welding, evaporation, refining, and thermal surface treatment process. In fact, electron beam and laser are the only ways of delivering large amounts of concentrated thermal energy to materials (maximum 10 8 W/cm 2 ). The heat absorption, the penetrations of the electrons in metal, focusing of beam are rather complicated, making their modeling a difficult task to solve. Also, the examination of the electron gun s variable is very difficult due to the nature of the process. Thus it is a necessity of a modern control strategy like artificial intelligence, adaptive and expert systems implemented on digital system to produce material processing at high quality and required standards. 1 Petru Maior University of Târgu-Mures, str. Nicolae Iorga 1, Târgu Mureş, soltean@upm.ro 2 Sapienta Hungarian University of Transilvania, Târgu Mureş Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 52, 1, p , Bucarest, 2007

2 44 Stelian-Emilian Oltean, Laszlo David, Mircea Dulău 2 The study of the documentation in this field and the experiments are currently being put into practice by the authors based on the electron beam equipment, CTW 5/60, developed by Petru Maior University of Tg-Mureş in partnership with Electrical Research Institute I.C.P.E. Bucharest. 2. ELECTRON BEAM PROCESSING SYSTEM The most common systems of this type used in manufacturing are of high vacuum design. The main parts of the equipment are the triode gun and the vacuum system that provides high vacuum environment, without which the beam cannot be generated. The triode gun design consists of the cathode, composed of the filament and the massive cathode, electrode or grid, anode, focusing and deflection coils. The vacuum system ensures a pressure level of Pa and it is controlled by a multitasking digital system implemented on the microcontroller and on PC. To avoid accidents, any error that may appear in this unit is pointed out and preparing sequences for material processing are halted. The emission of electrons from the incandescently heated termoemission filament, which is saturated during the process by a predetermined amount of electrical current, generates the main beam. A negative high voltage potential is applied to the filament cathode assembly, referred to as the accelerating voltage of kv. Another voltage, lower than the accelerating voltage is applied to the grid cup or bias assembly. In this way the grid cup acts as a valve that controls the volume of electron energy that can flow from the cathode to attracting targets [2, 3]. The first target, situated in the triode gun, is an anode at a positive potential, which forms the beam. Then the focused beam of electrons is led using focusing coil to a secondary target, situated in the workbox, consisting of a metallic workpiece, where the kinetic energy of the electrons is converted into thermal energy. The metallic workpiece offers a conductive path to earth to complete the circuit. This target can be stationary and the electron beam energy deflected using deflection coil or the workpiece can be moved using a CNC table. The magnetic focusing coil is located beneath the anode assembly and is circular in design and concentric with electron beam. An electrical current is passed through the coil, which produces magnetic fluxes that provides convergence of electron beam. The deflection coil is created with four wound coils positioned at right angles to the column. Another important part in experimental equipment is electron s collectors composed of the four electrodes used to capture reflected electrons from target surface (workpiece). The high power electron beam system with the classic triode gun is shown in the Fig. 1.

3 3 Considerations and perspectives in electron beam processing 45 Fig. 1 Electron beam system. The block diagram of the image capturing system is shown in Fig. 2. This part is important to reconstructing the image of the processing surface and to controlling the electron beam (EB) [2, 4]. Fig. 2 Image capturing system.

4 46 Stelian-Emilian Oltean, Laszlo David, Mircea Dulău 4 The lines amplification module contains four amplifiers for amplification of the signals provided by the four electrodes. After amplification the signal processing module assures the reduction of the additive noise. This module also combines the four signals for elimination of the distorsion generated by the asymmetric capture of the reflected electrons. The monitoring module contains the generators for EB deflection in a raster form (with RAX and RAY) and complex video formation block. Using the RAX sawtooth signal the digital horizontal synchronization (SYNC) for complex video is obtained. The horizontal frequency used is kHz. Dividing the digital horizontal synchronization with 256 the digital vertical synchronization is achieved. Via capturing and digitizing interface and PCL board images of pixels with workpiece surface are recorded using a special software on a personal computer. The capturing and digitizing interface, the PCL board and a capture and control software are currently in an stage of development and experiments in the Petru Maior University, based on CTW 5/60 equipment. 3. ELECTRON BEAM CONTROL PROBLEMS The accuracy of electron beam position control must be high to obtain any shapes on the surface of the processed material. Except vacuum environment and high voltage at least two other issues occur in this non-conventional technique: focus distance and target position point. A computer is used for processing control and for image of processing zone reconstruction based on information received from the electron s collectors, which captures reflected electrons from the workpiece surface. After digitization, some image processing methods are used for the enhancement and smoothing edge and line detection, region segmentation. Useful information is extracted from this image to control the coordinates of the trajectory and the optimal focus distance. At the specific power, the focus distance is a function of focal coil current intensity and beam current intensity. So, an intelligent controller can be made to control the focus distance (movement on the 0-z axis of the electron beam) with the aid of focal coil current using as inputs the image quality coefficient and its derivative. We consider that a defocused image has less information and using a depth from focusing idea an auto-focusing algorithm can be made. The objective of the focus distance control is to improve the image quality of the processing zone. In the control problem the image quality it is not the quality after the enhancement of image using special image processing methods. To find this image quality coefficient (IQC) several criteria presented in the following paragraph can be used [5].

5 5 Considerations and perspectives in electron beam processing 47 2D Fourier transform method is not a fast solution to evaluate the high frequency content in an image for real-time implementation. Gray level variance method (Akihiro Hori 1993) associates a sharp image to a high gray level variance and a blurring image to a low variance. Thus the criterion is to maximize the square of variance. Sum-modulus-difference criterion, which was also tested, means the maximum of the measure that is computed by summing the first intensity differences between neighboring pixels along a scan line. A fast and efficient algorithm was obtained using only four neighbors of every pixel and a maximum information zone criterion from image detected. So, image quality coefficient (IQC) can be determined with equation (1). Some images captured and a distribution of this coefficient (IQC) are shown in Fig. 3. n m k+ 1 l+ 1 IQC = abs,, 8. k= 1 l= 1i= k 1 j= l 1 ( p( kl) pi ( j) ) ( n m) (1) Fig. 3 Distribution of IQC in different focused captured images. Using a neuronal network with feed forward structure learned to find the image quality simplify the calculation of coefficient and then a fuzzy and/or an

6 48 Stelian-Emilian Oltean, Laszlo David, Mircea Dulău 6 adaptive neurofuzzy controller can be created to determine the best focus distance of electron beam. The modern and intelligent controller has as inputs image quality coefficient and his derivative and at output a value that is used by focusing drivers and interface block to generate focusing current I foc. The scheme from Fig. 4 illustrates the focus distance control using useful information provided by the electron s collectors and image capturing system. Fig. 4 Focus distance control. Once the sharp image (focused image by moving electron beam on the 0-z axis) have been aquired a computer extracts information for a visual tracking technique. Tracking control uses the trajectory coordinates from a database and the desired target point position is obtained with the aid of the magnetic field of deflection coil and mechanical workpiece movements (rototranslation) on the x-0-y plane controlled by an adaptive system. The resolution of trajectory depends on image resolution captured from electron s collectors. The objectives of the position point control is to follow an existing profile or diagram of the workpiece [1,5]. To cover all possible target point positions of the workpiece in almost all situations the positioning is realized by mechanical system (in workbox), which makes a movement of rotation on the 0-z axis with constant velocity, a movement of translation on the 0-x axis and movement of translation on the 0-y axis. In case of a trajectory with rectangular and acute-angle corners the control of electron beam deflection is necessary. So, mechanical system stops and using deflection current the special shape on the surface of the processed material is obtained (Fig. 5). A constant variation of the electron beam deflection needs a magnetic field with constant intensity generated by deflection coil. Equation (2) gives deflection x on the 0-x axis of the electron beam for a deflection current I defl and a constant value of the accelerating voltage U acc (40 60 kv). We neglect the variation of the electron beam velocity crossing through deflection coil.

7 7 Considerations and perspectives in electron beam processing 49 ( defl ) x I 0 ( 1) 0 e d + µ kb l Idefl =, 2 m b U (2) where e is elementary charge, m 0 electrons stationary mass, µ 0 vacuum permitivity, b a constant value, k b deflection coil configuration coefficient, d distance between deflection coil and material surface, l deflection coil width. acc Fig. 5 Target point position control. Of course an important part of every automatic system is to supervise the all operation with a central monitoring system (microcontroller and PC s). 4. CONCLUSIONS The advantages of using electron beam technologies like the possibility to generate high power energy density, high efficiency of electron s kinetic energy conversion in thermal energy, high productivity and flexibility, fast scanning possibility and high resolution make them able to perform special type of material processing. Because of the multivariable process complexity, which involve nonlinearity and undefined problems and parameters the importance of using modern techniques (artificial neuronal networks and fuzzy logic) in the electron beam control was also shown. Intelligent control based on information extracted from images create the possibility of extension of actual limits of electron beam welding equipment s in performance and diversity of trajectories. Controlling simultaneously the position of target and deflection of electron beam, and the focal distance relative to the current position of target offers better performance. The experiments was made on an electron beam equipment, in Petru Maior University of Tg-Mureş which has the following main characteriscs [2]: triode type

8 50 Stelian-Emilian Oltean, Laszlo David, Mircea Dulău 8 CTW5/60, U acc = 60 kv, P u = 5 kw; I foc = 85 ma, vacuum workbox dimensions (inside: mm, outside: mm), mechanical positioning system (rotation and translations on the x-0-y), vacuum system for triode gun and workbox environment (Fig. 6). Received on 17 January, 2006 Fig. 6 EB equipment (CTW5/60). REFERENCES 1. R. Bakish, Introduction to Electron Beam Technology, John Wiley & Sons, Inc., New York, * * *, Documentaţia Tehnică CTW 5/60, Universitatea "Petru Maior" Tg. Mureş. 3. A.H. Meleka, Electron Beam Welding: Principles and Practice, McGraw Hill, London, M. Dulău, Controlul procesării cu fascicul de electroni. Modelare. Simulare. Aplicaţii, Universitatea "Petru Maior" Tg. Mureş, L. Marton, L. David, Prelucrarea imaginii în tehnologiile cu flux de electroni, Buletin Ştiinţific, vol. V, pp , Universitatea Tehnică Tg. Mureş, N. Rykalin, I. Zuev, A. Uglov, Osnovy elektronnolucevoj obrabotki materialov, Moscova, F. Eichhorn, M. Panten, B. Spies, K. Depner, Application of Automatic On line Seam Tracking to High-power EB Welding, pp , Proceedings of the International Conference, Tokyo, Published on behalf of the International Institute of Welding by Pergamon Press, F.T. Tănăsescu, I. Costin, Electrotehnologii, vol. I, Institutul Politehnic Bucureşti, 1988.