Modelling and Control of the Nonconventional Material Processing Technologies with Electron Beam

Transcription

1 FACULTY OF AUTOMATION AND COMPUTER SCIENCE Abstract of the PhD Thesis Modelling and Control of the Nonconventional Material Processing Technologies with Electron Beam PhD Student: eng. Stelian Emilian OLTEAN Thesis advisor: Prof.dr.eng. Mihail ABRUDEAN Contents: I. Introduction II. Present stage in the nonconventional technologies for material processing III. Electron beam processing equipment control and heat transfer fundamentals IV. Modelling and adaptive control of the electron beam heating parameters V. Advanced control system of the nonconventional processing technology with electron beam VI. Conclusions and personal contributions Thesis outline: The thesis contains the author s results of the fundamental and applied researches in the domain of automation of the nonconventional material processing technologies. The main objectives are: the identification of the electron beam heating parameters and the study of the electron beam equipment components, modelling the heat transfer, the design of the control systems to improve the quality of the electron beam processing and the implementation of the advanced control system for nonconventional material processing technology with electron beam using the directing components of the equipments. The development in the material processing domain led to the new concept named nonconventional technologies. These technologies solve great tipical problems where conventional techniques proved to be inneficient. Electron beam material processing is an important non-conventional technique for industrial manufacturing. Nuclear technologies, aeronautics, microelectronics are some of the examples where this equipment is used.

2 The special electron beam s properties like high resolution, long depth of field attainable, high power energy density make them very useful in material handling. 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 ). However, the processing is very complex because the electron beam equipments are multivariable and have functional nonlinearities, the heat absorption; the penetration of the electrons in metal are rather complicated, making their modelling a difficult task to solve. Also, the examination of the electron gun s variables is very difficult due to the nature of the process. The heating process depends on the electrons emission, electromagnetic fields, X radiation and material properties. Thus it is a necessity for modern control strategies implemented on digital systems to produce high quality material processing at required standards. The originality of the thesis consists in using the fuzzy and adaptive systems to control the electron beam focusing and designing a 3d control system for the focal spot of the electron beam which modifies the heating and technological parameters. Chapter 1 contains the objectives, a short introduction and the structure of the thesis. Chapter 2 presents the equipments and the processing methods for the main special electro-technologies which use thermal action on the material. In this context, electron beam, laser beam and plasma are the three high power beams used in the field of nonconventional technologies, and are use to obtain products with various shapes, high precision and great productivity. For each of these technologies are presented the major historichal events, development stage, characteristics, block diagram of the equipments and some processing tecniques. From the constructive point of view we can find many similarities. Therefor, the structure of these equipments includes: the beam generator, the beam transportation and directing components on the material surface, the workpiece movement block. In the theoretical part of the electron beam technology are shown two types of equipments, one uses high vacuum ( Pa) in the workchamber and the other operates at small distances from the electron gun in normal air pressure. Actually, in normal conditions electrons scatter from the collision with air particles. Electron beam can be used in welding, melting, refining, evaporation, drilling, cutting and thermal surface treatment. For the laser beam technology, equipments which use carbon dioxide, neodim YAG (yttrium aluminium garnet) or diodes are presented. The laser effect and the constructive characteristics of these are also shown. The most representative processing methods are welding, cutting, drilling and engraving. 2

3 The plasma technology subsection details the plasma generation, the importance of the monoatomic or diatomic gases on the temperature distribution and processing power and the structures of the plasma arc or plasma jet equipments. Cutting and welding are the most used processing methods. Present stage of development in nonconventional technologies studied in this chapter consists in: - high performance, precision and speed; - material purity, strength and homogenousity; - high energy density on small areas; - advance control of the electro-technological parameters; - specialized devices dedicated on the desired operations; - undercovered software and technical control solutions. Based on the electron beam equipment for welding process, CTW 5/60 (5kW maximum power at 60kV accelerating voltage), developed by Petru Maior University of Târgu Mureş in partnership with the Electrical Research Institute I.C.P.E. Bucharest, the author studied and experimented new control solutions in the field of electron beam technology. Even from this chapter it is obvious the importance of the transportation and directing components on the quality of the material processing. Chaper 3 presents the constructive structure of the electron beam experimental equipment CTW 5/60 and its parameters which affect the thermal process on the material surface. Here the author identifies some methods and strategies to improve the technology control. An important final part of the chapter contains a description of the physical phenomena produced by the electron beam heating process and the modelling the temperature distribution in the material using the heat transfer equation. Figure 1. CTW5/60 electron beam processing equipment 3

4 The main parts of the CTW 5/60 equipment are the triode gun (provided by the Manfred von Ardenne Dresda Institute) 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, one focusing and two deflecting coils. The first target, situated in the triode gun, is the anode at a positive potential, which forms the beam. Then the focused beam of electrons is led using the focusing coil to the secondary target, situated in the workbox, which consists of a metallic workpiece, where the kinetic energy of the electrons is converted into thermal energy. The main performance requirements for these kinds of devices are to realize weldings with desired geometrical values (depth and width). The equipment is very complex and has many measurement, limiting and control circuits. Every control loop represents a way to modify the electro-technological parameters. Therefor, the author identified in this chapter the practical control possibilities referring to currents and voltages in the accelerating, grid cup, focusing, deflecting and positioning circuits. A major problem for feedback control of the electron beam heating process is given by the high temperature at the point of impact (10000 K). Block diagrams depending on the sensors were presented for the focusing and seam tracking systems. Qualitative and quantitative information about the processing can be obtained using: - the absorbed power from the workpiece and the power balance; - the penetrating current on some special weldings; - the X rays emission from the workpiece; - backscattered and reflected electrons from the surface of the material. The temperature s evolution and distribution in the workpiece determine the final shape and the processing performances. Multiple phenomena in the focal point, functional regime, the presence of nonlinearities and interdependencies between equipment parameters make impossible to determine the electron beam heating model and so the human operator experience becomes important. A subsection is allocated to the study of the temperature distribution using different hypotheses. The author proposed some analytical solutions for plane, punctiform and linear heating sources. The approximations are better if the source is considered surface (with one or two distributions parameters) or depth distributed (volumetric with three distributions parameters). However numerichal methods (finite element method or finite difference method) are needed in this case. The correct model for temperature distribution and weld shape determination is not unique and it is chosen depending on the functional regime and other factors. The complex process recommends advanced control of the electron beam processing. 4

5 Figure 2. Temperature distributions for differnet hypotheses Chapter 4 has two parts: modelling the electron beam directing components and designing advanced systems to control the electron beam heating and welding parameters. From this point of view the desired depth of pentration is obtained with a fuzzy adaptive control of the focusing system and the seam tracking is made with PI control of the deflecting system. In the final section a 3d control system of the electron beam is presented and tested, considering the equipment characteristics and practical experience. By including the magnetic field distribution and using some optics concepts from the dynamic equations of the electrons that cross through the electromagnetic coil the stationary model of the focusing system results. A first order model was added to express the coil dynamics. The final focusing model is an approximation, influenced by the electron beam equipment variables and nonlinearity. So, adaptive systems and human experience have been used and results a fuzzy model reference learning control FMRLC. This fuzzy adaptive system has the same structure as conventional model reference adaptive system and is capable to learn and to adapt to different unknown situations. The controller in the adaptive scheme is fuzzy and to develop the FMRLC a direct fuzzy controller has been designed first. Designing direct fuzzy controller means to choose and to process the inputs and outputs of the controller and to build its four elements: the rule base, the inference engine, the fuzzification interfaces and the defuzzification interface. The learning mechanism contains an additional fuzzy sys-tem, called fuzzy inverse model which acts like a second controller. This second controller updates the rule base of the direct fuzzy controller by acting upon the output variable. The two deflecting coils, located beneath the focusing coil, create two magnetic fields; both are perpendicular to the symmetry axis of the electron gun. A constant deflection on the linear axis (Ox or Oy) needs a magnetic field with constant intensity generated by one of the 5

6 deflecting coils (Ox or Oy). The magnetic fields created with both coils determine the position of the focal spot on the workpiece plane xoy. The mathematical model of the deflecting systems contains also two components: the stationary model and the dynamic model. The obtained stationary model is linear and in fact is a particular solution of the dynamic equations of the electrons when the electric and magnetic fields distributions are known. The dynamic model is characterized by a first order differential equation. Due to the nature of the final deflecting model a PI controller gives the desired performances (σ=4.3%, ε st =0). In the final subsection, an electron beam 3D control system is shown. This system contains the two types of control: classical PI for the deflecting components and fuzzy adaptive for the focusing component. Simulations for ideal material surface with 25cm 2 area and a 2cm/s welding speed were drawn. Figure 3. Electron beam 3D control system Chapter 5 presents the implementation of the control system of the electron beam processing technology using the directing components and the experiments obtained by the author. Because the temperature in the focal point reaches K the control implementation starts with the processing zone visualization block. This block offers the useful information for the electron beam control and builds the digital image with 256x256 pixels using the reflected electrons from the material s surface and a scanning process in a raster form. The distance from the surface material, the depth of penetration, the 3D seam 6

7 trajectory are determined by applying some image processing methods to the best primary image. The 3D control system of the electron beam is made in two stages. In the first stage a fuzzy controller (autofocusing controller) obtains the distance to the surface of the material corresponding to the best primary image catured. The image quality coefficient is calculated with the sum modulus difference. This controller offers the optimal focusing current to start the second stage. The second stage use the directing systems designed in the previous chapter. Between these two stages an intermediate procedure detects the 3d seam trajectory and gives the references for the focusing and deflecting systems. The best primary image obtained from the autofocusing system is processed using some image processing methods like: median filtering, binarization, complement of binary image, binary flood fill and skeletonization. Figure 4. Image processing and extraction of the 3D trajectory from the sharpest image The author proposed two functional regimes for the focusing system. The first one is named surface focusing regime and considers for the focusing system a constant reference determined from the autofocusing system in the first stage. The second regime named focusing in depth of the material considers for the focusing system a reference composed of the distance to the surface of the material and the seam depth. In conclusion the 3d seam trajectory from the binary image decomposed on the Ox and Oy directions gives the reference signals for the two deflecting systems. The reference signal for the focusing regime in depth of the material is obtained using the distance to the surface of the material and informations from the best primary image. The 8 bit values of the pixels on 7

8 the 2D seam trajectory corresponding to the depth of the seam is added to the constant distance from the focusing coil to the workpice. The preparing stages for the welding process are also important. The welding starts with the equipment parameter initialization, workpiece cleaning, vacuum generation and sincronization in the electron beam gun and workchamber, workpiece positioning. Figure 5. Signal evolution and electron beam 3D trajectory Chapter 6 contains a synthesis of the teorethical and applied researches from this thesis. The personal contributions are presented and possible future studies are proposed. The main personal contributions are: - determination of the mathematical models for the temperature distribution in the material for different hypotheses; - determination of the mathematical model for the focusing and deflecting systems composed of stationary and dynamic models; - design of the PI controller for the deflecting components and fuzzy adaptive for the focusing component; - design of the electron beam 3d control system; - detection and decomposition of the 3d seam trajectory using image processing methods; - design of the fuzzy autofocusing system; - proposing the two control regimes for the focusing system; - implementation of new solutions for the electron beam processing control. 8