電腦機(jī)箱側(cè)蓋沖壓工藝及模具設(shè)計(jì)含開題及8張CAD圖
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Automated Installation for Modification Surface Properties
of Details and Units of the Metallurgical Equipment
by the Electron Beam Facing
S.I. Belyuk, A.G. Rau, I.V. Osipov*, N.G. Rempe*
Institute of Strength Physics and Materials Science, 2/1 Akademicheskii Ave.,Tomsk, Russia
*Tomsk State University of Control Systems and Radioelectronics, 40 Lenin Ave., Tomsk, Russia,
Abstract : The electron-beam facing installation is designed for the production of coatings on the surface of metal articles. The coatings have protective, wear-resistive,and heat-resistive properties.The installation is capable of creating coatings on large-area surface with high efficiency.The technological process is automated.
There are two plasma-cathode e-guns in the facing installation. This makes it possible to increase the facing efficiency and productivity. The guns are placed in a vacuum chamber on a two-rectilinear manipulator and can operate simultaneously.
This installation is used in metallurgy for creating wear-resistant coatings on aerial and oxygen lances, on crystallizers of continuous casting of steel, on rolls, etc.
1. Introduction
Electron-beam facing in vacuum [1,2] allows coatings with unique properties to be produced. With this method of coating deposition there is no adhesion problem. The materials which can be treated by this method and the coatings which can be produced on their surfaces are widely diversified. The high repeatability of results in combination with the adaptable control of the technological process make it possible to produce coatings of required structure and preassigned properties.
We have developed an installation intended for deposition of heat and aerial blast-furnace lances with the purpose of increasing their operational durability and also for restoration of various machine parts and metallurgical equipment. It can also be used for welding various metals and alloys, including high-melting ones.
The installation makes it possible to produce mono-and multilayer coatings of various purposes (hardening, wear-resistant, heat-resistant, temperature-resistant, etc.) depending on the composition of the facing powder on the surface of articles made of any metals, steels, and cast iron.
With this installation it is possible to deposit coatings on plane surfaces of workpieces of length up to 2100 mm, width up to 900 mm, and thickness up to 200 mm and on bodies of revolution of diameter up to 1200 mm and length up to 2100 mm.
The technological process of coating deposition is full-automatic.
2. Electron-beam facing
The principle of electron-beam facing is shown in Fig. 1. The electron beam creates a molten metal pool on the surface of the workpiece. The powder whose particles form a coating with required properties on the surface is supplied to the molten metal by a dispenser. The workpiece is moved inside the vacuum chamber relative to the (immobile) e-gun and dispenser or the e-gun with the dispenser are moved relative to the (immobile) workpiece.
Fig.1.
The technology of multipass electron-beam facing is based on the phenomenon of "freezing" a powder into a melt pool. In every subsequent pass, a new portion of the powder is "frozen" and the previous portion is melted. The powder supplied to the pool speeds up the crystallization of the melt, thus promoting the formation of a fine grain structure and moderating the residual stresses in the deposited coating. The required thickness of the deposited layer is obtained by varying the rate of powder supply or by increasing the number of passes.
The process of facing is characterized by the following parameters: the accelerating voltage, the electron beam current, the distance from the focusing system to the surface of the workpiece, the electron beam scanning diameter and length, the velocity of motion of the workpiece, and the rate of powder supply.
3.Electron guns
The facing process is accompanied by intense ejection of vapors and gases from the facing zone. The refore, to produce an electron beam, plasma-cathode guns are used [3, 4]. These guns do not contain hot electrodes or components which would be heated in operation, and this makes them insensitive to reactive and high-melting vapors of the materials under treatment. They are capable of operating under the conditions of facing not taking special measures for protection of the emitter.
Figure 2
Figure 3
The electron emission in the guns occurs from the plasma of a hollow cathode low-voltage reflected discharge [4]. The electrons outgoing from the plasma get in a high-voltage electric field where they are accelerated, collected in a beam, and focused by the magnetic field of the focusing system. The electron emission current from the plasma is controlled by varying the discharge current.
In the design of the guns, metal used whose hermeticity and mechanical strength are provided by electron-beam weding.The gun housings are of in-chamber construction.The design of the housing provides easy and convenient access to the cathode assembly for periodic maintenance. Figure 2 presents the appearance of a gun mounted on the manipulator of the installation.
4. Power supply module
The power supply system of the equipment(Fig.3) consists of an accelerating voltage unit (AVU), a discharge power supply unit (DPU), a beam focusing and deflection control unit (BFDU),and a control unit of the gas flow controller. The units are controlled by a computer via an optical or an RS485 interface.
The accelerating voltage and discharge power supply units are made by the classical circuit design of a bridge inverter with the phase-shift control circuit. In the inverter, the resonance method of switching MOSFET transistors is realized that provides a low level of electromagnetic noise and reduced dynamic losses in switching power transistors. The high
conversion frequency (30 kHz) makes it possible to reduce the output capacitance of the power supplies to 10 nF and to increase the rate of processing of control signals.
Figure 4
The accelerating voltage unit can operate in one of the two modes: stabilization of the accelerating voltage and limitation of the output current. In the first mode, a given accelerating voltage is stabilized as the load current increases from 0 to 150 mA. This is the normal operation of the unit. As the load current increases to more than 150 mA, the accelerating voltage unit goes over to the current limitation mode within 50 s. This makes it possible to protect the load and to prevent the development of an arc discharge in case of breakdown in the electron gun. As the load current decreases, the accelerating voltage unit is back to normal operation. If the load current does not decrease, the unit is switched off for 20-100 m and then returns to normal operation. This algorithm provides fault-free operation in extreme transient and arcing environments.
The discharge power supply unit is a current source with the output voltage ranging between 50 and 1500 V. It operates in the current stabilization mode throughout the output voltage range.Structurally, the accelerating voltage and discharge power supply units are made as two sections: a low-voltage section containing inverters and an oil-filled high-voltage tank in which the output stages are housed (Fig. 4).
The control and stabilization of the beam current are performed by varying the discharge current with a control time constant no more than 0.1 s.
5. Arrangement and operation of the installation
Fig. 5.
Deposition of coatings is carried out in the vacuum chamber of the installation. Two e-guns mounted on the two-rectilinear manipulator are placed in the chamber. The manipulator, providing independent horizontal movement of the guns, is intended for deposition of coatings on large-area plane surfaces. The use of two simultaneously operating guns increases the productivity of the installation. For deposition of coatings on ring surfaces an additional manipulator is used which provides rotation of the workpieces. The appearance of the installation is shown in Fig. 5.
The operation of the vacuum system, the power supply, the movement of the guns, and the technological process are controlled with an automated computer system. The choice of the mode of operation and the monitoring of technological parameters are performed with the help of commercial displays. To change the mode or a parameter, it suffices to press the graphical label of controls on the display with a finger.
The control system can operate in one of the three modes: Vacuum System, E-Gun, and Manipulator.
In the Vacuum System mode, it is possible to switch the pumps on and off and to open and close the valves of the vacuum system. The display shows the readings of the vacuum meters at different points of the vacuum system and the state of the pump cooling system. In this mode, it is possible to program all sequences of switchings of the valves and pumps for automated pumpdown of the vacuum chamber.
Fig.6 Fig.7a
Fig.7b
The power supply of the electron-beam guns is controlled in the E-Gun mode (Fig.6).In this mode, it is possible to control the accelerating voltage,to change the magnitude of the beam current, and to control the gas flow rate and the parameters of the beam scanning over the surface of the workpiece.
The Manipulator mode (Fig. 7, a, b) is intended to control the movement of the workpiece and eguns. Depending on the properties of the workpieces, two modes of operation of the manipulator are possible. For deposition of coatings on large plane surfaces the Manipulator-Plane Body mode(Fig. 7, a) is intended. In this mode, the workpiece is immobile, and two eabove its surface along a prescribed trajectory.
The Manipulator-Body of Revolution mode(Fig. 7, b) is intended for deposition of coatings on axisymmetric surfaces. In this mode, the gun is immobile, and the workpiece is rotated at a certain angle with a given velocity.
Table. Principal characteristics of the installation
Voltage of the supply line, V
3805±5%
Power input, kW
30
Limiting pressure in the vacuum chamber, Pa
10-2
Number of simultaneously operating e-guns
2
Rate of powder supply by the dispenser, g/min.
10-50
Accelerating voltage, kV
up to 30
Beam current, mA
up to 150
Dimensions of the vacuum chamber:
diameter, mm
length, mm
2020
3500
Conclusion
The installation created by us is used on one of the world's largest metallurgical works-the West Siberian Iron and Steel Plant-for deposition of wear-resistant coatings on aerial and oxygen lances,on crystallizers of continuous casting of steel, and on rolls.
References
[1]V.E. Panin, S.I. Belyuk, V.G. Durakov, G.A. Pribytkov, and N.G.Rempe, Svarochnoe Proizvodstvo, 2, 34 (2000).
[2]V.E. Panin, V.G. Durakov, G.A. Pribytkov,I.V.Polev, and S.I.Belyuk, Fizika i Khimia Obra
botki Materialov, 6, 53 (1998).
[3]V.L. Galansky, V.A. Gruzdev, I.V. Osipov, and N.G. Rempe, J. Phys. D: Appl. Phys., 27, 953(1994).
[4]I. Osipov and N. Rempe, RSI, 1, 1638 (2000).
Designing Fault Tolerant Manipulators:
How Many Degrees-of-Freedom?
Christiaan J. J. Paredis and Pradeep K. Khosla
Department of Electrical and Computer Engineering and
The Robotics Institute,Carnegie Mellon University,Pittsburgh, Pennsylvania 15213.
Abstract
One of the most important parameters to consider when designing a manipulator is the number of degrees-of-freedom (DOFs). This article focuses on the question: How many DOFs are necessary and sufficient for fault tolerance and how should these DOFs be distributed along the length of the manipulator? A manipulator is fault tolerant if it can complete its task even when one of its joints fail sand is immobilized. The number of degrees-of-freedom needed for fault tolerance strongly depends on the knowledge available about the task. In this article, two approaches are explored. First, for the design of a General Purpose Fault Tolerant manipulator, it is assumed that neither the exact task trajectory, nor the redundancy resolution algorithm are known a priori and that the manipulator has no joint limits. In this case, two redundant DOFs are necessary and sufficient to sustain one joint failure as is demonstrate din two design templates for spatial fault tolerant manipulators. In a second approach, both the Cartesian task path and the redundancy resolution algorithm are assumed to be known. The design of such a Task Specific Fault Tolerant Manipulator requires only one degree-of-redundancy.
1 Introduction
As robots are being used in a growing range of applications, the issue of reliability becomes more and more important. Recently, with the Hubble telescope and the Mars Observer, NASA has experienced firsthand how devastating the consequences can be when a critical component fails during a multi-billion dollar mission. Space applications are particularly vulnerable to failure, because of the adverse environment (cosmic rays, solar particles etc.) and the demand for long term operation. In this context, NASA has started to incorporate fault tolerance in their robot designs (Wu et al. 1993). Reliability is also important in medical robotics, because of the risk of the loss of human life. Although medical staff will probably always be on standby to take over in the case of a manipulator failure, the robot should at least be fail-safe, meaning it should fail into a safe configuration. A third domain of robot applications in which reliability is a major issue is the Environmental Restoration and Waste Management (ER&WM) program of the Department of Energy. Consider, for instance, the use of a manipulator in a nuclear environment where equipment has to be repaired or space has to be searched for radioactive contamination. The manipulator system deployed in these kinds of critical tasks must be reliable, so that the successful completion of the task or the safe removal of the robot system is assured. In this article, we focus on fault tolerance as a technique to achieve reliability in manipulator systems. The traditional approach to reliability has been that of fault intolerance, where the reliability of the system is assured by the use of high quality components. However, increasing system complexity and the necessity for long term operation have proven this approach inadequate. The system reliability can be further improved through redundancy. This design approach was already advocated in the early fifties by von Neumann in connection with the design of reliable computers: “The complete system must be organized in such a manner, that a malfunction of the whole automaton cannot be caused by the malfunctioning of a single component, ... , but only by the malfunctioning of a large number of them”(von Neumann 1956, p. 70). This is the basic principle of fault tolerance: add redundancy to compensate for possible failures of components. However, this does not mean that any kind of redundancy added to a system results in fault tolerance. The main goal of this article is therefore to shed some light on the redundancy requirements for fault tolerant manipulators. That is, how much redundancy is needed and how should this redundancy be distributed over the manipulator structure?
The redundancy provisions needed for fault tolerance can be incorporated only at a price of increased complexity. This drawback can be overcome by a modular and structured design philosophy, as is advocated in (Schmitz, Khosla, and Kanade 1988; Fukuda et al. 1992; Sreevijayan 1992; Hui et al. 1993;Chen and Burdick 1995). Modularity in hardware and software has the advantage of facilitating testing during the design phase and therefore reducing the chances for unanticipated faults. Modules also constitute natural boundaries to which faults can be confined. By including fault detection and recovery mechanisms in critical modules, the effect of local faults remains internal to the modules, totally transparent to the higher levels of the manipulator system. Such a modular design philosophy is embodied in the Reconfigurable Modular Manipulator System (RMMS) developed in the Advanced Manipulators Laboratory at Carnegie Mellon University (Schmitz, Khosla, and Kanade 1988). The RMMS utilizes a stock of interchangeable link modules of different lengths and shapes, and joint modules of various sizes and performance specifications. By combining these general purpose modules, a wide range of manipulator configurations can be assembled. When one needs a different configurations for a specific task (Paredis, and Khosla 1993), a robot created with the RMMS can be easily taken apart and reconfigured suitably. This reconfigurability can be further exploited to reduce the complexity of fault tolerant manipulators, as is shown in Section 4. Over the past decade, a lot of research has been done in fault tolerance for computer systems (refer to Johnson (1989) for and overview), but only recently has the concept been applied in robotics. Most of the work in fault tolerant robotics is directly based on the results from computer science, and can be classified in three categories:
1. Design of fault tolerant robots,
2. Fault detection and identification (FDI),
3. Fault recovery and intelligent control.
When designing a fault tolerant manipulator, one should decide where to include redundancy so that the
overall reliability is maximum. One should distinguish between hardware, software, analytical, information, and time redundancy (Johnson 1989). Our focus will be on hardware redundancy, which consists of actuation, sensor, communication and computing redundancy. Each of these types of redundancies can still be implemented at different levels. In Sreevijayan (1992), for instance, a four-level subsumptive architecture for actuation redundancy is proposed:
Level 1: Dual actuators—extra actuators per joint,
Level 2: Parallel structures—extra joints per DOF,
Level 3: Redundant manipulators—extra DOFs per manipulator arm,
Level 4: Multiple arms—extra arms per manipulator system.
A system can possibly be designed with redundancies at all four levels, resulting in the ability to sustain
multiple simultaneous faults. An example of a fault tolerant design for the space shuttle manipulator is described in Wu et al. (1993). Fault tolerance is here guaranteed by using a differential gear train with dual input actuators for every DOF—an implementation of the first level of the four-level subsumptive architecture. In this article, we are more interested in achieving fault tolerance using redundant DOFs (Level 3). We envision the following scenario. A fault detection and identification algorithm monitors the proper functioning of each DOF of a redundant manipulator. As soon as a failure of a subcomponent is detected, one immobilizes the corresponding DOF by activating its fail-safe brake. Automatically, the joint trajectory is adapted to the new manipulator structure and the task is continued without interruption. The strength of this scenario resides in the fact that it can handle a large variety of possible faults, ranging from sensor failures to transmission and actuation failures. All these failures can be treated in the same manner, namely, by eliminating the whole DOF through immobilization. Although fault detection and identification (FDI) is an important part of our scenario for fault tolerance, we will not cover this subject in this article. Instead we refer to the following references (Chow and Willsky 1984; Stengel 1988; Ting, Tosunoglu, and Tesar 1993; Visinsky, Walker, and Cavallaro 1993;Visinsky, Walker, and Cavallaro 1994). In Visinsky, Walker, and Cavallaro (1993), using the concept of analytical redundancy, an FDI algorithm is presented along the lines of Chow and Willsky (1984). The result is a set of four simple equations which test for consistency between the measured position and velo
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