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Control of dynamic keyhole welding process (焊接方面的) Y.M. Zhang∗,Y.C. Liu Abstract Weld joint penetration control is a basic research topic in the welding research community. The authors propose using an innovative plasma arc welding process referred to as the quasi-keyhole process to achieve less application-dependent weld joint penetration sensing and control. To control the quasi-keyhole process, the peak current and keyhole sustaining current are selected as the control variables to maintain the keyhole establishment and sustaining periods at desired values. A linear model with interval parameters approximates the dynamic quasi-keyhole process. A control algorithm has been developed for the multivariable interval quasi-keyhole process based on a predictive control algorithm for interval SISO models. Experiments have been conducted to test the effectiveness of the control system developed. 2007 Elsevier Ltd. All rights reserved. 1. Introduction Gas tungsten arc welding (GTAW) has been the primary process for precision joining of metals and for critical applications such as the root pass where the weld joint penetration must be assured. The majority of research in arc welding process sensing and control has been devoted to GTAW especially to the sensing and control of the weld joint penetration in GTAW. The issue here is to assure the production of a desired full penetration as shown in Fig. 1(b) without occurrence of either partial penetration (Fig. 1(a)) or over-penetration (Fig. 1(c)), or maintain the backside bead width wb within a certain specified range. In the current practice, a highly skilled and experienced human welder is needed to observe the weld pool and adjust the welding parameters accordingly if the variations or changes in manufacturing conditions may exist. Unfortunately, human welders do not typically perform consistently because high concentration must be maintained during the labor-intensive operation in a difficult and arduous working environment. In addition, for new applications, the operators need significant practice in order to develop specific skills. Hence, automated sensing and control of joint penetration is an issue the welding research community must address and for which to find good solutions. One of the major criteria for a good solution is that the sensor used can be attached to and move with the welding torch to be qualified as a front-side sensor which measures the welding process from the front-side of the workpiece. Because the objective is to maintain the back-side bead width within a certain range using a front-side sensor, the invisibility of the back-side and the strong arc light radiation appear to be the major obscures and various methods have thus been proposed, including pool oscillation, ultrasound, infrared sensor, etc. The pioneering work in pool oscillation was conducted by Kotecki, Richardson, and Hardt found an abrupt change in the oscillation frequency of the pool during the transition from partial to full penetration. Ultrasound based weld penetration sensors have been extensively investigated at the Idaho National Engineering Laboratories. At Georgia Institute of Technology, Ume leads the development of non-contact ultrasonic penetration sensors based on laser-phased array techniques. Because the temperature distribution in the weld zone contains abundant information about the welding process, Chin at Auburn University has explored infrared sensing of welding processes. The penetration depth of the weld pool has been correlated with the infrared characteristics of the infrared image. At MIT, Hardt used an infrared camera to view the temperature field from the backside. The penetration depth was estimated from the measured temperature distribution and then controlled. Although existing methods can be effective for specific applications targeted and deserve further study, they are typically very application dependent and require sophisticated, application-oriented studies before being applied. To obtain technologies that can be more application independent and require much less application orientation studies before being applied, innovative methods are needed. To this end, the authors propose using an innovative keyhole plasma arc welding (PAW) process as a substitute for GTAW for more application-independent sensing and control of weld joint penetration. 2. Proposed method In melt-in processes like GTAW, the heat is applied on the surface of the base metal and the base metal is melted by heat transferred from the surface. While their major advantage is that they are relatively easy for human welders to operate and control because of the relatively slow process, their low heat efficiency and large distortion are often concerns. High-energy beam welding processes including electron beam, laser, and PAW process can vaporize or displace part of the molten metal in the weld pool to form a hole penetrating completely through the base metal as referred to as keyhole. The presence of the keyhole allows the heat/energy be directly imposed on the base metal underneath the surface of the base metal to greatly improve the heat efficiency and reduce the distortion. The major disadvantage associated with the keyhole process appears to be more difficult to control than the relatively slow melt-in GTAW process, but the authors will show below how the keyhole process can actually be taken advantage of to ease the weld joint penetration sensing and control. First, the authors propose to use PAW process to achieve keyhole because of its low cost. In name, PAW sounds quite different from GTAW; but PAW is actually a slight modification of GTAW by adding an orifice to restrict the arc in order to achieve a denser energy beam and the equipment cost is just slightly higher than that of GTAW. When the tungsten electrode locates within the torch nozzle and the orifice, the arc is restricted and the highly constrained arc or plasma jet can displace the molten metal in the weld pool to form a keyhole completely through the base metal. Second, the keyhole is associated with a fundamental change in the process. Before the keyhole is established, the gaseous cavity (front surface of the weld pool) produced by the arc pressure and the backside surfaces of the workpiece are divided by the weld pool and solid metal. The ionized shield gas or plasma jet must bounce back from the workpiece. After the keyhole is established, the gaseous passage from the front to the backside of the workpiece is formed. As a result, the plasma jet can pass from the front to the back and at least part of the plasma jet will not bounce back but exit from the backside of the workpiece as efflux plasma. This fundamental change associated with the keyhole establishment has led to the development of a few simple yet effective and application-independent sensors either by measuring the efflux plasma or the reflected plasma. Third, the welding current is applied at the peak level to provide peak level of heat input until the keyhole be established but is then switched to the base level after the keyhole establishment is confirmed or after the keyhole has been established for a specified short period. If the welding speed is faster or the plate is thicker, the peak current will be automatically applied for a longer time in order to establish the keyhole. Because the keyhole is only established very briefly and will close after the current is switched to the base level, this mode of operation is referred to as quasi-keyhole process. It is apparent that using the quasi-keyhole process, the control of the weld joint penetration to achieve the desired full penetration can be application-independent if (1) the peak level of the heat input is high enough so that the keyhole will be established in a reasonable period of time and (2) the base level of the heat input is low enough so that the keyhole will close in a reasonable period of time. While a few sensors have been developed, the control of the quasi-keyhole process has been focused on the establishment of the keyhole. To fully take advantage of the quasi-keyhole process, its full dynamics including the keyhole establishment and keyhole closure must also be controlled simultaneously. This paper thus addresses the control of the full dynamics of the quasi-keyhole process in order for the keyhole to be established and maintained in reasonable or desired periods. It is expected that the developed control method can be combined with different sensors to provide solutions to different applications. 3. Experimental system The experimental set-up is shown in Fig. 2. The power supply is an inverter designed for GTAW and PAW. Its current ranges from 10 to 400A and is controlled by an inner-loop controller. The controller adjusts the welding current through the analog output interface to the power supply. The torch, a regular commercial straight-polarity PAW torch rated at 200A, is attached to a manipulator. The motion of the manipulator is computer controlled. The keyhole sensor used in this study is referred to as the efflux plasma charge sensor (EPCS). As discussed earlier, after the keyhole is established, the plasma jet must exit from the keyhole. The efflux plasma establishes an electrical potential between the workpiece and the detection plate, which is electrically isolated from the workpiece, due to the phenomena of plasma space charge. However, if the keyhole is not established, there will be no efflux plasma between the workpiece and the detection plate, and thus no electrical potential established. Hence it is possible to measure the electrical potential, or the voltage between the workpiece and the detection plate, to determine if the keyhole is present. The experiments conducted in this study will be bead-on-plate welding on 3.6mm thick stainless steel (type 304). The diameter of the plasma orifice was 2.4 mm. Table 1 gives the values of those parameters being constant during experiments. Other welding parameters that are subject to change will be given later for the specific experiment or specific type of experiments conducted. [ Last edited by dongshanxin on 2007-6-11 at 19:56 ] |
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累死了，先翻译这些吧，最后2小段没时间了，麻烦他人吧 摘要 焊接联合渗透控制在焊接研究团队是一个基础研究题目，作者建议使用一种新的称为半锁孔进程的等离子弧焊过程，以实现应用较少依赖 焊缝渗透传感与控制. 控制准小孔过程中, 峰值电流和锁孔维持目前选定作为控制变量,以维持小孔的建立和维持，时段的预期价值. 线性模型参数区间约合动态准锁孔进程. 控制算法,已经开发了多变量区间半锁孔过程基于一种预测控制算法区间的输出模式.实验已经进行了高效率的控制系统研制.2007 Elsevier公司有限公司保留所有权利. 1介绍 钨极氩弧焊(氩弧焊)一直主要应用于工艺精密加入金属和譬如焊接联接渗透的根通行证。 作为根合格的焊缝渗透必须予以保证. 大多数研究弧焊过程传感与控制一直致力于氩弧焊尤其传感 和控制的焊缝熔氩弧焊. 这里的问题是向生产的是一种全新的全面渗透如图. 1 ( b )不发生或局部渗透 (图1 ( a )款)或超额突(图1 ( c )款) ,或是维持背面熔宽局现在某一指定范围内. 根据现行做法, 高技能和有经验的人力焊工需要观察熔池和调整焊接参数,因此，变异或改变生产条件是可能存在的. 不幸的是, 电焊工人通常并不一定注意,因为高浓度必须保持在劳动密集型，经营中的困难，艰苦的工作环境. 此外,对新申请的,经营者需要大量的实践,以发展技能. 因此, 自动传感与控制联合渗透率是焊接研究界必须正视和为上的一个问题，必须找到良好的解决办法. 一个很好的解决方法的重要标准之一,就是传感器可以附着，随着焊枪成为合格的侧前方传感器措施,其中的焊接工艺,从车头侧 工件. 因为它的目的就是要采用侧前方传感器维持后端熔宽在一定范围内，后端的隐蔽强烈弧光辐射似乎是一个主要的遮蔽方法。因此有人建议进行了kotecki,其中包括池振荡,超声,红外传感器, 等开创性工作,理查森 哈特并发现了急剧变化的振荡频率的集中过渡期间,从局部到全面渗透. 根据在爱达荷国家工程已有的实验室超声波焊接熔传感器广泛调查。 在佐治亚技术研究所, Ume 带领发展的non-contact 超音波渗透传感器根据laser逐步采用的列阵技术。由于温度发生的焊接区域包含关于焊接过程的丰富的信息, 下巴在赤褐色大学探索了红外感觉焊接过程。焊接水池的渗透深度被关联了以红外图象的红外特征。在麻省理工学院, Hardt 使用一台红外照相机从后侧方观看温度领域。深度估计了测量实测的温度分布，然后进行控制。 虽然现有的方法可以在具体应用和该当进一步研究是有效的, 他们在研究应用之前是非常依赖老练的学习。为了获得更大应用范围的技术，同时在应用前尽量减少培训，进行创新方法的研究是需要的。为此, 作者建议使用一种新的等离子弧焊的过程,以代替更多依赖于传感与控制焊缝渗透的氩弧焊. 2建议方法 在熔融过程中,像氩弧焊，热量加在金属基体的表面，金属通过表面穿过来的热量逐渐融化。 氩弧焊主要优点是：因为相对缓慢的过程使得电焊工人比较容操作和控制，但是其加热效率低,变形大的缺点,往往受到关注. 高能束焊接过程中,包括电子束,激光、等离子弧焊。可以由于汽化或部分熔化而在金属上形成一个洞，完全穿透金属时简称为小孔。金属小孔的存在可以使热量直接施加于的金属底下表面，大大提高热效率,减少变形。主要缺点在于氩弧焊过程中难以控制较相对缓慢融化。作者将在下面说明如何利用锁孔过程,以减轻焊缝 联合普及传感与控制. 首先,作者提出用等离子弧焊进程,以实现小孔,因为它的成本低. 在名义上, 等离子弧焊听起来不同于氩弧焊;其实它只是在氩弧焊的基础上稍加修改，加入一个小孔限制，以实现致密束能量。其设备费用只是略高于的氩弧焊。当钨电极定位在火炬喷嘴和孔板之间时，电弧是高度集中的，电弧或者等离子体完全通过的碱金属在焊接池中取代熔融金属而形成一个小孔。 第二，小孔的形成过程是与基础变化联系在一起的。在小孔形成之前，弧压力和背部表面形成的弧压力被熔池和固体金属所共同承担。电离气体或等离子体可以从固体金属反射回来，小孔的形成之后，从工件的前方到后方形成电离气体。结果等离子体可以从前到后穿越金属，很少量的电离气体会反射回来，大部分通过金属的后面流出了。关于小孔技术的更本性变革，引发了一些既简单又有效，且不依赖于等离子体反射和益处的传感器。 第三, 焊接应用高峰水平的电流，以提供最高水平的热输入,直到小孔形成了, 随后切换到基本电流，如果加热速度过快或者金属太厚，为了形成小孔，高峰水平的电流就要相应的延长时间。 因为小孔只是初步建立了，转换电流后，这种运作模式称为半穿孔过程。控制的焊缝渗透以达到预期的全面打通,可独立应用。条件是：（1）热输入的高峰水平足够高以在一个合理的时期将小孔建立。（2）热输入的基本的水平是足够在一个合理的时期将小孔打穿。 虽然有几个传感器已被开发，打孔的控制仍然集中在开始阶段。要充分利用现有的半打孔过程，它充分的动力学包括匙孔创立和匙孔关闭，必须同时并且被控制。本文因而描述了半打孔充分的动力学过程，以便顺利完成匙孔创立和在一段时间内的保持。它被期望，被开发的控制方法可结合不同的传感器提供对不同应用的解答。 3.实验系统 实验性设定被显示在图2 。电源是变换器被设计为GTAW 和等离子体。它的电流范围从10到400A，由内在圈控制器控制。控制器调整焊接电流通过和电源链接模拟接口输出。焊炬,规范商业平直极性等离子体焊炬估计在200A, 附有操作器。操作器的行动是计算机控制。 |
4楼2007-06-12 00:02:34
