Apparatus and Method for Viewing Molten Pools in Arc Welding

The invention relates to a novel method and apparatus for viewing the molten pool of metal in an arc welding process.

The process of arc welding is one of the most widely used manufacturing processes in the world. Efforts to automate the various aspects of the welding process have been especially great in recent years. The efforts have centered on attempting to automate the welding process to allow the use of robots which would be capable of producing very uniform, high quality welds. The reasons for seeking such automation are twofold: first, to increase productivity by increasing the speed and accuracy of the welding process in routine welding applications; and secondly, to increase the quality of the welds thus produced allowing automated welding to be utilized in high quality control industries such as the nuclear power industry.

In order to appropriately control the arc welding process and thereby allow for practical automation of the processes, numerous attempts have been made to objectively measure and discern precisely the pertinent parameters in the welding process. It has been said in the past that only the welding operator truly knows the quality of the weld and what is going on in the welding process. This is true because only he in the past has been able see precisely the quality and physical parameters of weld pool size and arc length. Also, only the operator can truly track the often irregular weld preparations or joints.

Attempts have been made to track welds by use of sensors which look ahead of the welding area. Such sensors attempt to discern where the weld preparation area or joint is and adjust the movement of the welding torch so as to coincide with the weld preparation or joint area. Such methods have utilized both direct contacting type sensors which are dragged through the weld preparation area ahead of the welding torch and by indirect measurement techniques, such as the use of infra-red detectors.

In recent years, use of television direct monitoring has increased. All of the methods of directly monitoring the arc welding area with a television camera have involved attempting to duplicate what the welding operator actually sees. Thus, the camera has been placed in such a position as to have an oblique view of the weld similar to the view enjoyed by the operator. Such an oblique view of the weld has the inherent problem of also viewing the arc that is very bright which tends to wash out the entire picture unless appropriate filters are utilized. Therefore, the methods of obliquely viewing the weld and arc area have also involved filtering the view by the camera to one or several discrete wave lengths of light, rather than viewing the broad spectrum of light that is available from the arc.

It has also been known for some time that there is a relationship between the actual width of the molten pool and the penetration being achieved by the welding process. Therefore, it has been thought to be desirable to be able to directly measure and also to control weld pool width. Precise control of the weld puddle width produces a correspondingly precise control of the penetration being achieved by the welding process.

Prior methods of analysis of video data being received from viewing the arc and weld pool area have made the assumption that the bright areas represent the weld pool and that once the light intensity has decreased to a certain value, then the edge of the weld pool has been approached. Such methods then employ a go/no-go type of binary logic system to establish the weld pool width. Unfortunately, there are oscillations to the weld pool caused by the fluctuation in voltage of the arc and also caused by the motion of the electrode forward along the weld preparation area. Such oscillatory motions cause the area of brightness to look considerably larger than it actually is. Attempts to mitigate the data error caused by the oscillatory motions have been precisely that--simply corrections of the data rather than actual measurement and evaluation of the true weld pool edge.

Thus, the problems of obliquely viewing the weld area with a television monitoring camera are that unforeseen obstructions of the camera in real welding situations can arise. Such obstructions can be caused by the weld preparation geometry and by constraints on the placement of the camera. Also, distortion of the image of the weld puddle is caused by the oblique viewing system. These are in addition to the over-exposure problems caused by direct arc viewing which are reviewed above.

It has been anticipated for some time that the best opportunity to provide reliable control to the welding process involves adequately and accurately measuring the process parameters at the point of welding. Welding parameters that can be measured directly at the point of welding seam tracking are voltage and current. The primary emphasis in this area has been tracking of weld seams or weld preparation areas. Unfortunately, in order to avoid damage to the sensor and also to keep the sensor from being obscured by the arc area and the melted metal, it has been necessary to sense the track some distance, even small, ahead of the weld torch. This immediately produces the requirement of having some delay in system response so that in fact the system responds to seam tracking changes at the time when the welding head is over the area of change.

Seam tracking devices have been of two types. First are those that use the arc itself as the sensor. Sensors of this type sense voltage and current variations in the arc when various surface features of the base metal are encountered. One such method oscillates the arc back and forth across the seam noting the voltage changes as the arc gets longer as it approaches the weld preparation. Various schemes have been proposed to allow for this oscillation. Both magnetic and mechanical motion devices have been utilized previously. The second method is the direct arc viewing method for seam tracking directly at the point of welding. Here the methods identify the edges of the weld preparation or weld groove from analysis of light from the arc reflected off the edges of the grooves or the side walls of the weld preparation area. A feedback system is then provided to respond to the reflected light to produce the desired effect of following the seam. Additionally, the problems of tracking the weld seam or weld preparation area by viewing the area ahead of the weld require that any information thus received be delayed before it is implemented so that the welding torch is indeed over the area detected, or the change detected, at the time the change instruction or signal is initiated. Also, any process control data whether it be used for weld seam tracking or for viewing welding parameters in the area of the arc and molten pool, are subject to error due to parallax from the oblique view of camera positions known in the prior art, and the undesirable masking of the far side of the weld pool by the arc itself.

The above noted problems are among those solved by reference to the present invention.

Therefore, a primary aspect of the present invention resides in the provision of a viewing system relying solely on reflected light. The light is reflected from the arc area off the mirror-like surface of the weld pool through an optical system into a television camera, or other camera type arrangement located at an image plane. The positioning of the optical system is such that the arc itself is not directly viewed, but is in fact shadowed by the end of the welding electrode. Another aspect of the present invention resides in the adaptation of the optical system for use with both fixed, non-consummable electrode welding processes, and consummable electrode welding processes, including automatic processes.

FIG. 1 is a schematic diagram of an optical viewing apparatus in accordance with the present invention;

FIG. 2 is schematic diagram of a second viewing apparatus according to the present invention; and

FIG. 3 is schematic diagram of a third viewing apparatus according to the present invention.

FIG. 1 depicts a schematic diagram of the optical viewing apparatus in place on a GTAW (gas tungsten arc welding) process welding torch. The welding torch has been modified so that the non-consummable tungsten electrode 32 has been bent at right angles to the electrode holder 30, thereby producing an upper electrode region 32 and a lower electrode region 34. Electrode region 34 extends from the point at which the electrode is bent to the point of the arc 36 or the end of the electrode. Also shown in FIG. 1 is the molten or liquid weld pool 38 which is being viewed by the optical apparatus shown. The shielding gas retention tube 54 has also been modified to conform to the bent configuration of the electrode 32. The tube 54 has been cut, and the portion remaining around the electrode holder 30 abutted to an extended piece of tube disposed at the same angle as the bend in the electrode 32, thereby allowing the tube 54 to be along the axis of lower electrode region 34, surrounding the electrode 34 so that argon gas or other inert purge gas can be maintained at the weld area. The reasons for inert gas purging are well known in the art. The top of the tube 54 along the axis extension of the lower electrode region 34 is open to allow viewing of the weld pool 38 area and of the base metal 40. Immediately above the tube 54 is lens 44, a collimating lens to gather the reflected light into a parallel beam 42 for transmission to the balance of the optical system.

During the arc welding process, the high intensity light of the arc resides only below the tip of the electrode and is reflected off the weld pool. In the FIG. 1 configuration, the lens 44 and the optical viewing path 28 down the center of shielding gas nozzle tube extension 54 is such that the tip of the welding electrode 34 blocks or shadows the actual arc from view. The only light that is observable is the light reflected from the mirror-like surface of the weld pool. Light is also reflected from the surrounding base material which is not molten and also from the resolidified weld area immediately behind the adjacent to the travel of the weld torch, but the light reflected from base metal and resolidified weld metal is more diffuse than the bright reflection from the mirror-like surface of the weld puddle. This light beam 42 is directed upwards and is received by lens 44 and in the FIG. 1 embodiment is reflected from mirror 46 which is disposed at 45.degree. to the direction of the welding electrode axis 34, but clearly reflectable at any usable angle. The light beam 42 is then directed to lens 48 for focusing at an image plane 50. In the embodiment shown in FIG. 1, lens 48 is a telephoto lens that has been focused at infinity and the image plane occupies the light sensitive array of a solid state television camera. The image plane can also occupy the light sensitive surface of a camera or can be directly viewed by eye with the use of a safety glass of some sort to protect from ultraviolet light that is also reflected up the light beam 42.

Therefore, the arrangement consists of a modified GTAW torch with a tungsten electrode bent at right angles. Light from the arc is reflected off the weld pool surface, through the shielding gas nozzle 54 and collimated by the lens 44 at the focal point of which coincides with the pool location. The collimated light is reflected by the mirror 46 and is brought to a focus in the image plane 50 by the focusing lens 48. This optical arrangement renders the image sharp and insensitive to actual camera placement along the optical path since the light beam 42 has been collimated. If only a single lens is utilized, the light is not columnated and precise location of the image plane 50, or camera 52, is important in order to avoid magnification or reduction of the image of the weld pool. Avoidance of such image distortion is most important since one of the uses made of this direct and undistorted view of the weld pool is the measurement of the weld puddle width.

However, when a consummable electrode is utilized as in the GMAW (gas metal arc welding) process, it is not practical to locate a lens directly in the path of the advancing consummable electrode.

Turning now to FIG. 2, which shows a schematic diagram of an alternative embodiment of the present invention as employed in a non-consummable electrode process, as for example GTAW, the the shielding gas tube 54' has been modified by inserting a mirror with a hole drilled in it about the non-consummable electrode and the opening of an optical window in the side of the tube 54'. During the welding process, light from the arc 36' is again obscured by the tip of the electrode 32'. The light from the arc 36' is reflected off the weld pool 38' and the base material 40' as previously described. This reflected light beam 42' is reflected upwards to mirror 56 which is disposed about the electrode. The light beam 42' is then reflected to a second mirror located at a coordinate angle to the first angle, to redirect light beam 42' in a direction parallel to the electrode holder 30'. The beam 42' is then directed through a lens 44' to an image plane 50'. It will be noted in this configuration that only one lens has been utilized. Therefore, the light has not been collimated before reaching by lens 44' and the precise location of image plane 50' is of vital importance as described above. In the embodiment shown in FIG. 2, the image plane 50' is actually intended to be occupied by a television camera or other camera or viewing device mounted directly on the welding torch. By placing the camera directly on the welding torch, the camera must follow all movements of the torch.

Turning now to FIG. 3, which shows schematically another embodiment of the invention for use with a consummable electrode process, as for example the GMAW process, the mirror 56' has been modified by having a hole drilled therein to allow for passage of the consummable electrode filler metal wire 64. Also, since this is an inert gas welding process, there is a gas tube 54" that is topped by a piece of plexiglass 66 with an hole drilled therethrogh also to allow for passage of the filler metal wire 64. The purpose of the plexiglass cover with the small hole is to prevent the inert gas purge from leaking out the top and also to provide a transparent window for the beam of light 42" which is reflected light originating at the arc 36" and reflected from the weld pool 38" and from the base metal 40" as described above. This, as in FIG. 2, is a single lens arrangement and therefore, the light is not collimated and the position of image plane 50" relative to lens 44" and the relationship between lens 44" and the mirror 56' and the distance from the mirror 56' to the weld pool 38" are strictly regulated distances. The distance from the weld pool 38" to the mirror 56' and hence to the lens 44" is equal to the focal length of the lens 44". Again, this is not the most advantageous relationship, but is necessary because the choice was made not to drill a hole through an operating lens. However, it will be appreciated that a hole could be drilled through the lens 44 or 44', or 44", thus avoiding the use of the mirrors to avoid the path of the weld filler metal 64 or of the fixed non-consummable tungsten electrode 34, and that the use of additional mirrors to avoid this lens drilling is simply a matter of choice. The precise location once achieved of the image plane, the lens and the other optics of this system will elminate or minimize any magnification problems. This can be set up in initial testing and thereafter does not need to be changed.

Additional advantages of directly viewing the weld pool from above and in line with the welding electrode, in addition to that of blocking of the arc by the electrode are that the entire weld pool may be viewed without blockage by the arc itself or by any retaining apparatus of the electrode. Any support structures for the electrode which are located near the molten weld pool are simply out of focus and do not block the view of the weld pool. Additionally, the viewing lenses in the optical systems are protected by the shield gas nozzle and by the purging gas itself from damage by the arc and any weld spattering that may be going on. Additionally, the viewing system does not intrude into the vital space immediately around the torch head and thus does limit access into tight areas.

It will be appreciated that the optical system can be easily integrated into a torch utilizing optical fiber methods to transmit the image to a remote viewing location. Additionally, the optical system can be easily utilized on welding process that has no inert gas purge or gas retaining tube, the light beam path being simply around the electrode.

This invention may be embodied in other specific forms without departing from the spirit and essential characteristics thereof. The present embodiments are therefore considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description and drawings and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.