Abstract:
A method and guidance system for accurately positioning a welding torch relative to a desired weld path. The method and system are based on ultrasonic sensing of information relating to the distance between the welding torch and surfaces on one or more components being welded, and then using this information to move the welding torch to a specific point in space relative to the desired weld path. The method and system maintain the welding torch over the weld path and at a predetermined distance from the weld path during movement of the torch along the weld path.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/320,132, filed Apr. 22, 2003. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to welding processes and equipment. More particularly, this invention relates to a method and equipment for performing a welding operation using ultrasonic sensing to position a welding head relative to a workpiece on which the welding operation is being performed.  
           [0004]    2. Description of the Related Art  
           [0005]    Automated welding operations make use of controls to accurately position a welding head or torch relative to the workpiece or workpieces being welded. For example, automated welders often employ a guidance system that includes a track on which the unit is mounted to maintain the position of the torch over the desire weld path, and the torch is mounted to a movable slide that enables automatic adjustment of the distance between the torch and the surface being welded. Currently, most guidance systems use various forms of mechanical and electronic systems to position the welding torch, from simple systems such as guide rollers to more elaborate artificial vision and laser controls. Due to the nature of the welding process, by-products such as intense light, x-rays, smoke, heat, and molten metal become interfering factors in the weld zone and often result in failure to position the weld torch accurately. Such systems also may require a large amount maintenance.  
           [0006]    In view of the above, there is a need for welding guidance systems that are not sensitive to by-products within the weld environment and can accurately position a welding torch relative to the surface being welded.  
         SUMMARY OF INVENTION  
         [0007]    The present invention provides a method and guidance system for accurately positioning a welding torch relative to a desired weld path. The method and system are based on ultrasonic sensing of information relating to the distance between the welding torch and surfaces on one or more components being welded, and then using this information to move the welding torch to a specific point in space relative to the desired weld path.  
           [0008]    According to a first aspect of the invention, a welding system comprises a welding unit including a welding torch positioned in proximity to a weld path on a component for forming a weldment along the weld path. A first means is provided for moving the welding torch along the weld path, at least one ultrasonic sensing device is operatively mounted with the welding unit for ultrasonically sensing the proximity of a surface and producing a sensor output in proportion to the proximity of the surface, and a second means is provided for moving the welding torch relative to the weld path in directions normal to and transverse to the weld path. Finally, control means is provided for receiving the sensor output from the ultrasonic sensing device, generating a control output based on the sensor output, and sending the control output to the second moving means to maintain the welding torch over the weld path and at a predetermined distance from the weld path during movement of the welding torch along the weld path by the first moving means.  
           [0009]    According to a second aspect of the invention, a method is provided for performing a welding operation. The method involves positioning a welding torch of a welding unit a predetermined distance from a weld path on a component by ultrasonically sensing the proximity of at least one surface, producing a sensor output in proportion to the proximity of the surface, generating a control output on the basis of the sensor output, and using the control output to move the welding torch relative to the weld path in directions normal to and transverse to the weld path so as to obtain the predetermined distance between the welding torch and the weld path. Thereafter, the welding torch is operated to form a weldment along the weld path while the welding torch is moved along the weld path, the surface is ultrasonically sensed to produce the sensor output, and the control output is generated to move the welding torch relative to the weld path in directions normal to and transverse to the weld path to maintain the predetermined distance between the welding torch and the weld path.  
           [0010]    A significant advantage of the invention is the ability to overcome problems encountered by other weld guidance systems. For example, ultrasonic sensing as employed by this invention is not affected by light, smoke, heat, or x-rays. Furthermore, ultrasonic sensing can be effective when positioned at distances from the surface being sensed so that the sensors are not located near the weld zone and thus are not affected by molten metal.  
           [0011]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    [0012]FIGS. 1 and 2 schematically represent welding systems in accordance with embodiments of the invention.  
         [0013]    [0013]FIG. 3 represents PLC programming instructions for carrying out a welding operation with systems of the type represented in FIGS. 1 and 2. 
     
    
     DETAILED DESCRIPTION  
       [0014]    A welding system  10  is illustrated in FIG. 1 as including a welding torch  18  whose position relative to a weld path  12  requires precise control to ensure the quality of the resulting weldment. The weld path  12  is depicted as being defined by a joint between a pair of workpieces  14  and  16  oriented perpendicular to each other, though the invention is not limited to such a configuration. The position of the torch  18  relative to the weld path  12  is controlled by a guidance system that comprises linear actuators  20  physically coupled to the torch  18  for moving the torch  18  relative to the weld path  12 , including movement toward and away from the path  12  and movement transverse to the path  12 . The guidance system further includes motion controllers  22  for controlling the actuators  20 , ultrasonic sensors  24  for sensing the proximity of surfaces of the workpieces  14  and  16 , a programmable logic controller (PLC)  26  for communicating with the motion controllers  22  and ultrasonic sensors  24 , and a data input device  28  through which instructions are inputted to the PLC  26 .  
         [0015]    As known, the operating principle of the ultrasonic sensors  24  is to direct a beam of sound at the targeted surfaces of the workpieces  14  and  16 . Sound waves are reflected by the surfaces back to receivers on the sensors  24 , and the distance the waves have traveled (and therefore the distance between sensor  24  and workpiece  14  or  16 ) is determined by the amount of time that was needed to travel to and from the surfaces. Each sensor  24  produces an analog output signal proportional to the sensed distance. The output signal may be either a voltage or current, and is delivered as an input to the PLC  26 . Various ultrasonic sensing devices could be used as the sensors  24  of this invention, a notable example of which is models UB500-18GM75-I-V1 and UB500-18GM75-U-V1 available from PepperI+Fuchs, Inc., of Twinsburg, Ohio. These sensors having sensing ranges of about 127 to 235 mm, allowing the sensors  24  to be placed well away from the workpieces  24 . The sensors  24  are shown as also being located some distance from the weld path  12 . This distance can vary depending on the geometry of the workpieces  14  and  16 . For example, the planarity of the workpieces  14  and  16  shown in FIG. 1 will determine in part how close the sensors  24  must be located to the weld path  12  in order to accurately indicate how far the torch  18  is from the weld path  12 .  
         [0016]    The actuators  20  are individually associated with the ultrasonic sensors  24  such that the operation of a given actuator  20  is based on the output of a given sensor  24 . In FIG. 1, the horizontally-oriented (lower) actuator  20  and the horizontally-oriented sensor  24  (adjacent workpiece  14 ) are associated, as are the vertically-oriented (upper) actuator  20  and the horizontally-oriented sensor  24  (adjacent workpiece  16 ). The sensors  24  are physically connected to the welding torch  18  (e.g., via a frame schematically represented at  17  in FIG. 1), such that movement of the torch  18  by the actuators  20  causes essentially an identical amount of movement of the sensors  24 . For example, downward movement of the torch  18  a vertical distance of one centimeter causes the vertically-oriented sensor  24  to move closer to the workpiece  16  a distance of one centimeter. In practice, series TMD100 actuators available from Duff-Norton have been found to perform well as the actuators  20  for the guidance system represented in FIG. 1. Suitable motion controllers  22  for the actuators  20  are model KBMG-212D available from KB Electronics, Inc.  
         [0017]    The PLC  26  converts the analog signals of the sensors  24  to a digital value within a range, e.g., between 0 and 32000 counts. A suitable PLC is a model MiniOCS or MiniRCS available from GE Fanuc. The data input device  28  is used to input a targeted digital value to the PLC  26  corresponding to the desired distance between the torch  18  and the weld path  12 . The digital values based on the signals from the sensors  24  are then compared to the targeted digital value corresponding to the desired torch distance. A mathematical algorithm is then performed by the PLC  26  to generate control outputs that are delivered to the motion controllers  22  for appropriately operating the actuators  20  to move the torch  18  horizontally and/or vertically as necessary to obtain the desired torch distance. For example, assuming the desired torch distance corresponds to a count value of 14000 for one of the sensors  24  and conversion of the analog signal from that sensor  24  yields a digital value of 15000 counts within the 0 to 32000 count range set by the PLC  26 , a comparison of the sensed count value and the targeted count value would evidence that the position of the sensor  24  is 1000 counts greater than the targeted count. To reposition the sensor  24  so that its position (and therefore its sensed count value) coincides with the targeted count value, the PLC  26  produces a control output (e.g., an ac or dc signal) that instructs the motion controller  22  for the actuator  20  associated with the sensor  24  to move the weld torch  18  toward the weld path  12  a distance corresponding to 1000 counts. Once the torch  18  has moved the specified distance, the PLC  26  terminates the control output, the output of the sensor  24  is again converted and a second computation is performed to verify that the torch  18  is now positioned at the targeted distance from the weld path  12 .  
         [0018]    In the above manner, the PLC  26  is able to achieve and maintain the desired distance between the torch  18  and the weld path  12  by an interpolation algorithm. FIG. 3 represents PLC programming instructions for carrying out a welding operation with the welding system  10  represented in FIG. 1. At line  15  of the instructions, a target torch distance is entered and values for the horizontal and vertical axes are stored in registers R 0015  and R 0020 . Line  16  performs the necessary conversions to count values within the count range (0 to 32000). At lines  23  to  27 , the signal (converted from analog to a count value at line  16 ) from the sensor  24  sensing in the vertical axis is compared to the targeted count value for the vertical position of the torch  18 . If repositioning is required based on a greater than (GT_INT) or less than (LT_INT) result from the comparison, a command for vertical movement of the torch  18  is made at either line  24  or line  25 , respectively, and lowering or raising of the torch  18  is initiated at line  26  or  27 , respectively. Lines  30  through  35  perform essentially the identical operations for the horizontal axis as lines  23 - 27  for the vertical axis. Notably, the instructions provide for positioning of the torch  18  in one axis before positioning is performed in the other axis (or axes). Positioning of the torch  18  is by interpolation because the signals from the sensors  24  are received and compared to the targeted count value continuously during movement of the torch  18  in response to previous signal comparisons, so that the desired torch distance is obtained quickly with extreme accuracy.  
         [0019]    Another example of an application for a welding guidance system in accordance with this invention is represented in FIG. 2, which is a top view of two plates  114  and  116  to be welded together with a torch  118  positioned directly above a weld path  112 . In FIG. 2, the plates  114  and  116  lie in the same plane and the weld path  112  is defined by a flat butt fillet joint formed by and between edges of the plates  114  and  116 . As such, the edges of the plates  114  and  116  facing the torch  118  and forming the weld path  112  are beveled. The torch  118  is part of a gantry-type welding system  110  in which the torch  118  is mounted to a carriage  117  (three carriages  117  are represented in FIG. 2 to illustrate how the position of the torch  118  adapts as the carriage  117  travels along the weld path  112 ). The carriage  117  is mounted on a track  119  attached to one of the plates  116  alongside the weld path  112 . The torch  118  and an ultrasonic sensor  124  are supported with an actuator arm  120  extending from the carriage  117 , so that the actuator arm  120  is operable to position both the torch  118  and sensor  124  directly over the weld path  112 .  
         [0020]    In FIG. 1, the single sensor  124  senses the distance between the torch  118  and weld path  112  as well as the location of the weld path  112  in the direction transverse to the direction of torch travel. To perform the latter, the sensor  124  locates the weld path  112  as a result of the beveled edges of the plates  114  and  116 . The output of the sensor  124  is operated on by a controller (not shown) which, in accordance with the previous embodiment, generates a control output by which the actuator arm  120  is operated to position and thereafter maintain the position of the torch  118  directly above the weld path  112 . When the guidance system is enabled and welding started, the carriage  117  travels the length of the track  119 , causing the torch  118  to travel the length of the weld path  112 . In FIG. 2, the variation in the position of the weld path  112  relative to the track  119  is recognized by the guidance system as a change in the output of the sensor  124 , which as previously noted occurs because of the shape of the beveled joint that forms the weld path  112 .  
         [0021]    As an optional feature of the invention, the welding system  110  represented in FIG. 2 includes an additional ultrasonic sensor  134  mounted to the carriage  117  for ultrasonically sensing the position of the carriage  117  along the weld path  112 , e.g., by directing sound waves toward one or more surfaces (not shown) along the length of the track  119  or at either end of the track  119 . As with the sensor  124 , the additional sensor  134  produces a sensor output in proportion to the position of the carriage  117 , and the controller receives the sensor output from the sensor  134  and generates a second control output by which movement of the carriage  117  along the track  119  can be controlled.  
         [0022]    In welds longer than about twelve inches (about 30 cm), distortion of the material being welded from heat produced in the process of welding can result in poor quality welds. The ultrasonic tracking capability provided by the sensors  24  and  124  of this invention enables the position of the torches  18  and  118  to be maintained at small tolerances (e.g., about 0.010 inch (about 0.24 mm)) from their weld paths  12  and  112  at all times during the welding operation, reducing variations in temperature that lead to distortion.  
         [0023]    While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.