Torch Alignment Verification Method and Apparatus

1. Field of the Invention

The present invention relates generally to robotic welding, and more specifically to torch alignment verification to assure quality welds in an automated welding process.

2. Related Art

Robotic welding techniques require relatively precise location of the welding torch relative to a workpiece. Typically a welding wire is fed through a welding tube, and any misalignment of the wire tip with respect to the workpiece can result in an inferior weld. If the wire tip is offset more than a very small fraction of an inch from the desired location in the path, the resulting weld can be uneven and weak since weld leg length and/or penetration are adversely affected by the offset. There are numerous sources of torch and wire tip misalignment (including non-rigid torch mounts, multiple articulation points on the robot arm, and deflection caused by heat in the welding tube itself), and less than optimum welds caused by torch and/or tip misalignment are not uncommon.

Several approaches have been utilized to deal with the problem of misalignment, but most of these have been relatively time-consuming or expensive or have not provided an accurate indication of wire tip alignment. One method in use involves clamping the robot wrist to a fixture and then determining if the tip is at a preselected location relative to the fixture. This and similar types of clamping methods are time-consuming and can usually detect only torch position variations resulting from problems arising between the clamped area and the tip. Variations caused by a problem elsewhere in the system may go undetected. Other methods, such as touch sensing utilizing a voltage source and contacting a specific area at the workstation, do not provide an indication of tip location which is sufficiently accurate to avoid poor welds due to wire tip misalignment.

It is therefore an object of the present invention to provide an improved torch alignment verification method and apparatus. It is another object to provide such a method and apparatus which overcome the aforementioned problems.

It is a further object of the present invention to provide a torch alignment verification method and apparatus for a robotic welding workstation which utilize simple and inexpensive construction and operation and yet which very reliably determine if torch position is acceptable for an optimum weld. It is yet a further object to provide such a method and apparatus which do not require a portion of the robot to be clamped during the position determination and wherein the determination is accomplished quickly with the robot in its free, normally operating state.

It is another object of the present invention to provide a torch alignment verification method and apparatus which can accurately detect an improperly located weld wire tip. It is a further object to provide such a method and apparatus which advantageously utilize the free movement of a robotic torch in a predetermined path before an automatic welding operation commences or continues.

A method and apparatus for checking the alignment of a torch on a robotic welder to assure that the torch is in the proper position before an automatic welding operation commences or continues include an alignment verification device having a plunger with a contact area of relatively small diameter positioned at the workstation in the path of the torch. The projecting wire tip of the torch is moved axially toward the plunger in a preselected path, and, if alignment is within the desired range, the tip will contact and depress the plunger. Upon depression of the plunger, a cam-operated limit switch is closed to send a signal to the robot that alignment is acceptable. If the torch is out of alignment for any reason, or if the welding wire is bent or otherwise misaligned, the wire will miss the contact area and the welding operation will cease so that faulty welds are prevented.

The verification device is very simple and inexpensive in construction and operation, and requires relatively little time to make a position determination. A more accurate position determination is made with the robot in its normal operating mode than if a portion of the robot is clamped or otherwise inhibited in movement. Since the wire tip is the element actually being detected, the method and apparatus is more reliable than those utilizing other parts of the torch or robot arm in the detection scheme. The range of acceptable alignment is easily set by selecting the size of the contact area on the plunger.

These and other objects, features and advantages of the present invention will become apparent to one skilled in the art upon reading the following detailed description in view of the drawings.

FIG. 1 is a perspective view of a robotic welding workstation including the torch verification device.

FIG. 2 is a enlarged side view, partially in section, of the torch verification device shown in FIG. 1.

Referring now to FIG. 1, therein is shown a robotic welding workstation 10 having a robot 12 and a workpiece support 14 fixed to a stationary member 16. A torch alignment verification device 18 is fixed to the member 16 by a tubular support 20.

The robot 12 is of conventional construction and includes selectively positionable arm structure indicated generally at 22 and supporting a torch 24 on a break-away mount. The mount 26 is connected to the arm structure 22 through wrist member 28. The robot 12 is connected to a conventional robotic controller (not shown) programmed to move the torch 24 through a precise path relative to the workpiece support 14 for automatically welding a workpiece such as shown at 32 in FIG. 1. In addition, the controller also moves the torch 24 in a path adjacent the torch alignment verification device 18 at preselected times before and/or during a weld cycle as will be discussed in detail below.

The torch 24 includes a welding tube 34 through which passes a welding wire 36 fed from a reel (not shown). The wire 36 includes a wire tip 38 projecting through a contact tip within a nozzle 42. The position of the wire tip 38 relative to the workpiece 32 is critical during welding to assure a strong, uniform weld. If the wire 36 is bent so that the tip is offset (see 38a of FIG. 2), or if the torch 24 is out of alignment, a poor weld will result.

To check for proper weld tip alignment, the torch alignment verification device 18 is supported in the programmed path of the torch 24 and is utilized to check tip position at a preselected portion of the weld cycle. If the tip position is within a preselected range of positions as determined by the device 18, the weld cycle will continue. However, if proper tip position is not sensed by the device 18 the weld cycle will be halted so that the problem can be corrected before welding is again commenced.

The torch alignment verification device 18 includes a tapered pin or plunger 50 (FIG. 2) having a shank slidably received within a vertical bore 51 in a circular block 52 fixed to the support 22 via a leg 53. A disk-shaped member 54 is secured near the upper end of the pin 50 and lies in a plane perpendicular to the axis of the pin. Projecting axially upwardly from the top of the pin is contact pin 56 having a flat test area or contact area 58 which lies in a plane perpendicular to the axis of the pin 50 and preferably is circular in configuration with diameter on the order of several times the diameter of the welding wire 36. A protective inverted cup-shaped cover 62 with a central aperture is received over the pin 50 directly below the disk-shaped member 54.

The device 18 also includes a coil spring 66 encircling the pin 50 between the cover 62 and the top of the block 52 to support the pin in a normal upwardly directed position as shown in FIG. 2 wherein the lower, tapered end is generally aligned with the bottom of the block 52. A limit switch 72 is fixed to the leg 53 below the pin 50 and includes a roller-type of actuator 74 aligned with the tapered portion so that when the pin 50 is urged downwardly against the bias of the spring 66, the limit switch is actuated to provide an alignment verification signal to the robot 12 via lines 78.

During one portion of the weld cycle, preferably before a weld commences or periodically during the welding operation, the robot 12 moves the torch to a position directly over verification device 18 (FIG. 1) such that the end of the wire 36 is in axial alignment with the contact pin 54 (provided, however, that the wire is not bent as shown at 38a in FIG. 2 or the torch 24 is not out of position for any reason). The robot 12 then moves the torch 24 downwardly in the direction parallel to the axis of the contact pin 54 such that the wire tip 38 contacts the test area 58 on the top of the pin 56 to force the tapered pin 50 downwardly against the bias of the spring 66. The pin 50 moves from the position shown in FIG. 2 to a depressed position wherein the actuator 74 is contacted by the tapered end to close the limit switch 72 and thereby provide the alignment verification signal to the robot 12. After the robot receives the verification signal indicating proper welding wire tip positioning, the weld cycle is continued and the pin 50 is returned by the spring 66 to the position shown in FIG. 2.

If for any reason the wire tip 38 is not properly positioned as, for example, would happen if the wire were bent (see 38a of FIG. 2) or the torch were out of alignment, the tip 38 would miss the relatively small contact area 58 on the pin 56 as the torch is moved downwardly in the direction of the axis of the pin 50. Preferably, the contact pin and test area 58 are circular in configuration and have diameters on the order of 0.13 inch. The circular configuration of the test area 58, which is centered at the ideal wire tip location and lies in a plane perpendicular to the torch path, assures that a uniform range of misalignments will be reliably detectible. The sensitivity of the verification device 18 may be modified easily by changing the size of the test are 56, preferably by replacing the contact pin 58 with another pin of a different diameter. In operations where weld location is not as critical, the diameter can be increased to increase the range of permissible tip deviation. The diameter can be decreased to reduce the permissible range of deviation in applications where a very precise weld location is critical.

Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.