Method for tracking joints and edges for welding operations

Details Method for tracking joints and edges for welding operations Method for tracking joints and edges for welding operations

1999-05-24

2001-06-19

Cuchlinski, Jr., William A. (Department: 3661)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Robot control

C700S246000, C700S248000, C700S249000, C700S251000, C700S253000, C700S254000, C700S259000, C700S260000, C318S568140, C219S130400

Reexamination Certificate

active

06249718

ABSTRACT:

TECHNICAL FIELD
This invention relates to automated (robot) welding systems and, more particularly, is for methods for simultaneously operating robot welders on the same workpiece so as to avoid collisions between the robot welders, for automatically determining how a weld is to be performed, for automatically keeping the weld on the desired path, and for automatically selecting a welding program based upon the particulars of the components.
BACKGROUND OF THE INVENTION
Robot welders are automated welding machines which operate under the control of a microprocessor. They are being used in more and more applications because they provide several advantages with respect to human welders. Some, but not all, of the advantages are that they do not need to take breaks, they can tolerate adverse environmental conditions, they can work in very tight areas, and they yield a consistent, quality weld.
However, a robot, like a human, must be told what to weld. A program for a robot welder can be written which specifies the exact spot or line that must be welded. Typically, an operator specifies the starting point of a weld and the length of the weld or the ending point of the weld. This information is given in precise coordinates, for example, start at point x=1.05 inches, y=1.77 inches, and z=0.0 inches and end at x=2.00 inches, y=2.00 inches, and z=0.5 inches. In this case the robot will weld on a seam defined by a line between the given starting and ending points. This procedure is simple and straightforward, but is very time consuming because the operator must determine the starting and ending points of the weld. This procedure is also prone to errors because the operator must manually enter these starting and ending points into the program and the operator may make errors in measuring the points or in entering the points into the program. Further, some points may be difficult or impossible for the operator to measure manually.
The robots have sensing devices, often referred to as touch sensors, which indicate that contact has been made with an object. In welding environments, the workpiece is typically a conductive material, such as iron or steel. Therefore, in these environments, the touch sensor is often the tip of the welding torch, or the welding wire, and the robot senses when the tip or wire has reached the workpiece by simply monitoring the output voltage or current at the tip or wire. When contact is made, the output voltage will drop and the output current will increase. The operator can therefore operate the robot to determine the coordinates of the starting and ending points of the weld, and then enter these coordinates into a welding program. This procedure is simple and straightforward, but is also very time consuming and prone to errors because, even though the robot makes the measurements precisely, the operator may make errors in entering the measurements.
Thus, each weld requires a substantial amount of attention by the operator. In some cases, the amount of time required to determine the coordinates of the weld and enter the points into the welding program can easily exceed the amount of time required for the robot to perform the weld. Also, because of human measurement and data entry, errors are quite possible. Therefore, there is a need for a method of operating a robot welder so as to reduce or eliminate the amount of attention required from the operator.
Further, once a welding program is written it will generally be useful for only one specified weld. That is, as long as the coordinates of the welds are identical (same starting and ending points), then the robot will faithfully repeat the weld. This is most useful on assembly lines where the robot performs the same welding operation on each workpiece which comes down the assembly line. However, if the coordinates of the welds are not identical then a different program is required. This may occur where one robot is required to make several welds on the same workpiece. Thus, the amount of memory required to store the several programs necessary for multiple welding operations can be substantial or even beyond the capability of the robot.
Robot welders, although useful, do only what the programmer instructs them to do. This is of little concern if there is only one robot performing a welding operation because, other than hitting the workpiece, there is little danger of the robot hitting anything. However, speed of assembly is frequently a customer requirement so it would be beneficial if two or more robots could be assigned to weld on the same workpiece at the same time. The problem with two or more robot welders is that they may try to occupy the same space at the same time. Two humans assigned to perform conflicting welding operations may be able to discuss and resolve the problem among themselves. They may also contact a supervisor to determine whose work will be done first. However, robots do not have the capability to recognize that a problem exists, much less discuss and resolve the problem or seek the guidance of a superior authority. Therefore, each robot follows its own program, mindless of the presence of any other robot. The result at some point, with some programs, will be a collision. The resulting damage to the robots and possibly even the workpiece can be extensive and expensive, and can cause serious delays.
It is difficult, if not impossible, to know in advance of the performance of an operation whether or not there will be a collision between the two robots. When programming a robot to perform a welding operation, the operator may specify a series of points to which the robot must move, with or without specifying exactly how the robot will get there. However, the microprocessor which controls the robot has its own set of manufacturer-installed operating instructions as to how the robot should operate. This problem is compounded by the fact that a robot typically has numerous degrees of freedom. For example, the Panasonic industrial robot model AW-010A has 6 degrees of freedom (rotation, upper arm, front arm, rotating wrist, bending wrist, twisting wrist), and two directions of linear motion (forward-backward, up-down). The particular moves generated by these operating instructions are frequently not the same moves, or in the same sequence, that the operator would have thought to use to accomplish the same result. Therefore, even if the operator specifies that a robot is to go from one point to another point, the operator has no information on what motions the robot may implement to do so. Further, the operator may have to particularly specify parts of the sequence to get the welding torch in the right position and at the right angle while avoiding parts of the workpiece, for example, an overhanging flange from a T-beam.
This problem is made worse if one attempts to simultaneously operate more than one robot on the same workpiece. None of the robots knows, or has the capability to know, where the other robots are located, what the other robots are doing, what the other robots are preparing to do, or how the other robots are going to accomplish whatever their next actions may be. Therefore, in the past, the only certain ways to avoid a collision were to put only a single robot on a workpiece, or to space the robots far enough apart to where it was impossible for one robot to intrude into the operating area of another robot. However, a single robot provides for slow operation. Further, in some situations, the small size of the workpiece may prevent spacing the robots such that they cannot hit each other. Also, operating space is like any other resource; it has a value and should be conserved whenever possible. There is therefore a need for a method which provides for simultaneous operation of the robots in close proximity to each other while preventing the collisions which, with the current state of the art, are probable.
A sophisticated spatial analysis computer program could be used to generate a computer model of each robot and the workpiece and perform a m

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