Software passes the torch to robots.

Welding jobs are complex. However, manufacturers of welding robots and third-party developers are steadily improving the capabilities and range of software for programming robotic welding operations.

WELDING IS AN orchestration of angles, movements, materials, voltages, and temperatures.

The process of following a seam or hitting a spot is complicated by numerous factors, each of which has to be precisely calibrated. On the factory floor, the welding torch has been passed from human to robotic hands. Where craftsmen once assured quality welds by virtue of their skill, software systems have been developed for automated control of industrial welding.

Unlike other computer-aided manufacturing operations such as milling and lathing, welding has not benefited from a wealth of third-party software development. Most of the control software for robotic welders is written by the manufacturers of the robots themselves. In general, this software is proprietary and engineered for specific machines. "There are not many generic controllers and none that handle all welding functions," said David Yapp, section manager for fusion welding and automation at the Edison Welding Institute (EWI) in Columbus, Ohio. He indicated that while CAM systems have a number of standards for modeling and data transfer inherited from their CAD forebears, such as NURBS, IGES, and ACIS, there are no corresponding standards for computer-aided welding software.

However, Yapp pointed out that organizations such as EWI and The Welding Institute (TWI) in Cambridge, U.K., as well as some private companies, are developing software systems for automating welding functions that are applicable to a broad range of welding robots. Advanced Manufacturing Engineering Technologies (AMET) in Rexburg, Idaho, for example, has developed a welding data-acquisition and control system for graphical program creation and editing of high-end welding operations. Matra Datavision in Paris markets a welding control program in Europe based on its Euclid-IS design system.

MIXED SIGNALS

Traditional programming of robotic welders is done through teach pendants and control panels on the machines themselves. Users effectively pantomime a desired welding operation with the teach pendant, which transfers the coordinate point logic of the motions to the controller's memory. At key points of the process the user inputs data via the control panel to regulate the amperage of the discharge from the welder's electrodes and the rate at which wire wielding material feeds, in the case of seam welding. Spot welding is similar, except that precise points rather than contours are programmed. In all cases, the angle and travel speed of the welding gun have to be specified. Equipment for spatter cleaning, venting, and cooling also have to be programmed. All of these variables may change dynamically during the course of the procedure, and the programming has to reflect this. In addition, other devices in the welding work area, called a cell, have to be programmed with instructions controlling the movement of the parts to be welded through the cell.

These functions involve a mix of analog and digital signals. Reference voltages for the current source and wire feed units are typically controlled by analog signals while on/off commands, contour following and coordinate point data, gun angles, and supervisory data re typically expressed digitally. According to Chris Anderson, an applications developer with Motoman, Inc., a welding robot manufacturer in West Carrollton, Ohio, the complex mix of analog and digital signal processing and programming specific to each robot is one of the key factors discouraging generic software solutions.

Robot welders have payload capabilities ranging from a dozen to hundreds of pounds and repeatability performances meausuring from hundredths to hundred-thousands of an inch. Since every robot and work cell is different, it is difficult for a generic program to capture all of the variables required for each installation. "Calibrating the power sources, positions, and other data for a welding job requires considerable expertise on the part of the operator," Anderson said.

However, the robot vendors do use software to expand the capabilities of their products and make tasks easier for their users. Motoman, for example, designs its controllers with CRT screens and keypads that assist operators when entering the welding data that accompanies the teach point data. Motoman's control software also allows operators to program welding operations on a personal computer, which can be downloaded to robot controllers.

The AS controller language developed by Kawasaki Robotics of Farmingham Hills, Minn., for its A Controller systems permits welding tasks to be defined by user-written programs. The programs can be based on block/step diagrams or a programming language. Programming is show on the controller plasma display or the teach pendant display screen. The software features high-level functions such as off-line program editing and multitask execution during robot operation. A process-control program that does not initiate robot motion can be executed simultaneously with a robot-control program, allowing greater flexibility in altering processes in progress and controlling peripheral equipment in the work cell. Signal-monitoring programs can be written that suspend operations when defined parameters are violated.

PC PLATFORMS

Efforts are being made by some robot vendors to move programming functions away from the teach pendants and control panels onto personal computer platforms. The OffLine software package from ABB Robotics Inc. of New Berlin, Wis., a division of Asea Brown Boveri that manufactures welding robots, allows users to develop programs for its robots on a PC. Control programs are created and modified in OffLine using commands similar to those used by ABB teach pendants. Alternatively, CAD data can be downloaded to OffLine and converted to robot position data. ABB's TagPoint product is an interface between OffLine and AutoCad from Autodesk Inc. of Sausalito, Calif. Control programs can be transferred to the robot welder's controller by disk or sent across a network using ABB's Computer Link communications software.

In addition to its programming software, ABB Robotics has developed its PreView software for simulating the motion of ABB robots and peripheral equipment on Iris workstations from Silicon Graphics Inc. of Mountain View, Calif. PreView has much the same function as tool path verification packages available from CAM software vendors. Users can create simulations of welding work cells to identify potential problems and optimize layout before initiating a job. Models of robots and equipment are positioned on the screen along with their intended motions. the models can be built from scratch or imported from CAD systems as IGES or DXF files. The PreView software automatically performs collision-detection calculations. Simulation programs can be converted to robot-control programs and transferred to robot controllers by disk.

These programs were developed by manufacturers of welding robots to improve the performance and usability of their particular systems. AMET developed its Advent welding process-control and data-acquisition system to be used with a broad range of welding equipment, with emphasis on high-end operations. Advent is a Macintosh-based system for graphic creation and editing of control programs. In addition, the program has data-acquisition and real-time performance-monitoring capabilities for quality assurance.

The Advent programs can be used to control most power sources and can be integrated with most torches, fixtures, feeders, seam trackers, and video equipment. Arc and beam welding processes can be programmed using teach pendant-like commands or by drawing contours that are converted to coordinate point data. All control parameters, such as current, voltage, torch travel speed, and wire feed rate, can be programmed using mouse clicks in the Macintosh user interface. Remote-control functions permit operators to override any welding parameter using joystick and mouse-driven commands.

Vision systems and seam trackers can be used to ensure that a welding operation is proceeding according to defined parameters. Advent concepts data from these systems to perform real-time quality-assurance monitoring. A data summary facility gathers information on parameters and reports on them. In addition, screens can be saved to a VCR during a welding operation, providing a record of each individual weld as part of a quality-control and quality-assurance program.

While Advent is compatible with most of the equipment common to welding operations, AMET engineers and systems integrators are required to tailor the software for particular operating environments. According to Don Schwemmer, vice president of engineering at AMET, the variety of components and the individual nature of welding operations defy off-the-shelf software solutions.

The Robot-CP program developed by Sediscad, a company acquired by Matra, is intended as a modeling package that can simulate any robotic operation, including spot and continuous welding. Robot-CP uses the Euclid-IS modeling entities and database. Euclid surface and solid modeling techniques can be used to construct a simulation of the welding work cell. Trajectories containing geometric information on robot and peripheral equipment movement are ascribed to the model in order to animate it. Specific modules for handling spot and continuous welding can be used to model functions particular to those operations. The parts to be welded are Euclid-IS surface models.

Simulations can be used to perform interference-checking and collision-avoidance analyses. It is possible to download simulation files to robot controllers that can recieve standard file formats, such as IGES and European data-exchange standards.

A family of software products developed by EWI and its U.K. partners is designed to automate welding procedures. Unlike control programs, the EWI/TWI suite of packages tends to focus on aspects of the welding process other than putting torch to metal. Nevertheless, EWI's Yapp indicated that many functions required by welders can be handled in a standard way.

Quality assurance is one such area where welding engineering software can be used with nearly any robot environment. The EWI's Weldspec Plus package is a database program designed to store welding procedures for analysis or future use. The program can be used to trace welding procedures from specific jobs. Traces are made using a sketching tool for drawing the sequence of passes in a given weld preparation. These procedures can be recalled to avoid duplication and for producing welding procedure documentation.

A series of expert system--based products is available from EWI. The Preheat Plus programs calculates the preheat welding temperature to minimize the risk of hydrogen cracking. Weldgen generates welding procedure guidelines for commonly used materials. Fatiguecalc is a spreadsheet-based expert system that calculates fatigue life, permissible stress ranges, and allowable joint configurations for specific welding applications. Weldcrack Expert is a diagnostic system that contains rules for predicting weld cracks in various steels. The program covers hydrogen cracking, reheat cracking, lamellar tearing, solidification cracking, and liquidation cracking. The user supplies data pertaining to the appearance and location of the crack and process and materials details. Weldcrack Expert then advises the user when a particular category of crack is likely.

Although the difficulties in programming a robotic work cell have slowed the development of broadly applicable third-party software solutions, manufacturers of welding robots have actively been improving techniques for programming their controlles. Interfaces to CAD systems, communications links to desktop computers, and improved control panels and teaching pendant displays all have made it easier for operators to tell their robots what to do. Modeling software for simulating work cells and graphically creating programs that can be downloaded to robot controllers also have made welding jobs easier to orchestrate.