How to select a robotic welding cell. (Welding/Brazing/Soldering).

Although horror stories are still heard about improperly specified robotic welders sitting idle, more and more welding managers are finding that prepackaged robotic solutions are highly cost-effective.

Improved communication between robot product manufacturers and users has meant that even small job shops are now comfortable with robotics, making prepackaged robotic cells very popular with both large and small companies. More and more, manufacturers and customers are speaking the same language, and the result is increasing awareness of the productivity and cost-saving benefits of robotics.

David Erbe, manager of the Applications Development Laboratory at Panasonic Factory Automation (Franklin Park, IL), provides insight into the selection process. According to Erbe, close communication between supplier and customer will be a key influence on the way a specific machine will achieve the desired weld quality and productivity in a specific application or range of applications. Particularly at the outset, selecting a robotic cell means providing the robotic product supplier with complete and detailed application information.

The Starting Paint

According to Erbe, at the outset application engineers will seek a great degree of detail about the customer's welding operation, including objectives for the robotic operation and specifics such as desired cycle times, quality levels, and consumable costs, and any preferences for welding process, or specific types of power source.

The range of questions should be extensive to achieve an optimum selection. What welding process is currently used? Is it being used semi-automatically or automatically? What shielding gas and filler metal are in use? What material is being welded? How thick is it? What is the joint design? Can weld spatter be tolerated for the application?

Even the physical site where the machine will be located provides important input data for the application engineers. "There must be sufficient space for the new equipment," comments Erbe.

Budgeting realistically and knowing what can and cannot be expected of a robotic welding cell will help the robotic supplier not only to recommend an appropriate system, but also forecast the likely return on investment.

Critical Input

"Not all parts lend themselves to robotic welding," explained Erbe. "Large, complex, heavy weldments may not fit the robotic mold, so the most critical input is the actual part or parts to be welded," he continued. Erbe suggests that customers provide the robotics supplier with the specific parts that will be welded, or share the drawings for parts still on the drawing board. In addition, it is important to share information about how the parts to be welded were processed prior to reaching the weld station. Previous steps such as machining, forming or heat-treating, may influence the suitability of robotics for a specific application.

Beginning with the fullest amount of information, the application engineer can begin to choose the appropriate workcell configuration. Options such as stationary tables, rotating turntables, multi-axis positioners, or even Ferris wheel-type handling systems offer a wide range of choices to meet each customer's unique requirements.

The Workcell's Components

When specifying components, there is an advantage to choosing a totally integrated system, engineered and produced by a single manufacturer. Erbe explains, "The robot itself is the central element of the workcell. But other components -- such as the power supply, controller, sensing systems, positioner, safety features, and the mounting base -- are no less critical to the system's efficient operation. Choosing an integrated system -- with robot controller, welding power supply, power transformer, main-panel control and programmable logic controllers all manufactured by a single company -- is often a successful strategy for robotic workcells. Integrated systems are unlikely to suffer the performance and functionality compromises that are inevitable in welding systems assembled from separately designed components.

Features to Look For:

* Teach Pendant -- Workcells equipped with a teach pendant provide users speed and productivity advantages. For robots without a teach pendant, the operator, in order to make changes in various pieces of equipment, must move from the robot controller, then to the power supply, and to the weld fixture control. With a teach pendant, the operator can easily program and edit both present and future applications from one position. Wasted time and wasted movements are eliminated.

* Power Supply -- The power supply is an extremely important component in a robotic workcell. Power supplies with advanced waveform control and advanced programming features adjust the arc characteristics in a fraction of a millisecond to compensate by for routine idiosyncrasies experienced in arc welding, such as variations in fit-up or electrode extension, the presence of weld tacks or wire-sticking. This is achieved by automated ultra-high speed adjustment of the arc and metal transfer characteristics. These systems include software that offers different modes of arc starting.

Other automated functions to consider include tool centerpoint calibration and torch tip shifting, which permit the robot to reach the weld joint without changing the program; torch alignment, overlap and resume functions; and statistical process control monitoring of current and voltage. Automation even provides a function that checks to make sure that the contact tip is in working order and notifies the operator whenever the contact tip is in need of cleaning or replacement.

* Safety -- To assure safe access to the power supply and controls, workcells equipped with sliding or swinging doors that have safety interlocks provide the technician or the operator with safe entry. An option is a special door that permits access to the main controls without having to enter the workcell. Safety-conscious shop managers should also verify that an operator cannot access the PLC controls to defeat safety-oriented programming.

* Sensors -- Automatic sensors are often found in robotic workcells, for keeping the arc on target within the weld joint. Sensors can detect the need to readjust the arc location whenever the parts being welded are out of dimension. A sensing system will also decrease the amount of time taken to program the robot.

Popular sensing systems include touch sensing, through-the-arc sensing, and vision sensing. Using touch sensing, the robot controller touches the part with the tip of the welding wire as a means of loading the weld joint. Through-the-arc sensing tracks the weld joint by monitoring weld current on both sides of the joint. This is accomplished because this sensor operates in a weaving motion. It is most popular for large weldments where the weld speed is slow but the amount of filler metal deposited is high.

Vision sensing is the most sophisticated system. When used in a robotic workcell, a vision system consists of a vision controller, a camera and appropriate software. One such vision system oscillates a light beam back and forth in front of the weld pool. This system makes use of a photoelectric eye to detect the weld joint. From the data picked up, the vision controller decides where the joint position should be in relation to the tip of the welding wire. If needed, the controller can even instruct the robot how to compensate for some irregularity.

* Positioner -- A wide range of sizes and types of positioners is available. A simple application might call for two stationary tables, one for welding and the other for loading. A more complex job might call for a turntable with automated 1800 indexing. For larger weldments -- in applications such as rotation of the workpiece in a horizontal plane -- servo-controlled positioners can be installed to provide smooth movement of workpieces weighing as much as 1,100 lb. Another advanced type of positioner is the servo-controlled headstock-tailstock positioner, which is mounted on a turntable.

* Safety -- Look for built-in safety precautions, such as use of fencing, interlock gates, and weld screen barriers so that if the door or gate is opened while the workcell is in operation, the interlock will cause a full-system stop. Anyone in the area is protected from accidental arc flashing by screens. Safety-oriented sensors may also be used within the workcell, to permit operators to go about the loading and unloading of fixtures and positioners while the system is in operation.

Conclusion

Advancements in robot, power supply and digital control technology have made today's robotic workcell a productivity-improvement tool for both large and small companies. Current workcells are now cost-effectively delivering outstanding production rates within an efficient and safe atmosphere. Panasonic Factory Automation

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