Robots: MORE CAPABLE, STILL FLEXIBLE

Today's robots possess a range of capabilities that can solve shop-floor manufacturing problems

Recent advances in technology now allow industrial robots to perform more, and different, applications than ever before. Some of these changeslike the advent of multiple robot control and application-specific robots, plus advances in vision-guided robot technology, connectivity enhancements, and improved laser seam-tracking and weld inspection-are available now for application in the production line.

Manufacturers looking for ways to streamline their processes and become lean invest in flexible automation that allows them to do more with the same equipment-while also using minimum floorspace. Multiple robot control is a trend that fits in well with these lean-manufacturing concepts.

Multiple robot control can be used in workcells dedicated to a single process, or in cells where the robots perform multiple processes. For example, due to built-in collision-avoidance features, multiple robot control enables up to four robots to simultaneously weld on a part, thereby reducing the number of stations or cells required for production.Multiple robot control is also useful in applications where one handling robot serves as a positioner to manipulate parts for as many as three processing robots. Any of the robots can also automatically exchange end-of-arm tooling to allow additional flexibility within the same workcell. For example, a handling robot can switch jigs or a processing robot can switch from gas metal arc welding (GMAW) to gas tungsten arc welding (GTAW) or from welding to grinding. The result is a truly flexible workcell capable of processing completely disparate parts with different manufacturing requirements.

Multiple robot control can lower overall fixture costs associated with a cell. To weld large automotive components, for example, a handling robot can present a pretacked assembly to two arc-welding robots for finish welding. The handling robot can serve as a highly flexible positioner, locating the part in the optimum position and orientation for the different welds. Instead of a fixed two-station cell with two sets of expensive, complex tooling to locate all the components, the multiple robot control cell requires only one set of complex tooling (for pre-tacking) and can use a simple holding fixture for the handling robot.

When manufacturing automotive exhaust systems, multiple robot control can leverage the coordination between a handling robot and a welding robot to process highly contoured parts that require continuous welds around compound angles. Such parts require a complex tacking fixture tended by a second arc-welding robot. Although this approach can also lower total fixture costs, the main benefit is that the handling robot can present the final welding robot with a pre-tacked part unencumbered by clamps, sensors, or brackets.

Benefits of multiple robot control with a single pendant include built-in automatic collision avoidance, greatly simplified programming structure, reduced integration cost, and ANSI/RIA R15.06-1999 safety-standard compliance. Besides lowering capital expense by potentially processing the parts with fewer workcells, additional cost reductions are realized by eliminating redundancy in I/O controls. The multiple-robot controller can control all of the I/O for the workcell-a single fieldbus connection rather than a connection for every robot manipulator.

Today's robot controllers support established fieldbus standards, such as DeviceNet and ProfiBus, and many also support emerging standards, such as EtherNet/IP. This off-the-shelf connectability improves reliability, promotes uptime, and permits a more seamless information infrastructure throughout the manufacturing facility. High-level supervisory controls, such as SCADA systems, can have ready access to any of the robots on the factory floor via TCP/IP protocol.

In many cases, the controls engineer has a single connection to the robot controller, and through it can access all I/O, logic, data structures, and files. The single connection means less hard-wired infrastructure for the line. In fact, the same is true for all services to a workcell that employs multiple robot control: for example, a single power drop for all the manipulators, or a single Ethernet drop for the workcell. This ease of installation is a side benefit to multiple robot control that should not be underestimated, because it represents a significant cost savings.

The advent of multiple robot control highlighted the problem of cable interference in arc-welding applications. With manufacturers now controlling up to four arc-welding robots-sometimes working in a space no larger than a mini-fridgecables created major interference, and also added complexity to the robot programming. Using generalpurpose robots in arc-welding applications presented shortcomings that needed to be addressed through a new approach.

Some robot manufacturers allowed the process to drive the manipulator design, and developed application-specific or purposedriven robots with built-in attributes that make them well-suited for arc welding. Changes include throughhole reducers, and an open-yoke design for the upper arm that allows the entire torch cable to be integrated and contained within the robot structure. Cable flop is eliminated, wire flip is greatly reduced, and interference between robots, torches, and fixtures is mitigated. New in-line torches allow access to welds that were previously not possible with offset torch mounts, and long welds and circumferential welding are no longer limited by wrist rotation restrictions.