Operating two or more robots from a single point of control
October 23, 2003
In many applications—such as large-component welding, press-tending lines, and multiprocess cells—running multiple robots from a single point of control assists in preventing collisions, simplifying the programming structure, and reducing integration cost. This approach also meets the American National Standards Institute/Robotic Institute of America (ANSI/RIA) R15.06-1999 safety standard.
Two robots can weld on the left side of a large vehicle frame, while two other robots weld on the right side, with all robot motion synchronized by a multiple-robot control.
Multiple-robot control can increase productivity in workcells in which two, three, or four robots perform the same process, such as welding, spot welding, press tending, assembly, or material handling. Adding extra robotic arms to a workcell can double, triple, or even quadruple productivity.
Robots 1 and 2 might weld on the left side of a large vehicle frame component, while Robots 3 and 4 weld on the right side, with all robot motion synchronized (see Figure 1).
In press tending, Robot 1 might take a part from an incoming conveyor and load Press A. Robot 2 then would unload Press A and load the part into Press B. Next, Robot 3 would unload Press B and load the part into Press C. Robot 4 then would unload Press C and either transfer the part to an outfeed conveyor or pass it to another multiple-robot cell.
In a flexible axle welding cell, bar codes indicate the axle model and determine which robot program to run.
When multiple robots work closely together in a small space, avoiding collisions becomes a major challenge. However, with a multiple-robot controller, one programmer creates the program for up to four robots, either via offline programming functions or using a programming pendant, a handheld control used to program robot paths. Programming is simplified and programming time is reduced through built-in collision avoidance software, editing features that allow cutting and pasting of program instructions, and cloning and mirror-image operations. Because there is only one controller for several robots, networks, personal computers, or external devices such as floppy disk drives are not needed for program transfer.
Multiple-robot control also is useful in robot cells with discrete robot models performing various process functions. For example, one large-payload material handling robot can serve as a flexible part positioner, manipulating a part through various stations for welding, spot welding, routing, grinding and sanding, inspection, or other processing. Robots also can hand off parts from one operation to another.
Multiprocess robot cells support lean manufacturing approaches because their flexibility allows manufacturers to produce more using the same capital equipment. For example, all truck axles are similar but have variations in mounting points and attachment brackets. One manufacturer uses a large material handling robot to position a pretacked axle for processing by two smaller welding robots. This flexible cell can handle any axle model without tooling changeover. Bar codes indicate the axle model and determine which robot program to run (see Figure 2).
|Figure 3A single point of control can run up to four different robot models with different features.|
Another manufacturer feeds axle blanks into a robot cell on a part positioner. A handling robot picks up brackets from a part magazine and positions them on the axle for processing by two welding robots. This allows brackets to be placed anywhere along the axle in any orientation without the use of part fixturing.
It now is possible to run up to four different robot models—each with different reaches, payloads, and number of axes (up to 36 axes total)—with a single point of control (see Figure 3).
A multiple-robot system can be configured by taking up to four standard robot controllers and connecting all of the servo amplifiers to a single processor board in one of the controllers. The other robots still have complete robot controllers with all required boards, cables, and pendants. However, only the servo controls and amplifiers in the Robot 2, 3, and 4 controllers are active.
Robots 2, 3, and 4 can be separated from a multiple-robot cell and redeployed as stand-alone units or in other multiple-robot cells. If necessary, unused boards and components can be used as spare parts for the Robot 1 multiple-robot controller.
A knowledgeable robotic systems integrator can evaluate an application and help to determine the number, type, and configuration of robots and controls that will meet the needs of a specific application. It is important to choose a robot vendor with experience and to make sure the robotic automation purchased has the necessary features to maximize the investment, including training and support.
Any automation system has critical components that can cause a production stoppage if they fail. A serious controller malfunction in a multiple-robot cell conceivably could shut down production in the entire cell, whereas partial production might be maintained in a cell with separate controllers. However, advanced controller software often can allow robots with mechanical problems to be detached so that production can continue. A robot can be repaired or replaced and resume its original programs when returned to service.
With multiple-robot controllers, initial teaching time can be longer in some cases because a single programmer is teaching versus having multiple programmers on individual robots. However, the ability to copy and edit programs between robots helps reduce the programming time for a single programmer, and safeguarding a workcell for multiple programmers adds complexity and cost. The single point of control and simpler programming structure of the multiple-robot controller also can help make systems easier to troubleshoot and maintain.