Vision, Robot Ease Antenna Fabrication

PC-based vision and robot automation control has allowed Lockheed Martin to solve problems in fabricating Aegis naval missile system radars while providing the company with a flexible architecture for assembling other antennas.

The Aegis system uses 12- x 12-ft aluminum plates to form the phased-array radar antennas placed on the four corners of a ship's superstructure. Under Lockheed Martin's previous fabrication procedure, mounting the 4,350, 1- x 3-inch ceramic antenna windows on each plate required excess RTV adhesive to hold them in their carriers within an assembly robot. This allowed retaining the windows in the carrier when the robot flipped the container prior to inserting them in their mounting openings. Once the robot centered on a mounting hole using a mechanical gauge, pins pushed each window out of the carrier into the array.

But some extra adhesive would flow onto the plate surface and had to be manually cleaned up. In addition, the carriers that were returned to the mounting station on the shop floor needed to be cleaned. Aside from the mess, if adhesive dripped onto an antenna phaser within a window opening, the phaser had to be replaced to the tune of $200,000.

To smooth the process, Lockheed Martin turned to Bristol, PA-based Advent Design (www.adventdesign.com) for its automation expertise. Bill Chesterson, Advent director of Manufacturing Automation, says his engineering team initially looked at a system that involved preprogramming the location in the plate for each window as well as the cover button for fasteners previously installed manually. Instead the engineers opted to use vision processing to scan for locations and install the proper part. "Not having to pre-configure for windows or buttons simplified the algorithms," he notes.

Eyes and arm

Key elements for the final system design are twin cameras in a Cognex (www.cognex.com) vision system and a Stäubli (www.staubli.com/web/robot/division.nsf) robot arm, says Chesterson. One camera is on the carriage that carries the window/button placement robot along a 14-ft track above the antenna plate and the other camera is on the robot's end effector. The cart is indexed in 7.2-inch steps to its initial position over the array. The camera in the carriage then checks: If a part has been successfully picked up from the window or button storage cassette; if the part is cracked or chipped; its exact location and orientation on the effector suction gripper; and, once the adhesive is applied, checks the size and continuity of the RTV bead. The effector camera then fine tunes the position of the hole and aligns the arm for window or button insertion.

Chesterson specifically cites the Cognex Checkpoint® II on-board vision processor and camera driving software that allows using PC control to integrate this extensive vision data with robot motion. Point-and-click tools in the package simplify combining vision with automation devices to develop applications and operator interfaces without the need to program in languages such as C++.

As for effecting motion, the Stäubli arm provides the best match of system size and reach, says Chesterson. Specifically, the robot controller is small enough to be mounted on the carriage, thus encoder and drive cables do not have to be run in the festoon to the carriage. And the 6-axis arm has the reach to go far below the carriage base to the plate.

One important tradeoff is that the new system extends the cycle time to complete an antenna, from a total process time of three days to four, Chesterson adds. The previous operation saved time by separating window adhesive application from the robot that mounted the pre-glued windows. The Advent Design method sequentially uses the same robot arm to obtain the part, apply adhesive, and then mount the part to the antenna plate. "We could have used a second robot to apply the adhesive while the first was attaching the part," he says. "But that would have made the system more complex, with the part hand-off from one robot to the other a potential point for dropping the part."

But while the system may take longer to complete an Aegis antenna, the control and vision flexibility built in doesn't require the precise leveling and alignment of the antenna plate that the old system did before the operation began.

Currenly, Lockheed Martin is also applying the system to other Naval antennas with different geometries and materials.