In aerospace, little things add up
Imaging is well known for applications in aerospace, from reconnaissance and remote sensing to weaponry and flight safety.
Vision system helps calibrate robotic drilling of critical aircraft components.
Imaging is well known for applications in aerospace, from reconnaissance and remote sensing to weaponry and flight safety. Less well known is the growing role of imaging-specifically machine vision-in manufacturing aerospace components and aircraft.
The F-22 Raptor is a stealth fighter aircraft that requires extreme precision at all stages of manufacture to maintain the lowest possible radar signature. One fastener protruding above the surface of the aircraft skin by only the thickness of a piece of paper could expose the craft to enemy radar. There are thousands of fasteners that attach the composite skin to the airframe, and every one needs to be perfect.
To illustrate the manufacturing challenge, consider the vertical stabilizers for the tail of the F-22, which are assembled by principal contractor Lockheed Martin at a production facility in Marietta, GA. Final assembly includes drilling 3459 holes, each of which must be countersunk to a specific depth within a few thousandths of an inch. As part of a research project to improve manufacturing performance, a custom gantry robot was designed and built by Applied International Motion (AIM; Verne, CA) to drill these holes. A critical role was played by machine-vision system integrator Delta Sigma (Kennesaw, GA), which developed the database that registers the vertical stabilizer to the robot and calculates the correct tool position and tool paths for each of the robot’s six axes.
Drilling and filling
The drilling gantry has an x-y travel of 12 x 12 ft, with 4 ft of z-axis travel. At the end of that positioner there is a drill head that has two rotary axes, plus quill and spindle axes capable of putting a 5/8-in.-diameter hole through titanium. Surrounding the drill bit is a spring-loaded probe, called a pressure foot, which includes a high-resolution encoder.
“The most challenging part of this project was getting the countersink depth,” says Dan DalColletto, vice president of engineering at AIM. Once the vertical stabilizer coordinate system is determined, getting on point and vector is relatively easy. He notes, “Unfortunately, the surface of the skin is somewhat ambiguous to this coordinate system. The skin surface tolerance is ±0.030 in., but we need to countersink the hole to an accuracy about 20 times better than that.”
The pressure foot is used to meet this challenge. The pressure foot measures the location of the skin surface just before the drill bit makes contact with the skin. These data are used to calibrate the drilling robot a few milliseconds before impact. The system works as long as the relative position between the skin and the probe, and the probe and the drill bit, are known-which is where the vision system comes in.
“Our tests show that the vision system can easily achieve sub-mil accuracy on these large cutting tools,” says Brett Haisty, vice president of engineering at Delta Sigma. “We are using a telecentric lens that gives us about a square inch of image area and a high-resolution smart camera with red LED lighting. The drill bits have the countersink about 0.9 in. up the shank, so we can drill and countersink in one motion. The most critical dimension is the pressure-foot face to the countersink. This has always been measured indirectly. Now we can measure it directly.”
By making direct measurements between the countersink and pressure foot with the vision system, performance and cost can be improved in several ways. The vision system removes the critical nature of the cutter in the tool holder. And reducing the tolerance on the tip-to-countersink specification of the drill bits may reduce their cost. Also, operators can determine that the drill diameter is correct in case the wrong cutter was loaded in the tool holder and can inspect for damage to the cutter, which can prevent a costly mistake.
“This technology looks very promising,” says Mark Bowen, the applications staff engineer at Lockheed Martin who heads the research project. “The vision drill-depth calibration project was initiated because a manual measure-and-adjust cycle was being repeated hundreds of times per day. It appears that we can take a task that takes one or two minutes each time and reduce it to just a few seconds. If we can do that and improve our accuracy at the same time, this project could be a tremendous benefit to the F-22 program.”
CONARD HOLTON is editor in chief of Vision Systems Design; e-mail: [email protected].