Removing Aircraft Paint Using Laser-Equipped Robots
Hexavalent chromium is not a term frequently associated with engineers. Residents of Hinkley, California, however, know all about it. So do workers tasked with the labor-intensive process of depainting aircraft.
Hexavalent chromium is a chemical compound used in electroplating, stainless steel production, leather tanning, textile manufacturing and wood preservation. The United States is one of the world's leading producers of chromium compounds, according to the National Toxicology Program.
Hinkley residents became familiar with hexavalent chromium in the 1990s. Pacific Gas and Electric Company (PG&E) used the compound in a compressor station for natural-gas transmission properties. A law clerk, Erin Brockovich, looked into illnesses in the area related to hexavalent chromium. After three years of litigation, PG&E settled a class action lawsuit for $333 million, the largest in American history at the time. Brockovich's story later became an Oscar-winning film.
Hexavalent chromium compounds have been known to cause lung cancer in humans when inhaled. Exposure also occurs by ingesting it in food or water, or direct contact with skin. Depainting aircraft produces a huge amount of hexavalent chromium.
Reducing risk of exposure was among the primary reasons Titan Robotics developed an automated solution for the U.S. Air Force to depaint military aircraft. The process includes two robots that use continuous-wave lasers to strip paint from the aircraft. The process is more efficient and safer than previous methods, which required workers to work in chemically hazardous environments.
“You had the human factors of a person working in a terrible environment for a long period of time,’' said Alex Klinger, a program engineer at Titan Robotics. “Then there was hazardous waste that had to be disposed of. With the laser processes, we take the people out of the hazardous environment, and we also contain the hazardous waste.”
Stripping Paint with Lasers
Automation infiltrates many business units, but automated laser depainting of aircraft is a new technique. The process includes robots that use lasers to strip paint from the aircraft. Titan can configure systems with mobile bases, rails, and fixed position robots. The rail-based systems allow the robots to reach very long parts of the aircraft. The rail system is preferred for off-aircraft components, while mobile base systems are used for full aircraft systems where the robots drive themselves around the aircraft.
“Rail systems operate in a manor where we can roll the part in, right next to the rail, and as long as the robot can reach it, it will figure out where that part is, how to go across it, and how to take the paint off with the high-powered lasers,’' Klinger said.
In two-robot full aircraft systems, one robot is positioned on each side of the aircraft, and LiDAR sensors — sometimes called 3-D laser scanning — convey detailed information about the airplane part under scrutiny. The robot will carefully apply the proper amount of laser pressure to remove the paint. The robots start with a rough outline of the plane, and then with the LiDAR sensors map the three-dimensional topography of the plane. LiDAR, which stands for light detection and arranging, is widely used in the autonomous vehicle market. While similar to radar, LiDAR uses laser light instead of bouncing radio waves to record measurements.
The robots are monitored by workers in a control room to ensure the process goes smoothly. Workers are not exposed to the chemicals generated by depainting. “It's really about safety,’' Klinger said. “They're really just being the supervisor of the system, making sure it's doing what it's supposed to be doing.”
Titan Robotics works in software development, unlike a traditional robotic integrator. Titan's robotic systems use the latest sensor technologies to analyze complex surfaces for data collection. Applications include 3-D scanning, non-contact thickness measurement and other inspection tasks.
Automated laser paint removal can be applied to any surface, regardless of complexity. The process also allows for the adaption to other surface treatments such as painting and sanding.
“We add more complexity and software control on to typical development of automated solutions,’' Klinger said. “With our software, the robots figure out where the plane is, how much paint is on the plane, what type of paint, and how to burn the paint off. And then once they know what they need to do, we plan how to move the robot over the surface to do that.”
The process needs to occur with extreme care. The planes are quite costly — one fighter jet can cost more than $130 million — and retaining the structural integrity of the plane is of foremost concern. Robots need to carefully monitor themselves during the laser removal process to ensure they do not damage the plane. Depainting too aggressively could lead to disastrous consequences when the plane returns to military duty.
“There is a tremendous amount of rigor on our side for our control systems and our robotics in making sure that we're not hitting the aircraft, and making sure that our laser processes are not damaging the aircraft. Our control system has a ton of protections in place for that, and it is all based on fundamental research that was done for the past 35 years. Every time we bring in a new aircraft or material, you go through the process again to learn the limits that we have to meet.”
Among the most distinctive features of the automated assemblies are cable carriers that contain an assortment of connections. Electrical cables, hoses, laser fibers and utilities are among the connections in the carrier. Manufactured by igus®, the triflex® system holds up to 16 cables that are used by the robots to remove paint from the plane.
“It is all about delivering utilities to the point of use,’' Klinger said. “It is a laser depainting system. But we also have systems that do sanding, machining, inspection, and x-rays. All of those have the exact same problem, which is getting those utilities to the point of use. It's important that we know where those cables are, know that they are protected, and not worry about them getting snagged on something.”
The primary advantage of triflex® is the capability to handle the weight and protect the cables. “The weight and the volume are substantial,’' Klinger said. “I think it's 12-to-16 distinct cables and they can be held very nicely. For us, it's all about having confidence in those cables. We know the product will contain them, will limit how they bend and it will hold them exactly where we want them to for the entire time.”
The other advantage of triflex® is the range of motion. “It can hold the cables, but it also has a three-degree freedom of movement,’' Klinger said. “A lot of cable chains move in a two-dimensional space, not a three-dimensional space. When you get to complex motions, where a robot is working in three-dimensional space, we need a flexible link that can contain the cables while the robot operates with six degrees of freedom. Triflex® can do that.”
Nasty and Noxious
An alternative to the automated solution for depainting planes, called plastic media blasting, is harmful in a variety of ways. The process includes shooting plastic beads out of hoses, similar to sandblasting, and the beads strip the paint off of the surface. Hand-sanding is also an alternative.
In plastic media blasting, workers wear protection, but the process generates thousands of pounds of hazardous waste. The plastic pellets mix with materials coming off the plane's surface, which include hexavalent chrome and cadmium, another known carcinogen.
“You have the human factor of a person in a terrible environment for a very long time,’' Klinger said. “And the hazardous waste at the end of the process is bad. In some instances, people were hand sanding the entire surface of the aircraft on their hands and knees. What we're able to do with the laser process is take the people out of the environment, away from manual labor and put them in an air-conditioned control room. It really is a much safer and healthier process.”
Depainting is just one process that can be accomplished with the automated solution. Klinger envisions a wide range of tasks which could be beneficial with similar automation using robotics.
“The robotics is the same, the software is the same,’' Klinger said. “We can remove the laser. Right now, we're taking on non-laser projects like sanding. We just changed the tool on the end of the robot from a laser to a sanding head and all of the sensing and control methodologies are still available and used. There's a lot of things that we can do with this system.”
This article was written by Thomas Renner, Freelance Technical Author. For more information, visit here .