Depainting By Numbers
An SwRI-developed robotic system removes coatings from off-airframe components of military aircraft
By Dan H. Weissling
Applying and removing coatings on military aircraft has become increasingly complicated by the widespread adoption of airframes with lightweight, fragile composite materials alongside traditional aluminum alloys, as well as the use of complex curves to maximize performance and minimize radar reflection.
At the same time, military maintenance centers have sought to take advantage of the greater precision and lower costs offered by automated, robotic systems.
The challenge has been to create robotic depaint systems that apply blast media in a way that removes the coating quickly and efficiently, yet does not damage the structure beneath.
Over time, Southwest Research Institute (SwRI) has become a world leader in automated paint removal, beginning with a project completed in 1992 to remove paint from F-16 fighter aircraft at Hill AFB, Utah. The Robotic Paint Stripping Cell (RPSC) at Hill remains in steady use today. A concurrent system, called the Robotic Depaint System (RDS), remains in use at Robins AFB, Georgia, for the stripping of F-15 fighter aircraft. Although there have been attempts by others to develop automated, full-aircraft depainting systems, none has approached the success achieved with both the RPSC and the RDS.
Over the years, robots have moved from the laboratory to commercial applications in a variety of industries. At the same time, aircraft materials have moved toward thinner metal skins and lighter-weight composite materials. Environmental, safety and ergonomic requirements have gained importance. However, much of the depainting of aircraft and aircraft components is still being performed either through labor-intensive manual blasting or chemical stripping. The U.S. Air Force is working proactively to implement processes that are more cost-effective and that reduce the hazardous waste stream and prevent occupational injuries.
In 2007, SwRI was contracted by the URS Corporation, prime contractor for the
project, to develop a system for depainting off-airframe components at Hill AFB.
Many of the C-130, A-10 and F-16 aircraft are typically disassembled and
reworked as part of the overhaul process. The resulting off-airframe components,
consisting primarily of flight control surfaces such as wings and flaps, are
stripped using conventional, manual means with either plastic media blasting or
chemical stripping. Leveraging the experience gained from a similar system
developed for Robins AFB, known as ADSOC, a team of SwRI engineers began a new
system that was dubbed ACRES (Automated Coating Removal System).
The blast subsystem includes a self-contained blast booth that uses a negative-pressure ventilation system and an operator control room. As is typical with many blast systems, the used media is recirculated through a floor recovery system and routed through a series of separators that remove broken-down media as well as ferrous and non-ferrous contaminants, including paint residue. Approximately 5 percent to 10 percent of the media is broken down per cycle, requiring a small amount of replenishment. Both new and cleaned media are transported to pressurized blast pots and metered through blast hoses to the nozzles at the robot’s end-effector.
One major difference between the off-airframe ACRES and its full-aircraft predecessors is the robot type. The full-aircraft system’s robots are free-standing and supported by a track in the floor, and the robot essentially “walks” around the aircraft. By comparison, the ACRES robot is supported by an overhead gantry structure which allows its manipulator to move around the aircraft components from above. Also, in contrast to previous systems, the ACRES robot is a combination of custom and commercial off-the-shelf (COTS) technologies. A FANUC Robotics manipulator with five degrees of freedom is the primary workhorse. To gain full access to the wide variety of aircraft components ACRES would depaint, SwRI engineers designed an additional three degrees of freedom to move the entire FANUC manipulator in three-dimensional space. Overall, the ACRES structure is 36 feet long, 22 feet wide and 20 feet tall, creating a work envelope approximately 25.5 feet long, 16.5 feet wide and 12.5 feet tall. ACRES can accommodate any component that fits within that space.
The total weight of the movable portion of the robot is approximately 14,300 pounds. To reduce the motor size required to move such a large mass, steel counterweights weighing approximately 3,500 pounds each hang inside the four support legs of the gantry.
The gantry’s up-and-down motion is referred to as the Z-axis. Side-to-side movement in a direction normal to the aircraft component’s surface is the X-axis. The manipulator moves side-to-side along the Y-axis that is parallel with the long axis of the component being stripped. These motions are accomplished by means of a large rack-and-pinion drive system on each axis.
The robot end-effector comprises an array of three flat, or “fan,” nozzles that project the media at high velocity onto the aircraft substrate, stripping about a 9-inch swath. The end-effector also has a built-in camera that allows the operator in the control room to see the strip zone from the perspective of the end-effector.
ACRES also features a carefully designed user interface that comprises three elements. The first is an operator console with easy-to-use functionality, which serves as the primary user interface during paint-stripping operations. The operator can start, stop, jog, back up and control the speed of the robot’s motion program. The second interface consists of PC-supported video monitors that allow the operator to select the component to be stripped, troubleshoot and view error logs, and monitor the status of the blast system. The third interface is a “teach” pendant that allows the advanced operator to manually jog the robot and create path files for new components.
A major consideration in the ACRES design was safety. Multiple safety features were implemented to protect aircraft components and the operator from any potential harm resulting from hardware or software failures of the robot. In addition to non-backdriveable drive components, the design includes other safety features such as large disk brakes located on each of the four Z-axes, tilt sensors, proximity and through-beam sensors, hardware and software interlocks, a “whisker switch” hoop surrounding the robot end-effector, and permanently mounted over-travel hard stops on each of the X, Y and Z-axes.
Although ACRES can be programmed to depaint any component that fits within its large work envelope, the system will initially be pre-configured to depaint 27 off-airframe components, including the A-10’s rudders, elevators, inboard flaps, outboard flaps, decelerons, outer wings and inner wing. F-16 components include horizontal stabilizers, flaperon, leading edge and wings; C-130 components include ailerons, elevators, rudder and floor panels. Unlike previous systems that required the operator to “teach” reference points to locate each component in three-dimensional space, ACRES takes advantage of absolute positioning by using a universal component support chassis that will affix each component in a repeatable position.
The component support chassis is a large steel frame, approximately 27 feet long and weighing over 5,000 pounds, that shuttles the aircraft components into and out of the blast booth. It is pneumatically driven on steel rails that are embedded in the floor. The chassis is designed for utmost flexibility; components can be base-mounted, hung and even rotated. A rotation mechanism mounted on one end of the chassis is designed to support and rotate the large A-10 wing.
Preparation and handling
ACRES includes a staging area outside the blast booth where components are prepared for blasting. This work includes masking areas that are not to be stripped and sealing all critical seams against media ingress. Support stands, similar to those used on the chassis, are replicated in the prep area. Due to very limiting space constraints and the need to protect aircraft components from damage, SwRI engineers developed effective procedures and handling tools to transfer parts into and out of the staging area.
Applications, present and future
ACRES will enable the Air Force to depaint off-airframe components more efficiently and effectively. The system will provide better consistency of the depaint process, reduce manpower and cost, and remove the operator from a potentially unsafe work environment.
The system is designed to use a new blast media, known as GPX, which has several advantages over the commonly used acrylic plastic media. With its cornstarch-based chemical matrix, it is not only less aggressive for sensitive substrates such as composites and thin-skinned aluminum, but it is also biodegradable. This more environmentally friendly media will result in additional savings to the government due to a smaller hazardous waste stream.
The ACRES design team at SwRI recognized that new processes may be introduced in the future for stripping paint. A major effort is under way to apply laser technology for this application. The ACRES robot has sufficient accuracy and repeatability to be retrofitted with a laser end-effector should the process be validated. Other paint removal processes could be incorporated as well.
Installation of ACRES at Hill AFB occurred during the autumn of 2008. SwRI plans to continue providing operational, training and maintenance support for the system under contract to Hill AFB.
Published in the Winter 2008 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.