Airplane Artists Aided by Advanced Motion Technology
Improvement of aircraft object modeling and better control technology permits the new aircraft painting system at Boeing to put operator cranes within 4 inches of the aircraft without risk of contacting it.
Ed Diehl and Scott VanDelinder, Concept Systems Inc. -- Control Engineering, 6/1/2009
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| The typical commercial airliner carries 800 pounds of paint, and designs like these are hand-applied. Source: Boeing |
Aircraft painting at Boeing’s Everett facility is a manually intensive operation performed by skilled artisans. Many of the paint schemes produced by the decorative painters at the facility are truly works of art. That art is now being facilitated by advanced motion technology that lets operator cranes reach within 4 inches of the aircraft without risk of contacting it.
The typical commercial airliner carries 800 pounds of paint. The paint’s primary function is corrosion protection to the aircraft skin. Requirements for the paint include: Durability to support the fuselage’s expansion with cabin pressurization, flexibility in all conditions, weather and temperature extremes, impact from hail and dust (at 600 mph); and resistance to salt spray and chemicals (hydraulic fluid, de-icer, etc.).
During operations to prep and paint the aircraft, painters navigate quickly around the aircraft on large working platforms mounted on cranes known as stackers. Each stacker (see picture) is an overhead-supported boom with four axes of movement: bridge, trolley, hoist, and rotate.
The painters carry supplies on the stacker platform and drive the stackers using pushbuttons on handheld pendants. A control system based on programmable automation controllers (PACs) accepts the pendant inputs and commands variable frequency motor drives to move the stacker as requested. Rapid movement is required because the entire fuselage must be painted in minutes to achieve uniform results.
While much care is taken by the painters in their operations, their primary focus is on the task at hand—producing a quality paint job. They are skilled painters, but not skilled precision equipment operators. With this in mind, Boeing seeks to make the equipment easy to use and safe to operate. To achieve these objectives, Boeing has been using a collision-avoidance system to prevent the stacker from coming in contact with the aircraft.
The motion of each platform is controlled by a PAC, working in conjunction with a supervisory computer, known as a collision avoidance module (CAM). The CAM examines the pendant requests and provides go/no-go responses to requests. When motion is permitted the system determines which preset speed of travel is allowable based on proximity to the aircraft. As the stacker approaches the aircraft, speeds are reduced until a nominal separation of 4 inches is achieved, at which further movement towards the aircraft is prohibited.
The proximity of the platform and boom to the aircraft is determined by using computer models of the aircraft and stacker surfaces, combined with position information from sensors attached to the motor drives that move the stacker.
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| View of the lower portion of a stacker platform (center left of photo). Source: Concept Systems |
The CAM system in use until 2007, though largely effective, was limited by the technology available when it was installed more than a decade ago. The software for this system was developed as a DOS application and was limited in modeling the aircraft structure to predict exactly where the surface of an aircraft being painted was.
The models were rendered as a collection of 2-in cubes, so curved surfaces were a gross approximation—similar in nature to a model constructed of Lego blocks. This resulted in model surfaces that had unnatural and sometimes baffling limitations on movement for the painters. Because the older digital models of various aircraft were very imprecise, it took a long time to develop and prove out the stacker motion plans to enable the painting of new aircraft.
In 2007 Concept Systems Inc. of Albany, OR, a systems integrator with considerable experience in developing motion systems, human interfaces, and custom real-time software solutions, was contracted to design and develop a new collision avoidance system for Boeing. The goal was to provide a modern software solution that would deliver performance and usability enhancements far beyond the capabilities of the old system. The system needed to support operations in three paint hangars, working with stacker systems ranging from new to ones in service for more than 20 years.
Building the new system
The new stacker systems, constructed by Oregon Iron Works of Clackamas, OR, are based on Rockwell Automation ControlLogix processors utilizing Ethernet communications, while the older systems are based upon Allen-Bradley PLC-5 controllers utilizing Data Highway Plus communications. Whereas the old collision avoidance system code varied from hangar to hangar and required separate compilations every time the systems were updated to support new aircraft, the new software system was designed 1396927554
with the flexibility to support the varying control architectures and communications methods within a common compiled codebase.
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Concept Systems put the initial focus on better 3D models (aircraft and stacker models are shown) generated from Boeing-supplied aircraft data. Source: Concept Systems. |
Concept Systems put the initial focus toward the improvement of aircraft object modeling. The new models would need to be easily generated from the Boeing-supplied aircraft data, modeling the surfaces of the aircraft more accurately, and supporting a 3D visualization utility. The Proximity Query Package (PQP) from the University of North Carolina at Chapel Hill was licensed to serve as the underlying engine for comparing model positions.
The Open Graphics Library (OpenGL), software for writing applications that produce 2D and 3D computer graphics, was selected as the rendering technology for visualizations. Both PQP and OpenGL work with models composed of meshes of triangles, and the Boeing design software was able to export surface models in a compatible format (STL). The PQP engine provides a mechanism to query the minimum distance between any two modeled objects, such as those of the stacker and an aircraft.
Concept developed a CAM runtime application around PQP that solves a complex decision tree to determine what movements are allowable. Through use of the triangle based models the system is able to provide highly accurate representations of the aircraft surfaces, eliminating the problems with curved surface interactions between the stacker and aircraft that had plagued the old system. With this improvement, the painters can work without losing time to unexpected movement limitations.
A central computer known as the engineering workstation (EWS) is utilized to configure the system and distribute updates to the CAMs. The EWS is able to convert Boeing-provided aircraft data into models for the system, as well as develop models for the stackers and other objects via a provided editor.
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| Example of an event log that's available on Boeing's company intranet. Source: Concept Systems |
The EWS provides an OpenGL-based tool to view any model as a 3D object with full rotate, pan, and zoom capabilities. The model viewer also allows the live interaction between any stacker and the aircraft to be viewed in real time. With this advanced viewing capability the model can easily be verified, while the interactive viewing provides confirmation that the aircraft is properly registered and located in relation to the stacker.
Since Boeing airplanes are not all the same size the system had to adapt to various models, including the 767, 747, 777 and the new 787 jets. With the old system, each time an airplane with different dimensions was introduced it took several days of hangar time to perform this process, during which painting production was interrupted. The old process was made very time consuming because it had to be implemented from scratch in each paint hangar, for each airplane type, due to limitations that required special aircraft model adjustments to conform to each hangar.
Simulation engine
To alleviate this issue, the system was designed with a simulation engine which emulates the function of the stacker PAC. Using the simulator, engineers can perform exhaustive testing of the aircraft model using the full software system in an offline environment. This has shortened the aircraft commissioning process in the hangar to a single work day. The design approach implemented has also allowed for the same model to be directly usable from hangar to hangar, eliminating the need to commission the same aircraft multiple times.
Concept Systems developed the collision avoidance software in Microsoft C++ .NET 2005, and created several class libraries to eliminate redundant coding in the various applications within the suite. All applications run on a Windows XP platform, and the CAM application is able to provide solutions within a 250ms scan.
Troubleshooting and diagnostic data for the system is accessible to Boeing engineers via a Web browser interface, allowing detailed event log viewing from anywhere within the Boeing intranet (see graphic). The event viewer provides category and keyword filtering capability, allowing engineers to rapidly narrow in on key information to support troubleshooting efforts. A Web server on the EWS hosts the event viewer, and queries a Web service on the CAMs across an isolated collision avoidance network to retrieve the data. Another page provides a quick status overview for all the CAMs in the hangar, and provides continuous updates to the status via AJAX technology.
ONLINE EXTRA
Why Not Just Use Sensors?
The conventional solution to Boeing airplane-painting problem might be to use non-contact sensors, such as laser-based photo-optic sensors, mounted around the platforms to detect when the stacker nears the aircraft. Why not use them now?
Boeing had actually tried using optical sensing techniques earlier, but there are a number of problems with the approach. The primary one is that, because this is a paint hangar, paint overspray on optical sensors is very likely, and it creates a maintenance nightmare.
Besides the overspray problem, maintaining the accuracy of multiple sensors created alignment and calibration problems. Also, small items on the aircraft surface, such as antennas and probes, might not be detected by optical sensors; however, the modeling approach described in the article can account for such small parts safely.
Further, as aircraft designs and dimensions change, it’s easier to change the software model of the aircraft than to make changes to a physical sensor-based control system.
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