Flexible automation for large aerospace assemblies

Citation

(1999), "Flexible automation for large aerospace assemblies", Aircraft Engineering and Aerospace Technology, Vol. 71 No. 5. https://doi.org/10.1108/aeat.1999.12771eab.002

Flexible automation for large aerospace assemblies

Flexible automation for large aerospace assemblies

Keywords: Meta Technology, Automation, Drilling, Fasteners, Lasers, Robots

A Boeing research project at the Southern California Prototype Centre (SCPC) has pulled together a number of technologies to dramatically improve productivity on what is often a labour intensive process.

The objective was to find an automated solution to carry out drilling and routeing operations for fastener systems on major sub-assemblies and final assembly for both commercial and military aircraft. A situation where conventional automated fastener solutions are not feasible. The solution developed incorporates robotics, off-line simulation and programming, photogrammetry measuring systems and a laser sensing system developed by UK-based Meta Technology.

At the heart of the system is a Tricept robot which is based on three arms that combine freedom of movement in six axes with sufficient rigidity for milling and drilling operations. The robot is attached to a column which in turn is mounted on a portable base plate to create a machining unit that can be rolled into position.

In practice the portable machining unit would be approximately positioned on the assembly floor in front of the components to be machined and assembled. For the research project, two pilot applications were selected; one for commercial and the other for military aircraft assembly.

The major civil assembly identified for investigation was the aft pressure bulkhead for the McDonnell Douglas Twin Jet. For the trials the bulkhead was mounted on a fixture in the vertical plane. Even though sophisticated hardware and software is used, the concept behind the process is elegantly simple.

The tooling at the end of the robot arms is positioned in three stages, each one achieving a quantum leap in accuracy. An analogy can be drawn with increasing the magnification on a microscope to zoom in on feature. Similarly, the system developed by Boeing uses the different technologies to home in on the precise position to drill or insert a fastener.

The process starts at the "lowest magnification" level by manually positioning the portable unit in front of the assembly in the approximate work area.

Twin digital photogrammetry cameras then view the work piece. This is compared with a CAD simulation in the master controller to determine the exact relationship between the robot and the work piece.

The digital photogrammetry metrology cameras are used to transform the robots' datum position on the actual component to a corresponding position on the CAD simulation. This "coffection factor" repositions the robot head from the crude manual positioning to a more precise position.

With this information, which represents a unique orientation between tool and component, the robot program can be positioned to perform the drilling or assembly operation. With this technique, the robot work area is effectively unlimited. As long as the CAD/simulation for the assembly exists, the robot can be manually positioned anywhere then corrected with the cameras.

Usually the fastener or drilling is positioned relative to a feature on the component such as an edge or a hole.

This is where the Meta laser tracking system comes in. It is used for the "fine focus" of the robot head. The Meta system uses "cross-hair" laser beams that can be used to "find" the feature.

The Meta sensor head used was specially developed for this process. There are two laser diodes in the sensor; each lays down a stripe at 90ú to the other. This forms the cross hairs similar to a gun sight.

A CCD camera in the sensor views the cross generated by the laser. When an edge or hole breaks a laser line, the exact location of the break can be identified by sophisticated software in the PC controller. For example, the centre of a hole can be determined from the four points on the circumference identified where the laser crosses the edge of the hole. The calculated position is compared with the nominal or reference position and the correction signals are then fed to the robot controller to adjust the position of the tooling.

For the sensor system a field of view of 25mm and an accuracy of 0.17mm was specified. In the event accuracy proved to be better than 0.1mm.

The sensor system is an important element in making the development a practical proposition.

With CAD model and cameras the robot can be directed to the theoretical position of a required hole. It cannot make allowances for component tolerance or any other permitted deviation from nominal dimensions.

In many instances it is more important to keep a set of drilled holes relative to a feature than a position solely dictated by drawing dimensions. For example fastening holes for a plate to reinforce an orifice through a bulkhead.

Once the principle had been proven practical further developments involved a quick-change tool changer at the end of the robot. This enabled drilling to be quickly followed by reaming then countersinking or insertion of fasteners.

Different machining heads are available for high speed drilling in aluminium and another for routeing titanium.

Compared with component manufacturing, assembly is a much more difficult operation to automate. It is the combination of visual co-ordination and manual dexterity that has to be resolved. For the aerospace industry this problem is only compounded by the size of the assemblies involved. The project is a very practical approach to addressing this very real problem.

For this project the Meta system was used solely for positioning the robot tooling relative to a feature. The laser technique also has the capability to alter the angle of the tooling so that it is normal to a compound surface in both planes.

This can be useful when counter-drilling holes for riveting. To maintain a uniform chamfer around the hole the tool must be normal, at 90ú in both planes. If not, the counter sink will not be concentric which leads to less than optimum fastening.

Other areas in aerospace where the laser sensing technology could make positive contributions are polishing and debarring; cutting and trimming; and the application of sealant and adhesives. These are all applications that have already been addressed in markets such as plastics and the automotive industry.

Further details are available from Meta Technology Ltd. Tel: + 44 (0) 1235 512215; Fax: + 44 (0) 1235 512115.