A new positioning system for aircraft structural assembly

  • 28-Jun-2012 06:49 EDT

Main sequences in the assembly tank.

Automated mating of large aircraft components offers an alternative solution for aircraft manufacturers to cut costs, improve process quality, and shorten time to market. More and more, flexible and automated systems are replacing hard jigs throughout the process of aircraft structural assembly—for instance, in the manufacturing of the fuselage sections from shell panels, the alignment of the sections to build the fuselage, and the joining of wings and tail units to the fuselage.

In the past decade, a few approaches were proposed for automated assembly systems consisting of multiple manipulators, which act simultaneously to handle the aircraft components. Especially in conjunction with the use of innovative composite materials (and particularly carbon-fiber-reinforced plastics, which find broad use in the new aircraft programs including the Boeing 787 and Airbus A350 XWB), the challenge for these assembly systems consists in the proper handling and accurate positioning of the aircraft components.

The EcoPositioner from Dürr Systems GmbH is a new conception of a modular and reconfigurable positioning technology that provides a holistic solution, taking into consideration the positioning task as well as the operational conditions and environmental influences. It is a mechanical system consisting of multimanipulators, or multipositioners, that pick up the aircraft component at defined attachment points and manipulate it in six degrees of freedom (three translational and three rotational).

The EcoPositioner includes a measurement module—a large metrology system and measurement software that provides information on the current and ideal poses (positions and orientations) of the aircraft component. The current and the ideal poses are determined according to the reference frame related to the CAD models of the components and the aircraft structure.

A reliable robot-control platform guarantees a high synchronization of up to 48 actuated axes per control unit. The EcoPRC control system is well-proven worldwide and is used in more than 6000 robot applications. With the aid of force sensors mounted on the end-effector of each positioner, the forces acting on the aircraft component can be monitored during the motion. This ensures strain-free handling of the components and thus prevents them from uncontrolled deflection.

The positioning of components and working tools is the essential task in the assembly of fuselage sections from single shell panels. For large and geometrically complex structural components, which by their very nature suffer from considerable physical distortion, the positioning task is even more crucial.

Classical assembly methods use large, complex jigs, which are specially tailored for an aircraft type. The jigs physically control the shape of the aircraft component and have to be moved together with the component to the assembly position. An advantage of hard jigs is that they do not require highly skilled workers; however, the jigs are expensive, have long manufacturing lead times, and cannot be modified economically for use on other aircraft types.

An alternative to hard jigs is the use of flexible manipulators. Combined with an accurate, large metrology system and an advanced control system, the positioning task can be structured in the following sequence:

• Pick up components using multipositioners at defined attachment points located on the component.

• Measure the geometry of the component using, for instance, a laser tracker and spherical-mounted retro-reflectors that are placed at defined locations.

• Adjust the component geometry with respect to product design (CAD data) using, for instance, Best-Fit strategies. This operation is sometimes necessary, especially for CFRP components.

• Measure the current pose of the component.

• Generate the motion path from current to ideal pose as corresponding to the CAD data of the product design. Advanced controls with robust path planning strategies are mandatory for this operation to avoid damage to the component.

• Move the component to ideal pose, taking into account the restrictions on the accuracy and the synchronicity of motion.

Four functionalities are needed to carry out the assembly task. First, an accurate measurement of points specifically placed on the component to recognize its shape and determine its pose must be taken. Various metrology systems can be used—for instance, systems based on laser or combined laser and IR technology such as an indoor global positioning system.

Second, acquisition, analysis, and processing of the measurement data must be ensured. This can be done via metrology software, which can analyze the measurement data and compare them with the CAD data of the components and their locations in the assembly site using robust Best-Fit strategies.

The third task consists of the generation of the trajectory along which the component will be moved from current to ideal pose. Since multiple positioners simultaneously act on the component, this task constitutes a tremendous challenge for any control system. For path generation, the control system must not only account for the restrictions related to the workspace of each positioner but also avoid any excessive strain induced by the positioner on the component during the motion.

The fourth and most crucial task is execution of the motion. Despite all additional impact in terms of external loads and environmental conditions such as temperature fluctuation, the single positioner should feature a positioning accuracy superior to that required for the component itself.

This article is based on SAE technical paper 2011-01-2637 by Taoufik Mbarek, Alexander Meissner, and Nihat Biyiklioglu of Dürr Systems GmbH.

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