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A measurement system for aircraft wing box sections

01 April 2015

More usually associated with oil & gas pipe measurement systems, Optical Metrology Services (OMS) has now developed a non-contact, laser-based dimensional measurement system for aircraft wing boxes and other awkwardly shaped systems.

Originally developed for a customer in the aerospace sector, the OMS Wing Box CHECKER measurement system may be purpose-built, but can be engineered to meet a variety of application needs. OMS' aerospace customer required a system that could accurately measure the internal profile and size of wing box sections designed for a commercial aircraft.

Made from a special carbon fibre composite, these wing box sections are complex in shape, with tapered ends down to 100mm x 100mm in dimension, and are split into multiple internal sections/compartments. In addition, the measurement system had to be capable of measuring the corner radii of the box wing sections, which are absolutely critical as these are stress concentration points. The specification called for a system capable of internally profiling four 12m long cavities inside the wing box section, within a four hour time frame.

Using expertise gained from the development of similar automatic internal weld scanning tools for oil and gas pipe lines, OMS completed the design of the Wing Box CHECKER system within 4 months.

It is a rugged system made from high grade aluminium, and comprises a tractor unit, automatic lifting/extension arms, three interchangeable sensors/measuring heads, a separate controller and a launch skid. The three sensor heads are interchangeable units that are mounted to the system as the wing box tapered section gets smaller. This was the only way of ensuring that a single measuring tool could access all the required internal sections of the wing box.

The tracked vehicle is driven by a dc servo motor geared drive and timing belt. The tractor unit can move forwards or backwards, and has a self-centering mechanism for alignment in the cavity, which ensures that the tool is kept on the wing box centre-line in three axes at all times (three laser displacement sensors are needed to accomplish this).

The extension arms are powered by an electronically-driven linear actuator. The three sensor heads incorporate laser profile scanners that provide a measurement accuracy of ±0.254mm and up to 2,048 measurement points digitally filtered to 600.

Data output is provided in the form of an SA point cloud for scans, and Microsoft Excel for radii and flanges. The angle of the tool with respect to gravity is also reported in one or two axes. Profile data is transferred to a PC via an umbilical cable and can be logged to a PC hard drive; all tool commands are controlled via a PC keyboard/mouse and software interface. OMS director, Hugh Davies concludes:

“Our mix of mechanical, electrical and software integration skills enabled us to engineer a complete measuring system for the customer. The real challenge was the shape of the wing box sections and devising a single system that was capable of performing all the measurements within the allotted time period. Because of our experience working on oil & gas pipe line projects, we were able to offer all the necessary design, manufacturing and software integration skills that were required for this project.”

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