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Luhmann/Müller (Hrsg.): Photogrammetrie – Laserscanning – Optische 3D-Messtechnik

Beiträge der Oldenburger 3D-Tage 2014 (ISBN 978-3-87907-536-2)

Measurement of Mould Tool for Laminar-flow Carbon-fibre Composite Airplane Wing Cover

Jan BURKE, Achim GESIERICH, Wansong LI and Ralf B. BERGMANN

Summary Within the framework of the European Union‘s CleanSky programme, carried out as BLADE (Breakthrough Laminar Aircraft Demonstrator in Europe) under management of Airbus, technology is being developed to build airplane wings for passenger planes that allow Natural Laminar Flow under cruise conditions. This will help reduce fuel consump-tion and emissions caused by air travel. Numerous technological challenges must be over-come to build wings that will comply with the necessary tight tolerances for profile wavi-ness and roughness.

We have built a customised and highly accurate fringe-projection stereo measurement sys-tem that complies with the specifications and discuss its design and performance. The very large amount of data generated by overlapping individual measurements must be reduced for stitching into one point cloud of the whole tooling surface. As a first example for such a procedure, we report on measurements of an Invar OML (Outer Mould Line) tool that will serve as the exterior form of the experimental wing cover, and give indications of the achievable 3-D resolution in individual and combined measurements, as well as first esti-mates on the match of the tooling surface against the CAD model.

1 Introduction

The BLADE project is an attempt to construct an airplane wing that will achieve Natural Laminar Flow (NLF) over its upper surface (aka wing cover). “Natural” in this context means unsupported by extra flow sheets through micro-bores or the like [JOSLIN 1998]. NLF would be very advantageous in terms of lift and therefore fuel efficiency for commer-cial aircraft, but to achieve it, the wing cover must be smooth on all spatial scales (from form to waviness to roughness [ISO 2012]), since at a typical cruise speed (say Mach 0.82 for an Airbus A340, the designated test airplane) even minute disturbances will nucleate turbulence that will spread out and grow in width over the wing surface, reducing efficien-cy. The manufacturing and metrology effort involves a large number of partners from many EU countries – Fig. 1 gives an overview of the project.

Depending on the scale of the feature investigated, NLF requires actual-vs-design toleranc-es from several mm to fractions of a mm, with commensurate measurement uncertainties. Given the size of the wing covers in this research effort (about 9 m in spanwise and 2 m in chordwise direction), it is clear that large surfaces must be measured with sufficient data density. A proven method of choice for this purpose is fringe projection [GORTHI 2010].

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Fig. 1: Overview of BLADE project and its partners. Source: http://www.cleansky.eu/content/page/sfwa-demonstrators

For measuring large plane or quasi-plane surfaces specifically, standard commercial fringe-projection systems would not be an optimal choice since their design involves some com-promises to provide versatility for a wide variety of technical specimens. Most importantly, the angle between the cameras is typically around 45° to avoid excessive obscuration. However, when no significant edges or steps are present in the field of view, one can afford to extend the triangulation base significantly, thereby reducing the depth uncertainty. Also, if the object distance can be kept constant, it is possible to lay out the system geometry for a specific distance and with the best possible usage of camera sensor area.

To ensure the best possible surface smoothness, the wing covers will be made from carbon-fibre reinforced polymer (CFRP) materials, with the necessary stringers and ribs integrated into the form before it is autoclaved. This avoids the need for bolting and thus introducing possible flow disturbances on the wing cover surface. Innovative production technologies will be trialled to build the wings: the starboard side will have a CFRP wing cover, with the leading edge of the wing fabricated from aluminium and later to be joined to the wing cover (where particular attention will need to be paid to the joint area to ensure no steps or gaps occur there). The port wing cover will be made from CFRP in one piece, including the leading edge.

For NLF, only the first half of the chordwise wing profile is important; hence the experi-mental wing covers do not include the trailing-edge section. Therefore, the Invar tools that will be used to generate the designed shape have dimensions of about 2.5 x 9 m². The entire wing cover fabrication and wing assembly is accompanied by a detailed measurement pro-cess to track the build-up of deviations on different length scales. In Section 2, we will describe the measurement system and show some representative performance data; Section 3 presents some measurement examples related to the wing cover fabrication.

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2 Metrology system

2.1 Design

When using a stereo fringe-projection system, in theory an angle of 90° between the camer-as should provide the smallest possible uncertainty volume xyz for measurements of 3-D co-ordinates. The price to pay lies in the large angle between the tested surface and the cameras’ optical axes, resulting in the necessity for a very large depth of focus (see Fig. 2 left) or even requiring the imaging system to be set up under the Scheimpflug condition.

Fig. 2: Left: set-up of wide-angle stereo system. Note that the opening angle of the pro-jector is larger than that of the cameras. Right: experimental data on position un-certainties as a function of camera angle. For details of the diagram, see text.

To minimise the disadvantage thus incurred, we conducted an experimental study of posi-tion uncertainty in phase maps. The phase maps were calculated in both x and y directions from a hierarchical sequence of phase-shifted fringe patterns [OSTEN 1996]. The values for x and y were determined from the standard deviation of the x and y phase maps against their respective versions filtered with an 11x11 Savitzky-Golay filter; z was then calculat-ed from x via the tangent of the camera angle. The uncertainty unit given in Fig. 2 is image pixels, since the system geometry was not re-calibrated for each angle. Since the pixel un-certainty is converted to co-ordinates with good linearity after calibration, the simplification is acceptable for this test. The quantity shown as “length of uncertainty volume” in Fig. 2 is the cubic root of xyz , although the actual shape of the uncertainty volume is an ellipsoid rather than a cube. It can be seen that there is a wide zone in which the uncertainty volume changes little; therefore we chose an off-normal angle of 40° as the best trade-off between uncertainty and acceptable imaging geometry.

The projector chosen is an LED type that combines the advantages of high reliability, free choice of attitude (as opposed to models with high-pressure lamps that can be operated only horizontally), large light flux (2500 ANSI lumen) and high resolution (1280×800 pixels). No attention needs to be paid to the quality of its imaging system, since the projector does not need to be calibrated. The nominal illuminated area is 1280×800 mm², which determines the working distance for which to set up the cameras. The 1mm/pixel setting is coincidental: the size of the image has been designed to match the rib spacing of the wings to be built.

Next, it is important to select suitable cameras with appropriate lenses to obtain the highest possible spatial resolution. We chose 6-megapixel cameras whose chip has excellent light sensitivity, allowing us to stop down the iris to increase the depth of focus. The wide-angle

Camera 1 Camera 2

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lenses selected still have an opening angle that is smaller than that of the projector optics, which means that the working distance of the system has an upper limit: if the system is too far away from a surface, the cameras will not be able to capture the complete illuminated area. Fig. 3 gives an impression of the geometry at correct working distance.

Fig. 3: Images from Camera 1 (left) and Camera 2 (right) of stereo system. The object is a wing tooling surface (see Section 3) with 3-D markers attached.

After setting the iris appropriately for the purpose, the camera/lens units are calibrated by generic camera calibration [BOTHE 2010], a model-free approach that is able to characterise all aberrations on lenses, including asymmetric and local flaws, and leads to uncertainties distinctly smaller than standard photogrammetric approaches. Fig. 4 shows error maps of camera calibrations.

Fig. 4: Calibration maps of camera 1 (left) and camera 2 (right) of stereo system. The quantity displayed as grey value is the position error on the sensor, and the scale ranges from 0 (black) to 10 µm (white).

The software used for calibration and measurement is the BIAS/VEW package “Fringe Processor”; it has been upgraded for the purpose of this work to allow high data rates and enable fast calculations on large images.

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2.2 Performance

The theoretical spatial resolution of the system is 0.3 mm/pixel, but due to the oblique viewing and the choice of an intermediate f-number for the lenses, some loss in resolution is expected. Since the system exceeds the required resolution of about 10 mm very safely, the investigation does not need to be quantitative. It is sufficient to demonstrate the detec-tion of sufficiently small spatial detail. Fig. 5 shows one corner of the outline of the wing cover tool. The outline has been milled into the surface and is approximately 0.8 mm wide; in addition, there is a registration cone in the corner whose diameter is about 4 mm. Both of these features are distinguished very clearly, and the cone is measured accurately enough to allow constructing a reference point from its geometry. This is important for later alignment of the measured point cloud with the CAD model.

Fig. 5: Left: white-light image of tool corner; right: same area from shaded point cloud.

The depth resolution, in particular the ability of the system to retrieve waviness, has been tested with a reference artefact that was designed in collaboration between Airbus and the National Physical Laboratory of the UK, and certified at the NPL. The artefact contains numerous features that are relevant for the BLADE metrology programme. We will not enumerate all of them here but restrict ourselves to the waviness and step standards.

The measurement system is required to detect waves of different lengths and amplitudes; the smallest wave present on the test artefact has an amplitude of 0.1 mm, and the smallest steps are 50 µm high.

During the measurements, the limitations of our purpose-built measurement system re-vealed themselves: the registration spheres around the area of measurement could not be measured with sufficient area coverage due to the large angle between the cameras; also, errors appeared on the milled surfaces (such as the step standard) that showed residual structure from milling and were partially reflective. Errors from excessive reflectivity are common to all fringe-projection systems and can partly be mitigated by aligning the meas-urement system appropriately. In our case, we used the freedom afforded by the LED pro-jector and tilted the measurement system 90° about the z axis to avoid obscurations of the waviness samples by the registration spheres and minimise direct reflections. Fig. 6 pre-sents the results thus obtained.

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Fig. 6: 2-D sections of sample areas on the . Diagrams (a) through (c) display the abso-lute vertical deviations from the reference plane: (a) Wave 4; (b) Waves 2 and 3; (c) Wave 1. Diagram (d) shows absolute vertical deviations of the step standard against the separate step reference surface.

As can be seen, all wave amplitudes can be easily detected, including the short 0.1 mm wave. The total height difference of 1 mm on the step standard is accurately reproduced despite the noise from reflections. The central four steps of 0.05 mm are also clearly distin-guished, and would come out even clearer when measured on a uniformly diffuse surface.

3 Measurement results

3.1 Mid-scale demonstrator

In order to characterise the additional deformations introduced by the attachment of string-ers and ribs on the rear surface of the wing cover, trials of the technology have been con-ducted on different scales. We have measured a mid-scale demonstrator with total dimen-sions of about 2 x 1.3 m² in four overlapping sub-areas. The results were then stitched and evaluated against the CAD model. The comparisons are shown in Fig. 7.

1.05 mm0.35 mm

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122 J. Burke, A. Gesierich, W. Li and R.B. Bergmann

Fig. 7: Top row: Visualisation of differences between CAD model and actual shape of mid-scale demonstrator. The stringer (spanwise) direction is into the plane of the paper; the rib (chordwise) direction is from left to right. Bottom row: two chordwise error profiles. The width of the plots is 1.3 m, the total height error displayed (magnified 100 times) is 2 mm.

In the direction of the stringers, the part is very stiff and very little deformation is discerni-ble. However, in the chordwise direction, it is evident that the overall form of the part does not match the CAD file well. This is due to the way the part was supported, which allowed it to sag under its own weight. However, even against the distinct bending of the part, it can be seen that the wing cover buckles slightly over the stringers. Without subtraction of the overall form error, it is not straightforward to quantify the disturbance, but by selective manipulation of the colour scales, the humps can be estimated to be at most 0.1 mm high.

A qualitative assessment of the surface is also possible using a shaded point cloud (with no CAD data subtraction) and a texture map that strongly amplifies small height differences, giving an effect similar to what would be observed in reflection from a reflective surface. Fig. 8 shows how sensitive this method is to surface irregularities.

Fig. 8: Visualisation of minor surface irregularities (< 0.1 mm) on the shaded and tex-tured point cloud.

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3.2 Wing cover mould tool

The mould tool had been polished to a high degree of lustre after milling and grinding; re-maining scratches gave some scattered light, but this was overpowered by direct reflections that were very difficult and sometimes impossible to suppress by aligning the measurement system with respect to the tooling surface. Hence we prepared the surface of the mould tool with water-based crack-detection developer spray, as is standard procedure for transparent or reflective surfaces. We attached custom-made 3D markers (flat domes cut from Styrofoam spheres that caused no obscurations, i.e. made no angle >40° with the tool surface) and meas-ured the entire part with an overlap of some 50% between individual sub-areas.

Fig. 9: Stereo system in measurement position over the tooling surface. Care was taken always to capture three common markers between successive measurements to ensure reliable alignment and stitching of sub-areas.

For complete coverage of the tooling surface, over 50 individual measurements were neces-sary, generating a very large collection of data amounting to over 170 million points. Using a powerful computer (Intel i7 with 16 GB RAM), Geomagic 2012 and an efficient strategy, it was nonetheless possible to combine all data into one file at full resolution. This is not strictly necessary, as the spatial resolution of our measurement system exceeds the require-ments by a large margin; however the philosophy here was to use all good data for stitching and only later to remove points from the combined data.

During stitching, the good calibration of the system became evident in the visual appear-ance of the results: different colours, indicating points from different measurements, were randomly interspersed in the overlapping zones, which showed that systematic errors in the reconstructed surfaces were at or below the random noise level. This included fine features such as dents on the Styrofoam markers. After removing the markers from the combined file, we created reference points in the CAD file and the point cloud (using three reference cones of the type shown in Fig. 5) and used them to align the data sets. A comparison with the CAD model gave the result shown in Fig. 10.

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Fig. 10: Comparison of measured mould tool surface with CAD data; frame included for easy visual orientation, dimensions approx. 9×2.1 m². Grey scale ranges from –2 mm to +2 mm; most important deviations denoted by text. Black ellipse shows location of weld between the two parts of the frame (see below).

Whereas good overall agreement with the specifications was found, our data showed a low area near the centre of the leading edge, and the outward end (left side in Fig. 10) twisted. As a result, the overall shape was re-checked with approximately 300 laser tracker measure-ments, and it was found that the two data sets agreed qualitatively. Residual discrepancies can partially be ascribed to the relative instability of the fit from 300 points as opposed to the large data set from the stereo system. After modifications were made to the support of the wing cover tool, all measured points showed the surface to be within specification.

Finally, it is very instructive to use the high spatial resolution of our measurement for detec-tion of polishing traces and other imperfections of the surface. Fig. 11 was prepared in a simi-lar way as Fig. 8 and shows print-through from the frame, some dips from polishing, and most interestingly, a distinct depression over the weld between the two frame parts. This is remarkable as the whole tool was assembled and welded together before grinding and polish-ing, and yet, the print-through from the underside of the tool surface, through several mm of Invar, still reveals part of the fabrication history.

Fig. 11: Visualisation of minor surface irregularities (< 0.2 mm) on a shaded and textured point cloud (dimensions approx. 4.5×2.1 m²). Black ellipse marks area above weld.

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4 Summary We have presented and discussed the design of a custom fringe-projection system for the measurement of airplane wing surfaces, including both the tooling required to make them and the actual wing covers. Performance parameters after the first few calibration runs indicate that the specifications will enable the BLADE consortium to build the experi-mental natural-laminar-flow wings to the required specifications, or at least to track errors in the fabrication and assembly process.

Examples from the first few measurements show that establishing overall shape deviation is difficult because it always depends on the support of the parts measured. The stitching of many sub-areas may also lead to the accumulation of small systematic errors, although this cannot be proven beyond doubt using the data thus far available. More experience will be gathered in later measurement campaigns, where all markers will be omitted and laser tracker support will be added to record all data in global so-ordinates, removing the need to align or stitch individual measurements.

The data presented show that the measurement system excels at its design purpose, which is to detect and quantify waviness at the mm to sub-mm level over various length scales. By virtue of the density of measuring points, even small flaws in the surface will not escape detection, and this is an important prerequisite for understanding later under flight condi-tions why laminar flow exists, or does not exist, in certain areas. We anticipate being able to reduce the amount of processed data significantly in later measurement campaigns, main-taining the highest resolution only where small-scale disturbances are likely to occur.

5 Acknowledgement This work was carried out within the EU’s CleanSky programme under JTI-CS-2010-5-SFWA-03-004 and Grant Agreement 286745. We sincerely thank the EU for support of this research. A special thankyou also goes to Alan McMillan of GKN for his support with equipment and supplementary data during and after our on-site measurement campaign.

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