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Monday, 25. September 2017

Image Pattern Correlation Technique (IPCT)

The Image Pattern Correlation Technique (IPCT) is an optical surface shape and deformation measurement technique. It is a special kind of DIC (Digital Image Correlation) and is based on photogrammetry in combination with modern correlation algorithms developed for the Particle Image Velocimetry (PIV). The simplest IPCT setup consists of one camera observing an object covered with a random pattern. After a reference image of the investigated object has been acquired (Figure 1a) the object will be set into a load condition which causes its deformation. The second image (Figure 1b) or a sequence of images will be recorded under such deformed states. The image(s) of the deformed object will be cross-correlated with the reference image. A 2D displacement vector field (Figure 1c) will be obtained as the result.

Figure 1: Principle of IPCT (a - reference image, b - deformed image, c - displacement vectors)

Using image pairs of the randomly patterned object acquired by a stereoscopic camera system, its 3D position and shape can be obtained. Figure 2 schematically shows the functionality of stereoscopic IPCT. During the first step the investigated surface H(X; Y) is recorded by two cameras (camera 1 and camera 2). Both cameras are looking at the same field of view, but under different viewing angles. A cross correlation algorithm identifies similar regions in the recorded images of both cameras. For best results a random dot pattern is applied onto the surface. The correlation procedure delivers the coordinates of areas with similar dot pattern in image 1 (coordinates x1, y1) and in image 2 (coordinates x2, y2).

Figure 2: Procedure of stereoscopic IPCT for 3D measurements

With known intrinsic parameters (e.g. focal length, distortion, principal point) and extrinsic parameters (position and orientation) of both cameras, the 3D coordinates of the recognized areas with the same dot pattern are determined by means of central projection and triangulation. The application of this algorithm to all areas in the image depicting the same dot pattern regions on the surface finally yields to a high accurate reconstruction of the complete 3D surface. By comparing the measured 3D surface under an unstressed reference state (e.g. the wing of the aircraft standing on ground) to the surface under load conditions (e.g. wing deformed during flight), the displacement vectors and thus deformations can be deduced with a high accuracy. If the material characteristics of the observed object are known the local stress can be calculated as well. The advantage of the stereoscopic setup is that the location of the surface under investigation can be measured and that motion in all directions can be determined.

Figure 3: Example of IPCT wing deflection measurement (left - recorded image pair of the outer wing of the Piaggio P180 during flight; right - resultant 3D surface of the outer wing calculated out of these images using IPCT)

The main advantage of IPCT is the simplicity of its basic experimental setup: In principle a foil furnished with a random dot pattern and two standard cameras are enough to determine the shape of the investigated surface with a high accuracy. Figure 3 shows as one example the outer wing surface measured during a flight test on the Piaggio P180.

If the geometry of the measurement setup is known and when assuming that the motion of each point on the wing with reference to the camera is only vertical and not span-wise a setup with only one camera, the “monoscopic” approach, can be applied. This setup has the advantage that the installation and the data processing are less complex. The distance from the camera to each point on the surface under investigation has to be known for the determination of movement as a single camera determines angular displacements instead of displacements through image correlation. Furthermore the motion in vertical direction and in the line-of-sight direction can not be distinguished.

In feasibility studies in the AIM project both the “monoscopic” and “stereoscopic” approach have been applied for mainly static deformation. The technique IPCT is able to give the local deformation on a larger area with a high accuracy. If only a part of the wing or a control surface is measured, solid body movements and rotations of the investigated area will occur. To separate the movements from the deformations additional (unique and larger) markers on the pattern can be used. If only the global deformation of the structure or the deflection of a control surface is requested, a sufficient amount of such markers can be used for deformation and shape measurements.

In combination with modern high speed recording techniques the technique can also be applied to fast rotating objects like propeller blades. Figure 4 shows an example of in-flight propeller deformation measurements with IPCT done in one fixed propeller phase angle. Within AIM² the recording system will be improved to be able to measure the propeller blade deformation for a complete revolution. The advantage of using the IPCT for fast rotating objects is the measurement of a large area on the blade without having sensors and wiring installed on the blade.

Figure 4: In-flight deformation measurements on a propeller blade of the Piaggio P180 (rotating with 2000 rpm, left- recorded image pair, right - deformed surface measured with IPCT)

The wing deformation measurements with IPCT on a large transport aircraft (AIRBUS A380) operated under industrial boundary conditions identified major challenges to be coped – e.g. the application of the right pattern to the surface, the calibration and recalibration of the camera systems, camera vibrations, large movements of the structure and the long data processing time after recording the images. Within AIM² the installation of the cameras on the aircraft with respect to an easy installation and the minimization of camera movements and compensation techniques for movements, the optimisation of the application of patterns and markers on the wing and the control and high lift surfaces, the calibration and recalibration procedures and the application of IPCT and marker techniques on rotating surfaces as well the application of IPCT and marker  techniques on vibrating surfaces will be improved. Furthermore the user friendliness of the post processing tools and the data processing time will be optimised. With the IPCT and marker based techniques the installation effort for deformation measurements can be reduced significantly. As the technique measures a complete area within one instant of time also the testing time can be reduced.

 
AIM² Advanced In-Flight Measurement Techniques, c/o German Aerospace Center (DLR), Bunsenstrasse 10, 37075 Goettingen, Germany, Tel: +49 551 709 2252, Fax: +49 551 709 2830, Email: aim.supportdlr.de