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75 1. Overview of the Initial Image System Before developing the assembly of the 45°-angled mirror and aggregate image particle tray, efforts were made for aggre- gate mounting systems to glue an aggregate at the very end of a needle. The aggregate mounting system is used for holding particles ranging from 1 mm to 50 mm in size. A preliminary fringe generator was set up to facilitate the system alignment and testing process. The fringes were uniformly spaced and fit the predictions based on the calculations in the design. Figure C-1 and Figure C-2 show the preliminary system in its entirety. They illustrate the camera track and carrier, to which the bellows is mounted. The fringe generator and coupler are shown along with the particle mounting and gluing systems. The laser and coupler are inside the wooden enclosure. 2. Particle Mounting System Figure C-3 shows the preliminary mount system. However, it is difficult to adjust this system accurately, and it has low stability during adjustment. The lack of a fine-tuning mecha- nism is especially apparent at high magnification, where the particle must be centered within a 3.5 à 4.5 mm area. Figure C-4 illustrates an improved particle mount design with both coarse and fine adjustments in the vertical and hor- izontal directions. Figure C-4(a) shows the improved design of the particle mounting system, and Figure C-4(b) presents the horizontal arm of the improved particle system. Coarse linear adjustments in the horizontal direction are made using a standard ½-in. post and post holder assembly. Fine hori- zontal adjustments can be made by turning the screw of the translating post, which provides up to 6 mm of travel, with 1 mm per turn resolution. The diameter of the vertical post has been increased from 0.5 in. to 1.5 in. for stability improvement. Positioning the large post clamp on the mounting post sets the coarse vertical position of the mount. Fine tuning of the particle height is A p p e n d i x C Initial Efforts for the Particle Mounting System performed by turning the base of the adjustable height collar, which has a range of 33 mm and resolution of 0.64 mm per turn. Adjustments along the camera imaging axis are made by rotating the boom, a motion that has shown experimentally to be both stable and accurate. Additionally, the post collar fixes the assembly height during boom rotation, allowing for independent x, y, and z axis positioning. Figures C-5, C-6, and C-7 illustrate the particle mounting system used to glue particles to the various sizes of needles. Large rocks, between 10 mm and 50mm in size, are held in the adjustable vise, while particles as small as 1 mm are immobi- lized on an adhesive pad during the gluing process. Mounting needles are held in place by a series of pin vises, and a movable, rotating arm allows the user to position the needle for mount- ing. A miniaturized version of this mounting system, involving precision positioning stages and a microscope-objective lens, is planned for mounting particles smaller than 1 mm in diameter. Figure C-8 and Figure C-9 show the particle mount and the system of targets used to align the object for imaging. The mounting arm and pin vise can be positioned and rotated to achieve virtually any viewing angle. Two targets are used to aid in alignment: one for the x-y position and one for the x-z position of the particle. Figure C-10 and Figure C-11 show the interference pat- tern produced by the fiber-based pinhole interferometer. The fringe period (i.e., the distance between two neighboring straight lines shown in Figure C-10) is approximately 2 mm, and the lines are straight and well aligned. Figure C-11 illus- trates the pattern on a medium-sized particle. 3. Limitations of Particle Mounting System and the Proposed Angled Mirror System After the development of the particle mounting system, the NCHRP project panel raised concerns about the practical usability of the imaging system, especially in the practice of
76 Figure C-3. Preliminary particle mount. Figure C-1. Front system view. Figure C-2. Top system view. pavement engineering. In particular, they indicated a strong need for the system to be able to quickly image multiple par- ticles at once. During the visit of the project panel, the angled- mirror design was proposed. A high-quality mirror, tilted at a 45° angle, is placed directly above the object. This mirror redirects the illumination rays emanating from the fringe gen- erator. It also redirects the imaging rays, which are received by the CCD camera. The object, no longer glued to an articulated mount, is placed on a flat plate for imaging. The orientation depicted in Figure C-12 has several advan- tages over the previous system design. It eliminates the need for the gluing of particles to needles, which saves time and makes the system easier to use. The angled-mirror design also allows multiple particles to be imaged at the same time. The user simply places several aggregate particles on the plate, and the FTI software can easily separate the data into those for individual particles. These advantages come at a price: the viewing angle of the image is limited. With the needle mount, the user is able to rotate the particle in any direction, providing the possibility of full 360â images. Using the plate-mounted system detailed here, particles can only be viewed directly from above. It is important to note that the system still generates a 3-D surface profile. Also, the user does have the ability to flip the aggregates over, gen- erating images of the bottom side as well. This configuration is similar to the one used on the commercial line-scan system.
77 (a) Overall system (b) Horizontal arm Figure C-4. Improved particle mount.
78 Figure C-6. Large/medium particle holders, detail. Figure C-5. Particle gluing station. Figure C-7. Mounting a large particle. Figure C-8. Particle mount.
79 Figure C-9. Aligning the particle mount. Figure C-10. Test fringes. Figure C-11. Particle illuminated by fringe pattern. virtual source fringe generator mirror x y z object CCD cameraIllumination rays Imaging rays Figure C-12. Angled mirror system.
