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3D Phase Measurement Technology

3D Phase Measurement is based on an existing optical metrology technique known as Phase Shifting. This generally involves sequentially projecting three or more line patterns onto a surface, capturing on camera an image of each pattern on the surface and then processing the pattern images using triangulation to produce a 3D map of the surface. With 3D Phase Measurement, the videoprobe projects sinusoidal phase shadow patterns onto the surface. These patterns are then analyzed using specific algorithms and a 3D surface map is generated from the X, Y and Z coordinates of the point cloud that is created.

Unlike conventional stereo, shadow or laser-based measurement systems that operate on a point-by-point basis, 3D Phase Measurement processes the image data to generate a full 3D map of the viewed surface before beginning the measurement process itself. The user can then simply place measurement cursors on a normal, full screen image without the point matching, shadow identification or dot selection steps that can be challenging with other measurement techniques. Areas where measurements cannot be made, because of shadowing or excessive distance to object, are clearly indicated by a red overlay.

3D Phase Measurement imagery provides more information about a flaw. For example, with a dent, the operator can perform an initial depth measurement by placing three cursors outside the area of the dent to establish a reference plane and then placing a fourth cursor within the dent reference plane in a position where the greatest depth would seem to be located. If the point cloud is then viewed, the system will indicate the location of the measurement cursors, focus on the region around the measurement and can optionally color code this region, using a depth scale relative to the reference plane. This can assist in more precise measurement by indicating whether or not the cursors or appropriately placed around the dent and if the fourth cursor is in fact at the deepest point.

3D map can also be rotated, zoomed and viewed to provide further information on the shape of a defect and the location of the measurement cursors. 3D Phase provides a Profile View of the defect as well. While working in Profile View, the user can move a cursor to obtain accurate and fast measurement of depth at points along the cross-section relative to the reference surface. This profile can then be graphically displayed regardless of viewing angle.

Applications An important application of the new technology is the measurement of aircraft engine tip to shroud clearance. Aircraft engines, and other axial flow turbomachinery, are typically designed to minimize the radial gaps between the blade tips and the blade housing or shroud. Gaps between tips and shrouds can reduce efficiency by allowing gas or air to leak into the downstream stages. Consequently, it is very important to check this clearance, both during manufacture and also during service as the gap changes during engine operation. (High operational rotating speeds and high temperatures can cause radial elastic growth of blades, as well as thermal expansion of the shroud.)

Historically, one method of measuring tip/shroud clearance has involved inserting a thin metal rod into an axially drilled bolt and attaching this assembly to the fan case so that the end of the rod is positioned where the blade tips should be. After the engine has been operated, the amount of wear on the rod is measured. Obviously, this is not a high accuracy technique and its execution often generates problems such as the liberation of metal from the rod, which can cause damage to the engine.