Optics: measuring and testing – Shape or surface configuration – Triangulation
Reexamination Certificate
2002-06-27
2004-08-31
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Shape or surface configuration
Triangulation
C356S623000, C250S236000
Reexamination Certificate
active
06785007
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of scanning devices. In particular, the present invention relates to the profiling of a component which includes scanning a spot of light through a range of angles. More specifically, the present invention relates to the measuring of the external surface profile of a component using a non-contact optical technique which has reduced sensitivity to anomalous off-axis reflections. Even more specifically, the present invention relates to the measuring of the external surface profile of a component using a non-contact optical technique which scans the field of view by utilizing a rotating mirror and which precisely determines the pointing angles using a time-based method.
BACKGROUND OF THE INVENTION
Freshly machined metallic (e.g. aluminum) parts, or components, have a highly reflective surface finish such that they can be considered a mirror with random grating marks. Conducting optical metrology on reflective surfaces of this type is difficult because the secondary reflections show up as bright anomalies that can severely complicate analysis. Although the initial measuring spot is visible, the anomalous light caused by scattering, diffraction, reflections, and multiple reflections off the part surface other than from the desired scan region show up as much brighter when recorded by a detector (or camera).
An existing form of non-contact profile measurement system that is currently commercially available includes the use of a laser fan that illuminates the part to be tested and a two-dimensional area detector that measures the profile of the part. This type of system uses no moving parts, includes the ability to operate with background ambient light, and results in cross-section measurement by simultaneously analyzing the entire area illuminated by the laser fan. The system has disadvantages associated with it that are significant, for example, the number of rows and columns in the area detector fundamentally limits depth resolution and cross-section resolution, respectively, of the system. Complicated tradeoffs in imaging performance occur because the area detector is rectangular in extent while the field of view of the area detector and the laser fan are both roughly trapezoidal in shape. This results in the area detector having a practical readout speed limitation of less than 60 frames per second which limits how fast this system can scan parts. The most significant drawback of this scanning technique is the system's sensitivity to spurious reflections from highly reflective parts because, as explained above, the image of spurious reflections of the laser fan can be brighter than the initial image where the fan illuminates the portion of the part. Such detection of the spurious reflections resulting from the laser striking highly reflective machined surfaces confuses the image processing of this type of scanning system and, therefore, renders the system completely ineffective at measuring highly reflective machined surfaces.
Another existing non-contact profile measurement system that utilizes a laser having a potential for less sensitivity to spurious reflections is a system comprising a single point of illumination light that is scanned across the part and measured using a “staring” area detector having a fixed field of view. Such a system, however, is not immune to detecting spurious reflections and can be easily confused when the image of the spurious light is brighter than the image of the initial laser spot. This system also has limitations to its depth resolution and cross-section resolution dependent on the characteristics of the area detector utilized. The most significant drawback to this particular approach is that it is extremely slow since it can measure only one point per frame of the area detector. This results in the area detector having a practical readout speed limitation of less than 60 points per second.
Even still another existing non-contact profile measurement system that utilizes a laser is a height gauge system which uses a single point laser illumination and a linear detector. There are several inexpensive and relatively fast single point laser scanners based on this technique that are commercially available for applications such as web inspections. Although this type of system has low sensitivity to off-axis spurious reflections due to the linear detector having a limited field of view, the main drawback to this technique is that the system only measures the height of the test part in one location and has no provision to provide a cross-sectional profile scan of the entire external surface of the part. It is possible to move the part under the single point scan or to move the system completely around the external profile of the part. This would be the optical equivalent of the single point touch probes used in coordinate measuring machines (CMMs). Although this technique can be accurate, it is also very slow. Since an excessive amount of time would be required to measure the external surface profile of the part with sufficient density, this technique is normally utilized to measure only a few representative points along the external surface of the part.
Polygon mirrors are well known in the art of applications such as printing and bar-code scanning. These polygon mirror scanners involve a metal disk with highly polished facets around the perimeter. In such implementations, the metal disk acts as both the structure of the rotor and the substrate of the mirror. This monolithic approach can yield a stable structure with very repeatable scan characteristics. One drawback to this method is that the surfaces of the mirrors are prone to defects left over from the machining process. If a post-machining polishing step is used to minimize the mirror defects, other undesirable defects such as edge turndown and wavy surfaces are likely to result. These common defects can result in unwanted scattering and out-of-plane wandering of the reflected light. As a result, high quality monolithic polygon mirror construction is time consuming and is therefore an expensive process that would be extremely prohibitive for the size of the rotor required in this system.
Another typical method for constructing these types of polygon mirror scanners involve adhering individual mirrors (typically first surface glass mirrors) to a supporting rotor structure. This is an inexpensive method of insuring good quality mirror surfaces that can be applied to rotors with large facets. The potential drawback to this technique is that it can be difficult to adhere the mirrors to the substrate in a fashion that insures common alignment of all the facets so as to minimize out-of-plane variation of the reflected light. Additionally, the mechanical stability and alignment can be adversely affected in the presence of effects such as temperature variations.
The most common motors used to drive precision scanner rotors are either AC brushless or DC brushless motors. Brushless motors are utilized primarily because they have minimized rotational “cogging” which is present to a small degree with all brushed motors and to a very large extent with stepper motors. At high rotational speeds, brushless motors can be controlled to yield extremely constant rotational velocities. However, these precision controllers are relatively expensive to implement.
A barrier to utilizing either brushless or brushed motors in this scanning system is that it is difficult to establish precise rotational control when the rotational velocities are as slow as 60 RPM. The torque delivered by such motors during rotation is centered on a few poles determined by the structure of the motors. To help even out the uneven application of torque, angular momentum (L) is typically utilized to smooth out the effects of the uneven forces applied during rotation. The angular momentum is related to the structure and motion of the rotor by L=I&ohgr; where I is the moment of inertia of the rotor and &ohgr; is the angular velocity. At high angular velocitie
Clary Thomas R.
Greenberg Heath M.
Johnston Kyle S.
Lock Tomas E.
Nelson Spencer G.
Esserman Matthew J.
McNichol, Jr. William J.
Metron Systems, Inc.
Pham Hoa Q.
Reed Smith LLP
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