Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1998-05-12
2003-10-21
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
Reexamination Certificate
active
06636310
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical metrology and, more particularly, to non-contact surface contour measurement systems and methods.
2. Related Art
Contoured, free-form surfaces and complex part geometries present special manufacturing challenges that have made conventional dimensional metrology approaches unsuitable for providing the necessary degree of control over the manufacturing process to insure that the workpiece dimensions are consistently within tolerances. Contoured, free-form surfaces, which generally have shapes that curve in the three coordinate axes, may be found on automobile bodies and jet engine blades; devices shaped for human use such as telephone hand sets, computer keyboards and tennis rackets; and artificial joints, such as replacement hips and knees. Complex geometries may be found on workpiece surfaces having very small dimensions, such as printed circuit boards, electronic assemblies and intricate machined parts.
In modern metrology it is important to determine not only the true position of a workpiece feature, but also size, run out, flatness, and other dimensional form characteristics of the entire workpiece that can indicate, for example, how well the part will perform as a component in a larger assembly, and how well intricate parts of a larger component have been assembled. Unlike prismatic shapes which require a minimum number of data points to establish dimensional information such as size and location, accurate form measurements require the compilation of a massive number of data points. For example, the diameter of a circle can be defined with a minimum of three data points. Form measurement of a circle, on the other hand, could require as many as 4,000 data points depending upon the level of definition desired.
Coordinate measuring machines (CMMs) have traditionally been used to gather dimensional data for inspection and process control purposes. In particular, scanning CMMs have been developed in response to the need to measure contoured, free-form surfaces. These CMMs automatically collect a large number of data points to define the three-dimensional shape and form of a workpiece or a workpiece feature, and use the information in conjunction with, for example, a CAD system to provide insight into the manufacturing processes. Two common scanning methods are continuous and stitch scanning. Continuous analog scanning is generally performed using an analog scanning probe or a passive probe. Analog probes are used on direct computer control (DCC) CMMs to measure, for example, changes in a workpiece's surface elevation as a proportion of the measuring tip's deflection over its full range. Continuous analog scanning typically sends an analog signal to a computer which processes the signal, converting it into a stream of discrete data points. However, continuous analog scanning probes experience large deflections with minimally-applied forces. Even when used with DCC CMMs, the displacement of analog scanning probes cannot be sufficiently controlled to yield the necessary accuracy to measure intricate and complex workpiece features.
Passive probes, which are generally used in manual CMMs, typically have solid shafts and a large probe tip diameter to reduce errors due to static bending. Unfortunately, conventional passive probes (also referred to as solid or hard probes) have a diameter that is generally too large to be used to measure small features of a workpiece. Furthermore, the diameter of passive probe shafts cannot simply be reduced to accommodate these smaller dimensions. Passive probes have deflections that significantly increase with a corresponding decrease in the diameter of the probe shaft. Thus, reducing the diameter of the passive probe shafts results in significant probe tip deflections. Another drawback of conventional passive probes is that the accuracy of the probes is dependent upon the operator and the speed at which the measurements are performed. As a result, passive probes in manual CMMs are incapable of meeting the requisite accuracy to define form errors of small workpiece features. Also, passive probes cannot be used on DCC CMMs because in such applications the force experienced by the probe is restricted.
In stitch scanning, data is acquired using a switching or touch trigger probe which often includes an electronic or electro-mechanical trigger that generates a signal each time the probe touches a surface of the workpiece. When the probe contacts individual points on a workpiece surface, the probe tip deflects and a switched signal is generated that causes the CMM to provide discrete data points to a controller. The probe tip must then be lifted to break contact with the workpiece surface, moved slightly, and contact reinitiated at another location along a measurement line to collect another measurement point. This approach may be effective for applications requiring a minimum number of data points to be gathered, such as where fairly smooth two-dimensional surfaces prevail. However, for three-dimensional form measurements, the single-point measurement technique, which generally collects data at a maximum rate of approximately 50-60 points per minute on a DCC machine and 20-30 points per minute on a manual scanning machine, cannot practicably collect the requisite number of data points to accurately define form errors. The time required to obtain such measurement points using a switching probe increases the cost of making such measurements due to the increased labor costs and adverse impact on the manufacturing speed. In addition, the time required to make such measurements increases the potential for measurement errors due to, for example, heat expansion during the measurement process.
One conventional approach that has been developed to overcome these limitations of CMMs includes the formation of an image of the workpiece, typically acquired by a video camera. The image is digitized and stored in a computer memory as a set of pixels. The computer then analyzes the image, such as by comparing it, pixel-by-pixel, with a stored reference image. However, there are drawbacks to such conventional techniques. For example, processing of a stored image requires a very large number of calculations. Even with high speed digital computers, processing of many such stored images requires considerable time, thereby limiting the ability of the system to generate immediate, real-time results. Also, the images are often of poor quality due to the inability of such conventional system to accurately obtain high resolution images of the workpiece which, in turn, reduces the accuracy of the resulting measurements.
More recently, other non-contact measurement techniques have been developed to measure surface contours. Such systems have been used to measure workpieces having complex surface configurations, contoured, free-form surfaces, and other complex workpieces such as printed circuit boards and the like. Typically, a single point range sensor using optical triangulation techniques is used to perform non-contact measurements. An illumination source projects a defined area of light onto the surface to be measured. Reflections received from the surface are used to form an image of the light reflected onto a light-sensing detector. As the distance from the sensor to the workpiece surface changes, the position of the reflected image on the detector plane shifts. This lateral shift of image position on the detector is used to measure the distance between the sensor and the surface, thereby providing the dimension of the measured surface.
A drawback to such conventional optical triangulation techniques is that in order to obtain high accuracy, the detector must be able to resolve small lateral shifts in the image position on the detector. This generally requires a high magnification in the direction of travel of the reflected image. However, the sensor is typically separated from the workpiece by a large stand-off distance. As a result, the sensor has a limited ran
Ben-Dov Shimshon
Kuperman Igor
Lanzet Igal
Metroptic Technologies, Ltd.
Rosenberger Richard A.
Wolf Greenfield & Sacks P.C.
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