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New Approaches in 3D Applications
by Winn Hardin, Contributing Editor - AIA Posted 06/12/2000
Although end users rarely have reasons to view it as such, machine vision is an optical technique, depending on light and the sensing of light in conjunction with silicon brains to perform inspection, process management and quality control operations once performed by human operators. In today's competitive markets, the need for tighter controls seems never ending. As such, the vision systems employed by companies have to provide more specific data as efficiency demands increase.
Perhaps nowhere is this trend more prevalent than in 3D systems. Once the exclusive domain of stereographic or twin CCD cameras that mimic the human eye, resolution requirements have far exceeded the better-than-human capabilities of electronic imaging systems. Today, companies are turning more and more to structured light through the use of coherent laser light or other optical techniques such as interferometry.
According to market watcher Nello Zuech of Vision Systems International, 3D vision inspection systems are finding their greatest demand in wood, automotive, semiconductor, electronic and general metrology applications. Although many of the systems offered for these various applications are similar in approach, slight variances reveal the different engineering approaches to solve a similar problem: namely, the fastest way to determine an object's dimension in full 3D Cartesian space.
According to Matt Collins of Perceptron's (Plymouth, MI) Emerging Business Unit, his company's business is delineated as 80 percent automotive, 15 percent lumber and related and 5 percent aerospace. In the lumber industry, the LASAR system is one part of a complete automated mill solution that helps clients be as efficient as possible when converting raw timber into lumber products.
LASAR provides the 3D imaging portion of the system, which includes tracking, ultrasound testing for rot and other internal defects, reporting and other functions. According to company officials, LASAR is a 3-Dimensional Large Field of View Scanner that utilizes a raster laser to scan the topology of an object within the scan beams line of sight. LASAR generates simultaneous Row, Column and Range Information for conversion into a users coordinate frame. LASAR has adjustable scanning patterns to accommodate different sized objects, speeds and resolutions; also several sensors can be linked to provide a larger Field -Of-View Coverage. To date, Perceptron's LASAR applications are limited to specific Lumber Scanning, Steel Processing and Research applications.
According to Kyle Johnston, a founder of Metron Systems, Inc. (Snoqualmie, WA), who also use a flying laser point scanning system for 3D measurements, single point data collection allows for greater depth of field, resolution and ability to tackle reflective surfaces with greater ease than laser fan approaches. Metron's G2X, which is expected to be officially released this month, provides a 60-arc degree, or maximum 21-inch horizontal coverage at maximum standoff from the inspection surface of 19-inches. Designed initially for aerospace applications, the system focuses on generating accurate data with a point to plane resolution of 0.0009-in. in the center of the scan area, a plane-to-plane accuracy of 0.001-in. and plane to plane repeatability of 0.0001-in.
Although high resolution laser scanning is not a speedy process, Johnston has created his system to incorporate full Ethernet capability, linking up to 12 sensor heads into a single data collection system that automatically normalizes data gathered from separate sensors into a single 3-D data set. This allows greater speed and coverage for large parts, such as the airplane wings and components the system was originally designed to inspect.
Focusing solely on the electronics industry, RVSI has incorporated a laser scanning mechanism into its LS-7700 semiconductor inspection system. Although RVSI's laser scanning system has been used for different purposes across the past several decades, a singular-focus on the semiconductor industry has allowed the company to beat spurious reflection problems associated with inspecting ICs during the fabrication process. The self-contained LS-7700 provides a total solution, including both final lead inspection and tape/reel packaging for shipping. Utilizing unique position sensitive detectors (PSD), RVSI eliminates spurious noise created by light reflecting off of the trays holding the ICs by only reading the portions of the PSD that relate to the location of the IC itself, excluding light that is reflected by the walls or bottom of the individual tray sections.
Line scanning & interferometry
To speed up the data collection process, Intelligent Automation's (Cambridge, MA) 4DI system projects up to 500 lines of light from a Class II diode laser on to the object under test. Incorporated into the sensor head along with the light source are two or three 640 x 480 CCD arrays that gather the reflected light from the entire field of view simultaneously within the standard 1/30th of second accorded to standard CCD cameras.
Turbin Ring Inspection
Intelligent Automation's 4DI 3D inspection system has been used in several applications including turbine ring inspection and coplanarity tests of IC connectors.
The stationary system can accommodate stand-off distances from several feet to 0.25 inches, yielding an accuracy of 1/1000th of the overall field of view or 1/4000th of the field of view using combined data points. Resolution is also dependent on stand off and field of view, typically a function of the light projector, or up to 100 horizontal by 500 vertical points.
According to Larry Chin, vice president of sales and marketing for Intelligent Automation, selecting between two or three cameras is a question of how much depth of field the application requires. "Adding a third camera allows you to triangulate the same line from different cameras with greater accuracy and yielding a greater depth of field," he said.
PPT Vision Inc. is offering a different approach to yielding high-resolution 3-D data sets. First incorporated in the PPT861 turnkey IC inspection system, white light interferometry and a unique sensor combination yield 3D data sets with an x/y resolution of 28 microns and 2.5 micron z resolution, or the equivalent of dividing the 5.5-cm scan width by the 2096 pixels in one of its three linear scan CCDs.
According to Arye Malek, vice president/general manager of PPT's Microelectronics Systems Division, the white light interferometry uses a trilinear array, or a combination of three linear arrays separated by a known distance. Light from a halogen light (selected for its stability across the visible spectrum) first passes through a grating that separates the light to many distinct sine waves. The individual rays passing through a condensing lens that projects the sine waves onto the IC tray at an angle. The trilinear CCD measures the phase changes along three known lines, producing x, y and z coorindates for the object under test, or in this case IC packages.
The system does not use an internal scanning mechanism and is capable of collecting data across 5.5-cm at 12.5-cm per second at speeds of 10 MHz or 10 million z-points per second. The system, which was recently sold into an OEM semiconductor test manufacturers line of inspection equipment, requires two passes to inspect a standard JEDEC tray.
"The interesting part of this technology is that you can change the sensor (configuration) to fit the application. If you change the angle of the sine wave projection, you change the density of the sine wave, the field view and the swath, giving a sensor with different combinations of x, y and z resolution," Malek explained.
Like his competitors, Malek expects the 3D systems to move towards more sophisticated optical arrangements. "In the past, projecting lines on an automotive part using a single array or stereogrpahic systems were adequate. Today, structured light approaches offer the higher resolutions our customers demand," he said.
About the Author: Prior to his recent appointment as managing editor of VerticalNet's Wireless Design Online, Winn Hardin wrote for a variety of vision-focused trade publications, including Vision Systems Design, OE Reports and Photonics Spectra.
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