Vision Meets Metrology
November 2, 2012
Vision metrology systems are driven by advances in optics, electronics and software.
Vision metrology systems with a moving stage and a video camera have been in use since the 1960s, when cameras were mounted on a toolmaker’s microscope. These systems have now proven themselves for feature measurements and surface inspection in a wide range of applications.
Early vision metrology systems made use of crosshair reticles or an external crosshair generator to superimpose the image of a crosshair on the video image. The stage was moved manually using micrometers, and stage positions were written down as image features passed across the crosshairs. The micrometers, and then the stage itself, became equipped with encoders to produce a numeric display of position on a digital readout (DRO) unit. In the late 1970s, the advent of the video frame grabber board and machine vision software allowed automatic edge detection, which led the way to the development of vision metrology software.
The early cameras were analog and produced a black and white image, with a theoretical resolution of 480 lines across the video monitor, but actual resolution was often closer to 300 lines. While primitive by today’s standards, these metrology systems set the standard by providing a highly magnified image of optical edges, and they allowed edge measurements with better than one micron resolution.
During the 30 years since their introduction, commercial vision metrology systems have branched into a wide range of products differing in size and capability for different applications and industries. The smallest bench-top systems have a compact stage commonly with 2 inch by 2 inch of manual X-Y travel, while larger systems can offer CNC travel to accommo-date large parts. The smaller systems tend to be manual, with X-Y stage positioning via hand wheels. The larger and more expensive systems are normally motorized and driven by CNC software, but they can also be operated manually for individual measurements and short jobs using a joystick and trackball pendant.
Vision metrology systems are now available in the form of multi-sensor CNC metrology systems, where the video lens “probe” can be augmented by a touch probe and/or a laser probe. In these systems, the video camera is used for high-speed measurement of edges within the field of view, while the touch probe handles contours, vertical surfaces and under hangs. The laser probes can rapidly collect a large number of data points to characterize complex shapes. Lasers may be suitable for translucent and soft surfaces, as well as difficult geometries such as narrow slots, grooves and blind holes. Metrology software can invoke all probe types as needed for measurements on the same part.
Zoom optics, with a continuously adjustable magnification ratio of 6.5:1 or 12:1, have long been used as the imaging optics for their effectiveness and convenience. These optics are first used at a low magnification and large field of view zoom setting to locate the features of interest. Then a high magnification zoom setting is used to measure these features with highest accuracy. The characteristic of zoom optics known as parcentricity ensures that the image remains centered at all magnifications. However, the convenience of the zoom optics is not without some limitations: the field of view is relatively small, even at the lowest zoom setting, optical distortion and optical resolution is limited.
A Look at Telecentric Lenses
The combination of telecentric optics with an ultra-high resolution video camera is now revolutionizing the field of video metrology by simultaneously offering a wide field of view, close to zero optical distortion, high-resolution, and real-time video imaging.
Telecentric optics are compound lenses that process the chief ray from each point in the field of view as if that point were on the optical axis of the lens, with the result that all chief rays are collimated and parallel to the optical axis. Parallel rays provide the benefit that magnification is the same for all distances from the center of the lens, eliminating perspective effects and most optical distortion. The image remains sharp with irregular, non-flat surfaces.
If the lens is to accept all parallel rays from within the field of view, the entrance aperture of the lens must be at least as large as the field of view, making telecentric lenses larger and more expensive than conventional lenses of the same focal length. While telecentric lenses have been available for many years, only recently have they come down in price to be affordable for vision metrology.
Telecentric optics are now also being offered in conventional vision metrology systems with a horizontal X-Y stage and a Z column. Current telecentric optics are fixed focus, but can be mounted so that they can be changed by the user in seconds.
Field-of-view (FOV) measurements, where all measurements are performed from video data within the field of view with no need for stage motion, have been made possible by the combination of telecentric optics with a high-resolution camera, high-speed processing, and new FOV metrology software. For example, a 5 megapixel camera divides the field of view into 2448 by 2058 picture elements and can provide 0.00075 inch (19 µm) of optical resolution across a 2.4 inch by 2.0 inch (60 by 51 millimeters) field view. FOV measurements on a small part or on features of a large part can be seamlessly integrated with stage motion for larger parts and be compared to CAD files by DXF/FOV metrology software.
History & Product Evolution
New, more powerful controllers are becoming available. We are now also seeing increasing overlap between optical comparators and vision metrology systems.
Optical comparators were patented in the United States as far back as 1929. Known as shadowgraphs and profilometers outside of the United States, these are rugged machines that are still being sold in large numbers for use in machine shops worldwide.
In the basic optical comparator, a part is brightly illuminated from the rear, and its profile, or silhouette, is projected on a screen at a precisely known magnification from 10X to 100X. The projected image is then compared to a screen overlay with pass-fail limits. Modern optical comparators also incorporate a moving stage with digital encoders so that dimensions can be read as the projected edges move past crosshairs etched on the screen. A common refinement is an optical edge detector, which measures brightness at a specific screen location and removes operator subjectivity in locating edges.
A new class of horizontal digital video comparators is a hybrid between an optical comparator and a vision metrology system. With a horizontal optical axis, an all-steel housing, and a stage with load capacity up to 110 pounds (50 kilograms), such a system is built like a traditional horizontal comparator for heavy-duty applications; however, it uses telecentric optics, a multi-megapixel video camera, and video image processing. It can import DXF CAD files and make direct high-speed electronic go/no-go comparisons to the engineering design, thereby eliminating the need for screen overlays. It can also measure and record deviations for statistical process control (SPC).
Driven by advances in optics, cameras, electronics and software, vision metrology systems have increased in performance, speed and versatility. Expect more from these systems for both measurement and quality assurance.
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