Quality 101: 21st Century Optical Comparators
Optical comparators have steadily evolved from the simple comparative shadowgraph devices of 60 years ago to the more complex, precision optical measuring systems of today. Some had predicted they would be replaced by other technologies such as camera-based (video) measuring systems. However, they continue to play an important role in noncontact measurement of profile dimensions. They also are practical alternative to video technology for many applications. In addition, there is a large installed base of optical systems so many people are already trained in their use.
While large floor-standing machines with very large screens and manual axis control provided the best accuracy 25 years ago, improved optics, readout devices, backlighting, software/computation systems and precisely controlled motorized work-stage travel combine to provide sophisticated, compact platforms. Projectors are available as benchtop machines or floor-standing models in either horizontal or vertical orientation with screen sizes ranging from 12 inches to 40 inches.
Advanced Software and ReadoutsToday’s machines are markedly different from the first shadowgraphs that could only compare projected images of the work piece to overlays for simple pass/fail inspections. While today’s projectors can still be used for overlay or comparative purposes, an ever-increasing demand for quantitative data has given rise to advanced optical comparators with a wide range of software and readout systems.
Readouts today vary in sophistication from traditional micrometer heads or indicators to digital readouts, microprocessor displays with geometric measurement capabilities and PC-based systems. In addition, an edge-detection capability, which allows the system to automatically detect and measure transition points of a profile image, has become a common industry feature. The combined effect of these capabilities has essentially transformed optical projectors from comparative tools to two-axis coordinate measuring machines.
Other automated features further enhance the capabilities of comparators. An operator may select a machine with manual, motorized or fully automatic operation. Fully automatic or computer numerical control (CNC) machines fitted with motors and edge-detection, allow part inspection to be completed under CNC control. Readout systems of today also are flexible because they allow for software expansion, as programs continue to add enhancements.
Super fast geometric processors make it possible for some projector software to interface to computer-aided design (CAD) to simplify part programming. This capability, combined with CNC control, allows features to be measured and constructed automatically with the results displayed on-screen or documented through various reporting options.
Quantitative DataTo get an idea of how much data today’s comparator can extract, consider the results that can be determined by measuring the diameter of a circle:
the maximum or minimum diameter.
the average diameter based on all measurement points taken, or the mean diameter based on the high and low values (ISO).
form (out-of-roundness) shown simply on a graphical display.
tolerances also can be applied to features, be they Cartesian, true position, maximum material condition (MMC) or least material condition (LMC).
Unique EnhancementsIncremental improvements based on new and emerging technologies continue to extend the utility of 21st century comparators. Here are two examples:
Optical-video adapter. This technology allows optical comparators to be converted into a basic two-axis manual video measuring systems by simply interchanging the comparator lens with a camera-based, video zoom package. The optical-to-video adapter utilizes the same quick-change bayonet mount that an operator would use to change from a 10X lens to a 50X lens, with no tools required. It allows operators to obtain excellent surface illumination at substantially higher magnifications than would normally be possible with comparators.
Touch-screen control. A new readout technology, analogous in function to tablet computers, provides a broad range of powerful, easy-to-use comparator functions via a compact, icon-based touchscreen interface. They provide instant information on feature form, tolerances and measurement data; coordinate display for X and Y axes and Q radius values for screen rotation and allow for easy part alignment and datum functions at the touch of a finger.
Some Things Never ChangeRegardless of how advanced a readout system is or how powerful the software, optical equipment will not perform to desirable standards without a high quality, stable machine platform, superb lighting and high-resolution optics. When selecting a system, it pays to focus on these critical basics, being careful to evaluate each element of the system.
That being said, modern optical projectors continue to be noncontact measurement workhorses because of their exceptional practicality, durability versatility, and ease of use.