Expert's Corner Fall 2020

Expert's Corner Fall 2020

CORNER

21 ST CENTURY OPTICAL COMPARATOR TECHNOLOGY The Quality Corner article in our Summer, 2020 issue provided an introduction to traditional optical comparator technology. As promised at the end of the article, we now present an update, detailing the latest technology advances that have been applied to this versatile inspection methodology, courtesy of CW Moran and the renowned L. S. Starrett Company.

The optical comparator has been around almost as long as the need for precision measurement has existed. With the advent of modern manufacturing came the need for strict quality control. From the world of telescopes and microscopy came the development of the first comparator. That was almost a hundred years ago, but, much like the modern bicycle with its opposing triangles, while technology has improved in almost every avenue available, the core principles have remained unchanged. The optical comparator is brilliant in its simplicity. Light casting a precise shadow is sent through a magnifying lens. It is then reflected off of a mirror and projected onto a screen at a much larger scale. An overlay or master is attached to the screen that corresponds to the magnification (10x, 20x, etc.), and just like that, minute details can be analyzed with extreme precision. It’s a technology that has stood the test of time because of its affordability, reliability, and simplicity.

everything. All of the wiring, mirrors, and computing hardware are safely away from the environment. The inspection stage itself is made of precision machined steel or aluminum. The light sources and electronics are also secured in an enclosed lamp house. All of these attributes contribute to both a sturdy and resilient design. As technology increas- es, the capabilities of manufacturers have grown exponentially. Optical comparators themselves have scaled with the demands. Systems are available in a full range of sizes; from small-stage bench top models to large floor-standing models that come equipped with large, heavy duty, stages with greater travel and weight capacity, as well as larger inspection screens. Both subdivisions of comparators, bench top and floor standing, are also available in both horizontal and vertical viewing orientations as well, depending on the needs of the operation.

This comparator features an integrated PC and edge detection

Alongside the actual improvements in the construc- tion of the systems themselves are improvements in the digital readouts (DROs) and software that are available at the operator interface point on the sys- tem. Available options for DROs have become as di- verse as the system offerings themselves. Depending on the specific needs of the end user, options start with a color touchscreen tablet for when simple, stan- dardized, measurements are required or basic geo- metric functions need inspection. Similar formats to the tablet interface also exist in self-contained units, sealed from the elements with a rubber keypad to ensure continued reliability under varying conditions.

A modern optical comparator

The first and most obvious improvements come from the actual equipment itself. Modern optical compara- tors are self-contained units. The body of the system, usually constructed from steel or aluminum houses

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Optical comparators, though, are no longer restricted to just optical magnification lenses. New advancements have allowed a comparator to be equipped with a video camera, similar to video inspection equipment. Much like a video inspection system, camera options come in both zoom and telecentric options. A zoom lens, capable of magnification levels of up to 240x, is useful for when smaller and more intricate features are the goal of inspection. Conversely, a telecentric option is more useful for when parts with larger features need to be fit within the field of view (FOV). Multiple telecentric lenses are now available depending on the specific FOV requirements of the user. An optical comparator that is equipped with a zoom or telecentric lens has all of the power of both a traditional comparator and video inspection system, but at a lower cost, while maximizing the initial investment in the comparator. The video cam- eras are designed to be interchangeable with traditional magnification lenses allowing the user to retrofit an existing system, quickly switch between interfaces, or equip a multi-lens system with a combination of the two. Modern comparators even have the ability to be equipped with a dedicated video system setup. Systems like the HDV Series from Starrett are essentially an optical comparator with a telecentric or zoom video lens, depending on the user’s needs. When an optical comparator is equipped with a video camera, it gains the ability to store digital overlays. Digital comparator technology eliminates the need for physical overlays and templates which can be costly to maintain and store.

For more advanced requirements there are a number of PC-driven options available. All in one touchscreen PCs are available and can mount directly to the comparator itself, as well as a traditional PC with monitor, keyboard and mouse for when full CNC control is required. All of these options are now also capable of supporting optical edge detection, a technology that makes use of a fiber optic edge sensor to identify transitions between light and dark fields, reducing operator subjectivity dra- matically when entering feature points to be measured. Second to none in terms of importance is the illumina- tion on an optical comparator. Most modern systems are equipped with LED lighting. LED illumination provides many advantages over more traditional options like fluorescent or halogen. They are far more durable and can withstand varying conditions across a wider range of temperatures making them far less likely to suffer a failure. They also boast a far longer lifespan than traditional bulbs and provide greater flexibility in design options, making them a more versatile resource. Along- side these important design features are many other benefits, including decreased energy requirements and lower waste production, that make them a very energy efficient and cost-effective design feature. The main feature set for any given optical compara- tor, however, is the optical equipment options that are available. There is a wide array of lens configurations available for both bench-top and floor-standing sys- tems. Lens options can range from 5x magnification all the way up to 100x in even magnification increments. Special lens options are also available, such as the 31.25x magnification, for when a special magnification specification is called for. Modern lenses are also easily interchangeable; a quick turn of the lens allows one magnification to be removed and another to be inserted in a matter of seconds.

Most companies also produce an extensive line of accessories that are designed to work in tandem with and enhance the capabilities of comparators. Storage cabinets are available in multiple sizes that are designed to work as a base for bench-top models or along- side floor-stand-

ing systems. Canopy and

curtain systems are available for

The Starrett HDV Series digital comparator

when the need for controlling, or minimizing, external light arises. Many different part-holding fixtures are also available depending on the specific requirements of each piece including multiple sizes of centers and vees, both fixed and rotary vises, rotary work stages, glass plate work holders, screen overlays, and calibration standards.

Lenses are readily interchanged

More advanced bench-top models, as well as floor-standing models, also have the capability of being equipped with two or more lenses simultaneously and indexed into position by means of a turret or slide.

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CORNER EXPERT’S

These are just a few of the available comparator accessories

The multitude of accessories also available with systems, along with optional CNC control and feature-packed operating systems and DROs, all increase the ability to provide accurate, repeatable results. Many years later, the modern optical compar- ator still looks very familiar to the systems of previous generations. Much like the bicycle, it is a brilliant initial design that has allowed innovation to keep it relevant and continue to ensure performance in today’s manufacturing environment.

A modern optical comparator has the potential to address nearly any inspection requirement that might arise in a modern manufacturing or quality control environment. The many different system and platform sizes available from current optical manufacturers ensure that a wide range of part types and sizes can be accommodated. A wide variety of lens options and configurations, both traditional and video, also ensures that nearly any feature can be inspected as well. Brilliant LED lighting along with larger inspection screens and features like optical edge detection make inspection far more accurate by reducing operator subjectivity.

www.starrett.com

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