50 Years of Quality: The Pros and Cons of Optical Comparators
January 1, 2011
The illustration pictured here should appear familiar to anyone who has spent any time in a quality control lab or on a factory floor. This is a picture of the venerable optical comparator from patent number 1,903,933, filed May 21, 1925. The fact that a modern comparator appears little different from one patented 85 years ago might raise several questions about its relevance.
The reason comparators have not changed much in 85 years is that the fundamental technology behind an optical comparator is elegantly simple, and it simply works. Since the physics behind optics have not changed, the only possible improvements in comparator technology will revolve around the quality of the optics themselves and the features added to the comparator to make taking measurements easier for the operator.
A good analogy for the principle behind an optical comparator is that it is somewhat similar to the old-school overhead projector. In fact, one can make a rudimentary comparator device with an actual overhead projector.
First, place any two-dimensional part on the stage of any overhead projector and project the image onto a large piece of paper taped to a wall. The resulting shadow projected on the paper can be outlined with a pen.
This pen outline becomes the reference to compare to any subsequent objects placed on the stage. If the parts do not match the drawing, they are not the same.
Projection BasicsAs a practical matter, we cannot really use an overhead projector to check parts. For one thing, if a projector is sitting on a cart, as most are, what happens if the cart is bumped or moved? The projection distance changes slightly, and as a result, the size of reference image on the wall is no longer accurate.
The basic concept of an optical comparator for quality control use is to take that idea of the overhead projector and package it all up inside a box so that the optical distance between the part and the screen is fixed, known and can be calibrated. A part is affixed to a stage, a light source shines on it, and the resulting shadow image of the part is magnified with lenses and bounced by mirrors, to be projected on the back of a screen for magnified viewing, pretty much just like the overhead projector example.
Based on the known magnification of the lenses, measurements of the part can be made directly off the screen, traditionally using a screen overlay or crosshairs as the reference point for projected points or edges. The operator centers a feature of interest on the crosshairs, records a point, then moves the image and records another point.
It is a process of taking multiple points that allows features such as circles, slots, radii and edges to be constructed mathematically. Typically this is done via a microprocessor-based digital display.
The size and magnification of the projected image on a comparator is dependent on the optics and screen size of the comparator itself-typical screen sizes range from 12 inches to 36 inches but ones up to 60 inches have been built.
However, the larger the screen size, the larger the enclosure becomes because a greater distance is required to throw the image. A comparator with an enormous screen is basically a giant, mostly empty box used to inspect small parts, as explained in the sidebar, “Pay No Attention to the Man Inside the Comparator.”
Straightforward, Easy-to-UseThe advantage of a comparator is that it is straightforward to use for simple operations with relatively little training. On the simplest comparator, an operator merely has to affix a part and move it with hand controls and observe the onscreen image. Advances in technology, such as computer displays that do the math automatically and remember all measured points, automatic triggering technologies and improvements in stage movement all have contributed to the venerable comparator continuing to serve a useful function in a quality lab.
If one reason for the continuing popularity of comparators is their basic simplicity, this also is their downfall.
As production parts become ever more complex, with more features to inspect to greater tolerances, with higher sampling rates or even 100% inspection, the advantages of the traditional comparator diminish significantly. The rise of vision-based inspection systems makes manual comparator technology, even equipped with modern capabilities, seem quaint in comparison.
This is particularly the case when the requirement is to inspect large quantities of parts at once, where the automation capabilities of a vision system make it the winner in speed and flexibility. Automatic moving stages, computer-aided design (CAD) programming capability, the ability to use multiple lighting techniques and 3-D inspection capabilities greatly surpass the limitations of traditional comparator technology.
So why would anyone still choose a comparator? For many simple, nonrepetitive tasks on two-dimensional parts with clearly defined edges, the optical comparator is still a great tool to have in the toolbox. As with any applied technology, knowing the correct tool to use and when to use it is essential.