Page 40 - MetalForming September 2010

George Keremedjiev has been writing this column for more than 20 years. He regularly consults with metalforming companies worldwide and provides metal- formers with training on the appli- cation and implementation of sen- sors for die protection. For more information on his seminars and consultancies, contact:
Tecknow Education Services, Inc. P.O. Box 6448
Bozeman, MT 59771
phone: 406/587-4751
fax: 406/587-9620
www.mfgadvice.com gk@mfgadvice.com
One of the nastiest surprises that can befall an optical sensor application is the interplay between focus, reflection and refraction. These three properties of light and their interac- tions with a target, lens and optical sen- sor can cause havoc and much frustra- tion if not properly accounted for in one’s bench top experiments. Let’s look at all three in detail.
corner must arrive at a specific point in space where they converge into a sharp image. As also stated above, this point is an ideal with the reality many times being a blur circle or an out-of-focus zone as opposed to a point of light. The optics of the fiberoptic, the camera or the sensor itself may be the cause of this, but also, the geometry of the part can cause the light beam to converge not into a point but rather into a zone. An irregular corner on that punch will more than likely provide a focus challenge.
METALFORMING ELECTRONICS GEORGE KEREMEDJIEV
Focus, Reflection and Refraction—Part 4
1) Focus—As defined on Wikipedia, In geometrical optics, a focus, also called an image point, is the point where light rays originating from
a point on the object
converge. Although
focus is conceptual-
ly a point, physically
the focus has a spa-
tial extent, called the
blur circle. This non-
ideal focusing may
be caused by aber-
rations of the imag-
ing optics. In other
words, whether it be
a vision-system
camera, a self-contained photoelectric sensor or a fiber optic/optical sensor combination, it is important that the object being detected appear as sharp as necessary to the detecting device.
Let’s say that you are interested in detecting breakage on the corner of a small punch. On your test bench you would mount the actual punch (or a very close simulation of the corner of that punch) and then carefully make sure that the light bouncing off that corner is in the best possible focus as it arrives into the lens of your detecting device. As stated above, the photons (or beam of light composed of pho- tons) bouncing off the surface of that
2) Reflection— Again using Wiki- pedia, reflection of light is either specu- lar (mirror-like) or diffuse (retaining the energy, but losing the image) depend- ing on the nature of the interface. Imag- ine your light source hitting the top of a form that you wish to detect. The light
reflected from that form can arrive at your sensor as either specular, a sharp image of the actual form, or as diffuse, a scattering of photons which diffuse or spread out as they bounce off the form’s surface. If your application involves a vision system, then a sharp image would be beneficial, especially if you are look- ing for edge-to-edge measurements, surface imperfections and the like. If, on the other hand, you are simply detect- ing the presence or absence of the form in the first place, then a diffuse reflection may be adequate to make that detection possible.
3) Refraction—Per Wikipedia, refraction is the change in direction of
Dust, shavings, galling, etc. can cause light to improperly reflect off the target’s surface.
This CD-ROM presents dozens of George’s columns as well as papers and exclusive new presentations covering all aspects of die protection and part-quality inspection, starting and maintaining sensor programs, the role of controls in in-die sensing, and the benefits of a sound sensor program. Order it online at www.metalformingmagazine.com.
36 METALFORMING / SEPTEMBER 2010
www.metalformingmagazine.com