Page 48 - MetalForming May 2016

Understanding Sensors
right or right-to-left in a circular pattern. When combined with the previously described multiple axes of rotation, this circular motion can prove indecipherable to a photoelectric sensor.
Simulate on a Test Bench
Personnel testing the photoelectric sensor must be fully aware of an exiting part’s potential rotations and vibrations. There is no excuse for not spending some productive R&D time to simulate all of the above conditions before commit- ting to a given sensor application and installation. I suggest first testing the photoelectric sensor to ensure that it will not pick up any oil mist ejected along with the part. Should the part refuse to exit, the photoelectric sensor may be fooled into thinking differently as it picks up oil mist instead, and incor- rectly issues a part-exit signal. This can lead to a double-hit die crash. Oil-detection testing is best accomplished by fill- ing a plastic spray bottle with the same oil, with the same vis- cosity, as will be used to produce that specific part. This test identifies any sensors that could be fooled by the oil.
After oil-testing, conduct a meticulous test of what the photoelectric beam can and cannot detect related to rotations and vibrations of the stamped part. Use a test bench with the transmitter and receiver mounted on adjustable, track- mounted towers. Mount the transmitter and receiver to mimic their positions on the die. Then, take a set of tweezers in the case of a small stamped part, or a pair of long-nosed
pliers for a larger part, and hold the part within the light beam. With much patience, rotate the part into every con- ceivable position within the light beam. Introduce slight vibration and jiggling by agitating the tweezers or pliers.
In other words, simulate every possible worst-case sce- nario regarding the position of the part within the light beam. You must find any blind spots that may arise with the part in a particular position, and then decide if the sensor can be adjusted electronically to compensate for such blind- ness. Increase sensor sensitivity to accommodate blind spots only after making sure that the increased sensitivity will not result in the sensor mistaking oil droplets for the part.
It’s Not Just Part-Out
Although this article focuses on part ejection, the very same concerns are valid for other photoelectric-sensor appli- cations, such as feed detection, cam/block returns, part-fea- ture detection, transfer operations, etc. Remember: All tar- get motions, no matter the application, should be simulated on a test bench first, before any photoelectric sensors are installed within the tooling. MF
Part 3 of this series will detail the various part-quality- measurement sensors that can be used within tooling to per- form 100-percent inspection−at running speeds—of critical dimensions.
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