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The Challenge of Part-Out Sensing, Part 2

By: George Keremedjiev

Wednesday, October 01, 2008
 
Parts exiting a die become miniature airplane wings—exhibiting lift and rotation as they freely move through the air and into the customer’s box. The parts may have many holes. Imagine an out-of-control airplane wing that has broken off an airplane in midflight—the wing is full of holes, spinning, turning, flipping, tipping, somersaulting and otherwise in motion. You get the picture. You have to deal with this every single day as a sensor applications specialist.

There is no magic in part-out sensing, only well-reasoned science. One needs to keep a handful of challenges in mind as the exiting parts are carefully analyzed. First and foremost is the trajectory or flight path of the part. As mentioned in last month’s column, a careful set of visual experiments must be conducted on the shop floor to ensure that the proper zone of coverage for the sensor(s) is determined. Whether doing it with one’s eyes or through the aid of a strobe light or high-speed camera, the crucial first step toward successful part-out detection is a thorough understanding of the motion of the part as it exits the tooling.

But here is the rub. The part usually has one or more holes, making it look like a flying piece of Swiss cheese. Actually, there are some parts that seem to have more holes than surface area with minute segments of sheetmetal holding the holes together into a recognizable shape. Who are these part designers and why would they do this to us? In any case, the part is what it is—holes and all—and we need to detect it and all others behind it that will follow out of the die. As the part flies out of the die and through the beam of light or large inductive field, be sensitive to the fact that with just the right speed, spin and number of holes, the part, for all practical purposes, appears as a jumble of holes within a randomly structured webbing of sheet. Set your sights on the webbing first and foremost. Ignore the overall shape of the part, it is irrelevant in as much as this shape becomes a blur as it travels through the sensing field. Keep thinking, “Swiss cheese in motion.”

Holes held together with minimum sheetmetal—that is what you are detecting in some of the more difficult parts. Thus your choices of sensors must be restricted to those types that can detect minimum metal. Don’t obsess with geometries; focus instead on the total solid mass that will be available for the sensor to detect at the moment that the part enters the sensing field. And then add motion, rotation and speed to that mass and you now have a solid understanding of the real challenge.

And don’t forget acceleration. Many dies have air blasts that propel the parts out of the die. The air is delivered via small tubing attached to an air solenoid that turns on the air or the tubing is directly connected via a pressure regulator to a line of shop air, making the die sound like a jet engine. Parts exiting in these types of dies don’t just leave, they accelerate out of the die unpredictably as their exit ranges from smooth to jerky, pending their stickiness to the oil in the die.

Let’s review. Part-out sensing, whether at the side of a die or out of its bottom is to be thoroughly understood from the perspective of holes in motion. Many styles of sensors need to be initially selected and then thoroughly tested on a bench to determine how good they are at detecting the minimal solid areas of a part as it exits the die.

On the test-bench simulations, take the part and, based on your patient observations of how it exits the tooling, present the part during this test-bench simulation in as close a recreation of the real world as possible. It may mean taking the part, rotating it with tweezers and determining the least amount of sheetmetal that would need to be detected. It may also means propelling the part through the sensing field on the test bench with an air blast.

You cannot be too paranoid when it comes to part-out sensing. Better to overtest on a bench than to have intermittent part-out issues during production runs. You need to find within yourself the patience for testing on a bench. This is my best advice on this issue. Do not be sucked into a false sense of security—assume the worst possible scenario where minimum material at maximum exit speeds needs to be detected.

Just keep in mind Thomas Edison’s famous saying, “Invention is 1 percent inspiration and 99 percent perspiration.” You need to sweat out the vast majority of part-out detection details on a test bench and not on the production floor. MF

 

Related Enterprise Zones: Sensing/Electronics

 


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