The Loop

Closed-loop control over the complete process is here. Then again, it’s been here all along.

Columns From: 9/1/2001 Modern Machine Shop, ,

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Peter Zelinski

Closed-loop control over the complete process is here. Then again, it’s been here all along.

Open-loop systems become closed-loop systems—loosely defined—whenever they’re put to use. A human closes the loop. The water pouring into your bathtub doesn’t zero in on the temperature you want the way your thermostat-controlled air conditioning does, so you feel the water yourself and adjust the knobs accordingly.

Machining processes employ a similar mechanism. The part is designed, machined, inspected . . . but then comes the hand in the bath water. The inspector compares the actual measurements to what they should be, and engineers analyze any discrepancies to determine how the process or part should change.

This loop can be slow to close. And just as with a CNC servo loop, the slow update time compromises either speed or accuracy. Lead time is lost while humans interpret the data, or else parts are scrapped if the process keeps running while the interpretation is going on.

One perspective on this problem comes from Xygent, a company that began its life as a software division of Brown and Sharpe. Xygent is working toward an open metrology operating system that would (among other benefits) make it practical to capture metrology data as a virtual model that could be used throughout the process and compared to the CAD ideal. (See article)

A representative of the company recently pointed out an area that stands to benefit: prototype manufacturing. Metrology-based models of mating parts could be given virtual loads to simulate fastening, then compared to see how well the parts fit.

Prototype factories do that same job today. Mating test parts may be shipped to a prototype assembly area from shops in various locations. All this assembly area needs is information—namely, whether the parts will fit together—but the most practical medium for conveying that information is the part itself! As a result, transport time is added to the duration of the loop. Metrology-based models could overcome this delay.

The same models could also speed the process inside the shop. With the machined part model and the CAD model brought together, CAM software might adjust the process automatically in response to any differences. Such a process would be “closed-loop” even by a strict definition of the term. It would also close the loop more quickly than the same loop can be closed today.

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