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When the sun shines on the machine

06 Dec 2011  | Clark S. Robbins

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Some time ago, I was involved in a project that used signals from various sensors to provide action inputs to a new electronically controlled hydraulic system that was part of a large industrial machine. Because of cost and design constraints, we had to use existing signals when possible. We slightly modified some of these signals to more closely meet our needs. Because these sensor signals were critical to the operation and success of the hydraulic-control system, we assimilated those people responsible for these other signals into a cohesive team. All subsystem and component owners had to report on a regular basis about the design and test progress relative to the new durability and reliability goals for their sensor subsystems. Initially, all tests went successfully with no unanswered questions from incidents that occurred during the testing.

After we committed to producing the system, however, a few puzzling incidents occurred. The safe operation of the hydraulic-control system required that we had to monitor a couple of the sensor signals 100% of the time for continuity, condition, and validity. The problem was that the system had detected the loss of a sensor signal for a longer time than the minimum allowable. We did in-depth testing and analysis of the sensor, the amplifier, the wiring, and the connections but could find nothing wrong and could not reproduce the problem. The sensor-subsystem team assumed a nonreproducible fault that would be of no concern. I disagreed with the findings but could offer no alternative cause or theory.

After we went into production, the same problem occurred but on only a few of the machines. However, because we had no root cause, we could not fix the problem and guarantee that those few machines would not have the problem again, so we had to replace each machine exhibiting the problem—an expensive approach.

An intermittent connection was obviously causing this sensor problem because the diagnostics reported the fault as a loss of continuity. According to the system's specifications, pin and sleeve connectors at the sensor amplifier and at the hydraulic-controller input could not be the source of the problem.

The project used some of these machines outdoors and sometimes from night into day. One of the machines had the problem a number of times on the same day, but the machine was sitting idle with energized controllers. When the problem occurred, the operator reset the fault because the machine was supposed to be up and running at a moment's notice. When I queried the operator about what had happened and when it had happened, he told me that the problem occurred whenever the sun was shining on the machine for a period of time. If clouds obscured the sun, the problem did not occur. The sensor amplifier's connector and surrounding support structure were facing directly into the sun.

It was now painfully obvious that thermal deformation of the support structure was moving the sensor amplifier, pin and sleeve connector. A close examination of the pin- and sleeve-connector terminals showed that the pin was slightly undersized; under certain conditions, it could lose contact.

You might be able to guess where this story is heading. The connector manufacturer admitted that it occasionally produced slightly under-tolerance pins, citing a study that indicated that this condition would not create a noticeable problem with any of the systems using this connector. Although that claim may have been true for other systems that were using this connector, it was a problem for our hydraulic-control system, which could not tolerate the loss of that signal beyond a short time.

About the Author
Clark S Robbins is a software-application engineer at GS Engineering (Houghton, MI).

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