1 January 2002
New sensor detects wire chafing
By Bob Felton
Tedious, time-consuming visual inspection of hard-to-reach electrical wires and hydraulic lines might soon be a thing of the past, thanks to a patent awarded in October to scientists at the Air Force Research Laboratory (AFRL). "Our technique," said Frank Born, one of the inventors, "basically involves wrapping a fiber-optic cable or piece of wire around the conduit. When the sensor breaks or short circuits, you know you have dangerous chafing."
AFRL licensed the patent to Kildeer Mountain Manufacturing Co. of Kildeer, N.D. The company has contracted with United Airlines and the FAA to develop a prototype.
"The outer surfaces of electrical, optical, and electromagnetic cable and cable bundles are frequently subject to wear caused by rubbing against external structures," the inventors said. "If allowed to proceed, this wear can cause grounding, shorting, or breaking of the cable's internal structures."
The inventors added further, "Currently nothing is in wide use to detect when a conduit experiences chafing and thus becomes subject to failure." The National Transportation Safety Board identified an electrical short circuit as the probable cause of the fuel tank explosion that destroyed TWA Flight 800 off Long Island in 1996.
The sensor consists, in part, of a conducting element or fiber-optic cable situated between the protected conduit and the surface against which it chafes; the conductor or cable is positioned so that it must be damaged before abrasion can damage the conduit. The remainder of the sensor consists of electronics that monitor the end-to-end integrity of the conducting element.
According to the patent application, "The sensing element is wrapped around the conduit so any external structure that causes chafing contacts the sensing element and causes it to break. The fragility of the sensing element is chosen so it breaks only after significant chafing but well before the conduit's functional failure."
Further, "If the sensing element comprises an optical fiber, care must be taken to assure that the losses due to bending are not great enough to make it appear as if the fiber is broken. Thus, optical fiber may not be usable on a small-diameter conduit."
Though wrapping the sensor about the exterior of the conduit is the preferred implementation of the sensor, "It is not necessary to spirally wrap the sensing element around the conduit. The sensing element can extend in the axial direction of the conduit, as long as the sensing element is in position to be chafed by any external structures that can cause chafing on the conduit." IT