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Time Domain Reflectometry Overheat Detection System.

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Final rept. Jan 70-Jan 71,

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The report is concerned with the utilization of thermally sensitive, electrical transmission lines for the detection and measurement of thermal hazard, or conditions in an aircraft. Use is made of a coaxial cable which changes its electrical properties drastically as the temperature passes through a predetermined alarm temperature. Short duration electrical pulses pass down the transmission line and are reflected by the overheated section, with the time delay proportional to the distance between the input and the overheated section. A system results which determines both the presence of the overheat condition and gives information relating to the position of the overheat. The latter may be displayed to an observer, or utilized in the system to improve discrimination against false alarms. Pursuant to the development of a prototype system design, several candidate cables and cable materials were investigated. These included brief tests of lines using semiconductor and lithium glass material, with more extensive testing of ferrite and slat dielectric lines. The prototype system employs salt dielectric cables it utilizes 100 percent redundancy of the pulse circuitry, and an independent cable fault detection circuit, built-in test functions, and a display which indicates alarms, equipment condition, and cable condition, and indicates position of overheat. It is concluded that the employment of the pulse reflection techniques for overheat detection is entirely feasible. Operational systems can result from development of improved cables. Author

Subject Categories:

  • Aircraft
  • Safety Engineering

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