Communications: electrical – Aircraft alarm or indicating systems
Reexamination Certificate
2000-11-17
2001-08-21
Lee, Benjamin C. (Department: 2632)
Communications: electrical
Aircraft alarm or indicating systems
C340S618000, C340S620000, C307S326000, C702S047000, C702S052000, C702S053000, C361S043000, C361S056000, C361S057000, C361S058000, C361S284000, C073S29000R, C073S30400R
Reexamination Certificate
active
06278381
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electronic systems which extend through the fuel tank of an aircraft, such as, systems for measuring the level of fuel in the tank of the aircraft, and more particularly, the present invention relates to a device for limiting current in wiring and electronic components located inside an aircraft fuel tank.
BACKGROUND OF THE INVENTION
For safety and economic reasons, aircraft must have the capability of determining accurately the amount of fuel onboard. Thus, various fuel gauging systems and other sensing apparatus have been utilized to deliver information concerning the status of fuel carried by the aircraft to the pilot. In general, all these systems require portions of electronic circuits, such as, wiring, probes, sensors, etc., to extend inside the fuel tank.
Examples of known aircraft fuel gauging systems are disclosed in U.S. Pat. No. 5,602,333 issued to Larrabee et al.; U.S. Pat. No. 4,918,619 issued to Orloff et al., U.S. Pat. No. 4,908,783 issued to Maier; U.S. Pat. No. 4,872,120 issued to Orloff et al.; U.S. Pat. No. 4,731,730 issued to Hedrick et al.; U.S. Pat. No. 4,451,894 issued to Dougherty et al.; U.S. Pat. No. 4,487,066 issued to Pardi et al.; U.S. Pat. No. 4,173,893 issued to Hedrick; and U.S. Pat. No. 4,090,408 issued to Hedrick.
Permitting excessive current to flow in wiring or electronic components located inside a fuel tank creates a potentially dangerous situation. Thus, the amount of current permitted in electronics in fuel tanks must be limited during normal operations of the systems and during unwanted fault conditions.
One example of a typical fault condition is a ground fault which results in a decrease in circuit resistance which, in turn, causes an increase In current. Another possible fault condition can occur between wiring of various circuits which are bundled together at locations exterior of the fuel tank along a given length of cableway. Such faults can result in greater voltages being applied on circuits extending into the fuel tank which, in turn, causes an increase in current in the electronics in the fuel tank.
Various surge protectors, circuit breakers and other current limiting devices are known which can be used to limit current through a circuit or a portion thereof For example, in the case of an aircraft fuel gauging system, transient suppressors can be electrically connected on each line entering or leaving the fuel tank to limit the voltage level, and a series resistor can be included on each line to set the resulting maximum current. However, since stray capacitance exists within an aircraft fuel tank, the addition of the series resistor causes a phase shift of the signal carried by the circuit, thereby degrading the accuracy of the fuel gauging system during normal operation.
Therefore, although various current limiting devices are known in the art which may be satisfactory for their intended purposes, there is a need for a current limiting device particularly useful in limiting the current through electronics located in and extending through the fuel tank of an aircraft. The device should be capable of limiting current without degrading the accuracy of the fuel gauging system.
OBJECTS OF THE INVENTION
With the foregoing in mind, a primary object of the present invention is to provide an improved current limiting device useful for limiting the amount of current through wiring, probes, sensors and the like, which are located inside the fuel tank of an aircraft.
Another object of the present invention is to provide an accurate aircraft fuel gauging system having a current limiting device which does not degrade the accuracy of the measurements provided by the fuel gauging system.
A further object of the present invention is to provide a current limiting device which is readily retrofittable for use in connection with an existing fuel gauging system on an aircraft.
SUMMARY OF THE INVENTION
More specifically, the present invention provides a fault condition protective device for use with an aircraft fuel gauging system having at least one probe disposed in an aircraft fuel tank for at least partial immersion in the fuel. The fuel gauging system includes a generator for applying an input signal at a predetermined frequency to the probe and instrumentation for receiving an output signal from the probe and for determining the quantity of fuel in the fuel tank. Both the generator and the instrumentation for determining the quantity of fuel in the tank are located exterior of the fuel tank and are electrically coupled to the probe via wiring extending through a wall of the fuel tank.
The fault condition protective device is electrically coupled to the probe at a location adjacent the wall of the fuel tank and has a voltage clamp and resonant circuit. The voltage clamp includes a resistor and a transient suppressor which limit the voltage applied to the probe. The resonant circuit includes an inductor and a capacitor which are connected in series and which resonate at a frequency substantially equal to the predetermined frequency of the input signal. The resonant circuit provides a low series impedance to the probe when the input signal frequency is substantially equal to the predetermined frequency and a high series impedance when the input signal frequency is not equal to the predetermined frequency as might be experienced during fault conditions.
REFERENCES:
patent: 4090408 (1978-05-01), Hedrick
patent: 4173893 (1979-11-01), Hedrick
patent: 4420976 (1983-12-01), Orloff et al.
patent: 4451894 (1984-05-01), Dougherty et al.
patent: 4487066 (1984-12-01), Pardi et al.
patent: 4545020 (1985-10-01), Brasfield
patent: 4589077 (1986-05-01), Pope
patent: 4731730 (1988-03-01), Hedrick et al.
patent: 4872120 (1989-10-01), Orloff et al.
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patent: 4918619 (1990-04-01), Orloff et al.
patent: 5121631 (1992-06-01), Koon
patent: 5128593 (1992-07-01), Gilbert
patent: 5142909 (1992-09-01), Baughman
patent: 5222010 (1993-06-01), Capan
patent: 5602333 (1997-02-01), Larrabee et al.
patent: 5613399 (1997-03-01), Hannan et al.
patent: 5712754 (1998-01-01), Sides et al.
patent: 5745327 (1998-04-01), Choo
patent: 5751534 (1998-05-01), DeBalko
patent: 5781386 (1998-07-01), Muelleman
patent: 5786972 (1998-07-01), Galipeau et al.
patent: 5793197 (1998-08-01), Shirai et al.
Howson and Howson
Lee Benjamin C.
Smiths Aerospace, Inc.
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