Shielding system for capacitance type fuel quantity gauge

A capacitance type fuel quantity gauge for an aircraft fuel tank including a probe capacitor whose capacitance varies in accordance with the level of fuel in the tank with a Lo-Z oscillator connected to the probe capacitor together with a pair of diodes for producing a DC output signal which is fed to signal processing circuitry, the diodes being disposed in a metal enclosure for isolation from interfering signals and the interconnecting leads to the diodes passing through feedthrough capacitors to block the entry of such interfering signals into the metal enclosure.

BACKGROUND OF THE INVENTION 
A widely used type of fuel quantity gauge for monitoring the level of fuel 
in a tank such as the fuel tank of an aircraft is the capacitance type 
gauge. One type of such a capacitance type fuel gauge system is shown in 
U.S. Pat. No. 3,747,407 entitled Probe Mounted Electronics for Fuel Gauge 
System, issued July 24, 1973 to Irwin Wallman. In such a capacitance type 
system, a probe capacitor is mounted in the fuel tank with signal 
generating means such as an oscillator connected to the capacitor to 
provide an output signal having a magnitude which varies in accordance 
with the variation of the capacitance of the probe capacitor in accordance 
with the level of fuel in the tank. This output signal is processed in 
suitable circuitry for indicating such fuel quantity in the tank. 
In the case of a DC type tank unit, a pair of oppositely poled diodes are 
connected to the output of the probe capacitor to provide a DC output 
signal which is fed to a remotely located signal processing circuitry for 
providing a readout of fuel quantity. 
One of the problems in such a capacitative type signal is the interference 
to such electronic circuitry caused by high electromagnetic fields in the 
fuel tank environment. Typically, in electronic circuits filter capacitors 
are used to bypass an interfering signal to "ground". However, in fuel 
quantity gauge systems of the type to which this invention is directed, 
there is no ground to which the interference can be bypassed. In an 
aircraft fuel tank utilizing such a system, bypassing to the aircraft is 
not possible since this would create an entirely new set of problems. 
Previous attempts in the field of aircraft fuel tanks to deal with high 
electromagnetic fields have utilized a third tube at ground potential with 
three bypass capacitors rather than two, by the use of shielded wires and 
the like. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, a primary object of the invention is to provide a new and 
novel capacitance type fuel quantity gauge for a fuel tank. 
Another object of this invention is to provide a new and novel capacitance 
type fuel quantity gauge which permits the gauge to operate in a high 
electromagnetic environment without interference with the operation of the 
electronic circuitry. 
A further object of this invention is to provide a new and novel 
capacitance type fuel quantity gauge which prevents the entry of 
interfering signals caused by a strong electromagnetic environment from 
affecting the operation of the fuel quantity gauge. 
Still another object of this invention is to provide a new and novel 
capacitance type fuel quantity gauge which eliminates the effect of 
interfering signals on the gauge and which permits existing such gauges to 
be easily modified with a minimum of parts and at low cost to filter and 
shield the gauge against such interfering signals. 
These and other objects of this invention will be better understood and 
further advantages thereof will become more apparant from the ensuing 
detailed description of the preferred embodiment taken in conjunction with 
the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown schematically a fuel quantity gauge 
of the capacitance type constructed in accordance with the invention and 
particularly adaptable for use on an aircraft fuel tank. The gauge of FIG. 
1 includes a probe capacitor designated generally by the reference numeral 
11 and as arranged to be disposed within a fuel tank designated generally 
by the reference numeral 12. The probe capacitor 11 includes an inner tube 
13 or what is referred to hereinafter as a Hi-Z tube and an outer tube 14 
or what is referred to hereinafter as a Lo-Z tube disposed in concentric 
coaxial relationship with the inner tube 13. 
As is well known, the tubes 13 and 14 form a capacitor the capacitance of 
which varies with the level of fuel in the tank 12. 
The outer tube 14 is arranged to be connected to an AC voltage source such 
as a Lo-Z oscillator 16 by means of a conductor 17, the oscillator 16 
being located at a remote location and preferably forming part of a signal 
processing unit 18. 
The inner tube or Hi-Z tube is connected through a capacitor 19 or what is 
referred to as a full height compensation (FHC) capacitor to the adjacent 
sides of a pair of oppositely poled diodes 21, 22, the other sides of 
which are connected by means of conductor 23, 24 to an amplifier 26 in the 
signal processing unit 18. The output of the amplifier 26 is connected to 
a suitable indicator 27 which provides a readout of the quantity of fuel 
in the tank 12. 
Referring now to FIG. 2, the tank unit designated generally in FIG. 1 by 
the letter U includes the inner Hi-Z tube 13 and the outer Lo-Z tube 14 
supported in the relationship shown on a mounting strap 31 preferably of 
plastic or the like and which may be formed as an extension of the tubular 
plastic housing in which the tubes 13, 14 are disposed. The mounting strap 
31 is provided with apertures 32 so that the tank unit U may be suitably 
attached to the aircraft structure. 
The tank unit U also includes a diode block designated generally by the 
numeral 33 having a top wall 34, a bottom wall 36, end walls 37, 38, a 
central partition and side walls 39, 41 as shown best in FIG. 5, all of 
the walls being formed of metal and defining an interior 42 that may be 
filled with a potting compound totally enclosing the diodes. The top wall 
34 is preferably provided with an opening 43 which is arranged to be 
closed by a removable cover 44. The outer tube 14 is also preferably 
closed at its end by means of metal end caps 46, 47 with the outer tube 14 
defining with the inner tube 13 an annular interior 48. 
As shown best in FIG. 3, the diode block 33 is provided with lateral 
grooves 51, 52 which are arranged to receive the arcuate ends of a 
mounting strap 53 to thereby permit the diode block 33 to be releasably 
clamped to the outer tube 14. A Hi-Z contact in the form of a pin 54 is 
secured to the bottom wall of the diode block 33 by means of a sleeve 56 
with its lower end portion extending through a suitable aperture 57 in the 
inner tube 13 in contact-making engagement therewith, preferably by means 
of a metal collar 58. The upper end of the Hi-Z contact 54 extends within 
the interior 42 of the diode block 33 and is connected to one side of the 
FHC capacitor 19 by means of a capacitor lead 19a as shown best in FIG. 5. 
The outer lead 19b of capacitor 19 is connected to one side of oppositely 
poled diodes 61, 62 through leads 61A, 62A, respectively, as shown best in 
FIG. 5. The other sides of diode 61, 62 are connected through leads 61B, 
62B to terminals 63, 64, respectively, which extend through the central 
partition 40 of the diode block 33 into the interior 45 on the opposite 
side of the partition 40 from the chamber 42. These terminals 63, 64 
represent the positive (+) and negative (-) leads connected to conductors 
23, 24 shown in FIG. 1. The ends of the terminal 63, 64 extending into the 
interior 45 are arranged to be connected to spring contacts 66, 67, 
respectively secured to the upper wall 34 of the diode block 33 by means 
such as screws 68, 69, respectively. 
As shown best in FIG. 4, a Lo-Z strap 71 is provided on the tank unit U, 
one end of which is connected by means of mounting screw 72 with the diode 
block 33 and the other end of which is connected to the Lo-Z outer tube 14 
by means of mounting screw 73. Thus, the Lo-Z strap 71 insures that good 
electrical contact is made between the metal diode block 33 and the outer 
tube 14 in the event of an insulating film developing on the contacting 
surfaces. Also, as shown in FIG. 4, a cable clamp 74 is provided on the 
diode block 33 oppositely of the Lo-Z strap 71 by means of which the 
conductors 17, 23 and 24 may be clamped to the diode block. 
In addition, the lead 19b of the full height compensator capacitor 19 is 
connected by means of a plate 81 to one side of a trimming capacitor 82, 
the other side of which is connected by mounting plate 83 by means of a 
lead 84 to a terminal 86 on the side 39 of the diode block 33 as shown 
best in FIG. 2. The trimming capacitor 82 is provided with an adjustment 
87 accessible through the upper opening in the diode block 33 when the 
cover 34 is removed. The adjustment knob 87 is used for a specific 
application when the tank unit U must be calibrated prior to installation 
on aircraft. 
Specifically illustrative of the invention, the terminals 63, 64 extend 
through feedthrough capacitors 91, 92, respectively, disposed within the 
central partition 40 of the diode block 33. The feedthrough capacitors 91, 
92 are of conventional construction but are utilized in the invention in a 
manner such that interfering signals picked up on the leads 66, 67 and/or 
terminals 63, 64 are bypassed by the feedthrough capacitors to a level 
that is below the threshold of the diodes 61, 62. In addition, by the use 
of a metal enclosure for the diode block 33 in which the diodes 61, 62, 
the FHC capacitor 19 and Hi-Z tube 13 are located, interfering signals are 
isolated from the diodes. As can be understood, the normal excitation 
signal can enter through the feedthrough capacitors 91, 92 due to the low 
frequency of such a signal which is below the bypass or cutoff frequency 
of the capacitors. Therefore, the interfering signals are bypassed to the 
Lo-Z conductor 17 which is considered as "ground" for the purpose of 
bypassing such interfering signals.