D.C. signal currents in capacitance gaging systems

A signal transmission system responsive to A.C. signals has rectifier means arranged to receive the A.C. signals. The rectifier means are connected to first and second circuit paths so as to establish respective unidirectional currents of opposite polarity and dependent on the A.C. signals, in the two circuit paths. Integrating means are connected to at least one of the circuit paths to integrate the current therein to thereby produce an output signal dependent on the A.C. signal but independent of any stray A.C. pickup on either circuit path.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to signal transmission systems responsive to A.C. 
signals. 
2. Brief Description of the Prior Art 
Known in the art are signal transmission systems for transmitting A.C. 
signals over a signal line in which the signal line is directly connected 
to the source of the A.C. signals. Such an arrangement suffers from the 
disadvantage that to avoid the possibility of false interpretation of 
signals received at the receiving end of the signal line due to spurious 
signals picked up by the signal line as a result of stray capacitive 
coupling and the like, either complex signal processing equipment must be 
used at the receiving end of the signal line, or the signal line must be 
shielded or made coaxial with its return line. These methods of avoiding 
misinterpretation of the received signals are expensive and reduce the 
overall system reliability. 
It is an object of the invention to provide an improved signal transmission 
system. It is a further object of the invention to eliminate the need to 
shield the signal line or the need to employ complex signal processing 
equipment for avoiding misinterpretation of received signals due to A.C. 
pick up in the signal line. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a signal transmission system 
responsive to A.C. signals, comprising first and second circuit paths, 
rectifier means connecting the first and second circuit paths to receive 
the A.C. signals so as to establish respective unidirectional currents of 
opposite polarity dependent on the A.C. signals in the two circuit paths, 
and integrating means connected to at least one said circuit path and 
integrating the current therein to produce an output signal dependent on 
the A.C. signals. 
According to the invention there is also provided a data transmission 
system responsive to low value A.C. data signals, comprising a circuit 
path including an unshielded signal line and rectifier means for 
establishing unidirectional current flow therein, integrating means 
connected to integrate the rectifier signal in the said signal line to 
produce a D.C. output signal dependent thereon, and output means connected 
to receive and amplify the said D.C. signals. 
According to the invention there is further provided an aircraft fuel 
contents measuring and display system, comprising a plurality of variable 
impedance type transducers each sensitive to the fuel content of a 
respective one of several aircraft fuel tanks, an A.C. energisation source 
for energising the transducers in parallel to produce A.C. signals 
dependent on the impedance of each transducer, a respective pair of 
rectifier means connected to each transducer and defining respective first 
and second circuit paths for that transducer which respectively carry 
unidirectional currents of opposite polarity both dependent on the A.C. 
signals of that transducer, first and second integrating means, selector 
means having inputs respectively connected to all the said first circuit 
paths and a single output for feeding any selected one of its inputs to 
the first integrating means to produce an output signal dependent on the 
fuel content sensed by the transducer feeding the selected one of the 
first circuit paths, and means additively connecting all the second 
circuit paths to the second integrating means to produce a second output 
signal dependent on the sum of the fuel contents sensed by all the said 
transducers.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the FIGURE, the aircraft fuel-level indicator system comprises 
capacitance transducers 1, each arranged in a different fuel tank of the 
aircraft. The capacitance of each capacitance transducer 1 varies with the 
fuel level in its associated tank, the average dielectric constant of the 
transducer being dependent on the quantity of fuel in the tank. The 
tansducers 1 are arranged to be excited by an 8kHz constant amplitude 
excitation voltage source 2. 
The output of each transducer 1 is connected to the anode of one diode 3 
and to the cathode of another diode 4. These diodes are preferably silicon 
diodes mounted on a terminal assembly of each tank. Thus two output signal 
tones, one each from the diodes 3 and 4, are connected to each transducer 
1. The signal lines from the cathodes of the diodes 3 are connected to a 
selector switch 5, which selectively connects each signal line in turn to 
a preamplifier 6 whose output is connected to an indicator channel 7 which 
indicates the fuel state in the fuel tank associated with the signal line. 
The other signal lines from the diodes 3 are grounded at that time. The 
signal lines from the anodes of the diodes 4 are connected together and 
connected to the input of a preamplifier 8 connected to an indicator 
channel 9 which indicates the total fuel state of all the fuel tanks. 
The preamplifiers 6 and 8 are arranged to act as integrators by use of 
feed-back circuits comprising respectively a resistor 10 parallelled by a 
capacitor 11 and a resistor 12 parallelled by a capacitor 13. The 
preamplifiers 8 and 9 are advantageously integrated-circuit differential 
amplifiers having one input connected to the signal line from the diodes 3 
or 4, and the other input (not shown) connected to the aircraft ground. 
The capacitors 11 and 13 are of a relatively large value to produce a 
steady D.C. voltage output from the preamplifiers 6 and 8. 
The output from each preamplifier 6 and 8 is fed to a comparator-chopper 
unit 14 or 15 which effects a comparison between the output voltage of the 
associated preamplifier and the voltage appearing on the wiper of a 
rebalance potentiometer 16 or 17 of a display 18 or 19 of each indicator 
channel 7 or 9 respectively. Any error signal resulting from this 
comparison is chopped to produce a 400 Hz signal which is then fed to a 
driver amplifier 20 or 21 which drives a servo motor 22 or 23 of the 
display 18 or 19. The rebalance potentiometer is driven by the servo 
motor. The comparator-chopper units 14 and 15 advantageously employ 
transistors. 
The indicator system operates as follows. 
An 8kHz A.C. signal appearing at the output of a transducer 1 is split into 
two half sine wave D.C. signals. A selected one of the positive sine waves 
is fed via the selector switch 5 to the preamplifier 6, there the positive 
sine wave is integrated into a steady D.C. voltage output which is fed to 
the indicator channel 7 where it is compared with the wiper voltage of the 
rebalance potentiometer 16. The error signal then produced is chopped and 
used to drive the servo motor 22 to correctly set the display 18. 
The negative half sine waves from all the transducers 1 are summed before 
being fed to the preamplifier 8, whose D.C. output is fed to the indicator 
channel 9 which operates in a manner similar to that of the indicator 
channel 7. 
The transducer outputs have high impedances and only relatively small 
signals are produced. However, since the high impedance signal lines from 
the diodes 3 and 4, carry D.C. currents and feed the preamplifiers 8 and 9 
acting as integrators, the signal lines are immune to stray A.C. currents 
which may be injected by capacitance coupling to other cables or from R.F. 
fields. Thus the signal lines need not be shielded as in conventional 
systems. 
A excitation frequency for the transducers 1 of 8kHz is used in order to 
reduce the capacitive reactance of the transducers by an order of 20 times 
compared to that produced by the more normally used 400 Hz of conventional 
systems. This has the effect of providing a degree of immunity to the 
effects of fuel conductance resulting from an excess of anti-static 
additive or other contaminants in the fuel for example. 
The above described aircraft fuel-level indicator system is advantageous in 
that elmination of the need for shielding on the signal lines leads to an 
increased signal line reliability and a reduction in weight and cost. The 
above described system also readily provides two independent signal lines 
from each transducer for discrete fuel tank fuel state and total fuel 
state indications. 
The arrangement of diodes connected to a line carrying an A.C. signal, to 
produce a D.C. signal that can be usefully carried on unshielded lines and 
later integrated to eliminate any stray A.C. signals picked up by the 
unshielded line, can be advantageously used in other applications where 
high impedances and low value current signals conventionally require the 
use of shielded or coaxial cables.