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BACKGROUND OF THE INVENTION
1. Field of the invention
This invention generally relates to devices for measuring the pressure
within gas-filled cables, especially cables used for data transmission and
more particularly a multiple sensor pressure transducer capable of
measuring the pressure level in a plurality of adjacent gas-filled cables.
2. DESCRIPTION OF THE PRIOR ART
The cable pressurizing technique is already known and is in widespread use
by telephonic Companies and consists of admitting compressed and
dehydrated air within the telephonic cables. This technique has the
purpose of maintaining a high insulation resistance along the copper
conductors, by preventing the outer moisture from entering the cable.
Further, the pneumatic phenomena correlated to and resulting from possible
air leakage from the cable are used to prevent the moisture from entering
the cable, which is often due to the greater outer pressure, and to locate
the place of these leakages.
The advantages resulting from the use of this technique are numberless, of
which some are already known and some are to be discovered or
demonstrated. The equipment and the devices used to attain the above
objectives are evolving and improving making the application of these
methods more and more attractive both from the standpoint of the technical
performances and from the standpoint of the costs.
The improvement in performance is obtained by the use of the electronic
techniques integrated by an effective system architecture and by a center
software devised for taking the maximal advantage from the measurements
and the surveys made by the equipment or devices installed in the field.
The optimization and the reduction of costs, especially of the labour for
installing these systems, are important factors for a quick and effective
diffusion thereof.
For measuring the pressure within gas-filled cables, pressure transducers
are normally used which convert pressure to a measurable electric signal.
At present, the use of electronic pressure transducers is known which make
use of a pair of wires belonging to the same gas-filled cable, the
pressure of which is to be measured for transmitting to a remote location
a signal having a frequency proportional to the pressure measured by the
transducer.
Such transducer uses as basic components a sensor and a timer.
Such systems use a plurality of pressure transducers arranged along the
gas-filled cable, in predetermined measuring locations. Each pressure
transducer is provided with one own timer allowing the signal transmission
for a limited time and in a predetermined time sequence so as to permit
the pressure transducers to transmit one at a time their signals according
to this sequence. Of course, these signals are supplied to a center unit
which is capable of indicating for each of the received electric signals
the associated pressure transducer in the same order as they are received
and is adapted to supply simultaneously through the transmission line all
the pressure transducers.
SUMMARY OF THE INVENTION
The present invention aims at providing a pressure transducer capable of
controlling a plurality of sensors and therefore of providing an important
cost reduction of each pressure measurement, of offering a substantial
reduction of the installation costs and of reducing the operating costs in
carrying out the pressure measurements in the desired locations along the
gas-filled cable.
More particularly, the sensor pressure transducer according to the present
invention is characterized in that it comprises:
a plurality of pressure sensors,
a multiplexer unit for enabling and selecting the sensors corresponding to
a coded addresses applied to an appropriate input thereof,
a clock generator,
a counting and timing circuit for forming the address to be delivered to
the multiplexer unit,
a power supply for delivering the operating voltage to the above mentioned
components, and
a line for delivering and returning the signal supplied by the sensors,
said signal having a frequency which is proportional to the pressure.
With this system a single pair of wires can be used for transmitting
measurements obtained in different pressure detecting location of other
gas-filled cables laid down in the neighbourhood so that there will no
longer be the necessity of having transducers in the same number as the
gas-filled cables and timers in the same number as the transducers,
thereby providing important savings both as to the component costs and as
to the operation thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a multiple pressure transducer in accordance
with the present invention.
FIG. 2 shows the circuit diagram of the clock generator;
FIG. 3 shows the circuit diagram of the address generator and the
programming device;
FIG. 4 shows the circuit diagram of the timing and selecting circuit and
the enabling circuit of the multiplexer unit;
FIGS. 5A and 5B show the circuit diagram of the multiplexer unit;
FIG. 6 shows the stabilized power supply circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, the transducer comprises a plurality of sensors
S0,S1,S2,S3 . . . S9 connected to a multiplexer unit 2 which is energized
by a stabilized power supply 5, a clock generator 3 and an address
generator 4. The address generator 4 is connected to the multiplexer unit
2 and to a device 7 for programming the address code by the operator or
installer.
SENSORS
The sensor is designed so as to generate at its output a signal of a
frequency which is proportional to the pressure in accordance with a well
know technique, as for example:
the integrated circuit of NATIONAL SEMICONDUCTOR LM 555 and its equivalents
applied as voltage-to-frequency converter (VCO) which permits the
conversion of the voltage to a frequency;
the VCO circuit disclosed in "Application Note 81" AN81-3 issued in June
1973 described in "LINEAR APPLICATION HANDBOOK" of the National
Semiconductor.
The voltage-to-frequency converter circuit provides as an output a
frequency signal which is proportional to the voltage applied to its
input.
Should a suitable resistor change its resistance, a voltage change
thereacross will occur.
This voltage, when applied to the VCO circuit, allows a signal to be
measured, and changes the frequency which is proportional to the electric
resistance.
The piezoresistive and extensometric pressure sensors used today in
commerce have the characteristic of changing their resistive value in a
manner proportional to a force applied and therefore to the pressure
applied on their surface.
CLOCK GENERATOR 3
As can be seen in FIG. 2, the clock generator 3 comprises a binary counter
10 and a quarz 11 having an oscillation frequency of 32768 Hz, which is
connected to the binary counter 10 through a set of two parallel connected
resistors R1,R2, and one series connected resistor R3 which are intended
to keep unchanged the characteristics of the oscillator 11 with respect to
the time and the temperature. The binary counter 10 has three outputs
12,13,14 each emitting a square wave of different frequency, for example 3
2768/2Hz.sup.7, 3 2768/2Hz.sup.8 and 3 2768/2Hz.sup.9. By bridging
terminals A,B,C of the binary counter outputs the desired clock frequency
of the system can be selected as output CLK.
The binary counter is supplied by a voltage Vc and is grounded at 15. Also
connected to the binary counter through the lead 16 is a reset circuit 19
comprising a NAND gate 17 the two inputs of which are connected to each
other through a resistor R4 and a diode D1 parallel connected to each
other and a capacitor C1 connected to ground. The output 18 of the NAND
gate 17 is connected to the address generator 4.
ADDRESS GENERATOR 4 AND PROGRAMMING DEVICE 7
As can be seen in FIG. 3, the address generator 4 comprises a binary
counter 20 having seven outputs 23 to 29. The outputs 23 to 29 of the
binary counter 20 are connected through diodes D2 to D8 to the programming
device 7.
The programming device 7 is formed by an interface 30 comprising seven
bridges BR which connect the outputs 23 to 29 to the wires 31 to 37 which
are connected to the output line 38 in order to provide in a binary code
the device address. In effect, the wires 31 to 37 supply bits of weight 1
to 6. Line 38 is connected at one end to a capacitor C2 connected to
ground. Lead 38 supplies the address signal H to the multiplexer unit and
to the two inputs of a NAND gate 22, the output of which supplies the
inverted address signal H, on the lead 39, also to the multiplexer unit 2
and to one input of a NAND gate 21, to the other input of which the signal
CLK is applied and the output of which is connected as input to the binary
counter 20.
MULTIPLEXER UNIT 2
The multiplexer unit 2 is comprised of four blocks, namely:
(1) Timing and selecting circuit
(2) Voltage level shifter
(3) Power supply enabling circuit
(4) Multiplexer.
As can be seen in FIG. 4, the timing and selecting circuit comprises a
binary counter 40 supplied by the voltage Vcc and having as an input the
inverted signal H coming from the programming device 30 through lead 39.
The outputs 41 to 44 of the binary counter 40 supply signals of binary
count as inputs to a voltage shifter 45 which gives on its outputs the
binary coded signals M,N,O,P,Q adapted for the multiplexer unit. Lead 39
of the inverted address signals H is also applied, through a diode D9, to
an input of the voltage shifter 45. Output 44 is also connected to ground
through a diode D11 and a resistor R7. From the binary counter 40 two
outputs 46,47 are connected to the two inputs of a NAND gate 48 the output
of which is connected, through a resistor R5, to one input of NAND gate
49, the output of which supplies a signal X to the power supply circuit.
Intermediate the resistor R6 and the NAND gate 49 a diode D10 is connected
through a lead 50 which supplies the address signal H. The outputs 42,44
of the binary counter 40 are also applied to the two inputs of a NAND gate
51 the output of which is connected to one input of another NAND gate 52,
the other input of which is connected to the output 44 of the binary
counter 40. The output of the NAND gate 52 supplies a signal which is
applied as one input to the voltage shifter 45. The output of the NAND
gate 51 is also connected to one input of the NAND gate 49 and to one
input of a further NAND gate 53 to the other input of which the clock
signal CLK is applied and the output of which is connected to the binary
counter 40.
The multiplexer unit 2 is illustrated in FIGS. 5A and 5B and comprises the
analogue multiplexers 60,61,62. The multiplexers 60,62 receive as inputs
the signals U0 to U9 which control the bases of transistors T1 to T10 of
the power supply enabling circuit. These multiplexers receive also as
inputs the coded signals M,N,O,P,Q coming from the voltage shifter 45. The
multiplexer 61 supplies the signals f0 to f9 and is set to receive the
frequency signal f.sub.out coming from the sensor selected by the
multiplexer unit by means of the coded signals M,N,O,P, from the voltage
shifter.
POWER SUPPLY CIRCUIT 5
The power supply circuit 5 supplies the voltage Vcc (+5V) and Ve (+12V) to
the system, these voltages are obtained from the line voltage. As can be
seen in FIG. 6, this circuit comprises two input lines provided with
parallel connected resistor R20,R21 and connected to a diode bridge 70
series connected through a resistor R9 to the emitter of a transistor T11,
the collector of which gives as output the voltage Ve. Upstream the
resistor R9 the collector of a transistor T12 is connected, the emitter of
which is connected to ground, through a resistor R12 and a capacitor C4,
while the base of this transistor is connected to the collector of a
transistor T13, the base of which is connected intermediate the emitter of
transistor T12 and the resistor R12 and the emitter of which is connected
to a diode Zener Z which is connected to ground and, through a resistor
R14 and a diode D12, to the output carrying the voltage Ve. The base of
transistor T12 is connected through a resistor R11 to c capacitor C3
parallel connected intermediate the base of transistor T11 and the
collector of transistor T12, between which also a resistor R10 is parallel
connected. The base of transistor T11 is connected to the collector of a
transistor T14, the emitter of which is connected to the output Ve through
a diode D13 and a resistor R15. Intermediate the diode D13 and the
resistor R15 a resistor R16 is connected to ground. The emitter of
transistor T14 is also connected through a resistor R17 and a resistor R18
to the terminal of signal X. Upstream the resistor R18 a capacitor C5 is
connected which, through a resistor R19, is connected to the frequency
terminal f.sub.out. The base of transistor T14 supplies the voltage Vcc.
The voltage Vcc obtained through the diode Zener Z is suitably filtered by
the capacitor C4. The limitation of current of the voltage Vcc is given by
the current generator formed of the transistors T12,T13 and the resistor
R12. This resistor is provided for deciding the current of the generator.
The resistor R13 is provided for discharging the capacitor C4 when there
is a lack of line voltage.
The voltage Ve is generated only when the system requires it, namely when
the signal X is at 0 volt. With the signal X is at 0 Volt the transistor
T14 becomes conducting, thereby biasing the base of transistor T11. The
current Ix flowing in the branch R17-R18 is constant since the voltage
drop across the resistors R17,R18 is constant. This voltage is given by
the following relation:
Vz.sup.2 -Vbe (T14) Ix=I.sub.1 +I.sub.2.
The voltage Ve is stabilized by the balance formed between the currents
I.sub.1 and I.sub.2 since if a decrease of Ve would occurs, there will be
a resulting decrease of the current flowing in the branch R15-D13 and,
since Ix is constant, this would cause a current increase at T14. This
current increase should bias the transistor T11 more strongly with a
resulting increase of the Ve value. In the presence of the voltage Ve, a
portion of the current necessary for the diode Zener Z is picked up by the
base of transistor T14. The current lacking to the diode Zener Z is
provided by the branch D12-R14. A second function of this power supply
circuit is to transform the frequency signal supplied by the terminal
f.sub.out in a current modulation. This transformation is always based
upon the current picked up by the branch R17,R18. The capacitor C5 acts as
a high pass filter.
OPERATION
The operation of the multiple pressure transducer according to this
invention is as follows.
When to the power supply circuit 5 is supplied with a the a.c. voltage, the
power supply 5 provides the necessary operating d.c. voltages Ve, Vc and
Vcc to the clock generator 3, the address generator 4 and the multiplexer
unit 2, respectively.
Then the clock generator 3 starts to deliver to address generator 4 a chain
of clock pulses CLK having a stable and precise frequency. By employing
this frequency the address generator 4 makes a count which generates a
different bit code for each pulse. The reset circuit 19 supplies a RESET
signal to the binary counter 10, for its initialization through lead 16,
i.e. to bring to a low logic level "0" the outputs 23 to 29 and to
initialize the address generator 4. The RESET signal remains at the high
logic level "1" until the voltage on the capacitor C1 overcomes the
treshold level of the NAND gate 17. Diode D1 is provided for quickly
discharging the capacitor C1 at the time where there would be a lack of
voltage Vcc supplying the reset circuit, thereby permitting a new
initialization of the binary counter 10.
When the generated code is the same as the code programmed by the operator
or installer through the programming device 7 (obtained by connecting one
or more of the bridges BR in this device), the address generator 4
generates on the output 38 the address signal H corresponding to the first
pressure sensor SO. This address signal H is supplied to the timing and
selecting circuit. The signal H suitably inverted through the NAND gate 22
is supplied to the multiplexer unit 2 in order to keep it to zero as long
as all the outputs 23 to 29 connected to the capacitor C2 through the
programming device 30 are in the logic state "1". Only with this precise
configuration the capacitor C2 is kept charged (logic level "1") thereby
bringing the signal H to a high level. The signal H, suitably inverted by
the NAND gate 22 starts the binary counter 40, the outputs 41 to 44 of
which are applied to the voltage shifter 45 which gives as output the
coded signals M,N,O,P,Q for the multiplexers 61,62. (An example of the
voltage shifter is the MC14504B level shifter sold by Motorola, the
purpose of which is to bring the voltage Vcc to the level of the voltage
Vc necessary for enabling the multiplexer unit). The signal H at the same
time disables the address generator 4 by locking the clock signal CLK to
the binary counter 20 by means of the NAND gate 21.
This address is decoded by the multiplexer unit 2 which diverts to the
selected one of the sensors SO to S9 the operating voltage Ve from the
power supply 5 and also diverts the frequency signal f.sub.out generated
by the sensor through the power supply 5 and to the signal output line 6.
The start of the measurings is provided by the inverted address signal H
coming from the address generator 4 through wire 39. The binary counter 40
remains with all the outputs 41-46 low as long as the inverted address
signal H is high, thereby desabling all the cascade connected blocks. By
bringing the H signal to the logic level 0 the binary counter 40 starts to
count thereby giving again on its outputs 41 to 44 a binary count which
through the voltage shifter circuit 45 form the "words" M,N,O,P,Q
necessary for the multiplexer unit to select the sensors S0 to S9. The
supplied voltage Ve is generated exclusively when the signal X coming from
the output of the NAND gate 49 is at a low lever "0". The signal X is
applied to the power supply circuit 5 for controlling it. This enabling
circuit is therefore intended to establish when the signal X is to be
brought to 0 through the NAND gates 48,51 and 49.
During the selection, the logic levels of these NAND gates bring the signal
X to a low logic level for 3/4 of the selection time of the sensor. The
high logic level of signal X is permitted, before the selection, by the
address signal H entering through wire 50 and, after the selection, by the
signal coming from the output of the NAND gate 51. Another function of
this enabling circuit is to distribute the supply voltages Ve0 to Ve9 to
the various sensors. This function is carried out by a set of transistors
T1 to T10 (FIG. 5B) controlled through their bases by the multiplexers
60,62. The bases of these transistors supply to the multiplexers 60,62 the
signals U0 to U9. By connecting the base of the pre-selected transistor to
the resistor R8 the voltage Ve is present on the collector of the same
transistor.
After a short predetermined time is elapsed, the address generator 4
interrupts the signal output and then the address generator 4 generates
the coded address corresponding to the following sensor.
This procedure will be repeated as many times as the sensors connected to
the multiplexer unit 2 are.
As can be seen, the pressure transducer comprises:
a multiplexer unit which enables and selects the sensor corresponding to
the coded address presented at a suitable input thereof;
a clock generator for the counting chain;
a counting and timing circuit which generates the address to be supplied to
the multiplexer unit for selecting one of the externally connected
sensors;
a power supply for delivering the supply voltage to the remaining
components of the circuit; and
the line on which the power is supplied and the signal proportional to the
pressure is generated by the sensors.
By arranging a transducer according to the invention in each of the
chambers or pits intended to receive it, the pressures of the other
adjacent gas-filled cables can be measured so that in a single measuring
location, a single transducer can measure the pressure of more than one
gas-filled cables which run through the same location and these pressure
values can be supplied to the control station in sequence by means of the
multiplexer unit. The advantages provided by the multiple pressure
transducer according to this invention with respect to the single pressure
transducers now available in commerce are the following:
(1) Cost reductions for each measurement because the costs of the common
components are divided by the number of used measurement locations.
(2) Substantial cost savings in the multiple pressure transducer
installation because it is sufficient to make a single electric connection
to the location in which it is desired to detect the pressure.
(3) Operating cost reduction of the pressure transducers because a single
loop is used.
* * * * *
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