Capacitance measuring circuit and method for liquid leak detection by measuring charging time
First Claim
1. A capacitance measuring circuit for use with a leak detection cable, said circuit comprising:
- (a) inverter means for receiving a DC input voltage and for inverting said DC input voltage to produce a mirror-image inverter DC voltage;
(b) current source means for receiving a DC voltage (VX) and for producing a constant current (IC) proportional to said DC voltage (VX), said constant current (IC) being defined by the equation IC =-k·
VX, where k is equal to a predetermined constant, and where VX is equal to one of said DC input voltage and said mirror-image inverted DC voltage;
(c) means for supplying said constant current (IC) to the leak detection cable such that the leak detection cable is charged from a first potential to a second potential;
(d) comparator means for comparing said second potential to the other one of said DC input voltage and said mirror-image inverted DC voltage and for producing a match signal when a predetermined relationship exists therebetween; and
(e) timer means responsive to said match signal for measuring a time required for the leak detection cable to be charged from said first potential to said second potential.
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Accused Products
Abstract
A leak detection system and method are disclosed. The capacitance of a leak detection cable is changed by a leaking liquid. A capacitance measuring circuit detects changes in the capacitance of the cable and thereby detects the presence of a leak. The capacitance measuring circuit includes an inverter, a current source, a comparator, and a timer. The inverter inverts a DC voltage to produce a mirror-image invented DC voltage. The current source produces a constant current proportional to the mirror image inverted DC voltage and supplies the constant current to the cable such that the cable is charged. A comparator compares the voltage across the cable to the input voltage and produces a match signal when they are equal. A timer which is responsive to the match signal measures the time required for the cable to be charged. The capacitance of the cable is directly proportional to the charge time.
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Citations
20 Claims
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1. A capacitance measuring circuit for use with a leak detection cable, said circuit comprising:
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(a) inverter means for receiving a DC input voltage and for inverting said DC input voltage to produce a mirror-image inverter DC voltage; (b) current source means for receiving a DC voltage (VX) and for producing a constant current (IC) proportional to said DC voltage (VX), said constant current (IC) being defined by the equation IC =-k·
VX, where k is equal to a predetermined constant, and where VX is equal to one of said DC input voltage and said mirror-image inverted DC voltage;(c) means for supplying said constant current (IC) to the leak detection cable such that the leak detection cable is charged from a first potential to a second potential; (d) comparator means for comparing said second potential to the other one of said DC input voltage and said mirror-image inverted DC voltage and for producing a match signal when a predetermined relationship exists therebetween; and (e) timer means responsive to said match signal for measuring a time required for the leak detection cable to be charged from said first potential to said second potential. - View Dependent Claims (2, 3, 4, 17, 18, 19, 20)
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5. An apparatus for measuring a capacitance, the apparatus comprising:
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(a) inverter means for receiving a DC input voltage and for inverting said DC input voltage to produce a mirror-image inverted DC voltage; (b) current source means for receiving a DC voltage (Vx) and for producing a constant current (Ic) proportional to said DC voltage (Vx), said constant current (Ic) being defined by the equation Ic =-k·
Vx, where k is equal to a predetermined constant, and where Vx is equal to one of said DC input voltage and said mirror-image inverted DC voltage;(c) means for supplying said constant current (Ic) to the capacitance such that the capacitance is charged from a first potential to a second potential; (d) comparator means for comparing said second potential to the other one of said DC input voltage and said mirror-image inverted DC voltage and for producing a match signal when a predetermined relationship exists therebetween; and (e) timer means responsive to said match signal for measuring a time required for the capacitance to be charged from said first potential to said second potential. - View Dependent Claims (6, 7, 8)
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9. A system for detecting leakage of a liquid, said system comprising:
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(a) a leak detection cable having two substantially parallel conductors with a permeable insulator disposed therebetween; and (b) a capacitance measuring circuit connected to a first end of said cable, said circuit including; (i) inverter means for inverting a DC voltage (VIN) to produce a mirror-image inverted DC voltage (-VIN); (ii) current source means for producing a constant current (Ic) proportional to said mirror-image inverted DC voltage (-VIN), Ic being defined by the equation Ic =k·
VIN, where k is equal to a predetermined constant;(iii) means for supplying said constant current (Ic) to said cable such that said cable is charged from a first potential to a second potential; (iv) comparator means for comparing said second potential to said DC voltage (VIN) and for producing a match signal when a predetermined relationship exists therebetween; and (v) timer means responsive to said match signal for measuring a time required for said cable to be charged from said frist potential to said second potential. - View Dependent Claims (10, 11, 12, 13)
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14. A capacitance measuring circuit for use with a leak detection cable having two substantially parallel conductors with a permeable insulator disposed therebetween, said circuit comprising:
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(a) an inverter which produces an output voltage -VIN from an input voltage VIN, wherein -VIN is a mirror-image of VIN ; (b) a current source, electrically connected to said inverter and to said leak detection cable, said current source accepting -VIN as input and outputting a constant current IC to the leak detection cable, wherein IC is defined by the equation IC =k·
VIN, and k is equal to a predetermined constant;(c) a voltage comparator having a first input and a second input, said first input being electrically connected to VIN and said second input being electrically connected to the leak detection cable, said voltage comparator comparing a voltage VX between the two substantially parallel conductors of the leak detection cable to VIN and producing a match signal when said voltage between the two substantially parallel conductors of the leak detection cable reaches VIN ; (d) a digitally controlled analog switch selectively having an OPEN position and a CLOSED position, said digitally controlled analog switch being electrically connected in parallel with the leak detection cable, wherein such that said constant current is shunted to ground and VX is substantially equal to zero when said digitally controlled analog switch is in said CLOSED position, and wherein said leak detection cable is charged by said constant current when said digitally controlled analog switch is in said OPEN position; and (e) a microcontroller unit having an internal timer, said microcontroller unit being electrically connected to said digitally controlled analog switch and to said voltage comparator such that said switch is controlled by said microcontroller unit and said internal timer is responsive to said match signal from said voltage comparator, wherein said switch is caused to OPEN and said internal timer is caused to start timing simultaneously, and wherein said internal timer is caused to stop timing in response to said match signal from said voltage comparator such that a time required for VX to be charged from zero to VIN is measured by said internal timer.
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15. A method for measuring a capacitance value, said method comprising the steps of:
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(1) receiving a DC voltage; (2) inverting said DC voltage to produce a mirror-image inverted DC voltage; (3) producing a constant current which is proportional to one of said DC voltage and said mirror-image inverted DC voltage, said constant current being defined by the equation I=-k·
V, where I is equal to said constant current, k is equal to a predetermined constant, and V is equal to said one of said DC voltage and said mirror-image inverted DC voltage;(4) providing said constant current to the capacitance such that the capacitance is charged from a first potential to a second potential; (5) comparing said second potential to the other one of said DC voltage and said mirror-image inverted DC voltage; (6) generating a match signal when a predetermined relationship exists between said second potential and said other one of said DC voltage and said mirror-image inverted DC voltage; (7) measuring a time required for the capacitance to be charged from said first potential to said second potential as indicated by said match signal; and (8) determining the value of the capacitance from said time.
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16. A method for monitoring a capacitance of a leak detection cable and for sounding an alarm signal when the capacitance is changed by permeation of a liquid into the leak detection cable, said method comprising the steps of:
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(1) receiving a DC voltage (VIN); (2) inverting said DC voltage (VIN) to produce a mirror-image inverted DC voltage (-VIN); (3) producing a constant current (IC) which is proportional to said mirror-image inverted DC voltage (-VIN), IC being defined by the equation IC =k·
VIN, where k is equal to a predetermined constant;(4) providing said constant current (IC) to the leak detection cable such that the leak detection cable is charged from a first potential to a second potential; (5) comparing said second potential to said DC voltage (VIN); (6) generating a match signal when a predetermined relationship exists between said second potential and said DC voltage (VIN); (7) measuring a time required for the leak detection cable to be charged from said first potential to said second potential as indicated by said match signal; (8) determining a value of the capacitance from said time; (9) comparing said value of the capacitance to a predetermined threshold; and (10) generating an alarm signal when said value of the capacitance exceeds said threshold.
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Specification