GB2223320A

GB2223320A - Capacitance measuring

Info

Publication number: GB2223320A
Application number: GB8921651A
Authority: GB
Inventors: Alexander J Binnie
Original assignee: Northern Engineering Industries PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 1988-09-28
Filing date: 1989-09-26
Publication date: 1990-04-04
Anticipated expiration: 2009-09-26
Also published as: GB2223320B; GB8822756D0; GB8921651D0

Abstract

Measurement of line capacitance, eg. of a telephone line, is performed using a known capacitance (6) in series with the line conductors (8). An oscillatory signal (1-4) is input (5) to the conductors through the known capacitance (6) and an output current signal is obtained (7b) that is dependent on the line capacitance. After passing the output through amplifying (10) and phase sensing (11) stages the output signal in phase with the input is obtained and is scaled (12) to drive (13) a meter (14). In Fig. 2 (not shown) an output voltage signal from the line conductors is used to determine line capacitance. <IMAGE>

Description

CAPACITANCE MEASURING METHOD & MEANS This invention relates to a method and means for measuring electrical capacitance.

The capacitance between insulated conductors in a cable is proportional to the length of the conductor. By measuring the capacitance between two conductors, therefore, the length of the conductors or the distance to an open circuit in one or both of the conductors can be determined if the unit capacitance between the two conductors is known.

This principle has been used in communications networks, where pairs of conductors are laid together to form telephone lines, to locate open-circuit faults in the conductors. For this purpose, a linesman's analog multimeter is provided with a reversing switch at its input. By selecting the appropriate resistance range and rapidly pressing the switch, the instrument meter scale can provide an indication of the capacitance of the pair of conductors to which the instrument is connected. The response obtained is proportional to the rate at which a reversing switch is activated and it is affected by the dynamics of the analog circuitry, which usually comprises a moving coil meter.The accuracy of the response as a measure of capacitance is, therefore, very poor and the meter reading can only be used to provide a very crude estimate of the length of the pair of conductors or the distance to an open circuit in one or both of them.

It is an object of the present invention to provide an improved method and apparatus for measuring the capacitance between a pair of conductors in a cable.

According to one aspect of the invention, in a method for measuring the capacitance between a pair of conductors, an oscillatory electrical signal is input into one of said conductors via a capacitance of known value in series therewith and an output is obtained from the other of the conductors related to said capacitance, and from the output deriving an analog signal component in known phase relation to said oscillatory electrical signal to provide said capacitance measurement, said analog signal being essentially proportional to the product of the input voltage and said known capacitance or the capacitance between said conductors, divided by the sum of the two capacitances.

Preferably, the oscillatory input signal is generated as a square-wave voltage signal at a fixed frequency.

According to another aspect of the invention, there is provided an apparatus for measuring the capacitance between a pair of conductors comprising means for generating an oscillatory signal, a pair of terminals for connection to the respective conductors, means for connecting the signal generating means to one of said terminals through a capacitance of known value, and measuring means for connection to at least one of said terminals to detect an electrical value produced in said conductors by the signal from said generating means, means for deriving from said value an analog measurement signal phase-related to the oscillatory signal from said generating means, said analog signal being essentially proportional to the product of the input voltage and said known capacitance or the capacitance between said conductors, divided by the sum of the two capacitances, and means for processing said measurement signal to provide a reading of the capacitance between said conductors.

By so placing the known capacitance in circuit with said pair of conductors, and relating the measurement signal to the combined values of the known and unknown capacitances, there is provided a relatively simple and convenient way of producing, for the widest range of values, measurement signals suitable for display in an analog indicator such as the scale of a moving coil meter.

Embodiments of the invention will be described by way of example with reference to the accompanying drawings, in which: Figs. 1 and 2 are schematic diagrams of two alternative forms of apparatus according to the invention.

The apparatus shown in Fig. 1 forms part of a hand-portable instrument (not shown) and can be integrated into an otherwise conventional analog multi-meter. A power source such as a 9 volt battery is connected to a stable voltage source 2 generating a DC reference voltage of, eg.

3 volts. The power source also provides power for operating the other electronic circuits in the apparatus (and the meter)7 although this is not specifically illustrated. The apparatus comprises a square-wave generator 3 which includes a free-running multivibrator oscillator with a frequency of, eg. 40Hz, the oscillator output providing the clock for a flip-flop whose output is thus a square wave with a frequency of 20Hz and a precise mark to space ratio of 1. The use of a square wave for the measurement input signal makes for simplification of the circuitry, lowering the manufacturing costs.

The output of the square wave generator operates an electronic switch 4 which acts as a single-pole changeover switch with a normally-open contact connected to the voltage source 2 and a normally-closed contact connected to the 0 volt supply rail (not shown). The switch output voltage, therefore, has a 20Hz square-wave with a mark space ratio of unity and a stable peak to peak voltage of 3 volts. This waveform is routed to terminal 7A of a pair of measurement terminals 7A,7B of the apparatus, via a protection network 5 and a capacitor 6.

The pair of conductors 8 whose capacitance is to be measured are connected to the terminals 7A,7B. A further protection network 9 is in the line from the terminal 7B. Both networks 5,9 serve to protect the apparatus against damage from voltages that may be present in the conductors 8 and may be formed by clamping diodes.

When the square-wave output is transmitted through the terminal 7A, the capacitance effect between the two conductors generates at the second terminal 7B a current with an amplitude inversely proportional to the sum of the impedance presented by the conductors and the capacitor 6 in series with them. This current passes, via the protection network 9, to a virtual earth amplifier 10 with feedback provided by a capacitor 10A, so that the voltage at the amplifier output is phase-corrected for the effect of the capacitor 6.

The amplifier output voltage is routed to a phase-sensitive rectifier 11 which has a reference input from the square-wave generator 3. The rectifier 11 transmits thus only the component of the phase-corrected voltage that is in phase with the reference voltage of the generator 3 and so produces a DC output voltage that is proportional to the reactive current flowing into the measurement terminal 7B. By this technique, the effect of any resistance between the pair of conductors connected across the measurement terminals 7A,7B can be substantially eliminated.

With the apparatus integrated in a multi-meter, to produce a reading the DC output voltage from the phasesensitive rectifier 11 is routed, preferably through a scaling circuit 12, to drive circuit 13 of the meter to drive the pointer of the meter scale 14.

If the capacitor 6 has a value Cs, and the capacitance between the conductors 8 has a value Cx, then assuming the effect of the resistance of the conductors 8 is eliminated by the phase-sensitive rectifier 11, with a voltage V from the electronic switch the voltage measurement signal output from the rectifier 11 is: kV.Cx Cs+Cx where k is a constant.

This analogue output signal follows a similar law to the signal for the resistance range on a multi-meter such as the Multistor. The resistance scale present on such a meter can thus be used for indicating the line capacitance of the conductors 8. Since the same scale can be used for both resistance and capacitance, it is possible to measure a circuit with the one scale both for resistance to a short circuit and for capacitance in open circuit.

If the scaling unit 12 is used to calibrate the drive circuit 13 to give the full scale value for an infinite capacitance across the terminals 7A,7B, and to give a zero reading with zero capacitance across the terminals, a mid-scale reading will be obtained when the capacitance being measured is equal to Cs. This arrangement therefore gives a scale form similar to that of a typical resistance measuring range on an analog multimeter which indicates full scale when the resistance being measured is infinite. The input signals can, by suitable scaling in the meter, indicate line distance to the fault.

The alternative apparatus illustrated in Fig. 2 is similar in many respects to that of Fig. 1 and parts already described are indicated by the same reference numbers. In this example, however, the conductor terminals 7A,7B are connected, respectively to an amplifier 15 via the protection network 9 and to the 0 volt supply rail.

The measuring process thus detects the voltage across the terminals 7A,7B rather than the current into one terminal.

For this purpose, the amplifier 15 is a non-inverting high input impedance amplifier. As before, the voltage signal is processed in a phase-sensitive rectifier 11 having an input from the square-wave generator. The measurement signal from the rectifier 11 in this circuit is: KV. Cs Cs+Cx where K is a constant.

It will be noted that the signal follows the same law as th first example. In this case, the meter drive circuit 13 can be calibrated by the scaling circuit 12 to give a full scale reading when the capacitance being measured is zero, a zero reading when the capacitance is infinite and, therefore, a mid-scale reading when the capacitance being measured is equal to Cs. This also results in a scale of similar form to that of a typical resistance measuring range on an analog multi-meter, but in contrast to the first example, one which indicates full scale when the resistance being measured is zero.

Claims

1. A method of measuring the capacitance between a pair of conductors comprising placing the conductors in series with a capacitance of known value, inputting an oscillatory voltage signal via said known capacitance into one of said conductors and obtaining an output therefrom dependent upon said capacitance, and from said output deriving an analog signal in known phase relation to said oscillatory electrical signal to provide said capacitance measurement, said analog signal being essentially proportional to the product of the input voltage and said known capacitance or the capacitance between said conductors, divided by the sum of the two capacitances.

2. A method according to claim 1 wherein said input signal has a square wave form.

3. A method according to claim 1 or claim 2 wherein said oscillatory signal is input to the conductors through said known capacitance and said output signal is processed to provide a scale reading in an intermediate region of a scale range when the known capacitance is equal to the capacitance between the pair of conductors.

4. A method according to claim 1 or claim 2 wherein said oscillatory signal is input to the conductors through said known capacitance and the output signal is obtained as a current signal from the second of the conductors and is phase-corrected to remove the phase shift imposed by the known capacitance.

5. A method according to claim 4 wherein the measurement signal is formed by part of said phasecorrected output signal in predetermined phase relationship to the oscillatory input signal.

6. A method according to any one of claims 1 to 3 wherein the output signal is formed by the voltage across the conductor.

7. Apparatus for measuring the capacitance between a pair of conductors comprising means for generating an oscillatory signal, a pair of terminals for connection to the respective conductors, means for connecting the signal generating means to one of said terminals through a capacitance of known value, and measuring means for connection to at least one of said terminals to detect an electrical value produced in said conductors by the signal from said generating means, means for deriving from said value an analog measurement signal phase-related to the oscillatory signal from said generating means, said analog signal being essentially proportional to the product of the input voltage and said known capacitance or the capacitance between said conductors, divided by the sum of the two capacitances, and means for processing said measurement signal to provide a reading of the capacitance between said conductors.

8. Apparatus according to claim 7 wherein the known capacitance is placed between and in series with said signal generating means and said one terminal and the measuring means comprise a measurement scale so calibrated that said measurement signal produces a scale reading in an intermediate region of the scale range when the known capacitance is equal to the capacitance of the pair of conductors.

9. Apparatus according to claim 7 or claim 8 arranged to sense said electrical value as the current in the other conductor terminal and having means for producing a phase-changed signal from said current value to reverse the phase-change imposed by the capacitance in series with the conductors before said phase-related component is separated.

10. Apparatus according to claim 7 or claim 8 arranged to sense the voltage across said terminals and said voltage value is processed in a non-inverting high input impedance amplifier before said phase-sensitive component is produced.

11. Apparatus according to any one of claims 7 to 10 wherein protection means are provided before each of said terminals to protect the said generating and measuring means from over-voltage in the conductors.

12. A method of measuring the capacitance between a pair of conductors substantially as described herein.

13. Apparatus for measuring the capacitance between a pair of conductors substantially as described herein with reference to Fig. 1 or Fig. 2 of the accompanying drawings.