WO1986007155A1

WO1986007155A1 - Two body capacitance measurement instrument remotely connected to probe via transmission channel

Info

Publication number: WO1986007155A1
Application number: PCT/AU1986/000151
Authority: WO
Inventors: Warwick Kemp
Original assignee: The Commonwealth Of Australia Care Of The Secretar
Priority date: 1985-05-28
Filing date: 1986-05-28
Publication date: 1986-12-04

Abstract

A two body capacitance bridge for accurate capacity measurement which is affected by the presence of the measurement instrument in which a probe (1) is located at the measurement site and includes measuring means, and a measurement instrument (2) is remotely positioned from the probe and interconnected by a transmission channel (3), having a capacitance-to-measurement signal converter (4) connected to a measurement signal transmitter (5) and arranged to transmit a measurement signal and a measurement signal receiver (6) forming part of the measurement instrument (2) and connected to a measurement-signal-to-voltage converter (7) using a standard-component-to-voltage converter (10) arranged to generate a calibrated voltage from a standard component (9) and a detector (8) to measure the voltage differential between the voltage of the measurement-signal-to-voltage converter (7) and the voltage of the standard-component-to-voltage converter (10).

Description

TITLE: TWO BODY CAPACITANCE MEASUREMENT INSTRUMENT REMOTELY CONNECTED TO PROBE VIA TRANSMISSION CHANNEL

This invention relates to a two body capacitance bridge.

With the type of measuring equipment generally used, other than in special circumstances, it has 5. been difficult to accurately measure the capacitance between two conducting bodies where one body does not fully enclose the other. In this case of the open bodied capacitance, the associated electric field extends far beyond the confines of the bodies and 10- hence encompasses the measuring instrument and/or its measuring cables or connections.

With the measuring equipment in use, virtually all measurements of capacitance between two open con¬ ducting bodies have been inaccurate three body 15. measurements where the measuring instrument or its cable or cables have been the corrupting third body. These inaccuracies can be gross and result from two causes:

(1) The presence of the measuring instrument or its 20. cable or cables which modifies the capacitance between the two bodies by changing the distribution of the field around the bodies.

(2) The capacitance between the bodies and the measuring instrument and/or its connecting cable or cables

25. and even the operator of the instrument.

One problem encountered with present techniques of measuring capacitance, closed body or otherwise, on the end of a coaxial cable is that there is a practical limit to the length of cable or cables that can be 30. used. This limits how remotely the main measuring instrument can be placed. A further problem is caused by interference and many instruments malfunction or become inaccurate when external signals are superimposed on the capacitance to be measured.

5. It has been proposed in Australian Patent

Specification No. 77739/81 in the name of Dennis W. Burt to use modified light fibres to connect a remote measuring device to measuring means but the proposal requires a series of such light fibre channels to 10. compensate for light variations, and the present invention has as its objective the overcoming of problems of the presently known and used systems and the invention achieves this by using a probe which communicates with the measuring instrument through a single channel.

15. The instrument, the subject of this invention, can make these two body capacitance measurements with very greatly improved accuracy and there is virtually no limit to how remotely the main instrument can be placed. Furthermore, quite high levels of superimposed

20. signal can be tolerated before significant inaccuracies occur.

The accurate measurement of capacitance between two bodies is achieved according to this invention by the use of a small measuring probe which is connected

25. to the capacitance to be measured by means such as very fine wire leads, while the probe is connected to a remotely placed measuring instrument, which we refer to as the main instrument, by the special transmission means of this invention, such as light transmission means,

30. for example a fibre optic cable, or in selected instances where this does not significantly corrupt the measurement, electrical transmission means, for instance a coaxial cable. The measurement error is minimised because the very small conducting area and volume particularly of the electrically isolated probe causes very little disturbance to the field around the bodies. The small probe size also 5. means that its capacitance to the bodies is also very small. In most cases the probe can be enclosed within or shielded by one of the bodies which completely removes the sources of error mentioned above.

The two body capacitance bridge according to this

10. invention comprises a capacitance-to-signal converter forming part of the probe and connected to a measurement- signal transmitter also forming part of the probe and arranged to transmit a signal froπi the capacitance-to- signal converter to the measurer^nt instrument over a

3_5 transmission channel from the probe to the measurement instrument, a measurement signal receiver forming part of the measurement instrument and connected to a measurement- signal-to-voltage converter also forming part of the said instrument, a standard-component-to-voltage converter also

20. forming part of the measurement instrument and arranged to generate a selected voltage from the standard component, and a detector coupled between the measurement-signal-to- voltage converter and the standard-component-to-voltage converter to measure the voltage differential between the

25. voltage of the measurement-signal-to-voltage converter and the voltage of the standard-component-to-voltage converter.

The method of transmitting the signal can vary and can comprise a light signal or an electrical signal but to enable the invention to be fully understood embodiment of

30, the invention will now be described with reference to the accompanying drawings, but it is to be clear that the invention is not necessarily to be limited to the specific embodiment shown as the transmission channel and means can be varied in form within the spirit of the invention. In the drawings:

FIG. 1 is a block diagram showing the basic form of the instrument,

FIG. 2 shows the instrument using a fibre optic 5« transmission channel and capacitance standards,

FIG. 3 again uses fibre optics but conductance standards, and

FIG. 4 shows how a coaxial cable may replace fibre optics signal transmission channel.

10- Referring first to the basic form illustrated in Fig. 1, the probe 1 is shown connected to the measuring instrument 2 by a transmission channel 3.

The probe 1 has in it a capacitance-to-signal converter 4 coupled to a measurement signal transmitter 15. 5 which transmits the measurement signal to a measurement signal receiver 6 over the transmission channel 3.

The measurement signal receiver passes the measure¬ ment signal to a measurement-signal-to-voltage converter 7 which passes the voltage signal to a detector 8 20. which measures the voltage signal against a voltage derived from a standard component 9 connected to the detector 8 through a standard-component-to-voltage converter 10. The measurement-signal-to-voltage converter 7 and the standard-component-to-voltage converter 10 share a common reference voltage 11.

The measurement signal transmitter 5 is selected to 5. give minimum interference with the probe 1 and can be of any form which gives this result, but as examples embodiments shown in Figs 2,3 and 4 will now be described, but the invention is not to be limited to this form as other light paths or electromagnetic or in special cases 10. electrical paths can be used as the transmission channel 3.

In the case of the "photonic probe" as described in Fig. 2, the probe 1 is in the form of a photonic measuring probe 14 and contains a capacitance-to-inverse- frequency (C/IF) converter 15 which, in the case of the

15. prototype instruments consists of a precision RC

(resistance capacitance) oscillator whose frequency of oscillation is precisely inversely proportional to the total amount of capacitance (the effective residual internal capacitance and Cx, the external capacitance

20. under measurement) at its input. This oscillator is of the dual threshold type with a symmetrical waveform to eliminate dielectric absorbtion errors for capacitances with other than air or vacuum dielectrics.

The signal from the capacitance-to-inverse-frequency 25. converter 15 is fed to an optical transmitter 16, and the power for the capacitance-to-Inverse-frequency converter 15, and the optical transmitter 16 is supplied by batteries 17 such as silver oxide watch batteries. The signal from the optical transmitter 15 is transmitted along an optical fibre cable 18 which in this embodiment forms the transmission channel 3. The electronics is fabricated using thick film integrated 5. circuit techniques to achieve a very small volume.

In the remote measurement instrument 19 the light signal from the remote photonic probe 14 is reconverted to an electrical signal in a sensitive optical receiver 20 to minimise the transmitter current requirement in the 10. photonic probe in order to minimise the battery size required.

This signal is now fed to a first frequency-to- voltage (F/V) converter 21, the output voltage of which is then proportional to the frequency of the oscillator 15. in the photonic probe 14 and hence inversely proportional to the total capacitance at the probe input. This voltage is then passed to one input of the detector 22.

A second detector input is fed from a second

20. identical frequency-to-voltage converter 23 which shares the same reference voltage source 24 as the first frequency-to-voltage converter 21.

The detector 22 is used to determine when the output voltages from the two F/V converters 21,23 are 25. identical and hence when this 'bridge' configuration is balanced. The input to this second F/V converter 23 comes from another precision RC oscillator which is a capacitance-to-inverse-frequency converter 25 and is identical in configuration to the capacitance-to-inverse- 5. frequency converter 15 in the probe. The internal RC oscillator of this converter 25 has its frequency determined by a variable capacitor 26 (zero control) and a set of switched standard capacitors 27. The zero control capacitor 26 is adjusted to balance the bridge before the

10. capacitance under measurement is connected, or any standard capacitors selected. Under this condition the residual capacitances at the probe 14 and internal oscillator inputs are exactly equal. When the unknown capacitor 27 is connected to the probe an identical amount

15. of standard capacitance has to be switched onto the internal oscillator to balance the bridge. In practice the two oscillators are arranged to have different constants of proportionality to allow higher and hence more practical values for the standard capacitors.

20. ^ϋne prototype instrument uses sixteen standard capacitors, weighted in binary coded decimal (BCD) fashion so that a set of four BCD thumbwheel switches can be used to appropriately connect the capacitors. This allows the unknown capacitance to be read directly from the thumb-

25. wheel switches when the bridge has been balanced.

This configuration of instrument has the advantage that any inherent non linearities in the oscillators or frequency-to-voltage converters 21,23 are cancelled out by the balanced nature of the system. In Fig. 3 an instrument is shown in which conductive standards are used in preference to capacitors. In this embodiment, components similar to these refered to in Fig. 2 are given the same reference numerals.

5 For this method the second F/V converter 23 and its RC oscillator are replaced by an inverse conductance-to- voltage IG/V converter 30 to which are connected conductances 31 with analogous values to the variable capacitance, zero control 26 and the switched standard 0 capacitances 27 of Fig. 2. The IG/V converter 30 is arranged to have an exactly analogous transfer function from the conductances to the detector 22 as exists between the capacitance at the probe input to the detector 22. This means the output voltage of the IG/V converter 30 is 5 _m inversely proportional to the total conductance at its input. The conductances 31 are switchable, and a variable conductance 32 provides a zero control.

This configuration has the advantage that stray capacitance does not have to be taken into account 20. in the switching arrangements of the conductances

31. This is not the case for the switched capacitors 27.

As the capacitance-to-Inverse-frequency transfer function of the probe is precisely known, an alternative

2 could be the use of a microprocessor based system where all the signal processing beyond the optical receiver including calibration is done in software. In this case again the microprocessor is programmed to compare the unknown capacitance to the standard but details are not

_3Q described herein as this would be in the scope of experts in this field.

A further alternative could make use of the precisely linear relationship between the period of oscillation of the oscillator and the capacitance using digital ,₅ techniques to measure this period and display it directly as capacitance. In Fig. 4, in which again components similar to Figs 2 and 3 have similar reference numerals, is shown a variation in which a coaxial cable 35 is used for situations where remote measurements are required and a 5. coaxial or other electrically conducting cable can be tolerated. This probe 34 contains the same capacitance-to- inverse-frequency converter 15 as the photonic probe 14 and is connected to the measuring instrument 19 through coaxial cable interfaces 36 and 37. This has the advantage 10. of allowing the probe 34 to be powered via the cable 35 from the measuring instrument 19 thus obviating the need for batteries 17 in the probe 34. It also allows the probe 34 to be made much smaller.

While details may be varied as stated earlier 15. herein, the invention has been applied using four

BCD thumbwheel switches for both the capacitive standard system shown in Fig. 2 and 4 and conductive standard system shown in Fig. 3 with both photonic and coaxial probes.

2o. In each of the illustrations the bridge is designated

40.

Test instruments have been designed to measure capacitances in two ranges: 0 to 1000 pF with 0.1 pF resolution and 0 to 100 pF with 0.01 pF resolution. 25. Basic measurement accuracies of 0.57. have been achieved but it is expected this can be improved to below 0.17..

Two design features have made the instruments very Insensitive to external RF (radio frequency) fields which superimpose a signal on open bodied 30. capacitances:

(1) The use of relatively high residual capacitance at the probe input provides a low input impedance at RF frequencies. (2) The relatively large voltage swings applied by the oscillator to the capacitance under measurement tend to swamp the effect of the relatively small field-induced voltage.

5. The invention can be applied in various fields but the following are typical uses:

(a) In the design and development of Active Receiving Antennas, an important parameter to determine is that of receiving element capacitance. The

10. receiving elements of active antennas are usually small compared with the wavelengths of the signals they are to receive. It can be shown that the element capacitance at these signal frequencies is the same as would be measured at DC or low

15. frequencies. This has been confirmed experimentally.

It is therefore appropriate to measure the element capacitance at low frequencies using this instrument. It is especially difficult to accurately measure the capacitance of small dipoles using existing

20. techniques, errors of several hundred percent often being experienced. A prototype instrument has been used to accurately measure the capacitance of a small tubular dipole by placing the photonic probe inside one half. Accurate results are

25. also achieved with the photonic probe connected externally.

An example of very large measurement error made using existing measurement techniques on a dipole can be seen in a paper from the International Conference 30. on Antennas and Propagation, ICAP 83, IEE Conference

Publication 219, titled "An Active Antenna for TV Bands 3,4 and 5" by R.N. Velev and R.W. King, University of Southampton, U.K. (b) A prototype instrument has been successfully used to refine mathematical computer models of complex antenna structures. This has been achieved by using the instrument to determine the capacitance

5 between complicated small components, impossible to model mathematically, removed from the antenna for measurement and by making physical models of components for measurement. The measured capacitances were then included in the computer 0. models.

(c) The instrument can be used to accurately investigate the element capacitance of ground based active

(or otherwise) monopole antennas for various soil conditions e.g. dry sand, wet earth etc. If the 5 antenna under measurement utilizes a ground mat, a coaxial probe could be used. An instrument has been successfully used for this type of measurement. The insensitivity to RF fields is useful in this situation.

20. (d) The instrument is useful where measurements need to be made at extreme distances from the capacitance under measurement. This can be done with the photonic or coaxial probe with no inherent limit on separation.

-c So far as the "bridge" nomenclature is concerned, although the configuration used in this instrument is of necessity, not identical to that of a conventional bridge circuit, they have many properties in common:

(a) The component under measurement is compared to a standard component of the same or analogous

30. type (e.g. Comparing and unknown capacitance to a standard capacitance or conductance). (b) The component under measurement can be compared, in calibrated fashion, to a standard reference component of a quite different value.

(c) A detector is used to compare and balance the 5_^ voltages on each side of the bridge.

(d) The final balance point is insensitive to variations in the reference voltage applied to the bridge.

(e) The voltages applied to the detector inputs can range between the limits of the reference voltage o. applied to the bridge.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A two body capacitance bridge for accurate capacity measurement affected by the presence of the measurement instrument in which a probe (1,14,34) is located at the measurement site and includes measuring means, 5. and a measurement instrument (2,19) is remotely positioned from the probe (1,14,34) and interconnected by a trans¬ mission channel (3,18,35) characterised by;

(a) a capacitance-to-measurement signal converter (4,15) forming part of the said probe (1,14,34) and 10. connected to a measurement signal transmitter

(5,16,36) also forming part of the probe (1,14,34) and arranged to transmit a measurement signal from the said capacitance-to-measurement signal converter (4,15) to the said measurement instrument (2,19)

15. (b) a measurement signal receiver (6,20,37) forming part of the measurement instrument (2,19) and connected to a measurement-signal-to-voltage converter (7,21) also forming part of the said measurement instrument (2,19)

20. (c) a standard-component-to-voltage converter

(10,23,30) also forming part of the measurement instrument (2,19) arranged to generate a calibrated voltage from a standard component (9,26-27,31) and

(d) a detector (8,22) coupled between the 25. measurement-signal-to-voltage converter (7,21) and standard-component-to-voltage converter (10,23,30) to measure the voltage differential between the voltage of the measurement-signal-to-voltage converter (7,21) and the voltage of the standard-component-to-voltage 30. converter (10,23,30) 14 .

2. A two body capacitance bridge according to claim 1 wherein the transmission channel (3) is a light channel driven by an optical transmitter (16) connected to the said capacity-to-inverse frequency converter

5. (15) of a photonic probe (14).

3. A two body capacitance bridge according to claim 1 characterised by a photonic probe (14) incorporating a capacitance-to-inverse frequency converter (15) and our optical transmitter (16) both powered by batteries 5. (17) in the photonic probe 14 whereby the photonic probe

14 is selfcontained and isolated from the said measurement instrument (19) excepting by a light channel driven by the said optical transmitter 16, further characterised by an optical receiver (20) forming part of the measure- 10. instrument (19).

4. A two body capacitance bridge according to claim 2 or 3 wherein the said light channel comprises a fibre optic cable (18) connected between the optical transmitter (16) and an optical receiver (20)

5. A two body capacitance bridge according to claim

1 wherein the said probe (1) comprises the said capaci¬ tance-to-measurement-signal converter (4) coupled to a measurement signal transmitter (5) to provide a 5. transmission channel (3) between the said signal transmitter (5) and the measurement signal receiver (6) in the measurement instrument (2). 6. ^*A two body capacitance bridge according to claim

5 wherein the said transmission channel (3) is a coaxial cable (35) connected between a coaxial cable interface (36) which forms the measurement signal transmitter (5) and a coaxial cable interface (37) which forms the measurement signal receiver (6).

7. A two body capacitance bridge according to claim 1 wherein the measurement instrument (2,19) comprises a measurement-signal-to-voltage converter (7,21) adapted to receive the measurement signal from the measurement signal receiver (6,20,37), and is connected to one side of a detector (8), and a standard-component-to- voltage converter (10,23,30) is connected to the other side of the detector (6), and wherein the measurement- signal-to-voltage converter (7,21) and the standard- component-to-voltage converter have a common reference voltage (24,29)

8. A two body capacitance bridge according to claim 7 wherein the standard component (9) of the standard- component-to-voltage converter (10,23,30) comprises capacitances (26,27)

9. A two body capacitance bridge according to claim 7 wherein the standard component (9) of the standard- component-to-voltage converter (10,23,30) comprises conductances (31) 10. The method of obtaining accurate capacity measurement which would be affected by presence of the measurement instrument which comprises,

(a) locating a probe (1,13,34) at the measurement 5. site containing a capacitance-to-measurement signal converter (4,15) connected to a measurement signal transmitter (5,16,36),

(b) activating the said probe (1,13,34) to energise transmission channel from the said measurement

10. signal transmitter (5,16,36),

(c) receiving the said measurement signal in a remote measurement signal receiver (6,20,37) and changing it to a form suitable to derive the capacitance value by reference to a standard.

11. The method of claim 10 wherein a microprocessor is used to derive the value of the capacitance being measured.

12. The method of obtaining accurate capacity measurement which would be affected by presence of the measurement instrument which comprises,

(b) activating the said probe (1,13,34) to energise a transmission channel from the said measurement

10. signal transmitter (5,16,36), (c) receiving the said measurement signal in a remote measurement signal receiver (6,20,37) and changing it to a voltage signal in a signal to-voltage converter (7,21),

15. (d) comparing the said received measurement signal voltage with a voltage derived from a standard- component-to-voltage converter (10,23,30) in the detector (8) of a bridge (40) while maintaining a common reference voltage to the signal-to-voltage

20. converter (7,21) and the standard-component-to- voltage converter (10,23,30).