AU4274985A - Method and circuit for the measurement of capacitance - Google Patents

Description

2:2, iii

FORM 1

COMMONWEALTH OF AUSTRALIA

PATENTS ACT 1952

APPLICATION FOR A STANDARD PATENT

I/We, VAISALA OY

Vanha Nurmijarventie 21 Mst~ ~a~o~ir 00420 HELSINKI 7 MAYft FINLAND.

hereby apply for the grant of a standard patent for an inve]ntion entitled "METHOD FOR THE MEASUREMENT OF CAPACITANCES. IN PARTICULAR OF LOW CAPACITANCES7 WHEREIN TWO REFE]RENCES ARE USED" which is described in the accompanying Hrxxamin/complete specification.

Details of basic application(s): Number of basic Name of Convention country in Date of basic application which basic application was filed application

84 2193 FINLAND May 31, 1984

roVtGOF

"A"ij.o~ieu 44

My/our address for service is care of CLEMENT HACK CO., Patent Attorneys, 140 William Street, Melbourne, Victoria, 3000, Australia.

DATED this 21st day of May, 1985 I VAISALA OY '1

C d

To: The Commissioner of Patents. Y17 ;X.Pk il-~c~~

PF/App/ 3 82

(12) AUSTRALIAN PATENT ABSTRAC'T (19) All (11) AU-A-112749

(54) METHOD FOR THE MEASURENENT OF GAPAT.Ai E, IN PA.RTI,'CLAP OF LOW CAPACITANCES, WHEREIN TWO 4RE JSZO

(71) VAISALA OY. (21) 42749185 (2Z) 22.5.85 (31) 842193 (3Z) 31-5.834 (43) 5.12-85 (51)4 GOIR 17/02 (72) NOT GIVEN (74) HA (57) Ia i

-tances, in aa':.cX; c

inc ludc a rasureme.: L -i is a funiwtcn X:c' input term~inals of qu e nc the s a t-f C two z z i a e 1e ctL' !Df7o~a alterot ai c c a Pa c t~ s

signals o: i~ wiJthe rs n oa ooto-., o th '-esuemnt -lction ,os a:id, r

D2 17

1145/7 y4 U

(11) AIPA-42749/85 ence -Capa,-,itan ces (CRl ICR2)t that signals (U VU 2 representing the difference s between the said output signais (U1 and the sa.id sigznalis RU R2) comina from the external auxiliary references anxs formed, by means of which signals thle feedback 6ignals (U 1 U~2 controlling the circuit h. are forme the measurement electronics (16) b~ccontrolled by means of the said feedback signals iin s!ch a direction that the said dif: 'rential s ignals (U1 1 or equivalent approacn zero or a preset corresponding value, and tht* determi1nation of the output signal (U1 U ou cres,'odn to the Icapacitance (CM)to be measur,3d Is i:erforxned by means of the measurement ecc-trctnics (16) adjusted correctly by means of the method s p defined above.

;'45/8~

At~ibiALIA PATENTS ACT 1952

COMPLETE SPCIFI CATION (ORIGINAL)

FOR OFFICE USE: Application Number; q/ Class Int. Class

Complete Specification Lodged: Accepted: Published:

RelateJ Arv

Name of Applcants): VAISALA OY

Address of Applicant(s): Vanha Nurmijarventie 21 00420, HELSINKI FINLAND,

Actu ai It. ,entor(s):

Address for Service:- CLEMENT HACK CO., 140 William Street, Melbourne, Victoria, 3000 Australia

Complete Specification for the invention entitled: "'METHOD FOR THE MEASUREMENT OF CAPACITANCES91 IN PARTICULAR OF LOW CAPACITANCES WHERFREN',! TWO REFERENCES ARE'USED" 1

The followingstatement is a full description of this invention, including the best method of performing it known to me! ,4s L -ujLUI

Method for the measurement of capacitances, in particular of low capacitances, wherein two references are used

The invention is concerned with a method for the measurement of capacitances, in particular of low capacitances, in which said method measurement electronics are used that include a measurement oscil- lator, whose output frequency is a function of the capacitance to be connected to the input terminals of the circuit determining the frequency of the said oscillator, and in which method two reference capaci- tances are used, whose electrical values are placed a ,within the range of measurement and which are con- 15 nected, being alternatingly exchanged with the capa- Scitance or capacitances to be measured, to the measure- ment oscillator while making use of a switching arrangement. ao One starting point for the present invention S 20 has been the prior-art technology that comes out, e.g., from the applicant's FI Patents 54,664 and 57,319 (cor- responding US PLenits 4,295,090 and 4,295,091). In the said patents, a method is suggested for the measure- ment of low capacitances, as well as an electronic change-over switch to be used in this connection, in particular for telemeter use in sondes. S- In radios'tdes, for the measurement of various parameters,, in particular of atmospheric pressure, temperature and/or humidity, capaciftive detectors are used, the magnitude of whose capacitance depends on the parameter being measured. The capaci- tances of these detectors are often relatively low, from a few pF to some dozens or, at the maximum, about 100 pF. The measurement of low capacitances is prob- lematic, ,pwing to stray capacitances, variations in supply voltage, and other disturbances. Moreover, the said detectors have to some extent varying proper-

G 'i ties, so that they have, an individual non- linearity and dependence on temperature. In particular in telemeter applications, wh temperature, humidity or pressure or other, corresponding parameters are being measured by means of electric or mechano-electric detectors, it is known in prior art that, in connection with the measurement electronics, one or several references are provided which are stable and precisely known and by means of which it is possible to compensate individual properties of the measurement circuit and/or of the detector as well as their variations in time. In connection with capacitive detectors, it oa. is known in prior art to use a reference capacitance, 15 which is, alternatingly with the measuring capacitance, connected to the measurement circuit, usually the input circuit determining the frequency of the RC- oscillator. By appropriately adjusting the measure- ment circuit or in some other way, the output variable of the measurement circuit, derived from the reference capacitance, can be brought to the.correct level at each particular time. It is known in prioroart to use measurement circuits of one reference, in particular bridge con- nections, in which the measurement is, however, precise only when the electrical value of the reference is close to the value of the detector, when the bridge is in equilibrium. The moru distant the value of the detector becomes from the reference, the larger will also the various errors be, e.g. errors caused by changes in the dynamics of the electronic measurement circuit. An advantage of connections with one refer- ence is, however, the simplicity of the measurement circuit. The foundationc, of this prior-art method will be described in the following in more detail with reference to Figure 1.

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An advantage in measurement arrangements with two or more references is accuracy of the measurement even within wide ranges of measurement, but a drawback is the complexity of the measurement method and of the related computation. The foundations of measurement with two references will be described in the following in more detail with reference to Figure 2. An objective of the present invention is a further development of the said measurement methods and circuits for low capacitances, e.g. about 0 to 100 pF, 4 applied by the applicant and known in prior art, so that the said methods and circuits become more precise. An objective of the present invention is to S, .provide such a measurement method and measurement cir- 15 cuit with two references in which the complicated computation operations are avoided that were necessary in determining the results of capacitance measurements in prior art. An objective of the invention is to provide such a measurement method and such "self-adjusting" o- measurement electronics in which the output variables on. corresponding to the reference detectors remain Sinvariable even if the measurement electronics should creep, owing to variations in temperature or c~ 25 other circumstances. In view of achieving the above objectives and those that will come out in the following, the invention is mainly characterized in that in the method two external auxiliary references are used, the auxiliary reference signals obtained from them being compared with the corres- ponding output signals of the measurement electronics, derived from the said reference capacitances, that signals representing the-differences between the said output signals and the said signals coming from the external auxiliary references are formed, by means of which signals the feedback signals controlling the circuit are formed, the measurement electronics being controlled by means of the said feed- back signals in such a direction that the said differ- ential signals or equivalent approach zero or a preset corresponding value, and that determination'of the output signal cor- responding to the capacita3-e to be measured is per- formed by means of the measurement electronics adjusted correctly by means of the method steps defined above. The objectives of the invention are achieved by in the measurement circuit fitting external auxiliary Sreferences corresponding to the output variables t. obtained in response to the capacitance references proper, which said auxiliary references are stable and independent from the measurement electronics and from its creep as well as from various interference sources. The variables obtained from these auxiliary references are compared with the output variables derived from the capacitance references proper, and on the basis of 20 the said references, differential signals are formed, o which are allowed, for one part, to act in the way of a constant term upon the measurement electronics and, for the other part, to act upon the steepness of the measurement electronics summingly during so many measurement cycles as a sort of an iteration process, so that the difference between the capacitance reference and the external reference becomes zero or sufficiently close to zero. The comparison takes place preferably so that the differential signal derived from the first external auxiliary reference and from the first capacitance reference is made to act upon the measurement electron- ics in the way of a constant term, in other words, it is ntade to act upon the off-set of the measurement electronics. The differential signal derived from the second external auxiliary reference and from the second capacitance reference is, in the way described above, nt made to ajct upon the steepness of the measurement feE'd- electronics, e.g. on its amplification. If a measure- f f er- ment circuit or a neasurement method is concerned whose resktt output variable is a variable frequency, one of the said differential signals is made to act upon the car- basic frequency of the measurement electronics and )er- the other differential signal upon its dynamics, i.e. justed upon the change in frequency per a certain unit of )ove. change in capacitance. iieved 10 In the following, the invention will be ixiliary described in detail with reference to the figures in the accompanying drawing, wherein some of the back- as ground of the invention and some of its preferred Le and 3 embodiments are illustrated. o Figure 1 illustrates the characteristic 320 1) 0 curves (straight lines) of a single-reference measure- rences ment method in a system of xy-coordinates. rom Figure 2 shows the characteristic curves of sis of a two-reference measurement system, in a way corres- rmed, 030 2) ponding to Fig. 1. way of Figure 3 shows a measurement method and and, circuit in accordance with the invention-as a block the O;C diagram. Figure 4 shows an exemplifying embodiment ocess, 25 of the.method and circuit in accordance with the inven- eference c; 4 3) tion as a wiring diagram. ciently 3) 3)3

Fig. 1 illustrates a single-reference 35 measurement method in a system of xy-coordinates. By so that means of a reference, which is, a capacitance xternal whose electrical value is at the middle of the range ce ectron- X1 of measurement, a point x 0 ,y0 is fixed, through which the straight line k a illustrating the measure- S, it ment electronics runs. The coordinate x stands for the at. input variable, i.e. in the present case'the magnitue Dm the of the capacitance to be measured, and y stands for the s'econd output variable, i.e. in the present case, a DC Dove, voltage or a variable frequency. However, owing to the

0430 0 004 4 0 o 0 0*0 0 0 0 4 0P 0 00 4 0 0 00 4 o 00 boo 0 000 0 0 00r 0 044 CoO 0o 0 0 0 0 0 4 0 000 0 0 00L creeping of the measurement electronics and to other circumstances, the straight line illustrating the pro- perties of the system diverges from the basic straight line k between the example lines k1 and k 2 depicted with dashed line and dot-dashed line. Thereby, within the permitted margins of error, the measurement range X around x becomes relatively Pnarrow. In a corresponding way, Fig. 2 illustrates a two-reference measurement method, which is the starting point of the present invention, in a way cor- responding to Fig. 1. In the method, two references are applied, viz. references 1. and which fix two points in the system of xy-coordinates, viz. points o x1 ,1 and x 2'Y 2 the straight line k placed through 15 the said points being the linear basic operation line of the system. In practice, owing to changes in temper- ature or to other circumstances, the characteristic curves of the system vary at both sides of the straight ao line k between the curves f1 and f2. Thereby, within 0o 20 the margins of error, it is possible to accomplish a range of measurement X 2 which is larger than x 2 -x I o Thereby, as compared with the single-reference measure- ment method, a range of measurement X 2 can be accom- plished that is at least by one order larger. S 25 Since it is possible, by means of the inven- Stion to be described in more detail in the following, to eliminate the complicated computation operations occurring in prior art in connection with a measurement circuit of two references, by means of the present invention an advantageous measurement method and mea- surement circuit are accomplished, which can be carried into effect quite simply and an exemplifying embodiment of which will be described in the following with reference to Figures 3 and 4. According to Fig.-3, the system comprises a capacitance 12 to be measured and two reference capacitances 10 and 11 (CRl and CR 2 The values'

Ni

fi CRYSTAL C 4024 OSC. C 8 O- 7 Ca between the reference capacitances 10 and 11 corres- pond to the points x1 and x 2 shown in Fig. 2, between which said points, and even outside them, the measure- ment range X 2 extends. The measurement circuit includes an electronic change-oVer switch 13, its control circuit which is controlled by a clock 14. Being controlled by the control signals a,r,r 2 from the control circuit the change-over switch 13 connects the capacitance C M to be measured and the reference capacitances CR1 and CR2 alternatingly to the measurement electronics 16. a In a way in itself known, the measurement electronics 16 includes, a RC oscillator, the .on capacitance to be measured, which is as a rule within °a the range of 0 to 100 pF, and the reference capacitances o 15 being alternatigly connected to the input circuit that determines the frequency of the said oscillator. The measurement electronics 16 may also include distri- butors and other known switching arrangements, so that oa.o from the measuremett electronics, an output variable. S 20 is obtained, e.g. a DC voltage or a frequency which varies substantially linearly on the basis of the ao 0 electrical value of the capacitance C M It is assumed that the output variable of the measurement electronics .16 is a voltage U 1 This volt- n 25 age U 1 is passed to the first comparison circuit 19 o and to the second comparison circuit 20. According to the present invention, in the measurement circuit and method, two external auxiliary reference circuits 17 and 18 are used. From these reference circuits 17,18, DC voltages URi and UR 2 are obtained, which are passed each of them to its own comparison circuit 19 or 20. The differential voltages U 1 and U 2 of the said external auxiliary reference voltages UR1 and UR 2 and of the output voltage U1 of the measurement electronics are input voltages of the comparison cir- cuits 19 and 20, The comparison circuits 19 and are controlled by means of the control pulse sequences

The invention is preferably accomplished so that the first control signal Ucl acts, in the way of a constant term, upon the measurement electronics 16, i.e. upon the so-called off-set of the measurement electronics. The second control signal Uc2 again acts upon the steepness of the measurement electronics, e.g. Co upon its amplification. The control signals Ucl and Uc2 affect the ci c2 1 15 measurement electronics in such a direction that the said differential voltages U 1 and U 2 are reduced step- wise, and this feedback effect is repeated, e.g. as controlled by the control circuit 15 of the change-over switch, for the duration of so many measurement cycles I 20 that the said differential voltages U 11 and U 22 can be ,o made to approach zero stepwise. After the said differ- Sential voltages U and U 2 have been brought sufficiently close to the zero point, the measurement electronics 16 has been adjusted "correctly". Thereby, controlled by the control circuit 15 of the change-over switch 13, the change-over switch 13 connects the capacitance 12 to.be measured to the measurement electronics. At the same time, by means of the control circuit 15 of the change-over switch 13, a holding element 23 or another, corresponding component is con- trolled so that the output signal U 1 of the measu 'ment electronics 16 is connected,as such or as appropriately scaled, so as to make the outlet signal Uout. Fig. 4 shows a wiring diagram of an embodiment of a practical wiring system, which constitutes the measurement electronics for little capacitive detectors (0 to 100 pF). The measurement frequency is about 100 kHz, which frequency has not been processes as such, but it has been divided by means of circuits 25 and 26 to a sufficient extent to a lower frequency in order that the delays in gates etc. .and the changes in them should not affect the measurement result. In respect of the construction and operation of the multicap circuit 27, which is an essential part in the wiring system and which is a patented special circuit expressly for the measurement by capacitive detectors, reference S 10 is made to the said FI Patents 57,664 and 57,319 (cor- responding US Patents 4,295,090 and 4,295,091). -The 0Vvariable that is being examined is time. The auxiliary "reference consists of a time that is obtained from the crystal oscillator 24 and further from the pin 3 of the Co 15 distributor microcircuit 25 (4024). The zeroing takes place a little later (pins 3 and 6 are ones). A second auxiliary reference is not needed, because a different distribution number has been taken from the other distributor microcircuit 26 (4040) by means of the S 20 references CR and CR. R1 R2 The comparison of the time differences is Scarried out by the gates 31 and 32. The gates 33 and 34 take care that the correction currents may act upon the voltages of the capacitors C1 of 47 nF only when Seo 25 the references CR1 and CR2 are being measured. The a. output signal U is, in the wiring of Fig. 4, a frequency burst, whose frequency contains the infor- mation on the electrical magnitude of the detector capacitance CM to be measured. The output may also be connected in the same way as has been done in respect of the refbrences CR1 and CR2' whereby a pulse is obtained whose width (duration of half cycle) contains the information on the electrical value of the detec- tor capacitance. From the wiring in Fig. 4, it is possible to calculate tfe number of pulses in the frequency burst or, with little modifications, a burst is obtained from which the number of pulses corresponding to the value of one of the references C 1;CR 2 has been deducted. The latter alternative embodiments do not require a crystal oscillator, because the pulse numbers are abstract numbers. It the following, the'patent claims will be given, whereby the various details of the invention may show variation within the scope of the inventive idea defined in the 'said claims and differ from what has been stated above.

"45/7

THE CLAIMS DEFINING THlE INVENT~ION WEA LQW:

a: tances, inl parti21 la 0 said method sasurenet eietrY include a meas=U2\2 no asall W- -rv

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electronics (10) adjuated ccir~vinl nf iq the method steps defined above.

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Claims (1)

1. The followingstaternent is a full description of this invention, i1rcuding the best method of performing it known to me/,1,-

   change-over switch which connect the reference 1 r'aaa I C ),aro ulied Ooe fr fr- control- ,,tt io 1 he cthei onIr for 2 ~ot I~no the other com irate,-r Moth od IAlmdzUin any 1AI0he claims 4. ,1 -1r a cter iz e d an that as the x .frcnincy, -a basic irequenx.-y oAthe order o: ki:' 1,isofd, which said basic frequency F,3 wr&.a'I: a ;of d ir ibu't~trs (2,5,26) to such a lo,-w I evs<.*,hat th iel1avs, zAqatos ec;. anld changes in t-hom -l-not hcivt. :an 'n ~~qeffoct on the measure-

   s al: -irmea ,ha in any of the claims to 5,c ra z eYi zc d in that the output clt~ C voltage. D.te MthA isn me in any c-f the claims r a t r i z o d i that the output s i,7:n a t'.e circiiit appl.ying t1hc- method is a fre- quienci-yIbirst whose frequency contains the information cn 't he m a tut',, of the capacitance (C of the b, he rmasured. 0)on Mcth d as claimed in claimp 7, c h a r and a acit r 3 T e d3 in tbat the method the number of mthe frequency burst is compui-ted, the result ofte said! coriputation being used as the measure of the canacitance (CM) to be measured. 9. Method as claimed in claim 8, c h a r- ciua I a c 41- ri z e d in that the number of pulses corres- pending to the value of one of the referenoqes is ims 30 deducted from the number of palses in the said fre- -ed and quency burst, and the result of computation obtained in this way is utilized in the determination of the -ovur capaciance to be measured. nic s Use of any method as claimed in claim 1 10. tch 35 or oif a circuit operating as per said method in tele- och metry of radiosondes for the measurement of atmospheric oc k pressure, temperature and/or huimidity. tro 1. Dated this 21st day of May, 1985 VAISALA OY e By Its Patent Attorneys CLEMENT HACK CO. Fellows Institute of Patent Attorneys of Australia cnown to mejrP;S rice tro 0r

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