SE424776B

SE424776B - ELECTRONIC CAPACITY METHOD

Info

Publication number: SE424776B
Application number: SE7801208A
Authority: SE
Inventors: K W Ripka
Original assignee: Elin Union Ag
Priority date: 1977-02-03
Filing date: 1978-02-01
Publication date: 1982-08-09
Also published as: ATA67577A; NL7801303A; AT359599B; SE7801208L; DE2804696A1

Description

7801208-s I 2 och kapacitansändringen arbeta med mycket små växelspänningar över de kondensatorer, som skall mätas. Den exakta vidarebehandlingen' och mätningen av dessa små spänningar är i princip problematisk. Ändamålet med uppfinningen år att åstadkomma en krets för mät- ning av små kapacitanser av storleksordningen pikofarad och särskilt en krets för jämförande mätning av två variabla kapacitanser, vilka ändrar sig i beroende av fysikaliska storheter, såsom exempelvis temperatur, tryck, en fluids volym etc., varvid den ena kapacitan- sen förändras av den storhet, som skall mätas, medan den andra kapa- citansen användes för kompensering av oönskade effekter. 7801208-s I 2 and the capacitance change work with very small alternating voltages across the capacitors to be measured. The exact further processing and measurement of these small voltages is in principle problematic. The object of the invention is to provide a circuit for measuring small capacitances of the order of picofarads and in particular a circuit for comparative measurement of two variable capacitances, which change depending on physical quantities, such as for example temperature, pressure, volume of a fluid etc. , whereby one capacitance is changed by the quantity to be measured, while the other capacitance is used to compensate for undesirable effects.

Uppfinningen avser således en elektronisk kapacitansmätanord- ning, särskilt för mätning av små kapacitanser av storleksordningen pikofarad, speciellt för jämförelse av två likartade kapacitanser, varvid kapacitansändringar omvandlas till proportionella ström- res- pektive spänningsändringar och varvid en av rektangelpulser triggad, monostabil multivibrator användes för pulsformning, varjämte respek- tive kondensator, vars kapacitans skall mätas, utgör del av en mono- stabil vippas belastningskrets, vilka vippors angivna pulser uppvisar en mot kapacitansen, som skall mätas, proportionell bredd, varvid en- ligt uppfinningen dessa vippor triggas_av en växelspänning av extremt konstant frekvens och efter integrering av den resulterande, puls- breddsmodulerade signalen av konstant frekvens matas denna utsignal till en indikator, varvid-två monovippor, vilkas utpulsers pulsbredd på i och för sig känt sätt i ena fallet bestämmes av en kondensator, vars kapacitans skall mätas, och i andra fallet av en kompensations- kondensator, triggas samtidigt, vilka båda monovippors utpulser för bildande av skillnaden mellan dessas pulsbredder matas till en koin- cidenskrets, som innefattar en inverterare och en OCH-krets, eventu- ellt med efterföljande filtrering; närmare bestämt matas pulserna från den monovippa, i vars belastningskrets kondensatorn, vars kapa- citans skall mätas, befinner sig, först till inverteraren och därefter imatas dess pulser och pulserna från den monovippa, i vars belastnings- krets kompensationskondensatorn befinner sig, till OCH-kretsen, var- efter utpulsen, eventuellt via ett motstånd, matas till indikatorn.The invention thus relates to an electronic capacitance measuring device, in particular for measuring small capacitances of the order of picofarad, especially for comparison of two similar capacitances, whereby capacitance changes are converted to proportional current and voltage changes and whereby one of rectangular pulses is pulsed for mono pulverizer, monost , and each capacitor, the capacitance of which is to be measured, forms part of a load circuit of a monostable flip-flop, which pulses indicated by flip-flops have a proportional width to the capacitance to be measured, wherein according to the invention these flip-flops are triggered by an alternating voltage of extreme constant frequency and after integration of the resulting pulse width modulated signal of constant frequency, this output signal is fed to an indicator, two mono-flip-flops, the pulse width of which pulses are in a manner known per se in one case determined by a capacitor, the capacitance of which is to be measured. , and in other cases of a compensation capacitor, are triggered at the same time, which pulses of both monovipers to form the difference between their pulse widths are fed to a coincidence circuit, which comprises an inverter and an AND circuit, possibly with subsequent filtering; more specifically, the pulses are fed from the mono rocker, in whose load circuit the capacitor, whose capacitance is to be measured, is located, first to the inverter and then its pulses and pulses are fed from the mono rocker, in whose load circuit the compensation capacitor is located, to the AND circuit , after which the output pulse, possibly via a resistor, is fed to the indicator.

Enligt uppfinningen kan koincidenskretsen bestå av en inver- Lvvnndv och vn ickw-inverterande Lrunuiuturförutärkurv med öppcnkol- lektorutgäng, vilka är parallellkopplade på utgångssídan.According to the invention, the coincidence circuit may consist of an inverting and non-inverting lruniuturförutär curve with open collector outputs, which are connected in parallel on the output side.

För noggrann kalibrering av skillnadspulsernas höjd kan en- ligt uppfinningen en zenerdiod förefinnas, vilken är parallellkopp- lad med utgångarna från den inverterande och den icke-inverterande transistorförstärkaren. I I QUÅL/ïy i 7ßo12os-S 3 Enligt uppfinningen kan vidare för arbetsomrâdesinställning respektive nollpunktsundertryckning en motståndskombination förefin- nas, vilken är förbunden med den icke-inverterande ingången hos en integrationsförstärkare.For accurate calibration of the height of the difference pulses, according to the invention a zener diode can be present, which is connected in parallel with the outputs of the inverting and the non-inverting transistor amplifier. I I QUÅL / ïy i 7ßo12os-S 3 According to the invention, furthermore, for working area setting and zero point suppression, a resistance combination can be present, which is connected to the non-inverting input of an integration amplifier.

Fördelen med kretsen enligt uppfinningen består i att de ovannämnda problemen elimineras. Dessutom ernås en hög noggrannhet på förhållandevis enkelt sätt genom den omfattande användningen av digitala kretsar. Eftersom integrerade kretsar redan finnes på marknaden, vilka i sig förenar ett flertal funktioner, är kostna- den vld realiseringen av den till grund för uppfinningen liggande idén förhållandevis ringa jämfört med de hittills kända metoderna med stor noggrannhet.The advantage of the circuit according to the invention is that the above-mentioned problems are eliminated. In addition, a high degree of accuracy is achieved in a relatively simple manner through the extensive use of digital circuits. Since integrated circuits already exist on the market, which in themselves combine a number of functions, the cost of realizing the idea underlying the invention is relatively small compared with the hitherto known methods with great accuracy.

Uppfinningen beskrives närmare nedan under hänvisning till bi- fogade ritning, på vilken fig. 1 schematiskt visar en principiell krets, fig. 2 visar en krets, för jämförelse av kapacitansvärdena hos två likartade kondensatorer, fig. 3 visar ett kopplingsschema över en i kretsarna enligt fig. 1 och 2 använd monostabil vippa, fig. 4 visar kopplingsschemat över den monostabila vippan för omvandling av kapacitansen, som skall mätas, till en pulsbreddsmodulerad rek- tangelväxelspänning och fig. 5 visar kopplingsschemat för koincidens- _ kretsen enligt fig. 2.The invention is described in more detail below with reference to the accompanying drawing, in which Fig. 1 schematically shows a principal circuit, Fig. 2 shows a circuit, for comparison of the capacitance values of two similar capacitors, Fig. 3 shows a circuit diagram of one of the circuits according to Figs. 1 and 2 show a monostable flip-flop, Fig. 4 shows the wiring diagram of the monostable flip-flop for converting the capacitance to be measured into a pulse-width modulated rectangular AC voltage, and Fig. 5 shows the wiring diagram for the coincidence circuit according to Fig. 2.

I fig. l befinner sig kondensatorn CX, som skall mätas, i den monostabila vippans MF belastningskrets. Denna vippa triggas medelst exempelvis en högfrekvens- eller lågfrekvensgenerator med konstant frekvens fo. Den monostabila vippans MF utpuls matas efter integre- ring över motståndet R och kondensatorn C till utgången A, till vil- ken exempelvis ett indikatorinstrument kan anslutas.In Fig. 1, the capacitor CX to be measured is in the MF load circuit of the monostable flip-flop. This flip-flop is triggered by, for example, a high-frequency or low-frequency generator with a constant frequency fo. The MF output pulse of the monostable flip-flop is fed after integration across the resistor R and the capacitor C to the output A, to which, for example, an indicator instrument can be connected.

I fig. 2 kopplas kapacitansen, som skall mätas, och en jämfö- relsekapaeitans i var sin monostabil vippas (monovippas) belastnings- krets. Denna krets finner användning särskilt för jämförande mät- ning av två likartade kapacitanser, varvid den ena förändras genom yttre påverkan, såsom exempelvis tryck, temperatur, volymändring etc., medan den andra kapacitansen användes för kompensationsändamål. Ut- pulserna från de monostabila vipporna MF matas till en koinoidens- krets, som innefattar en inverterare I och en OCH-krets U, varvid utpulserna från den till kondensatorn CX, som skall mätas, mörande monostabila vippan matas till inverteraren I, varefter dessa utpul- ser tillsammans med pulserna från den till kompensationskondensatorn Ccomp hörande, monostabila vippan matas parallellt till OCH-kretsen U. Via lntegratorn (motståndet R och kondensatorn C) är OCH-kret- sens utgång ansluten till indikatorn (utgàngsklämman A).In Fig. 2, the capacitance to be measured is coupled, and a comparative capacitance in each of the monostable load (monovip) load circuits. This circuit finds use especially for comparative measurement of two similar capacitances, one of which changes through external influences, such as, for example, pressure, temperature, volume change, etc., while the other capacitance is used for compensation purposes. The pulses from the monostable flip-flops MF are fed to a coinidens circuit comprising an inverter I and an AND circuit U, the pulses from the flammable monostable flip-flop to the capacitor CX to be measured being fed to the inverter I, after which these pulses - looks together with the pulses from the monostable flip-flop belonging to the compensation capacitor Ccomp is fed parallel to the AND circuit U. Via the integrator (resistor R and the capacitor C) the output of the AND circuit is connected to the indicator (output terminal A).

Claims

7801208-5 4 Fig. 3 shows a wiring diagram for a monostable rocker MF. All components shown with the exception of the components CO, Ru and Rs wiring diagram were also used to generate a constant frequency fo. i are available for purchase in the trade in the form of integrated circuits. The same _ The output signal consists of a rectangular alternating voltage fo with a relatively low pulse ratio. Fig. Ä shows the circuit diagram of the monostable flip-flop for converting the capacitance to be measured into a pulse-width modulated rectangular AC voltage. The triggering of the monostable flip-flop takes place via the capacitor C1 and the voltage divider R6 and R7. P1 is used to fine-tune the time constant of the monostable flip-flop. Fig. 5 shows an embodiment of the coincidence circuit used in practice. The inputs E1 and E2 correspond to the outputs A1 and A2 in Fig. 2, respectively. The circuit consists of three transistors T2, TB and TB, T3 providing the inversion, while the parallel-connected transistors T2 and Tu provide the AND function. R8 and R9 were used for current limitation, while R constitutes the working resistance of the transistor Tar The exact calibration of the height of the difference pulses is achieved by means of the zener diode Z1 together with the resistance Río. The subsequently connected amplifier V operates with the capacitor C as an integrator, whereby the gain can be set by means of the potentiometer P3 in combination with the resistors R11 and Hin. The potentiometer P2 is used for displacement of the working area and for zero point suppression, respectively. The Zener diode Z2 was used to protect the amplifier against external interference voltages. P a t e n t k r a v

Electronic capacitance measuring device, in particular for measuring small capacitances of the order of picofarad, in particular for comparing two similar capacitances, wherein capacitance changes are converted into proportional current and voltage changes and one of the rectangular pulses is triggered; monostable multivibrator was used for pulse shaping, and the respective capacitor, the capacitance of which is to be measured, forms part of a load circuit of a monostable flip-flop, which pulses emitted by flip-flops have a proportional width to the capacitance to be measured, characterized in that these flip-flops (MF) are triggered by an alternating voltage of extremely constant frequency (fo) 7 and after integration of the resulting pulse width modulating signal of constant frequency, this output signal is fed to an indicator (terminal A), whereby two mono flip-flops (MF), which are pulsed out pulse width 7801208-5 5 in a manner known per se is determined in one case by a capacitor (CX), the capacitance of which is to be measured, and in the other case by a compensation capacitor (Ccomp), triggered simultaneously, which two monovipotors (MF) pulses to form the difference between their pulse widths are fed to a coincidence circuit, which comprises an inverter (I) and an AND circuit (U) optionally with subsequent filtering (C), n more specifically, the pulses are fed from a mono rocker, in whose load circuit the capacitor (CX), whose capacitance is to be measured, is located, first to the inverter (I) and then its pulses and the pulses from the mono rocker, in whose load circuit the compensation capacitor (Ccomp) is located, to the AND circuit (U), after which the output pulse, possibly via a resistor (R), is supplied to the indicator.

Device according to claim 1, characterized in that the coincidence circuit consists of an inverting (TB) and a non-inverting transistor amplifier (T2, Tu) with open collector output, which are connected in parallel on the output side.

3. Device according to claim 2, characterized in that for accurate calibration of the height of the difference pulses there is a zener diode (Z1) which is connected in parallel with the outputs of the inverting and the non-inverting transistor amplifiers. H. Device according to claim 2 or 3, characterized in that for working area setting and zero point suppression there is a resistance combination (P2, B12, H3), which is connected to the non-inverting input of an integration amplifier (V). BEFORE PUBLICATIONS: