DE2346307C2 - Electrical circuit arrangement for the linear conversion of changes in capacitance depending on the measured variable - Google Patents

Description

mechanical quantities «by Christof Rohrbach, _{Die s} j _c h _on the capacities C ₁ and C, building up

VDE Verlag, Düsseldorf 1967, in particular from Ka- tensions
pitel G 10 measuring cells are known in which z. B. ₅₅
the plate spacing of plate capacitors
a measured variable (force, pressure, etc.) is changed.
However, these measuring cells have extremely high internal ₌ 2_. Γ / d / = + (1 - X) - ^ - ■ [

Furthermore there are electronic circuits for βα * C, 7 * C ₀ J

analog measurement of capacitive detuning specified. However, these have the disadvantage that they are very sensitive as well as throw themselves by means of a summing comparison against disturbances and changes in the amplifier SV . To be based on discharged line capacities. State of capacities C ₁ and C _{j (} takes the

Furthermore, these are circuits for conversion 65 total voltage

the capacitive detuning in frequencies known. , ..

From the brochure blue about the tType 4-460 Digital _U} . - _UCi + _Uc% + _Us - _ 2 X - - I u _s dt - \ - us Pressure Transducer "from Bell & Howell is C J

1 C- .. n + jn ^ - fu dt

^{C |} C ₁ 'J C ₀ "J

however, have extremely high internal resistances. ₌ 2_. Γ / d / = + (1 - X) - ^ - ■ [usdt

* J

(assuming χ > Q) until zero is reached. In other words, the current source for the capacitor Bm, the following comparator K switches in this satorC "from a differential amplifier OK" with a moment by means of a switch S the voltage ""output> and feedback resistors R *, H _n t, from the positive reference voltage + U ₀ on the R &, Ryt and a current resistance Η% as well as negative reference voltage - U ₀ of preferably 5 a buffer amplifier PV _% . The output voltages roughly the same amount. The polarity reversed in this way from PV ₁ and PV _% , namely Mc ₁ and "c", as well as Hs itself currents "\ and i _t , now discharge C _x and C, and are charged via summing resistors As ₁ , Ä _{S | t} and As ₄ them in the opposite direction, until again in a summing amplifier SV switched back to + U ₀ with a feedback resistor * Rs: rum »ε = Q is reached and irs sums. Whose is switched. These voltage curves are shown in Fig. Ib * o output voltage - Mr is sketched in a comparator K. A complete cycle of the resulting supplied, which via a switch S the switching oscillation is concerned after a time of Ms.

The Integrator IV is another special feature.

C ₀ This conventionally with a capacitor Ct and

"Tx k ^{l5 e 'nem} resistance Rt wired integrator generates

an integration voltage that corresponds _{to KC 1}

κ "t, u". _n "y,!. ,, · cu · t is. It is also added to Mr _{via As 8.} To this

closed, so that the oscillation frequency _wdse ^ _reachi ³ _{that schou at} χ = ₀ a oscillation

,, generation is generated with a finite frequency (»life zero · -

/ = - = X. - _- so Effela), so that X can also become negative.

T 2 C ₀ The proposed arrangement brings

the following advantages. First of all it is

is proportional to the measurand. Relationship between measured variable and frequency

F i g. 2 shows an embodiment. The current Z _{1 is} linear. Parasitic parallel capacitances generate errors is generated by a differential amplifier DV ₁ . 25 before. only higher order. Offset voltages of all this is wired with resistors R _ia , R _lb , R ₁₀ and R _ld so the amplifier and the comparator rise in that the voltage drop of the output corresponds to the opposite half-cycles of the dynamic current at a resistor A _{1 of} the input span oscillation itself on. After all, have also Vernung is proportional. A buffer amplifier PV ₁ changes the reference voltages + U ₀ and - U ₀ enables the capacitor voltage to be decreased, and thus also changes to the operating voltage without loading the capacitor C ₁ . Either of the converter has no influence on the oscillation frequency.

1 sheet of drawings

Claims (7)

I 2 also known, jfopt; pF-QszH (atQren, with the same claims: basic frequency? $ Jren $ nd, which is determined by two partial capacities of the Meö | ufiiehmers, which are to be 1. Electrical attitude adjustment to the linear correct measured variable are changed in opposite directions, Conversion of capacities dependent on measured variables - 5 are detuned in the opposite direction. the Changes in frequency changes, containing the difference frequency, are further processed as a measurement signal oppositely controlled capacities and one works. However, this results in a non-linear order frequency generator, this results in a characteristic curve, and in addition, <; in more exact shows that the generator as a triangular oscillator requires temperature synchronization of the two oscillators is formed, whose integrating members are different. the two capacities (C ₁ ,. G ₂ ), g ^e * Λ The object of the present invention is therefore to create one! ¥ fe * ausjeTeiner current source <against- ^ circuit arrangement of the type mentioned at the beginning too sensible to create with a constant current (/ \, I ₂ ) alternating, which brings about a linear transformation and throws in with polarity, and for polarity only one oscillator circuit gets by,
15 According to the invention, this task raises the voltage of the capacitances switching grain by the fact that the generator is available as a triangular oscillator separator (K) . forms whose integrators ai> s the two 2. Circuit arrangement according to claim 1, there measuring transducer capacitances are formed from each characterized in that the transducer of a current source is opposite to a constant Double plate capacitor is formed. «Currents of alternating polarity flow in, and 3. SchaltungsanordnungnachAnspruchloder2, for polarity change at certain sums, characterized in that a summing amplifier (SV) is provided upstream of the comparator for summing the charging voltages of the capacitances switching charging voltages. tying is. On the basis of the drawing, an implementation example 4. Circuit arrangement described in more detail according to one of the examples of the invention. In it shows
Claims 1 to 3, characterized in that the F i g. 1 a PnnzipschaltbiJd with associated span current sources can be controlled. voltage diagrams and 5. Circuit arrangement according to one of the preceding figs. 2 a detailed circuit example,
Claims 1 to 4, characterized in that C ₁ and C ₂ are the capacities of a measurement intermediate between the capacities (C ₁ , C ₂ ) and the receiver, the summing amplifier \ SV) t buffer amplifier (PV ₁ , A measured variable X change the plate PV ₂ ) are connected. the distance between the two capacitors is opposite 6. Circuit arrangement me one of the on and linear so that
Proverbs 1 to 5, characterized in that as Current sources differential amplifier (DV ₁ , DV ₃ ) before- 35 J =. (χ + χ) ^ are in hand. C ₁ C ₀ 7. Circuit arrangement according to one of claims 1 to 6, characterized in that a 1 _ i (\ - Y \
voltage generated in the integrator (IV) (Inte- qq
voltage) to the total voltage of the cable 40 capacities is added. where C _{0 is} the quiescent capacitance of both capacitors. Controlled current sources, together with the measuring capacitors C ₁ and C _{1, form} integrators. The current sources are designed in such a way that their 45 currents / | and i _{2 are} of the same size but of opposite sign if both current sources The invention relates to an electrical system to be controlled by the same control voltage «5: circuit arrangement for | linear conversion of measured variable dependent capacitance changes in frequency- / = - k · us]
changes, containing two oppositely controlled _5o 'I £ _ _CO nst.
Capacities and a frequency generator. , · _ _J_ β. _Us J From the »Handbook for Electrical Measurement