JP3025837B2 - Capacitance measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitance measuring device used for a capacitance type liquid level meter and the like, and more particularly to a technique for correcting a measurement error caused by a measuring cable.

[0002]

2. Description of the Related Art A conventional capacitance measuring apparatus 1 of this type will be described with reference to FIG. For example, 1 from the measurement signal generator 2
KHz, 3. 6V approximately sinusoidal voltage V ₁ is the output terminal A, A '
It is supplied to the near end of the first cable 3. One end of the capacitance to be measured 4 is connected to the core wire at the far end of the first cable 3,
A core wire at the far end of the second cable 5 is connected to the other end.
The near end of the second cable 5 is connected to the measurement terminals B and B '. The first and second cables 3 and 5 have substantially the same length, for example, about several hundred meters, and are cables of the same standard. The measured capacitance 4 is a capacitance that changes according to the liquid level of the liquid level sensor, for example.

The measuring terminal B is a resistor (the resistance value is R).
6 is connected to an inverting input terminal of an operational amplifier 7.
A parallel feedback capacitor (capacitance value is assumed to be _Cf ) 8 is connected between the inverting input terminal and the output terminal of the operational amplifier 7. The AC output of the operational amplifier 7 is converted into a DC voltage by a rectifying / smoothing circuit 9, further converted into digital data by an A / D converter, and input to a capacitance calculator 11, and the capacitance of the capacitance 4 to be measured is measured. The value Cx is calculated.

A resistor / operational amplifier 7 and a capacitor 8 constitute a capacitance / voltage conversion circuit 12, and a rectifying / smoothing circuit 9, A / D conversion means 10 and capacitance value calculation means 11.
Constitutes the capacity calculation unit 13. Measurement signal generator 2
Is the output voltage V ₁ , the voltage at the measurement terminal B is V ₂ , and the output voltage of the operational amplifier 7 is V ₃ . From the nature of the operational amplifier,
As is well known, V ₃ / V ₂ = −Zc / R (1) Zc = 1 / jωC _f (2) ∴V ₃ / V ₂ = −1 / jωC _f R... (3) holds. Where ω is the angular frequency of the measurement signal,
Zc is the impedance of the parallel feedback capacitor.

The gain of the operational amplifier 7 itself is extremely large,
Since the input voltage is very small and almost zero, the inverting input terminal is a virtual ground point, and the potential at the point P is zero (common potential).
Therefore, the current I flowing to the point P through the capacitor 4 and the resistor 6 due to the voltage V ₁ is as follows: I = V ₁ / {R + 1 / jωC _f } (4) Here, since the resistor 6 is set so as to satisfy R << 1 / ωCx (5), I ≒ jωCxV ₁ (6) ΔV ₂ = RI = jωCxV ₁ R ......... (7) (7) equation (3) is substituted into _{equation, V 3 / jωCxV 1 R =} -1 / jωC f R V 3 = -CxV 1 / C f ............ (8) to be measured The capacitance Cx is converted to the voltage V by the capacitance / voltage conversion circuit 12.
It can be considered that it has been converted to ₃ . From equation (8), C
x is obtained as follows: Cx = −C _f V ₃ / V ₁ (9)

In the above, the expression (9) is obtained by using the expression (1), that is, the relationship of the expression (3). The input impedance of the operational amplifier is extremely high, and the current flowing through the input terminal can be ignored.
The current I that has flowed into the point P flows through the capacitor 8. Accordingly, the output voltage V ₃ of the amplifier 7 is equal to the terminal voltage of the capacitor 8, and V ₃ = (− I) / jωC _f −I = −jωC _f V ₃ (10) (6), (10) _{jωCxV 1 = -jωC f V 3 V} 3 = -CxV 1 / C f ∴Cx = -C f V 3 / V 1 becomes (9) is the same formula as formula obtained from equation.

[0007]

Experiments and examinations on the measurement error of the conventional measuring device have revealed that it increases according to the cable length L as shown in FIG. 5, for example. The error ε in the figure is defined as: ε = (true value−measured value) / true value (11)

An object of the present invention is to provide a high-precision measuring device in which a measurement error caused by a measuring cable is corrected.

[0009]

[Means for Solving the Problems]

(1) According to the first aspect of the present invention, the capacitance to be measured connected between the core wires at the far ends of the first and second cables having substantially the same length is measured from the near ends of the cables. In the capacitance measuring device, a measurement signal generator, a resistor connected between an output end of the measurement signal generator and a core wire at a near end of the first cable, and a cascade connection to a near end of the second cable A capacitance / voltage conversion circuit for converting a capacitance value to be measured into a voltage, a capacitance calculation means for calculating an approximate value of the capacitance to be measured from an output of the capacitance / voltage conversion circuit, and a terminal of the resistor. Means for measuring the voltage between the terminals, means for calculating the lengths of the first and second cables from the measured voltage between the terminals, and correction capacitance based on the calculated cable length and the approximate capacitance value. Means for calculating a value, and a capacitance using the calculated correction capacitance value. And correcting and calculating the approximate value, and correcting means are provided to obtain measurements.

(2) In the invention according to claim 2, in the above (1), the cable length calculating means has a memory storing a correspondence table between the lengths of the first and second cables and the voltage between terminals of the resistor. Things. (3) In the invention of claim 3, in (1), the correction amount calculating means has a memory storing a correspondence table between the correction amount and the cable length.

(4) The invention according to claim 4 is based on (1), wherein the capacitance / voltage conversion circuit is an operational amplifier, an input resistor connected to a negative-phase input terminal of the operational amplifier, and A capacitor connected between the negative-phase input terminal and the output terminal. (5) The invention according to claim 5, wherein in the above (4), the capacity calculation means rectifies and smoothes the output of the capacity / voltage conversion means, means for A / D-converting the output of the circuit, and And capacitance value calculating means for calculating an approximate value of the capacitance to be measured from the converted digital data.

(6) In the invention according to claim 6, in the above (4) and (5), the capacitance value calculating means is configured to output the output data of the A / D converting means and the voltage (V ₁ ) at the output terminal of the resistor. Information and
The approximate value of the measured capacitance is calculated from the capacitance of the capacitor of the capacitance / voltage conversion circuit.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the embodiment shown in FIG. In FIG. 1, parts corresponding to those in FIG. 4 are denoted by the same reference numerals, and redundant description will be omitted. As a result of various experiments and studies, the distributed capacity between the core wire of the cable and the outer conductor is, for example, 44000 in the case of a coaxial cable.
pF / Km and the order of the measured capacitance value Cx (2000
pF about below) very large compared to, since most of the current I ₀ flowing from the terminal A to the core wire through the outer conductor (ground) through a distributed capacitance from the core, the measured capacitance value Cx
Regardless of this, it was found that the value increased according to the length of the first cable 3 and was determined by the length.

According to the present invention, a resistor 21 is connected in series to the output side of the measurement signal generator 2 in order to measure a voltage Vr proportional to the current I ₀ sent to the core wire at the near end of the first cable 3. You. The voltage V ₁ at the output terminal of the resistor 21 is converted into digital data by the A / D converter 26 and supplied to the capacitance value calculating means 11. The capacitance value calculating means 11 calculates the approximate value Cx 'of the measured capacitance 4 in the same manner as in the prior art.

The voltage Vr between the terminals of the resistor 21 is measured by a difference voltage measuring circuit 22 and inputted to a cable length calculating means 23. In the cable length calculating means 23, as shown in FIG. 2, the cable length L vs. the voltage Vr characteristic L
= F (Vr) to estimate the cable length L. The table of L vs. Vr is stored in a built-in memory. In the correction amount calculating means 24, the capacity calculating means 13 similar to the conventional one (FIG. 4) is used.
From the approximate value Cx 'of the capacitance to be measured 4 obtained in step (c) and the cable length L estimated by the cable length calculating means 23, as shown in FIG. Values, eg, 2000, 1500, 1000, 500, 1
Correction amount ΔC with 00pF as a parameter vs. cable length L
It is determined from the characteristic ΔC = Gi (L). When the approximate value Cx 'is, for example, between 2000 pF and 1500 pF, an interpolation operation is performed to obtain a correction amount ΔC. The table of the correction amount ΔC versus the cable length L is stored in the memory in the correction amount calculation means 24.

The correction means 25 adds the correction amount ΔC to the approximate value Cx ′, and the corrected capacitance value Cx = Cx ′ + ΔC
Is obtained as final measurement data. In the above description, the data of the voltage V ₁ at the output terminal of the resistor 21 is
It is assumed to be supplied to the capacitance value calculating means 11 through, in the initial setting, so as to set the output voltages V ₁ to a predetermined value, the V ₁ and known data, by omitting an A / D converter 26 Is also good.

[0017]

As described above, according to the present invention,
The cable length L is estimated from the voltage Vr corresponding to the transmission current I ₀ , and the correction amount ΔC is obtained and corrected from the cable length L and the approximate value Cx ′ of the measured capacitance measured in the same manner as in the related art. As a result, a high-precision measuring device with almost no error caused by the measuring cable can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a graph showing a relationship between a length L of a measurement cable 3 (4) in FIG. 1 and a voltage Vr between terminals of a resistor 21;

FIG. 3 is a graph showing a relationship between a length L of a measurement cable 3 (4) in FIG. 1 and a correction amount ΔC.

FIG. 4 is a block diagram of a conventional capacitance measuring device.

5 is a graph showing the relationship between the measurement error of the apparatus shown in FIG. 4 and the cable length L.

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-3-44568 (JP, A) JP-A-2-17459 (JP, A) JP-A-7-198765 (JP, A) JP-A-7-1987 20172 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01R 27/26 G01D 5/24 G01F 23/26

Claims (6)

(57) [Claims] 1. A capacitance measuring device for measuring a capacitance to be measured connected between core wires at far ends of first and second cables having substantially the same length from near ends of the cables. A measurement signal generator, a resistor connected between an output end of the measurement signal generator and a core wire at the near end of the first cable, cascaded to the near end of the second cable, A capacitance / voltage conversion circuit for converting a capacitance value into a voltage, a capacitance calculation means for calculating an approximate value of a capacitance to be measured from an output of the capacitance / voltage conversion circuit, and a voltage between terminals of the resistor. A means for measuring, a means for calculating the length of the first and second cables from the measured voltage between terminals, a correction capacitance value based on the calculated cable length and the approximate capacitance value Using the calculated correction capacitance value And correcting and calculating the capacitance approximate values, the capacitance measuring apparatus characterized by comprising a correction means for obtaining a measurement value. 2. The static electricity storage device according to claim 1, wherein said cable length calculation means has a memory storing a correspondence table between the lengths of said first and second cables and a voltage between terminals of said resistor. Capacitance measuring device. 3. The capacitance measuring apparatus according to claim 1, wherein the correction amount calculating means has a memory storing a correspondence table between the correction amount and the cable length. 4. The operational amplifier according to claim 1, wherein the capacitance / voltage conversion circuit includes an operational amplifier, an input resistor connected to a negative-phase input terminal of the operational amplifier, and a negative-phase input terminal and an output terminal of the operational amplifier. And a capacitor connected between the first and second capacitors. 5. The circuit according to claim 4, wherein said capacitance calculation means includes a circuit for rectifying and smoothing an output of said capacitance / voltage conversion means, a means for A / D converting an output of said circuit, and a digital signal obtained by the conversion. A capacitance measuring device comprising: capacitance value calculating means for calculating an approximate value of a capacitance to be measured from data. 6. The capacitance / voltage conversion device according to claim 4, wherein said capacitance value calculation means includes output data of said A / D conversion means, voltage (V ₁ ) information at an output terminal of said resistor, and said capacitance / voltage conversion means. An electrostatic capacitance measuring device for calculating an approximate value of an electrostatic capacitance to be measured from a capacitance of a capacitor of a circuit.