JPH07113566B2 - Level detector - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a level detecting device for detecting a continuous level by changing a capacitance between a pair of electrodes.

[0002] There are a pair of electrodes between the electrostatic capacitance change detected and the level detection device as, for example, Japanese official Akira 62 over 18004 JP detecting the level device with a level change of the liquid. This device discloses that the capacitance change is detected as a change in the frequency of the CR oscillation circuit.

[0003]

However, when detecting the capacitance change between the electrode pairs due to the level change described above as the change of the oscillation frequency, ω = (LC) ^−1/2 , and the frequency and static Since the capacitance C does not have a linear relationship,
For example, the f / V converted signal voltage does not have a linear relationship with the liquid level (which has a linear relationship with the capacitance between the electrodes), and the subsequent signal processing becomes complicated.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a level detection device which is excellent in linearity between a fluid level change and an output signal voltage change and which has a simple structure. Its purpose is.

The level detecting device of the first invention is composed of a pair of elongated electrodes which hang down at a predetermined position in a container for containing a fluid to be measured, with one electrode being grounded. includes a level detecting capacitor, an oscillation circuit for applying an AC voltage of Jo Tokoro frequency through the impedance element for signal voltage detected at the other of the electrode pair, and an amplifying circuit for amplifying a potential difference across said impedance element, wherein
The impedance of the impedance element is Z, the level
The capacitance of the detection capacitor is C, and the angular frequency of the AC voltage is
When the number is ω, ωCZ is set to 0.1 or less .

[0006] In favorable optimal embodiment, the impedance element comprises a capacitor, the signal voltage at the connection point between the level sensing capacitor and the impedance element, to an input of the amplifier circuit through a DC blocking capacitor for blocking direct current it is intended to be input.

The level detecting device of the second invention is a fluid to be measured.
It hangs at a predetermined position in the container that houses
Consists of a pair of long electrodes, one of which is grounded
Level detection capacitor and the other of the electrode pair.
Predetermined frequency through impedance element for signal voltage detection
Oscillator circuit for applying the AC voltage of the
An amplifier circuit for amplifying a potential difference between both ends of the element, the level detection capacitor has a coaxial cylinder shape, an outer electrode formed of an outer cylinder is grounded, and an inner electrode housed coaxially in the outer cylinder is It is connected to one end of the impedance element.

[0008]

The AC voltage output from the oscillation circuit is applied to the series circuit of the impedance element and the level detection capacitor, the other end of the level detection capacitor is grounded, and the potential difference between both ends of the impedance element is differentiated by the differential amplifier circuit. Is amplified. Here, when the impedance of the impedance element is Z, the electrostatic capacity of the level detection capacitor is C, and the angular frequency is ω, by making ωCZ as small as possible, the linearity of the signal voltage output from the amplifier circuit can be reduced. improves.

[0009]

As described above, the level detecting device of the present invention applies the AC voltage output from the oscillation circuit to the series circuit including the impedance element and the level detecting capacitor, and grounds the other end of the level detecting capacitor. However, since the potential difference between both ends of the impedance element is differentially amplified, it is possible to output a signal voltage having a high linearity with respect to a fluid level change with a simple circuit. Further, since one electrode of the level detecting capacitor can be grounded, superposition of high frequency noise and fluctuation of DC level are small.

[0010]

FIG. 1 shows an embodiment of the level detecting apparatus of the present invention. This device comprises an oscillation circuit 1, a level detection capacitor circuit 2, a differential amplifier circuit 3, a detection circuit 4, and a smoothing circuit 5. The oscillation circuit 1 is a circuit that outputs a sine wave AC voltage having a constant frequency, but may be an oscillation circuit having another waveform.

The capacitor circuit 2 has an impedance element 7 having one end connected to the output terminal of the oscillation circuit 1, a level detection capacitor 6 having one end connected to the other end of the impedance element 7 and the other end grounded, and a DC cutoff. And a capacitor 22. The DC blocking capacitor 22 connects the connection point between the impedance element 7 and the level detection capacitor 6 and the negative input end of the differential amplifier circuit 3. Here, as the impedance element 7, a capacitor having a capacitance of 1/20 or less than that of the level detection capacitor 6 is adopted.

The amplifier circuit 3 is a linear amplifier circuit having a predetermined input impedance and is composed of an operational amplifier. Since the configurations of the detection circuit 4 and the smoothing circuit 5 are well known, description thereof will be omitted. Other parts except the level detection capacitor 6 are housed in a sealed case (not shown). The capacitor circuit 2 will be further described.

As shown in FIG. 2, the level detecting capacitor 6 is an external electrode made of a coaxial cylinder which is hung at a predetermined position in a container (not shown) which contains a dielectric liquid to be measured (not shown). 62 and an internal electrode 61, and a lid 63 made of resin is fitted on the upper part of both. This lid 63 has a central cylindrical portion and an outer tubular portion that hang down coaxially from the upper lid portion,
The central cylindrical portion is fitted in the inner electrode 61, and the outer cylindrical portion is fitted in the outer electrode 62. A resin spacer 64 is fitted under the gap between the internal electrode 61 and the external electrode 62 to keep the gap constant.

Claws 61a and 62a extend upward at the upper ends of the inner electrode 61 and the outer electrode 62, respectively.
a and 62a are individually soldered to both ends of the pair of signal cables. Each of the signal cables 64 is composed of a wire mesh shield wire and extends to the above-mentioned sealed case. A cylindrical resin or metal cap 65 with a bottom is fitted and fixed to the lower end of the external electrode 62, and a small hole 66 for fuel inflow and outflow is formed in a side wall of the cap 65. This small hole 66 acts as a mechanical low pass filter.

The operation of this device will be described below. The AC voltage of the oscillator circuit 1 is divided by the impedance element 7 and the level detection capacitor 6. It is assumed that the impedance of the DC blocking capacitor 22 or the input impedance of the negative input terminal of the differential amplifier circuit 3 viewed from the DC blocking capacitor 22 is large, and the signal current flowing through the DC blocking capacitor 22 can be ignored.

When the level change of the dielectric liquid occurs, the capacity change ΔC of the level detecting capacitor 6 including the electrodes 61 and 62 is substantially proportional to the level change amount ΔL. The signal current i flowing through the impedance element 7 is C, the impedance of the impedance element 7 is Z, the AC voltage output by the oscillation circuit 1 is Vo, and the output impedance of the oscillation circuit 1 is small. If it can be ignored, i = Vo / (Z + (1 / jωC)).

Therefore, the potential difference ΔV across the impedance element 7 is ΔV = ZVo / (Z + (1 / jωC)) = jωCZVo / (1 + jωCZ) Now, for example, O.V. If it is 1 or less, it can be considered that ΔV = jωCZVo, the signal voltage Vc is proportional to the capacitance C of the level detection capacitor 6, and the change ΔC of C is proportional to the level change ΔL. Can be regarded as proportional to the level change ΔL.

That is, ω, C and Z may be reduced. The signal voltage Vc at the connection point is amplified by the differential amplifier circuit 3, detected by the detection circuit 4, converted into DC by the smoothing circuit 5, and output. The DC cutoff capacitor 22 described above prevents a DC current from being applied to the level detection capacitor 6, and in this case, the impedance element 7 is used.
Is also a capacitor. By doing so, it is possible to prevent an accident such as leakage of direct current between the electrodes of the level detection capacitor 6.

In this embodiment, the outer electrode 62 of the coaxial cylindrical level detecting capacitor 6 is grounded, and the inner electrode 61 is connected to the input end of the differential amplifier circuit 3 through the DC blocking capacitor 22. In this way, the external electrode 62
The parasitic capacitance between the tank and the tank (not shown) can be ignored, and the parasitic capacitance between the connection point and the tank is significantly reduced. Even if it changes, there is an advantage that the signal voltage Vc does not easily change.

Another modification of this embodiment will be described below. The DC blocking capacitor 22 can be omitted. The impedance element 7 can be replaced with a resistor or a reactance in addition to the capacitor. Just impedance element 7
If the DC cutoff capacitor 22 is used as a capacitor,
Since no DC current is transmitted to the level detection capacitor 6, it is excellent in safety.

Further, the internal electrode 61 of the level detecting capacitor 6 may be a cylinder instead of a cylinder. The fluid to be measured may be powder.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram showing an embodiment of the present invention.

FIG. 2 is a sectional view of a level detection capacitor,

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Oscillation circuit, 2 ... Capacitor circuit, 3 ... Amplification circuit, 4 ... Detection circuit, 5 ... Smoothing circuit, 6 ... Level detection capacitor, 7 ... Impedance element, 22 ... DC blocking capacitor,

Claims (3)

[Claims] 1. A level detection capacitor, which is composed of a pair of long electrodes that hang down at a predetermined position in a container containing a fluid to be measured at a predetermined interval, and one electrode of which is grounded, and the electrode. comprising an oscillation circuit for applying the other to signal voltage AC voltage Jo Tokoro frequency through the impedance element for the detection of pairs, and amplification circuit for amplifying a potential difference across said impedance element, the impedance of said impedance element Z, wherein Les
The capacitance of the bell detection capacitor is C, and the angle of the AC voltage is
When the frequency is ω, ωCZ is set to 0.1 or less
A level detection device characterized by the above. 2. The impedance element comprises a capacitor, and a signal voltage at a connection point between the impedance element and the level detection capacitor is input to one input terminal of the amplifier circuit through a DC blocking capacitor for DC blocking. The level detection device according to claim 1, wherein 3. At a predetermined position in a container containing a fluid to be measured.
Consists of a pair of long electrodes that hang down at a predetermined interval
Detection capacitor with one electrode grounded
And an impedance element for signal voltage detection on the other side of the electrode pair.
Oscillation circuit that applies an AC voltage of a specified frequency through a child
And an amplifier for amplifying the potential difference across the impedance element
A circuit, the level detection capacitor has a coaxial cylindrical shape, an outer electrode composed of an outer cylinder is grounded, and an inner electrode coaxially housed in the outer cylinder is connected to one end of the impedance element. A level detection device characterized by being a thing .