JP3420635B2 - Liquid 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 liquid detection device for detecting the presence or passage of a liquid in a pipe or tube, which is used in a control device for storing, transporting and supplying various liquids.

[0002]

2. Description of the Related Art As one of conventional liquid detecting devices, a "window" is provided in one part of the pipe, or the pipe is made of a transparent material so that the presence or absence of liquid in the pipe can be directly visually recognized from the outside. Alternatively, there is one in which a float is inserted inside a pipe to visually recognize the float floating on the liquid surface.

As a second example, there is a method in which an electrode is inserted into a pipe and liquid is brought into contact with the electrode to detect the presence or absence of conduction. Further, as a third example, a light emitting element and a light receiving element are arranged outside the pipe, and a light beam axis formed between the two elements is passed through the pipe to detect a difference in received light energy depending on the presence or absence of liquid. There is something I did.

[0004]

The above-described first example of the conventional liquid detecting device requires a dedicated manager or maintenance person for visual recognition, and is not suitable for automation of the device or system. In addition, in the second example, the liquid to be detected is limited to a substance having good conductivity, and is applied to a ignitable substance (energy / chemical agent) for conducting an electric current, though it is small. It is not possible, and because the electrode comes into contact with the object to be detected, the material of the electrode is limited, and there is concern about the influence on the physical properties of the object to be detected (food, chemicals, chemistry), so the applicable range is limited. Was done.

The third example is of two types, a light transmission system and a light reflection system. In the former case, the light beam is blocked by the liquid to reduce the received light energy, and in the latter case, the presence or absence of the liquid is detected by receiving the light energy due to the reflection and refraction phenomenon of the liquid. In this example, since the transparency, straightness, reflection, and refraction effect of light are used, the material of the pipe is transparent (material with good transparency) and the transmission depending on the type of liquid,
Since the reflection and refraction effects are different, it is required to strictly configure and set the light beam axis (optimum setting condition).

The present invention has been made in view of the above problems and has the following objects. It is to solve the drawbacks and inconveniences of the conventional method, and to realize a labor-saving and optimal liquid detection device for an automated system. While the conventional liquid detection device is a relatively large pipe (φ20 or more) and is difficult to apply to a small diameter pipe (φ10 or less) or a tube, in the present invention, a smaller diameter pipe or tube (φ3 to It is to provide a liquid detection device that can be applied to liquid detection of φ6) (washing machine, vending machine, medical device).

In the conventional method, the type of liquid to be detected is limited, and the setting conditions of the detector are limited.
The present invention is applicable to all liquids, and does not require severe adjustment in the setting of the detector,
It is to realize a liquid detection device that is easy to install and maintain. Another object of the present invention is to realize a performance of separating and extending the sensor head part and the amplifier part (controller) by 1 to 2 m or more in the liquid detection device.

[0008]

According to the liquid detecting device of the present invention, two concentric cylindrical conductors having different outer diameters are arranged in a pipe to be detected, and the inner cylindrical conductor is used as a detection probe. The outer cylindrical conductor is used as a shield probe, and the liquid in the pipe to be detected is detected by detecting the capacitance change of the detection probe.

[0009]

The present invention will be described in more detail with reference to the following examples. FIG. 1 shows an application example of the liquid detection apparatus of the embodiment, FIG. 2 is a block diagram of the apparatus, and FIGS. 3 and 4 show the structure of a sensor head. The apparatus of this embodiment includes a sensor head unit 10 attached to a pipe or tube 1 through which an object to be detected (liquid) passes, and an amplifier unit 2 including a signal processing circuit.
0, both of which are connected by a dedicated extension cord (coaxial cord) 30 between them.

The sensor head 10 includes a detection probe 11,
The detection probe 11 is connected to the central conductor of the coaxial cord, and the shield probe 12 is connected to the outer peripheral shield conductor of the coaxial cord. The amplifier unit 20 includes an oscillator 21 that drives the sensor head unit, a T / V converter 22 that processes an output signal from the oscillator, a comparator 23, and an output circuit 24.

The sensor head portion 10 is composed of two concentric cylindrical conductors 11 and 12 having different outer diameters. The inner cylindrical conductor having a small outer diameter is a detection probe 11, and the outer cylindrical conductor having a large outer diameter is a shield probe. Function as 12.
The two cylindrical conductors 11 and 12 are arranged concentrically, and the pipe 1 through which the liquid to be detected passes through inside the detection probe 11.
And the tube penetrates, and the outer diameter of each probe is set according to the pipe to which it is applied.

The cylindrical length L ₁ of the shield probe 12 is
It is set to be large with respect to the cylindrical length L ₂ of the detection probe 11,
To prevent potential coupling between the detection probe 11 and the ground,
The shielding effect is realized (L ₁ / L ₂ ≧ 1.5).
FIG. 5 shows the configuration of the oscillator 21 and its operation explanatory diagram. This circuit is composed of a wide band amplifier 211, a filter 212 and a buffer amplifier 213.
1 is a CR in which a feedback resistor R _f and a phase compensation capacitance C _p are arranged.
It is used as an oscillator circuit. The buffer amplifier 213 sets the gain to 1 and sets the outer circumference shield conductor 31 of the coaxial cord 30.
Is an excitation amplifier that supplies a balanced voltage to Due to this balanced voltage, the outer peripheral shield conductor 31 and the central conductor 32
And are driven with the same phase and the same amplitude, and the potential difference between both conductors is ideally zero (equilibrium state). Further, a Zener diode ZD is connected between the outer peripheral shield conductor 31 and 0V, and with respect to pulse noise that enters through (detection probe-shield probe),
It has a protective effect on the oscillator circuit side. Since the stray capacitance C _k formed between the detection probe 11 and the shield probe 12 is common to the central conductor 32 and the outer peripheral shield conductor 31 of the coaxial cord 30, the potential is in a balanced state like the coaxial cord 30. , Its capacitance C _k is not formed (C _k = C _L ≈0). Further, the detection probe 11 and the ground (ground) are almost completely shielded by the shield probe 12, so that the capacitance component is Not formed. Therefore, the oscillation cycle T of the CR oscillator circuit
_x can be expressed by the following equation.

[0013] _{T x ≒ R f (C x} // C p) _{where, C x = C s1 // (} C s2 + C k) ≒ C s1 // C s2 (C G1,2 »C s1, C s2 ) C _s1 : capacitance formed between the probe 11 and the pipe 1. C _s2 : Capacity formed between the pipe 1 and the liquid 2 when the liquid 2 is in the pipe 1.

C _G1 : The capacity formed between (liquid-ground). Here, the cycle depending on the presence / absence of liquid in the pipe 1 is each capacitive component C _s1 formed by (detection probe-pipe-liquid),
It varies depending on C _x determined by the combination of C _s2 and C _G1 . Assuming that the capacitance change due to the presence or absence of liquid is ΔC _x , the periodic change ΔT _x is ΔT _x = T _x −T _x '= K · R _f {(C _x // C _p ) − (C _x ' / _{/ C p)} = K ·} R f (C x -C x ') = K · R f · ΔC x (K: proportional constant) that is, the cycle changes in proportion to the capacitance change [Delta] C _x due to the presence or absence of liquid is obtained .

The capacitance C detected by the detection probe 11
_xIs the capacitive component C formed by the pipe 1._s1,as well as
Capacitance component C formed by the presence of liquid_s2Is dominated by
It C formed between liquid and ground_G1Is C_s2To serial
And the capacitance value is also large C _G1≫ C_s2To be
Therefore, when viewed from the detection probe 11, C is approximately_s2+ C_G1
≈ C _s2Becomes

On the other hand, the capacitance C _s1 between the detection probe 11 and the pipe 1 and the capacitance C _s2 between the pipe 1 and the liquid 2 are added in parallel, and the additional capacitance C _x viewed from the detection probe 11 is C
_x = C _s1 // C _s2 Here, the capacitance C _s2 formed by the liquid 2 varies depending on the type of liquid, but most of it is determined by the magnitude of the relative dielectric constant ε _s inherent to the liquid. FIG. 6 shows the types of liquids and the relative dielectric constants that are widely used in the general industry. The relative dielectric constant epsilon _s relative capacitance value C ₀ in a vacuum (air), when the capacitance value C _s of a substance is represented by the ratio C _s / C _0. ε _s = C _s / C ₀ ∴C _s = C ₀ ε _s Here, since the relative permittivity of vacuum (air) ε _s = 1.0, the capacitance change ΔC _x depending on the presence or absence of liquid in the pipe.
Is ΔC _x = C _s2 −C _s2 ′ = ε _s C _s2 ′ −C _s2 ′ = C _s2 ′ (ε _s −1) C _s2 : the capacity formed in the presence of a liquid.

C _s2 ': Capacity formed in the absence of liquid. In other words, it is governed by the size of the relative permittivity ε _s of the liquid to be detected, and most liquids are several times to several tens times the size of vacuum (air). In, it is possible to detect the presence or absence in the pipe. The filter 212 disposed between the wide band amplifier 211 and the coaxial cord 30 constitutes a low pass filter or a band pass filter, and is considered to pass only the oscillation frequency region. This is the outer peripheral shield conductor 31 of the coaxial cord 30.
While the Zener diode ZD connected between 0 V and 0 V protects the circuit components from damage due to noise, it has an effect of preventing unnecessary noise from being superimposed on the output T _{x of the} oscillation circuit, which is favorable. The signal state can be maintained.

Here, the function and effect of the phase compensation capacitance C _p will be described. The oscillation cycle T _x is determined by the feedback resistance R _{f, the} capacitance C _x obtained by the detection probe 11, and C _p inserted on the amplifier side. In order to secure the periodic change ΔT _x obtained by the detection capacitance component ΔC _x with a high gain, it is necessary to suppress the added capacitance formed between the oscillator 21 and the detection probe 11 to be small (ideally zero). Further, in consideration of the application under the installation conditions, it is important to secure the extension length of the coaxial cord 30 in order to attach the detection probe 11 to the pipe 1 containing the liquid 2 which is the object to be detected. Generally, there is a capacitance component of about 100 _PF / m between the center conductor and the outer circumference shield conductor of the coaxial cord, and the detection probe 11 is 1 m to 2 m from the amplifier.
When extended, the additional capacity is about 100-200 _PF .

On the other hand, the detection capacitance ΔC _x obtained by the detection probe 11 is 0.2 to 0.5 _PF or less, and sufficient detection sensitivity (ΔC _x / C _x ) cannot be secured considering the influence of the above code. I understand. In this embodiment, the coaxial code is in a balanced state and the additional capacitance is canceled to ensure high detection sensitivity. That is, it is important to operate the detection probe 11 in a good balanced state with the extended coaxial cord 30. Therefore, in this embodiment, the frequency characteristic of the wide band amplifier 212 is made good (high frequency characteristic), and the phase compensation is further performed. This is realized by selecting the capacitance C _p as an optimum value.

Since this C _p is arranged in parallel with the additional capacitance C _x of the detection probe 11, it is generally judged that it is better not to insert it in order to obtain a high gain of the periodic change ΔT _x . The equilibrium state of the coaxial cord is ideally that the potential between the center conductor and the outer shield conductor is the same in either amplitude or phase, but the components that make up the device,
Complete equilibration cannot be expected as it is affected by uncontrollable factors of the circuit elements.

In the present embodiment, it was found that the addition of the optimum value of C _p can improve the equilibrium state of the potential in the phase component. As a result, the deterioration of the detection sensitivity (ΔC _x / C _x = ΔT _x / T _x ) due to the extension of the coaxial cord is minimized, and the cord extension length of 2 m or more is realized. By the inventors experiments, to obtain the optimum value = 1 to 2 _PF of C _p. The measured data are shown in FIG. 7.

The period / voltage conversion circuit 22 includes an oscillator 2
A T / V converter circuit that receives the oscillation output from the unit 1 and converts it into a DC voltage proportional to its cycle T _x is configured.
A circuit configuration example is shown in FIG. 8 and an operation chart is shown in FIG. T _x A obtained from the oscillator 21 drives the analog switch S via the differentiating circuits C ₁ and R ₁ . During the rising portion T _{s of} T _x , the switch S is turned on, which causes the charge of the capacitor C ₂ to be rapidly discharged. After T _{s, the} switch S is turned off, the capacitor C ₂ is charged through the resistor R ₂ , and its terminal voltage rises in proportion to the time constant R ₂ C ₂ (waveform C). This signal is smoothed and amplified by the integrating circuits R ₃ , C ₃ and the amplifier A, and V ₀ having a predetermined level value is output (waveform D). Here, R ₂ is a variable resistor, and the output voltage V ₀ is adjusted by changing the time constant of R ₂ C ₂ .

The comparator / output circuit 23 receives the output V ₀ of the T / V conversion circuit 22 and outputs a detection signal when V ₀ > E _s by comparing with the reference voltage E _s . Here, the level of outputting the detection signal is adjusted by changing the resistance R ₂ in the T / V conversion circuit, but the method of changing the gain of the amplifier A and the change of the reference value E _s of the comparator. It goes without saying that any of the above methods can be used.

[0024]

According to the present invention, two concentric cylindrical conductors having different outer diameters are arranged in a pipe to be detected, the inner cylindrical conductor is used as a detection probe, and the outer cylindrical conductor is used as a shield probe. Since the liquid inside the pipe to be detected is detected by detecting the change in capacitance of the detection probe, the pipe material is non-conductive (all insulators other than metal).
As long as it is transparent, and there is no restriction on transparency or color.

Most liquids can be detected. The liquid in the pipe can be applied to the ground regardless of whether it is grounded or ungrounded, and it is also possible to detect the liquid in the pipe or container completely insulated from the ground. Any pipe or tube that can be stored in the inner diameter of the detection probe may be used, and there is no restriction on application depending on the size of the pipe outer diameter.

It reacts only with the substance inside the detection probe and is not affected by the substance outside the shield probe. Therefore, a stable detection operation can be obtained without being affected by surrounding structures. Since the sensor head part can be separated and extended (1 to 2 m or more) from the amplifier part and can be downsized, there is no restriction on the mounting location. Also, a mounting mechanism (clamp) for pipes and tubes
If it is provided, one-touch mounting can be easily realized.
And so on.

[Brief description of drawings]

FIG. 1 is a schematic perspective view showing an example to which a liquid detection device of the present invention is applied.

FIG. 2 is a block diagram of a liquid detection device according to an embodiment of the present invention.

FIG. 3 is a vertical cross-sectional view of a sensor head portion of the liquid detection device according to the embodiment.

FIG. 4 is a transverse cross-sectional view of a sensor head portion of the liquid detection device according to the embodiment.

FIG. 5 is a circuit diagram for explaining the configuration and operation of the oscillator of the device of the embodiment.

FIG. 6 is a diagram showing types of liquids and relative dielectric constants.

FIG. 7 is a diagram showing a detection characteristic of the apparatus of the embodiment.

FIG. 8 is a circuit diagram showing a specific example of a cycle / voltage conversion circuit of the device of the embodiment.

FIG. 9 is a waveform diagram for explaining the operation of the same period / voltage conversion circuit.

[Explanation of symbols]

1 Detected pipe 11 Inner cylindrical conductor (detection probe) 12 Outer cylindrical conductor (shield probe) 21 oscillators 30 coaxial cord 31 Peripheral shield conductor 32 center conductor

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Claims (3)

(57) [Claims] 1. A pipe to be detected is provided with two concentric cylindrical conductors having different outer diameters, the inner cylindrical conductor is used as a detection probe, and the outer cylindrical conductor is used as a shield probe. A liquid detection device configured to detect a liquid in a pipe to be detected by detecting a change in capacitance. 2. The electrostatic capacitance change detection comprises an oscillation circuit by connecting the two cylindrical conductors and a main body circuit section,
The liquid detection device according to claim 1, which detects a change in a signal period of the oscillation circuit. 3. The two cylindrical conductors and the main body circuit portion of the oscillation circuit are connected by a coaxial cord, the detection probe is connected to the central conductor of the coaxial cord, and the shield probe is connected to the outer peripheral shield conductor of the coaxial cord. The liquid detection device according to claim 2, wherein the outer peripheral shield conductor is excited by a balanced voltage to bring the coaxial cord into a balanced state.