JPH02290516A - Water level sensor - Google Patents

Abstract

PURPOSE:To prevent the effect of noise and to detect a water level with high accuracy by providing a conductive film for preventing noise which is disposed between at least one conductive film of a set of conductive films and a reservoir. CONSTITUTION:The conductive film 11a which is one of a set of opposed conductive films 11a and 11b is disposed on a side which is near to the reservoir and the other conductive film 11b is disposed on a side which is near to the outside case of a pot. Besides, near the conductive film 11a disposed on the side which is near to the reservoir, in the concrete, between the conductive film 11a and the reservoir, the conductive film for preventing noise 13 is disposed so as to cover the conductive film 11a. Thus, the conductive film 11a is shielded and the noise is prevented from being induced to the conductive film 11a. Simultaneously, the effect of the stray capacity of the noise generated between the conductive film and the reservoir, the case of the pot, the potential of the earth or the like is reduced.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a water level sensor used for detecting the water level of a container such as a pot (prior art). The type of water level sensor is used to dispense hot water at a predetermined location according to the water level of the hot water contained in the pot. A pair of conductive films are placed facing each other on the outer circumferential surface of a water-resistant tube made of a glass pipe or the like that communicates with the water tank and extends from the bottom of the tank toward the ijLL section, and the inside of the pot is placed inside this tube. There is a capacitive type water level sensor that utilizes the fact that the capacitance between a pair of conductive films changes depending on the water level when a liquid enters the water level. It is incorporated into the oscillation circuit, and the oscillation circuit oscillates at an oscillation frequency that corresponds to the capacitance detected by the water level sensor, so the water level can be detected by measuring this oscillation frequency. Figure 6 is a graph showing changes in oscillation frequency with respect to water level in an oscillation circuit using such a water level sensor. The oscillation frequency shown in the figure is as shown by multiple curves,
The water level fluctuates for the same water level, but this is because the conductive film provided on the outer circumferential surface of the pipe that makes up the water level sensor is exposed. Stray capacitance occurs between the electric potential, etc., and external noise tends to enter the conductive film that is connected to the high impedance side. Therefore, as mentioned above, the oscillation of the oscillation circuit using the water level sensor The frequency changes. (Problems to be Solved by the Invention) In conventional water level sensors, as mentioned above, stray capacitance occurs between the conductor tM and the water tank, pot case, ground potential, etc. There is a problem in that noise enters the conductive film connected to the water level sensor, which causes the oscillation frequency of the oscillation circuit that uses the water level sensor to fluctuate, making it impossible to accurately detect the water level. The present invention has been made in view of the above, and its purpose is to provide a water level sensor that can prevent the influence of noise and detect water level with high accuracy. [Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the water level sensor of the present invention is a water level sensor that detects the water level of liquid in a water tank, and includes a water level sensor that detects the water level of a liquid in a water tank. an insulating pipe communicating with the water tank through the water tank and extending from the bottom of the water tank to approximately the same height as the top;
a pair of conductive films provided oppositely on the outer circumferential surface of the insulating tube body so as to change electrical characteristics according to the level of liquid entering the insulating tube body from the water storage tank; The present invention further comprises a conductive pipe for preventing H sound, which is connected to the conductive film and arranged between at least one of the pair of conductive films and the water tank. (Function) In the water level sensor of the present invention, a noise prevention conductive film is provided between at least one conductive film of a pair of conductive KHs and a water tank, and this noise prevention conductive film is held at a fixed potential. (Embodiment) Below, an embodiment of the present invention will be explained using the drawings. FIG. 1 is a sectional view showing the interior layout of the main parts of a pot to which a water level sensor according to an embodiment of the present invention is applied. 1
Below this water storage tank 1, a heat retention heater 3 and a boiling ILK overheating heater 5 are provided. In order to dispense hot water, the pot shown in Fig. 1 dispenses a fixed amount of liquid in accordance with the level of the liquid stored in the water tank 1 when the operator operates the operating section provided on the top. In order to detect this water level, a pipe 7, which is an insulating pipe body, is provided. This vibrator 7 communicates with the water tank 1 from the bottom of the water tank 1, and also extends from the bottom of the water tank 1 toward the -E part. 4 liters of liquid is stored from the bottom of the water tank 1! It will enter to the same height as the water level inside 1. This pipe 7 is shown enlarged in FIG. 2, and a pair of conductors i[1la, llb are provided on the outer peripheral surface of the pipe 7 to face each other. This pair of conductive jig 11a,
flb, when the liquid in the water storage tank 1 enters the vibrator 7, air or the entered liquid will exist in addition to the insulating pipe 7 between the pair of opposing conductive M11. Air and liquid, specifically water, have different dielectric constants, so when liquid enters pipe 7, a pair of conductors tJJl11
When the water level between these two opposing conductive films 11 changes,
The #j capacitance between is changed. This change in capacitance is then converted into, for example, voltage or frequency by a signal processing circuit, which will be described later, and the water level is thereby detected. A pair of opposing conductive patterns 11a
, llb, one conductive M 1 1 a is connected to the water tank 1
The other conductive film 11a is arranged on the side near the outer case of the pot. Specifically, conductivity 71. Between 1 1 a and water tank 1, there is a second
As shown in the figure, the conductive M13 for noise prevention is connected to the conductive JBI11.
a, thereby shielding the conductive film 11a and preventing the induction of noise to the conductive M 1 1 a, as well as preventing interference between the conductive film and the water tank, pot case, earth potential, etc. This reduces the effects of stray capacitance that occurs in Furthermore, as explained in the signal processing circuit n'''c' of FIG. FIG. 3 shows the change in capacitance between the pair of conductive films 11a and llb attached to the outer peripheral surface of the pipe 7 as a result of the liquid in the water storage tank 1 entering the pipe 7. This shows a signal processing circuit that generates a signal with a corresponding frequency.
In the signal processing circuit shown in the figure, as shown in FIG.
3 is used, the conductive film 13 for noise prevention is connected to the lowest potential ground of the circuit, and one conductive film 11a is connected to the output terminal 18 via an inverter 15.17 connected in series. The other conductive Mllb is connected to the output of the inverter 17, that is, the output terminal 18, thereby forming an oscillation loop. Further, a resistor 19 is connected in parallel to the inverter 15, and the other output terminal 20 is connected to ground. In the signal processing circuit configured in this way, a pair of conductive JP! I11a. A signal with a frequency corresponding to the capacitance of 1 lb is output from the output terminal 18.20, and by detecting this output frequency, the water level of the liquid in the water tank 1 can be detected. Figure 4 is a graph showing the relationship between water level and frequency. As shown in the figure, when the water level is low, the frequency is high, and when the water level is high, the frequency is low. In addition, when the water level sensor of this embodiment shown in the figure is used, the oscillation frequency corresponding to the water level does not fluctuate as in the conventional case and corresponds accurately to the water level. Therefore, by using this water level sensor having such characteristics, the water level in the water tank 1 can be accurately detected. This is due to the effect of the noise-prevention conductor 813, as described above. FIG. 5 shows a signal processing circuit similar to FIG. 3 that outputs a voltage that varies in level instead of frequency. Fifth
Also in the signal processing circuit shown in the figure, the noise prevention conductor tM
1 3 are connected to the lowest potential ground, and one conductor [
111 1 a is one input of the operational amplifier 25 and the capacitance C
o is connected to the connection point with one end of the reference capacitor 27,
The other conductive C film 1lb is connected to one input of the operational amplifier 25 via a resistor 23.
The other input of the MA device 25 is connected to the output, the curved end of the reference capacitor 27 is connected to ground, and the resistor 23
and one conductive t-film 11a are connected to a DC voltage source 31 via a voltage signal source 29. In this configuration, the voltage of the voltage signal source 29 is connected to a pair of conductors tg11a, tg11a, which changes depending on the water level in the water tank 1.
The capacitance between l l b and the capacitance C of the reference capacitor 27
o, and this divided voltage is output via the operational amplifier 25 to the output terminals 33 and 35. This output voltage varies depending on the water level in the water tank 1.
'rXM Since it changes depending on the capacitance between Ila and Ilb, the water level in the water tank 1 can be detected by detecting this voltage. Note that this divided voltage V is
If the capacitance between the js film 11a and Ilb is Cs, and the voltage of the voltage signal source is ■, then V=Cov/(Co-t-C
s). [Effects of the Invention] As explained in 1 below, according to the present invention, a noise prevention film is provided between at least one of the pair of conductive films and the water tank, This noise-prevention conductive film is connected to a fixed potential, so it is free from the effects of stray capacitance that occurs between the conductive film that forms the water level sensor and the water tank, pot case, earth potential, etc., and from external sources. It can reduce the influence of noise and detect water level with high accuracy.

[Brief explanation of drawings]

Fig. 1 is a sectional view showing the internal structure of a pot to which a water level sensor according to an embodiment of the present invention is applied, and Fig. 2 is a #J of the pipe top and conductive film used in the water level sensor of Fig. 1. Fig. 3 is a circuit diagram showing an example of a signal processing circuit used in the water level sensor shown in Fig. 1, and Fig. 4 shows the relationship between water level and oscillation frequency in the signal processing circuit shown in Fig. 3. graph,
FIG. 5 is a circuit diagram showing another example of the signal processing circuit, and FIG. 6 is a graph showing the relationship between the water level detected by a conventional water level sensor and the oscillation frequency. 1... Water tank, 7... Pipe, 11a, Ilb- - -\P Reito, 13... Conductive film for noise prevention.

Claims (1)

[Claims] A water level sensor that detects the water level of liquid in a water storage tank,
an insulating pipe communicating with the water tank through the bottom of the water tank and extending from the bottom of the water tank to approximately the same height as the top; and a water level of liquid entering the insulating pipe from the water tank. a pair of conductive films provided opposite to the outer circumferential surface of the insulating tube so as to change electrical characteristics according to A water level sensor comprising: a conductive film for noise prevention disposed between a conductive film and the water tank.