JPS63286720A - Water level detector using piezoelectric vibrator - Google Patents

Abstract

PURPOSE:To detect a water level highly accurately with simple circuit constitution, by scanning a frequency centering around a resonance frequency of a piezoelectric vibrator by using a microcomputer, and detecting an impedance variation of the piezoelectric vibrator at that time. CONSTITUTION:A frequency of an output of a V/F converting part 3 is detected by a counter part 12 in a microcomputer 9. Subsequently, by opening or closing a switch 11 being an output terminal of the microcomputer 9, and controlling a capacitor voltage (namely, an input of the converting part 3) of a CR charge/ discharge part 8, the frequency of the output of the converting part 3 can be scanned with high accuracy. An output voltage of the converting part 3 detects an impedance variation of a piezoelectric vibrator 5 by an impedance variation detecting part 4, thereafter, goes to a DC voltage by a smoothing part 6, and inputted to an A/D converting part 13 of the microcomputer 9. A value of a DC voltage is read by the converting part 13, and when it becomes lower than a prescribed voltage, it is decided that a resonance point exists, and it is decided that the vibrator 5 exists in the air. On the contrary, unless the resonance point exists, it is decided that the vibrator 5 exists in the water. In such a way, a water level can be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a device for detecting water level in washing machines, dishwashers, and other appliances using piezoelectric vibrators.

Conventional technology Conventionally, a piezoelectric vibrator was used as a method for detecting water level.
It is known that a change in the medium of the vibration surface (underwater←air) is detected as a change in the impedance of a piezoelectric vibrator. However, there was no mention of specific means for actually detecting it.

Therefore, the present inventors have proposed a method of scanning the frequency around the resonance frequency of the piezoelectric vibrator and detecting the impedance change of the piezoelectric vibrator at that time. Specifically, as shown in FIG. 4, a sawtooth wave generator 1 generates a sawtooth wave, and its output is input to a V/F converter 3 via a buffer section 2. As an output from the V/F converter 3, a pulse having a frequency corresponding to the input voltage is generated. The output of the V/F conversion section 3 is input to an impedance change detection section 4 in which a resistor and a piezoelectric vibrator 6 are connected in series, and a voltage corresponding to the impedance of the piezoelectric vibrator 5 is input to a smoothing section 6 and becomes a DC voltage. . This is then amplified by the amplification section 7 and compared by the voltage comparison section 8. The output of the voltage comparator 8 changes between H and L depending on the presence or absence of a resonance point of the piezoelectric vibrator (that is, the presence or absence of water). This utilizes impedance characteristics due to changes in the medium of the vibration surface of the piezoelectric vibrator 6 (underwater or in the air).

As shown in Fig. 5, which shows the frequency-impedance characteristics of the piezoelectric vibrator, in the air A there is a resonance point fa and an anti-resonance point fa.
r exists, but when immersed in water B, the resonance characteristic no longer exists. This resonant frequency changes somewhat due to production variations of piezoelectric vibrators, etc., and also changes due to temperature changes. In other words, f in Fig. 6 (lni minimum frequency) and fmaX
(Maximum frequency) may be set so as to sufficiently cover the above-mentioned resonant frequencies even if they vary. The characteristics of the V/F converter 3 are particularly important because they define the fmi and f, and it is necessary to improve the accuracy. This is important because it regulates the voltage.

Problems to be Solved by the Invention As described above, the configurations proposed so far require the sawtooth wave generating section 12, the amplifying section 13, the voltage comparing section 14, etc., and this also affects the cost. It was something that came.

In view of the above-mentioned points, it is an object of the present invention to provide a low-cost water level detection device with a simplified circuit configuration.

Means for Solving the Problems In order to solve the above-mentioned problems, the water level detection device using the piezoelectric vibrator of the present invention has a counter unit that can count external frequencies, and converts the external voltage into digital data. A
a microcomputer with a built-in control section that operates according to signals from the A/D conversion section, the counter section, and the A/D conversion section;
A CR charging/discharging section where the charging and discharging of a capacitor is controlled by the output terminal of a microcomputer, and a V/
It has an F conversion section, an impedance change detection section in which a resistor and a piezoelectric vibrator are connected in series to which the output voltage of the V/F conversion section is applied, and a smoothing section that converts the output into a DC voltage, The output of the V/F conversion section is input to a counter section within the microcomputer, and the output of the smoothing section is input to an A/D input section within the microcomputer.

Operation In the above configuration, the frequency of the output of the V/F conversion section is detected by the counter section in the microcomputer, and the output terminal of the microcomputer is opened or closed to adjust the capacitor voltage (
In other words, by controlling the input of the V/F converter), V
The frequency of the output of the /F converter can be scanned with high accuracy.

After the impedance change detecting section detects a change in the impedance of the piezoelectric vibrator, the V/F change amount voltage is converted into a DC voltage by a smoothing section and is input to an A/D converting section of a microcomputer. The microcomputer's A/1) converter reads the DC voltage value, and if it becomes lower than the predetermined constant voltage, it is determined that there is a resonance point. Depending on how the constant voltage is determined, there may be an anti-resonance point. ), it is determined that the piezoelectric vibrator is in the air. Conversely, if there is no resonance point (or anti-resonance point), it is determined that the piezoelectric vibrator is underwater. This allows the water level to be detected.

Embodiment FIG. 1 shows an embodiment of the present invention. In the figure, the same parts as in FIG. 3 are given the same reference numerals, detailed explanations are omitted, and only the differences will be explained. 8 is the CR charging/discharging section,
It is connected to the output terminal of the microcomputer 9 and controls charging and discharging of the capacitor. V/ depending on the charging/discharging voltage of this capacitor.
The frequency of the F converter 3 changes. Microcomputer 9
is a counter section 12 that can count external frequencies, an A/D converter section 13 that converts external voltage into digital, and a control that operates based on signals from the counter section 12 and A/D converter section 13. It incorporates a switch 11 controlled by a section 10 and a control section 1o. The CR charging/discharging section 8 is connected to the push end of the switch 11, the output of the V/F converting section 3 is input to the counter section 12, and the output of the smoothing section 6 is input to the A/D converting section 13. The output is the input. 14 is control section 1
This is a water supply valve drive circuit that operates by a signal from o.

The operation of the above configuration will be explained below.

First, when the switch 11, which is the output terminal in the microcomputer 9, is turned from the closed state to the open state, the voltage at section A in FIG.
The state is from t0 to t1. Then, the output voltage of the CR charging/discharging unit 8 (section B in Figure 1) becomes the charging curve of the capacitor, and that voltage becomes the vin input of the V/F conversion unit 3 (t0 to 11 in Figure 2B). The frequency f of the output of section C in FIG. 1 depends on the input voltage. U, changes (Tsum fi
If K is a constant, the formula fout=Kxviu holds true).

When fout is counted by the counter section 12 inside the microcomputer 9 and reaches the maximum value (FIG. 27C), the control section 10 inside the microcomputer 9 closes the switch 11. Then, section A in FIG. 1 becomes the voltage from t1 to t2 in A in FIG.
part) decreases to the state from t1 to t2 in FIG. 2B. And f of the output of the V/F converter 3. U, is f. From ax to a low frequency (from fma to fmi in Figure 2 C)
, by the counter section 12 in the microcomputer 9. ut is f, n
If it is detected that the
Switch 12 is closed. After that, repeat the same process. In this way, the counter unit 12 in the microcomputer 9
The output of the converter 3 is detected and the switch 11 in the microcomputer 9 is activated.
By controlling the input of the V/F converter 3 with the following, a highly accurate V/F converter 3 can be constructed. The output of the high-precision V/F converter 3 created by the above operation is applied to the impedance change detector 4, and the impedance change of the piezoelectric transducer 6 is detected by the smoother 6 as a DC voltage (D in Fig. 2). ) after converting to
The signal is taken into the A/D converter 13 of the microcomputer 9. Inside the microcomputer 9, the voltage of the A/D converter 13 is checked several times while 'out is transitioning from fmiyu to ffnax (or from 'max to fmin), and the voltage becomes lower than the predetermined constant voltage. For example (in the case of a voltage as shown in the second diagram), the control unit 1o determines that there is a resonance point fr, and determines that the piezoelectric vibrator 5 is in the air (depending on how the constant voltage is determined, there may be an anti-resonance point far). This is K because it can also be determined
It is also possible to determine that the piezoelectric vibrator 6 is in the air.

Conversely, when the resonance point fr or the anti-resonance point far disappears,
The control unit 1o determines that the piezoelectric vibrator 5 is underwater, that is, determines that the water level has reached a predetermined value, and operates the water supply valve drive circuit 14 to stop the water supply.

FIG. 3 shows a specific example of detecting the water level in the water tank 15, which means that when the piezoelectric vibrator 5 is submerged in water, the water supply valve 17 is closed by the action of the control unit 16 shown in FIG. This will stop the water supply. This makes it possible to obtain a predetermined water level.

Effects of the Invention As described above, the present invention uses a microcomputer having a counter section and an A/D conversion section, scans the frequency around the resonance frequency of the piezoelectric vibrator, and measures the impedance change of the piezoelectric vibrator at that time. Since the water level is detected, the water level can be detected with high precision, a simple circuit configuration, and at a low cost.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a water level detection device that is an embodiment of the present invention, Fig. 2 is a waveform diagram of each part of Fig. 1, Fig. 3 is a diagram showing an application example of the device in Fig. 1, and Fig. 4 is a diagram showing an application example of the device in Fig. 1. The figure is a block diagram of the device proposed so far, and FIG. 6 is a frequency-impedance characteristic diagram of the piezoelectric vibrator. 3... V/F conversion section, 4... Impedance change detection section, 6... Piezoelectric vibrator, 6...
...Smooth part, 8...CR charging/discharging part, 9...
...Microcomputer, 10, old control section, 12
. . . Counter section, 13 . . . A/D conversion section. Name of agent: Patent attorney Toshio Nakao and 1 other person2-
--Buffer color Fig. 2 5- Oscillator 15-1-pe 16- Control a capital 17-7 to valve Fig. 3 Fig. 5-tY Thin Long 1 0222222 Niko Yield

Claims (1)

[Claims] A counter section that can count external frequencies,
A microcomputer with a built-in A/D conversion section that converts external voltage into digital, a counter section, and a control section that operates based on signals from the A/D conversion section, and a CR whose capacitor charging and discharging is controlled by the output terminal of the microcomputer. A charging/discharging section,
A V/F converter whose input voltage is the charge/discharge voltage of the capacitor and whose frequency changes according to the voltage, and a resistor and piezoelectric vibrator connected in series to which the output voltage of the V/F converter is applied. The output of the V/F converter is sent to the counter section in the microcomputer, and the output of the smoothing section is sent to the A/F converter in the microcomputer. A water level detection device using a piezoelectric vibrator that is input to the D input section.