JP2009063484A - Liquid level detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid level detector preventing DC power from being applied to a liquid level sensor via an AC voltage generation circuit when a microcomputer stops its operation. <P>SOLUTION: The liquid level detector 1 includes a CPU 55 for controlling a main unit and also generating an AC signal, the AC voltage generation circuit 70 for supplying AC voltage in response to the AC signal to the liquid level sensor 63 for sensing a level 62a of a liquid reserved in a predetermined vessel 61, a liquid level detection circuit 52 connected with the circuit 70 for detecting a change in voltage of the liquid level sensor 63 due to the change in the liquid level 62a and informing the CPU 55 of the change, and a power-off part 2 for cutting the power supply from the circuit 70 to the sensor 63 when the AC signal stops. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a liquid level detection device that detects the water level of a liquid stored in a tank or the like, and more specifically, when a circuit that AC drives an electrode that detects the water level fails, a DC voltage is not applied to the electrode. And a circuit that prevents corrosion of the electrode and prevents the stored liquid from being electrolyzed.

  Conventionally, a liquid level control device 100 shown in the block diagram of FIG. 3 is disclosed as one using a liquid level detection device. In addition to the function as the liquid level detection device, the liquid level control device 100 also has a function as a control device that adjusts the water amount (liquid level) of the tank based on the detected water level data.

  FIG. 3 is an overall configuration diagram showing the liquid level control device 100. Reference numeral 61 denotes a container. The container 61 is a semi-insulating or conductive medium, and acts to corrode the metal due to the movement of electric charges due to the difference in ionization tendency from the metal in the liquid, that is, electrolytic corrosion. The liquid 62 having an action is supplied to the position of the liquid level 62a. Examples of the liquid 62 include water, beverages, saline, chemicals such as acids and alkalis, CFCs containing water, and fats and oils.

  The liquid level 62a is the liquid level of the liquid 62 to be controlled, and a liquid level sensor 63 is attached to the side surface of the container 61 corresponding to the position of the liquid level 62a. Reference numeral 64 denotes a gas (air or vapor of the medium), and the gas 64 is adjacent to the liquid surface 62a of the liquid 62 supplied to the container 61 and is another medium having a different thermal conductivity from that of the liquid 62.

  Reference numeral 50 denotes a control device, and a liquid level sensor 63 is connected to the control device 50 via an AC voltage generation circuit 70. The control device 50 includes a water level detection circuit 52 connected to the power supply unit 51, an A / D converter 53 that performs A / D conversion on a signal from the liquid level sensor 63, and a CPU 55 that is a control unit of the liquid level control device 100. The oscillator 56 connected to the CPU 55 and an A / D converter 54 for A / D converting the signal from the outside air temperature sensor 66 for correcting the outside air temperature of the liquid level sensor 63 data. The CPU 55 is connected to a ROM 57 that stores a control program and a RAM 58 that stores various data.

  The AC voltage generation circuit 70 includes four switching transistors 71 and an inverting transistor 72 connected in a flip-flop manner. Reference numeral 68 denotes a liquid supply electromagnetic valve disposed in a pipe for supplying the liquid 62 into the container 61, and 69 denotes a drainage electromagnetic valve disposed in the pipe for discharging the liquid 62 from the container 61.

  FIG. 4 is an enlarged longitudinal sectional view of the liquid level sensor 63. The outer body 31 of the liquid level sensor 63 has a screw portion 31a for attaching to the side surface of the container 61 at a part of the outer periphery thereof. A terminal block 32 is fitted inside the cylinder of the outer body 31. The terminal block 32 is provided with a sealing glass 33, and the outer body 31 is filled with a filler 34 in its inner space, and the electrodes 35 and 36 are positioned and fixed. A negative thermistor 37 for detecting the liquid level is connected between the opposing electrodes 35 and 36 via lead wires 38, respectively. The thermistor 37 has a characteristic that the electrical resistance decreases as the temperature rises.

  The electrodes 35 and 36 are made of a Kovar wire having a thermal expansion coefficient substantially equal to that of the sealing glass 33, and the lead wire 38 is made of a platinum wire. Further, lead wires 39 for connecting the liquid level sensor 63 to the AC voltage generating circuit 67 are connected to the opposite sides of the electrodes 35 and 36 from the thermistor 37 at the respective crimp terminals 39a and 39b.

  Next, the operation will be described. When the level of the liquid level 62 a of the liquid 62 in the container 61 is lower than the position of the thermistor 37 of the liquid level sensor 63, the liquid level sensor 63 is in the gas 64. When the control device 50 starts operating, a voltage is applied to the thermistor 37 from the power supply unit 51 via the water level detection circuit 52. At this time, the voltage to be applied from the water level detection circuit 52 to the liquid level sensor 63 is output as a direct current. Then, an AC signal (rectangular wave) having a frequency to be applied to the liquid level sensor 63 is output from the CPU 55 of the control device 50 and output to the inverting transistor 72 of the AC voltage generation circuit 67.

  Then, the inverting transistor 72 sends a signal to each switching transistor 71 according to the frequency output from the CPU 55. As a result, the DC voltage output from the water level detection circuit 52 becomes an AC rectangular voltage waveform at the frequency of the AC signal output from the CPU 55, and is applied to the thermistor 37 of the liquid level sensor 63.

  Then, the thermistor 37 becomes a temperature higher than the temperature of the gas 64 in the container 61 and is in an equilibrium state. That is, when the thermistor 37 is in a gas having a low thermal conductivity, Joule heat generated from the thermistor 37 has a small amount of heat transfer to the gas 64, and the thermistor 37 easily rises in temperature and reaches a constant temperature at a low voltage. The voltage applied from the water level detection circuit 52 at this time corresponds to a value at which the thermistor 37 is present in the gas. Therefore, the CPU 55 of the control device 50 determines that the thermistor 37 of the liquid level sensor 63 is in the gas 64.

  Then, the CPU 55 of the control device 50 opens the liquid supply electromagnetic valve 68 via the interface 59 and starts supplying the liquid 62 into the container 61. When the liquid 62 continues to be supplied into the container 61, the liquid level 62a of the liquid 62 eventually reaches the position of the liquid level sensor 63, that is, the position of the thermistor 37.

  Then, the amount of heat transfer from the thermistor 37 to the liquid 62 increases rapidly, and the temperature of the thermistor 37 itself rapidly decreases. That is, the resistance value increases. As in the case described above, a voltage is applied to the thermistor 37 from the water level detection circuit 52 of the control device 50, and the thermistor 37 is in an equilibrium state at a temperature higher than the temperature of the liquid 62 in the container 61.

  That is, when the thermistor 37 is in a liquid having a high thermal conductivity, the Joule heat generated from the thermistor 37 is dissipated in the liquid having a higher thermal conductivity than the gas 64, and the temperature of the thermistor 37 is difficult to rise. High voltage is required to reach a certain temperature. The voltage applied from the water level detection circuit 52 at this time corresponds to the value at which the thermistor 37 is present in the liquid. That is, when the thermistor 37 reaches from the gas to the liquid, the amount of heat transfer increases and the resistance value increases, so that the applied voltage increases.

  Then, the CPU 55 of the control device 50 detects the increase in the voltage value and determines that the liquid level 62a has reached the position of the thermistor 37 of the liquid level sensor 63. Then, the liquid supply electromagnetic valve 68 is closed to stop the supply of the liquid 62 into the container 61 (see, for example, Patent Document 1).

  While such control is being performed, the CPU 55 always outputs an AC signal to the AC voltage generation circuit 70 so that the voltage applied to the liquid level sensor 63 does not remain DC. In general, when an electrode is placed in a liquid such as water and direct current is applied to the electrode, electrolysis may occur and hydrogen or oxygen may be generated, and long-term direct current application may be dangerous. Moreover, the application of direct current to the electrode may induce electrode corrosion and shorten the life of the electrode. Therefore, such an inconvenience is prevented by applying an alternating current to the electrode.

However, when the drive signal to the AC voltage generation circuit is stopped for some reason, the above-described electrolysis or corrosion may occur. In recent years, the drive signal to the AC voltage generation circuit is often generated by the CPU (microcomputer). If the microcomputer runs out of control or the standby power is reduced, the microcomputer may enter sleep mode. There has been a demand for means for preventing electrolysis and corrosion as described above even when the operation is stopped.
Japanese Patent No. 3044776 (page 3-4, FIG. 4)

  An object of the present invention is to solve the above-described problems and to provide a liquid level detection device in which a DC power supply is not applied to a liquid level sensor via an AC voltage generation circuit when the operation of a microcomputer stops. To do.

In order to solve the above-described problem, a liquid level detection device according to the present invention supplies an AC voltage to a liquid level sensor driven by an AC voltage, and the liquid level sensor via the AC voltage generation unit. A water level detection circuit that monitors the voltage of the liquid, a control unit that controls the AC voltage generation unit and the water level detection circuit, and a power source that supplies power to the liquid level sensor, the AC voltage generation unit, and the water level detection circuit A liquid level detecting device that generates the AC voltage based on a signal output from the control unit,
When the AC voltage is stopped, a power cutoff unit is provided that shuts off the power supply from the power source to the liquid level sensor.

  In addition, the power shut-off unit includes an AC signal cut-off detection circuit that detects generation or stop of the AC voltage, and the power supply unit when the detection result of the AC signal cut-off detection circuit is that the AC voltage is generated. And a switch circuit that shuts off the power supply of the power supply unit when the detection result of the AC signal cutoff detection circuit is the stop of the AC voltage.

  The power shut-off unit supplies power to the water level detection circuit and the AC voltage generation circuit together with the liquid level sensor.

By using the above means, according to the liquid level detection device of the present invention,
The invention according to claim 1 automatically shuts off the power supply to the liquid level sensor when, for example, a failure occurs in the control unit, or when the control unit shifts to the low power consumption mode and the AC signal stops. Further, corrosion of the liquid level sensor and electrolysis of the stored liquid due to the control unit being stopped for a long time can be prevented.

  The invention according to claim 2 can be configured by a one-shot timer capable of retriggering the AC signal cutoff detection circuit or a monostable multi-circuit, and the switch circuit can be configured by a relay or a transistor, so that the circuit configuration becomes easy.

  In the invention according to claim 3, when the control unit shifts to the low power consumption mode and the AC signal is stopped, the power supply to the liquid level sensor, the water level detection circuit, and the AC voltage generation circuit is performed simultaneously and automatically. Therefore, the power consumption of the water level detection related circuit can be reduced in the low power consumption mode.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail as examples based on the attached drawings. In addition, about the part demonstrated by background art, the same number is provided and detailed description is abbreviate | omitted. Further, the liquid level detection device 1 in FIG. 1 is one in which the control of the electromagnetic valve is omitted from the liquid level control device 100 described in FIG. 3 of the background art, and only the liquid level is detected. It is. Therefore, the interface 59 of the circuit of the present invention of FIG. 1 is deleted from the conventional circuit of FIG. 3. However, if the solenoid valve is also controlled by a single CPU, it remains connected as before. Good.

  FIG. 1 is an overall configuration diagram showing a liquid level detection device 1 including a control device 50 ′ and an AC voltage generation circuit 70, and a container 61 having a liquid level sensor 63 connected thereto. The liquid 61 is stored in the container 61, and a water supply pipe having a liquid supply electromagnetic valve 68 and a drain pipe having a liquid discharge electromagnetic valve 69 are connected to the container 61. ing.

  In the container 61, the liquid level 62a is the liquid level of the liquid 62, and the liquid level sensor 63 is screwed and attached to the side surface of the container 61 corresponding to the position of the liquid level 62a. Reference numeral 64 denotes a gas (air or vapor of the medium), and the gas 64 is adjacent to the liquid surface 62a of the liquid 62 supplied to the container 61 and is another medium having a different thermal conductivity from that of the liquid 62.

  Reference numeral 50 ′ denotes a control device, and a liquid level sensor 63 is connected to the control device 50 ′ via an AC voltage generation circuit 70. The control device 50 ′ includes a power shut-off unit 2 to which a power unit 51 is connected, a water level detection circuit 52 to which power is supplied from now on, and an A / D converter 53 that performs A / D conversion on a signal from the liquid level sensor 63. The signal from the outside air temperature sensor 66 for correcting the outside air temperature of the CPU 55 which is the control unit of the liquid surface detecting device 1, the oscillator 56 connected to the CPU 55, and the data of the liquid surface sensor 63 is A / D converted. And an A / D converter 54. The CPU 55 is connected to a ROM 57 that stores a control program and a RAM 58 that stores various data.

  Further, when the AC signal output from the CPU 55 is input to the power shut-off unit 2, and the AC signal is interrupted, the water level detection circuit 52 and the AC voltage generation circuit 70 and the liquid level supplied via this are detected. A function of cutting off the power supply to the sensor 63 is provided.

  The AC voltage generation circuit 70 includes four flip-flop-connected switching transistors 71 and inversion transistors 72. The AC voltage output from the CPU 55 alternately inverts the supply voltage to the liquid level sensor 63. It is supposed to let you.

  The voltage to the liquid level sensor 63 is supplied from the water level detection circuit 52, and the water level detection circuit 52 transmits the change in the supply voltage to the CPU 55 via the A / D converter 53. The CPU 55 determines whether or not the liquid level 62a has reached the liquid level sensor 63 by monitoring this voltage change.

  The detailed structure of the liquid level sensor 63 and the water level detection method using the above configuration are described in detail in the background art, and the detection method is also the same in the present invention, so the description thereof is omitted. Next, the power cutoff unit 2 that is a feature of the present invention will be described.

  FIG. 2 is a block diagram of the power shut-off unit 2 that shuts off the power to the water level detection circuit 52 when the AC signal output from the CPU 55 is interrupted. The power cutoff unit 2 includes an AC signal cutoff detection circuit 15 that detects the cutoff of the input AC signal, and a switch circuit 16 that turns on / off the input power source to the output side based on the detection result of the AC signal cutoff detection circuit 15. It consists of and.

  In FIG. 2, the power applied to the power input terminal 3, that is, the voltage of the power supply unit 51 is input to the switch circuit 16, subjected to on / off control, and output from the switch circuit 16 to the power output terminal 17. On the other hand, an AC signal (output from the CPU 55) input to the AC signal input terminal 4 is input to the AC signal interruption detection circuit 15, and an ON / OFF signal from the AC signal interruption detection circuit 15 is output to the switch circuit 16. ing.

  The AC signal cut-off detection circuit 15 does not specify a circuit system as long as it constantly inputs an AC signal with a constant period and outputs a signal of a certain level when the AC signal stops. For example, a retriggerable monostable multi-circuit or a one-shot timer.

  Similarly, the switch circuit 16 may be of any type as long as the power supply unit 51 can be controlled to be turned on / off by the output signal of the AC signal interruption detection circuit 15. A possible stabilized power supply may be used.

  In this embodiment, an example in which a discrete component having a low cost is used will be described.

  2 includes a capacitor 5, a diode 6, a diode 7, and a capacitor 8. One end of the capacitor 5 is connected to the AC signal input terminal 4, and the other end is connected to the cathode terminal of the diode 6 and the diode. 7 are respectively connected to the anode terminal. The anode terminal of the diode 6 is connected to the ground (earth), and the cathode terminal of the diode 7 is connected to the other end of the capacitor 8 having one end connected to the ground. The cathode terminal of the diode 7 is an output to the AC signal interruption detection circuit 15.

  Next, the operation of the AC signal interruption detection circuit 15 will be described. When an AC signal is applied to the AC signal input terminal 4, the signal passing through the capacitor 5 is half-wave doubled rectified by the diodes 6 and 7 to charge the capacitor 8. When the capacitor 8 reaches a certain voltage or higher, the switch circuit 16 is turned on. On the other hand, when there is no signal at the AC signal input terminal 4 or when it becomes a DC potential, the electric charge of the capacitor 8 is gradually discharged, and the switch circuit 16 is turned off when the voltage drops below a certain voltage.

  Therefore, by determining the time constant between the capacitor 8 and the discharge resistance of the switch circuit 16 to be described later in consideration of the period of the input AC signal, the capacitor 8 remains at a predetermined voltage or more while the AC signal continues. Thus, the capacitor 8 is set to a predetermined voltage or less when the AC signal is interrupted. The diode 6 is for voltage doubler rectification, and there is no problem even if it is not provided. In place of the diodes 6 and 7, a bridge diode may be used.

  Next, the switch circuit 16 will be described. The switch circuit 16 includes a PNP transistor 14 having an emitter terminal connected to the power input terminal 3 and a collector terminal connected to the power output terminal 17, and an NPN transistor 11 for controlling the base voltage of the transistor 14. The base terminal of the transistor 14 and the collector terminal of the transistor 11 are connected via a resistor 12, and a resistor 13 is connected between the emitter terminal and the base terminal of the transistor 14.

  The emitter terminal of the transistor 11 is connected to the ground, and the resistor 10 is connected between the base terminal of the transistor 11 and the ground. Between the base terminal of the transistor 11 and the capacitor 8 of the AC signal cutoff detection circuit 15, A resistor 9 is connected. Therefore, the resistor 9 and the resistor 10 connected in series serve as the discharge resistor of the switch circuit 16 described above.

  Next, the operation of the switch circuit 16 will be described. When the capacitor 8 is equal to or higher than a predetermined voltage, a predetermined base current flows to the transistor 11 via the resistor 9, and the transistor 11 is turned on. Then, the base current of the transistor 14 flows to the ground through the resistor 12 and the transistor 11, and the transistor 14 is turned on. That is, a voltage is applied to the power output terminal 17.

  On the other hand, when the capacitor 8 is equal to or lower than the predetermined voltage, the voltage between the base and the emitter terminal of the transistor 11 is equal to or lower than the predetermined voltage, and the transistor 11 is turned off. Then, the voltage between the base terminal and the emitter terminal of the transistor 14 becomes a predetermined voltage or less, and the transistor 14 is turned off. That is, no voltage is applied to the power output terminal 17.

  As described above, for example, when a failure occurs in the control unit (CPU), or when the control unit shifts to the low power consumption mode and the AC signal stops, the power supply to the liquid level sensor is automatically cut off. Therefore, it is possible to prevent corrosion of the liquid level sensor and electrolysis of the stored liquid due to the control unit being stopped for a long time.

  In addition, the AC signal cut-off detection circuit can be configured with a one-shot timer capable of retriggering or a monostable multi-circuit, and the switch circuit can be configured with a relay or a transistor, thereby simplifying the circuit configuration.

  Furthermore, when the control unit shifts to the low power consumption mode and the AC signal stops, the power supply to the liquid level sensor, the water level detection circuit, and the AC voltage generation circuit is cut off simultaneously and automatically. In the power consumption mode, the power consumption of the water level detection related circuit can be reduced.

  In the present embodiment, the control unit is configured using a CPU, but is not limited thereto, and may be configured with a logic IC. Further, the outside air temperature sensor may not be provided, and the liquid level sensor and the present liquid level detection device may be integrated.

  In this embodiment, the power supply to the water level detection circuit is cut off. However, the present invention is not limited to this, and the power supply to the AC voltage generation circuit may be cut off, You may make it interrupt | block the power supply to a surface sensor.

  In this embodiment, the thermistor is used to detect the non-conductive liquid, but it goes without saying that if the stored liquid is a conductive liquid, the electrode can be detected alone.

  In this embodiment, the power cutoff unit monitors the state of the AC signal output from the CPU and controls the power cutoff. However, the present invention is not limited to this, and the AC voltage applied to the liquid level sensor is not limited thereto. May be monitored. In this case, since the power cutoff unit can shut off the power even when the AC voltage generation circuit fails, the fault monitoring capability is higher than when monitoring the state of the AC signal output from the CPU.

It is a block diagram which shows the Example of the liquid level detection apparatus by this invention, and the water storage tank of a detection target. It is a circuit diagram which shows an example of the power-supply-cutoff part of the liquid level detection apparatus by this invention. It is a block diagram which shows the liquid level control apparatus which adjusts a water level with the detection information of the conventional liquid level detection apparatus. It is sectional drawing of the conventional liquid level sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Liquid level detection apparatus 2 Power supply interruption | blocking part 3 Power supply input terminal 4 AC signal input terminal 5 Capacitor 6 Diode 7 Diode 8 Capacitor 9 Resistance 10 Resistance 11 Transistor 12 Resistance 13 Resistance 14 Transistor 15 AC signal interruption detection circuit 16 Switch circuit 17 Power supply output Terminal 31 External body 31a Screw part 32 Terminal block 33 Sealing glass 34 Filler 35 Electrode 35 Electrode 37 Thermistor 38 Lead wire 39 Lead wire 39a Each crimp terminal 50 Controller 50 'Controller 51 Power supply 52 Water level detection circuit 53 AD conversion 54 AD converter 55 CPU
56 Oscillator 57 ROM
58 RAM
59 Interface 61 Container 62 Liquid 62a Liquid level 63 Liquid level sensor 64 Gas 66 External temperature sensor 67 AC voltage generation circuit 68 Liquid supply electromagnetic valve 69 Liquid discharge electromagnetic valve 70 AC voltage generation circuit 71 Switching transistor 72 Inversion transistor 100 Liquid level Control device

Claims (3)

An AC voltage generator that supplies the AC voltage to a liquid level sensor driven by an AC voltage, a water level detection circuit that monitors the voltage of the liquid level sensor via the AC voltage generator, and the AC voltage generator. A control unit that controls the water level detection circuit; and a power supply unit that supplies power to the liquid level sensor, the AC voltage generation unit, and the water level detection circuit, and the AC voltage generation unit includes: In the liquid level detection device that generates the AC voltage based on the output signal,
A liquid level detection device comprising a power cutoff unit that cuts off power supply from the power source to the liquid level sensor when the AC voltage is stopped.   The power cut-off unit includes an AC signal cut-off detection circuit that detects generation or stop of the AC voltage, and a power source of the power supply unit when the detection result of the AC signal cut-off detection circuit is that the AC voltage is generated. 2. The liquid according to claim 1, further comprising: a switch circuit that shuts off a power source of the power source unit when the detection result of the AC signal cutoff detection circuit is a stop of the AC voltage. Surface detection device.   The liquid level detection device according to claim 1, wherein the power cutoff unit supplies power to the water level detection circuit and the AC voltage generation circuit together with the liquid level sensor.

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