JPH1114433A - Object detector and liquid level detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid level detector which can detect the surface level of a liquid without receiving any influence from the ambient temperature. SOLUTION: In a liquid level detector in which high-frequency signals from a high-frequency oscillating section 2 are supplied to a sensor section 1 and a reflected wave sensor section 3 detects reflected waves from the oscillation of the magnetic field which is generated from a core wound with the detecting coil 11 of the sensor section 1 and varies when the surface level of a liquid passes, and then, a signal processing section 7 discriminates the passing of the surface level of the liquid from the quantity of the reflected waves, the thermistor of a temperature detecting section 6 is provided in the core so as to measure the ambient temperature and correct the detected quantity of reflected waves in accordance with the ambient temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an object detector and a liquid level detector using a sensor including a resonance circuit including a detection coil wound around a core and a capacitor.

[0002]

2. Description of the Related Art Conventionally, in order to detect, for example, a liquid level in an opaque container, as shown in FIG. 1, a sensor unit 1 including a resonance circuit comprising a detection coil wound around a core and a capacitor.
, An oscillator 2 that generates a high-frequency signal and transmits it to the sensor unit 1, a reflected wave sensor 3 that detects a reflected wave reflected from the sensor unit 1, and detects a liquid level based on a detection signal from the reflected wave sensor 3. A liquid level detector provided with a signal processing unit 7 that performs the processing is used. When detecting the liquid level, the detection coil of the sensor unit 1 is arranged in the vicinity of the container 10, and when the object in the area through which the magnetic field of the detection coil passes due to the change of the liquid level changes from liquid to air, the sensor unit Since the impedance of the first resonance circuit changes and the level of the reflected wave changes, the signal processing unit 7 detects the liquid level based on the voltage change.

[0003]

In the above-described conventional liquid level detector, the electronic components constituting the resonance circuit have temperature characteristics, and the resonance of the LC circuit of the liquid level detection unit is caused by a change in the temperature of the component or the surrounding environment. Since the frequency changes, the amount of reflected waves obtained by comparison at an arbitrary fixed frequency also changes according to the temperature. For this reason, the change in the liquid level to be detected is such that the reflection level level value to be obtained by the detection substance has a temperature characteristic, and the liquid level change cannot be detected as an absolute level value. The liquid level is detected by the relative change of the wave amount. In this method, it is necessary to set an initial value such as whether or not there is a liquid as an initial detection value when the power is turned on, and there is a possibility that an erroneous determination may be made depending on a use situation.

The present invention has been made in view of the above problems, and has as its object to provide a liquid level detector capable of detecting a liquid level with high accuracy without being affected by ambient temperature. .

[0005]

An object detector according to the present invention detects a reflected wave amount by giving a high frequency signal of a predetermined frequency to a high frequency resonance circuit comprising a detection coil and a capacitor wound around a core, In a method for determining the presence or absence of an object based on the amount of reflected waves that vary depending on whether or not an object is present in a magnetic field emitted from the detection coil, a temperature sensing element is provided at or near the core of the detection coil. A temperature correction unit is provided for correcting a change in the amount of reflected waves due to a temperature drift based on an ambient temperature measured by the temperature element.

Further, the liquid level detector of the present invention detects a reflected wave amount by giving a high frequency signal of a predetermined frequency to a high frequency resonance circuit comprising a detection coil wound around a core and a capacitor. In a device for determining a liquid level based on an amount of reflected wave that varies depending on a liquid surface passing through a magnetic field generated, a temperature-sensitive element disposed at or near a core of the detection coil, and an ambient temperature output from the temperature-sensitive element. Temperature correction means for correcting a change in the amount of reflected waves due to a temperature drift with a signal corresponding to the temperature.

In this object detector and liquid level detector, even if the ambient temperature changes, the reflected wave amount is corrected according to the change, and the presence or absence of an object or the liquid level is detected based on the corrected reflected wave amount. I do. Therefore, the presence or absence of the object or the liquid level can be detected without being affected by the ambient temperature.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to embodiments. FIG. 2 is a circuit block diagram showing a configuration of the liquid level detector according to one embodiment of the present invention. The liquid level detector of this embodiment includes a detection circuit section 5, a temperature detection section 6, and a signal processing section 7.

The detection circuit section 5 includes a sensor section 1, an oscillating section 2 for transmitting a high-frequency signal to the sensor section 1 via a transmission section 4, and a high-frequency signal supplied to the sensor section 1.
And a reflected wave sensor unit 3 for detecting a reflected signal which is reflected back to the oscillation unit 2 side. The reflected wave sensor 3 and the sensor unit 1 as an example of the detection unit are composed of only passive elements and do not include active elements.

The sensor section 1 has a detection coil 11 wound around a core 11a, a resonance capacitor 12 forming a series resonance circuit with the detection coil 11, and a primary side connected to an input side resonance circuit. , Secondary side is high frequency input terminal 1
4 and a transformer 13 for the real number connected to the ground GND. Here, the oscillation unit 2 employs an oscillation circuit using a crystal oscillator.
As long as it is a known high-frequency oscillation circuit, another circuit may be used. For example, an LC oscillator or a PLL may be used.
Here, the output frequency of the oscillator 2 is 40.68 MHz, but 10 MHz to 300 MHz is appropriate for detecting the non-magnetic material and keeping the device small.

The reflected wave sensor unit 3 includes a directional coupler, detects a reflected wave from the sensor unit 1 as electric power, and converts it into a voltage. That is, the capacitor 31 connected to the transmission line 4 and the resistor 3 connected to one end of this capacitor
2 is a parallel circuit of the coil 33 and the coil 33 is M-coupled to the transmission path 4, the other end of the parallel circuit is connected to the anode of the diode 34, and the cathode of the diode 34 is connected to GND via the capacitor 35. , An analog output signal from the cathode, that is, a reflected wave converted into a voltage is derived. Although the reflected wave sensor used here uses the CM-coupled sensor as described above, other sensors such as those using the MM coupling method may be used.

The signal processing unit 7 includes an A / D conversion unit that converts an analog signal voltage from the reflected wave sensor unit 3 into a digital signal, and an A / D conversion unit that converts an analog signal from the temperature detection unit 7 into a digital signal. And a CPU for performing liquid level passage detection processing based on the taken-in reflected wave signal and temperature signal. The temperature detection unit 6 includes a temperature sensing element such as a thermistor for detecting the ambient temperature, and a temperature detection circuit for outputting a temperature signal corresponding to the detected temperature.

Each circuit of the sensor section 1, the oscillation section 2, the reflected wave sensor section 3, the temperature detection section 6, and the signal processing section 7 is shown in FIG.
Is mounted on the printed circuit 8 shown in FIG. 1 and the gap of the C-shaped toroidal core 11a around which the detection coil 11 is wound is fitted into the liquid container 10. Further, a temperature sensing element 61 is provided above the toroidal core 11a.

In this embodiment, a high-frequency signal is supplied from the oscillating unit 2 to the sensor unit 1 via the transmission line 4. Now, for example, the resonance frequency of the resonance circuit is set so as to match the frequency of the oscillating unit 2, and when the detection liquid is not present, the impedance of the sensor unit 1 (here, the imaginary part is zero and 50Ω only in the mounting unit) ) And the impedance of the transmission path 4 are matched, the reflection of the transmitted high-frequency signal is almost zero, and the output from the reflected wave sensor 3 is zero.

If a liquid is present, the sensor unit 1 is magnetically affected by the difference in the magnetic permeability between air and the object.
The impedance changes. When the impedance of the sensor unit 1 is greatly deviated from the matching impedance, the reflected signal also becomes large, the reflected signal is detected by the reflection sensor unit 3, and a reflected wave, that is, an analog signal corresponding to a change in impedance is output. The signal processing unit 7 can detect the passage of the liquid level based on a change in the signal voltage from the reflected wave sensor unit 3. When the ambient temperature changes, the characteristics of the resonance circuit of the sensor unit 1 change, and the amount of reflection also changes. Therefore, the signal processing unit 7 uses the signal corresponding to the ambient temperature input from the temperature detection unit 6 to reflect the signal. Correct the amount.

FIG. 4 shows a frequency characteristic of a liquid level detector for detecting a liquid level based on the magnitude of a reflected wave. Here, the temperature characteristics at the time of adjustment, the frequency characteristics at the time of object detection at high temperature and low temperature, and the frequency characteristics at the time of adjustment at the temperature, high temperature and low temperature without object detection are shown. In FIG. 4, (b) is a reflected wave level at the time of object detection at the temperature at the time of adjustment, (a) is a reflected wave level at the time of object detection at a high temperature, and (c) is a reflected wave level at object detection at a low temperature. The reflected wave level at the time of object detection varies in the range of (a) to (c) depending on the temperature at the time of detection. Unless the variation in the reflected wave amount due to the temperature is corrected, it cannot be determined whether the difference in the reflected wave amount is due to a temperature change, a difference in a detected substance, or a passage through a liquid surface. Therefore, in each embodiment of the present invention,
The reflection amount is corrected based on the temperature.

FIG. 5 is a block diagram showing a functional configuration of the signal processing unit 7 of the liquid level detector according to the embodiment. In FIG.
A / D value 72 of reflected wave input from A / D converter 71
And the temperature A / D value 74 input from the A / D converter 73
Is input to the correction table 75, the correction value is read from the correction table 75 with reference to these, the addition or subtraction is performed on the reflected wave A / D value 72, and the corrected A
/ D value 76 is compared with the liquid level passage determination threshold 77,
A determination output 78 indicating the presence / absence of liquid level passage is output. FIG. 6 shows an example of a correction table for subtraction, and FIG. 7 shows an example of a correction table for multiplication. Each data of the correction table 75 is stored in a ROM.
Is stored in 6, the temperature A / D value is an A / D conversion value of the temperature read into the CPU of the signal processing unit 7, and the reflected wave A / D value is the value of the reflected wave read into the CPU of the signal processing unit 7. This is an A / D conversion value.

For the correction, instead of a table, the relationship between the temperature and the amount of reflection may be stored in the signal processing unit 7 in an approximate expression, and the approximate calculation may be performed by inputting the temperature and the amount of reflection. Here, the temperature correction by the thermistor will be described. First, there is a case where an approximate calculation is performed. Thermistor resistance R
The temperature characteristic of _T is given by the following equation.

[0019]

(Equation 1)

Here, B = 3950K, R ₃₇ = 49.8
Using a 51 KΩ thermistor, the output voltage V _{0 of the} circuit shown in FIG.

[0021]

(Equation 2)

FIG. 9 is a graph showing the temperature characteristics of the output voltage. Next, the result of measuring the reflected wave temperature characteristic (without a detection object) when the reflected wave is adjusted so as to minimize the reflected wave at −10 ° C. (example of resonance frequency: 40.68 MHz) is obtained by the approximate expression V _T = a · exp {b (T + c)} a = 0.024334, b = 0.05205, c = 15 and the graph of FIG. 10 is obtained.

As described above, in both cases, the temperature characteristic of the voltage can be represented by an exponential function, and approximation is relatively easy. When the output voltage temperature characteristic (approximate calculated value) of the reflected wave detection circuit was approximated by the temperature sensor output temperature characteristic V ₀ (calculated value), the approximate expression could be given as follows. V ₀ ′ = aV ₀ ² + b a = 0.065, b = 0.03 Also, when this approximation was performed using the measured data for both the reflected wave and the temperature, a = 0.057, b = 0.03 became.

This characteristic graph is shown in graph 2 of FIG. Assuming that the level of the reflected wave that changes due to the movement of the LC circuit resonance point at the time of object detection is on the same characteristic curve as the fluctuation characteristic due to temperature, and that the characteristic near the level at the time of detection is linear, The correction can be performed only by subtracting the correction amount obtained by the above equation as an offset.

Next, a method of creating a correction amount table will be described. The relationship between the reflected wave detection circuit output V _T with respect to the temperature detection output _{V 0, V T = aV 0} 3 + bV 0 2 + c a = 0.00000085, b = 0.00068, c =
Assuming that the approximation is 0.19, the correction table is
-10 ° change in output when an object is detected at any temperature
It is a subtraction table for converting into an output to be obtained when detected at C (adjusted temperature).

The correction value was obtained as follows. (Correction value) = {f (T2 + ΔT) −f (T2)} − ｛f (T1 +
ΔT) −f (T1)｝ Function f (T): The above approximate expression f (T), f (T2): Temperature T1 (−10 ° C.) °, detection output at no temperature obtained at temperature T2 f (T2 + ΔT): Object detection value obtained at temperature T2 f (T1 + ΔT): Object detection value to be obtained at temperature T1 (−10 ° C.) ΔT: Detection output fluctuation obtained by object detection is replaced by fluctuation due to temperature FIG. 11 is a block diagram showing another example of a functional circuit of the signal processing unit 7. The signal processing unit 7 determines the default value of the correction table in the adjustment, that is, how much the temperature of the reflected wave output at the temperature at the time of writing in the nonvolatile memory corresponds to the temperature change from the adjustment. .

The adjustment performed here will be further described. The correction table is stored in the ROM of the CPU in the relation between the temperature value and the reflected wave A / D value as shown in FIG.
Now, the reflected wave output to be adjusted at a temperature T ₀ V _0, V ₁ a reflected wave output at a temperature T ₀ of the unregulated condition, the reflected wave output when adjusted to V ₀ at a temperature T ₀ The reflected wave output is V ₁
Assuming that the temperature at which the temperature changes becomes T ₁ , the reflected wave output when not adjusted at an arbitrary temperature T is the temperature when the temperature changes by T ₁ −T ₀ from the reflected wave output when adjusted. This corresponds to the reflected wave output. Therefore, T ₁ −T ₀ is obtained from the reflected wave output obtained at the temperature T _{0 in the} unadjusted state,
The value is stored in a non-volatile memory, and when the table of FIG. 12 is read at a temperature obtained by adding T ₁ −T ₀ to the temperature value detected at the time of the detection operation, the same table as the case where the temperature is adjusted is obtained. A correction value can be obtained. According to this method, manual work such as volume adjustment at the time of product adjustment can be omitted, and the efficiency of adjustment automation can be increased. In FIG. 12, the temperature value is the temperature A read by the CPU of the signal processing unit.
This is the value calculated and converted from the / D conversion value to the temperature value. The calculation here may be either a calculation using a mathematical expression or reading from a relationship table between the temperature and the temperature A / D value. Unlike FIG. 6, the reason for using the temperature value in FIG. 12 is that the temperature A / D value does not have a linear characteristic with respect to the temperature when a thermistor is used for the temperature sensor. This is because if it is taken along the axis of the table, it will not be possible to read the table value by shifting the axis of the table by the temperature correction change.

FIG. 13 is a block diagram showing the configuration of a liquid level detector according to another embodiment of the present invention. The temperature detector of this embodiment also includes a detection circuit section 5, a temperature detection section 6, and a signal processing section 7. The detection circuit unit 5 includes an oscillator 51
, An amplifier 52, a low-pass filter 53, a reflected wave sensor 54, a matching device 55, and a detection coil wound around a C-shaped core 57. The matching unit 55 and the detection coil 57 correspond to the sensor unit in FIG. The temperature detecting section 6 includes a temperature sensing element 61 and a temperature detecting section 62. The signal processing unit 7 includes an amplifier 81 that amplifies the detected reflected wave signal and an amplifier 8 that amplifies the detected temperature signal.
2, a CPU 83 including an A / D converter for converting the input reflected wave signal and temperature signal into digital values, and an output transistor 84. In the CPU 83,
It has a correction table and also stores correction values corresponding to the ambient temperature. The correction operation is not particularly different from that of FIG.

FIG. 14 is a block diagram showing the structure of a liquid level detector according to still another embodiment of the present invention. 13 is different from that of FIG. 13 in that the core 1a has a ring shape. In this embodiment, when liquid is present, the magnetic flux generated in the core 1a by the detection coil 1 leaks into the liquid. In addition, the leakage magnetic flux becomes large and the amount of reflection is large. When the liquid level passes and there is no liquid, most of the magnetic flux passes through the ring because of low magnetic permeability, and the amount of reflection decreases. Therefore, passage of the liquid surface can be detected based on a change in the amount of reflection. Further, the same temperature correction can be performed on an object detector that detects an object by sweeping the frequency of a high-frequency signal provided from the oscillation unit to the sensor unit and determining the amount of reflection at the time of resonance.

In the above embodiment, the liquid level detector has been described. However, the present invention is not limited to this liquid level detection.
It can also be applied to the detection of powder surfaces and the like.

[0031]

According to the present invention, the ambient temperature of the core is detected and the amount of reflection is corrected in accordance with the ambient temperature. Therefore, the amount of reflected waves obtained from the object to be detected depends on the temperature change. Therefore, the state of the detection target can be determined by only one measurement without monitoring the amount of change. Therefore, the initial value setting required for the relative change measurement of the reflected wave becomes unnecessary, and the state determination immediately after the power is turned on can be accurately performed.

Further, since the absolute level of the detection level can be detected, it is possible to determine the material, state, property, and the like of the substance to be detected.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a detection principle of a liquid level detector.

FIG. 2 is a circuit diagram showing a configuration of a liquid level detector according to one embodiment of the present invention.

FIG. 3 is a diagram illustrating an installation state of a temperature sensing element and an arrangement state of a core of the liquid level detector according to the embodiment.

FIG. 4 is a diagram for explaining a difference in frequency characteristics between when the temperature is high and when the temperature is low when the liquid level detector does not detect an object when detecting the object.

FIG. 5 is a block diagram illustrating a functional configuration of a signal processing unit of the detector according to the embodiment.

FIG. 6 is a diagram illustrating an example of a correction table of the detector according to the embodiment.

FIG. 7 is a diagram showing another example of the correction table of the detector according to the embodiment.

FIG. 8 is a circuit connection diagram showing a temperature sensing element and a temperature detection circuit.

FIG. 9 is a diagram showing a relationship between a temperature and an output voltage in the temperature detection circuit.

FIG. 10 is a diagram for explaining the relationship between temperature and output voltage by approximation calculation.

FIG. 11 is a block diagram showing another example of the function of the signal processing unit of the detector according to the embodiment.

FIG. 12 is a diagram illustrating an example of a correction table of the signal processing unit.

FIG. 13 is a block diagram showing a liquid level detector according to another embodiment of the present invention.

FIG. 14 is a block diagram showing a liquid level detector according to still another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sensor part 2 Oscillation part 3 Reflection wave sensor part 6 Temperature detection part 7 Signal processing part

Claims (9)

[Claims] 1. A high frequency signal of a predetermined frequency is applied to a high frequency resonance circuit comprising a detection coil and a capacitor wound around a core to detect an amount of reflected waves, and an object exists in a magnetic field generated by the detection coil. In an object detector that determines the presence or absence of an object with a reflected wave amount that varies depending on whether or not, a temperature-sensitive element is provided at or near the core of the detection coil, and the ambient temperature measured by the temperature-sensitive element is: An object detector, comprising: a temperature correction unit that corrects a change in the amount of reflected waves due to a temperature drift. 2. A liquid surface passing through a magnetic field generated by a detection coil by applying a high frequency signal of a predetermined frequency to a high frequency resonance circuit including a detection coil and a capacitor wound around a core to detect a reflected wave amount. In the liquid level detector that determines the liquid level with the reflected wave amount that changes according to the following, a temperature-sensitive element is provided at or near the core of the detection coil, and a temperature drift is caused by an ambient temperature measured by the temperature-sensitive element. A liquid level detector comprising temperature correction means for correcting a change in the amount of reflected waves. 3. The liquid level detector according to claim 2, wherein said temperature correction means is a correction table for storing a correction value corresponding to temperature information obtained by said temperature sensing element. 4. The liquid level detector according to claim 3, wherein the correction table is an addition / subtraction table for converting a reflected wave detection amount at the time of detecting an object into a value to be obtained when the temperature is at an arbitrary point. 5. The correction table according to claim 1, wherein the table correction value for the detected temperature information is a multiplication coefficient table of a ratio to a change to be obtained when a change in the reflected wave detection amount of the object detector is at an arbitrary temperature. The liquid level detector according to claim 3. 6. An apparatus according to claim 2, wherein said temperature correction means obtains, by an approximate calculation formula, that the amount of reflected wave detected by the object detector changes with temperature from temperature information obtained by the temperature sensing element. Liquid level detector. 7. A liquid surface which passes a high-frequency signal having a variable frequency to a high-frequency resonance circuit comprising a detection coil and a capacitor wound around a core and which passes through a magnetic field generated by the detection coil having an oscillation frequency characteristic of a reflected wave. A temperature sensor, such as a thermistor, is attached to or near the core of the detection coil in a liquid level detector that determines the passage of the liquid level based on the change due to the temperature. A liquid level detector comprising temperature correction means for correcting a change in the amount. 8. The liquid level detector according to claim 7, wherein the temperature correction means corrects the temperature using a correction table of a resonance point frequency and a resonance point reflection coefficient which change with temperature. 9. The liquid level detector according to claim 8, wherein the data is written in a non-volatile memory at the time of adjusting the frequency characteristic of the reflected wave amount obtained at the time of production adjustment, and is used as a comparison reference at the time of correction.