JP4083038B2 - Ultrasonic level meter and liquid level detection method using the level meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic level meter and a liquid level detection method using the level meter, more specifically, an ultrasonic level for detecting the liquid level of contents such as liquefied gas contained in a container from the outside of the container. The present invention relates to a meter and a liquid level detection method using the level meter.
[0002]
[Prior art]
An ultrasonic level meter that uses an ultrasonic sensor for both transmission and reception measures the time until an ultrasonic wave is transmitted from the ultrasonic sensor and the reflected wave is received, and the distance to the object is calculated based on that time. Therefore, since the distance to the object can be measured accurately and easily, it is used for various purposes. Since this ultrasonic level meter has different pass frequency bands depending on the thickness of the container or storage tank (hereinafter referred to as the container), several types of ultrasonic sensors with different applicable frequency ranges are prepared. Although an ultrasonic sensor suitable for the range was selected, it would be difficult to obtain a mass production effect and to reduce the cost because it would be a variety of products. Therefore, it is considered possible to avoid a wide variety of products by using an ultrasonic sensor with a wide adaptive frequency range to expand the applicable container thickness range.
[0003]
However, S / N deterioration occurs as a side effect due to the wide band of the ultrasonic sensor, and stable liquid level measurement cannot be performed. Depending on the plate thickness, especially when the plate thickness increases, the S / N decreases in the direction in which the reverberant wave level becomes more dominant than the reflected echo level, so that simple gain control is sufficient. S / N cannot be secured.
[0004]
Conventionally, as an object detection method using an ultrasonic sensor, when detecting an object with a probe attached to the outer part of the container, it is used for any container with any thickness and material. There is provided an apparatus that can provide a versatile ultrasonic sensor that can be used. This is done by inputting the emission waveform and reflection waveform data from the probe and detecting the resonance frequency between the piezoelectric element of the probe and the container based on the difference in the attenuation characteristics of the waveform. The resonance frequency is registered as the operation frequency. (For example, see Patent Document 1)
[0005]
[Patent Document 1]
JP 2000-213979 A
[0006]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and even when an ultrasonic sensor having a wide frequency band that can be applied to a wide range of plate thicknesses is used, the band of the band-pass filter is switched. / N is not reduced, and stable liquid level detection is possible. Further, in addition to the band switching of the bandpass filter, the amplification factor of the signal increases with time starting from the driving timing of the ultrasonic sensor. In addition, by changing the oscillation time of the pulse signal input to the ultrasonic sensor, the dead zone time (distance) that cannot be detected due to reverberation is adjusted, so that stable reverberation characteristics and sensitivity characteristics can always be realized. It is possible to apply to a wide range of plate thicknesses, and it is possible to measure the liquid level from low to high liquid level position. There is provided a liquid level detection method using the wave level meter and ultrasonic level meter.
[0007]
[Means for Solving the Problems]
The invention of claim 1 is an ultrasonic sensor that is attached to the outer wall surface of the bottom of the container and that transmits ultrasonic waves toward the liquid surface of the liquid contained in the container and receives reflected waves from the liquid surface. In the ultrasonic level meter connected to the control unit for controlling the detection operation of the liquid level based on time information from the time when the ultrasonic wave is transmitted by the ultrasonic sensor to the time when the reflected wave is received, The ultrasonic sensor transmits ultrasonic waves based on a pulse signal input from the control unit, The controller is The ultrasonic propagation time from when the ultrasonic sensor is driven by the input of the pulse signal to when the reflected echo from the liquid surface is received is measured, and is detected immediately after the ultrasonic sensor is driven from the driving start time. Reverberation margin detection means for measuring a drive reverberation time until the reverberation of the ultrasonic wave is reduced to a predetermined level, and detecting a difference between the measured ultrasonic propagation time and the drive reverberation time as a reverberation margin; A signal corresponding to the reflected wave with respect to the ultrasonic wave transmitted by the ultrasonic sensor is input. The From the frequency component of the input signal Permeates the bottom plate of the container Selectively extract transmission frequency components put out A variable bandwidth filter, an amplification means for amplifying a signal extracted by the bandwidth variable filter at a predetermined amplification rate, and a reflected echo signal from the liquid surface among signals amplified based on the amplification rate of the amplification means. Echo timing detection means for detecting the echo timing by specifying, and liquid level detection means for detecting the position of the liquid level based on the detected echo timing. When detecting the position of the liquid level, the reverberation margin detection unit detects a reverberation margin based on a predetermined amplification factor when the ultrasonic sensor is driven, and the detected reverberation margin is equal to or less than a predetermined value. In this case, the control unit switches the frequency band of the band-variable filter in accordance with the frequency component of the reverberation at the container bottom plate, the detected reverberation margin becomes larger than the predetermined value, and from the liquid level. Adjust the frequency band to identify the reflected echo signal It is characterized by doing.
[0011]
Claim 2 The invention of claim 1 In the invention, the amplifying means changes the amplification factor of the signal extracted by the band-variable filter to the position of the liquid level by changing the amplification factor so as to increase with time starting from the time of driving the ultrasonic sensor. Set amplification factor according to Do It is characterized by that.
[0012]
Claim 3 The invention of claim 1 Or 2 In the invention, the control unit changes an oscillation time of the pulse signal by changing a pulse number or a pulse width of the pulse signal, and inputs the pulse signal to the ultrasonic sensor according to the changed oscillation time. Ultrasonic sensor driving means for transmitting ultrasonic waves while changing a driving time at a driving frequency corresponding to a pulse signal input from the ultrasonic sensor driving means. Do It is characterized by that.
[0013]
Claim 4 The invention of claim 3 In this invention, when detecting the position of the liquid level, the reverberation margin detecting means detects a reverberation margin based on a predetermined amplification factor when the ultrasonic sensor is driven, and the detected reverberation margin is a predetermined value. In the following cases, the amplification means But The amplification factor is reduced at the time of driving the ultrasonic sensor, the reduced amplification factor is changed so as to increase at a predetermined rate according to time, and the ultrasonic sensor driving means But While changing the oscillation time of the pulse signal input to the ultrasonic sensor, and further switching the frequency band of the variable band filter according to the frequency component of the reverberation in the container bottom plate, the detected reverberation margin is the predetermined value Bigger And any one or more of the amplification factor, oscillation time, and frequency band are adjusted so that the reflected echo signal from the liquid surface can be specified.
[0014]
Claim 5 The invention of claim 3 In the invention, when detecting the liquid level position in the container, the ultrasonic sensor driving means transmits ultrasonic waves from the ultrasonic sensor a plurality of times while changing the oscillation frequency, and the control unit From ultrasonic sensor By analyzing the state of reverberation in the container bottom plate for each transmitted ultrasonic wave, a resonance frequency suitable for transmission of ultrasonic waves on the bottom outer wall surface of the container is detected, and a drive frequency including the detected resonance frequency Pulse signal for transmitting ultrasonic waves in the band is input to the ultrasonic sensor Do It is characterized by that.
[0015]
Claim 6 The invention of claim 5 In the invention, the control unit extracts a frequency component including the resonance frequency from a frequency component of the signal input from the ultrasonic sensor by switching a frequency band of the band variable filter. Do It is characterized by that.
[0016]
Claim 7 The invention includes an ultrasonic sensor that is attached to a bottom outer wall surface of a container, transmits ultrasonic waves toward a liquid surface of the liquid contained in the container, and receives reflected waves from the liquid surface; Liquid level detection using an ultrasonic level meter connected to a control unit that controls the liquid level detection operation based on time information from the time when ultrasonic waves are transmitted by the ultrasonic sensor to the time when the reflected waves are received. In the method The ultrasonic level meter transmits an ultrasonic wave from the ultrasonic sensor based on a pulse signal input from the control unit, and the liquid level from a driving start time of the ultrasonic sensor by the input of the pulse signal. Measures the ultrasonic propagation time until the reflected echo from the receiver is received, and measures the drive reverberation time until the ultrasonic reverberation detected immediately after the start of driving of the ultrasonic sensor is reduced to a predetermined level. Detecting a difference between the measured ultrasonic propagation time and the drive reverberation time as a reverberation margin; A signal corresponding to the reflected wave with respect to the ultrasonic wave transmitted by the ultrasonic sensor is input. The From the frequency component of the input signal Permeates the bottom plate of the container An extraction step for selectively extracting a transmission frequency component, an amplification step for amplifying the extracted signal at a predetermined amplification factor, and an echo timing by identifying a reflected echo signal from the liquid surface from the amplified signal An echo timing detecting step for detecting the liquid level, and a liquid level detecting step for detecting the position of the liquid level based on the detected echo timing. When detecting the position of the liquid level, a reverberation margin is detected based on a predetermined amplification factor when the ultrasonic sensor is driven, and when the detected reverberation margin is a predetermined value or less, While switching the frequency band of the band variable filter according to the frequency component of reverberation, the frequency band is set so that the detected reverberation margin becomes larger than the predetermined value and the reflected echo signal from the liquid surface can be specified. Adjustment It is characterized by doing.
[0020]
Claim 8 The invention of claim 7 In this invention, the amplification factor according to the position of the liquid surface is changed by changing the amplification factor of the signal extracted by the band-variable filter so as to increase with time starting from the time when the ultrasonic sensor is driven. Setting Do It is characterized by that.
[0021]
Claim 9 The invention of claim 7 or 8 In the invention, the oscillation time of the pulse signal is changed by changing the number of pulses or the pulse width of the pulse signal, and the pulse signal is input to the ultrasonic sensor according to the changed oscillation time. Sending ultrasonic waves from the ultrasonic sensor while changing the driving time at the driving frequency according to the pulse signal Do It is characterized by that.
[0022]
Claim 10 The invention of claim 9 In this invention, when detecting the position of the liquid level, a reverberation margin is detected based on a predetermined amplification factor when the ultrasonic sensor is driven, and the ultrasonic wave is detected when the detected reverberation margin is less than or equal to a predetermined value. Decreasing the amplification factor at the time of driving the sensor, changing the reduced amplification factor to increase at a predetermined rate according to time, and changing the oscillation time of the pulse signal input to the ultrasonic sensor, Furthermore, the detected reverberation margin is the predetermined value while switching the frequency band of the band variable filter according to the frequency component of the reverberation at the container bottom plate. Bigger And any one or more of the amplification factor, oscillation time, and frequency band are adjusted so that the reflected echo signal from the liquid surface can be specified.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram for explaining a configuration example in which an ultrasonic level meter to which the present invention is applied is installed in a container, in which 10 is a controller (control unit), 20 is an ultrasonic sensor (piezoelectric sensor), 21 is an ultrasonic wave transmitted from the ultrasonic sensor 20, 17 is a display unit of the controller 10, 30 is a storage tank or container (hereinafter referred to as a container) for storing a liquid such as liquefied gas or kerosene, and 31 is a liquid level. . The ultrasonic level meter includes a controller 10 and an ultrasonic sensor 20, and the ultrasonic sensor 20 is installed on the outer bottom surface of the container 30 by a magnet (not shown) and connected to the controller 10 by wire. Note that the communication between the ultrasonic sensor 20 and the controller 10 may be performed wirelessly. In this case, it is possible to avoid malfunction due to wiring damage or intentional disconnection by a third party. Is possible. In this example, the case where the liquid level inside the container is detected as a detection target will be described as a representative example. However, the target that can be detected by the ultrasonic level meter is not limited to the liquid level. It can also be applied to the detection of objects.
[0024]
In the above configuration, for example, when detecting the liquid level 31 of the liquid, the operator operates the controller 10 to transmit the ultrasonic wave 21 from the ultrasonic sensor 20, and mainly at the liquid level 31. A reflected echo that has been reflected back is received, a liquid surface position, that is, a distance to the liquid surface 31 (hereinafter referred to as a liquid surface distance) is calculated based on the received reflected echo, and the calculated liquid surface distance is further calculated. Based on this, the liquid volume in the container 30 can be calculated.
[0025]
When detecting the height of the liquid surface 31, the ultrasonic sensor 20 transmits an ultrasonic wave upward toward the bottom wall of the container 30. The transmitted ultrasonic wave is reflected and returned by the liquid surface 31, but the delay time until the ultrasonic sensor 20 receives the reflected wave differs depending on the height of the liquid surface 31. The controller 10 holds data for calculating the liquid level distance from the delay time of the received reflected wave, for example, sound speed data according to temperature and propagation medium (air or liquid, etc.), and based on this data. The liquid level distance is calculated.
[0026]
Here, in the case where the bottom wall of the container 30 is, for example, an iron plate, when the ultrasonic sensor 20 is attached to the iron plate, reverberation occurs due to the influence of sound waves reflected at the iron plate boundary as the thickness of the iron plate increases. The characteristic becomes remarkable, and as a result, the minimum detection distance margin is lowered, so that it is difficult to detect a stable liquid level particularly on the short distance side. Moreover, since the frequency solution reflected at the boundary of the iron plate also increases and approaches the approach of the transmission frequency solution of the steel plate due to the increase in the plate thickness, the boundary with respect to the free vibration region due to the reverberation wave of the ultrasonic sensor is approached. The echoes reflected at the top of each other overlap, making it appear to have a large reverberant wave characteristic. The ultrasonic level meter according to the present invention is stable even when an ultrasonic sensor having a wide frequency band applicable to a wide range of plate thicknesses is used, without reducing the S / N by switching the band of the band pass filter. It is possible to detect the liquid level, to attenuate the reverberant wave on the short distance side where the liquid level is low, and to provide a band switching band-pass filter for obtaining an effect as a band filter in a region necessary as a received signal.
[0027]
Further, due to the acoustic coupling between the ultrasonic sensor 20 and the iron plate, the vibration state of the sensor vibration system including the iron plate, that is, the state of free vibration due to the reverberation wave, changes from the relationship between the sensor resonance frequency band and the transmission frequency spectrum of the iron plate. Resulting in. For this reason, when measuring the liquid level when the liquid level is low, the reflected wave from the liquid level may overlap the free vibration, that is, the reverberant wave. In such a case, a dead zone time (distance) in which the reflected wave cannot be captured and the liquid level distance cannot be measured occurs. The ultrasonic level meter of the present invention can measure the liquid level from when the liquid level is low to when it is high so that the sensitivity required to receive the reflected echo can be maintained while reducing the influence of the reverberant wave. To do.
[0028]
FIG. 2 is a diagram showing a circuit configuration example of the ultrasonic level meter of the present invention. The controller 10 of this ultrasonic level meter includes a receiving circuit 11, a variable band-pass filter (hereinafter referred to as a variable BPF) 12, an amplifying means 13, a detecting circuit 14, an echo timing detecting means 15, a liquid level detecting means 16, and a display. Unit 17, reverberation margin detection means 18, ultrasonic sensor driving means 19, and CPU (not shown). The ultrasonic sensor 20 transmits an ultrasonic wave based on a pulse signal input from the controller 10, and this pulse signal is preferably a rectangular wave pulse signal including all frequency components. Even when the thickness and material change, it is possible to cope. In the present embodiment, the receiving circuit 11 and the variable BPF 12 are illustrated separately, but the variable BPF 12 may be physically included in the receiving circuit 11.
[0029]
Hereinafter, the operation of the controller 10 will be described in detail.
The variable BPF 12 inputs a signal corresponding to the reflected wave with respect to the ultrasonic wave transmitted by the ultrasonic sensor 20, and switches the frequency band according to the transmission frequency component transmitted through the bottom plate of the container 30. The transmission frequency component can be selectively extracted from the frequency component. For example, in a state where the ultrasonic sensor 20 is attached to the bottom iron plate of the container 30, constants for obtaining an attenuation effect due to the transient response of the LC filter shown in FIG. By appropriately switching the filters, the reverberation characteristics and the reflection sensitivity characteristics are compatible with each other over the entire frequency band corresponding to the plate thickness within a predetermined range.
[0030]
FIG. 3 is a diagram showing a circuit configuration example of the variable BPF 12 corresponding to a predetermined thickness range to which the present invention is applied. The variable BPF 12 has a switching unit 12a. The variable BPF 12 of this embodiment has three types of frequency bands corresponding to a plate thickness of about 4.5 mm to 15 mm: 650 to 850 kHz (filter A), 800 to 1050 kHz (filter B), and 1000 to 1300 kHz (filter C). It is assumed that it is assigned to and used as appropriate. In this case, wideband characteristics can be realized without damaging the band by using a photo moss with a capacitance value of 100 P or less when OFF in the switching unit 12a. Also, by simultaneously turning on the photo moss, depending on the capacitance value, it is possible to design a higher-order band, and it is possible to realize a wider band by combination.
[0031]
As described above, the variable BPF 12 assigns constants capable of obtaining an attenuation effect due to the transient response of the LC filter in three types of frequency bands, and achieves both reverberation characteristics and reflection sensitivity characteristics for all frequency bands according to the plate thickness. Can be made. Based on the bottom plate thickness of the container 30, the controller 10 sets the variable BPF 12 to a band where the reverberation level is maximum due to the relationship between the resonance frequency of the ultrasonic sensor 20 and the plate thickness natural frequency that is actually transmitted. Switch to make a significant damping effect. Further, based on the range of the plate thickness and the absolute value level of reverberation, the switching of the variable BPF 12 is suppressed to the minimum number and combined. In the case of the above plate thickness range (about 4.5 to 15 mm), the switching is made in a minimum of three steps. Thus, high speed stability can be improved by minimizing the number of switching.
[0032]
The amplifying unit 13 amplifies a signal corresponding to the frequency component extracted by the variable BPF 12 with a predetermined amplification factor. The echo timing detection means 15 identifies the reflected echo signal from the liquid surface 31 based on the amplification factor set in the amplification means 13 and detects the echo timing. The liquid level detection unit 16 detects the position of the liquid level 31 based on the echo timing detected by the echo timing detection unit 15.
[0033]
The reverberation margin detection means 18 measures the ultrasonic propagation time from the start of driving the ultrasonic sensor 20 to the reception of the reflected echo from the liquid surface 31 by the input of a pulse signal, and the start of driving of the ultrasonic sensor 20. The reverberation of ultrasonic waves detected immediately after driving, that is, the reverberation due to the reflected wave at the bottom plate of the container 30, or the reverberation due to the self-vibration due to the inertia after the driving is stopped, etc. The drive reverberation time until reduction is measured, and the difference between the measured ultrasonic wave propagation time and the drive reverberation time is detected as a reverberation margin.
[0034]
The ultrasonic sensor driving means 19 changes the oscillation time of the pulse signal by changing the number of pulses or the pulse width of the pulse signal, and inputs the pulse signal to the ultrasonic sensor 20 according to the changed oscillation time. Can do. The ultrasonic sensor 20 can transmit ultrasonic waves while changing the driving time at a driving frequency corresponding to the pulse signal input from the ultrasonic sensor driving means 19. In this case, the ultrasonic sensor driving means 19 generates a pulse signal corresponding to each oscillation frequency within a range of the oscillation frequency including the resonance frequency of the container 30 and the ultrasonic sensor 20 from, for example, 50 μs to a maximum of 100 μm every 5 μs. It may be inputted to the ultrasonic sensor 20.
[0035]
Here, in order to improve the transmittance of the ultrasonic wave with respect to the plate thickness of the container 30, the drive frequency of the ultrasonic wave transmitted from the ultrasonic sensor 20 is optimized, that is, resonance according to the plate thickness and material of the container 30. It is necessary to set so that the ultrasonic sensor 20 is driven at a frequency. Usually, the ultrasonic sensor 20 and the drive frequency are customized in accordance with the container 30 so as to optimally drive at the resonance frequency. However, when the ultrasonic sensor 20 is variably driven and attached to the container, the resonance frequency between the container 30 and the ultrasonic sensor 20 is obtained, and the resonance frequency is used as the drive frequency of the ultrasonic sensor 20. You may make it set as. In this case, the ultrasonic sensor driving means 19 transmits the ultrasonic wave from the ultrasonic sensor 20 a plurality of times while changing the oscillation frequency of the pulse signal, and analyzes the reverberation state of each transmitted ultrasonic wave, thereby A resonance frequency suitable for transmitting ultrasonic waves through the bottom outer wall surface 30 is detected, and a pulse signal for transmitting ultrasonic waves in a driving frequency band including the detected resonance frequency is input to the ultrasonic sensor 20. As an example of the technique for analyzing the reverberation state, the resonance frequency can be detected by looking at the attenuation level because the attenuation level of the reverberation is different between the resonance time and the non-resonance time.
[0036]
Usually, since the resonance frequency is an eigenvalue of the material, it can be obtained from the plate thickness and material of the container 30. However, the connection between the ultrasonic sensor 20 and the container 30 is generally performed by grease or the like. In this case, the matching layer of the ultrasonic sensor 20 and the container 30 are ideally combined. 30 and the resonance frequency. Therefore, it does not necessarily match the resonance frequency of the container 30. Therefore, when the coupling between the ultrasonic sensor 20 and the container 30 is made of a resin material (silicon sheet), the influence of the coupling between the ultrasonic sensor 20 and the container 30 is small, and the ultrasonic sensor 20 may be driven at the resonance frequency of the container 30. The resonance frequency can be easily obtained.
In addition, when the material of the container 30 is known, the plate thickness of the container 30 can be obtained by reverse calculation. For example, as described above, when the liquid level is converted into the remaining amount and displayed, Not only the volume of the container 30 can be obtained from the dimensions of the container 30, but also the volume of the container 30 can be obtained in consideration of the plate thickness information. The amount can be determined.
[0037]
Further, the display unit 17 displays information related to the liquid level position detected by the liquid level detection unit 16.
[0038]
Here, when detecting the distance to the liquid level 31, the reverberation margin detection means 18 calculates the reverberation margin based on a predetermined amplification factor (in the previous setting or initial setting) when the ultrasonic sensor 20 is driven. When the detected reverberation margin is less than or equal to the predetermined value, the controller 10 switches the frequency band of the variable BPF 12 in accordance with the frequency component of the reverberation at the bottom plate of the container 30, and the detected reverberation margin is the predetermined value. The frequency band is adjusted so that the reflected echo signal from the liquid surface 31 can be specified.
[0039]
According to the present invention, even when a broadband ultrasonic sensor is used, only the transmission frequency component corresponding to the thickness of the container bottom plate can be extracted, so that unnecessary noise can be removed.
In addition, the band performance type BPF is configured by a simple circuit including a relatively inexpensive LC filter circuit and a switch element, so that cost performance can be improved.
Moreover, since sufficient S / N more than the case where a narrow-band sensor is used can be ensured, it is possible to reduce power consumption while ensuring sufficient S / N when driving the ultrasonic sensor. As a result, a battery with a small capacity can be used, and the life can be extended.
[0040]
FIG. 4 is a diagram showing another circuit configuration example of the ultrasonic level meter of the present invention. The controller 10 of the ultrasonic level meter in the present embodiment includes a receiving circuit 11, a variable BPF 12, a time varying characteristic amplifying unit 13a, a time varying characteristic correcting unit 13b, a detecting circuit 14, an echo timing detecting unit 15, and a liquid level detecting unit 16. , A display unit 17, a reverberation margin detecting unit 18, an ultrasonic sensor driving unit 19, and a CPU (not shown). The ultrasonic sensor 20 transmits an ultrasonic wave based on a pulse signal input from the controller 10, and this pulse signal is preferably a rectangular wave pulse signal including all frequency components. Even when the thickness and material change, it is possible to cope. In the present embodiment, the receiving circuit 11 and the variable BPF 12 are illustrated separately, but the variable BPF 12 may be physically included in the receiving circuit 11.
[0041]
The difference between the circuit configuration of the controller 10 in this embodiment and the circuit configuration of the controller 10 shown in FIG. 1 is that it has time-varying characteristic amplifying means 13 a and time-varying characteristic correcting means 13 b instead of the amplifying means 13. It is. Accordingly, the other means are the same, and the description thereof will be omitted here.
The time-varying characteristic amplifying unit 13 a and the time-varying characteristic correcting unit 13 b receive a signal corresponding to the reflected wave with respect to the ultrasonic wave transmitted by the ultrasonic sensor 20, and determine the amplification factor of the input signal when the ultrasonic sensor 20 is driven. By changing the starting point so as to increase with time, the amplification factor is adjusted according to the position of the liquid surface 31, and the adjustment value is set. The ultrasonic level meter according to the present embodiment controls the time-varying characteristic amplifying unit 13a, the time-varying characteristic correcting unit 13b, the ultrasonic sensor driving unit 19, and the variable BPF 12 as appropriate, thereby controlling stable reverberation characteristics and reflection sensitivity. This realizes the characteristics and enables even the liquid level measurement when the liquid level position is low.
[0042]
Here, when calculating the distance to the liquid surface 31, the reverberation margin detection means 18 calculates the reverberation margin based on a predetermined amplification factor (in the previous setting or initial setting) when the ultrasonic sensor 20 is driven. When the detected reverberation margin is less than or equal to the predetermined value, the time-varying characteristic amplifying unit 13a and the time-varying characteristic correcting unit 13b reduce the amplification factor at the time of driving the ultrasonic sensor 20, and the reduced amplification factor is changed to time. In accordance with the above, it is changed to increase at a predetermined rate. At this time, the ultrasonic sensor driving means 19 changes the oscillation time of the pulse signal input to the ultrasonic sensor 20. Further, the frequency band of the variable BPF 12 is switched according to the frequency component of reverberation at the bottom plate of the container 30. Conditions for changing the amplification factor and the oscillation time can be arbitrarily set, and the oscillation time may be fixed and changed by the amplification factor. The controller 10 adjusts any one or more of the amplification factor, the oscillation time, and the frequency band so that the detected reverberation margin becomes equal to or greater than the predetermined value and the reflected echo signal from the liquid surface 31 can be specified. .
[0043]
Hereinafter, the adjustment of the amplification factor and the oscillation time will be specifically described with reference to FIG.
In the time-varying characteristic amplifying means 13a and the time-varying characteristic correcting means 13b shown in FIG. 4, the AMP circuit of the transistor Q1 includes the emitter resistor R10 of the first stage AMP and the coupling of the emitter resistor R10 in the first stage base. When the ring C7 is appropriately adjusted to give a time dead band time (distance) based on the time constant of the first stage AMP, and when the reflected wave delay time is early (in the case of a short distance), the transistor Q2 does not operate completely. The AMP gain is changed with respect to time (distance). Thereby, the ultrasonic level meter of the present invention can achieve both reverberation characteristics and sensitivity characteristics.
[0044]
In the time-varying characteristic amplifying means 13a, 1.0 V is applied as the DC bias voltage of the transistor Q2, and the voltage after the variable BPF 12 is passed between the terminal: TEST1 and the ground, that is, by the bidirectional diode D1 on the input side. Since the level is clamped to ± (0.6 to 0.7) V, the current flowing in or out of the capacitor C1 changes. At this time, the base bias potential of the transistor Q2 periodically changes in the range of about 0.3 to 1.7 V in accordance with the drive frequency. For the capacitor C7, the time constant differs depending on the magnitude relationship between the impedance on the transistor Q2 side and the impedance on the diode D1 side, and the time constant on the transistor Q2 side is long. Therefore, the bias of the transistor Q2 is more than 1.0V with time. It tends to be biased to a low potential.
[0045]
A low bias is applied to the capacitor C7 in this cycle until the sensor is driven by forced vibration due to the driving of the ultrasonic sensor 20, the charge is charged, and when the forced vibration ends, the same bias is applied in the cycle of free vibration of the circuit. Even after the change is biased, even after the potential on the diode D1 side converges to 0V, the transistor Q2 is biased for a while due to the input impedance on the transistor Q2 side and the time constant of the capacitor C7. It converges to 0.0V. Due to the slope of the bias at which the low bias potential with respect to 1.0 V returns with time, the signal amplification factor with respect to time can be gradually increased with respect to the liquid level distance.
[0046]
The time-varying characteristic correction means 13b has a subtle gain gradient by arbitrarily determining the number of resistors and the resistance value as necessary with respect to the main resistor R10 as the emitter resistor R11, R12, R13,. Can be given. Furthermore, instead of switching the capacitor C7, the switch S7 may be switched to the capacitor C8 and the resistor R14 for use. In this case, the range of the gain gradient can be further expanded. At this time, the switches S4, S5, S6,... Are switched by a signal from a CPU (not shown).
[0047]
Here, the reflection from the liquid surface 31 based on the amplification factor adjusted by the time-varying characteristic amplifying means 13a and the time-varying characteristic correcting means 13b and / or the oscillation time of the pulse signal and the frequency band switched in the variable BPF 12. After specifying the echo signal and measuring the ultrasonic propagation time according to the specified reflected echo signal, the liquid level detection means 16 calculates the distance from the measured ultrasonic propagation time to the liquid level 31 and calculates it. The calculated distance value is displayed on the display unit 17. At this time, the liquid level detection unit 16 can calculate the volume of the liquid stored in the container 30 based on the calculated distance calculation value, and causes the display unit 17 to display the calculated volume of the liquid as liquid volume information. You may do it.
[0048]
At the reverberation level and sensitivity level based on the amplification factor, oscillation time, and frequency band after adjustment as described above, for example, operations from transmission of an ultrasonic wave from the ultrasonic sensor 20 to reception of a reflected echo signal are performed. When a predetermined number of times are performed and it is confirmed that there is no variation in the measured ultrasonic propagation times and the average value thereof is within the allowable value, the calculated distance value may be output. . At this time, if the average value does not fall within the allowable value, any one or more of the amplification factor, oscillation time, and frequency band are repeatedly adjusted, and the reverberation level and the sensitivity level are adjusted again.
[0049]
FIG. 5 is a diagram showing characteristic curves in the relationship between the frequency band of the ultrasonic sensor 20 and the bottom plate thickness of the container 30, 41 is a filter C, 43 is a filter B, and 45 is a filter A. In this example, the applicable frequency band of the ultrasonic sensor 20 is set to 880 to 1350 kHz, and a three-stage filter is prepared for this frequency band. These three types of filters are referred to as a filter C, a filter B, and a filter A, respectively. 42 is a band where the filter C (41) and the filter B (43) overlap, and 44 is a band where the filter B (43) and the filter A (45) overlap.
[0050]
As described above, it has been difficult to secure a reflection sensitivity above a necessary level and control the minimum detection distance below a certain level (for example, 100 mm or less). This is because the frequency solution increases as the bottom plate thickness of the container 30 increases, and the interval between the solutions becomes narrower. At the same time, since the reverberation characteristic becomes conspicuous due to the influence of the ultrasonic wave reflected at the bottom boundary of the container 30, the minimum detection distance margin, that is, the reverberation margin is lowered due to this influence. As a result, it is difficult to perform stable liquid level detection when the liquid level position is low.
[0051]
Therefore, the three types of filters A, B, and C are appropriately switched. For example, the reverberation margin decreases at a thickness of about 9.0 mm. The frequency band is divided into two parts around the sensor anti-resonance frequency, and a sufficient attenuation characteristic can be obtained by adapting the transmission frequency band transmitted through the bottom of the container 30 and the LC filter when receiving ultrasonic waves. Thus, the filter is switched to either the filter B (43) or the filter C (41). Thereby, the reverberation wave due to the increase in the bottom plate thickness of the container 30 can be reduced and controlled to stable reflection sensitivity characteristics. At this time, since the sensitivity characteristic is important in the selection of the filter B (43) or the filter C (41) at the frequency band boundary, the reverberation characteristic is secured without adjusting the time constant of the first-stage AMP to impair the short-range sensitivity. Adjust to a frequency band that can.
[0052]
Basically, the filter B (43) is used as a reference to switch to the filter C (41) or the filter A (45). However, the variable BPF 12 can perform optimum filter selection based on frequency information. By combining the time-varying characteristic amplifying means 13a and the time-varying characteristic correcting means 13b based on information, current liquid level position information, etc., an optimum adjustment value, that is, an optimum frequency band for an arbitrary liquid level position , The gain can be adjusted.
[0053]
FIG. 6 is a waveform diagram showing an example of a voltage waveform with respect to time in the ultrasonic level meter of the present invention, in which 51 is a collector output waveform and 52 is a reflected wave waveform. This example will be described based on the circuit configuration shown in FIG. In the reflected wave waveform 52 based on the collector output waveform 51 shown in FIG. 6 (A), the result of dealing with the increase in the reverberation level due to the increase in the plate thickness by switching the variable BPF 12 is shown. It can be seen that the reverberation level is large. Further, FIG. 6B shows the state of the collector output waveform 51 after changing the gain gradient by changing the emitter resistance to the collector output waveform 51 shown in FIG. 6A and changing the gain gradient. It can be seen that the reverberation of the reflected wave waveform 52 can be reduced according to the change in the gain gradient.
[0054]
FIG. 7 is a waveform diagram showing another example of a voltage waveform with respect to time in the ultrasonic level meter of the present invention, in which 53 is a reflected wave waveform. This example will be described based on the circuit configuration shown in FIG. From the reflected wave waveform 53 shown in FIG. 7A, in the natural frequency solution on the low frequency side by switching the frequency band of the variable BPF 12, when the broadband ultrasonic sensor 20 is used, the reverberation is caused by the influence of the high frequency solution. Will occur. Therefore, as shown in FIG. 7B, by switching the filter of the variable BPF 12, it is possible to converge to the center of the band of the ultrasonic sensor 20 (about 1070 kHz) and to suppress the occurrence of reverberation.
[0055]
FIG. 8 is a flowchart for explaining an example of a liquid level detection method using an ultrasonic level meter to which the present invention is applied. In this example, the flow of the filter switching process of the variable BPF 12 will be described based on the circuit configuration shown in FIG. First, the ultrasonic sensor 20 and the controller 10 are installed in the container 30, and the controller 10 performs initial setting (step S1). Next, a pulse signal is input from the controller 10 to the ultrasonic sensor 20 to drive the ultrasonic sensor 20 (step S2), and a waveform of a reflected wave (reflected echo) with respect to the ultrasonic wave transmitted from the ultrasonic sensor 20 is detected. Then, the echo timing is measured based on the ultrasonic wave propagation time until the detected reflected wave is received (step S4). Next, the reverberation state when the ultrasonic sensor 20 is driven is checked (step S5). If there is no problem in the reverberation state (in the case of OK), that is, if the reverberation margin is equal to or greater than a predetermined value, the liquid level detection value (liquid level) (Distance) is output (step S7), and the process proceeds to a liquid level detection request standby state (step S8).
[0056]
In step S5, if there is a problem in the reverberation state (NG), filter switching processing is performed (step S6), and the result is returned to step S1 and set in the controller 10. Here, in step S6, as the filter switching process, for example, in the case of the filter characteristics shown in FIG. 5 described above, filter B (43) → filter A (45), filter B (43) → filter C (41). The two types of switching are performed.
[0057]
FIG. 9 is a flowchart for explaining another example of the liquid level detection method using the ultrasonic level meter to which the present invention is applied. In this example, the flow of the time-varying characteristic correction process and the filter switching process will be described based on the circuit configuration shown in FIG. First, the ultrasonic sensor 20 and the controller 10 are installed in the container 30, and the controller 10 performs initial settings (step S11). Next, a pulse signal is input from the controller 10 to the ultrasonic sensor 20 to drive the ultrasonic sensor 20 (step S12), and a waveform of a reflected wave (reflected echo) with respect to the ultrasonic wave transmitted from the ultrasonic sensor 20 is detected. The echo timing is measured based on the ultrasonic wave propagation time until the detected reflected wave is received (step S14). Next, the reverberation state when the ultrasonic sensor 20 is driven is checked (step S15). If there is no problem in the reverberation state (in the case of OK), that is, if the reverberation margin is equal to or greater than a predetermined value, the liquid level detection value (liquid level) (Distance) is output (step S17), and the process proceeds to a liquid level detection request standby state (step S18).
[0058]
In step S15, if there is a problem in the reverberation state (in the case of NG), time-varying characteristic correction processing and filter switching processing are performed (step S16), and the result is returned to step S11 and set in the controller 10. In step S16, the following processes (1), (2), and (3) are appropriately combined as the time-varying characteristic correction process and the filter switching process.
(1) AMP gain is switched. This is performed by, for example, switching in the order of switches S4 → S5 → S6 → S7 → (S4) → shown in FIG.
(2) Change the sensor drive time (tn). This is performed by sequentially switching, for example, tn (unit: μs) = 50 → 55 → 60 → 65 → (maximum 100).
(3) In the case of the filter characteristics shown in FIG. 5 described above, two types of switching are performed: filter B (43) → filter A (45) and filter B (43) → filter C (41).
[0059]
FIG. 10 is a flowchart for explaining another example of the liquid level detection method using the ultrasonic level meter to which the present invention is applied, and FIG. 10 (A) is the process flow shown in FIG. 9 described above. Is described in detail. FIG. 11 is a diagram showing an output waveform and a gain gradient change parameter corresponding to the processing flow shown in FIG. In FIG. 10A, first, a sensor driving burst pulse time to be input to the ultrasonic sensor 20 is set (step S21). At this time, the sensor drive frequency variable Vco is operated. Further, the peak search comparator level is set to the comparator level (1) shown in FIG. 11B, the AMP gain is set to the switch S4 in the circuit configuration shown in FIG. 4, and the variable BPF 12 is set to the above-described FIG. Filter B (43) shown in FIG. Next, based on the conditions set in step S21, the ultrasonic sensor 20 transmits an ultrasonic wave, performs reflection echo analysis processing / statistical processing on the ultrasonic wave, and obtains a peak voltage equal to or higher than the comparator level (1). A search is made for a reflected echo indicating (step S22).
[0060]
As a result of searching the reflected echo in the step S22, the reflected echo showing the peak voltage equal to or higher than the comparator level (1) is determined as the reflected first echo, that is, the liquid surface distance is determined (step S23). The comparator level is set to the comparator level (2) shown in FIG. 11B (step S24). Next, a reverberation margin, that is, a minimum detection distance is determined based on the comparator level (1) and the comparator level (2). The reverberation margin is a portion circled by a dotted line shown in FIG. Next, it is checked whether or not the reverberation margin is greater than or equal to a predetermined value (step S25). If the reverberation margin is greater than or equal to the predetermined value (in the case of OK), the liquid level distance measurement value determined in step S23 is output (step S29). ).
[0061]
In step S25, if the reverberation margin is less than or equal to a predetermined value (NG), in step S26,
(1) AMP gain is switched. This is performed by, for example, switching in the order of switches S4 → S5 → S6 → S7 → (S4) → shown in FIG.
(2) Change the sensor drive time (tn). For example, tn (unit: μs) = 50 → 55 → 60 → 65 → (maximum 100).
(3) In the case of the filter characteristics shown in FIG. 5 described above, two types of switching are performed: filter B (43) → filter A (45) and filter B (43) → filter C (41).
The gain gradient change parameter at this time is shown in FIG. FIG. 11D shows a waveform in a state where tn = 55 shown in FIG. 11C and the switch S7 is set as a parameter. The reverberation margin is larger than the waveform shown in FIG. 11B. Thus, it can be seen that the reverberation is reduced. Thus, by adjusting the gain, the sensor drive time (tn), and the filter by the gain gradient change parameter, the optimum value of the gain (AMP gain) is set and the sensor drive time (tn) is set ( Step S27).
[0062]
Based on the measurement conditions set in step S27, the ultrasonic sensor 20 is driven to detect waveform detection and echo timing, and then confirm the sensitivity and reverberation level of the acquired reflected echo (step S28). If there is no problem in the level (in the case of OK), the liquid level distance measurement value calculated based on the echo timing is output (step S29). If there is a problem in the sensitivity and reverberation level of the reflected echo in the above step S28 (in the case of NG), the process returns to step S26, and the time-varying characteristic correction process and the filter switching process are repeated.
[0063]
【The invention's effect】
According to the present invention, even when an ultrasonic sensor having a wide frequency band applicable to a wide range of plate thicknesses is used, stable liquid level detection is achieved without switching S / N by switching the band of the band-pass filter. Is possible.
In addition to switching the band of the bandpass filter, the ultrasonic sensor driving timing is changed so that the amplification factor of the signal increases with time, and the oscillation time of the pulse signal input to the ultrasonic sensor is changed. By changing, the dead zone time (distance) that cannot be detected due to reverberation is adjusted, and stable reverberation characteristics and sensitivity characteristics can be realized at all times.
In addition, it is possible to provide an ultrasonic level meter that can be applied to a wide range of container plate thicknesses and that enables liquid level measurement from a low level to a high level.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a configuration example in which an ultrasonic level meter to which the present invention is applied is installed in a container.
FIG. 2 is a diagram showing a circuit configuration example of an ultrasonic level meter according to the present invention.
FIG. 3 is a diagram showing a circuit configuration example of a variable BPF according to a predetermined thickness range to which the present invention is applied.
FIG. 4 is a diagram showing another circuit configuration example of the ultrasonic level meter of the present invention.
FIG. 5 is a diagram showing a characteristic curve in the relationship between the frequency band of the ultrasonic sensor and the bottom plate thickness of the container.
FIG. 6 is a waveform diagram showing an example of a voltage waveform with respect to time in the ultrasonic level meter of the present invention.
FIG. 7 is a waveform diagram showing another example of a voltage waveform with respect to time in the ultrasonic level meter of the present invention.
FIG. 8 is a flowchart for explaining an example of a liquid level detection method using an ultrasonic level meter to which the present invention is applied.
FIG. 9 is a flowchart for explaining another example of a liquid level detection method using an ultrasonic level meter to which the present invention is applied.
FIG. 10 is a flowchart for explaining another example of a liquid level detection method using an ultrasonic level meter to which the present invention is applied.
11 is a diagram showing an output waveform and a gain gradient change parameter corresponding to the processing flow shown in FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Controller (control part), 11 ... Reception circuit, 12 ... Variable type BPF, 12a ... Switching part, 13 ... Amplification means, 13a ... Time-varying characteristic amplification means, 13b ... Time-varying characteristic correction means, 14 ... Detection circuit, DESCRIPTION OF SYMBOLS 15 ... Echo timing detection means, 16 ... Liquid level detection means, 17 ... Display part, 18 ... Reverberation margin detection means, 19 ... Ultrasonic sensor drive means, 20 ... Ultrasonic sensor (piezoelectric sensor), 21 ... Ultrasonic, 30 ... container, 31 ... liquid level, 41 ... filter C, 43 ... filter B, 45 ... filter A, 42, 44 ... overlap band, 51 ... collector output waveform, 52, 53 ... reflected wave waveform.

Claims (10)

An ultrasonic sensor that is attached to the outer wall surface of the bottom of the container, transmits ultrasonic waves toward the liquid surface of the liquid contained in the container, and receives reflected waves from the liquid surface; In the ultrasonic level meter connected to the control unit that controls the detection operation of the liquid level based on the time information from the time of transmitting the sound wave to the time of receiving the reflected wave,
The ultrasonic sensor transmits ultrasonic waves based on a pulse signal input from the control unit,
The control unit measures an ultrasonic propagation time from when the ultrasonic sensor is driven by the input of the pulse signal to when it receives a reflected echo from the liquid surface, and from when the ultrasonic sensor starts to be driven. Reverberation margin detection means for measuring a drive reverberation time until the reverberation of ultrasonic waves detected immediately after driving is reduced to a predetermined level, and detecting a difference between the measured ultrasonic propagation time and the drive reverberation time as a reverberation margin; ,
The inputs a signal corresponding to the reflected wave against the ultrasonic wave transmitted by the ultrasonic sensor, a band variable filter that out selectively extract the transmission frequency component passing through the bottom plate of the container from the frequency components of the input signal,
Amplifying means for amplifying the signal extracted by the band variable filter at a predetermined amplification rate;
Echo timing detection means for detecting an echo timing by specifying a reflected echo signal from the liquid surface from the amplified signal based on the amplification factor of the amplification means;
Based on the echo timing the detected possess a liquid level detecting means for detecting a position of the liquid surface,
When detecting the position of the liquid level, the reverberation margin detection means detects a reverberation margin based on a predetermined amplification factor when the ultrasonic sensor is driven, and the detected reverberation margin is less than a predetermined value, The control unit switches the frequency band of the band variable filter according to the frequency component of the reverberation at the container bottom plate, the detected reverberation margin becomes larger than the predetermined value, and the reflected echo from the liquid surface An ultrasonic level meter , wherein the frequency band is adjusted so that a signal can be specified . 2. The ultrasonic level meter according to claim 1 , wherein the amplifying means changes the amplification factor of the signal extracted by the band variable filter so as to increase according to time starting from the time when the ultrasonic sensor is driven. ultrasonic level meter and sets the amplification factor in accordance with the position of the liquid surface by. 3. The ultrasonic level meter according to claim 1, wherein the control unit changes an oscillation time of the pulse signal by changing a pulse number or a pulse width of the pulse signal, and responds to the changed oscillation time. Ultrasonic sensor driving means for inputting a pulse signal to the ultrasonic sensor,
The ultrasonic level meter, wherein the ultrasonic sensor transmits an ultrasonic wave while changing a driving time at a driving frequency corresponding to a pulse signal input from the ultrasonic sensor driving means. The ultrasonic level meter according to claim 3 , wherein when detecting the position of the liquid level, the reverberation margin detection means detects a reverberation margin based on a predetermined amplification factor when the ultrasonic sensor is driven, when reverberation margin the detected is less than a predetermined value, said amplifying means to decrease the amplification factor in driving time of the ultrasonic sensor, the change to be greater by a predetermined ratio in accordance with the time the amplification factor that has made as low together is, the ultrasonic sensor drive means is a change of the oscillation time of the pulse signal input to the ultrasonic sensor, further, while switching the frequency band of the band-variable filter in accordance with the frequency components of the reverberation in the container bottom plate reverberation margin in the detection becomes larger than the predetermined value, and the amplification factor to be able identify the reflected echo signals from the liquid level, the oscillation time, the frequency band Ultrasonic level meter and adjusting any one or more, respectively. 4. The ultrasonic level meter according to claim 3 , wherein when detecting the liquid level position in the container, the ultrasonic sensor driving means transmits ultrasonic waves from the ultrasonic sensor a plurality of times while changing the oscillation frequency. is, the control unit, said by analyzing the state of the reverberation in the container bottom plate of each ultrasonic wave is transmitted from the ultrasonic sensor, a resonance frequency suitable for ultrasound transmission bottom outer wall surface of the container An ultrasonic level meter, wherein a pulse signal for detecting and transmitting an ultrasonic wave in a driving frequency band including the detected resonance frequency is input to the ultrasonic sensor. The ultrasonic level meter according to claim 5 , wherein the control unit changes a frequency component including the resonance frequency from a frequency component of a signal input from the ultrasonic sensor by switching a frequency band of the band variable filter. An ultrasonic level meter characterized by extracting. An ultrasonic sensor that is attached to the outer wall surface of the bottom of the container, transmits ultrasonic waves toward the liquid surface of the liquid contained in the container, and receives reflected waves from the liquid surface; In the liquid level detection method using the ultrasonic level meter connected to the control unit that controls the detection operation of the liquid level based on time information from the time when the sound wave is transmitted to the time when the reflected wave is received,
The ultrasonic level meter transmits ultrasonic waves from the ultrasonic sensor based on a pulse signal input from the control unit;
The ultrasonic propagation time from when the ultrasonic sensor is driven by the input of the pulse signal to when the reflected echo from the liquid surface is received is measured, and is detected immediately after the ultrasonic sensor is driven from the driving start time. Measuring a drive reverberation time until the reverberation of the ultrasonic wave is reduced to a predetermined level, and detecting a difference between the measured ultrasonic propagation time and the drive reverberation time as a reverberation margin;
An extraction step of inputting a signal corresponding to a reflected wave with respect to the ultrasonic wave transmitted by the ultrasonic sensor , and selectively extracting a transmission frequency component that passes through the bottom plate of the container from the frequency component of the input signal;
An amplification step of amplifying the extracted signal at a predetermined amplification rate;
An echo timing detection step of detecting an echo timing by identifying a reflected echo signal from the liquid surface from the amplified signal;
Possess a liquid level detection step of detecting a position of the liquid surface based on the echo timing the detected,
When detecting the position of the liquid level, a reverberation margin is detected based on a predetermined amplification factor when the ultrasonic sensor is driven, and if the detected reverberation margin is less than a predetermined value, the reverberation margin in the container bottom plate is detected. While switching the frequency band of the band variable filter according to the frequency component, the frequency band is adjusted so that the detected reverberation margin becomes larger than the predetermined value and the reflected echo signal from the liquid surface can be specified. A liquid level detection method using an ultrasonic level meter. 8. The liquid level detection method using an ultrasonic level meter according to claim 7 , wherein an amplification factor of a signal extracted by the band variable filter is increased according to time starting from driving of the ultrasonic sensor. A liquid level detection method using an ultrasonic level meter, wherein an amplification factor corresponding to the position of the liquid level is set by changing the gain. The liquid level detection method using the ultrasonic level meter according to claim 7 or 8 , wherein the oscillation time of the pulse signal is changed by changing the number of pulses or the pulse width of the pulse signal, and the changed oscillation time. ultrasonic level meter pulse signal input to the ultrasonic sensor, and transmits ultrasonic waves from said ultrasonic sensor while changing the drive time at a driving frequency corresponding to the pulse signal the input in response to Liquid level detection method using The liquid level detection method using the ultrasonic level meter according to claim 9 , wherein when detecting the position of the liquid level, a reverberation margin is detected based on a predetermined amplification factor when the ultrasonic sensor is driven, When the detected reverberation margin is less than or equal to a predetermined value, the amplification factor is reduced at the time of driving the ultrasonic sensor, the reduced amplification factor is changed to increase at a predetermined rate according to time, and While changing the oscillation time of the pulse signal input to the ultrasonic sensor, and further switching the frequency band of the band variable filter according to the frequency component of the reverberation at the container bottom plate, the detected reverberation margin is more than the predetermined value become large, and, characterized in that adjusting the amplification factor to be able identify the reflected echo signals from the liquid level, the oscillation time, either of the frequency bands 1 or more, respectively Liquid level detection method using the ultrasonic level meter that.

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