JP2000035357A - Ultrasonic level gage - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrasonic level gage in which a driving frequency is adjusted by a simple control circuit and in which the amplitude of a received signal is made maximum. SOLUTION: In an ultrasonic level gage, a drive circuit 10 which outputs a driving signal at a variable driving frequency fd is provided, a piezoelectric element 20 which radiates ultrasonic waves according to the driving signal of the drive circuit 10 is provided, and a receiving circuit 30 which receives reflected waves received by the piezoelectric element 20 is provided. A driving- frequency adjusting circuit 40 which adjusts the driving frequency fd is provided in such a way that the driving frequency fd agrees with the natural frequency of the piezoelectric element 20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic level meter used as a liquid level gauge, a snow depth gauge, a vehicle detector, and the like in a flow path, a tank, and the like, and particularly to an improvement in reception sensitivity of an ultrasonic sensor. .

[0002]

2. Description of the Related Art An ultrasonic level meter is disclosed in Japanese Unexamined Patent Publication No. Hei.
As disclosed in Japanese Unexamined Patent Application Publication No. 10-82684, the level of the liquid level and the like is determined by using the time until the ultrasonic wave emitted from the transmitter is reflected by the reflector and reaches the receiver. It is to be measured.

FIG. 11 is a block diagram showing the configuration of a conventional apparatus. The drive circuit 10 operates at a drive frequency fd of several tens of kHz.
A pulse train drive signal having a voltage of about 000 V _PP is output. The piezoelectric element 20 transmits an ultrasonic signal in accordance with a drive signal and also functions as a sensor for receiving reflected ultrasonic waves. For example, PZT or the like is used. Receiving circuit 30
Receives the reflected wave. The signal processing circuit 32 uses the point where the envelope of the received signal intersects the threshold voltage Vth as the receiving position, and calculates the distance from the transmission time to the reception and the sound speed to the object to be reflected from the sound speed. According to such a configuration, although it does not have a function of tracking the fluctuation of the resonance frequency f ₀ , it is automatically adjusted to have the highest sensitivity.

[0004]

By the way, the piezoelectric element P
ZT has a resonance frequency f ₀₁ and an anti-resonance frequency f ₀₂ . In general, the ultrasonic level meter attention to anti-resonance frequency f ₀₂ and the resonance frequency f _01, defines the drive frequency fd to maximize sensitivity in transmission and reception. This maximum sensitivity is
It is determined by detecting the amplitude of the received signal. However,
Even if it is attempted to maximize the amplitude of the received signal by adjusting the drive frequency fd, there is a problem that the amplitude does not increase or decrease in one direction, so that complicated control is required.

[0005] The speed of sound depends on the temperature, for example, +0.0183 [% / deg] in air. -20 to + outdoors
Since the sound speed fluctuates in the range of 50 ° C., the sound speed changes by about 1.3%. Therefore, the temperature is corrected using a resistance thermometer having an accuracy of about ± 0.5 to 1.0 ° C. In this case, there are an integral type that is housed in the same case as the piezoelectric element, and a separate type that is installed independently. The integrated type is easy to install, but there is a problem that the temperature of the space where ultrasonic waves propagate cannot be measured,
It is often used because the construction cost is low. Here, in the case of the integrated type, the temperature sensor may be exposed to the outside in the field, and may cause a temperature detection error of several degrees or more due to the influence of solar radiation or the like.

FIG. 12 is an explanatory diagram for connecting a piezoelectric element and a signal converter with a cable. The drive circuit 10 drives the piezoelectric element 20 at a high voltage of 1000 V _PP or more and at a frequency of about several tens of kHz. Drive cable 2
When 2 is 100 m or more, the capacitance between the cores becomes large, and the transmission / reception energy is greatly attenuated. For example, 50
A 0 m cable attenuates by 20 dB or more, which is not preferable in signal processing. The limit of the cable capacity is to reduce the general-purpose 60 [pF / m] to about 40 [pF / m] even if the core wire and the insulator are changed.

[0007] Then, transmission is performed at low impedance and low pressure,
A general measure is to convert to high voltage at the tip of the piezoelectric element 20 to compensate for the attenuation due to the cable capacity. For example 1: 5
When the transformer 60 is driven at 1200 V, the transmission capacity between the cables is 240 V and the transmission capacity can be apparently reduced to (1/5) ² = 1/225. However, the reflected wave
Since the signal is sent to the receiving circuit 30 through 0, the amplitude of the reflected wave is also attenuated to 1/5. If the cable is long, there is some loss on the way, so the attenuation of the reflected wave amplitude due to the transformer must be accepted. On the other hand, when the length of the cable is short, it is desired that the reflected wave amplitude be as high as possible.

A first object of the present invention is to provide an ultrasonic level meter in which the drive frequency fd is adjusted by a simple control circuit to maximize the amplitude of a received signal. is there.

A second object of the present invention is to provide an ultrasonic level meter which is not easily affected by solar radiation or the like and can perform temperature correction even when the resistance thermometer is integrated into the same case as the piezoelectric element.

A third object is to transmit a drive signal at a low impedance and a low voltage, convert the drive signal to a high voltage at the tip of the piezoelectric element, and compensate for the attenuation of the reflected wave by the transformer while compensating for the attenuation due to the cable capacitance. It is to provide an ultrasonic level meter which can be avoided.

[0011]

In order to achieve the first object, an ultrasonic level meter according to the first aspect of the present invention comprises a driving circuit 10 for outputting a driving signal having a variable driving frequency fd;
In an ultrasonic level meter having a piezoelectric element 20 that emits an ultrasonic wave according to a drive signal of the drive circuit and a receiving circuit 30 that receives a reflected wave received by the piezoelectric element, the drive frequency is unique to the piezoelectric element. The drive frequency adjusting circuit 4 adjusts the drive frequency so as to match the frequency.
It is characterized by having 0.

According to such a configuration, even if the natural frequency of the piezoelectric element fluctuates due to a temperature change or the like, the driving frequency adjusting circuit 40 matches the fluctuated natural frequency so that the driving frequency adjustment circuit 40 matches the fluctuated natural frequency. Since the frequency is adjusted, the amplitude of the reflected wave in the receiving circuit is favorably maintained.

Here, as in claim 2, the drive frequency adjusting circuit includes a circuit 42 for synchronizing and integrating at a first phase of the drive frequency, and a second phase shifter and a second phase separated from the first phase by a quarter period.
And an amplitude calculating circuit 46 for obtaining an increase or decrease in the amplitude of the received wave from the outputs of the two synchronous integrating circuits.
When the amplitude calculation circuit has an adjustment circuit 48 that increases the drive frequency when the amplitude of the received wave increases and decreases the drive frequency when the amplitude of the received wave decreases, the circuit configuration becomes simple. The adjustment circuit may be configured to select the drive frequency stepwise in the vicinity of the natural frequency of the piezoelectric element.

In order to achieve the second object, an ultrasonic level meter according to a fourth aspect of the present invention includes a drive circuit 10 for outputting a drive signal having a variable drive frequency fd, and a drive circuit 10 for outputting a drive signal having a variable drive frequency fd. In an ultrasonic level meter having a piezoelectric element 20 that emits an ultrasonic wave and a receiving circuit 30 that receives a reflected wave received by the piezoelectric element, a temperature-dependent function that stores the relationship between the natural frequency and the temperature of the piezoelectric element is used. And a temperature measuring unit that measures the temperature from the relationship between the natural frequency of the piezoelectric element and the temperature-dependent storage unit.

According to such a configuration, the temperature is measured by utilizing the fact that the natural frequency of the piezoelectric element is almost uniquely determined by the fluctuation of the temperature. The measurement value of the sensor does not fluctuate or generate an error, and the measurement error can be reduced. here,
The measured temperature may be used to correct the sound speed of the propagation medium of the reflected wave.

In order to achieve the third object, an ultrasonic level meter according to a sixth aspect of the present invention includes a driving circuit 10 for outputting a driving signal having a variable driving frequency fd, and a driving circuit for outputting a driving signal having a variable driving frequency fd. In the ultrasonic level meter having the piezoelectric element 20 that emits the ultrasonic wave and the receiving circuit 30 that receives the reflected wave received by the piezoelectric element, a cable 50 that connects the driving circuit and the receiving circuit to the piezoelectric element is provided. , A transformer 6 for boosting the drive signal sent through this cable
0, and a diode circuit 70 for transmitting a reception signal via the cable independently of the transformer.

According to such a configuration, since the drive signal is boosted by the transformer, the drive voltage of the drive circuit can be low. Since the received signal is transmitted to the cable by a separate circuit using a diode circuit without passing through a transformer, the signal attenuation of the reflected wave can be reduced.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a configuration block diagram showing one embodiment of the present invention. In FIG. 1, components having the same functions as those in FIG. 11 are denoted by the same reference numerals, and description thereof will be omitted. Receiving circuit 3
0 indicates that the preamplifier A1, the time increasing gain unit TVG, and the main amplifier A2 are connected in series. Preamplifier A1
Reads the reflected wave sent from the piezoelectric element 20 while removing the influence of the drive signal of the drive circuit 10. The time step-up gain unit TVG compensates for the influence of the distance between the piezoelectric element 20 and the object to be reflected, because the amplitude of the reflected wave decreases when the distance between the piezoelectric element 20 and the object to be reflected increases. The gain is determined so that The main amplifier A2 receives the signal of the time increasing gain unit TVG and distributes the signal to the signal processing circuit 32 and the drive frequency adjustment circuit 40.

The drive frequency adjusting circuit 40 includes a circuit 42 for synchronously integrating at a first phase of the drive frequency, and a circuit 44 for synchronously integrating at a second phase which is a quarter cycle apart from the first phase.
And an amplitude calculating circuit 46 for obtaining an increase or a decrease in the amplitude of the received wave from the outputs of the synchronous integrating circuits. The amplitude calculating circuit 46 increases the driving frequency when the amplitude of the received wave increases, and lowers the driving frequency when the amplitude decreases. It has an adjustment circuit 48.

Here, the first synchronous integration circuit 42 includes a flip-flop, an X-phase switch, an X-phase integration circuit, and an A / D converter. Second synchronous integration circuit 44
Is composed of a flip-flop, a Y-phase side switch, a Y-phase integrating circuit, and an A / D converter. Drive circuit 10
Is applied to the flip-flop. The output of the flip-flop is sent to the piezoelectric element 20 via a transformer, the P output is applied to the X-phase switch, and the Q output is applied to the Y-phase switch.
The P output and the Q output are separated by a quarter cycle, and the on / off duty ratio is 50%. The X-phase integrating circuit synchronously integrates the reflected wave in the cycle in which the X-phase side switch is turned on, performs A / D conversion, and reads it with a microprocessor or the like. The Y-phase integrating circuit synchronously integrates the reflected wave in the cycle in which the Y-phase-side switch is turned on, performs A / D conversion, and reads it with a microprocessor or the like.

The amplitude calculating circuit 46 includes a first synchronous integrating circuit 4
2 of the A / D converter output X and the A / D converter
The output Y of the D converter is input, and the amplitude of the reflected wave is calculated.
That is, since the amplitude of the reflected wave can be accurately represented using the sine wave component and the cosine wave component, the amplitude of the reflected wave can be calculated by X and Y. The adjustment circuit 48 adjusts the driving frequency of the driving frequency signal oscillator based on the amplitude information of the reflected wave of the amplitude calculation circuit 46 so that the amplitude of the reflected wave is maximized.

FIGS. 2A and 2B are explanatory diagrams of the driving signal, the residual vibration, and the reflected wave. FIG. 2A is a waveform diagram, and FIG. 2B is an operation timing diagram of the driving frequency adjusting circuit 40. The drive signal of the piezoelectric element 20 is driven at a drive frequency fd, for example, continuously for 64 pulses. At this time, the drive frequency adjustment circuit 40 also performs synchronous integration. Next, since the residual vibration remains in the piezoelectric element 20, the drive frequency adjusting circuit 40 does not perform synchronous integration in this portion. Then, near the time window where the reflected wave appears, the drive frequency adjustment circuit 40 also performs synchronous integration.

FIG. 3 is an explanatory view of a reflection wave amplitude and the drive frequency fd, (A) if the drive frequency fd is compatible with the resonance frequency f ₀₁ of the piezoelectric element, (B) is shown a case of nonconformity I have. When the driving frequency fd is 39.60 kHz, the amplitude of the reflected wave is gradually increased by the driving pulse, so that the amplitude of the reflected wave is maximized. On the other hand, when the driving frequency fd is 40.38
At kHz (= 39.60 + 0.78), the drive pulse interferes with the resonance of the piezoelectric element, and a beat appears in the amplitude of the reflected wave. Therefore, the reflected wave amplitude does not increase so much.

4A and 4B are diagrams for explaining the adjustment of the amplitude of the reflected wave and the drive frequency fd. FIG. 4A is a circuit diagram of a main part, and FIG.
FIG. 7C is a diagram showing changes in the amplitudes of the converter outputs X and Y, and FIG.
This is the set value of d. If the drive frequency fd is they match the resonance frequency f _01, the oscillating voltage of the drive waveform and the piezoelectric element and is constant phase relationship is constant. Therefore, the output X of the A / D converter becomes constant, and the driving frequency fd is maintained as it is.

[0025] On the other hand, if the drive frequency fd is larger than the resonance frequency f _01, A / D converter output X is so increased, lowering the drive frequency fd. Further, if the drive frequency fd is smaller than the resonance frequency f _01, A / D converter output X is due to the reduced, increasing the drive frequency fd. Drive frequency fd
Is divided into 14 steps here, 38.38 kHz
To 40.96 kHz at approximately 0.15 kHz intervals.

FIG. 5 is a flowchart of the adjustment mode of the drive frequency fd. Initially, the initial value of the drive frequency fd is shown in FIG.
F7 (39.80 kHz) which is the center of all settable frequencies shown in
(S10). Then, the piezoelectric element is driven at the drive frequency f7, and the received Y-phase integration circuit signal of the reflected wave is A / D converted (S14). Repeat this three cycles,
The output Y of the A / D converter is added three times (S16). if,
If the sum is greater than the upper threshold, the drive frequency fd is increased by one step (S18). If the sum is smaller than the lower threshold, the drive frequency fd is lowered by one step (S20). Then, the setting of the drive frequency fd is changed in S18 and S20 (S12), and the optimum drive frequency fd is maintained by repeating S14 and subsequent steps.

FIG. 6 is an explanatory diagram of the adjustment mode of the drive frequency fd. If the sum of the three outputs of the A / D converter output Y is between the upper threshold (+600) and the lower threshold (-300) for the current driving frequency fn, the driving frequency fn is maintained as it is. Is done. On the other hand, when the value obtained by adding the A / D converter output Y three times exceeds the upper threshold value, the drive frequency fn is increased by one step to f (n + 1). When the three-time sum of the A / D converter output Y falls below the lower threshold value, the drive frequency fn is lowered by one step to f (n-1).

FIG. 7 is a flowchart of the distance measurement mode. First, the drive frequency fd is set to the set value fn in the adjustment mode (S22). Then, the piezoelectric element is driven at this drive frequency fn, and the received Y-phase integration circuit signal of the reflected wave is converted into an A / D signal.
Conversion is performed (S26). By repeating this for 100 cycles, A
The output Y of the / D converter is added 100 times (S28). If the added value is larger than the upper threshold, the drive frequency fd is increased by one step (S30). If the sum is smaller than the lower threshold, the drive frequency fd is lowered by one step (S32). Then, the setting of the drive frequency fd is changed in S30 and S32 (S24), and the optimal drive frequency fd is maintained by repeating S26 and subsequent steps.

FIG. 8 is an explanatory view of the temperature dependence of the resonance frequency f ₀₁ of the piezoelectric element. Since the ultrasonic level meter is used in a wide range from −40 degrees Celsius to +60 degrees Celsius or more, considering that the temperature dependence of the resonance frequency f ₀₁ of the piezoelectric element is −0.026 [% / deg], it is 41 kHz to 39 kHz. Fluctuate up to Drive frequency fd from to follow the variation of the resonance frequency f ₀₁ of the piezoelectric element, capable of measuring the ambient temperature of the ultrasonic level meter using the drive frequency fd. If this is used to compensate for the propagation speed of the ultrasonic wave, the error in the distance measurement will be ± 2%
It can be suppressed to the extent.

FIG. 9 is a circuit diagram when the piezoelectric element and the signal converter are connected by a cable. The drive circuit 10 is connected to the cable 22 via the diode D3. The piezoelectric element 20 is connected to the cable 22 via the diode D2, the transformer 60 and the diode D1, and the resistor R1 is connected in parallel between the diode D2 and the diode D1. The receiving circuit 30 includes a diode D4
And a cable 2 through an LC tank circuit and a resistor R2.
2 is connected.

The operation of the apparatus having the above-mentioned configuration will be described below. FIGS. 10A and 10B are equivalent circuit diagrams of the circuit of FIG. 9, in which FIG. 10A shows a transmission time and FIG. 10B shows a reception time. At the time of transmission, the diodes D1, D2, and D3 are short-circuited, so that the drive voltage of the drive circuit 10 is applied to the piezoelectric element via the transformer 60. At the time of reception, diodes D1, D2, D3
Is turned off, so that the reflected wave of the piezoelectric element 20 has a resistance R
1 and R2 to the receiving circuit 30. The diodes D1, D2, and D3 constituting the diode circuit 70 work effectively, and the reflected wave does not pass through the transformer, so that the amplitude of the reflected wave does not decrease.

In the above embodiment, the driving frequency f
Although the case where d is prepared in advance in several steps and the adjustment is performed step by step in the adjustment mode has been described, the present invention is not limited to this, and the drive frequency fd may be determined continuously, and The width of the frequency adjusted in the mode may be determined by appropriately calculating using the amplitude of the reflected wave.

[0033]

As described above, according to the first aspect of the present invention, a drive circuit 10 for outputting a drive signal having a variable drive frequency fd and an ultrasonic wave corresponding to the drive signal of the drive circuit are generated. In an ultrasonic level meter having a radiating piezoelectric element 20 and a receiving circuit 30 for receiving a reflected wave received by the piezoelectric element, the driving frequency is adjusted so that the driving frequency matches the natural frequency of the piezoelectric element. Since the configuration includes the drive frequency adjustment circuit 40 for adjustment, even if the natural frequency of the piezoelectric element fluctuates due to a temperature change or the like, the drive frequency adjustment circuit 40 matches the fluctuated natural frequency. In addition, since the driving frequency is adjusted, the amplitude of the reflected wave in the receiving circuit is favorably maintained.

According to the fourth aspect of the present invention, a drive circuit 10 for outputting a drive signal having a variable drive frequency fd.
And a piezoelectric element 20 that emits an ultrasonic wave in accordance with a drive signal of the drive circuit, and a receiving circuit 30 that receives a reflected wave received by the piezoelectric element, wherein the natural frequency of the piezoelectric element A temperature dependency storage unit 40 for storing the relationship between the temperature and the temperature, and measuring the temperature from the relationship between the natural frequency of the piezoelectric element and the temperature dependency storage unit. Then, since the temperature measurement is performed using the fact that the natural frequency of the piezoelectric element is almost uniquely determined by the fluctuation of the temperature, the measured value of the temperature sensor fluctuates rapidly due to solar radiation or the like, as in the related art, Without causing errors,
No large measurement error occurs.

According to the sixth aspect of the present invention, a drive circuit 10 for outputting a drive signal having a variable drive frequency fd.
And a piezoelectric element 20 that emits an ultrasonic wave according to a drive signal of the drive circuit, and a receiving circuit 30 that receives a reflected wave received by the piezoelectric element. A cable 50 for connecting the piezoelectric element, a transformer 60 for boosting a drive signal transmitted through the cable, and a diode circuit 7 for transmitting a reception signal via the cable independently of the transformer
Since the driving signal is boosted by the transformer, the driving voltage of the driving circuit is low.
Since the received signal is transmitted to the cable by a separate circuit using a diode circuit without passing through a transformer, the signal attenuation of the reflected wave can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration block diagram showing one embodiment of the present invention.

FIG. 2 is an explanatory diagram of a drive signal, a residual vibration, and a reflected wave.

FIG. 3 is an explanatory diagram of a driving frequency fd and a reflected wave amplitude.

FIG. 4 is a diagram illustrating adjustment of a reflected wave amplitude and a driving frequency fd.

FIG. 5 is a flowchart of an adjustment mode of a driving frequency fd.

FIG. 6 is an explanatory diagram of an adjustment mode of a driving frequency fd.

FIG. 7 is a flowchart of a distance measurement mode.

8 is an explanatory view of the temperature dependence of the resonance frequency f ₀₁ of the piezoelectric element.

FIG. 9 is a circuit diagram when a piezoelectric element and a signal converter are connected by a cable.

FIG. 10 is an equivalent circuit diagram of the circuit of FIG. 9;

FIG. 11 is a configuration block diagram of a conventional device.

FIG. 12 is an explanatory diagram for connecting a piezoelectric element and a signal converter with a cable.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Drive circuit 20 Piezoelectric element 22 Cable 30 Receiving circuit 32 Signal processing circuit 40 Drive frequency adjustment circuit 50 Temperature dependence storage unit 60 Transformer 70 Diode circuit

Continued on the front page (72) Inventor Toshiro Kurihara 2-9-132 Nakamachi, Musashino-shi, Tokyo Inside Yokogawa Electric Corporation (72) Inventor Akira Kataoka 2-9-132 Nakamachi, Musashino-shi, Tokyo Yokogawa Electric Co., Ltd. In-company (72) Inventor Toshio Aga 2-93-2 Nakamachi, Musashino-shi, Tokyo Yokogawa Electric Co., Ltd. (72) Inventor Takao Hirose 85, Hisamatsu-cho, Ashikaga-shi, Tochigi Pref. Inventor Yoshinori Ishii No.85 Hisamatsu-cho, Ashikaga-shi, Tochigi Yokogawa Wezac F-term (reference) 2F014 AA04 FB01 GA01 5J083 AA02 AC08 AC40 AD04 AE06 BA01 BB03 BE06 BE47 CB03 CC02

Claims (6)

[Claims] A drive circuit for outputting a drive signal having a variable drive frequency (fd); a piezoelectric element for emitting ultrasonic waves in accordance with the drive signal of the drive circuit; In an ultrasonic level meter having a receiving circuit (30) for receiving a received reflected wave, a drive frequency adjusting circuit (40) for adjusting the drive frequency so that the drive frequency matches the natural frequency of the piezoelectric element. 2. An ultrasonic level meter, comprising: 2. The driving frequency adjusting circuit according to claim 1, wherein the driving frequency adjusting circuit performs a synchronous integration at a first phase of the driving frequency and a circuit integrating at a second phase separated by a quarter period from the first phase. 44), an amplitude calculating circuit (46) for obtaining an increase or a decrease in the amplitude of the received wave from the outputs of the synchronous integrating circuits, and when the amplitude of the received wave is increased by the amplitude calculating circuit, the driving frequency is increased and when the amplitude is decreased. 2. An ultrasonic level meter according to claim 1, further comprising an adjusting circuit for lowering the driving frequency. 3. The ultrasonic level meter according to claim 2, wherein said adjustment circuit selects said drive frequency in a step-like manner near a natural frequency of said piezoelectric element. 4. A drive circuit (10) for outputting a drive signal having a variable drive frequency (fd); a piezoelectric element (20) for emitting ultrasonic waves in accordance with the drive signal of the drive circuit; An ultrasonic level meter having a receiving circuit (30) for receiving a received reflected wave, comprising: a temperature-dependent storage unit (50) for storing a relationship between a natural frequency of the piezoelectric element and a temperature; An ultrasonic level meter for measuring a temperature from a relationship between a natural frequency and a temperature-dependent storage unit. 5. The ultrasonic level meter according to claim 4, wherein the measured temperature is used for correcting a sound speed of a propagation medium of the reflected wave. 6. A driving circuit (10) for outputting a driving signal of a variable driving frequency (fd), a piezoelectric element (20) for emitting ultrasonic waves according to the driving signal of the driving circuit, An ultrasonic level meter having a receiving circuit (30) for receiving a received reflected wave, comprising: a cable (22) connecting the driving circuit and the receiving circuit to the piezoelectric element; and a driving signal transmitted through the cable. An ultrasonic level meter, comprising: a transformer (60) for boosting a voltage; and a diode circuit (70) for transmitting a reception signal via the cable independently of the transformer.