JPH08166439A - Detector with ultrasonic sensor - Google Patents

Abstract

PURPOSE: To ensure a stabilized operation regardless of the length of coaxial cable by setting the driving frequency of a circuit on the drive side at such value as the impedance of the circuit on the load side, including the coaxial cable and an ultrasonic element, becomes constant. CONSTITUTION: An ultrasonic element 1 is connected through a coaxial cable 3 with a circuit 2 on the drive side thereof. The circuit 2 comprises a transmission system including an oscillation circuit 21 and a drive circuit 22, and a receiving system including a receiving circuit 23, a detection circuit 24 a signal processing circuit 25 and an output circuit 26. The driving frequency of the drive circuit 22 is set at such value as the impedance of a circuit 13, including the coaxial cable 3 and the ultrasonic element 1, is kept constant regardless of the length of the coaxial cable 3. With such arrangement, the operation can be stabilized regardless of the length of the coaxial cable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a detecting device equipped with an ultrasonic sensor for transmitting an ultrasonic pulse to an object and receiving a reflected wave from the object.

[0002]

2. Description of the Related Art Conventionally, as an ultrasonic sensor, there is known an ultrasonic sensor in which an ultrasonic element such as a piezoelectric element is driven at a high frequency by using impulse or constant current. Further, there is also known one in which an inductor is connected in series to an ultrasonic element to form a series resonance system, thereby improving transmission / reception efficiency (see, for example, Japanese Patent Laid-Open No. 61-220591).

[0003]

By the way, in the ultrasonic diagnostic apparatus for medical use, the length of the coaxial cable connecting the ultrasonic element and the circuit on the drive side is short and constant, whereas In liquid level meters such as storage tanks in factories, the length of the coaxial cable is long, and it is necessary to change the length of the coaxial cable each time depending on the situation of the factory and the site.

Here, when the ultrasonic element is driven with a constant current, the coaxial cable has an electric capacitance, so that when the coaxial cable becomes longer, the current supplied to the ultrasonic element decreases and the amplitude is reduced. It becomes smaller and the detection sensitivity decreases.

On the other hand, when the ultrasonic element is driven by the series resonance system, the resonance matching the natural frequency of the ultrasonic element is taken into consideration in consideration of the capacitance between the electrodes of the inductor and the ultrasonic element and the capacitance of the coaxial cable. The frequency is set. However, in such a resonance system, as the length of the coaxial cable changes, the impedance of the circuit changes significantly as the capacitance of the entire circuit changes, so the resonance point deviates and the ultrasonic element vibrates. Disappear.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to provide a detection device equipped with an ultrasonic sensor in which an ultrasonic element and a drive-side circuit are connected via a coaxial cable. It is to obtain stable operation regardless of the length of the cable.

[0007]

In order to achieve the above-mentioned object, the present invention has the following advantages even when the length of the coaxial cable is changed.
The drive frequency of the drive side circuit is set to a frequency at which the impedance of the load side circuit including the coaxial cable and the ultrasonic element is constant.

[0008]

Next, the principle of the present invention will be described. Figure 6
Shows a value obtained by an experiment in which, when a coaxial cable is connected to the ultrasonic element, the length La of the coaxial cable is changed and the change of the impedance Z with respect to the driving frequency is obtained. Here, as shown in FIG. 5, since the capacitance C2 of the coaxial cable 3 is connected in parallel to the capacitance C1 of the ultrasonic element 1, the ratio of the L component to the C component increases, When the length La of the coaxial cable 3 is increased, the impedance Z of the entire load-side circuit 13 generally decreases, as shown in FIG. However, the present inventor has found that the impedance Za does not change even if the length La of the coaxial cable 3 is changed at a specific frequency fa, and completed the present invention. It is presumed that the reason why the impedance Za does not change is that the coaxial cable 3 can be regarded as having the resistor R2 and the inductor L2 in series.

According to the present invention, even if the length La of the coaxial cable 3 changes, the impedance of the circuit on the load side becomes constant. Therefore, for example, by driving at constant voltage, the supply current to the ultrasonic element 1 becomes constant. Can be

[0010]

Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, a case where the liquid level meter is applied will be described. As shown in FIG. 1, two ultrasonic elements 1 ₁ , 1 ₂ are provided on the outer wall surface of the storage tank 5 for the food liquid 6.
Is attached. The upper ultrasonic element 1 ₁ is attached to the upper limit level position of the liquid surface, and the lower ultrasonic element 1 ₂ is attached to the lower limit level position of the liquid surface.

In FIG. 3, the ultrasonic element 1 is connected to the drive-side circuit 2 via a coaxial cable 3. The drive side circuit 2 includes a transmission system including an oscillation circuit 21 and a drive circuit 22, a reception circuit 23, a detection circuit 24,
The reception system includes a signal processing circuit 25 and an output circuit 26. The oscillation circuit 21 outputs an oscillation signal a having the same frequency as the natural frequency of the ultrasonic element 1 to the drive circuit 22. The drive frequency of the drive circuit 22 is the impedance of the load side circuit 13 (FIG. 3) including the coaxial cable 3 and the ultrasonic element 1 even when the length La of the coaxial cable 3 changes, as shown in FIG. The frequency fa is set so that Z is constant Za.

An example of the drive circuit 22 is shown in FIG. In FIG. 4, the transformer T is driven by receiving the oscillation signal a on the primary side, and when a voltage is induced in the arrow direction on the secondary side of the transformer T, the base voltage of the NPN transistor Q1 is higher than the emitter voltage. When the voltage becomes higher, the transistor Q1 is turned on and a voltage of 120 V appears at the point b. On the other hand, when a voltage is induced in the direction opposite to the arrow on the secondary side of the transformer T, the base voltage of the PNP transistor Q2 becomes lower than the emitter voltage, so that the transistor Q2
Turns on and the potential at point b becomes 0V. Thereafter, similarly, both transistors Q1 and Q2 are alternately turned on by the oscillation signal a.
Turned off. That is, the drive circuit 22 acts as a constant voltage source for the load side circuit 13 including the ultrasonic element 1 and the coaxial cable 3 shown in FIG.

Thus, both transistors Q1,
The base-emitter voltage of Q2 changes via the transformer T by the oscillation signal a which is a low voltage, and the push-pull operation is performed. Therefore, both transistors Q1 and Q2 can be switched with respect to a high frequency of several MHz without delay. In addition, power C-MOS
The electric field transistors Q3 and Q4 are provided for quickly stepping up to a high voltage of 120V and stepping down to 0V.

In FIG. 3, the oscillation signal a is amplified to 120 V as described above and then applied to the ultrasonic element 1 through the coaxial cable 3. As a result, an ultrasonic pulse is generated, is transmitted toward the inside of the tank 5, and the reflected pulse reflected by the wall surface (boundary surface) of the tank 5 is converted into an electric signal again by the ultrasonic element 1. This electric signal is input to the receiving circuit 23 through the coaxial cable 3 and amplified. The amplified received signal c is detected by the detection circuit 24.
Is output as a detection signal d, and the signal level is determined by the signal processing circuit 25. The output circuit 26 receiving the discrimination signal e output from the signal processing circuit 25 outputs a control signal f corresponding to the discrimination signal e.

Next, the operation of the liquid level meter will be described. When the ultrasonic element 1 _1, 1 ₂ of the liquid level 6a than the mounting of Figure 1 is low, the ultrasonic pulse emitted from the ultrasonic element 1 are reflected alternately at the inner wall surface and the attachment the outer wall surface of the storage tank 5 That is, the reflection is repeated in the range of D1. on the other hand,
When the liquid level 6a is higher than the mounting position of the ultrasonic element 1, the ultrasonic pulse emitted from the ultrasonic element 1 is alternately reflected on the outer wall surface of the storage tank 5 on which it is attached and the outer wall surface on the opposite side, That is, the reflection is repeated in the range of D2.

[0016] Therefore, when the liquid 6 is not in the mounting position of the ultrasonic element 1 _1, since the reciprocating distance of the ultrasonic pulses is short, as shown in FIG. 2 (a), the attenuation between the reflected pulse is short To do. On the other hand, when the liquid 6 is present in the mounting position of the ultrasonic element 1 _2, since the reciprocating distance of the ultrasonic pulses is long,
As shown in FIG. 2B, it takes time for the reflected pulse to attenuate. Therefore, the transmission timing t ₀ shown in FIG.
From a certain time (4mse depending on the tank shape
In measurement period t ₂ which is provided on c about) t ₁ after, by comparing the detected signal d and the threshold level, it is possible to determine the presence or absence of the liquid 6 in the ultrasonic element 1 _1, 1 ₂ of the mounting position .

FIG. 5 shows an electrical equivalent circuit of the ultrasonic transmission system of FIG. In FIG. 5, the ultrasonic element 1 can be replaced with the interelectrode capacitance C1 when the voltage is applied, and the coaxial cable 3 can be replaced with the capacitance C2. The internal resistance of the ultrasonic element 1 and the coaxial cable 3 is extremely small and can be neglected when viewed as the impedance of the entire circuit, so it is not shown. The applied voltage across the ultrasonic element 1 is the voltage obtained by subtracting the voltage drop of the coaxial cable 3 from the voltage of 120V set in the drive-side circuit 2. Therefore, even if the length of the coaxial cable 3 is arbitrarily set, the applied voltage across the ultrasonic element 1 hardly changes, so that the current flowing through the ultrasonic element 1 becomes almost constant, and stable operation can be obtained. Also,
The length of the coaxial cable 3 can be freely increased.

However, when the length of the coaxial cable 3 changes, it can be considered that the coaxial cable 3 has a resistor R2 and an inductor L2 (not shown) in series in addition to the capacitance C2, and therefore the drive circuit 22. Even if is set to a constant voltage, generally, the current flowing through the ultrasonic element 1 slightly changes. On the other hand, in the present invention, the drive frequency of the drive circuit 22 is set to a frequency at which the impedance Z of the load-side circuit 13 becomes constant even when the length of the coaxial cable 3 changes. Therefore, the value of the current flowing through the ultrasonic element 1 becomes constant, so that the sound pressure level to be transmitted can be made substantially constant regardless of the length of the coaxial cable 3.

In the present invention, as shown in FIG.
Even in a resonance circuit provided with the inductor L, since the resonance frequency does not change, the ultrasonic element 1 can be driven at the same frequency, and thus is included in the scope of the present invention. However,
Since the voltage across the ultrasonic element 1 changes as L2, R2, C2 of the coaxial cable 3 change, the length of the coaxial cable 3 is limited.

The driving frequency is preferably about 2 MHz in the case of a metal tank, while it is preferably about 300 KHz in the case of a resin tank.

In addition to the piezoelectric element, a piezoelectric polymer or the like may be used as the ultrasonic element.

[0022]

As described above, according to the detection device having the ultrasonic sensor of the present invention, the ultrasonic device is connected to the drive side circuit including the oscillation circuit through the coaxial cable, Since the drive frequency of the drive side circuit is set to a frequency at which the impedance of the load side circuit becomes constant even if the length of the cable is changed, stable operation can be obtained regardless of the length of the coaxial cable.
Also, if the drive side and load side circuits are set so that the applied voltage across the ultrasonic element is almost constant even when the stray capacitance of the coaxial cable changes, the length of the coaxial cable can be adjusted according to the site of use. The length can be lengthened.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a tank to which an embodiment of the present invention is applied.

2 (a) and 2 (b) are characteristic diagrams of a reception signal of the same liquid level meter with and without liquid.

FIG. 3 is a block diagram of a detection device including an ultrasonic element according to an embodiment of the present invention.

FIG. 4 is an electric circuit diagram of a drive circuit in the example.

FIG. 5 is an electrical equivalent circuit diagram of the embodiment.

FIG. 6 is a characteristic diagram showing changes in impedance with respect to frequency when the length of the coaxial cable is changed.

FIG. 7 is a circuit diagram showing another circuit to which the present invention can be applied.

[Explanation of symbols]

1, 1 ₁ , 1 ₂ : Ultrasonic element 2: Drive side circuit 3: Coaxial cable 13: Load side circuit 21: Oscillation circuit 22: Drive circuit

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location G01N 29/22 501

Claims (3)

[Claims] 1. A detection device including an ultrasonic sensor in which an ultrasonic element is connected to a drive-side circuit including an oscillation circuit via a coaxial cable, wherein the coaxial cable is used even when the length of the coaxial cable is changed. A detection device provided with an ultrasonic sensor, wherein a drive frequency of the drive side circuit is set to a frequency at which impedance of a load side circuit including the cable and the ultrasonic element is constant. 2. The detection device according to claim 1, wherein the circuit on the drive side includes an ultrasonic sensor provided with a drive circuit for amplifying the voltage of the oscillation circuit. 3. The drive side circuit according to claim 1, wherein the applied voltage across the ultrasonic element by the drive side circuit is substantially constant even when the stray capacitance of the coaxial cable changes. A detection device including an ultrasonic sensor in which a circuit on the load side is set.