JP2001086587A - Ultrasonic wave transducer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrasonic wave transducer that has desired impedance characteristics and desired frequency pass characteristics over a broadband frequency with a simple configuration. SOLUTION: An inductor L and capacitors Ca, Cb among a resistor R, the inductor L and the capacitors Ca, Cb being constants of an equivalent circuit of a piezoelectric element 10 are used for parts of circuit constants of a butterworth filter and inductors L1, L2 and capacitors C1, C' required to configure the remaining parts of the butterworth filter are connected to the piezoelectric element 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention can be used for an ultrasonic flaw detector, an ultrasonic flow meter, an ultrasonic medical instrument, and the like, and converts an electric signal into an ultrasonic wave or receives an ultrasonic wave to generate an electric signal. The present invention relates to an ultrasonic transducer having a piezoelectric element that converts a current into an ultrasonic wave.

[0002]

2. Description of the Related Art In conventional ultrasonic transducers, when used in a narrow band, there are many examples in which special consideration is not given to frequency pass characteristics such as matching of electric impedance. If impedance matching is to be performed, a configuration shown in FIG. 18 is generally considered. That is, an inductance L0 is connected in series (FIG. 18 (a)) or in parallel (FIG. 18 (b)) to the piezoelectric element 30 of the ultrasonic transducer, and the inductance L0 reduces the reactance in the impedance of the piezoelectric element 30. This is a method of canceling out a pure resistance and matching the resistance of the piezoelectric element 30 to a target impedance by using a transformer T0. Alternatively, as another configuration, FIG.
As shown in (d), a method is conceivable in which the inductance L0 and the capacitance C0 are connected to the piezoelectric element 30 to simultaneously cancel the reactance and match the resistance.

[0003]

However, in such a general impedance matching method, in principle, very good impedance matching is performed for a single frequency, but when a signal having a wide frequency band is handled. In this case, the reactance changes greatly with frequency, and the resistance also changes drastically.Since sufficient impedance matching is not performed in a wide frequency range, the conversion efficiency of electric-ultrasonic waves or vice versa is poor. Also, at frequencies where impedance matching has not been performed, there is a problem that signal reflection may occur, causing unnecessary reverberation.

In addition, for example, reverberation does not become a significant problem due to the use of a delay material, and impedance matching may not be important. Even in such a case, a desired frequency transmission characteristic or the like for an ultrasonic transducer is required. There is a case where it is desired to have characteristics, and there is a problem that such a request cannot be answered with the conventional configuration.

The present invention has been made in view of the above problems, and has as its object to provide an ultrasonic transducer that can have desired characteristics with a simple configuration.

[0006]

According to one aspect of the present invention, there is provided an ultrasonic transducer having a piezoelectric element for converting an electric signal into an ultrasonic signal or converting an ultrasonic signal into an electric signal. By applying a part of the equivalent circuit constant of the piezoelectric element to a part of a filter circuit having a desired characteristic, a circuit element having a circuit constant necessary for forming the remaining part of the filter circuit is used as the piezoelectric element. It is characterized by being connected.

[0007] The equivalent circuit of the piezoelectric element is regarded as a part of the filter circuit, and the concept and design of the ordinary filter circuit are reversed, and a part of the equivalent circuit constant is applied to a part of the filter circuit. Then, a circuit element such as an inductance or a capacitance having a circuit constant necessary for forming the remaining part of the filter circuit is connected to the piezoelectric element, so that a desired filter circuit is formed as a whole. For example, if the resistance in the equivalent circuit is used as the characteristic impedance of the filter circuit and a filter circuit that cancels out the reactance is selected, impedance matching can be achieved.

[0008] The filter circuit may be a Butterworth type filter. The Butterworth filter is also called a maximum flat characteristic filter, and the pass characteristic of the filter is almost flat in the pass band. In this connection, the resistance component of the impedance characteristic in many parts of the passband is equal to a constant value, for example, the characteristic impedance, and the reactance component has a characteristic of being substantially 0Ω. Therefore, a wide band pass characteristic is obtained, and impedance matching is performed over a wide band.

When the equivalent circuit constants are a series inductance L, a series capacitance Ca, and a resistance R connected in series and a parallel capacitance Cb connected in parallel to the series inductance L, the filter circuit has a third order or higher. T-type Butterworth type band-pass filter. Alternatively, when the equivalent circuit constant is a resistance R and a capacitance Cb connected in parallel, the filter circuit may be a third-order or higher π-type Butterworth bandpass filter. A third-order Butterworth-type bandpass filter is preferable because it has a wide frequency pass characteristic and can be easily configured. However, a third-order Butterworth-type bandpass filter cannot be used when a filter circuit cannot be configured in a third order depending on the resistance, inductance, or capacitance of an equivalent circuit. It is advisable to increase the order of the filter in order of 4th and 5th.

If the resistance R of the equivalent circuit is too large for the configurable filter circuit, the resistance R '
Is connected in parallel to the piezoelectric element, and a small resistance combined with R and R ′ can be obtained as compared with the case where only the resistor R in the equivalent circuit is used.

Also, the parallel capacitance C in the equivalent circuit
If b is too small to form a filter circuit, a capacitance C 'is connected in parallel with the piezoelectric element as one of the circuit elements forming the rest of the filter circuit,
Compared to the case where only the parallel capacitance Cb in the equivalent circuit is used, a large capacitance obtained by combining Cb and C ′ can be obtained.

The filter circuit is not limited to the above-mentioned Butterworth type band-pass filter, but may be a Chebyshev type, Bessel type, phase linear type or the like filter. These components constitute the rest of the filter circuit. This can be realized by changing necessary inductance or capacitance parameters according to the respective filter circuits. Then, it is also possible to select any one of the above filter circuits in consideration of the frequency pass characteristic, the impedance characteristic, and the group delay characteristic specific to each filter circuit.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing a first embodiment of the ultrasonic transducer of the present invention. In the figure, the ultrasonic transducer has a lead titanate-based piezoelectric element 10. As shown, the equivalent circuit constants of the lead titanate-based piezoelectric element 10 are a series inductance L, a series capacitance Ca, and a resistor R connected in series, and a parallel capacitance Cb connected in parallel to these.

The resistance R of this equivalent circuit constant is defined as a load resistance, and the series inductance L and the series capacitance C
a, the parallel capacitance Cb is part of the filter circuit, and the inductance L1 and the capacitance C
1, an inductance L2, and a capacitance C 'are circuit elements constituting the remaining part of the filter circuit, and these are connected to the piezoelectric element 10 to form a third-order T-type Butterworth bandpass filter as a whole. In case of third order,
If load resistance = input resistance, L = L1 and Ca = C1. Further, the value of L2 can be obtained from a known formula or a numerical table of a Butterworth bandpass filter. The value of C ′ may be such that the combined capacitance with Cb is equal to the required second-stage capacitance from the formula or numerical table. That is, if Cb is smaller than the required capacitance, C 'can be added to substantially increase the value of Cb. Thus, if Cb is equal to the required capacitance, then C 'is unnecessary, while if Cb is greater than the required capacitance, the filter cannot be constructed in 3rd order, so the order of the filter Up, 4th, 5th
By performing the following, any high-order Butterworth filter can be configured.

FIG. 2 and FIG. 3 show calculated values of the frequency pass characteristic and the impedance characteristic when a third-order Butterworth filter is constructed. As shown in FIG. 2, the characteristic is almost flat within the pass band, and a wide band pass characteristic is obtained. Further, as shown in FIG. 3, the resistance component of the impedance characteristic is, for example, 50Ω and the reactance component is almost 0Ω in many parts of the passband of FIG. Accordingly, the Butterworth filter cancels out the reactance component, and further uses the transformer T to match the resistance component to a characteristic impedance of, for example, 50Ω. Unnecessary electric reflection is suppressed as much as possible, unnecessary reverberation is reduced, and the sensitivity of the ultrasonic transducer can be improved.

On the other hand, in the ultrasonic transducer shown in FIG. 1, L1 and C1 can be eliminated to form a second-order Butterworth filter. In the case of the second order, the pass characteristic is not flat. However, when such a pass characteristic is acceptable, a second-order Butterworth filter may be configured.

FIG. 4 is a circuit diagram showing an ultrasonic transducer according to a second embodiment of the present invention. In the figure, the ultrasonic transducer is a lead niobate-based piezoelectric element 20.
have. The lead niobate-based piezoelectric element 20 is composed of a resistor R and a parallel capacitance Cb connected in parallel without exhibiting electric resonance characteristics.

The resistance R of the equivalent circuit is a load resistance, the parallel capacitance Cb is a part of the filter circuit, and the inductance L21, the capacitance C21, the inductance L22, the capacitance C22, and the inductance L23 form the remaining part of the filter circuit. These circuit elements are connected to the piezoelectric element 20 to form a third-order π-type Butterworth bandpass filter as a whole. In the case of the third order, if load resistance = input resistance, L
21 = L23 and C21 = Cb. Also, L21, L
The values of 23, L22, and C22 can be obtained from known formulas or numerical tables of Butterworth bandpass filters. If the value of R is too large, the value of R can be substantially reduced by adding a resistor R '.
When it is desired to substantially increase the value of the capacitance Cb, the capacitance C ′ can be connected in parallel by the same method as in the first embodiment.

In the second embodiment configured as described above, the same effects as in the first embodiment can be obtained.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An actual configuration example will be described below.

Example 1 As a result of measuring a lead titanate-based piezoelectric element of a 1 MHz ultrasonic transducer with an impedance analyzer, as shown in FIG. 5, a resistor R, an inductance L and a capacitance Ca constituting an equivalent circuit thereof were obtained. , Cb is R = 1.95 kΩ,
L = 476 μH, Ca = 55.5 pF, Cb = 210 p
F. FIG. 6 shows the values of the respective circuit constants when a third-order Butterworth filter is designed using a part of these filters. In FIG. 6, L3 = L and C3 = Ca. L and C
When the cut-off frequency of the band-pass filter is calculated from the value of “a”, the low-frequency cut-off frequency FLc = 706
kHz, high frequency cutoff frequency FHc = 1.358MH
z. In FIG. 6, C2 is 250 pF, but since Cb is 210 pF, C '= 39 pF is added as a shortage, and L1, C1, and L2 are 470 μH, which are close to the standard number within a range where no inconvenience occurs. 56
pF, 100 μH, and with a 1: 6 transformer T,
Example 1 shown in FIG. 7 was configured by impedance matching to approximately 50Ω. FIG. 8 shows the impedance characteristic of the first embodiment in FIG. 7 and FIG. 9 shows the impedance characteristic of the ultrasonic transducer without forming the filter circuit as the first comparative example. As shown in FIG. 8, in the first embodiment, the calculated value considerably matches, and it can be seen that good impedance matching is performed.

FIG. 10 shows the result of measurement of the signal transmission characteristics by acoustically adhering two ultrasonic transducers similar to the first embodiment, and FIG. 11 shows the transmission characteristics of a conventional example without a filter circuit. It is. In FIG. 11, 3 dB
While the bandwidth was 443 kHz and the insertion loss was 30.0 dB, in FIG. 10, the 3 dB bandwidth was 320 kHz and the insertion loss was about 10 dB. As can be seen, the sensitivity of the ultrasonic transducer was improved by approximately 20 dB without sacrificing much bandwidth.

Example 2 As a result of measuring a lead niobate-based piezoelectric element of a 1 MHz ultrasonic transducer with an impedance analyzer, as shown in FIG. 12, the equivalent circuit constant was R = 3.8 kΩ and Cb = 480 pF. Met. In this example, the product of R and Cb is so large that an appropriate bandpass filter cannot be constructed. Therefore, in preparation for a decrease in sensitivity, a resistor R ′ = 470Ω is connected in parallel, and the combined resistance of R and R ′ is set to 418Ω.
FIG. 13 shows the values of the respective circuit constants when a third-order Butterworth filter is designed using a part of them. FIG.
C23 ≒ Cb. In this example, the low-pass cutoff frequency FLc of the band-pass filter is 600 kHz.
z, the high cutoff frequency FHc = 1.4 MHz. In FIG. 13, the standard number of 63 μL, which is close to L21, C21, L22 and C22 within a range that does not cause inconvenience
H, 470 pF, 166 μH, 180 pF, and
Example 2 shown in FIG. 14 was configured by impedance matching of approximately 50Ω with a transformer T2 of 1: 3. FIG. 15 shows the impedance characteristic of the second embodiment of FIG. 14 and FIG. 16 shows the impedance characteristic of the ultrasonic transducer not forming the filter circuit as the second comparative example. It can be seen that also in Example 2, the impedance is matched to 50Ω over a wide band.

FIG. 17 shows the pass characteristics. The solid line shows the pass characteristics according to the second embodiment, and the dotted line shows the pass characteristics of the comparative example 2. In Comparative Example 2, the 3 dB bandwidth was 748 kHz and the insertion loss was 44.5 dB.
The 3dB bandwidth was 598 kHz and the insertion loss was 30.9 dB. In the second embodiment, a resistance of R '= 470Ω is added, and a loss of approximately 10 times is given to R, but as in the first embodiment, the frequency bandwidth is not sacrificed so much. It was possible to improve the sensitivity by about 13.5 dB.

[0026]

As described above, according to the present invention,
A part of the equivalent circuit constant of the piezoelectric element is applied to a part of a filter circuit having a desired characteristic, and a circuit element having a circuit constant necessary for forming the remaining part of the filter circuit is connected to the piezoelectric element. By doing so, the desired characteristics can be imparted to the ultrasonic transducer.

In particular, when the filter circuit is a Butterworth type filter, its frequency pass characteristic becomes almost flat in the pass band, a wide band pass characteristic can be obtained, and impedance matching can be performed over a wide band. Therefore, the bandwidth of the ultrasonic transducer can be widened, and sufficient information can be transmitted. Further, it is possible to obtain an effect that the electro-acoustic conversion efficiency is increased and the sensitivity is improved, and unnecessary reflection of the electric signal is reduced and the signal waveform is not disturbed.

The remaining portion of the above filter circuit can be realized by using general-purpose circuit elements, so that a rise in cost can be suppressed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a first embodiment of an ultrasonic transducer according to the present invention.

FIG. 2 is a frequency pass characteristic of a third-order Butterworth filter.

FIG. 3 is an impedance characteristic of a third-order Butterworth filter.

FIG. 4 is a circuit diagram illustrating a second embodiment of the ultrasonic transducer of the present invention.

FIG. 5 is an equivalent circuit of the piezoelectric element of the first embodiment.

FIG. 6 is a circuit diagram when a third-order Butterworth filter of the first embodiment is designed.

FIG. 7 is an actual circuit diagram of the first embodiment.

FIG. 8 is an impedance characteristic of the first embodiment.

FIG. 9 is an impedance characteristic of Comparative Example 1.

FIG. 10 shows transmission characteristics of the first embodiment.

FIG. 11 shows a transmission characteristic of Comparative Example 1.

FIG. 12 is an equivalent circuit of the piezoelectric element of the second embodiment.

FIG. 13 is a circuit diagram when a third-order Butterworth filter according to the second embodiment is designed.

FIG. 14 is an actual circuit diagram of the second embodiment.

FIG. 15 shows impedance characteristics of the second embodiment.

FIG. 16 shows impedance characteristics of Comparative Example 2.

FIG. 17 shows transmission characteristics of Example 2 and Comparative Example 2.

FIGS. 18A to 18D are circuit diagrams showing a method of performing a conventional general impedance matching.

[Explanation of symbols]

 Reference Signs List 10 piezoelectric element 20 piezoelectric element L series inductance in equivalent circuit Ca series capacitance in equivalent circuit R resistance in equivalent circuit Cb parallel capacitance L1, L2 inductance C2, C 'capacitance T transformer L21, L22, L23 inductance C22, C 'capacitance R' resistance

 ──────────────────────────────────────────────────続 き Continued on front page F term (reference) 2G047 EA00 EA14 GB00 4C301 EE20 GB40 5D019 AA07 AA23 BB00 EE06 FF01 FF02 FF04 FF05

Claims (6)

[Claims] 1. An ultrasonic transducer having a piezoelectric element and converting an electric signal into an ultrasonic signal or an ultrasonic signal into an electric signal, wherein a filter circuit having a part of an equivalent circuit constant of the piezoelectric element having a desired characteristic. An ultrasonic transducer, wherein a circuit element having a circuit constant necessary for forming the remaining part of the filter circuit is connected to the piezoelectric element by applying a part of the circuit constant of the filter circuit. 2. The ultrasonic transducer according to claim 1, wherein said filter circuit is a Butterworth type filter. 3. The equivalent circuit constant is a series inductance L, a series capacitance Ca, and a resistance R connected in series, and a parallel capacitance Cb connected in parallel with the series inductance L, the filter circuit having a third order or higher. 3. The ultrasonic transducer according to claim 2, wherein the ultrasonic transducer is a T-shaped Butterworth bandpass filter. 4. The filter circuit according to claim 1, wherein said equivalent circuit constant is a resistance R and a parallel capacitance Cb connected in parallel, and said filter circuit is a π-type Butterworth bandpass filter of third order or higher. 2. The ultrasonic transducer according to 2. 5. A resistor R ′ is connected in parallel to the piezoelectric element, and a combined resistor obtained by combining the resistor R ′ connected in parallel with the resistor R in the equivalent circuit is connected to a resistor R in the equivalent circuit. 5. The ultrasonic transducer according to claim 3, wherein said ultrasonic transducer is smaller than said ultrasonic transducer. 6. A capacitance C ′ is connected in parallel to said piezoelectric element, and said capacitance C ′ is connected in parallel.
5. The ultrasonic transducer according to claim 3, wherein a combined capacitance combined with the capacitance Cb in the equivalent circuit is larger than the capacitance in the equivalent circuit.