JPH0123767B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention performs pulse compression using an acousto-optic method to reduce the peak value of a transmitted pulse, and
Sonar method (method) using acousto-optic pulse compression to improve the signal-to-noise (S/N) ratio of received signals
Regarding.

Traditionally, radar, sonar, ultrasonic flaw detectors,
In order to improve the S/N ratio when receiving pulse signals in pulse signal transmission lines and the like, time axis compression processing of pulse signals has been realized by many methods. The principle of time-base compression of pulse signals is to output a temporally integrated pulse input signal all at once. For this reason, two methods are considered to be basic: a method of temporarily storing pulse input signals, and a method of simultaneously releasing these stored signals. In terms of electrical circuits, for example, there is a method of instantaneously discharging a signal stored in a capacitor or the like using a trigger signal, but the circuit configuration is difficult in the high frequency range, and the trigger signal with appropriate timing is difficult. There are drawbacks that are not easily obtained. Currently, the most common and effective method is pulse time compression using a linear FM signal and a surface acoustic wave filter, but in this method, the usable signal frequency is limited depending on the shape and physical properties of the surface acoustic wave element. Number 10M
It is limited to high frequencies above Hz. In order to use this method in the ultrasonic frequency range (several tens of kilohertz to several megahertz), it is necessary to add special signal processing, but compared to the complexity of the signal processing, we cannot expect much improvement in the S/N ratio. . From these points, it is desired to investigate a simple and effective pulse time compression method using the surface acoustic wave element described above in a sonar method (method) in the ultrasonic frequency domain.

In view of the above, the present invention compresses the time axis of a pulse signal by performing acousto-optic spatial signal processing to reduce the signal peak value during transmission, and when the received signal contains many noise components. It is an object of the present invention to provide a sonar system (method) that can obtain a good S/N ratio even when the received signal itself is weak.

For this purpose, in the present invention, a received signal is converted into an ultrasonic signal, which is a spatial signal, and the signal is processed.
At this time, the received signal is radiated as an ultrasound signal into two ultrasound propagation media having different sound velocities, so that the information of the received signal is equally contained, and the spatial axis of the signal is By simultaneously forming two types of spatial signals with different spatial lengths, holding them spatially, and performing real-time correlation calculations on these two signals at an appropriate time,
We devised a method to obtain the received signal with time axis compression. Furthermore, we investigated the waveform of the spatial signal so that the real-time correlation calculation output can be efficiently pulsed, and devised the properties of the signal waveform used for pulse time axis compression.

Since the main part of the present invention is an acousto-optic pulse time axis compression method, this method will be explained first. The spatial phase grating forming signal used for pulse time axis compression is configured as follows. First, the ratio v _k / of ultrasonic propagation velocities v ₁ , v ₂ , ... v _o (n2)
v _k-1 (k=2, 3,...n) is a constant value R (0<R
Assume at least two types of ultrasound propagation media such that <1). FIG. 1 shows an example of the spatial phase grating forming signal 1 when the ratio R is determined, and the horizontal axis shows time and the vertical axis shows the amplitude value. This spatial phase grating forming signal 1 is composed of r consecutive segments (segments), and each of these segments is composed of signals of mutually different time frequencies. Set the time frequency f ₁ of the first segment 2 and the segment duration T ₁ to arbitrary values, and set the time frequency of the second segment 3 to
f ₁ /R, the duration time T ₁ and R, and each of the third segment 4 as f ₁ /R ² and T ₁ ·R ² , and so on. This produces a signal 1 for forming a spatial phase grating in which the temporal frequency of each segment increases by a ratio of 1/R and the duration of each segment decreases by a ratio of R.

Next, an example of a liquid cell for acousto-optical electrical signal processing for inputting the ultrasonic spatial phase grating forming signal 1 and simultaneously realizing two types of ultrasonic signals having spatially different lengths. The configuration diagram in the second
As shown in the figure. An ultrasonic transducer 6 and an ultrasonic absorbing member 7 are arranged to face each other on one pair of opposing wall surfaces of the rectangular parallelepiped cell 5, and a pair of light transmitting windows is provided on the other opposing wall surface of the rectangular parallelepiped. 8a and 8b are provided. Monochromatic plane wave light from outside the cell passes through this light transmission window, enters the cell, and is further emitted outside the cell. The inside of the cell is divided into two chambers by a single transparent partition plate 9, and the portion where this partition plate and the vibrator come into contact, that is, the portion corresponding to the edge of the partition plate, prevents the vibration of the vibrator. In order to prevent this, the two chambers are separated using an elastic member 10 that does not allow liquid to pass through. These two chambers are filled with liquid ultrasonic propagation media 11a and 11b having different ultrasonic propagation velocities, respectively.
These two liquids are kept liquid-tight without leaking to each other by the partition plate and the elastic member. An input terminal 12 for supplying the spatial phase grating forming signal 1 is connected to the ultrasonic transducer. Note that the outer shape of the cell is not limited to a rectangular parallelepiped.

FIG. 3 shows an embodiment of an acousto-optic pulse time-base compressor constructed using the liquid cell. In a Fourier transform optical system that combines the liquid cell 5, a lens 13, an optical filter 14, and a photodetector 15, the ultrasonic transducer in the cell is formed with a spatial phase grating having the segment period and frequency described above. When a liquid signal 1 is applied, the two ultrasonic signals in the cell caused by this signal cause the liquid to enter the cell due to the spatial phase grating, which is a regular density change generated in the propagation medium. Monochromatic plane wave light undergoes spatial phase modulation. Therefore, if the monochromatic plane wave light emitted from the liquid cell is focused by passing through the lens 13, a diffraction image is generated at the focal plane of the lens. Of this diffraction image, only a specific diffraction bright spot generated as a result of passing through the two spatial phase gratings together is detected by the optical filter 14 and converted into an electric signal by the photodetector 15. , it is possible to obtain a time-compressed signal of the electrical signal applied to the vibrator by the principle of pulse time axis compression, which will be described later.

Next, the principle of pulse time axis compression will be described. Fourth
The figure shows two superimpositions generated in the acousto-optic electrical signal processing liquid cell shown in FIG. 2 when the above-described spatial phase grating forming signal 1 is inputted from the electrical signal input terminal 12. The states of the acoustic wave phase gratings 16a and 16b are shown. The ultrasonic signals emitted from the ultrasonic transducer 6 propagate through the ultrasonic propagation media 11a and 11b at different velocities v _a and v _b, respectively. At this time, if it is assumed that the ultrasonic signal is composed of, for example, five segments, the length l ₁ of each segment of the ultrasonic spatial phase grating 16a traveling at a speed v _a in the medium 11a is calculated by the formula
It is given by l _i =T・R ^i-1 , l ₁ =T・v _a , l ₂ =T・R.
v _a , l ₃ = T・R ²・v _a , l ₄ = T・R ³・v _a , l ₅ T・
R ⁴・v _a . Moreover, the spatial frequency k _i in each segment is given by the formula k _i =f・R ^1-i / v _a , k ₁ = f/v _a , k ₂ = f/R・_va , k ₃ = f/
R ² · v _a , k ₄ = f/R ³ · v _a , k ₅ = f/R ⁴ · v _a . Similarly, the segment length in the medium 11b
l' _i and spatial frequency k' _i can be obtained by replacing v _a with v _b in the formulas for l _i and k _i . Here, if the values of l' _i and k' _i in the medium 11b are calculated using the condition of R=v _b /v _a , then l' ₁
= T・R・v _a , l′ ₂ = T・R ²・v _a , l′ ₃ = T・R ³・
v _a , l′ ₄ = T・R ⁴・v _a , l′ ₅ = T・R ⁵・v _a , and,
k′ ₁ = f/R・_va , k′ ₂ = f/R ²・_va , k′ ₃ = f/
R ³ · v _a , k' ₄ = f/R ⁴ · v _a , k' ₅ = f/R ⁵ · v _a are obtained. That is, the second to fifth segments of the ultrasonic spatial phase grating 16a traveling in the medium 11a have different spatial lengths and spatial frequencies, respectively, to the first to fourth segments thereof traveling in the medium 11b. Both are in perfect agreement. Furthermore, these two ultrasonic spatial phase gratings each have different speeds.
Since it is progressing at v _a and v _b , the spatial phase grating 1
6b reaches a position a distance x from the vibration surface of the ultrasonic transducer 6, the spatial phase grating 16a moves further in the traveling direction by a distance x(1-R)/R from the position x. I'm here. Therefore, the difference in position between the spatial grids 16b and 16a is x(1-
At the time when R)/R becomes equal to the first segment length _l1 of the spatial phase grating 16a, as shown in the figure, the second to fifth segments of the spatial phase grating 16a are spatially the same. 16b 1st to 4th
Segments completely overlap. Based on the principle of optical cross-correlation calculation, the brightness of the diffraction bright spot of the acousto-optic pulse time axis compressor shown in FIG. 3 is proportional to the degree of overlap of the two spatial phase gratings in the liquid cell. (Japanese Patent Application No. 178589, filed on the same day by the same applicant as the present application,
No. 27696 “Ultrasonic time axis compression method, patent no.
1434803) As shown in FIG. 4, the maximum value is reached at the time when the two spatial phase gratings 16a and 16b completely overlap.

Next, it will be explained using simple computer simulation results that the output from this cross-correlation calculation becomes a time-base compressed signal of the input signal.

FIG. 5 is an example of a computer simulation result of the time base compression operation in the acousto-optic pulse time base compressor shown in FIG. For simplicity,
These are the results obtained when there are two ultrasonic propagation media. The frequency-modulated signal shown at the bottom of the figure is a modeled input signal, and is the ultrasonic spatial phase grating forming signal 17. The uneven waveform 18 in the upper part of the figure shows the intensity of the diffraction bright spot, and at the point showing the maximum peak value, the degree of overlap between the two ultrasonic signals generated in the cell by the input signal, that is, Indicates that the cross-correlation value is maximum. It can be seen that the half-width of the waveform indicating the peak value is sufficiently short compared to the duration of the input signal, indicating that pulse time axis compression is performed satisfactorily. The peak value of the time-axis compressed output pulse wave is proportional to the product of the input signal's peak value (amplitude value) and duration, so even if the input signal has a small peak value, if its duration is long, the pulse time A large peak value can be obtained by compression. Furthermore, when there are two or more media, the number of ultrasonic spatial phase gratings generated in the liquid cell increases, making it possible to calculate higher-order cross-correlation values, making it possible to obtain even steeper output waveforms. can. Moreover, if the segment length of the spatial grating forming signal 1 is shortened to about the ultrasonic period, there is no need to divide it into segments and perform frequency modulation, and frequency modulation can be applied by approximating an appropriate nonlinear curve. It is also possible to use electrical signals as said input signals.

Next, a sonar using the pulse time axis compression method according to the present invention will be explained by showing configuration diagrams of some specific embodiments. The configuration of the sonar can be roughly divided into two types depending on whether the detection method (method) uses transmitted ultrasonic signals or reflected signals. It is subdivided into several parts.

FIG. 6 shows the basic configuration of a transmission sonar. Detection ultrasonic signal 2 for object to be measured 19
The transmitting transducer 21 and the receiving transducer 22 are placed in a transducer holder 23 so that
is held by. This vibrator holder can change its position with respect to the object to be measured by a mover 24, and generates an electric signal indicating the amount of movement at that time or the relative position of the object to be measured and the holder. be able to. The electric signal applied to the transmitting vibrator is a sine wave signal frequency-modulated using the above-described specific segment period and frequency, and further amplitude-modulated into a rectangular pulse having an appropriate duration. This signal is obtained by modulating a sine wave signal generated by the first oscillator 25 using a modulation signal from a modulation signal generator 26 in an ultrasonic/amplitude modulator 27 . At this time, the modulation signal generator generates a trigger signal representing the start time of amplitude modulation, that is, the emission time of the detection ultrasonic signal 20. The signal output from the frequency/amplitude modulator is sent to an amplifier, where it is amplified to a predetermined amplitude value, and then applied to the transmitting transducer, where it becomes an ultrasonic wave and is radiated toward the object to be measured. Ru. The ultrasonic signal transmitted through the object to be measured and detected by the receiving transducer has a time delay depending on the movement and material of the object to be measured. This time-delayed detection signal passes through an amplifier and is sufficiently amplified in amplitude before being input to the amplitude modulator 28. On the other hand, a resonant frequency sine wave of the ultrasonic transducer 6 placed in the liquid cell is sent from the second oscillator 29 to this amplitude modulator,
This sinusoidal signal is amplitude modulated in the amplitude modulator by the time-delayed detection signal. The output signal of the amplitude oscillator is power amplified by an amplifier and then applied to the ultrasonic transducer in the liquid cell. an acousto-optic pulse time-base compressor 30 using the liquid cell and a Fourier transform optical system;
The configuration and operation of the system have already been described in detail. The output from the acousto-optic pulse time base compressor is sent to the display device 31, where it is combined with the position signal from the moving device and the trigger signal from the modulation signal generator to display the state of the object to be measured. Display measurement values.

The above is the basic configuration and operating principle of a transmission sonar.The components of this type of sonar are roughly divided into the acousto-optic pulse time base compressor 30 and the transmission signal generator 32, which are surrounded by dotted lines in the figure. , and a received signal processing section 33. It is also possible to realize a reflection sonar using exactly the same operating principle. In this case, it goes without saying that the receiving transducer is placed at a position where the transmitted ultrasonic signal hits the object to be measured, is reflected, and returns. Furthermore, in the reflection sonar method, a transducer having the same frequency characteristics as the transmitting/receiving transducer is often used. In such cases, the transmitting and receiving transducers can be covered by a single transducer, simplifying the equipment and
Also, it is useful for downsizing the transmitting/receiving section of the ultrasonic signal for detection. FIG. 7 shows a configuration diagram of a reflection sonar using one such transceiver transducer 34. The basic configuration is the same as that of the transmission sonar shown in FIG. Therefore, a new switch 35 is added which is operated by a trigger signal from the modulation signal generation phase. Further, in place of the transmissive sonar mover 24, a transducer direction controller 36 is provided which changes the direction of the ultrasonic transducer and determines the emission direction of the detection ultrasonic wave. An electrical signal is generated that represents the direction in which the ultrasound waves are emitted. The two types of sonar described above are examples in which signals of different frequencies are used for the ultrasonic signal for detection and the ultrasonic signal for compressing the pulse time axis within the liquid cell, and the ultrasonic signal for detection uses signals of different frequencies. There is an advantage that an ultrasonic signal in a frequency band suitable for the material of the object to be measured and the measurement item can be used.

On the other hand, by using ultrasonic signals in the same frequency band for the detection ultrasonic waves and the compression ultrasonic waves, the configuration of the apparatus can be simplified. FIG. 8 shows a configuration example in which the same waveform signal is used for the two types of ultrasonic signals in a transmission sonar. In the configuration shown in this figure, compared to the configuration of the transmission sonar shown in FIG. It is configured to be input to an optical pulse time-base compressor 30.
Therefore, the electrical signal applied to the transmitting vibrator 21 must be a signal having the segment period and frequency described above and which can be directly subjected to correlation processing in the compressor. This signal is transmitted by the output signal of the sine wave oscillator 37 which generates the resonance frequency sine wave of the vibrator in the liquid cell, by the modulation signal of the modulation signal generator 26.
It is created by modulating it in the frequency/amplitude modulator 27. In this configuration, the usable frequency band of the detection ultrasonic signal is determined by the ultrasonic signal used in the compressor, but the device can be simplified in this way by pulse time axis compression. This is one of the major features of the sonar system of the present invention, which uses ultrasonic signals.

FIG. 9 shows an example of a configuration in which a reflection sonar is simplified. The basic configuration is similar to the configuration of the reflection sonar shown in FIG. 7, and the simplification is done by omitting the received signal processing section 33, as in the case of FIG. 8.

Comparatively considering the four embodiments shown in FIGS. 6 to 9, in the embodiments shown in FIGS. 6 and 7, after the detection ultrasonic signal is irradiated onto the object to be measured, Child 2
The signal from the second oscillator 29 is converted into an electric signal by the transducer 2 or the transceiver 34, and is guided to the received signal processing section 33, where the signal from the second oscillator 29 is modulated by the vibration modulator 28 to generate a spatial phase grating. It is used as a formation signal. That is, a frequency suitable for the operation of the acousto-optic pulse time-base compressor 30 is also used as a signal.

On the other hand, in the embodiments shown in FIGS. 8 and 9, after the detection ultrasonic signal is irradiated onto the object to be measured, it is converted into an electric signal by the receiving transducer 22 or the transceiver transducer 34. However, this electric signal (signal for forming a spatial phase grating) is directly guided to the acousto-optic pulse time base compressor 30, and the received signal processing section 33 is not required.

In either case, a signal for forming a spatial phase grating to the acousto-optic pulse time compressor 30 is generated from an ultrasonic signal that has passed through or reflected from the object to be measured.

In addition to the embodiments described above, several different embodiments can be considered depending on the method of amplitude modulation of the detection ultrasonic signal, that is, analog modulation or digital modulation. For example, by digitizing the modulation signal generator 26 used in the above embodiment,
A logic circuit with a simpler configuration than an analog circuit can realize an externally programmable modulation signal generator. As a result, the frequency, repetition period, waveform, etc. of the ultrasonic detection signal can be adjusted according to the properties of the object being measured. It is also possible to change it arbitrarily according to the situation.

As described above, in the sonar method using acousto-optic pulse compression according to the present invention, signals in the ultrasonic range (several KHz to several MHz) are compressed on the time axis by real-time correlation calculation, and as a result, the signal waveforms are S/N by increasing the high value and selecting only the signal in the noise
It has the effect of improving the ratio. This type of sonar can reduce the amplitude intensity of the emitted detection ultrasonic signal, so it is effective for measuring and inspecting objects to be measured, living organisms, etc. that are easily affected by ultrasonic waves. Furthermore, the transducer for transmitting and receiving can be miniaturized and can be applied to the measurement of minute objects to be measured, and many practical effects can be expected.

[Brief explanation of drawings]

FIG. 1 shows the configuration of a signal for forming a spatial phase grating, FIG. 2 shows the configuration of a liquid cell for acousto-optic electrical signal processing, and FIG. 3 shows an acousto-optic pulse time axis compressor. FIG. 4 is a diagram showing the state of two ultrasonic phase gratings in a liquid cell. FIG. 5 is a diagram showing computer simulation results of pulse time axis compression, and FIGS. 6 to 9 The figure is a block diagram of an embodiment of a sonar based on the method of the present invention. In the figure, 1 is a signal for forming a spatial phase grating, 11a,
11b is an ultrasonic propagation medium, 13 is a lens, and 14 is a spatial filter.

Claims (1)

[Claims] 1 In the sonar method that detects the shape, position, material, etc. of a measured object from an ultrasonic signal that passes through or reflected from the measured object, the propagation velocity V _o
(n2) ratio V _k /V _k-1 (k=2, 3, 4,...)
is a constant value R (0<R<1), by simultaneously emitting the spatial phase grating forming signal 1 formed from the ultrasonic signal into at least two types of ultrasonic propagation media 11a and 11b in the same direction. forming an ultrasound spatial phase grating in each of the ultrasound propagation media;
A monochromatic plane wave light is passed in a direction perpendicular to the ultrasonic spatial phase grating and is focused by a lens 13 to create a diffraction image; a bright spot of the diffraction image at a predetermined position is detected via a spatial filter 14; The method consists of detecting the shape, position, material, etc. of the object to be measured based on the difference between the emission time of the ultrasonic signal emitted towards the object and the time when the bright spot of the diffraction image is detected, and the spatial phase grating The forming signal is an ultrasonic signal of time frequency f for T time, hereinafter, an ultrasonic signal of time frequency f/R for RT time, and an ultrasonic signal of time frequency f/R ² for time T.
R ² T time, time frequency f/R ³ ultrasonic signal
R ³ A sonar method using acousto-optic pulse compression, which is characterized by sequentially switching and emitting signals for T time... (T is an arbitrary time).