RU2648292C1 - Resonance method of ultrasonic thickness measurement - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: application: for ultrasonic thickness measurement. Invention consists in the fact that the transmitting and receiving transducers are installed on the surface of the monitoring object at the main monitoring point with the possibility of acoustic contact, and the transmitting transducer is excited by broad-base linear-frequency-modulated oscillations. Main echo signal excited at this point of the monitoring object is received and recorded by the receiving transducer. Further, the position of the transmitting or receiving transducers is changed N times on the surface of the monitoring object, in each additional position an additional echo signal is recorded which is used as probing one in the next position of the transducers for excitation of acoustic oscillations; in N position, the amplitude-frequency characteristic of the monitoring object is determined from the detected signal, the value of the frequency corresponding to the maximum amplitude-frequency resonance characteristic of the monitoring object is measured, and the thickness of the object is determined from the frequency value of the maximum of the resonance of the amplitude-frequency characteristic.

EFFECT: technical result: providing the possibility of increasing the accuracy and reliability of ultrasonic thickness measurements.

1 cl, 4 dwg

Description

The invention relates to the field of non-destructive testing by ultrasonic methods and can be used in various branches of engineering for thickness measurement of samples of materials and products, mainly large-sized and with high attenuation of ultrasound.

A known method of resonant ultrasonic thickness gauge using forced oscillations, adopted as a prototype and described in [Non-destructive testing: Reference: in 7 t. Under the general ed. V.V. Klyueva. T. 3 Ultrasonic control / I.N. Eromolov, Yu.V. Lange. - M.: Mechanical Engineering, 2004, p. 292-293]. The method consists in the fact that forced vibrations of a continuously changing frequency are excited in the control object, the amplitude-frequency characteristic of the control object is determined, the values of the frequencies corresponding to the maximum amplitude-frequency characteristic are measured, on which an even number of half-waves fit along the thickness of the control object, and the thickness value H the control object is calculated by the following formula:

where n is the number of harmonics;

C is the speed of ultrasound;

ƒ _n is the resonance frequency corresponding to harmonics n.

The maximum resonant frequency determines the thickness of the control object.

The disadvantage of this method is the low accuracy and reliability of the control, because with a complex configuration of controlled products, several frequency resonances can be excited in them in a narrow frequency range, the values of which are determined by the geometric dimensions of the steps, protrusions, tides, steps, supporting platforms, etc., and which do not allow identification of the desired resonance associated with the measured size, and accurately measure the value of the frequency corresponding to its maximum.

The known resonant method of ultrasonic thickness measurement using forced oscillations, adopted as a prototype and described in [RF patent No. 2354932, IPC G01B 17/02, publ. 05/10/2009]. The method consists in the fact that continuous harmonic oscillations of varying frequency are excited in a monitoring object at one point by a radiating electro-acoustic transducer. An echo signal is received by the receiving electro-acoustic transducer, oscillations excited at this point of the test object are recorded, and the main resonance amplitude-frequency characteristic of the test object is measured. Then, at another point, they excite, emitting harmonic oscillations of a continuously changing frequency, or receive an echo signal in a new position. Thus, for several points of emission or reception of an electro-acoustic signal on the surface of the test object, N additional amplitude-frequency characteristics are measured. All N amplitude-frequency characteristics are mutually multiplied, forming the final resonant amplitude-frequency characteristic, the maximum at which the thickness of the test object is determined at the resonant frequency.

The disadvantage of this method is the low productivity of the method of ultrasonic thickness gauge, associated with the need to register N + 1 of the main and additional amplitude-frequency characteristics, as well as N times the operations of the mutual multiplication of amplitude-frequency characteristics.

The technical problem of the proposed method is to increase the productivity of the method of ultrasonic thickness measurement.

The technical result of the method lies in the fact that interference resonances are suppressed, which significantly increases the reliability of ultrasonic testing while improving accuracy.

This task is achieved by the fact that in the known method of ultrasonic testing, which consists in the fact that at the main point of radiation on the surface of the test object, a radiating transducer is located, and at the main point of reception on the surface of the test object there is a receiving transducer, a probe signal with a continuously changing carrier frequency is supplied to the emitting transducer, which excites acoustic vibrations in the control object, the main transducer receives and registers the main echo signal, N times and change the position of the emitting and (or) receiving transducers on the surface of the control object, in each additional position, excite acoustic oscillations and register additional echo signals, determine the amplitude-frequency characteristic of the control object and measure the frequency value corresponding to the maximum amplitude-frequency resonance characteristic of the control object, and the thickness of the object is determined by the value of the frequency of the maximum resonance of the amplitude-frequency characteristic, as a probe signal, stimulating forced oscillations at the main point of radiation, use a linearly frequency-modulated signal with a large base B, as an probing signal in each subsequent additional position of the emitting and (or) receiving transducers on the surface of the control object use an echo signal recorded in the preliminary additional position of the radiating and (or) receiving converters, the value of N is chosen equal to or greater than 1, and the amplitude-frequency characteristic of the controlled object is determined by the echo registered in the N-th additional monitoring point.

The invention is illustrated by drawings, where in FIG. 1 shows a structural diagram of a device that implements the method of ultrasonic thickness measurement; in FIG. 2.a presents the main echo signal and the main amplitude-frequency characteristic of the controlled object (Fig. 2.b.); in FIG. 3.a, FIG. 3.b and FIG. 3.c shows additionally recorded additional echo signals of three additional measurement cycles, and in FIG. 4 shows the final amplitude-frequency characteristic of the controlled object.

The resonance method of ultrasonic thickness gauge consists in the fact that on the surface of the test object in the main point of radiation set the emitting transducer, and in the main point of reception set the receiving transducer. Further, a probing signal is excited in the control object, emitting harmonic oscillations of a continuously changing frequency. The main echo signal is received: U _main (t) = K _approx. ⋅ U _probe (t), where K _ok. = K _{approx. Base} (ƒ) and the vibrations present at this point of the monitoring object, the amplitude of which depends on the resonance properties of the object of control and which, in turn, are determined by the configuration and properties of the material of the product (see Fig. 2). Next, the position of the emitting and (or) receiving transducers on the surface of the control object is changed, positioning the combination of transducers in the first additional position (N = 1). They again excite mechanical vibrations in the control object, emitting a probe signal of continuously changing frequency, and receive an echo signal U ₁ (t) = K _{approx. Ос} K _approx. ⋅ K _{approx. 1} ⋅ U _probe (t) in a new additional position converters. Similarly, the procedure "repositioning of the transducers + registration of the echo signal" is performed N times. In this case, in each new position of the emitting and (or) receiving transducers on the surface of the control object, in addition to the main position, the sounding signal recorded at the previous stage is an echo signal, and the transducers are moved along a plane that limits the measured thickness. Thus, in the N position of the emission and reception of the electro-acoustic signal on the surface of the test object, the signal registered is U _N (t) = K _approx. ⋅ K _{approx. 1} ⋅ K _ok . ₂ ⋅ K _ok . ₃ .... ⋅ K _{ok .N} ⋅ U _probe (t) (see Fig. 3). As a result, all registered echo signals, and hence the amplitude-frequency characteristics of the test object, characteristic of the selected control points, are mutually multiplied K _approx. ⋅ K _{approx. 1} ⋅ K _ok . ₂ ⋅ K _ok . ₃ .... ⋅ K _{approx. N} , forming the final resonant amplitude-frequency characteristic (see Fig. 4), on which there are almost no interference resonances associated with the complex shape of the control object. Next, the maximum thickness of the resonant frequency determines the thickness of the object of control.

A device that implements a resonant method of ultrasonic thickness gauging comprises an electro-acoustically series-connected linear-frequency-modulated signal generator 1, a switch 2, a radiating electro-acoustic transducer 3, a receiving electro-acoustic transducer 4, an input amplifier 5, a band-pass filter 6, a memory unit 7, an amplitude detector unit 8 and indicator 9. The second output of the memory unit 7 is connected to the second input of the switch 2. The emitting electro-acoustic transducer 3 and the receiving electro-acoustic transducer 4 is placed and acoustically fix them on the surface of the object 10 of the control in the field of controlled size.

The resonant ultrasonic thickness gauge device operates as follows.

The whole measurement procedure consists of N + 1 cycles (main and N additional) corresponding to N + 1 position of the electro-acoustic transducers 3 and 4 on the surface of the measured object 10 (N≥1). In the main measurement cycle, the emitting electro-acoustic transducer 3 is excited once by an extended pulsed linear frequency-modulated (LFM) signal. The nature of the frequency dependence of the spectral density modulus of a rectangular chirp pulse completely depends on the dimensionless number B equal to the product of the frequency deviation Δƒ _c and the duration T _{s of the} pulse and is called the base of the chirp signal. The spectrum of chirp signals with a base of B >> 1 has a number of specific features. First, the spectral density modulus is almost constant here within a frequency band of width Δƒ _s . Secondly, there is a gradual disappearance of the oscillations of the spectral density modulus with an increase in the B signal base. This means that for large B (the value of B is not less than 10 ⁴ ), the spectral density modulus is constant in the frequency band Δƒ _s and vanishes outside this band, i.e. is the best known signal, suitable for use as a test signal in the study of amplitude-frequency characteristics. In the monitoring object 10, the primary probing (LFM) signal is excited at the main monitoring point by a radiating converter 3. Duration T _C.S. linear frequency-modulated signal used as a probe during experimental testing of the proposed technical solution was T _C.s. = 3.4 s, and the frequency deviation Δƒ _s = 5.3 kHz, which corresponds to the value of the base B = 1.8⋅10 ⁴ . The receiving electro-acoustic transducer 4 receives the main echo signal (see Fig. 2.a), which is the oscillations excited in the control object 10, which, after amplification in the amplifier 5 and the filtering band in the band-pass filter 6, enter the memory unit 7. Thus, at the end of the main measurement cycle, the main echo signal is recorded in the memory unit 7, which carries information about the main resonance amplitude-frequency characteristic of the monitoring object 10 (see Fig. 2.b). Next, the emitting electro-acoustic 3 or (and) receiving electro-acoustic transducer 4 is moved to another position on the surface of the control object 10 and they are again fixed. At the command of the operator, the switch 2 disconnects the input of the emitting transducer 3 from the output of the linear frequency-modulated signal generator 1 and connects it to the output of the memory unit 7, in which the main echo signal is detected at this point in time.

In the control object 10 during the first additional measurement cycle, a probing signal from the 2nd output of the memory unit 7 is supplied to the emitting electro-acoustic transducer 3 and harmonic oscillations of a variable frequency are again excited in the control object, the amplitude of which is modulated in accordance with the amplitude-frequency characteristic of the object 10 control at the main point of control. The receiving electro-acoustic transducer 4 registers vibrations, amplifies them in the amplifier 5, filters them in a band-pass filter 6, and the first additional signal, carrying information about the product of the main and first additional amplitude-frequency characteristics (see Fig. 3.a), is fed to the input of block 7 memory in which it is remembered. As a result of the implementation of the first additional measurement cycle, a signal is stored in the memory unit 7 corresponding to the multiplied main and additional amplitude-frequency characteristics of the control object, measured at the main and first additional control points. Then, the second and third additional measurement cycles are implemented similarly, remembering the second and third additional signals (see Fig. 3.b and Fig. 3.c). Thus, sequentially changing the position of the emitting 3 and (or) receiving 4 transducers on the Nth measurement stage on the surface of the monitoring object, the Nth additional signal is recorded that carries information about the final resonant-multiplicative amplitude-frequency characteristic, which is almost completely absent interference resonances associated with the complex shape of the control object. The signal recorded during the Nth additional control cycle is read from the output of the memory unit 7 and, after amplitude detection in the unit 6 of the amplitude detector (see Fig. 4), is fed to the input of indicator 9.

The use of the invention allows to almost completely suppress interference resonances, which significantly increases the reliability of ultrasonic testing while increasing the accuracy of thickness measurement by 3-5 times.

Claims (1)

The resonance method of ultrasonic thickness gauge is that a radiating transducer is located at the main point of radiation on the surface of the test object, and a receiving transducer is located at the main point of reception on the surface of the test object, a probing signal with a continuously changing carrier frequency is fed to the radiating transducer, which in the object acoustic vibrations are excited by the control, the main echo signal is received and recorded by the receiving transducer, the emitting position is changed N times of the receiving or receiving transducers on the surface of the control object, in each additional position, acoustic vibrations are excited and additional echo signals are recorded, the amplitude-frequency characteristic of the control object is determined and the frequency value corresponding to the maximum amplitude-frequency resonance characteristic of the control object is measured, and the thickness of the object is determined by the value of the frequency of the maximum resonance of the amplitude-frequency characteristic, characterized in that, as the probe signal, the excitation stimulated oscillations at the main point of radiation, use a linear-frequency-modulated signal with a large base B, as an probing signal in each subsequent additional position of the emitting or receiving transducers on the surface of the object of control use an echo signal recorded in the preliminary position of the radiating or receiving transducers , the value of N is chosen equal to or greater than 1, and the amplitude-frequency characteristic of the controlled object is determined by the echo signal, registered at the Nth additional control point.

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