RU2593444C1 - Method and device for measuring vibrations parameters of ultrasonic waveguide tip - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention can be used for measuring parameters of ultrasonic waveguide tip longitudinal vibration. This invention consists in that waveguide tip vibration acceleration is transmitted to measuring converter, converting energy of mechanical oscillations into electric signal with known by calibration results conversion factor by acceleration, transmitting said signal via cable to measuring device, measurement of said signal with its further conversion into acceleration value, wherein transmission of acceleration is performed on hydrophone with piezo ceramic element freely resting by its piezo on vibrating tip.

EFFECT: technical result is possibility of taking measurements by waveguide tip under conditions when vibration sensor securing on it is not possible.

12 cl, 5 dwg, 1 tbl

Description

The invention relates to the field of measuring equipment, namely to vibrometry, and can be used to measure the acceleration, speed and amplitude of mechanical vibrations of the surfaces of solids in the range of sound and ultrasonic frequencies, in particular for measuring the parameters of the longitudinal vibrations of the radiating surface of the waveguide in the apparatus intended for the intensification of technological processes and the treatment of various diseases.

Low-frequency ultrasonic treatments are widely used in industry and healthcare. In this case, the ultrasound generated by the electro-acoustic transducer (ultrasonic, magnetostrictive, etc.) is brought to the place of application (the place of welding, cutting, treatment, etc.) using a waveguide - a long, and sometimes thin, straight or curved rod of constant or variable cross section . The effectiveness of the impact depends on the parameters of the ultrasonic vibration of the working part (tip) of the waveguide, such as its vibration acceleration, vibration velocity, and the amplitude of movements in the medium being processed. In healthcare, such a scheme for transmitting ultrasonic exposure is mainly used in devices for ultrasonic contact lithotripsy in the treatment of urolithiasis and cholelithiasis by fragmentation (destruction) of stones in the ureter, kidneys, and biliary tract. These devices use rigid or flexible probes, one end of which is mounted on a generator of ultrasonic vibrations, and the other (tip) contacts the calculus and performs longitudinal vibrations of ultrasonic frequency (usually 20-30 Hz), thus destroying this calculus.

When measuring the vibration parameters of such instruments, the following features must be taken into account:

1) the range of amplitudes of vibration displacements - from fractions of microns to 20 microns;

2) the oscillation frequency (20-30 kHz) determines high values of vibration acceleration (from 100 to 1000 g);

3) the small size of the oscillating surface (from 0.5 to 15 mm ² ) and a slight hesitating mass of the end face of the probe.

To measure vibration displacements generated by surgical ultrasonic instruments, the IEC standard [1] recommends the use of optical methods: by “spreading” the points on the lateral surface of the probe end in a line (observed under a microscope) or using a laser interferometer. The first of these methods is not applicable for measuring small displacements, since even a maximum magnification (x1600) of optical microscopes is not enough to measure a line with a length of about 1 μm. The interference method requires a rather complicated hardware implementation, and commercially available laser vibrometers are very expensive. In addition, the frequency range of interference methods is limited from above by a frequency of 20 kHz, and the dynamic range of the measured vibration accelerations is insufficient (the upper limit is not more than 100 g) [2].

Currently widely used methods of vibration measurements using vibration sensors are focused mainly on the vibration frequencies of machines and mechanisms (up to 1000 Hz). Therefore, the frequency range of vibrometers as working measuring instruments is usually limited from above by a frequency of 20 kHz [2]. However, there are vibration sensors with a wider frequency range [3]. Given the small amplitudes of vibration displacements, but the high values of vibration accelerations inherent in the above waveguides (probes) at frequencies of 20-30 kHz, it is advisable to use vibrometers with widely used piezoelectric acceleration transducers (piezo accelerometers) [3-5] to measure their vibration parameters [3-5], characterized by a large dynamic range, small size and weight. By integrating the measured value (vibration acceleration) over the frequency, it is possible to determine the vibration velocity of the measured object, and after the second integration, the amplitude of the vibration displacements.

The closest in technical essence to the proposed technical solution is a device for measuring vibration acceleration described in [5 (Fig. 36 and 40)] and taken as a prototype. This device consists of a primary measuring transducer (accelerometer), matching amplifier, recording device (or measuring device). The measuring transducer consists of two piezoceramic elements in the form of washers mounted between the body (base) and the inertial mass, elastically pressed against the pair of piezoelectric elements by a screw. At the base of the transducer there is a threaded hole for mounting it on a measured (vibrating) surface using a stud.

However, the method and device for measuring vibration acceleration, adopted as a prototype, has the following disadvantages:

1) the need for rigid mounting of the piezo-accelerometer on the measured object, which ideally requires the presence of a threaded hole for the pin on the latter, as well as a flat smooth surface, in the area coinciding with the landing area of the base of the accelerometer. For testing circular waveguides with a diameter of 0.8-4 mm (typical probes for contact lithotripsy devices) with a flat, convex or concave end, this requirement is not feasible;

2) the use of other methods of attaching the accelerometer to the test object (waxing, using a magnetic pad, etc. [5]) is almost impossible due to the small size of the waveguide and the use of non-magnetic material (stainless steel), from which it is usually made;

3) the mass of the accelerometer mounted on the measured object should be significantly less than the mass of the waveguide, so as not to introduce distortions in its vibration characteristics inherent to the waveguide when working for its intended purpose. Such a requirement is not always feasible.

The technical effect obtained from the implementation of the invention is to enable the measurement of vibration acceleration of the tip of the waveguide under conditions where it is not possible to fix the vibration sensor on it. This technical result is achieved due to the fact that instead of the piezoelectric vibration sensor used in the prototype, a piezoelectric hydrophone coated with an elastic polymer material (elastic modulus of not more than 1 GPa) is used. In this case, the acoustic contact between the tip of the waveguide and the piezoelectric element of the hydrophone is carried out not by rigidly fixing the base of the vibration sensor to the waveguide (as in the prototype), but by passing an ultrasonic wave through the soundproof coating of the piezoelectric element and pressing the hydrophone into the end of the waveguide with a measured load determined by the mass of the hydrophone. The mass of the hydrophone and the elasticity of the polymer coating of the piezoelectric element must be such as to ensure contact over the entire surface of the end face of the tip of the waveguide. The calculated value ε of the relative compression of the elastic coating, defined as

ε = m _g g / (S _in · E _g ),

where m _g is the mass of the hydrophone;

g is the acceleration of gravity (g = 9.8 m / s ² );

S _in - the surface area of the waveguide;

- модуль упругости материала полимерного покрытия (ρ - плотность материала, с_зв - скорость звука в нем), должно быть не менее 0,015%. $$E_g(=\rho \cdot {\mathrm{from}}_{s\mathrm{at}}^2)$$

- the modulus of elasticity of the material of the polymer coating (ρ is the density of the material, with _sv is the speed of sound in it) should be at least 0.015%.

The hydrophone is mounted vertically and coaxially with the measured object.

As a sensitive element in the hydrophone, a spherical piezoceramic shell mounted on an elastic (rubber) leg and not in contact with any solid (for example, metal) structural elements that create additional vibrational interference is used. Thus, the ultrasonic wave from the end of the waveguide freely passes through the sound-transparent coating of the piezoelectric element and exerts a local vibrational effect on it. An additional advantage of this solution is the fact that ultrasonic vibrations act not on the rigid base of the sensor (as in the prototype), but on the elastic coating of the piezoelectric element. In a hydrophone, such a coating in terms of specific acoustic impedance practically coincides with the impedance of water, and therefore with the impedance of the working medium for most applications (for example, medical) of ultrasonic waveguides. Therefore, the proposed method measures the ultrasonic vibrations of the waveguide practically in the operating conditions of its application.

Calibration of the hydrophone (determining its sensitivity to voltage to acceleration transmitted to the piezoelectric element when the longitudinal vibrations of the waveguide tip is applied locally to it) is determined using a standard (reference) vibration bench, in the table of which a nozzle is fixed (using a standard threaded connection), identical in shape to the tip waveguide. During calibration, the hydrophone is pressed against the nozzle with the same force and in the same place as when measuring vibration acceleration on the waveguide. In FIG. 1 shows a diagram of the installation of a hydrophone when measuring vibrations of a waveguide and when calibrating a hydrophone on a vibration stand.

The technical result is achieved due to the fact that in the device for measuring the vibration parameters of the tip of an ultrasonic waveguide containing a measuring transducer that converts the energy of mechanical vibrations into an electrical signal with a known acceleration conversion coefficient, a hydrophone with a piezoceramic element is used as a measuring transducer in the form spherical shell.

The spherical piezoceramic shell is mounted on an elastic (rubber) leg and does not come into contact with any solid (for example, metal) structural elements that create additional vibrational noise.

The piezoelectric element has an elastic polymer coating, elastically deformable under the weight of the hydrophone mounted on the tip of the waveguide.

The polymer coating of the piezoelectric element has a specific acoustic impedance close to the impedance of water.

May contain a tripod with a holder that provides free movement without the play of the hydrophone under its own weight in the vertical direction.

It may also contain a tripod with a hinged device for the exact installation of the measured object (the tip of the waveguide) in a vertical position.

To create identical conditions when measuring an object and calibrating the hydrophone, it can have a special nozzle that is identical in shape and material to the tip of the waveguide. For rigid attachment of the nozzle to the vibration stand, it has an external thread corresponding to the internal thread of the table of the used vibration stand.

The invention is illustrated by drawings and photographs:

In FIG. 1 - layout of the hydrophone when measuring the vibration acceleration of the waveguide (a) and when calibrating the hydrophone (b);

In FIG. 2 is a diagram of a stand for measuring vibration acceleration of a waveguide;

In FIG. 3 is an image of a contact ultrasound lithotripter;

In FIG. 4 - image of the placement of the hydrophone at the end of the probe;

In FIG. 5 is a measurement image of a signal at the output of a hydrophone using an oscilloscope.

In FIG. 1 shows the layout of the hydrophone when measuring the vibration acceleration of the waveguide (a) and when calibrating the hydrophone (b), where the hydrophone 1 with a spherical piezoelectric element 2 mounted on an elastic (rubber) leg (not shown in the diagram) has an elastic coating 3 of the piezoelectric element. When performing measurements (Fig. 1A)), the hydrophone 1 is mounted vertically and coaxially with the working part (tip) of the waveguide 4. When performing calibration (Fig. 1b)), a special nozzle 7 is rigidly fixed to the vibrating table table 5 by means of a threaded connection 6, identical in shape and material to the tip of the waveguide. The direction of vibration 8 is shown in the diagram.

The proposed method is implemented in the device schematically shown in FIG. 2. In this scheme, the hydrophone 1 with a spherical piezoelectric element is located vertically on a tripod 9 in the holder 10 so that it can freely move vertically. The hydrophone piezoelectric element coated with an elastic material rests on the end of the waveguide 4, which transmits ultrasonic vibrations from the emitter 11 excited from the generator 12. The ultrasonic emitter 11 with the waveguide 4 is mounted on a tripod with a hinge 13 so as to provide a vertical (and coaxial relative to the hydrophone) position of the tip waveguide. Own weight of the hydrophone (weight about 0.3 kg) provides a constant compression force of the polymer coating of the piezoelectric element and the stability of the acoustic contact between the end of the waveguide and the piezoelectric element of the hydrophone. The signal from the hydrophone 1 is measured using a digital oscilloscope 14.

To convert the voltage values at the output of the hydrophone into the vibration parameters at the end of the waveguide, the hydrophone must be calibrated. The hydrophone, as a means of measurement, converts the acoustic pressure acting on it in water into an electrical signal, and this pressure is perceived by the entire spherical shell of the active element, regardless of the direction of reception of the acoustic signal, since the length of the acoustic wave in water (6 cm at 25 kHz) significantly larger than the size of the piezoelectric element (diameter of not more than 10 mm). Therefore, the calibration of the hydrophone for its intended purpose is carried out in water when an acoustic wave is exposed to it of a significantly longer length than the dimensions of the active element, and this will be a calibration by acoustic pressure.

In our case, the hydrophone works in a different way: it perceives only local effects from vibration of the waveguide tip in a certain area of the active element, and here we are talking about the impact of a variable force only on this area. Such an effect is usually characterized by acceleration. In order to calibrate the hydrophone by vibration acceleration, it should be installed on a certified vibrostand acting on the active element of the hydrophone in the same way as the waveguide under test. To calibrate the hydrophone on a standard vibration bench that reproduces the specified values of vibration acceleration in the required frequency range, a special nozzle should be made that accurately reproduces the shape and material of the waveguide tip. The back of the nozzle must have a thread for its rigid fastening on the table of the vibrating stand.

Measurement of the parameters of the waveguide vibration is performed as follows:

1) install the emitter 11 with the waveguide 4 on a tripod with a hinge 13 so that the working part of the waveguide is in a strictly vertical position;

2) above the upper end of the waveguide on a tripod 9, a hydrophone 1 is installed in the holder 10 so that its axis coincides with the axis of the upper part of the waveguide, and the hydrophone itself can freely move vertically in the holder and rest against the tip of the waveguide with a piezoelectric element;

3) the end of the waveguide is lubricated with a thin layer of grease;

4) the output connector of the hydrophone cable is connected to the input of the digital oscilloscope 14;

5) turn on the emitter excitation generator 12 with a given frequency f and power and measure the rms (effective) voltage U _d at the output of the hydrophone at a given frequency of harmonic oscillations of the waveguide using an oscilloscope 14;

6) with the help of a threaded connection, the nozzle 7 is fixed on the table of the reference vibrating table (see Fig. 1), which accurately reproduces the shape and material of the waveguide tip;

7) using a tripod 9, set the hydrophone over the vibrating stand so that its axis coincides with the axis of the nozzle, and the piezoelectric element abuts against its end face (see Fig. 1);

8) the output connector of the hydrophone cable is connected to the input of a digital oscilloscope;

9) set the vibration frequency setpoint f on the vibrating stand and adjust the amplitude of its vibrations so that the signal at the output of the hydrophone is equal to the value of U _g-in ;

10) record the amplitude of vibration acceleration a in, which reproduces the vibration stand at the rms voltage U _{g in} the output of the hydrophone. This amplitude will be the desired value of vibration acceleration of the waveguide end vibrations;

11) integrating the measured value of vibration acceleration over the frequency f, it is possible to determine the amplitude of the vibration velocity of the measured object as ν _in = a _in / (2πf), and after the second integration, the amplitude of vibration displacement as r = a _in / (2πf) ² .

Testing of the invention

The proposed method and device for measuring vibration parameters were tested during testing of a contact ultrasonic lithotripter for the destruction of stones in the common bile duct [6]. The authors of [6] created a lithotripter, the waveguide of which is a metal rod made of stainless steel, with a total length of 400 mm, with a radially curved inoperative part of 40 °, a diameter of 6 mm, which conically passes into the working part of the waveguide, intended for introduction into the lumen of the choledoch, about 60 mm long and 4 mm in diameter (see Fig. 3). Using a threaded connection, the waveguide is connected to an ultrasonic transducer, which receives a signal from an ultrasonic oscillation generator. The ultrasonic generator is designed to convert industrial-frequency current (50 Hz) to a current with a frequency in the range of 17-30 kHz and serves as a power source for the ultrasonic transducer of magnetostrictive type.

To implement the proposed methodology, a hydrophone with an active element in the form of a hollow sphere with a diameter of 10 mm made of PZT-19 piezoceramics coated with a soft polyurethane compound (coating thickness 2 mm) was chosen. The upper frequency of the working range of the hydrophone is 160 kHz. The mass of the hydrophone (without cable) is about 300 g, which was sufficient to create a stable acoustic contact between the active element and the end face of the probe under test. During measurements, the hydrophone was mounted on a tripod in a special holder that allows the hydrophone to move freely in the vertical direction until it contacts the tip of the waveguide (see Fig. 4a)). The ultrasonic transducer was mounted on a tripod with a rotation unit providing vertical alignment of the waveguide end (see Fig. 4b). The waveguide did not touch any foreign objects.

The signal from the hydrophone was measured with a GDS-820C digital storage oscilloscope (frequency band - up to 150 MHz). With stable capture of the signal by the oscilloscope, the image was frozen (by pressing the STOP button) and the peak (rms) values of the measured voltages on the hydrophone, as well as the frequency values f (see Fig. 5), indicated by them, were recorded from the oscilloscope screen ) Then the oscilloscope was switched to the measurement mode (by pressing the START button) and measurements were again taken. Measurements for which any of the above parameters (U _pp , U _rms or F) were not identified were not counted. To assess the repeatability of the results, 10 measurements were carried out, after which the hydrophone was reinstalled and another 10 measurements were repeated.

The results of these measurements are shown in table 1.

Presented in the table. 1, the results of processing the measurement data showed a rather high convergence of the results: the standard deviation of the estimate of the measured voltage does not exceed 4%, and the frequency - 0.5%.

Despite the fact that the spread in the results of measuring peak-to-pick voltage values is somewhat lower than the rms average, it is recommended to measure the mean square voltage values, since they are more resistant to distortion of the waveform caused by electromagnetic interference and noise effects on the measuring circuit.

Calibration of the used hydrophone (GI-10 plant No. 045) was carried out on the standard of the 2nd category at FSUE VNIIFTRI. The upper working frequency of the standard is 30 kHz. Under the hydrophone loading conditions shown in FIG. 1b), the acceleration sensitivity coefficient a at a frequency of 26.5 kHz was

K _a = U _rms / a = 0.01 mV m ^-1 s ² .

Moreover, the linearity of the response of the hydrophone in the range of output voltages from 10 to 100 mV (rms) is not worse than ± 2%.

From here and from table. 1 you can find the main output parameter of the vibrational effect of the waveguide - the acceleration amplitude at its end

For sinusoidal vibrations, such an acceleration at a frequency f = 26.5 kHz (see table) corresponds to a range of displacements of 2 a _s / (2πF) ² = 0.81 μm.

LITERATURE

1. IEC 61847: 1998 Ultrasonics - Surgical systems - Measurement and declaration of the basic output characteristics.

2. GOST P 8.800-2012. State system for ensuring the uniformity of measurements. State verification scheme for measuring instruments for vibration displacement, vibration velocity and acceleration in the frequency range from 1 · 10 ^-1 to 2 · 10 ⁴ Hz.

3. J.T. Broch Mechanical Vibration and Shock Measurements, Ed. Brtiel & Kjasr, 1984. ISBN 8787355345.

4. Devices and systems for measuring vibration, noise and shock. Ed. V.V. Klyueva, M., "Engineering", 1978

5. B.C. Shkalikov et al. Measurement of vibration and shock parameters, M., Ed. Standards, 1980

6. E.V. Razmakhnin, S.L. Lobanov, B.S. Khyshiktuev. Contact ultrasound lithotripsy in the lumen of the common bile duct, Bulletin of Experimental and Clinical Surgery, Volume VII, No. 2, 2014, p. 159-164.

Claims (12)

1. The method of measuring the longitudinal vibration parameters of the tip of the ultrasonic waveguide, which consists in transmitting vibration acceleration of the tip of the waveguide to a measuring transducer that converts the energy of mechanical vibrations into an electrical signal with the acceleration conversion coefficient known from the calibration results, transmitting this signal via cable to the measuring device, measuring this signal with its subsequent conversion into the value of acceleration, characterized in that the transmission of acceleration is carried out on hydro it is a piezoceramic element, freely based on its piezo vibrating tip. 2. The measurement method according to p. 1, characterized in that the hydrophone is mounted vertically and coaxially with the measured object. 3. The measurement method according to claim 1, characterized in that the acoustic contact of the hydrophone with the measured object is carried out under the action of the weight of the hydrophone on the elastic polymer coating of the hydrophone piezoelectric element, the calculated value of the relative compression ε of the elastic coating, defined as
ε = m _g g / (S · E _{in g)}
where m _g is the mass of the hydrophone;
g is the acceleration of gravity (g = 9.8 m / s ² );
S _in - the surface area of the waveguide;
E _r $$(=\rho \cdot c_{s\mathrm{at}}^2)$$

- the elastic modulus of the material of the polymer coating (ρ is the density of the material, c _sv is the speed of sound in it),
must be at least 0.015%. 4. The measurement method according to p. 1, characterized in that the hydrophone used as a measuring transducer is calibrated on a standard vibrating stand, installing the hydrophone on a nozzle simulating the tip of the waveguide. 5. A device that implements the measuring method according to claim 1, comprising a measuring transducer that converts the energy of mechanical vibrations into an electrical signal with an acceleration conversion coefficient known from the calibration results, characterized in that a hydrophone with a piezoceramic element in the form of a spherical shell is used as a measuring transducer . 6. The device according to claim 5, characterized in that the spherical piezoceramic shell of the hydrophone is mounted on an elastic (rubber) leg and does not come into contact with any solid (eg, metal) structural elements that create additional vibrational noise. 7. The device according to p. 5, characterized in that the piezoceramic element of the hydrophone has an elastic polymer coating, elastically deformable under the weight of the hydrophone mounted on the tip of the waveguide. 8. The device according to claim 7, characterized in that the polymer coating of the piezoelectric ceramic element of the hydrophone has a specific acoustic impedance close to the impedance of water. 9. The device according to p. 5, characterized in that it contains a tripod with a holder that provides free movement without the play of the hydrophone under its own weight in the vertical direction. 10. The device according to p. 5, characterized in that it contains a tripod with a hinged device for accurate installation of the measured object (the tip of the waveguide) in a vertical position. 11. The device according to claim 5, characterized in that for creating identical conditions when measuring the object and calibrating the hydrophone has a special nozzle that is identical in shape and material to the tip of the waveguide. 12. The device according to p. 11, characterized in that for rigidly fixing the nozzle on the vibrating stand, it has an external thread corresponding to the internal thread of the table of the used vibrating stand.

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