RU2724296C1 - Molecular-electronic hydrophone with static pressure compensation - Google Patents

Abstract

FIELD: acoustic metrology.SUBSTANCE: molecular-electronic hydrophone with static pressure compensation contains a molecular-electronic converter rigidly fixed inside a sealed housing, filled with easily compressible liquid and divided into two chambers with a rigid wall with a capillary, such that each of the chambers communicates with one of the membranes of the molecular-electronic converter, wherein the outer chamber of the chambers is separated from the surrounding environment by the impermeable membrane, and the inner chamber is separated from the surrounding environment by the rigid casing of the housing. Chamber separated from the environment by a soft membrane has a much smaller volume than the other chamber. Compressible fluid used is polymethylsiloxane or polydimethyl siloxane fluids of different viscosity. Note here that negative feedback is introduced to stabilize hydrophone parameters.EFFECT: high accuracy of measurements at large depths due to compensation of external static pressure, which will allow the hydrophone to have the same response regardless of depth.4 cl, 6 dwg, 1 tbl

Description

Hydrophones, devices for receiving / emitting, and recording acoustic signals in a liquid medium have a wide range of applications. These devices are used for scientific research of the underwater biosphere, for navigation and location of ships, as a receiver of seismic signals, including in the exploration of mineral resources on shelves and transitions from land to sea zones, in the fishing industry and in other areas. Hydrophones can be used as part of floating and bottom seismic streamers, multicomponent measuring complexes, for example, dual sensors such as geophon / hydrophone, vector acoustic receivers or bottom seismic stations. In addition to the scope and method of application, hydrophones differ in the mechanism of converting an acoustic signal into an electric current (piezoelectric, fiber-optic, electro- and magnetostrictive, electro- and magnetodynamic and others) type of execution (submersible, mortise) [1]. The use of hydrophones in a marine environment imposes serious requirements on instruments for tightness and resistance to high static pressures. Unlike shallow-water hydrophones, in which the immersion depth does not exceed several tens of meters (corresponds to a range of hydrostatic pressures from zero to several atmospheres), deep-sea hydrophones should work at depths from tens of meters to hundreds of meters and even several kilometers (this corresponds to a large range of hydrostatic pressures from units to several tens of atmospheres). Therefore, deep-sea hydrophones should have characteristics that are weakly dependent on static pressure over a wide range, or have a mechanism for compensating static pressure. For example, deep-sea hydrophones are widely used in the field of seismic exploration on the sea shelf, where they are used as part of bottom seismic streamers and bottom autonomous seismic stations. At the same time, the depth of immersion can be more than 500 meters.

Today, piezoelectric [1-4] and fiber optic [5-6] hydrophones are the most common. There are also hydrophones with a conversion mechanism based on the principle of molecular electron transfer [7–8]. Sensors based on this technology have successfully established themselves in seismic exploration, navigation, monitoring the status of complex engineering structures and buildings and some other areas. Their distinguishing feature is the high sensitivity and low level of intrinsic noise in the low frequency region (from fractions to hundreds of Hertz) characteristic of seismic exploration, as well as low power consumption and low cost. The main element of sensors based on this technology is a molecular-electronic converter, the principle of operation and device of which is described in detail in [9].

The molecular-electronic transducer is a channel with a rigid body, filled with an electrolyte solution, divided into two symmetrical relative to the cross section of the half electrode assembly and closed from the ends with liquid-tight elastic membranes. The electrode assembly consists of four electrodes arranged in the order cathode-anode-anode-cathode, between which there are dielectric porous partitions. When a constant voltage is applied to the cathode-anode pairs as a result of redox reactions on the surfaces of the electrodes, a stationary distribution of the concentration of electrolyte ions in the space between the electrodes occurs. The symmetric arrangement of the pairs of electrodes gives in this case a zero difference cathode current. When the electrolyte solution moves along the channel, the stationary distribution is violated and the difference cathode current becomes nonzero. The interelectrode current linearly depends on the speed of the electrolyte solution.

In the molecular-electronic hydrophones described in patents [7] and [8], one of the transducer membranes communicates with the environment, and the other with a chamber separated from the environment by a rigid shell. In [7], the chamber is filled with air and has a small volume of the order of several cm ³ , and in [8] it has a volume of the order of several hundred cm ³ and is filled with an easily compressible liquid. To ensure a linear response, hydrophones are equipped with a feedback mechanism, which is a coil mounted on the hydrophone body and a magnet connected to the membrane of the molecular-electronic sensor. In this case, the sensor output signal controls the magnitude of the current in the coil, and special control circuits adjust the current in the coil so that the interaction force between the coil and the current and the magnet rigidly fixed to the membrane compensates for the effect of external pressure on the membrane. The disadvantage of the proposed technical solutions is a significant dependence of the response on static pressure, since static pressure deforms the membrane and changes the relative position of the magnet and the coil. As a result, with increasing static pressure, the strength of the interaction between the coil and magnet and the overall sensitivity of the hydrophone change.

The prototype of the invention is a deep-sea hydrophone, presented in the patent [8], the disadvantage of which is the lack of compensation of static pressure, which leads to a significant change in the response of the sensor when immersed at great depths.

The objective of the invention is to improve the design of a deep-sea hydrophone, which provides compensation for external static pressure, which will allow the hydrophone to have the same response regardless of depth.

The problem is solved in that a molecular-electronic hydrophone with static pressure compensation, in which the molecular-electronic transducer is rigidly fixed inside a sealed enclosure filled with easily compressible liquid and divided into two chambers by a rigid wall with a capillary, so that each of the chambers communicates with one from the membranes of the molecular-electronic transducer, while the outer chamber from the chambers is separated from the environment by an impermeable membrane, and the inner chamber is separated from the environment by a rigid shell of the housing. Moreover, the chamber, separated from the environment by a soft membrane, has a much smaller volume than the other chamber. Moreover, polymethylsiloxane or polydimethylsiloxane liquids of various viscosities are used as easily compressible liquids. At the same time, negative feedback was introduced to stabilize the hydrophone parameters.

In FIG. 1 is a diagram of the invention, where one of the membranes 6 divides the channel of the molecular-electronic transducer 3 of the inner volume chamber 1 under a rigid shell 12 filled with an easily compressible non-conductive electric current. Another membrane separates the channel and the outer chamber 2, filled with the same easily compressible liquid as the inner chamber 1. The outer chamber 2 communicates with the environment through an impermeable membrane 8. The chambers 1 and 2 are separated by a rigid wall 9, in which there is a thin capillary 7. Through the capillary 7, chambers 1 and 2 communicate with each other, due to this, the liquid can slowly flow from one chamber to another. In addition, a cylindrical magnet 11 is mounted on one of the membranes of the molecular-electronic converter, which can freely move inside the coil 10, which is stationary relative to the body of the molecular-electronic converter. The magnet 11 and the coil 10 are elements of the electromagnetic power negative feedback of the hydrophone. Feedback works in this way: under the action of an external signal, the output electric current from the electrode assembly 4 is supplied to the coil 10 through the conversion electronics, as a result of which a magnetic field appears inside the coil 10, which acts on the magnet 11 and, as a consequence, on the membrane, with a certain force partially compensating for the external impact.

An external chamber with an external membrane and a capillary perform the function of compensating the external static pressure in the environment. A capillary allows fluid to flow from one chamber to another, and a soft outer membrane allows you to freely change the volume of the outer chamber. Therefore, the compression of the liquid with a change in the external static pressure will occur due to a change in the volume of the external chamber and deformation of the external membrane, while the membranes of the molecular-electronic transducer, when the pressures are equalized between the chambers, can return to the equilibrium position due to their own elastic forces.

Let us describe mathematically the conversion of environmental pressure variations into the movement of an electrolyte solution and the mechanism for compensating the static environmental pressure.

The process of converting pressure variations into the movement of the electrolyte solution can be represented using the equation describing the volume flow of the electrolyte solution through the channel section of the sensor electrode assembly. In this equation, we neglect the inertia of the liquid, since it contributes to the volumetric flow rate only at high frequencies outside the range required for the sensor; moreover, from the condition of the incompressibility of the electrolyte solution, the membranes of the molecular-electron transducer will move identically.

- давление раствора электролита в половине канала, которая сообщается с внешней камерой, р₀ - статическое давление в окружающей среде, p(t) - воздействие, поданное на внешнюю мембрану, p_in - давление во внутренней камере,

- давление раствора электролита в половине канала, которая сообщается с внутренней камерой, Δp_in - изменение давления во внутренней камере при подаче возмущения, х - смещение мембран молекулярно-электронного преобразователя, R_h - коэффициент гидродинамического сопротивления канала молекулярно-электронного преобразователя.where Q is the volumetric flow rate of the electrolyte solution through the channel cross section of the molecular electronic transducer, s is the membrane area of the molecular electronic transducer, p _out is the pressure in the external chamber,

is the pressure of the electrolyte solution in half of the channel that communicates with the external chamber, p ₀ is the static pressure in the environment, p (t) is the effect applied to the external membrane, p _in is the pressure in the internal chamber,

is the pressure of the electrolyte solution in half of the channel that communicates with the inner chamber, Δp _in is the change in pressure in the inner chamber when a disturbance is applied, x is the displacement of the membranes of the molecular-electronic transducer, R _h is the hydrodynamic resistance coefficient of the channel of the molecular-electronic transducer.

The membrane, through which the external chamber communicates with the environment, will be considered soft and assume that the pressure in the external chamber coincides with the pressure in the environment.

Since the fluid in the chambers is compressible, then, considering the relative change in pressure in the inner chamber to be small, we can write the expression for the density of the fluid in the inner chamber in the form:

where ρ ₀ , ρ are the densities of the fluid in the inner chamber in the absence of exposure and when applied, respectively, β _T is the isothermal coefficient of compressibility of the fluid in the chambers.

Express Δp _in from (4):

where m is the mass of fluid in the inner chamber, V = V ₀ - sx is the volume of the inner chamber.

Hereinafter, index 0 for the parameters of the internal chamber corresponds to the state in the absence of external disturbance.

Assuming the relative change in the mass of liquid in the inner chamber to be small, we obtain:

where Δm is the mass of fluid entering the inner chamber through the capillary.

We substitute (6) into (5) and discard the terms of the 2nd and higher order of smallness:

If you act on the outer membrane with a harmonic signal p (t) = p _ω e ^iωt ,

then the pressure difference between the ends of the channel of the molecular-electronic transducer is equal to:

After substituting (8) in (1), we obtain:

Now we write the mass flow rate of the liquid through the capillary:

where q (t) is the volumetric flow rate of the fluid through the capillary. From Poiseuille's Law:

- коэффициент гидродинамического сопротивления капилляра, r - радиус капилляра,

- длина капилляра, μ₀ - динамическая вязкость сжимаемой жидкости, заполняющей внешнюю и внутреннюю камеры.Where

is the coefficient of hydrodynamic resistance of the capillary, r is the radius of the capillary,

is the length of the capillary, μ ₀ is the dynamic viscosity of the compressible fluid filling the external and internal chambers.

After substituting (11) into (10), taking into account (8), we obtain:

We represent x (t) and Δm (t) in the form of harmonic signals:

Substituting (13) into (12), and then differentiating with respect to time, we obtain:

We express from here Δm _ω / m ₀ :

Hence:

where Q _ω = sX _ω iω is the amplitude of the oscillations in the volumetric flow rate of the electrolyte solution through the channel cross section of the sensor electrode assembly, with Q = Q _ω e ^iωt .

Substituting (16) in (9), we obtain:

From (17) we obtain the transfer function of the mechanical system of the hydrophone:

As can be seen from (18), the expression for the transfer function of a mechanical system does not include parameters that depend on the static pressure of the environment; therefore, the response of the mechanical system also does not depend on the static pressure of the environment.

The expression for the transfer function module of the hydrophone mechanical system has the form:

The lower transmission frequency of the mechanical hydrophone system is equal to:

As can be seen from (20), for R _h <r _{h, the} lower transmission frequency is practically independent of the coefficient of hydrodynamic resistance of the capillary and is determined by the coefficient of hydrodynamic resistance of the channel of the molecular-electronic converter.

In addition, the lower transmission frequency is inversely proportional to the volume of the internal chamber V ₀ . Thus, to reduce this parameter and improve the response of the hydrophone, it is necessary to make the volume of the inner chamber large, and since the volume of the outer chamber does not affect the response of the hydrophone, it can be made significantly smaller than the volume of the inner chamber to reduce the dimensions of the hydrophone.

We show now that the membranes remain undeformed after changing the value of the external static pressure. To do this, apply a constant signal p (t) = Δp to the external membrane, where Δр is the value by which the external static pressure changes. In this consideration, we take into account the action of the elastic force on the membranes of the molecular-electronic transducer. With this in mind, expression (9) and time-differentiated expression (12) will take, respectively, the form:

, получим систему линейных дифференциальных уравнений:Having made the change in (21) and (22)

, we obtain a system of linear differential equations:

The general solution (23) has the form:

- собственные векторы матрицы системы (23), λ_{1, 2} - собственные числа матрицы системы (23), причем λ_{1, 2} < 0, так как

where C _{1, 2} are constants determined by the initial conditions,

are the eigenvectors of the matrix of system (23), λ _{1, 2} are the eigenvalues of the matrix of system (23), and λ _{1, 2} <0, since

As can be seen from (24), x → 0 as t → ∞, i.e., when the external static pressure changes, the membranes of the molecular-electron transducer under the action of elastic forces will return to their original position. As a result, static pressure will be compensated in the hydrophone under consideration.

Thus, the technical effect of the proposed solution is that due to the presence of a capillary, static pressure is compensated and the membranes of the molecular-electronic transducer return to their original position when the static pressure changes. The latter allows you to maintain the relative position of the coil and magnet attached to one of the membranes, unchanged at various static pressures. As a result, the response of the negative feedback hydrophone remains unchanged. In addition, the capillary practically does not affect the response within the passband of the hydrophone.

An example implementation of the invention.

We carry out calculations according to formulas (19) and (20) for the system parameters presented in Table 1. The obtained frequency dependence is shown in FIG. 2.

In this example implementation, the lower transmission frequency is ƒ _lower = 9.9 Hz.

For practical implementation (Fig. 3, Fig 4, Fig. 5) of the technical solution found, a stainless steel base 13 was made having an internal volume and three openings emanating from this volume. Three of them go to the side surface of the cylinder and are designed for access of water to the central cavity when the hydrophone is immersed. The fourth hole serves to transmit pressure to the sensing element. This hole is closed by the external rubber membrane of the hydrophone 8. The membrane is hermetically pressed against the body by an aluminum flange 9, the walls of which form a partition separating the inner and outer cavities 7, which forms an capillary during assembly that connects the outer and inner volumes of the hydrophone. A molecular-electronic transducer is installed on the flange in the housing 3 with membranes 6, on one of which a magnet 11 is mounted, which interacts with the coil 10. Then, a cylindrical stainless housing is fixed on the base, which is closed by an outer cover, with special holes to fill the internal volume with an easily compressible liquid. PMS-5 polymethylsiloxane liquid was used as such a liquid.

In FIG. 6. shows the amplitude-frequency characteristics of the hydrophone, measured at different static pressures from 1 to 3 atmospheres. It can be seen that the characteristics completely coincide in the main part of the frequency range. The scatter of data in the high-frequency region is due to measurement errors.

Graphic materials and a brief description of graphic materials explaining the essence of the invention:

FIG. 1 - Diagram of a hydrophone with electromagnetic feedback with compensation of external static pressure. 1 - inner chamber under a rigid cylindrical shell 12 filled with compressible fluid, 2 - outer chamber communicating with the environment through a soft outer membrane 8, 3 - molecular-electronic transducer (4 - electrode assembly, 5 - electrolyte solution, 6 - elastic membranes ), 7 - capillary, 9 - rigid walls separating the inner and outer chambers, 10 - coil, 11 - permanent magnet.

FIG. 2 - The calculated frequency dependence of the transfer function module of the mechanical system of the hydrophone.

FIG. 3 - The base of the hydrophone with an external membrane and flange. The process of filling an external chamber of a volume with a capillary with an easily compressible liquid is also shown.

FIG. 4 - The base of the hydrophone with the installed molecular-electronic transducer.

FIG. 5 - Hydrophone assembly mounted on a stand for measuring frequency response at various hydrostatic pressures.

FIG. 6 - frequency response of the hydrophone at different pressures (blue line - 1 atm, burgundy - 2 atm, red - 3 atm), taken by an external signal relative to the reference ZetLab BC-311 hydrophone with a sensitivity of 0.477 mV / Pa.

Sources of information

1. S.N. Sherman and J.L. Butler // Transducers and arrays for underwater sound. Springer, 2007

2. Patent of the People's Republic of China 104486705

3. Patent of the People's Republic of China 201348661

4. RF patent for utility model 168468

5. RF patent for utility model 58216

6. US patent 8094519

7. RF patent 2678503

8. RF patent 2696060

9. Krishtop V.G., Agafonov V.M., Bugaev A.S. // Electrochemistry. 2012.V. 48. No. 7. S. 820

10. Zaitsev, D.L., Avdyukhina, S.Y., Ryzhkov, M.A., Evseev, I., Egorov, E.V., and Agafonov, V.M .: Frequency response and self-noise of the MET hydrophone, J. Sens. Sens. Syst, 7, 443-452, https://doi.org/10.5194/jsss-7-443-2018, 2018

11. Ivan V. Egorov, Anna S. Shabalina, and Vadim M. Agafonov Design and Self-Noise of MET Closed-Loop Seismic Accelerometers IEEE SENSORS JOURNAL, VOL. 17, NO. 7, APRIL 1, 2017

12. V.M. Agafonov, I.V. Egorov, and A.S. Shabalina, "Operating principles and specifications of small-size molecular electronic seismic sensor with negative feedback," Seismic Instrum., Vol. 49, no. 1, pp. 5-19, 2013.

Claims (4)

1. Molecular-electronic hydrophone with static pressure compensation, characterized in that the molecular-electronic transducer is rigidly fixed inside a sealed enclosure filled with an easily compressible liquid and divided into two chambers by a rigid wall with a capillary, so that each of the chambers communicates with one of the membranes molecular-electronic Converter, while the outer chamber of the chambers is separated from the environment by an impermeable membrane, and the inner chamber is separated from the environment by a rigid shell of the housing. 2. The hydrophone according to claim 1, characterized in that the chamber, separated from the environment by a soft membrane, has a much smaller volume than the other chamber. 3. The hydrophone according to claim 1, characterized in that polymethylsiloxane or polydimethylsiloxane liquids of various viscosities are used as easily compressible liquids. 4. The hydrophone according to claim 1, characterized in that a negative feedback is introduced to stabilize the hydrophone parameters.

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