RU2193172C2 - Pressure sensor with frequency output - Google Patents

Abstract

FIELD: information and measurement technology; measurement of pressures and pressure differentials of liquid and gaseous media. SUBSTANCE: pressure sensor includes self-excitation system and sensitive element in form of oscillatory system 23 performing transversal oscillations and dividing the interior of sensitive element into two communicating working chambers 21 and 22; walls of working chambers which are opposite to oscillatory system are made in form of barometric sensitive diaphragms 11 which may be provided with flexural centers 19 and 20. Interior of sensitive element is filled with inert gas. Sensor is provided with thermocorrecting sensor made in form of self-excited system with sensitive element in form of oscillatory system dividing interior of sensitive element into two chambers. Sensitive element may be made in form of two oscillatory systems: one system forms pressure sensor and other system forms thermocorrecting sensor. EFFECT: possibility of measuring pressures within wide range irrespective of chemical composition and density of media; enhanced accuracy and reliability. 10 cl, 12 dwg

Description

 The invention relates to the field of information and measurement technology and may find application in the measurement of pressures and pressure differences between liquid and gaseous media.

 Known frequency pressure sensor containing a self-excitation system and an oscillating system in the form of a flat rigid plate, reinforced with elastic braces and located in the internal cavity of the sensor between the electrodes with a gap whose transverse dimension does not exceed twice the penetration depth of a viscous wave [1].

 The disadvantages of this design include a significant mass of the oscillatory system, which leads to a low sensitivity of the sensor, especially in the field of low measured pressures, as well as the limited functionality of the sensor, which makes it unsuitable for measurements, for example, pressures of liquid and contaminated gas environments.

 A pressure sensor with a frequency output is also known, containing an oscillating system in the form of a flat membrane dividing the internal cavity of the working chamber into two symmetric volumes communicating with each other through openings along the periphery of the membrane and with a controlled medium through capillary channels in the housing, while the inner surfaces of the working chambers facing the membrane are made in the form of parts of a sphere [2].

 The lower limit of the operating range of the sensor is determined by the ratio of the stiffness of the gas spring to the stiffness of the membrane. A decrease in the stiffness of the membrane in the sensor is achieved by reducing its thickness, as well as by performing perforations in the peripheral part. The upper limit of the working range of the sensor is determined by the quality factor of the oscillatory system, which depends on the amount of viscous friction of the gas in the working volume. Due to the execution of the inner surfaces of symmetric volumes in the form of parts of a sphere whose radius is similar to the radius of the oscillating membrane in extreme positions, the radial pressure gradient decreases significantly, since the relative deformations of the ring elements of the gas spring turn out to be approximately equal, and with a decrease in the radial pressure gradients, the radial gas flows and significantly increases the quality factor of the oscillatory system.

 However, the known sensor has significant drawbacks in the sensitivity of the sensor to the density of the medium being measured, and therefore to its chemical composition and humidity, as well as the possibility of contamination of the sensor’s working gap by small mechanical particles (dust), which does not allow to fully realize the potential the possibility of achieving high accuracy of the sensor, due to the principle of converting pressure into frequency, incorporated in it, and sharply reduces the reliability of the sensor. It is also impossible to measure the pressure of liquid media, which sharply limits the functionality of the sensor.

 The closest in technical essence to the present invention is a pressure sensor with a frequency output, containing a self-excitation system and an oscillating system in the form of a flat elastic membrane that divides the internal volume of the sensor into two symmetrical working chambers, communicating with the medium through two capillaries, the time constant of which is an order of magnitude above half oscillation period [3].

 The main disadvantage of the known design is its low accuracy and reliability, due to the possibility of clogging of the capillaries and working chambers of the sensor with liquid or mechanical impurities in the gas, which, in addition to reducing accuracy, can lead to loss of the sensor’s performance.

Other disadvantages of the known design, which lead to low accuracy and reliability, include:
a) non-linear distortions of the pressure-to-frequency conversion characteristics when measuring pressures over a wide range, due to the possibility of excitation at the operating frequencies of secondary low-Q acoustic resonances in the sensor chambers, while the radial gas movement occurs at the secondary frequencies, which reduces the elasticity of the working chambers and leads to an increase in losses vibrational energy due to the viscosity of the gas;
b) a sharp non-linearity of the conversion of pressure to frequency when measuring high pressures due to the possible excitation of high-quality mechanical resonances in the body parts of the sensing element, which is caused by periodic mechanical stresses on the membrane fastening circuit during its oscillations;
c) a significant value of the temperature error when measuring even relatively low pressures due to the temperature dependence of the viscosity of the gas and the temperature dependence of the relaxation energy loss of vibrations, which are caused by thermodynamic processes that accompany periodic gas compression / rarefaction in the working chambers.

 In addition, the known design does not provide pressure measurements of liquid media and is not suitable for measuring relative pressures, which significantly limits the functionality of the sensor.

 The objective of the invention is to create a new design of a pressure sensor with a frequency output, with the possibility of realizing high accuracy of pressure measurements of liquid and gaseous media in a wide range, as well as providing high reliability of the sensor and the expansion of its functionality.

 The problem is solved in that in a pressure sensor with a frequency output containing a self-excitation system and a sensing element in the form of a flat oscillating system that transversely vibrates and separates the internal volume of the sensing element into two communicating working chambers, according to the invention, the walls of the working chambers opposite the oscillating system, made in the form of pressure-sensitive membranes, and the internal volume of the sensitive element is filled with an inert gas and sealed.

 The pressure-sensitive membranes can be made in the form of membranes with rigid centers, and the area of each of them can exceed the size of the oscillatory system.

 The rigid centers of the pressure-sensitive membranes can be rigidly interconnected by protrusions located outside the oscillation region of the oscillatory system, and the working chambers can be made with different transverse dimensions.

 It is possible, along with the implementation of the pressure-sensitive membrane with an area exceeding the dimensions of the oscillatory system, the execution of the wall of the working chamber outside the vibrational system in the form corresponding to the deflection of the pressure-sensitive membrane in the extreme position.

 It is possible to make one of the opposite walls of the working chambers in the form of a pressure-sensitive membrane, while the other working chamber is rigid.

 A thermocorrection sensor can be located in the housing of the pressure sensor, made in the form of a self-excitation system with a sensitive element in the form of a flat oscillating system that transversely vibrates and divides the internal volume of this sensitive element into two working chambers, while the internal volume of the sensitive element is filled with inert gas and sealed .

 The sensitive element of the sensor can be made in the form of two oscillatory systems, one of which forms a pressure sensor, and the other a thermocorrection sensor.

 The working chambers of the sensing element can be made in the form of slots, the transverse size of which does not exceed twice the thickness of the viscous friction layer, and the oscillating system can be made in the form of a rectangular plate fixed along the nodal lines of bending vibrations.

 The oscillation system can be made in the form of a membrane equipped with holes located at its periphery.

 This design of the sensor due to the execution of the walls of the working chambers opposite the oscillatory system in the form of pressure-sensitive membranes, filling with inert gas and sealing the internal volume makes it impossible to fill the working chambers of the sensor with a measured medium and, therefore, eliminates contamination of its internal volume, which significantly increases the reliability of the sensor. At the same time, it is possible to measure pressures of not only gaseous, but also liquid media.

 The possible implementation of pressure-sensitive membranes with rigid centers and an area exceeding the size of the oscillating system allows to obtain a high sensitivity of the sensor to the measured pressure both by increasing the compression ratio of the inert gas in the internal volume of the sensing element and by changing the transverse dimensions of the working chambers, which increases the accuracy measurements. At the same time, supplying the pressure sensitive membrane with a rigid center increases its stability under pressure, increases its effective area, and also provides the same change in the transverse size of the working chamber over the area of the oscillating system during membrane deformation, which leads to an increase in the accuracy of the sensor. At the same time, a possible constructive choice of the intrinsic elasticity of pressure-sensitive membranes makes it possible to coordinate the range of measured pressures with the range of operating frequencies of the sensor, in particular, to exclude its operation at high frequencies at which high-quality mechanical resonances in case parts can be excited, and thereby eliminate non-linear distortions of the pressure conversion characteristics in frequency, which ensures high accuracy of the sensor in a wide pressure range.

 The possible rigid connection between the rigid centers of the pressure-sensitive membranes with the help of protrusions located outside the oscillation region of the oscillating system, as well as the execution of the working chambers of the sensing element with different transverse dimensions, allows the sensor to be used for measuring absolute or relative (excess) pressures due to various methods of summing the measured pressures , as well as for measuring the pressure difference, which significantly expands the functionality of the sensor.

 The possible implementation of a pressure sensitive membrane with an area exceeding the dimensions of the oscillating system and the walls of the working chamber outside the area of the vibrational system with a shape corresponding to the deflection of the pressure sensitive membrane in the extreme position, allows to increase the overload capacity of the sensor and, therefore, its reliability.

 The possible implementation of only one of the walls of the working chamber opposite to the oscillatory system in the form of a pressure-sensitive membrane, while the other rigid working chamber simplifies the design of the sensor, which increases its reliability.

 The location in the housing of the pressure sensor of the thermal correction sensor, as well as its possible implementation in the form of a self-excitation system with a sensitive element in the form of a flat oscillatory system that transversely vibrates and divides the internal volume of the sensitive element into two working chambers, as well as filling the internal volume of the thermal correction sensor with inert gas and its sealing ensures unity and simplification of structural and technological solutions in the manufacture of a pressure sensor, ensuring They offer the possibility of both temperature correction of changes in the frequency of the sensor due to thermal changes in the pressure of an inert gas in the internal volume of the sensing element, and the possibility of introducing additional temperature corrections arising due to other related factors. In this case, both the additive temperature error (zero error) and the multiplicative component of the error (sensitivity error) are sharply reduced, which greatly increases the accuracy of the sensor measurements, and as a side result, the possibility of temperature measurement arises, which significantly expands the functionality.

 The possible implementation of the sensitive element in the form of two oscillatory systems, one of which forms a pressure sensor, and the other a thermocorrection sensor, simplifies the design of the sensor and its electronic circuit due to the possible use of the same self-excitation systems, which creates the conditions for the same thermal state of the sensitive elements, increases reliability sensor and provides high accuracy of temperature corrections, while the same physical nature of the frequency signals of the pressure sensor and thermocorrection sensor increases the accuracy of their subsequent algorithmic processing and thereby increases the accuracy of measurements.

 The possible implementation of the working chambers in the form of slots, the transverse dimension of which does not exceed twice the thickness of the viscous friction layer, provides conditions under which periodic gas compression / rarefaction occurs mainly in the direction perpendicular to the plane of the oscillatory system, since the gas movement along its surface is sharply inhibited by the intrinsic viscosity of the gas and its “sticking” to surfaces forming a thin slit gap. The consequences of this design solution are the constancy of the gas mass in the working chambers over a half-cycle of oscillations and the inability to excite low-Q side acoustic resonances in the working chambers, which eliminates nonlinear distortions and increases the accuracy of converting pressure to frequency over a wide range.

 The possible implementation of the oscillating system in the form of a rectangular plate fixed along the nodal lines of bending vibrations can drastically reduce the possibility of exciting high-quality mechanical resonances in the body parts of the sensitive element under the action of periodically occurring mechanical stresses on the fastening circuit of the oscillating system and thereby increase the accuracy of converting pressure to frequency in wide range of frequencies.

 The possible implementation of the oscillatory system in the form of a membrane, equipped with holes located at its periphery, makes it possible to use an oscillatory system (membrane) with a low surface density and eliminates the possibility of a difference in quasistatic pressures in the working chambers of the sensor, which allows, on the one hand, to obtain high sensitivity sensor and thereby increase the accuracy of pressure conversion, and on the other, due to the rapid equalization of quasistatic pressures in the working chamber , Eliminate their impact on the membrane and thereby increase reliability of the sensor.

 A comparative analysis of the proposed sensor and prototype reveals the distinctive features of the claimed sensor in comparison with the closest analogue, which allows us to conclude that the proposed solution meets the criterion of "novelty."

 The presence of distinctive features makes it possible to obtain a positive effect, expressed in the possibility of realizing a wide range of measured pressures, achieving improved accuracy and reliability, measuring pressures regardless of the chemical composition and density of the measured media, as well as significantly expanding the functionality of the sensor.

 Since when examining the object of the invention in patent and scientific literature, no solutions were found containing the features of the claimed invention other than the prototype, it should be concluded that the claimed invention meets the criterion of "significant differences".

 The use of the claimed invention in the information-measuring technique ensures that the invention meets the criterion of "industrial applicability".

 The invention is illustrated in figures 1 to 10, the structural diagram in figure 11 and the graph in figure 12.

 Figure 1 presents a General view of the possible designs of the pressure sensor in the context of BB, and figure 2 presents the same design, but in the context of AA.

 FIG. 3 - 5 explain one of the possible options for the sensor element, which is structurally combined with the sensor element thermocorrection sensor. In this case, Fig. 3 shows a section along CC of the sensor element of the pressure sensor, and Fig. 4 is a section along D-D of the sensor element of the temperature-correcting sensor. Figure 5 shows the F-F pull-out, showing the possible location of the oscillatory systems of the pressure sensor and the temperature-correcting sensor, as well as the possible location of thin-film piezoelectric transducers that provide excitation and removal of oscillations.

 FIG. 6 and 7 illustrate another possible embodiment of a pressure sensor sensing element, which is also structurally combined with a temperature-correcting sensor sensitive element. At the same time, Fig. 6 shows an additional section along L-L of the sensing element of the pressure sensor, and Fig. 7 shows a pull-out along M-M, which explain the possible structural connection of the rigid centers of the pressure-sensitive membranes with the help of the protrusions provided with the rigid centers.

 In FIG. 8 to 10 show an example of yet another possible design of a pressure sensor sensing element using a single pressure sensitive membrane and a thin membrane oscillating system. In this case, Fig. 8 shows a section along E-E of the sensing element of the pressure sensor, Fig. 9 is a section along PP of the sensing element of the temperature-correcting sensor, and in Fig. 10 is a section along K-K, showing the possible location of thin-film piezoelectric transducers that provide excitation and removal of oscillations of a thin membrane.

 Figure 11 presents the structural diagram of self-excitation systems of the pressure sensor.

 In FIG. 12 is a graph explaining a change in inert gas pressure that fills the internal volume of the sensing element formed by the working chambers of the sensor under the influence of the measured pressure and temperature.

 A sensor element 2 is installed in the housing 1 (Figs. 1 and 2) of the sensor, in which the sensitive elements of the pressure sensor and the thermocorrection sensor are structurally combined, and the electronic chips of their self-excitation systems 3 and the output connectors 4 and 5 are placed. Using the microconductors 6, the sensitive elements connected to the electronic chips of the self-excitation systems 3, which are connected via electrical conductors 7 to the connectors 4 and 5, through which the supply voltage supply and the removal of the electric frequency signals generated by self-excitation systems 3.

 The housing 1 of the sensor is equipped with a cover 8, which is rigidly connected to the housing, for example, by welding along the circuit 9.

 The sensing element 2 is sealed by its flat surfaces with the housing 1 and the cover 8, for example, using thin layers 10 of cold-curing adhesive, while in the region of the pressure sensitive membranes 11 in the housing 1 and the cover 8, samples 12 are made into which through the hermetically connected input fittings 13 and 14 are measured pressure.

 The practical implementation of the design of the sensitive element 2 is possible using various materials and technologies, however, the most promising is its manufacture on the basis of crystalline silicon using planar and thin-film technologies, as well as dimensional etching technologies, while constructive combination of the sensitive elements of the pressure sensor and thermocorrection sensor.

 One of the possible designs of the sensitive element 2 (Figs. 3 - 5) can be made in the form of a sensor chip formed by a substrate plate 15, an intermediate plate 16, and a cover plate 17, which are hermetically connected to each other, for example, by thin melted low-melting films 18 glass.

 In plates 15 and 17, pressure-sensitive membranes 11 are made by dimensional etching methods, provided with rigid centers 19 and 20, which are the walls of the working chambers 21 and 22 and form an internal volume filled with an inert gas under a certain initial pressure.

 Rigid centers 19 and 20 increase the stability of the membranes 11 when exposed to pressure and increase their effective area.

 Two bending vibration cavities are formed in the thickness of the intermediate plate 16 by dimensional etching methods, for example, in the form of thin rectangular plates 23. During etching operations, four holders 24 are formed on the periphery of each of them, which are arranged in pairs in the plane of the plates 23 on the nodal lines of their own bending fluctuations.

 One of the plates 23 is located in the internal volume of the sensor element of the pressure sensor (figure 3), and the other in the internal volume of the sensor element of the temperature-correcting sensor (figure 4).

 The plate 23 is an oscillatory system with distributed parameters and has many natural resonant frequencies. The specific arrangement of the holders 24 defines the shape (mode) of the resonant bending vibrations of the plate 23 and sharply limits the possibility of exciting its vibrations in other bending forms. For example, to ensure bending resonant vibrations of the plate 23 in the first mode, it is necessary that the holders 24 are located at a distance of 0.224 from the edges of the plate (in the direction of its length).

 In addition, the installation of the plate 23 with the help of holders 24 located on the nodal lines of its bending vibrations practically eliminates the influence of mechanical stresses arising from vibrations of the plate 23 on the body parts of the sensing element (intermediate plate 16) and, therefore, excludes the conditions for excitation of high-quality mechanical resonances in these details.

 The internal volume of the sensitive element of the thermal correction sensor, formed by the working chambers 25 and 26 with constant geometric dimensions, is also sealed, filled with inert gas under a certain initial pressure and isolated from the internal volume of the pressure sensor.

 Filling of the internal volumes of the sensitive elements of the pressure sensor (chambers 21 and 22) and the temperature correction sensor (chambers 25 and 26) with inert gas can be performed, for example, during the operation of connecting parts 15, 16 and 17 by melting the films of glass 18 in an inert gas atmosphere.

 As an inert gas, for example, nitrogen or argon can be used.

где h - поперечный размер щели; μ - динамический коэффициент вязкости инертного газа; ρ - плотность инертного газа; f - рабочая частота датчика.The working chambers 21 and 22 of the pressure sensor and the working chambers 25 and 26 of the thermal correction sensor are made in the form of slots, the transverse size of which (in the direction perpendicular to the plane of the plate 23) does not exceed twice the depth of the viscous friction layer and is selected from the ratio

where h is the transverse size of the slit; μ is the dynamic coefficient of viscosity of an inert gas; ρ is the density of the inert gas; f is the operating frequency of the sensor.

 The limitation of the transverse dimension of the working chambers within the doubled depth of the viscous friction layer provides the conditions under which periodic gas compression / rarefaction during oscillations of the oscillatory system occurs mainly in the direction perpendicular to its plane (excluding a small peripheral region along the plate contour), since the gas moves along the surface of the vibrational system is sharply inhibited by the viscosity of the gas and its “sticking” to surfaces forming thin gap gaps, which prevents Gas leakage into the peripheral part of the slots. For this reason, there are no conditions for exciting low-Q side acoustic resonances in the working chambers of the sensor, and the mass of gas enclosed in slotted volumes cannot change significantly and remains constant over a time of the order of a half-period of oscillations. Under these conditions, the energy loss of gas compression during oscillations of the oscillatory system becomes minimal, and gas compression / rarefaction is close to isothermal.

 Each pressure-sensitive membrane 11 has an area that exceeds the size (area) of the oscillatory system (plate 23 with holders 24), while outside the region of the oscillatory system the shape of the walls of the working chambers 19 and 20 can correspond to the maximum possible deflection of the pressure-sensitive membrane. This correspondence, based on dimensional etching technologies, can be achieved by making the walls of the working chambers in the form of steps 27, and the height of the edges of the steps 27 corresponds to the maximum deflection of the membrane 11 at points opposite these edges. The edges of the steps 27 form the contours repeating the opposite contours on the membrane 11 in its extreme position. It is obvious that the more steps will be performed on the wall opposite the membrane 11, the smaller the height of each of them will be and the more the shape of the wall will correspond to the maximum deflection of the membrane 11.

 The edges of the steps 27 restrict the movement of the membranes 11 with rigid centers 19 and 20 after reaching the extreme position, which protects the sensor element from overload by measured pressure. At the same time, the steps 27 restrict the movement of the rigid centers 19 and 20, leaving, after reaching the maximum deflection of the membranes 11, the minimum permissible transverse dimensions of the slots 21 and 22, which does not allow the rigid centers to exert a mechanical effect on the oscillatory system and prevent its vibrations.

 Another possible embodiment of the pressure sensor sensing element is shown in FIGS. 6 and 7. In many details, this design option repeats the above described, including the design of the temperature-sensitive sensor element. The fundamental difference is that the rigid centers 19 and 20 of the pressure sensitive membranes 11 are provided with protrusions 28 and 29 located outside the oscillation region of the plates 23 in the windows 30 etched in the plate 16. The protrusions 28 and 29 are rigidly interconnected by fusible films 31 of fusible glass. Such a connection can be made simultaneously with the connection of parts 15, 16 and 17 of the sensor chip.

 Another structural difference in the design of the sensor in FIG. 6 from the sensor in FIG. 3 is a different value of the transverse dimensions of the working chambers 21 and 22, which differ from each other by 2 to 5 times, but each is made within the doubled depth of the viscous friction layer.

 The edges of the steps 27, as in the construction of FIG. 3, limit the movement of the membranes 11 with rigid centers 19 and 20 after they reach the extreme position.

 An example of another possible implementation of the pressure sensor sensing element is the design shown in Figs. 8-10, in which only one of the walls of the working chamber 33 opposite to the oscillating system is formed using a pressure sensitive membrane 11c with a rigid center 20, and the working chamber 32 is made rigid with using the housing part 17. As an example of another type of oscillation system, this design uses a thin membrane 34 made by etching in the thickness of the intermediate plate 16. In the periphery hydrochloric membrane portion 34 is provided with several openings 35, which serve to align the quasi-static pressure of inert gas in the working chambers 32 and 33 when pressure changes in the chamber 33 by deformation of membrane 11.

 The transverse dimensions of the chambers 32 and 33, as in the structures described above, are made within the doubled depth of the viscous friction layer, and the edges of the steps 27 limit the movement of the membrane 11 with a rigid center 20 after reaching the extreme position.

 Obviously, the use of only one pressure-sensitive membrane 11 with a rigid center 20 allows you to exclude one of the chambers 12 and the corresponding fitting 13 (Fig. 2), which simplifies both the sensitive element and the design of the sensor as a whole.

 The sensitive element of the thermal correction sensor (Fig. 9) is similar to the sensitive pressure element (Fig. 8), but has unchanged geometric dimensions of the working chambers 36 and 37.

 Thin-film piezoelectric transducers 38, for example, of zinc oxide, can be used to excite and remove bending vibrations of oscillatory systems (plates 23 or membranes 34) in all the described examples of sensor designs.

 The location of the piezoelectric transducers 38 on the plane of the oscillatory system, their size and quantity are determined by the shape of the oscillatory system and the transmission coefficient of the sensing element, acceptable under the operating conditions. The implementation of the piezoelectric transducers 38 in the form of small rectangular sections oriented in the direction of propagation of the bending wave may be due to the need to reduce mechanical stresses and deformations that may occur in the bending vibration resonator (plate 23 or in the membrane 34) during the manufacturing process, in particular, due to cooling parts after high-temperature vacuum spraying of a piezoelectric film.

 On the surface of each of the rectangular sections of the piezoelectric transducers 38, thin-film electrodes 39 are applied by vacuum spraying and etching, connected by thin-film microconductors 40, which end in contact pads 41. To prevent the electrical conductivity of the microconductors 40 with silicon, the surface of the intermediate plate 16 may be exposed before applying microconductors 40, for example thermal oxidation to create an insulating film.

 In parallel, connected by microconductors 40, the electrodes 39 of the piezoelectric transducers 38 form two groups, one of which serves to excite, and the other to remove the bending vibrations of the oscillatory system.

 The pads 42 are used to connect a common ("grounding") conductor, for which the insulating film under the pads 42 is removed before application, and they have electrical contact with the thickness of the plate 16, which serves as the second (common) electrode for both groups of piezoelectric transducers 38 .

 Microconductors 6 are connected to the contact pads 41 and 42, by which the electronic chips of the self-excitation systems 3 are connected to the sensitive elements of the pressure sensor and thermocorrection sensor.

 To provide access to the contact pads 41 and 42 in the part 17, grooves 43 are made by etching methods.

 The self-excitation systems 3 of the pressure sensor and the thermal correction sensor are the same (Fig. 11) and each of them contains a conventionally broadband amplifier 44 having a high input impedance and equipped with an automatic gain control circuit 45 that maintains a constant amplitude of the output frequency signal of the amplifier 44. Each sensitive element is turned on in the positive feedback circuit of the corresponding amplifier 44, which thereby turns into a self-sustained oscillator.

 The operation of the sensor is as follows.

 When supplying voltage, self-excitation of the pressure sensor and thermal correction sensor occurs at the natural resonant frequencies of their sensitive elements. At the outputs of amplifiers 44, frequency output signals appear which are fed to the inputs of automatic gain control circuits 45, with the help of which control signals are generated that act on amplifiers 44 and control their amplification factors in accordance with the conditions of self-excitation of the oscillator. Due to the operation of the automatic gain control circuits 45, nonlinear restrictions on the amplitudes of the output frequency signals are eliminated, and the amplifiers 44 operate within their dynamic ranges.

где f₀ - собственная резонансная частота колебательной системы в вакууме (условно принято, что в силу одинаковых конструкций колебательных систем датчика давления и термокорректирующего датчика их собственные частоты в вакууме одинаковы);
γ - показатель адиабаты (отношение газовых теплоемкостей инертного газа), величина которого при поперечных размерах рабочих камер, не превышающих удвоенной толщины вязкого слоя трения, близка к единице;
Р_д - давление инертного газа во внутреннем объеме чувствительного элемента датчика давления;
Р_т - давление инертного газа во внутреннем объеме чувствительного элемента термокорректирующего датчика;
σ - поверхностная плотность колебательной системы;
d_экв - эквивалентный поперечный размер рабочих камер, который определяется как среднее геометрическое из их поперечных размеров d₁ и d₂

Давление Р_т инертного газа во внутреннем объеме чувствительного элемента термокорректирующего датчика определяется уравнением состояния газа в замкнутом объеме (например, уравнением Менделеева-Клапейрона) и является однозначной функцией абсолютной температуры. Поэтому, полагая, что при изготовлении датчик был заполнен инертным газом под начальным давлением Р₀ при температуре Т₀, и используя (3), упрощенно можно записать

где Т - абсолютная температура; k_т - коэффициент чувствительности, величина которого тем больше, чем выше начальное давление инертного газа в полости чувствительного элемента термокорректирующего датчика.The frequencies of the output signals of the pressure sensor f _p and thermocorrection sensor f _{t are} determined by the following expressions:

where f ₀ is the natural resonance frequency of the oscillatory system in vacuum (it is conventionally assumed that, due to the identical designs of the oscillatory systems of the pressure sensor and the thermocorrection sensor, their natural frequencies in vacuum are the same);
γ is the adiabatic index (the ratio of the gas heat capacity of the inert gas), the value of which is close to unity with the transverse dimensions of the working chambers not exceeding twice the thickness of the viscous friction layer;
R _d - inert gas pressure in the internal volume of the sensing element of the pressure sensor;
P _t - inert gas pressure in the internal volume of the sensing element thermocorrection sensor;
σ is the surface density of the oscillatory system;
d _equiv - the equivalent transverse size of the working chambers, which is defined as the geometric mean of their transverse dimensions d ₁ and d ₂

The inert gas pressure Р _t in the internal volume of the sensing element of the thermocorrection sensor is determined by the equation of state of the gas in the closed volume (for example, the Mendeleev-Clapeyron equation) and is a unique function of the absolute temperature. Therefore, assuming that in the manufacture of the sensor was filled with an inert gas under the initial pressure P ₀ at a temperature T ₀ , and using (3), we can write

where T is the absolute temperature; k _t is the sensitivity coefficient, the value of which is greater, the higher the initial inert gas pressure in the cavity of the sensing element of the thermocorrection sensor.

When the temperature changes, the inert gas pressure in the internal volume changes and the frequency f _{t of the} thermocorrection sensor changes accordingly, which in this case is an absolute (thermodynamic) temperature meter.

The change in frequency f _p depends on the design of the deformable box, as well as on the method of supplying the measured value.

 The design with two pressure-sensitive membranes (Figs. 3 - 5) is designed to measure the absolute pressure that is supplied simultaneously to both receiving nipples 13 and 14 (Fig. 2).

In the absence of a measured pressure P _{x, the} pressure-sensitive membranes 11 with rigid centers 19 and 20 experience elastic deformation under the influence of an inert gas pressure P _d , which leads to the largest possible value of the internal volume of the sensitive element and, accordingly, the minimum value of the inert gas pressure.

где W_p - собственная жесткость барочувствительной мембраны 11 по давлению, а λ - ее деформация (смещение жесткого центра).When applying the measured pressure, the internal volume of the sensitive element decreases, and the inert gas pressure increases, while the deformation values of the pressure-sensitive membranes 11, determined by the displacement of their rigid centers 19 and 20, depend on the pressure difference P _x and P _d

where W _p is the intrinsic stiffness of the pressure-sensitive membrane 11 in pressure, and λ is its deformation (displacement of the rigid center).

If the pressures P _x and R _d are equal, the pressure-sensitive membranes 11 are in the initial (undeformed) state, and the inert gas pressure is equal to the initial value of P ₀ .

Simultaneously with the change in inert gas pressure, the deformation of the pressure-sensitive membranes 11 with rigid centers 19 and 20 leads to a corresponding change in the transverse dimensions d ₁ and d _{2 of the} working chambers 21 and 22. Assuming that at P _x = P _{0 the} transverse dimensions of the slit chambers are equal (d ₁ = d ₂ = d ₀ ), and the intrinsic stiffnesses of the pressure-sensitive membranes are the same, such a change can be represented as
d ₁ = d ₂ = d ₀ -λ (7)
The inert gas pressure P _d in the internal volume of the sensing element depends on the measured pressure P _x and the elastic reaction of the pressure sensitive membranes 11 with rigid centers 19 and 20:
P _d = P _x -w _p • λ, (8)
Moreover, if P _x = P _d , then λ = 0, and P _d = P ₀ , i.e. the inert gas pressure is equal to the initial pressure P ₀ that was created during the manufacture of the sensor. It is obvious in this case that the pressure P _{d of the} inert gas depends on temperature.

Equation (8) is illustrated by the graph in Fig. 11, which shows in a conventional scale the change in inert gas pressure R _d depending on the measured pressure P _x at three temperatures.

Line B conditionally corresponds to the real characteristic of the pressure change P _d depending on P _x at the nominal (average) temperature in the conditional temperature range in which the sensor can be operated, while the dashed lines B _in and B _n correspond to the conditionally maximum (upper) and minimum ( lower) operating temperatures. The dash-dot line A characterizes the equality P _d = P _x and serves as a guide when reading the graph.

An increase in inert gas pressure under the influence of the measured pressure can occur until the pressure sensitive membranes 11 and their rigid centers 19 and 20 abut against the steps 27, which restrict the movement of the membranes 11 and their rigid centers 19 and 20 after reaching the extreme position, which corresponds to the “fracture” of line B at point D. A further increase in pressure P _d at P _x ≥P _Xmax becomes impossible.

где Р_д и λ определяются соответственно (8) и (6).Given both factors, namely, the change in inert gas pressure and the change in the transverse dimensions of the working chambers, and also assuming that γ = 1, the characteristic of the conversion (2) of the measured pressure into frequency can be represented as

where P _d and λ are determined respectively (8) and (6).

 From (9) it follows that an increase in the measured pressure leads to an increase in the frequency of the sensor both due to an increase in inert gas pressure and due to a decrease in the transverse dimensions of the working chambers of the sensing element.

The operation of the sensor under varying temperatures leads to a change in the initial pressure P ₀ in the internal volume of the sensing element of the pressure sensor, which determines the corresponding temperature dependence (error) of the conversion characteristic (9). This error is additive in nature and can conditionally be divided into two components, one of which is caused by a change in inert gas pressure, and the other by a change in deformations of pressure-sensitive membranes, which, in accordance with (6), depends on the pressure difference. The additive nature of this error is reflected in the value of the maximum inert gas pressure in the internal volume of the sensing element (points D _n and D _in figure 11).

 Since the temperature error is of a natural nature, the simultaneous measurement of the frequencies of the pressure sensor and the thermocorrection sensor allows one to take into account (correct) this error in the procedures of the measurement algorithm and obtain a measurement result whose accuracy depends on the accuracy of the algorithmic introduction of the corresponding temperature correction.

The operational value of the maximum measured pressure P _max can be set at point D _n (11), corresponding to the structural restriction of deformations of pressure-sensitive membranes at a minimum operating temperature of the sensor. Moreover, the required value of P _max depends on the purpose of the sensor and its operating conditions and can be ensured by the corresponding design and elastic properties of the pressure-sensitive membranes, as well as the choice of the inert gas initial pressure in the internal volume of the pressure sensor sensitive element.

где Р_д и λ определяются соответственно (8) и (6); d₀₁ - поперечный размер рабочей камеры 32 (фиг.8) с неизменными геометрическими размерами; d₀₂ - поперечный размер камеры 33, который изменяется под действием деформации барочувствительной мембраны 11 (смещения жесткого центра 20).The operation of the sensor with a sensitive element, the internal volume of which is deformed under the action of only one pressure-sensitive membrane 11 with a rigid center 20 (Fig.8 - 10), occurs in a similar way. The main difference is the conditionally lower sensitivity ("steepness") of the pressure-to-frequency conversion characteristic, which is due to the lesser compressibility of the inert gas under the action of deformation of only one pressure-sensitive membrane 11, as well as the change in the transverse dimension of only one of the working chambers 33

where P _d and λ are determined respectively (8) and (6); d ₀₁ - the transverse size of the working chamber 32 (Fig.8) with constant geometric dimensions; d ₀₂ - the transverse size of the chamber 33, which changes under the action of deformation of the pressure-sensitive membrane 11 (displacement of the rigid center 20).

 When changing the measured pressure and the corresponding deformation of the volume of the working chamber 33 through the holes 35 located on the periphery of the membrane 34, the quasistatic pressure of the inert gas in the working chambers 32 and 33 is equalized, which eliminates the possibility of undesirable deformation of the membrane 34.

 The operation of the temperature-correcting sensor (Fig. 9) corresponds to that described above, and a change in its operating frequency is described (5).

The conditionally lower sensitivity of the design of the sensor with one pressure-sensitive membrane is largely compensated by the use of an oscillating system in the form of a thin membrane 34 with a low surface density. In addition, as follows from (10), the sensitivity of the sensor can be increased due to the various transverse dimensions of the slotted chambers. In particular, due to a slight increase in the transverse dimension d ₀₁ compared with d _{02, the} sensitivity of the sensor can be increased several times, since in this case, a change in the smaller of the transverse dimensions has a predominant effect on the frequency of the sensor.

 The operation of the sensor design with a sensitive element, in which the internal volume is formed by two pressure-sensitive membranes, the rigid centers of which are rigidly connected to each other (Fig.6 and 7), has some difference from the work of the structures described above. One of the differences is due to the constant value of the total volume of the working chambers during deformation of the pressure-sensitive membranes, since the deformation of the internal volume occurs at the same time and the same magnitude of their displacement from the initial position. Moreover, a decrease in the volume of one of the working chambers leads to the same increase in the volume of another working chamber, and vice versa.

 The consequence of this is the independence of the inert gas pressure on the measured pressure and the dependence of this pressure only on temperature. In addition, the transverse dimensions of the slotted working chambers 21 and 22 are interconnected.

где Р₀ - начальное давление инертного газа в рабочих камерах датчика; d₀₁ и d₀₂ - поперечные размеры камер 21 и 22 при отсутствии измеряемых давлений.The conversion characteristic can be represented as

where P ₀ is the initial inert gas pressure in the working chambers of the sensor; d ₀₁ and d ₀₂ are the transverse dimensions of the chambers 21 and 22 in the absence of measured pressures.

где w_p - собственная жесткость по давлению одной барочувствительной мембраны.The displacement λ of the rigid centers 19 and 20 during deformation of the pressure-sensitive membranes 11 is determined by the difference in the measured pressures P ₁ and P ₂ which can be independently supplied to the receiving nipples 13 and 14 (Fig. 2)

where w _p is the intrinsic pressure stiffness of one pressure sensitive membrane.

 The functionality of a sensor with a sensitive element with a rigid connection of the rigid centers of the pressure sensitive membranes is determined by the method of supplying the measured pressures.

 If vacuum is applied to one of the nozzles, for example, to the nozzle 13, and the measured pressure is applied to the other (nozzle 14), then the sensor works as an absolute pressure meter.

 When applying atmospheric pressure to the nozzle 13, and the measured pressure to the nozzle 14, the sensor becomes a relative (gauge) pressure meter.

 When applying to the fittings 13 and 14 different measured pressures, the sensor measures the difference of these pressures.

As follows from (11) and (12), the frequency of the sensor is determined by the ratio of the transverse dimensions of the slotted working chambers, while the radical expression in (11) is hyperbolic regardless of the direction of displacement of the rigid centers of the baro-sensitive membranes, which allows one to obtain both increasing and the decreasing nature of the frequency of the sensor with, for example, increasing the measured pressure. The nature of the change in the output frequency of the sensor is determined by the selected ratio of the transverse dimensions d ₀₁ and d _{02 of the} chambers 21 and 22 in the absence of measured pressures and the method of their supply during measurements.

The high sensitivity of the sensor is achieved by working chambers with different transverse dimensions d ₀₁ and d ₀₂ so that one of them is 2 to 5 times smaller than the other, while changing the frequency of the sensor occurs mainly due to a change in the smaller of the transverse dimensions. The initial value of the frequency of the sensor (at P ₁ = P ₂ ) corresponds to a point on a sharply ascending section of the hyperbolic dependence, characterizing the change in the radical expression in (11) under the influence of the difference in the measured pressures.

 The high sensitivity of the sensor both over the entire operating range and in the region of small differences in the measured pressures, as well as the possibility of using the sensor for measurements of pressures of a different nature sharply expands its functionality. In particular, the sensor can be used to measure the flow rate of a substance by the pressure difference occurring on the measuring diaphragm installed in the pipeline for transporting this substance.

 Correction of the temperature dependence of the frequency of the sensor due to a change in the initial pressure of the inert gas under the influence of temperature can be performed by simultaneously measuring the frequencies of the pressure sensor and the temperature-correcting sensor and introducing appropriate corrections in the procedures of the measurement algorithm, which ensures the receipt of measurement results, the accuracy of which depends on the accuracy of the algorithmic temperature input amendments.

 A variety of various side factors that manifest themselves to a greater or lesser extent depending on the operating conditions and purpose of the sensor and accompanying the conversion of pressure (or pressure difference) to frequency, including, in particular, such as the difference in temperature coefficients of linear expansion and elastic moduli of materials that used for the manufacture of the sensor, lead to the fact that the frequency conversion along with the additive temperature error is characterized by a certain multiplicative error variable over the range of measured pressures. The magnitude of this additional multiplicative error depends on the range of measured pressures, the range of operating frequencies, the structural dimensions of the sensor parts, and other factors. Moreover, the multiplicative error, in addition to the possible systematic component, almost always has a temperature-dependent component. Therefore, high measurement accuracy can only be achieved using special temperature correction algorithms for the output frequency of the pressure sensor.

 For this purpose, it is advisable to use intelligent algorithms that can repeatedly increase the accuracy of measurements.

 In the process of joint algorithmic processing of the output frequencies of the pressure sensor and the temperature correction sensor, the functions of linearization, scaling, subtraction of the initial (nonzero) values of the sensor frequencies, calculation of additive and multiplicative temperature corrections and their automatic accounting in the procedures of the intelligent algorithm to compensate for errors accompanying the measurements can be implemented. The practical implementation of an intelligent algorithm is possible, for example, using a programmable controller, in an information-computer control complex, or using a microprocessor, which can be equipped with a pressure sensor.

An example of joint algorithmic processing of frequency signals that provides the measurement results in pressure units is an algorithm based on, for example, polynomials of 3–5 degree:
P _X = a ₀ + a ₁ • f _P + a ₂ • f _P ² + a ₃ • f _P ³ + a ₄ • f _P ⁴ + a ₅ • f _P ⁵ , a _i = b _i0 + b _i1 • f _T + b _i2 • f _T ² + b _i3 • f _T ³ ,
where f _p is the frequency of the pressure sensor; f _t - the frequency of the thermal correction sensor; and _i (i = 0, 1, 2 ... 5) and b _ij (j = 0, 1, 2, 3) are the coefficients of the polynomials.

The determination of the current values of the polynomial coefficients, which can linearly or nonlinearly depend on temperature (frequency f _t ), can be done automatically using an intelligent algorithm that uses data that can be written to the microprocessor read-only memory (ROM) during certification of the pressure sensor.

SOURCES OF INFORMATION
1. Copyright certificate 667840, G 01 L 11/00, publ. 06/15/79.

 2. Copyright certificate 1000805, G 01 L 11/00, publ. 02/28/83.

 3. Copyright certificate 228992, IPC G 01 l, G 08 s, publ. 11/07/69.

Claims (10)

 1. A pressure sensor with a frequency output, containing a self-excitation system and a sensing element in the form of a flat oscillatory system, transversely oscillating and dividing the internal volume of the sensing element into two communicating working chambers, characterized in that the walls of the working chambers opposite the oscillating system are made in the form pressure sensitive membranes, and the internal volume of the sensitive element is filled with an inert gas and sealed.  2. A pressure sensor with a frequency output according to claim 1, characterized in that the pressure-sensitive membranes are made in the form of membranes with rigid centers, while the area of each of them exceeds the size of the oscillatory system.  3. A pressure sensor with a frequency output according to claim 2, characterized in that the rigid centers of the pressure sensitive membranes are rigidly interconnected by protrusions located outside the oscillating system, and the working chambers are made with different transverse dimensions.  4. A pressure sensor with a frequency output according to claim 2, characterized in that the wall of the working chamber outside the oscillating system has a shape corresponding to the deflection of the pressure sensitive membrane in the extreme position.  5. A pressure sensor with a frequency output according to claim 1, characterized in that the wall of one of the working chambers opposite the oscillating system is formed by a baro-sensitive membrane, and the other working chamber is rigid.  6. A pressure sensor with a frequency output according to claim 1, characterized in that a thermocorrection sensor is located in the pressure sensor housing, which contains a self-excitation system with a sensitive element in the form of a flat oscillatory system that transversely vibrates and divides the internal volume of the sensitive element into two working chambers while the internal volume of the sensing element is filled with inert gas and sealed.  7. A pressure sensor with a frequency output according to claim 6, characterized in that the sensing element is made in the form of two oscillatory systems, one of which forms a pressure sensor, and the other a temperature-correcting sensor.  8. A pressure sensor with a frequency output according to claim 1, characterized in that the working chambers are made in the form of slots, the transverse dimension of which does not exceed twice the thickness of the viscous friction layer.  9. A pressure sensor with a frequency output according to claim 8, characterized in that the oscillating system is made in the form of a rectangular plate fixed along the nodal lines of bending vibrations.  10. A pressure sensor with a frequency output according to claim 8, characterized in that the oscillating system is made in the form of a membrane equipped with holes located at its periphery.

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