Gas pressure sensor

Abstract

FIELD: measuring equipment.SUBSTANCE: invention is intended for monitoring gas pressure in pipelines. The pressure sensor comprises a cylindrical body with a cover and a bottom. A central pin with connection loops is installed on the bottom of the body. The cover has a hole for communication between the resonator cavity and the measured gas environment. The measuring circuit is made in the form of an autogenerator connected to a communication loop and an electronic frequency meter.EFFECT: increasing the accuracy of measurements, improving the temperature stability and linearity of the sensor characteristics while simplifying its design.1 cl, 1 dwg

Description

[0001] The invention relates to measuring equipment and can be used to measure gas pressure in pipelines.

[0002] Pressure sensors are widely used in industry, performing the functions of monitoring, regulation and prevention of emergency situations. Depending on the purpose, many types of sensors have been developed, which use various physical principles. Classification of sensors by type is given in the source [Zamaletdinova E.Yu., Egorychev A.I., Comparative analysis of pressure measurement methods. Bulletin of the Technological University. 2014, Vol. 17, Issue 8, pp. 124-127]. The operation of the sensor is usually based on the conversion of the medium pressure into the movement of the actuator, which is somehow connected to the sensor. Along with sensors such as manometers in the form of deformable tubes, bellows and diaphragms, sensors with piezoelectric, strain gauge, capacitive, inductive, optical and similar sensitive elements are known. The sensor, as a rule, is isolated from the external environment by an elastically deformable membrane.The growing volume of information required to be collected from pressure sensors necessitates their convenient integration with data transmission networks. Sensors and their sensing elements must be integrated with digitalization and network communication nodes.

[0003] Among the sensors that meet these requirements, capacitive sensors in various modifications are of interest. In these, an elastically deformable membrane is connected to a movable capacitor plate and, when the pressure changes, brings it closer to the fixed plate, causing a corresponding reaction in the electrical circuit. Historical examples include capacitive pressure sensors from [Ash Zh. et al., Sensors of Measuring Systems. Book 2. Moscow: Mir, 1992, pp. 196-201].

[0004] Advances in high-frequency technology and microelectronics make it possible to increase the frequency range of capacitive sensors, thereby improving resolution, signal generation speed, and other metrological characteristics.

[0005] A pressure sensor is known according to patent [Russian Federation 2221228 C2], selected as a prototype. The pressure sensor is designed as a coaxial microwave resonator. It contains a cylindrical housing with a lid and a bottom. A central pin with coupling loops for connecting a measuring circuit is mounted on the bottom of the housing. The housing cover is designed as a flexible membrane that undergoes deformation under gas pressure. The resonant frequency in the prototype depends on the capacitance between the end surface of the pin and the cover. Changes in resonant frequency are recorded by the measuring circuit and associated with the pressure value. The known sensor has the following characteristics in the pressure range of up to 40 MPa: resonant frequency of 670 MHz, quality factor of 80-90 units, temperature drift of 0.3% per 100°C.An analysis of the metrological characteristics reveals that frequency determination accuracy is significantly limited by the low Q factor of the microwave resonator, and its temperature frequency stability is inferior to that of a copper resonator, which has a thermal expansion coefficient of 0.17% per 100°C. It can be assumed that, despite the authors' choice of specialized alloys, the combination of their disparate characteristics leads to unsatisfactory temperature stability. Fabricating the sensor from several dissimilar components complicates its design. Furthermore, the degree of nonlinearity of the pressure-capacitance conversion, caused by the membrane deformation pattern, has not been determined for the existing sensor. Accounting for and overcoming nonlinearity poses a serious challenge for membrane sensors. Therefore, the existing sensor requires improvement in its metrological characteristics, including accuracy, temperature stability, and linearity.

[0006] The technical result of the claimed invention, in accordance with the shortcomings of the prototype, consists in improving the characteristics of accuracy, temperature stability and linearity while simplifying the design.

[0007] The technical result is achieved in that in a pressure sensor containing a cylindrical housing with a cover and a bottom, wherein a central pin with communication loops for connecting a measuring circuit is installed on the bottom of the housing, the difference is that an opening is made in the cover for communicating the resonator cavity with the gas medium, and the measuring circuit is represented by an autogenerator connected to a communication loop, and an electronic frequency meter.

[0008] The achievement of the technical result is due to the following factors.

[0009] Communication between the resonator cavity and the gas medium ensures a direct effect of the medium's characteristics on the resonant frequency. As is known, the frequency of a hollow resonator is proportional to the square root of the relative permittivity of the medium filling it. In turn, the permittivity of natural gas, represented mainly by methane, under normal conditions is δ = 1.0009, where the small addition of 0.0009 is due to the polarizability of the gas molecules. The change in polarizability is directly proportional to the number of gas molecules in the resonator, i.e., the pressure. The relative smallness of the addition allows for the use of a linear approximation when calculating the square root using the formula .

[0010] At pressures of approximately 100 bar, typical for pipeline transport, the value of ε will already be 1.09. With an initial resonant frequency of 1 GHz, the resonator detuning will reach 45 MHz, which is easily and accurately recorded by modern electronic circuits. For example, the proposed electronic circuit, represented by an oscillator, generates oscillations at a frequency tied to the resonator frequency, and the frequency stability is higher, the higher the resonator's Q factor. If the resonator is made of copper, the Q factor can reach 4000 [Lebedev I.V. Microwave Technology and Devices. Moscow: Higher School, 1970, pp. 333-334]. The frequency accuracy, within the resonator's half-width, will be approximately 1000 MHz / (2⋅4000) = 0.125 MHz, or a fraction of a percent of the total detuning of 45 MHz. This provides a significant improvement in accuracy compared to the prototype, which has a Q factor of 60-80.The resonator can be easily manufactured from copper. Compared to a prototype made from dissimilar materials, this allows for temperature stability comparable to that of copper resonators.

[0011] A high degree of linearity with relatively small deviations allows for a significant improvement over the prototype in terms of design simplicity. This is confirmed by the following circumstances. With regard to linearity, the characteristics of a known capacitive sensor with a deformable membrane are affected by the nonlinear dependence of the membrane deflection on pressure. The dependence of the membrane center deflection w0 on pressure p is given in the monograph [Andreeva L.E. Elastic elements of devices. Moscow: Mashgiz, 1962, p. 255]. It is expressed by the formula

[0012]

[0013] where R is the membrane radius, h is its thickness, E is Young's modulus, and μ is Poisson's ratio. For typical alloys, Poisson's ratio is close to 0.3, and the formula takes the approximate form

[0014]

[0015] Considering the value x=w0 / h as an argument, it can be determined that the deviation from the linear law is expressed by the formula

[0016] l + 0.61x 2 .

[0017] For adequate pressure-to-displacement conversion, deflection values ​​that represent small fractions of the membrane thickness are relevant. For example, at x = 0.05, the error at the upper measurement limit will be 0.15%, while at x = 0.1, it will be 0.61%. With a membrane thickness of 1 mm and an initial gap between the membrane and the pin of 0.5 mm, a reduction of 0.05 mm can be expected. It is easy to see that the membrane design of the known sensor is essentially subject to precision requirements, expressed in hundredths of a mm. The proposed sensor is free of these requirements, confirming the technical result in terms of design simplification.

[0018] The error due to the nonlinearity of the membrane deformation of a known sensor is superimposed on the error associated with the nonlinearity of the square root function, which is expanded in a Taylor series according to the formula

[0019]

[0020] It should be noted that the nonlinearity of the frequency-to-capacitance conversion is inherent to both the existing and proposed sensors. It is analytically defined and can be accounted for with any accuracy. For example, a relative change in the proposed sensor's capacitance at a pressure of 100 bar, equal to 0.09, leads to a relative change in the resonant frequency in the linear approximation of 0.045, and taking into account the second term of the expansion, to 0.044, resulting in an analytically accountable error of approximately 2%. A membrane sensor exhibits an error of the same order of magnitude, but is additionally subject to the aforementioned deformation nonlinearity error, so the linearity is clearly worse.

[0021] The accuracy characteristic is related to the thermal expansion coefficient of the resonator material. The absence of dissimilar metal assemblies reduces the frequency temperature coefficient to the material's thermal expansion coefficient, eliminating movement and hysteresis at the joints. Specifically, when using copper, the temperature coefficient is 17-10 -6 / °C, or the above-cited value of 0.17% per 100°C. In the case of using the 29N26KHBTYu alloy proposed in the prototype, the temperature coefficient decreases to a value of 8.5-10 -6 / °C, or twice as much. The maximum quality factor of such a more thermally stable resonator is achieved by copper plating, which is technologically feasible. This provides an advantage over the prototype, which achieved a stability of 0.3% at 100°C, or 3010 -6 / °С.

[0022] Constructing a measuring circuit as an oscillator loaded onto a resonator via a coupling loop ensures minimal frequency error, which would be introduced by alternative measuring circuits, since the resonator directly determines the oscillation frequency. Due to the high Q factor of the resonator, the residual influence of the circuit elements on the oscillator frequency is kept to less than the half-width of the resonant characteristic, i.e., in relative units, about 10 -4The design of the oscillator, loaded onto a resonator, allows for the use of a single coupling loop, eliminating the need for a second loop. This reduces losses in the resonator due to external coupling, which also contributes to an increase in its quality factor. The need for a second coupling loop hypothetically arises when preliminary frequency adjustment is required. The presence of a digital frequency counter ensures consumer applicability of the sensor by allowing it to be interfaced with a data transmission system via any wired or wireless interface.

[0023] The invention is illustrated by Fig. 1, which shows a cross-section of the pressure sensor. The numbers indicate: 1 - cylindrical body, 2 - bottom, 3 - cover, 4 - pin, 5 - communication loop, 6 - hole, 7 - oscillator, 8 - digital frequency meter.

[0024] The pressure sensor (Fig. 1), according to the claimed technical solution, comprises a cylindrical body 1 with a bottom 2 and a cover 3. A pin 4 with communication loops 5 for connecting a measuring circuit is mounted on the bottom coaxially with the body. A hole 6 is made in the cover 3 for communicating the resonator cavity with the gaseous medium. The sensor is equipped with a measuring circuit represented by an auto-oscillator 7, connected to communication loop 5, and an electronic frequency meter 8.

[0025] During sensor operation, the gaseous medium enters the resonator cavity through hole 6, changing its resonant frequency proportionally to the gas pressure. This changes the oscillation frequency of oscillator 7, connected to the resonator via loop 5. Electronic frequency meter 8 generates a digital code reflecting the frequency value, which is transmitted to the data acquisition system.

[0026] The improvement of the sensor characteristics relative to the prototype is due to the following.

[0027] Increasing the resonator's Q factor to several thousand units and, accordingly, strictly locking the oscillator frequency to the resonant frequency plays a significant role in determining the frequency accuracy. In this study, a Q factor of 2600 was obtained at a resonant frequency of 900 MHz.

[0028] In terms of thermal stability, it's crucial to construct the resonator from a single metal, preferably copper, or from more thermally stable copper-coated alloys. This significantly eliminates slippage and hysteresis.

[0029] The absence of a membrane, which is an integral element of the prototype, eliminates errors associated with the nonlinearity of its deformation characteristics. This also eliminates the requirement for precise adjustment of the gap between the membrane and the pin, simplifying the sensor design.

Claims

A pressure sensor comprising a cylindrical housing with a cover and a bottom, wherein a central pin with communication loops for connecting a measuring circuit is mounted on the bottom of the housing, characterized in that an opening is made in the cover for communicating the resonator cavity with a gaseous medium, and the measuring circuit is represented by an autogenerator connected to the communication loop and an electronic frequency meter.

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