RU2488125C1 - Hydrostatic accelerometer - Google Patents

Abstract

FIELD: instrument making.

SUBSTANCE: accelerometer comprises a metering cell in the form of a rectangular parallelepiped, a porous sphere, inside of which there is a sealed bellows with additional weights. In the lower part of the cell there is an additional bellows fixed, an electromagnet coil, a core, and also three pairs of piezoelements (emitters and receivers in pairs).

EFFECT: invention makes it possible to simplify design and to increase accuracy of measurements.

2 cl, 2 dwg

Description

Technical field

The invention relates to instrumentation, namely to accelerometry, the measurement of microaccelerations in space objects.

State of the art

Piezoelectric type accelerometers are widespread, but with all their advantages (high sensitivity, impact resistance, small size) have a minimum recording frequency of at least 0.3 Hz [Friden J. Modern sensors. Directory. Ed. "Technosphere." M., 2005, 592 p.).

This makes them unsuitable for recording ultra-low frequencies, and even more so for recording constant small accelerations. Brucl & Kjaer piezoelectric accelerometers [Freescale Semiconductor Small Acceleration Sensors. Syrov L. Components and Technologies, No. 2, 2010] guarantee the operation of some of their sensors starting from 0.1 Hz, but most types of these sensors work starting from 1 Hz. Similar data are provided by the Piezopribor company, whose sensors have a lower frequency of 0.3 ... 0.7 Hz, and ZETLab with a frequency range of 0.5 ... 5000 Hz. MEMSIG dsgeerftn company has two-axis acceleration sensors in the form of complementary microchips MXD2020G / H and MXD2020M / N with a sensitivity of 1 · 10 ^-3 g and a frequency range of 17 ... 160 Hz.

Although the frequency range from 0 Hz is stated in the brochures of some companies, but this is not possible for piezoelectric accelerometers, but only accelerometers with capacitive as well as strain gauges. The disadvantages of capacitive accelerometers include low sensitivity - 1200 mV / g, which in terms of yields a value close to 1 · 10 ^-3 g.

Recently, convective accelerometers and seismographs have been increasingly used, and in some cases such devices are designed in the form of microcircuit housings (for example, MEMSIG products). The first devices had a sensitivity of the order of 1 Mg (10 ^-3 g), later this value was increased by two orders.

In addition to seismic receivers using convection, a DAKON-2 convective sensor designed for a sensitivity of 10 ^-6 g was created for measuring accelerations in the orbit of the ISS (patent RU 2379693, MHK G01P 15/08, published on January 20, 2010).

The average time for establishing the readings of the convection sensor DAKON-2 is at least one hour (US Pat. RF No. 2421735 MHK G01P 15/08, publ. 06/20/11).

Therefore, the task was set to create a highly sensitive instrument with a short set-up time on the basis of the so-called Cartesian diver (KB).

There is a design of a pendulum float accelerometer with pendulums integrated in the floats, magnetic angle sensors and friction compensators in the pins. The accelerometer is hydrostatic, contains a housing, inside of which there is a float pendulum sensing element and a measurement system. We took this accelerometer as a prototype [SU 1839858 A1 MHK G01P 15/08, publ. 06/20/06].

This accelerometer has two drawbacks, especially manifested in the process of measuring small overloads with the vertical position of the measuring axis of the sensing element:

- insensitivity, determined by the film formed on the stone support as a result of the use of various organic substances in the design of the sensitive element;

- an error due to the relatively long time it takes to reach the accelerometer output information when changing the orientation of the measuring axis due to the high viscosity due to the immiscibility of the liquid filling the sensor at low speeds of its moving system ("exit to the mode").

The objective of the invention is to develop a simple device design with greater measurement accuracy.

The problem is solved using the features specified in the 1st paragraph of the claims common to the prototype, such as a hydrostatic accelerometer containing a housing, inside which a float pendulum sensing element and a measurement system are placed, and distinctive, essential features, such as the housing is filled with a working fluid, in which a sensitive element is placed in the form of a porous sphere, inside of which a sealed bellows with additional loads in the form of a pendulum is placed, providing a float consisting An approach close to neutral buoyancy, while for changing the pressure inside the body in order to change the speed and direction of movement of the float-sphere, the accelerometer is equipped with an additional bellows installed in the lower part of the body between the coils and the core of the electromagnet connected to the control and registration unit, and for measuring the speed of the movement of the sphere on the walls of the body are installed emitting and receiving piezoelectric elements of radiation and reception of ultrasonic waves, connected to the reception and registration unit.

According to claim 2 of the claims, distilled water is used as the working fluid.

The above set of essential features allows you to get the following technical result - simplification of the design, improving the accuracy of the device due to the low viscosity of the working fluid.

The invention is illustrated by the following drawings. Figure 1 shows a diagram of the proposed device; figure 2 - placement of the piezoelectric elements, top view.

The hydrostatic accelerometer (Fig. 1) contains a housing 1, inside which a float pendulum sensing element and a measurement system are placed. The housing 1 is filled with a working fluid, in which a sensing element in the form of a porous sphere 2 is placed, inside of which a sealed bellows 3 with additional loads 4 in the form of a pendulum is placed, providing the float with a state close to neutral buoyancy.

To change the pressure inside the housing in order to change the speed and direction of movement of the float-sphere 2, the accelerometer is equipped with an additional bellows 5 mounted in the lower part of the housing 1 between the coils 6 and the core 7 of the electromagnet connected to the control and registration unit 8.

To measure the speed of movement of the sphere 2 on the walls of the housing 1 are installed emitting 9 and receiving 10 piezoelectric elements of radiation and reception of ultrasonic waves, connected to the receiving and recording unit 8. In the accelerometer, distilled water 11 is used as the working fluid.

In the proposed design, the working cell 1, in contrast to the prototype, where a diamagnetic liquid is required (usually organic liquids such as benzene, octane) is filled with distilled water, which simplifies the selection of materials for the manufacture of the measuring cell and the gaskets inside it.

The action of the Cartesian diver (KB) is based on the following principle. The sum of the external and hydrostatic pressures compresses the bellows 2 inside the KB cavity, while the gas volume decreases, as a result of which the Archimedes force also decreases. The relationship between gravity and Archimedes is obtained from the equation of state of an ideal gas. The total pressure in the liquid P _sum at an immersion depth z is equal to:

$$P_{\mathrm{from}\mathrm{at}m}=P_0+{\rho }_{\mathrm{well}}g\cdot z,\left(\mathrm{one}\right)$$

where P ₀ is the gas pressure at the liquid – gas interface, ρ _Ж is the liquid density, g is the gravitational acceleration in the system. The dimensions of the body KB in comparison with the immersion depth z are considered small.

The process is considered isothermal. For an ideal gas, the condition of constant mass gives the following ratio of volumes and pressures

$$P_{\mathrm{from}\mathrm{at}m}{V}_z=P_0{V}_{z0}\left(2\right)$$

here V _z0 and V _z are the volumes of gas in the bellows near the surface of the liquid (z = 0) and when the body is immersed to a depth of z. To take into account the elasticity of the bellows, we estimate the pressure difference inside and outside the bellows. We assume that the bellows is in compression, that is, an increase in external pressure reduces the volume of the bellows, which is equivalent to compression of the spring. Summary differential between the external pressure on the base of the bellows and the side wall and the inner P _ext we assume proportional to the pressure outside the bellows _nar P = _sum P:

$$P_{n\mathrm{but}R}-P_{\mathrm{at}n\mathrm{at}tR}=\kappa \cdot P_{n\mathrm{but}R}\left(3\right)$$

where κ is the coefficient of proportionality, or

$$P_{\mathrm{at}n\mathrm{at}tR}=P_g=P_{n\mathrm{but}R}\left(\mathrm{one}-\kappa \right)\left(\mathrm{four}\right)$$

Given (2), we calculate the relationship of the pressure in the bellows R _g with the immersion depth z as follows

$$P_g=P_{n\mathrm{but}R}\cdot \left(\mathrm{one}-\kappa \right)=\frac{m_{g}RT(\mathrm{one}-\kappa )}{V_g{\mu }_g}=\frac{m_{g}RT\left(P_0+{\rho }_{\mathrm{well}}g\cdot z\right)\left(\mathrm{one}-\kappa \right)}{{\mu }_g\cdot P_0{V}_0}.\left(5\right)$$

The equilibrium equation taking into account the change in the volume of the bellows is written as follows (we omit the value of g in the expression)

$${\rho }_m{V}_m+m_{\mathrm{from}}+{\rho }_g{V}_g-{\rho }_{\mathrm{well}}{V}_m-{\rho }_{\mathrm{well}}{m}_{\mathrm{from}}/{\rho }_{\mathrm{from}}-{\rho }_{\mathrm{well}}{V}_g=0\left(6\right)$$

Here, V _m is the volume of the body (mesh sphere 2), ρ _{m is the} density of the body material, m _c is the mass of the bellows; ρ _c is the density of the material of the bellows, V _g is the volume of gas inside the bellows, ρ _g · V _g = m _g = conct is the mass of gas inside the bellows. The mass of the additional load 4 is also included in the body volume.

The relationship of the acceleration observed in the system with the immersion depth and other parameters receives the following expression

$$g=\frac{\mathrm{one}}{{\rho }_{\mathrm{well}}z}\left[\frac{m_g{\rho }_{\mathrm{well}}RT/{\mu }_g}{\left[\left({\rho }_m-{\rho }_{\mathrm{well}}\right)V_m+m_c\left(\mathrm{one}-\frac{{\rho }_{\mathrm{well}}}{{\rho }_{\mathrm{from}}}\right)+m_g\right]\left(\mathrm{one}-\kappa \right)}-P_0\right]\left(7\right)$$

Achieving equilibrium can be a lengthy process, so the goal was to measure acceleration in a more operational way. For this we use the equation of motion of an immersed body. Suppose that the friction force for a spherical body at low speeds is proportional to the speed of motion and the viscosity coefficient of the fluid: F _mp = -χ · η · W, where χ is the proportionality coefficient, η is the dynamic viscosity coefficient, W is the speed of the sphere in the fluid . The balance of gravity and Archimedes forces in this system has the form:

$$\left({\rho }_m{V}_m+m_c+{\rho }_g{V}_g-{\rho }_{\mathrm{well}}{V}_m-{\rho }_{\mathrm{well}}{V}_g\right)g=F_{\mathrm{one}}\left(8\right)$$

Here F ₁ - the total force acting on the float KB (sphere 2). The mass of fluid undergoing acceleration is

$$\left({\rho }_m{V}_m+m_c+{\rho }_{\mathrm{well}}{V}_{\mathrm{well}}^{d\mathrm{about}P}\right)=M\left(9\right)$$

- дополнительный объем жидкости, находящийся внутри прибора между сильфоном и корпусом (в балансе сил тяжести и Архимеда не участвует). Второй закон Ньютона должен быть записан в виде:Here $$V_{\mathrm{well}}^{d\mathrm{about}P}$$

- An additional volume of liquid located inside the device between the bellows and the body (does not participate in the balance of gravity and Archimedes). Newton’s second law should be written as:

$$F_{\mathrm{one}}+\chi \eta W=M\frac{dW}{dt}\left(10\right)$$

The speed of the steady motion of the body is calculated at zero acceleration:

$$W_{\mathrm{at}\mathrm{from}t}=\frac{g}{\chi \mu }\left[\left({\rho }_m-{\rho }_{\mathrm{well}}\right)\cdot V_m+\left(m_c-{\rho }_{\mathrm{well}}{V}_{\mathrm{from}}^{d\mathrm{about}P}\right)+\left({\rho }_g-{\rho }_{\mathrm{well}}\right)V_g\right].\left(\mathrm{eleven}\right)$$

From this expression, the acceleration g

$$g=\frac{W_{\mathrm{at}\mathrm{from}t}\chi \eta }{F_{\mathrm{one}}}\left(12\right)$$

The experiments with KB showed that the friction forces during the movement of the mesh body are quite large, therefore we will assume that the steady-state value of the velocity W _{mouth is} reached in a negligible time.

The absence of supports simplifies the proposed device, friction in the supports is excluded; the source of vibrations is also excluded, which entails an increase in the accuracy of the device.

The accelerometer works as follows.

At certain intervals, the electronic control unit 8 supplies current to the coil of the electromagnet 6, the core of the electromagnet 7 is drawn into the coil 6 and compresses the additional bellows 5. The pressure in the measuring cell increases, the bellows 3 decreases in volume, and the float 2 begins to sink (but moves direction of action of acceleration). At the same time, the control unit 8 includes three ultrasonic emitters 9 and receivers 10. Each of them determines the distance from the sphere 2 to each of the sensors along the coordinate axes. Block 8 thus determines the speed W _mouth , the values χ, η are constants, the value of F ₁ is determined by preliminary measurements and calculations.

The speed of immersion determines the magnitude and direction.

Ultimately, 3 acceleration components are calculated through 3 steady-state components. After turning off the electromagnet 5, the float 2 pops up, moving already against the direction of the total acceleration and thereby returning to its original state.

From the description and practical application of the present invention, other particular forms of its implementation will be apparent to those skilled in the art. This description and examples are considered as material illustrating the invention, the essence of which and the scope of patent claims are defined in the following claims, a combination of essential features and their equivalents.

Claims (2)

1. A hydrostatic accelerometer, comprising a housing, inside which a float pendulum sensing element is placed, and a measurement system, characterized in that the housing is filled with a working fluid, in which the sensing element is placed in the form of a porous sphere, inside which a sealed bellows with additional loads in the form of a pendulum is placed providing the float a state close to neutral buoyancy, while changing the pressure inside the body in order to change the speed and direction of movement of the float, s The accelerometer is equipped with an additional bellows installed in the lower part of the body between the coils and the core of the electromagnet connected to the control and registration unit, and to measure the speed of the sphere moving on the body walls, the emitting and receiving piezoelectric elements of radiation and reception of ultrasonic waves are connected to the reception and registration unit . 2. The accelerometer according to claim 1, characterized in that distilled water is used as the working fluid.

Images