RU1795315C - Variable-capacitance pressure transducer and method of manufacture - Google Patents

Abstract

The invention relates to measurement and technology and can be used to measure small variables and quasi-static pressures in gaseous and liquid media. The goal is to increase the sensitivity and accuracy of measurements. Essence of ebrenis: a piezoelectric quartz sloth sensor contains a housing 1 with a quartz piezoelectric element 2 placed in it in the form of a circular X-slice plate on which electrodes 6 and 7 are applied, connected, respectively, with the housing 1 and the sensor output 5. A feature of the sensor is that in it the plate 2 is rigidly clamped around the periphery in the sensor housing 1, forming a membrane, and the electrode 7 connected to the output 5 of the sensor is located in the central part of the plate 2, limited by a radius of 0.63 from the radius of the plate. To simplify fastening in the housing 1, the piezoelectric element 2 can be made in the form of a cap with a bottom, which is a membrane, and walls fixed to the base of the housing 1, wherein the wall thickness is not less than twice the thickness of the bottom, and the height exceeds the radius of the cap. 1 s.p. f-ly. 4 ill. (L C

Description

The invention relates to information measurement technology and may find application for accurate measurements of variable and quasi-static pressures.

Known types of domestic piezo-piezoelectric pressure sensors are constructed using piezoelectric elements from quartz X-section. operating on a direct piezoelectric effect With longitudinal or transverse polarization of an element. The most important advantages of these sensors are high Reliability, a wide range of workers. temperatures, high tread frequencies (on the order of hundreds of kilohertz), small dimensions and mass. The quartz elastic elements used in these sensors cause low intrinsic noise and low sensitivity thresholds, which allows obtaining a dynamic range of more than 100 dB on a single piezoelectric package design. The use of a charge amplifier as an intermediate converter allows these sensors to be used in the most difficult operating conditions.,

It is well known that quartz has practically perfect elastic properties, which leads to its use in the manufacture of elastic elements of precision devices. On the direct piezoelectric effect of quartz, it is potentially possible to create high-precision sensors characterized by simplicity of design and measuring circuit. At the same time, the measurement accuracy of known sensors is limited to 0.5-1.0%, which is primarily associated with errors in power input and power distribution. The disadvantages that limit the accuracy of measurements are associated with the construction of the power circuit according to the traditional scheme, namely: medium - an intermediate element (bellows or membrane) - smolevodyaschie details - piezoelectric elements. Thus, in the sensor power circuit there are a number of joints with unstable rigidity, which lead to distortion of the mechanical fields in the piezoelectric elements and the appearance of contact stresses. In addition, the presence of devices, pre-tensioning piezoelectric elements, which can not be dispensed with, and sensors with built-in piezoelectric packages, leads to a bypass of the sensor siphon and the emergence of additional mechanical stresses in the piezoelectric elements, which is generally inhomogeneous and causes instability of the calibration sensor specifications.

The closest technical solution (prototype) should be considered the design of the pressure sensor described in the Swiss patent. This sensor, patented by Kistler, is designed to meet a number of recommendations for reducing power input errors. This is primarily the use of piezoelectric elements operating on the transverse piezoelectric effect of quartz, and an analysis of the methods for manufacturing these piezoelectric elements from a single crystal, aimed at

reduction of random components of the sensor errors from influencing factors. The aim of the present invention is to increase the sensitivity and accuracy of the conversion of pressure sensors based on the direct piezoelectric effect of quartz.

The potential of the quartz will be fully realized by the sensor design, in which there are no intermediate links that transmit the measured load to the piezoelectric element. Since among the known types of quartz piezoelectric elements there are no such for which the natural input value is pressure, the author analyzed

possible methods for constructing quartz elastically sensitive elements arising from a matrix of quartz piezoelectric modules. In FIG. 1-4 depict: 1 - sensor housing; 2 - quartz piezoelectric element in the form

a circular plate of radius R and thickness h, the base of which is perpendicular to the crystallophysical axis X of quartz; 3-element of the plate volume with the current coordinates r, p in the polar coordinate system, limited by the area dS and the plate thickness h, which are subjected to mechanical stresses Oi and az; quartz axis in a rotated coordinate system X, Y, Z: b, 7 - electrodes, one of which (6) is connected to

the housing 1, and the second (7) with an electrical terminal 5, isolated from the housing 1 using an insulating sleeve 4; P is the measured pressure; a seal 8, which is a sealant layer; on figs

The plane of the piezoelectric element in the crystallophysical axes of quartz X, Y, Z and the surface characterizing the distribution of the specific charge density over the area of the piezoelectric element during its bending are shown.

The sensor operates as follows.

Under the action of the measured pressure P (see Fig. 1), the piezoelectric element 2 is deflected, which is rigidly clamped in the sensor body 1.

in a disk piezoelectric element 2 (see Fig. 2), which is used in the proposed sensor, when deflected by the pressure difference x; mechanical stresses occur on both sides of the disk.

(1; - +. / I.) - r2- (3 + / Ol

 + / 0 (1 + 3/0,

0) where P is the external (measured) pressure;

Cr is the radial normal stress;

i cup - circumferential normal stresses;

| q is the Poisson's ratio; ; R d / 2 is the radius of the plate; j r is the current coordinate in the polar coordinate system;

i Under the influence of these voltages, polarizing electric charges are formed on each elementary part of the surface of the piezoelectric element dS (see Fig. 2).

dQ- (di2 02 + di3 u) dS dS rdrdy

where 03, c,

 di2 and di3 are the values of the piezoelectric modules in

the coordinate system XY Z1 (when turning the X axis on an arbitrary angle p), equal

respectively:

di2 di2 cos2 p + dn sin pcos # dis di2 sin2 (p- di4 J,

(3

Moreover, di2 and dn are tabular values of the quartz modules of quartz,

I After substituting in (2) the values of og, oz, CJ12, dis and intermediate transformations, we obtain:

dQ 3 4 cH2R3 (1 +,) 2di2r2R (1 + p) +

Oh h

+ di2T2Rcos2 (1 - /) + + -fifldrd p

An analysis of expression (4) shows that the presence of periodic functions of elementary charges is unevenly distributed over the surface and can vary by 4, which, when directly integrated, will lead to mutual compensation of charges. The surface charge depends on the coordinate r and angle p (with a period of variation of 180 °). We introduce the notation; r / R or, for quartz dij / di2e 0.29; ns 0.3, then:

d Q 3. D R3

gd, 2P

where g is the specific density of surface charges,

g 1.3 + u (0.7cos2 p + 0.2sln2 p. - 2.6) (5)

10

The three-dimensional surface, constructed on the basis of the obtained formula (5), is shown in Fig. H, the hatching shows the part of the surface of the piezoelectric element, on which ge, (- non-irradiated charges are positive. Value (О, at which the sign changes in charge at the maximum distance from the center, equal to 0.63. Therefore, to remove the charges should be limited to the central

20 around the surface of a piezoelectric element with a radius of 0.63R, The total charge generated on the electrodes of the piezoelectric element under the action of pressure P can be determined by integrating the elementary charge dQ by the area of application of the electrode 25 on the piezoelectric element:

0.63 R 1pl P4

Ovikh. / / dQ di2 (1 + ft) -P (6) oo ° h

30 Thus, the output value of the sensor linearly depends on the measured pressure and the maximum measured

laziness

pressure is determined by the formula:

35

h

. Pmax g- Odop. (fifteen }

whereadop. Permissible mechanical stresses

40:. The need to crush the edges of a brittle quartz disk can be avoided by replacing it with a sensitive element in the form of a cap (see Fig. 4), which is secured to the sensor with a sealant. Under the influence of the measured pressure, the bottom of the cap bends, the repeated stress state of the disk clamped along the edges, and the imperfect attachment will affect the stability of the sensor sensitivity in

50 much less so. For this, it is necessary that the wall thickness of the cap (see Fig. 4) is at least twice the bottom thickness and the cap height is H D / 2. If necessary, for example, in the case of

55 for measuring pressure in aggressive environments, the quartz piezoelectric element from the external pressure side can be protected in a hollow membrane or chemically resistant coating.

The present invention, a pressure sensor, can be used to measure with high accuracy variable and pulsating pressures in gaseous and liquid media over a wide temperature range. Simplicity of sensor design

provides its high reliability. The calculated sensitivity of the sensor with a cap piezoelectric element is up to C / Pa; as standard, such a sensor will have a random component and linearity error of not more than 0.1-0.2%.

Claims (2)

1. A piezoelectric quartz pressure sensor, comprising a housing with a quartz piezoelectric element placed in it in the form of a circular X-slice plate, on which electrodes are applied, connected to the housing and the output of the sensor, so that, in order to increase sensitivity and measurement accuracy, in it the plate is rigidly clamped around the periphery in the sensor body, forming a membrane, and the electrode connected to the sensor output is located in the central part of the plate, limited by a radius equal to 0.63 of the radius of the plate. 2. The pressure sensor according to claim 1, with the exception of the fact that, in order to simplify mounting in the housing, the piezoelectric element is made in the form of a cap with a bottom, which is a membrane, and walls fixed to the base of the body, and the wall thickness is not less than twice the thickness of the bottom, and the height exceeds the radius of the cap.