RU2486646C1 - Surface acoustic wave sensor for wireless passive measurement of displacements - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: surface acoustic wave (SAW) sensor for wireless passive measurement of displacements has a reading device and a SAW device. The reading device consists of a transceiver and an antenna, and the SAW device has an antenna and SAW resonators, each having an interdigital transducer which is electrically connected to the antenna of the SAW device, and two reflecting structures; the sensor also includes a rod, two diaphragms, two covers, two glass/metal insulators, spacers with two fluoroplastic films, a spacer made from solid substance, and mountings for the SAW device.

EFFECT: high accuracy of measuring displacements in real operating conditions associated with the effect of environmental factors.

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Description

The invention relates to the field of radio engineering and can be used in monitoring systems of the stress-strain state of objects.

A known sensor of mechanical stresses containing a device on surface acoustic waves (SAW) in the form of a console of piezoelectric quartz, the external impact of which leads to a change in the parameters of the device (US Patent No. 3888115 of 07/10/1975, IPC G01b 7/16). The disadvantages of this device is that the sensor is wired, i.e. the signal from the device is transmitted via cable, in addition, the sensor is active and requires batteries, in addition, the device uses delay lines for SAWs, and not resonators for SAWs, which impairs the accuracy and sensitivity of the device, in addition, the proposed device lacks structural elements transmitting external influence to the console and ensuring the tightness of the device on the surfactant, therefore, this device cannot be used for measurements under environmental factors: precipitation, dust, moisture, etc.

Known radio-interrogated passive sensor on surface acoustic waves, containing a reading device and a device for a SAW with an antenna, and a device for a SAW can be made in the form of resonators for a SAW (RF Patent No. 2105993 C1 dated 12/21/1992). The disadvantages of this device is that it lacks structural elements for transmitting measured values, as well as elements for fastening the device to a surfactant to transmit measured effects to it, in addition, since the sensor housing is absent, the device is not operable under environmental factors: precipitation dust, moisture, etc.

Known devices for converting motion based on the delay of surface acoustic waves, including a device on a surfactant in the form of a cantilever fixed plate, and to measure external influence, a relative change in the propagation characteristics of the surfactant is used when the console is bent (US Patent No. 3848144 of 11/12/1974). The disadvantages of this device is that the sensor is wired, i.e. the signal from the device is transmitted via cable, in addition, the sensor is active and requires batteries, and as a device for a SAW, delay lines for a SAW are used rather than resonators for a SAW, which degrades the accuracy and sensitivity of the sensor, finally, there are no design elements in the proposed device transmitting external influence to the device on the surfactant and ensuring its tightness, therefore, this device cannot be used under the influence of environmental factors: precipitation, dust, moisture, etc.

A housing device for a sensor of mechanical stresses is known, including a housing, inside of which there is a device for a SAW with resonators (US Patent No. 7886607 of 02.15.2011). The disadvantages of this device is that the sensor is suitable for measuring mechanical stresses caused by power loads, for example the torque of the shaft on which it is mounted, and is not suitable for accurate measurements of displacements, since it is known that the exact measurement of the force causing the relative movement of individual parts of the sensor , and the exact measurement of the movement itself, which is caused by this force, conflict with each other, therefore the design of this sensor is focused on accurate measurement of torque and not at suitable for accurate measurement of displacement.

A device of the housing and the sensor on surface acoustic waves is known, including a device for a SAW, an antenna and a reader, in addition, the sensor may also contain a RFID tag (US Patent No. 7730772, B2 dated 06/08/2010). The disadvantages of this device is that the sensor is suitable for measuring mechanical stresses caused by power loads, in this case, external atmospheric pressure and is not suitable for accurate measurement of displacements. Since it is known that the exact measurement of the force causing the relative movement of individual parts of the sensor, and the exact measurement of the movement itself, which is caused by this force, conflict with each other, therefore the design of this sensor is suitable for accurate pressure measurement and not suitable for accurate measurement of movement.

A known sensor element of the linear compression-tensile forces sensor comprising a surfactant device consisting of two surfactant resonators mounted on a console (RF Patent No. 2401999 of 20.10.2010). The disadvantages of this device is that the device is suitable for measuring mechanical stresses caused by power loads of the console, and is not suitable for accurate measurements of displacements, since it is known that the exact measurement of the force causing the relative movement of individual parts of the sensor, and the exact measurement of the displacement itself, which is caused by by this force, they conflict with each other, therefore, the design of the sensor can provide either an accurate measurement of force or an exact measurement of displacement, in addition, this design e is technological, as it involves soldering SAW devices to the console, poorly compatible with the requirements for high cleanliness on the surface of SAW resonators required to provide a high Q SAW resonators, which in turn provides a high sensitivity of the sensor.

Closest to the technical nature of the proposed technical solution is a sensor and / or identification device containing a reader and a device for SAW, which can perform the functions of a tag or a sensitive element of the sensor, while the reader consists of a transceiver and antenna, and the device The SAW contains an antenna and resonators on the SAW, each of which contains an interdigital converter (IDT), which is electrically connected to the antenna of the device on the SAW, and two reflectors ln structures (OS) (US Patent No. 5691698 dated November 25, 1997). The disadvantages of this device is that the sensor of this design without additional structural elements cannot be used to measure displacements, in addition, since the sensor does not have a housing that ensures the tightness of the device on the surfactant, the sensor is not functional in real conditions associated with the influence of environmental factors: precipitation , dust, moisture, etc., finally, the use of SAW resonators in the RFID tag significantly limits the possible number of RFID tags with different codes.

The objective of the invention is the creation of a sensor on surface acoustic waves for passive wireless measurement of displacements, allowing measurements of displacements with high accuracy in real conditions associated with the influence of environmental factors: precipitation, dust, moisture, etc.

The technical result that will be obtained during the implementation of the invention is to increase the accuracy of measuring displacements in real operating conditions associated with the influence of environmental factors: precipitation, dust, moisture, etc.

To achieve this technical result, a sensor on surface acoustic waves for passive wireless movement measurement, comprising a reader and a SAW device, the reader consists of a transceiver and antenna, and the SAW device contains an antenna and SAW resonators, each of which contains an interdigital transducer, which is electrically connected to the antenna of the SAW device, and two reflective structures, is equipped with a sealed housing, including, including casing, two flexible membranes made, for example, of beryllium bronze and rigidly connected to the element transmitting movement - a metal rod, and two membranes are soldered or glued into two covers, and the covers are soldered or glued to the base of the body, and the rod has a protrusion, which is in contact with a gasket made of a solid substance, such as leucosapphire, and the gasket is glued to the device on the surfactant in the place of contact with the protrusion of the rod, and the device on the surfactant is pressed to the base of the housing by mounting, and in place is attached between the base of the case and the device on the surfactant, as well as between the mount and the device on the surfactant, two thin fluoroplastic films are placed, in addition, a radio tag in the form of a delay line on the surfactant with reflective structures is used to identify the sensor, electrically connected to the antenna of the device on the surfactant through matching elements, in addition, the electrical connection of the sensor elements located inside the sensor housing with the sensor antenna is carried out through two metal-glass insulators soldered into the housing cover.

The introduction of a sensor on surface acoustic waves for passive wireless measurement of displacements, a rod, two membranes, two covers, two metal-glass insulators, a gasket with two fluoroplastic films, a gasket made of solid material, and mounting the device on a surfactant allows us to provide a new property, which consists in the tightness of the sensor and the accuracy of measuring displacement under environmental factors, and the use of a delay line with reflective structures in the RFID tag Surveying surfactant resonators will increase the number of possible sensors with different identification codes.

The invention is illustrated by drawings, where

figure 1 presents a drawing of a device for a surfactant in the housing, where indicated

1 - housing base;

2 - board device on the SAW;

3 - elements matching resonators;

4 - antenna device for SAW;

5 - mount;

6 - fluoroplastic film;

7 - a rod with a protrusion;

8 - a plate of a solid (for example, leucosapphire);

9 and 10 - membranes;

11 and 12 covers;

13 - radio tag;

14 - elements matching RFID tags;

15 - metal-glass insulators;

P - soldering places;

figure 2 presents the device on the surfactant, where indicated

1 - part of the base of the housing;

2 - board device on the SAW;

3 - elements matching resonators;

4 - antenna device for SAW;

F - direction and place of influence of external displacement;

ΔX is the amount of movement of the edge of the plate with the device on the surfactant;

figure 3 presents one of the possible designs of the RFID tag in the form of a delay line with reflective structures with a diagram of its connection to the antenna, where indicated

4 - antenna device for SAW;

13 - RFID tag in the form of a delay line with reflective structures;

14 - elements matching RFID tags;

figure 4 presents a block diagram of a sensor on surface acoustic waves for wireless passive measurement of movements, where indicated

2 - board device on the SAW;

4 - antenna device for SAW;

16 - device for surfactants in the housing;

17 - antenna reader;

18 - reading device;

19 - transceiver;

20 - computer;

figure 5 presents a graph of the calibration of one of the sensors on surface acoustic waves for wireless passive measurement of displacements.

A drawing of a device for a SAW in the housing 16 is shown in figure 1. On the basis of the housing 1, the device board is cantilevered on the surfactant 2 in the form of a plate of single-crystal quartz, on which two resonators on the surfactant R1 and R2 are formed (Fig. 2). It is also possible to use other single crystal materials, for example lithium niobate, lithium tantalate, langasite, etc. The resonators R1 and R2 through the matching elements 3 and the metal-glass insulators 15, ensuring the tightness of the internal cavity of the case, are connected to the antenna 4. The resonator R1 is located on the undeformable part of the board 2, and the resonator R2 on the deformable part of the board 2. The board 2 is tightly pressed to the base of the case 1 using clamp 5 with screws. To protect against damage to the board 2 during its deformation during measurements, two thin fluoroplastic films 6 with a thickness of 5-10 μm each are placed between the base of the case 1 and the board 2, as well as between the board 2 and the clip 5. The film 6 should be thin in order to exclude its influence on the measurement results, since the compression of the film during prolonged or short-term bending of the board 2 can lead to distortion of the measurement results.

The magnitude of the measured movement is transmitted to the device board on the surfactant 2 using the rod 7 through a thin plate 8 of solid material, such as leucosapphire. The plate 8 eliminates the wear of the material of the board 2 at the point of contact with the rod 7 during operation of the sensor. In the absence of plate 8, abrasion or micro-scratches of plate 2 can lead to distortion of the measurement results with an increase in error during operation. The Mohs hardness according to Mohs is 9, while that of quartz is only 7. To ensure the tightness of the sensor, if the rod 7 can be moved, two elastic membranes 9 and 10 are used, made, for example, of beryllium bronze and mounted on the covers 11 and 12. The rod 7 is fixed in the center of membranes 9 and 10 by soldering or glue. Places for soldering or applying glue are indicated P. The membranes 9 and 10 provide for the movement of the rod in the direction of its axis by the required amount (for example, + -1 mm) with a minimum force applied to the rod 7. In addition, the elastic membranes 9 and 10 together with the guides the base of the housing 1 provide a rigid retention of the rod 7 from movement in horizontal directions relative to the axis of the rod and, thus, stability and accuracy of the measurement results.

The design of the circuit board of the device for SAW 2 is shown in Fig.2 and contains two resonators for SAW R1 and R2, each resonator consisting of an interdigital transducer, to the right and left of which are reflective structures in the form of grooves or metal strips. The board 2 is made of a piezoelectric single crystal. The resonator on the surfactant R1 is located on the non-deformable part of the board 2, and the resonator on the surfactant R2 on the deformable part of the board 2. The board 2 is fixed to the base of the housing 1 by pressing 5 with screws. The resonant frequency f _{R1 of the} resonator R1 depends only on the external temperature, and the resonant frequency f _{R2 of the} resonator R1 depends on the offset ΔX of the cantilever edge and the external temperature, the difference Δf = f _R2 -f _R1 depends on the offset ΔX of the cantilever edge.

The sensor may also contain a RFID tag 13, made in the form of IDT with a number of reflective structures (Fig. 3), each OS consists of a group of grooves formed on a plate of piezoelectric material. The RFID tag 13 through matching elements 14 and metal-glass resonators 15, ensuring the tightness of the internal cavity of the housing, is also connected to the antenna 4.

The sensor on surface acoustic waves for wireless passive measurement of displacements works as follows.

Let the rod 7 be displaced by ΔX under external influence F. The free movement of the stem is provided by the membranes 9 and 10. This movement transmitted through the plate 8 causes a displacement of the edge of the board 2 also by ΔX (Fig. 2). Under the influence of the deflection of the board 2, the resonant frequency of the resonator R2 will change, which is recorded by polling the sensor using a reader 18.

The interrogation of the sensor by the reading device 18 is as follows. The block diagram of the sensor on surface acoustic waves for wireless passive measurement of displacements is presented in figure 4. At a command from computer 20, reader 18 generates N polling radio pulses, where N is, for example, 5, with fixed, equally spaced carrier frequencies f ₀₁ ... f _N (for example, f ₀₁ = 433 and f _0N = 434 MHz). The duration of each interrogation pulse depends on the quality factor of the resonators, and for a loaded quality factor, for example, ~ 8000 can be, for example, ~ 10 μs. Interrogation pulses through the antenna 17 are emitted in the direction of the device on the SAW in the housing 16. The response signals of the device on the SAW in the housing 16 for each interrogation radio pulse through the antenna 17 are fed to the reader 18. The reader 18 amplifies, filters and digitizes the received signals. The digitized signals are transmitted to a computer 20, where they are processed in accordance with the algorithms described below. As a result of processing in the computer 20, the resonant frequencies of the resonators f _R1 and f _R2 are determined, the values of which determine the amount of displacement ΔX.

The algorithm for determining the displacement ΔX includes the following procedures.

Entering the initial data obtained as a result of digitizing by the reader N (for example, 5) the pulse responses of the sensor obtained by polling N with radio pulses with fixed filling frequencies f ₀₁ ... f _N.

Calculation of resonant frequencies of resonators from digitized impulse responses:

$$f_{R\mathrm{one}}^i=Max\left\{\mathrm{Re}{\displaystyle \underset{0}{\overset{T_{\mathrm{one}}}{\int }}{S}_{\mathrm{one}}^i\left(t\right)\cdot \exp \left(\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="00000008.png,00000010.png"\; state="\{\{state\}\}">j</figure-callout>}2\pi ft\right)dt}\right\},PR\mathrm{and}{f}_{\mathrm{one}}<f<{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="00000008.png,00000010.png"\; state="\{\{state\}\}">f</figure-callout>}}_2;\left(\mathrm{one}\right)$$

$$f_{R2}^i=Max\left\{\mathrm{Re}{\displaystyle \underset{0}{\overset{T_{\mathrm{one}}}{\int }}{S}_{\mathrm{one}}^i\left(t\right)\cdot \exp \left(\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="00000008.png,00000010.png"\; state="\{\{state\}\}">j</figure-callout>}2\pi ft\right)dt}\right\},PR\mathrm{and}{\mathrm{<figure-callout\; id="3"\; label="f"\; filenames="00000008.png"\; state="\{\{state\}\}">f</figure-callout>}}_3<f<f_{\mathrm{four}};\left(2\right)$$

- импульсный отклик датчика; T₁ - длительность импульсного отклика (зависит от добротности используемых резонаторов); частоты f₁ и f₂ задают диапазон частот для поиска частоты f_R1, при которой имеет место максимум преобразования (1); частоты f₃ и f₄ задают диапазон частот для поиска частоты f_R2, при которой имеет место максимум преобразования (2).where i is the number of the impulse response from 1 to N; $$S_{\mathrm{one}}^i\left(t\right)$$

- impulse response of the sensor; T ₁ - the duration of the pulse response (depends on the quality factor of the resonators used); the frequencies f ₁ and f ₂ set the frequency range for searching the frequency f _R1 at which the maximum of conversion (1) takes place; the frequencies f ₃ and f ₄ specify the frequency range for searching the frequency f _R2 at which the maximum of conversion (2) takes place.

As a result of calculations according to (1) and (2), N values of frequency f _R1 and f _R2 can be obtained at which there is a maximum of transformations (1) and (2). For the values of the frequencies f _R1 and f _R2 , those values are selected for which the maximum of transformations (1) and (2) has a maximum value.

, $$f_{R\mathrm{one}}=Max\left\{f_{R\mathrm{one}}^i\right\}$$

,

. $${\mathrm{<figure-callout\; id="2"\; label="f\; R"\; filenames="00000008.png,00000010.png"\; state="\{\{state\}\}">f\; </figure-callout>}}_R=Max\left\{f_{R2}^i\right\}$$

.

Finally, the strain ΔX is determined by the frequency difference Δf _R = f _R2 -f _R1 and the sensor pre-calibration data:

$$\Delta X=\Delta X_n+\frac{\Delta X_{n+\mathrm{one}}-\Delta X_n}{\Delta f_{n+\mathrm{one}}-\Delta f_n}\left[\Delta f_R-\Delta f_n\right],PR\mathrm{and}\Delta f_n<\Delta f_R<\Delta f_{n+\mathrm{one}},\left(3\right)$$

where Δf _n , Δf _{n + 1} and ΔX _n , ΔX _{n + 1} are calibration data in the form of a set of the difference in the frequencies of the resonators Δf _n and the corresponding strain values ΔX _n . Calibration is carried out by setting known displacements (for example, using the PPG-3 device) and determining, according to the above-described algorithm, the corresponding frequency difference Δf _R.

Figure 5 presents an example graph of the calibration of the sensor on the surface of the acoustic waves for passive wireless motion measurements made to test the present invention.

As a result of testing a prototype of the sensor on surface acoustic waves for wireless passive displacement measurement, made in accordance with the proposed technical solution, we obtained positive results confirming the high accuracy of displacement measurement. So, with a distance between the sensor antenna and the reader antenna equal to 5 meters, an error in determining the displacement of 1 μm was obtained with a measured displacement of 50 μm to 450 μm. The exact value of the displacement was determined using the PPG-3 device.

We believe that the proposed device has all the criteria of the invention, since:

- a sensor on surface acoustic waves for passive wireless measurement of displacements together with the restrictive and distinctive features of the claims is new to well-known devices and, therefore, meets the criterion of "novelty";

- the totality of the features of the claims of the device is not known at this level of technology and does not follow well-known rules for the construction of displacement sensors, which proves compliance with the criterion of "inventive step";

- the constructive implementation of the sensor on surface acoustic waves for wireless passive measurement of displacements does not present any structural, technical or technological difficulties, since it has been manufactured and tested, from which it meets the criterion of "industrial applicability".

Information sources

1. US patent No. 3888115 of 07/10/1975, IPC G01b 7/16.

2. RF patent No. 2105993 C1 dated December 21, 1992, IPC G01S 13/75.

3. US patent No. 3848144 of 12/12/1974, IPC H01v 7/00.

4. US patent No. 7886607 B2 of 02.15.2011, IPC G01L 13/02.

5. US patent No. 7730772 B2 dated 06/08/2010, IPC G01L 9/00.

6. RF patent No. 2401999 C1 of 10.20.2010, IPC G01L 1/16.

7. US patent No. 5691698 of 11.25.1997, IPC C08B 13/14 - prototype.

Claims (1)

A surface acoustic wave sensor for wireless passive displacement measurement, comprising a reader and a SAW device, wherein the reader consists of a transceiver and antenna, and the SAW device contains an antenna and SAW resonators, each of which contains an interdigital transducer, which electrically connected to the antenna of the device on the SAW, and two reflective structures, characterized in that it is equipped with a sealed housing, including, including the base of the housing, two flexible m membranes made, for example, of beryllium bronze and rigidly connected to the element transmitting movement — a metal rod, with two membranes soldered or glued to two covers, and the covers soldered or glued to the base of the body, and the rod has a protrusion that contacts the gasket, made of a solid substance, for example leucosapphire, and the gasket is glued to the device on the surfactant at the point of contact with the protrusion of the rod, and the device on the surfactant is pressed to the base of the housing by mounting, and in the place of attachment between the base of the housing and a device for a SAW, as well as between the mount and a device for a SAW, two thin fluoroplastic films are placed, in addition, a RFID tag in the form of a delay line for a SAW with reflective structures is used to identify the sensor, electrically connected to the antenna of the device for the SAW through matching elements, in addition , the electrical connection of the sensor elements located inside the sensor housing with the sensor antenna is carried out through two metal-glass insulators soldered into the housing cover.

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