RU2387051C1 - Detector of physical value on surface acoustic waves - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: detector comprises piezoelectric acoustic line 1, on working surface of which there are the following components arranged - transceiving interdigital transducer (TIT) 2, two reflecting unidirectional TIT 3 and 4. TIT 3 is loaded by impedance 6, value of which is sensitive to measured physical value. At the ends of acoustic line there are acoustic absorbers 7. TIT 2 is loaded for transceiving antenna 5. Acoustic line together with TIT is placed in tight body 8.

EFFECT: increase of measurement accuracy.

1 dwg

Description

The invention relates to piezoelectric sensors designed for remote monitoring of various physical quantities.

Sensors based on surface acoustic waves (SAWs) are known, containing a housing inside which a piezoelectric sound duct is located, on the working surface of which there are two interdigital transducers (IDT) and an acoustic absorber deposited on the ends of the sound duct [1, 2] (Dias JF Hewlett-Packard J. - 1981 / - V.32, N 12. - P.21-37. [1], Kostromin AC, Rozanov IA, Chernykh EB, Kuvahara Hiroyuki, Tomilova LG, Zefirov N.S. The sensor on surface acoustic waves for the detection of carbon dioxide (RF Patent RU 2132584 C1, IPC6 H01L 41/18 from 1999.06.27 [2]). In one of the sensors, the sound duct with IDT on the working surface is a delay line, which is included in the feedback circuit of the amplifier and is an electric oscillation generator, the frequency of which depends on the temperature or the magnitude of the sound duct deformation [1]. The signal from the sensor using a transmitting antenna connected to the generator is transmitted to the receiving device, which performs remote monitoring. The device of another sensor [2] is similar, only between the IDT there is a film that can selectively absorb various substances. In this case, the sensor can monitor the appearance of various substances. In this case, the housing cannot be made airtight, which reduces the reliability of the sensor, since various aggressive substances can destroy the metal film of which the IDT is made. Since the sensor includes an amplifier, the sensor needs a power source that must be changed periodically and which may fail (discharge) at an unexpected time, which reduces the reliability of the sensor. In addition, the presence of semiconductor elements in the amplifier can lead to its failure in the presence of ionizing radiation, which also reduces the reliability of the sensor.

To eliminate these shortcomings allows the device in which the housing is sealed, one of the IDT is unidirectional and loaded on the transceiver antenna located outside the sealed housing, and the other IDT is made with split pins and loaded on the impedance, the value of which depends on the physical impact, the magnitude of which must be monitored and which is located outside the sealed enclosure, and the magnitude of the impedance may be sensitive to temperature, pressure, humidity, ionizing radiation June, electromagnetic radiation, the presence of various substances [3] (Bagdasaryan AS, Bagdasaryan SA, Gulyaev Yu.V., Karapetyan G.Ya. Physical quantity sensor on surface acoustic waves. RF patent 2296950 C2, IPC G01D 5/00 (2006.01) from 10/04/2007), taken as a prototype.

In this device, the reflection coefficient depends on the magnitude of the impedance, the magnitude of which depends on the measured physical quantity. Since the housing is sealed, the IDT and the substrate are isolated from the environment, which increases the reliability of the sensor. The absence of semiconductor elements in the sensor makes this sensor insensitive to ionizing radiation. The lack of a power source allows you to place this sensor in hard-to-reach places only once. The sensor is interrogated using a reader sending an interrogating electromagnetic pulse, which is received by the sensor antenna and converted into surface acoustic waves (SAWs), which, reflected from the reflective IDT, are received by the IDT transceiver and converted again into the electromagnetic signal, which is received by the reader receiver. The magnitude of this signal, obviously, depends on the reflection coefficient, which, in turn, depends on the magnitude of the impedance loaded on the reflective IDT. This impedance, in turn, depends on the measured physical quantity. Thus, by the magnitude of the pulse reflected from the sensor, one can judge the measured physical quantity. However, the amplitude of the pulse received by the reader will depend not only on the reflection coefficient, and therefore on the impedance value, but also on the distances and relative positions of the sensor and reader antennas, which can lead to significant errors in measuring the physical quantity and is a significant drawback of this sensor.

The problem to which the invention is directed, is to increase the measurement accuracy by creating a sensor on a surfactant in which the data on the measured physical quantity will not depend on the distances and relative position of the sensor and reader antennas. The technical result that the implementation of the invention provides is to receive not one reflected pulse from the sensor, but two, due to the introduction of another reflective IDT and to perform reflective IDTs unidirectional, and the amplitude of only one of them depends on the load,

This is achieved by the fact that the transceiver IDT is located in the center of the sound duct and is bi-directional, and on either side of it there are unidirectional reflective IDTs with internal reflectors with the same number of periods, one of which is loaded on the impedance, the value of which is sensitive to the measured physical quantity, and the distance to the loaded reflective IDT is 2L + Nλ, where L is the distance between the transceiver IDT and the unloaded reflective IDT, N is the number of periods in the reflective IDT, λ is the SAW length and the center frequency of IDT. The introduction of the second reflective IDT, the reflection of which does not depend on changes in the physical quantity, just leads to an increase in the measurement accuracy.

Figure 1 shows the topological structure of a device for surfactants in accordance with the invention.

The sensor contains a piezoelectric sound duct 1, on the working surface of which there is a transceiver IDT 2, a reflective unidirectional IDT 3, which is loaded on the impedance Z 6, another reflective unidirectional IDT 4. Acoustic absorbers 7 are located at the ends of the sound duct, and IDT 2 is loaded on a transceiver antenna 5 Sound duct together with IDT, placed in a sealed enclosure 8.

The sensor operates as follows. When a polling electromagnetic pulse is applied to the transceiving antenna 5, it is converted by means of IDT 2 into SAW pulses that are reflected from IDT 3 and IDT 4. Reflected SAW pulses from IDT 3, 4 are converted back by IDT 2 to electromagnetic pulses that are emitted by antenna 5 Since a unidirectional IDT 4 is not loaded, the surfactants incident on it will almost completely be reflected from it, since a unidirectional IDT in idle mode (in the absence of load) should reflect all surfactants incident on it back.

In the mode of full coordination of IDT 3 with the load, surfactants reflected from active electrodes and surfactants reflected from internal reflectors are in antiphase. In this case, IDT 3 will not reflect SAW. But it’s very difficult to achieve full coordination, and this can be done only on one frequency. Therefore, the interrogating pulse will be reflected from such IDT much less than from unloaded IDT 4, and the reflection coefficient will depend on the degree of matching IDT 3 with the load, i.e. from the magnitude of this load. This value, in turn, will depend on the measured physical quantity. Therefore, the reflection coefficient will depend on the measured physical quantity. Since the reading pulse reflected from the unloaded reflecting IDT 4 will have a constant amplitude, comparing this amplitude with the amplitude of the reading pulse reflected from IDT 3, one can judge the measured physical quantity. Obviously, the ratio of these amplitudes will not depend on the relative position of the sensor reader antennas, but will depend only on the ratio of reflection coefficients from IDT 3 and 4, which depends on the magnitude of the impedance loaded on IDT 3, and therefore, on the measured physical quantity. The impulse from reflective IDT 3 arrives earlier, because the distance from it to IDT 2 is less than from IDT 2 to IDT 3. Since IDT 2 is bi-directional, some part of the surfactant that comes to it from IDT 3 and 4 will be reflected on back to these IDTs, and then again reflected on IDT 2. Since the distance between IDTs 2 and 4 is L, and the distance between IDTs 2 and 3 is 2 (L-Nλ), the reflected pulse from IDT 4 will arrive at IDT 2 for time 2L / V _SAW , and the pulse reflected from IDT 3 will arrive at IDT 2 during time 2 (L-Nλ) / V _SAW , i.e. these pulses will be divided in time by the value of 2Nλ / V _SAW and will not overlap. The pulse reflected from IDT 4 will be located behind the pulse reflected from IDT 3. This is very important, because reflected from loaded IDT 3 may have an amplitude smaller than reflected from IDT 4. If these impulses were superimposed, this would lead to significant errors in determining the measured physical quantity. The pulses reflected from IDT 3 will arrive after time 4 (L-Nλ) / V _SAW , i.e. much later reflected from IDT 3 and IDT 4 pulses. Thus, all re-reflected pulses will come later than the reflected pulses and will not affect the measurements.

, где В=3100 К, Т - абсолютная температура, R_∞ - сопротивление терморезистора при больших температурах. Нетрудно посчитать, зная сопротивление терморезистора при 25°С (Т=298 К), что при 100°С это сопротивление (Т=373 К) равно 122 Ом. Если соединить параллельно два таких терморезистора, то получим 61 Ом, т.е. близко к 50 Ом, где коэффициент отражения будет минимальным. При нагревании сопротивление терморезистора падает и в районе 100°С становится близким к 50 Ом, что приводит к уменьшению коэффициента отражения и амплитуды отраженного от ВШП 3 импульса в 6-7 раз (16-17 дБ) по отношению к импульсу, отраженному от ВШП 4.Execution example. The sensor is made on sound duct 1 from YX - a cut of lithium niobate with dimensions of 8 × 2 × 0.5 mm. IDTs 3 and 4 are made with internal reflectors with a period of two SAW lengths at the center frequency f ₀ = 870 MHz and a length of 33 SAW lengths at the center frequency, which provides a unidirectional mode of 15 dB. IDT 2 is made bidirectional with the width of the electrodes equal to a quarter of the length of the surfactant at the center frequency, and has a length equal to 20 lengths of the surfactant. IDT 4 is located at a distance of 2.15 mm from IDT 2, and IDT 3 is at a distance of 4 mm, which provides a delay between reflected pulses of 1 μs, and reflected from IDT 4 and reflected from IDT 3 pulses - 0.074 μs with a reading pulse length 0.037 μs, i.e. these pulses do not overlap. The IDT apertures 2,3 are chosen equal to 80 SAW lengths at the central frequency, which allows neglecting diffraction losses at given distances. The insertion loss when applying a signal to IDT 2 and removing it from IDT 3 was 10 dB. The reflection coefficient R from IDT 3, 4 at Z = ∞ (open IDT) was not less than 95%, and with closed IDT 3 (Z = 0) R≤9%. The sound pipe along with the IDT is placed in a sealed enclosure. Antenna 5 in the form of a half-wave vibrator and impedance 6 are located outside the housing. Impedance 6 is a NAT102V thermistor, which has a resistance of 1000 Ohms at 25 ° C, as well as a small capacity. The dependence of the resistance of the thermistor is determined by the formula

where B = 3100 K, T is the absolute temperature, R _∞ is the resistance of the thermistor at high temperatures. It is easy to calculate, knowing the resistance of the thermistor at 25 ° C (T = 298 K), that at 100 ° C this resistance (T = 373 K) is 122 Ohms. If two such thermistors are connected in parallel, we get 61 Ohms, i.e. close to 50 ohms, where the reflection coefficient will be minimal. When heated, the resistance of the thermistor drops and near 100 ° C it becomes close to 50 Ohms, which leads to a decrease in the reflection coefficient and amplitude of the pulse reflected from IDT 3 by 6-7 times (16-17 dB) with respect to the impulse reflected from IDT 4 .

Information sources

1. Dias J.F. Hewlett-Packard Journal, V.32, N 12, 1981, p.21-37.

2. RU 2132584 C1, IPC6 H01L 41/18 dated 06/27/1999.

3. RU 2296950, IPC G01D 5/00 (2006.01) dated 04.10.2007 - prototype.

Claims (1)

A sensor for remote control of physical quantity on surface acoustic waves (SAW), containing a sealed enclosure, inside which a piezoelectric sound duct is located, on the working surface of which there is a transceiver interdigital transducer (IDT) loaded on an antenna that is located outside the sealed enclosure, reflective IDT, loaded on an impedance located outside the sealed enclosure, the magnitude of which is sensitive to the measured physical quantity, and an acoustic absorber, is applied at the ends of the sound duct, characterized in that the transceiver IDT is located in the center of the sound duct and is bi-directional, and on either side of it there are unidirectional reflective IDTs with internal reflectors with the same number of periods, one of which is loaded on an impedance, the value of which is sensitive to the measured physical quantity, and the distance to the loaded reflective IDT is 2 (L-Nλ), where L is the distance between the transceiver IDT and the unloaded reflective IDT, N is the number of periods in the reflection Yelnia IDT, λ - SAW length at the center frequency IDT.