EP1828736A1 - Power-free/wireless sensor based on surface acoustic wave with energy collecting type - Google Patents

Power-free/wireless sensor based on surface acoustic wave with energy collecting type

Info

Publication number
EP1828736A1
EP1828736A1 EP05740839A EP05740839A EP1828736A1 EP 1828736 A1 EP1828736 A1 EP 1828736A1 EP 05740839 A EP05740839 A EP 05740839A EP 05740839 A EP05740839 A EP 05740839A EP 1828736 A1 EP1828736 A1 EP 1828736A1
Authority
EP
European Patent Office
Prior art keywords
acoustic wave
surface acoustic
unit
idt
power
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP05740839A
Other languages
German (de)
French (fr)
Other versions
EP1828736A4 (en
Inventor
Wang Sup 105-1402 Sinil Apt. 319 Yuljeon-do KIM
Bum Kyoo Choi
Jae Geun Oh
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
MDT CO Ltd
Original Assignee
MDT CO Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by MDT CO Ltd filed Critical MDT CO Ltd
Publication of EP1828736A1 publication Critical patent/EP1828736A1/en
Publication of EP1828736A4 publication Critical patent/EP1828736A4/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
    • G01L9/0001—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means
    • G01L9/0008—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means using vibrations
    • G01L9/0022—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means using vibrations of a piezoelectric element
    • G01L9/0025—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means using vibrations of a piezoelectric element with acoustic surface waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L11/00—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00
    • G01L11/04—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00 by acoustic means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L11/00—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00
    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
    • G05D19/00—Control of mechanical oscillations, e.g. of amplitude, of frequency, of phase
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q9/00—Arrangements in telecontrol or telemetry systems for selectively calling a substation from a main station, in which substation desired apparatus is selected for applying a control signal thereto or for obtaining measured values therefrom

Definitions

  • the present invention generally relates to a power-free/wireless sensor, and more specifically, to a power-free/wireless sensor for converting mechanical energy generated by changes of peripheral physical environment (vibration) into electric energy, self-generating a surface acoustic wave (SAW) in the sensor using the converted electric energy, and then sensing a pressure using the generated surface acoustic wave to transmit the sensed data wirelessly, thereby enabling long-distance transmission of sensed data without additional power supply.
  • SAW surface acoustic wave
  • a sensor can be operated when the sensor receives a DC power.
  • power is externally supplied through electric wires or internally supplied through batteries comprised in the sensor.
  • the conventionally used passive sensor employs a method for detecting a resonant frequency by electromagnetic inductive coupling, a method using a reflecting wave of a surface acoustic wave, or a wireless power transmitting method such as RFID.
  • the method using a LC resonant frequency has been restrictively used because a sensing distance is within several centimeters.
  • the wireless power transmitting method read/write operations can be sufficiently performed by low power with the current semiconductor technology to for identification but the sufficient amount of power for driving a sensor (temperature, humidity, deflection, pressure, etc.) cannot be supplied.
  • the method using a surface acoustic wave shows the most excellent performance of the passive sensor, but cannot transmit data to a long distance because its sensing distance is short.
  • FIG. 1 is a diagram illustrating a structure of a conventional power-free/wireless sensor using a surface acoustic wave.
  • the power-free/wireless sensor of Fig. 1 comprises a plurality of Inter Digital
  • IDT Inducers arranged in parallel along a propagating direction of a surface acoustic wave on a substrate (LiNbO ) having piezoelectricity.
  • an external transmitting/receiving device wirelessly transmits a requesting signal which is a high-frequency pulse signal to a sensor
  • the pulse signal is applied through an antenna 3 of the sensor to a SAW device unit 1.
  • a stress-power generation process is repeated by the piezoelectric substrate, thereby generating a surface acoustic wave.
  • the surface acoustic wave generated from the SAW device unit 1 propagates toward a sensor unit 2. A part of the wave propagates continuously toward the wave- propagating direction and the other part of the wave is reflected to be a reflecting wave PI l in an opposite direction to the wave-propagating direction while passing through a reflector.
  • the reflecting wave Pl 1 is applied to the SAW device unit 1, which converts the applied reflecting wave PI l into a pulse signal (wireless responding signal) as an electric signal to transmit them wirelessly through an antenna 4 to the external transmitting/receiving device (not shown).
  • the external transmitting/receiving device analyzes data measured in the sensor using shapes of the reflecting wave (variation of amplitudes).
  • the above-described power-free/wireless sensor has a reciprocating structure to receive the pulse signal having the high frequency wirelessly from the external transmitting/receiving device and transmit a responding signal corresponding to the received signal wirelessly to the external transmitting/receiving device.
  • the sensing distance becomes shorter by energy loss, so that it is impossible to transmit sensed data to a long distance.
  • its maximum sensing distance is 5m (about 20m in case of RFID tag).
  • a power-free/wireless sensor based on a surface acoustic wave with an energy collecting type comprises: an energy converting unit for converting mechanical energy generated by changes of peripheral physical environment into electric energy; an energy storing unit for rectifying and storing the electric energy generated from the energy converting unit; a pulse generating unit for receiving the electric energy stored in the energy storing unit to output a RF signal; a sensing unit for sensing an externally applied pressure; and a SAW transponder for receiving the RF signal from the pulse generating unit to output a surface acoustic wave, varying the wave acoustic wave depending on the pressure applied to the sensing unit to output the varied surface acoustic wave as the RF sensor signal.
  • the energy converting unit is a piezoelectric power generator for converting pressure applied to a piezoelectric material by external vibration into electric energy
  • the sensing unit is a variable capacitance pressure sensor whose capacitance is varied depending on externally applied pressure.
  • FIG. 1 is a diagram illustrating a structure of a conventional power- free/wireless sensor using a surface acoustic wave
  • FIG. 2 is a diagram illustrating a structure of a power- free/wireless sensor using a surface acoustic wave with an energy collecting type according to an embodiment of the present invention
  • FIG. 3 is a diagram illustrating the concept of piezoelectric power generation using a vibration-electric energy converter as an example of an energy converting unit of Fig.2;
  • FIG. 4 is a circuit diagram illustrating an energy storing unit of Fig. 2;
  • Fig. 5 is a diagram illustrating an experimental result where electric energy is stored in a capacitor depending on the number of vibration applied to the piezoelectric material (PZT);
  • FIGs. 6-9 are diagrams illustrating an operating structure of a sensing unit of Fig. 2;
  • Fig. 10 is a diagram illustrating a result obtained by simulating deformation of a membrane depending on application pressure through FEA on Equation 2, wherein the thickness of the TM is 4.5 ⁇ m, the pole plate gap is 5.8 ⁇ m and the length of one side in the membrane is 400 ⁇ m;
  • FIG. 11 is a diagram illustrating a simulation measuring result on boundary behavior in TM contact of Fig. 10;
  • Fig. 12 is a diagram illustrating a simulating measuring result by the FEA in Fig.
  • Fig. 13 is a diagram illustrating the actual manufacturing process of IDT electrodes in the SAW transponder of Fig. 2;
  • FIGs. 14-15 are diagrams illustrating etching characteristics of an Al thin film depending on the composition ratio of an etching solution. Best Mode for Carrying Out the Invention
  • FIG. 2 is a diagram illustrating a structure of a power-free/wireless sensor using a surface acoustic wave with an energy collecting type according to an embodiment of the present invention.
  • the power-free/wireless sensor based on a surface acoustic wave with an energy collecting type comprises an energy converting unit 10, an energy storing unit 20, a pulse generating unit 30, a sensing unit 40 and a SAW transponder 50.
  • the energy converting unit 10 is a piezoelectric power generator for converting mechanical energy generated by changes of peripheral physical environment such as vibration into electric energy .
  • the energy converting unit 10 generates a voltage by a force applied to an piezoelectric material (hereinafter, abbreviated as "PZT"), and the amount of the generated voltage is changed depending on the size of the applied force.
  • Fig. 3 is a diagram illustrating the concept of piezoelectric power generation using a vibration-electric energy converter as an example of the energy converting unit 10 of Fig.2.
  • the energy converting unit 10 has a structure in which the mechanical energy generated by vibration is converted into electric energy by the PZT.
  • the voltage generated from the energy converting unit 10 is transmitted into the energy storing unit 20 and stored therein.
  • the energy storing unit 20 transforms the high voltage generated from the energy converting unit 10 into a voltage having a predetermined level, rectifies the transformed voltage to a DC voltage, and stores the DC voltage.
  • Fig. 4 is a circuit diagram illustrating the energy storing unit 20 of Fig. 2. The voltage generated by the energy converting unit 10 is lowered to a predetermined level through a transformer 21. The lowered voltage is rectified to DC voltage (full wave rectification) through a bridge rectifier 22 and the DC voltage is stored in a capacitor Cl.
  • Fig. 5 is a diagram illustrating an experimental result where electric energy is stored in the capacitor Cl depending on the number of vibration applied to PZT. If electric energy stored in the capacitor Cl reaches a predetermined level, the energy storing unit 20 emits the stored electric energy to drive the pulse generating unit 30.
  • the pulse generating unit 30 generates a RF signal by an oscillating frequency regulated depending on the voltage Vcc outputted from the energy storing unit 20, and applied the RF signal to the SAW transponder 50 so that a surface acoustic wave may be generated in the SAW transponder 50. That is, in an embodiment, the surface acoustic wave is induced by self-generating the RF signal without receiving the RF signal wirelessly from the outside.
  • a Voltage Controlled Oscillator (VCO) is used as the pulse generating unit 30.
  • the sensing unit 40 varies impedance of a sensor IDT 52 of the SAW transponder 50 depending on the varied capacitance. That is, depending on the pressure applied to the sensor, the impedance of the sensor IDT 52 is varied, and the amplitude of the surface acoustic wave passed through the sensor IDT 52 is changed depending on the change of the impedance. As a result, the pressure applied to the sensor 40 can be obtained by calculating the change of the amplitude.
  • MEMS Micro Electro Mechanical System
  • the SAW transponder 50 receives the RF signal from the pulse generating unit 30 to generate a surface acoustic wave, and outputs a sensing value (pressure information) of the sensing unit 40 measured by the self-generated surface acoustic wave as a wireless signal through an antenna 55. That is, the SAW transponder 50 according to an embodiment of the present invention does not generate a surface acoustic wave by receiving the RF signal externally like the conventional SAW transponder, but generates a RF signal internally by using electric energy generated by the piezoelectric power generator 10 comprised in the sensor to generate a surface acoustic wave. As a result, insertion loss of the SAW transponder 50 is decreased by about 20 ⁇ 30dB.
  • the SAW transponder 50 Since the SAW transponder 50 is not a reciprocating type using the requesting signal and the responding signal , the SAW transponder 50 can obtain a profit of more than 3OdB in an aspect of the system. Additionally, since the SAW transponder 50 has a profit by about 1,000 times in an aspect of power, the sensor comprising the SAW transponder 50 has a profit by about more 5.6 times on distance than the reciprocating sensor comprising a common SAW.
  • the above-described SAW transponder 50 comprises a plurality of IDT metal electrodes that are arranged in parallel on a substrate (LiNbO ) having piezoelectricity.
  • the SAW transponder 50 comprises a SAW Launching IDT 51, a sensor IDT 52, a reference IDT 53, an output IDT 54 and antenna 55.
  • the SAW Launching IDT 51 receives the RF signal from the pulse generator 30, and converts the RF signal into a surface acoustic wave to be outputted to the sensor IDT 52.
  • the sensor IDT 52 is connected electrically to the sensing unit 40, and positioned on a propagating path of the surface acoustic wave which is generated from the SAW Launching IDT 51 and transmitted to the output IDT 54 between the SAW Launching IDT 51 and the output IDT 54. That is, the surface acoustic wave generated from the SAW launching IDT 51 is passed through the sensor IDT 52 and applied to the output IDT 54.
  • impedance of the sensor IDT 52 is varied depending on the change of the capacitance, so that the amplitude of the surface acoustic wave passed through the sensor IDT 52 is changed.
  • the SAW Launching IDT 51 based on the output IDT 54, receives the RF signal from the pulse generator 30, the reference IDT 53 converts the RF signal into a surface acoustic wave and outputs it to the output IDT 54.
  • the surface acoustic wave generated from the reference IDT 53 has the same amplitude as that generated from the SAW Launching IDT 51, and it is a basic surface acoustic wave to find how much the surface acoustic wave generated from the SAW Launching IDT 51 is changed in the sensor IDT 52 depending on the pressure applied to the sensing unit 40.
  • the SAW Launching IDT 51 and the reference IDT 53 are designed to generate the surface acoustic wave having the same amplitude.
  • the reference IDT 53 is positioned so that the distance between the reference IDT 53 and the output IDT 54 may be shorter than that between the SAW Launching IDT 51 and the output IDT 54.
  • the output IDT 54 converts the surface acoustic wave applied from the reference
  • the IDT 53 and the sensor IDT 52 into a RF signal (RF sensor signal), respectively.
  • the RF sensor signals converted by the output IDT 54 are transmitted into an external measuring device (not shown) wirelessly through the antenna 55.
  • the external measuring device sequentially receives the RF sensor signal corresponding to the surface acoustic wave generated from the reference IDT 53 and the RF sensor signal corresponding to the surface acoustic wave generated from the oscillating IDT 52 wirelessly, and then signal-processes the received signal to compare their amplitudes, thereby measuring the pressure applied to the sensing unit 40.
  • FIGs. 6-9 are diagrams illustrating the operating structure of the sensing unit 40 of
  • a capacitance pressure sensor is performed to detect capacitance varied depending on a distance d between two plates, a dielectric constant ⁇ and an area of a pole plate.
  • the capacitance between the two pole plates is represented by
  • Equation 1 Equation 1
  • d(x,y) is the quantity of change in coordinates depending on deflection of the membrane.
  • the change of the capacitance is a nonlinear function which is changed depending on the distance between the two plates.
  • the linearity one of the characteristics of the sensor is an important factor.
  • the nonlinearity is not a preferable characteristic. Therefore, the sensing unit 40 of the present invention is designed to have a touch mode (hereinafter, referred to as "TM"). While characteristics of the sensing unit 40 are not changed depending on temperature change like a conventional capacitance sensor, non-linearity and a small difference between the maximum and the minimum capacitance that are shortcomings of the conventional capacitance sensor are solved in the sensing unit 40 of the present invention. Also, the sensing unit 40 is suitable for a characteristic pattern of a SAW device to be applied to a wireless transponder.
  • Fig. 6 shows the deformation of the membrane before touch (capacitance oc 1 /pole plate interval)
  • Fig. 7 shows the deformation of the membrane in touch
  • Fig. 8 shows the deformation of the membrane after touch (capacitance oc plate area touched by pressure)
  • Fig. 9 shows the shape of the membrane in a x-y aspect, respectively.
  • the pressure applied to the membrane is P ⁇ P ⁇ P .
  • D represents flexural rigidity, and its value is
  • Fig. 10 shows a result obtained by simulating the deformation of the membrane depending on the application pressure through the FEA on the Equation 2.
  • the thickness of the TM is 4.5 ⁇ m
  • electrode separation is 5.8 ⁇ m
  • the length of one side in the membrane is 400 ⁇ m.
  • Fig. 11 shows behavior of TM contact boundary of Fig. 10
  • Fig. 12 shows a simulation measuring result by the FEA.
  • Fig. 13 is a diagram illustrating an actual manufacturing example of IDT electrodes in the SAW transponder 50 of Fig. 2.
  • the IDT electrodes of the SAW transponder 50 are formed by depositing Al thin films on a piezoelectric substrate (LiNbO ) as shown in Fig. 13.
  • the deposition process of the Al thin film and a lithography process for forming the IDT electrodes are important to affect the quality of a device.
  • the Al thin film is depos Q JL ited thinner than as required in the design, the impedance of the whole IDTs rises to obstruct the flow of the RF signal.
  • the Al thin film having a predetermined thickness should be deposited.
  • the deposition of the Al thin film is performed through processes such as thermal deposition, ion-beam deposition or sputter. In the above-described deposition process, it is important to remove the effect by residual stress on the piezoelectric substrate after the deposition process.
  • the Al thin film is deposited on the piezoelectric substrate, a lithography process and an etching process are performed to form the IDT electrodes 51-54.
  • the deposited Al thin film is rapidly etched by an etching solution, the Al thin film is over-etched below its pattern.
  • the applicant performed the process by changing the composition ratio of the etching solution to reduce the over-etch effect, and Figs. 14 and 15 show the etching phenomenon depending on the composition ratio of the etching solution.
  • the etching rate depends on the content of the phosphoric acid, for example, corresponding to a fast etching condition when the content of phosphoric acid is large, and the etching rate ranges from 300 to 3000/min depending on the composition ratio.
  • AZ1512 or AZ7220 as photo-resist (PR) can be used for a photo process.
  • the minimum line width of the pattern is 2 ⁇ m, and the etching process of the Al film is performed under the fast etching condition and a slow etching condition, respectively.
  • Fig. 14 shows the etching state of the Al thin film depending on the fast etching condition
  • Fig. 15 shows the etching state of the Al thin film depending on the slow etching condition.
  • the Al IDT electrode is over- etched under the fast etching condition, but the Al IDT electrode is formed to be similar to the actual designed shape under the slow etching condition.
  • the piezoelectric power generator 10 converts vibration energy into electric energy as shown in Fig. 3.
  • the electric energy generated from the piezoelectric power generator 10 is stored in the capacitor Cl of the energy storing unit 20. If the amount of the stored electric energy reaches a predetermined level, the capacitor Cl emits the stored electric energy into the pulse generator 30 to drive the pulse generator 30.
  • the pulse generator 30 When the pulse generator 30 receives the electric energy from the energy storing unit 20, the oscillating frequency is regulated depending on the voltage of the received electric energy, so that the pulse generator 30 generates the RF signal depending on the oscillating frequency.
  • the RF signal generated from the pulse generator 30 is applied to the reference IDT 53 and the SAW Launching IDT 51, respectively.
  • the reference IDT 53 and the SAW Launching IDT 51 convert the received the RF signal into surface acoustic wave, respectively.
  • the surface acoustic wave generated from the reference IDT 53 (hereinafter, referred to as “reference surface acoustic wave”) has the same amplitude as that of the surface acoustic wave generated from the SAW Launching IDT 51 (hereinafter, referred to as “sensing surface acoustic wave").
  • the reference surface acoustic wave generated from the reference IDT 53 and the sensing surface acoustic wave generated from the oscillating IDT 51 propagate in an arrow direction, respectively, shown in Fig. 2, and are applied to the output IDT 54. That is, the reference surface acoustic wave is instantly applied to the output IDT 54, and the sensing surface acoustic wave is applied through the sensor IDT 52 to the output IDT 54.
  • the reference surface acoustic wave is applied to the output IDT 54 earlier than the sensing surface acoustic wave.
  • the reference surface acoustic wave is not passed through other IDT metals, its wave form generated from the reference IDT 53 is kept as it is.
  • the sensing surface acoustic wave is converted into the RF signal and converted again into the surface acoustic wave in the sensor IDT 52 while moving toward the output IDT 54, unlike the reference surface acoustic wave.
  • the amplitude of the sensing surface acoustic wave is changed, and the degree of the change is differentiated depending on the impedance of the sensor IDT 52, that is, the capacitance varied by the pressure applied to the sensing unit 40.
  • the capacitance of the sensing unit 40 is changed depending on the external pressure applied to the pressure sensor 40, and variation in the amplitude of the sensing surface acoustic wave is determined by the degree of the change in the capacitance.
  • the external measuring device signal-processes the RF sensor signal corresponding to the sensing surface acoustic wave and the RF sensor signal corresponding to the reference surface acoustic wave respectively, and compares the amplitudes of the two RF sensor signals to calculate its difference, thereby obtaining the pressure applied to the pressure sensor 40.
  • a power-free/wireless sensor based on a surface acoustic wave with an energy collecting type can perform a sensing operation without external power supply and transmit sensed data wirelessly to a long distance.
  • the sensor can coincide with a tendency of miniaturizing, intelligent and wireless sensors, and may be semi-permanently used after installation without maintenance.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Automation & Control Theory (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Measuring Fluid Pressure (AREA)
  • Arrangements For Transmission Of Measured Signals (AREA)
  • Surface Acoustic Wave Elements And Circuit Networks Thereof (AREA)

Abstract

A power-free/wireless sensor based on a surface acoustic wave with an energy collecting type comprises: an energy converting unit for converting mechanical energy generated by changes of peripheral physical environment into electric energy; an energy storing unit for rectifying and storing the electric energy generated from the energy converting unit; a pulse generating unit for receiving the electric energy stored in the energy storing unit to output a RF signal; a sensing unit for sensing an externally applied pressure; and a SAW transponder for receiving the RF signal from the pulse generating unit to output a surface acoustic wave, varying the wave acoustic wave depending on the pressure applied to the sensing unit to output the varied surface acoustic wave as the RF sensor signal. The power-free/wireless sensor performs a sensing operation without external power supply and transmits sensed data wirelessly to a long distance. As a result, the power-free/wireless sensor coincides with a tendency of miniaturizing, intelligent and wireless sensors, and is semi-permanently used after installation without maintenance.

Description

Description
POWER-FREEAVIRELESS SENSOR BASED ON SURFACE ACOUSTIC WAVE WITH ENERGY COLLECTING TYPE
Technical Field
[1] The present invention generally relates to a power-free/wireless sensor, and more specifically, to a power-free/wireless sensor for converting mechanical energy generated by changes of peripheral physical environment (vibration) into electric energy, self-generating a surface acoustic wave (SAW) in the sensor using the converted electric energy, and then sensing a pressure using the generated surface acoustic wave to transmit the sensed data wirelessly, thereby enabling long-distance transmission of sensed data without additional power supply.
Background Art
[2] Generally, a sensor can be operated when the sensor receives a DC power. To apply the DC power to the sensor, power is externally supplied through electric wires or internally supplied through batteries comprised in the sensor.
[3] However, when the sensor is installed in an untouchable place like the inside of concrete of a bridge, a power supply method using a wire has a power loss problem by conducting wire resistance if the sensor is located remote from a power source, and its maintenance is impossible if the wire is disconnected. In addition, a sensor applied in a method using a battery cannot be used if the battery goes dead.
[4] As a result, the development of sensors that are semi-permanently usable and do not require the maintenance has been immediately necessary. To solve the above- mentioned problem, a passive sensor which is a power- free sensor has been conventionally used.
[5] The conventionally used passive sensor employs a method for detecting a resonant frequency by electromagnetic inductive coupling, a method using a reflecting wave of a surface acoustic wave, or a wireless power transmitting method such as RFID. Of these methods, the method using a LC resonant frequency has been restrictively used because a sensing distance is within several centimeters. In the wireless power transmitting method, read/write operations can be sufficiently performed by low power with the current semiconductor technology to for identification but the sufficient amount of power for driving a sensor (temperature, humidity, deflection, pressure, etc.) cannot be supplied. The method using a surface acoustic wave shows the most excellent performance of the passive sensor, but cannot transmit data to a long distance because its sensing distance is short.
[6] Fig. 1 is a diagram illustrating a structure of a conventional power-free/wireless sensor using a surface acoustic wave.
[7] The power-free/wireless sensor of Fig. 1 comprises a plurality of Inter Digital
Transducers (hereinafter, referred to as "IDT") arranged in parallel along a propagating direction of a surface acoustic wave on a substrate (LiNbO ) having piezoelectricity.
[8] If an external transmitting/receiving device (not shown) wirelessly transmits a requesting signal which is a high-frequency pulse signal to a sensor, the pulse signal is applied through an antenna 3 of the sensor to a SAW device unit 1. When the pulse signal having a high frequency is projected into the SAW device unit 1, a stress-power generation process is repeated by the piezoelectric substrate, thereby generating a surface acoustic wave.
[9] The surface acoustic wave generated from the SAW device unit 1 propagates toward a sensor unit 2. A part of the wave propagates continuously toward the wave- propagating direction and the other part of the wave is reflected to be a reflecting wave PI l in an opposite direction to the wave-propagating direction while passing through a reflector. The reflecting wave Pl 1 is applied to the SAW device unit 1, which converts the applied reflecting wave PI l into a pulse signal (wireless responding signal) as an electric signal to transmit them wirelessly through an antenna 4 to the external transmitting/receiving device (not shown). The external transmitting/receiving device (not shown) analyzes data measured in the sensor using shapes of the reflecting wave (variation of amplitudes).
[10] The above-described power-free/wireless sensor has a reciprocating structure to receive the pulse signal having the high frequency wirelessly from the external transmitting/receiving device and transmit a responding signal corresponding to the received signal wirelessly to the external transmitting/receiving device. As a result, in the conventional power-free wireless sensor, its sensing distance becomes shorter by energy loss, so that it is impossible to transmit sensed data to a long distance. Currently, its maximum sensing distance is 5m (about 20m in case of RFID tag).
Disclosure of Invention Technical Problem
[11] Accordingly, it is an object of the present invention to provide an improved power- free/wireless sensor which enables wireless long-distance transmission of sensed data without power supply or additional electric circuits. Technical Solution
[12] In order to achieve the above-described object, a power-free/wireless sensor based on a surface acoustic wave with an energy collecting type comprises: an energy converting unit for converting mechanical energy generated by changes of peripheral physical environment into electric energy; an energy storing unit for rectifying and storing the electric energy generated from the energy converting unit; a pulse generating unit for receiving the electric energy stored in the energy storing unit to output a RF signal; a sensing unit for sensing an externally applied pressure; and a SAW transponder for receiving the RF signal from the pulse generating unit to output a surface acoustic wave, varying the wave acoustic wave depending on the pressure applied to the sensing unit to output the varied surface acoustic wave as the RF sensor signal.
[13] In the above-described power- free/wireless sensor based on a surface acoustic wave with an energy collecting type, the energy converting unit is a piezoelectric power generator for converting pressure applied to a piezoelectric material by external vibration into electric energy, and the sensing unit is a variable capacitance pressure sensor whose capacitance is varied depending on externally applied pressure. Brief Description of the Drawings
[14] Other aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:
[15] Fig. 1 is a diagram illustrating a structure of a conventional power- free/wireless sensor using a surface acoustic wave;
[16] Fig. 2 is a diagram illustrating a structure of a power- free/wireless sensor using a surface acoustic wave with an energy collecting type according to an embodiment of the present invention;
[17] Fig. 3 is a diagram illustrating the concept of piezoelectric power generation using a vibration-electric energy converter as an example of an energy converting unit of Fig.2;
[18] Fig. 4 is a circuit diagram illustrating an energy storing unit of Fig. 2;
[19] Fig. 5 is a diagram illustrating an experimental result where electric energy is stored in a capacitor depending on the number of vibration applied to the piezoelectric material (PZT);
[20] Figs. 6-9 are diagrams illustrating an operating structure of a sensing unit of Fig. 2;
[21] Fig. 10 is a diagram illustrating a result obtained by simulating deformation of a membrane depending on application pressure through FEA on Equation 2, wherein the thickness of the TM is 4.5μm, the pole plate gap is 5.8μm and the length of one side in the membrane is 400μm;
[22] Fig. 11 is a diagram illustrating a simulation measuring result on boundary behavior in TM contact of Fig. 10;
[23] Fig. 12 is a diagram illustrating a simulating measuring result by the FEA in Fig.
10. [24] Fig. 13 is a diagram illustrating the actual manufacturing process of IDT electrodes in the SAW transponder of Fig. 2; and
[25] Figs. 14-15 are diagrams illustrating etching characteristics of an Al thin film depending on the composition ratio of an etching solution. Best Mode for Carrying Out the Invention
[26] The present invention will be described in detail with reference to the accompanying drawings.
[27] Fig. 2 is a diagram illustrating a structure of a power-free/wireless sensor using a surface acoustic wave with an energy collecting type according to an embodiment of the present invention.
[28] In an embodiment, the power-free/wireless sensor based on a surface acoustic wave with an energy collecting type comprises an energy converting unit 10, an energy storing unit 20, a pulse generating unit 30, a sensing unit 40 and a SAW transponder 50.
[29] The energy converting unit 10 is a piezoelectric power generator for converting mechanical energy generated by changes of peripheral physical environment such as vibration into electric energy . The energy converting unit 10 generates a voltage by a force applied to an piezoelectric material (hereinafter, abbreviated as "PZT"), and the amount of the generated voltage is changed depending on the size of the applied force. Fig. 3 is a diagram illustrating the concept of piezoelectric power generation using a vibration-electric energy converter as an example of the energy converting unit 10 of Fig.2. The energy converting unit 10 has a structure in which the mechanical energy generated by vibration is converted into electric energy by the PZT. The voltage generated from the energy converting unit 10 is transmitted into the energy storing unit 20 and stored therein.
[30] The above-described piezoelectric power generator employs conventionally used piezoelectric devices alternatively, and the detailed explanation thereof is omitted.
[31] The energy storing unit 20 transforms the high voltage generated from the energy converting unit 10 into a voltage having a predetermined level, rectifies the transformed voltage to a DC voltage, and stores the DC voltage. Fig. 4 is a circuit diagram illustrating the energy storing unit 20 of Fig. 2. The voltage generated by the energy converting unit 10 is lowered to a predetermined level through a transformer 21. The lowered voltage is rectified to DC voltage (full wave rectification) through a bridge rectifier 22 and the DC voltage is stored in a capacitor Cl. After the DC voltage stored in the capacitor Cl is boosted in a boosting circuit 24, the DC voltage is regulated as a voltage Vcc for driving the pulse generating unit 30 by the voltage regulating unit 25 and then outputted to the pulse generating unit 30. Fig. 5 is a diagram illustrating an experimental result where electric energy is stored in the capacitor Cl depending on the number of vibration applied to PZT. If electric energy stored in the capacitor Cl reaches a predetermined level, the energy storing unit 20 emits the stored electric energy to drive the pulse generating unit 30.
[32] The pulse generating unit 30 generates a RF signal by an oscillating frequency regulated depending on the voltage Vcc outputted from the energy storing unit 20, and applied the RF signal to the SAW transponder 50 so that a surface acoustic wave may be generated in the SAW transponder 50. That is, in an embodiment, the surface acoustic wave is induced by self-generating the RF signal without receiving the RF signal wirelessly from the outside. A Voltage Controlled Oscillator (VCO) is used as the pulse generating unit 30.
[33] As capacitance of the sensing unit 40 is varied depending on externally applied pressure, the sensing unit 40 varies impedance of a sensor IDT 52 of the SAW transponder 50 depending on the varied capacitance. That is, depending on the pressure applied to the sensor, the impedance of the sensor IDT 52 is varied, and the amplitude of the surface acoustic wave passed through the sensor IDT 52 is changed depending on the change of the impedance. As a result, the pressure applied to the sensor 40 can be obtained by calculating the change of the amplitude. For the sensing unit 40, a variable capacitance pressure sensor using a Micro Electro Mechanical System (hereinafter, abbreviated as "MEMS") technology is employed.
[34] The SAW transponder 50 receives the RF signal from the pulse generating unit 30 to generate a surface acoustic wave, and outputs a sensing value (pressure information) of the sensing unit 40 measured by the self-generated surface acoustic wave as a wireless signal through an antenna 55. That is, the SAW transponder 50 according to an embodiment of the present invention does not generate a surface acoustic wave by receiving the RF signal externally like the conventional SAW transponder, but generates a RF signal internally by using electric energy generated by the piezoelectric power generator 10 comprised in the sensor to generate a surface acoustic wave. As a result, insertion loss of the SAW transponder 50 is decreased by about 20~30dB. Since the SAW transponder 50 is not a reciprocating type using the requesting signal and the responding signal , the SAW transponder 50 can obtain a profit of more than 3OdB in an aspect of the system. Additionally, since the SAW transponder 50 has a profit by about 1,000 times in an aspect of power, the sensor comprising the SAW transponder 50 has a profit by about more 5.6 times on distance than the reciprocating sensor comprising a common SAW. The above-described SAW transponder 50 comprises a plurality of IDT metal electrodes that are arranged in parallel on a substrate (LiNbO ) having piezoelectricity.
[35] The SAW transponder 50 comprises a SAW Launching IDT 51, a sensor IDT 52, a reference IDT 53, an output IDT 54 and antenna 55.
[36] The SAW Launching IDT 51 receives the RF signal from the pulse generator 30, and converts the RF signal into a surface acoustic wave to be outputted to the sensor IDT 52.
[37] The sensor IDT 52 is connected electrically to the sensing unit 40, and positioned on a propagating path of the surface acoustic wave which is generated from the SAW Launching IDT 51 and transmitted to the output IDT 54 between the SAW Launching IDT 51 and the output IDT 54. That is, the surface acoustic wave generated from the SAW launching IDT 51 is passed through the sensor IDT 52 and applied to the output IDT 54. Here, when the capacitance of the sensing unit 40 is changed by the pressure applied to the sensing unit 40, impedance of the sensor IDT 52 is varied depending on the change of the capacitance, so that the amplitude of the surface acoustic wave passed through the sensor IDT 52 is changed.
[38] When the reference IDT 53, which is positioned in an opposite direction of the
SAW Launching IDT 51 based on the output IDT 54, receives the RF signal from the pulse generator 30, the reference IDT 53 converts the RF signal into a surface acoustic wave and outputs it to the output IDT 54. Here, the surface acoustic wave generated from the reference IDT 53 has the same amplitude as that generated from the SAW Launching IDT 51, and it is a basic surface acoustic wave to find how much the surface acoustic wave generated from the SAW Launching IDT 51 is changed in the sensor IDT 52 depending on the pressure applied to the sensing unit 40. As a result, the SAW Launching IDT 51 and the reference IDT 53 are designed to generate the surface acoustic wave having the same amplitude. The reference IDT 53 is positioned so that the distance between the reference IDT 53 and the output IDT 54 may be shorter than that between the SAW Launching IDT 51 and the output IDT 54.
[39] The output IDT 54 converts the surface acoustic wave applied from the reference
IDT 53 and the sensor IDT 52 into a RF signal (RF sensor signal), respectively. The RF sensor signals converted by the output IDT 54 are transmitted into an external measuring device (not shown) wirelessly through the antenna 55.
[40] The external measuring device sequentially receives the RF sensor signal corresponding to the surface acoustic wave generated from the reference IDT 53 and the RF sensor signal corresponding to the surface acoustic wave generated from the oscillating IDT 52 wirelessly, and then signal-processes the received signal to compare their amplitudes, thereby measuring the pressure applied to the sensing unit 40.
[41] Figs. 6-9 are diagrams illustrating the operating structure of the sensing unit 40 of
Fig. 2.
[42] In an embodiment, a capacitance pressure sensor is performed to detect capacitance varied depending on a distance d between two plates, a dielectric constant ε and an area of a pole plate. The capacitance between the two pole plates is represented by
. In case of a membrane type, the rate of change in the capacitance is differentiated depending on deformation, and represented by the following Equation 1 with integration type. [43] [Equation 1]
[44]
[45] Here, d(x,y) is the quantity of change in coordinates depending on deflection of the membrane.
[46] As shown in Equation 1, the change of the capacitance is a nonlinear function which is changed depending on the distance between the two plates. Generally, the linearity one of the characteristics of the sensor is an important factor. However, in case of a capacitance sensor, the nonlinearity is not a preferable characteristic. Therefore, the sensing unit 40 of the present invention is designed to have a touch mode (hereinafter, referred to as "TM"). While characteristics of the sensing unit 40 are not changed depending on temperature change like a conventional capacitance sensor, non-linearity and a small difference between the maximum and the minimum capacitance that are shortcomings of the conventional capacitance sensor are solved in the sensing unit 40 of the present invention. Also, the sensing unit 40 is suitable for a characteristic pattern of a SAW device to be applied to a wireless transponder.
[47] Fig. 6 shows the deformation of the membrane before touch (capacitance oc 1 /pole plate interval), Fig. 7 shows the deformation of the membrane in touch, Fig. 8 shows the deformation of the membrane after touch (capacitance oc plate area touched by pressure), and Fig. 9 shows the shape of the membrane in a x-y aspect, respectively.
[48] Here, the pressure applied to the membrane is P < P < P .
[49] While the deformation of the membrane or the plate is generally analyzed under small deformation, the touch mode corresponds to large deformation. However, it is difficult to precisely analyze the deformation by a general governing equation because the contact area becomes broader as the border condition is continuously changed depending on application pressure after the membrane touches the bottom. Therefore, the applicant performed analysis on the TM deformation through Finite Element Analysis (hereinafter, referred to as "FEA") on a general governing equation such as Equation 2. [50] [Equation 2] [51]
[52]
1 4 4 1 4
, α w d F2 d W 9 W η 4 - h ( P + 2 - + d 2F (5 2w
T 2 - -2- d y2 d X σ ^ c J x2 d x dy d x dy
[53] Here, D represents flexural rigidity, and its value is
. P represents application pressure, w represents deflection in the coordinate (x,y), E represents Young's modulus, v represents Poission's ratio, and F represents stress function. Fig. 10 shows a result obtained by simulating the deformation of the membrane depending on the application pressure through the FEA on the Equation 2. Here, the thickness of the TM is 4.5μm, electrode separation is 5.8μm and the length of one side in the membrane is 400μm. Fig. 11 shows behavior of TM contact boundary of Fig. 10, and Fig. 12 shows a simulation measuring result by the FEA.
[54] Fig. 13 is a diagram illustrating an actual manufacturing example of IDT electrodes in the SAW transponder 50 of Fig. 2.
[55] The IDT electrodes of the SAW transponder 50 are formed by depositing Al thin films on a piezoelectric substrate (LiNbO ) as shown in Fig. 13. Here, the deposition process of the Al thin film and a lithography process for forming the IDT electrodes are important to affect the quality of a device. When the Al thin film is depos QJLited thinner than as required in the design, the impedance of the whole IDTs rises to obstruct the flow of the RF signal. On the other hand, when the Al thin film is deposited thicker than as required in the design, the impedance is decreased but a weight of an AL electrode increases to generate "mass loading effect" which obstructs a propagating of the surface acoustic wave. As a result, the Al thin film having a predetermined thickness should be deposited. The deposition of the Al thin film is performed through processes such as thermal deposition, ion-beam deposition or sputter. In the above-described deposition process, it is important to remove the effect by residual stress on the piezoelectric substrate after the deposition process.
[56] After the Al thin film is deposited on the piezoelectric substrate, a lithography process and an etching process are performed to form the IDT electrodes 51-54. Here, since the deposited Al thin film is rapidly etched by an etching solution, the Al thin film is over-etched below its pattern. As a result, the applicant performed the process by changing the composition ratio of the etching solution to reduce the over-etch effect, and Figs. 14 and 15 show the etching phenomenon depending on the composition ratio of the etching solution. Here, the composition ratio of the etching solution is phosphoric acid : nitric acid : acetic acid : DI water = 4-20 : 1 : 1-5 : 1-2. The etching rate depends on the content of the phosphoric acid, for example, corresponding to a fast etching condition when the content of phosphoric acid is large, and the etching rate ranges from 300 to 3000/min depending on the composition ratio.
[57] Generally, AZ1512 or AZ7220 as photo-resist (PR) can be used for a photo process.
Here, the minimum line width of the pattern is 2μm, and the etching process of the Al film is performed under the fast etching condition and a slow etching condition, respectively.
[58] Fig. 14 shows the etching state of the Al thin film depending on the fast etching condition, and Fig. 15 shows the etching state of the Al thin film depending on the slow etching condition. As shown in Figs. 14 and 15, the Al IDT electrode is over- etched under the fast etching condition, but the Al IDT electrode is formed to be similar to the actual designed shape under the slow etching condition.
[59] Hereinafter, the operation of the power-free/wireless sensor based on a surface acoustic wave with an energy collecting type according to an embodiment of the present invention is explained.
[60] When vibration is applied to the piezoelectric power generator 10 by peripheral physical environment (for example, vibration) in a structure having the power- free/wireless sensor according to an embodiment of the present invention, the piezoelectric power generator 10 converts vibration energy into electric energy as shown in Fig. 3.
[61] The electric energy generated from the piezoelectric power generator 10 is stored in the capacitor Cl of the energy storing unit 20. If the amount of the stored electric energy reaches a predetermined level, the capacitor Cl emits the stored electric energy into the pulse generator 30 to drive the pulse generator 30.
[62] When the pulse generator 30 receives the electric energy from the energy storing unit 20, the oscillating frequency is regulated depending on the voltage of the received electric energy, so that the pulse generator 30 generates the RF signal depending on the oscillating frequency. The RF signal generated from the pulse generator 30 is applied to the reference IDT 53 and the SAW Launching IDT 51, respectively.
[63] The reference IDT 53 and the SAW Launching IDT 51 convert the received the RF signal into surface acoustic wave, respectively. Here, the surface acoustic wave generated from the reference IDT 53 (hereinafter, referred to as "reference surface acoustic wave") has the same amplitude as that of the surface acoustic wave generated from the SAW Launching IDT 51 (hereinafter, referred to as "sensing surface acoustic wave").
[64] The reference surface acoustic wave generated from the reference IDT 53 and the sensing surface acoustic wave generated from the oscillating IDT 51 propagate in an arrow direction, respectively, shown in Fig. 2, and are applied to the output IDT 54. That is, the reference surface acoustic wave is instantly applied to the output IDT 54, and the sensing surface acoustic wave is applied through the sensor IDT 52 to the output IDT 54. Here, since the distance between the reference IDT 53 and the output IDT 54 (movement distance of the reference surface acoustic wave) is shorter than that between the SAW Launching IDT 51 and the output IDT 54 (movement distance of the sensing surface acoustic wave), the reference surface acoustic wave is applied to the output IDT 54 earlier than the sensing surface acoustic wave.
[65] As a result, the reference surface acoustic wave generated from the reference IDT
53 is firstly converted into the RF sensor signal in the output IDT 54, and then transmitted through the antenna 55 wirelessly to an external measuring device (not shown). Here, since the reference surface acoustic wave is not passed through other IDT metals, its wave form generated from the reference IDT 53 is kept as it is.
[66] Next, the sensing surface acoustic wave generated from the SAW Launching IDT
51 is converted into the RF sensor signal in the output IDT 54, and then transmitted through the antenna 55 wirelessly to an external measuring device (not shown). However, since there is the sensor IDT 52 between the SAW Launching IDT 51 and the output IDT 54, the sensing surface acoustic wave is converted into the RF signal and converted again into the surface acoustic wave in the sensor IDT 52 while moving toward the output IDT 54, unlike the reference surface acoustic wave. As a result, the amplitude of the sensing surface acoustic wave is changed, and the degree of the change is differentiated depending on the impedance of the sensor IDT 52, that is, the capacitance varied by the pressure applied to the sensing unit 40. In other words, the capacitance of the sensing unit 40 is changed depending on the external pressure applied to the pressure sensor 40, and variation in the amplitude of the sensing surface acoustic wave is determined by the degree of the change in the capacitance.
[67] The sensing surface acoustic wave whose amplitude is changed by the sensor IDT
52 is converted into the RF sensor signal in the output IDT 54, and transmitted through the antenna 55 to an external measuring device (not shown). The external measuring device (not shown) signal-processes the RF sensor signal corresponding to the sensing surface acoustic wave and the RF sensor signal corresponding to the reference surface acoustic wave respectively, and compares the amplitudes of the two RF sensor signals to calculate its difference, thereby obtaining the pressure applied to the pressure sensor 40.
Industrial Applicability [68] As described above, a power-free/wireless sensor based on a surface acoustic wave with an energy collecting type according to an embodiment of the present invention can perform a sensing operation without external power supply and transmit sensed data wirelessly to a long distance. As a result, the sensor can coincide with a tendency of miniaturizing, intelligent and wireless sensors, and may be semi-permanently used after installation without maintenance.

Claims

Claims
[1] A power-free/wireless sensor based on a surface acoustic wave with an energy collecting type, comprising: an energy converting unit for converting mechanical energy generated by changes of peripheral physical environment into electric energy; an energy storing unit for rectifying and storing the electric energy generated from the energy converting unit; a pulse generating unit for receiving the electric energy stored in the energy storing unit to output a RF signal; a sensing unit for sensing an externally applied pressure; and a surface acoustic wave(SAW) transponder for receiving the RF signal from the pulse generating unit to output a surface acoustic wave, varying the wave acoustic wave depending on the pressure applied to the sensing unit to output the varied surface acoustic wave as a RF sensor signal.
[2] The power- free/wireless sensor according to claim 1, wherein the energy converting unit is a piezoelectric power generator for converting a pressure applied to a piezoelectric material by an external vibration into electric energy.
[3] The power- free/wireless sensor according to claim 1, wherein the energy storing unit comprises: a transformer for lowering a voltage generated from the energy converting unit to a predetermined level; a rectifier for rectifying an output voltage from the transformer; a charging unit for charging an output voltage from the rectifier; a boosting unit for boosting an output voltage from the charging unit; and a voltage regulating unit for regulating an output voltage from the boosting unit to output a driving voltage of the pulse generating unit to the pulse generating unit.
[4] The power- free/wireless sensor according to claim 1, wherein the pulse generating unit is a voltage controlled oscillator (VCO) for generating an oscillating frequency depending on the output voltage from the energy storing unit.
[5] The power- free/wireless sensor according to claim 1, wherein the sensing unit is a variable capacitance type pressure sensor which has capacitance varied by an externally applied pressure.
[6] The power- free/wireless sensor according to claim 1, wherein the SAW transponder comprises: a reference inter digital transducers(IDT) for receiving the RF signal from the pulse generator to convert the RF signal into a first surface acoustic wave; a SAW Launching IDT for receiving the RF signal from the pulse generator to convert the RF signal into a second surface acoustic wave; a sensor IDT for changing amplitude of the second surface acoustic wave outputted from the SAW Launching IDT by impedance varied depending on the pressure applied to the sensing unit; an output IDT for converting the first surface acoustic wave applied from the reference IDT and the second surface acoustic wave applied from the sensor IDT sequentially into the RF sensor signal; and an antenna for wirelessly transmitting the RF sensor signal of the output IDT.
[7] The power-free/wireless sensor according to claim 6, wherein the output IDT is interposed between the reference IDT and the oscillation IDT which are separated at a different distance apart from the output IDT.
[8] The power-free/wireless sensor according to claim 6 or 7, wherein the first surface acoustic wave have the same amplitude as that of the second surface acoustic wave.
[9] The power-free/wireless sensor according to claim 6, further comprising a measuring device for receiving sequentially the first RF sensor signal corresponding to the first surface acoustic wave and the second RF sensor signal corresponding to the second surface acoustic wave wirelessly and comparing their amplitude to calculate the pressure applied to the sensing unit.
EP05740839A 2004-12-04 2005-05-03 Power-free/wireless sensor based on surface acoustic wave with energy collecting type Withdrawn EP1828736A4 (en)

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