FIELD 
       [0001]    The present invention relates to a wireless sensor, in particular to a temperature and humidity sensor using surface acoustic wave resonators. 
       BACKGROUND 
       [0002]    Surface Acoustic Wave (SAW) sensor is a kind of sensor in which the SAW acts as an environment dependent information carrier. This kind of sensor has properties of high precision, fast response, small size, etc., and is widely applied in fields of aerospace, food hygiene, environmental monitoring, pharmaceutical chemicals, process monitoring, military security, etc., in which SAW sensor displays its unique technical advantages. 
         [0003]    Currently, a variety of SAW multi-parameter sensors have been disclosed. For example, Chinese Utility Model CN201837405U disclosed a backscattering surface acoustic wave sensor with pulsed excitation, wherein wireless pulse signals emitted by a remote sensing device are received by a sensor antenna and drives an interdigital transducer to generate radio frequency surface acoustic wave. The acoustic wave pulse is reflected by a reflector and reversely reflected to the remote sensing device by the connected antenna. Temperature and other sensitive variations are extracted during detection of changes in reflection i.e. the delay in the sensor. Chinese patent application CN102313614 disclosed a surface acoustic wave thermometer with improved accuracy, wherein a plurality of transmitters are used to generate backscattering pulse sequences with time intervals, and the temperature detection accuracy is improved by combining time delay differences among those sequences of pulses and phase differences of a plurality of pulses at a single frequency. The patents described above employ a SAW device comprising piezoelectric materials. It should be noted that a drift will appear in backscattering time delay with the aging of piezoelectric materials or electrical connectors, which affects the long-term stability of the measurement. Meanwhile, the structure comprises delay line of larger size, and thus is uneasy to be miniaturized. Also, reliable wireless communication distance is reduced since such high path loss has been introduced. 
         [0004]    In addition, China Patent CN201348624Y further disclosed a multi-channel surface acoustic wave chemical sensor, wherein such sensor comprises a plurality of parallel surface acoustic wave detecting channels and a reference channel. Frequency difference between detecting channels and the reference channel is generated by a frequency mixer and delivered to an analyzing device via cables. The invention needs to deliver frequency domain differential signals at low frequency via cable connection, and contains an active peripheral circuit, making it impossible to obtain a passive wireless sensor. Further, the sensor does not include function(s) of temperature or humidity measurement. 
       SUMMARY 
       [0005]    The objective of the present invention is to provide a miniaturized wireless temperature and humidity sensor which maintains measurement accuracy against the aging of the devices. 
         [0006]    According to the first aspect of the present invention, it provides a wireless temperature and humidity sensor, comprising a substrate, a feeding network, an antenna and surface acoustic wave resonators, wherein said surface acoustic wave resonators are fed by said feeding network through said antenna; 
         [0007]    said surface acoustic wave resonators comprise a reference resonator and measuring resonators; and 
         [0008]    the resonant frequency difference between said reference resonator and said measuring resonators is used to modulate the temperature and/or humidity to be measured. 
         [0009]    In a preferred embodiment, in said wireless temperature and humidity sensor, said measuring resonator comprises a humidity measuring resonator and a temperature measuring resonator, wherein said reference resonator, said humidity measuring resonator and said temperature measuring resonator are connected in parallel; 
         [0010]    said temperature measuring resonator has different frequency-temperature coefficients from those of said reference resonator; a hydrophilic coating is applied on the surface of said humidity measuring resonator such that said humidity measuring resonator has different frequency-humidity coefficients from those of said reference resonator. In a preferred embodiment, said temperature measuring resonator is placed in a closed packaging structure, so as to monitor temperature selectively. 
         [0011]    In a preferred embodiment, said humidity resonator is packaged by a hydrophilic thin-film coating, so as to selectively monitor humidity. 
         [0012]    In a preferred embodiment, said sensor is able to monitor both temperature and humidity simultaneously, or to monitor temperature or humidity selectively. 
         [0013]    In a preferred embodiment, said reference resonator and said measuring resonator are made on single or separate pieces of piezoelectric substrates with single layer or stratified layers. 
         [0014]    In a preferred embodiment, said reference resonator and said measuring resonator have different rotation angles relative to the substrate&#39;s crystal orientation. 
         [0015]    In a preferred embodiment, said film coating is applied on the surface of said substrate and said humidity measuring resonator by vapor or liquid phase deposition. 
         [0016]    According to the second aspect of the present invention, it provides a wireless temperature and humidity sensor system, comprising a substrate, a feeding network, an antenna, surface acoustic wave resonators and a remote sensing device, wherein 
         [0017]    said surface acoustic wave resonators are fed by said feeding network through said antenna; 
         [0018]    said surface acoustic wave resonators comprise a reference resonator and a measuring resonator, wherein said reference resonator and said measuring resonator have different rotation angles relative to the substrate&#39;s crystal orientation; 
         [0019]    the resonant frequency difference between said reference resonator and said measuring resonator is used to modulate temperature and/or humidity to be measured; and 
         [0020]    said remote sensing device scans the resonant frequency or the phase of the backscattering signal from said surface acoustic wave resonators and calculates the difference values therebetween, so as to extract temperature or humidity to be measured. 
         [0021]    In a preferred embodiment, said temperature and humidity sensor operates at a frequency range different from that of adjacent sensors of same type, so as to facilitate simultaneously query by said remote sensing device. 
         [0022]    In a preferred embodiment, said remote sensing device includes a human-machine interface to provide temperature and/or humidity readings, and a processing means to provide an alarm signal of preset threshold. 
         [0023]    In a preferred embodiment, said remote sensing device comprises a wired or wireless repeater or hub, so as to achieve cluster and maintain simplex or duplex communications with a remote monitoring device. 
         [0024]    According to the third aspect of the present invention, it provides a method for measuring temperature and/or humidity, wherein the wireless temperature and humidity sensor and its system according described above is employed in the method, and the frequency difference between said reference resonator and said measuring resonator is used to modulate temperature and/or humidity. 
         [0025]    According to the fourth aspect of the present invention, it provides a method for measuring temperature and/or humidity, comprising the following steps: arranging said wireless temperature and humidity sensor described above on an object to be measured; obtaining frequencies of said reference resonator and said measuring resonator and calculating the frequency difference between said reference resonator and said measuring resonator; and calculating the temperature and/or humidity of the object based on said frequency difference. 
         [0026]    In the wireless temperature and humidity sensor according to the first aspect of the present invention, all surface acoustic wave resonators are fed by said feeding network through said antenna. All resonators operate at adjacent or the same resonant frequency, and resonant frequency difference between resonators is used to modulate the temperature and/or humidity to be measured. The reference resonator and each measuring resonator are processed on a same substrate along preferred orientations, achieving different rotation angles relative to the substrate&#39;s crystal orientation respectively, thereby said reference resonator has a different frequency-humidity coefficient from that of each humidity measuring resonator. Resonators may be packaged in a same package structure or separate package structures. Meanwhile, a hydrophilic coating is applied on the surface of one of the measuring resonators and contacts with the external environment directly so that the measuring resonator has a different frequency-humidity coefficient from that of the reference resonator. The resonator with the hydrophilic coating provides the measurement of humidity, while the resonator without hydrophilic coating provides the measurement of temperature. Besides, the measurement of temperature can be further used to compensate the temperature influence in humidity measurement. Furthermore, frequency drift or phase drift caused by aging of the resonator material and the connector is effectively suppressed during differential modulation. 
         [0027]    In the wireless temperature and humidity sensor system according to the second aspect of the present invention, said sensor cooperates with the remote sensing device. Said remote sensing device employs band-limited frequency modulated continuous electromagnetic wave to illuminate and interrogate said sensor, after the sensor antenna receives the wave, the SAW resonator is motivated and feeds back to the remote sensing device through the antenna. When the measured temperature or humidity is lower or higher than the preset threshold, the device will alarm and deliver an alarm signal to a remote receiver. 
     
    
     
       DRAWINGS 
         [0028]      FIG. 1  is a schematically top view of the structure of a wireless passive temperature and humidity sensor according to the present invention; 
           [0029]      FIG. 2  is a schematically section view of the structure of a wireless temperature and humidity sensor according to the present invention; 
           [0030]      FIG. 3  is a schematic view showing the frequency spectrum and frequency difference according to an embodiment of the sensor of the present invention; and 
           [0031]      FIGS. 4 and 5  is a graph showing the relationship between humidity, temperature and frequency of an embodiment, wherein  FIG. 4  shows the change of frequency with that of relative humidity during humidity measurement and  FIG. 5  shows the changes of frequency and frequency difference with that of temperature and humidity. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings so that the purpose, features and advantages of the present invention will be better understood. It should be understood that the embodiments shown in the drawings are not to limit the scope of the invention, but merely to illustrate the true spirit of the technical solution of the present invention. 
         [0033]      FIG. 1  shows a schematically top view of the structure of a wireless passive temperature and humidity sensor  100  according to the present invention. As shown in  FIG. 1 , the sensor  100  comprises a piezoelectric substrate  106 , an integrated antenna  104 , a feeding network  105 , a temperature measuring resonator  101 , a reference resonator  102 , and/or a humidity measuring resonator  103 , wherein the temperature measuring resonator  101  and the humidity measuring resonator  103  are measuring resonators. The piezoelectric substrate  106  is a temperature dependent crystal or film. The hardness, density and size of the piezoelectric will be changed with temperature. In the practice, material, slice orientation and thickness of the piezoelectric substrate  106  may be selected according to a specific application. Preferably, the piezoelectric substrate  106  is made of lithium niobate, quartz, zinc oxide, aluminum nitride, cadmium sulfide, or lanthanum gallium silicate or the like. In addition, the bottom of the substrate further comprises a supporting layer. Preferably, the support layer is made of ceramics or metals, and the thickness of the supporting layer can be determined as desired. 
         [0034]    The integrated antenna  104  is a miniaturized integrated antenna. In this embodiment, it may be a meander line dipole antenna, a microstrip patch, an inverted-F or a slot antenna. The radio frequency ground of the antenna is connected with the ground of the sensor. Meanwhile, the feed efficiency can be improved via an optional impedance matching network. The integrated antenna is used for delivery or feedback of temperature and/or humidity signal, and also used for the power supply of the humidity measuring resonator, temperature measuring resonator and the reference resonator. 
         [0035]    The feeding network  105  is connected to the antenna port, for example, through microstrip lines or bonding wires. The length, diameter or width of microstrip lines or bonding wires can be determined as desired. 
         [0036]    The resonators  101 ,  102  and  103  are surface acoustic wave resonators, which respectively comprise interdigital transducers  101   b ,  102   b  and  103   b , and respectively comprise short circuit reflectors  101   a ,  102   a  and  103   a . Interdigital transducers work at the same frequency with corresponding reflectors, and resonators. The resonators operate at resonance frequencies f 1 , f 2  and f 3  which are adjacent but not overlapping respectively. Resonators  101 ,  102  and  103  are connected in parallel, and the reference resonator  101   h  as a different angle θ r  relative to the substrate&#39;s crystal orientation as compared to measuring resonators  102  and  103 . Preferably, the angle θ r  ranges from 25° to 45°, 
         [0037]      FIG. 2  shows a schematically section view of the structure of the wireless passive temperature and humidity sensor  100  according to the present invention, wherein the substrate  107  is a passive substrate acting as a supporter. In this embodiment, the substrate  107  is a metal bracket. The piezoelectric substrate  106  is a temperature dependent piezoelectric crystal which can be formed by fixed the slices prepared by single crystal growth process on the bracket, or the piezoelectric substrate  106  is a piezoelectric film which can be coated on the surface of the bracket via physical or chemical vapor phase film deposition. Then, metal film structures  202  (i.e., the interdigital transducers of the resonators) with specific shape and thickness are processed on the piezoelectric substrate via surface micromachining processes, such as, photolithography, laser engraving, printing or bonding, etc. Said metal film structures may be made of aluminum, gold, tungsten, copper, titanium and its alloys. It should be noted that each of the resonators  101 ,  102 ,  103  may be made on a single or a group of piezoelectric substrates with a single layer or stratified layers. 
         [0038]    Resonators  101 ,  102  and  103  may be packaged by two packaging methods, namely by a film coating  201  or a housing  203 . Wherein, the film coating  201  may be applied on the surfaces of the piezoelectric substrate and the metal film structure via vapor or liquid phase film deposition. In this embodiment, alumina oxide material (or silicon oxide and other materials) is used to form a passivated surface so that the temperature measuring resonator  101  and the reference resonator  102  are sealed. Alternatively, porous materials with high porosity, such as, zinc oxide or aluminum nitride, are used to form a hydrophilic thin-film coating  201  absorbing vapor in order to package the humidity measuring resonator  103 , wherein the upper surface of the hydrophilic thin-film coating  201  is exposed to external environment. In the embodiment shown in the drawings, porous materials with high porosity, such as, zinc oxide or aluminum nitride, are used to form a hydrophilic thin-film coating  201  absorbing vapor to package the humidity measurement resonator  103 , while a metal, ceramic or plastic housing is fixed over the temperature measuring resonator  101  and the reference resonator  102  via bonding or welding to package the temperature measuring resonator and the reference resonator. 
         [0039]    In another preferred embodiment, an interdigital transducer with aluminum or copper layer, comprising a lithium niobate substrate is used in the sensor  100 , wherein thickness of the aluminum or copper layer is 160 nm. A metal bracket is provided at the bottom of the piezoelectric substrate, wherein thickness of the metal bracket may range from 600 micrometers to a few millimeters. Preferably, in this embodiment, the thickness of the metal bracket is 1 mm. Resonators  101 ,  102 , and  103  respectively contains 50 pairs of electrodes and reflection gratings with 100 period length symmetrically distributed at both sides of the resonator  100 . The designed operating frequency of the sensor is about 860 MHz. First, the humidity measuring resonator  103  is packaged by porous zinc oxide film, and then the temperature measuring resonator  101  and the reference resonator  102  are inertly packaged by silicon oxide film. 
         [0040]      FIG. 3  is a graph showing the frequency spectrum structure and frequency difference according to an embodiment of the sensor  100  of the present invention. In this example, resonators operates at frequencies f 1 , f 2  and f 3  respectively, wherein the temperature measurement resonator works at frequency f 1 , the reference resonator works at frequency f 2 , and the humidity measurement resonator works at frequency f 3 . Frequency difference Δf T  is used for modulating the temperature and frequency difference Δf H  is used for modulating humidity. 
         [0000]      Δ f   T   =f   2   −f   1   (1)
 
         [0000]      Δ f   H   =f   3   −f   2   (2)
 
         [0041]    It should be noted that when there are sensors of same type nearby, preferred frequency range is required to avoid frequency conflicts and ensure that the remote sensing device can simultaneously query (i.e. detect and identify) these sensors. 
         [0042]      FIGS. 4 and 5  is a graph showing the relationship between humidity, temperature and frequency of an embodiment, wherein  FIG. 4  shows the change of frequency with that of relative humidity during humidity measurement and  FIG. 5  shows the change of frequency and frequency difference with that of temperature and humidity. 
         [0043]    Specifically, the coating (film) on the upper surface of the humidity measuring resonator changes its density ρ with outside environment vapor partial pressure C V  according to the following rule: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0044]    Wherein, k is a constant determined by film thickness, ρ v  is vapor density. Moreover, the thickness of the film becomes thicker with the adsorption of steam. 
         [0045]      FIG. 5  shows that these two changes cause the change of high-frequency sound waves velocity of the humidity measuring resonator coated with the film, thereby causes the frequency change. When relative humidity changes within the range of 0% to 100%, frequency change can reach 150 ppm. It should be noted that, at this time, frequency difference Δf H  for modulating humidity is a total frequency difference including frequency changes caused by possible temperature change ΔT=T−T 0  with respect to room temperature and humidity change ΔH=H−H 0  with respect to reference ambient humidity (wherein T 0  is room temperature, H 0  is reference ambient humidity), the relative humidity is obtained in the following manner: 
         [0000]      Δ H−H   C =√{square root over (Σ i=0   M β i (Δ f   H ) i +Σ i=0   P γ i (Δ f   T ) i )}{square root over (Σ i=0   M β i (Δ f   H ) i +Σ i=0   P γ i (Δ f   T ) i )}  (4)
 
         [0046]    Wherein, H C , β i (i=0 . . . M), and γ i (i=0 . . . P) are calibration coefficients, and the calibration coefficients are calibrated through curve fitting or solving vector equations via iterative method in the factory. The process can be divided into two steps: 
         [0047]    Firstly, the frequency difference Δf H  between humidity measuring resonator  103  and reference resonator  102  is measured under preset humidity H 0  . . . H (M+1)  and ambient temperature T 0 , then the following equation can be established and H C , β i (i=0 . . . M) can be iteratively solved 
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         [0048]    Then, the frequency difference Δf H  between humidity measuring resonator  103  and reference resonator  102  as well as the frequency difference Δf T  between reference resonator  102  and temperature measuring resonator  101  are measured based on the preset temperature T 0  . . . T P  and ambient humidity H 0 , establishing the following equation and solve 
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         [0049]    Further, a larger number of measurement sampling points than the length of the undetermined vector (i.e. more than M+2 humidity sample points or more than P+1 temperature sample points) may be taken so as to be fitted to obtain the calibration parameters described above. Humidity measurement process and calibration methods described above enable the sensor to extract humidity directly. It should be noted that during the above humidity measurement, frequency drift caused by aging of devices has been compensated by frequency difference Δf T  and Δf H , thereby there is no need to re-calibrate the coefficients when used, thus ensuring the long-term stability of humidity measurement. Temperature change ΔT with respect to calibrated ambient temperature in the factory can be extracted via high order polynomial of Δf T : 
         [0000]      Δ T−T   C =√{square root over (Σ i=0   N α i (Δ f   T ) i )}  (7)
 
         [0050]    Wherein, calibration coefficients T c , α i  (i=0 . . . N) are calibrated through solving vector equations via iterative method in the factory, comprising the following step: measuring the frequency difference between the reference resonator  102  and the temperature measuring resonator  101  based on the preset temperature T 0  . . . T N+1 , establishing the following equation and solving T C , α i (i=0 . . . N) via iterative method: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0051]    Similarly, in the above temperature measurement, frequency drift caused by aging of devices has been compensated by the frequency difference Δf T , thereby there is no need to re-calibrate the coefficients when used, thus ensuring the long-term stability of humidity measurement. 
         [0052]    In another embodiment of the present invention, said temperature and humidity sensor (not shown) collaborates with a remote sensing device to constitute a sensor system, wherein the remote sensing device can be an appropriate one known in the art and will not be elaborated here. Said remote sensing device employs band-limited frequency modulated continuous electromagnetic wave to illuminate and interrogate said sensor. After the sensor antenna receives the wave, the SAW resonators (including the temperature measuring resonator  101 , the reference resonator  102  and the humidity measuring resonator  103 ) are motivated. At this time, the piezoelectric film substrate deforms and is charged, thereby it remains transient oscillations and feeds frequencies of resonators back to the remote sensing device via antennas after the irradiation is stopped. Said remote sensing device may include a human-machine interface and have the function of data processing, thereby it is able to directly display the temperature and humidity values locally and to alarm according to the preset threshold. Moreover, the remote sensing device may also include a wired or wireless repeater or hub, so as to achieve cluster and maintain simplex or duplex communications with a remote monitoring device, thereby deliver the temperature and/or humidity data to the remote control device to perform the cluster network networking capability. 
         [0053]    Further, it should be noted that, when only temperature is to be measured, the sensor system of the present invention may not include a humidity measuring resonator. Similarly, when only humidity is to be measured, the sensor system of the present invention may not include a temperature measuring resonator. Moreover, depending on the application, more temperature measuring resonators and/or more humidity measuring resonator and/or more reference resonators may be provided in one sensor so as to improve accuracy and reliability of measurement. 
         [0054]    Preferred embodiments of the present invention has been described in detail, while it is to be understood that, after reading the above teachings of the present invention, those skilled in the art may make various modifications to the present invention. All these equivalent forms also fall into the scope limited by attached claims of the present application.
The present invention provides a wireless temperature and humidity sensor and system, and measurement method. The wireless temperature and humidity sensor comprises a substrate, a feeding network, an antenna and surface acoustic wave resonators, wherein the surface acoustic wave resonators are fed by said feeding network through said antenna. Said surface acoustic wave resonator comprises a reference resonator and measuring resonators. The resonant frequency difference between said reference resonator and said measuring resonators is used to modulate the temperature and/or humidity to be measured. Said system can monitor both temperature and humidity simultaneously, or monitor humidity or temperature selectively. Furthermore, frequency drift caused by aging of the sensor material and the connector is effectively suppressed by the differential modulation, thereby improving long-term stability of measurement and avoiding recalibration.