Method for optimizing the design of a device comprising interrogation means and a remotely-interrogatable passive sensor

Abstract

A method for optimizing the design of a device includes interrogation means and a differential passive sensor, including a generator connected directly or indirectly to a reader antenna, a passive sensor including at least two resonators, a sensor antenna connected to the sensor. The method includes determining a set of curves P.sub.SAW as a function of the frequency of interrogation of the sensor, each curve being defined for a given impedance Z.sub.T representing the impedance of the Thevenin equivalent generator dependent on the impedance of the reader antenna, on the impedance of the sensor antenna and on the coupling between the two antennas, for a given sensor impedance Z.sub.SAW; selecting at least one curve P.sub.SAW from the set of predefined curves meeting two criteria: exhibiting two frequency peaks representative of a coherent differential sensor behavior; having a width at mid-height of the two the peaks below a threshold value; and determining the sensor antenna exhibiting the sensor antenna impedance correlated to the curve P.sub.SAW selected for the predefined SAW sensor.

Claims

1. A method for optimizing the design of a device comprising interrogation means and at least one differential passive sensor, comprising a generator connected directly or indirectly to a reader antenna, at least one passive sensor comprising at least two resonators, a sensor antenna connected to said sensor wherein it comprises the following steps: the determination of a set of curves P.sub.SAW corresponding to the received power/transmitted power ratio as a function of the frequency of interrogation of the sensor, for a reader antenna and a set of sensor antennas; each curve P.sub.SAW being defined for a given impedance Z.sub.T representing the impedance of the Thevenin equivalent generator dependent on the impedance of the reader antenna, on the impedance of the sensor antenna and on the coupling between the two antennas, for a given sensor impedance Z.sub.SAW; the selection of at least one curve P.sub.SAW from the set of predefined curves meeting two criteria: exhibiting two frequency peaks representative of a differential sensor; having a width at mid-height of the two said peaks below a threshold value; the determination of the sensor antenna exhibiting the sensor antenna impedance correlated to the curve P.sub.SAW selected for the predefined SAW sensor.

2. The method according to claim 1, wherein the curves P.sub.SAW are determined by using a quadripole as matching circuit between the generator and the reader antenna, so as to increase the number of curves P.sub.SAW by varying the impedance of the reader antenna and the coupling between the reader antenna and the sensor antenna.

3. The method according to claim 1, in which the threshold value of the mid-height width is approximately 100 kHz.

4. The method according to claim 1, in which the resonators are acoustic resonators that can be surface wave resonators or volume wave resonators.

5. The method according to claim 1, in which the resonators are dielectric resonators.

6. The method according to claim 1, in which the set of the curves is generated for a set of operating temperatures of said device, the sensor being a temperature sensor.

7. The method according to claim 1, in which the sensor is a strain/deformation sensor.

Description

[0067] The invention will be better understood and other advantages will become apparent on reading the following description given in a nonlimiting manner and through the attached figures in which:

[0068] FIG. 1 illustrates the surface of a solid in the presence of a conventional surface acoustic wave: the Rayleigh wave;

[0069] FIG. 2 illustrates the electrodes of an SAW resonator seen in cross section in the sagittal plane in the presence of a surface wave;

[0070] FIG. 3 illustrates the electrodes of a surface wave resonator seen from above;

[0071] FIG. 4 illustrates the typical frequency response of an SAW resonator on quartz;

[0072] FIG. 5 illustrates the principle of RF interrogation of an SAW resonator;

[0073] FIG. 6 illustrates the Butterworth-Van Dyke (BVD) electric model;

[0074] FIGS. 7a and 7b show the measurement-BVD model superposition for the real and imaginary parts of the impedance of an SAW sensitive element comprising two resonators meeting the conditions given in the patent DE102009056060 A1;

[0075] FIG. 8 illustrates the equivalent circuit seen at the terminals of an SAW sensor connected to a sensor antenna coupled to a reader antenna;

[0076] FIG. 9 illustrates the variation of P.sub.SAW (vertical axis dB) as a function of the frequency and of the imaginary part of Z.sub.T for a real part value of Z.sub.T=5 ;

[0077] FIGS. 10a and 10b respectively illustrate the frequency responses associated with two acceptable antenna operating points and with an errored antenna operating point;

[0078] FIG. 11a illustrates the graphic representation of the conjugate of the imaginary part of Z.sub.T and of the imaginary part of the impedance of the differential sensor, and FIG. 11b illustrates an example of frequency response of a differential sensor in the case of a significant frequency variation of Z.sub.T;

[0079] FIGS. 12a and 12b show the trend of the imaginary part Im (Z.sub.T)* as a function of the frequency in the ISM band respectively for respectively an impedance Z.sub.T=50 and an impedance Z.sub.T=150 ;

[0080] FIG. 13 illustrates examples of typical P.sub.SAW curves as a function of the frequency showing the satisfaction of the parameters required, according to the optimization method of the invention.

[0081] The optimization method comprises the following different steps in the context of a differential sensor, that can typically comprise two resonators with acoustic waves that can be surface waves (SAW). It should be noted that the sensor could also comprise volume acoustic wave resonators (BAW) or dielectric resonators.

[0082] Step 1: [0083] The determination, for a given differential sensor, of its impedance Z.sub.SAW.

[0084] Step 2: [0085] The definition not simulation of a set of curves P.sub.SAW in a frequency band of interest for a reader antenna and a set of sensor antennas, that can for example be of dipole type.

[0086] Step 3: [0087] The selection of the curve or curves which meet the following two criteria: exhibiting two frequency peaks representative of a coherent differential sensor behavior with maximum power peaks; [0088] having a width at mid-height of said two peaks below a threshold value, that can typically be less than or equal to 100 kHz.

[0089] FIG. 13 illustrates examples of curves that can be obtained with different sensor antenna values.

[0090] The curve 13a meets the selection criteria: a good overvoltage and a good power transmission; [0091] The curve 13b presents: a bad overvoltage; [0092] The curve 13c presents a good overvoltage and a bad power transmission.

[0093] It is thus possible, after having selected the subset of curves 13a and 13c, to ultimately select the curve 13a.

[0094] Example of method for optimizing a temperature probe comprising a differential sensor having two elastic wave resonators and that can typically be used in the metal walls of an oven.

[0095] The device comprises: [0096] a differential passive sensor comprising two SAW resonators coupled to a sensor antenna placed at 2 cm from a ground plane; [0097] a reader antenna exhibiting, for example, an impedance with a real part of 50 ohms, and imaginary part equal to 0; [0098] The distance between the reader antenna and the sensor antenna is 30 cm.

[0099] The Applicant performed 21 acquisitions with antenna frequencies characteristic of the sensor antenna chosen.

[0100] These measurement points make it possible to define the antenna frequencies and therefore the antennas that make it possible to meet the optimization criteria defined in the present invention, i.e. a high power and a sufficiently small resonance peak width, characterized by a sufficiently high quality factor.

[0101] More specifically, Table 1 below provides 21 acquisitions performed for antennas associated with the sensors whose frequency lies, theoretically, between 300-500 MHz. More specifically, Table 1 lists, for a given curve PSAW correlated to a given antenna, the maximum powers of the two resonance peaks of the two resonators, their resonance frequencies (that can vary slightly, hence the benefit of performing differential measurements) and the widths at mid-height of the two resonance peaks reflected by the quality factors.

[0102] In Table 1: [0103] Fr_antenna is the resonance frequency of the receiving antenna determined from the direct measurement of the antenna not charged by the SAW sensor; [0104] Fr_reso1 and Fr_reso2 correspond to the frequencies calculated by the interrogation unit for the resonator 1, respectively for the resonator 2; [0105] Power_reso1 and Power_reso2 correspond to the index of the received power given by the interrogation unit.

[0106] Table 1 below lists all the data obtained from the interrogation unit for each resonator and for different resonance frequencies of the antenna associated with the sensor.

[0107] The quality factors Q_1 and Q_2 are the quality factors of the two resonators and are representative of their frequency bandwidth fr_resol1/f and fr_resol2/f. The higher these quality factors are, the smaller the mid-height width becomes.

TABLE-US-00001 TABLE 1 # Acquisition Fr_antenna Fr_reso1 P_reso1 Q_1 Fr_reso2 P_reso2 Q_2 ACQ001 301.6 433.6135411 2 7430 434.3826943 2 5187 ACQ002 310.58 433.6141191 5 7926 434.3836148 3 5139 ACQ003 319.56 433.6159531 1 8650 434.3850105 1 5478 ACQ004 328.53 433.6182821 2 5319 434.3869932 2 5348 ACQ005 337.51 433.620741 4 5660 434.3897185 4 5691 ACQ006 346.49 433.6227145 5 3532 434.3921891 5 3541 ACQ007 355.47 433.6267515 6 3460 434.3960006 5 3472 ACQ008 364.44 433.6363957 8 2344 434.4058471 8 2352 ACQ009 373.42 433.6529192 1 3346 434.4239706 1 1961 ACQ010 382.4 433.7046978 1 1363 434.4855054 1 1365 ACQ011 391.38 433.372663 1 1046 434.1985667 1 1067 ACQ012 400.35 433.5279973 1 1380 434.3055715 1 1488 ACQ013 409.33 433.5661504 1 3460 434.3374469 9 3747 ACQ014 418.31 433.5740617 9 4579 434.3450286 8 4810 ACQ015 427.29 433.5827963 6 4743 434.3525812 5 4930 ACQ016 436.26 433.5871137 4 9646 434.3564237 3 9931 ACQ017 445.24 433.5926757 3 9671 434.3604782 2 9918 ACQ018 454.22 433.59452 2 8819 434.3625106 1 9026 ACQ019 463.2 433.5952762 4 9384 434.363593 2 9460 ACQ020 472.17 433.596497 2 9285 434.3648226 2 9296 ACQ021 481.15 433.5986819 2 8493 434.3662842 2 8502

[0108] It can be deduced from Table 1 that the best results are obtained in terms of received power for antennas resonating at 409 MHz and at 418 MHz, in correlation with high quality factors.