**4. Study of the geometric parameters of the sensor**

There are parameters that can modify the dielectric characteristics of a material such as temperature, for example. In our case, the aim is for this modification to generate a frequency variation in the microwave domain. The basic element is the sensing material whose dielectric constant varies with temperature. The choice of the sensitive material is not obvious because of the requirements imposed, which complicate the integration of dielectric materials into microelectronic circuits or their use in the manufacture of the sensor. To achieve this, we have carried out an in-depth bibliographical study in order to find the appropriate material that meets our specifications. Among the materials proposed in the literature, we chose lead-lanthanum-zirconatetitanate (Abbreviated in PLZT). As a result, and as part of our collaboration with the Faculty of Computer Science and Materials Sciences, Silesian University of Poland (see **Figure 3**) [8, 9], we have several samples taken in his laboratory.

Also referred to as lanthanum-doped lead zirconate titanate, this material meets our needs in terms of temperature range and operating frequency. In particular, it has a dielectric property that depends on the change in temperature (see **Figure 4**) [10, 11]. Indeed, the variation in temperature has an impact on the properties of this material since it causes a relatively large change in its dielectric permittivity [12]. This PLZT will then detect the temperature change through its integration into the microwave circuit described in **Figure 1**, with a radius of RPLZT = 3.25 mm and a thickness of HPLZT = 10 μm.

The system is based relatively on the use of a dielectric resonator and two coplanar membrane lines. These lines serve as an excitation support for the RD gallery modes. The study of the mechanism of operation of these two components was widely discussed in the literature [13–15]. Thus, the RD has a radius of RRD = 3.25 mm, a thickness of HRD 360 μm, and a relative dielectric permittivity of 80. This dielectric resonator is held on the line plane by an Alumina (Al2O3) wedge with RSupport = 0.8 mm radius and HSupport = 230 μm height.

The RD's gallery mode excitation mechanism and sensitive material were selected, and a radar interrogation was carried out to transmit temperature information.

**Figure 4.** *Dielectric constant of PLZT as a function of temperature.*

## **5. Sensor interrogation method**

The transduction mode, size, and frequency of operation of this sensor are important characteristics that represent a technological break with the existing systems of passive wireless temperature sensors RFID and SAW.

To be remotely accessible, the sensor requires a reader that is compatible with its operating characteristics. Technical criteria for the use of a reader must be defined to satisfy the detection but also to ensure that the interrogation range is as long as possible. The existing readers for passive sensor interrogation, present in RFID and SAW technologies, do not meet the problems imposed on our study in terms of high frequencies of use and a range greater than 10 m.

As a result, the characteristics of our sensor (wide range of detection, analysis, and processing of high frequency signals) guide us to consider a radar technology reader. Its operating principle, as with any radar, is to send a flow of electromagnetic waves to the sensor, which will return an echo whose power amplitude and will depend on the measured temperature. Indeed, radar is used in many applications such as level measurement, obstacle detection for automobiles, meteorology, or the military [16]. Its use for passive sensor network interrogation with RF transduction presents an innovative solution.

The proposed temperature sensor uses a millimeter radiofrequency transduction. The resonant frequencies of the sensor are included in the Ka band and shift from a bandwidth of a few hundred MHz to a few GHz. An antenna to communicate remotely with the reader will connect the sensor. To interrogate this sensor, we turned to a radar technology reader developed during Chebila's thesis [17], according to precise technical criteria in terms of operating frequency satisfying wireless communication over a range greater than 20 m. This distance remains a key point because many applications in the aeronautics, construction, and nuclear sectors refer to it for the installation of sensor networks. The modulation technique of this radar and its architecture based around a voltage-controlled oscillator (VCO) facilitated its realization and adjustment (see **Figure 5**).

The radar developed in 2011 is a frequency-modulated continuous radar (FMCW), used in the Ka band around 30 GHz (see **Figure 6**) [17]. This HF radar will be used to remotely detect the temperature sensor measurements. The signals received by the reader must therefore inform us about the distance between the radar and the sensor but also about the temperature value coming from the

**163**

ing cells within a network.

**Figure 5.**

**Figure 6.** *Picture of the radar.*

*Synoptic diagram of the 30 GHz radar.*

*Optimal Temperature Sensor Based on a Sensitive Material*

questioned measuring cell. In conclusion, the radar in question satisfies three important parameters for remote reading: its range is greater than 20 m, works at a frequency compatible with the proposed sensor, and contains a system for identify-

• A significant reduction in signal losses thanks to the direct modulation of the

• A high sensitivity of the electromagnetic propagation to the environment used

• A more flexible choice of operating frequency that can be adapted to the differ-

• High spatial and temporal resolution due to the high operating frequency

The following section is devoted to the results of microwave measurements made using a high-performance simulator, allowing the frequency offset to be

• Easy integration into a measurement chain (radar and antennas)

The potential advantages of this type of transducer are:

microwave signal by the quantity to be measured

to perform the sensor function

ent operating constraints of the sensor

*DOI: http://dx.doi.org/10.5772/intechopen.90733*

*Optimal Temperature Sensor Based on a Sensitive Material DOI: http://dx.doi.org/10.5772/intechopen.90733*

**Figure 5.**

*Perovskite and Piezoelectric Materials*

**5. Sensor interrogation method**

*Dielectric constant of PLZT as a function of temperature.*

**Figure 4.**

The transduction mode, size, and frequency of operation of this sensor are important characteristics that represent a technological break with the existing

To be remotely accessible, the sensor requires a reader that is compatible with its operating characteristics. Technical criteria for the use of a reader must be defined to satisfy the detection but also to ensure that the interrogation range is as long as possible. The existing readers for passive sensor interrogation, present in RFID and SAW technologies, do not meet the problems imposed on our study in terms of high

As a result, the characteristics of our sensor (wide range of detection, analysis, and processing of high frequency signals) guide us to consider a radar technology reader. Its operating principle, as with any radar, is to send a flow of electromagnetic waves to the sensor, which will return an echo whose power amplitude and will depend on the measured temperature. Indeed, radar is used in many applications such as level measurement, obstacle detection for automobiles, meteorology, or the military [16]. Its use for passive sensor network interrogation with RF

The proposed temperature sensor uses a millimeter radiofrequency transduction. The resonant frequencies of the sensor are included in the Ka band and shift from a bandwidth of a few hundred MHz to a few GHz. An antenna to communicate remotely with the reader will connect the sensor. To interrogate this sensor, we turned to a radar technology reader developed during Chebila's thesis [17], according to precise technical criteria in terms of operating frequency satisfying wireless communication over a range greater than 20 m. This distance remains a key point because many applications in the aeronautics, construction, and nuclear sectors refer to it for the installation of sensor networks. The modulation technique of this radar and its architecture based around a voltage-controlled oscillator (VCO)

The radar developed in 2011 is a frequency-modulated continuous radar (FMCW), used in the Ka band around 30 GHz (see **Figure 6**) [17]. This HF radar will be used to remotely detect the temperature sensor measurements. The signals received by the reader must therefore inform us about the distance between the radar and the sensor but also about the temperature value coming from the

systems of passive wireless temperature sensors RFID and SAW.

frequencies of use and a range greater than 10 m.

transduction presents an innovative solution.

facilitated its realization and adjustment (see **Figure 5**).

**162**

*Synoptic diagram of the 30 GHz radar.*

**Figure 6.** *Picture of the radar.*

questioned measuring cell. In conclusion, the radar in question satisfies three important parameters for remote reading: its range is greater than 20 m, works at a frequency compatible with the proposed sensor, and contains a system for identifying cells within a network.

The potential advantages of this type of transducer are:


The following section is devoted to the results of microwave measurements made using a high-performance simulator, allowing the frequency offset to be

monitored and a direct relationship to be established between the temperature variation and the observed frequency offset. In this way, a temperature measurement is carried out via an electromagnetic transduction.
