The RFID wireless system to monitor the internal temperature of concrete sample during the setting reaction is structured as depicted in the block diagram in
Figure 1. During the pouring stage of concrete, the RFID sensors consisting of an EM4325-IC (Integrated Circuit) temperature sensor matched to a bow-tie broad band antenna and conformed around a 3D printed PLA structure with a cylindrical shape, are poured inside the concrete mixture as part of the aggregates. A commercial RFID reader formed by a circularly polarized patch antenna and a reader ID eNUR-10W from Nordic is placed outside the concrete. This RFID reader is software controlled to make systematic readings of the temperature registered by the wireless sensors previously placed inside the concrete sample. A detailed description of the main design features of the RFID sensor and reader, as well as an analytic study of the propagation path between the external antenna and embedded device, are provided in this section. The block diagram in
Figure 1 illustrates the operation principle of the RFID system. Further technical information of the EM4325 chip and the NORDIC ID eNUR-10W reader can be found in [
12,
13], respectively.
2.1. Wireless RFID Temperature Sensor
When designing a RFID sensor to work embedded in a medium whose properties change over time, there are certain constrains that must be taken into account as they will have an impact on the performance of the device that cannot be neglected.
From an industrial point of view, it is desired to come up with a design that is as compact and small as possible. In the construction industry for example, robustness is a must, as a device embedded in a block of concrete has to withstand a tremendous amount of weight and pressure. Taking this into account, a thin planar design is quickly off the table, and a shift towards a more “rock-like” device is preferred as it mimics the shape of aggregates already present in concrete mixtures. A considerable amount of water will be present at the first stages of the setting process of concrete, so it is important to assure that the device is waterproof to prevent damage on the electronics. Nevertheless, the main goal of this device is to monitor the temperature of the surrounding medium, which implies that the sensor must be exposed to the medium somehow for accurate temperature measurement. RFID tags, typically operate as passive devices, which means that they are powered by the RF energy transmitted from a reader. However when they incorporate “sensing” capabilities, working in battery operated mode generally provides more processing capabilities and higher reading range and reliability. The RFID sensor layout must be then designed in a way that simultaneously ensures compactness and robustness, isolation, and an accurate temperature measurement.
The EM4325 RFID chip is a Radio Frequency Identification Integrated Circuit from EM Microelectronic that can be either battery powered or beam powered by RF (Radio Frequency) energy transmitted from a reader. In a Battery Assisted Passive (BAP) configuration, the EM4325 chip offers better sensitivity (−30 dBm) that translates into a superior reading range, compared to purely passive operation. When designing sensors intended to operate inside of lossy materials, as it is the case of concrete, maximizing the Dynamic Range (DR) between the transmitting and receiving antenna is of great interest to cope with the high propagation losses produced as a consequence of concrete’s attenuation. This can be either achieved by increasing the transmission power, or as done in this case, by improving the sensitivity of the embedded passive device. The integrated temperature sensor supports temperatures in the range of −40
C to +60
C, which widely covers typical concrete temperatures reached during placement [
12].
The EM4325 RFID chip does not include the capability of storing temperature measurements. Meaning that these should either be acquired on demand, or by installing a micro-controller. For the current application, the chip has been configured to operate as a radio-frequency front-end and to perform on demand temperature readings when requested by the external reader.
This chip, when configured in BAP operation mode, has an input impedance of
. In accordance, the RFID antenna conceived to operate matched to the EM4325 chip, is designed to provide a
impedance at its input. To provide the required low resistance and high reactance input impedance, a T-match structure is employed between the bow-tie antenna and RFID chip [
14,
15].
In order to provide the aforementioned required compactness and robustness, the proposed design is printed on top of a bendable substrate of 0.125 mm thickness from Rogers (
). This is then wrapped around the external walls of a semi-holed 3D printed PLA cylinder with a diamter of 30 mm. To provide a planar section to place the RFID chip and for isolation purposes, a vertical slice is made on the wall of the cylinder where both the EM4325 RFID chip and the T-match section of the antenna are placed. The removed section is then placed back on top, covering the electronics and isolating it from the surrounding medium. In this way, we are able to protect the electronics from the high water concentration present in the concrete mixture at the beginning of the setting reaction. An additional advantage of placing back on top of the T-match the removed PLA section, is that it enables a reduction of the T-match dimensions, as PLA’s relative permittivity is higher than air’s (
), hence contributing to the compactness of the sensor. A 3V battery is placed inside the PLA cylinder, connected to the RFID chip through biasing lines, for BAP operation. The proposed RFID sensor is represented in
Figure 2.
An important factor when assessing the performance of the antenna is the Impedance Coupling Factor (ICF), computed as:
where
is the antenna impedance and
is the impedance of the RFID chip,
in this case. The Impedance Coupling Factor measures the fraction of power that is effectively transferred from the antenna to the RFID chip (and vice versa), and the design goal is to obtain a value close to one.
Figure 3 represents the the behavior of the proposed T-match bow-tie antenna in terms of input impedance and Coupling Factor, when placed in the embedding scenario. These results have been obtained through EM (Electromagnetic) simulations using CST Microwave Studio, considering the nominal value for the embedding medium’s relative permittivity
at 0.868 GHz.
It is known from [
1] that the real part of the relative permittivity of concrete throughout the setting reaction varies exhibiting maximum values around
at the beginning of the reaction and lowering to
by the eighth day of the process. With this phenomenon in mind, the Impedance Coupling Factor between the wrapped T-match bow-tie antenna and the RFID-IC has been evaluated for real part relative permittivities ranging between 4 and 18 (
Figure 4).
Figure 4 shows that at the operation frequency of 868 MHz, despite the changes produced in the sensor’s antenna impedance as a consequence of the permittivity variations in the embedding medium, the Impedance Coupling Factor is above 0.4 for the expected dielectric permittivity evolution of the concrete sample throughout its setting process.
To enable the temperature sensing capability of the chip, a small hole is drilled on the section of the cylinder that acts as a “cover” for the T-match and chip. This hole is placed right on top of the chip and has dimensions equivalent to those of the sensor. Then a small aluminum cylinder is placed right on top of the temperature sensor (
Figure 5b). One face of the aluminum cylinder is left exposed to the external medium while the other one is laying on the surface of the chip where a thermal conductive paste is applied in advance. The walls of this aluminum cylinder are glued to the cover to ensure fixation. Aluminum’s good thermal conductivity makes it a suitable choice to act as an interface between the temperature sensor and embedding medium.
To assure that the whole device is completely isolated from the surrounding medium, the sensor is dipped in a liquid solution of PlastiDip and an air-dry rubber solution is applied to provide at the same time for protection and isolation to the RFID sensor.
Figure 5 displays the final manufactured prototype. The main technical features of the proposed RFID temperature sensor are summarized in
Table 1.
2.3. Monitoring Software
The software in charge of establishing, controlling, and monitoring communication between the reader and RFID sensor has been developed by WiTekLab following Nordic ID semiconductors’ recommendations for the development of software solutions for their RFID Embedded Reader Modules.
When the reader is connected to the computer and powered on, the control software scans for tags within the reading distance of the RFD reader, and displays parameters such as tag configuration, sensor data, and current temperature. As the EM4325 sensor does not have the capability of storing temperature measurements, the software tracks the temperature readings of a given tag, and stores it on a CSV file in the computer.
2.4. Propagation Loss between the RFID Reader and the Embedded RFID Sensor
The propagation loss between a RFID sensor located inside a concrete structure, and a RFID reader placed outside is represented through Equation (
2). This expression, extracted from [
16], has been conceived from the classical approximation for free-space spherical wave propagation, considering that there are several propagation media with relative permitivities much higher than air in the propagation path between the receiving and transmitting antenna:
In (
2), the first two terms account for the free-space spherical wave propagation in air (
) and inside the embedding medium (concrete) (
), respectively.
represents losses due to the attenuation of concrete,
is the reflection efficiency (computed according to [
14]) and it accounts for the losses at the interface between the mediums, and
are the circular to linear polarization losses.
We have extracted from [
1] the relative permittivity values measured at 868 MHz for a concrete sample at days 0.5, 1, 2, 4, and 8 of the setting reaction.
Table 2 and
Table 3 summarize all communication link parameters, while
Figure 7 represents the propagation loss in the considered scenario over distance
computed through Equation (
2). The radiation efficiency (
) in
Table 2, has been obtained from the simulations conducted in CST of the proposed RFID sensor. This parameter represents the ratio of the power radiated over the total input power [
14]. The parameter
(dB) in
Table 3 accounts for the total propagation loss in the embedding medium (
(dB) +
(dB)).
From
Table 2, we can extract that the dynamic range of the current scenario, considering the transmission power of the RFID reader (
), the sensitivity of the RFID sensor (
), and the gain of both transmitting and receiving antennas, is of −68dB. The analytical results presented in
Figure 7 provide an estimation of the maximum distance where the external RFID reader can be placed with respect to the interface with the concrete medium, considering that the maximum propagation loss would correspond to the available dynamic range. Hence, at the beginning of the setting reaction, the reader should be placed no further than 10 cm from the medium interface, while at the eighth day of setting the mixture has dried enough that it is possible to place the reader over 1 m away from the embedding medium’s interface.