This work provides an overview of the current state-of-the-art of the RFID technology in the Wireless Sensor Network (WSN) field. The different RFID alternatives are described with their pros and cons. The benefits and limitations of battery-free wireless sensors based on RFID technology are presented. Subsequently, the paper describes the design of an UHF RFID integrated circuit (IC) designed to harvest and store the power from an incoming RF signal in order to supply low power analog or digital sensors. The IC has been implemented using a low cost 0.35 μm CMOS process and it incorporates a serial peripheral interface (SPI) in order to communicate with the sensor. Battery-free wireless operation for industrial monitoring using the designed IC has been demonstrated by implementing two wireless sensors that monitor both temperature, using a thermistor, and pressure, using a commercial digital sensor. The wireless temperature sensor shows an error below ±1°C from -25°C to 105°C, whereas the pressure sensor error is below ±2 mbar from 325 to 1000 mbar. A battery free communication range of 1.5 meters is demonstrated for both sensors using a 2-W effective radiated output power reader.

The number of wireless medical wearables has increased in recent years and is revolutionizing the current healthcare system. However, the state-of-the-art systems still need to be improved, as they are bulky, battery powered, and so require maintenance. On the contrary, battery-free wearables have unlimited lifetimes, are smaller, and are cheaper. This paper describes a design of a battery free wearable system that measures the skin temperature of the human body while at the same time collects energy from body heat. The system is composed of an UHF RFID temperature sensor tag located on the arm of the patient. It is assisted with extra power supply from a power harvesting module that stores the thermal energy dissipated from the neck of the patient. This paper presents the experimental results of the stored thermal energy, and characterizes the module in different conditions, e.g., still, walking indoors, and walking outdoors. Finally, the tag is tested in a fully passive condition and when it is power assisted. Our experimental results show that the communication range of the RFID sensor is improved by 100% when measurements are done every 750 ms and by 75% when measurements are done every 1000 ms when the sensor is assisted with the power harvesting module.

This paper presents a comprehensive overview and analysis of the state-of-the-art (SoA) in semi-passive or Battery-Assisted (BAP) Ultra-High Frequency (UHF) Radio Frequency Identification (RFID) sensor tags compliant with EPC Global G2/ISO-18000C. These tags operate on the same communication principle as fully passive sensor tags but incorporate a battery or an energy harvesting module. This additional power source extends communication ranges and enables power demanding applications using low power microcontrollers (MCUs) and higher-end sensors. This article also analyzes various key features, including tag integrated circuit (IC) architecture, types of energy harvesting modules, and communication range. The main conclusions are threefold. Firstly, selecting the appropriate tag IC requires a careful analysis its features such as sensitivity, sensor interfaces or data logging capabilities. For instance, among the solutions examined in the SoA, half of them opted for a tag IC capable of MCU communication via SPI or I2C buses. Secondly, it is essential to assess both the forward and backward communication links to leverage the sensitivity of the tag IC in BAP mode. Interestingly, only one third of the solutions in the SoA achieved the theoretical communication range anticipated by the tag IC sensitivity. Finally, an energy budget analysis is required to ensure that the energy generation suffices to meet the energy requirements of the tag. While most solutions rely on batteries as the energy source and analyze battery lifespan, only a few studies employing energy harvesters conduct an energy budget analysis due to the additional complexity involved.

A full passive UHF pressure RFID tag is presented in this work. Special attention is paid to the novel capacitive sensor-interface and the analog FE which were designed and implemented on a low cost 0.35µm standard CMOS technology. The characterization of the fabricated sensor interface connected to a pressure MEMS transducer shows a fully digital output with an ENOB of 7.27 bits and an FOM of 5.47pJ. The fabricated analog front-end, assembled to a matched dipole antenna, was connected to the digital core and the implemented pressure sensor. The RFID pressure sensor tag was characterized inside a PVC pressure chamber. Successful ID and pressure measurement (30kPa–120kPa) communication with a commercial RFID reader over a distance of 1.5m was achieved using the EPC Gen 2 standard. These characteristics allow the use of the proposed system in long range wireless pressure sensing and motivates the implementation of the proposed capacitive sensor interface in passive wireless sensors.

In this paper, a novel Radio-Frequency Identification (RFID) tag for “pick to light” applications is presented. The proposed tag architecture shows the implementation of a novel voltage limiter and a supply voltage (VDD) monitoring circuit to guarantee a correct operation between the tag and the reader for the “pick to light” application. The feasibility to power the tag with different photovoltaic cells is also analyzed, showing the influence of the illuminance level (lx), type of source light (fluorescent, LED or halogen) and type of photovoltaic cell (photodiode or solar cell) on the amount of harvested energy. Measurements show that the photodiodes present a power per unit package area for low illuminance levels (500 lx) of around 0.08 μW/mm², which is slightly higher than the measured one for a solar cell of 0.06 μW/mm². However, solar cells present a more compact design for the same absolute harvested power due to the large number of required photodiodes in parallel. Finally, an RFID tag prototype for “pick to light” applications is implemented, showing an operation range of 3.7 m in fully passive mode. This operation range can be significantly increased to 21 m when the tag is powered by a solar cell with an illuminance level as low as 100 lx and a halogen bulb as source light.

A novel wireless temperature monitoring system for electric machines is presented based on fully passive UHF RFID sensor tags. The system is composed of a custom designed IC UHF RFID sensor tag connected to an external μP and ADC through a master-slave series-peripheral interface (SPI) connection. The sensing element is a NTC 10kOhm thermal resistor (-40° to 300°C). The sensor tag, which includes an optimized meander dipole antenna, an analog front-end with a power consumption of 4.3uA and a digital core, communicates the temperature value to two EM optimized PIFA antennas located inside the stator of the electrical machine. The signal received by the PIFA antennas is demodulated by a commercial 1W RFID reader. The sensor is able to sense from -40° to 150°C and robust to the presence of the high electromagnetic disturbances generated by the electrical machine. Measured data rates are higher than 1 sample per second.