Simulation based verification of energy storage architectures for higher class tags supported by energy harvesting devices

Abstract

Enhanced RFID tag technology especially in the UHF frequency range provides an extended functionality like high operating range and sensing and monitoring capabilities. Such complex functionality requires extended system structures including data acquisition units, real time clocks and active transmitters that cause a high energy consumption of the tag and require an on-board energy store (battery). Since the lifetime is a key parameter for the reliability of an RFID system, the energy budget of the higher class tag has to be as balanced as possible. This can be achieved by using energy harvesting devices as additional power supplies. The PowerTag¹ project and thus this paper propose special energy storage structures, which interface energy harvesting devices and deal with their special requirements to be used with battery-driven higher class UHF RFID tags. Different implementation variants of such structures are compared by using accurate simulation models of various parts of the system. The results of the simulation are compared to provided manufacturer performance parameters of a state-of-the-art higher class UHF RFID system.

Introduction

The next generation of UHF RFID tags (called higher class UHF RFID tags – HCT [2]) provides extended functionality in terms of independent sensing and monitoring, high communication range and large memory areas. Such functionality requires a continuous tag operation, which is not possible if the tag is solely powered by the reader as in the former pure passive UHF RFID systems [3]. Thus, an on-board energy supply is necessary, which forces the renaming of the previously called passive UHF RFID technology to semi-passive and active technology. This change in the tag architecture causes new issues in the area of UHF RFID: the limited operational lifetime and the need of long lasting power saving techniques. The energy, which can be stored on a tag is limited by the state-of-the-art energy storage technology and so even with profound power saving techniques the achievable lifetime is often dissatisfying. The proposed way to solve this issue is to harvest energy from the environment by energy harvesting devices, which due to a massive research and development are available in small sizes and high efficiencies satisfying limitations of UHF RFID tags. As the energy converted by such devices comes from the environment, it is generally unpredictable, discontinuous and unstable [6].

This work presents novel, special developed energy storage architectures dealing with those special requirements by proposing novel interface- and energy storage structures, which are able to store and handle the largely varying provided power. Fig. 1 shows the structure of an active higher class tag including energy harvesting device(s) and the energy storage architecture. It divides the tags architecture into three subgroups: (i) the communication and data processing subsystem, which deals with the communication and processing of the measured data; (ii) the sensing subsystem, which includes sensors and data acquisition units; (iii) the power subsystem, which is the key focus of this paper and describes power supply and energy storage modules of the tag.