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Nowadays, wireless sensor networks are becoming increasingly important in several sectors including industry, transportation, environment and medicine. This trend is reinforced by the spread of Internet of Things (IoT) technologies in almost all sectors. Autonomous energy supply is thereby an essential aspect as it decides the flexible positioning and easy maintenance, which are decisive for the acceptance of this technology, its wide use and sustainability. Significant improvements made in the last years have shown interesting possibilities for realizing energy-aware wireless sensor nodes (WSNs) by designing manifold and highly efficient energy converters and reducing energy consumption of hardware, software and communication protocols. Using only a few of these techniques or focusing on only one aspect is not sufficient to realize practicable and market relevant solutions. This paper therefore provides a comprehensive review on system design for battery-free and energy-aware WSN, making use of ambient energy or wireless energy transmission. It addresses energy supply strategies and gives a deep insight in energy management methods as well as possibilities for energy saving on node and network level. The aim therefore is to provide deep insight into system design and increase awareness of suitable techniques for realizing battery-free and energy-aware wireless sensor nodes.

Abstract. Safe localization of trains via GPS and wireless sensors is essential for railway traffic supervision. Especially for freight trains and because normally no power source is available on the wagons, special solutions for energy supply have to be developed based on energy harvesting techniques. Since vibration is available in this case, it provides an interesting source of energy. Nevertheless, in order to have an efficient design of the harvesting system, the existing vibration needs to be investigated. In this paper, we focus on the characterization of vibration parameters in railway application. We propose an electromagnetic vibration converter especially developed to this application. Vibration profiles from a train traveling between two German cities were measured using a data acquisition system installed on the train's wagon. Results show that the measured profiles present multiple frequency signals in the range of 10 to 50 Hz and an acceleration of up to 2 g. A prototype for a vibration converter is designed taking into account the real vibration parameters, robustness and integrability requirements. It is based on a moving coil attached to a mechanical spring. For the experimental emulation of the train vibrations, a shaker is used as an external artificial vibration source controlled by a laser sensor in feedback. A maximum voltage of 1.7 V peak to peak which corresponds to a maximum of 10 mW output power where the applied excitation frequency is close to the resonant frequency of the converter which corresponds to 27 Hz.

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Abstract. In order to maximize energy from ambient vibration sources, wide band harvesters working at a range of frequencies are important. This paper presents an electrodynamic energy harvester model working for a frequency band from 25 Hz to 45 Hz. The developed converter consists of a magnetic spring formed by one moving magnet placed between two fixed magnets. A ring magnet is placed around the moving magnet leading to additional nonlinear stiffness to increase the power output. A comparison to a basic configuration electrodynamic converter was carried out by finite element analysis to show that a significant increase in power output was realized. Simulation results have been confirmed by experimental investigations under harmonic excitations. Based on the experimental time series, we have examined the frequency spectrum and phase portraits to identify the dynamic response of the system. In conclusion, the generator is able to harvest 1.5 times more energy than the simple generator for the bandwidth of 20 Hz with the resonant frequency of 35 Hz and the excitation amplitude of 2 mm.

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Integration of smart materials to harvest energy from low vibration sources for powering wireless sensor networks has been of significant interest and still is. Owing to large magnetostriction and energy density of Terfenol- D, the use of piezoelectric/Terfenol-D composites exhibit great potential for the realization of vibration converters. In this work, a magnetoelectric vibration converter is dpresented. The interaction between the magnetic circuit and the magnetoelectric transducer is employed to tune the resonance frequency of the converter.