A credit card sized self powered smart sensor node

Abstract

This paper reports a self powered smart sensor node (also called ‘smart tag’) consisting of a piezoelectric vibration energy harvester, a power conditioning circuit, sensors and an RF transmitter. The smart tag has dimensions similar to a credit card and can be easily integrated into various applications such as the surface of the aircraft. The smart tag is powered by an integrated bimorph piezoelectric generator that extracts energy from ambient vibrations. The generator is fabricated using thick film printing technology. Experimentally, the generator produced a maximum RMS output power of 240 μW when excited at vibration with a frequency of 67 Hz and peak amplitude of 0.4 g (3.9 m s⁻²). This generated power is sufficient to enable periodic sensing and transmission. Details of the experimental results of the piezoelectric generator and the power conditioning circuit are presented. Test shows that the waiting time of the system between two consecutive transmissions is around 800 s.

Introduction

Wireless sensor networks have been of great interests over the last few decades [1]. Compared to wired systems, wireless systems can be used more widely due to their flexibility. Once deployed, wireless sensor nodes can be routed and connected without changing their physical layout. One concern about the wireless sensor nodes is their power supply. Conventional power source for wireless sensor nodes is batteries. However, replacing batteries is a tedious and costly job. Therefore, it is preferred to make the sensor nodes have the ability to generate energy from ambient environment, for example, energy harvesting from photonic, thermal and mechanical energy.

Vibration energy harvesting, as a promising solution to powering wireless sensor nodes, has been studied comprehensively over the recent years. Piezoelectric, electromagnetic, electrostatic and magnetostrictive transduction mechanisms are commonly used to convert mechanical energy into electrical energy [2]. While conventional vibration energy harvesters can only work at one fixed frequency, advanced strategies have been studied to increase the operating frequency range, offering a more extensive range of applications [3]. Among all transduction mechanisms, piezoelectric is the most common one due to its simplicity [4], [5], [6], [7].

Although most research present standalone vibration energy harvesters, some self-powered wireless sensor systems have also been reported. James et al. [8] reported a self-powered system which was powered by a mechanical to electrical energy converter for condition monitoring applications. Torah et al. [9] presented a wireless sensor node powered by an electromagnetic vibration energy harvester. The node integrated an accelerometer and an AM transmitter. It could measure and transit data every 3 s. One potentially important application of wireless sensors is Structure Health Monitoring (SHM) in aircraft [10], [11], [12], [13], [14], [15]. In recent years, many self-powered systems for structural health monitoring have also been reported [16], [17], [18], [19]. These systems were powered by various energy harvesters that produced enough energy regular measurement.

This work is part of the EU Framework 7 project TRIADE. The aim of the TRIADE project is to develop a low-profile, planar, self powered intelligent sensor device (also known as smart tag) that can be fabricated within a composite material, and thus embedded in the structure of an aircraft to monitor structural health and transmit measurement data wirelessly to the base station during the flight. As a partner of this project, researchers in University of Southampton work mainly on energy harvesting. Details of work of other partners in this project, including SHM scheme, can be found in [20].

Although our work focuses on the generator, it is useful to demonstrate its use in a real application, and this involves defining a target environment, and then using the generator to power an example sensor node. Our initial target for this generator is the vertical fin of a PZL SW4 helicopter. Due to size restrictions for this application, a thick film piezoelectric generator was selected as the power source. An initial prototype of the thick film piezoelectric generator designed for this application has already been reported [21]. The generator produced a peak power of 117 μW when excited at 6.9 m s⁻² (RMS) and 70 Hz with an optimum load of 140 kΩ and an output voltage of 2.9 V, but for the target application the generator was redesigned to be active at 67 Hz with an expected acceleration of 2.8 m s⁻² (RMS) from measured vibration spectra.

This paper describes a simple sensor node, using off the shelf sensors to demonstrate the practicality of the generator in relation to current low power systems. It should be noted that this in only an example system, and that other sensors can be designed into the system. The paper then describes the design of the thick-film generator for this application, and discusses the power conditioning and storage system needed to allow the operation of standard electronics. This results in a credit card sized self powered smart sensor node consisting of an improved piezoelectric vibration energy harvester, its power conditioning circuit, sensors and an RF transmitter. Finally, performance of the system is investigated experimentally and is discussed.