Experimental Characterization of Optimized Piezoelectric Energy Harvesters for Wearable Sensor Networks

Abstract

The development of wearable devices and remote sensor networks progressively relies on their increased power autonomy, which can be further expanded by replacing conventional power sources, characterized by limited lifetimes, with energy harvesting systems. Due to its pervasiveness, kinetic energy is considered as one of the most promising energy forms, especially when combined with the simple and scalable piezoelectric approach. The integration of piezoelectric energy harvesters, generally in the form of bimorph cantilevers, with wearable and remote sensors, highlighted a drawback of such a configuration, i.e., their narrow operating bandwidth. In order to overcome this disadvantage while maximizing power outputs, optimized cantilever geometries, developed using the design of experiments approach, are analysed and combined in this work with frequency up-conversion excitation that allows converting random kinetic ambient motion into a periodical excitation of the harvester. The developed optimised designs, all with the same harvesters’ footprint area of 23 × 15 mm, are thoroughly analysed via coupled harmonic and transient numerical analyses, along with the mostly neglected strength analyses. The models are validated experimentally via innovative experimental setups. The thus-proposed f = 50 mm watch-like prototype allows, by using a rotating flywheel, the collection of low-frequency (ca. 1 to 3 Hz) human kinetic energy, and the periodic excitation of the optimized harvesters that, oscillating at their eigenfrequencies (~325 to ~930 Hz), display specific power outputs improved by up to 5.5 times, when compared to a conventional rectangular form, with maximal power outputs of up to >130 mW and average power outputs of up to >3 mW. These power levels should amply satisfy the requirements of factual wearable medical systems, while providing also an adaptability to accommodate several diverse sensors. All of this creates the preconditions for the development of novel autonomous wearable devices aimed not only at sensor networks for remote patient monitoring and telemedicine, but, potentially, also for IoT and structural health monitoring.

Analysis of Influencing Parameters Enhancing the Plucking Efficiency of Piezoelectric Energy Harvesters

“…The clamping mechanism allows the plectrum overlap over the PEH to be adjusted, while the clamping force is kept consistent by carefully tightening the two clamping bolts with a 1.5 Nm (±6%) torque by using a micro-torque wrench. The rotating speed of the motor, and thus, the plucking frequency, is controlled via a laboratory power supply (4) [ 41 ], while the output voltage generated by the PEH is measured via an Agilent ® DSO-X 2012A oscilloscope (5) [ 42 ] connected to the PEH electrodes via a TE ® 1051 resistance decade box (6) [ 43 ], set to the optimal load resistance of the PEH, thus enabling the assessment of the PEH power output [ 5 , 6 , 14 ].…”

“…The excitation can be induced via the clamped base (harmonic excitation) and by plucking or impacting the free end (plucking/impact excitation). As the conversion efficiency of a cantilever PEH is highest when the excitation frequency matches its coupled electromechanical eigenfrequency, the highest voltage (and power) outputs are generated at these exact operating conditions [ 5 , 6 , 14 ]. A typical voltage output of a cantilever PEH under harmonic excitation is shown in Figure 1 b.…”

“…The described phenomenon is particularly relevant if a PEH device, subjected to base excitation, is to be utilized for collecting kinetic energy from human motion, due to the random nature of such excitation [ 14 , 15 , 16 ]. Studies have, in fact, shown that the kinetic energy from human motion during the activities of daily living (ADLs), particularly from arm movement and footfalls, can result in a maximal power output of ~60 W [ 17 ], but the frequencies of the thus occurring excitations typically randomly range from 1 to 4 Hz, with relatively low acceleration values of <1 g [ 14 , 15 , 16 , 18 ].…”

“…The described phenomenon is particularly relevant if a PEH device, subjected to base excitation, is to be utilized for collecting kinetic energy from human motion, due to the random nature of such excitation [ 14 , 15 , 16 ]. Studies have, in fact, shown that the kinetic energy from human motion during the activities of daily living (ADLs), particularly from arm movement and footfalls, can result in a maximal power output of ~60 W [ 17 ], but the frequencies of the thus occurring excitations typically randomly range from 1 to 4 Hz, with relatively low acceleration values of <1 g [ 14 , 15 , 16 , 18 ]. This results, in turn, in a less than optimal operation of the cantilever PEH, and therefore, in a rather limited power output [ 6 , 14 ].…”

“…In order to overcome this limitation, several approaches have been suggested in the literature [ 19 , 20 , 21 , 22 , 23 ], such as the flywheel-based plucking or impacting of the PEH free end [ 19 ], the use of magnetic excitation [ 20 ], multiple cantilevers or nonlinear geometries [ 21 ], active tuning of the PEHs [ 22 ] or nonlinear force customization technologies [ 24 ]. One of the most promising approaches is the frequency up-conversion (FUC) mechanisms, i.e., the conversion of random ambient motion into periodical plucking excitation of the PEH, that induces the deflection (bending) of the free end of the cantilever and suddenly releases it, allowing the cantilever to freely oscillate [ 14 , 15 , 19 ]. Such an excitation mechanism generally comprises a flywheel collecting the random kinetic energy from the environment, equipped with single or multiple plectra aimed at inducing the excitation of the free end of the PEH.…”