MEMS Generator Study for an Embedded Energy Harvesting System

This work has been triggered by an industrial project targeting the development of a novel regulation system for a mechanical watch. Mechanical watches have been known for over a century as one of the finest example of energy autonomous devices, embodying an exceptional amount of human knowledge and high craftsmanship. Nevertheless, the accuracy of a mechanical timepiece keeps being significantly lower with respect to a quartz watch powered by a chemical battery. The aim of the study is thus filling such gap without compromising energy independence. The new regulation concept revolves around a small scale electromagnetic generator, the development of which is severely constrained at different levels. A major key point is miniaturization, which is needed in order allow the embedding of the generator in a watch movement. In fact, the overall size of the generator falls in the millimeter range, while some inner features might reach a micrometric critical dimension. A first important consequence is that not all the mathematical models that are used for conventional scale applications are suitable when it comes to the design of small scale devices. Another fundamental aspect concerns the technology associated to the fabrication of the device, which heavily affects the solutions that can actually be considered. In this sense, the adoption of the ensemble of the latest MEMS (acronym for Micro Electro Mechanical System) technologies plays a fundamental role. The research presented within this thesis aims to address such topics in the most comprehensive way as possible, with the ambition of providing scientific tools of general use. After a brief review of the state of art in the vast domain of microscale power generation, an electromechanical model for the time-dependent description of the dynamics of synchronous machines is derived. In this context, the main issues associated with size reduction are discussed in detail, with a particular emphasis on how the manufacturing technique affects the overall functionality of this class of devices. The model is then refined accordingly and used for the analysis of an existing MEMS machine. Then, the design of the MEMS generator for the watchmaking application is addressed. On the basis of the theoretical structure defined, an algorithm is conceived in order to perform the optimization of the device. The dissertation will then digress on the modeling and manufacturing of single layer planar coils. In this framework, two different fabrication processes, based on copper and aluminum respectively, are explored and the limits of each technology are compared. The experimental data resulting from this pilot study are then used for finalizing the design of the MEMS generator, in particular by determining the most suitable configuration among the ones identified by the optimization routine. The last part of the thesis will be dedicated to the fabrication and characterization of a functional prototype. In order to pursue this objective, the copper process that was already used for the single layer planar coils is upgraded for enabling the manufacturing of multilayer structures. Morphological and electrical measurements will be performed throughout all the fabrication process as well as on the final prototype.