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.

Small scale very high speed slotless permanent magnet motors

Guillaume Paul-André Burnand

This research work has been triggered by an industrial project aimed at modelling and manufacturing a small-scale very high speed motor. Very high speed electrical motors offer the main advantage of reducing the size of a system. Indeed, at constant output mechanical power, increasing the rotational speed enables to decrease the electromagnetic torque, hence the size of the motor. When a system already possesses very high speed devices, it becomes attractive to combine them with an electrical machine. Combined with very high speed, the miniaturisation of motors benefits limited weight and space applications. Used as hand tools, they provide solutions for otorhinolaryngology or dental surgery. Because they can be freely arranged in the airgap, slotless windings represent a substantial capacity for enhancement. Thanks to new manufacturing technologies, the design of windings can be rethought. Different shapes and topologies can be embraced leading to improved performances for electrical motors. The aim of this thesis is consequently twofold: to give multiphysics models enabling the design and the optimisation of small scale very high speed permanent magnet synchronous motors, and to propose an innovative solution for a new type of slotless winding topology entailing the performances of electrical motors. By essence, very high speed motors are pushed to their limits: they experience high mechanical stresses in the materials of the rotor and the bearings, are subject to critical speeds, operate sometimes at high temperature and harsh environments and, due to high electrical frequencies, produce significant iron and winding losses. As a result, robust and reliable multiphysics models have to be established. In this thesis, a very complete set of both mechanical and electromagnetic analytical models is presented. Analytical models are preferred to numerical models as they bring more insight in understanding physical phenomena occurring in the motors and require less computational effort for optimisation. All the aforementioned analytical models are used in the context of the design and the optimisation of small scale very high speed slotless permanent magnet motors with ball bearings. With an appropriate optimisation algorithm, fixed constraints on the maximal size and the operating point, optimal designs are obtained for several rotational speeds. In particular, an original contribution comes from the comparison of Litz-wire and rectangular wires as well as hollow and solid magnet rotors used in small-scale very high speed slotless motors. In order to experimentally validate the models, a 400 krpm 12.7 mm diameter prototype is manufactured and tested up to 475 krpm. In addition, it demonstrates the feasibility of miniaturised very high speed motors. An astute experimental method is implemented in order to separate the losses components related to the rotation of the rotor. Therefore, every model can be obtained and/or validated independently. Finally, the research work emphasis on the modelling and the optimisation of a novel slotless winding topology. By challenging the traditional way of manufacturing windings, the use of nonconstant wire sections and an optimised shape enables the performances of the winding, and thereby the motor, to be significantly improved. A prototype of the new winding topology is manufactured and leads to the validation of the theoretical model.

EPFL2020

Analyse et mise en oeuvre d'un moteur électromagnétique microfabriqué

Sebastiano Merzaghi

Until now, the preferred solution for MEMS (Micro Electro Mechanical Systems) actuation is the electrostatic one. The main reason is that this kind of actuators can be easily manufactured following the microfabrication rules, as their geometry perfectly fits to the characteristics of this technology. On the other hand, electromagnetic systems are rarely developed at small scale. Two explanations can be given. First, ferromagnetic materials are not available in standard cleanroom processes and secondly, adapting the typical three-dimensional geometry of electromagnetic drives to a planar technology is quite difficult. This thesis addresses the design of a new electromagnetic MEMS micromotor. The aim is to develop a new motor, which is able to satisfy the specifications of the watchmaker industry. The state of the art and the scaling laws show that a permanent magnet is compulsory to obtain small scale high performances motors. This is one of the reasons why, according to the project specifications, a permanent magnet synchronous motor (BLDC) seems to be the best solution. The designed motor is hybrid because it combines a microfabricated stator and a common magnet obtained with standard macroscopic fabrication processes. Its geometry is characterized by the overlapping of the rotor over the stator and it is well suited to the microsystems manufacturing principle which is based on the design of stacked layers. In order to design the motor, an analytical electromagnetic model has been developed. The accuracy of this mathematical model has been validated by means of finite elements simulations before using it to find the optimal design. Optimization results are very interesting and they demonstrate the suitability of such electromagnetic micromotor for the watch industry. At least, the same performances as the Lavet motor, which actually drives the clock hands, are predicted. Prototypes manufacturing is indispensible for theoretical analysis validation. Moreover, for the current project, this part has a higher importance because the feasibility of the stator microfabrication must also be demonstrated. These components are made following a process flow that has been especially developed for this application by combining several methods available in cleanrooms. It allows carrying out coils with two copper layers. Even if it seems to be complicated and with many steps, a great effort has been made to obtain the simplest and most reliable process flow. Prototypes have been assembled using standard watchmakers bearings. The goals are to validate the theoretical results and to highlight the critical fabrication steps as well as secondary phenomena, which were not considered during the design phase. Once again, the motors characterization demonstrates the great potential of electromagnetic MEMS.

EPFL2010

Internal electric fields and electrode effects in ferroelectric thin films for piezoelectric energy harvesting

Robin Nigon

The continuing decrease of power requirements of electronic circuits offers the potential to deploy wireless systems as embedded sensors for cars or industrial tools, and implanted medical devices. Harvesting ambient vibrations by an appropriate energy harvesting (EH) device allows to avoid an undesirable battery replacement. At the scale of micro-electromechanical systems, where severe size constraints must be met, microfabricated EH devices with piezoelectric thin films offer the best energy density. Ferroelectric lead zirconate titanate (PZT) thin films with interdigitated electrodes (IDE) appear as the most promising device design for this purpose. Accurate characterization of the thin film response is necessary to determine the PZT composition and doping, the electrode geometry, and the stack design for maximum EH efficiency. Unfortunately, there is no rigorous description of the physical behavior of the IDE system to date. One goal of this thesis was thus to provide a better understanding of the experimental observations made by previous researchers. In addition, only a limited PZT composition and doping range has been investigated in this configuration. It was the second goal of this thesis to widen this range in order to determine the combination that yields the best EH efficiency. Finally, the risk of partial or total depoling over the device lifetime is always present in ferroelectric materials. The phenomena of aging and self-poling are of great interest to ensure proper retention of the poled state and, thus, the reliability of the harvesting device. Neither of the two are well understood. It was the third goal of this thesis to investigate the aging behavior and methods to promote self-poling. In this thesis work, we have have proposed a description of the physical behavior of the IDE system, and we have developed an analytical model for extracting the effective material properties from standard characterization measurements, which is well supported by both finite element (FE) simulations and experimental data. We found that if the substrate is conductive enough, a parasitic capacitance is present in parallel to the material response. We have provided a method to subtract the contribution of the parasitic capacitance, which has an accuracy of better than 4% in a wide range of IDE geometries as determined by FE simulations. We have investigated the performances of doped PZT thin films with IDE for several combinations of dopant and composition. We have improved an existing fabrication route to obtain textured PZT films on an insulating MgO layer. We found that dopants systematically reduced the piezoelectric response and retention capability, and increased the dielectric constant. All three are detrimental for EH. Undoped compositions should be chosen. We have studied methods to improve the stability of the poled state through aging and self-poling. Introducing the latter into the IDE configuration did not provide sufficiently strong effects to be of practical interest. On the contrary, the aging process may allow to tune the extrinsic contributions to the dielectric and piezoelectric response. It is likely caused by polarization discontinuities at grain boundaries. Further work is needed for fully optimizing this phenomenon. Finally, from the previous investigations, we could deduce and propose golden rules for the design of IDE structures, and discuss typical applications where they are advantageous.

EPFL2017