Analyse et mise en oeuvre d"un moteur électromagnétique microfabriqué

Abstract

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.