Analyse et mise en oeuvre d'un moteur électromagnétique microfabriqué

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.

Modélisation analytique multiphysique pour la conception optimale de moteurs synchrones à aimants permanents

Patrick Ragot

During the last years, permanent magnets synchronous motors have become a reference in the industrial world, both from the economical and the technical point of view, because of their very good performance in both regards. Thus, application of specific methodologies and algorithms during the design phase has become a must. In this thesis, the analytical modelling of permanent magnet synchronous motors for stator configurations with and without teeth is presented. Several physical aspects, such as magnetic, thermal and electrical phenomena, are taken into account. For the magnetic part, several models are presented which all have their own areas of application. Some of them take the magnetic saturation of the ferromagnetic parts into consideration. The simplified thermal model is based on an equivalent Kirchhoff circuit in which convection and radiation are included. Concerning the determination of the electrical characteristics of slotless machine configurations, the determination of the back-emf and of the inductances using the method of images are presented. Special emphasis has been placed that the tendencies of the variations of the modelled phenomena as they are observed with changes in other parameters or criteria are correctly represented. This is almost more significant that the accuracy of the model itself. The models are validated and improved by using finite element simulations. The mathematical expressions of the design problems are taken into account to allow the mathematical optimisation applied to converge towards the optimal solution without failure. Thereby, the objective is to avoid non-continuity of the functions and their first derivatives and to reduce the space where the functions are not mathematically defined. In the "Solar Impulse" project, which aims to realise an around-the-world flight with an airplane using only the solar radiation as energy source, a prototype is optimised and built. A prototype motor has been built and experimentally analysed, to be able to further validate and improve the analytical model used for the optimisation of the machine. The originalities of this thesis can be summarised as follows : modelling of the magnetic phenomena, with several specific models adapted for different configurations ; formulations of the physical phenomena adapted for the design phase ; comparison between analytical and finite element simulations in order to validate or/and improve the analytical models ; use of the method of images to determine the inductances in a slotless motor ; design optimisation by taking into account several physical phenomena where the free parameters are analysed globally ; design and manufacturing of a prototype for a concrete project which allows to completely validate the results of the optimisation and the analytical models.

EPFL2008

Design methodology and numerical optimization of ultrasonic transducers for spinal surgery

Daniel Porto

Nowadays, the general trend towards to minimally invasive interventions is present in all the medical domains. For the surgical intervertebral spinal disc cutting or removal domain, it is particularly a necessity because the manual methods currently employed are tedious, time consuming and taxing on the hands of the practitioner. In that context, new devices that are small enough to pass through a small opening in the skin or through a small portal are required. A detailed analysis of current cutting methods that are or could be used for disc and disc nucleus removal provided that ultrasonics technology should be investigated as a possible solution. The study of ultrasonics technology to fulfil the overall cutting function needed for spinal annulus and nucleus disc material removal is based on a design methodology that breaks down the overall cutting function in many partial functions. The applied design methodology consists in drawing a complete catalogue of solutions for each partial function. Based on a predetermined choice of criteria for each partial function, the evaluation and the classification of each solution allows determination of the best solutions for each partial function. A new ultrasonic transducer designed device composed of a piezoelectric stack for the source of energy and movement, a transmission partial function with rods or discs, an amplification partial function with exponential horns and the cutting partial function solutions is detailed. Existing analytical methods for the design of ultrasonic transducers are mostly based on quarter wavelength segments used to build the transducer. The modeling of that transducers with a finite element method (FEM) avoids building the prototypes and constitutes progress. Analytical models different from the quarter wavelength approach have already been developed and are very useful when used in an optimization process. Furthermore, when the geometry of the analyzed model is not straightforward, a FEM optimization approach to solve that kind of problems can be a valid solution too. Some existing optimization algorithms and other already developed pseudo-gradient methods applied to optimize the analytical models of the ultrasonic transducers are not valid for numerical optimizations where the computing time is a key factor. This leads to the development a new genetic algorithm (GA) optimization methods. One advantage being that the number of parameters to be optimized does not change the complexity of the algorithm unlike other algorithms. Three GAs with improvements done on different parts are discussed, implemented and tested. One GA is chosen to optimize the transducer model with numerical methods. As the main drawback with FE optimizations is the amount of computation time spent for each simulation, this creates a need to develop numerical 2D models that can be quickly simulated but with accurate results. Ultrasonic transducer prototypes are also built and measured. As the prototype has to be used for the cutting or the removal of spinal disc material, the cutting effect of the prototypes has been tested and evaluated.

EPFL2009

Modeling and Simulation of Energy Efficient Milling Process Plans for Prismatic Parts

Nenad Nesic

The milling process has been continuously optimized from many aspects: forces, velocities, stability, quality. Numerous papers have been published that report results in the domain of toolpath optimization with criteria such as raising the quality of the machined part, obtaining a stable machining process and maximizing the material removal rate. However, only recently researchers have started investigating the ecological impact of machining processes. With the constantly increasing prices of electrical energy and all environmental problems caused by the production and waste of the energy, it has become indispensable to look for ways to optimize machining processes from the energy consumption aspect, too. It is in this domain, then, that the work described in this dissertation contributes – to add a new criterion for the sustainable operation of machine tools: reducing the energy consumption during the use phase of the machine tool. The total energy spent in a machining process is the sum of power spent by all machine tool subsystems multiplied by the time that they are working. Therefore, the operating strategies for increasing energy efficiency for the high-speed machining processes must be oriented towards the reduction of the total machining time (productive and unproductive) and reduction of instantaneous consumed power in the machining process or, preferably, both. There are many causes for milling process time and energy inefficiency. The major sources of potential time reductions include: (i) the issues related to the geometry and topology of the toolpath („air-time”, overlapping segments, gouge and uncut regions, orientation of the toolpath), (ii) the issues related to the kinematics of the feed drives (the feed velocity/acceleration profile). The possible domains for power consumption reduction are related to the forces that exist in the machining process (cutting, inertial, gravitational and friction forces). The power invested to overcome the cutting force is necessary for the machining process but its consumption should be optimized through constant cutting engagement and feedrate scheduling strategies. The other forces are responsible for the pure mechanical power loss and they should be minimized. The optimization variables for inertial and gravitational forces include the feed motion profile and moving mass configuration, while friction losses are load and/or velocity dependent. In addition, there are some power losses in the electric components of machine tool drives (electrical power losses), whose dependency on velocity and mechanical load can also be modeled. Despite many research efforts to analyze and model the phenomena causing inefficiency of the machining process, they are usually not taken into account during milling process planning. In fact, rare are the attempts to use this knowledge to optimize the milling process with more than one of the above mentioned criteria. The reason for that lies in the fact that increasing the instantaneous values of tool engagement and feedrate in order to minimize total machining time also increases the consumed power and, indirectly, deteriorates the tool and/or destabilizes the cutting process. The existence of conflicting criteria in the objective functions imposes the use of evolutionary algorithms for multiobjective optimization. The process of experimental validation of such an optimization model requires costly and complex external hardware setup and it is also time consuming. Hence, there is an emerging need for the development of an accurate model, which would be used for the simulation of the milling process and testing the competing optimization strategies. The ultimate goal of contemporary manufacturing science and engineering is the development of a holistic virtual machine tool system. The motivation of this PhD research is to contribute to this objective by developing the “virtual milling process for prismatic parts (produced by 2.5D milling)”. This category of objects is selected, because the majority of CNC milling tasks can be performed using 2.5D milling. A very large number of mechanical parts are prismatic (their geometry consists exclusively of features that represent 2D contours extruded in a perpendicular direction). Parts that are even more complex are usually produced from a blank by a 2.5D roughing and 3D–5D finishing. Therefore, development of an accurate model of a 2.5D milling process represents a significant contribution to manufacturing engineering. Starting with above given problem description, we have defined the following research objectives: 1. Model power and energy consumption of a 2.5D milling machine as a function of the toolpath geometry (curvature and continuity), cutting engagement, real feedrate profiles and spindle rotational speed, taking into account moving mass configuration, all power losses (mechanical and electrical) in feed-axis and spindle drives and electromechanical constraints of servo drives; 2. Design an integrated software environment for simulation of the developed models; 3. Validate the model experimentally; 4. Demonstrate the potential of its use for multi-objective process optimization. Cutting engagement calculation is performed by using two competing methodologies, in order to make a comparison of their performances. The first one, the pixel based method, is based on discrete image processing, while the other, the exact method, is based on Boolean operations and vector geometry. The tests performed in more than 20 feature/toolpath combinations demonstrate that the exact method is more precise whereas the pixel-based method is computationally much faster. The machine tool operator commands a constant feedrate value. During the process, based on his/her subjective judgment, he/she can manually tune it to keep the process in the desired boundaries. One of the objectives of this research is to predict the real feedrate values so that they can be integrated in the part program before the process start. The feasible profiles of feed velocities for each toolpath segment are analyzed by answering the following questions: can the commanded feedrate be achieved and with what sequence of acceleration and deceleration phases, how much the tool has to slow down in corners and how does the feedrate profile degenerate in segments where the commanded feedrate cannot be reached? In order to determine all forces and torques, a machine tool model is developed. The following subsystems are modeled as rigid bodies (without vibrations): feed motor shaft, transmission, lead screw/nut pair, lead screw bearings, guides (sliding and rolling design), table with the workpiece and electrospindle (rotor, stator and bearings). All kinetic variables (linear and angular speeds; cutting, inertial and friction forces and torques) are then calculated in the joints of this multi-body system. The modeling of all speeds, forces and torques that occur in the machine tool system during the machining process, allows for calculation of various indicators of process time and energy efficiency: material removal rate, total machining time, useful power (invested in cutting), power loss due to the friction in the joints, power losses in electrical motors, electrical energy consumption of feed and spindle drives. In order to assess the accuracy and the reliability of the developed model two sets of experiments have been performed. The model validation is performed on several subsystem levels and the simulation results show a very good compliance with the experimental results. The first experimental setup consists of the force dynamometer, to measure cutting force components in linear axis directions, two laser optical position sensors, to measure the real feedrate and the power sensor, to measure the total power consumption of the machine tool electrical motors. The machining process was performed on the 6-axis machining center C.B. Ferrari A152 and the data acquisition platform was developed in LabVIEW 2010. The second experimental setup consists of the torque dynamometer, to measure the cutting torque directly on the spindle, the built-in current sensor that measures the motor current proportional to the electrical power consumed by the spindle and, again, the power sensor, to measure the total power consumption of the machine tool electrical motors. This time, MIKRON HPM 600U, a 5-axis milling machine equipped with the controller iTNC530, was used for the experimentation. The developed model is ready for practical implementation. Several examples have been prepared to demonstrate its capabilities in the domains of milling process simulation, sensitivity analysis and prospective optimization strategies. These examples include the prediction of various kinetostatic variables for given toolpaths, choice of the optimum machining strategy (minimization of machining time, moving mass and energy consumption) and process optimization by feedrate scheduling (reduction of cutting force fluctuation). As a conclusion, the major contribution of this PhD research is the development of a comprehensive rigid-body dynamics model of a machine tool system, aimed for the generation, simulation and optimization of 21⁄2D milling process plans. The model features several novelties compared to existing approaches: (i) it takes into account the machine tool rigid body dynamics and real tool kinematics in toolpath planning, (ii) it predicts all mechanical and electrical power losses in the machine tool system (iii) it allows the modeling of machine tool system electromechanical constraints. The model was experimentally validated on two real machine tools in different workshops and its capabilities for the simulation of different toolpath patterns and optimization of process parameters were successfully demonstrated using several examples.

EPFL2012