Design methodology and numerical optimization of ultrasonic transducers for spinal surgery

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.

Modeling of Inductive Contactless Energy Transfer Systems

Pascal Meyer

In the domain of electronic devices and especially desktop peripherals, there is an industrial trend which consists in removing the cables that pollute our domestic and professional environments. In this sense, wireless communication protocols are already massively widespread while the power supplies still use wires or batteries. To address this problem, alternative solutions must be investigated such as contactless energy transfer (CET). In a broad sense, CET is a process that allows to bring electrical energy from one point to another through a given medium (generally air or vacuum) and at a certain distance. Inductive CET means that the intermediate form of energy is the magnetic induction, generated from primary coils excited by high-frequency alternating currents and collected in secondary coils by induced voltages. Most of existing approaches to design CET systems are applicable to only single applications and do not include an optimization method. For this reason, the present thesis focuses on the modeling, design and optimization of inductive CET systems. Using the coreless transformer as the central part of CET systems, an equivalent electric model is derived from the theory of conventional transformers. The absence of ferrite core gives rise to a specific characteristic, which is to have large leakage inductances compared to the main one. In order to circumvent this issue, using a high frequency together with a resonant circuit allow to enhance the effect of the mutual inductance and to transfer power with an excellent efficiency. Different parts of the coreless transformer are addressed separately. First, an accurate modeling of DC resistances, self and mutual inductances is proposed. Then, the equivalent electric circuit is resolved and the different compensation topologies for the resonant circuit are discussed. Finally, the AC resistance is computed using a 2D finite element modeling that takes into account the skin and proximity effects in the conductors. So as to exploit optimally FEM simulations, a complete output mapping together with a specific interpolation strategy are implemented, giving access to the AC resistance evaluation in a very short time. As a result, all the models are implemented in a way that makes them highly adaptable and low-consuming in term of computing resources. Then a sensitivity analyzis is performed in order to restrict the variation range of different parameters and to provide a general and intuitive understanding of inductive CET. After that, an optimization method using genetic algorithms (GAs) is presented. The main advantage of GAs is that the number of free parameters does not change the complexity of the algorithm. They are very efficient when a lot of free parameters are involved and for optimizations where the computing time is a key factor. As existing GAs failed to converge properly for different tested CET problems, a new one is developed, that allows to optimize two objective functions in the same time. It is thus a multiobjective genetic algorithm (MOGA) and has been successfully applied to the design of different CET systems. Finally, in order to validate the models and optimization methods proposed along the thesis, several prototypes are built, measured and tested. Notably, a CET table that allows to supply simultaneously different peripherals is fabricated. By analyzing in real time the current amplitude in the primary coils, an efficient sensorless detection of the peripherals is implemented. Digital control techniques have enabled the autonomous management of the detection and the local activation of the table. These results contribute to the future development of robust and efficient CET tables.

EPFL2012

Méthodologie de conception et optimisation d'actionneurs intégrés sans fer

Igor Stefanini

The growing demand for new technologies in several fields : recreation, medicine, transport, telecommunications and the space industry, etc. is increasingly related to the field of the management, transformation and use of electric power. In this domain, the development of no-ferromagnetic integrated electromechanical actuators finds wide application and thus implies an usefulness over a large diversity of geometries. The absence of ferromagnetic materials does not allow for quick determination of an equivalent electromagnetic circuit for this type of actuator. The interaction of the magnetic induction fields produces phenomenas like the skin effect and the proximity effect which degrade its performance. In such circumstances, there is a compromise to be made between the analytical model's complexity and the precision of the results. Often, a modeling based on numerical approaches, which uses finite element methods, is the best adapted approach. Nevertheless, this type of analysis requires much experience and in particular, the parameterization of the actuator specifications and the exploitation of such a model in the optimized design is not very practical and requires considerable computing time. The use of discretized analytical models constitutes a good compromise between the degree of complexity and the model's correspondence to reality. Accordingly, in this thesis, we propose a suite of discretized analytical models allowing for the design and the optimization of this type of actuator. The development of a general rule to determine the optimal discretization step allows an arbitration between computing time, degree of complexity, and the precision of the results. The identification and the validation of the analytical models describing the skin and proximity effects have made it possible to improve the computation of self-inductance and winding resistance. The development of a design and optimization methodology for this type of actuator permits each application to be optimized in a systematic and efficient way, by proposing one or more possible solutions with their advantages and their disadvantages. Two distinct applications were conceived and optimized adopting this design methodology : the Montrac® system and the Iglus® system. In the Montrac® project, it was a question of modifying the power system of a modular assembly line. This system is composed of a fixed track with motorized shuttles for use in a clean room environment. The replacement of the electrical contacts by a contactless energy transmission system was dimensioned and optimized in order to guarantee an operation compatible with the requirements of the above mentioned environment. In the Iglus® project, a subcutaneous biomedical actuator for glycemic level detection, with the aim of measuring the glucose level of diabetes patients was optimized. The measurements and results from these two applications, made it possible to confirm the validity of the models expounded in this thesis.

EPFL2006

Analytical design and optimization of ultrasonic vibrational transducers for spinal surgery

John Murphy

Just about every other technical publication you pick up these days makes sweeping statements concerning the pressures on scientists, engineers, and industries as a whole to get to market in less time, with an improved, less expensive product. I am reminded of a slogan used rather ubiquitously by a prior employer : Cheaper, Faster, Better. The mention of this race has become almost blase for most, but for the individuals taking the brunt of the pressure, can be disconcerting if not outright painful. Expected to live up to this ambition, the design engineer is faced with the fact that the door is closing on antiquated methods of trial and error engineering. Unfortunately, most individuals have not been equipped with the necessary skills to enact a more intelligent design process and in many cases when the skills are present, industry, a bit paradoxically, is wary of expending the time and resources needed to fit into this new streamlined paradigm. This thesis could be considered as a case study of a somewhat mature technology coming to grips with this new rushed reality. Typical design methods for ultrasonic transducers, while based on analytical models, have relied heavily on building a transducer from quarter wave segments that have somewhat known characteristics. Prototypes are built, tested and modified until the desired results are obtained. Advances have been made to this method with the introduction of finite element softwares that allow modeling without realizing a prototype. The finite element methodology also permits a limited amount of system optimization, and while this is a definite improvement, it can still take months to realize an acceptable design. In this work, we draw on the analytical models, but abandon the quarter wave approach. We also use finite elements, to a limited extent, to validate results obtained from the analytical models, but do not attempt to use the finite element analysis to perform optimizations. On the contrary, a novel optimization method is developed which uses the analytical models to calculate values for the various cost functions necessary to optimize a transducer and its drive electronics. In this way, system parameters are left free to vary in the search for a maximum or minimum solution. While some systems are not candidates for system optimization as a means to get to market faster with a better product, this is not the case for ultrasonic transducers. The design of ultrasonic transducers is an application that begs for models, for optimization, and characterization. The investment made here in understanding the mathematical models, and in the development of optimization routines and softwares has allowed for a modified method of development for ultrasonic handpieces and their drive electronics. This thesis shows that, given a set of design specifications, the design of a transducer can be done in a very short period of time, a few days, even-and furthermore, the resulting design will function as predicted by the simulations. This work presents analytical models for the transducers elements with analysis and discussion. It then introduces a new method of optimization that is well adapted to optimizing the transducers as well as the drive electronics. The control electronics and a simple method of transducer control, which allow for the operation of the system for testing purposes, are also laid out.

EPFL2007