Essays on Economic Inequality

Economic inequality has been rising according to various measures and in many countries over the last four to five decades. This thesis contributes to understanding the forces that shape income inequality in a market-based economy and studies their implications for the design of redistributive policies. The first chapter presents a generalization of central results from the theory of directed technical change, showing that these results hold much more generally than suggested by previous work. Most importantly, the restriction to labor-augmenting technologies required by previous results can be dropped without replacement. The second chapter studies the implications of directed technical change theory for the design of redistributive income taxes. It finds that, for a social objective that values redistribution from high- to low- income earners, the optimal income tax is more progressive, featuring higher marginal tax rates for high incomes and lower marginal tax rates for low incomes, when accounting for directed technical change. The third chapter provides a theoretical argument against the deregulation of markets for credence goods. It shows that a joint regulation of entry and prices can create Pareto gains in a credence good market if experts (the producers on a credence goods market) have an empirically reasonable form of social preferences and there is a common agency structure, whereby many consumers are served by the same expert

Interface micromotion in cementless hip prostheses.

The most commonly reported failure modes of cementless hip stems are loosening and thigh pain both are attributed to the relative motion at the bone-implant interface due to failure to achieve sufficient primary fixation. The main aim of the current study is to investigate, using Finite Element Analysis, various factors that could affect micromotion and could compromise the stability of cementless femoral stems. We propose a novel technique for predicting hip stem instability to analyse these problems. The designs of cementless hip stems are crucial to its success. We first categorize them into three major types based on the overall geometry and they are all found to be stable under physiological loadings. Tsoelastic' stems are found to increase interface micromotion, but if tight fit is achieved distally, the stem would still be stable. Having shorter stems for primary arthroplasty is beneficial if revision surgery is required, but these produce larger relative motion. The results from this study show that if sufficient cortical contact is achieved distally, stability is not impaired. Two types of hip stems' fixation are also compared the proximal fixation design is found to be less stable than the distal fixation design, but stability can be improved with the use of proximal macrofeatures. The strength of primary fixation also depends on surgical parameters imprecise surgical procedures can cause interfacial gaps, implant undersizing and implant malalignment. The FE results show that undersizing should be avoided because it increases micromotion and instability, especially in stems with cylindrical design. Hip stems with varus malalignment are found to be relatively stable compared to the normally aligned undersized stem. Interfacial gaps due to surgical error are not found to impair the stem's fixation as long as maximum press-fit is achieved. Successful implant fixation also depends on the quality of the bone. Bone with skeletal diseases of osteoporosis and osteoarthritis are analysed and compared with the results from a normal healthy bone. The hip stem in the osteoporotic bone is found to have the largest micromotion and is the most unstable, especially during stair-climbing activity

Six Degree of Freedom Force/Torque Sensor

The use of robots and manipulators in many kind of applications, such as scientific, medical or industrial ones, requires efficient multi-component force sensing schemes to control the force exerted by the robot end-effector on a human or an object. A multiaxis force sensor can be used to measure the contact force as accurately as possible, and to feed it back to the command signal so that the robot can achieve the pre-specified contact force. As the commercial force sensors are complex and expensive, the goal of this work is to make a multiaxis force sensor that could rThis work describes the design, development and calibration of a complete six?degree-of-freedom force and torque sensor. Compared to commercial sensors, this design has the advantage of simplicity and low cost. The sensor was machined from aluminium, and sensed by an array of commercial low-cost strain gauges. As a sensor, it could be applied in multi-DOF industrial, scientific and medical robotic systems, for instance

Improved tribology and materials for a new generation of hip prostheses

Not availabl

Sixth Goddard Conference on Mass Storage Systems and Technologies Held in Cooperation with the Fifteenth IEEE Symposium on Mass Storage Systems

This document contains copies of those technical papers received in time for publication prior to the Sixth Goddard Conference on Mass Storage Systems and Technologies which is being held in cooperation with the Fifteenth IEEE Symposium on Mass Storage Systems at the University of Maryland-University College Inn and Conference Center March 23-26, 1998. As one of an ongoing series, this Conference continues to provide a forum for discussion of issues relevant to the management of large volumes of data. The Conference encourages all interested organizations to discuss long term mass storage requirements and experiences in fielding solutions. Emphasis is on current and future practical solutions addressing issues in data management, storage systems and media, data acquisition, long term retention of data, and data distribution. This year's discussion topics include architecture, tape optimization, new technology, performance, standards, site reports, vendor solutions. Tutorials will be available on shared file systems, file system backups, data mining, and the dynamics of obsolescence

Computational Design of Compositionally Graded Alloys

In this work, a new computational methodology is presented for the design of compositionally graded alloys. Compositionally graded alloys are a class of functionally graded materials, or materials which exhibit spatially varying properties. While the introduction of additive manufacturing has accelerated interest in these materials, there are many challenges that impede their development like the formation of deleterious phases and material compositions that are incompatible with manufacturing processes. Previous design methods have attempted to design gradients that avoid these issues, but such methods have been limited to the analysis and interpretation of two-dimensional diagrams and are therefore hindered by the limits of human visualization and ideation. The proposed methodology is made possible by the novel formulation of gradient design as a path planning problem. This formulation allows the use of path planning algorithms to optimize gradient paths in composition space. Such algorithms can optimize gradients with any number of constituent elements to meet specified design requirements or objectives. To make the gradient design problem tractable for such algorithms, surrogate modeling techniques are employed to represent design constraints and objectives. Constraints, like deleterious phase formation, can be predicted by CALPHAD software and then modeled by a machine learning classifier. Similarly, regression models can be trained to evaluate cost functions in an efficient manner. Several unique problem formulations are demonstrated to showcase the advantages of the methodology in gradient design. Among these are constraints to avoid deleterious phase regions and other regions of the state space with poor predicted manufacturability. Common cost functions in the path planning community, like path length and obstacle clearance, are shown to be useful for some problems, while including constraint violation as penalty term is demonstrated to satisfy constraints that might otherwise be unachievable. Lastly, a novel cost function is proposed to design gradients with monotonic properties, which can achieve nearly any bounded property distribution on a gradient part. All proposed problem formulations are demonstrated in the design of authentic compositionally graded alloys and experiments are used to validate predicted results

Orthèses fonctionnelles à cinématique parallèle et sérielle pour la rééducation des membres inférieurs

Robotics applied to rehabilitation requires specific manipulators: Powered Orthoses. They are orthopedic devices equipped with motors and captors that enable locomotor assistance. These powered orthoses must be capable of reproducing physiological articular trajectories and taking over or simulating the segmentary charges of a movement, mainly walking. One needs to obtain rather high dynamic performances with the help of small activators enabling mechanical integration bearable for its user. This doctoral thesis deals with the conception of such POs as well as it proposes original parallel kinematics that are compared with serial kinematics, i.e. ordinary exoskeletons. We have chosen to limit our study to motor re-education of lower limbs associated to neurological disorders: paratetraplegia, hemiplegia, cerebral palsy, etc. This project was initiated by the Fondation Suisse pour les Cyberthèses (Swiss Foundation for Cyberthoses) in 1999, in collaboration with the Laboratoire de Systèmes Robotiques (Laboratory of Robotic Systems) of the Ecole Polytechnique Fédérale de Lausanne (EPFL). It aims at developing systems of motor re-education and walking assistance, associating powered orthosis with trans-cutaneous and closed-loop electrical muscle stimulation: the MotionMaker™ and the WalkTrainer™. Its goal is to create active muscular participation that respects body dynamics of movements and to quicken re-education, whenever possible. In cases of total paralysis, its purpose is to activate lower limbs in order to reduce side-effects and complications resulting from immobilization. The FSC also makes it its ambition to conceive powered orthoses for autonomous walking with functional electric stimulation in its research programme: the WalkMaker™. To begin with, this doctoral thesis defines the biomechanical bases relative to lower limbs and pelvis. Anthropometrical, kinematic and dynamic data of body segments, as well as space-time parameters of walking have been specified. These data have been used in the theoretical models of the conception of the Powered Orthoses, and applied to the numerical simulations carried out in this study. Then we have presented the state of research on orthoses. The number of projects and the diversity of technologies offer a good illustration of the challenge posed when conceiving Powered Orthoses. So far there are no autonomous Powered Orthoses for re-education of ground walking subsequent to neurological trauma. If it is possible to find treadmills on the market, they are however deficient for medical purposes because of the subject's passivity and lack of pelvis mobility. We have then conceived two Powered Orthoses for a stationary training device: firstly, a serial orthosis of the exoskeleton type has been conceived with three articulations: hip, knee and ankle. Its activators consist in connectingrod and crank systems. Secondly, we have devised a device of leg manipulation with a parallel structure (in the shape of the Greek letter lambda λ). We have modelled these two powered orthoses and carried out a numerical simulation of two movements in order to compare their performances: leg press and cycling. The λ parallel powered orthosis gives better results. Before these findings, we built the prototype of the serial powered orthosis for clinical tests for feasibility of closed-loop controlled electric stimulation. The thesis then offers the design of an unprecedented powered orthosis to be integrated into a walker. We have analyzed and modelled a parallel structure with orthogonal connections and another one with λ connections. The comparative results of the numerical simulations of normal speed walking show that the orthogonal powered orthosis is optimal for an autonomous walker. We have built a prototype, and walking tests with healthy subjects prove the feasibility of such a concept. Finally we carried out two studies for a leg-powered orthosis, compatible with a pelvic PO and the walker. An exoskeleton is compared with a parallel structure. For these two systems, the numerical simulations and models give all the kinematic and dynamic features of the activators. Following these results, we chose the parallel PO so as to design an experimental prototype. It is currently being built as we are writing these lines. To complete the procedures, a chapter deals with the integration of the POs into a walker, with the motorization of the mobile frame and with a system of active body relieving. We also present a system of optical measurements of pelvic movements for diagnosis or for biomechanical studies. The last chapter offer guidelines for development in powered orthosis

Experimental and Numerical Studies on the Structural Dynamics of Flapping Beams

The nonlinear structural dynamics of slender cantilever beams in flapping motion is studied through experiments, numerical simulations, and perturbation analyses. A flapping mechanism which imparts a periodic flapping motion of certain amplitude and frequency on the clamped boundary of the appended cantilever beam is constructed. Centimeter-size thin aluminum beams are tested at two amplitudes and frequencies up to, and slightly above, the first bending mode to collect beam tip displacement and surface bending strain data. Experimental data analyzed in time and frequency domains reveal a planar, single stable (for a given flapping amplitude-frequency combination) periodic beam response with superharmonic resonance peaks. Numerical simulations performed with a nonlinear beam finite element corroborate the experiments in general with the exception of the resonance regions where they overpredict the experiments. The discrepancy is mainly attributed to the use of a linear viscous damping model in the simulations. Nonlinear response dynamics predicted by the simulations include symmetric periodic, asymmetric periodic, quasi-periodic, and aperiodic motions. To investigate the above-mentioned discrepancy between experiment and simulation, linear and nonlinear damping force models of different functional forms are incorporated into a nonlinear inextensible beam theory. The mathematical model is solved for periodic response by using a combination of Galerkin and a time-spectral numerical scheme; two reduced order methods which, along with the choice of the inextensible beam model, facilitate parametric study and analytical analysis. Additional experiments are conducted in reduced air pressure to isolate the air damping from the material damping. The frequency response curves obtained with different damping models reveal that, when compared to the linear viscous damping, the nonlinear external damping models better represent the experimental damping forces in the regions of superharmonic and primary resonances. The effect of different damping models on the stability of the periodic solutions are investigated using the Floquet theory. The mathematical models with nonlinear damping yield stable periodic solutions which is in accord with the experimental observation. The effect of excitation and damping parameters on the steady-state superharmonic and primary resonance responses of the flapping beam is further investigated through perturbation analyses. The resonance solutions of the spatially-discretized equation of motion (via 1-mode Galerkin approximation of the inextensible beam model), which involves both quadratic and cubic nonlinear terms, are constructed as first-order uniform asymptotic expansions via the method of multiple time scales. The critical excitation amplitudes leading to bistable solutions are identified and are found to be consistent with the experimental and numerical results. The approximate analytical results indicate that a second harmonic is required in the boundary actuation spectra in order for a second order superharmonic response to exist. The perturbation solutions are compared with numerical time-spectral solutions for different flapping amplitudes. The first-order perturbation solution is determined to be in very good agreement with the numerical solution up to 5° while above this angle differences in the two solutions develop, which are attributed to phase estimation accuracy