A simple footskate removal method for virtual reality applications

Footskate is a common problem encountered in interactive applications dealing with virtual character animations. It has proven difficult to fix without the use of complex numerical methods, which require expert skills for their implementations, along with a fair amount of user interaction to correct a motion. On the other hand, deformable bodies are being increasingly used in virtual reality (VR) applications, allowing users to customize their avatar as they wish. This introduces the need of adapting motions without any help from a designer, as a random user seldom has the skills required to drive the existing algorithms towards the right solution. In this paper, we present a simple method to remove footskate artifacts in VR applications. Unlike previous algorithms, our approach does not rely on the skeletal animation to perform the correction but rather on the skin. This ensures that the final foot planting really matches the virtual character's motion. The changes are applied to the root joint of the skeleton only so that the resulting animation is as close as possible to the original one. Eventually, thanks to the simplicity of its formulation, it can be quickly and easily added to existing framework

Flexure-Pivot Oscillator Restoring Torque Nonlinearity and Isochronism Defect

Flexure pivot based oscillators can advantageously replace the hairspring and balance wheel, the time base used in mechanical watches, by drastically reducing friction. However, flexure pivots have drawbacks including gravity sensitivity and restoring torque nonlinearity. In previous work, we introduced a novel gravity insensitive flexure pivot (GIFP) to solve the problem of gravity sensitivty, but no analytical formulation for the restoring torque nonlinearity was found. In this paper, we use numerical simulation to find an empirical expression for restoring torque nonlinearity. We use this expression to find an analytical formula for the rotational stiffness of GIFP. This formula gives an explicit relationship between restoring torque nonlinearity and the isochronism of the corresponding harmonic oscillator. The results also apply to the widely used generalized cross-spring pivot

High Performance Control of a Corner Cube Reflector by a Frequency-Domain Data-Driven Robust Control Method

The linear motion of the Corner Cube Mechanism developed for the infrared sounder of the third generations of Meteosat weather satellites requires a high level of accuracy. The system is subject to external micro-vibration perturbations from surrounding instruments, which cannot be rejected with the current PID controllers with notch filters. A data-driven H-infinity robust controller design method is proposed to improve the control performance. The method uses only frequency-domain data and satisfies the constraints on the weighted infinity-norm of sensitivity functions using the convex optimization algorithms. The frequency response of the system is identified from the finite element model of the system. The designed controller is validated in simulation. The performance improvement with respect the PID controller with notch filters is illustrated via experimental results

Gravity-Insensitive Flexure Pivot Oscillators

Classical mechanical watch plain bearing pivots have frictional losses limiting the quality factor of the hairspring-balance wheel oscillator. Replacement by flexure pivots leads to a drastic reduction in friction and an order of magnitude increase in quality factor. However, flexure pivots have drawbacks including gravity sensitivity, nonlinearity, and limited stroke. This paper analyzes these issues in the case of the cross-spring flexure pivot (CSFP) and presents an improved version addressing them. We first show that the cross-spring pivot cannot be simultaneously linear, insensitive to gravity, and have a long stroke: the 10 ppm accuracy required for mechanical watches holds independently of orientation with respect to gravity only when the leaf springs cross at 12.7% of their length. But in this case, the pivot is nonlinear and the stroke is only 30% of the symmetrical (50% crossing) crossspring pivot’s stroke. The symmetrical pivot is also unsatisfactory as its gravity sensitivity is of order 104 ppm. This paper introduces the codifferential concept which we show is gravity-insensitive. It is used to construct a gravity-insensitive flexure pivot (GIFP) consisting of a main rigid body, two codifferentials, and a torsional beam. We show that this novel pivot achieves linearity or the maximum stroke of symmetrical pivots while retaining gravity insensitivity

Echappements à impulsion virtuelle

L’échappement à détente est reconnu pour sa performance chronométrique, mais il n’est pas sécurisé pour la montre-bracelet. Plusieurs échappements ont été proposés pour adapter cet échappement à la montre, dont l’échappement Robin récemment sécurisé par Audemars Piguet. George Daniels a poursuivi une démarche qui a mené à l’échappement coaxial. Nous proposons un nouveau concept, l’impulsion virtuelle, qui pourrait réunir tous les avantages de ces échappements. Notre solution est une simple modification de l’échappement Robin, nous ajoutons seulement une dent d’impulsion indirecte. Le principe de l’impulsion virtuelle consiste en une impulsion indirecte qui ne se fait qu’à l’arrêt et à faible amplitude. Ceci ajoute la contrainte du double coup, donc sécurise, et assure l’auto-démarrage. Un tracé et un démonstrateur ont été réalisés. Des observations du démonstrateur, à l’aide d’une caméra haute vitesse, démontrent la validité du concept de l’impulsion virtuelle

Next-generation museomics disentangles one of the largest primate radiations

Guenons (tribe Cercopithecini) are one of the most diverse groups of primates. They occupy all of sub-Saharan Africa and show great variation in ecology, behavior, and morphology. This variation led to the description of over 60 species and subspecies. Here, using next-generation DNA sequencing (NGS) in combination with targeted DNA capture, we sequenced 92 mitochondrial genomes from museum-preserved specimens as old as 117 years. We infer evolutionary relationships and estimate divergence times of almost all guenon taxa based on mitochondrial genome sequences. Using this phylogenetic framework, we infer divergence dates and reconstruct ancestral geographic ranges.We conclude that the extraordinary radiation of guenons has been a complex process driven by, among other factors, localized fluctuations of African forest cover. We find incongruences between phylogenetic trees reconstructed from mitochondrial and nuclear DNA sequences, which can be explained by either incomplete lineage sorting or hybridization. Furthermore, having produced the largest mitochondrial DNA data set from museum specimens, we document how NGS technologies can "unlock" museum collections, thereby helping to unravel the tree-of-life. [Museum collection; next-generation DNA sequencing; primate radiation; speciation; target capture.] © The Author(s) 2013.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Open Science In Practice summer school - Lessons learnt

This presentation summarizes the notes I have taken during the Open Science In Practice summer school 2017 at EPFL. https://osip2017.epfl.ch

Design of a Flexure Rotational Time Base with Varying Inertia

Flexure oscillators are promising time bases thanks to their high quality factor and monolithic design compatible with microfabrication. In mechanical watchmaking, they could advantageously replace the traditional balance and hairspring oscillator, leading to improvements in timekeeping accuracy, autonomy and assembly. As MEMS oscillators, their performance can rival that of the well-established quartz oscillator. However, their inherent nonlinear elastic behavior can introduce a variation of their frequency with amplitude called isochronism defect, a major obstacle to accurate timekeeping in mechanical watches. Previous research has focused on addressing this issue by controlling the elastic properties of flexure oscillators. Yet, these oscillators exhibit other amplitude-related frequency variations caused by changes of inertia with amplitude. In this article, we not only improve existing models by taking into account inertia effects but also present a new way of using them to adjust the isochronism defect. This results in a better understanding of flexure oscillators and an alternative way of tuning isochronism by acting on inertia instead of stiffness. This also opens the door to new promising architectures such as the new Rotation-Dilation Coupled Oscillator (RDCO) whose symmetry has the advantage of minimizing the influence of linear accelerations on its frequency (the other major limitation of flexure oscillators). We derive analytical models for the isochronism of this oscillator, show a dimensioning with compensating inertia and stiffness variations and present a practical method for post-fabrication isochronism tuning. The models are validated by FEM and mock-ups serve as preliminary proof-of-concept

Conceptual design of a rotational mechanical time base with varying inertia

Flexure pivot oscillators have the potential to advantageously replace the traditional balance wheel-spiral spring oscillator used in mechanical watches due to their significantly lower friction. However, they have inherent nonlinear elastic properties that can introduce a variation of their frequency with amplitude called isochronism defect. Previous research has focused on controlling the elastic behavior of flexure pivot oscillators to reach isochronism. We present a new way of minimizing the isochronism defect of rotational oscillators by varying their inertia. This principle is embodied in a new family of oscillators we call rotation-dilation coupled oscillator (RDCO). Their architecture also presents a rotational symmetry that is advantageous for minimizing the effects of gravity on their period. We present a description of this new oscillator family, give conceptual tools for tuning its isochronism and show examples of physical implementations

Flexure Pivot Oscillators for Mechanical Watches

It appears that the concerted efforts of the watchmaking industry are leading towards a limit in mechanical watch accuracy. The general consensus in horology is that the time base's quality factor, a dimensionless number that characterizes the oscillator damping, needs to be improved in order to significantly increase timekeeper accuracy. The three classical mechanical time bases all suffer from limitations for mechanical watch applications. The pendulum used in precision mechanical clocks reaches quality factors of order 100000 but is too sensitive to the orientation of gravity to be used in portable timekeepers (watches). The balance and hairspring oscillator used in classical mechanical watches sees its quality factor limited to about 300 (450 in partial vacuum) by the friction in its bearings. The tuning fork used in electronic watches can reach quality factors of order 100000 but its high frequency and very small oscillation amplitude makes it very challenging to sustain mechanically. The solution appears to be flexure-pivot oscillators in silicon. Flexure-pivot oscillators use the elastic deformation of thin beams to guide the rotational motion of an inertial body and to exert a force opposed to its displacement, thus providing all the elements for a one degree-of-freedom mechanical oscillator. The use of flexures instead of bearings eliminates contact friction and monocrystalline silicon minimizes internal friction, leading to significant improvements in quality factor in comparison to balance and hairspring oscillators. Moreover, the rotational amplitude is compatible with existing sustaining mechanisms (escapements). Accurate timekeeping requires the period of oscillation of the time base to stay as regular as possible regardless of changes in operating conditions such as amplitude of oscillation, orientation with respect to gravity, temperature and shocks. This thesis focuses on minimizing the effects of amplitude and gravity that arise from the use of flexures. First, the nonlinear elastic behavior of flexures introduces a dependence of oscillation period on amplitude called isochronism defect. Second, the weight-bearing function of the flexures and their deviation from the motion of ideal linkages result in a contribution of gravity to their effective stiffness and hence an effect on their frequency. It is assumed that the other effects, i.e., temperature and shocks, can be solved with existing techniques. The first technical contribution of this thesis is to note that the isochronism defect is a second order phenomenon, and to deal with it by modifying the second order behavior of flexure spring stiffness or inertia. The second technical contribution is a design method to reduce the effect of gravity for all orientations of the time base to within the specifications of current mechanical watches, this by ingeniously placing the flexures and exploiting the position of the center of mass. These findings are embodied in three new flexure pivot architectures called Gravity Insensitive Flexure Pivot, co-RCC flexure pivot oscillator and Rotation-Dilation Coupled Oscillator. A silicon co-RCC prototype satisfying typical mechanical watch specifications is manufactured. The technical contributions of the thesis are validated by finite element simulations and experimentally. This thesis only deals with time bases and a new method has been developed to measure their chronometric performance without a sustaining mechanism