Direct Sparse Odometry with Rolling Shutter

Neglecting the effects of rolling-shutter cameras for visual odometry (VO) severely degrades accuracy and robustness. In this paper, we propose a novel direct monocular VO method that incorporates a rolling-shutter model. Our approach extends direct sparse odometry which performs direct bundle adjustment of a set of recent keyframe poses and the depths of a sparse set of image points. We estimate the velocity at each keyframe and impose a constant-velocity prior for the optimization. In this way, we obtain a near real-time, accurate direct VO method. Our approach achieves improved results on challenging rolling-shutter sequences over state-of-the-art global-shutter VO

Slow dynamics and aging of a confined granular flow

We present experimental results on slow flow properties of a granular assembly confined in a vertical column and driven upwards at a constant velocity V. For monodisperse assemblies this study evidences at low velocities ($1<V<100 \mu m/s$) a stiffening behaviour i.e. the stress necessary to obtain a steady sate velocity increases roughly logarithmically with velocity. On the other hand, at very low driving velocity ($V<1 \mu m/s$), we evidence a discontinuous and hysteretic transition to a stick-slip regime characterized by a strong divergence of the maximal blockage force when the velocity goes to zero. We show that all this phenomenology is strongly influenced by surrounding humidity. We also present a tentative to establish a link between the granular rheology and the solid friction forces between the wall and the grains. We base our discussions on a simple theoretical model and independent grain/wall tribology measurements. We also use finite elements numerical simulations to confront experimental results to isotropic elasticity. A second system made of polydisperse assemblies of glass beads is investigated. We emphasize the onset of a new dynamical behavior, i.e. the large distribution of blockage forces evidenced in the stick-slip regime

Stability Analysis of Optimal Velocity Model for Traffic and Granular Flow under Open Boundary Condition

We analyzed the stability of the uniform flow solution in the optimal velocity model for traffic and granular flow under the open boundary condition. It was demonstrated that, even within the linearly unstable region, there is a parameter region where the uniform solution is stable against a localized perturbation. We also found an oscillatory solution in the linearly unstable region and its period is not commensurate with the periodicity of the car index space. The oscillatory solution has some features in common with the synchronized flow observed in real traffic.Comment: 4 pages, 6 figures. Typos removed. To appear in J. Phys. Soc. Jp

Acoustic energy measurement for a standing acoustic wave in a micro-channel

An optical method to measure the acoustic energy density of standing acoustic waves in microfluidic devices has been proposed and applied by Barnkob et al. (Lab Chip, 10 (2010) 563) to a 2 MHz standing acoustic wave in a 250 μm high micro-channel, with the acoustical axis orthogonal to the optical axis. The method is based on the motion of a particle under Acoustic Radiation Force (ARF). However, this configuration is not always possible. We present a new optical method to measure the acoustic energy density when acoustical and optical axis are parallel. The main difference is that it is not possible to track directly the trajectory of the particles anymore, since they move along the optical axis. The method is based on the information contained in the defocused particle images. The variation of the diameter of defocused particle images is used to measure the vertical velocity along the vertical direction. From the axial velocity profile induced by ARF, it is possible to evaluate the acoustic energy density in our experimental setup. This result is important for every application using ultrasounds in micro-channels