On contact numbers in random rod packings

Random packings of non-spherical granular particles are simulated by combining mechanical contraction and molecular dynamics, to determine contact numbers as a function of density. Particle shapes are varied from spheres to thin rods. The observed contact numbers (and packing densities) agree well with experiments on granular packings. Contact numbers are also compared to caging numbers calculated for sphero-cylinders with arbitrary aspect-ratio. The caging number for rods arrested by uncorrelated point contacts asymptotes towards <γ> = 9 at high aspect ratio, strikingly close to the experimental contact number <C> ≈ 9.8 for thin rods. These and other findings confirm that thin-rod packings are dominated by local arrest in the form of truly random neighbor cages. The ideal packing law derived for random rod–rod contacts, supplemented with a calculation for the average contact number, explains both absolute value and aspect-ratio dependence of the packing density of randomly oriented thin rods

Model of tactile sensors using soft contacts and its application in robot grasping simulation

In the context of robot grasping and manipulation, realistic dynamic simulation requires accurate modeling of contacts between bodies and, in a practical level, accurate simulation of touch sensors. This paper addresses the problem of creating a simulation of a tactile sensor as well as its implementation in a simulation environment. The simulated tactile sensor model utilizes collision detection and response methods using soft contacts as well as a full friction description. The tactile element is created based on a geometry enabling the creation of a variety of different shape tactile sensors. The tactile sensor element can be used to detect touch against triangularized geometries. This independence in shape enables the use of the sensor model for various applications, ranging from regular tactile sensors to more complex geometries as the human hand which makes it possible to explore human-like touch. The developed tactile sensor model is implemented within OpenGRASP and is available in the open-source plugin. The model has been validated through several experiments ranging from physical properties verification to testing on robot grasping applications. This simulated sensor can provide researchers with a valuable tool for robotic grasping research, especially in cases where the real sensors are not accurate enough yet

Data-Augmented Contact Model for Rigid Body Simulation

Accurately modeling contact behaviors for real-world, near-rigid materials remains a grand challenge for existing rigid-body physics simulators. This paper introduces a data-augmented contact model that incorporates analytical solutions with observed data to predict the 3D contact impulse which could result in rigid bodies bouncing, sliding or spinning in all directions. Our method enhances the expressiveness of the standard Coulomb contact model by learning the contact behaviors from the observed data, while preserving the fundamental contact constraints whenever possible. For example, a classifier is trained to approximate the transitions between static and dynamic frictions, while non-penetration constraint during collision is enforced analytically. Our method computes the aggregated effect of contact for the entire rigid body, instead of predicting the contact force for each contact point individually, removing the exponential decline in accuracy as the number of contact points increases.Comment: 7 pages, 7 figures. Submitted to ICRA 2019. Added video attachment with full 3D experiments: https://youtu.be/AKSD8TabDV

Collision Resolutions in Cloth Simulation

We present a new collision resolution scheme for cloth collisions. Our main concern is to find dynamically convincing resolutions, i.e. positions and velocities of cloth elements, for any kinds of collisions occuring in cloth simulation (cloth-cloth, cloth-rigid, and cloth-cloth-rigid). We define our cloth surface as connected faces of mass particles where each particle is controlled by its internal energy functions. Our collision resolution method finds appropriate next positions and velocities of particles by conserving the particles’ momentums as accurately as possible. Cloth-cloth collision resolution is a special case of deformable N-body collision resolution. So to solve deformable N-body collision resolutions, we propose a new collision resolution method, which groups cloth particles into parts and resolves collisions between parts using the law of momentum conservation. To resolve collisions, we solve a system of linear equations derived from the collision relationships. A system of linear equations is built using a scheme adapted from the simultaneous resolution method for rigid N-body collisions [1]. For the special case where we can find cyclic relationships in collisions, we solve a system of linear inequalities derived from the collision relationships

Numerical implementation of the exact dynamics of free rigid bodies

In this paper the exact analytical solution of the motion of a rigid body with arbitrary mass distribution is derived in the absence of forces or torques. The resulting expressions are cast into a form where the dependence of the motion on initial conditions is explicit and the equations governing the orientation of the body involve only real numbers. Based on these results, an efficient method to calculate the location and orientation of the rigid body at arbitrary times is presented. This implementation can be used to verify the accuracy of numerical integration schemes for rigid bodies, to serve as a building block for event-driven discontinuous molecular dynamics simulations of general rigid bodies, and for constructing symplectic integrators for rigid body dynamics.Comment: Shortened paper with updated references, 28 pages, 3 figure