Space Of Contact Forces Research Articles

Investigating the nature of discontinuous shear thickening: Beyond a mean-field description

Dense suspensions can undergo a dramatic increase in viscosity at a critical value of the shear stress. This phenomenon, termed discontinuous shear thickening (DST), has been attributed to an increase in the fraction of particle interactions becoming frictional with increasing shear stress, and a successful mean-field theory has been developed to explain various accompanying rheological properties. On a microscopic scale, however, conventional structural analysis measures such as the grain-position pair correlation function show no significant changes with the onset of DST, though recent work has shown that similar analysis in the dual space of contact forces does lead to marked changes at this transition. Furthermore, experimental results have suggested the existence of higher-order microscopic correlations and the importance of incorporating fluctuations away from a mean-field description. To this end, we use a higher-order cluster analysis tool to study the force networks obtained from simulations of dense suspensions to construct an effective interaction potential in force space. We show that there are significant changes occurring in this potential as a function of density and stress close to DST. We discuss the implications of these observations on an emergent field theory of the DST transition.

Modeling of combined slip and finite sliding at spherical grain contacts

The present paper is aimed at developing the analytical description of the interaction of two contacting spheres for several classes of slip and sliding trajectories, typical in the experimental testing. The analysis accounts for memory effects in the slip regime and configurational effects in the sliding regime, expressed in terms of an active loading surface and memory surfaces within the space of contact forces. Analytical relations for contact response are derived for linear and piecewise-linear motion trajectories of the sphere. The problem of multiple contact interaction of the sphere moving over the regularly packed granular bed is also considered analytically. It is demonstrated that the dual contact activation-separation processes occur within the combined slip–sliding modes, essentially affecting the distribution of contact tractions. The results obtained are relevant for the class of contact problems requiring analysis of interaction of slip and sliding displacements, in particular in testing grain contact interaction aimed at specification of elastic, frictional and wear parameters.

A bond failure criterion for DEM simulations of cemented geomaterials considering variable bond thickness

SUMMARYA series of micromechanical tests were conducted to investigate the bond failure criterion of bonded granules considering the effect of bond thickness, with the aim of enhancing the bond contact model used in the distinct element simulations of cemented geomaterials. The granules were idealized in a two‐dimensional context as one pair of aluminum rods bonded by resin epoxy or cement. The mechanical responses of nearly 500 rod pairs were tested under different loading paths to attain the yield loads of bonded granules at variable bond thickness. This study leads to a generic bond failure criterion incorporating the effect of the bond thickness. The results show that the bond compressive resistance largely decreases with increasing bond thickness owing to the presence of the confinement at the bond‐particle interface. The strength envelopes obtained from the combined shear compression tests and combined torsion compression tests have identical functional form, and they decrease in size with increasing bond thickness but remain unchanged in shape. Given the same cementation material, the generic bond strength envelope in a three‐dimensional contact force space under different loading paths remains the same in shape but shrinks with the increase of bond thickness. Copyright © 2014 John Wiley & Sons, Ltd.

On the analysis of force closure and force optimization in fixtures and robotic manipulation

This paper deals with mechanical systems with passive contact elements, such as whole-hand grasps and fixture systems. The force closure problem in fixtures and manipulation systems with hybrid (active and passive) contacts is shown to be quite different from the pure active force closure in a multi-fingered robot hand. We discuss the conditions of active and passive force closure problems in such devices based on a rigid-body model with frictional contact. The contact force space is decomposed into three orthogonal subspaces: the space of external forces, the space of active internal forces, and the space of passive internal forces. The space of active internal force and the space of passive internal force play important roles in achieving the closure grasping. This paper presents an algorithm for computing the basis of each subspace. Based on this force decomposition, the force closure problem is formulated as linear matrix inequality feasibility problems, and minimization of the joint torque is also studied.

Force Attractor in Confined Comminution of Granular Materials

We reveal a novel attractor in the space of contact forces that bounds the behavior of granular materials during confined comminution. The attractor is reached asymptotically as the porosity reduces and the grain size distribution attains an ultimate power law scaling. The ultimate distribution of the contact forces follows a clear log-normal distribution, distinctively different from previous observations in uncrushable systems. Supporting evidence comes both from comprehensive discrete element simulations and a theoretical Apollonian model.

Extent of force indeterminacy in packings of frictional rigid disks

Static packings of frictional rigid particles are investigated by means of discrete element simulations. We explore the ensemble of allowed force realizations in the space of contact forces for a given packing structure. We estimate the extent of force indeterminacy with different methods. The indeterminacy exhibits a nonmonotonic dependence on the interparticle friction coefficient. We verify directly that larger force indeterminacy is accompanied by a more robust behavior against local perturbations. We also investigate the local indeterminacy of individual contact forces. The probability distribution of local indeterminacy changes its shape depending on friction. We find that local indeterminacy tends to be larger on force chains for intermediate friction. This correlation disappears in the large friction limit.

A parametric approach to numerical modeling of TKR contact forces

In vivo knee contact forces are difficult to determine using numerical methods because there are more unknown forces than equilibrium equations available. We developed parametric methods for computing contact forces across the knee joint during the stance phase of level walking. Three-dimensional contact forces were calculated at two points of contact between the tibia and the femur, one on the lateral aspect of the tibial plateau, and one on the medial side. Muscle activations were parametrically varied over their physiologic range resulting in a solution space of contact forces. The obtained solution space was reasonably small and the resulting force pattern compared well to a previous model from the literature for kinematics and external kinetics from the same patient. Peak forces of the parametric model and the previous model were similar for the first half of the stance phase, but differed for the second half. The previous model did not take into account the transverse external moment about the knee and could not calculate muscle activation levels. Ultimately, the parametric model will result in more accurate contact force inputs for total knee simulators, as current inputs are not generally based on kinematics and kinetics inputs from TKR patients.

Robot multiple contact control

SUMMARYThis paper addresses the problem of contact force control for multiple contacts distributed over multiple links in a robot. This is of importance when performing complex tasks in unstructured environment, particularly in humanoid robot applications. The proposed multicontact control framework provides a new way of defining the operational space coordinates, which facilitates the specification of multiple contact control. The contact force space on multiple links is constructed as an operational space for the highest priority task. Motion control, given lower priority, can be executed using the rest of degree of freedom within the null-space of the force control. The dynamic control structure, then, provides a means to control each contact force and motion independently. This dynamic decoupling enables each contact force controller to utilize linear control theories. In particular, the contact force controllers adopt full state feedback control and estimation methods to produce robust performance with respect to modeling and parameter uncertainties. The effectiveness of the multiple contact control framework was demonstrated using a PUMA560 manipulator, with multiple contacts on the end-effector and third link. The demonstrated tasks involved controlling each of the contact forces with null-space motion.

Force Indeterminacy in the Jammed State of Hard Disks

Granular packings of hard disks are investigated by means of contact dynamics which is an appropriate technique to explore the allowed force realizations in the space of contact forces. Configurations are generated for given friction coefficients, and then an ensemble of equilibrium forces is found for fixed contacts. We study the force fluctuations within this ensemble. In the limit of zero friction, the fluctuations vanish in accordance with the isostaticity of the packing. The magnitude of the fluctuations has a nonmonotonous friction dependence. The increase for small friction can be attributed to the opening of the angle of the Coulomb cone, while the decrease as friction increases is due to the reduction of connectivity of the contact network, leading to local, independent clusters of indeterminacy. We discuss the relevance of indeterminacy to packings of deformable particles and to the mechanical response properties.

Classification of grasps by robot hands

Grasping robotic hands is classified into three categories based on the object connectivity. We decompose the space of contact forces into four subspaces and develop a method to determine the dimensions of the subspaces with respect to the connectivity of the grasped object. The relationships we obtain reveal the kinematic and static characteristics of three categories of grasps. It indicates how contact forces can be decomposed corresponding to each type of grasp. The technique also provides a guideline for determining the distribution of contact forces on grasped objects. We analyze how power grasps are identified from the object connectivity and used to synthesize hand configurations for grasping and manipulation tasks. A physical interpretation of the subspaces and the determination of their dimensions are illustrated by examples.

A force-closure test for soft multi-fingered grasps

This paper deals with the problem of force-closure analysis for soft multi-fingered grasps. The first step is the study of the relationship between the external wrench space and the manipulation force space at any contact. Constraint force set, strictly constraint force set and normal force set are defined in the contact force space, followed by an investigation of their relationships. Based on the convexity of the friction constraints for soft finger contact, the necessary and sufficient conditions for force-closure grasps are derived. Accordingly an efficient algorithm for testing force-closure is presented. Some illustrative examples are given.

Multiple whole-limb manipulation: An analysis in the force domain

It is expected that future applications of robotic manipulators will require the skilled use of multiple mechanical systems interacting with the environment in more flexible and powerful manners than those allowed by a single device equipped with a standard end effector. In fact, these multiple systems (e.g. multi-fingered hands, legged robots, cooperating manipulators) can interact with the environment not only by a specific part of their bodies, i.e. the gripper, but more generally with any part of their external surfaces. In this paper, the problem of the analysis of the force systems acting in static conditions in such mechanical devices is considered. The goals of the analysis are the following: to understand which sets of forces, belonging to the joint torque space, the contact force space, and the external wrench space, are present or applicable to/by the system, to know in which manner forces in these sets can be obtained, and to extend to this class of manipulators well known definitions of manipulability measures. The solutions to these problems, crucial in prob]ems of grasp optimization and control, are not trivial in case of limited mobility (of some) of the manipulators present in the system, a common situation when multi-arm cooperating systems are considered. A technique is proposed for the solution of these problems, and examples are presented and discussed.

不完全な接触をもつ把持の力学的多面凸解析

Contacts with insufficient degrees of freedom are defined as defective contacts. Although defective contacts are found in the power-grasp and the enveloping grasp of a multifingered robot hand, the mechanical characteristics are not yet studied in depth. In this paper, an approach to the mechanical analysis of grasps with defective contacts is presented. The relations among joint torque space, contact force space, friction cone, admissible external force space are made clear. By using the convex analysis, we represent these sets in the external or internal representation of polyhedral convex set. Finally, the validity of our approach is illustrated by a numerical example.

Dynamic force simulator for multifinger force display

In this paper, the authors present a dynamic force simulator (DFS) for force feedback in human-machine systems. They propose a virtual world model with two force flows: one is the force flow from human to an object, the other is the force flow from an object to human. To use this model, the DFS simulates object dynamics, contact models, and friction characteristics of the human hand interacting with the object in virtual reality. After the derivation of kinematic and force relations between hand and object space, they balance the two forces: one from the human and the other from the object in the contact force space in virtual world and then realize the adequate feedback forces to human operator. Interaction with the DFS allows calculation and feedback of appropriate forces to the force controlled actuators of the sensor glove they have developed. In this paper grasping of a cylinder in the virtual world is presented. During object grasping, they measure joint angles and torques using the sensor glove system. In the future, they will use this system to analyze human dextrous manipulations called human skill.