Bodies¶

There are two main types of bodies that exist in a GraspIt! simulation world: static bodies (also known as obstacles) and dynamic bodies (such as robot links and objects). Static objects do not participate in the dynamics, but provide collision surfaces for dynamic bodies. Note that this difference mainly applies when the dynamic engine is being used; otherwise, dynamic bodies can be used as static bodies as well. This document describes what makes up a basic GraspIt! body, and what a dynamic body adds to that definition.

Every GraspIt! body has the following data associated with it:

Geometry:

this describes the shape of the body. It is stored as an Inventor scene graph, a format similar to VRML. This structure can contain pure shape nodes such as cubes, spheres, cylinders, and cones, as well as sets of 2D polygons that define a surface. Although GraspIt! (through the Inventor scene graph) can display all of these geometry types, the collision detection system only works with triangles. When any body is imported to a GraspIt! world, it is faceted into triangles, and these are used for detecting collisions and finding contact points. The units in these files are assumed to be millimeters. Note that the origin of a body’s coordinate system is the origin of its geometry, as loaded from a file or created by the user. This can be a tricky aspect, as the origin of a body’s geometry is fairly arbitrary. To counter this, dynamic bodies also use the notion of center of mass, explained below. However, the origin of a body’s coordinate system is always the origin of it’s geometry.

Material:

the material affects the amount of friction possible at contacts on this body. For each pair of materials we define a coefficient of friction. When the dynamic engine is not used, GraspIt! uses a static coefficient of friction for all bodies. When not using dynamics, this coefficient only affects grasp quality computations, not the relative motion of the bodies. During the dynamic simulation, the coefficient of friction affects the relative motion of the bodies in contact. We also use a dynamic coefficient: if the relative speed at a contact point exceeds a threshold currently set at 1mm/sec, a kinetic coefficient of friction is used.

Transform:

each body keeps track of the 3D position and orientation of its body frame relative to the GraspIt! world coordinate system.

Name:

a body’s name is currently derived from the its filename, except in the case robot links, which are named using the robot’s name and their kinematic chain and link numbers.

Dynamic bodies also have the following properties:

Mass:

this is expressed in grams.

Center of mass:

this is a 3D position expressed relative to the body coordinate system. There are two main uses for this. The first one is to provide a stable point for grasp quality computations to use as reference. The second one is to be used as a reference point for transformation between forces and torques in dynamics. It should be more stable than the origin of the body’s coordinate system, which is arbitrary.

Inertia tensor:

this is the standard 3x3 mass distribution matrix. It is expressed relative to a coordinate frame that is aligned with the coordinate system of the body, but positioned at the center of mass. When stored in a file, it is scaled by 1/mass so that changes to the mass can be made by changing only the mass value above.

Dynamic state:

two values, q and v, store the current position and velocity of the body’s center of mass relative to world coordinates. q is expressed as a 7x1 vector: the first three values are the position, and the last four are the rotation in quaternion form. v is expressed as a 6x1 vector: the first three values are the linear velocity of the body, and the last three are the rotational velocity. When the dynamics updates each body state, the body transform is also updated. If a body is moved in the static mode, the position value of the dynamic state is also updated.

<?xml version="1.0" ?>
<root>
    <material>glass</material>
    <mass>300</mass>
    <cog>0 0 0</cog>
    <inertia_matrix>4853.0 -1.1196 -6.5156 -1.1196 4853.0 47.542 -6.5156 0.0 2357.6</inertia_matrix>
    <geometryFile type="Inventor">flask.iv</geometryFile>
</root>