SimMechanics

Representing the Bodies

The information that you use to specify body properties in a SimMechanics model is enough to represent each body in certain special, simplified ways. SimMechanics does not have the information about the bodies needed to represent their full geometries.

Equivalent Ellipsoids

A rigid body has a unique equivalent ellipsoid, a homogeneous solid ellipsoid with the same inertia tensor. For more about this representation figure for rigid bodies, see Rendering Body Shapes in SimMechanics.

Because the rod has an axis of symmetry, the x-axis in this case, two of its three generalized radii are equal: a_y = a_z. The generalized radii of the equivalent ellipsoid are a_x = = 0.646 m and a_y = a_z = = 1.12 cm.

Convex Hulls

Every Body has at least one Body coordinate system (CS) at the CG. A Body also has one or more extra Body CSs for the attached Joints, as well as possible Actuators and Sensors. Each Body CS has an origin point, and the collection of all these points, in general, defines a volume in space. The minimum outward-bending surface enclosing such a volume is the convex hull of the Body CSs, and this is the alternative way that SimMechanics has to represent a body.

You created the pendulum body with only two Body CSs, CG and CS1. The convex hull for the pendulum rod is thus the minimum connecting figure, a line joining the two Body CS origins.

Choosing the Body Representation

You can choose which displayed representation of the pendulum rod or any machine bodies SimMechanics uses, by configuring the Visualization tab in the Mechanical Environment Settings dialog box. In the Represent bodies as menu, choose Convex hulls or Equivalent ellipsoids.

Continue to keep the Mechanical Environment Settings dialog box open.

Visualizing a Simple Pendulum

Visualizing with Handle Graphics