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## Specifying Solid Geometry

SimMechanics™ provides a set of shapes that you can use to represent rigid bodies. You can specify the shapes directly in the Solid block dialog box.

### Simple Shapes

Shapes range from simple to advanced. Simple shapes require a small number of dimensional parameters. The following simple shapes are available.

• Cylinder — Cylinder with custom dimensions, centroid at the solid reference frame origin, and symmetry axis along the solid reference frame z-axis.

• Sphere — Sphere with custom dimensions and center located at the solid reference frame origin.

• Brick — Brick with custom dimensions along the three Cartesian axes and centroid located at the block reference frame.

• Ellipsoid — Ellipsoid with custom dimensions with centroid located at the block reference frame.

• Regular Extrusion — Extruded solid with constant cross-section along the z-axis and centroid located at the block reference frame. The constant cross-section is a regular polygon with a custom number of sides.

Simple shapes are easier to use than advanced shapes. When modeling a rigid body, consider using a simple shape as a first approximation. After successful model assembly, you can add detail to the rigid body. The following figure shows the four simple shapes, ordered left to right: cylinder, Sphere, Brick, and Ellipsoid.

The Regular Extrusion shape is more versatile than other simple shapes. With this shape, you can model solids with constant cross sections. Cross-sections can have any number of sides, but all lengths and internal angles are equal.

The following figure shows a set of shapes you can model with the Regular Extrusion shape.

• General Extrusion — Extruded solid with custom cross-section swept along the z-axis and centroid located at the block reference frame.

• Revolution — Solid of revolution with constant cross-section revolved about the z-axis and centroid located at the block reference frame.

The shapes require a MATLAB® cross-section matrix. To be valid, the matrix must observe a set of rules. See Cross-Section Coordinates.

#### General Extrusions

For extrusions with irregular cross-section, SimMechanics provides a General Extrusion geometry. This geometry is among the most versatile in SimMechanics. You can use it to model shapes with an increased level of detail.

This shape requires a MATLAB matrix that contains the cross-section coordinates. The matrix must follow a set of rules that are specific to the shape. See Revolution and General Extrusion Cross-Sections.

The following figure shows some shapes you can model with General Extrusion.

For General Extrusion examples, see:

#### Solids of Revolution

Solids that have a constant cross-section about an axis are solids of revolution. To model these solids, use the Revolution shape.

The Revolution shape requires a MATLAB matrix that contains the cross-section coordinates. The matrix must follow a set of rules specific to the Revolution geometry. See Revolution and General Extrusion Cross-Sections.

The following figure shows some shapes you can model with Revolution.