Body Elements

A body is a collection of simpler body elements: solids with geometry and inertia and, less often, plain inertias (without assigned geometries) and plain geometries (without associated inertias). The body elements that you model and the ways in which you connect them determine the overall attributes of the body.

Bodies with simple shapes often require a single solid. Those with complex shapes or inertias may require several solids and the occasional plain inertia or plain geometry. Like bricks in a modular build set, the properties of the various body elements can differ from each other. It is up to you to combine them in a way that produces the desired body.

Each body element comprises one or more frames and a set of material attributes. The frames determine the placement of the body elements relative to each other and provide the attachment points for joints and constraints. The attributes factor in the dynamic behavior of the body elements and, in those with geometry, help determine their visual appearance.

Relevant Blocks

You can model most bodies using the solid blocks and Rigid Transform block. The solid blocks represent their namesake—a solid element of a certain type. The block parameters set the attributes of the solid and a frame port, labeled R, provides a reference frame for connection to a model. You can create additional, custom, frames and position them using a variety of solid features.

The Rigid Transform block represents a fixed spatial relationship between two frames. Whenever you add a Rigid Transform block to a frame connection line, you replace the coincidence relationship originally set by that line with a rotational and translational offset that you specify in the block dialog box.

Only rarely do bodies contain other blocks. Those that remain in the Body Elements library—Graphic, Inertia, and all in the Variable Mass sublibrary—serve special cases. Use them, for example, to add graphic markers, adjust inertia through compounding, or allow inertia to vary during simulation.

Body Visualization

You can visualize individual solids and complete models. The right visualization tool to use depends on which of the two you want to visualize. You can visualize individual solids directly through their respective solid blocks. In its dialog box, The solid blocks provide a visualization pane that shows the geometry, frames, and color of the solid that it represents. The visualization works even if the block diagram is incomplete or invalid.

You can visualize a model, and all the bodies within it, using the Simscape Multibody visualization utility, Mechanics Explorer. The model must be free of kinematic conflicts, such as those due to mutually incompatible joints and constraints. It must also contain one Solver Configuration block for every topologically distinct multibody network—each a group of Simscape Multibody blocks that connect without breaks in their frame connection lines.

Mechanics Explorer opens a static model visualization in its initial configuration whenever you do a diagram update (in the Debug tab, click Update Model). The initial configuration is the aggregate result of all initial joint positions and angles, whose values you can specify using joint state targets. The visualization becomes dynamic when you run a simulation, although this is a task you are unlikely to perform while still modeling bodies.

For more information about visualization, see Visualize a Model and Its Components.

See It: A Typical Body

At the MATLAB^® command prompt, enter openExample("sm/PointOnCurveExample"). A model of a flapping wing mechanism opens up. Look inside the mask of the subsystem block named RightWing. You can do this by clicking the down arrow at the bottom left corner of the block. The blocks inside the subsystem are a typical representation of a body.

The solid blocks each represent a section of the wing body. The frame connection lines between the solid blocks, and the Rigid Transform blocks that some of the connection lines contain, define the spatial relationships that exist between the solid sections. Open the dialog box of the Cylindrical Solid block named CrossPin and explore its attributes:

Boundaries of Bodies

Direct connection lines and Rigid Transform blocks unite body elements into a single body. Such connections are treated as internal to the bodies they belong to. Joint and constraint blocks in turn separate solids into different bodies. These blocks identify the boundaries of the bodies that they connect. This distinction has practical consequences in models with Gravitational Field blocks.

The figure shows a flattened portion of the PointOnCurveExample block diagram. The joint blocks namedRFJ and RWJ separate the blocks that compose the body to their right (identified by the shaded area) from the neighboring bodies to which it connects (not shown).

Bodies as Simulink Subsystems

It is common practice to enclose the blocks that belong to a body inside a Simulink Subsystem block. You can save the resulting Subsystem block in a Simulink^® library for later use in different models. The mask of the Subsystem block provides a place to define the variables and parameters that are common to the enclosed blocks—often some length, density, and color—without cluttering the workspace of a model.

You can model a body and convert it to a Simulink subsystem at any time prior to its use in a multibody model. For example, when modeling a piston engine, you can defer work on a piston body until you are ready to connect the piston to the engine housing or to the connecting rod. However, given that the shape and size of one body often depend on those of another, you should at least consider those attributes prior to focusing on assembly.