Bodies

Model body geometries, inertias, colors, and frames

Model the bodies of an articulated mechanical assembly. Bodies can be rigid or flexible, the latter being free to deform when acted upon by a force or torque. All bodies are characterized by their physical properties, among them geometry, inertia, and color. Flexible bodies have the additional properties of stiffness, damping, and discretization level.

Rigid bodies are based on the various solid blocks, the File Solid block, or, in special cases, their equivalents of variable mass and geometry. You can find the latter in the Body Elements > Variable Mass library. Use the File Solid block to import a body from a 3-D part file. Flexible bodies are based on the General Flexible Body block—a representation of a slender body with a specified cross section.

A single Brick Solid, File Solid, or General Flexible Beam block may suffice to completely model a body. More often, several are required. The body is then a composite of simpler body elements fixed to one another. Frame connection lines between the blocks establish the necessary rigid connections between the body elements. Rigid Transform blocks, normally inserted in the connection lines, provide the relative positions and orientations required for proper assembly.

Simscape Blocks

expand all

Body Elements

File Solid	Solid element with properties derived from external file
Graphic	Marker with graphic properties
Inertia	Mass element with fixed inertial properties
Inertia Sensor	Sensor to measure the inertial properties of body groups or mechanisms
Brick Solid	Solid brick element with geometry, inertia, and color
Cylindrical Solid	Solid cylindrical element with geometry, inertia, and color
Ellipsoidal Solid	Solid ellipsoidal element with geometry, inertia, and color
Extruded Solid	Solid extruded element with geometry, inertia, and color
Revolved Solid	Solid revolved element with geometry, inertia, and color
Spherical Solid	Solid spherical element with geometry, inertia, and color
General Flexible Beam	Slender extrusion with elastic properties for deformation
General Variable Mass	Mass element with variable inertial properties
Reduced Order Flexible Solid	Flexible body based on a reduced-order model
Variable Brick Solid	Solid brick with variable mass and size
Variable Cylindrical Solid	Solid cylinder with variable mass and size
Variable Spherical Solid	Solid sphere with variable mass and size

Curves and Surfaces

Spline

Cubic interpolating plane curve or space curve

Frames and Transforms

Reference Frame	Non-inertial reference frame
Rigid Transform	Fixed spatial relationship between frames
World Frame	Inertial reference frame

Functions

`smnew`	Open Simscape Multibody model template and block library
`sm_lib`	Open the Simscape Multibody block library

Topics

Bodies

Bodies Workflow

Bodies, the core constituents of a multibody model, comprise body elements, each in turn comprising frames and attributes. Modeling a body is more than adding a block to a model. Here are some general steps to keep in mind when modeling one.

Modeling Bodies

Bodies are representations that you create of physical parts—gears, pistons, levers—for later assembly into multibody systems, a piston engine serving as an example. Here is an introduction to bodies as well as the blocks and tools commonly used to represent one in a model.

Compounding Body Elements

A typical body is a collection of solids and other body elements. In this sense, it is a compound unit. Learn how you can use compounding to create complex geometries and inertias from simpler ones.

Geometry

Representing Solid Geometry

Geometry is a key attribute of solids and of the bodies they comprise. You can specify the shape and size of a solid using the Solid block. Here is an introduction to the types of shapes that you can specify (or import) using this block.

Modeling Extrusions and Revolutions

General Extruded Solid and Revolved Solid blocks are the most versatile of the preset solid shapes. Their cross-sections are custom and set by coordinate matrices that you specify. Here is an introduction to both the shapes and the cross-sections that define them.

Model an Excavator Dipper Arm as a Flexible Body

Use the Reduced Order Flexible Solid block to model a deformable body of arbitrary geometry. Start with the CAD geometry of the body, produce a finite-element mesh, and generate reduced-order data to use with the block.

Inertia

Representing Solid Inertia

Inertia quantifies the resistance of a body to changes in motion and is among the most important attributes in a model. Here is an introduction to the types of inertias that you can represent in a model as well as the blocks and parameterizations that you can use to do so.

Specifying Custom Inertias

"Inertia" is a general term often used to mean mass, center of mass, the moments of inertia, and the products of inertia. Learn more about these inertia parameters and how they are defined in the Simscape Multibody environment.

Specifying Variable Inertias

Not all inertias remain constant during simulation. Some can vary and it is often their changing parameters that most matter in a model. Learn more about variable inertias and how you can specify their parameters in a model.

Frames

Working with Frames

Frames are axis triads that encode the position and orientation of body elements in a body. Learn about frames as a means to connect body elements and about rigid transforms as a means to offset those elements.

Creating Connection Frames

Often, you must create new frames for use in joint and constraint connections. Learn how you can create such frames and how joints and constraints affect their placements in a body.

Creating Custom Solid Frames

All solids have a local reference frame but you can create other, custom, frames, for connection in a model. Learn how you can perform this task using the frame creation panel of the Solid block.

Visualization

Manipulate the Color of a Solid

Color helps to differentiate bodies and contributes to the effectiveness of a multibody visualization. Learn about the color parameters that you can specify and their impact on the appearance of your bodies.

Visualize a Model and Its Components

Visualization is not only a central part of a multibody simulation, it is an essential tool in modeling bodies and verifying their shapes, sizes, frame placements, and colors. Here is an overview of the visualization utilities available in the Simscape Multibody environment and the roles they play in your modeling workflow.

Featured Examples

Creating Frames Using Rigid Transforms

The correspondence of coordinate frames to connection lines and frame ports. It highlights the Rigid Transform block as the fundamental method to rigidly relate nonidentical frames. The network reference frame is the world frame, located at the center of the bottom face of the cube. The other frames are the frames at the eight vertices of the cube, one at the center of the top face, and two on the upper curved section.

Open Model

Creating a Simple Part

The first step in modeling a rigid body. This model is a simple body (brick) with a frame at each of two ends and a reference frame at the center of mass. The inertia for the body is specified as Geometric Inertia with a constant density. The block automatically computes the appropriate inertia components. This serves as a first approximation of the actual rigid body. In subsequent iterations more detail can be added to obtain a more accurate model of the actual rigid body. See the example sm_compound_body for a more detailed model of the rigid body.

Open Model

Creating a Complex Part

A moderately complex mechanical link with a frame at each end and a reference frame at the center. The link has a hole at one end and a peg at the other end. The link is a composite of three simple solids.

Open Model

Flexible Dipper Arm

Model a flexible dipper arm, such as the arm for an excavator or a backhoe, by using the Reduced Order Flexible Solid block. This block captures the mechanical behavior of a deformable body through reduced-order stiffness and mass matrices that are associated with interface frame locations. In this example, the Partial Differential Equation Toolbox™ is used to create the reduced-order model.

Open Model