SimMechanics

Modeling Bodies

The basic components of any machine are its constituent rigid bodies. In SimMechanics, the term body refers to any point or spatially extended object that has mass. SimMechanics bodies, unlike physical bodies, do not have degrees of freedom. The SimMechanics Bodies library contains two blocks for representing bodies in a Simulink model:

• Ground

• Models a point on an ideal body of infinite mass and extent that serves as a fixed environment for machines (see Modeling Grounds following).

• Body

• Models rigid bodies of finite mass and extent, including their attached body coordinate systems (see Modeling Finite Rigid Bodies following).

Modeling Grounds

In SimMechanics, ground refers to a body of infinite mass and size that acts both as a reference frame at rest for a system as a whole and as a fixed base for attaching system components, e.g., the factory floor on which a robot stands. SimMechanics Ground blocks enable you to represent points on ground in your model. This in turn enables you to specify the degrees of freedom that your system has relative to its environment. You do this by connecting Joint blocks representing degrees of freedom between the Body blocks representing parts of your machine and the Ground blocks representing ground points.

Each Ground block has a single Connector Port to which you can connect a Joint block that can in turn be connected to a single Body block. Each Ground block therefore allows you to represent the degrees of freedom between a single part of your model and its environment. If you want to specify the motion of other parts of your machine relative to the environment, you must create additional Ground blocks.

Note    To be valid, a SimMechanics model must contain at least one Ground block connected to a Body block via a Joint block.

The World Coordinate System

SimMechanics contains an internal master coordinate system and reference frame called World. All grounds are at rest in World. The Connector Port of each Ground block defines a grounded coordinate system called GND. The GND coordinate system's axes are parallel to World. By default the origin of the grounded coordinate system coincides with the origin of the World coordinate system. The Grounded point field of a Ground block's dialog box allows you to move the origin of GND to some other point in the World coordinate system, as in the example Building a Simple Pendulum.

The GND coordinate system allows you to specify the positions and motions of parts of your machine relative to fixed points in the environment.

Modeling Machines

Modeling Finite Rigid Bodies