SimMechanics

The World Coordinate System and Gravity

Before you configure a Ground block, you need to understand SimMechanics' internally defined fixed or "absolute" coordinate system (CS) called World. The World CS sits at rest in the inertial reference frame also called World. The World CS has an origin (0,0,0) and a triad of right-handed, orthogonal coordinate axes.

The default World coordinate axes are defined so that

• +x points right

  +y points up (gravity in -y direction)

  +z points out of the screen, in three dimensions

The vertical direction or up-and-down is determined by the gravity vector direction (acceleration g) relative to the World axes. Gravity is a background property of a model that you can reset before starting a simulation, but does not dynamically change during a simulation.

See Mechanical Environment Settings Dialog Box in Running Mechanical Models for displaying global mechanical properties of SimMechanics models.

Configuring a Ground Block

World serves as the single absolute CS that defines all other CSs. But you can create additional ground points at rest in World, at positions other than the World origin, by using Ground blocks. Ground blocks, representing ground points, play a dynamical role in machine models. They function as immobile bodies.

Minimum Ground Block    Every machine model must have at least one Ground block.

Figure 2-1: A Ground Point Relative to World

Steps to Configuring the Ground Block

Now place a fixed ground point at position (3,4,5) in the World CS:

1. In the SimMechanics library, open the Bodies library.
2. Drag and drop a Ground block from the Bodies library into the model window. Close the Bodies library.
3. Open the Ground block dialog box. Into the Grounded point/Location [x y z] field, enter the vector [3 4 5]. Click OK to close the dialog box.

   

Properties of Grounds

At every ground point, a Grounded CS is automatically created:

• The origin of each Grounded CS is the ground point itself.
• The Grounded CS axes are always fixed to be parallel to the World CS axes, as shown in Figure 2-1.
• You can use these Grounded CSs in setting up later Body CSs.

Building a Simple Pendulum

Configuring a Body Block