Rigid Bodies

In a multibody system, a rigid body is an ideal representation of solid body/part of fixed size and shape in which deformation is insignificant or neglected, or in other words, the distance between any two points of a rigid body remains unchanged (irrespective of the external forces acting on it). A rigid body will have six degrees of freedom (DOF) and therefore every additional rigid body in a multibody system adds an additional six DOF to the system.

  1. If the Bodies panel is not currently displayed, select the desired body by clicking on it in the Project Browser or in the modeling window.
    The Bodies panel is automatically displayed.
  2. Select the Properties tab and specify the mass and inertia properties of the body by entering in the values in the appropriate fields.
    Note: If the selected body is a pair entity, first distinguish between the Left and Right tabs in the panel, and then edit the properties.
    • Check Flex Body (CMS) to convert a rigid body definition to a flexible/deformable body.
    • Check Symmetric properties to make the body properties symmetric. Once this option is activated, MotionView will ask you which side of the values of the pair entity (Left or Right) is to be used. Selecting any one side will make values of that side as “master” and the values of the other side will gray out and follow the values on the master side.
    • Check Get Properties from associated Graphics to get the mass and inertia properties from the graphic associated with the body. Activating this option will deactivate the Mass and Inertia properties fields, and the CM Coordinates and Inertia Coordsys tabs.
  3. Click the CM Coordinates tab.
  4. Click the Use center of mass coordinate system check box to select a point as the origin of center of mass for the body.
    Note: If a center of mass coordinate system is not specified, MotionView assumes that it coincides with the global frame of the model.
  5. Click the Point collector and select the appropriate center of mass point from the modeling window, or double-click Point to display the Model Tree (from which the desired point can be selected).
  6. Use the orientation options to orient the coordinate system.
  7. Following the same steps as the center of mass, specify the inertia coordinate system and the body coordinate system by using the Inertia Coordsys and Body Coordsys tabs respectively.
    Note: The inertia coordinate system is the reference frame with respect to which the inertia properties of the body are specified. If a unique inertia coordinate system is not specified, then the specified moment of inertia values are assumed to be defined about the center of mass marker. If a unique body coordinate system is not specified, then it is assumed to be at the global reference frame. This is valid for both single and pair body types.
  8. Define initial conditions.
    1. Click the Initial Conditions tab on the panel.
    2. Activate the desired check boxes and enter values for translational and rotational velocity.
    3. Select a marker for VM and WM.
      The translational initial conditions for a body are assumed to be with respect to the global reference frame in absence of a unique VM (linear velocity reference marker), and the rotational initial conditionals are applied about the center of mass marker in absence of a unique WM (angular velocity reference marker).
  • The inertia values are specified with regard to the inertia coordinate system.
  • MotionView does not check for the physical correctness of the properties entered in the field (for example: I1 +I2 >= I3 where I1, I2, and I3 are the principal moment of inertia of the body).
  • Specifying symmetric properties makes both pairs have the same values. Verify the correctness of data by considering the orientation of the Inertia coordinate system.
  • When the Get Properties from associated Graphic(s) option is activated:
    • The property values (mass, inertia, and CM origin) are obtained from the following type of graphics: cylinder (both ends capped), box, sphere and CADGraphic. File Graphics (H3D) is currently not supported.
    • The CM marker orientation is aligned with Global Frame. The Inertia coordinate system is deactivated, which means that the inertia properties displayed are with respect to the CM marker.