World

Model World represents a global coordinate system fixed in ground. This model serves several purposes:

• It is used as inertial system in which the equations of all elements of the MultiBody library are defined.
• It is the world frame of an animation window in which all elements of the MultiBody library are visualized.
• It is used to define the gravity field in which a multi-body model is present. Default is a uniform gravity field where the gravity acceleration vector g is the same at every position. Additionally, a point gravity field can be selected.
• It is used to define default settings of animation properties (e.g. the diameter of a sphere representing by default the center of mass of a body, or the diameters of the cylinders representing a revolute joint).
• It is used to define a visual representation of the world model (= 3 coordinate axes with labels) and of the defined gravity field.
  

Since the gravity field function is required from all bodies with mass and the default settings of animation properties are required from nearly every component, exactly one instance of model World needs to be present in every model on the top level. The basic declaration needs to be:

    inner Modelica.Mechanics.MultiBody.World world

Note, it must be an inner declaration with instance name world in order that this world object can be accessed from all objects in the model. When dragging the "World" object from the package browser into the diagram layer, this declaration is automatically generated (this is defined via annotations in model World).

All vectors and tensors of a mechanical system are resolved in a frame that is local to the corresponding component. Usually, if all relative joint coordinates vanish, the local frames of all components are parallel to each other, as well as to the world frame (this holds as long as a Parts.FixedRotation, component is not used). In this "reference configuration" it is therefore alternatively possible to resolve all vectors in the world frame, since all frames are parallel to each other. This is often very convenient. In order to give some visual support in such a situation, in the icon of a World instance two axes of the world frame are shown and the labels of these axes can be set via parameters.