Modelica.Mechanics.MultiBody

Information

Library MultiBody is a free Modelica package providing 3-dimensional mechanical components to model in a convenient way mechanical systems, such as robots, mechanisms, vehicles. Typical animations generated with this library are shown in the next figure:

For an introduction, have especially a look at:

• MultiBody.UsersGuide discusses the most important aspects how to use this library.
• MultiBody.Examples contains examples that demonstrate the usage of this library.

Copyright © 1998-2013, Modelica Association and DLR.

This Modelica package is free software and the use is completely at your own risk; it can be redistributed and/or modified under the terms of the Modelica License 2. For license conditions (including the disclaimer of warranty) see Modelica.UsersGuide.ModelicaLicense2 or visit https://www.modelica.org/licenses/ModelicaLicense2.

Package Content

Name	Description
UsersGuide	User's Guide of MultiBody Library
World	World coordinate system + gravity field + default animation definition
Examples	Examples that demonstrate the usage of the MultiBody library
Forces	Components that exert forces and/or torques between frames
Frames	Functions to transform rotational frame quantities
Interfaces	Connectors and partial models for 3-dim. mechanical components
Joints	Components that constrain the motion between two frames
Parts	Rigid components such as bodies with mass and inertia and massless rods
Sensors	Sensors to measure variables
Visualizers	3-dimensional visual objects used for animation
Types	Constants and types with choices, especially to build menus
Icons	Icons for MultiBody package

Modelica.Mechanics.MultiBody.World

Information

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 or no gravity can be selected. Also, function gravityAcceleration can be redeclared to a user-defined function that computes the gravity acceleration, see example Examples.Elementary.UserDefinedGravityField.
• 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.

Parameters

Name	Description
enableAnimation	= true, if animation of all components is enabled
animateWorld	= true, if world coordinate system shall be visualized
animateGravity	= true, if gravity field shall be visualized (acceleration vector or field center)
label1	Label of horizontal axis in icon
label2	Label of vertical axis in icon
gravityType	Type of gravity field
g	Constant gravity acceleration [m/s2]
n	Direction of gravity resolved in world frame (gravity = g*n/length(n)) [1]
mue	Gravity field constant (default = field constant of earth) [m3/s2]
driveTrainMechanics3D	= true, if 3-dim. mechanical effects of Parts.Mounting1D/Rotor1D/BevelGear1D shall be taken into account
replaceable function gravityAcceleration	Function to compute the gravity acceleration, resolved in world frame
Animation
if animateWorld = true
axisLength	Length of world axes arrows [m]
axisDiameter	Diameter of world axes arrows [m]
axisShowLabels	= true, if labels shall be shown
axisColor_x	Color of x-arrow
axisColor_y
axisColor_z	Color of z-arrow
if animateGravity = true and gravityType = UniformGravity
gravityArrowTail[3]	Position vector from origin of world frame to arrow tail, resolved in world frame [m]
gravityArrowLength	Length of gravity arrow [m]
gravityArrowDiameter	Diameter of gravity arrow [m]
gravityArrowColor	Color of gravity arrow
if animateGravity = true and gravityType = PointGravity
gravitySphereDiameter	Diameter of sphere representing gravity center (default = mean diameter of earth) [m]
gravitySphereColor	Color of gravity sphere
Defaults
nominalLength	"Nominal" length of multi-body system [m]
defaultAxisLength	Default for length of a frame axis (but not world frame) [m]
defaultJointLength	Default for the fixed length of a shape representing a joint [m]
defaultJointWidth	Default for the fixed width of a shape representing a joint [m]
defaultForceLength	Default for the fixed length of a shape representing a force (e.g., damper) [m]
defaultForceWidth	Default for the fixed width of a shape representing a force (e.g., spring, bushing) [m]
defaultBodyDiameter	Default for diameter of sphere representing the center of mass of a body [m]
defaultWidthFraction	Default for shape width as a fraction of shape length (e.g., for Parts.FixedTranslation)
defaultArrowDiameter	Default for arrow diameter (e.g., of forces, torques, sensors) [m]
defaultFrameDiameterFraction	Default for arrow diameter of a coordinate system as a fraction of axis length
defaultSpecularCoefficient	Default reflection of ambient light (= 0: light is completely absorbed)
defaultN_to_m	Default scaling of force arrows (length = force/defaultN_to_m) [N/m]
defaultNm_to_m	Default scaling of torque arrows (length = torque/defaultNm_to_m) [N.m/m]

Connectors

Name	Description
frame_b	Coordinate system fixed in the origin of the world frame
replaceable function gravityAcceleration	Function to compute the gravity acceleration, resolved in world frame