Modelica.Mechanics.MultiBody

Library to model 3-dimensional mechanical systems

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.

This Modelica package is free software; it can be redistributed and/or modified under the terms of the Modelica license, see the license conditions and the accompanying disclaimer here.

Extends from Modelica.Icons.Library (Icon for library).

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

Types Constants and types with choices, especially to build menus

Visualizers 3-dimensional visual objects used for animation

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
Types	Constants and types with choices, especially to build menus
Visualizers	3-dimensional visual objects used for animation

Modelica.Mechanics.MultiBody.World

World coordinate system + gravity field + default animation definition

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 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.

Parameters

Type Name Default Description

Boolean enableAnimation true = true, if animation of all components is enabled

Boolean animateWorld true = true, if world coordinate system shall be visualized

Boolean animateGravity true = true, if gravity field shall be visualized (acceleration vector or field center)

AxisLabel label1 "x" Label of horizontal axis in icon

AxisLabel label2 "y" Label of vertical axis in icon

GravityTypes gravityType GravityTypes.UniformGravity Type of gravity field

Acceleration g 9.81 Constant gravity acceleration [m/s2]

Axis n {0,-1,0} Direction of gravity resolved in world frame (gravity = g*n/length(n)) [1]

Real mue 3.986e14 Gravity field constant (default = field constant of earth) [m3/s2]

Boolean driveTrainMechanics3D true = true, if 3-dim. mechanical effects of Parts.Mounting1D/Rotor1D/BevelGear1D shall be taken into account

Animation

if animateWorld = true

Distance axisLength nominalLength/2 Length of world axes arrows [m]

Distance axisDiameter axisLength/defaultFrameDiame... Diameter of world axes arrows [m]

Boolean axisShowLabels true = true, if labels shall be shown

Color axisColor_x Modelica.Mechanics.MultiBody... Color of x-arrow

Color axisColor_y axisColor_x

Color axisColor_z axisColor_x Color of z-arrow

if animateGravity = true and gravityType = UniformGravity

Position gravityArrowTail[3] {0,0,0} Position vector from origin of world frame to arrow tail, resolved in world frame [m]

Length gravityArrowLength axisLength/2 Length of gravity arrow [m]

Diameter gravityArrowDiameter gravityArrowLength/defaultWi... Diameter of gravity arrow [m]

Color gravityArrowColor {0,230,0} Color of gravity arrow

if animateGravity = true and gravityType = PointGravity

Diameter gravitySphereDiameter 12742000 Diameter of sphere representing gravity center (default = mean diameter of earth) [m]

Color gravitySphereColor {0,230,0} Color of gravity sphere

Defaults

Length nominalLength 1 "Nominal" length of multi-body system [m]

Length defaultAxisLength nominalLength/5 Default for length of a frame axis (but not world frame) [m]

Length defaultJointLength nominalLength/10 Default for the fixed length of a shape representing a joint [m]

Length defaultJointWidth nominalLength/20 Default for the fixed width of a shape representing a joint [m]

Length defaultForceLength nominalLength/10 Default for the fixed length of a shape representing a force (e.g. damper) [m]

Length defaultForceWidth nominalLength/20 Default for the fixed width of a shape represening a force (e.g. spring, bushing) [m]

Length defaultBodyDiameter nominalLength/9 Default for diameter of sphere representing the center of mass of a body [m]

Real defaultWidthFraction 20 Default for shape width as a fraction of shape length (e.g., for Parts.FixedTranslation)

Length defaultArrowDiameter nominalLength/40 Default for arrow diameter (e.g., of forces, torques, sensors) [m]

Real defaultFrameDiameterFraction 40 Default for arrow diameter of a coordinate system as a fraction of axis length

Real defaultSpecularCoefficient 0.7 Default reflection of ambient light (= 0: light is completely absorbed)

Real defaultN_to_m 1000 Default scaling of force arrows (length = force/defaultN_to_m) [N/m]

Real defaultNm_to_m 1000 Default scaling of torque arrows (length = torque/defaultNm_to_m) [N.m/m]

Type	Name	Default	Description
Boolean	enableAnimation	true	= true, if animation of all components is enabled
Boolean	animateWorld	true	= true, if world coordinate system shall be visualized
Boolean	animateGravity	true	= true, if gravity field shall be visualized (acceleration vector or field center)
AxisLabel	label1	"x"	Label of horizontal axis in icon
AxisLabel	label2	"y"	Label of vertical axis in icon
GravityTypes	gravityType	GravityTypes.UniformGravity	Type of gravity field
Acceleration	g	9.81	Constant gravity acceleration [m/s2]
Axis	n	{0,-1,0}	Direction of gravity resolved in world frame (gravity = g*n/length(n)) [1]
Real	mue	3.986e14	Gravity field constant (default = field constant of earth) [m3/s2]
Boolean	driveTrainMechanics3D	true	= true, if 3-dim. mechanical effects of Parts.Mounting1D/Rotor1D/BevelGear1D shall be taken into account
Animation
if animateWorld = true
Distance	axisLength	nominalLength/2	Length of world axes arrows [m]
Distance	axisDiameter	axisLength/defaultFrameDiame...	Diameter of world axes arrows [m]
Boolean	axisShowLabels	true	= true, if labels shall be shown
Color	axisColor_x	Modelica.Mechanics.MultiBody...	Color of x-arrow
Color	axisColor_y	axisColor_x
Color	axisColor_z	axisColor_x	Color of z-arrow
if animateGravity = true and gravityType = UniformGravity
Position	gravityArrowTail[3]	{0,0,0}	Position vector from origin of world frame to arrow tail, resolved in world frame [m]
Length	gravityArrowLength	axisLength/2	Length of gravity arrow [m]
Diameter	gravityArrowDiameter	gravityArrowLength/defaultWi...	Diameter of gravity arrow [m]
Color	gravityArrowColor	{0,230,0}	Color of gravity arrow
if animateGravity = true and gravityType = PointGravity
Diameter	gravitySphereDiameter	12742000	Diameter of sphere representing gravity center (default = mean diameter of earth) [m]
Color	gravitySphereColor	{0,230,0}	Color of gravity sphere
Defaults
Length	nominalLength	1	"Nominal" length of multi-body system [m]
Length	defaultAxisLength	nominalLength/5	Default for length of a frame axis (but not world frame) [m]
Length	defaultJointLength	nominalLength/10	Default for the fixed length of a shape representing a joint [m]
Length	defaultJointWidth	nominalLength/20	Default for the fixed width of a shape representing a joint [m]
Length	defaultForceLength	nominalLength/10	Default for the fixed length of a shape representing a force (e.g. damper) [m]
Length	defaultForceWidth	nominalLength/20	Default for the fixed width of a shape represening a force (e.g. spring, bushing) [m]
Length	defaultBodyDiameter	nominalLength/9	Default for diameter of sphere representing the center of mass of a body [m]
Real	defaultWidthFraction	20	Default for shape width as a fraction of shape length (e.g., for Parts.FixedTranslation)
Length	defaultArrowDiameter	nominalLength/40	Default for arrow diameter (e.g., of forces, torques, sensors) [m]
Real	defaultFrameDiameterFraction	40	Default for arrow diameter of a coordinate system as a fraction of axis length
Real	defaultSpecularCoefficient	0.7	Default reflection of ambient light (= 0: light is completely absorbed)
Real	defaultN_to_m	1000	Default scaling of force arrows (length = force/defaultN_to_m) [N/m]
Real	defaultNm_to_m	1000	Default scaling of torque arrows (length = torque/defaultNm_to_m) [N.m/m]

Connectors

Type Name Description

Frame_b frame_b Coordinate system fixed in the origin of the world frame

Type	Name	Description
Frame_b	frame_b	Coordinate system fixed in the origin of the world frame

Modelica definition

model World 
  "World coordinate system + gravity field + default animation definition"

  import SI = Modelica.SIunits;
  import Modelica.Mechanics.MultiBody.Types.GravityTypes;
  import Modelica.Mechanics.MultiBody.Types;

    Interfaces.Frame_b frame_b 
    "Coordinate system fixed in the origin of the world frame";

  parameter Boolean enableAnimation=true 
    "= true, if animation of all components is enabled";
  parameter Boolean animateWorld=true 
    "= true, if world coordinate system shall be visualized";
  parameter Boolean animateGravity=true 
    "= true, if gravity field shall be visualized (acceleration vector or field center)";
  parameter Types.AxisLabel label1="x" "Label of horizontal axis in icon";
  parameter Types.AxisLabel label2="y" "Label of vertical axis in icon";
  parameter Types.GravityTypes gravityType=GravityTypes.UniformGravity 
    "Type of gravity field";
  parameter SI.Acceleration g=9.81 "Constant gravity acceleration";
  parameter Types.Axis n={0,-1,0} 
    "Direction of gravity resolved in world frame (gravity = g*n/length(n))";
  parameter Real mue(
    unit="m3/s2",
    min=0) = 3.986e14 
    "Gravity field constant (default = field constant of earth)";
  parameter Boolean driveTrainMechanics3D=true 
    "= true, if 3-dim. mechanical effects of Parts.Mounting1D/Rotor1D/BevelGear1D shall be taken into account";

  parameter SI.Distance axisLength=nominalLength/2 
    "Length of world axes arrows";
  parameter SI.Distance axisDiameter=axisLength/defaultFrameDiameterFraction 
    "Diameter of world axes arrows";
  parameter Boolean axisShowLabels=true "= true, if labels shall be shown";
  input Types.Color axisColor_x=Modelica.Mechanics.MultiBody.Types.Defaults.FrameColor 
    "Color of x-arrow";
  input Types.Color axisColor_y=axisColor_x;
  input Types.Color axisColor_z=axisColor_x "Color of z-arrow";

  parameter SI.Position gravityArrowTail[3]={0,0,0} 
    "Position vector from origin of world frame to arrow tail, resolved in world frame";
  parameter SI.Length gravityArrowLength=axisLength/2 "Length of gravity arrow";
  parameter SI.Diameter gravityArrowDiameter=gravityArrowLength/
      defaultWidthFraction "Diameter of gravity arrow";
  input Types.Color gravityArrowColor={0,230,0} "Color of gravity arrow";
  parameter SI.Diameter gravitySphereDiameter=12742000 
    "Diameter of sphere representing gravity center (default = mean diameter of earth)";
  input Types.Color gravitySphereColor={0,230,0} "Color of gravity sphere";

  parameter SI.Length nominalLength=1 "\"Nominal\" length of multi-body system";
  parameter SI.Length defaultAxisLength=nominalLength/5 
    "Default for length of a frame axis (but not world frame)";
  parameter SI.Length defaultJointLength=nominalLength/10 
    "Default for the fixed length of a shape representing a joint";
  parameter SI.Length defaultJointWidth=nominalLength/20 
    "Default for the fixed width of a shape representing a joint";
  parameter SI.Length defaultForceLength=nominalLength/10 
    "Default for the fixed length of a shape representing a force (e.g. damper)";
  parameter SI.Length defaultForceWidth=nominalLength/20 
    "Default for the fixed width of a shape represening a force (e.g. spring, bushing)";
  parameter SI.Length defaultBodyDiameter=nominalLength/9 
    "Default for diameter of sphere representing the center of mass of a body";
  parameter Real defaultWidthFraction=20 
    "Default for shape width as a fraction of shape length (e.g., for Parts.FixedTranslation)";
  parameter SI.Length defaultArrowDiameter=nominalLength/40 
    "Default for arrow diameter (e.g., of forces, torques, sensors)";
  parameter Real defaultFrameDiameterFraction=40 
    "Default for arrow diameter of a coordinate system as a fraction of axis length";
  parameter Real defaultSpecularCoefficient(min=0) = 0.7 
    "Default reflection of ambient light (= 0: light is completely absorbed)";
  parameter Real defaultN_to_m(unit="N/m", min=0) = 1000 
    "Default scaling of force arrows (length = force/defaultN_to_m)";
  parameter Real defaultNm_to_m(unit="N.m/m", min=0) = 1000 
    "Default scaling of torque arrows (length = torque/defaultNm_to_m)";

  /* The World object can only use the Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape model, but no
     other models in package Modelica.Mechanics.MultiBody.Visualizers, since the other models access
     data of the "outer Modelica.Mechanics.MultiBody.World world" object, i.e., there are
     mutually dependent classes. For this reason, the higher level visualization
     objects cannot be used.
  */
protected 
  parameter Integer ndim=if enableAnimation and animateWorld then 1 else 0;
  parameter Integer ndim2=if enableAnimation and animateWorld and 
      axisShowLabels then 1 else 0;

  // Parameters to define axes
  parameter SI.Length headLength=min(axisLength, axisDiameter*Types.Defaults.
      FrameHeadLengthFraction);
  parameter SI.Length headWidth=axisDiameter*Types.Defaults.
      FrameHeadWidthFraction;
  parameter SI.Length lineLength=max(0, axisLength - headLength);
  parameter SI.Length lineWidth=axisDiameter;

  // Parameters to define axes labels
  parameter SI.Length scaledLabel=Modelica.Mechanics.MultiBody.Types.Defaults.FrameLabelHeightFraction*
      axisDiameter;
  parameter SI.Length labelStart=1.05*axisLength;

  // x-axis
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape x_arrowLine(
    shapeType="cylinder",
    length=lineLength,
    width=lineWidth,
    height=lineWidth,
    lengthDirection={1,0,0},
    widthDirection={0,1,0},
    color=axisColor_x,
    specularCoefficient=0) if enableAnimation and animateWorld;
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape x_arrowHead(
    shapeType="cone",
    length=headLength,
    width=headWidth,
    height=headWidth,
    lengthDirection={1,0,0},
    widthDirection={0,1,0},
    color=axisColor_x,
    r={lineLength,0,0},
    specularCoefficient=0) if enableAnimation and animateWorld;
  Modelica.Mechanics.MultiBody.Visualizers.Internal.Lines x_label(
    lines=scaledLabel*{[0, 0; 1, 1],[0, 1; 1, 0]},
    diameter=axisDiameter,
    color=axisColor_x,
    r_lines={labelStart,0,0},
    n_x={1,0,0},
    n_y={0,1,0},
    specularCoefficient=0) if enableAnimation and animateWorld and axisShowLabels;

  // y-axis
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape y_arrowLine(
    shapeType="cylinder",
    length=lineLength,
    width=lineWidth,
    height=lineWidth,
    lengthDirection={0,1,0},
    widthDirection={1,0,0},
    color=axisColor_y,
    specularCoefficient=0) if enableAnimation and animateWorld;
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape y_arrowHead(
    shapeType="cone",
    length=headLength,
    width=headWidth,
    height=headWidth,
    lengthDirection={0,1,0},
    widthDirection={1,0,0},
    color=axisColor_y,
    r={0,lineLength,0},
    specularCoefficient=0) if enableAnimation and animateWorld;
  Modelica.Mechanics.MultiBody.Visualizers.Internal.Lines y_label(
    lines=scaledLabel*{[0, 0; 1, 1.5],[0, 1.5; 0.5, 0.75]},
    diameter=axisDiameter,
    color=axisColor_y,
    r_lines={0,labelStart,0},
    n_x={0,1,0},
    n_y={-1,0,0},
    specularCoefficient=0) if enableAnimation and animateWorld and axisShowLabels;

  // z-axis
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape z_arrowLine(
    shapeType="cylinder",
    length=lineLength,
    width=lineWidth,
    height=lineWidth,
    lengthDirection={0,0,1},
    widthDirection={0,1,0},
    color=axisColor_z,
    specularCoefficient=0) if enableAnimation and animateWorld;
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape z_arrowHead(
    shapeType="cone",
    length=headLength,
    width=headWidth,
    height=headWidth,
    lengthDirection={0,0,1},
    widthDirection={0,1,0},
    color=axisColor_z,
    r={0,0,lineLength},
    specularCoefficient=0) if enableAnimation and animateWorld;
  Modelica.Mechanics.MultiBody.Visualizers.Internal.Lines z_label(
    lines=scaledLabel*{[0, 0; 1, 0],[0, 1; 1, 1],[0, 1; 1, 0]},
    diameter=axisDiameter,
    color=axisColor_z,
    r_lines={0,0,labelStart},
    n_x={0,0,1},
    n_y={0,1,0},
    specularCoefficient=0) if enableAnimation and animateWorld and axisShowLabels;

  // Uniform gravity visualization
  parameter SI.Length gravityHeadLength=min(gravityArrowLength,
      gravityArrowDiameter*Types.Defaults.ArrowHeadLengthFraction);
  parameter SI.Length gravityHeadWidth=gravityArrowDiameter*Types.Defaults.ArrowHeadWidthFraction;
  parameter SI.Length gravityLineLength=max(0, gravityArrowLength - gravityHeadLength);
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape gravityArrowLine(
    shapeType="cylinder",
    length=gravityLineLength,
    width=gravityArrowDiameter,
    height=gravityArrowDiameter,
    lengthDirection=n,
    widthDirection={0,1,0},
    color=gravityArrowColor,
    r_shape=gravityArrowTail,
    specularCoefficient=0) if enableAnimation and animateGravity and gravityType == GravityTypes.UniformGravity;
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape gravityArrowHead(
    shapeType="cone",
    length=gravityHeadLength,
    width=gravityHeadWidth,
    height=gravityHeadWidth,
    lengthDirection=n,
    widthDirection={0,1,0},
    color=gravityArrowColor,
    r_shape=gravityArrowTail + Modelica.Math.Vectors.normalize(
                                                n)*gravityLineLength,
    specularCoefficient=0) if enableAnimation and animateGravity and gravityType == GravityTypes.UniformGravity;

  // Point gravity visualization
  parameter Integer ndim_pointGravity=if enableAnimation and animateGravity
       and gravityType == 2 then 1 else 0;
  Modelica.Mechanics.MultiBody.Visualizers.Advanced.Shape gravitySphere(
    shapeType="sphere",
    r_shape={-gravitySphereDiameter/2,0,0},
    lengthDirection={1,0,0},
    length=gravitySphereDiameter,
    width=gravitySphereDiameter,
    height=gravitySphereDiameter,
    color=gravitySphereColor,
    specularCoefficient=0) if enableAnimation and animateGravity and gravityType == GravityTypes.PointGravity;

  function gravityAcceleration = gravityAccelerationTypes (
      gravityType=gravityType,
      g=g*Modelica.Math.Vectors.normalize(
                                     n),
      mue=mue);

protected 
  function gravityAccelerationTypes 
    "Gravity field acceleration depending on field type and position"
    import Modelica.Mechanics.MultiBody.Types.GravityTypes;
    extends Modelica.Icons.Function;
    input SI.Position r[3] 
      "Position vector from world frame to actual point, resolved in world frame";
    input GravityTypes gravityType "Type of gravity field";
    input SI.Acceleration g[3] 
      "Constant gravity acceleration, resolved in world frame, if gravityType=1";
    input Real mue(unit="m3/s2") 
      "Field constant of point gravity field, if gravityType=2";
    output SI.Acceleration gravity[3] 
      "Gravity acceleration at point r, resolved in world frame";
  algorithm 
    gravity := if gravityType == GravityTypes.UniformGravity then g else 
               if gravityType == GravityTypes.PointGravity then 
                  -(mue/(r*r))*(r/Modelica.Math.Vectors.length(
                                                r)) else 
                    zeros(3);
  end gravityAccelerationTypes;
equation 
  Connections.root(frame_b.R);

  assert(Modelica.Math.Vectors.length(
                       n) > 1.e-10,
    "Parameter n of World object is wrong (lenght(n) > 0 required)");
  frame_b.r_0 = zeros(3);
  frame_b.R = Frames.nullRotation();
end World;

Modelica.Mechanics.MultiBody.World.gravityAccelerationTypes

Gravity field acceleration depending on field type and position

Information

Extends from Modelica.Icons.Function (Icon for a function).

Inputs

Type Name Default Description

Position r[3] Position vector from world frame to actual point, resolved in world frame [m]

GravityTypes gravityType Type of gravity field

Acceleration g[3] Constant gravity acceleration, resolved in world frame, if gravityType=1 [m/s2]

Real mue Field constant of point gravity field, if gravityType=2 [m3/s2]

Type	Name	Description
Position	r[3]	Position vector from world frame to actual point, resolved in world frame [m]
GravityTypes	gravityType	Type of gravity field
Acceleration	g[3]	Constant gravity acceleration, resolved in world frame, if gravityType=1 [m/s2]
Real	mue	Field constant of point gravity field, if gravityType=2 [m3/s2]

Outputs

Type Name Description

Acceleration gravity[3] Gravity acceleration at point r, resolved in world frame [m/s2]

Type	Name	Description
Acceleration	gravity[3]	Gravity acceleration at point r, resolved in world frame [m/s2]

Modelica definition

function gravityAccelerationTypes 
  "Gravity field acceleration depending on field type and position"
  import Modelica.Mechanics.MultiBody.Types.GravityTypes;
  extends Modelica.Icons.Function;
  input SI.Position r[3] 
    "Position vector from world frame to actual point, resolved in world frame";
  input GravityTypes gravityType "Type of gravity field";
  input SI.Acceleration g[3] 
    "Constant gravity acceleration, resolved in world frame, if gravityType=1";
  input Real mue(unit="m3/s2") 
    "Field constant of point gravity field, if gravityType=2";
  output SI.Acceleration gravity[3] 
    "Gravity acceleration at point r, resolved in world frame";
algorithm 
  gravity := if gravityType == GravityTypes.UniformGravity then g else 
             if gravityType == GravityTypes.PointGravity then 
                -(mue/(r*r))*(r/Modelica.Math.Vectors.length(
                                              r)) else 
                  zeros(3);
end gravityAccelerationTypes;

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