WOLFRAM SYSTEMMODELER
UniversalSphericalUniversal  spherical joint aggregation (1 constraint, no potential states) 
SystemModel["Modelica.Mechanics.MultiBody.Joints.UniversalSpherical"]
This information is part of the Modelica Standard Library maintained by the Modelica Association.
This component consists of a universal joint at frame_a and a spherical joint at frame_b that are connected together with a rigid rod, see default animation figure (the arrows are not part of the default animation):
This joint aggregation has no mass and no inertia and introduces the constraint that the distance between the origin of frame_a and the origin of frame_b is constant (= Frames.length(rRod_ia)). The universal joint is defined in the following way:
The definition of axis 2 of the universal joint is performed according to the most often occurring case. In a future release, axis 2 might be explicitly definable via a parameter. However, the treatment is much more complicated and the number of operations is considerably higher, if axis 2 is not orthogonal to axis 1 and to the connecting rod.
Note, there is a singularity when axis 1 and the connecting rod are parallel to other. Therefore, if possible n1_a should be selected in such a way that it is perpendicular to rRod_ia in the initial configuration (i.e., the distance to the singularity is as large as possible).
An additional frame_ia is present. It is fixed in the connecting rod at the origin of frame_a. The placement of frame_ia on the rod is implicitly defined by the universal joint (frame_a and frame_ia coincide when the angles of the two revolute joints of the universal joint are zero) and by parameter vector rRod_ia, the position vector from the origin of frame_a to the origin of frame_b, resolved in frame_ia.
The easiest way to define the parameters of this joint is by moving the MultiBody system in a reference configuration where all frames of all components are parallel to other (alternatively, at least frame_a and frame_ia of the UniversalSpherical joint should be parallel to other when defining an instance of this component). Since frame_a and frame_ia are parallel to other, vector rRod_ia from frame_a to frame_b resolved in frame_ia can be resolved in frame_a (or the world frame, if all frames are parallel to other).
This joint aggregation can be used in cases where in reality a rod with spherical joints at end are present. Such a system has an additional degree of freedom to rotate the rod along its axis. In practice this rotation is usually of no interest and is mathematically removed by replacing one of the spherical joints by a universal joint. Still, in most cases the Joints.SphericalSpherical joint aggregation can be used instead of the UniversalSpherical joint since the rod is animated and its mass properties are approximated by a point mass in the middle of the rod. The SphericalSpherical joint has the advantage that it does not have a singular configuration.
In the public interface of the UniversalSpherical joint, the following (final) parameters are provided:
parameter Real rodLength(unit="m") "Length of rod"; parameter Real eRod_ia[3] "Unit vector along rod, resolved in frame_ia"; parameter Real e2_ia [3] "Unit vector along axis 2, resolved in frame_ia";
This allows a more convenient definition of data which is related to the rod. For example, if a box shall be connected at frame_ia directing from the origin of frame_a to the middle of the rod, this might be defined as:
Modelica.Mechanics.MultiBody.Joints.UniversalSpherical jointUS(rRod_ia={1.2, 1, 0.2}); Modelica.Mechanics.MultiBody.Visualizers.FixedShape shape(shapeType = "box", lengthDirection = jointUS.eRod_ia, widthDirection = jointUS.e2_ia, length = jointUS.rodLength/2, width = jointUS.rodLength/10); equation connect(jointUS.frame_ia, shape.frame_a);
frame_a 
Type: Frame_a Description: Coordinate system fixed to the component with one cutforce and cuttorque 


frame_b 
Type: Frame_b Description: Coordinate system fixed to the component with one cutforce and cuttorque 

frame_ia 
Type: Frame_a Description: Coordinate system at the origin of frame_a, fixed at the rod connecting the universal with the spherical joint 
animation 
Value: true Type: Boolean Description: = true, if animation shall be enabled 

showUniversalAxes 
Value: true Type: Boolean Description: = true, if universal joint shall be visualized with two cylinders, otherwise with a sphere (provided animation=true) 
computeRodLength 
Value: false Type: Boolean Description: = true, if distance between frame_a and frame_b shall be computed during initialization (see info) 
n1_a 
Value: {0, 0, 1} Type: Axis () Description: Axis 1 of universal joint resolved in frame_a (axis 2 is orthogonal to axis 1 and to rod) 
rRod_ia 
Value: {1, 0, 0} Type: Position[3] (m) Description: Vector from origin of frame_a to origin of frame_b, resolved in frame_ia (if computeRodLength=true, rRod_ia is only an axis vector along the connecting rod) 
sphereDiameter 
Value: world.defaultJointLength Type: Diameter (m) Description: Diameter of spheres representing the universal and the spherical joint 
rodShapeType 
Value: "cylinder" Type: ShapeType Description: Shape type of rod connecting the universal and the spherical joint 
rodWidth 
Value: sphereDiameter / Types.Defaults.JointRodDiameterFraction Type: Distance (m) Description: Width of rod shape in direction of axis 2 of universal joint. 
rodHeight 
Value: rodWidth Type: Distance (m) Description: Height of rod shape in direction that is orthogonal to rod and to axis 2 
rodExtra 
Value: 0.0 Type: ShapeExtra Description: Additional parameter depending on rodShapeType 
cylinderLength 
Value: world.defaultJointLength Type: Distance (m) Description: Length of cylinders representing the two universal joint axes 
cylinderDiameter 
Value: world.defaultJointWidth Type: Distance (m) Description: Diameter of cylinders representing the two universal joint axes 
kinematicConstraint 
Value: true Type: Boolean Description: = false, if no constraint shall be defined, due to analytically solving a kinematic loop 
checkTotalPower 
Value: false Type: Boolean Description: = true, if total power flowing into this component shall be determined (must be zero) 
rodLength 
Value: Type: Distance (m) Description: Length of rod (distance between origin of frame_a and origin of frame_b) 
eRod_ia 
Value: Modelica.Math.Vectors.normalizeWithAssert(rRod_ia) Type: Real[3] () Description: Unit vector from origin of frame_a to origin of frame_b, resolved in frame_ia 
e2_ia 
Value: Modelica.Math.Vectors.normalize(cross(n1_a, eRod_ia)) Type: Real[3] () Description: Unit vector in direction of axis 2 of universal joint, resolved in frame_ia (orthogonal to n1_a and eRod_ia; note: frame_ia is parallel to frame_a when the universal joint angles are zero) 
e3_ia 
Value: cross(eRod_ia, e2_ia) Type: Real[3] () Description: Unit vector perpendicular to eRod_ia and e2_ia, resolved in frame_ia 
world 
Type: World Description: 


R_rel_ia1 
Type: Orientation Description: 

R_rel_ia2 
Type: Orientation Description: 

R_rel_ia 
Type: Orientation Description: Rotation from frame_a to frame_ia 

rodShape 
Type: Shape Description: 

sphericalShape_b 
Type: Shape Description: 

sphericalShape_a 
Type: Shape Description: 

universalShape1 
Type: Shape Description: 

universalShape2 
Type: Shape Description: 
One kinematic loop with four bars (with UniversalSpherical joint; 1 nonlinear equation) 