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Constraint-Joint Mapping in SimMechanics First Generation

In Autodesk® Inventor®, unconstrained parts have six mechanical degrees of freedom (DoFs) that describe how the parts move with respect to each other. Of the six degrees of freedom, three are rotational and three are translational. Applying a constraint between two parts eliminates degrees of freedom between the two parts. Constraints can remove between zero and six degrees of freedom.

Degrees of Freedom in SimMechanics

SimMechanics™ First Generation assigns zero degrees of freedom to an unconstrained rigid body. Connecting the rigid body to a joint or constraint block increases the mechanical degrees of freedom available to the rigid body.

Rigid Body ConditionFirst-Generation DoF
Not connected to joints, constraints, or World Frame0
Connected to Joints or Constraints blocksAdd degrees of freedom according to joint or constraint

CAD Constraint – SimMechanics Joint Mapping

During CAD export, SimMechanics Link maps Inventor constraints between parts to SimMechanics joints between rigid bodies. CAD constraints and SimMechanics joints do not follow a one-to-one correspondence — multiple constraints can map into a single joint. All SimMechanics joints contain a combination of three joint primitives: Prismatic, Revolute, and Spherical. The Weld Joint block contains zero joint primitives, and therefore zero degrees of freedom. The following table identifies the degrees of freedom of each joint primitive.

PrimitiveAbbreviationMotion TypeNumber of DoFs
PrismaticPTranslational1
RevoluteRRotational1
SphericalSRotational3

Supported Constraint Entity

Depending on the constraint combination, SimMechanics Link utility supports the following Inventor constraint entities:

Entity Description
Circle/ArcCircular edge/arc sketch segment*
Ellipse/ArcElliptical edge/arc sketch segment*
ConeConical face
Cylinder Cylindrical face
Line Linear edge/sketch segment/reference axis
Plane Reference plane or planar face
Point Vertex/sketch point/reference point

* A complete circle or ellipse is a special case of a circular or elliptical arc.

Supported Constraint Entity Combinations

The following sections list the constraint-entity combinations that SimMechanics Link supports for different constraint types.

    Note:   If the SimMechanics Link exporter cannot translate a constraint–constraint entity combination into a supported SimMechanics joint with DoFs, it converts the combination into a weld (W) primitive.

Coincident Constraint

The following table identifies supported constraint-entity combinations for the Coincident constraint. A ✓ indicates the combination is supported.

Constraint-Entity 2
PointLinePlaneCylinderConeCircle/Arc
Constraint-Entity 1Point     
Line    
Plane   
Cylinder   
Cone   
Circle/Arc  

Concentric Constraint

The following table identifies supported constraint-entity combinations for the Concentric constraint. A ✓ indicates the combination is supported.

Constraint Entity 2
PointLinePlaneCylinderConeCircle/Arc
Constraint Entity 1Point      
Line    
Plane      
Cylinder   
Cone  
Circle/Arc  

Distance Constraint

The following table identifies supported constraint-entity combinations for the Distance constraint. A ✓ indicates the combination is supported.

Constraint Entity 2
PointLinePlaneCylinderConeCircle/Arc
Constraint Entity 1Point    
Line     
Plane   
Cylinder      
Cone      
Circle/Arc      

Angle Constraint

The following table identifies supported constraint-entity combinations for the Angle constraint. A ✓ indicates the combination is supported.

Constraint Entity 2
PointLinePlaneCylinderConeCircle/Arc
Constraint Entity 1Point      
Line     
Plane     
Cylinder      
Cone      
Circle/Arc      

Supported SimMechanics Joints

The SimMechanics Link utility supports the following SimMechanics joint-primitive combinations.

Primitive CombinationSimMechanics Block
PPrismatic
PPIn-Plane
PPPCustom Joint
PPPRCustom Joint
SSpherical
R-SRevolute-Spherical
RRevolute
PRCylindrical
PPRPlanar
PPPSSix-DoF
R-RRevolute-Revolute
S-SSpherical-Spherical
WWeld

Tips for Specific Constraints

  • The point-point coincident constraint maps onto a spherical joint.

  • The point-point distance constraint maps onto a spherical-spherical massless connector.

Limitations

The following limitation applies to CAD export from Inventor.

Weld is Default Joint

If the SimMechanics Link utility fails to translate a CAD constraint, a Weld joint replaces the constraint.

Restriction on Point-Point Distance Mate

For SimMechanics Link to successfully map the CAD point-point distance constraint onto a SimMechanics spherical-spherical massless connector, the constraint must not connect to any other constraint.

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