Creating a Simple Mechanism

DMU Kinematics Simulator

Creating a Simple Mechanism

This macro shows you how to create a very simple mechanism, including joint, fixed part and commands, so that you can simulate it afterwards. The macro assumes that a CATProduct has been loaded, with 2 parts, each containing a line object .

After running the macro, the document contains a mechanism, with one joint, one fixed part, and two commands.

CAAKiiMechanismCreation is launched in CATIA [1]. You need to previously open the provided document.

CAAKiiMechanismCreation.CATScript is located in the CAAScdKiiUseCases module. Execute macro (Windows only).

CAAKiiMechanismCreation includes five steps:

Prolog
Creation of the mechanism
Creation of the fixed part
Creation of the joint
Creation of the commands

Prolog

Some constants are initialized :

oRootProduct: it represents the root of the product graph.

oFixedProduct: one of the parts of the assembly. It is initialized with the first child of the root product through RootProduct.Product.Item(1).

Two references are created using a string describing the path from the root product to a geometric object. The syntax for references is described in the Referenceobject documentation.

Creation of the mechanism

cTheMechanisms: it represents the mechanism collection for this product. This collection is retrieved using the GetTechnologicalObject method, with the string "Mechanisms",

The Add method is used on the cTheMechanisms collection to create the oNewMechanism object.

Some properties and side methods for collections are demonstrated at this level: Count, Item.

Creation of the fixed part

A product is used to define the fixed part of the mechanism, using the FixedPart property.

Creation of the joint

Creating a joint is done by using the AddJoint method on the mechanism. The method requires a string representing the type of joint to be created, and an array of References to geometric objects to be used to locate the joint.

The following table summarizes the strings and geometric objects to be used for the supported joint types.

Note that for example "Line (1)" means that the geometry type must be a line and must belong to the 1st product of the joint.

Joint type	Degree of freedom	Type string	Reference.1	Reference.2	Reference.3	Reference.4	Reference.5	Reference.6	Reference.7	Reference.8
Prismatic	1T	CATKinPrismaticJoint	Line (1)	Line (2)	Plane (1)	Plane (2)	-	-	-	-
Revolute	1R	CATKinRevoluteJoint	Line (1)	Line (2)	Plane (1)	Plane (2)	-	-	-	-
Rigid	-	CATKinRigidJoint	Product (1)	Product (2)	-	-	-	-	-	-
Spherical	3R	CATKinSphericalJoint	Point (1)	Point (2)	-	-	-	-	-	-
PointCurve	3R+1T	CATKinPointCurveJoint	Point (1)	Curve (2)	-	-	-	-	-	-
RollCurve	1R+1T	CATKinRollCurveJoint	Curve (1)	Curve (2)	-	-	-	-	-	-
SlideCurve	1R+1T	CATKinSlideCurveJoint	Curve (1)	Curve (2)	-	-	-	-	-	-
Cylindrical	1R+1T	CATKinCylindricalJoint	Line (1)	Line (2)	-	-	-	-	-	-
Screw	1T or 1R	CATKinScrewJoint	Line (1)	Line (2)	-	-	-	-	-	-
Universal	3R	CATKinUJoint	Line (1)	Line (2)	Line (1 or 2), optional	-	-	-	-	-
CVJoint	5R	CATKinCVJoint	Line (1)	Line (2)	Line (3)	-	-	-	-	-
PointSurface	3R+2T	CATKinPointSurfaceJoint	Point (1)	Surface (2)	-	-	-	-	-	-
Planar	1R+2T	CATKinPlanarSurfaceJoint	Plane (1)	Plane (2)	-	-	-	-	-	-
Gear	2R (coupled)	CATKinGearJoint	Line (1)	Line (3)	Plane (1)	Plane (3)	Line (2)	Line (3)	Plane (2)	Plane (3)
Rack	1R + 1T (coupled)	CATKinRackJoint	Line (1)	Line (3)	Plane (1)	Plane (3)	Line (2)	Line (3)	Plane (2)	Plane (3)
Cable	2T (coupled)	CATKinCableJoint	Line (1)	Line (3)	Plane (1)	Plane (3)	Line (2)	Line (3)	Plane (2)	Plane (3)

Notes :

for Prismatic Joint : Plane 1 must contain Line 1 and Plane 2 must contain Line 2

for Revolute Joint : Line 1 must be normal to Plane 1 and Line 2 must be normal to Plane2

for Universal Joint : Connector 3 is the cross-pin axis. It must belong to product 1 or 2 and must be orthogonal to Line (1) or Line (2). it is optional.

for CV Joint : the angle between Line (1) and Line (2) must be the same as the angle between Line (2) and Line (3)

for Gear Joint : it is seen as a Revolute joint between Product 1 and Product 3, and a Revolute joint between Product 2 and Product 3

for Rack Joint : it is seen as a Prismatic joint between Product 1 and Product 3, and a Revolute Joint between Product 2 and Product 3

for Cable Joint : it is seen as a Prismatic joint between Product 1 and Product 3, and a Prismatic Joint between Product 2 and Product 3

The NbJoints property of the Mechanism allows to get the count of joints.

Creation of the commands

In the example, a single cylindrical joint is created. Two commands are required to simulate this mechanism.

The type name for commands is MechanismCommand.

Creating a command is done by using the AddCommand method on the mechanism. The method requires a string representing the type of command to be created, and the joint that must be driven.

The following table summarizes the strings and joints objects to be used for the supported command types:

Joint type	CATKinAngleCommand	CATKinLengthCommand
Revolute	YES	-
Prismatic	-	YES
Spherical	-	-
Cylindrical	YES	-
Planar	-	-
Rigid	-	-
PointCurve	-	YES
RollCurve	-	YES
SlideCurve	-	-

The NbCommands property of the mechanism allows to get the count of commands.

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In Short

This use case has shown how to create a very simple mechanism, so that it can be simulated. Specifically, it has illustrated how to:

retrieve the mechanisms' collection
create a mechanism, its joints, commands and fixed part
analyze structural properties of the mechanism

You can derive a standard mechanism creation for any mechanism structure from this use case plus some knowledge on wireframe and/or knowledgeware macro resources.

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References

[1]	Replaying a Macro
[2]	DMU Kinematics Automation Objects

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