Redundancies in COSMOSMotion 2008 – Part I

July 18th, 2008

I taught Motion 2008 this week to a group of engineers from one of the larger accounts that we handle in Cincinnati, Ohio. As you might have gathered from my previous post, Motion 2008 has a new revamped interface that makes the tool easier to use. Yet, the ubiquitous problem with rigid body kinematics and dynamics – Redundancies, continues to haunt the user and make them think twice about integrating simulation directly into their conceptual stages.

BACKGROUND: What are redundancies?

Apart from a single-word definition (nuisance!!), redundancies can be understood easier by taking note of an example. Consider the door to your room that is mounted to the frame by three hinges. From Newton’s falling apple theory, the weight of the door because of gravity is opposed by equally distributed reaction forces in the opposite direction at all the three hinges. Now, if we remove the two lower hinges and let the door be mounted just with the top hinge, two things happen:

  1. The opposing reaction force is seen at the top hinge, and is equal and opposite to the weight of the door. This leads to the hinge material (aluminum/steel) bending, and ultimately deforming/breaking because of its inability to carry the moment acting about the center of mass of the door.
  2. The person mounting that hinge is rushed to the ER!!

The same event happening in COSMOSMotion would give the ultimately desired effect of the door standing upright without any problem! The reason for this is that COSMOSMotion considers treats every part in the assembly as rigid bodies, and every joint as a rigid joint. In other words, the hinge being used is not made of a material with a finite stiffness, but has infinite stiffness and can carry any load by itself without any deformation! What a pain – since the door motion can now be simulated by just one hinge, and the other two hinges become redundant!!!

Thus, the output in COSMOSMotion would show the opposing reaction force to be equal and opposite to the weight of the door on the top hinge, and show zero reaction force on the other two hinges!!

THE PROBLEM: What does a user do when his SolidWorks model has three hinges?

We know that any part held in 3-D space has six degrees of freedom – three rotations and three translations. When you create mates in an assembly, you are removing degrees of freedom from this part. In other words, when you mate the door to one hinge, you ensure that the only degree of freedom remaining for the door is the rotation about the axis of the hinge. The other five degrees of freedom are constrained by the mates created in SolidWorks.

Redundancies are created if the same five degrees of freedom for the door are repeatedly constrained along the same axis (even if at different locations, such as the other two co-axial hinges). In other words, if you have constrained all five degrees of freedom at one hinge, then you cannot re-account for the same at the other locations.

(To be Continued…Part II : The Solution)

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2 Responses to “Redundancies in COSMOSMotion 2008 – Part I”

  1. Larry says:

    dear Vikram,
    I am having some problems with cosmos motion analysis. I tried to ignore the redundancies but the motion analysis kept giving me the same results for different materials.

    Please how can i reduce this redundacies and also how can i define the joints as well as the moving parts.

    Your response will highly be appreciated.



  2. Larry,

    Redundancies are a complicated phenomenon and is a by-product of the fact that this is rigid body kinematics/dynamics. All the joints between bodies are infinitely stiff and hence duplicate themselves unless resolved. For instance, a door can be supported just by one hinge in Motion as against 3-4 hinges in real life, because that one hinge is rigid enough to withstand the inertial weight of the door.

    Redundancies can lead to wrong answers, and hence it is important for you to fix them. One way to fix them would be to replace the redundant joint with a bushing (edit the corresponding mate and assign a bushing in the analysis tab of the mate). The other way would be to delete the mates, and re-create mates between all the parts while taking precautions on not duplicating constraints for parts if they have been constrained in a specific direction already by a previously created mate. A model with no redundancies will lead to a correct solution.

    Do consult your SolidWorks reseller to see if they can give you some personal assistance on the files that you are working on. They might even offer a Motion training class that specifically has a lesson on Redundancies and how to eliminate them.


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