Monday, August 23, 2010

How to simulate rolling with CATIA DMU Kinematics?

Slide 1
A reader recently asked if I could put together a tutorial for the kinematics rolling simulation that I created. This may not be extremely detailed but should give you a pretty good idea of the steps to take when trying to simulate rolling in CATIA. This was one of the preliminary tests I ran before attempting to simulate my entire roller coaster model.


(Download the rolling in kinematocs tutorial here)

Every time you use kinematics something needs to be fixed, so the rest of the assembly moves in relation to the fixed part. In this case, the track will be fixed. Set the assembly up in assembly design then switch to the DMU Kinematics workbench when you are ready to begin simulating. You can see the constraints I used in the screen shot. It is very important to pay attention to the degrees of freedom of the system. When dealing with large assemblies, I often fully constrain the entire assembly and then go back in later and delete the constraints where I want a part to be able to move, in order to not be confused. 

Slide 2
Here are the joints I used to create the connected wheel rolling kinematics simulation.  To create these constraints, click “Assembly Constraints Conversion” wizard.  Create a new mechanism then click “Auto Create” and it will automatically create joints based on the constraints in the assembly. Next, you need to manually create the roll curve joints between the wheels and the track.
 Slide 3
In order to create a roll curve joint you must have two curves that are tangent to one another. Here, you can see the sketch on the wheels is tangent to the sketch on the corner of the track. Double click on one of the roll curve joints and toggle the “Length Driven” option. This is your input. After you click ok you should get a message that reads “The machanism can now be simulated” and the degrees of freedom of the system  (DOF) should be zero.
Slide 4
In order to apply gravity to your simulation you need to create a sketch  called a “trace law.” This is essentially a graph of distance which respect to time, with the horizontal axis being time and the vertical axis being distance. A straight line gives a linear relationship, or a constant speed. Change the graph to an exponential curve and you now have acceleration.

Now you should be able to simulate two connected wheels rolling down a hill with gravity. Any questions?


2 comments:

  1. Worked perfectly! Now I just need to play with the law a bit :)
    Thank you.

    ReplyDelete
  2. link is dead :(

    ReplyDelete

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