How to model a free multibody in simmechanics?

Hi!

We got a trial version of simMechanics at our departement to compare it to Adams and other simulation software. So no real experience with the software here...

I modeled a simple 2-body 1 revolute joint mechanism which is free to move in the plane (no gravity or gravity perpendicular to the plane). I think the model created by the SolidWork plugin is right, as far as I understand simMechanics (see mdl attached), but the simulations results aren't good. Since both links are identical it should 'flap' the two of them around the joint without moving the mass center. In this case the mechanism starts translating in 'y' quite a lot.

thanks you.

[David St-Onge]

I think I found the solution. It seems my motion input weren't good for simulation. using transfer function to derive the position as in: http://blogs.mathworks.com/seth/2012/02/01/applying-motion-to-simmechanics-models/ and it worked.

Thank you.

Answer 1

[Sebastian Castro]

Did you modify the Mechanism Configuration block included?

When you import from CAD, the block is automatically set to have a gravitational acceleration of -9.81 m/s^2 in the y (or z) direction. If you haven't already, you should try changing that vector to [0 0 0] (so, no gravity).

- Sebastian

Answer 2

[David St-Onge]

Thank you for your answer. I did change the acceleretion to [0,0,0]. Same results.

[Sebastian Castro]

It's likely numerical integration errors building up, then.

If you go to Simulation > Model Configuration Parameters and go to the Solver pane , try reducing the Max step size, Relative Tolerance, and Absolute Tolerance parameters.

This will likely help reduce the drift, but may not give perfect results since it's all numerical integration and computers have their limits too. Try it out, though!

- Sebastian