Coupled Body Motion

[Robert Steidl]

Dear Open Sim Professionals,

I do have a simple model but a huge problem! The model consists of a ground and two bodies! The first body is connected via a free joint to the ground, the second body is connected via a free joint to body one! One marker is attached to body one and via a simple trc-file I do add a motion to that marker!

Unfortunately, the second body is moving along with the motion provided by the marker attached to body one, like they are fixed and coupled! How can I get rid of this coupled motion, so that only body one is moving?

Hope you can help me soon,
greetings from Vienna - Rob

[Michael Sherman]

Hi, Robert. If you connect body2 to body1 you are saying you want body2's motion to be relative to body1. That means if you set body1's position body2 will keep the same relative position with respect to body1. That will look like motion with respect to Ground.

If you want the two bodies to be positioned independently, connect body2 to Ground by a Free joint rather than connecting it to body1.

Regards,
Sherm

[Robert Steidl]

Perfect, thank you for your answer,

I will try that step and implement the ground as the 2nd body relative! But I do have another question for interest:

"What would happen if I keep the body-connection and add a new marker to the second body?" Can I control then the 2nd Body Movement via the new attached Marker or is the joint-connection of the two bodies prior to the Marker-Movement?

Kind regards,
Rob

[Michael Sherman]

For IK either configuration would work if you have a marker on body2. The solution for the joint coordinates will be different in the two cases but the final positions of the markers should be the same either way.

I suggest you try it both ways if you can so that you can get a good feeling about how you can parameterize a system's motion in a variety of ways, all essentially equivalent but having different properties. Then for any particular application you can arrange the joints so that they provide the most convenience for you.

Sherm

[Robert Steidl]

Hello again,

at first I just want to say that it works as you suggest - if i keep the joint connection between body two and body one and add an additional marker to body 2, I can now move both indepentendly controlled by the trc-file related to marker movement!

But I have another query - let me say that the default position of body one is (0,0,0) and body two (0.5,0,0) - marker both attached to the center of the bodies! To check if the marker react to the trc-file - the first frames of the trajectories of the marker 1 is (0, 0.5, 0.5) and marker 2 is (0.5, 0.5, 0.5)! As expected, both bodies "jump" to the position provided by the marker's trc-file - unfortunately I get a rotation to the bodies too. Is that usual because the joints are 6DoF and if how can I suppress this unwanted rotation!?

Robert

[Robert Steidl]

Hello,

in addition to the rotations:

I have found a solution, that works for me now, but i do not know if that is an optimal way to solve it! I have just set the default lock to true, therefore no rotation is allowed and the translation works! Is there another way to probably fix it?

Yours, Rob

[Michael Sherman]

Rob - adding more markers would be another way to constrain rotations. - Sherm