Is it possible to define 2 joints on single body?

[Mahshida hamid]

I am trying to attach a walker on an OpenSim model and as such I need to attach both the left and right hands to the handle of the walker. i tried various methods but it isn't working.

[Aaron Fox]

Hi Mahshida,

I recall someone having a similar problem on this forum (it might have been Bas van Hooren) when trying to model the connection between two hands to a barbell for weight lifting exercises; and I think the problem in doing this is that it 'closes' the loop in the model joint definition tree. I'm not sure what the end solution was, but I'm pretty sure what you're proposing (joining both hands to the walker frame) isn't possible. One solution I recall being talked about was to have the object (i.e. walker frame) as two bodies and join these with a weld joint, and then you can attach your hands to each side of the frame individually. I think you still might run into an issue with closing the joint definition tree with this though. I think you get a similar problem when trying to join feet to pedals when simulating cycling, and have seen people address this issue using constraints rather than joints.

Aaron

[Mohammadreza Rezaie]

Hi Mahshida, thanks for reaching out to me via LinkedIn. I worked on your files, and this is what I'm thinking in your case. Please note that I'm not an expert.

1) separated your walker geometry into two parts (right and left geometries),
2) simulated 90 deg wrist pronation,
3) attached each hand to each walker part using a weld join.
4) converted the walker parts into one rigid body using a weld constraint.

This is not perfect. You may want to adjust the translations/orientations. Now, it's working, if you change the arm, elbow, or hand coordinates, you will see how it moves. You may get assembly solver error. If IK tool couldn't find a good solution, you may need to add more DoF to the joints and/or unclamping/increasing the RoM. Also, don't forget to adjust the mass and inertial properties of the walker parts.

Mahshida.zip

(211.31 KiB) Downloaded 39 times

Also, this software may help build your models efficiently: https://opensimcreator.com/

Hope this helps.
Mohammadreza

[Mahshida hamid]

Thanks a lot rezza