stiffness in a joint

[Roger Mathys]

Hi,

I want to give a certain stiffness to a Custom Joint about one rotational axis. How can I do this?

Thanks for your help

Roger

[Roger Mathys]

I am actually looking for a torsional spring where the resistance to rotate a body away from it's normal position depends on the angular displacement. I found a class 'torsional spring' but with the information given in the documentation I am not able to implement it in my XML document. For example, where and how can I specify the axis about which the torsional spring acts?

Thanks for any help...

Roger

[Ayman Habib]

Roger,

Did you check the Bushing Force? I'm aware of a few other OpenSim users who used it to model the back/neck. It also has better documentation. The torsional springs in OpenSim were used for some specific purpose a while ago and will likely be rewritten..

Good luck and please let me know if you have questions,
-Ayman

[Roger Mathys]

Hi Ayman,

Unfortunately I have no idea about bushing force. Could you help me how to implement it or where to find documentation about.

So far I tried a GeneralizedSpringForce with certain stiffness but this is actually not what I am looking for.

Thanks a lot...

Roger

[Ayman Habib]

Hi Roger,

The distribution should have the Docygen documentation for the class (although it may not have the XML representation). Please follow the links under the installation directory/opensim/API/doc/index.html and look for BushingForce. According to the documentation:

* A Bushing Force is the force proportional to the deviation of two frames. One can think of the Bushing as being composed of 3 linear and 3 torsional spring-dampers, which act along or about the bushing frames. The underlying Force in Simbody is a SimtK::Force::LinearBushing

An example XML representation looks like this:
<BushingForce name = "L_Bushing">
<body_1> pelvis </body_1>
<body_2>lumbar5</body_2>
<location_body_1>0.0 0.0 0.0</location_body_1>
<orientation_body_1>0.0 0.0 0.0</orientation_body_1>
<location_body_2>0.0 0.0 0.0</location_body_2>
<orientation_body_2>0.0 0.0 0.0</orientation_body_2>
<rotational_stiffness>10.0 20.0 30.0</rotational_stiffness>
<translational_stiffness>0.0 0.0 0.0</translational_stiffness>
<rotational_damping>0.0 0.0 0.0</rotational_damping>
<translational_damping>0.0 0.0 0.0</translational_damping>
</BushingForce>

Hope this helps,
-Ayman

[Roger Mathys]

Thank you very much!

Exactly what I was looking for!

Roger

[Anne Schmitz]

Use a Generalized Spring Force. This can be about any generalized coordinate, including rotational ones.