Joint Stiffness

[Moe Curtin]

I am interested in adding stiffness properties to joints, i.e. how much force to apply to a joint in order to move it a certain distance. Is there a way to set this up in the osim file?

[Patrick Riley]

Related question, I think. How is the stiffness of the range-of-motion constraint set?

[Ajay Seth]

There are no stiffness properties associated with a joint. A joint represents the kinematic relationship between two bodies. Stiffness and viscosity are properties of forces such as torsional spring-dampers. You can write a force plugin to be a joint torque that is a function of joint angle to resist deflection and it can be used as a way to enforce limits on the joint range of motion. We are in the process of providing a PassiveJointTorque to do this that can be added to the ForceSet.

It also sounds like you want the stiffness to be a time varying input to drive the joint to a particular "desired" state/position, then perhaps what you want is a torque actuator where the torsional stiffness value is its control. Then you need something (a controller) to determine the stiffness input and that gets a bit more complicated. But with the API you can write a controller as well.

There is documentation on creating your own actuator and controller from the last Jamboree:
https://simtk.org/docman/?group_id=442

[Patrick Riley]

Are the range of motion limits, then absolutely rigid? I seem to recall in SIMM, they were enforced by a stiff spring.