What is the purpose of CreateAnyGeneralForce6DOF?

[Taejun001]

Hi, I find that you use CreateAnyGeneralForce6DOF to build some interaction between exos and humans. I want to know the purpose of these forces. Is it the force to attach the exoskeleton to the user's body? Besides, could you give me some hints about how you chose the value of it. For example, in the following figure, strength_Fy = 0 while others equal to 1000. Best， TJ

[melund]

Hi Taejun

I am not an expert on exoskeleton design, but I think I can speculate why it is done like this.

If something is rigidly attached to the body you could just have created a reaction force between the two segments. But a reaction force is infinitely strong, so the system will not care how much force they take up. In some cases it may result in some unwanted behavior of the model.

However, the class template (CreateAnyGeneralForce6DOF) creates the attachment between the two segment using recruited actuators. These behave like the virtual muscles in the model, so they become part of the recruitment algorithm. So the system will try to minimize their activation.

How big the penalty for the activation is depends on the strength of the those actuators. In your example the strength is set high (1000N), which means that the contact acts almost like a reaction force.

Maybe @divyaksh-chander can better elaborate a bit on why it smart to use this approach.

[divyaksh-chander]

Hi Taejun,

Further to the comments from Morten, I would like to add a few points:

• Using reaction forces would eventually lead you to the problem that the additional reaction forces that you introduce in the model must be equal to the additional degrees of freedom in the model. Then, you would not be able to add reaction forces in 6DOF at each interface as you would exceed the DOFs of the exoskeleton. So, CreateAnyGeneralForce6DOF uses the recruited actuators to provide the 6DOF interaction forces at the interface. And as you may know, these are minimized in the recruitment algorithm and more importantly they are not limited by the number of DOFs in the model, just as there are many physiological muscles in the human body.
• Of course, on the other hand, you do have the issue of how to set the parameters, e.g., strength of the recruited actuators in this case. The guideline is to keep the strength large enough (a few kN) so that the activation of the recruited actuator is very low (maybe less than 1%).
• I believe strength_Fy has been set to 0 so that the human-exoskeleton interface cannot generate axial force. In this rigid body exoskeleton, you would expect the exoskeleton to bear the axial loads and not through the human-exo interface.

I hope this answers your questions. Best Divyaksh

[Taejun001]

Thanks for everything you have done！Best wishes!