How to adjust/position joint axes in an exo/skate?

[adamkewley]

This question was ported from akewley@tudelft.nl, some details are REDACTED because it was initially a private email.

I am writing to seek your expertise and assistance regarding the use of your "OpenSim creator" program for integrating 'klapschaatsen' into the existing 'Rajapogal' musculoskeletal model. I have encountered a few challenges.

My primary issue pertains to the construction of the klapskates. These skates consist of two bodies connected by a hinge (pinjoint). After adding the 2 bodies and pinjoint to my musculoskeletal model, I am unable to adjust the axis of rotation. Consequently, the hinge movement is occurring along the wrong axis, as depicted in the attached video (REDACTED). When I attempt an alternative approach by importing the bodies as meshes (mesh importer), I can select the axis of rotation, but I don't know how to 'connect' this skate.osim to the mskmodel.osim.

Furthermore, when I export a model in OpenSim Creator to a .osim file, I am unable to open this file in my OpenSim program. I suspect there may be an issue with the export process/settings/mismatch in versions? Have you come across this problem before?

Lastly, I am trying to make a compelling video of the skating simulation and export this video without background. I am struggling to keep the camera locked on the subject/skater during recording, as the subject tends to leave the screen. Ajay mentioned that this functionality existed in older OpenSim versions but appears to have disappeared in recent updates. He suggested OpenSim Creator still has the ability to lock the camera on the subject, but I haven't been able to locate it.

[adamkewley]

Adjusting the axis of a joint can be tricky to do in OpenSim because all children of that joint will then move (they're expressed w.r.t. the parent of the joint). This is to say, you can adjust the joint's axis--as you desire--by selecting the parent frame of the joint and then adjusting that frame (assuming that the parent frame is an adjustable offset frame). If using a PinJoint, the Z direction of the parent frame of that joint should be where you want the joint to pivot. However, adjusting a parent frame like this is also going to move everything else - probably not what you want.

I think where this gets tricky (and, rightly, very confusing for users) is when you have mesh data that is not aligned the same way as the joints you're trying to add. E.g. if your 'klapschaatsen' mesh file is designed such that the X axis is the pivot, you will find that you run into all kinds of "fun" issues when you attach that mesh file to the child (moving) part of your PinJoint in OpenSim. This is because the PinJoint will always (effectively) force the Z axis of the child frame (and, via the attachment, of your mesh) to be the thing that's pivoting.

So, I'd keep that in mind and design around it:

• Always have offset frames on either side of a Joint (I think this is OSC's default, these days, when adding a body). This lets you adjust the pivot location/direction of a Joint.
• Adjusting a PinJoint's parent offset frame effectively adjusts the position/direction of the PinJoint.
• Adjusting a PinJoint's child offset frame effectively adjusts the position/direction of the thing that is being mobilized by the joint.
• One of the easiest ways to align meshes to joints is to design the mesh with OpenSim in mind. Open your skate mesh in a mesh editor (e.g. Blender) and translate the whole thing such that (0, 0, 0) is equal to your joint's pivot point. Rotate the whole thing so that Z in the mesh file is the pivot axis. When this mesh is attached to any frame in the OpenSim model (incl. a joint child), it will already be aligned correctly.
• Alternatively, use the mesh importer, which lets you move things around free-form, but automatically does the necessary maths (installing transforms, rotations, offset frames, etc.) to render a valid .osim file at the end - however, this produces a new model that needs to be merged (as you've experienced). The reason the mesh importer was developed was to side-step these joint issues with new models.
  • Because the mesh importer leaves you with just one working part (the skate), you will then need to attach the skate to the existing model. The only way to merge two model files is to manually copy the XML from one model file into the other - and to ensure that the merged bit (your skate) has a relevant joint to the existing model (i.e. you copy the joint+body+mesh into Rajagopal and then change the joint's parent from Ground/ground_offset to 'ankle', or similar, to attach it to the Rajagopal foot).

"I am unable to open this file in my OpenSim program"

Insufficient information for me to help, given that I don't know the version of either piece of software (the OSC version you're using looks over a year old), and I don't know what you mean by "unable to open". If you mean "it opens, but I can't see my skate", then I would guess that the mesh file paths haven't been updated to create a portable osim file. The mesh importer creates absolute paths to mesh files (C:\Users\Adam\Desktop\Mesh.obj) because users have a habit of importing meshes from all kinds of locations on their computer.

When sending the osim to other computers, the mesh files need to be gathered up into a Geometry/ folder, the paths need to be updated, and the osim+Geometry/ folder need to be sent together (your email only supplies the osim, so I can't see your skates) - this is legacy behavior from OpenSim.