Search found 807 matches

by Michael Sherman
Sat Oct 14, 2023 4:11 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Rotation Matrix notaion
Replies: 2
Views: 1295

Re: Rotation Matrix notaion

Hi, Amit. Good question! Terminology for transforms and rotation matrices is tricky, because they serve as both "verb" and "noun" depending on your point of view. As a verb, we have v_B=R_BG * v_G, mapping G to B (as long as you multiply from the left!). The notation ensures you get that correct bec...
by Michael Sherman
Tue Sep 05, 2023 8:07 am
Forum: OpenSim
Topic: Rolling on Surface Constraint Usage
Replies: 3
Views: 1387

Re: Rolling on Surface Constraint Usage

According to the OpenSim documentation here it uses a Simbody PointInPlane constraint for the normal contact. That is described here . I'm not sure why it wasn't implemented by a Simbody SphereOnPlaneContact constraint, but I believe it produces a similar result. Ajay Seth might have further insight...
by Michael Sherman
Mon Sep 04, 2023 8:52 pm
Forum: OpenSim
Topic: Rolling on Surface Constraint Usage
Replies: 3
Views: 1387

Re: Rolling on Surface Constraint Usage

The constraint is applied at the point of contact.
- Sherm
by Michael Sherman
Tue Aug 15, 2023 8:37 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Unstable equilibria
Replies: 3
Views: 1499

Re: Unstable equilibria

Thanks for your kind remarks -- I'm glad you're finding Simbody useful. Yes, minimizing PE only finds stable equilibria. You should be able to find everything you need in SimbodyMatterSubsystem . For example you can calculate accelerations udot directly with calcAccelerations() and you can convert t...
by Michael Sherman
Sun May 28, 2023 10:20 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Drone
Replies: 1
Views: 1276

Re: Drone

Yes, a Free (6dof) joint for the body and Pin (1dof revolute) joints for the propellers.
by Michael Sherman
Thu Apr 20, 2023 10:39 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Ragdolls
Replies: 2
Views: 1081

Re: Ragdolls

Hi, Nat. There is one ragdoll among the Simbody examples that I know of. It's called "Gazebo2Simbody" and is mostly about reading .sdf files. But the .sdf it reads by default is a ragdoll -- you can run it and see the ragdoll fall down.

Regards,
Sherm
by Michael Sherman
Fri Feb 24, 2023 11:53 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Running Simbody on Apple Silicon
Replies: 3
Views: 1637

Re: Running Simbody on Apple Silicon

Definitely possible since we don't currently build Simbody on arm64 Macs in CI. If you have access to one and are able to find the problem we would welcome a pull request so others can benefit!
by Michael Sherman
Tue Feb 21, 2023 5:05 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Running Simbody on Apple Silicon
Replies: 3
Views: 1637

Re: Running Simbody on Apple Silicon

Possible workaround: grab the latest Simbody source from GitHub and build it locally.
by Michael Sherman
Thu Jan 26, 2023 10:12 am
Forum: Simbody: SimTK multibody dynamics API
Topic: time derivative of the ball constraint
Replies: 5
Views: 5391

Re: time derivative of the ball constraint

Agreed, there is no one-size-fits-all perfect solution. In Simbody I biased my choices to modeling some physical system at all times. That allows validation based on physical laws like f=ma, conservation of energy, etc. A comment on finding feasible solutions: Simbody has an Assembler study for find...
by Michael Sherman
Wed Jan 25, 2023 12:42 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: time derivative of the ball constraint
Replies: 5
Views: 5391

Re: time derivative of the ball constraint

Another good question, thanks Kaiwen. I had to think about this one for a while since it has been a long time! This comes from Simbody's definition of the position constraint matrix P, based on the intended use in computation. You can see the definition in Doxygen here (scroll down to "Theory discus...