Search found 804 matches

by Michael Sherman
Wed Jul 17, 2024 1:44 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: SimTK Initial Velocity Analysis:
Replies: 2
Views: 47

Re: SimTK Initial Velocity Analysis:

Sadly, no. There is a nice Assembly analysis in Simbody but no InitialVelocity analysis (there should be!) You can work around that by adding a velocity constraint (or Motion object) to one of the dofs (assuming a net-1-dof system). Then constraint projection will figure out all the other velocities...
by Michael Sherman
Wed Jul 17, 2024 7:40 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Leaf spring substitute system
Replies: 3
Views: 107

Re: Leaf spring substitute system

You could try:
- CPodes limited to Order 1 (that makes it implicit Euler)
- SemiExplicitEulerIntegrator
- Use fixed step or very loose accuracy requirement, like 10%
- Use less damping or large masses to avoid extreme stiffness

Optimization is still probably the most direct approach.
by Michael Sherman
Tue Jul 16, 2024 8:40 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Leaf spring substitute system
Replies: 3
Views: 107

Re: Leaf spring substitute system

Assembly analysis is for satisfying position constraints. In this case it sounds like you would like the spring to minimize its stored potential energy, that is have the system relax so that it is in a steady state. That's a "static" or "equilibrium" analysis, if I understand your question. Simbody ...
by Michael Sherman
Thu Jul 11, 2024 10:20 am
Forum: OpenSim
Topic: Using SimTK Assemblar object to assemble Closed-Chain systems
Replies: 2
Views: 180

Re: Using SimTK Assemblar object to assemble Closed-Chain systems

I think all you need is a C-style cast like
SimTK::MobilizerQIndex(int)
by Michael Sherman
Mon Jun 24, 2024 6:15 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: GeneralContactSubsystem
Replies: 2
Views: 305

Re: GeneralContactSubsystem

Hi, Andrea. The entire System (including all its Subsystems) needs to be built before any computations are done. It looks like you are adding a Subsystem later, would be good to get that sorted out (might not be the problem but could cause trouble). Also, the GeneralContactSubsystem is old and super...
by Michael Sherman
Mon Jun 24, 2024 6:00 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Fixed step size and end-of-step event
Replies: 2
Views: 328

Re: Fixed step size and end-of-step event

Hi, Simone. Apologies for the delayed reply. (b) There is currently no provision in Simbody to call a function at step completion. This would be a nice addition to the TimeStepper class in case anyone would like to submit a Simbody PR! (a) This is a great question. It's not clear to me whether a fix...
by Michael Sherman
Wed Mar 13, 2024 7:44 am
Forum: OpenSim
Topic: computing jacobian: matlab and c++ api difference #3735
Replies: 3
Views: 991

Re: computing jacobian: matlab and c++ api difference #3735

A quick look suggests you need to get a reference to the existing SimbodyMatterSubsystem rather than a copy:

Code: Select all

const SimbodyMatterSubsystem& smss = model->getMatterSubsystem();
Regards,
Sherm
by Michael Sherman
Wed Mar 13, 2024 7:44 am
Forum: OpenSim
Topic: computing jacobian: matlab and c++ api difference #3735
Replies: 3
Views: 991

Re: computing jacobian: matlab and c++ api difference #3735

A quick look suggests you need to get a reference to the existing SimbodyMatterSubsystem rather than a copy:

Code: Select all

const SimbodyMatterSubsystem& smss = model->getMatterSubsystem();
Regards,
Sherm
by Michael Sherman
Wed Nov 22, 2023 8:39 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Get internal forces for a MobilizedBody
Replies: 14
Views: 2695

Re: Get internal forces for a MobilizedBody

Right! Sorry I didn't think of that earlier. Mobilizer forces are conceived as being generated by a motor in the joint so act equal and opposite on the connected bodies.
by Michael Sherman
Tue Nov 21, 2023 4:01 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Get internal forces for a MobilizedBody
Replies: 14
Views: 2695

Re: Get internal forces for a MobilizedBody

Oh, I think I understand now. If you don't want the generalized force Qa included in the reaction, have you considered applying it as a body force to A rather as a mobility force? Then all you would see in the reaction is the coordinate coupler force and the sliders' reactions to gravity. Otherwise ...