Search found 809 matches

by Michael Sherman
Fri Dec 13, 2024 6:25 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: MobilizedBody::Custom to off-the-shelf solution?
Replies: 1
Views: 49

Re: MobilizedBody::Custom to off-the-shelf solution?

Hi, Simone. Congratulations on getting a custom mobilizer to work -- very nice! I think you are correct that that's the only way to implement the singularity-free behavior you want. The fact that MobilizedBody::FunctionBased assumes 3 angles for rotation dooms it. (BTW that is itself a custom mobili...
by Michael Sherman
Sun Dec 08, 2024 2:57 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Understanding Linear Velocities in a Rotational System Simulation Using Simbody
Replies: 2
Views: 146

Re: Understanding Linear Velocities in a Rotational System Simulation Using Simbody

Sorry for the delay in responding to this. I ran your program to see what's happening. My guess is you didn't realize that when you apply a mobility force it is applied equal and opposite to the two connected bodies. So a torque is being applied both to the actuated wheel and the free-floating chass...
by Michael Sherman
Wed Sep 04, 2024 11:40 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Integrate over short transients
Replies: 4
Views: 6171

Re: Integrate over short transients

Hi, Simone. Simbody's integrators interpolate when a reporting time doesn't coincide with a step completion.

That's on by default but can be controlled by Integrator methods like setAllowInterpolation().

Regards,
Sherm
by Michael Sherman
Wed Aug 07, 2024 2:20 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Integrate over short transients
Replies: 4
Views: 6171

Re: Integrate over short transients

Hi Simone. If you use one of SImbody's variable step integrators, it should be able to figure out to take small steps over the rapid transition automatically.

Regards,
Sherm
by Michael Sherman
Tue Jul 23, 2024 9:15 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Leaf spring substitute system
Replies: 5
Views: 6466

Re: Leaf spring substitute system

Awesome! Thanks, Walter.
by Michael Sherman
Wed Jul 17, 2024 1:44 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: SimTK Initial Velocity Analysis:
Replies: 2
Views: 2588

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: 5
Views: 6466

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: 5
Views: 6466

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: 2930

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: 3092

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...