Search found 808 matches
- Thu Nov 24, 2022 10:29 am
- Forum: OpenSim
- Topic: Difference joint vs constraint
- Replies: 8
- Views: 3028
Re: Difference joint vs constraint
You are correct that kinematics doesn't consider forces so the force elements don't affect IK. IK always respects joints and should satisfy constraints also. "To a tolerance" means that (unlike joints) constraints won't be satisfied perfectly (that is, to machine precision). Instead there will be so...
- Wed Nov 23, 2022 12:01 pm
- Forum: OpenSim
- Topic: Difference joint vs constraint
- Replies: 8
- Views: 3028
Re: Difference joint vs constraint
Tom beat me to answering and gave a better answer than I would have! (Hi, Tom) To elaborate a little -- in your example of a Weld joint vs. Weld constraint: Say we weld body A to body B. With a weld joint, A and B essentially become the same body, with B attached exactly as specified by the joint. T...
- Tue Nov 08, 2022 9:45 am
- Forum: Simbody: SimTK multibody dynamics API
- Topic: Code is not running due to inertia values
- Replies: 5
- Views: 2946
Re: Code is not running due to inertia values
Hi, Harshal. An important thing to know is that Simbody expects inertias to be provided taken about the body origin, NOT the body mass center. I suspect your specification of the motor inertia was intended to be about the COM (0,0,.4) but Simbody interpreted it about (0,0,0) which, when shifted to (...
- Sun Oct 23, 2022 9:45 pm
- Forum: Simbody: SimTK multibody dynamics API
- Topic: Suitability for large branching structures?
- Replies: 1
- Views: 1165
Re: Suitability for large branching structures?
Of course there is a limit to how big a system can run in real time, but I don't think you're close to it yet. Take a look at "ChainExample". That's an interactive 3d example with 100 bodies connected by ball joints so 300 dofs. Depending how stiff your springs are, you might find an explicit integr...
- Tue Sep 27, 2022 9:29 am
- Forum: Simbody: SimTK multibody dynamics API
- Topic: Constraints between massless bodies
- Replies: 2
- Views: 1214
Re: Constraints between massless bodies
The rule to follow is this: Imagine the system with just the bodies and joints (constraints removed). That tree-structured system defines the mass matrix and must be non-singular. So you can weld massless bodies together if they are part of the tree system (i.e. using a weld joint rather than a weld...
- Wed Jul 06, 2022 10:33 pm
- Forum: Simbody: SimTK multibody dynamics API
- Topic: MobilizedBody_FunctionBased - mapping between qdot and u
- Replies: 4
- Views: 1693
Re: MobilizedBody_FunctionBased - mapping between qdot and u
No, sorry Nicos. Angular velocity is not the derivative of Euler angles (except in one special case: when all three Euler angles are zero). Otherwise you have to use Euler angle derivatives for u to get N=identity. Angular velocity is in fact not the derivative of any set of q's -- there is always a...
- Wed Jul 06, 2022 3:28 pm
- Forum: Simbody: SimTK multibody dynamics API
- Topic: MobilizedBody_FunctionBased - mapping between qdot and u
- Replies: 4
- Views: 1693
Re: MobilizedBody_FunctionBased - mapping between qdot and u
Hi, Nicos. No, that's a very reasonable question! The answer is that FunctionBased Mobilizer is a special case of Custom Mobilizer provided for convenience. The author of that class designed it to work only for mobilizers where qdot=u (that is, N=identity). The general Custom mobilizer supports N !=...
- Sat Jan 08, 2022 12:10 pm
- Forum: Simbody: SimTK multibody dynamics API
- Topic: How to create a new friction force with special properties?
- Replies: 2
- Views: 1349
Re: How to create a new friction force with special properties?
Nice! Congratulations.
- Thu Nov 25, 2021 11:42 am
- Forum: Simbody: SimTK multibody dynamics API
- Topic: torque-free motion not giving correct output
- Replies: 6
- Views: 3200
Re: torque-free motion not giving correct output
You're welcome!
- Tue Nov 23, 2021 3:46 pm
- Forum: Simbody: SimTK multibody dynamics API
- Topic: torque-free motion not giving correct output
- Replies: 6
- Views: 3200
Re: torque-free motion not giving correct output
Here is a plot of the three components of the angular velocity of the shaft, expressed in the shaft frame. I set the initial conditions to (10, 1000, 1000) degrees/s. The y & z velocities vary between +/- 1414.21 degrees/s. plot.png The modified code I used is here: #include "Simbody.h" #include <io...