Search found 808 matches

by Michael Sherman
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...
by Michael Sherman
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...
by Michael Sherman
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 (...
by Michael Sherman
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...
by Michael Sherman
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...
by Michael Sherman
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...
by Michael Sherman
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 !=...
by Michael Sherman
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...