Search found 807 matches

by Michael Sherman
Tue Jan 24, 2023 11:21 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

Thanks for this great question, Kaiwen. Here is our thinking behind this constraint: Simbody constraints always model some physically-possible system, always assuming that the position and velocity constraints can't be exactly satisfied (they never are in practice). It is easiest to think of this by...
by Michael Sherman
Mon Dec 05, 2022 11:41 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Few Question regarding setting initial angular velocity to body?
Replies: 6
Views: 3166

Re: Few Question regarding setting initial angular velocity to body?

Hi, Harshal. Simbody is not doing anything. The user applied an angular velocity to one body in the assembly of two bodies (a cylinder and a brick). Every point of both bodies that is not aligned with the angular velocity vector will have a translational velocity also (think of a spinning propeller ...
by Michael Sherman
Sun Dec 04, 2022 3:16 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Few Question regarding setting initial angular velocity to body?
Replies: 6
Views: 3166

Re: Few Question regarding setting initial angular velocity to body?

3. From the example "ExampleMotorWithSpeedControl": MyTorqueLimitedMotor (const MobilizedBody& mobod, MobilizerUIndex whichU, Real gain, Real torqueLimit) What is the use of MobilizeUIndex. According to my understanding U stand for generalized speed, which contains 6 elements ( 3 angular and 3 linea...
by Michael Sherman
Sun Dec 04, 2022 3:01 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Few Question regarding setting initial angular velocity to body?
Replies: 6
Views: 3166

Re: Few Question regarding setting initial angular velocity to body?

In DzhanibekovEffect, you were surprised that changing the angular velocity from (10, 0, 1e-10) to (10, 10, 1e-10) caused translation to occur. That's because the angular velocity value you chose gave the system center of mass a non-zero translational velocity. You can see that by adding the followi...
by Michael Sherman
Sun Dec 04, 2022 2:54 pm
Forum: Simbody: SimTK multibody dynamics API
Topic: Few Question regarding setting initial angular velocity to body?
Replies: 6
Views: 3166

Re: Few Question regarding setting initial angular velocity to body?

These are great questions, thanks Amit. I'll answer one per post: I want to provide Initial angular velocity to the whole body , not one body. is function "setUToFitAngularVelocity" (https://simbody.github.io/3.5.0/classSi ... de27544411), but this function only provide the angular velocity between ...
by Michael Sherman
Mon Nov 28, 2022 10:45 am
Forum: Simbody: SimTK multibody dynamics API
Topic: unable to provide initial angular velocity in URDF file
Replies: 3
Views: 1345

Re: unable to provide initial angular velocity in URDF file

I think you wanted:

Code: Select all

// Obtain a reference to the mobilized body of interest.
MobilizedBody& shaft = myRobot.updBody("base_link");
so that you are working with the actual MobilizedBody in your system rather than a copy.
by Michael Sherman
Thu Nov 24, 2022 11:42 am
Forum: OpenSim
Topic: Difference joint vs constraint
Replies: 8
Views: 2910

Re: Difference joint vs constraint

I'm not familiar with OpenSim's IK interface so this is just Thanksgiving speculation in case no one else is available: - assuming there is a way to loosen the tolerance, you may find you can solve the problem using joints and constraints with a little more slop allowed - you may have a problem with...
by Michael Sherman
Thu Nov 24, 2022 11:34 am
Forum: Simbody: SimTK multibody dynamics API
Topic: Code is not running due to inertia values
Replies: 5
Views: 2831

Re: Code is not running due to inertia values

This diagram may help (scroll down to "Mobilizer Terminology and Notation") -- notice that a mobilizer is a relationship between two frames. One (F) is on the parent (inboard) body and the other (M) is on the child (outboard) body. A Pin mobilizer aligns the Z axes of those two frames. Each frame c...
by Michael Sherman
Thu Nov 24, 2022 10:29 am
Forum: OpenSim
Topic: Difference joint vs constraint
Replies: 8
Views: 2910

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...