Dynamic hybrid joints

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atoz
Posts: 57
Joined: 18 Oct 2013, 09:02

Dynamic hybrid joints

Post by atoz »

Hi,

I'm simulating a dynamic LBR4 robot arm and controlling it via IK. The problem I'm having is that when dynamic mode is on, the redundant joint is moving by itself for a while before stopping. Looks like some kind of effect of gravity. I've tried playing around with the torque values but it doesn't seem to have any effect.

I've linked to the scene here: https://www.dropbox.com/s/cmzx4iueze6dk ... t.ttt?dl=0

Thanks in advance for your help.

coppelia
Site Admin
Posts: 10336
Joined: 14 Dec 2012, 00:25

Re: Dynamic hybrid joints

Post by coppelia »

Hello,

this happens mainly with the Bullet engine (and to a certain extent with the ODE engine). But not with the Vortex or Newton engine. This is directly linked to how the engine can hold a position with the implemented control loop, and the way the IK hybrid mode works in V-REP.

The IK hybrid mode works like this:
  • IK computes angles based on current manipulator configuration
  • Above angles will be used by the physics engine as values to hold (i.e. position control)
  • In next simulation loop, we start at point 1 above
For redundant manipulators and if the physics engine cannot precisely hold/reach the desired angular positions, the next IK calculation steps starts with a different manipulator configuration, thus the drift.

You can try to play with the position controller for the joints, but best would be to have a second manipulator purely kinematic and invisible, non-collidable, etc. Then, use the invisible manipulator for kinematics calculation, and then use the calculated values in a script to drive the joints of the second, dynamic robot, in position.

Cheers

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