Robot hand grab objects

Discussion in 'Support requests' started by George Zhao, May 19, 2019.

  1. We recently try to simulate a robot hand which grabs objects. The issue we encounter is that when robot hand touchs object, the object will fly away. We tried to increase the weight of the object but it flies away anyway. What shall we do so that the object will not fly away when the robot hand touches it?
  2. Emo Todorov

    Emo Todorov Administrator Staff Member

    In general this can happen for two reasons: (1) the simulation is inaccurate; (2) the simulation is accurate and the same thing would have happened in the real world.

    (1) could be due to large time step, or poor solver convergence. Make sure you use the Newton solver with sufficient number of iterations (say 100) and reduce the time step to something very small, and see if the problem is still there. Also, what geometry are you using? Box-box contacts suffer from collision detection artifacts in some rare cases. You could try to step through the simulation and see if very large contact forces appear all of a sudden.

    (2) could be due to large forces being applied by your robot, combined with a large mass ratio. Have you looked at the contact forces and actuation forces? Note that if there is penetration and the contacts are hard, the contact forces will be very large -- this is supposed to happen. The question is, why did the penetration get large in the first place. Usually this happens due to initialization or due to large time steps.

    You could also download the HAPTIX package. It contains the MPL hand model with multiple scenes that have been fine-tuned for object manipulation (under tele-operation). It may not be what you need, but still, it will provide an example environment where grasping is possible without any issues. The HAPTIX documentation chapter has links to videos illustrating this.