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How to lock gripper joints?

Posted: 16 Jan 2019, 05:29
by jamesp

I am having a small issue with the Robotiq85 gripper: when the whole gripper is moving, the trailing gripper pad seems to be subject to some kind of inertia or drag effect.


In the above picture the gripper is moving to the left, causing the right pad to trail behind.
The pic is also available at ... _loose.png.

I first tried setting

Code: Select all

    sim.setObjectInt32Parameter(j1, sim.jointintparam_velocity_lock,1)
    sim.setObjectInt32Parameter(j2, sim.jointintparam_velocity_lock,1)
in the gripper's sysCall_init(), followed by zeroing the target velocities of j1/j2 a few seconds after the gripper shut on the nut. I also followed the suggestion provided in viewtopic.php?t=2567, but neither seems to work.
Somehow even supposedly locking all the joints within the gripper structure doesn't remedy this issue.

Any help would be appreciated.

Re: How to lock gripper joints?

Posted: 16 Jan 2019, 08:21
by coppelia

the Robotiq85 gripper is unfortunately very difficult to simulate properly. The model is close to the real gripper, but behaviour is not always as one could expect. Trying to tweak a few joints parameters can only give you minor improvements. We highly recommend building a simplified model of that gripper. Or to additionally attach the object to grip via a force sensor to the gripper in order to mimic a perfect grip.


Re: How to lock gripper joints?

Posted: 17 Jan 2019, 04:03
by jamesp
Thanks for the reply!

A work-around was simply temporarily attaching the fingertip meshes to the force sensor connection point.
Not the most elegant solution, but I can appreciate the complexity behind a model like this.