Picking up a block with a gripper - Friction issues

Discussion in 'Priority support' started by rubstar, Feb 20, 2020.

  1. Hi Emo,

    I'm having issues with gripping a simple cube with a 2 jaw gripper.

    With `friction="1 0.05 0.01"` the cube unstably shakes and launches out of the gripper

    With `friction="10 0.05 0.01"`, the jaw of the gripper move into the cube and nothing happens as shown in the photo:

    This is the geom for the two gripper contact surfaces (connected using an equality constraint):
    <geom type="box" pos=".02 .031 0" euler="0 0 0" size="0.003195 0.019 0.011" condim="6" friction="1 0.05 0.01" solimp="0.99 0.99 .001" solref="-10000000 -100"/>

    And the green cube:
    <geom name=test1 type="box" rgba="0.0 0.0 0.0 0.0" material="block_mat" mass="1" condim="6" friction="1 0.05 0.01" />
    <joint name=joint1 type="free"/>

    Gripper actuator:
    <position ctrllimited="true" ctrlrange="-1.047 0.015" joint="gripper_RJ0_outer"
    kp="100" name="gripper_finger_joint"
    forcelimited="true" forcerange="-50 50"/>

    Any ideas? I've tried varying the solref, friction, forcerange on the actuator, but I haven't found any parameters that can grip the cube and pick it up.
  2. Emo Todorov

    Emo Todorov Administrator Staff Member

    Friction value of 10 is very unrealistic, and results in counter-intuitive artifacts. It is better to keep it closer to 1. The solref and solimp values you are using correspond to very stiff contacts which are difficult to integrate and cause instabilities. How about this: start from the default values given in the documentation, and try changing them a little if necessary. People use mujoco for grasping simulations all the time and the defaults usually work.

    If you are seeing slip and want to avoid it, you could enable the noslip option.