cliport/environments/assets/ur5/gripper/README.md

Robotiq 2F 85 gripper

For this gripper, the following Github repo can be used as a reference: https://github.com/Shreeyak/robotiq.git

mimic tag in URDF

This gripper is developed for ROS and uses the mimic tag within the URDF files to make the gripper move. From our research mimic tag within URDF is not supported by pybullet. To overcome this, one can use the createConstraint function. Please refer to this example from the bullet3 repo to see how to replicate a mimic joint:

#a mimic joint can act as a gear between two joints
#you can control the gear ratio in magnitude and sign (>0 reverses direction)

import pybullet as p
import time
p.connect(p.GUI)
p.loadURDF("plane.urdf",0,0,-2)
wheelA = p.loadURDF("differential/diff_ring.urdf",[0,0,0])
for i in range(p.getNumJoints(wheelA)):
    print(p.getJointInfo(wheelA,i))
    p.setJointMotorControl2(wheelA,i,p.VELOCITY_CONTROL,targetVelocity=0,force=0)


c = p.createConstraint(wheelA,1,wheelA,3,jointType=p.JOINT_GEAR,jointAxis =[0,1,0],parentFramePosition=[0,0,0],childFramePosition=[0,0,0])
p.changeConstraint(c,gearRatio=1, maxForce=10000)

c = p.createConstraint(wheelA,2,wheelA,4,jointType=p.JOINT_GEAR,jointAxis =[0,1,0],parentFramePosition=[0,0,0],childFramePosition=[0,0,0])
p.changeConstraint(c,gearRatio=-1, maxForce=10000)

c = p.createConstraint(wheelA,1,wheelA,4,jointType=p.JOINT_GEAR,jointAxis =[0,1,0],parentFramePosition=[0,0,0],childFramePosition=[0,0,0])
p.changeConstraint(c,gearRatio=-1, maxForce=10000)


p.setRealTimeSimulation(1)
while(1):
    p.setGravity(0,0,-10)
    time.sleep(0.01)
#p.removeConstraint(c)

Details on createConstraint can be found in the pybullet getting started guide.

Files in folder

Since parameters like gear ratio and direction are required, one can find the robotiq_2f_85_mimic_joints.urdf which contains the mimic tags as in original URDF, which can be used as a reference. It was generated from robotiq/robotiq_2f_robot/robot/simple_rq2f85_pybullet.urdf.xacro as so:

rosrun xacro xacro --inorder simple_rq2f85_pybullet.urdf.xacro
adaptive_transmission:="true" > robotiq_2f_85_mimic_joints.urdf

The URDF meant for use in pybullet is robotiq_2f_85.urdf and it is generated in a similar manner as above by running:

rosrun xacro xacro --inorder simple_rq2f85_pybullet.urdf.xacro > robotiq_2f_85.urdf