Deformable collapse with 1N difference in MAX_FORCE

[PeterQiu0516]

I tried to move a deformable cloth by:
(1) Using a cube as the gripper;
(2) Use createSoftBodyAnchor to tie one vertex of the cloth to the cube;
(3) Use positional constraint to move the cube.

However, the cloth collapsed when I increased the parameter maxForce for the positional constraint by 1N and called changeConstraint twice.

This is a huge problem because when this breaks the simulation stays broken. Furthermore it's not clear what the right values of MAX_FORCE is, or what calls to changeConstraint cause it to break. This means I cannot know reliably that the simulation won't break after the 1000th random action (imagine using this for data collection)

Problem GIF:

• When MAX_FORCE = 51N
  

• When MAX_FORCE = 52N
  

To reproduce:

import pybullet_data
import pybullet as p
import time
import numpy as np

def run(broken, mass=1):

    if broken:
        MAX_FORCE = 52
    else:
        MAX_FORCE = 51

    TIME_STEP = 1/240.
    DURATION = 1000

    # basic setup
    physicsClient = p.connect(p.GUI)
    p.resetSimulation(p.RESET_USE_DEFORMABLE_WORLD)
    p.resetDebugVisualizerCamera(
            cameraDistance=5, cameraYaw=20, cameraPitch=0, cameraTargetPosition=[0.0, 0.0, 0.0])

    gravZ = -10
    p.setGravity(0, 0, gravZ)
    p.setPhysicsEngineParameter(sparseSdfVoxelSize=0.25)
    p.setTimeStep(TIME_STEP)

    # load cloth
    p.setAdditionalSearchPath(pybullet_data.getDataPath())
    cloth = p.loadSoftBody("cloth_z_up.obj", basePosition=[0, 0, 2], scale=0.5, mass=mass, useNeoHookean=0, useBendingSprings=1, useMassSpring=1,
                           springElasticStiffness=40, springDampingStiffness=.1, springDampingAllDirections=1, useSelfCollision=0, frictionCoeff=.5, useFaceContact=1)

    # load cube (serve as a gripper)
    scalingFactor = 0.10
    cubeStartPos = [0, 1, 2]
    cube = p.loadURDF("cube.urdf", cubeStartPos, globalScaling=scalingFactor)

    # tie cloth to the cube
    p.createSoftBodyAnchor(cloth, 15, cube, -1,
                           scalingFactor*np.array([0.5, -0.5, 0]))

    # create positional constraint
    gripperConstraint = p.createConstraint(cube, -1, -1, -1, p.JOINT_FIXED,
                                           jointAxis=[0, 0, 0],
                                           parentFramePosition=[0, 0, 0], childFramePosition=cubeStartPos, childFrameOrientation=p.getQuaternionFromEuler([0, 0, 0])
                                           )

    # execution
    def settle():
        for i in range(DURATION):
            time.sleep(0.001)
            p.stepSimulation()

    p1 = [0, 0, 1]
    p2 = [0.3, 0, 1]

    # let it settle
    settle()

    # move the cloth to prove it's not broken
    p.changeConstraint(gripperConstraint, p1, maxForce=MAX_FORCE)
    settle()

    # break it by calling changeConstraint twice
    p.changeConstraint(gripperConstraint, p2, maxForce=MAX_FORCE)
    settle()
    p.changeConstraint(gripperConstraint, p2, maxForce=MAX_FORCE)
    settle()

    p.changeConstraint(gripperConstraint, p1, maxForce=MAX_FORCE)
    settle()
    time.sleep(5)
    
    p.disconnect()


if __name__ == "__main__":
    run(False)
    run(True)
    input("Press Enter")

Has anyone else encountered similar issue before? > <