Real-Time Physics Simulation Forum 2022-02-16T14:45:22+00:00 https://pybullet.org/Bullet/phpBB3/app.php/feed/topic/13379 2022-02-16T14:45:22+00:00 2022-02-16T14:45:22+00:00 https://pybullet.org/Bullet/phpBB3/viewtopic.php?p=43862#p43862 <![CDATA[Fixed joint constraint pendulum problem]]>
My aim is to create a simple mid-air suspended oscillating pendulum.
Here is the URDF code:

Code: 

<?xml version="1.0" ?><robot name="pendolo"> <link name="base">    <visual>      <origin rpy="0 0 0" xyz="0 0 0"/>      <geometry>        <box size="0.001 0.001 0.001"/>      </geometry>    </visual>    <!-- added so pybullet doesn't complain and set mass 1 diagintertia 1-->    <inertial>      <origin rpy="0 0 0" xyz="0 0 0"/>      <mass value="0.00001"/>      <inertia ixx="1e-5" ixy="0" ixz="0" iyy="1e-5" iyz="0" izz="1e-5"/>    </inertial>  </link>  <joint name="floating_base" type="fixed">    <origin rpy="0 0 0" xyz="0 0 0"/>    <parent link="base"/>    <child link="supporto"/>  </joint>  <link name="supporto">      <inertial>          <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>          <mass value="0.001"/>          <inertia ixx="0.0001" ixy="0.0" ixz="0.0" iyy="0.001" iyz="0.0" izz="0.0001"/>      </inertial>      <visual name="">          <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>          <geometry>              <box size="0.0 0.0 0.0"/>          </geometry>          <material name="">              <color rgba="1.0 0.0 0.0 1.0"/>              <texture filename=""/>          </material>      </visual>      <collision>        <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>          <geometry>              <box size="0.0001 0.0001 0.0001"/>          </geometry>      </collision>  </link>          <joint name="pendulum_joint" type="revolute">        <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0"/>        <parent link="supporto"/>        <child link="sbarra"/>        <axis xyz="0.0 1.0 0.0"/>        <limit lower="-10.0" upper="10.0" effort="0.0" velocity="0.0"/>    </joint>  <link name="sbarra">      <inertial>          <origin xyz="0.0 0.0 -0.5" rpy="0.0 0.0 0.0"/>          <mass value="1.0"/>          <inertia ixx="0.001" ixy="0.0" ixz="0.0" iyy="0.001" iyz="0.0" izz="0.001"/>      </inertial>      <visual name="">          <origin xyz="0.0 0.0 -0.5" rpy="0.0 0.0 0.0"/>          <geometry>              <box size="0.1 0.1 1"/>          </geometry>          <material name="">              <color rgba="1.0 0.0 0.0 1.0"/>              <texture filename=""/>          </material>      </visual>      <collision>        <origin xyz="0.0 0.0 -0.5" rpy="0.0 0.0 0.0"/>          <geometry>              <box size="0.1 0.1 1"/>          </geometry>      </collision>  </link>  </robot>
If I use a constraint to fix the joint base to be in a certain position it works and the pendulum is suspended exactly where I want.
Now if I use

Code: 

 resetJointState
to change pendulum initial angle it starts to oscillate in an unstable way, even with no gravity. Besides the pendulum rotate on further axis.
Here is the code to reproduce the issue:

Code: 

import pybulletimport timeimport pybullet_dataimport pybullet_utils.bullet_client as bcimport numpy as npp = bc.BulletClient(connection_mode=pybullet.GUI)p.setAdditionalSearchPath(pybullet_data.getDataPath())# p.setGravity(0,0,-10)planeId = p.loadURDF("plane.urdf")startPos = [0,0,1.5]startOrientation = p.getQuaternionFromEuler([0,0,0])pendId = p.loadURDF("pendulum.urdf", startPos, startOrientation)p.resetJointState(pendId, 1, -np.pi/4, 0)p.createConstraint(pendId, -1, -1, -1,                   p.JOINT_FIXED,                   [0,0,0],                   [0,0,0],                   [0,0,2],                   childFrameOrientation=[0,0,0,1])for i in range (100000):    p.stepSimulation()    time.sleep(1./240.)p.disconnect()
Besides, if the position of the constrain is [0,0,1.5] exactly as the initial one of the pendulum the system behaves as desired.

Can anyone explain why this weird behavior ?

Statistics: Posted by francevezzi — Wed Feb 16, 2022 2:45 pm

]]>