stepSimulation() not equivalent to setRealTimeSimulation(1)

[avandekleut]

I want to control a procedurally generated robot. Consider the following code:

Code: Select all

import time
import pybullet as p
import pybullet_data
import sys

physicsClient = p.connect(p.GUI)
p.setPhysicsEngineParameter(enableFileCaching=0)
p.setAdditionalSearchPath(pybullet_data.getDataPath())
p.resetSimulation()
p.setGravity(0, 0, -10)
planeId = p.loadURDF("plane.urdf")
robot = p.loadURDF(sys.argv[1], [0, 0, 2])
p.setRealTimeSimulation(1)
for t in range(200000):
    p.getKeyboardEvents()

p.resetSimulation()
p.setGravity(0, 0, -10)
planeId = p.loadURDF("plane.urdf")
robot = p.loadURDF(sys.argv[1], [0, 0, 2])
p.setRealTimeSimulation(0)
for t in range(200000):
    p.stepSimulation()
    time.sleep(1. / 240.)
p.disconnect()

The first half runs fine. The second half, my URDF file goes crazy and flies off. Example URDF files that have this behaviour:
https://raw.githubusercontent.com/avand ... ter/2.urdf
https://raw.githubusercontent.com/avand ... ter/1.urdf

Am I misunderstanding something here? What is going on? Why are the two so different?


Edit:

I tried this code using some of the sample URDF files provided and they seem to work alright. There must be some issue in processing the physics of my URDF files.

[Erwin Coumans]

Testing here there is no difference between stepSimulation and setRealTimeSimulation, both show instable motion due to a degenerate urdf file.

Those are urdf files with degenerate values. Avoid too small mass (keep it either 0 or in range 0.01 to 100 or so)
avoid large rpy angles, don't use collision shapes with extreme small radius (use 0.05 smallest for a start)

Here is a working version:

Code: Select all

<?xml version="1.0"?>
<robot name="robots/1">
  <link name="8325804">
    <inertial>
      <origin xyz="-5.421010862427522e-20 -1.6263032587282567e-19 0.0010000000000000002" rpy="0 0 0"/>
      <mass value="1"/>
      <inertia ixx="1.675516081914556e-17" ixy="0" ixz="0" iyy="1.675516081914556e-17" iyz="0" izz="1.675516081914556e-17"/>
    </inertial>
    <collision>
      <origin xyz="-5.421010862427522e-20 -1.6263032587282567e-19 0.0010000000000000002" rpy="0 0 0"/>
      <geometry>
        <sphere radius="0.1"/>
      </geometry>
    </collision>
  </link>
  <link name="4735681">
    <inertial>
      <origin xyz="-3.469446951953614e-18 -1.214306433183765e-17 0.08472517387841258" rpy="0 0 0"/>
      <mass value="0.6946108199226944"/>
      <inertia ixx="0.0019944652455508296" ixy="0" ixz="0" iyy="0.0019944652455508296" iyz="0" izz="0.0019944652455508296"/>
    </inertial>
    <collision>
      <origin xyz="-3.469446951953614e-18 -1.214306433183765e-17 0.08472517387841258" rpy="0 0 0"/>
      <geometry>
        <sphere radius="0.08472517387841255"/>
      </geometry>
    </collision>
  </link>
  <link name="4064029">
    <inertial>
      <origin xyz="-0.013392437970720003 6.841951323297693e-05 0.036824153984054794" rpy="0 0 0"/>
      <mass value="0.32657421908334666"/>
      <inertia ixx="0.0003600348756754367" ixy="0" ixz="0" iyy="0.0003600348756754367" iyz="0" izz="0.00022142031140993636"/>
    </inertial>
    <collision>
      <origin xyz="-0.013392437970720003 6.841951323297693e-05 0.036824153984054794" rpy="0 0 0"/>
      <geometry>
        <cylinder radius="0.0368241539840548" length="0.09571551589530464"/>
      </geometry>
    </collision>
  </link>
  <link name="5314437">
    <inertial>
      <origin xyz="-8.890457814381136e-18 -1.0408340855860843e-17 0.04736004193466575" rpy="0 0 0"/>
      <mass value="0.14423106891506815"/>
      <inertia ixx="0.00012940259033819833" ixy="0" ixz="0" iyy="0.00012940259033819833" iyz="0" izz="0.00012940259033819833"/>
    </inertial>
    <collision>
      <origin xyz="-8.890457814381136e-18 -1.0408340855860843e-17 0.04736004193466575" rpy="0 0 0"/>
      <geometry>
        <sphere radius="0.047360041934665746"/>
      </geometry>
    </collision>
  </link>
  <link name="1844789">
    <inertial>
      <origin xyz="-0.01302201858425922 5.847181364028571e-06 0.009928035035897393" rpy="0 0 0"/>
      <mass value="0.022543488066187885"/>
      <inertia ixx="1.8224520051453555e-05" ixy="0" ixz="0" iyy="1.8224520051453555e-05" iyz="0" izz="1.111009366082133e-06"/>
    </inertial>
    <collision>
      <origin xyz="-0.01302201858425922 5.847181364028571e-06 0.009928035035897393" rpy="0 0 0"/>
      <geometry>
        <cylinder radius="0.09928035035897387" length="0.09698090677467489"/>
      </geometry>
    </collision>
  </link>
  <link name="3416624">
    <inertial>
      <origin xyz="0.006343136646001411 0.018183987579849615 0.06582443936436896" rpy="0 0 0"/>
      <mass value="0.8541329457565685"/>
      <inertia ixx="0.0020295629099782607" ixy="0" ixz="0" iyy="0.001257342235037602" iyz="0" izz="0.0020295629099782607"/>
    </inertial>
    <collision>
      <origin xyz="0.006343136646001411 0.018183987579849615 0.06582443936436896" rpy="0 0 0"/>
      <geometry>
        <capsule radius="0.05818503294385344" length="0.04143685882263688"/>
      </geometry>
    </collision>
  </link>
  <joint name="2951283" type="continuous">
    <origin xyz=" 0.0001594  -0.00093199  0.00132557" rpy="0 0 0"/>
    <parent link="8325804"/>
    <child link="4735681"/>
    <axis xyz="0 0 1"/>
    <dynamics damping="0.1" friction="0.1"/>
    <limit lower="0" upper="0" effort="1" velocity="1"/>
  </joint>
  <joint name="2574580" type="fixed">
    <origin xyz=" 0.04142879 -0.06828364  0.0564523 " rpy="0 0 0"/>
    <parent link="4735681"/>
    <child link="4064029"/>
    <axis xyz="0 0 0"/>
    <dynamics damping="0" friction="0"/>
    <limit lower="0" upper="0" effort="10" velocity="10"/>
  </joint>
  <joint name="9958721" type="revolute">
    <origin xyz="-1.94187710e-02  2.18095641e-04  1.91914661e-07" rpy="0 0 0"/>
    <parent link="4064029"/>
    <child link="5314437"/>
    <axis xyz="0 0 1"/>
    <dynamics damping="0.1" friction="0.1"/>
    <limit lower="0" upper="6.283185307179586" effort="1" velocity="1"/>
  </joint>
  <joint name="3283812" type="continuous">
    <origin xyz="-0.00912397 -0.04638031  0.04442856" rpy="0 0 0"/>
    <parent link="5314437"/>
    <child link="1844789"/>
    <axis xyz="0 0 1"/>
    <dynamics damping="0.1" friction="0.1"/>
    <limit lower="0" upper="0" effort="1" velocity="1"/>
  </joint>
  <joint name="7781930" type="fixed">
    <origin xyz="-5.21927538e-02  3.11575316e-05  3.00292219e-09" rpy="0 0 0"/>
    <parent link="1844789"/>
    <child link="3416624"/>
    <axis xyz="0 0 0"/>
    <dynamics damping="0" friction="0"/>
    <limit lower="0" upper="0" effort="10" velocity="10"/>
  </joint>
</robot>