This time, we will experiment with a parameter called spinning_friction. According to the quickstart guide, the difference from the previous rolling friction is as follows. I don't know if it's correct, but I understood it as a coefficient of friction with different orientations.

rolling friction coefficient orthogonal to contact normal
spinning friction coefficient around contact normal

environment

Windows 10 Home
Python 3.6.8
PyBullet 2.5.5
Pillow 6.2.0

What i did

Set spinning_friction in the urdf file.

I want to know the effect of the parameters, so I will make 3 sets. --a set <spinning_friction value =" 0.004 "/> in test_box07a.urdf <spinning_friction value =" 0.001 "/> in test_sphere05a.urdf --b set <spinning_friction value =" 0.004 "/> in test_box07b.urdf <spinning_friction value =" 0.004 "/> in test_sphere05b.urdf -C set <Spinning_friction value =" 0.001 "/> in test_box07c.urdf <spinning_friction value =" 0.004 "/> in test_sphere05c.urdf

The urdf file of a set is as follows. (B set, c set) And in test‗sphere05a.urdf, visual is box and collision is sphere.

`test_box07a.urdf`


<robot name="robot_name">
  <link name="link_name">
    <contact>
      <lateral_friction value="1.0"/>
      <rolling_friction value="0.0"/>
      <spinning_friction value="0.004"/>
    </contact>
    <inertial>
      <origin xyz="0.0 -1.0 -0.1" rpy="0.0 0.0 0.0"/>
      <mass value="0.0"/>
      <inertia ixx="0.0" ixy="0.0" ixz="0.0" iyy="0.0" iyz="0.0" izz="0.0"/>
    </inertial>
    <visual>
      <origin xyz="0.0 -1.0 -0.1" rpy="0.0 0.0 0.0"/>
      <geometry>
        <box size="0.8 0.8 0.2"/>
      </geometry>
      <material name="red">
        <color rgba="1.0 0.0 0.0 1.0"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0.0 -1.0 -0.1" rpy="0.0 0.0 0.0"/>
      <geometry>
        <box size="0.8 0.8 0.2"/>
      </geometry>
    </collision>
  </link>
</robot>

`test_sphere05a.urdf`


<robot name="robot_name">
  <link name="link_name">
    <contact>
      <lateral_friction value="1.0"/>
      <rolling_friction value="0.0"/>
      <spinning_friction value="0.001"/>
    </contact>
    <inertial>
      <origin xyz="0.0 -1.0 0.5" rpy="0.0 0.0 0.0"/>
      <mass value="1.0"/>
      <inertia ixx="0.1" ixy="0.0" ixz="0.0" iyy="0.1" iyz="0.0" izz="0.1"/>
    </inertial>
    <visual>
      <origin xyz="0.0 -1.0 0.5" rpy="0.0 0.0 0.0"/>
      <geometry>
        <box size="0.4 0.4 0.4"/>
      </geometry>
      <material name="l_red">
        <color rgba="1.0 0.5 0.5 1.0"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0.0 -1.0 0.5" rpy="0.0 0.0 0.0"/>
      <geometry>
        <sphere radius="0.2"/>
      </geometry>
    </collision>
  </link>
</robot>

applyExternalTorque

shpere05aid = p.loadURDF("test_sphere05a.urdf")
p.applyExternalTorque(shpere05aid, -1, [0,0,100], flags=p.LINK_FRAME)

I wanted to make it move like a top, so I use applyExternalTorque. Obtain the unique id at the time of loadURDF and give it to applyExternalTorque. The next -1 is base, [0,0,100] is torque, and flags = p.LINK_FRAME is the flag that gives torque in the local coordinate system.

The entire script is below.

import pybullet as p
import time
from PIL import Image

phisicsClient = p.connect(p.GUI)
p.setGravity(0,0,-10)
p.loadURDF("test_box07a.urdf")
p.loadURDF("test_box07b.urdf")
p.loadURDF("test_box07c.urdf")
sphere05aid = p.loadURDF("test_sphere05a.urdf")
sphere05bid = p.loadURDF("test_sphere05b.urdf")
sphere05cid = p.loadURDF("test_sphere05c.urdf")
p.applyExternalTorque(sphere05aid, -1, [0,0,100], flags=p.LINK_FRAME)
p.applyExternalTorque(sphere05bid, -1, [0,0,100], flags=p.LINK_FRAME)
p.applyExternalTorque(sphere05cid, -1, [0,0,100], flags=p.LINK_FRAME)

camera_img = p.getCameraImage(320,320)
imgs = [Image.fromarray(camera_img[2])]

for i in range(1200):
    p.stepSimulation()
    if i % 10 == 0:
        camera_img = p.getCameraImage(320,320)
        imgs.append(Image.fromarray(camera_img[2]))
    time.sleep(1./240.)
p.disconnect()

imgs[0].save("test_pybullet_09.gif", save_all = True, append_images = imgs[1:], duration=10, loop = 0)

Execute

The execution result is the following GIF animation. From the front, a set, b set, c set.

The a set and c set kept spinning longer, and the b set was shorter than them. This time, it seems that the magnitude of friction is determined by the sum of the parameters given to the two sides. ∵a set: 0.004 + 0.001 = 0.005 b set: 0.004 + 0.004 = 0.008 c set: 0.001 + 0.004 = 0.005 However, it seems that lateral_friction also has an effect, so I do not know how to use it ...

[PYTHON] Set spinning_friction in pybullet