Uncontrollable damping/friction after adding planar constraint

[LemonPi]

After adding in 2 prismatic joint to constrain motion in the xy plane in #2336 , there now seems to be damping/friction that I can't remove.

For the ball (urdf below), I'm trying to get it to move in the xy plane with a very small force. I set gravity to 0 in all dimensions, friction and damping in the tag of the main link to be 0, and made sure all link damping was 0 with p.changeDynamics(ball, i, lateralFriction=0, linearDamping=0, angularDamping=0)

However the ball still refuses to move under a certain applied force (behaviour seems like countering static friction), and comes to a stop very quickly.

This wasn't happening before I added the planar movement constraint; how are these affecting the damping and what can I do to control this damping/friction?

ball urdf:

<?xml version="1.0" encoding="utf-8"?>
<!-- =================================================================================== -->
<!-- |    This document was autogenerated by xacro from models/ball.xacro              | -->
<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                                 | -->
<!-- =================================================================================== -->
<robot name="ball" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <link name="base_link">
    <contact>
      <lateral_friction value="0.0"/>
      <rolling_friction value="0.0"/>
      <spinning_friction value="0.0"/>
      <stiffness value="30000"/>
      <damping value="0"/>
    </contact>
    <visual>
      <geometry>
        <cylinder length="0.5" radius="1.0"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0.2 1.0 1"/>
      </material>
    </visual>
    <visual name="angle_visualizer">
      <origin xyz="0.98 0 0.2"/>
      <geometry>
        <box size="0.1 0.1 0.7"/>
      </geometry>
    </visual>
    <collision>
      <surface>
        <bounce restitution_coefficient="1.0" threshold="0.01"/>
      </surface>
      <geometry>
        <cylinder length="0.5" radius="1.0"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="20"/>
      <inertia ixx="8.0" ixy="0" ixz="0" iyy="8.0" iyz="0" izz="8.0"/>
    </inertial>
  </link>
  <link name="ball_world">
    <inertial>
      <mass value="0"/>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
    </inertial>
  </link>
  <link name="ball_y_prismatic">
    <inertial>
      <mass value="0"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
      <origin rpy="0 0 0" xyz="0 0 0"/>
    </inertial>
  </link>
  <joint name="ball_y_to_world" type="prismatic">
    <parent link="ball_world"/>
    <child link="ball_y_prismatic"/>
    <axis xyz="0 1 0"/>
    <limit effort="0.0" lower="1" upper="-1" velocity="1000.0"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
  </joint>
  <link name="ball_x_prismatic">
    <inertial>
      <mass value="0"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
      <origin rpy="0 0 0" xyz="0 0 0"/>
    </inertial>
  </link>
  <joint name="ball_x_to_y" type="prismatic">
    <parent link="ball_y_prismatic"/>
    <child link="ball_x_prismatic"/>
    <axis xyz="1 0 0"/>
    <limit effort="0.0" lower="1" upper="-1" velocity="1000.0"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
  </joint>
  <joint name="ball_base_to_x" type="continuous">
    <parent link="ball_x_prismatic"/>
    <child link="base_link"/>
    <axis xyz="0 0 1"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
  </joint>
</robot>

[DarrenLevine]

I just ran into potentially the same issue, where damping/friction would seemingly be very high even when set to zero in the URDF and in the p.changeDynamics() API.

The issue was that each joint I added was not, as I had first assumed, a dumb constraint. The joint was instead modeled as a motor/actuator, with a default motor force that needed to be exceeded for any movement to occur in a force/torque controlled application.

So, to enable torque/external-force control, I had to first set all the motor forces to zero to turn it back into a dumb constraint, like so:

# Disable the motors:
joint_indexes = [j for j in range(p.getNumJoints(robot_id))]
joint_forces_zero = [0.0 for j in range(p.getNumJoints(robot_id))]
p.setJointMotorControlArray(robot_id, joint_indexes, p.VELOCITY_CONTROL, forces=joint_forces_zero)