Spring-damper prismatic joint system plugin

[damanikjosh]

Desired behavior

Simulating spring-damper physics on prismatic joint. https://en.wikipedia.org/wiki/Mass-spring-damper_model. In Gazebo classic it was implemented by modifying ODE parameters https://github.com/arpg/Gazebo/blob/master/test/worlds/spring_damper_test.world, but I can't get it to work on gz-sim. A similar issue was posted in #1961, but instead of modifying the physics, I suggest adding a system plugin to simulate this.

Implementation suggestion

1. Create JointPosition and JointVelocity for prismatic joint
2. Send JointForceCmd to the joint entity with spring-damper equation
3. Add plugin on sdf file similar like below

<plugin filename="libSpringSystem.so" name="spring_system::SpringSystem">
      <spring_constant>1200.0</spring_constant>
      <damping_constant>150.0</damping_constant>
      <joint_name>left_front_shock</joint_name>
      <joint_name>right_front_shock</joint_name>
      <joint_name>left_rear_shock</joint_name>
      <joint_name>right_rear_shock</joint_name>
</plugin>

[azeey]

Have you tried using the damping and spring_stiffness parameters in the <dynamics> tag of a joint axis (http://sdformat.org/spec?ver=1.11&elem=joint#axis_dynamics)? I believe those work for prismatic joints, so I don't see the value of adding a separate system to simulate this.

[damanikjosh]

I finally solved this. The problem was the model loaded using URDF and I found out that URDF doesn't support spring_stiffness parameter natively (http://wiki.ros.org/urdf/XML/joint). But this one works (https://github.com/gazebosim/sdf_tutorials/blob/master/urdf/sdf_extensions.md)

<gazebo>
    <springStiffness>1200</<springStiffness>
</gazebo>

I will close this issue, thank you.