[Proposal] Add compliant stiffness and damping randomization when randomizing materials

[ParlitsisG]

Proposal

Add compliant stiffness and damping randomization when randomizing materials. Currently physics material randomization only supports friction and restitution.

Motivation

Manipulation tasks such as a manipulator pushing and picking require randomizations in the behaviour of the object during contact to simulate the diversity of object material properties. During impact and contacts a compliant contact model which is randomized could be a good method for domain randomization when training an rl agent.

Alternatives

I guess using Deformable objects is possible but at much higher computational cost.

Additional context

I am currently working on a project using a dual arm manipulation franka manipulator with flat siliconde end effectors instead of a gripper to pick and place large objects. In order to describe contacts I want to randomize the contact model as part of domain randomization to make my RL agent more robust during the Sim-to-real.

Checklist

• [x] I have checked that there is no similar issue in the repo (required)

[RandomOakForest]

Thank you for posting this proposal. Could you elaborate on what you mean by "compliant contact model" and give an example or elaborate in what would be a good implementation? Thanks.

[ParlitsisG]

Thank you for posting this proposal. Could you elaborate on what you mean by "compliant contact model" and give an example or elaborate in what would be a good implementation? Thanks.

In Isaac lab documentation it is defined this way under the physics material configuration in documentation

compliant_contact_stiffness: float = 0.0
  Spring stiffness for a compliant contact model using implicit springs. Defaults to 0.0.
  A higher stiffness results in behavior closer to a rigid contact. The compliant contact model is only enabled
  if the stiffness is larger than 0.
  compliant_contact_damping: float = 0.0
  Damping coefficient for a compliant contact model using implicit springs. Defaults to 0.0.
  Irrelevant if compliant contacts are disabled when :obj:`compliant_contact_stiffness` is set to zero and
  rigid contacts are active.
 

What I propose is to allow for randomization of this values the same way one can already randomize friction and restitution from this the physics material essentially add to randomize_rigid_body_material class

This can be used for any contact rich task with random objects for domain randomization as it alters the behaviour of the object during contact.

[RandomOakForest]

Thanks. The team has taken note of this.