How do I set the friction properties (or any GeometryProperties) of a specific body in MultibodyPlant?

Question

I would like to know how I can set the frictional properties of a body in MultibodyPlant.

More specifically, I have a model instance that has been read in from an SDF and I would like to add or change the friction properties of a particular body in that model instance.

Answer 1

There are several ways to set the frictional properties. In addition to your very specific question, I'm going to outline multiple workflows. (As a side note, while some of this is documented, it isn't necessarily discoverable -- or even rendered -- in the doxygen. That's an outstanding issue that needs to be addressed, for which an issue exists).

• Define in the urdf/sdf.
  • Drake provides custom tags associated contact properties (including friction and more) with the <geometry> tag. The tags are the same across URDF/SDF but are exercised slightly differently (based on the style of SDF vs URDF). Details can be found here for sdf and here for urdf.
• Add new geometry via the API and include friction in the definition.
  • MultibodyPlant has an API for assigning geometry to a body. There are multiplpe variants:
    • Adding a geometry to a body and explicitly defining the friction
      • This is a legacy sugar API. Favor the API that takes ProximityProperties as the preferred API.
    • Generally defining ProximityProperties
      • In this case, the only question is, where do my proximity properties come from?

  #include "drake/geometry/proximity_properties.h"
  #include "drake/multibody/plant/multibody_plant.h"
  
  // Omitting drake:: namespaces for clarity.
  MultibodyPlant<double> plant;
  // Populate plant with bodies.   
  ProximityProperties contact_props;
  AddContactMaterial({}, {}, CoulombFriction<double>(mu_s, mu_d), &contact_props);
  // Assume we've defined body, X_BG, shape, and name elsewhere.
  plant.RegisterCollisionGeometry(body, X_BG, shape, name, contact_props);
  

• Add/Update to geometry defined in a urdf/sdf
  • In this case, we need to change the geometry definition in SceneGraph based on its association with a body. That means, starting with the Body, we need to get the GeometryId. We're going to assume that has been done and you have the GeometryId for this workflow.

  SceneGraph<double> sg;
  const GeometryId g_id = ...;
  CoulombFriction<double> friction(mu_s, mu_d);
  ProximityProperties new_props;
  const ProximityProperties* curr_props = 
      sg.model_inspector().GetProximityProperties(g_id);
  if (curr_props != nullptr) {
    new_props = *curr_props;
  }
  // Replaces old friction definition, or adds it if previously absent.
  new_props.UpdateProperty("material", "coulomb_friction", friction);
  sg.AssignRole(g_id, new_props, RoleAssign::kReplace);