Physics Material

Dynamic friction (or kinetic friction) is the resistive force that opposes the relative motion of two surfaces already sliding against each other. It is defined by a coefficient (μ_d) typically between 0.0 (frictionless) and 1.0 (high friction).

• Key Role: Governs the deceleration of sliding objects, affecting robot locomotion and object manipulation.
• Simulation Impact: A low coefficient makes surfaces slippery, while a high coefficient causes objects to stop quickly. Accurate modeling is critical for predicting how a robot arm will slide a part across a table or how a mobile robot will traverse different floor types.

Static friction is the resistive force that must be overcome to initiate sliding motion between two stationary surfaces in contact. It is defined by a coefficient (μ_s), which is usually equal to or greater than the dynamic friction coefficient.

• Key Role: Determines the minimum force required to start moving an object from rest.
• Simulation Impact: A higher static friction coefficient means more force is needed from an actuator to begin moving a grasped object. This parameter is vital for simulating realistic pick-and-place scenarios and preventing unrealistic object "sliding" on inclined surfaces under gravity.

Restitution is a scalar value between 0.0 and 1.0 that defines the "bounciness" of a collision, representing the ratio of relative speed after impact to relative speed before impact.

• 0.0 (Inelastic): Objects stick together or stop with no bounce.
• 1.0 (Perfectly Elastic): Objects rebound with no loss of kinetic energy.
• Simulation Impact: Critical for modeling dropping, throwing, or any dynamic contact. Low restitution is used for soft materials like clay; high restitution is used for rigid balls. Misconfiguration can lead to unrealistic, perpetually bouncing objects or overly "dead" interactions.

The friction combine mode is an algorithm that determines how the friction coefficients of two contacting surfaces are combined to calculate the effective friction for the interaction.

Common modes include:

• Average: Uses the mean of the two coefficients.
• Minimum: Uses the lower of the two coefficients.
• Maximum: Uses the higher of the two coefficients.
• Multiply: Uses the product of the two coefficients.

Simulation Impact: This mode dictates interaction realism. For example, using 'Minimum' for a metal-on-ice interaction ensures low friction, regardless of the metal's high friction property. It provides fine-grained control over complex material interactions.

The restitution combine mode is an algorithm that defines how the restitution values of two colliding objects are combined to determine the overall bounciness of the collision.

It uses the same set of operations as friction combine modes (Average, Minimum, Maximum, Multiply).

Simulation Impact: This parameter prevents unrealistic energy gain in collisions. For instance, if two highly bouncy objects (restitution ~0.9) collide using a 'Maximum' combine mode, the effective restitution could be 0.9, leading to near-perfect bounce. Using 'Average' or 'Multiply' typically yields more physically plausible and stable simulations by damping the collision energy.

Damping (or contact damping) is a parameter that models energy loss during contact, simulating effects like softness, deformation, or internal friction within the material.

• Key Role: Suppresses high-frequency oscillations or "jitter" at contact points, which are numerical artifacts common in rigid body simulators.
• Simulation Impact: Adds stability to complex multi-contact scenarios, such as a robot hand grasping a pile of parts or a vehicle driving over rubble. While not a direct physical analog like friction, it is an essential numerical parameter for achieving robust, real-time simulation necessary for training reinforcement learning policies.

A friction model mathematically represents the resistive forces opposing motion between contacting surfaces. It is a core component of a physics material, defining parameters like:

• Static friction (stiction): The force threshold that must be overcome to initiate motion.
• Coulomb friction: The constant kinetic friction force during sliding motion.
• Viscous damping: A velocity-dependent resistive force. Accurate friction models are critical for simulating realistic manipulation, locomotion, and object sliding behaviors.

Contact dynamics is the simulation subsystem that resolves forces and collisions between rigid bodies. It uses the properties defined in physics materials—coefficient of restitution (bounciness) and friction coefficients—to compute collision responses. This engine determines:

• Impulse-based or penalty-based force calculations during collisions.
• Stacking and resting behavior of objects.
• Joint limits and constraint stabilization. The fidelity of contact resolution directly impacts the stability and physical plausibility of a simulation.

An actuator model defines the internal dynamics of a simulated motor or linear actuator. While a physics material governs external surface interactions, the actuator model dictates how joint commands are realized, including:

• Torque/speed curves and electrical current limits.
• Gearbox backlash and efficiency losses.
• Rotor inertia and winding resistance.
• Saturation and rate limits. High-fidelity actuator models are essential for simulating torque control, impedance control, and the electromechanical delays present in real hardware.

These are the core computations for predicting and controlling robot motion:

• Forward Dynamics: Calculates the resulting acceleration of a robot given applied joint torques/forces, mass properties, and external forces (including those from physics material contacts).
• Inverse Dynamics: Calculates the joint torques required to achieve a desired acceleration, accounting for inertial, Coriolis, gravitational, and external contact forces. Both rely on an accurate mass matrix and solve the equations of motion, with contact forces informed by physics material properties.

Impedance control is a high-level strategy that makes a robot's end-effector behave like a programmable mass-spring-damper system. It regulates the dynamic relationship between position error and output force. This is distinct from, but interacts with, low-level physics material properties:

• The controller's virtual stiffness and damping define the robot's compliance.
• Upon contact, the interaction force is a function of both the controller's impedance and the object's physics material (e.g., its surface friction). It enables safe, compliant interaction with uncertain environments.

These are file formats for describing robots and simulation worlds:

• URDF (Unified Robot Description Format): An XML format used in ROS to define a robot's kinematic tree, links, joints, visual/collision geometries, and inertial properties. Physics material properties are often assigned to collision elements here.
• SDF (Simulation Description Format): A more comprehensive XML format used by simulators like Gazebo/Isaac Sim. It can describe entire worlds, nested models, lights, and includes explicit tags for <surface> properties where <friction> and <bounce> (restitution) are defined, directly mapping to physics material parameters.