Soft Contact Model

A contact model that allows a small amount of interpenetration and generates contact force like a spring-damper.

Soft contact is a family of contact-modeling approaches used in physics engines. The classical hard-contact approach requires that bodies never interpenetrate at all, describing this with a complementarity condition (contact force can only exist when bodies are not separating, and separation implies zero contact force), which is difficult to solve and prone to numerical instability. Soft contact relaxes this requirement: it allows a small amount of penetration and treats contact as a spring-and-damper constraint, where force grows with penetration depth. MuJoCo is the leading example — it drops the strict complementarity condition from the linear complementarity problem and instead formulates contact as a convex optimization problem, tuned with two parameter groups: solref (a time constant and damping ratio, defaulting to 0.02 and 1) and solimp (constraint impedance, which controls how soft the contact is). The benefits are stable solving, smooth results, and a unique inverse-dynamics solution; the cost is that poorly chosen parameters cause visible interpenetration or bounciness, so sim-to-real transfer work often has to tune these parameters to match real materials.

ExampleIn MuJoCo, increasing the time constant of solref for a gripper fingertip's contact makes the contact softer and allows deeper penetration, smoothing out how contact force changes during a grasp; setting the time constant smaller than the simulation timestep tends to make contact unstable.

Also called

Soft Contact, Compliant Constraint

Related

Contact Model · Linear Complementarity Problem · MuJoCo (Multi-Joint dynamics with Contact) · Interpenetration · Projected Gauss-Seidel · Simulation Timestep

Sources

MuJoCo 文档 - Computation (Chinese)
MuJoCo 文档 - Modeling（solref / solimp） (Chinese)