2 research outputs found
A pressure field model for fast, robust approximation of net contact force and moment between nominally rigid objects
We introduce an approximate model for predicting the net contact wrench
between nominally rigid objects for use in simulation, control, and state
estimation. The model combines and generalizes two ideas: a bed of springs (an
"elastic foundation") and hydrostatic pressure. In this model, continuous
pressure fields are computed offline for the interior of each nominally rigid
object. Unlike hydrostatics or elastic foundations, the pressure fields need
not satisfy mechanical equilibrium conditions. When two objects nominally
overlap, a contact surface is defined where the two pressure fields are equal.
This static pressure is supplemented with a dissipative rate-dependent pressure
and friction to determine tractions on the contact surface. The contact wrench
between pairs of objects is an integral of traction contributions over this
surface. The model evaluates much faster than elasticity-theory models, while
showing the essential trends of force, moment, and stiffness increase with
contact load. It yields continuous wrenches even for non-convex objects and
coarse meshes. The method shows promise as sufficiently fast, accurate, and
robust for design-in-simulation of robot controllers.Comment: (revised in accordance with the IROS camera ready
A Transition-Aware Method for the Simulation of Compliant Contact with Regularized Friction
Multibody simulation with frictional contact has been a challenging subject
of research for the past thirty years. Rigid-body assumptions are commonly used
to approximate the physics of contact, and together with Coulomb friction, lead
to challenging-to-solve nonlinear complementarity problems (NCP). On the other
hand, robot grippers often introduce significant compliance. Compliant contact,
combined with regularized friction, can be modeled entirely with ODEs, avoiding
NCP solves. Unfortunately, regularized friction introduces high-frequency stiff
dynamics and even implicit methods struggle with these systems, especially
during slip-stick transitions. To improve the performance of implicit
integration for these systems we introduce a Transition-Aware Line Search
(TALS), which greatly improves the convergence of the Newton-Raphson iterations
performed by implicit integrators. We find that TALS works best with
semi-implicit integration, but that the explicit treatment of normal compliance
can be problematic. To address this, we develop a Transition-Aware Modified
Semi-Implicit (TAMSI) integrator that has similar computational cost to
semi-implicit methods but implicitly couples compliant contact forces, leading
to a more robust method. We evaluate the robustness, accuracy and performance
of TAMSI and demonstrate our approach alongside relevant sim-to-real
manipulation tasks.Comment: Published in IEEE RA-L and accepted to ICRA 2020. The first two
authors contributed equally to this work. Copyright 2020 IEEE. Personal use
of this material is permitted. Permission from IEEE must be obtained for all
other uses, in any current or future media. The supplemental video is
available publicly at https://youtu.be/p2p0Z1Bf91Y . 8 pages with 9 figure