Implicit frictional dynamics with soft constraints

TL;DR

This paper evaluates lagged friction models in simulation, identifies inaccuracies near the stick-slip threshold, and proposes methods to improve implicit frictional contact systems using automatic differentiation and soft constraints.

Contribution

It introduces a detailed analysis of lagged friction inaccuracies, demonstrates how to correctly incorporate implicit friction with higher-order integration, and extends the method to simulate complex phenomena with soft constraints.

Findings

Lagged friction models have inaccuracies near the stick-slip threshold.

Implicit friction systems can be improved with higher-order time integration.

The proposed methods effectively simulate complex phenomena while maintaining system smoothness.

Abstract

Dynamics simulation with frictional contacts is important for a wide range of applications, from cloth simulation to object manipulation. Recent methods using smoothed lagged friction forces have enabled robust and differentiable simulation of elastodynamics with friction. However, the resulting frictional behavior can be inaccurate and may not converge to analytic solutions. Here we evaluate the accuracy of lagged friction models in comparison with implicit frictional contact systems. We show that major inaccuracies near the stick-slip threshold in such systems are caused by lagging of friction forces rather than by smoothing the Coulomb friction curve. Furthermore, we demonstrate how systems involving implicit or lagged friction can be correctly used with higher-order time integration and highlight limitations in earlier attempts. We demonstrate how to exploit forward-mode automatic differentiation to simplify and, in some cases, improve the performance of the inexact Newton method. Finally, we show that other complex phenomena can also be simulated effectively while maintaining smoothness of the entire system. We extend our method to exhibit stick-slip frictional behavior and preserve volume on compressible and nearly-incompressible media using soft constraints.