Mass Spring Cloth Example

This example demonstrates the dynamics of a cloth modeled as a collection of particles connected by springs. The cloth, fixed at two corners and initially horizontal, drapes down and swings under gravity. The purpose of this example is to demonstrate the ability of Drake to enable simulation of customized dynamical systems through user-defined time derivative update for continuous systems and discrete states update for discrete systems. Note that this example is a particle system connected by springs whose parameters are tuned to exhibit cloth-like behaviors without considerations of environmental forces or collisions and contact. In particular, it does not indicate that Drake supports cloth simulation at this point.

The system runs in discrete mode when the timestep dt is set to be positive, and it runs in continuous mode when the timestep dt is set to be zero or negative. As the discrete and the continuous mode share many common features, we implement them in a single system to minimize code duplication. Alternatively, one could implement them as two separate systems.

The mass-spring model is adapted from [Provot 1995]. The discrete time integration scheme follows that of [Bridson, 2005].

The following instructions assume Drake was built using bazel.

All instructions assume that you are launching from the drake workspace directory.

cd drake

Prerequisites

Ensure that you have the visualizer and the demo itself built:

bazel build //tools:drake_visualize //examples/mass_spring_cloth:run_cloth_spring_model

Mass Spring Cloth Simulation

In one terminal, launch the visualizer

bazel-bin/tools/drake_visualizer

In another terminal, launch the mass spring cloth simulation

bazel-bin/examples/mass_spring_cloth/run_cloth_spring_model

The discrete mode is run by default. Since the discrete solver uses a conditionally stable time integration scheme. Using too large a dt may lead to instability. Usually, you will need to decrease dt when you:

1. increase the elastic stiffness (accessible via the parameter file drake/examples/mass_spring_cloth/cloth_spring_model_params_named_vector.yaml),
2. decrease h, or
3. decrease the mass of the particles.

To switch to the continuous mode, add the flag --dt=0. The number of particles in the x-direction and the y-direction can be configured with the flag --nx and --ny respectively, and the separation between particles can be set with the flag --h. Use --help to get the full list of flags.

References

• [Bridson, 2005] Bridson, Robert, Sebastian Marino, and Ronald Fedkiw. "Simulation of clothing with folds and wrinkles." ACM SIGGRAPH 2005 Courses. 2005.
• [Provot 1995] Provot, Xavier. "Deformation constraints in a mass-spring model to describe rigid cloth behaviour." Graphics interface. Canadian Information Processing Society, 1995.