Rethinking Contact Simulation for Robot Manipulation

Hydroelastic Contact in Drake

Figure 1: Predictive simulation of subtle manipulation effects requires detailed modeling of contact pressure distribution — treating contact as though it occurs at a point is not sufficient. Shown is torsional slip control with TRI’s bubble grippers, simulated in Drake.

Problems with point contact

Figure 3: Point contact (top) uses minimum penetration distance
to determine force direction (green arrow), resulting in
non-physical discontinuities. In contrast, the net force produced
by a contact patch (bottom) evolves smoothly. (3 views)

What about FEA?

Figure 4: Contact stress modeled with the Finite Element Method. (Image copyright ANSYS 2013)

A brief history of Hydroelastic Contact

  • Results must be calculated from the undeformed overlap region, without requiring calculation of the deformed mesh.
  • Effects must be local, that is, independent of size and shape of objects far from the contact region.
  • Force magnitude and direction must be continuous with respect to configuration.
  • Artifact free: no weird behavior, at least in normal use.
  • Accurate in simple cases, reasonable in others.
  • Uses familiar theory-of-elasticity parameterization where possible.
  • Fast enough to be useful to roboticists.
Figure 5: Hydrostatic pressure
  • the pressure at any point on the hull is calculated locally just from knowing the depth of that point (it’s the weight of the displaced column of water above that point),
  • the force direction is locally perpendicular to the hull, and
  • no matter what shape or sharp edges on the hull, the forces are always continuous — small changes in position and orientation of the boat produce small changes in force magnitude and direction.
Figure 6: Elastic Foundation model
Figure 7: Local calculation of contact pressure (orange arrows) between an irregular compliant object and a rigid block using a precalculated pressure field (gray contours). Net force is shown as a green arrow.

How to calculate a pressure field

Figure 8: A pressure field calculated like a temperature gradient (e=strain).

Fun with pressure fields

Figure 10. This Drake visualization shows a full-physics simulation using hydroelastic contact to generate contact forces. The sphere is compliant, the plate is rigid, and the tabletop is compliant. Note the detailed contact patches under the plate and ball, and the smooth evolution of forces (red arrows) even as contact crosses sharp edges.



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