Soft Fingers

The GraspIt! engine never computes geometry deformation explicitly, therefore can not find exact contact areas between soft bodies. However, the frictional implications of soft fingers in contact are too important to be completely ignored for grasp quality computations. The most important effect is that contacts over an area (as opposed to point contacts) can also apply torsional friction. The soft finger model in GraspIt! attempts to capture at least an approximation of this effect, without explicitly computing the contact deformation. See the section for complete details on the theoretical aspects of our soft finger contact computation.

In order to designate a body as “soft”, specify it’s Young’s modulus in the Body XML file (see the section of this manual for a description of the Body XML files). The XML tag that should be added is named youngs, and it’s value should be the value of the Young’s modulus in Pa. For example, an entry can have the following form:

<youngs>1500000</youngs>

Analytical contact area model

Any body that has such an entry in the properties section, including robot links, is treated by GraspIt! as a soft body. When a soft body is found to contact another body (irrespective of whether the second body is also soft or not), the contact engine does the following:

  • find a set of vertices on the surface of each body in a small area around the contact points. These vertices define the “soft neighborhood” of the contact
  • if multiple point contacts are found close to each other, only one of them is kept, and their soft neighborhoods are merged
  • fit an analytical surface to the soft neighborhoods. This is done by fitting a surface of the form z=ax^2+by^2+cxy to the vertices in the neighborhoods
  • the radii of curvature of the analytical surfaces on each body are used to approximate the amount of torsional friction that can be applied at the contact
  • the Contact Wrench Space is built in order to also contain the effect of torsional friction. This wrench space is 4D (unlike the Coulomb friction cone which is 3D). Therefore, it can not be displayed as a contact marker. Instead, soft contacts are indicated by displaying a small patch of the analytical surface fit to each body around the contact.
  • the resulting Contact Wrench Space affect both grasp quality computations and the behavior of the contact in dynamic simulations.

All of this functionality is encapsulated in the SoftContact class; see the code and documentation of this class for details.

Intuitively, this entire computation has the following effect: if the bodies are locally “flat”, or if their curvatures match in a small region around the contact, they will produce a larger area of contact for a given normal force. This will in turn lead to larger torsional friction. Conversely, sharp edges in contact, even on soft bodies, will create small torsional friction. The amount of torsional friction is also influenced by the value of Young’s modulus specified for each body.

This method captures much of the effects of soft contacts on the kind of simulations that are of primarily interest in GraspIt!. It is important to note though that it is only an approximation of the real-life phenomenon. It relies on fitting analytical surfaces to each of the bodies in a small region around the contact. On bodies with very complex or degenerate geometry the fitting procedure can fail leading to incorrect amounts of torsional friction applied. The fitting procedure also is not very good at handling very sharp features such as corners or edges.