2. Grasp classifications and definitions

The two most important classes of manipulator grasps are known as form closure and force closure grasps [20]. These terms were first used by Reuleaux in his studies of machine design [21], and were later applied to robotic grasping by Salisbury [22], though recent inconsistent use of these terms has confused their meanings.

A fixed set of contacts on a rigid object is said to exhibit form closure if the object's equilibrium is maintained despite any possible externally applied wrench (i.e. force, torque, or combination thereof). In other words, the geometries and relative poses of the object and the contacting body (e.g. a gripper) are sufficient to hold the object in place with complete kinematic constraint.

In our automatic tooling design research, we strive to achieve form closure. In certain important instances, however, form closure is impossible. For example, any part which is axisymmetric (defined by a surface of revolution) cannot be kinematically constrained in form closure. Under such circumstances, a robotic manipulation strategy should strive for what Trinkle refers to as frictional form closure instead [20]. Under frictional form closure, friction contributes to immobilize a grasped part which is otherwise not fully kinematically constrained by a gripper.

Our basic (naive) approach to gripper and fixture design is equivalent to performing a constructive solid geometry subtraction of a part shape from a uniform volume, thus defining a complementary cavity equivalent to a casting mold. This conceptualization has the appealing quality of resulting in complete form closure in all instances where form closure is possible. Necessary variations on this naive theme violate the assurance of form closure, and we must subsequently test our candidate designs computationally for grasp stability.

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