Non-prehensile manipulation

Move an object without enclosing it#

A robot may push a box across a table, pivot an object against a fixture, roll a cylinder, or slide a tool into a better grasp pose. The object is not held in a stable grasp throughout the motion. MIT's manipulation notes use the term for manipulation without grasping and point out that environmental contact can also help with tasks that later include a grasp.

Non-prehensile does not mean contact-free. It often requires deliberate contact-rich manipulation, with motion determined by contact geometry, normal force, friction, and object inertia.

Pushing depends on contact mechanics#

Mason's pushing analysis develops mechanics and planning for manipulator pushing operations. A commanded pusher path does not uniquely determine object motion unless the contact and friction behavior are known well enough. Sticking, sliding, and rotation can change as the contact point and pressure distribution change.

This differs from prehensile manipulation, where a maintained grasp constrains the object's motion relative to the hand. It also differs from simply losing a grasp. A non-prehensile action is planned around the absence of a stable enclosure or force-closure grasp.

Small uncertainty can change the outcome#

Surface friction varies with material, wear, dust, and normal load. The object's center of mass and pressure distribution may be unknown. A small pose error can move the contact to a different edge and produce a different rotation.

Feedback from vision, touch, or force control can correct some errors, but occlusion and intermittent contact remain difficult. Successful pushing of one object on one surface does not establish transfer to another geometry or friction condition. Reports should state whether object properties were known, estimated online, or covered by feedback and repeated trials.