ISR research accomplishments

Modular Dexterous Hand
US Patent 5,052,736    Oct. 1, 1991

Research team

Josip Loncaric (ISR)
Fabrice de Comarmond (ISR) Currrently at INRIA (French Institute for Research in Computer Science and Control)

Accomplishment

The need for the hand. Many devices have been proposed as grasping hands for use with robot arms. The simplest consist of simple parallel jaw grippers. However, most of the time the resulting two-point grasping contact is insufficient for preventing rotation of the grasped object.

More complex systems include designs that mimic the human hand. These are suitable for master/slave control by a human operator. However, anthropomorphic designs are relatively complex. Since fingers are multi-jointed, the fingertips pivot about axes perpendicular to the length of the fingers. Because the fingertips do not always move within a single plane, it is difficult to grasp small objects lying on flat surfaces.

Another problem with anthropomorphic designs is that both grasping and fine manipulation are provided by the same structure. This coupling implies that fine manipulation of the grasped object will involve rolling the object between the fingertips, and this can result in a loss of positional certainty of the object if a precise knowledge of the shape of the object near the contact points is not well known.

U.S. Pat. No. 4,653,793 (Guinot et al) discloses a multiple point contact gripper in which three grasping fingers are movable linearly toward and away from one another to form a deformable grasping triangle at their tips. Two of the finger tips are formed by rotatable balls while the other is formed by a ball fixed to the finger tip. However, since the fingertips must move linearly, it is difficult to grasp small objects, and obstructions in the path of movement of the fingertips can make grasping difficult. In the embodiment of FIG. 8 of Guinot et al, the fingertips pivot about axes perpendicular to the length of the fingers, and so suffer from the same shortcomings as do those of anthropomorphic hands, i.e., the fingertips do not remain in a plane as they move, which makes it difficult to pick up objects, especially small objects, from tabletops.

What does the hand accomplish? This modular dexterous hand comprises a Stewart platform, which may be attached to a robot arm. The platform has a mounting plate, a plate attachable to the robot arm and six telescoping legs that connect the mounting plate to the attachment plate. Three rotational axes are mounted to the mounting plate. They extend parallel to each other and perpendicular to the mounting plate, and define an equilateral triangle.

The hand consists of a base and at least three mounted grasping fingers that rotate about parallel axes. Each finger is made up of a base end that intersects one of the axes and a distal end. Each distal end has a grasping element, is spaced perpendicularly from the axis, and can describe a circle.

The fingers grasp a randomly shaped object by rotating about axes perpendicular to a base. The hand also can grasp objects lying on a flat plane, and its fingertips define randomly shaped grasping triangles. There are separate stages for grasping and providing fine manipulation of the object. The hand grasps using three points of contact with friction.

The object can be manipulated in fine positions by varying the length of the legs of the Stewart platform, and in gross positional manipulations by moving the robot arm.

For more information

View this patent at Delphion.