The figure depicts a tendon-driven, three-fingered robot hand (or foot, depending on one's perspective) designed to be installed on the end of the left front leg of the six-legged robot described in the preceding article. This hand, which was undergoing development at the time of reporting the information for this article, is intended to satisfy the need for a small, lightweight mechanism that can grasp objects of various shapes (e.g., balls or cylinders of differing diameters), without need for an elaborate electronic control system or for multiple actuators. The hand also houses rudimentary sensors: the terminal optics of a fiber-optic camera are incorporated into the palm of the hand, and the housing for the optics doubles as a contact-sensor switch.

This Three-Fingered Robot Hand features a relatively simple actuation mechanism, yet it can grip irregularly shaped objects. When fully open, it spans a diameter of about 2 in. (≈5 cm).

Each finger includes two links, with knucklelike joints. The knucklelike joint at the inner end of the innermost link is mounted on a base that pivots around an axis perpendicular to the joint axis, thereby allowing the finger to rotate into conformation with the object to be gripped. The three fingers are spring-biased into planes of action 120° apart, but forces and torques of contact with a gripped object overcome the spring biases, causing the finger to pivot into the aforementioned conformation.

For the sake of simplicity, the tendons that drive all three fingers are, in turn, driven by only one motor. More specifically, the motor drives a winch to which is coupled a set of tendons arranged as finger flexors. When these tendons are drawn in, the fingers close around the gripped object. For opening the fingers to release the gripped object, the fingers are equipped with a second set of spring-loaded tendons arranged as finger extensors. Each flexor tendon is connected to a spring stiffer than that of the corresponding extensor tendon; this provision prevents the full contact of one finger with the object from acting as a hard stop against the further closure of the other fingers, thereby enabling the fingers to conform to an irregularly shaped object.

This work was done by Brett Kennedy of Caltech for NASA's Jet Propulsion Laboratory.


This Brief includes a Technical Support Package (TSP).
Three-Fingered Robot Hand with Self-Adjusting Grip

(reference NPO20907) is currently available for download from the TSP library.

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