Robot Arm Actuated by Electroactive Polymers

NASA's Jet Propulsion Laboratory, Pasadena, California

The figure shows a robot arm that is essentially a miniature crane actuated by electroactive polymers. This mechanism has been constructed as part of a continuing effort to develop lightweight, compact, low-power-consumption telerobots.

Electroactive polymers have been chosen as the actuator materials for this development because they offer advantages over such competing actuator materials as electroactive ceramics (both piezoelectric and electrostrictive). For example, whereas the maximum actuation strains of electroactive ceramics range between 0.1 and 0.3 percent, those of electroactive polymers exceed 10 percent; and whereas the densities of electroactive ceramics range from is 4 to 6 g/cm³, those of electroactive polymers range from 1 to 2.5 g/cm³. Electroactive polymers can be formed into almost any shape, are flexible and tough, and damp vibrations. Like other polymers, electroactive polymers can be mass-produced at relatively low cost. Unlike piezoceramics, electroactive polymers need not be poled during manufacturing; this helps keep production costs low.

This Miniature Crane has been used to demonstrate the feasibility of small, lightweight, low-power-consumption robot arms containing electroactive-polymer actuators. In an experiment, the crane lifted a rock by 3/4 in. (19 mm). It should be possible to increase the lifting distance by optimizing the design of the ropelike LEAs.

The lever arm of the miniature crane is a hollow graphite/epoxy rod 15 in. (38.1 cm) long, with an inner diameter of 1/4 in. (6.4 mm) and an outer diameter of 1/3 in. (8.5 mm). The pivot point divides the rod into two parts with length ratio of 5:1. The electroactive-polymer actuators are of two types:

There are two linear electrostatic actuators (LEAs). These are ropelike objects made from an electrostatically activated polymer (a silicone) with carbon surface coats as electrodes. LEAs function analogously to muscles in that they act by shortening or lengthening. One of the LEAs lies along the top and over the outer end of the longer part of the lever arm, where it hangs down and holds a gripper. The other LEA is fastened between a fixed point and the outer end of the shorter part of the lever arm. The two LEAs are electrically activated in synchronism so that their actuation effects add to maximize the stroke in lifting or lowering an object held by the gripper.
The gripper contains four fingers that are not jointed but nevertheless bend and thus function similarly to human fingers. The fingers are perfluorinated-ion-exchange-membrane/platinum composites. When a voltage is applied across the thickness of such a finger, electrostriction in the ion-exchange polymer in the membrane causes the finger to bend; the direction of bending depends on the polarity of the voltage. Hooks on the ends of the fingers help to secure the grip on the object, which can be picked up and carried once the fingers close around it. [The gripper was described previously in more detail in "Robot Hands With Electroactive-Polymer Fingers" (NPO-20103) NASA Tech Briefs, Vol. 22, No. 10 (October 1998), page 78.]

This work was done by Yoseph Bar-Cohen and Tianji Xue of Caltech, and Brian Lucky, Cinkiat Abidin, Marlene Turner, and Harry Mashhoudy of UCLA for NASA's Jet Propulsion Laboratory. NPO-20393