A compact actuator generates rotary or linear motion with a large torque or force, respectively. The original version of this actuator is designed to pull a wedge that, until pulled, prevents retraction of the proposed extended nose landing gear of the space shuttle. The original version is also required to fit into a volume that is severely limited by the size of the landing-gear assembly. The basic actuator design could be adapted to other applications in which there are requirements for compact, large-force actuators with similar geometries.
The transducer portion of this actuator is a ribbon of a nickel/titanium shapememory alloy. A component made of such an alloy undergoes a pronounced deformation to a “remembered” shape (in the present case, the ribbon becomes shorter) when its temperature rises through a transition value, causing a transformation in its metallurgical structure from a martensitic phase to an austenitic phase. The component resumes its previous shape (in the present case, the ribbon lengthens) when its temperature falls below a lower transition temperature (there is hysteresis in the transformation). In this case, the transition temperatures are somewhat above room temperature.
To obtain sufficient length in order to obtain sufficient stroke, the shape-memory ribbon is wrapped three times around a cylinder. One end of the ribbon is attached to the cylinder, the other to an object that does not move, relative to the axis of the cylinder. The ribbon is heated by passing an electrical current through it. When the temperature of the ribbon rises above the first transition temperature, the resultant shape-memory shrinkage exerts a torque, causing the cylinder to turn. In the original application, the rotary motion of the cylinder is converted to linear motion by use of ramps, attached to the cylinder, that roll along wheels. In a different application, the rotary motion of the cylinder could be the desired output.
The primary difficulty encountered in initial tests was that the sliding friction engendered by wrapping the ribbon three times around the cylinder was so large as to impair actuation. The friction was reduced to an acceptably low level by placing the ribbon on nonconductive roller assemblies to enable the ribbon to move around the cylinder by rolling instead of sliding.
This work was done by Bradley Files and James W. Akkerman of Johnson Space Center.