Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
18.104.22.168 Stewart Platform Manipulators
Another class of simple manipulators is the 6-DOF Stewart platform.1237,1238 The Stewart platform exploits the geometry of an octahedron -- one triangular face serves as a mobile platform and the opposing triangular face serves as the fixed base. Six struts of the octahedron connect the base to the platform and can be varied in length, thus controlling platform position and orientation with respect to the base. The six struts carry only compressive or tensile, but no bending, forces. Figure 9.10 shows a two-dimensional analog with three struts and 3 DOF to illustrate a portion of the available range of motions. This 5 x 10-20 kg device has pressure-actuated vernier ratchets that allow strut lengths to be altered in ~0.1 nm increments across a 100-nm work envelope, providing excellent positional control.10 Merkle1239 has examined a novel family of 6-DOF positional control devices -- the double tripod configuration -- which offers greater stiffness for a given structural mass than a robotic arm and significantly greater range of motion than a Stewart platform. Tradeoffs between stiffness and range of motion in the double tripod can be continuously adjusted by altering simple design parameters.
Assembly of parts while immersed in a fluid eliminates electrostatic and surface tension effects.1150 In the earliest, most primitive Stewart platform type nanomanipulators operating in aqueous suspension, R. Merkle suggests that leg extension and retraction may be controlled by manufacturing the legs using a repeating sequence of binding sites on the surfaces of two opposing struts. Chemical binding molecules are designed that can selectively link one strut to another, but only between one specific pair of binding sites (one site on each strut). Varying the linking agent controls which part of a strut links to the adjacent strut; cycling of link molecule types in solution forces the linking and unlinking in such a way as to move the struts along.1303
Last updated on 20 February 2003