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 Telescoping Manipulators
Another useful class of manipulators is the telescoping design (Fig. 5.14), in which snugly fitted or threaded components extend or retract via sliding motions induced either pneumatically or by shaft-driven rotations of linked elements.
The best-known telescoping nanomanipulator design has been proposed by Drexler10 in the context of a stiff mechanism that may be used for precision molecular positioning and assembly work. As depicted in Figure 9.8, the mechanism features a central telescoping joint whose extension and retraction is controlled by a 1.5-nm diameter drive shaft. The rapid rotation of this drive shaft (up to ~1 m/sec tangential velocity) forces a transmission gear to quickly execute a known number of turns, causing the telescoping joint to slowly unscrew or screw in the axial direction, thus lengthening or contracting the manipulator. Additionally, two pairs of canted rotary joints -- one pair between the telescoping section and the base, the other pair between the telescoping section and the working tip -- are controlled by toroidal worm drives. These joints enable a wide variety of complex angular motions and give full 6-DOF access to the work envelope (Fig. 9.9; isolated planetary gear (Section 2.4.1) shown at lower left, for comparison). By engaging and disengaging various drive shafts using clutches, these shafts can be made to turn through a known number of rotations between locked states giving odometer-like control of manipulator joint rotations. The power supply and drive shaft control systems originate outside the manipulator, thus are not subject to tight geometric constraints.
Drexler's telescoping manipulator is approximately cylindrical in shape with an outside diameter of ~35 nm and an extensible length from 90 nm to 100 nm measured from top of base to working tip. The manipulator includes a hollow circular channel 7 nm in diameter to allow tool tips and materials to be moved from below the manipulator through the base up to the working tip. At the tip, a slightly larger region is reserved for a mechanism to allow positioning and locking of tool tips. Device mass is ~10-19 kg, and the manipulator is constructed of ~4 x 106 atoms excluding the base and external power and control structures. Device stiffness (which scales roughly with arm length10) is ks ~ 25 nN/nm, so that a force of 1 nN may be applied with an elastic deflection in the arm of only 0.04 nm. The classical positional variance of the manipulator at T = 310 K is Dx ~ (kT / ks)1/2 ~ 0.01 nm, although with a manipulator tube segment thread pitch of 0.5 nm and drive shafts that execute ~700 turns to produce a single tube segment rotation, segment extension may be altered in ~0.001 nm steps. The workspace includes a 180° hemispherical-shell volume ~100 nm in diameter and ~10 nm deep. Conveyance of the tip through a full 100-nm arc requires ~10 microsec at a conservative ~1 cm/sec arm speed. Efficient task planning permits much smaller motion arcs, allowing ~MHz operating frequencies. The manipulator dissipates 0.1 pW of power (power density ~109 watts/m3) while in motion under no-load conditions. Energy dissipated per unit mass of delivered payload is scale-invariant, and the design of telescoping manipulators generally becomes easier with increasing size.10 Power dissipation scales roughly as the square of arm length and as the square of tip velocity;10 power density scales inversely with arm length.
Like the ciliary and pneumatic designs, the telescoping manipulator is hermetically sealed, thus maintaining a controlled internal environment while allowing leakproof operation in vivo.
Last updated on 20 February 2003