The Shape of Things to Come
By Forrest Bishop
Shape-Shifting Matter, Self-replicating Interstellar
Nanoprobes, Drexler Universal Assemblers... Appeared in NanoTechnology
Magazine - April 1996 Issue. Pepper T. Kim layout, Copyright (c) 1996
Forrest Bishop, All Rights Reserved.
Introduction
Our ability to imagine must always exceed our physical grasp; every
conscious action is preceded by thought. The following is an overview
of several interrelated ideas, not all of which may be realizable. If
there is validity in the general concepts here, as well as in the diamondoid
manu- facturing paradigm [1], we may see Shape-Shifting Terminators, self-replicating
interstellar nanoprobes, Drexler Universal Assemblers, the terraformation
of the Solar System, and more, within our lifetimes.
The Shape Shifter
The Shape Shifter is an aggregate of "standard active cells",
and other specialty cells of various dimensions and descriptions. An "active
cell" is a space-filling polyhedral construct having power, signal,
drive, and mechanical interfaces on each face of the cell [2]. A connected
collection, or aggregate, of these cells form a "kinetic cellular
automaton" capable of radically changing its shape. It is important
to realize that a connected group of cells can be moved as a single unit
some distance, then some new group can be formed and moved, perhaps in
a different direction.
The size of the individual active cell is arbitrary, just as is the size
of a pixel on a two dimensional monitor screen. The highest quality monitors
use very many, very tiny, pixels to achieve their superior resolution.
By analogy, the most useful and intriguing active cell would be very small,
and atomically precise. A properly constructed "MNT Active Cell"
should be able to move around without ever wearing out.
If you had a bread-box size piece of this "material", it might
take the place of most of your electronic and household appliances by
simply turning into the machine you wish to use at the time. Perhaps you
would have it walk the dog (or simply be the dog), clean the house, and
fix dinner in its free time. Tendrils of it might be used to perform microsurgery
and other very delicate operations, such as removing plaque from an artery
wall. As this is a machine that can radically alter its shape and surface
composition, most applications haven't even been dreamed of.
An MNT Active Cell
For the purpose of illustration, a particular "XY cube" MNT
active cell was designed and characterized. Here 'XY' (and 'Z') refers
to the local coordinate system of the particular face of the cube under
consideration, not to any global coordinates. "XY" refers to
the fact that the cells have mechanical interfaces on each of their six
faces, that allow sliding two interfaced cubes in one of the two directions
'X' or 'Y'. The cubes are unable to detach the two joined faces by movement
normal to the plane of the joint. They can only move in one of the two
permitted directions at a time, and must be aligned with four faces flush
in order to change direction. .It has been shown [2] that the restriction
to only sliding does not incur significant penalties on the possible movements
of the cells, while providing a number of advantages.
This baseline design has a nominal edge length, or cell metric,
of 167 nanometers. The principle reason for the selection of the cell
metric was to adhere to the design rules in [1], especially regarding
size and spacing of conductors. It should not represent a lower bound
on the size of an active cell. Secondary constraints include structural
considerations, as well as the size of the included motor and controller.
The structural material is of the diamondoid class, assembled by
the putative methods of molecular manufacturing [1]. Each active cell
contains an internal Drexler "rod logic" [1] controller to perform
various housekeeping functions, as well as to communicate with its neighboring
cells.
When two XY cubes are interfaced and aligned, some method of locking
them together is desirable. This might be done piezoelectrically, but
the baseline design uses four tapered, retractable locking pins extending
from one cell to complementary holes in another. There are other mechanical
methods of accomplishing the same thing.
There is, in addition to the controller, an internal electromechanical
interface switch for power and signals. Energy is delivered to the cell
electrically, via roller contacts. The drive system looked at in some
detail is a linear electrostatic motor derivative. The conductor material
is unfortunately not specified. As the electrical lead and contact resistances
cannot be fully characterized, a programmable voltage multiplier is incorporated
to keep the cell-to-cell energy transfer at the nominal voltage. This
imposes a major signal propagation delay, which might be decreased, even
to the theoretical limit (with superconductors), in a more refined design.
It should be noted that a system composed of these kinds of active
cells may form the core of (in addition to a Universal Assembler) a research
facility inquiring into many aspects of atomic-scale engineering. Some
of the research would, in turn, produce data for better design of the
active cell.
Geometrical, Mechanical:
The mechanical interfaces consist of orthogonal "T-slots" cut
into a face of a cube. The slots are parallel to the edge of the face,
and actually form "T-posts" when both sets of slots are considered.
There are two complementary sets of these T-posts, and each cube has one
type ("active faceplate") on three of its adjoining faces, and
the other type ("passive faceplate") on its other three adjoining
faces. Although the number of T-posts is somewhat arbitrary, the reference
design uses nine T-posts on the active face, and 16 T-posts on the passive
face.
These features of an XY cube introduce a chirality to the aggregate,
and a specific orientation for mutual interfacing. As there are no permitted
modes of rotation, this internal orientation is maintained regardless
of the configuration of the aggregate. An active face is therefore always
adjoining a passive face.
In order to move in the 'X' or 'Y' direction, the two pins on the axis
perpendicular to the desired movement are first withdrawn. With the drive
engaged, the two pins in line with the movement, which have tapers in
this direction, are then withdrawn at some controlled rate.. Owing to
this taper, the cell begins its movement before the pins are fully withdrawn.
The purpose of all this activity is to prevent the cell from wedging by
rotating about the local 'Z' axis. Once the cell moves out of the aligned
rest position, this is no longer a major issue.
With the electrostatic drive systems outlined below, an active cell can
be built that has no breaches, or holes, through the cell wall, an advantage
in environments such as air. This particular design is composed of six
individual faceplates (three of each type) which are then assembled, along
with the internal parts, to form the active cell. The inclusion of these
joints, along with the mechanically operated locking pins may limit their
use to in vacuo.
Each of the active and passive faceplates has the same kind of edge treatment,
consisting of a finger joint and other standardized features. As the joint
surfaces are parallel to the cube faces, the faceplates can be assembled
in any order. A pin introduced at one end of the formed edge joint holds
the two faceplates together. The pins are always inserted through a hole
in an active faceplate. In addition to simplifying the manufacture, this
modularity physically allows a modicum of self repair (remove and replace),
in the case of a radiation damaged space probe, for example. The "Overtool"
concept also makes use of this feature.
Drive Systems:
It is possible to use rotating motors, clutches, pinions, racks on the
faces, and so forth to drive an XY cube. The T-posts on the active faceplate
might house the drive pinions and their associated bearings. The toothed
racks are then incorporated on the channel surfaces of the passive faceplate.
These pinions might also serve as rolling electrical contacts.
There are several alternative linear electric motor types of interest
for the mesoscopic active cell. One is a linear version of the electrostatic
motor presented in [1], Section 11.7. The tunneling contacts and variable
work function surfaces would be on the faces of the active T-posts, with
the conductors embedded in the channels of the passive faceplate.
This particular design uses the "Dielectric Drive". Consider
two parallel, charged conductive plates as in a capacitor. If a slab of
dielectric material, thin enough to fit in the gap between the plates,
is introduced at one edge of the plates, it will experience a force tending
to draw it into the gap.
When the dielectric slab reaches the far edge of a charged plate, no
further motive force is available. It is then necessary to switch to another
set of plates further along the path of motion. This is not unlike the
switching in an ordinary forced-commutation electromagnetic motor.
To implement this drive system, the undersides of each of the nine
active T-posts are divided into several conductive regions, separated
by insulating gaps. The "channel surface" of the active faceplate
contains the ground planes, which together with the conducting plates
embedded in the undersides of the T-post shelves forms the powered portion
of the dielectric drive system. The passive faceplate is a solid piece
of dielectric material (diamondoid), with interfacing conductors embedded
in its channel surface.
The operating voltage range for the dielectric motor is quite variable,
from greater than zero, to some value below the dielectric strength of
the specified 3nm diamondoid insulating gap.For this particular design,
at one volt applied, this yields an acceleration of about two million
gravities, neglecting damping.
Note that the cell being moved relative to the power supply isn't
necessarily the one providing the motive power. For this and other reasons,
it is desirable to utilize sliding electrical contacts in this design.
This is another area where more research is needed.
Power and Signal:
The four locking pins also serve as the cell-to-cell electrical contacts
when they are extended into their matching holes. They are spring- loaded
so as to be extended when no power is applied (i.e. "deadman"
switches). A lever from the rod logic engine operates the pins. Two of
these pins form the sliding contacts when they are in their partially
retracted position. The other two are not in use when the cell is moving.
A fifth pin at the center of the active faceplate is for a more direct
and lower resistance power transmission path. More such pins and holes
can be added if a finer step resolution or greater shear strength is needed.
Note that the dielectric drive is capable of stopping in many places,
depending on the layout of the conductive plates.
The power and signal interface switch is located at the center of the
cell. A set of power and signal busbars from each face terminate in a
mechanically operated, conductive sleeve (or roller contact) that can
be extended to contact a conducting "routing" cube.
A serial signal bus may be implemented by modulating the power, but the
voltage boosters add some complications to this. An alternative is a separate
line for signal . Another alternative is acoustic transmission via the
locking pins. These pins should include contact "switches",
levers actually, to verify the locked and unlocked conditions. These same
levers could be operated directly by the logic engine for serial signaling.
Research Facility
The ability to manipulate several very small objects in three dimensions
is very limited at present. It may be possible to build active cells with
current silicon micromachining techniques, perhaps combined with some
"bottom up" components. These could be very useful in studying
the behavior of materials at the mesoscopic scale. Any face of a cell
can be replaced with some other tool besides the standard faceplate. It
may be possible to use these microtech cells to construct an even smaller
family of active cells. An intriguing variant would use larger active
faceplates, connected to an external computer, to manipulate smaller passive
faceplates not burdened with onboard microtech processors.
Space Probes
A Shape Shifter makes for a very interesting space probe. The ability
to act as the structural material, computer system, attitude control,
and reaction mass for the probe's various instruments and propulsion systems
significantly decreases the amount of mass required. The Starseed/Launcher
electrostatic accelerator proposal is built around these notions.
The Overtool
This is a proposed method of constructing a Drexler Universal Assembler.
The essential features include the MNT Active Cell, described above, and
a manipulator mounted on an active cell. An aggregate of these devices
should provide the necessary functionality, variability, and means of
transport to form the core of a general assembler, for macroscopic assembly
as well as for diamondoid mechanosynthesis.
The active cells and manipulators are further broken down into a set
of standardized parts. The assembler should be capable of making and assembling
all of these parts, and thereby replicating itself.
The essence of this proposal is to reduce the number of individual parts
(the active cells) to a small set of identical components governed by
a few simple rules of interaction. A collection, or aggregate, of these
cells then form a device of arbitrary size which can change its configuration
to fit the desired task.
A second type of cell has much the same functionality as the "standard
cell", except that one of the faces of the cube is replaced by an
"XYZ gantry", similar to a contemporary Coordinate Measuring
Machine, to permit the fine control and generate the forces needed for
diamondoid mechanosynthesis. This simple type of three-dimensional manipulator
may seem to be too restricted in its motion to perform the many necessary
maneuvers. However, when incorporated in an active cell aggregate, and
programmed to work in concert with others of its kind, this machine can
indeed execute a large class of rotational and translational movements
of interest to the mechanosynthesist.
The XYZ Gantry has a base configuration that allows mating to the open
side of the otherwise standard active cell. There are six possible gross
orientations for this gantry (+-('x','y','z')). Additionally, since the
faceplate joints are four-fold symmetrical, two or four 'XY' orientations
are possible, depending on the symmetry of the gantry itself.
The tip of this mechanical arm may have a holder for interchangeable
tools, or perhaps a dedicated tool. As there are six faces on a cube,
there are six possible orientations for this "Gantry Cell".
This means a number of Gantry Cells can be simultaneously engaged in a
particular process. Some of the cells can be holding, straining, and rotating
an arbitrary workpiece, while others fetch reactive species or perform
abstraction reactions.
Although it is not necessary to use the XYZ gantry here, a number of
advantages incur. Its geometry is quite compatible with the cubicle active
cell, simplifying the design. In addition, the Cartesian kinematics are
trivial, for both forward and backward solutions. This means the development
time, as well as the real-time computation, are significantly less than
what would be required for a more elaborate robot arm.
It may be possible to allow a portion of the 'Z' axis actuator to extend
into the included volume of the active cell, depending on the arrangement
of the internal parts of the five-faced standard cell. This is not strictly
necessary. A Gantry Cell can be two or more times the standard cell edge
length in its local '-Z' direction without putting undue constraints on
its versatility. The gantry cell of figure (Gantry Cell) is designed to
fit within a volume of two standard cells without interfering with the
interior components of its active cell base.
A secondary feature is the "Moiety Palette", which is based
on an active cell "passive faceplate". What would be the interior
surface of this component is made flat, with receptors incorporated for
the desired molecules. A similar device is used for waste removal.
With the above specifications, as many as 14 gantry cells of the six
possible gross orientations can be brought together such that their working
tips are confined to the same small volume (say 1000 nm^3, depending on
the tip geometry). For example, four each of +-'z', two each of +-'y',
and one each of +-'x', are brought together as depicted in the "14
gantry cells" figure. One of the 'z' cells is shown with a different
'XY' orientation than the other three, as mentioned above.
This configuration, or some part of it, would be useful in straining
a bearing sleeve such as the one depicted in [1], Figure 1.1. The +-'x'
tips are brought together to form a mandrel. Some portion of the remaining
12 available gantry cells (say, some of the four +'z') then bring a pre-assembled
flat strip up to the mandrel and wrap (and strain) the strip around it.
The four +-'y' cells can assist in this. While these cells hold the strip
in place, another available gantry cell(s) (the four -'z') performs the
necessary abstractions and reactions needed to finish the joining. The
+-'x' tips then retract, freeing the finished part.
The architecture of an active cell aggregate is eminently scaleable,
from the assembly of mesoscopic systems, to the mechanosynthesis and assembly
of quite large structures using the same active cell family. The diameter
of the shell in the "Strained Shell"
figure is intentionally omitted. It may be 1700 nanometers, or perhaps
1700 kilometers (space-based). It would be necessary to build such a large
structure of something very strong and cheap, like diamond.
Using the "Strained Shell"
figure as an example, consider now that the active cells are not being
used to strain the structure, but rather to transport gantry cells and
moiety palettes to and from the entire exterior surface of the (unstrained)
shell. This shell is now being constructed as a single unit, in a manner
akin to crystal growth. This principle can be extended to more elaborate
morphologies, as well as being combined with the traditional convergent
assembly processes.
Conclusion
Our machines will come to resemble biological systems in their complexity,
adaptability and agility. The above is only one genus. It is instructive
to cast these directed, replicating machines in the light of a new form
of intelligent life.
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