Giant Helium Catom Design Notes
A Giant Helium Catom is designed to rotate by extending a flap to the facing flap of an adjacent catom in order to form an electrostatic latch between both catoms. At the same time, the catom in motion releases a flap latched to an opposing edge of another adjacent catom. A shape memory alloy (SMA) in a spring-loaded mechanism manipulates the catom's position. When heated, the SMA contracts and through a pulley raises its corresponding flap. As the SMA cools, a constant force spring slowly closes the flap and returns the SMA to its initial length. This application of force creates the motion of these low mass modules.
Using its array of 24 electrostatic latches for alignment and direction, the catom can rotate across any edge of an adjacent module in order to reach any location on the perimeter of an ensemble of GHCs.
The Giant Helium Catom was conceived as a large-scale platform to test the effects of surface tension and related electrostatic effects on low-mass devices. Such testing is necessary to understand the behavior of modular robots at sub-millimeter - or nano - scale where electromagnetic forces are more important than gravitational forces. During macroscale operations, normal gravitational forces have relatively stronger effects than normal electromagnetic forces. However, nanoscale operations reverse the relative strengths of these forces, producing effects that have not been often observed in engineered systems.
The change in the relative importance of the two forces occurs at the nanoscale because a sub-millimeter device has a small mass in relation to its electron structure. Thus, because of the greater effect of electrons on a minimal concentration of physical mass, the sub-millimeter catom becomes more susceptible to normal electromagnetic forces, such as surface tension, while the normal force of gravity proportionally subsides.
Direct observation of these phenomena in a low mass environment is necessary because intuition of microscale behaviors based upon an understanding of physical forces in macroscale settings rarely amounts to more than guesswork. However, other than very costly investigations involving scanning electron microscopes, few instruments are available to provide such direct observations. The Giant Helium Catom was created to fill this gap for the Carnegie Mellon-Intel Claytronics Research Project.
Research into claytronics and other modular robots for use in nanotechnology requires the invention of the research devices that provide approximations of the conditions expected at nanoscale. In the earlier stages of these investigations, the emphasis falls upon the rapid testing of prototypes in order to derive the greatest insights. This is a cost-effective way to drive engineering concepts along the most promising path toward the design of catoms that can be produced at less-than-millimeter scale.
Prototypes for the GHC testbed were developed from balsa wood, glue and plastic parts printed with a fused deposition modeler. Once a design proved satisfactory, it was adapted to a working model constructed from carbon fiber box beams, as shown in the image to the left.
At 2 meters on a side, the GHC provided eight cubic meters of surface area. However, filled with helium, it held a very low mass and was effectively weightless - thus providing a test bed where the electrostatic forces on its large surfaces theoretically represented the dominant force.
Despite its large-scale replication of these theoretical relationships, the Giant Helium Catom proved to be an ineffective tool for the study of nano scale behaviors. Because of the effects of inertia, its motion was unpredictable. In spite of its low mass-to-surface ratio, its moment of inertia caused erratic movement that overwhelmed the binding force of its electrostatic latches and made the devices too cumbersome and slow for the study of motion within an ensemble of catoms.
Although this robot weighs less than 50 grams, its effective mass was closer to 6 kilograms, and it had a center of mass almost 1 meter from its rotation point, which created a very unstable operating condition for the device. Its initial hinge design could not cope with the torques that resulted. In motion, the catom twisted and rolled off its axis when there were minute imbalances or wind currents. Adding additional hinges alleviated those problems. However, the spring-activated return system actuated ponderously slow payloads.
Even so, experimentation with this design revealed an important lesson about the performance of electrostatic latches. Although theoretically designed to provide a level of force that would hold the catoms in place, the mating flaps proved to be susceptible to a peeling effect in which points of insufficient contact between the matching sheets of aluminum foil would separate, and these points of separation would then spread, breaking the latch.
The prototype electrodes were composed of floppy conductors coated with a dielectric material. Although excited electrodes established a good seal, the flexible conducting surfaces allowed the electrodes to separate under small amounts of external force. This caused plates to separate and break the latch. This separation resulted from a peeling of two electrodes from each other at the boundary of their contact. This happened because of slight irregularities in the degree of separation between the two latching surfaces, which then begin to propagate across the entire boundary of contact and cause the electrodes to peel apart.
Study of this peeling effect, which is illustrated in the diagram to the left, led to a more robust latch design and the engineering of a passive mechanical reinforcement of the electrostatic latch. This subsequent design can be seen in the electrostatic latch
employed on the cube-shaped modular robot
Claytronics research seeks to create modular robots that will operate on a scale - less than one millimeter in diameter - at which mechanical devices have never before been manufactured. Testing the performance of devices conceived for this micro-scale poses unusual challenges because physical forces whose effects at the macroscale are well understood often reveal unpredictable effects at the micro scale.
The Giant Helium Catom project didn't produce the results originally intended from its conception. However, it produced results that gave the claytronics research project the insight that inspired a more effective design for electrostatic latches.