Planar Catom Design Notes
The Evolution of a Modular Robot
As the planar catom has evolved through eight versions, one principal goal has guided its development -- sustaining robotic motion without moving parts by reliable contact of one catom with another across the magnet faces that encircle each device.
To optimize these contacts, the research team has steadily improved the geometry and power of the planar catom's magnet system, increased the density and capacity of its solid state circuitry, integrated its controls and drivers, improved wireless communication between modules and built better containments to make the robot more stable.
This dynamic evolution of the device can be seen in the photo at the top of this page, which presents, from right to left, Planar Catom versions 2 through 6. More detail on these version changes is presented below.
The focus of these modifications and upgrades can also be seen at the midpoint in the planar catom's design life, with the closeup, right, of Planar Catom V4, the version that introduced densely packed, custom-made circuit boards with a full set of drivers to power all 24 magnets. This version created the first fully equipped system whose features provided a basis for testing and modification in the project's search for a reliable demonstration of planar catom motion.
The closeup of Planar Catom V4 shows a board with 24 controllers at the top of the stack and a connector board below it with wires to each of the 24 magnets on the two magnet rings located at the base of the catom. The two magnet rings appear as the round steel faces braceleting the body of the device, each cylinder wrapped by copper magnet wire. The rings discharge electro-magnetic force in rapid contact with their counterparts on an adjacent catom and thus induce cooperative movement -- creating motion without moving parts.
To drive the high power electromagnet array, subsequent versions adopt multiplex lines that route signals to groups of four magnets on each ring. The two robust rings of 12 electromagnets are designed to permit full 360 degree rotation. As magnets on adjacent catoms come close, they energize before they touch, and the attractive force rotates the catom in a rapid charge and release around the circumference of each catom managed by the controllers.
The evolution of these systems continued through four subsequent versions to improve upon the module's capacity and performance. A brief history of these modifications follows below with pictures of individual models and their magnet types.
A Close-up of Planar Catom Version Changes
Planar Catom V2 [Magnet Version 1]
With its stack-of-pancakes appearance, Planar Catom V2 emphasizes the initial reliance of the claytronics research team on ready-made parts to build quickly a hardware device that corresponded to the general concept of a self-actuating modular robot that moves without moving parts.
Catom V2 has a wireless top-mounted LCD and receives power from a power plane through conductive pads on its base that elevate the device from the table.
For the critical contact device that transfers force to an adjacent catom, the V2 adopted a prefabricated magnet shown in the inset photo.
The V2 provided the first practical understanding of the challenge of building hardware that places a sufficient electromagnetic force in a precise location on the very small target of an adjacent catom’s magnet-sensor.
Although it gave original form to the planar catom, the V2’s magnets proved to be too weak to generate sufficient torque to move one V2 against another.
Planar Catom V3 [Magnet Version 2]
With the failure of prefabricated magnets to actuate the planar catom, the team conducted a magnetostatic analysis to calculate requirements for flux density, field intensity and inductance and then made magnets by hand from magnet wire and steel cores.
The result was the first of two barrel-shaped designs shown in the inset photo and used in planar catoms V3 through V5.
V3 also introduced two rings of 12 magnets as a more efficient geometry for electromagnetic force to generate motion. Improved magnet design also inspired more development of the planar catom’s controllers, resulting in a prototype design for a high-power electronics system to drive the electro-magnets and the communication devices on the module.
As a prototype, the V3 magnet driver board was designed to power 6 magnets, which was sufficient to produce a back and forth motion between two adjacent V3s. After modelling the concept with this design, the team went back to the drawing board to create a controller board with a complete set of drivers and other features for the first fully equiped prototype of the planar catom.
Planar Catom V4 [Magnet Version 2]
In order to boost performance of the magnet configuration installed in Version 3, its successor Version 4 carried the first generation of fully loaded electronics drivers on very densely packed controller boards, equipped to charge 24 magnets.
The upgrade in the electronics package included an individual controller ring dedicated to each of the two magnet rings. The V4's fully-powered magnet rings produced the power needed to create motion from the exchange of force between adjacent magnets. However, the round faces on the magnets, retained from the V3, proved to be sloppy terrain on which to seat the pull and push of electromagnetic forces while the catoms rotated against each other. As a consequence, it produced a discontinuous movement and was not able to achieve complete rotations.
So the team redesigned the module to alter the structure of the magnets and test a configuration that would improve the contact between magnets on adjacent catoms.
Planar Catom V5 [Magnet Version 3]
To improve mechanical contact, the barrel-shaped magnets introduced in the two preceding versions were given a flat-faced core. This enabled the magnets to find a better seating during contacts between adjacent catoms.
The shift to flattened magnet faces required a reconfiguration of the magnet coils and in turn called for a longer chamber in which to install them. That realignment required a wider diameter for the catom containment, which gives the V5 the puckish appearance of its studded rim.
Despite the change in its physical profile, Planar Catom V5 carried the same components as its preceding version, including high-density controller boards, drivers and magnets.
These mechanical changes settled the finicky behavior of the electro-magnetic package, and the V5 proved able to create motion on a reliable basis although it failed to deliver the full spin of 360-degree rotation.
Planar Catom V6 [Magnet Version 4]
Encouraged by improvements in magnet performance, the research team created Planar Catom V6 to optimize the delivery of power through the magnet face from the high-density electronic controllers.
This objective led to the design of a horseshoe magnet bearing two flat faces at the ends of a steel column that holds the copper wire coil where the magnetic field is generated. The magnet is shown next to the V6 in the inset picture to the left.
This innovation delivered a dramatic improvement over the barrel-shaped magnets of earlier versions. It nearly doubled the torque (or rotational force) of the catom. As a result, the V6 met a critical benchmark for this stage of the hardware program in claytronics research. Increased turning power and improved seating of magnet contacts enabled Planar Catom V6 to perform full rotations. This result was the first engineered demonstration of the concept of “motion without moving parts” in a pair of modular robots.
The V6 also returned to the smaller diameter of earlier versions of the planar catom, even though it used the larger magnet coils developed for the V5. The need for more space was accommodated skyward, as this device introduced a new line of taller planar catoms.
Satisfaction with the improved performance, however, was not unqualified. A bug came to light in the high-density, high-powered driver board, which illuminated reliability issues present in the planar catom from V4. This discovery motivated the team to rework the electronics and introduce other improvements to the planar catom.
Planar Catom V7 [Magnet Version 5]
Coaxing more power from the horseshoe magnets, Planar Catom V7 featured a new design for its magnetic coils.
Additional improvements were also made in this version to the physical stability of the device through increased precision for the seating of magnets and electronic controls in the interlocking acrylic casings of the module.
With more secure fitting for its parts, the V7 also adopted a new connector system that allows easier assembly and disassembly of controls and magnet rings. As have earlier versions, the electronic system for the V7 also carried MOSFET semiconductors. Its driver boards introduced Pulse-Width Modulation and multiplexing of signals to allow the magnet groups to operate on a 1-of-4 basis. The device performed well.
Nonetheless, while providing the electronics that create a well-modulated electro-magnetic connection between catoms, the V7 experienced a continuation of issues related to mechanical alignment and physical contact between catoms.
Planar Catom V8 [Magnet Version 5]
Planar Catom V8, shown below with the horseshoe magnet, is the current version. It was developed to eliminate mechanical misalignment in the positions of magnets on adjacent catoms in earlier versions of the planar catom. Misalignments cause breaks in the rapid sequence of attraction-repulsion that leverages motion between catoms.
This issue has followed the engineering of the planar catom through every version. Its persistence reflects the extreme precision with which a very small magnetic force -- 12 milliNewton-meters -- must be delivered to generate the torque that turns the devices. The tolerance for these contacts is dictated by a natural decay of magnetic force -- measured at a cubed rate as distance occurs between them.
This physical limit of the force requires one magnet to be placed exactly against another while the devices rotate. The alignment of magnets thus presents the challenge of two magnets meeting at an exact point -- multiplied by the matching of each magnet and 12 others on the two rings of adjacent catoms.
To create Planar Catom V8, the project developed a special jig to align and glue each of the 5.925 mm wide magnets into the snug, laser-cut space of their acrylic rings. This improvement to construction of the planar catom produced a more precisely arranged set of magnets and a resolution of issues related to the physical connection between these devices.
The evolving design of the planar catom reflects the complexity of engineering for creating a new paradigm of mobility for nanoscale devices – one that depends upon the mechanics of cooperation among robots to induce motion without moving parts.
The knowledge from these test-of-concept modules prepares engineers for the fabrication of even smaller nano-scale modules and provides a learning curve that has moved the claytronics project closer to construction of millimeter-scale catoms.
