A spacecraft lands on a distant planet. From it, a four-legged spider-like robot ventures forth to survey the rocky terrain. The robot's long legs gingerly move it across a span of large cobbles and small boulders.
But the robot soon encounters a rock wall that's impassible except for a narrow crevice. Undeterred, the robot reconfigures itself into a serpentine shape and slithers through the crevice to the other side. Safely through, it reconfigures as a spider and continues its mission.
Shape shifter. That scenario could lie in the future for robots patterned after PolyBot, a reconfigurable robot under development at Palo Alto Research Center (PARC). PolyBot contains tens—and soon to be hundreds—of intelligent cubical modules that detach from and attach to each other to create several robotic forms. Each configuration—spider, snake, or rolling track (like a bulldozer tread, but without internal support)—operates most efficiently over a particular type of terrain.
With its modules in a chain configuration, PolyBot can move like a snake or caterpillar.
PolyBot is also learning to find its way around. "We have an algorithm that does some pathfinding. Once PolyBot has a path plan, it runs an algorithm to decide how to place the feet of a spider, say, to get from point A to point B," says Kimon Roufas, a PARC research engineer. Eventually, he says, PolyBot will even know when it needs to reconfigure itself and how to do it. The robot has already performed some reconfigurations and PARC researchers are teaching it to do others.
Adroit engineering and powerful computing are be-hind these advanced feats. PolyBot has only two types of modules, and they've been ingeniously designed to allow for easy, automatic connection and flexible orientation. For computing and control, each module contains at least one 32-bit MCP555 microcontroller from Motorola (Austin, TX) for running algorithms and providing control functions. Two autonomous time-processor units (TPUs) within the 555 perform infrared ranging and motor control, and integrated analog-to-digital converters (ADCs) accept sensor input. Also, because the 555 is widely used in automotive applications, it has an integrated Controller Area Network (CANbus) module that the PolyBot building blocks use to communicate with each other.
E pluribus unum. The two PolyBot module types, called segments and nodes, allow the robot to be connected in myriad configurations. Segments, each containing a motor, join to each other and to nodes and perform the movements and work required of PolyBot. Each segment has two connecting plates on opposite sides, enabling segments to join together in chains or loops. Nodes, on the other hand, are basically six-sided connectors that do no physical work, but can connect segments in complex forms such as the spider configuration.
The cleverly designed connection plates on segments and nodes also contribute to configuration versatility. The plates are "hermaphroditic" (each has both pins and sockets for mechanical connection), so the four pins of any plate can plug into the four sockets of any other plate to mechanically connect the two. In addition, the square plates have four-way rotational symmetry, so any two plates can attach to each other at 90-degree intervals. When two plates attach, they also establish a control link via quadruple-redundant, hermaphroditic, electrical connectors.
The motor and a hinged joint in each segment module enable PolyBot to move. Via the hinge, the motor changes the orientation of one connection plate relative to the other, opposing plate, so that half of the segment moves up and down or side to side with respect to the other half. Multiple, linked segments—in a chain, for instance—perform these simple motions in a choreography that gives PolyBot more complex motions, such as a snake slither or a caterpillar crawl. In most cases, segments are alternately connected for up-and-down and side-to-side motion, so that a caterpillar-configured robot, for example, can crawl forward and also change its direction.
To do useful work, PolyBot must achieve a critical strength level by connecting a certain, minimum number of modules. Below that number, says Roufas, "You have this expensive rug that can't get up!" On the other hand, he adds, if PolyBot cantilevers too many modules together, the modules will either collapse from their own weight or expend all their motors' energy just holding themselves up. Even if some motor energy is leftover for useful work, power efficiency plummets—a bad thing for a robot that eventually will be battery powered.
Brake dancing. The solution to critical strength comes in the form of brakes. Modules in the next generation of PolyBot, now in development, apply brakes that can hold a module in place without using motor energy. In PolyBot's rolling track configuration, for example, only four modules at a time need to actuate their motors—those that are in transition from a straight track section to a curved section, or from a curved section to a straight section. Brakes can lock the remaining modules in place to save energy.
PolyBot's next generation, the G3, will also be smaller and lighter. PARC engineer David Duff has replaced G2's cylindrical motors with pancake motors from Maxon Precision Motors (Burlingame, CA) and added a harmonic-drive gearhead from HD Systems (Hauppauge, NY) that provides a large gear reduction in a very small size. With Duff's design, "We got a huge mechanism, 9.5 centimeters in length, down to approximately 2.5 centimeters," Roufas says. As a result, G3 cubic modules are 5 cm on each side, whereas a G2 module measures 11 × 7 × 6 cm. In addition, G3 has accelerometers and cat-whisker-like contact sensors.
Although PolyBot is still experimental, videos on PARC's website (www2.parc.com/spl/projects/modrobots) provide impressive demonstrations of its potential. One shows some of the steps of reconfiguration from snake to loop to spider. Another shows automatic docking. Still others show PolyBot in action, riding a tricycle and climbing stairs.
Amazingly, PolyBot seems very lifelike in the videos. In one clip showing its chain configuration, it moves so much like a caterpillar that it's easy to overlook the actual hardware and think of PolyBot as a living creature. PARC engineers apparently thought the same thing—they attached a smiley face to one end of the chain. Or maybe they just wanted their project to have a happy ending.