This is an excerpt from Make: Soft Robotics, A DIY Introduction to Squishy, Stretchy, and Flexible Robots. Find it at here.
As NASA-funded soft robotic and compliant system concepts begin to attract attention, we jokingly refer to the rover concepts as the “Menagerie” since so many take research inspiration directly from animals. These bioinspired exploration robots include soft elements.
Eye-catching with its squid-like form, Mason Peck and Robert Shepherd’s Soft-Robotic Rover with Electrodynamic Power Scavenging (Cornell University) serves as a perfect illustration of applying the mechanics of biology to exploration. This rover can travel where wheeled vehicles aren’t suited to the terrain and where solar power and nuclear power are not viable solutions. The moons Europa and Enceladus, whose oceans lie beneath a thick layer of ice, are two potential targets. The concept behind the bioinspired squid form is to provide efficient motion in a fluid medium using an incredibly low-power system.
This robot’s architecture opens up the possibility of escaping dependence on limited-life batteries, and scavenging small amounts of power from locally changing magnetic fields and directly applying it to electrolyzing water. Electrolysis produces a mixture of H2 and O2 gas, which can be used to power the robot pneumatically or, even more excitingly, by combustion. When the gas ignites, internal chambers expand causing shape change or even providing direct water-jet propulsion. Although it is still in early research stages, it is not hard to imagine the advantages and appeal of a robotic squid jetting through an alien ocean.
If you’ve ever seen a beetle crawl up a wall or a fly rest effortlessly on the ceiling, you’ve seen a complex compliant mechanism in action. Certain insects have lots of tiny hooked hairs called setae on their feet. As a fly brushes its feet across a surface, the hairs flex and hook into microscopic crevices. One on its own could never support the fly’s weight, but only a small fraction of them need to catch for the fly to get a firm grip. The hairs are biased in a particular direction so the fly can easily let go by just brushing its foot backward.
When this concept is shifted to the flexible engineering space, the hairs become hinged parts that rely on compliant materials for the flex feature and they have a literal metal hook on the end. At NASA’s Jet Propulsion Laboratory (JPL), roboticist Aaron Parness has been experimenting with mass production of such mechanisms.
The hook “hairs” get bundled together into a rover foot, and the result is a rover that can climb upside down and sideways. These Anchoring Foot Mechanisms have been attached to JPL’s LEMUR IIb climbing robots, and tested in conjunction with rock coring drills. Now, suddenly, missions to other planets and moons can include the option of high-value cliff science. Such systems could also be used for asteroid capture and drilling.
ROVERS THAT BOUNCE,
ROLL, OR CLING
The challenges associated with getting a rover onto a planetary surface are huge. Landings typically occur at high speeds and temperatures, relying on single-use hardware and precision timing. New concepts would cleverly circumvent these problems by letting the innate softness of compliant materials absorb the impact. Ideally, the rovers can be launched from orbit, and they will bounce and roll when they contact the planetary surface. The intended impact velocity can be as fast as 15 miles per second without external support, and the rovers will keep on going undamaged. The ability to take a fall without injury also contributes to the possibility of high-value cliff exploration, as was mentioned for the fly-footed climbing robot. This way, rover missions can be carried out on an astronomical body in which a parachute system could never work, such as one with no atmosphere. Other advantages of soft rovers are that they can be packed flat, which maximizes space, and they are relatively light, which drops launch cost.
Tensegrity refers to a structure made of multiple rigid components (none of which touch each other) held together with continuous tension from flexible components. The Super Ball Bot robotic rover harnesses the flexibility of tensegrity structures to traverse challenging terrain.
In this bot, and the other tensegrity projects that have sprung up in its wake, the rigid components shorten and lengthen. The alteration of the center of gravity results in the robot tipping over. The repetition of this tilt generates a bumpy rolling motion, which can be directed toward its target.
These tensegrity rovers have the innate ability to compensate for a single-line component failure of either a hard or flexible part by altering its movement patterns. The flexibility to keep on working beyond component failures leads to a graded capability that can extend missions. Current testing indicates that there can be multiple individual failure points in the tensegrity rover before total loss of useful motion.
How these soft/hard suspensions are bioinspired may be less obvious than other examples from soft robotics. Remember that the bones in our skeletons are held in tension by our ligaments. These aren’t the only soft components doing functional work in our skeletal system, either. Cartilage serves as a protective cushion in the space between our bones, and tendons attach muscle to bone.
Another concept for tensegrity planetary exploration are the Moballs from the California Institute of Technology. These bouncing balls would pack flat for launch and would then inflate and rigidize when deployed. On the planet’s surface, they would be semipassive, collecting kinetic energy from wind or downhill slopes to discharge as magnetic force to add stopping ability and directional driving. Their form takes inspiration from passive biological systems such as tumbleweeds.
Both genres of rolling rover concepts aim to take the network advantage of a small and distributed technology, similar to what we see from CubeSat miniaturized satellites, and bring it to the ground level. By making them relatively small and cheap, you can send more robots, cover more ground, and take more risks, collecting a larger array of data. More data means broader scientific opportunities.
Using an entirely different mode of locomotion, the AoES (Area-of-Effect Soft-Bots) by Jay McMahon of the University of Colorado, Boulder, is specifically designed for the environment on and around rubble-pile asteroids. This bot takes advantage of the large, flexible surface of this type of asteroid to gain adhesion-based anchoring, surface mobility via crawling without pushing itself off the asteroid, and a fuel-free orbit and hopping control using solar radiation pressure (SRP) forces. By adding the options of clinging to the surface, hopping, or orbiting, these soft robots aim to perform on low-gravity asteroids that can be unstable due to their low density.