Nature’s Mechanical Secrets Could Help Build Faster Robots
The most impressive jaws in nature belong not to a bear or a shark but an insect called Odontomachus bauri. Popularly known as the trap-jaw ant, its mandibles, which it uses to snatch prey and catapult itself away from danger, accelerate shut at 1 million meters per second squared. The force from each jaw exceeds the insect's body weight more than 300 times over, propelling the ant to heights as lofty—for an bug, anyway—as eight centimeters, and distances of close to 40 centimeters.
The insect's secret is a spring-latch system that allows it to store large amounts of energy and release it almost instantaneously. Such systems are common in small organisms, including animals (like the infamously pugilistic mantis shrimp), plants (like the infamously carnivorous Venus flytrap), and even mushrooms, many of which eject their spores with phenomenal fungal power.
But not every organism’s spring-latch system works the same way. "We've known for a long time that small biological things are capable of producing power that muscles alone cannot—and we've known that springs and latches are involved, because we could see them," says Sheila Patek, an evolutionary biomechanist at Duke University. "What we don't necessarily know is how biology does it." And if the biologists don’t understand those mechanisms, engineers can’t translate them into synthetic systems like robots.
To better understand the mechanical principles that govern tiny, fast things, Patek and a multidisciplinary team of researchers spent half a decade standardizing researchers' non-standardized measurements of mass, speed, and acceleration in more than 100 biological and synthetic systems—e.g. Venus flytraps, as well as robots inspired by Venus flytraps—and modeling the interactions of minuscule springs, latches, projectiles, and motors. Their results, which they document in the latest issue of Science, lay out the general principles underpinning small, speedy, mechanical systems, providing biologists a systematized resource for studying biomechanics, and engineers their clearest view yet of what they can achieve with their synthetic designs.
In fundamental terms, the researchers describe how motors, springs, and latches can be tuned to optimize their power output. It's one thing to perfectly mimic the mechanics of a trap-jaw ant's maw; it is a much better and more useful thing to grasp the principles underlying those mechanics. By understanding evolution's finely tuned solution to an individual problem, you can apply the rules that govern those solution to any problem.
Among the researchers' greatest contributions is a detailed examination of the points at which small, spring-based systems become more useful than ones powered entirely by muscles. See, muscles can only move so fast, and the faster they move the less force they exert. This puts a cap on their power output (you know, force times velocity). Spring and latch systems offer a way to circumvent the force-velocity tradeoffs of muscles. And while scientists generally understand that the benefits of springs fall off at a certain size limit (you can use a bow to launch an arrow, but you'd never use one to propel, say, a hefty rock), Patek and her colleagues have done the tedious work of characterizing that limit by fiddling with the individual components of these teeny systems.
And as it turns out, that limit varies considerably based on what you want a system to achieve: Do you want to maximize the power delivered to your projectile? The duration of your projectile's take-off? The velocity at which the projectile launches? In each instance, the mass at which a muscle-driven system becomes preferable to a spring-driven one is different.
Let's pause, here, to talk examples. Consider a locust. Consider also a mantis shrimp. Both organisms use spring based systems—the locust to jump, the mantis shrimp to demolish the shells of snails with a toothpick-sized hammer—but their systems address very different problems.
The locust needs to develop the force and momentum required for its leap while its feet are in contact with the ground, so its spring and latch system has evolved to develop that acceleration relatively slowly, lest the insect break its legs. The mantis shrimp, on the other hand, needs to annihilate its prey, and so must expel as much energy via its hammer as quickly as it can. Its spring and latch system has evolved to deliver peak acceleration, for a near-instantaneous impact.
What the locust and mantis shrimp illustrate is that these systems can be tuned and timed to achieve dramatically different mechanical tasks. "Each organism has developed a unique solution to a specific problem," says Mark Ilton, a soft matter physicist at UMass Amherst who oversaw the study's modeling efforts. In a series of mathematical simulations, he and his colleagues showed how minute adjustments to individual components—the material properties of a spring, the shape of a latch, the speed at which that latch is removed—can translate to surprising differences in the performance of these small, fast systems.
"There's so much more here than we realized," Patek says. Muscles aren't the only things that experience fore-velocity tradeoffs; every component of the motor-spring-latch system does. Understanding the synergistic effects of those tradeoffs will help biologists better understand how species have evolved, and engineers develop smaller, faster, more robust synthetic systems. "On a certain level this is super basic stuff, but now we can all start tweaking these models and experimenting with, I don't know, weird electromagnetic motors and squishy viscoelastic latches," Patek says. "We've thrown down the gauntlet—now we get to go have fun."
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