Arthrobots & Spiders: Why 6 Legs are Better Than 4

June 5, 2017
Spider Robots

In a recent paper from Soft Robotics, a team of scientists including Woodford L. and Ann A. Flowers University Professor George M. Whitesides and Alex Nemiroski presented a class of robots called "arthrobots." Why "arthrobots"? These soft robots mimic the musculoskeletal system of arthropods (spiders and insects, for example) and use pneumatic joints in their many-limbed prototypes.

Read the paper, "Arthrobots," here

Read the related Harvard Gazette article, here

One of the lead authors, Alex Nemiroski, answered a few questions about how their spiders might assist in search-and-rescue missions, how the design originated, and what comes next:
Q: How did this project originate?
A: The initial idea originated from George observing that straws have an excellent strength-to-weight and that they would make for a good exoskeleton for soft-robots. We experimented with all sorts of ways of cutting notches and inserting balloons until we figured out the optimal combination that gave us our fundamental building block of the joint. From that point, we kept challenging ourselves to make bigger and bigger robots with more and more limbs to see what the limits of this approach were. As we increased the complexity of the robots, we found that we had to make modifications to the joints (e.g., re-enforcements) to support the load of larger robots. We are proud to have made it all the way up to 8-legs, but think that more limbs than that would yield a heavier robot that may need tougher materials to support its own weight.
Q: What differentiates these robots from similar creations?
A: This fits our soft-robots work and actually our low-cost diagnostics philosophy, in that we are using bio-inspired approaches and also, in this work, using very low-cost, off-the shelf materials to engineer quite complex motions. One of the main goals is to make robots out of materials that are inherently safe to work with (unlike hard, metallic robots).
Q: What kinds of applications do you envision?
A: We believe that there are potential applications of larger, more rugged versions of these robots, particularly in search and rescue operations, or where low-cost (disposable) robots may be an advantage. Right now, this is a first-stage work where we have focused on simple materials to prove the concept that these types of simple, bio-inspired actuation mechanisms can yield complex motions. The next step would be to use denser materials for the exoskeleton, tougher rubbers for the joints, and higher pressures - this would enable the robots to be autonomous, carry a load, and navigate complex obstacles, similar to insects.
Q: Why are the number of legs so important?
A: 6-8 legs yields extra stability and maneuverability, especially when climbing something like rubble. With 4 or less legs, the robot needs to be very careful about how it positions its center of gravity and which legs are off the ground, so as not to fall over. With 6 or more legs, the the robot (like insects) can lift multiple legs up without tipping over, and does not need to exert much effort try not to fall. It also means, from a robotics perspective, the algorithms for locomotion do not necessarily need to be closed loop, and do not necessarily need to take into consideration the momentum or center of gravity of the robot. Therefore, controlling such a robot, even though it has more limbs, is actually easier. Also, having more legs can also help spread the weight of a load to more points of contact, for example, if carrying a person. 
Still curious about the mechanics of insect legs? This 1951 article from Science explains why the overwhelming majority of the several million species of insects rely on three pairs of legs.

Open the Article Here:

Arthrobots.pdf483 KB