Monday, August 5, 2013

Exploiting Bio-Inspired Limbless Locomotion in Robots

Limbless locomotion is utilized by several creatures in the nature.  Snakes have perfected this mode of locomotion through millions of years of evolution. Limbless locomotion has several distinguishing characteristics. It enables the creature to move through extremely rugged and cluttered terrains.  The obstacle height simply does not matter! The creatures can just go around them (or over them in some cases). It is also quite versatile and can be used to climb trees and jump over gaps. It also appears to work for a wide variety of size scales from tiny worms to huge pythons. At least in theory, this mode of locomotion is highly fault tolerant due to the high degree of redundancy in joints needed to locomote.  

Limbless locomotion has fascinated roboticists for decades. They have created very impressive platforms that present remarkable advances in robotics. However, currently robots that use limbless locomotion do not come close to their natural counterparts in terms of capabilities. Unfortunately, we do not yet have access to engineered actuators that can match natural muscles founds in biological creatures. We also do not simply have highly distributed fault-tolerant self-calibrating multi-modal sensors and materials with highly anisotropic friction properties. So our design options for limbless locomotion are limited and truly mimicking nature is simply not possible right now.

In the short term, we are better off taking a different approach to exploit inspiration from biological creatures in field of robotics. I believe that we should try to find a useful feature in the nature and exploit it to the fullest extent. This often means that the feature will be highly exaggerated or amplified in the engineering context. Ultimately, this might make our bio-inspired robots look like a caricature of their natural counterpart. We need to keep in mind that the nature imposes constraints on the size of features due to the inherent biological processes for realizing them. For example, so far naturally evolved flying creatures do not come close the size of engineered jumbo jet. So the notion of our robots looking like caricatures of animals should not a viewed as a disappointment. We should simply “borrow” ideas from nature and “distort” them to fully exploit their engineering potential based on the technological constraints.

James Hopkins, a graduate student in my lab has been trying hard to overcome speed limitations of engineered limbless locomotion. He decided to take his inspiration from rectilinear gaits utilized by some snakes. He then decided to dramatically exaggerate rectilinear gaits to increase the speed. This gait is ultimately implemented by expanding and contracting the body.  The current prototype can achieve the speed of one mile per hour. In this design, the speed is linearly proportional to the length of the robot. So by doubling length we should be able to easily achieve the speed of two miles per miles. Unfortunately, the motors used in the current design won’t allow us to go much beyond 2 miles per hour. For that, we will need to utilize better motors! James used actively actuated friction pads near the head and tail of the robot to improve traction. He has found that different terrains require different friction pads. He has used pads ranging from bed of nails for traversing over grass to rubber for carpets. This robot required us to use 3D printing to realize a novel mechanism for expanding and contracting the body and maintain a small body cross section. You can check out the video of the current prototype below:

When people see this video the first time, they often say “this looks nothing like what we have seen in nature”. Some of them look disappointed and perplexed! Some people have suggested that we should try to make our robot move like a real snake. I am against creating a search and rescue robot that moves and looks like a real snake. Imagine that you are trapped in building destroyed by an earth quake. I don’t know about you but the last thing that I want to see in that situation is a snake crawling towards me. For the record, I like birds but I am not fond of snakes! 

We wanted to come up a clever name for James’s new robot. I wanted to call it LiLo (Limbless Locomotor). But unfortunately I found that Ms. Lindsay Lohan, a paparazzi magnet celebrity in Hollywood is known by that moniker. So we decided to call our robot R2G2 (Robot with Rectilinear Gait for Ground operations). It may come as a surprise to you, but James and I are Star Wars fans!


  1. Fascinating work! Have you tried navigating through rubble with this robot yet? Will the friction pads currently in use be able to grab onto rubble like that?

    1. To date, we have not attempted to navigate through rubble, however, I have been working with a new anchor design that I believe would be effective in that enviroment. Use of the new anchor was observed in beginning of the video where the robot navigates grass/sand enviroment. Once we locate a realistic rubble environment, we will perform trials and document the results.

  2. Sir, what are your beliefs with respect to the development of anthropomorphic features in robots. I understand that the development of humanoids is plagued with exceedingly complicated issues of kinematics and controls, but don't you think they would offer superior locomotive capabilities? Especially, when you think about "exaggerating" the engineering capabilities of such a robot! Between, I quite like the play on R2D2, clever!!!

  3. This is indeed a neat piece of work and demonstrates that some features of biological systems could potentially inspire albeit similar but new type of functionality.
    A robot made up of actuators like servo motors and discrete modules cannot have "exactly" same mode of locomotion as that of its biological counterpart. Imitating the bottom-up (cell-tissue-organ-body) biological processes is still not a technological reality. This severely restricts the realizable shapes and motions of the inspired robots. Human creativity however circumvents this limitation of robot fabrication process to attain interesting functionalities as demonstrated by James's work.
    In order to get closer functional similarities we need to find ways to mimic process of physical structure fabrication from the nature as well. In other words, we strive for a deeper bio-inspiration!

  4. R2G2 looks great - much sleeker than the last time I saw him! I'm glad you're getting away from the "snake" association, as he moves much more like an earthworm or caterpillar.

    I would tend to agree with your perspective on bioinspired engineering. In addition to material constraints, the organisms we mimick have many other external pressures on their morphology that may be irrelevant to the engineering application. For example, the robots we build will probably not need to worry about maintaining a constant body temperature, capturing prey, or avoiding predators. Then again, we do see things like Ecobot, which is designed to "digest" food via a microbial fuel cell. And let's not forget the bird robot's encounter with the hawk!

    Of course as you point out, when building something out of circuits and actuators instead of nerves and muscles, there is a completely different set of constraints to deal with. So why compound them with additional, extraneous constraints from the biological domain?