Morphing Morphology --- From Caterpillar Locomotion to Soft-Bodied Robots

The rise of "soft robotics"

2011-04-26T17:24:00.000-07:00

Today, my first publication on soft robots appears in the journal "Bioinspiration and Biomimetics". It is indeed very difficult to publish both in biology and robotics at the same time. Please check out the supplemental videos going along with my paper first. For more stories about the development of my other robots, feel free to browse some older posts.

Almost 5 years ago, I finished my B.A. in a hurry to plunge myself into the research of animal locomotion. As I accepted Tufts' Ph.D student position, I knew that I was caught in a wave of modern robotics innovation. Five years ago, "soft robotics" is a funny term. Even today, roboticists still debate what soft robots really are. In any case, "soft robotics" has transformed from a crazy talk to becoming an active field of research in the past few years. You will hear a lot more about soft robotics this year from me and from many other research teams around the world.

What really triggered this movement is not really clear. But I would venture to say that the DARPA ChemBot project certainly motivated and cultivated the first generation of soft roboticists in the United States. Many Ph.D students PostDocs alike were supported by this program to pursue soft robotics in many aspects. I was one of them.

Although I have received my Ph.D at Tufts and am now working as a postdoc at Harvard, I will continue to blog about caterpillars and soft robots as long as I am still publishing my thesis work throughout the summer. Beyond that point I will probably have to decide where to begin telling a new series of stories about my new pursuit in animal flight.

It's just the beginning!

Caterpillar acoustics

2011-03-27T18:41:00.000-07:00

I have been trying to write a new post with real scientific materials in the past two weeks. Frankly, it was very difficult to decide where to start after such a long break. In any case, I finally decided to start with a literature review about acoustics in caterpillars.

Yes, some caterpillars do make sounds and use acoustics for communications. Sounds are essentially mechanical vibrations transmitted via some medium. Any organism capable of creating vibration has the potential to transmit information acoustically. In recent years, acoustic communication has become one of the hot topics among the caterpillar biologists. We know that soft tissues are extremely good at absorbing vibrations. How do caterpillars make a sound with such soft bodies?

Well, there are a number of rigid parts in caterpillars where significant vibration can be generated. For example, the mandibles (the mouthparts) are made of tanned cuticle (chitin composite). They can be as hard as some aluminum or carbon-fiber reinforced polymers (Vincent 2004). To signal territory, cherry leaf roller caterpillars (Caloptilia serotinella) scrap their mandibles across the leaf surface to create vibrations on the leaf. Sometimes, plucking the leaf would be sufficient to generate the vibration. These behaviors are observed more frequently when a conspecific (another caterpillar) stumbles onto the leaf roll (Fletcher et al 2006). Some direct body vibration was observed in these caterpillars but with much rare occurrence. Most acoustic signals are still generated by working the hard mandibles against the substrate.

Alternatively, caterpillars can drag their rear ends across the leaf to produce vibrations. Yes, “anal scraping ” is the technical term used in the original literature that reports this behavior (Yack et al 2001). As much as it sounds indignant for the caterpillars, the methodology is quite effective. Larvae of the hook-tip moth (Drepana arcuata) develop a pair of modified oar-like setae instead on their anal segments (they do not have terminal prolegs). These stiff “anal oars” are less than half a millimeter and bit into the leaf as the animal drag the anal segment across the leaf surface to produce relatively long and audible noise (Bowen et al 2008). Such adaptation has been observed exclusively in the family Drepanae, and it can be shown that the anal oars are derived morphologies in these small leaf-rollers as a special adaptation (Scott et al 2010).

The acoustics in caterpillars continue to make headlines. In February, the Journal of Experimental Biology features a research article reporting whistling behaviors in caterpillars. Again this is not a metaphoric description but an illustrative one. Caterpillars of the North American walnut sphinx (Amorpha juglandis) can produce sounds by expiring forcefully through the spiracles (caterpillar’s air valves) (Bura et al 2011). When a caterpillar contracts forcefully, the body compression can squeeze air out of the tracheal system. I reported this phenomenon in one of my earlier posts as I put crawling caterpillars under water. In the walnut sphinx, however, the spiracles of the anal segment are elongated to produce wider lips for whistling. The caterpillar produces three types of audible acoustic signals in response to simulated predation and effectively startles an attacking bird.

The diverse morphologies and behaviors of caterpillars continue to amaze biologists in different contexts. For caterpillars that are vocal, they certainly have expanded their abilities to relay signals important for survival.

Caterpillars, make some noise!!

References

Bowen, J. L., Mahony, S. J., Mason, A. C. and Yack, J. E. (2008). Vibration‐mediated Territoriality in the Warty Birch Caterpillar Drepana Bilineata. Physiol. Entomol. 33, 238-250.

Bura, V. L., Rohwer, V. G., Martin, P. R. and Yack, J. E. (2011). Whistling in Caterpillars (Amorpha Juglandis, Bombycoidea): Sound-Producing Mechanism and Function. J. Exp. Biol. 214, 30-37.

Fletcher, L. E., Yack, J. E., Fitzgerald, T. D. and Hoy, R. R. (2006). Vibrational Communication in the Cherry Leaf Roller Caterpillar Caloptilia Serotinella (Gracillarioidea: Gracillariidae). J. Insect Behav. 19, 1-18.

Scott, J. L., Kawahara, A. Y., Skevington, J. H., Yen, S. H., Sami, A., Smith, M. L. and Yack, J. E. (2010). The Evolutionary Origins of Ritualized Acoustic Signals in Caterpillars. Nature Communications 1, 1-9.

Vincent, J. F. V. and Wegst,U.G.K. (2004). Design and Mechanical Properties of Insect Cuticle. Arthropod Struct.Dev. 33, 187-199.

Yack, J. E., Smith, M. L. and Weatherhead, P. J. (2001). Caterpillar Talk: Acoustically Mediated Territoriality in Larval Lepidoptera. Proc. Natl. Acad. Sci. U. S. A. 98, 11371.

Huai-Ti's Ph.D and beyond

2011-02-17T19:39:00.000-08:00

Dear friends,

I apologize for the lack of update for such a long time. In the past 5 months, I've been busy with publications, thesis and postdoc interviews. I have successfully defended my thesis on January 26th and am now starting my first postdoctoral research at Harvard University. Many colleagues inquire whether I would keep on blogging. The answer is yes, as this is my major contribution in science outreach. However, given the among of sleep I've been missing and the number of duties I have, I could probably only update once a month.

In the world of scientific research, the researchers are "the projects". There is no way to separate "work" from what people call "life" because research is part of a scientist's life. In fact, it is what makes a scientist "a scientist".

Sincerely
HTL

My primary Ph.D projects

2010-09-19T21:05:00.001-07:00

It gets pretty confusing about what I do for Ph.D. I am in the Biology Department at Tufts University, getting my Ph.D in biology. However, I've worked with people from biomedical engineering, mechanical engineering, electrical engineering, and computer sciences in the course of my Ph.D training. In fact, most of my colleagues start to think that I'm from "their department" (whatever it is). What exactly am I? That's a very good question which I actually does not have an answer to. Perhaps after I summarize my primary Ph.D research projects below, you may call me whatever you wish with a better understanding of what you are referring to.

My first project at Tufts was to design a force beam array to collect ground reaction forces from all the leg contacts from Manduca caterpillars in two directions simultaneously. This system involves synchronized video tracking and alternative force transducer design. The results suggested that caterpillars may use the substrate to transmit force much like an external skeleton. I'm currently working on the manuscript for the second part of the animal study.

For soft-bodied animals, tissue material properties can contribute greatly to the overall behaviors. My second project at Trimmer Lab was to instrument a lever-arm system to perform uniaxial tests on soft cuticle from the Manduca caterpillar. These soft specimens were often 5mm long and less than 1mm wide. Real-time video extensometry was necessary to control the strain. We then attempted an constitutive model for the material.

To understand how tissue mechanical properties affect the overall animal behaviors, one must go to the structural level. My third project was to investigate the overall structural properties of the Manduca caterpillar over a scaling range. We tracked down all the body tissue that could take mechanical loads and simulate the hydrostatic skeleton in a FEA model. We found a dramatic increase of flexural stiffness associated with body miniaturization. This implies several evolutionary constraint on caterpillar body plans.

Going beyond the Manduca crawling, I wanted to explore some clues about how caterpillars developed inching gaits in the course of evolution. I first simulated different crawling and inching gaits in my first soft-bodied robot lineage. Then I created a second robot lineage to simulate some ballistic behaviors which were supposed to derive from the normal locomotor patterns. Finally, I conducted a field work in Costa Rica to examine all these caterpillar behaviors in nature across different species.

In summary, my practical skills in the lab allowed me to design systems, acquire all the parts/supplies, manufacture/implement devices, and program systems to execute experiments. I would summarize my scientific contribution in my little niche in three statements.

1) During locomotion, soft-bodied animals can use the substrate as their external skeleton, and therefore gain stability and robustness.

2) The mechanical scaling of hydrostatic skeleton may limit the prolegs configurations and locomotor modes in caterpillars.

3) Soft-bodied robots, by definition, cannot rely on exact postural control. The very minute such a robot force a conformation, it becomes rigid.

Mechanics of a ballistic roll - Part 2

2010-09-15T07:03:00.000-07:00

GoQBot is a soft-bodied robot designed to simulate the ballistic rolling escape behavior in caterpillars. The robot mimics the scale, timing and morphing of the behavior while allowing us to explore the control parameters and perform dynamics tracking. Please refer to earlier posts for more background information.
Kinematics tracking can be very tricky especially with high speed erratic movements. In order to compute the angular momentum of GoQBot, we must know the mass distribution of the robot as it deforms quickly within the 0.2 second window. One way to do it is to break down the body into many segments and track each segment individually. However, that requires installing more than twenty 1-mm size IR emitters on a small soft body no longer than 12cm. The soldering would be simply a torture, not to mention the wiring. Luckily, there is an alternative: kinematics-based model extrapolation.The idea is to constrain a virtual model of the GoQBot using the five IR-marked coordinates as the reference. By applying some deformation characteristics on the model, I can easily extrapolate the positions of every bit of the robot. This is exactly what I have done.

Sharing research tools and links

2010-09-06T20:09:00.000-07:00

Hi there,

A new section has been added to my blog called "Huai-Ti's research tools". There you would find some useful links from general referencing to good online stores. There are also some good deals such as a free SEM imaging program (but the image will be public). In addition, you may find some useful software for making presentations or publications. For example, there is a Screen Capture Software that streamlines the print-screen function in most computers. Anyways, stuffs like this could be useful when you need it. I will populate this list gradually.

Over the past four years of my R&D, I have designed over 12 instruments, written more than 5 systems of feedback control programs, placed more than 85 orders for supplies and parts. I guess the number of projects and pet projects in experimental science directly correlates to the spending and instrumentation. Also, people in my research team tend to leave purchasing jobs to me since I've been really good at communicating with vendors and company associates to get the right parts for laboratory research. But I have to say such job is really stressful, although all the spending came from various research grants and not from my pocket.

Mechanics of a ballistic roll - Part 1

2010-09-03T05:58:00.000-07:00

First off... I would like to remind you of the behavior that I'm studying. See the ballistic rolls and flips in some leaf roller caterpillars, and check out my biomimetic GoQBot's performance.

To study the the dynamics of such impressive behavior, the robot has been a great tool to test hypotheses and facilitated the collection of mechanical and control data. Back in April this year, I successfully collected the kinematics of GoQBot using my own adaptation of the VICON motion capture system. However, the kinematics data alone only tells half of the story. We could compute the angular momentum as well as the linear momentum from pure motion tracking. However, there is no data on the mechanical power output and loading condition. To extend the analysis, I took one of the force beams I used for caterpillar ground reaction forces measurement and implemented it for robot GRF collection. In fact, VICON system has built-in capabilities to take in force-plate measurements and any other analog signals.With some modification and instrumentation, I obtained GRF in two directions from the head anchor and simultaneous recording of the actuator current draw during the high speed kinematics recording. Here are some very preliminary data.I'm currently processing the data to calculate the center of mass and the development of angular momentum. More results are coming!!

Stop crawling... pace up!!

2010-08-18T16:31:00.000-07:00

My Ph.D defense is coming up in December. I better hurry and pace up. When I was 7, I thought about a great deal of things about academic research and becoming a Ph.D. But I always thought of them in future tense. Time is truly incredible.

In any case, I just sent out another manuscript to Journal of Experimental Biology about my studies on the hydrostatic skeleton on caterpillars... three chapters of my thesis done... now onto the fourth one: soft-bodied robots.

Well, thesis writing requires a lot of integration and I've been organizing all my research files in my multiple hard drives as well as lab notes on paper. It's a very tedious yet enlightening process. I realize that my blog updates have been slowing down and infrequent. Well, that's because I begin to invest most of my writing power on my publications and thesis. Sometimes, after going through so much organization, I am left with no energy and patience to write anymore.

Nevertheless, hosting a blog is a commitment. I will post some update on my kinematics-dynamics simultaneous recording of ballistic rolling caterpillar robots in a week. In the mean time, I just release the Costa Rica Fieldwork Journal Part 3, which has been in my draft box for months. Sorry about that!! I will do better...

Quick clarification for the media

2010-07-26T08:55:00.000-07:00

I got some inquiries regarding the recent press release on caterpillar gut movement. It is entirely my colleagues' X-ray work and I have not taken any part in that project. Thus I am not responsible for any claims in the NPR interview or other press manuscript. Many online media translations started to deviate from the original release and the announcement does not have any bearing on the on-going or past soft robotic projects.

If you really want to see something in the news, the Boston Globe has a more conservative release, and the NSF promotional video was roughly accurate. As a scientist, I am always concerned about interviews from the mass media because the public can always see sciences in some funny ways. Want the real story? Go straight to the researcher(s) who work(s) on the specific project that interests you. You might be surprised!

A short review of my European resonances

2010-07-24T12:24:00.000-07:00

I've been away for three weeks in Europe for the Society for Experimental Biology annual meeting and several academic visits . I met quite a few cool people on the road and in meetings. Here I would only highlight a few topics that I found relevant to my current research theme.

At the SEB conference, there were a significant number of people involved with the EU project "Locomorph". I found this new term intriguing. It has been widely recognized that morphology plays a key role in locomotion, and many studies have focused on how the default body configuration function and coordinate to produce desirable kinematics. Well, this project combines "locomotion" and "morphing" into a word so to emphasize the effects of "changing" body morphologies during locomotion. For soft-bodied animals, morphing can be a means to locomotion, but that is the extreme case I'm currently concerned with. For most skeletal systems, coping with a changing morphology is a process of adaptation, and it is this very strategy that biologists and roboticists are interested in. To find out more about this huge collaborative project, see the project website. It sounds awful lot like a DARPA project in the US. I wish good health and good progress to all the people who are involved.

My visit to the EPFL in Switzerland was quite an experience. I heard so many things about it even since I started following robotics stuffs. That's when I was still a teenager. It's quite amazing that I actually get to visit and actually gave a talk about my work. Again, I won't even attempt to summarize people's research, in case I don't do a good job. But one recurring concept in many robotic efforts was localized intelligence and the emergent properties. Indeed, intelligence does not have to live in a central processing unit. Localized simple logic goes a long way when certain information sharing occur between the functional nodes. This can be seen at the sensory reflex level such as the optical flow control drones, or on the coordination level as in the salamander robot's coupled oscillators, or at the navigation level where swarms of MAVs create a communication network. In a way, I think this is how intelligence arise in any systems. Even our complex brain employ millions of neurons to generate a gross computation. Each single neuron has its own logic depending on the excitability and pre-conditioning. To exchange information, neurons rely on the varied synapses which can be modulated dynamically as well. In fact, I would go so far to say that all forms of intelligence are emergent properties of certain collection of simple logic units. This subject is indeed fascinating and I think using robots is often a more efficient way to study it. Browse their website to find out more.

U. Jena Lauflabor is known for the development of the SLIP (Spring Loaded Inverted Pendulum) model for biped locomotion. More specifically, this group of physicists attempt to find dynamic stability in a open-loop control scenario. In contrast to the reflex model for locomotion as employed in many traditional robots, researchers at Jena really investigate how inherent walk-run mechanics and leg properties can produce stability. Implementing robots from these simulations may lead to much more elegant solutions to legged locomotion, far from the reflex control as demonstrated by Boston Dynamics' "Big Dog" (which is very impressive, but not very smart). Their goal is actually similar to my soft-bodied robotic trials, except that our subjects are really quite distinct.

In any case, these are only three things that seem the most relevant to my current research goals. The field of biomechanics is really growing bigger each year and the use of robots has become a standard procedure now (for one purpose or the other).

The European Tour

2010-07-14T01:16:00.000-07:00

Sorry everybody!!

I've been traveling across Europe for more than two weeks already. The internet access and schedule doesn't really allow me to do any serious blogging. However, I will be reviewing some of the things I learned from all the amazing biomechanics researchers in Europe next week when I go back to Boston. Stay tuned!!

Huai-Ti (YT)

Field Work Journal (Part 3)

2010-06-12T20:15:00.001-07:00

6/7
Working in reverse, I started to examine how inching caterpillars may revert to crawling gait. Behaviorally it's quite feasible and we've observed some tiny step crawling when inchworms adjust their bodies on the foliage. My discovery of the day was that by constraining the uplifting body bend, I can actually induce crawling in many caterpillars that inch as their default gait. What I did was a very simple behavioral experiment: take a sheet of heavy duty plastic bag and lay it on top of a inching caterpillar on a flat surface. The result was quite stunning: the weight of the plastic prevent body upward bulking, so the inchworms start the stereotypical anterior-grade crawling. The very second plastic sheet constraint is gone (when I remove the plastic sheet or when they simply crawl out from under the sheet) they start inching again. This transition can happen at any phase of a gait cycle and so reversible that I can't imaging any other gait transition mechanism other than a biomechanical one. The implication is quite mind-bogging: inching gaits came naturally when mid-abdominal segments are not locked down to the substrate.6/8
I went back to the memorial behind the casona today for the sunset. Actually I went with my friend Ian. The casona is a big historic farm house that has been converted to a little museum to document the development of ACG. The memorial was built for many worriers and political leaders who defended Costa Rica against an ambitious Nicaragua war-lord in 80's. This memorial is about two stories high at the top of a hill, overlooking the entire Santa Rosa sector and the two main volcanoes on the other side of the intercontinental highway. We walked up there briskly right after dinner and caught glimpse of sunset. Before it became pitch-dark, I looked around and wonder where the scorpion I met earlier went, and whether it also lingered around at sunset.
6/9
My Costa Rica stay is drawing to an end, but I feel that everything has just become part of my life... three meals of rice and beans everyday, new trails everywhere, mosquito swarms at certain spots, incomprehensible language that sounds very familiar, WiFi domain guided by the trees, checking e-mail late at night in pitch-dark with some frogs as companies. The list goes on and on, making my experience quite unforgettable.
Today I decided to stop looking for caterpillars, otherwise I can't really wrap up the study. I went back to the caterpillar barn to examine all the caterpillars I've been working with and start to let them lose. If there is a video that is so amusing and that I can release freely, it is this video that I captured today of two noctuidae inching along a jar cover. Why is it funny? Well, I love the way they negotiate with each other. This circular walk lasted for about 6min until I got tired of filming. Apparently, you don't need very fancy enclosure to keep inching caterpillars. They would happily stay on a nice upright track even if it has a periodic boundary -- never ending. Isn't that what we call "inertia"?

Field Work Journal (Part 2)

2010-06-12T20:12:00.000-07:00

6/4
I started another day with fried rice and beans plus scramble eggs and cheese. Just when I got to my second coffee around 7:30am the bus brought in all the park rangers in for breakfast. People streamed into the Comidor with greetings and jokes. I watched them eat and talk and set out to work with a mouthful of Costa Rican rice coffee aroma. Somewhere in my heart, I envied such life: so simple, so natural and communal.
It’s time to organize data and find out what and what else can I get out of this trip. I started up with three aspects: Biometry, Kinematics, and Behavior. The biometry project was targeted to compare the biometry proportions of different caterpillar species as well as tracking down the ontogenetic scaling of some species. The only way to perform measurements on soft-bodied animal is to use photography. Unfortunately, caterpillars are wild animals after all. There is no easy way to get them to sit still in a specific posture while I photograph. Weighing them is also impractical because many of them do not relinquish their substrate (often their food as well). Brute force can injure caterpillars and decrease their survival rate dramatically, making ontogenetic tracking impossible. However, the general biometry can be still obtained from the video frames I collect in the kinematics project. One of the measurements I was looking for is the aspect ratio of the cylindrical body. Compare the two caterpillars before and after this paragraph to see what I mean.

6/5
In a forest full of activities, it is very difficult to stay put for more than a day. Although I haven’t finished the image organization on my two EEE PCs, I decide to head out to the field anyways. As the sun journeyed pass 10am, the wet “dry forest” turned into a steamer. I could smell many things around me, from fresh leaves to fermentation in the rotten woods. The strongest of all was a pungent smell that reminded me of steamed peanuts. I never figured out what that was, but it definitely imprinted in my memory of Santa Rosa. Anyways, the most memorable discovery from today’s field work was a leaf craftily “eaten” into a beautiful symmetric pattern. I couldn’t help but respect the "minds" of these wild caterpillars.

Over time, I found myself very tuned in to looking for caterpillars. I could distinguish leaves damaged by caterpillars from those eaten by ants or beetles. I was able to spot the caterpillar feces and trace the source to a plant, and I became pretty picky about what caterpillar I get. One of the tasks for today’s field work was to collect some cydista plants for my Manduca lanuginose. This was the only Manduca I found so far so I really should keep them alive. Just as I was full with plastic bags of caterpillar and plant harvest, something lighted up my eyes. It was a huge Manduca sexta gorging up a Solanum hayesii. I was very excited to see such a familiar body even though my memory of Manduca has been this obstinate stupid animal in the lab. For some reason, the wild type looked much brighter in color. It’s got a puffy body with clear healthy white lateral strips. Maybe the organic food really made a difference.

6/6
The English speaking researchers tend to cluster in one table at dinner, although many of them speak perfect Spanish. I have been meaning to learn to speak a few words, but the data organization work every night really crushed my ambition. Over some rice and beans with pork stomach, we talked about what we encountered during the day and frustration with the animals. Indeed, field work is a very different mode of research. We are studying the organisms in the great nature which is beyond our power to control. We cannot force any activities or interactions. We must let them come to us. Patience is the key and letting things be is the attitude.

Due to my field work during the day, I started to shift my caterpillar photography work to after dinner. Tonight, I found a keystone to solving the gait transition mystery in caterpillars. So far I’ve found caterpillars that inch with reduced prolegs and caterpillar that crawl with full prolegs. This caterpillar I picked up today inches with full prolegs, displaying how exactly a inching gait can be derived from a crawling pattern. This spotted caterpillar had a full set of functional prolegs from abdominent 3rd. However, when it picked up speed to run away from me, it lifted very large proportion of its body and actually cut under itself to gain the maximum step length. The whole gait pattern resembled a very conservative inching which can be shifted into a crawl at any moment in a cycle.

Field Work Journal (Part 1)

2010-06-03T20:46:00.000-07:00

Part 1 --- 6/1~3

6/1
I was so busy figuring out experimental protocols and data analysis during the first week of my stay. Every night I barely had energy to brush my teeth, not to mentioned writing in my journal book. My advisor left yesterday so I’m literally on my own now. But I think I am in good hands. The dormitory house keeper Lily helped me with my laundry on Sunday even though I didn’t have any soap. When I came back in 40min, she was already folding my clean dried clothes in the laundry room. The cook Aida came up with some food for me tonight when I worked overtime and forgot about the dinner time. Somehow I think they could empathize this poor young Asian kid who doesn’t even know how to say “por favor”. Of course, Dan and Winnie continued to bring me interesting caterpillars when they encounter them. What more could I wish for in the care of these people.
All the inching caterpillars I worked with so far tend to be very active. Some of them moved with impressive speed (up to ~4cm/s). In addition, they can also perform various acrobatic moves especially in the situation of disturbance. This Anomis I picked up today demonstrated one of the most memorable moves in front of my camera. It simply “disappeared” when I poked it on the rear back. 300fps high speed video showed exactly what it did. The caterpillar first span some silk around the thoracic legs, then it flip its whole body sideway with extremely high speed. The prolegs release was nicely coordinated to let go of the momentum it built up. The result was a ballistic lateral jump. The caterpillar landed on another leaf below the substrate I provided. It then used the silk line to climb back to the exact same spot where it jumped off. I simply couldn’t say a word but marvel such innate skill.

6/2
I took today off for a hike with my new friends, mainly to explore the conservation area and also to get some exercises. The focus of field collection was never about walking, and I found myself so out of shape. Nevertheless, I carried my big CASIO EXLIM camera in case my SONY Cybershot can’t do some animals justice. Field exploration is very much part of the field work. You never know what you would find by wondering about without a particular search criterion in mind. We started out right after breakfast at 7:30am and headed straight down to the valley. It was a pretty damaging road for most cars, but an easy one for hikers.
I met more butterflies than caterpillars on my way down to the coast line. They all cluster under the sun sucking liquid on the mud or some rocks. Wing flaps by wing flaps just like having group meetings. The blazing sun started to steam up the water from yesterday’s rain. Each water puddle contained thousands of tadpoles and supported tens of water surface insects. I found another beetle larva moving upside down by peristaltic on the rocky ground. I wonder why they still keep their thoracic legs if they don’t even use it for locomotion. At the bottom of the valley we crossed two rivers. We met a gang of monkeys after we waded across the second one. My friend was somewhat aggressive on photo shooting, that the monkeys decided to protest. Several of them started breaking branches to drop on top of us, and many more gather over. We left soon after these demonstrations. Sometimes, communication can be so effective. We were caught in the pouring rain when we reached the beach. It didn’t bother us much since we were all soaking wet in sweat anyways. Sweat, rain, and Pacific Ocean all mixed together as we headed back to the research station.

6/3
Today I had to move out of my room to stay with other researchers. It was a bit of a hassle, but I don't mind joining the party. Having a room with four double bunkers for myself is too luxurious out in the forest. It was always an adventure to interact with the ACG staffs, because I pretty much don't speak Spanish and many of them don't speak English. In any case, we all managed to came to the same conclusion on our subject whatever it was. Still, I wish I spoke Spanish. I’m missing so much.
My caterpillars in the barn are doing pretty well now. But I need to finish up filming these caterpillars before I can get more. I can never predict when I will lose them to stress, parasite, or pupation. The inchworm Sphacelodes I picked up before breakfast was the largest geo I’ve even seen. It’s about 3.5 cm long and weighed 0.146g. However, it had every bit of athleticism of Geometridae. Most geo’s preferred to inch on top of the branches if possible, but this one had much stronger preference. When I turned it upside down on my dowel, it started to have trouble pulling the body in. After a few steps it just spiraled around to the top of the dowel again. Well, counter-levering a hydrostatic body up to 3.5cm long does become pretty difficult with only ~0.5cm leverage.

Costa Rica Field Work

2010-05-30T10:03:00.000-07:00

It's been over a month since I last updated my blog. I truly apologize for those who follow my blog. But honestly, this is my first chance since mid April to catch a breath and log in to my blog. In any case, I'm currently in Costa Rica for a field work, tracking down an amazing collection of caterpillars in the tropics. Pictures and field work journals will follow. Stay tuned in the next 10 days.

The week before I set off to Costa Rica, I questioned myself once about this trip. What exactly do I expect to get out of the forests? Do I really need so many projects for my Ph.D? With two manuscripts pending and an international conference travel coming up, setting up my first field work at a distant foreign place was the last thing I needed. However, as my adviser and I arrived at the Area de Conservación Guanacaste, Costa Rica, my doubt dissolved instantly. It is totally worth the sleepless April and May. For the following week, I will be posting my field work journal. They all consist of two paragraphs. The first one contains something about my field experience. In the second paragraph you will find some portraits of what I saw in the field and animal interactions. For my research, I will focus on locomotion in different caterpillars and some associated behaviors.

For those who track 3D

2010-04-20T19:07:00.000-07:00

Folks in the field of animal locomotion would know how kinematics data are usually obtained. But allow me to summary the general procedure in a few sentences. To track anything in 3D, at least two camera views have to be available at all time. After space calibration, one can calculate the 3D configurations of the objects in the analysis software of his/her choice. Ideally, video tracking can use any inherent features of the subject. However, to facilitate automatic tracking, high contrast makers are often attached to the subject. Infrared markers offer a way to highlight the features of interest without compromising the lighting for the normal video acquisition. After I created two families of soft-bodied robots, I was challenged by the need of quantitative data. These kinematics data are critical for any mechanical analysis on the robot locomotion.I set up our VICON 3D system to track my robot kinematics at Tufts Advanced Technology Laboratory. VICON is a company that makes 3D tracking systems for research in locomotion and animation industry. It employed several near infrared high speed cameras which would detect the IR signals coming off the retro-reflective markers attached to the subject. Unfortunately, retro-reflective marking is really not the way to track small animals such as insects or robots of the same scale. After going through many types of IR florescent chemicals, I finally decided to go with semi-conductor IR emitters (or infrared LEDs).
This works out really great for my application because I do not need to worry about IR light flooding or bad camera focus. These surface mount IR emitters produce point-source lighting smaller than 1mm. The IR cameras pick them up like many distant stars. In fact, a little out of focus actually increase the pixel numbers from which the centroid positions are derived.
Data are coming alright, but my data crunching techniques are still too slow for the rate by which these high speed cameras acquire data. I better work on that!

Gait transition and embeded AI

2010-04-02T19:08:00.001-07:00

A couple of years back, I was deeply impressed by probably the most well-known bio-inspired robot which demonstrated the effects of central pattern generator on gait transitions. This is the EPFL salamander robot with coupled non-linear oscillators. In this research amphibious robot, smooth gait transitions were accomplished by tuning the gain of oscillators coupling.

As I started working on soft-bodied robots, I discovered that many non-linear characteristics of the soft materials and actuators can be exploited to engineer behaviors. So I took a completely different approach to robot control. Instead of programming complex behaviors on a micro-processors, I "tuned" the body and actuators so they create desirable behaviors when I switch on a behavioral circuit. Amazingly, when the motor variations and body properties reach a certain domain, the robot was able to achieve gait transitions with a simple scaling of motor-pattern. This is a very intriguing demonstration because it provokes a rather radical inquiry: how much logic/intelligence can we embed in a piece of material? To what extent can we use morphing morphologies to perform computation (or thinking if you will)?

Multi-threading...

2010-04-01T04:33:00.000-07:00

A lot has happened in the past few weeks. Besides my secondary injury during my recovery of my bone fracture, everything else seems to progress in a positive direction.

First of all, my paper on caterpillar ground reaction forces was finally printed. It's been really over-due for a year now. Most data were collected by Christmas 2008, and I actually presented the major finding at the SICB 2009 January. I felt pretty bad about this delay but the robotics project last year really took my life from March through October. To summarize the findings in a few sentences: large caterpillars such as Manduca sexta load their bodies in constant tension when they are attached to a substrate. Locomotion was achieved by progressing the body tension/deformation forward. Biomechanically speaking, these critters use the substrate as their external skeletons. We call this strategy: environmental skeleton. For more details on this radical view of soft-bodied animal body control, check out the April 1st issue of the Journal of Experimental Biology. If you would like a PDF copy of my paper, simply e-mail me at huai-ti.lin@tufts.edu and I will gladly send you one.

Besides my old new paper, I've been planning a field trip to Costa Rica for this May and June. Last spring at the SICB conference, I bought a few books about caterpillars. Among them, I was really impressed by a couple of books regarding tropical caterpillar diversity. So I contacted the authors Dr. Daniel Janzen et al and was struck by the idea of visiting the home of caterpillars in the wild. Lab animals are always somewhat unnatural. This idea was incubated in the back of my mind for many months until I finally formulated it into a more concrete field study project. My mentor Dr. Barry Trimmer was very supportive of the idea and quickly decided to make it happen. In any case, we have now arranged a 17 days field work at a conservation in Santa Rosa, collaborating with Dr. Janzen's team from UPenn.

Finally, to continue the imaging theme from last time, let me share a few images from our histology for Manduca caterpillars. Working together with my great undergraduate lab-mate Dan, we've been able to produce very clean cross-sections of caterpillar abdomens.Through some imaging techniques, we can enhance the cuticular folds.Or we can also highlight the muscles! So awesome... the biology I mean (but we're not bad either)

Take a hard look at the soft morpholgies

2010-03-15T16:09:00.000-07:00

Every so often when I need to get some details about the caterpillars, I would do some electron microscopy. I especially love to browse specimens under a good scanning electron microscope (SEM). It feels like entering a different world: a microscopic one.

Recently I decided to look at more surface features of Manduca caterpillar bodies with SEM. This time I wanted to explore the folding structures on the soft cuticle.

The first thing I noticed was how hairy these cute caterpillars really are. No wonder people call caterpillars "fussy worms" in tropical Taiwan where I grew up. If vision is weak and proprioception is irrelevant, then tactile sensing must be dominant.

Then I couldn't help focusing my electron beam on the spiracles.... well, they look like tiny stadium to me. This the the hairs around the air slit can't be for tactile functions, or are they?
I think these are hairs that help repel moisture and particles to keep the air flow smooth. Maybe I should look into the literature.

Finally, I must show you at least one image of the crochets (microscopic claws) on the caterpillar prolegs. They are simply gorgeous!! I got many more images with higher magnification, but it's hard to explain what you are looking at in such close-up photos. This image was actually taken three weeks ago.
These caterpillars relay on these double array of crochets to grip on to any substrate. When a proleg retracts, these crochets are pulled into the cuticle pocket on the left side of the image. And the whole leg closes like a purse to prevent any unwanted hooking. It's so simple but reliable. I wonder if there is any better strategies for controlling these hooks array with large surface deformation... (another long night of restless dream)

Morphing Morphologies...

2010-03-01T08:17:00.000-08:00

Some people asked me how I got into rolling locomotion from soft-bodied animals. This is actually a subtle point which perhaps I didn't make it explicit in my previous post. Although my current study system is one without any well-defined articulation, my interest is really about morphologies that function through morphing. All animals in the wild have to undergo dramatic transformation to switch the mode of locomotion (e.g. from crawling, swimming, running or whatever to wheeling/rolling). Their bodies are definitely morphing morphologies.

Similarly, soft-bodied robot can be defined by its ability to morph regardless of its material. Indeed, the definition of "soft-bodied robot" has been a indefinite argument in my research group. What is really considered "soft"? Isn't it all relative?

After four years of contemplation, I have only recently come to the conclusion that a soft-bodied robot is a robotic device that can conform to the environment without active control. In other words, soft-bodied robots do not maintain any definite posture. Instead they allow the environment to determine its shape in conjunction with the internal control of body properties. This definition was really an inspiration from my study of caterpillar locomotion. Proprioception (perception of body posture) is therefore insignificant by definition. If we translate this definition of soft-bodiedness back to the animal kingdom. A true soft-body is one that does not force any posture. This will exclude all the hydrostatically controlled bodies especially muscular hydrostats. How heretic? Octopus arms are not soft? Well, we must also recognize that tissues can be tuned to different states. An octopus arm can be a very well-controlled muscular hydrostat when performing a manipulative task but highly compliant when relaxed. From this concept, I urge the biology community to take on more specific terms when describing animals or organismic bodies:

Articulated body [lever-linkage system with joint actuation]
Celumic hydrostat [the pressurized fluid-filled body as a skeleton]
Muscular hydrostat [muscles as the skeleton and actuators]
Environmental skeleton [substrate as skeleton on which muscles act]

A short review of rolling locomotion

2010-02-14T17:53:00.000-08:00

For my rolling GoQBot publication, I've been doing literature reviews and dug out some interesting information about animals with rotary locomotion. While I wrote a formal literature review in my manuscript, here allow me to share my thrills in a visually guided relaxed format! For in depth information, see my list of references at the end of the post.

The earliest documented rotary locomotion I could find was from this shrimp like creature living on sandy beaches. It has short legs specialized for swimming. So when there is no water, they flip on their backs and performed a slow body rolling motion (Caldwell 1979).Of course, a much more dynamic gymnast has to be this somersaulting spider in Sahara desert. This little guy can perform amazing somersaults across the dessert sand after a running start up to 2 m/s, according to the discoverer Dr. Ingo Rechenberg. Check out some of his videos on YouTube: Short intro; Extended
Despite the amazing gymnastic moves, the above two creatures don't really roll in a circular form. The stomatopod really just flips its body by reaching the head with the tail, and the somersaulting spider actually got airborne in their strides. A true wheel is one that relies on the continuous contact of same radius spokes. The following two examples are animals that form quite perfect circles for downhill passive rolling. They are really very cute and circular... (Henschel 1990, 1995; Garcia-Paris et al 1995)Finally, the true powered wheeler is still my favorite rolling mother-of-pearl caterpillar. These caterpillars would curl into a wheel ballistically and catapult themselves into free-wheeling objects when disturbed (Bruckenbury 1997, 1999). According to the scientist who characterized this motion Dr. John Bruckenbury, there are a few more species of caterpillars that perform this behavior. It's really quite an effective way to escape. [pictures below are from Bruckenbury 1997 publication]
Armour, R. H. and Vincent, J. F. V. (2006). Rolling in Nature and Robotics: A Review. Journal of Bionic Engineering 3, 195-208.

Brackenbury, J. (1997). Caterpillar Kinematics. Nature 390, 453.

Brackenbury, J. (1999). Fast Locomotion in Caterpillars. J. Insect Physiol. 45, 525-533.

Deban, S. M. (1995). A Novel Antipredator Mechanism in Salamanders: Rolling Escape in Hydromantes Platycephalus. J. Herpetol. 29, 149-151.

Full, R., Earls, K., Wong, M. and Caldwell, R. (1993). Locomotion Like a Wheel? Nature 365, 494.

Gould, S. J. (1981). Kingdoms without Wheels. Natural History 90, 42-48.

LaBarbera, M. (1983). Why the Wheels Won't Go. Am. Nat. 121, 395-408.

Siegwart, R., Lamon, P., Estier, T., Lauria, M. and Piguet, R. (2002). Innovative Design for Wheeled Locomotion in Rough Terrain. Robotics and Autonomous systems 40, 151-162.

Biomimetic vs. Bio-inspired Robots

2010-01-30T07:47:00.000-08:00

Whenever I tell people that I do robotics, the first question that bounces back is: "That's the application?" Then I would have to start making things up quickly...

Well, to be honest, the true motive for my robotic effort was purely academic. I want to learn about how animals move by making physical models of them. That's why I insist to stick with biomimcry and try to be loyal to the biological system at least in the functional level. The ways biology deals with mechanics are not always optimal and often contain constraints, but I want to have those in my robots as well so I can discover them, feel them, and characterize them.

Recently the discussion of biomimetics came up in my research group because there are a good proportion of engineers who design for functions. Of course, in the name of good engineering, we should not make biomimetic robots but bio-inspired robots. The difference is that we could learn the principles of operations in biological systems and apply these ideas and only these ideas to robotic applications. In this approach, we can leave all the biological constraints behind and attempt to optimize specific functions in our devices. For example, Dr. Shimoyma in Japan has a project that aims to recreate the swallow tail butterfly wings on his micro-ornithopter (see below).

Taking the inspiration, they also created a tail-less micro-ornithopter with all the control electronics and sensors on-board. (very impressive works!!)

Of course, that's not to say I don't think about applications and moving on to bio-inspired robotics. Besides, my robots are still performing well beyond average among so many soft-bodied robotic platforms up to date. I believe that sticking to biomimetics would help us understand the nature of our biological inspirations. Surely, there is still much to learn from soft-bodied animals.

SICB 2010 Seattle

2010-01-18T08:14:00.000-08:00

Welcome to the first post of 2010!!
I just got back from Seattle one week ago from the annual meeting for the Society of Integrative and Comparative Biology, or SICB in short. So here is a short report about some of the inspirations I received over the course of the 10 days Seattle visit. For the limited space, I will only mention three topics with: 1) ideas that resonates with my current research; 2) something about Manduca sexta; 3) the most entertaining contents.

To start with, I would like to acknowledge those researchers working on plant biomechanics. After staring at the quasi-static locomotion of caterpillars for 4 years, I can really appreciate the intricate movements without dynamics as also in plants. Really, caterpillar crawling is a "static problem" only that people don't like to hear about "static locomotion" so "quasi-" makes it sound better. Of course, plants movements are all driven hydraulically, but cellulose fibers can define the deformation. The morphology very much depends on the material properties and internal pressure. For climbing plants, stem development and attachment scheme are highly correlated. Those that attach to host substrate tightly can be soft, while the ones that hang on to forest tapestry loosely would need stiffer stems to self support. This principle of substrate interaction really resonates with the "environmental skeleton" hypothesis I proposed. For a big fat caterpillar with many prolegs attached to the substrate, no internal structural support is needed. In fact, compliance is imperative to allow conforming to the substrate, just like those tightly attached plants.

This year's SICB also had a lot of animal flight stuffs. In particular, Manduca hawkmoth flight has been a highlight. Thanks to the modern high-speed videography and kinematics tracking software, wing strokes can be digitized at many thousands of frames per second rate. Much attention has been delegated to turning maneuvers, especially in the yaw direction. In general, hawkmoth and other big insects create asymmetric effective wing angle of attack to turn. This change of stroke plane can generate a fairly acute turn, and we can find this strategy in many current radio controlled micro-ornithopters. After watching so many slow-motion of moth flying, it occurred to me that body weight shift must play a key role for stability as well. While most micro-ornithopters don't use tail for turning anymore, they still need it to smooth out the unsteady air flow from the wing. To create a tail-less flapping flight robot, we might want to model the body coordination as well. For any flapping flight agent on the order of a few grams, it seems to me that weight shift is as effective (if not faster) for stability compensation as wing stroke modification.

Finally for the topic with the most entertaining contents, I would like to mention some work on maximum performance of musculoskeletal systems. Although I am currently working with a critter without any skeleton, my original biology training was functional morphology of skeletal systems. In plain English, that means I held scalpels more often than pipettes. I was the student helper at this session called "Terrestrial Locomotion -- Jumping" with the session chair Steve Reilly. It's a strange feeling to see Dr. Reilly because I once read a lot of his work and almost did my undergraduate thesis on frog jumping. Anyways, the first talk of this session was probably the most entertaining talk I went to in SICB 2010. It was about why jumping frogs contests produced much better jump distance record than the scientific research. Well, apparently it's all in the arts of these professional "frog jockeys", which are unfortunately kept secret. However, the investigators in this research did find out one well-tuned factor that affects maximum muscle performance: temperature. This is probably a general issue for all poikilotherms (animals don't actively maintain a constant body temperature) which can be easily affected by climate change. In any case, although it's ambiguous what these "frog jockeys" were doing to their frogs, it was absolutely hilarious to see them "jump their frogs" with the utmost seriousness.

Of course, there was a lot more impressive research presented in this meeting. I was simply overwhelmed by Wednesday afternoon that I had to stop going to the talks in order to recall my own presentation scheduled on Thursday morning. It's good to be at a conference like this and feel connected to this fun community of scientists.

Soft robots preview videos come online!!

2009-12-31T00:12:00.000-08:00

Happy New Year!!
Finally, my robot videos are here, on the last day of the year 2009!

After searching around for a host server, I decided that the good old YouTube was still the best so far. My last post was some preliminary tests on video linking. Now let's check out some preview videos of my soft-bodied caterpillar robots!!

First off, the InchBot I-III are the early versions of the my soft robot implementation, dated back to March 2009. It's a process by which I developed inching gaits and learned about frictional control. My colleague Chris successfully modeled these Early InchBots in a finite element environment as well. He really spiced up the video, too.

Then, here come the InchBot IV-VII which twitch, inch, burrow, and climb with much smaller body size. Chris also implemented the InchBot-V in FEA. These soft robots featured open-loop robust inching/crawling/climbing gaits.

Finally, the newest class of caterpillar soft robots, GoQBot, have an escape ballistic rolling behavioral inspired by the caterpillar of Pleuroptya ruralis (mother-of-pearl moth). This class of robots can initiate a rolling behavior within 300ms and hit top speed over 15cm/s. In addition, the updated versions have include almost all the previous InchBot series capabilities and are radio controlled fully untethered. Simple intelligence is implemented into the body structures and active sensing will be next. To hear more, stay tuned to my publications coming soon.

Interesting videos of caterpillers!!

2009-12-16T04:20:00.000-08:00

Everybody in my lab knows me for training caterpillars to perform various sportive activities. Well, inducing caterpillars to crawl underwater was a true story. My motivation was to test the role of gravity and external pressure on a behaving animal. As it turned out, 5th instar caterpillars float in water and thus experienced a force negative to gravity, but none of the kinematics characteristics changed. See my video "Manduca underwater walk" on YouTube.

Looking closely, I found that bubbles could be seemed to come out of the spiracles as the animal compressed itself. This observation illustrated the potential change of body volume due to tracheal compression (see video). If caterpillars can squeeze air out under the influence of water pressure, they must perform quite a lot of gas exchange in the air. In other words, locomotion facilitates gas exchange by compressing and squeezing the air out of the trachea.

Finally, I would like to share a video I shot the other day when one big caterpillar was crawling on top of a smaller one. It's quite an pathetic scene because the smaller caterpillar was actually in the molting process and could not fight back. Nevertheless, as the big caterpillar crawled along, I observed appropriate deformation on the substrate (in this case another caterpillar) as illustrated by my new ground reaction forces paper (to appear in Journal of Experimental Biology).

The missing post released!!

2009-12-08T17:11:00.000-08:00

Dear readers,

There has been one missing post about the DARPA meeting which I started back in October but never finished. The reason was quite simple: I wanted to wait for better graphics. In any case, since I was orchestrating the live robotic demos for Tufts, there was no way I could take photographs as I always do. It turned out that it was not allowed anyways. DARPA actually hired prefessional film crew and photographers for the event.

Two months has gone by since the event so it's not news anymore. Nevertheless I thought some of you might be interested in reading my story a few days before the meeting! Now, according to the non-disclosure document I signed, I am not supposed to share anything I saw at the meeting. So in this post, I simply described what happened the very last week before the meeting in the labs. The ME professor that I worked with on this project said: "(It was) the most stressful/intense academic experience that I've gone through". Indeed, for the last month we worked at least 15 hours everyday. To hear more, see October 11 post which is newly released.

Hydrostatic skeleton model for caterpillars

2009-11-29T07:57:00.000-08:00

As my regular readers, you might have noticed my dual role as a biologist and a roboticist. Indeed, I currently have 6 projects running in parallel, three in locomotion and other three in robotics. Of course, there are also many more side projects. In any case, the posting about caterpillar prolegs configuration was the motivation of a locomotion project. Here let me present to you another locomotion project of mine with some literature review. This one concerns modeling hydrostatic skeleton in caterpillars. The following photos show how an anesthetized caterpillar can lose turgor and fail to "stay in shape".

Modeling mechanics of biological soft structures has been a long time endeavor for functional morphologists as well as theoretical biologists. In general, this field focuses on morphologies without any rigid skeleton (internal or external). They are usually soft tissues supported by some fluid which allows very large deformation. The notion of “hydrostatic skeleton” became well-known by the 50’s largely due to research on worms (cnidarians, annelids, and nematodes). Clark and Cowey established how soft-bodied animals achieved extreme extension with helical reinforcing fibers in the body wall (Clark and Cowey, 1958). The oblique fibers winding around the body allows very large longitudinal stretching. Soon this type of fiber reinforcement was found in many other cylindrical biological structures including those of plants. In 1980’s, a new wave of theoretical investigation of soft-bodied animal locomotion began. Keller and Falkovitz attempted a model of worm crawling using finite difference method which calculated the transverse traveling wave along the body and its associated contact friction (Keller and Falkovitz, 1983). A few years later, Dobrolyubov generalized this line of reasoning to traveling deformation (both transverse and longitudinal). He claimed that the transverse traveling wave can represent caterpillar locomotion while the longitudinal traveling wave resembles crawling worms. Then he gave an example on how this model could describe snake’s locomotion (Dobrolyubov, 1986). These models proposed credible mechanisms for locomotion, but did not explain how animals achieved those body deformations. In another Journal of Theoretical Biology paper, Wadepuhl presented probably the first comprehensive finite element hydrostatic skeleton model based on medical leech which had been well studied by then (Muller et al., 1981; Sawyer, 1986; Stern-Tomlinson et al., 1986; Wadepuhl and Beyn, 1989). This model included geometry, elastic properties of the body wall, internal volume, and body pressure. It revealed some principles of antagonism in worm-like structures as well as the pressure-volume interactions (265 Wadepuhl, M. 1989). At about the same time, Wainwright nicely summarized the mechanics of cylindrical biological structures in his famous little book “Axis and Circumference” (Wainwright, 1988).
Before the turn of the 21st century, Journal of Theoretical biology continued to host models of hydrostatic skeleton. However, experimental data gradually dominated the modeling efforts. Skierczynski et al constructed an updated leech model empirically based on dimensions of animals in limiting cases, passive properties of the tissues, muscle responses to activation, and the transform from motor-neurons to muscles. It assumes elliptical shapes for cross-sections, constant volume, and that the shape tends to minimize the potential energy. It simulates the vermiform elongation and predicts the pressure changes (Skierczynski et al., 1996). Similarly Alscher and Beyn simulated the motion of leech using Lagrangian mechanics and a large system of differential-algebraic equations (Alscher and Beyn, 1998).
While leech models seemed to develop with fast pace, earthworm studies were thriving as well. Dobrolyubov refined his mass transfer wave model and published another paper in JTB with Douchy on peristaltic transport. This general model attempted to explain the digestive transport as well as locomotion by caterpillars, earthworms, snake and snails (Dobrolyubov and Douchy, 2002). Accoto et al added to JTB another earthworm kinematics model, again based on constant volume and simple friction (Accoto et al., 2004). With these numerous hydrostatic skeleton models, it was thought that soft-bodied animal locomotion is more or less realized and what we learned from worms can be applied to others such as caterpillars. Unfortunately, caterpillars are simply not worms in all biomechanical respects.
Caterpillar’s body differs from that of a worm in several essential features: 1) Extension in the longitudinal direction is accounted by numerous inter-segmental folds instead of body wall stretching. 2) Body pressure is highly variable and less predictable. 3) It contains more compressible volume in the body. 4) There is no segmental septum that compartmentalizes the animals. 5) Caterpillars are legged systems with discrete and on-off attachments. As the results, the helical fiber-reinforced cylinder model does not apply. The constant volume assumption does not hold, and real-time pressure recording lacks correlation to body movements. Frictional model based on mass transfer is useless in this system. What’s more, caterpillars don’t move with one single gait and/or body configurations. In this study, we seek an alternative approach to model this worm-like structure that is so much unlike worms.

References

Accoto, D., Castrataro, P. and Dario, P. (2004). Biomechanical Analysis of Oligochaeta Crawling. J. Theor. Biol. 230, 49-55.
Alscher, C. and Beyn, W. J. (1998). Simulating the Motion of the Leech: A Biomechanical Application of DAEs. Numerical Algorithms 19, 1-12.
Clark, R. B. and Cowey, J. B. (1958). Factors Controlling the Change of Shape of Certain Nemertean and Turbellarian Worms. J. Exp. Biol. 35, 731.
Dobrolyubov, A. I. (1986). The Mechanism of Locomotion of some Terrestrial Animals by Travelling Waves of Deformation. J. Theor. Biol. 119, 457-466.
Dobrolyubov, A. I. and Douchy, G. (2002). Peristaltic Transport as the Travelling Deformation Waves. J. Theor. Biol. 219, 55-61.
Keller, J. B. and Falkovitz, M. S. (1983). Crawling of Worms. J. Theor. Biol. 104, 417-442.
Muller, K. J., Nicholls, J. G. and Stent, G. S. (1981). Neurobiology of the Leech: Cold Spring Harbor Laboratory Pr.
Sawyer, R. T. (1986). Leech Biology and Behaviour: Clarendon Press Oxford.
Skierczynski, B. A., Wilson, R. J. A., Kristan Jr, W. B. and Skalak, R. (1996). A Model of the Hydrostatic Skeleton of the Leech. J. Theor. Biol. 181, 329-342.
Stern-Tomlinson, W., Nusbaum, M. P., Perez, L. E. and Kristan, W. B. (1986). A Kinematic Study of Crawling Behavior in the Leech, Hirudo Medicinalis. Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology 158, 593-603.
Wadepuhl, M. and Beyn, W. J. (1989). Computer Simulation of the Hydrostatic Skeleton. the Physical Equivalent, Mathematics and Application to Worm-Like Forms. J. Theor. Biol. 136, 379-402.
Wainwright, S. A. (1988). Axis and Circumference: The Cylindrical Shape of Plants and Animals. Cambridge: Harvard University Press.

Research odds and ends...

2009-11-20T15:39:00.000-08:00

Sorry for the delayed posting again! I've been occupied with the end of the year academic madness: grants writing. One can tell how bad the economics is by looking at how people panic about funding. That's very much the case in academia. In any case, I didn't find these experiences extremely appealing to talk about so I blanked out last Sunday when I was supposed to update my blog.

Regarding the robot videos many people inquired about, I've done my best to push the school administration for video hosting. Unfortunately Tufts is still an university and all administrations eat time. Actually, that's the main reason why I started this blog. Our labs websites update never caught up with our research pace, because we researchers cannot access the webpages ourselves.

By the way, I do have a facebook account, but I don't use it very often. My colleague asked me about it a couple of weeks ago here is how I responded:

Caterpillar prolegs diversity

2009-11-01T21:56:00.000-08:00

While we are waiting for the caterpillar robot video links, let's come back to some morphological discussion of lepidoptera larvae. All my biomechanics studies so far are based on a well-known model system tobacco hornworm (Manduca sexta). It is a fair size macro-lepidopera species commonly found in the America. It has 4 pairs of abdominal prolegs plus 1 pair of anal prolegs (or terminal prolegs). This is thought to be the ancestral form.
However, we are perfectly aware of the diversity of lepidoptera species. Caterpillars really vary in numbers and arrangement of prolegs. They also adopt different gait patterns accordingly. So how do we generalize what we learned from Manduca? Or can we?

To find out why, I am planning a field study to compare body overall scaling across different species of caterpillar. Although much work has been done on comparing morphological changes in the evolutionary context of species interaction, little is known about the physical constraints during evolution. I believe that there is a link between locomotor biomechanics and the evolution of prolegs configuration.

From InchBot to GoQBot - video links coming soon!

2009-10-18T11:55:00.000-07:00

Dear readers,

If you see this message, then I must thank you for your loyalty to my blog. I apologized for missing out almost a month of blogging, but really I can't afford to get into trouble. In any case, welcome back to my rapid pace of locomotion research and biomimetics. Currently I have two robotic lineages: InchBot[7 generations] and GoQBot[4 generations]

Each robot generation has a very specific research aim and target performance. I think it's time for me to present them to you in a video format. While all the videos have been edited and annotated, I still need to link them from the Tufts Media Server. Please be patient! They are coming soon.

DARPA Review

2009-10-11T23:45:00.000-07:00

It's peculiar how things evolve. I never meant to get into this DARPA project until I realized how far I've been sucked in. So here I am working on my robots in the second room of Biomimetic Devices Laboratory every minutes of my awareness.

System engineering requires a lot of collaboration but creation really has to be solo. A roboticist has to be able to work with people as well as alone. I was fully aware of the importance of this final review, because a lot of people depend on this funding at Tufts, including many friends and colleagues. For me, that's enough at stack even if I had alternative funding source for my primary research. I simply had to do everything in my power to make this robotic demonstration rock solid and fail-proof. I was really up for the job!

In September, I produced a plan for preparing this robot demonstration. Following is a rough list of the issues we had to consider:

1. Transportation protection
2. Demonstration platforms
3. Displays cases and proper Logos
4. Portable power supply and charging units
5. The robots (obviously) and their doubles
6. The control system and a secondary backup
7. Primary robot operator, backup operator
8. Demonstration program and rehearsal
9. Robot specification data
10. Backup video shots in case some live demos fail
11. Tool boxes

This list just went on... My job was to coordinate the designs, search for the parts, arrange the purchases, find appropriate help, and do as much as possible to complete my robots and their accessories. In a typical day, I would spend 5+ hrs doing micro-soldering and bonding, 2+ hrs programming, 2+ hrs literature research, 3+ hrs robot testing, 2+ hrs casting/molding. On top of that, I had to purchased about $500 worth of components everyday on average. Actually, I found shopping the most stressful task among all. In order to get the right thing on time, I had to check the mechanical/electric compatibility, availability, pricing, shipping, and make payments. That's probably why R&D companies always have a person or a group of people specifically do purchasing. It's such a tough job.

Anyways, it's Sunday night now! We are up for the show in two days. I better get some sleep and hope for the best.

Intelligence embeded in morphology?

2009-09-27T10:12:00.000-07:00

Recent years, the bio-inspired robotic community became very interested in the idea of embedding control strategies into the structures or morphology of the robots. In a fast ballistic locomotion, many animals have some structural designs which allow them to tolerate certain degree of perturbation. This control method has been named "pre-flex" as oppose to neural "re-flex". The phenomenon is well represented by the example of cockroach recovering from lateral perturbation. Sprawl biomimetic hexapod robot nicely demonstrated this damping effect in polypedal running locomotion. In a way, the morphology effectively reduces the computation required to control balance and recovery. Thus some people like to call this effect: "morphological computation".

The power of "morphological computation" relies on the structure's ability to response "differently" to varying conditions. A linear transform is hardly any good because its result only depends on one order of the input. For example, an Hookean spring produces certain tension at certain stretch regardless of how fast it is stretched. Therefore most interesting logic components are non-linear. As the results, the input can be processed conditionally, taking more things into account.

Now, what exactly can be computed in morphology and what parameters are critical? That's a fundamental question for the field of functional morphology and biomechanics. I think I might have found my version of the answer in my robots. Let me get back to this topic in a few weeks after consulting with some caterpillars.

Unleash the robots!!

2009-09-20T08:45:00.000-07:00

A complete robotic agent has to be independent. Going untethered is a very important step in any robotic development. This week, my robot controller pulled free!!

Taking the 3D wiring approach, I successfully customized a micro-RC system for my robots. Traditional electronics are limited by the 2D wiring of PCB boards. Making thin electronics is easy as long as the integrated circuits utilize the surface on the PCB efficiently. That's why cellphones and iPods can be so thin. However, if bulk size is an issue (my robots need to fit through a small hole), traditional circuit layout cannot accommodate that down to a critical size. The only solution to improve packing is to do away with any sort of PCB and stack integrated circuits one on top of each other. By fitting these electronics components carefully according to their geometry, one can minimize the overall dimensions. This is exactly what I did.

Well, piecing 3D puzzles is only the first step. Since overall dimension has to be as small as possible, I worked with the smallest of everything from diodes to wires. I started with the World's smallest RC receiver and built on top of that. Typical soldering dot size is less than 500 micron and most soldering joint spacing is 800 micron. Due to the complex 3D structure, wiring and insulation became very complicated. All the wiring and soldering has to be done manually under a microscope. My micro-dissection skills really came to the rescue!! This is perhaps a good example of how human beats machines. [the scale in the following images is mm]

The result was quite satisfactory. I could radio control 7 actuators on my robot with all the electronics packed into a ~6mm cube volume (not including external wires) over a 300 meters range. The customized transmitter can receive commands from a computer interface via a USB cable. This allows the robot operator to gain various computer support including an library of CPG gaits. Image below showed a test setup where actuators were distributed across several different worm-like bodies.

Huai-Ti is up for the DARPA challenge -- 21 days countdown!

2009-09-15T18:01:00.000-07:00

With all these robotics attempts, Huai-Ti has taken up the DARPA challenge together with several fellow Tufts roboticists. According to the ChemBot Phase-I challenge, a soft robot has to be produced for covert access.

Primary challenges:
1. Cover 5m in 20min (average speed at 25cm/min)
2. Reduce the largest dimension via morphing (10 fold)
3. Traverse an arbitrary 1cm opening
4. Reform original functions and capabilities

This effort will be evaluated in a live robot demonstration in exactly three weeks or 21 days. Other ChemBot teams include Harvard, MIT, and U. Chicago. We must not look bad in front of them. Start the count down, and wish me good luck. Go Jumbo!!

Intelligent Design versus Evolution in Robotics

2009-09-13T19:06:00.000-07:00

Ok, between posts of robotics, I'm going to risk my neck and touch on the famous American debate on Intelligent Design vs Evolution. Since I'm not only a physicist but also a biologist, my position is hard to be neural. Thus I decide to speak in the context of robotics only.

Robotics is a multi-disciplinary engineering by itself. A roboticist not only need to know mechanical design and to be familiar with materials, but also need to understand control strategies and to learn electronic interfacing. It is an integrated system design process. All robotic systems must be designed through our superior intelligence then. Is there any question?

More recently, the Genetic Algorithm (GA) approach to engineering optimization became very popular. The idea is that by introducing variations similar to mutation in a model, we could come up with very unintuitive solutions quickly. This approach can be very powerful for complex systems, which we don't fully understand. Some people start put much hope on this especially in tough engineering problems.

Being a philosophical person, I ask: is there a fundamental difference between human intelligent design and an optimization process such as GA. The working behind GA is try-and-error via simulation. Now let's analyze the process of engineering design to find out how it could be different.

First of all, I would like to quote a saying in the community of mechanical engineering: "you design by intuition, model for conscience, or don't model at all." In many well-designed human artifacts, there was no modeling involved during the designing process. Often times, the designer "simply knew" what would be a better configuration without knowing exactly why. And this knowledge came from experiences. These experiences include personal observations, hands-on works, conceptual reflections, and communication with others. Our engineering knowledge is not legislative. We often go with rules of thumb.

Secondly, the key to good engineering is to build a good sense of physics. In fact, personal observations and hands-on experiences provide physical episodes of how things work. Then conceptual reflection and communication setup virtual simulations of physical systems in our brain. Every good engineer has a physics engine inside hisz/her brain working out solutions by simulating ideas. And the predictive power of this simulator is directly proportional to experiences and our understanding of physics.

Finally, we call our ability to create and design “intelligence”! This ability is how we can determine what could work and fail without actually building the entity. In the ultimate sense, it is our ability to simulate different situations in our brain quickly and come to a good but non-perfect decision. We can avoid physical try-and-error because we did that quickly in our mental simulator. We can often skip many unnecessary simulations because we remember the results from similar episodes before.

Now, let me come back to my question: is there a fundamental difference between human intelligent design and an optimization process such as GA? The difference can only be in the processing units. While we simulate with biological neural networks, GA relies on computers and programming. For a robot, designing a locomotor gait is the same as evolving one. The only difference is brain vs. computer algorithm. So if you can build a computer that interprets our physical understanding well for the system of interest, then robot evolution by GA saves your brain labor. Otherwise, human design and try-and-error is more efficient. We teach computers how to “think” remember?

This is how I solve the question of Intelligent Design vs. Evolution in robotics by arguing their equivalency, or how I create a debate with my radical views.

A Brief Review of Huai-Ti's Caterpillar Robot

2009-09-07T19:51:00.000-07:00

It's time for a review of all my robotic attempts with soft materials. I think I am starting to grasp the core principle of making a piece of rubber move. Of course, it's the "rubber soul"!

Coming up... it's GoQBot-II(Roll-n-Jump) and GoQBot-III(Shake-n-Glide). All future robots will be tether-less because I just finished the micro-RC system with dual control [PC+Mannual]. More will be coming up in the near future. Stay tuned!!

Special post on rolling caterpillar robot!!

2009-09-04T17:53:00.000-07:00

If any of you remember in April when I featured the fastest locomotion by caterpillar (15" per second), then you would recall this impressive ballistic roll performed by "mother-of-pearl moth" larvae. BBC had a nice little video introduction about it. I posted the YouTube link on my blog under "Interesting Links". In any case, here is that link if you want to refresh your impression on what caterpillars can do.

Well, this August when I came back from the UK, I was compelled to consider replicating this biological feat with my robot. It was the week right after I developed a gait for my caterpillar robot to crawl through a 1cm hole all by itself. The Tufts robotic team was concerned with meeting the DARPA speed metric. Crawling gaits are robust, but rather slow in general. To defended my love of biological inspirations, I introduced this more dramatic locomotor mode. A few days later I had lunch with two of my best colleagues again at WholeFood, it came across our minds that it might not be too crazy to replicate what the rolling caterpillar has done. So the idea of "GoQBot" came into being, but it took us another month of hard work to translate what's on a napkin blueprints to a physical entity (yes, the original ideas were outlined on two WholeFood napkins).

Why is it called "GoQBot"? That's another interesting story. My buddy Tim is really good at drawing comics around engineering schematics. He was also one of the two colleagues in that brainstorming lunch break, the other being Dr. Gary Leisk. Tim envisioned this robot being so quick that it will go on and on as it rolls. So he drew a robot rider who shouted "GoGoGo". Gary later interpreted that into Go to the cubic power like "G^3". I thought the name could be a little more sexy and turned it into "Go-Q". The letter "Q" pictorially resembles the configuration when the caterpillar rolls into shape, and phonetically retained the cubic power of Go. Interestingly, the first generation of GoQ-Bot literally rolled three rotations (Go^3) in the Q configuration. See the brief introduction below!

"GoQBot-I" retains the same body plan from the InchBot series (InchBot IV~VII) which could performed three kinds of inching gaits, two variations of crawling gaits and a spacial climbing gait. This time, the robot has two additional flexible tail appendages that provided stability and guided curling trajectory. How does it perform a ballistic roll? See the following snapshots for yourself! It's quite obvious why it has to "Go-Q"I apologize for the bad contrast on my robot. I simply forgot to mix in rubber dye and sparkles! Next GoQBot will definitely dress up in a flamboyant color with pink sparkles (maybe some fluorescent markers for kinematics analysis as well). It might be Go-Cute Bot instead!

My discipline does not fracture

2009-08-30T13:25:00.000-07:00

This past fortnight was a tough one, for I managed to fracture my right foot two weeks ago. I was immobilized for 4 days before I couldn't stand for anymore home arrest and went back to the laboratory anyway. Indeed, it was a hard time for everything from financial impact to simply getting somewhere. I deeply thank all my friends who helped me survive. I shall get back on my feet very soon.

Well, I am not about to continue this unfortunate story although that was part of what I had to deal with last two week. Since I spent more time at home due to the injury, I started to work on that pile of papers that I "was" supposed to read... and guess what? These were papers about the kind of skeleton that doesn't fracture: hydrostatic skeleton!!

According to my ground reaction forces analysis of caterpillar crawling, hydrostatic skeleton was not active during normal locomotion (manuscript still under review at J Expt Biol). However, we know that Manduca caterpillar can pressurize itself substantially and cast about with precise control. Then where is that switch that "turns on" the hydrostatic skeleton?
Well, I thought I would look into discussions on body pressure of all the major soft-bodied animal systems. In annelids [leeches, earthworms, sandworms...etcc], the cylindrical body is covered by very tough layers of circumferential and longitudinal muscles. Segments are separated by septum muscles which constrict the flow of incompressible coelomic fluid. This type of hydrostat has been modeled and studied for a long time. Deformation can be mapped given that the body volume and pressure are known. Nematodes, on the other hand, does not have circumferential musculature. But they tend to have much higher internal pressure and some evidence suggests that their body wall has very high residual stress. In this model, longitudinal muscles are working against the highly stressed elastic cuticle through the incompressible body fluid. Now, what is the case in caterpillars?
Unlike worms, caterpillar cannot breath through the skin; it has to ventilate via an extensive trachea system. Morphologically, these are significant air cavities open to the exterior. The body cannot be "incompressible" even if the body fluid is. In fact, air bubbles are expelled when a caterpillar crawls underwater. In addition, caterpillars don't have circumferential muscles. To pressurize the body, a caterpillar can do two things: close the spiracles, and compress the body fluid. Indeed, when we try to tear an caterpillar off its substrate, the animal gets shorter and stiffer. However, what can we say about other caterpillar behaviors where the body seems to extend beyond resting length and still maintain body pressure. I suggest a flattening action via the oblique muscles that cause the animals to compress body fluid. Some EMG data might support it, but there is no sufficient proof at this moment. How much compression is require to pressurize the hydrostatic skeleton is totally unclear?

There will be more robotics coming up in the next posting in two weeks I promise! I've been working my leg off (... almost) on the robot radio control system and other robot behavioral integration.

InchBot-VII starts to climb!!

2009-08-16T14:34:00.001-07:00

After two weeks of endeavor (sleeping in the lab etc...), I finally convinced the new InchBot to climb up a steep incline. According to the animal locomotion literature, climbing is defined as moving up a incline over 45 degree. Well, currently this InchBot-VII can handle just over 45 degree... so it's a climbing robot for sure!!
This 124mm long robot has two batteries in the head capsule and the rear capsule respectively. So it does not need to be tethered really. It weighed less than 4g if we include the R/C control circuits. It climbs with three sticky pads and several morphological features. The climbing gait was adopted directly from the principles of motion in Manduca caterpillar. Stay tune to my upcoming JEB paper titled: Substrate as skeleton: ground reaction forces from a soft-bodied legged animal for details.
Oh...this robot also got some pink sparkles in the body which made it kind of cute!
Go InchBot!!

Sticky Pads... for climbing robots

2009-08-07T16:55:00.000-07:00

In order to improve crawling efficiency, we started to develop controllable grippers for the caterpillar robot. There are three common mechanisms for gripping: Hooking, Adhesion, and Suction. For the scale of our robot, micro-hooks array and adhesion pads seemed most probable. Manduca caterpillars use the former(crochets) while most insects employ both. However, there are great challenges in both systems.

To effectively dig into substrate of non-uniform stiffness, the micro-hooks array has to vary in hook size and compliance. Up to date, we have very little knowledge about these hooks arrangement and properties. Let alone the task of manufacturing such an micro-array for load bearing.
On the other hand, sticky pads are easier to produce. There are hundreds if not thousands of different adhesive materials to choose from. However, release of sticky pads may be quite a challenge for small robots.

After evaluating the situation, I decided to design a membrane with sticky substance on it. Once I master how to control such a membrane, I could control the sticky pads. Then, micro-hooks array can be incorporated into the system.

Akimidis caterpillars

2009-07-26T19:53:00.000-07:00

People often ask me what I do with caterpillars and poke their head toward my various apparatus. Well, this one is obvious! "Why are you dipping caterpillars into water?"

Together with a hard working undergraduate research student, we perform an simple experiment what Akimidis would have appreciated very much. By dipping the caterpillar progressively, we could map the volume contribution from head to tail by quantifying the buoyancy. This simple method easily beats the fancy 3D laser scanning, MRI imaging, and X-ray data our predecessors attempted before us in this lab. The only special skill is to tam the caterpillars to stay still during the measurment. Well, this is what caterpillars do in nature -- "I'm not here!!" Nevertheless, cold water can be irritating, thus controling the temperature is critical. The total emersion directly gives the overal body density of any given caterpillar.

Another collateral experiment was something Sir Issac Newton would have been quite interested. We balance a lightweight beam with the caterpillar on it by providing a pivot support below the beam. By measuring the moment force at one end, and shift the pivot across the animal, the mass distribution can be easiliy calculated. The first immediate result is the center of mass for the animal. Combining this data with the above volume map, we can reconstruct a density map across the animal body length.

Density map and mass distribution are two crucial parameters for any biomehcnaical model of a soft body. In addition, the density change across different animal sizes indirectly reflect the trachael volume and respiratory capacity, since most other tissues in the body are similar to water density. Open gas cavities also affect the use of hydrostatic skeleton for a soft-bodied animal. If some caterpillars don't use their hydrostatic skeleton for locomotion, maybe their bodies are too leaky for economic pressurization.

PS: The caterpillar in the lower photo was groomming after a water bath!!

My UK trip

2009-07-09T16:59:00.000-07:00

Having been back to Boston just last night, my first surprise this morning was seeing my chaotic room from my bed. I guess the hectic preparation for this UK trip had sped up the entropy evolution since the second week of June. Nevertheless, I was not discouraged by the mess a bit. What I gained the most from this trip was "motivation". Perhaps meeting more people who appreciate my research made my doing more meaningful. After transfering the dirty clothes from the suitcase to the laundry machine, I started cleaning my room. In the afternoon, I headed straight to the lab to pick up my projects. It's so good to feel motivated.

By the way, today I observed a caterpillar that curves around a rod, trying to crawl onto its own back. As it encountered its dorsal horn, it took a bite. I have seen dogs chasing their own tails but seeing a hornworm trying to eat its own horn was really something more attractive than a gossip. Perhaps this particular caterpillar was somewhat unusual, but it certainly made me doubt caterpillars' self-awareness.

A new behavioral experiment --- walk the substrate

2009-06-21T16:18:00.000-07:00

From my caterpillar GRF data, I discovered that in almost any instance the substrate is stretching the animal axially. I was intrigued by the idea that such a soft-bodied animal might be actually "depending" on the substrate stiffness to achieve neccessary deformation during locomotion. So, I thought maybe the stiffness of the substrate would affect the crawling performance in a very predictive way. The first test that came to my mind was to suspend the animal from the head capsule and the rear horn (see photos). By feeding the animal different substrates, they could "walk the substrate backward" by doing their normal forward locomotion in my suspension setup. Interestingly, they crawl just fine with a balsa wood. As I reduced the stiffness of the substrate by changing from wood to rubber and eventually to a soft wire, the caterpillar started to buckle the substrate and failed to produce the stereotypic locomotion pattern. This is the first behaviroal support for the "environmental skeleton hypothesis".

The roles of thoracic legs in caterpillar locomotion

2009-06-07T05:36:00.000-07:00

We all know that the six thoracic legs are the true legs that will be retained through metamorphosis in lepidoptera. However, amputation of these limbs does not seem to prevent a Manduca caterpillar from crawling around. Nevertheless, caterpillars appear to use them constantly during each crawl cycle. Originally we considered them as probing devices for sensing the substrate ahead, until I analized the GRF data from thoracic legs. Besides that fact that each leg pair could take up as much body weight as any pair of prolegs, they also exert quite significant amount of forward pull during each crawl. What happened to the amputated caterpillars I cannot figure, but thoracic legs definitely have a important role in normal locomotor performance.

Soft-bodied robotics

2009-05-24T14:12:00.000-07:00

For the past few weeks, robotics has taken over my life once more. This time, I challenged the scale and stability with my new designs of soft-bodied inchworm robot. The previous crawling green foam known as InchBot-III (classic): FoamBot. It has been successfully modeled in a FEA implementation with high accuracy.

In order to scale down and maintain the same mechanics, I needed to change the material, or body bending mechanism. For that, I developed the next generation robot InchBot-IV (mini): LeechBot that reversed the working principles of Mckibben artificial muscle. By weaving the SMA into the tubular braid, the structure could create suction durint contraction at two ends, very much like a leech. The suction adhesion morphology is currently under design.

By changing the material, I was able get another inching robot. This little guy measured no longer than 80mm and no wider than 5mm. A newly featured membranous wing allows better adhesion and lateral stability. This winged inchworm was the InchBot-V (nano): InchFly. It does not fly at this moment. But one could easily imaging such a gliding potential from the wing area to mass ratio.

Predatory caterpillars

2009-04-26T16:24:00.000-07:00

Recently, I came across some literature about predatory caterpillars. I guess lepidopteran larvae are not strictly herbivorous as I assumed. Several ambush predator caterpillars have been found in Hawaii including the green grappler Eupithecia. A YouTube video demonstrates how this kind of caterpillar modifies the strike reflex for ambushing a pasing termite. They have modified thoracic legs and A6 prolegs for grabing the prey and manuveuring the mostly airborn body.

More recently, a paper in Science reported a case-bearing caterpillar named "Hyposmocoma molluscivora" which feeds exclusivly on snails. This kind of caterpillar spins silk over a resting snail in a spiderlike fashion in order to anchor the prey. Then it would start digging into the shell opening all the way to consume the snail, even if that means leaving its silk case behind.

Another class of caterpillar carnivorousity is consipecific cannibalisim when the population density is too high. Semlitsch and West described the relationship between body size and caterpilalr cannibalism in the journal Oecologia. It was found that smaller caterpillars are more likely to become the victims simply because they are not capable of killing the bigger conspecifics.

Reconsidering "caterpillar locomotion"

2009-04-19T18:14:00.000-07:00

Want to guess the highest speed by which a caterpillar can move?
Answer: ~15" per second!!!
No way... what species and how?

I recently reviewed a few papers about rolling locomotion in nature, and Mother-of-Pearl moth caterpillar (Pleurotya ruralis) is one of the two active rollers ever discovered. Check out a YouTube clip by BBC Aniamls and you will believe what I claimed. In this movie, the caterpillar was rolling downhill, but the rolling action is actually initiated by rapid muscle contraction. According to Dr. John Brackenbury reported in his paper "Fast locomotion in caterpillars", this active rolling could hit a top speed of 39cm/s.

Here are a couple of references for those who want to read into this subject.
J. Brackenbury. Fast locomotion in caterpillars. Journal of Insect Physiology. 45:525-533 (1999)
J. Brackenbury. Caterpillar Kinematics. Nature. 390:453 (1997)

Three forms of caterpillar and their locomotion

2009-04-14T05:20:00.000-07:00

Manduca caterpillar only represents a class of Lepidoptera with four abdominal prolegs. Many other species have reduced the size or number of prolegs. Geometrids (loopers, inchworms, and spanworms), for example, retain only the A6 abdominal prolegs which work in conjunction with the terminal (anal) prolegs to facilitate the rear end attachment during a looping action. Lepidoptera larvae in the limacodid-group have very short legs at the belly that their modes of locomotion resemble slug creeping in apparence. For these so called "slug caterpillars", there can be abdominal prolegs from A2 to A7 with a combination of crochets and suckers. After starring at the "typical" 4-Abd prolegs caterpillars for three years, I start to wonder how the differences among these locomotor modes are associated with the evolution of these three forms of caterpillars. First thing that came to my mind was the mass scaling issue. According to my experiences with live caterpillars in the wild, 4 Abd body plan is common in species that can get really big and juicy. Inchworms are rarely bigger than 5cm. Slug caterpillars seems even smaller.

Larger caterpillars --- 4 Abd prolegs (use as many body anchors as the locomotion allows)
Loopers/Inchworm --- 1~3 Abd prolegs (reduce unneccessary attachements for efficiency)
Slug caterpillars --- short prolegs close to the ventrum (increase contact surface for fluid/silk adhesion mechanism)

This line of reasoning is very consistent with my current biomechanical finding: prolegs=attachments. (results from the caterpillar substrate reaction force analysis) Since soft legs cannot provide effective leverage, they are just acting as anchors for caterpillars. And it follows that the evolutionary modifications of prolegs morphologies should be determined by the maximum body size and mechanisms of attachement.

Body pressure controled by the smart integuement...

2009-04-11T16:48:00.000-07:00

Integument of soft-bodied animals is often a very interesting biomaterial. Besides unfolding and stretching to accommodate growth, it may function as a body pressure monitor in some instances. More specifically, the viscoelastic effect of the Manduca body wall prevent stress from building up as the body volume increases. When the caterpillar lost body volume, the body wall will automatically shrink to maintain the body pressure. The above effect might sound like a typical barometric reflex in mammals, except that this is done via the passive properties of the cuticular integument. Indeed, a simple reflex may be substituted by a smart material! The following graph shows the "adaptation" response found in the body wall, muscles, and the stretch receptor organ lining along the body axis. More detailed experimentation on caterpillar body pressure is ongoing.

Manduca goes aquatic!!

2009-04-07T08:31:00.000-07:00

Caterpillar locomotion is slow and quasi-static. However, it is very robust over a wide range of environmental conditions. In addition to maneuvering in highly branched vegetation, caterpillars can burrow underground and even crawl underwater. In my experiment, water submersion converts gravity into buoyancy and increases the external pressure by a factor of two. Nevertheless, a Manduca 5th instar caterpillar can continue to function up to ~3min before the hypoxia kicks in.

InchBot-III comes alive!

2009-04-07T08:10:00.000-07:00

InchBot is a biomimetic soft-bodied inchworm robot actuated by shape memory alloy springs, controlled by a pair of coupled neural oscillators, and powered by a Li-Po battery. It is completely autonomous with the ability to conform to the substrate as a soft body. The exploitation of this compliance allows the robot to operate without feedback over a range of terrain variations, much like the real animal. InchBot-III can crawl up to 5mm/s on a tabletop and carry a payload backpack up to 15g (bottom photo). The whole robot can be rolled up and compressed into one palm (top picture).

Welcome to Huai-Ti's research blog!!

2009-04-07T06:18:00.000-07:00

Hi there!

In the field of animal locomotion, fast ballistic motions are always eye-catching. However, on the other end of the spectrum, slow soft body movements can be equally amazing. My research focuses on force transmission in soft-bodied animal locomotion. In particular, we use Manduca sexta caterpillar as the model system.

In response to the feedbacks and requests from recent encounters at professional conferences, I decided to start this blog as a window for people to check on my research progress. I will do my best to update new cool graphics and info about my research. Please note that this site is not intended for formal publication. I cannot post any unpublished data or engineering details. I apologize if the materials cannot satisfy your curiosity, but I promise to publish my research as fast as I can. Thanks for the interest.

HTL