Interview with Mark Yim

Featured Engineer

Mark Yim

Mark Yim - Professor, Mechanical Engineering and Applied Mechanics, University of Pennsylvania

How did you get into electronics/ engineering and when did you start?

That is kind of a hard question. You can go all of the way back to when I was 11 years old, reading science fictions books. I thought to myself that it would be really cool to build robots.

More practically, my development in robotics started while I was working on my graduate degree at Stanford, around 25 years ago. The ability and resources have changed a lot over the last 25 years. The field is growing like crazy.

What is on your bookshelf?

One thing that may be unusual for a professor is the lack of books in my office. However, I do have many shelves on which sit some past projects that I am lucky to have been a small part of.

This includes an Airjet Paper Mover prototype I was involved with at Xerox PARC. This is an approximately 1m x 1m PCB with about 200 electrostatic flap valves and 1,000 sensors that control 200 angled air jets to float paper in different directions.

I also have a three foot long Tyrannosaurus Rex head made out of flexible foam that I made as part of a Halloween costume for my kids. This head sits on top of a Nerf machine gun that was modified for a class project to shoot down small robot blimps.

Past student projects sitting on my shelves include a robotic skydiver (actually a 6 foot wide controllable parachute as a Radiosonde Recovery Project), satellite dishes (hacked to make microwave transceiver based scanning devices), and a one-wheel Segway-like device hangs from the ceiling. A six-foot tall vertical axis wind turbine, and a four foot 3D linkage based device that looks like a cross between a giant claw and a flower, which we call a poison gourd that was part of a robotic version of a Shakespearian play, are also in my office.

Shape-changing robots are my primary research. Many versions of these devices sit on my desk. There are some baseball-sized cube shaped devices that bend and can attach or detach from each other. I have mockups of a chain of sugar cube-sized robots that can form different shapes as well as a sheet of plastic with box-pleated folds in it which is a mockup of a shape-memory alloy based self-folding origami project I was recently part of.

Can you tell us more about the Robotic Skydiver Project?

This was a senior design project. I like to call it extreme robotics, but that is not at all what it is. The student started by studying radiosondes, the instrumentation carried by weather balloons, and some of the problems they face. One of the problems is that when the weather balloons have reached their highest point, they pop and the instruments crash to the ground. They have notes on the packages that say if they are found to please return them. They get maybe 1% of the packages returned. This student came up with the idea of putting a steerable parachute on the packages, so that when the balloon pops, the parachute deploys and can direct the radiosonde to a specific location, a collection site. It is technically a environmentally hardened autonomous control system using GPS, inertial sensors and some control algorithms to guide the device under different wind conditions. It was actually a really cool project, and won all kinds of awards from the university. And when the student graduated, he left the project with me.

Do you have any tricks up your sleeve?

The T-Rex head was made with 1” thick foam. Foam is a great material for making costumes (flexible and light) but turns out to be very hard to manually cut in a clean fashion. Because the foam bends, scissors or a knife will bend the material as it cuts it distorting the cut. However, laser cutting works really well. Most schools have laser cutters. If you don’t have access to one, there are many internet based service places that can cut for you.

In my lab we use a process called shape-deposition manufacturing which was developed at Stanford over a decade ago. It involves a two-part urethane molding process in which complex moving parts can be made with varying compliance. We can also embed other materials like PCB’s or SMA wires or sensors, all without using any screws or other fasteners.

What has been your favorite project?

We are studying lateral undulation in snakes, which turns out to be a surprisingly unintuitive mechanism.

There is a funny story where the inspiration for this work comes from. I had been working on robots for search and rescue. Typically for search and rescue you want robots to go into cluttered environments to see if they can find a victim, someone that has been injured from either natural or manmade disasters.

I was actually driving home from a family vacation on a highway, and my kids were playing in the back and one of them starts screaming, “Snake! Snake!” And I was thinking that they were just playing around. When I turned around there was actually a snake that had somehow gotten into the car and was lying across the back seat. We pulled over and when I opened the back door I saw the snake which had been across the seam of the seat, it had somehow went up the vertical seam of the back seat, got into a crack in the back and disappeared into the rear of the car. Later on I looked at seat and there was a tiny hole that you could barely stick a couple of fingers in, but somehow the snake had gotten in there. This snake had somehow gotten from the ground into the car, we think through the trunk area, and could maneuver in that tiny area inside the back seat. I was so amazed that something could have such maneuverability in such tiny places. The snake is ideal for things like search and rescue.

We have built many robot snakes. If you make a chain of 1DOF rotary motors and have them make do traveling waves in the horizontal plane (like you might see a snake do as it travels on the ground), the robot snake will not move as you might expect. It will typically wriggle in place – often moving slightly backwards. Many people have discovered if you put passive wheels on the snake, the snake will move very much like a real snake does using lateral undulation. However, real snakes don’t have wheels!

How does the robotic snake work?

It turns out that there are multiple factors that come into play including anisotropic friction on the snake skin (David Hu at Georgia Tech has a great paper on this). As well, the snake lifts its body in certain locations as it moves (Hu also has some work on this, as does Shigeo Hirose using robot snakes).

If you look at snakes and at how they the most commone slithering motion, they use lateral undulation, moving in a horizontal plane. It is just recently that biologists are looking more deeply at how snakes move, using the friction of their skin.

In my lab, we are studying the snake skin mechanism a bit more in detail, trying to see if we can replicate it in some fashion. We may get a pet snake at some point, but haven’t done it yet.

Do you have any note-worthy engineering experiences?

I’ve spent many hours in an acid mine (a superfund site in California where acidity averages around 1 to -2 pH where it also gets very hot). This was a NASA sponsored project to see if robots could help in the search for extremophiles—typically bacteria that thrive in these hazardous conditions.

What are you currently working on?

As part of the shape changing, self-assembly work, we’ve started to explore expanding foams as an element for rapidly building robot parts. Expanding foams are often used as insulation in homes. There are some types of foam that have good stiffness properties. We have built a robot that can pour out this expanding hardening foam into shapes that can be parts of bigger robots. We’ve demonstrated this robot building a large crab-like robot with a foam body and legs, as well as a snake.

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