Engineers Produce a Rocket-Powered Human Arm

Aug. 28, 2007 — -- Engineers in Michael Goldfarb's creative lab at Vanderbilt University have come up with a possible solution to a whopping technological challenge: How do you build a synthetic human arm that will restore full functions to an amputee, and where do you get the kind of power needed to run it?

Batteries aren't a good solution because they weigh too much and they don't pack enough punch.

The answer, Goldfarb said, is a miniature rocket motor.

Yup. The kind of rocket that is used to control the attitude of the space shuttle while it's on orbit may be just the ticket, he added.

And furthermore, he's done it. His lab has produced a robotic human arm that comes surprisingly close to replicating the complex functions of the human arm, and it is powered by a tiny rocket motor. It isn't ready for prime time yet, but as a proof of concept, it's pretty impressive.

For several years Goldfarb, a professor of mechanical engineering at Vanderbilt, has been pioneering in the development of incredibly small robots that can do things like slip behind enemy lines and sniff out chemical weapons, and he has returned to his original inspiration for his ideas. If you want to make a robot that can mimic nature, look at biological systems.

Amazing Arm Degrees

Goldfarb's research is part of a large scale program supported by the Defense Advanced Research Project Agency. Two other prosthetic arms are under development at the Advanced Physics Laboratory at Johns Hopkins University in Baltimore, which is running the project. But Goldfarb's approach is very different.

Although the current research is directed toward development of a prosthetic arm, there would be many applications for a device that is as sophisticated as something we tend to take for granted. The human arm is, indeed, an amazing piece of equipment.

From the elbow to the fingertips, the human arm has "26 degrees of freedom, which means it can move that many joints," Goldfarb said. By contrast, the robotic arm aboard the space shuttle, which has been used for everything from launching satellites to repairing the orbiter, has only six degrees of freedom. So how do you get from six to 26?

Human muscle is also lightweight and packs a tremendous punch, Goldfarb said. Engineers are interested in the amount of power you get per unit of weight, and any kind of biological system is five times better, in terms of power per unit of weight, than the most powerful rare earth electric motors, Goldfarb said.

"We don't just have a lot of degrees of freedom, but we've got a lot of power and speed and force packed into a small space and into a fairly lightweight object," he added. "So there's this huge gap, and our task in building this arm was to close that gap as much as possible."

Batteries Won't Cut It

The engineers needed something much better than batteries and electric motors, he added. So they looked at biology, which has an enormous advantage in that biological systems convert chemical energy directly into mechanical energy. And Goldfarb concluded that the best imitation of that is a rocket motor.

The arm in his Nashville lab can pick up a bottle of water and pour it into a cup. It can take a ball from a human hand and toss it back. And since it is powered by a monopropellant rocket motor system, it can be turned on and off quickly, and is very versatile.

"I can control it well enough that I can lift a heavy weight with it [up to 25 pounds,] but I can also screw in a light bulb," Goldfarb said. "I can do delicate things with it."

The power source is hydrogen peroxide, which burns when it comes into contact with a catalyst, producing pure steam. The steam is then used to open and close various valves, causing the arm to respond to commands. The power source is about half the size of a small pencil, and a tiny sealed cylinder of hydrogen peroxide can power the arm for up to 18 hours.

The chemical reaction produces heat — up to 450 degrees Fahrenheit — which is vented through a system that mimics normal sweat. Goldfarb said you could hold your hand in front of the vent and barely feel the warmth.

So how soon are we going to see this technology in the marketplace? Goldfarb releases a deep sigh at that question, because the road from the lab to the market can be very long and bumpy.

There's such a thing as cost, probably high at this point due largely to the very precise engineering to produce the equipment. But the age of robotics is upon us, and systems like Goldfarb's will probably have a place.

However, don't expect anything soon like you see in the movies.

"People really misunderstand where robotics is at right now," Goldfarb said. "They see these animated robots in the movies that have superhuman capabilities. The reality is just the opposite. Robots are much heavier and weaker than any biological system."

So if some day soon you find yourself being chased by an evil robot and you want to escape, Goldfarb offers this advice.

"Walk just a little bit faster."