Iron Man's Hands

Building an Iron Man suit is hard (of course). That's why Sarcos/Raytheon's suit disappeared into vaporware, why the HULC exoskeleton barely registers google searches, and why no one is pulling them out of suitcases out at grand prix race and defeating guys with plasma-powered whips.

But if you're going to build it, you have to start somewhere. Here's something I find completely astounding, for two reasons. Reason Number One is that it is almost the exact design I proposed to my friend Josh a month ago and am pleased to see that I am good at conjecturing how to build powered arms. Reason Number 2 is that its not just a prototype - people are actually using it.

Festo's design has a key feature: pneumatics. This presents one advantage and one disadvantage. The advantage is that a pneumatic actuator and its peripheral support systems are much smaller than hydraulics. The disadvantage, as any good engineer will tell you, is that gas is compressible, so controlling a pneumatic actuator is a nightmare compared to a hydraulic one.

"What do you mean?" says the non-engineering-background reader. Well, let me explain. Let's say you had a bike pump and you wanted to air up your tires. The bike pump is basically an pneumatic actuator: you apply a force and the air moves. Ever pushed down really fast on your bike pump? It bounces back. That basically means it is "hard to control." Now let's move to hydraulics. If you've ever used a hydraulic jack to lift up a car (because you had a flat) you know that there's not that bounce. You apply a force, the jack moves. Apply a force quickly and the jack moves quickly.
This is an over-simplified case, but the truth is universal. People have been messing with pneumatics for prosthetics for years. McKibben Artificial Muscles are basically Really Well Engineered Balloons that are rapidly inflated or deflated to simulate muscle motion.

So the Festo hand system intrigues me. But at the same time, as an anatomist I see a problem: their actuators are two way.

"What do you mean?" says the non-anatomy-background reader. Well, let me explain. In the human body, every muscle does the same thing: it contracts. So our bodies have an "antagonist" muscle to pair with every "agonist." The simplest example is the bicept in your arm (flex it tough guy, I dare you) and the tricep, whose job is to do the exact opposite thing the bicep does. In your thigh, the quadriceps and the hamstring act as an agonist-antagonist pair.
This Festo system uses a single actuator for both. And therein lies the reason for the convoluted design. This is an unnatural way to create an artificial hand. I mean, I laud Festo's efforts. Their powered glove is beautiful and functional.

But its not perfect.


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