Monday, May 3, 2010

Where Does Iron Man Get His Strength?

Last week I outlined the four technologies that a high school student entering college could focus on in terms of their career path if they wanted to be part of a future in which Iron Man was feasible.
Of those four, we've covered power supplies and control systems. Today I want to focus on the 700 lb. gorilla in the room: the artificial muscle.

As a mechanical engineer by training and a biomechanist by preference, I've become intimately aware just how incredibly well-designed skeletal muscle is. Frankly, if it didn't work so well, why has the entire animal kingdom adopted it as a locomotive method? Muscle, as it turns out, is elegant in its complexity of construction while being simple in its purpose: to pull as hard as possible. Most people don't realize just how strong muscle is; during your hardest muscle flex, only about a third of your muscle fibers are firing at any given moment...much more than that and your muscles would literally rip themselves off your bones. Muscles come in a wide variety, from quickly-tiring fast-twitch to endurance slow-twitch. They, at some point in history, captured some bacterial precursor to mitochondria and enslaved them into their own built-in energy factories.

A quick math lesson: typing 120 words a minute, with an average of 6 letters per word, takes about 2,500 individual muscle actions a minute. And that's just to sit there and type!

But the real marvel of muscle is how energy dense it is. Muscle can produce about 0.35 MPa of stress, roughly 50 psi for us Americans. So a muscle with a one inch diameter can pull with about 40 lbs of force. Want to be stronger? Add muscle thickness.

Creating an artificial version of muscle, however, has been thwarting scientists and engineers for decades. Mechanical actuators, driven by motor/gear sets are promising, especially as brushless permanent magnet motors get smaller and stronger. But those assemblies trade speed for strength. Pneumatic actuators work really well at the same strength as muscle, but suffer from control issues due to the compressibility of air. Futuristic materials like "electroactive polymers" and "shape memory alloys" require high voltage, high temperature, or suffer from short lifespans.
Oak Ridge National Laboratory seems to have thrown their weight behind "mesofluidics" which is their term for hydraulic pistons the size of human muscles. They report hydraulic systems that can basically mimic human muscle in energy density in this paper. The researchers suggest the artificial hand could be used for telerobotic disassembly of roadside bombs in war areas.

TAE suggests a more likely use for these systems: powered exoskeletons. Here's two reasons why it would be better for exoskeletons: auxiliary systems and danger.
Unlike a standalone mechanical servo actuator, a hydraulic piston requires valving, pumps, and a motor assembly. These are all heavy. The ORNL team waxes over the fact that their prosthetic hand will require these systems, instead focusing on how light the actual hand it. Of course, if any artificial muscle technology could have auxiliary systems of unknown weight elsewhere, those AMT's would look amazing too!
But imagine instead, the mesofluidic actuators used as a powered the user 3-5 times their normal strength, and the system would carry its power generator, pump, reservoir, filter, valving, and control systems on itself somewhere, unloading it from the user. This is essentially what the SARCOS/Raytheon exoskeleton intends to do; the exoskeleton is a hydraulic system that carries the weight of the auxiliary hydraulic components on its back.
The second problem with hydraulic actuation, as opposed to some of the other actuation technologies, is danger. Personally, I don't know that I want to have a mechanical component intimately attached to my body that has 3,000 psi compressed parts in it. The last time I got in the way of a power-washer (at ~200 psi), I found the sensation a little unpleasant. I can't imagine getting blasted at 15X that intensity.
Once again, though, I turn to exoskeletons. It would be a relatively simple process to plate the exoskeleton wearer with light plastic or carbon fiber that would protect that soldier/user from exposure to failed hydraulic components. Those protective plates, however, are unsuitable for an 80-year-old who lost his hand in the war.

What ORNL and SARCOS have hit on here is that really, if we are honest with ourselves, hydraulic pistons are the only things capable of producing the power, pound for pound, that muscle does.

That's not to say Tony Stark uses hydraulics in his Iron Man armor. Given that he invented a new power source for it, invented a new control system technology, and is responsible for inventing all the weapons in his company and on the wouldn't be a leap to assume he invented a novel artificial muscle too.


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