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Adventures in Engineering
The wanderings of a modern ronin.

Ben Cantrick
  Date: 2006-02-12 19:46
  Subject:   The "Big Dog" four legged robot by Boston Dynamics.
  Mood:der uber-nerd
  Music:Kraftwerk: The Robots
I jumped a couple of links from a Slashdot story and found this:

BigDog is the alpha male of the Boston Dynamics family of robots. It is a quadruped robot that walks, runs, and climbs on rough terrain and carries heavy loads. BigDog is powered by a gasoline engine that drives a hydraulic actuation system. BigDog's legs are articulated like an animal’s, and have compliant elements that absorb shock and recycle energy from one step to the next. BigDog is the size of a large dog or small mule, measuring 1 meter long, 0.7 meters tall and 75 kg weight.

Check out the latest video (12.4 MB) (Their FTP s(w)erver is either massively overloaded, or throttled down to nothing, so you may have trouble logging in.) You can put your foot on the thing and give it a big push, and it won't fall over. This is dynamic stability implemented better than I've ever seen before.

The waspy model airplane engine is very obnoxious, but this is just a test platform. A real one might be able to use a silent direct methanol fuel cell to generate power. Of course that increases the IR signature, but if it's using a combustion engine already maybe not by much.

The part about the legs having "compliant elements that absorb shock and recycle energy from one step to the next" sounds verrryy much like Yobotics' Series Elastic Actuators. If so, a very, VERY smart and cutting edge move on the part of Boston Dynamics.

Looks like they're working on exoskeletons, too. (18.7 MB) Damn you guys, using series-elastic actuators in an exoskeleton was my idea! ;]
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  User: ohmisunao
  Date: 2006-02-13 10:10 (UTC)
  Subject:   (no subject)
OMFG! Hardsuits next! I'd settle for a Geuges-D ;)
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Ben Cantrick
  User: mackys
  Date: 2006-02-13 17:58 (UTC)
  Subject:   Well, simulation isn't reality...
The technical problems are the same as ever. Building a metal skeleton to fit outside the wearer is easy. We've been able to do that since GE built "Hardiman" in the 1960's. The real trick is to find a power source light enough and at the same time powerful enough to move it around. I was doing some math the other month on deep knee bends, which is roughly equivalent to climbing stairs...

Let's say we're moving 80 kg (175 lb) a vertical distance of 2.5 ft (.75m).
Going down is free, but coming back up requires moving that 80 kg 0.75m against gravity. Gravity has an acceleration of 9.8 m/s^2. Work is force times distance, and force is mass times acceleration, so...

F = ma = (80)(9.8) = 784 N

W = Fd = (784)(0.75) = 588 Nm

Assuming you only want to do one deep knee bend per second, that comes out to approx 600 Watts of power, which is a little more than 3/4 HP. Human beings can generally only sustain about 1/10th of a HP if they have to do some activity for more than a few minutes.

Then you consider that whatever engine you use, you're probably only going be able to get half of its output energy to the actuators, so now you're looking at 2 HP just to do simple stuff. Fast, energetic motion (running, jumping) drives up the energy requirements by a factor of 5 or so. So now you have to find a 10 HP engine (double most lawn mowers) small and light enough to fit on someone's back, and then enough fuel to run it for more than a few minutes... it gets heavy pretty fast. And every time you add to the weight, you have to go back and re-run the equations above and see how much more energy you're going to need to move not just the wearer, but the exoskeleton's weight too. A good electric motor to drive one joint can easily weight 10-20 lbs. Multiply by two ankles, two knees, and two hips... (that's just for simplification; if you want to be able to run, the toes joints need to articulate too, and there probably need to be two degrees of freedom on the hips.) This is why most of the successful exoskeletons right now are using hydraulics. The pump weighs about as much as a motor, but you only need one of them. The cylinders weigh about 2-3lbs each, empty of fluid. The fluid overall can weigh as much as much as the pump, but again that's still only the equivalent weight of two electric motors, as opposed to maybe eight or ten.

Currently most people looking for a lightweight power source are using model airplane engines, and for good reason. I was poking around yesterday and found a piston engine that weighs 9 lbs and puts out 11 HP. Hard to beat that, though I bet a model airplane jet turbine could... at the price of eating fuel three times as quickly.

And this isn't even going into the kind of materials you need for the armor, and synchronizing the actuators with the wearer's body, and the sensor systems, and etc...

The more you look at the engineering of hardsuits, the more impressed you become. Sylia pulled off an amazing bit of tech with them. They're still just a little beyond our technical means. But then 2030 is still more than 2 decades away. ;] (Hint - don't be in Tokyo in 2018 when BGC predicts the big kanto quake will hit. ;])
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May 2015