Building a Big Dog in a smaller scale.

As a design exercise, it's worth thinking about what it would take to do a Big Dog like robot, with good balance and rough terrain capabilities,
in toy size.
If you know nothing about how Big Dog really works, read these papers:
    http://www.bostondynamics.com/img/BigDog_IFAC_Apr-8-2008.pdf     http://www.bostondynamics.com/img/BigDog_Overview.pdf
So what's needed on the hardware front?
Big Dog is 1 meter high and weighs about 100Kg. A quarter-scale model would be 25cm high and weigh about 1.5Kg. (That's probably light). Its actuators would have to move 4x as fast in angular motion.
Big Dog's legs are simpler than they look. Each leg has four actuators (two at the hip, one each at knee and ankle). All actuators are identical. All four legs are identical, too. So this doesn't take very many different parts.
Each leg has force feedback on all actuators, and force sensing in three axes at the "foot". Big Dog's "foot" is a small rounded rubber object, mounted at the end of a shock absorber/spring combination.
A foot sensor with 3DOF strain information is rare in hobbyist robotics, but parts are available. See "http://www.tactologic.com/doc/datasheet_tactopoint1x1.pdf "
Big Dog is hydraulic, but that's not necessary in a small size. What is necessary is rather fast actuators. Remember, time scales down with size. Bioloid AX-12 servos are 300 degree/sec devices. It's not clear if this is fast enough.
The system-wide sensors on Big Dog are a 6DOF inertial unit and a laser line scanner. Big Dog uses a laser ring gyro, but that's overkill. An ordinary 6DOF inertial unit (3 accelerometers, 3 rate gyros) is good enough. A stereo camera pair could replace the LIDAR.
Big Dog doesn't have that much compute power. One ruggedized Pentium 4 class machine runs the thing. (It runs QNX, the hard real time microkernel OS.) A WiFi connection could be used to move vision processing off-board to a larger computer.
The control loops have to be fast. Big Dog runs the servo inner control loops at 1KHz, and the main balance control loop at 200Hz. For a quarter scale machine, these all have to be speeded up by 4x. And the inertial information has to come in at comparable rates.
None of this is totally out of reach. The $1000 hobbyist robots have comparable complexity, although far less compute power and much slower control loops.
                John Nagle
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