Boston Dynamics BigDog

What BigDog is

BigDog is a four-legged robot built by Boston Dynamics, the company led by Marc Raibert. Boston Dynamics' own innovation timeline lists BigDog under 2004 and calls it the company's first legged robot to leave the lab^[1]. The IEEE Spectrum ROBOTS database gives its year as 2005^[2]. The machine was 100 cm long, 30 cm wide and 125 cm tall, weighed 113.4 kg and could reach a speed of 11.26 km/h^[2].

Where it came from

According to IEEE Spectrum's ROBOTS database, Boston Dynamics began developing BigDog in 1995 with funding from DARPA, and researchers from NASA's Jet Propulsion Laboratory and Harvard University collaborated on the project^[2]. The funding came with a concrete aim: a robotic pack mule that could accompany soldiers over terrain too rough for conventional vehicles^[3]. That aim shaped the design from the start — the job was to carry cargo across ground where wheels and tracks struggle, and the legs were the means to that end.

How it worked

BigDog had 16 custom hydraulic actuators and 20 degrees of freedom, 16 of them powered and 4 passive^[2]. A hydraulic actuator is a cylinder that extends and retracts under fluid pressure, which makes it possible to get large forces out of a compact joint — useful for a machine that has to push off the ground while carrying a load. The hydraulic pump was driven by a 15-horsepower internal combustion engine, and the control computer was a PC/104 board with a Pentium 4 class CPU running the QNX real-time operating system^[2]. A real-time operating system is one that guarantees the control program runs on a fixed schedule instead of being interrupted arbitrarily; for a walking robot, whose joints must be commanded many times per second, that predictability is what keeps the machine upright.

Legs also change the balancing problem. A wheeled vehicle is stable as long as it is on the ground; a legged one is always falling slightly and catching itself, adjusting each step to the terrain underneath. This is what "dynamic" walking means — the robot stays up by continuously placing its feet rather than by standing on a wide, static base.

What it could do

BigDog could carry payloads of 150 kg^[2]. It could walk dynamically on rubble, mud, snow, water and ice, and on slopes up to 35 degrees^[2]. These are exactly the conditions a pack mule was meant for: ground that is uneven, slippery or soft enough that a conventional vehicle would lose traction or simply not fit.

The combination of a 150 kg payload and rough-terrain mobility is what made the machine notable. The legs were not decoration on a vehicle; they were the part that let it go where the vehicle could not.

Where it sits in the line of Boston Dynamics robots

BigDog occupies an early place in the company's history: by Boston Dynamics' own account it was the first legged robot to leave the lab^[1]. The line continued. In 2011 Boston Dynamics announced a larger, stronger quadruped called AlphaDog^[2], built on the same idea of a cargo-carrying walking machine. Looking back at BigDog from that later work, its importance is easy to state plainly: it was the demonstration that a hydraulic quadruped could carry a heavy load over ground that vehicles cannot handle, and the starting point for the four-legged machines that followed it at the company.