BigDog

Lore & Background

BigDog was funded by DARPA in the hope it would accompany soldiers as a pack mule over terrain too rough for wheeled vehicles. It uses four legs for movement, each equipped with four low-friction hydraulic cylinder actuators powering the joints. The legs contain sensors for joint position and ground contact, and the robot features a laser gyroscope and stereo vision system. BigDog is 3 ft long, stands 2.5 ft tall, and weighs 240 lb, roughly the size of a small mule. It can traverse difficult terrain, run at 4 mph, carry 340 lb, and climb a 35-degree incline. Locomotion is controlled by an onboard computer receiving input from approximately 50 sensors that measure attitude, acceleration, joint motion and force, engine speed, temperature, and hydraulic pressure. The robot can stand, sit, crawl, trot, or run with a trotting gait, with travel speeds from 1 km/h to 5.3 km/h.

The project was headed by Dr. Martin Buehler, who received the Joseph Engelberger Award from the Robotics Industries Association in 2012 for the work. In September 2011, Boston Dynamics released footage of a new generation called AlphaDog, showing its ability to walk on rough terrain and recover balance when kicked. In February 2012, with further DARPA support, the militarized Legged Squad Support System (LS3) variant demonstrated capabilities during a hike over rough terrain. DARPA planned to complete development by summer 2012, refining key capabilities over 18 months, including completing a 20-mile trail in 24 hours without refueling while carrying a 325 lb load. At the end of February 2013, Boston Dynamics released footage of a modified BigDog with an arm that could pick up and throw objects, lifting around 55 lb, funded by the United States Army Research Laboratory.

BigDog is powered by a small two-stroke, one-cylinder, 15 bhp engine operating at 9,000 RPM, driving a hydraulic pump that powers the leg actuators. Each leg has four actuators (two for the hip, two each for knee and ankle), totaling 16. Each actuator unit includes a hydraulic cylinder, servo valve, position sensor, and force sensor. Onboard computing uses a ruggedized PC/104 board stack with two computers: one running a Pentium M processor with QNX for sensor data processing, and another running a Core Duo processor for visual data processing.

Reader's Guide

BigDog's significance lies in its demonstration of dynamic quadruped locomotion for military logistics, pushing the boundaries of legged robotics in rough terrain. The project was funded by DARPA to create a pack mule that could accompany soldiers where wheeled vehicles could not go, and it successfully showed capabilities such as carrying 340 lb, climbing 35-degree inclines, and recovering balance when kicked. However, the gasoline-powered engine proved too noisy for combat, audible from hundreds of meters away, leading to the project's discontinuation at the end of December 2013. The legacy of BigDog includes the development of the militarized Legged Squad Support System (LS3) variant, which demonstrated rough-terrain hiking in February 2012, and the modified version with an arm that could lift 55 lb, paving the way for integrating manipulators with quadrupeds. The all-electric successor Spot, released in 2020, was much quieter but could only carry 45 lb. Both projects are no longer in progress, but BigDog's work informed later legged robot designs and highlighted the trade-off between power and noise in military robotics.