Less than half the Earth’s landmass is accessible to wheeled  and tracked vehicles, yet people and animals can go almost anywhere on Earth. This situation motivates the development of robot vehicles that use legs for their locomotion, thereby embracing nature’s mobility solution. The goal is to achieve animal-like mobility on rough and rugged terrain, terrain too difficult for any existing vehicle.

BigDog is the alpha male of the Boston Dynamics robots. It is a rough-terrain robot that walks, runs, climbs and carries heavy loads. BigDog is powered by an engine that drives a hydraulic actuation system. BigDog has four legs that are articulated like an animal’s, with compliant elements to absorb shock and recycle energy from one step to the next. BigDog is the size of a large dog or small mule; about 3 feet long, 2.5 feet tall and weighs 240 lbs.

BigDog’s on-board computer controls locomotion, servos the legs and handles a variety of sensors. BigDog’s control system keeps it balanced, navigates, and regulates its energetics as conditions vary. Sensors for locomotion include joint position, joint force, ground contact, ground load, a gyroscope, LIDAR and a stereo vision system. Other sensors focus on the internal state of BigDog, monitoring the hydraulic pressure, oil temperature, engine functions, battery charge and others.

In separate tests BigDog runs at 4 mph, climbs slopes up to 35 degrees, walks across rubble, climbs a muddy hiking trail, walks in snow and water, and carries a 340 lb load. BigDog set a world’s record for legged vehicles by traveling 12.8 miles without stopping or refueling.

The ultimate goal for BigDog is to develop a robot that can go anywhere people and animals can go. The program is funded by the Tactical Technology Office at DARPA.


An onboard computer controls BigDog’s behavior, manages the sensors, and handles communications with a remote

human operator. The control computer also records large amounts of engineering data for performance analysis, failure analysis and operational support. BigDog has about 50 sensors. Inertial sensors measure the  attitude and acceleration of the body, while joint sensors measure motion and force of the actuators working at the joints. The onboard computer integrates information from these sensors to provide estimates of how BigDog is moving

in space. Other sensors monitor BigDog’s homeostasis: hydraulic pressure, flow and temperature, engine speed and temperature, and the like.


That is a snapshot of where BigDog stands today. While we are happy with the progress BigDog has made so far, much higher performance is possible and many practical problems remain to be solved. Our immediate goals are to focus on

Rougher Terrain: Although BigDog is doing well on rough terrain, it should be possible to traverse rougher and

steeper terrain with more load. It will require a mechanical design that is stronger and has larger ranges of limb motion. Advanced terrain sensing and locomotion planning will also be needed to enable travel on rougher terrain.

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