In many situations, the lowly legs of people and animals are superior to the most sophisticated sets of wheels, especially when it comes to getting around difficult landscapes. Considering that less than half of Earth's solid surface is traversable by wheeled vehicles, the flexibility and mobility of legged creatures to go just about anywhere is inspiring.

That's what spurred engineers at Boston Dynamics, Waltham, Mass., to develop BigDog, the “rough-terrain quadruped robot.” The company got its start as a spin-off of MIT where National Academy of Engineering member Marc Raibert and his colleagues first developed robots that run and maneuver like animals. Raibert, president of Boston Dynamics, answers a few questions about BigDog and other pet projects in an exclusive interview with Motion System Design.

Back story

MSD: Who came up with the idea for BigDog?

Raibert: Our team at Boston Dynamics had been working on legged robots for some time. When DARPA solicited a proposal for “Biologically Inspired Robots” in 2002, we cooked up the idea of BigDog and submitted a proposal for it. DARPA selected three proposals, one being BigDog, and started funding them in 2003. The DARPA program is called Biodynotics, which stands for Biologically Inspired Dynamic Robots.

We were inspired by legged animals, which can travel on all sorts of rough terrain. The purpose of BigDog was to see if a robot with animal-like legs and an advanced control and balance system could achieve rough-terrain mobility approaching that of an animal.

Design dilemmas

MSD: What were some of BigDog's greatest engineering challenges?

Raibert: BigDog proceeded through a series of designs with a variety of major and minor changes. We tried out four different engines and made many changes to the legs, including the numbers of leg joints and how they were oriented. Some designs were more successful than others, but our approach to building robots is “design, build, test/break, redesign,” so I don't think we ever lost confidence that the system was moving in the right direction.

One of the primary challenges was developing a control system that moves the legs and body to keep the robot balanced as it travels. We, and others, had made progress on this problem in the past, but no one had ever done it in a legged robot that works on real-world rough terrain. So the process of developing sensor-based dynamic controls that let BigDog walk, first in the lab, then in a grassy field, then in mud and snow, then on rocky hiking trails, took a lot of hard work, smart engineers, field testing, trips back to the drawing board, innovation, and perseverance.

MSD: What about BigDog's power system?

Raibert: We chose internal combustion because useful missions will require many miles of range. We knew from the outset that keeping the engine and hydraulic system cool would be a challenge in such a small, energetically dense design. We were happy when BigDog first walked for five minutes without stopping or overheating; when it went for 19 minutes, we were practically swinging from the chandeliers. Now BigDog can walk for more than eight hours without stopping or refueling.

MSD: What was your biggest surprise?

Raibert: We designed BigDog's control system to make the robot smart about balance and have reflexes that help it recover balance when there are disturbances, such as stepping on an unexpected rock. But we hadn't anticipated how well it would work until we inadvertently drove BigDog across a patch of black ice. Seeing BigDog struggle to catch itself when it hits the ice really made it come alive.

MSD: What's next for Boston Dynamics?

Raibert: Much of the BigDog design has broader application, especially the control and balance principles and the high-performance actuation system. We took advantage of that generality to accelerate the PETMAN biped robot program. By leveraging BigDog leg designs and controls, PETMAN hit the ground running, going from program start to human-like heal-to-toe walking in just seven months.

On February 1, we were awarded a DARPA contract to develop LS3, the first Legged Squad Support System, based on the BigDog research. Each robot will carry up to 400 lb of gear and enough fuel for missions covering 20 miles and lasting 24 hours. LS3 will follow a leader using computer vision or will travel to designated locations using sensors and GPS. Development will take 30 months at a cost of $32 million, with first walk out scheduled for 2012.

Tech specs

Power supply: A water-cooled, two-stroke internal combustion engine delivers about 15 hp. The engine drives a hydraulic pump that sends high-pressure hydraulic oil through a system of filters, manifolds, accumulators, and other plumbing to the robot's leg actuators.

Actuators: Low-friction hydraulic cylinders regulated by two-stage aerospace-quality servovalves; each actuator has sensors for joint position and force; each leg has four hydraulic actuators to power the joints.

Cooling: A heat exchanger mounted on BigDog's body cools the hydraulic oil; a radiator cools the engine for sustained operation.

Control: An onboard computer controls BigDog's behavior, manages the sensors, and handles communications with a remote human operator.

Sensing: BigDog has about 50 sensors. Inertial sensors measure things like acceleration, while joint sensors measure motion and force of the actuators working at the linkages.

Locomotion: BigDog can stand, squat, walk with a crawling gait lifting one leg at a time, use a trotting gait that lifts diagonal legs in pairs, trot with a running gait that includes a flight phase, and bound in a special gallop gait.

Max speed: 7 mph

Weight: 240 lb

Technical wish list: Handle rougher terrain, self-right if it falls over, quieter operation, greater autonomy

More info: www.BostonDynamics.com