Combining electrical and mechanical technology to design a legged robot can be challenging. However, Boston Dynamics (Waltham, MA) started confronting this challenge 25 years ago. Initially, they found that they could control these robots by breaking the required behavior into three primary activities: supporting the body with a vertical bouncing motion, controlling the attitude of the body by servoing the body through hip torques during each leg’s stance phase, and by placing the feet in key locations on each step by using symmetry principles to keep the robots balanced as they moved.
One design limitation was the need for on-board power so the robot could operate in the field without hoses and wires. Another was the need for control algorithms that provide locomotion and stability on rough terrain. These limitations were met with BigDog, a self-contained quadruped robot that addresses the practical problems of onboard power and rough-terrain controls.
With funding from DARPA, the company’s goal was to build unmanned legged vehicles with rough-terrain mobility superior to existing wheeled and tracked vehicles. The ideal system would travel anywhere a person or animal could go using their legs, run for many hours at a time, and carry its own fuel and payload. It had to be smart enough to negotiate terrain with a minimum of human guidance and intervention.
Existing BigDog robots have taken steps toward these goals, though a great deal of work remains. Onboard systems provide power, actuation, sensing, controls and communications. Power for movement comes from a water-cooled two-stroke internal combustion engine that delivers about 15 hp. The engine drives a hydraulic pump that delivers high-pressure hydraulic oil through a system of filters, manifolds, accumulators and other plumbing to the robot’s leg actuators. The actuators are low-friction hydraulic cylinders regulated by two-stage aerospace-quality servo valves. Each actuator has sensors for joint position and force. Each leg has four hydraulic actuators that power the joints, as well as a fifth passive degree of freedom.
A heat-exchanger mounted on BigDog’s body cools the hydraulic oil and a radiator cools the engine for sustained operation. An onboard computer controls BigDog’s behavior, manages the sensors, and handles communications with a remote human operator. The control computer also records engineering data for performance analysis, failure analysis and operational support.
BigDog’s 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.
BigDog can stand up, squat down, walk with a crawling gait that lifts just one leg at a time, walk with 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. Travel speed for the crawl is about 0.2 m/s, for the trot is about 1.6 m/s (3.5 mph), for the running trot is about 2 m/s (4.4 mph) and BigDog briefly exceeded 3.1 m/s (7 mph) while bounding in the laboratory.
BigDog weighs about 109 kg (240 lbs), is about 1 meter tall, 1.1 meters long, and 0.3 m wide.
BigDog is usually driven by a human operator who works through an operator control unit (OCU) that communicates with the robot via IP radios. The operator can also tell the robot to start or stop its engine, stand up, squat down, walk, trot, or jog. A visual display provides the operator operational and engineering data. The operator provides high-level input, leaving BigDog’s onboard control system to operate the legs, provide stability on rough terrain, and reflex responses to external disturbances.