Ambler is a legged robot built to traverse extreme terrain with high reliability. It is a unique mechanical device with six legs, each consisting of two segments that move horizontally, and one segment that moves vertically. One horizontal segment rotates and the other extends to permit planar motion of the leg. The vertical segment extends to place the foot on the ground. The rotational segments are stacked vertically, attaching three legs to one shaft and the other three to another shaft. The legs can rotate continuously, enabling Ambler to place any leg at any location within its workspace.
Ambler has an orthogonal leg design, which decouples horizontal and vertical motions for energy and planning efficiency. Each vertical segment adjusts to the terrain roughness so that Ambler remains constantly level, providing a stable platform for sensing and sampling operations. Efficiency, stability, and reliability are important for mechanisms destined for highÐrisk, lowÐresource exploration on Earth, the Moon, and on other planets.
Ambler walks by lifting a leg vertically, swinging it horizontally to its desired position, and then extending it down to the ground. The body glides forward at level elevation, by coordinating the motion of the supporting legs.
Ambler can perform any of the gaits used by naturallyÐoccurring hexapods such as insects. These include the wave, ratchet, and tripod gaits. In the wave gait, the front pair of legs is moved, then the middle pair, and then the rear pair. In the ratchet gait, an alternating wave is created by pairingÐup each front leg with the opposite rear leg. In the tripod gait, three legs move at a time: the middle leg on one side and the front and rear legs on the other side.
Ambler's primary gait, however, is a circulating wave. The circulating wave gait, which has no natural counterpart, is performed by lifting a rear trailing leg, swinging it between Ambler's stacks (the two vertical columns to which the legs are connected), and placing it out in front of a leading leg.
The circulating wave is Ambler's primary gait because it is the most productive gait available to the Ambler configuration. Its gait planner generates a uniform circulating wave on smooth terrain. For autonomous walking in rough terrain, however, it is not sufficient to use a fixed gait; the robot must operate to the limits of its ability to maximize its performance and to achieve reliability and efficiency. When necessary, Ambler can reÐsequence leg motions into a free gait. Through proper selection of gait, Ambler can follow an arc of any radius, from zero for a point turn, to infinity for a straight line
Ambler's gait generation system can be conceptualized as a deliberative planner. This planner takes a trajectory specification and then reasons about the constraints of configuration, support, and stability that are imposed on the range of possible motions. The planner also utilizes sensor information and on-line maps of its environment to satisfy terrain constraints. The gait planner produces a complete plan of actions in the form of a detailed sequence of motion commands. This sequence, comprising a free gait, is sent to the robot, which then moves. Position and status are then noted to indicate whether or not the motion was performed.
In implementation, a number of deliberative planners work together to produce a gait plan. Specialized planners advise in such areas as leg trajectory, footfall selection, and body motion. They interact within the Task Control Architecture [Simmons 91] and produce a single gait plan.
Ambler is designed for exploration of extremely rough terrain. It possesses a unique circulating gait that improves energy efficiency and enables tight turning maneuvers. By accounting for configuration, collision, terrain, support and stability constraints, a free circulating gait can be generated for Ambler that propels the robot along the desired path. This gait generation has succeeded in demonstrating long-duration autonomous walking in rough terrain.
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Maintained by: Dave Lavery
Last updated: May 10, 1996