Gait Generation For Legged Robots

Abstract

Ahstruct-Gait generation is the formulation and selection of a sequence of coordinated leg and body motions that propel a legged robot along a desired path. Approaches to gait generation can be classified into control, behavioral, rule-based, and constraintbased paradigms. We survey these models of gait generation and introduce the Ambler, a hexapod robot that can circulate its legs to produce unique gaits. Then we present kinematic, collision, terrain, support, and stability constraints available for gait generation, and discuss our progress using a constraint-based method to generate the Ambler's gait. I. INTRODUCTION In comparison to wheeled mechanisms, legged rnechnisms require complex design, move slowly, and are difficult to control. However, for locomotion over rough or discontinuous terrain, legged mech'anisms are potentially superior to wheeled mech,uiisms.[ 11 Legged mechanisms make discrete terrain contacts and avoid undesirable footholds while wheeled mechanisms have rollers in continuous contact with the ground. The posture of a wheeled mechanism is dependent upon the terrain, but a legged mechanism can isolate its body from tlie ten ani- ' achieving a more stable stunce' and allowing smooth level motion. Legged locomotion is theoretically more energy efficient because body propulsion does not expend as much energy in soil compaction and body motion can occur in a level plane decoupled from the effects of gravity. Legged mechanisms can actively position their center of gravity to maximize stability. Dead reckoning (estimating position by integrating motion over time) can be more accurate in a legged machine where feet make discrete contacts and do not slip or skid like a wheel. The complexity in the design of legged mechanisms can be achieved, a$ is demonstrated by the working systems examined in this paper. We believe that the slow, smooth motion of legged devices is advantageous in rough terrain where caution (and, sometimes, sensing) will limit the speed of wheeled and legged mechanisms alike. The significant drawback to legs has been the difficulty in control. For a slow-moving walker, this challenge is not in moving the individual legs, a$ this is directly analogous to well-unders tood manipulator control, but in the coordination of leg and body motions. The legged robot, either with a real-time controller, low-level planner, or as an artifact of its architecture, must generate a sequence of leg and body motions, a gait, that will propel it along some path. Gait yeneralion is the formulation and selection of a sequence of coordinated leg and body motions that propcl the robot along a desired path. In this paper we survey several approaches taken toward gait generation aid present some of and discuss our work on gait generation for the Ambler.