Toward a Bipedal Robot with Variable Gait Styles: Sagittal Forces Analysis in a Planar Simulation and a Prototype BallTray Mechanism Variable walking styles in bipedal robots may be used to communicate information about purpose andor personality to human viewers and may also help accommodate features of the environment. This paper presents variable gait in a simulation and results from a hardware prototype for gravitydriven rolling ball mechanism in a previously proposed bipedal design. First, optimal control inputs that produce a range of variable feasible gaits is generated on a simplified underactuated planar model. Then, a traylike mechanism that provides a curved path for a ball to roll on is presented. This mechanism is designed to replicate a notion of pelvic shift, described in Bartenieff Fundamentals, with movement of the ball (which is shaped by the tray) creating a shift of weight. Analysis of two gait styles and two tray designs shows comparable ranges of forces in the direction of travel between the simulated planar model and the hardware mechanism. This work is an important first step in generating feasible, stable and variablexe2x80x93 bipedal gaits using this hardware design.

Stability and Gait Switching of Underactuated Biped Walkers This paper introduces a new gait switching approach for underactuated biped walkers. The switching condition relies on a reduced region of attraction xe2x80x94 that of the unactuated dynamics xe2x80x94 which is shown to be sufficient to predict falls. A gait transition is accordingly stable (as in xe2x80x9cthe robot does not fall if the bipedxe2x80x99s state is within the reduced region of attraction of the switchedin gait when switching takes place. Two and fivelink biped models complete a sequence of random gait transitions using the switching logic. The condition is also used to enlarge the full region of stability of the fivelink model by embedding feedbackstabilized trajectories from a gait library in a mapping. The mapping stitches stable trajectories to the orbit of the desired gait based on each gaitxe2x80x99s reduced region of attraction. This improves the robustness of the fivelink model walking on uneven terrain without ground perception regardless of the size of the gait library.