CMU Bow Leg Hopper

Overview

A pogo-stick robot whose whole leg is a single curved strip of archery-bow fiberglass. During flight a hobby servo pulls the bow string to bend the leg and store energy. At touchdown the string goes slack and the spring releases that energy, so no motor carries load while the robot is on the ground. Since the energy is stored slowly during the long flight phase, small low-power servos are enough. The hip pivots freely and sits just above the body's center of mass, so the body hangs like a pendulum and stays upright with no active attitude control. This removes the stance-phase hip torque that Raibert's hoppers relied on, and the leg returns about 70–80% of its energy each bounce. The planar prototype circled a 1.5 m radius boom, with an elastic cord to the ceiling cutting effective gravity to about 0.35 G. It crossed patio-brick stepping stones and shallow stairs using a real-time graph-search planner. A later 3D version without a boom, refined with help from Jean-Christophe Zufferey of EPFL, used a two-axis gimbal hip and a flywheel-driven thrust mechanism. It managed short hops under remote control (CLAWAR 2002). The authors noted that RHex later used similar curved spring legs, though without a bow string for energy storage. Spring-powered jumpers such as SALTO continue the same theme of storing energy in an elastic leg.