Peter writes -
The devices shown here were created to explore aspects of regular, step-wise locomotion where the only energy input required is gravity. The first models were attempts to find a non-collision version of the “Wilson Walkie” type of ramp walker. These maintain constant surface contact and move with a rolling, non-scrubbing motion. Another goal was to see how this movement could be achieved with as few moving parts, or degrees of freedom as possible (see “single piece walker”). The hopping models demonstrate passive, persistent hopping with regulated descent down the ramp. A future goal is to incorporate this motion into a passive bipedal running robot.
Passive-Dynamic models – Link.