The typical way to make a bipedal robot walk is to actuate its leg joints, strap a bunch of sensors to measure its state and add a tight control loop to make sure it is performing the desired steps.
In a radically different approach, passive dynamic walkers can step down slopes without the need for sensing, control or energy. Their driving force comes from gravitation pushing them down the hill. If well designed, and started with adequate initial conditions, the walker will reach a rhythmic and stable walking gait that prevents it from falling on its nose.
Of course, always walking downhill is hardly a viable solution. To make robots walk on level ground, Dong et al. propose to trick the robot into thinking it’s walking on a slope. This is done by extending the back leg of the robot (stance leg) while shortening its front leg (swing leg) before it hits the ground as shown in the figure below (steps I through IV).
The authors propose an analytical model to predict the energy efficiency and speed of the walker based on easy to tune parameters. The result is an energy efficient walker that can move at high speeds. To validate their model, experiments were done on the real walker below. The robot was able to top at a full 1.12 m/s speed, or 4.48leg/s, which is the fastest walking gate demonstrated so far. The leg length was changed by bending and unbending the knee joints.