bipedal walking
Bipedal walking is moving on two legs the way humans do — taking steps by lifting one foot, swinging it forward, and catching the body's weight on it before lifting the other. It sounds effortless because you have done it since you were a toddler, but for a robot it is one of the hardest tricks in all of robotics. With only two narrow feet for support, a two-legged machine is almost always on the edge of toppling over, and a single mistimed step sends it crashing down.
The trouble is balance. When both feet are on the ground the robot is fairly stable, but to actually move it must lift one foot, and in that instant it is standing on a single small foot, like balancing on one leg. Walking is really a controlled, repeated near-fall: the robot tips its weight forward, then quickly swings the free leg out to catch itself before it hits the floor, over and over. To do this it must sense its tilt many times a second and constantly nudge its joints and shift its weight to stay upright — a tireless juggling act that never stops while it is on its feet.
Engineers describe a stride by which feet are down. During the double-support phase both feet touch the ground and the robot is steadier; during the single-support phase only one foot is planted while the other swings through the air, and balance is most precarious. A smooth walk is just these two phases alternating in a steady rhythm — left single-support, double-support, right single-support, double-support — repeating with every step. The faster and more aggressively this rhythm runs, the more the robot relies on motion rather than a wide stance to stay up.
Honda's ASIMO walked, climbed stairs, and even jogged on two legs, but it had to compute its balance and footstep timing constantly; pause that control loop for an instant and it would simply fall over.
Two-legged walking is a controlled fall caught one step at a time.
Because a walking biped is constantly catching a fall, two-legged robots usually rely on dynamic stability — they stay up by moving, not by standing on a wide, stable base.