Because the first joint has no motor, the robot is . It cannot simply "lift" itself; it must use precisely timed "kicks" at the elbow to build up energy, eventually swinging into an inverted vertical position—a feat known as the "swing-up" task. The Challenge of Control
Underactuated systems are often more energy-efficient because they utilize natural physics (like gravity and inertia) rather than fighting against them with heavy motors. Acrobots
Unlike a standard robotic arm where every joint has its own motor, the Acrobot has only one powered joint. It consists of two links and two joints: Because the first joint has no motor, the robot is
The Acrobot: Balancing Science and Skill The "Acrobot"—a portmanteau of "acrobatic" and "robot"—is a fascinating classic in the world of control theory and robotics. It is a two-link, underactuated planar robot designed to mimic the movement of a gymnast swinging on a high bar. While it may look simple, the Acrobot represents one of the most significant challenges for engineers and roboticists: mastering complex movement with limited control. What Makes an Acrobot Unique? Unlike a standard robotic arm where every joint
Modern robots like Boston Dynamics' Atlas use similar principles of momentum and balance to perform flips and navigate rough terrain.
This joint is unpowered (passive). It hangs freely from a fixed pivot point, much like a gymnast's hands on a bar.
Advanced prosthetic limbs must often react to the body's natural momentum without having a motor at every possible point of movement.