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 . 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.
Advanced prosthetic limbs must often react to the body's natural momentum without having a motor at every possible point of movement.