Autonomous Swing-Up Control of a Three-Link Robot Gymnast

In this paper the problem of swing-up control of an under-actuated, three-link robot gymnast (Robogymnast) is discussed. Robogymnast mimics the human acrobat who hangs from a high bar and tries to swing-up to an upside-down position with his/her hands still on the bar. Unlike human acrobats, Robogymnast's hands are firmly attached to a freely rotating high bar mounted on ball bearings. Although this helps during the swing-up phase, it poses a great challenge to balancing the robot at the upright position. The motion of Robogymnast was initiated by applying regularly changing sinusoidal torques to the two motors located respectively at its shoulder and hip joints. As the swing angle increases, the amplitudes of the applied sinusoidal torques were increased while their respective frequency was reduced proportionately. Experimental results showed successful swing-up of Robogymnast from the stable downwards position to the inverted configuration.