Control of two-link robot attached to a mass-spring using disturbance observer

The dynamics of a robot arm is highly non-linear. The acceleration, velocity and angle of a single joint affect other joints. Moreover, other external forces including Coriolis force, centrifugal force, gravity, friction, etc are present, and they can influence the states of a robot. The states of a robot have an effect on themselves as well. So, in order to control the arms, the torque at each joint must be calculated every moment. But, as the degree of freedom is increasing, the calculating time is growing in proportion to its square number. Moreover the unknown external force can be applied to robot. In this paper, the effects from other joints and external forces are considered as disturbance. And the disturbance observers are used at each joint to detect and cancel them. Then the system can be controlled as decoupled simple motors. The effects of modeling error or system uncertainties are also minimized by the disturbance observer. The two link arm robot is used to apply the disturbance observer, and a mass spring is attached to the end effector of the robot to give unknown external force.