Longitudinal Motion Control Method For An Underactuated Spherical Robot

Spherical robot, which is also called spherical vehicle, is a new special kind of underactuated mobile robot. It has the characteristics of strong environmental adaptability and low energy consumption owing to its good sealing ability, underactuated characteristic and better flexibility, for which the spherical robot has a broad application prospect in the industrial field, military field and unknown space exploration etc. Therefore, it attracts many researchers’ attraction all over the world recently. The movement of the spherical robot in this study can be separated as longitudinal movement and turning movement; and particularly the longitudinal movement is a main mode of motion. The control performance of the longitudinal motion is more important because it directly determines the exploration effectiveness. This investigation mainly focuses on the control problem of spherical robot when it performs longitudinal movement.Firstly, the dynamical model of spherical robot which performs longitudinal movement is established through vectorial mechanics. After analyzing the influence of uncertain rolling resistance in unstructured environment, the balance movement condition of spherical robot is presented.Secondly, considering that the spherical system is a typical underactuated system and sliding mode control is of better robustness and parameter insensibility characteristics, a hierarchical sliding mode control scheme is presented. Meanwhile, to deal with the unavoidable rolling resistance in unstructured environment, an adaptive control scheme is addressed to estimate the uncertain rolling resistance. The stability of the proposed controller is testified by choosing a proper Lyapunov function. The proposed control approaches are verified to be effective by numerical simulation for a spherical robot.Thirdly, in view of the reality that the states of the spherical robot and the uncertainty of rolling resistance could not be able to be directly measured, a hierarchical sliding mode control based on extended state observer is presented. The system states and the estimated uncertain rolling resistance can be obtained through the observer, and then the controller can be derived by the observed states. The stability of the proposed extended state observer and the effectiveness of the proposed control scheme are verified by choosing a proper Lyapunov function. In order to achieve satisfactory control performances, the proposed control approaches are verified to be effective by numerical simulation.Finally, considering that the underactuated characteristic and strong coupling of spherical robot system make the controller design much more difficult and owing to the rolling resistance the inner suspension undergoes dynamic oscillation behavior which has a bad effect on the image quality, therefore, a magnetorheological (MR) damper is creatively introduced to the spherical robot system to suppress the irregular oscillation of inner suspension. The concrete implementation plan is presented and the dynamic model is established by vectorial mechanics. The control law and adaptive law which is design to estimate the uncertain item are obtained by Lyapunov method, and the proposed control methods are preliminary verified to be effective by numerical simulation for the spherical robot system.