This paper proposes a study on the linearization of a four wheeled mobile robot with wheel arms, its behavior with a variable structure control for both linearized system and nonlinear system when it transform to two wheeled inverted pendulum state from four wheeled state. It also proposes a control technique to stabilize the system in inverted pendulum mode after the system transforms. Since the system have to reach the target stable state from initial state a nonlinear sliding mode controller is used for transformation mean while taking the velocity of the system into consideration and sub-optimal methods are used to minimize switching-function chattering. To stabilize the system in the inverted pendulum mode, an LQR controller has been implemented. Also the position tracking and behavior of the system using a ramp signal is studied. The transformation and movement of the robot could be achieved by two DC motors on each side of the robot.
J. .S.Lekshmi and Nandagopal J. L., “Control for transformation of a four wheeled to two wheeled mobile robot”, Materials Today: Proceedings , vol. 5, no. 1, pp. 1425-1431, 2018.