Control Engineering Practice 85 (2019) 50–58 Contents lists available at ScienceDirect Control Engineering Practice journal homepage: www.elsevier.com/locate/conengprac Omnidirectional mobile robot robust tracking: Sliding-mode output-based control approaches L. Ovalle a , H. Ríos b,∗ , M. Llama a , V. Santibáñez a , A. Dzul a a Tecnológico Nacional de México/I.T. La Laguna, División de Estudios de Posgrado e Investigación, Blvd. Revolución y Cuahutémoc S/N C.P. 27000, Torreón, Coahulia, Mexico b CONACYT-Tecnológico Nacional de México/I.T. La Laguna, División de Estudios de Posgrado e Investigación, Blvd. Revolución y Cuahutémoc S/N C.P. 27000, Torreón, Coahulia, Mexico ARTICLE INFO Keywords: Robust control Output-feedback control Omnidirectional mobile robot Position tracking Continuous sliding-mode control ABSTRACT This work deals with the robust position tracking control problem for an omnidirectional mobile robot. To this aim, four continuous Sliding-Mode Control strategies are presented. The position and orientation of the platform are assumed to be the only available information about the system. To implement the controllers as output-feedback controllers, a High-Order Sliding-Mode Observer is implemented for each output signal. The proposed robust control strategies are able to deal with some classes of external disturbances. The closed-loop stability of each controller is proved by means of Lyapunov functions and homogeneity concepts. Simulations and experiments validate the applicability of the proposed controllers. 1. Introduction The velocity tracking control problem for mobile robots is usually done by means of kinematic controllers, assuming that the driver of the motors can achieve perfect velocity tracking (Dong & Xu, 2001). However, as mobile robots perform tasks where a heavy workload is employed, or a high velocity is needed, this assumption might not be fulfilled (Li & Ye, 2014) and therefore the dynamics of the mechanism should be considered. For mobile robots under highly changing loads, or vehicles where more than one surface is traveled, and therefore the friction of the ground might change, it is important to consider the presence of disturbances, something that is not possible with kinematic controllers. When a feedback control loop for robotic mechanisms is designed, it is often assumed that the drivers provide a perfect response to a general- ized force, meaning that the actuator dynamics is often neglected (Kelly, Santibáñez, & Loría, 2006). In this sense, a better accuracy might be achieved if the effect of this dynamics is considered. Both position and velocity tracking control of omnidirectional mo- bile robots have already been reported in the literature. For instance in Li, Chen, Hung, and Yeh (2008), a kinematic controller based on fuzzy logic is presented to solve the velocity tracking problem for a three wheeled robot. In Lin and Shih (2013), an adaptive controller is shown; in this paper the generalized forces are assumed to be the control inputs to solve the velocity tracking problem in a four wheeled mobile robot. In Treesatayapun (2011) a fuzzy neural network-based controller is proposed for a class of discrete-time nonlinear systems; ∗ Corresponding author. E-mail address: hriosb@correo.itlalaguna.edu.mx (H. Ríos). the controller ensures an ultimate bound of the position tracking error without knowledge of the mathematical model and experiments are carried out in a three wheeled platform. In Bigelow and Kalhor (2017) an adaptive extended Sliding-Mode controller (SMC) is proposed. The scheme makes use of an evolving linear model; asymptotic stability of the position tracking errors are achieved for four wheeled robot. In Wang, Liu, Yang, Hu, Jiang, and Yang (2018), a model predictive scheme is presented for a three wheeled robot to track a position trajectory. In Li and Zell (2009), a kinematic controller, considering the actuators dynamics and actuator saturation is presented to deal with the position tracking problem for a three wheeled robot. In Peñaloza-Mejía, Márquez-Martínez, Alvarez, Villarreal-Cervantes, and García-Hernández (2015) a position tracking controller that ensures boundedness of the velocities is presented for a three wheeled robot. In Alakshendra and Chiddarwar (2017a), an adaptive sliding-mode controller is proposed for a four wheeled platform; the sliding-mode controller is based on a first-order algorithm and the position tracking error converges to zero asymptotically. In Alakshendra and Chiddarwar (2017b), the position tracking problem for a four wheeled robot is tackled; in this work, a cylinder is mounted atop the robot and the objective is to follow a path while maintaining the cylinder on the surface of the robot, this objective is achieved by a sliding-mode controller and a special type of switching sliding surface. In Ren, Sun, and Ma (2016) a passivity based approach is utilized for the position tracking control of a three wheeled omnidirec- tional mobile robot considering the generalized forces as control inputs. In Huang and Tsai (2008) a robust controller, based on an integral https://doi.org/10.1016/j.conengprac.2019.01.002 Received 27 July 2018; Received in revised form 16 November 2018; Accepted 1 January 2019 Available online xxxx 0967-0661/© 2019 Elsevier Ltd. All rights reserved.