Torque Distribution and Slip Minimization in an Omnidirectional Mobile Base Yuan Ping Li, Denny Oetomo, Marcelo H. Ang Jr.* National University of Singapore 10 Kent Ridge Crescent, Singapore 119260 *mpeangh@nus.edu.sg Chee Wang Lim Singapore Institute of Manufacturing Technology 71 Nanyang Drv, Singapore 638075 Abstract—Two forward kinematic models which are used in the control of an omnidirectional mobile base are evaluated. These two models result in different sensitivities to joint position error. The analysis and experimental results in this paper demonstrate the capabilities of dynamic model in improving the sensitivity of the forward kinematic model, resulting in a more even distribution of joint torques and in minimizing the amount of slip between wheels. I. I NTRODUCTION A mobile robot with four Powered Caster Wheels (PCW) (Fig. 1) is developed in Singapore Institute of Manufacturing Technology (SIMTech). Mobile robots with Powered Caster Wheels (or “off-centered orientable wheels”) have omnidirec- tional motion capability [1] [2] i.e. have three controllable degrees-of-freedom in the planar case. Omnidirectional motion capability simplifies the trajectory planning of the mobile base: all degrees-of-freedom in the planar workspace are controllable simultaneously. Having full maneuverability, the applicability of the mobile base as a service robot in human and unstructured environment is improved. The kinematic and dynamic modelling of PCW-based mobile robots are well studied in the literature [2]–[6]. Optimal design and analysis of this kind of mobile base are studied in [7] [8]. Kinematic and dynamic control strategies were formulated and implemented on the omnidirectional base. Various meth- ods were investigated in order to obtain accurate models of the kinematic and dynamic characteristics, therefore achieving better motion control performance. This paper presents the formulation of kinematic and dynamic models of the mobile base and analyzes two forward kinematic models which result in different sensitivities of the base position errors to joint position errors. Using experimental results carried out on the actual robot, the effectiveness of dynamic compensa- tion is demonstrated by achieving a more even joint torque distribution and minimized amount of slip between wheels. These results improve wheel-floor traction, hence decrease the chances of wheel-floor slippage. This in turn produces a more accurate odometry of the base. The strategies described were implemented on the omnidirectional mobile robot and real time experimental results are presented in this paper. Fig. 1. An omnidirectional mobile base with 4 Powered Caster Wheels. B C 1 C N rρ i φ i β 1 β i b r x B y B x Ci y Ci φ 1 φ N C i t C i ω p Ci h i b i v Fig. 2. Frame assignment and parameter definition of the mobile base. II. KINEMATICS The frame assignment and parameter definition of a mobile base with N PCWs are shown in Fig. 2. Frame B is defined as the frame attached to the center of the mobile base and moves with the base. The location of the wheel with respect to Frame B is defined by vector h i , of length h i , forming an angle β i with the x axis of Frame B. The contact point between the wheel i and the ground is defined as point C i and its position with respect to Frame B is defined as p Ci . A Frame C i is defined with its origin at 567 0-7803-9177-2/05/$20.00/©2005 IEEE