Research article How to ensure stable motion of suspended wheeled mobile robots Khalil Alipour Faculty of Industrial and Mechanical Engineering, Islamic Azad University, Qazvin Branch, Qazvin, Iran, and S. Ali A. Moosavian Department of Mechanical Engineering, K.N. Toosi University of Technology, Tehran, Iran Abstract Purpose – A suspended wheeled mobile robot (SWMR) that consists of one or more manipulators can be exploited in various environmental conditions such as uneven surfaces. The purpose of this paper is to discuss the requirements for stable motion planning of such robotic systems to perform heavy object manipulation tasks. Design/methodology/approach – First, a systematic procedure for dynamics modelling of such complicated systems for planar motion is presented and verified using ADAMS simulation software. Next, based on the new dynamic moment-height stability (MHS) measure, the stability of such systems will be investigated using the obtained dynamics. To this end, introducing the concept of a virtual frame, the obtained model of SWMR has been employed for investigating the effect of the base suspension characteristics as well as terrain roughness on the stability of the system. Next, the stability evaluation of the system is investigated after toppling down which has been rarely addressed in the literature. In addition, using the aforementioned model, the effect of stiffness is examined after instability. Findings – First, a systematic procedure for dynamics modelling of such complicated systems for planar motion is presented and verified using ADAMS simulation software. Next, based on the new dynamic MHS measure, the stability of such systems will be investigated using the obtained dynamics. To this end, introducing the concept of a virtual frame, the obtained model of SWMR has been employed for investigating the effect of the base suspension characteristics as well as terrain roughness on the stability of the system. Next, the stability evaluation of the system is investigated after toppling down which has been rarely addressed in the literature. In addition, using the aforementioned model, the effect of stiffness is examined after instability. Originality/value – A general procedure for dynamics modelling of SWMRs is presented. To verify the obtained dynamics model, another model for the considered system has been developed by ADAMS software. Next, using the obtained dynamics, the postural stability of such systems is investigated, based on the new postural MHS measure extended for SWMRs. The obtained simulation results show that by decreasing the stiffness coefficients of suspension subsystem the stability of the system weakens. Keywords Robots, Dynamics, Modelling, Motion, Stability (control theory) Paper type Research paper Nomenclature a ¼ A scalar quantity that denotes the MHS measure for the whole mobile robotic system. a i ¼ Dynamic stability measure associated with the ith edge of support boundary polygon. u 11 ¼ The angle between first link of the first manipulator arm and the positive direction of x 0 . u 21 ¼ The angle between first and second links of the first manipulator arm measured counterclockwise. u 12 ¼ The angle between first link of the second manipulator arm and the positive direction of x 0 . u 22 ¼ The angle between first and second links of the second manipulator arm measured counterclockwise. w ¼ Pitch angle of the robot platform from its static configuration. w 0 ¼ Pitch angle of the robot platform due to the initial static configuration of the robot. c ¼ Base rotation about front axle or front contact point during transient phase of toppling down. a x0 C0 ¼ Acceleration of center of mass of platform in direction of x 0 . a y0 C0 ¼ Acceleration of center of mass of platform in direction of y 0 . a XI C0 ¼ Acceleration of center of mass of platform in direction of X I . a YI C0 ¼ Acceleration of center of mass of platform in direction of Y I . ^ a i ¼ The unit vector of ith edge of support boundary polygon. b ¼ Track width. C f ¼ Damping coefficient of front suspension element. C r ¼ Damping coefficient of rear suspension element. F fs ¼ Force exerted on the platform by front suspension. F rs ¼ Force exerted on the platform by rear suspension. f fx ¼ The x component of the force exerted on the platform by front arm. The current issue and full text archive of this journal is available at www.emeraldinsight.com/0143-991X.htm Industrial Robot: An International Journal 38/2 (2011) 139–152 q Emerald Group Publishing Limited [ISSN 0143-991X] [DOI 10.1108/01439911111106354] 139