Design and dynamic simulation of mobile manipulators incorporating tribological analysis of 16MnCr5 and EN19 steels.

Abstract

Search and Rescue (SAR) robotics is a growing research area in spite of which reports show that heavy equipment like excavators are still used in SAR operations despite their unsuitability. Literature reveals the need for new designs for application-specific manipulators in time-critical SAR operations. To this end, the Computer Aided Design (CAD)-Multibody dynamics (MBD) simulation technique was applied instead of traditional numerical modelling and roofs-of-concept for two types for manipulators, i.e. anthropoid and complex closed-loop manipulators are presented. The combined friction model (CFM) was incorporated into this simulation considering the importance of friction as a tribological component affecting the dynamic performance of such mobile manipulators. To surmount the limitations of friction models, scaled-down tribological experiments were conducted to determine the coefficient of friction (COF) for two contact geometries - sliding cylinder and sliding pin, selected as approximations of manipulator joint contact. Oil lubricant was used to generate reference COF characteristics against which COF of grease-lubricated contacts were compared, and oil-lubrication showed better COF than grease-lubrication. However, oil is a fluid and it cannot be used in the application environment. Subsequently, the effect of nickel alumina nanocomposite coating on COF deposited on 16MnCr5 specimens using pulse electrodeposition (PED) was analysed. The results showed that this coating only reduced the COF by 3.1% compared to uncoated specimens in sliding cylindrical contacts, while in the case of sliding pin contacts the coating proved to be detrimental with a 22.1% increase in COF. The values of COF were used in a linear model for computing joint torque. Results of surface characterisation carried out using white light interferometry, digital microscopy and scanning electron microscopy substantiate the COF measurements. Microhardness of the surface was also analysed and showed that coatings improved the surface hardness by 19.7% to 55.9%. Therefore, this work contributes to the SAR robotics through design and simulation, and tribology.