An-Najah Staff

Using vapor deposition as a tool, novel coating structures have been developed for low friction coefficient, fretting wear resistant and high temperature stable electrical contacts. Thin films of Ag-Ni nanocrystalline composites between 100 and 500-nm thick were deposited by electron beam evaporation onto sputter-cleaned 301 stainless steel substrates. The structure and composition of the films were studied in detail using x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and Auger depth profiling. The contact properties, such as contact resistance, fric-tion coefficient, fretting wear resistance, and thermal stability of these coatings have been measured. Both the Ag and Ag 81 Ni 19 composite coatings about 500-nm thick passed the 1,000,000 cycle fretting wear test. These coatings also showed good high temperature stability during heat-aging at 150 º C in air, especially the Ag 81 Ni 19 composite coating. This study shows that vapor deposition is a powerful technique which can be used to discover new coating compositions and structures for electrical contact applications.

Dynamic simulation in robotic systems can be considered as a useful tool not only for the design of both mechanical and control systems, but also for planning the tasks of robotic systems. Usually, the dynamic model suffers from discontinuities in some parts of it, such as the use of Coulomb friction model and the contact problem. These discontinuities could lead to stiff differential equations in the simulation process. In this paper, we present an algorithm that solves the discontinuity problem of the Coulomb friction model without applying any normalization. It consists of the application of an external switch that divides the integration interval into subintervals, the calculation of the friction force in the stick phase, and further improvements that enhance its stability. This algorithm can be implemented directly in the available commercial integration routines with event-detecting capability. Results are shown by a simulation process of a simple 1-DoF oscillator and a 3-DoF parallel robot prototype considering Coulomb friction in its joints. Both simulations show that the stiffness problem has been solved. This algorithm is presented in the form of a flowchart that can be extended to solve other types of discontinuity.

In this paper, the dynamic parameters, both inertial and frictional, of a 3-DOF RPS parallel manipulator are identified considering two important issues: the physical feasibility of the identified inertial parameters and the use of nonlinear friction models in the identification process in order to model the friction phenomenon at robot joints. The dynamic model of the parallel manipulator is obtained starting from the Gibbs–Appell equations of motion along with the Gauss principle of Least Action, and these equations of motion are rewritten in a/their linear form with respect to the inertial parameters of the mechanical system. At this point, in accordance with the friction model considered, either linear or nonlinear, two types of dynamic models are dealt with: the totally and the partially linear with respect to the parameters to be identified. In order to solve the identification problem when nonlinear friction models are included, a nonlinear constrained optimization problem will be formulated and solved, instead of the Least Square Method, which is valid only for linear identification problems. It must be mentioned that the above-mentioned optimization problem will include the physical feasibility of the identified parameters in its formulation. The proposed procedure will be verified against a virtual parallel manipulator and finally, experimental identification processes are carried out over an actual parallel manipulator and a comparison is made between the LSM and the optimization process in the case of linear friction models, and between the linear and nonlinear friction models in the optimization process.

Since no universal friction model exists and the practical measurement of friction is not straightforward, this paper presents an experimental method of identifying friction in mechatronic systems. Friction is perhaps the most important nonlinearity that may found in any mechatronic system of moving partsand influences the system in all regimes of operation. For the the purpose of improving the performance of mechatronic systems and solving their servo problem, a better understanding of friction behaviour in its two basic regimes is needed. In this paper, the two basic friction regimes, viz., presliding with its hysteresis behaviour, which is predominantly position dependent, and gross sliding, which is predominantly velocity dependent, are well exposed and identified.