AbstractsEngineering

The design of innovative epicyclic mechanical transmissions : application to the drives of wheeled mobile robots

by Chao Chen




Institution: McGill University
Department: Department of Mechanical Engineering.
Degree: PhD
Year: 2006
Keywords: Engineering, Robotics.
Record ID: 1782841
Full text PDF: http://digitool.library.mcgill.ca/thesisfile100335.pdf


Abstract

Epicyclic mechanisms have found wide applications in industry, especially in automobiles and robotics. Low efficiency due to the high gearing power occurring in an epicyclic train is an important problem. This thesis develops a novel family of epicyclic transmissions, based on cams and rollers. This kind of cam-based mechanical transmissions, Speed-o-Cam (SoC), offers features such as high stiffness, low backlash, and high efficiency. We develop multi-lobbed cam profiles, the sun cam and the ring cam, which comprise an epicyclic cam train (ECT) with the roller follower. New design criteria are established: the generalized transmission index (GTI) and the contact ratio in cam transmissions. The GTI is an index that quantifies the force transmission quality in a mechanism, thereby generalizing the pressure angle, the transmission angle, and the transmission index (TI) proposed by Sutherland and Roth in 1973. The contact ratio is an index of the quantity of overlap occurring between two conjugate cams during transmission. A contact ratio greater than unity guarantees smooth motion during operation. In order to avoid "poor" transmission, we apply an undercutting technique on the cam profile to achieve a smooth motion. We introduce two new concepts, virtual power and virtual power ratio, and derive an original algorithm to compute the efficiency in an epicyclic train upon the assumption that power loss is due only to friction upon meshing. The results show that friction has a larger effect on the total efficiency of an epicyclic train than on a simple train. Examples are given to validate this algorithm, by comparison of our results with previous works. The dual-wheel transmission (DWT), proposed elsewhere using epicyclic gear trains (EGTs), is designed here with epicyclic trains of cams and rollers. We optimize the DWT to achieve a compact design and a high transmission performance. Furthermore, we define the total transmission index (TTI), which allow us to evaluate the final DWT design. Two virtual prototypes of the DWT, the central and the offset versions, are generated: the former is capable of quasi-omnidirectional mobility, the latter of full omnidirectional mobility. Finally, we include a general kinematic analysis of wheeled mobile robots (WMRs) with single-wheel drives and apply this method to WMRs with DWT units; then, we obtain symbolic solutions to the direct kinematics (DK) and inverse kinematics (IK) problems, for both central and offset types of units.