In recent years, the motors and actuators with three degrees-of-freedom have attracted special interests as novel direct drive type actuators for many modern devices applications, such as robotic joints, computer vision, transporting elements and omnidirectional wheels etc. The spherical actuator as one of them can provide advantageous features over traditional drive mechanisms which are usually constructed by several conventional drive motors or actuators, each having one degree of freedom and reducing the position accuracy, stiffness, dynamic performance and efficiency of the system [1-7]. Unlike the conventional cylindrical motor, the permanent magnet spherical actuator (PMSA) can implement 3 degrees-of-freedom motion and rotate with any axis in space. The electromagnetic field distribution and torque characteristics are important aspects for application and research, the rational torque analysis can provide the bases for structural optimization and control system modeling and application. Genetic algorithm (GA) is a stochastic and parallel search technique based on the mechanisms of natural selection, genetics and evolution, which was first developed by Holland in 1970s [8-10]. GA is known to be a powerful tool for performing search in complex spaces. In recent years, GA has been widely applied to different areas such as fuzzy systems, neural networks etc. The only drawback is that when dealing with multi-modal functions with peaks of unique value, ordinary GA is characterized by converging to the best peak of the space (or to a space zone containing several other best peaks) and to lose adequate individual sampling of other peaks in other space zones. This is called the genetic drift and is not a correct behavior for many kinds of problems in which other locations of functions' optimal values are more interested to know. The niche and species concepts have been introduced for overcoming this behavior [11]. Niche is viewed as an organism's task in the environment and species is a collection of individuals with similar features. In this way, its main purpose is to form stable subpopulations of organisms surrounding separate niches by forcing similar individuals to share the available resources. The niche genetic algorithm aims at gathering the individuals on several peaks of fitness function in the population according to genetic likeness and they permit GA to investigate those peaks in parallel. The fittest individual in the niche is kept unchanged or with high fitness value, while others in the niche are changed to reduce their fitness values sharply. So the individuals in the population may be dispersed into the whole search space. Thus some diversity can be maintained effectively during the generations in the population. The key problem for solving in the optimization is to avoid falling into local minimum, improve the convergence speed. Thus, new optimization techniques are used to solve the constrained problem. 3D electromagnetic systems are the core elements of many electric devices such as motors or actuators. The main indexes such as efficiency, force or torque capability and other performances can be optimized by some schemes. However, compared to the conventional motors or actuators, the three dimensional structures of PMSA make the structure optimization process more complicated and hard to implement. The aim of this study is to calculate and evaluate a novel PMSA by 3D finite element analysis of electromagnetic field and torque characteristics, also with detailed discussion on the effects of some key factors, then the improvement on the niche genetic algorithm for optimization applications is performed by using the combination of sharing method and the exclusion mechanism to derive better effects. The actual situations of PMSA optimal design are introduced with application of niche genetic algorithm to meet the final practical requirements.
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