Spherical electric motor design
To date, electric motors have been rotary and linear with one degree of freedom (DOF) motion. Advancements in technology have brought new, complex machines requiring multiple degrees of freedom motion; however, this is currently being provided by connecting single-DOF motors together, such as in a robotic ankle joint, where three servo motors are combined and 3- DOF motion are obtained. However, the end-product is heavy, complex, and inefficient in many ways. Several researchers have been studying spherical motors in order to obtain a 3-DOF motion motor. The first attempt was in 1959 by Williams et al. . Then other researchers began investigating 3-DOF motion using a single actuator. Unfortunately, a spherical electric motor (SEM) has not been commercialized yet because of some difficulties in realizing 3-DOF with a single motor. Position resolution accuracy is very difficult to achieve in an SEM, and the bearing system is another challenge. Transfer and magnetic bearings have been introduced as the solution; however, under high torque, transfer bearings create high friction, and magnetic bearings cannot handle the load. The actuation method remains the biggest challenge in developing an SEM. In this research, the goal was to develop an SEM that could be applied to industry relatively soon. First, a new actuation method was introduced by concentrating on position accuracy. Similar methods have been used in liquid crystal display (LCD) and magnetic suspension systems, whereby an array of coils on the stator surface are controlled to create a moving stator pole coupled with a rotor pole that moves together, thus realizing 3-DOF. Second, an electromagnetic torque model was derived for an SEM. Finally, a dynamic model was developed, and a dynamic decoupling control system was designed. Results show that position accuracy was achieved reasonably because the torque model is promising for calculating the required currents based on torque values calculated by the computed torque model.
Thesis (Ph.D.)--Wichita State University, College of Engineering, Dept. of Electrical Engineering and Computer Science