Linear actuators for locomotion of microrobots
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The successful development of the miniaturisation techniques for electronic components and devices has paved the way for the miniaturisation in other technological fields. In the past two decades, the research achievements in micromechatronics have spurred fast development of micro machines and micro robotic systems. Miniature or micro actuators are the critical components to make these machines more dexterous, compact and cost effective. The main purpose of this dissertation is to develop micro actuators suitable for the locomotion of an in-pipe or endoscopic microrobot. The content of the thesis covers the selection of the actuation principle, robotic system design, actuator design and prototype construction, performance analysis, and design, analysis, and implementation of the appropriate drive control system. Among different types of actuation principles, piezoelectric and electromagnetic actuators are the two major candidates for the micro robotic systems. In order to find a suitable actuation principle for the desired robotic application, a comparative study was conducted on the scaling effects, attainable energy density, and dynamic performances of both types of actuators. Through the study, it was concluded that the electromagnetic actuator is more suitable for the endoscopic microrobot. Linear actuators are the common design used for the locomotion of microrobots due to many advantages compared to their rotational counterparts. Through a thorough review and comparison of the electromagnetic linear actuator topologies, a moving-coil tubular linear actuator was chosen as the first design due to its simplest structure. Via the magnetic circuit analysis and numerical magnetic field solutions, the actuator was designed for optimum force capability, and the electromagnetic force and the machine parameters of the actuator were predicted. According to the results obtained from the magnetic field analysis, the dynamic model of the actuation system with a driving control scheme was established and used in the actuation performance analysis of the robotic system. Based on the experience achieved through the first design, a new moving-magnet tubular linear actuator was designed. The methodology developed in the design and analysis of the moving-coil linear actuator was adopted for the moving-magnet actuator design. However, the optimal design is more complicated due to the multi-pole and multi-phase structure of the moving-magnet actuator. The electromagnetic force of the actuator was analysed under the condition of different excitation methods. An enhanced parameter computation method is proposed for predicting the actuator parameters. Based on the results of magnetic field analysis, a comprehensive dynamic model of the actuator was developed. Through the coupled field-circuit analysis, this model can predict accurately the dynamic performance of the actuator. The characteristics analysis shows that the performance of the moving-magnet actuator is much better than that of the moving-coil actuator. Two prototypes of the moving-magnet tubular linear actuator with different dimensions were constructed to verify the performance and the scaling theory. Various precision machining techniques were employed during the fabrication. The performances and parameters of the two different prototypes were measured and the results agree substantially with the theory. The brushless DC drive method was chosen for the driving control of the proposed linear actuator because of the compact circuit topology and simple implementation, which are two essential factors for micro applications. A sensorless control scheme based on the back EMF was developed as physical position sensors are not permitted in such a micro system. The control scheme was then applied to the locomotion control of the proposed microrobot. The system simulation shows that the control performances of both the actuator and microrobot are satisfactory. A dSPACE prototyping system based driving control hardware was designed and implemented to experimentally verify the control design. The experimental results agree substantially with the theoretical work.
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