Title : DESIGN AND FABRICATION OF ELECTROTHERMAL MICROMOTORS AND COMPLIANT MECHANISMS FOR SPATIAL PARALLEL MICROMANIPULATORS
Department of Mechanical Engineering
This dissertation addresses the development of microscale spatial mechanisms capable of taking advantage of these benefits offered by established microfabrication methods. A parallel manipulator is defined by its kinematic structure, which takes the form of a closed-loop kinematic chain, unlike serial manipulators which consist of open-loop chains. In the macro world, many kinds of parallel manipulators have been developed, for example those depicted. Such parallel mechanisms offer several key advantages, such as high stiffness, low inertia, and high payload capacity. Although serial manipulators can offer larger workspace limits, such devices are not easily ported to the microscale world. The primary disadvantage of serial manipulators is that actuators must be mounted on the mechanism’s moving joints. This is a major constraint for MEMS based devices due to practical fabrication limitations. By contrast, actuators in a parallel manipulator can always be attached to the fixed base, and are thus well suited to MEMS manufacturing processes. For a planar mechanism, the loci of all points in all links can be conveniently drawn on a plane, but actually they lie on parallel planes. A mechanism is said to be a spherical mechanism if the motions of all particles in the bodies lie on concentric spheres. A manipulator is called a spatial manipulator if at least one of the moving links in the mechanism possesses a general spatial motion that cannot be characterized as planar or spherical motion [1]. Since the devices of interest in this work consist of micro-scale parallel mechanisms providing spatial motion, with integrated actuators for driving the mechanism located within the base link, they may be termed spatial parallel micromanipulators.
In this dissertation a new class of spatial micromechanisms employing compliant joints and electrothermal motors has been developed. The spatial micromechanisms contain three limbs driven by individual electrothermal linear motors to form multiple degree-of-freedom (DOF) manipulators. At the coaxial point of the actuated limbs, a platform acts as the end effector of the device. Each limb in this spatial mechanism interconnects compliant pseudo-revolute joints, which are capable of providing either in-plane or out-of-plane rotations. Mechanisms are demonstrated using polysilicon surface micromachining, and a new four-layer UV-LIGA fabrication process is also presented for future production of high aspect ratio spatial micromechanisms. Linear motors are developed to provide bi-directional continuous motions to drive the spatial mechanism. Individual electrothermal actuators within a linear motor employ saw-toothed impactors to provide a synchronized locking/pushing motion without needing a secondary clamping actuator. These saw-toothed linear motors provide a platform for accurate open-loop position control, continuously smooth motion, high motion resolution, and long life operation. Electrothermal V-beam actuators using multiple arrayed beams have been shown to provide large output forces up to several mN, sufficient for the spatial micromechanisms developed in this work. Taking advantage of a modeling approach based on the pseudo-rigid-body model, a new force and displacement model for the electrothermal V-beam actuators is developed and shown to provide good agreement with experimental results. The optimization design for the thermal actuators is also discussed to reduce actuation power. Pseudo-rigid-body modeling is used to simplify the designed compliant spatial mechanisms, allowing the well-known rigid body method to replace the cumbersome matrix method for compliant mechanism analysis. Based on the pseudo-rigid-body model, inverse kinematics is used to find the workspace of a typical microscale mechanism, together with the required movement for each linear motor to allow the end effector to reach a desired position. Dynamic analysis of the mechanism is applied to determine the maximum required forces for each actuator. The manipulator workspace volume defined by maximum link lengths and joint rotation angles is determined by using the Monte Carlo method. A systematic design procedure is finally proposed to enable effective compliant micromanipulator designs.
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