William S. N. Trimmer

Microrobots and micromechanical systems

Abstract The domain of micromechanical systems is an extensive, useful and yet largely unexplored area. It is likely that, in many applications, small mechanisms will prove to be faster, more accurate, gentler and less expensive than the macro systems presently used. This paper explores the advantages of micromechanical systems and analyzes the scaling of forces in the micro domain.

Rarefaction and Compressibility Effects in Gas Microflows

Gas microflows are encountered in many applications of Micro-Electro-Mechanical Systems (MEMS). Computational modeling and simulation can provide an effective predictive capability for heat and momentum transfer in microscales as well as means of evaluating the performance of a new microdevice before hardware fabrication. In this article, we present models and a computational methodology for simulating gas microflows in the slip-flow regime for which the Knudsen number is less than 0.3. The formulation is based on the classical Maxwell/Smoluchowski boundary conditions that allow partial slip at the wall. We first modify a high-order slip boundary condition we developed in previous work so that it can be easily implemented to provide enhanced numerical stability. We also extend a previous formulation for incompressible flows to include compressibility effects which are primarily responsible for the nonlinear pressure distribution in microchannel flows. The focus of the paper is on the competing effects of compressibility and rarefaction in internal flows in long channels.

Design considerations for a practical electrostatic micro-motor

Abstract Magnetic motors and actuators dominate the large-scale motion domain. For smaller, micro-mechanical systems, electrostatic forces appear more attractive and promising than magnetic forces. Despite their distinguished history, electrostatic motors have found few practical applications because of the high voltages and mechanical accuracies traditionally required. This paper explores the design of electrostatic motors utilizing the advances in silicon technology. Using silicon wafers, and the associated insulators, conductors, anisotropic etching and fine-line photolithographic techniques, it is possible to develop large electrostatic fields with moderately high voltages (≈100 V) across insulators of well-controlled thickness. We present two preliminary designs and numerical simulations: one for a linear electrostatic motor and one for a rotary electrostatic motor.

Integrated fabrication of polysilicon mechanisms

The integrated fabrication of planar polysilicon mechanisms incorporating lower and higher kinematic pairs (or joints) is described. The two lower kinematic pairs (revolute and prismatic) commonly used in macrorobotic systems are compatible with silicon microfabrication technology. The mechanisms are fabricated by surface micromachining techniques using polysilicon as the structural material and oxide as the sacrificial material. Turbines with gear and blade rotors as small as 125 mu m in diameter and 4.5 mu m in thickness were fabricated on 20- mu m-diameter shafts. A clearance as tight as 1.2 mu m was achieved between the gear and the shaft. Gear trains with two or three sequentially-aligned gears were successfully meshed. A submillimeter pair of tongs with 400- mu m range-of-motion at the jaws was fabricated. This structure incorporates a single prismatic joint and two revolute joints, demonstrating linear-to-rotary motion conversion. >

Silicon micromechanics: sensors and actuators on a chip

The techniques used to fabricate micromechanical structures are described. Bulk micromachining is routinely used to fabricate microstructures with critical dimensions that are precisely determined by the crystal structure of the silicon wafer, by etch-stop layer thicknesses, or by the lithographic masking pattern. Silicon fusion bonding has been used to fabricate micro silicon pressure sensor chips. Surface micromachining, based on depositing and etching structural and sacrificial films, allows the designer to exploit the uniformity with which chemical vapor deposition (CVD) films coat irregular surfaces as well as the patterning fidelity of modern plasma etching processes. Silicon accelerometers, resonant microsensors, motors, and pumps made by these techniques are discussed. Measuring the mechanical properties of silicon, which are important to these applications, is examined.<<ETX>>

Harmonic electrostatic motor

An electrostatic motor comprising a stator and a rotor, one of which is equipped with a plurality of lands to which a voltage is sequentially applied. The other of the stator and rotor is made of conductive material. Rolling contact is established between the stator and rotor along at least one line of contact. The rolling contact is controlled to repetitive first and second paths along first and second surfaces of the stator and rotor, respectively, in which the lengths of the first and second paths are different. Embodiments include a cylindrical motor, a flexible disk motor and a conical motor.

Microactuators for aligning optical fibers

Abstract This paper describes two microactuators used to align fiber optics. One, an actuator using a thin strand of shape memory alloy, is used to align an input fiber with one of two output fibers. This component is useful for switching fiber-optic signals. The second is an electrostatic actuator capable of switching optical fibers, and also of making fine adjustments to correct for misalignments.

An operational harmonic electrostatic motor

An operational harmonic electrostatic motor is described. A cylindrical rotor is placed inside a hollow cylindrical hole of slightly larger diameter. Electrodes on the circumference of the hole electrostatically attract the rotor and cause it to roll inside the stator. The harmonic motion of the rotor produces a gear reduction between the electrical drive frequency and the shaft rotation rate. This motor design has the advantage of increasing the torque of the motor. This motor has several other advantages. First, it uses the clamping force, normally larger than the tangential force used by most electrostatic motor designs, to generate the motion. Second, the sliding friction between the rotor and stator, a source of hindrance for most micro electrostatic motors, helps be keeping the rotor and stator from slipping. Third, this motor uses rolling surfaces that dissipate less energy in fiction than sliding surfaces.<<ETX>>

Micro gears and turbines etched from silicon

Abstract Using silicon etching techniques, free-standing micro gears and turbines were fabricated from silicon. Silicon gears with geometries similar to those of standard watch gears were fabricated, but with outer diameters as small as 300 μm. A micro air-turbine was built out of discrete, silicon micro-mechanical components and operated at speeds of approximately 400 rps (24000 rpm). Limitations to further size reduction are not the photolithographic or etching processes, but the difficulty in handling such small components without loss or damage. Micro-mechanical structures, on the scales of tens of microns, can be fabricated inexpensively, accurately and repeatedly. Once these micro-structures are cut free from their silicon substrates, these silicon components can be used to build a wide array of micro-mechanical systems. Moreover, the use of silicon processing technology makes possible the eventual fabrication and integration of sensors, actuators, mechanical structures and control circuitry on the same substrate.

Actuators for micro robots

The problem of scaling robot actuators down to microscopic scale is examined in a review. It is shown that several forces scale well into the micro domain. These include electrostatics, hydraulics, pneumatics, and biological forces. Electromagnetic forces have a less favorable scaling. The scaling of electrostatics and electromagnetics is explained, and two electrostatic actuators are presented. The first actuator is a harmonic electrostatic motor. This motor uses rolling surfaces that reduce friction, and has an integral gear reduction that increases the torque output of the motor. The second actuator is designed to move fibers into alignment with other fibers.<<ETX>>