Il-Han Hwang

Modeling and experimental characterization of the chevron-type bi-stable microactuator

A compliant bi-stable micromechanism allows two stable states within its operation range to remain at one of the local minimum states of potential energy. Bi-stable energy characteristics offer two distinct and repeatable stable states that require no power input to maintain. In this paper we suggest a new theoretical model of the chevron-type bi-stable microactuator using equivalent stiffness in the rectilinear and rotational directions. From this model, the range of the spring stiffness in which the bi-stable mechanism can be operated is analyzed and compared with the results of finite element analysis (FEA) for buckling analysis. The analysis of the equivalent stiffness model shows that the forces necessary for the forward and backward actuation are almost linearly proportional to the equivalent stiffness, also in agreement with that of FEA. Based on the analysis, a novel chevron-type bi-stable microelectromechanical systems (MEMS) actuator with hinges and coupling bars is proposed for the improvement of a stable latch-up operation. The thickness and orientation of the hinge is determined through FEA in the light of reliable operation, stroke requirement, mechanical stress, and process constraint. The change in the cross-sectional area during the fabrication process is also considered to take into account its effects on the reduction of equivalent stiffness of the bi-stable MEMS actuator. The fabricated chevron-type microactuator showed a reliable bi-stable operation with a 60 µm stroke at 36 V input voltage, in agreement with the results of the equivalent stiffness model. Therefore, these results confirm that the chevron-type bi-stable MEMS actuator using hinges with coupling bars is applicable to optical switches.

Application of Reliability-Based Topology Optimization for Microelectromechanical Systems

In reliability-based design optimization, the constraints consider the probability of the satisfaction/failure of critical events. Lately, reliability-based design optimization has been applied to topology optimization, resulting in the development of reliability-based topology optimization. And though reliability-based topology optimization can be a useful and meaningful method, it requires excessive computational resources. Therefore, this research proposes a parallel-computed reliability-based topology optimization using the response surface method. This paper demonstrates that the proposed method greatly reduces the computation requirement of reliability-based topology optimization. The proposed methodology is then applied to design microelectromechanical systems. Specifically, in microelectromechanical systems, reliability-based topology optimization can be highly effective because of randomness generated during the etching process and scaling effect. The proposed method successfully designs new devices and verifies the designs via experiment.

Self-actuating biosensor using a piezoelectric cantilever and its optimization

This paper presents a self-actuating biosensor based on the piezoelectric cantilever optimized by Taguchi method for the label-free detection of specific biomoleculars. Until now, even though the optimisation of the cantilever-based biosensors have been performed by changing dimensions one by one, few researches have performed the geometric optimisation considering the overall configuration of the piezoelectric cantilever. This paper suggests a method of optimising the piezoelectric cantilever by the Taguchi method. The first resonance frequency, the separation factor, and the sensing signal of the piezoelectric cantilever were selected as the object functions. The resonator driving circuit will keep the cantilever resonance on the binding of the antigen with antibody. By real-time monitoring of the resonance frequency shift by the frequency sensing circuit, the effects of the protein binding on the change of the cantilever stiffness and mass will be investigated.

High-resolution inchworm linear motor based on electrostatic twisting microactuators

A new inchworm micromotor using new electrostatic in-plane twisting microactuators has been designed, fabricated and characterized for nano-resolution manipulators. The proposed twisting mechanism was implemented employing a pair of differential electrostatic actuators with a high stiffness in the driving direction for stable positioning. The electromechanically coupled motion of the voltage–displacement relation was analyzed using a finite element method (FEM), confirming that the twisting actuator makes a tiny step movement efficiently. The proposed actuator was fabricated on a silicon-on-insulator (SOI) wafer with the device footprint of 2.2 × 2.8 mm2, and its nano-stepping characteristics were measured by an optical interferometer consisting of an integrated micromirror and optical fiber. The fabricated inchworm motor showed a minimum step displacement of 5.2 ± 3.8 nm (2σ) and 4.1 ± 2.9 nm (2σ) for cyclic motion in the +y- and the −y-directions, respectively, with the gripping voltage of 15 V and differential voltage of 1 V. As a result, the proposed inchworm micromotor could operate with a stroke of 3 µm and a bi-directional step displacement of less than 10 nm. The step displacement is the smallest value of in-plane-type micromotors so far, and its magnitude was controllable up to 120 nm/cycle by changing the differential voltage.

A micromachined friction meter for silicon sidewalls with consideration of contact surface shape

A friction meter with consideration of contact surface shape is proposed for the evaluation of the static and dynamic friction coefficients on the sidewalls of micromachined structures. In order to validate the proposed friction measurement method, a friction meter for sidewalls was designed employing simple beam springs with holding and driving comb actuators fabricated using a silicon deep reactive ion etching process. In experiments to assess the meter, a shuttle was placed at a certain position by the driving actuator, and a symmetric normal holding force was subsequently applied to the sidewalls of the shuttle. After increasing the driving voltage with a ramp slope, the sliding distance was measured so as to determine the static and dynamic friction coefficients with consideration of the spring nonlinearity. To characterize the suggested friction meter, experiments were performed to investigate the effects of the normal force and the contact surface shape on friction coefficients by varying the contact widths and the number of contact points. The results indicate that the friction coefficients increased with the normal holding force, whereas the contact surface shape did not show a noticeable effect on the friction coefficients.

A pulse-operating electrostatic microactuator for bi-stable latching

We propose a novel microactuator with spade comb fingers designed for pulse-operating bi-stability. When the broad parts of the movable comb fingers are positioned facing the broad parts of the fixed comb fingers, the direction of electrostatic force is inward, like a conventional comb actuator. However, the spade comb fingers generate outward electrostatic force when the broad end part of each movable comb finger is positioned facing the narrow part of the fixed comb fingers. The proposed spade comb is combined with chevron-type springs to effectively operate the actuator in a bi-stable latching. For a driving input, a fast electrical pulse was applied to one driving electrode, and its operability was investigated in terms of voltage magnitude and duty time. Through an FEA (finite element analysis) and a numerical analysis, the dynamic characteristics were theoretically investigated including transient time response and operable bi-stability, which were compared with the experimental values. The bi-stable actuator was fabricated using a silicon-on-insulator (SOI) wafer, and successfully operated by a pulse voltage input of 55 V with a duty time of 0.32 ms.

A novel micropump with fixed-geometry valves and low leakage flow

A novel micropump with fixed-geometry valves was designed and tested with a leakage barrier to reduce leakage flow. Conventional micropumps with fixed-geometry valves have achieved net positive fluid flow from different fluid resistances in diffuser/nozzle channels. However, those micropumps are susceptible to leakage flow even at low pressure differences between the inlet and the outlet because the channels remain normally open state when the pumps are not in operation. Therefore, a leakage barrier in the chamber was designed to reduce leakage flow without interfering with the net positive fluid flow of the diffuser/nozzle channels. The diffuser/nozzle channels, the chamber and the leakage barrier were fabricated on the silicon substrate by KOH etching and the silicon substrate was anodically bonded with a Pyrex glass plate. A PZT disk was bonded on the glass plate by epoxy and was actuated to oscillate the glass diaphragm for flow generation. When the micropump is not operating, the leakage barrier removes most of the gap between the glass plate and the bottom of the chamber. It was experimentally confirmed that the leakage barrier reduced the leakage flow by 96% compared to the case of no leakage barrier at a pressure difference of −400 Pa. Moreover, by applying the holding dc voltage to the PZT disk, a smaller gap can be obtained reducing the leakage flow further down to 0.043 µL min−1 at a holding dc voltage of 100 V. The maximum flow rate was 3.9 µL min−1 at a peak-to-peak driving voltage of 150 V at 20 Hz with a maximum back pressure of around 800 Pa. The approximate device size was 18 × 25 mm2.

Novel Measurement System of the Friction Coefficients for the Drie Sidewalls

A silicon friction meter employing simple beam springs with holding and driving comb actuators was suggested to evaluate the static and the dynamic friction coefficients of the DRIE silicon sidewalls. After moving the shuttle to a certain position, a symmetric normal force was applied on the sidewalls of the shuttle by the holding actuators. By ramp increasing the driving voltage, the slide distance was measured to evaluate the static and dynamic friction coefficients considering the spring nonlinearity. The effects of the contact width, the number of contact points and the normal force on friction coefficients were investigated.

Contact resistance of micromachined electrical switches incorporating a chevron-type bi-stable spring

Micromachined electrical switches with bi-stable springs, which can stay at one of the two stable states without consuming energy, are proposed. Cascaded bent beams are incorporated as thermoelastic microactuators and are characterized through a coupled electro-thermo-mechanical analysis using ANSYS. For improved electrical switch performance, the contact resistances should be kept as low as possible. Therefore, the shape of the contact head needs to be optimized, though to date there have been few studies pertaining to the contact heads of electrical switches reported, except for a flat contact shape. In this paper, the effects of contact angle on the electrical resistance are investigated for contact angles of 30°, 45°, and 60°. It is subsequently observed that the contact resistance decreases with the contact angle due to a greater normal contact force; the minimum contact resistance is 0.22 Ω at a contact angle of 60°. The contact resistance shows negligible change during repeated ON/OFF switching operations.

A micromachined reaction force actuator (RFA) for a nanomanipulator preparation

A reaction force actuator (RFA) was fabricated to translate a microstage with nanostep movement, and its performance was experimentally evaluated using an optical fiber based built-in microinterferometer. The proposed RFA consists of a shuttle mass, movable electrode, fixed electrode, springs, and spring anchor, all of which reside on the movable substrate. The RFA placed on the platform is free to move when the driving force is larger than the static friction. The fixed electrodes are gold-wired to the external electrodes on the platform covered with a dielectric layer for electrical isolation. When external voltage is applied to the electrodes, the springs experience deflections, and the electrostatic force and restoring force react on the movable substrate through the spring anchor and the fixed electrode, respectively. If the driving voltage is large enough that the resultant force overcomes the friction from the platform, the RFA including the movable substrate can make a displacement with no physical collision between the movable and fixed electrodes. In order to suppress the drift motion due to external noise, electrostatic pressure was applied between the movable substrate and the platform on which a 100-/spl mu/m-thick dielectric thin film is positioned. The nanomotion of the fabricated actuator was evaluated with various voltages using an optical fiber interferometer. The minimum step movement 1.21/spl plusmn/0.24 nm was experimentally obtained at the driving voltage of 18 V, and the estimated total displacement was 450 nm at the highest affordable driving voltage of 85 V.