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.
A valveless micropump, actuated by a PZT disk bonded to a glass plate, can generate positive flow. In order to estimate flow characteristics of micropumps, it is necessary to theoretically analyze the radial expansion (more specifically, the equivalent moment) of the PZT disk according to the voltage input. Using the equivalent moment, deflection equations are derived for the tri-layer disk (PZT, epoxy bonder and glass plate) and are confirmed to match well with experiments. The flow rate of the valveless micropump is also theoretically and experimentally investigated in terms of input voltage and oscillation frequency. The flow increased at a rate of 0.1 lL/min/V, and the maximum flow rate was obtained at the driving frequency of around 225 Hz.
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 mm 2 , 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 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.
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