Flexure-Based Two Degree-of-Freedom Legs for Walking Microrobots

This work examines the design of legs for a walking microrobot. The parameterized force-displacement relationships of planar serpentine flexure-based two degree-of-freedom legs are analyzed. An analytical model based on Euler-Bernoulli beam theory is developed to explore the design space, and is subsequently refined to include contact between adjacent beams. This is used to determine a successful leg geometry given dimensional constraints and actuator limitations. Standard comb drive actuators that output 100 μN of force over a 15 μm bi-directional throw are shown able to drive a walking gait with three legs on a 1 cm2 silicon die microrobot. If the comb drive suspensions cannot withstand the generated reaction moments, an alternate pivot-based leg linkage is proposed.

Rubber-Assisted Hot Embossing for Structuring Thin Polymer Film Polymeric Films

Precision structured polymer thin films with microstructures comparable to or greater than the film thickness are highly desired in many applications. Such micro-patterned thin films, however, are difficult to fabricate using the standard hot embossing technology where both halves of the mold are made of hard materials. This study investigated a rubber-assisted embossing process for structuring thin polymer films. The advantages of the rubber backup instead of a hard support include but are not limited to 1) simplifying the embossing tool, 2) protecting the embossing master, 3) facilitating embossing pressure buildup, and 4) accommodating conformal forming of microscale shell patterns. Several design and process variables including rubber hardness, embossing temperature, embossing pressure and holding time were carefully studied. Thin polystyrene films in a thickness of 25 μm were accurately patterned with microgrooves of characteristic dimensions on the order of 100 μm.

Surface Topography Induced Ultrahydrophobic Behavior: Effect of Three-Phase Contact Line Topology

The wettability of silicon surface hydrophobized using silanization reagents was studied. The advancing and receding contact angles were measured with the captive needle approach. In this approach, a drop under study was held on the hydrophobized surface with a fine needle immersed in it. The asymptotic advancing and receding angles were obtained by incrementally increasing the volume added and removed, respectively, until no change in angles was observed. The values were compared with the previously published results. Further, the wetting behavior of water droplets on periodically structured hydrophobic surfaces was investigated. The surfaces were prepared with the wet etching process and contain posts and holes of different sizes and void fractions. The surface geometry brought up a scope to study the Wenzel (filling of surface grooves) and Cassie (non filling of the surface grooves) theories and effects of surface geometry and roughness on the contact angle. Experimental data point to an anomalous behavior where the data does not obey either Wenzel or Cassie type phenomenology. This behavior is explained by an understanding of the contact line topography. The effect of contact line topography on the contact angle was thus parametrically studied. It was also inferred that, the contact angle increased with the increase in void fraction. The observations may serve as guidelines in designing surfaces with the desired wetting behavior.

Rapid Silicon-to-Steel Bonding via Inductive Heating

In this work, we present a rapid, low temperature process for the bonding of silicon to steel through the use of inductive heating for MEMS sensor applications. The bonding process takes as short as three seconds with a maximum bonding temperature as low as 230°C at the steel surface. The bonding strength is strong, and causes minimal damage to steel. The process has also been shown to work using leaded and leadfree bonding solder with minimal surface preparation to the steel. Four characterization experiments – tensile and compressive 4-point bend, axial extension, and fatigue tests – have been performed to validate the bonding process and materials. As such, this work illustrates the promise of applying inductive heating for the rapid silicon bonding to steel components for MEMS sensing applications.

Hydoroil-Based Micro Pin Fin Heat Sink

An experimental study on thermal-hydraulic performance of de-ionized water over a bank of shrouded NACA 66-021 hydrofoil micro pin fins with wetted perimeter of 1030-μm and chord thickness of 100 μm has been performed. Average heat transfer coefficients have been obtained over effective heat fluxes ranging from 4.0 to 308 W/cm2 and mass velocities from 134 to 6600 kg/m2s. The experimental data is reduced to the Nusselt numbers, Reynolds numbers, total thermal resistances, and friction factors in order to determine the thermal-hydraulic performance of the heat sink. It has been found that prodigious hydrodynamic improvement can be obtained with the hydrofoil-based micro pin fin heat sink compared to the circular pin fin device. Fluid flow over pin fin heat sinks comprised from hydrofoils yielded radically lower thermal resistances than circular pin fins for a similar pressure drop.

Restoring Force Surface Analysis of Nonlinear Vibration Data From Micro-Cantilever Beams

The responses of micro-cantilever beams, with lengths ranging from 100-1500 microns, have been found to exhibit nonlinear dynamic characteristics at very low vibration amplitudes and in near vacuum. This work seeks to find a functional form for the nonlinear forces acting on the beams in order to aide in identifying their cause. In this paper, the restoring force surface method is used to non-parametrically identify the nonlinear forces acting on a 200 micron long beam. The beam response to sinusoidal excitation contains as many as 19 significant harmonics within the measurement bandwidth. The nonlinear forces on the beam are found to be oscillatory and to depend on the beam velocity. A piecewise linear curve is fit to the response in order to more easily compare the restoring forces obtained at various amplitudes. The analysis illustrates the utility of the restoring force surface method on a system with complex and highly nonlinear forces.

Simultaneous Measurement of Temperature and Velocity Using µPIV

In μPIV, for a uniform velocity field the displacement of the cross-correlation function gives the velocity of the fluid and the broadening of the peak-width represents the amount of Brownian motion present. In the presence of a linear or a parabolic shear, the shape of the cross-correlation function would have both the Brownian motion information as well as the velocity distribution information. In the present work, the broadening of the cross-correlation function caused by the velocity gradient was subtracted from the total peak broadening in order to isolate the Brownian motion information and thus infer temperature. To the authors' knowledge, this technique has not been applied to measure the temperature of a moving fluid. The experiments were conducted in a gravity driven flow through a tube surrounded by a constant temperature water bath.

On-Off Control for Full-Gap Positioning of Parallel-Plate Electrostatic MEMS

Electrostatic parallel-plate actuators are classically limited to displacements up to 1/3 of the gap due to the pull-in effect [1],[2]. A closed-loop feedback based method presented in [3] is recently introduced to overcome this limitation. Optimized structures are designed to minimize residual position ripple while maintaining bandwidth, effectively reducing the required device size by a factor three.

Design and Verification of a Low-Powered Pre-Programmable In-Package Temperature Controller

A zero power passive temperature regulator has been studied and designed to maintain electric chip operating temperature using a variable thermal resistor. Apart from the passive temperature regulator design, we also present active variable thermal resistors using electrostatic force to actuate the device. Test samples were fabricated to verify these two designs and we observed the temperature change of a heated chip due to thermal resistance changes. This study estimated and measured the thermal contact resistance and the force required to remove it.

Experimental Verification of Rarefied Gas Squeezed-Film Damping Models Used in MEMS

Existing compact parallel-plate squeeze-film models including rarefaction and border effects are verified using the experimental results of a new electrostatic MEMS actuation technique that enables full gap positioning. Measurements at high Knudsen numbers ranging from 0.03 to 0.18 are performed and results compared to the models. The simulation errors are confirmed to be lower than 20%. The experiments also indicate that both gas rarefaction and border effects have to be included in any model.