Kuang-Shun Ou

Viscoelastic Characterization and Modeling of Polymer Transducers for Biological Applications

Polydimethylsiloxane (PDMS) is an important polymeric material widely used in bio-MEMS devices such as micropillar arrays for cellular mechanical force measurements. The accuracy of such a measurement relies on choosing an appropriate material constitutive model for converting the measured structural deformations into corresponding reaction forces. However, although PDMS is a well-known viscoelastic material, many researchers in the past have treated it as a linear elastic material, which could result in errors of cellular traction force interpretation. In this paper, the mechanical properties of PDMS were characterized by using uniaxial compression, dynamic mechanical analysis, and nanoindentation tests, as well as finite element analysis (FEA). A generalized Maxwell model with the use of two exponential terms was used to emulate the mechanical behavior of PDMS at room temperature. After we found the viscoelastic constitutive law of PDMS, we used it to develop a more accurate model for converting deflection data to cellular traction forces. Moreover, in situ cellular traction force evolutions of cardiac myocytes were demonstrated by using this new conversion model. The results presented by this paper are believed to be useful for biologists who are interpreting similar physiological processes.

Viscoelastic mechanical behavior of soft microcantilever-based force sensors

Polydimethylsiloxane (PDMS) microcantilevers have been used as force sensors for studying cellular mechanics by converting their displacements to cellular mechanical forces. However, PDMS is an inherently viscoelastic material and its elastic modulus changes with loading rates and elapsed time. Therefore, the traditional approach to calculating cellular mechanical forces based on elastic mechanics can result in errors. This letter reports a more in-depth method for viscoelastic characterization, modeling, and analysis associated with the bending behavior of the PDMS microcantilevers. A viscoelastic force conversion model was developed and validated by proof-of-principle bending tests.

Command-Shaping Techniques for Electrostatic MEMS Actuation: Analysis and Simulation

Precision positioning of microelectromechanical systems (MEMS) structures using electrostatic actuation has been widely used for optical and radio-frequency MEMS. How to achieve fast switching without exciting excessive residual vibration or structural impact is an important issue for these applications. This paper presents the analysis and simulation of applying command-shaping techniques for controlling MEMS electrostatic actuation. According to the nature of application fields, electrostatic actuators are classified into three categories: 1) lateral linear actuation; 2) vertical nonlinear actuation; and 3) pull-in actuation. Their corresponding linear or nonlinear command-shaping schemes are developed and presented. Both lumped element and continuous models of typical MEMS electrostatic actuated structures are simulated using Simulink and the finite-element method, and results indicate that the shaped command would yield a much superior response than that by the unshaped commands. Essential sensitivity studies are also conducted to examine the robustness of these shaping schemes, and results shows that within a certain level of parameter variation, these shapers are robust enough to retain the performance.

On the development of autonomously manipulation of group mobile robots for smart living and biomimetic applications

In this work, by integrating omni-wheel mobile robots with X-Bee communication protocol, Arduino control, IR range finders, and CMOS camera, as well as wiimote multizone localization, tasks such as obstacle and collision avoidances, following, autonomously movement, and indoor localization of group robots are implemented as the first step toward an autonomously control of group robots for smart living and biomimetic applications. In conjunction with hardware design, novel algorithms are also developed to realize these tasks for future group robot applications in indoor service and biomimetic tasks.

On the influence of roller misalignments on the web behavior during roll-to-roll processing

Roll-to-Roll (R2R) web transportation has been widely applied in numerous industrial material processes, such as nano-imprinting, continuous plasma deposition, paper rolling, and steel sheet forming. During a typical web transportation process, a pre-applied tensile stress is required for overcoming possible compressive buckling failure due to geometrical misalignments. However, the applied tensile stress must be limited to within the material yield strength. As a result, the relationship between the minimum required pre-tensile, geometrical misalignments, and material properties, must be clarified. The traditional approach to handle the relationship is based on a beam theory, which is not practical in typical wide web structures. Therefore, a modified formula for finding the required minimum tensile load is presented, using the finite element method and plate theory in this article. In addition, the effects of in-plane and out-of-plane misalignments of rollers on the web buckling are also investigated and quantified. It was found that web stress distribution is more sensitive to in-plane misalignment. On the other hand, although out-of-plane misalignment is not critical, it could trigger web buckling with the presence of an in-plane misalignment. Finally, material characterization to investigate the relationship among the allowable pre-tension, the misalignment angle, and the web width was performed. It was found that a misaligned web could carry less pre-tension than a perfectly aligned web. In addition, the situation deteriorates once the web width is further increased. These mechanical analyses, combined with specific material characterizations presented in this study, provide crucial information for improving the next generation of R2R applications.

Design of Wiimote indoor localization technology for omni-directional vehicle trajectory control

Accurate motion sensing is essential for trajectory control of indoor service mobile carriers for realizing smart living. In this work, a modified Wiimote-based 2D localization scheme is proposed and experimentally validated for indoor mobile robots tracking and control tasks. This scheme utilizes one Wiimote to monitor the position of at least two IR LEDs simultaneously for determining both translation and rotation motions of the Wiimote-mounted carrier. An algorithm is developed for converting the Wiimote readouts to the position and orientation of the corresponding mobile unit based on major axes identification, coordinate transformation, and position updating. The scheme and the algorithm are validated by tracking the Wiimote location on a two-axis linear servomotor. Finally, by integrating this global positioning scheme with feedback control, trajectory tracking of an omni-wheel based mobile robot is performed to demonstrate the importance of the scheme in indoor smart living technology.

Performance evaluation of drift-free integration for navigation and velocity feedback applications

In many applications, such as navigation or control, the vital localization or feedback information can be obtained via inertial sensors with subsequent time integration. However, the intrinsic near-DC noise existing in virtually all signals would cause signal drifting after integration and eventually disables the associated applications. Various approaches have been proposed to solve this problem. In this study, the existed drift-integration approach is re-examined for evaluating its performance and optimal design criterion through both simulation and experimental characterizations. A drift-free integrator based on a first-order low-pass filter and two second-order Sallen–Keys high-pass filters was realized using commercially available OP 741 and associated circuit elements. An electromagnetically actuated vibrating system was designed and realized for providing vibration excitation, and its acceleration and displacement were measured separately by a PCB accelerometer and a Micro-Epsilon displacement probe. Both signals are subsequently converted to velocities using the drift-free integration and standard signal differentiation for the purpose of the performance evaluation and the optimal design of the integrator. The experimental results indicated that the integrator can effectively suppress the signal drifting due to the near-DC components commonly existing in all inertial signals. However, it also was found that a trade-off exists between drifting suppression and the extent of signal distortion, and there exists an optimal design criterion based on the dynamic parameters of both circuit and mechanical systems. The conclusion drawn from this study would be useful for a designed drift-free integrator for related applications such as an accelerometer-based velocity feedback control and the MEMS accelerometer-based inertial navigation systems.

Mechanical Characterization of Atomic Layer Deposited (ALD) Alumina for Applications in Corrosive Environments

In this work, thin ALD alumina films were fabricated for evaluating their capabilities as a barrier material for corrosive environments. The fracture toughness and the corrosion-resisting properties after fatigue cycle of these thin ALD alumina films have been characterized. Indentation tests indicate that the ALD alumina/Al structures could enhance both the yield strength of the metal and the effective fracture toughness of the coated ALD alumina films and this result could be useful for designing nanocomposite structures. However, the test results also indicate that the interfacial strength of the ALD/Al structures was prone to degrade under fatigue loading under corrosive environment. This could potentially be a problem for the long term reliability of related devices operated under a harsh environment. In addition, the strong correlation between indentation behavior and fatigue loading for the structure indicate that nanoindentation response could be possibly used to indicate the damage level of microstructures for future reliability evaluations.