Gih-Keong Lau

Electrothermal Microgripper With Large Jaw Displacement and Integrated Force Sensors

This paper presents a novel sensing microgripper based on silicon-polymer electrothermal actuators and piezoresistive force sensing cantilever beams which can monitor the displacement of the microgripper jaws and also the contact force between the tips and the gripped object. A jaw motion up to 32 mum with an output sensing voltage of 49 mV and 114 mW power consumption is measured at a driving voltage of 4.5 V. The working temperature of the structure is then 177degC. The force sensitivity is 1.7 V/N and the corresponding displacement sensitivity is 1.5 kV/m. A minimum detectable displacement and a minimum detectable force of 1 nm and 770 nN are estimated, respectively.

Use of functional specifications as objective functions in topological optimization of compliant mechanism

Abstract Instead of using the concepts of minimum compliance or maximum mutual compliance, alternative formulation, based on functional specification is proposed to solve for topology of compliant mechanism. Among mechanical advantage, geometrical advantage and work ratio, one of the functional specifications is posed as objective function while displacement constraint and material constraint are imposed. The topological design of compliant mechanism is then solved as a problem of material distribution using spring and SIMP model without filtering technique. The solution of this problem is obtained by using the method of moving asymptotes (MMA). The effect of problem formulation on final topology of compliant mechanism is then illustrated using two examples and the results are compared against their rigid-body counterparts.

Topological optimization of mechanical amplifiers for piezoelectric actuators under dynamic motion

Topological optimization is used to systematically design mechanical amplifiers that magnify the limited actuation stroke of a piezoelectric actuator. The design problem is posed as a material distribution problem using a variable thickness method. Two design goals are formulated for the design of the mechanical amplifier. They are the maximum dynamic stroke and the maximum dynamic magnification factor. The optimization problems are then solved using a method of moving asymptotes. The design domain is modelled as a plane-stress solid, and is actuated by harmonic excitation without the inclusion of damping. To model the actuator and workpiece, their stiffness is included and idealized as rod elements in the finite-element analysis. To show the capability of the design methodology, an elliptic amplifier and a magnification mechanism, used in a dot-matrix printer head, are reinvented using topological optimization. The dynamic effects on the optimum topology are also studied using different excitation frequencies.

Polymeric Thermal Microactuator With Embedded Silicon Skeleton: Part II—Fabrication, Characterization, and Application for 2-DOF Microgripper

This paper presents the fabrication, characterization, and application of a novel silicon-polymer laterally stacked electrothermal microactuator. The actuator consists of a deep silicon skeleton structure with a thin-film aluminum heater on top and filled polymer in the trenches among the vertical silicon parts. The fabrication is based on deep reactive ion etching, aluminum sputtering, SU8 filling, and KOH etching. The actuator is 360 mum long, 125 mum wide, and 30 mum thick. It generates a large in-plane forward motion up to 9 mum at a driving voltage of 2.5 V using low power consumption and low operating temperature. A novel 2-D microgripper based on four such forward actuators is introduced. The microgripper jaws can be moved along both the x- and y-axes up to 17 and 11 mum, respectively. The microgripper can grasp a microobject with a diameter from 6 to 40 mum. In addition, the proposed design is suitable for rotation of the clamped object both clockwise and counterclockwise.

Powerful polymeric thermal microactuator with embedded silicon microstructure

A powerful and effective design of a polymeric thermal microactuator is presented. The design has SU-8 epoxy layers filled and bonded in a meandering silicon (Si) microstructure. The silicon microstructure reinforces the SU-8 layers by lateral restraint. It also improves the transverse thermal expansion coefficient and heat transfer for the bonded SU-8 layers. A theoretical model shows that the proposed SU-8/Si composite can deliver an actuation stress of 1.30?MPa/K, which is approximately 2.7 times higher than the unconstrained SU-8 layer, while delivering an approximately equal thermal strain.

Finite element modelling and experimental characterization of an electro-thermally actuated silicon-polymer micro gripper

This paper presents simulation and experimental characterization of an electro-thermally actuated micro gripper. This micro actuator can conceptually be seen as a bi-morph structure of SU-8 and silicon, actuated by thermal expansion of the polymer. The polymer micro gripper with an embedded comb-like silicon skeleton is designed to reduce unwanted out-of-plane bending of the actuator, while offering a large gripper stroke. The temperature and displacement field of the micro gripper structure is determined using a two-dimensional finite element analysis. This analysis is compared to experimental data from steady-state and transient measurements of the integrated heater resistance, which depends on the average temperature of the actuator. The stability of the polymer actuator is evaluated by recording the transient behaviour of the actual jaw displacements. The maximum single jaw displacement of this micro gripper design is 34 µm at a driving voltage of 4 V and an average actuator temperature of 170 °C. The transient thermal response is modelled by a first-order system with a characteristic time constant of 11.1 ms. The simulated force capability of the device is 0.57 mN per µm jaw displacement.

Polymeric Thermal Microactuator With Embedded Silicon Skeleton: Part I—Design and Analysis

This paper presents the modeling of a new design of a polymeric thermal microactuator with an embedded meander-shaped silicon skeleton. The design has a skeleton embedded in a polymer block. The embedded skeleton improves heat transfer to the polymer and reinforces it. In addition, the skeleton laterally constrains the polymer to direct the volumetric thermal expansion of the polymer in the actuation direction. The complex geometry and multiple-material composition of the actuator make its modeling very involved. In this paper, the main focus is on the development of approximate electrothermal and thermoelastic models to capture the essence of the actuator behavior. The approximate models are validated with a fully coupled multiphysics finite element model and with experimental testing. The approximate models can be useful as an inexpensive tool for subsequent design optimization. Evaluation, using the analytical and numerical models, shows that the polymer actuator with the embedded skeleton outperforms its counterpart without a skeleton, which is in terms of heat transfer and, thus, response time, actuation stress, and planarity.

Very high dielectric strength for dielectric elastomer actuators in liquid dielectric immersion

This letter reported that a dielectric elastomer actuator (3M VHB), which is immersed in a liquid dielectric bath, is enhanced tremendously in dielectric strength up to 800 MV/m, as compared to 450 MV/m for the actuator operated in air. The bath consists of silicone oil (Dow Corning Fluid 200 50cSt), which is 6.5 times more thermally conductive than air, and it is found able to maintain the actuator at a stable temperature. As a result, the oil-immersed dielectric elastomer actuator is prevented from local thermal runaway, which causes loss of electrical insulation, and consequently avoids the damage by electromechanical instability.

Systematic design of displacement-amplifying mechanisms for piezoelectric stacked actuators using topology optimization

Displacement-amplifying mechanisms can be systematically designed using topology optimization. Due to the special need for large displacement amplification of piezoelectric stacked actuators, suitable objective functions should be formulated. Among various possible formulations, this work considers maximum output stroke, magnification factor and mechanical efficiency as objective functions. As the formulation of maximum magnification factor performs better numerically than the rest, it is recommended as a suitable design goal for displacement amplifiers undergoing undamped harmonic response. The design problem is solved as a 2D material distribution problem using Method of Moving Asymptotes (MMA). Plane stress solid is assumed for the design domain. Stiffness of the actuator and the workpiece is included in the analysis. To show the viability of this design methodology, two examples of magnification mechanisms for printer heads driven at different excitation frequencies were presented.

Can DC Motors Directly Drive Flapping Wings at High Frequency and Large Wing Strokes

This paper proposes and experimentally validates a method for driving flapping wings at large wing strokes and high frequencies with a DC motor, based on direct, elastic transmission. The DC motor undergoes reciprocating, rather than rotary, motion avoiding the use of nonlinear transmissions such as slider-crank mechanisms. This is key to compact, easy to fabricate, power efficient, and controllable flapping mechanisms. First, an appropriate motor based on maximum power transfer arguments is selected. Then, a flapping mechanism is prototyped and its experimental performance is compared with simulations, which take into account the full dynamics of the system. Despite inherent nonlinearities due to the aerodynamic damping, the linearity of the direct, elastic transmission allows one to fully exploit resonance. This benefit is best captured by the dynamic efficiency, close to 90% at larger wing strokes in both experimental data and simulations. We finally show a compact flapping mechanism implementation with independent flapping motion control for the two wings, which could be used for future autonomous micro-aerial vehicles.