Analysis and Design of Thermally Actuated Micro-Cantilevers for High Frequency Vibrations Using Finite Element Method

Vibrational behavior of thermally actuated cantilever micro-beams and their mechanical response at moderately high frequency under a non-harmonic periodic loading is studied in this paper. Two different configurations are considered: 1) a straight beam with two actuation layers on top and bottom which utilizes the bimorph effect to induce bending;2) a uniform beam with base excitation, where the beam is mounted on an actuator which moves it periodically at its base perpendicular to its axis. Generally, vibrating micro-cantilevers are required to oscillate at a specified frequency. In order to increase the efficiency of the system, and achieve deflections with low power consumption, geometrical features of the beams can be quantified so that the required vibrating frequency matches the natural frequencies of the beam. A parametric modal analysis is conducted on two configurations of micro-cantilever and the first natural frequency of the cantilevers as a function of geometrical parameters is extracted. To evaluate vibrational behavior and thermo-mechanical efficiency of micro-cantilevers as a function of their geometrical parameters and input power, a case study with a specified vibrating frequency is considered. Due to significant complexities in the loading conditions and thermo-mechanical behavior, this task can only be tackled via numerical methods. Selecting the geometrical parameters in order to induce resonance at the nominal frequency, non-linear time-history (transient) thermo-mechanical finite element analysis (using ANSYS) is run on each configuration to study its response to the periodic heating input. Approaches to improve the effectiveness of actuators in each configuration based on their implementation are investigated.

DESIGN AND FABRICATION OF A MICRO THERMAL ACTUATOR FOR CELLULAR GRASPING

The development of a novel polymer-based micro robotic gripper that can be actuated in a fluidic medium is presented in this paper.Our current work is to explore new materials and designs for thermal actuators to achieve micromanipulation of live biological cells.We used parylene C to encapsulate a metal heater,resulting in effectively a tri-layered thermal actuator.Parylene C is a bio-compatible dielectric polymer that can serve as a barrier to various gases and chemicals. Therefore,it is suitable to serve as a thermal/electrical/chemical isolation material for protecting the metal heater from exposing to an aqueous environment.We have demonstrated parylene actuators (2mm×100μm×0.5μm)to operate in an aqueous environment using 10 to 80mW input power.The temperature of these actuators at full deflection was estimated to be～60℃,which is much lower than the typical requirement of>100℃ to actuate other conventional MEMS actuators.Danio rerio follicles in fluidic medium were captured successfully using these actuators.Moreover,these actuators were found to be responsive to moderate rise in environmental temperature,and hence,we could vary the fluidic medium temperature to actuate trimorphs on a chip without any input of electrical energy, i.e.,raising the fluidic temperature from 23℃ to 60℃ could actuate the trimorphs to grasp follicles of ～1mm size in diameter.At 60℃,the embryos inside the follicles were observed to be alive,i.e.,they were still moving in the biological fluid isolated by the follicle membrane.The smallest follicles grasped were～500μm in diameter using 800μm×130μm×0.6μm actuators.The fabrication process,modeling, and optimization of the trimorph actuators are presented.Based on the successful operation of these polymer-based actuators,we are currently developing multifinger thermal microgrippers for cellular grasping and manipulation,which can potentially be hybridly integrated with circuits for computer control.

Design of a test structure based on chevron-shaped thermal actuator for in-situ measurement of the fracture strength of MEMS thin films

A novel test structure to characterize the fracture strength of MEMS(Micro-electro-Mechanical Systems)thin films is presented.The test structure is comprised of a micro fabricated chevron-shaped thermal actuator and test specimen.The test structure is capable of producing large displacement and stresswhile keeping a relatively low temperature gradient across the test specimen.A voltage is applied across the beams of the chevron-shaped actuator,producing thermal expansion force to fracture the test specimen.Actuator deflection is computed based on elastic analysis of structures.To verify the test structure,simulations have been implemented using COMSOL Multiphysics.A 620μmlong,410μm wide,10μm thick test structure produced stress of 7.1 GPawhile the applied voltage is 5 V.The results indicate that the test structure is suitable for in-situ measurement of the fracture strength of MEMS thin films.

Influence of low temperature on the surface deformation of deformable mirrors

The two factors which influence the low temperature performance of deformable mirrors（DMs） are the piezoelectric stroke of the actuators and the thermally induced surface deformation of the DM. A new theory was proposed to explain the thermally induced surface deformation of the DM： because the thermal strain between the actuators and the base leads to an additional moment according to the theory of plates, the base will be bent and the bowing base will result in an obvious surface deformation of the facesheet. The finite element method（FEM） was used to prove the theory. The results showed that the thermally induced surface deformation is mainly caused by the base deformation which is induced by the coefficient of thermal expansion（CTE） mismatching; when the facesheet has similar CTE with the actuators, the surface deformation of the DM would be smoother. Then an optimized DM design was adopted to reduce the surface deformation of the DMs at low temperature. The low temperature tests of two 61-element discrete PZT actuator sample deformable mirrors and the corresponding optimized DMs were conducted to verify the simulated results. The results showed that the optimized DMs perform well.

On-demand Direct Design of Polymeric Thermal Actuator by Machine Learning Algorithm

The design optimization of thermal-driven actuators is a challenging task because the performance depends on multiple materials parameters,structural parameters,and working conditions.In this work,we adopted large scale finite element simulation together with machine learning algorithm to fulfill the on-demand design of thermal actuators.Finite element analysis was used to simulate the performance of thermal actuator with two-layer structure,which generated large amount of dataset by considering the variation of parameters including the moduli,thermal expansion coefficient,sample thickness and length,and temperature.Support vector regression(SVR)was adopted to establish the relationship between multiple input parameters and the resulting contact pressure.Thereafter,a simple interior point algorithm was used to achieve the on-demand design based on the SVR model.The contact pressures of thermal actuator constructed from the optimized parameters deviated less than 15%of the target values.