A nodal analysis method for simulating the behavior of electrothermal microactuators

This paper presents a novel approach to verify and optimize surface micromachined electrothermal microactuators by using a nodal analysis method. The nodal analysis method for the mechanical and electrostatic devices is a schematic-based method which simplifies the design of MEMS devices significantly. A variety of the surface micromachined electrothermal microactuators have been widely applied in various areas due to the high force provided at a relatively low input voltage. These electrothermal microactuators can also be decomposed into essential elements of beams and anchors. This paper presents the nodal analysis method for the electrothermal microactuators. The temperature dependent properties for the thermal conductivity, electrical resistivity and thermal expansion coefficient of polysilicon beams are included. The effect of the effective axial length for the beams due to lateral deflection and large axial stress is also taken into account. This approach is verified by ANSYS and the simulation data agrees well with each other. It extends the general nodal analyses method to simulate the electrothermal microactuators.     

硅基压电多层膜悬臂梁的偏转模型和优化设计

利用压电多层膜悬臂梁的形变曲率半径,导出了简洁的压电悬臂梁型微执行器的偏转模型.采用此模型,对硅基PZT压电悬臂梁型微执行器进行了模拟和分析.分析获得了各层薄膜结构参数与微执行器偏转位移的关系.根据此分析结果,进一步优化了压电悬臂梁型微执行器的设计,并给出了优化的方法.     

Optimal design analysis of electrothermally driven microactuators

This paper explores a comparative study between different designs of electrothermal microactuators with emphasis on optimal design and performance key factors. For this purpose, two typical designs for electrothermal microactuators with the same material properties are studied: one with different beam lengths (design A), other one with different beam sections and a flexure part (design B). Analytical model and finite element model (FEM) have been developed and validated by comparison of simulation results with experimental results in literature. Optimal geometrical dimensions to achieve maximum deflection have been obtained using genetic algorithm (GA). As the key factors, temperature distribution, power consumption and deflection of these microactuators have been compared in the range of microactuator functionality. Design B is more sensitive to geometrical dimension variation. Using optimal geometrical dimensions, an increase of almost 40 and 55% has been achieved for design A and B tip deflections, respectively. The modified design A with a gold layer results to an increase of 70% for tip deflection comparing to its optimal design.     

Influence of material stiffness and geometrical variations on the electro-thermally driven microactuator performance

In order to make robust design of microdevices, it is important to implement a model considering uncertainty analysis. The high cost of experimentation and product development has led to an emphasis on simulated based design to achieve the success in the first-pass design and reliability. In this paper, finite element model is developed to investigate the effects of geometrical and material stiffness variations on an electro-thermally driven microactuator performance as an example. These microactuators are generally composed of two suspended beam (arm) joined at the free end. This device generates deflection through asymmetric heating of the hot and cold polysilicon arms with variable length or cross-section. These microactuators based on the force and deflection characteristic of elastic members are particularly sensitive to uncertainty in material properties. Polysilicon as a microdevice material shows a wide variation rang for Young’s modulus from batch to batch fabrication process. Also, the microfabrication process that are utilized for microdevice production can yield sometimes shapes that are not geometrically perfect, either due to the microfabrication process limitations itself or because of phenomena that take place during or after microfabrication. These geometrical errors can decrease the net cross section and will affect both modeling and experimental results. The effect of fillet radius at the juncture of this flexible microactuator and contact pads is investigated in this model. The residual stresses due to the fabrication process are taken into account too. The analysis is performed at multiple input voltages to estimate uncertainty bands around the deflection curve of the microactuator. Simulation results are compared with experimental results in literature. The results demonstrate how each of these factors affects the microactuator performance and justifies the deviation of previous nominal results from experimental results.     

衔铁平行运动的磁微执行器Pull-in机理分析

与静电微执行器类似,磁微执行器中也存在重要的Pull-in失稳问题.但相对于静电微执行器详尽的模型相比,研究磁微执行器的文献很少.对于宏观器件,由于磁芯磁阻可以忽略,所以可利用电磁类比,得到Pull-in失稳参数.而对于微观器件,磁芯磁阻不可以忽略.不可利用电磁类比.基于小信号模型对衔铁平行运动的磁微执行器Pull-in的失稳现象首先进行了分析和建模,考虑了微磁芯磁阻,在建立磁微执行器动态小信号模型的基础上,分析发现衔铁平行运动的磁微执行器中不存在Pull-in现象.然后又采用能量法研究了同样的Pull-in失稳问题,得出了和力法相同的结论.最后,给出了一个分析实例,以反映微磁芯磁阻对Pull-in的影响,同时证实了衔铁平行运动的磁微执行器中不存在Pull-in现象的结论.结论对于磁微执行器的设计具有重要意义.     

Fabrication and characterization of DRIE-micromachined electrostatic microactuators for hard disk drives

In this paper, deep micromachined three-dimensional (3-D) electrostatic microactuators used for dual-stage positioning system of hard disk drives are reported. Actuators with parallel-arranged comb drives enhance the electrostatic driving force. By using proper flexures, secondary stage actuators will drive the magnetic head with fast response and high accuracy. Fabrication of the actuators starts with a 200-μm-thick n-type silicon wafer, and it is subsequently bonded to a Pyrex glass substrate, which can be called silicon-on-glass process. This process is more cost-effective than the SOI wafer process, and the high aspect ratio structures with large thickness also provide good strength and reliability for the microactuators. Deep RIE and wafer bonding techniques were utilized to fabricate the electrostatic actuators. The fabricated actuators were statically and dynamically characterized for three different designs of straight-flexures, folded-flexures and quad-flexures with bandwidth of 7.15, 5.85 and 15.85 kHz, respectively. With proper designed flexures, the proposed microactuators would fulfill the requirements of the dual-stage servo of hard disk drives.     

Creep behavior of undoped and La–Nb codoped PZT based micro-piezoactuators for micro-optical modulator applications

Time-dependent deformation (creep) behaviors of piezoelectric microactuators have been investigated. Position (or gap height) drift of microbridged actuator beam and its displacement amplitude change with time could be attributed to the anelastic behavior of the driving unit itself based on lead zirconate titanate (PZT) in the actuator, and as well as to the mechanical stress states established in the microactuator beam after the surface release of the micromachined actuator structure. From creep analyses of a simple microcantilever and microbridge beam structures, it was turned out that the overall creep behavior in microbridged piezoactuator structure was mainly dependent on the actuator beam initial deflection configuration governed both by residual stress of the actuating part Pt/PZT/Pt stack and the stress gradient through the silicon nitride (SiNx) microbridge beam.     

Laterally driven electrostatic repulsive-force microactuators usingasymmetric field distribution

We present a new electrostatic actuation method using a lateral repulsive-force induced by an asymmetric distribution of planar electrostatic field. The lateral repulsive-force has been characterized by a simple analytical equation, derived from a finite element simulation. Quality-factors are estimated from the computer simulation based on creep flow model. A set of repulsive-force polycrystalline silicon microactuators has been designed and fabricated by a four-mask surface-micromachining process. Static and dynamic response of the fabricated microactuators has been measured at the atmospheric pressure for the driving voltage range of 0-140 V. The static displacement of 1.27 μm is obtained at the dc voltage of 140 V. The resonant frequency of the repulsive-force microactuator increases from 11.7 kHz to 12.7 kHz when the dc induction voltage increases from 60 V to 140 V. The measured quality-factors are increased from 12 to 13 in the voltage range of 60-140 V. Fundamental characteristics of the force, frequency and quality-factor of the electrostatic repulsive-force microactuator have been discussed and compared with those of the conventional electrostatic attractive-force microactuator     

Microsized Piston-Cylinder Pneumatic and Hydraulic Actuators Fabricated by Lithography

Future microrobotic applications require actuators that can generate a high actuation force in a limited volume. Up to now, little research has been performed on the development of pneumatic or hydraulic microactuators, although they offer great prospects in achieving high force densities. In addition, large actuation strokes and high actuation speeds can be achieved by these actuators. This paper describes a fabrication process for piston-cylinder pneumatic and hydraulic actuators based on etching techniques, UV-definable polymers, and low-temperature bonding. Prototype actuators with a piston area of 0.15 mm² have been fabricated in order to validate the production process. These actuators achieve actuation forces of more than 0.1 N and strokes of 750 mum using pressurized air or water as driving fluid.     

A MEMS conical spring actuator array

A new MEMS conical spring actuator array is proposed. Previously, we have developed conical spring microactuators having a long stroke (180 /spl mu/m) in the out-of-plane direction. However, the maximum output force and the packing density were not satisfactory. In the present paper, mechanical and electrical models of a conical spring are described for the calculation of the maximum output force and the driving voltage. Geometrical parameters were optimized using these models and a new geometry for the actuator was derived. The new geometry incorporates a wider and thicker spring that increased the maximum output force from 0.087 mN up to 0.83 mN. The packing density was increased up to 1 actuator/mm/sup 2/ using an additional interconnect layer. In addition, the driving voltage was decreased using a thinner insulating layer. The use of an ac drive prevented the sticking of the actuator during operation. A detailed investigation of the ac drive was also performed.