Piezoelektrik tutucunun dizayn ve analizi
The design and analysis of piezoelectric gripper
- Tez No: 75284
- Danışmanlar: YRD. DOÇ. DR. HİKMET KOCABAŞ
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Makine Teorisi ve Kontrol Bilim Dalı
- Sayfa Sayısı: 81
Özet
Sonlu eleman metodu mühendislik problemlerinin birçoğunun nümerik olarak çözümlerinde önemli yer almaktadır. Bilgisayar teknolojisindeki ve bilgisayarla dizayn sistemlerindeki gelişmeler, karmaşık problemlerin bile kolayca modellenip çözülebilmesini sağlamaktadır. Örnek model üretilmeden önce, bilgisayar üzerinde değişik konfıgürasyonların denenerek en uygun modelin dizaynı gerçekleştirilir. Model sonlu eleman kurallarına göre küçük parçalara ayrılarak değişik yükleme durumları uygulanır. Elde edilen denklemler çözülerek gerçek durumdakine yakın sonuçlara ulaşılabilmesi sağlanır.Birleşik alan analizi bir sonlu eleman analiz yöntemidir. Bu yöntemde iki değişik fiziksel niceliğin etkileşimi incelenmektedir. Örnek olarak bizim de bu çalışmada ele aldığımız piezoelektrik analizde; elektrik alan ile yapısal değişikliğin arasındaki etkileşim ele alınmaktadır. Termal-gerilme analizi, termal-elektrik analizi ve basınç-yapısal analiz gibi birçok örnek de bu analizin içindedir.Piezoelektrik malzeme birim çift kutupların dizilmesiyle ortaya çıkan bir malzemedir. Bu malzemenin uçlarına bir voltaj uygulandığında uçlardaki yük yoğunluğu değişecektir. Yük yoğunluğundaki bu değişiklik uygulanan voltajın yönünde malzemenin boyutlarında bir değişikliğin ortaya çıkmasına sebep olur.Piezoelektrik malzemenin bu özelliğinden faydalanarak geliştirdiğimiz tutucuyu analiz etmek için ANSYS analiz programından faydalandık. ANSYS'de piezoelektrik analiz için gerekli bilgiler ayrıntılı olarak tezin bölümlerinde ele alınmıştır.Sonuç bölümünde ANSYS'de analiz ederek bulduğumuz sonuçlarla, benzer şekilde dizaynlar üzerinde yapılmış çalışmalardan elde edilen sonuçlar karşılaştırılmıştır ve tutucunun kullanılabileceği sahalar belirtilmiştir.
Özet (Çeviri)
The smart structures field has grown rapidly över the last few years. One factor enabling tfais grovvth has been the development of small actuators based on smart materials such as piezoelectrics and shape memory alloys. Piezoelectric actuators are currently more widely used for smart structure applications because they are small, have low-power requirements, and respond quickly. However, commonly used piezoelectric actuators produce either high force coupled with small deflections (stacks) or large deflections coupled with low force (bimorphs). For piezoelectric actuation, this leaves a gap in force-deflection capabilities in which many smart structures applications operate.There exists an ongoing need in the area of robotics and prosthetics for light fast compact actuators. A review of current actuation systems such as electrical, hydraulic, and pneumatic systems that are commonly used for robotic and effectors reveals that these systems are too bulky, too heavy, too slow or too complex for use in lightweight anthropomorphic end effectors and prosthetic devices.Trimmer, defines a device in the range of 2 cm or smaller to be a microactuator. Many microactuator designs exist based upon shape memory alloy materials, electromagnetic materials, magnetic materials, electrostatic materials and piezoelectric materials. These devices were not applicable to the artificial hand problem because either they could not be cotnbined to form a macroactuator, they were too slow or the additional equipment required to power the microactuator was too bulky or heavy. It became clear that the final microactuator would need to have a geometry that allowed it to be combined and also be capable of a direct electrical to mechanical energy conversion.Barium titanate and many other ceramic materials exhibit what is called the piezoelectric efiFect, illustrated schmetically in Figüre 1. Let us consider a sample of a ferroelectric ceramic material which has a resultant dipole moment due to alignment of many small unit dipoles as indicated in Figüre 1-a. In this material there will be an excess of positive charge at one and negative charge at the other end in the direction of the polarization. Now let us consider the sample when compressive stresses are applied, as shown in Figüre 1-b. the compressive stresses reduce the length of the sample between the applied stresses and thus reduce the distance between the unit dipoles, which in turn reduces the overall dipole moment per unit volume of the material. The change in dipole moment of the material changes the charge density at the ends of the sample and thus changes the voltage difference between the ends of the sample if they are insulated from each other On the other hand, if an electric field is applied across the ends of the sample, the charge density at each end of the sample will be changed (Fig. 1-c). This change in charge density will cause the sample to change dimensions in the direction of the applied field. In the case of Figüre 1-c the sample is slightly elongated due to an increased amount of positive charge attracting the negative poles of the dipoles, and reverse at the other end of the sample. Thus the piezoelectric effect is an electromechanical effect by which mechanical forces on a ferroelectric material can produce an electrical response, electrical forces a mechanical response.t ) (r* T111© 6(EKDI(bl© ©©©©0idT© ©© ©e ©TTt t t(a)(c)Figüre 1 : (a) Schematic illustration of electric dipoles within a piezoelectric material. (b) Compressive stresses on material cause a voltage difference to develop due to change in electric dipoles. (c) Applied voltage across ends of sample causes dimensional change and changes the electric dipole moment.The finite element method has become a powerful tool for the numerical solution of a wide range of engineering problems. Applications range from deformation and stress analysıs of automotive, aircraft, building and bridge structures to field analysis of heat flux, fluid flow, magnetic flux, seepage and other flow problems. With the advances in computer technology and CAD systems, complex problems can be modeled with relative ease. Several alternative configurations can be tried out on a computer before the first prototype is built. Ali of this suggests that we need to keep pace with these developments by understanding the basic theory, modeling tecniques and computational aspects of the finite element method. In this method of analysis, a complex region defining a continiuum is discretized into simple geometric shapes called finite elements. The material properties and governing relationship are consıdered över these elements and expressed in terms of unknown values at element corners. An assembly process, duly cosidering the loading and constraints, results in a set of equations. Solution of these equations gives us the approximate behavior of the continuum.A coupled field analysis is one that takes into account the interaction (coupling) between to or more disciplines of engineering. A piezoelectric analysis, for example, handles the interaction between the structurel and electric fields: it solves for the voltage distribution due to applied displacement, or vice versa. Other examples of coupled field analysis are thermal-stress analysis, and electromagnetic analysis.The coupled field element contains ali the necessary degrees of freedom and handles the field coupling by calculating the appropriate element matrices ( matris coupling ) or element load vectors ( load vector coupling ). In linear problems with matrk coupling, coupled field interaction is calculated in one iteration. With load vector coupling, at least two iterations are required to achieve a coupled response. Nonlineer problems are iterative for both matrk and load vector coupling.There are certain advantages and disadvantages inherent with coupled field formulations:Advantages:-Allows for solutions to problems othervvise not possible with usual finite elements.-Simplifies modeling of coupled field problems by permitting one element type to be used in a single analysis pass.Disadvantages:-Increases wavefront (unless a segregated solver is used).-Inefficient matrk reformulation (if a section of matrk associated with one phenomena is reformed, the entire matrk will be reformed).-Larger storage requirements.Piezoelectrics is the coupling of structural and electric fields, which is a naturel property of materials such as quartz and ceramics. Applying a voltage to a piezoelectric material creates a displacement, and the reverse is also true: vibrating a piezoelectric material generates a voltage. Possible piezoelectric analysis types (available in the ANSYS/Multiphysics product only) are static, modal, harmonic, and transient.If a model has at least one element with piezoelectric degrees of freedom (displacement and volt) activated, then ali elements where a volt degree of freedom is needed must be one of the piezoelectric types, and they must ali have the piezoelectric degrees of freedom activated. If the piezoelectric effect is not desired in these elements, simply define very small piezoelectric material properties for them.To do a piezoelectric analysis, you need to use one of these element types: PLANE13 2-D coupled-field quadrilateral solid SOLID5 coupled-field brick SOLED98 coupled-field tetrahedronWe used SOLID5 in our analysis. SOLID5 has a three-dimensional magnetic, thermal, piezoelectric, and structural field capability with limited coupling between the fields. The element has eight nodes with up to six degrees of freedom at each node.The geometry, node locations, and the coordinate system for this element are shown in Figüre 2.Figüre 2 : SOLID5 Coupled Field SolidThe main goal of a finite element analysis is to examine how a structure or component responds to certain loading conditions. Specifymg the proper loading conditions is, therefore, a key step in the analysis. You can apply loads on the model in a variety of ways in the ANSYS program. Also, with the help of load step options, you can control how the loads are actually used during solution.The word loads in ANSYS terminology includes boundary conditions and externally or internally applied forcing functions. Loads are divided into six categories: DOF constraints, forces (concentrated loads), surface loads, body loads, inertia loads and coupled field loads.You can apply most loads either on the solid model (on keypoints, lines, and areas) or on the finite element model (on nodes and elements). For example, you can specify forces at a keypoint or a node. Similarly, you can specify convections (and other surface loads) on lines and areas or on nodes and element faces. No matter have you specify the loads, the solver expects ali loads to be in terms of the finite element model. Therefore, if you specify loads on the solid model, the program automatically transfere them to the nodes and elements at the beginning of solution.In this study we devised a gripper by using one property of piezoelectric material, that is the displacement of the material in the direction of applied voltage (Figüre 3). We used the ANSYS computer program in the design and analysis of the gripper. In the project and ANSYS's catalog, the necessary information exists in detail for the design process.In conclusion, we specified the application area of the gripper by comparing the results found by ANSYS and those retrieved from similar studies in literatüre.
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