中英文翻译几何在机械设计中的作用.docx

1、中英文翻译几何在机械设计中的作用 本科毕业论文（设计）翻译 题 目 新型超声波洗碗机 学 院 制造科学与工程学院 专 业 机械设计及其自动化 学生姓名 学 号 年级 08 指导教师 蔡鹏 教务处制表二O一二年五月二十八日On the Role of Geometry in Mechanical DesignVadim Shapiro Herb VoelckerThe Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York, USAA complete design u

2、sually specifies a mechanical system in terms of component parts and assembly relationships. Each part has a fully defined nominal or ideal form and well defined material properties. Tolerances are used to permit variations in the form and properties of the components, and are used also to permit va

3、riations in the assembly relationships. Thus the geometry and material properties of the system and all of its pieces are fully defined (at least in principle). Henceforth we shall focus on geometry and, for reasons that will become evident, will not deal with materials despite their obvious importa

4、nce.Mechanical systems specified in the manner just described meet functional specifications that appeared initially as design goals. The process of design can be thought of as generating the geometry the breakdown into components with coarsely specified geometry, and then the detailed specification

5、 of the component forms and fitting relationships. Design seems to proceed through simultaneous refinement of geometry and function I. An important line of design research seeks scientific models for this refinement process and systematic procedures for improving and perhaps automating it.At present

6、 we have tools for dealing with two widely separated stages of the refinement process.For single parts, function is usually specified through loads on pieces of surface (e.g. a force distribution over a support surface, a flow rate through an orifice, a radiation pattern over a cooling fin); specifi

7、cation of the solid material that pro-vides a carrier for the pieces of surface may be viewed as a constrained shape optimization process.At the higher level of unit functionality, where one deals with springs, motors, gear boxes, heat exchangers, and the like, geometry usually is abstracted into re

8、al numbers if acknowledged at all, and function is cast in terms of ordinary differential or algebraic equations (for heat flow, motor torque as a function of field current, and so forth).Systems of such equations describe the composite functionalism of networks of functional units. There is a big g

9、ap between these islands of understanding, and intermediate stages of abstraction are needed which acknowledge the partial geometry and spatial arrangement topology of subassemblies. Broadly speaking, geometry is faring badly in contemporary design research; many investigators either sweep it under

10、the carpet ordeal with it syntactically, e.g. through features defined in ad hoc ways. Clearly we need more systematic ways to address the relationship between geometry and function, and we suggest below some initial steps toward this goal.Energy Exchange as a Mechanism for Modeling Mechanical Funct

11、ionMechanical artifacts interact with their environments through spatially distributed energy ex-changes, and we argue below that mechanical functionalism can be modeled in terms of these exchanges. The initial cast of the argument draws heavily on seminal work by Henry Paynter 2.We shall regard mec

12、hanical artifacts as systems that range from single solids or fluid streams, which usually are the lowest level of natural system that exhibit important properties of mechanics, to complex assemblies of solids and streams. A closed boundary, which may be physical or conceptual, is a distinguishing c

13、haracteristic of a system: the sys-tem lies within (and partially in) the boundary, the environment lies outside, and interaction occursthrough the boundary. We distinguish the following:S : the physical system under discussion;8S : the boundary of S;V : a spatial region containing S whose complemen

14、t isthe environment;8 V : the boundary of V.S may coincide with V, and 8S and 8 V are closed surfaces (usually 2-mainfolds) in E 3. We distinguish S from V because S may be partially or wholly un-known (recall that this note is about design) but bound able by a known V. The principle of continuity o

15、f energy applies tall levels of system abstraction. If no energy is generated by the system, thenThe surface integral on the left describes the total energy flux (instantaneous power) through the boundary; P is a generalized Poynting vector describing the instantaneous rate at which energy is transp

16、orted per unit area, and n is the normal at a point in the boundary 8 V. On the right, Oe/Ot is the(volumetric) density of energy stored in the system, and g is the rate of energy loss or dissipation. A system interacts with its environment by ex-changing energy through its physical boundary: for ex

17、ample, by radiating energy stored in the system over a portion of its area, or by providing support to an external mating part and thereby inducing storage of deformation energy in the system. The sub-sets of the physical boundary over which such ex-changes occur will be called (following Paynter) e

18、nergy ports. If s is the physical boundary subset (piece of surface) associated with the it u port, thenThus the total energy flux through the boundary is as um of signed fluxes through the ports. We note that a boundary subset si may belong to several ports, and that body forces, such as those indu

19、ced by gravitational and magnetic fields, may be accommodated by taking S as the associated port.Geometrical and Functional Refinement in the LimitThe left side of Eq. (2a) specifies energy exchanges through the systems ports and requires that the flux vector(s) and port geometries be known. The ter

20、ms on the right cover internal energy (re)distribution and/or dissipation. The physical effects implied by these terms depend on the energy regime(s) and the geometry of the system; there may be rigid body motion, elastic or plastic deformation, temperature redistribution, and so forth. Mathematical

21、 evaluation requires the solution of 3-D boundary- and/or initial-value problems. Very marked simplifications ensue if one assumes that 1) the ports are spatially localized and idealized so that the integrals on the left of Eq. (2a)may be evaluated individually to yield terms Pi, and2) internal ener

22、gy storage and dissipation are similarly localized in disjoint discrete regions, thereby permitting the right-hand integrals to be decomposed into sums of local integrals which may be evaluated individually. With these assumptions, Eq. (2a) may be rewrittenwhere Pi is the power through the discrete

23、port, E is the instantaneous energy stored in the discrete region, and Gk is the dissipation rate in the k discrete region. A limiting form of this refinement(in Paynters terminology-reticulation) is a Dirac-delta limit wherein the ports shrink to spots of zero area and the volumetric regions shrink

24、 to point masses, idealized resistors, and the like. Equation (3) is the basis for Paynters energy bond diagrams, or bond graphs. It describes a sys-tem that may transfer, transform, store, and dissipate energy through elements whose geometry has been refined into a few real numbers-the spatial l po

25、sitions of the discrete ports and lumped regions(which generally are not carried in bond-graph representations), and integral characterizations of the discrete ports and regions (for example the value, in kilograms, of a point mass). This higher view enables one to analyze the dynamics of the ideali

26、zed (discreet) system, but one can deduce little about the geometry of feasible distributed (i.e., real) systems from such analyses; essentially all geometry must be induced. Apparently we have gone too far, i.e., have thrown away too much geometry.Fig. (1) Design of simple bracketToward an Appropri

27、ate Role for Geometry We would like to step back from the limiting refinement just discussed, where all notions of form have been lost, and includes in the problem some continuous geometry-but not the full-blown field problem covered by Eq. (I) unless this is unavoidable. We shall suggest below thre

28、e principles governing the interaction of form and function that we believe will yield geometrically well defined (but not necessarily optimum) designs. A simple but common example drawn from practice-design of a bracketwill motivate the discussion (Fig. 1).The design begins with three holes of know

29、n diameter and configuration that are to be carried by an unknown solid (Fig. la); these mate with other parts (two screws and a pivot pin). Bosses are created to contain the holes (Fig. lb) because of concern about interference with other components passing between the holes. Finally the holes and

30、bosses are bound together into a single part as in Figs. lc and ld, with the final shape being governed by criteria for clearance, strength, weight, and aesthetic and manufacturing simplicity. Two simple but important inferences may be drawn from the example. Firstly, the initial holes(plus some imp

31、lied constraint surfaces in the third dimension) are the brackets energy ports; they are fully specified geometrically and specify by implication what the bracket is to do-maintain the relative position of ports whose geometry admits rotational motion. In principle the associated energy regimes(forc

32、e, torque: elasticity) can be fully specified as well, but in practice they are often only implied or understood. Secondly, the remaining geometry is discretionary but constrained by requirements that the holes be bound into a connected solid, that the solid not interfere with other components, and so forth. We note that, at the single-component level of the bracket, shape optimization usually does require solution of the full 3-D field problem covered by Eq. (2a). Fig (2) position-fixing of the character bracketFrom this example an