外文翻译--浅析CAFD中的自动装夹规划系统

Analysis of automated modular fixture configuration design system RONG Yi ming， LI Jie， MA Wei-dong （ Worcester Polytechnic institute， MA 01609， U. S. A） Abstract To counter the fixture planning of computer-aided fixture design (CAFD) an automated modular fixture configuration design system is developed. Having fixture surface accessibility analysis as the core, the optimum selection of fixturing surfaces and points on workpiece is fullfilled. Firstly, several basic criteria for evaluating the eligibility of a surface being a preliminary candidate fixturing surface are discussed. Secondly, by applying a discretization technique, an accessibility model of fixturing surfaces is established based on an overall evaluation of the accessibility of discrete points on the surface. Finally, the implementation issue and an analysis/design example are presented. Key words computer-aided fixture design； fixture planning； fixturing surface 1 INTRODUCTION Important manufacturing activity in the production cycle. Computer-aided fixture design (CAFD) technique has been developed and become part CAD/CAM integration. The development of CAFD contributes to the reduction manufacturing lead time, optimization of manufacturing operations, and verification of manufacturing process designs. CAFD plays an important role in flexible manufacturing system(FMS) and computer-integrated manufacturing system (CIMS) Figure one outlines the activities of fixture design in manufacturing systems which basically include three major aspects: setup planning, fixture planning, and fixture configuration design. The objective of setup planning is to determine the number of setups needed, the position and orientation of workpiece in each setup, and the machining surfaces in each setup. Fixture planning is to determine the locating, supporting, and clamping points on workpiece surfaces. The tasks of fixture configuration design is to select fixture components and place them into a final configuration to fulfill the functions of locating and clamping the workpiece. An automated modular fixture configuration design system has been developed where when fixturing surfaces and points are selected on the workpiece model fixture units are automatically generated and placed into position with the assistant of fixture component assembly relationships. In the development of automated fixture planning, it is desired that the fixturing surfaces and positions on workpiece be selected automatically. As shown in Figure2, several factors which attribute influences on fixture planning should be taken in- to consideration, ie., workpiece geometric information and operational information need to be extracted and retrieved， accuracy relationships and surface accessibility of workpiece need to be analyzed, fixturing stability and easiness of workpiece loading/unloading operation need to be verified. In this paper, the research focus is on resolving the problem of fixturing surface accessibility analysis. 2 BASIC REQUIREMENTS ON FIXTURING SURFACES Fixturing surfaces are the surfaces on workpiece used to locate and clamp the workpiece where functional fixture components (locators and clamps) are in contact with these surface. As the focus of this research is on analyzing the accessibility of fixturing surface on workpiece, first of all, the study is started with the discussion on the basic requirements for a surface on workpiece to be eligible as a preliminary candidate fixturing surface. In automated fixture planning, once the primary locating direction is determined in setup planning, the accessibility property of each candidate fixturing surface should be assessed so as to help the fixture planning fulfill the optimum selection of fixturing surfaces and point distributions. In this research, the accessibility analysis is investigated briefly on the basis of pure geometric information of the workpiece and its surfaces which can be extracted from the CAD solid model. Other information such as surface finish and tolerance are excluded from concern because they are the factors considered in the accuracy analysis of fixture planning. On a complex workpiece, some surfaces might be obviously ineligible to be the candidate fixturing surfaces and should be filtered out at first. In our current research, only the surfaces which satisfy the following basic requirements can be selected as preliminary candidate fixture surfaces, imachining surfaces,planar surfaces,surfaces with accessible normal directions, and surfaces which are large enough. The main purpose of identifying these requirements is to filtez out those obviously ineligible surfaces on workpiece and assume all re-maining surfaces as preliminary candidate .fixturing surfaces. 2.1 Non-machining Surfaces In a real fixture design, it is well known that the surfaces to be machined at one setup should not be used as fixturing surfaces and hence are definitely inaccessible to any fixture component Therefore a candidate fixturing surface must be a non-machining surface. 2.2 Planar Surfaces The fixturing surface types are commonly divided into the planar surface type and the cylindrical surface type. However, the accessibility analysis approach developed in this research is limited to planar surfaces on workpiece, though the method presented may be applicable to cylindrical surfaces. In many cases, planar surfaces are selected as fixturing surfaces with high priority in fixture design. 2.3 Surfaces with Accessible Normal Direction In most fixtures, the primary locating surfaces is perpendicular to other locating surfaces, which can be defined as the bottom-locating and side-locating form while the common clamping forms are top-clamping and side-clamping. This assumption is especially true when modular fixtures are employed in production. For these locating and clamping forms, a constraint is valid that the side-locating direction n S and side-clamping direction nsc are perpendicular to the bottom-locating direction nL, and the top-clampin direction nTC is negative to the bottom-locating direction, and nsc are all normalized vectors and could be regarded as the accessible directions in one setup. Generally, nBL is always set equal to the primary locating direction which is obtained from setup planning. Unlike curved surfaces, a planar surface on workpiece has a unique normal direction, which is written as n.r. If n.t is not coincident with any accessible directions mentioned above, the surface is regarded inaccessible to fixture components and ineligible to be the candidate fuxturing suface. In another word, the fixturing surface should have an accessible normal direction. 2.4 Surfaces Which are Large Enough It is a common sense of fixture design that the surfaces with too small size or a too slim shape are also inelegible to be the candidate fixutring surface. To roughly determine whether the size of a surface is large enough for fixturing, a simple rule can be applied. The eule states that a surface is eligible in size if the smaller edge length of its bounding rectangle is bigger than a threshold lT. The value of lT is set based on the sized of fixture components used in fixture de-sign, which can be specified and modified by user. After filtering out apparently ineligible surfaces according to above requirements, the remaining surfaces on the workpiece can be regarded as the preliminary candidates of fixturing surface and their accessibility properties needs to be evaluated. 3 ACCESSIBILITY ANALYSIS Fixturing surface accessibility is a vague concept, which is associated with the fixture components used in fixture design. To determine accessible to a regular fixture component whether a candidate fuxturing surface of workpiece is and figure out a numerical value to represent the corre. A. Geometry of the fixturing surface which contains the information of surface area and shape B .Possible obsturctive workpiece geometry along the normal direction of fixturing surface or around the geometric region of fixturing surface. C. The size and shape of functional fixture components. Factor A merely refers to the geometric representation of the fixturing suface. In a feasible fixture design, the selected fixturing points usually locate inside the region of fixturing surface and the contact area between this region and the fixture component should be over half of the area of relevant functional surface of the fixture component. In fact, the accessiblility analysis result should reflect the real effective accessible area of the fixturing surface especially when there exists obstructive workpiece geometry along the normal direction of fixturing surface or around the geometric region of fixturing surface. Factor B also greatly affects the actual accessibility of the fixturing surface because possible obsturctive workpiece geometry along the normal direction of fixturing surface or around the geometric region of the fixturing surface may block the approaching of the fixture component to fixturing surface in some sub-areas of suface region and hence lead to a decrease of the effective accessible area. For a workpiece as shown in Figure4, even though the face F, is large enough in size and not complex in shape, its accessibility to a regular fixture component reduces a lot because of the inherent obstructive geometry of workpiece. It is obvious that the accessibility analysis can not be made without considering the functional sizes and features of fixture components. To obtain a more accurate evaluation of accessibility to guide the later fixture configuration design, factor must be involved into the comprehensive analysis. However, in the real circumstance, before the fixture configuration design is finished, the fixture component selected from the library is unknown at the stage of fixture planning. To circumvent this problem, a least accessing unit size, T, is applied to represent t 卜 e minimum functional size of fixture components, which can be specified and modified by user. It implies that if a fixturing suface is accessible, at least a fixture component with the functionally bounded surface size of Tx T can be placed in contact with the surface. To establish the accessibility model for a fixturing surface, several basic facts of evaluating the property of the accessibility are considered. 1)With the same shape and no obstruction along/around the surface by the workpiece, the surface with larger area will have a higher accessibility value. 2) With the same area and no obstruction, the surface with simple shape complexity will have a higher accessibility value. 3) With the same area and same shape, the surface with less obstruction along or around it will have higher value. In complex gion. It practical accessibility situations, it is very possible that the planar surface of the workpiece has a shape and fully/partially obstruction along its normal direction or around its geometric reis thus required tinned above date surface that a that the accessibility model should reasonably comparable accessibility comprehensively reflect the facts men- value can be applied to every the workpiece no matter how complex the grometry of the surface might can be. A discretization modeling method is preferred since it is generic in principle and the algorithm is easy to implement on computer. The methodology is made up of three steps: 1)The surface is sampled into a set of discrete point, 2) Both individual and neighbor related accessibility of each sample point is assessed, and 3) The overall accessibility of the surface is evaluated based on the results of all sample points. 3.1 Surface Discretization As the accessibility analysis is prior to the fixture planning, the accessibility model between an arbitrary planar surface and fixture component surface is difficult to be established if the fixturing point is undetermined. Before the final position of workpiece on the baseplate is settled down, the possible candidate fixturing points on a planar surface may be enormous in number and hard to handle by a continuous model. Thus in our approach, the surface is sampled into grid-arrayed discrete points with equal interval length T. In order to make the sampling algorithm generic, the outer-bounding rectangle of the surface is used as the sampling region instead of the surface region itself. When a certain set of fixture components are used in fixture design, T can be reasonably selected in terms of the smallest functional surface size of fixture components The outer-bounding rectangle of a planar surface can succinctly information of the exterior shape limitations, and also is very helpful for provide the geometric restraining the sampling region and enabling the sampling algorithm more generic. The rule for extracting the outer bounding rectangle is very simple. For a bottom-locating/top-clamping surface which normal direction ns is identical/negative to two edges of the outer-bounding rectangle must be paralled to X axis and two other edges parallel to Y axis since the bottom-locating direction is identical to the negative vector of Z axis in the workpiece coordinate system, as illustrated in Figure. For a side-locating/clamping surface where ns_n, there must be two edges parallel to Z axis, while the other two edges should be parallel to the cross product of ns and nBL, as shown in Figure Sb. In such a way the surface can be sampled into a set of discrete points within the outer-bounding rectangle, as shown in Figure 6. Some points are sampled outside the outer-bounding rectangle. In this approach, these points are not exorbitant because they may be useful to check the possible obstructions around the fixturing surface. 3.2 Point Accessibility (PA) of Sample Fixturing Point In our model, the surface accessibility is a statistical value based on the Point Accessibility(PA) of every valid sample point. PA consists of two parts: the point Self Individual Accessibility (STA) and the point Neighbor Related Accessibility (NRA). The SIA is mainly corresponding to the isolated accessibility of the fixturing point while the NRA reflects the extended accessibility of the fixturing point. A sample point cab be regarded valid if it is tested to be at least not inaccessible to a fixture component with a functional surface size T x T. The definitions and calculation methods of SIA and NRA are given below. SIA of a virtual sample point is defined on the basis of three attribute tags which are separately assigned by s, representing the position status of a sample point on the surface, representing the obstruction status of the surface in the normal direction at the sample point, ands, representing the contact area matching extent in the text area.The tag of s, is used to indicate whether the square tset grid with a center at current sample. 浅析 CAFD 中的自动装夹规划系统 融亦鸣 ， 李杰 ， 马卫东 （ 伍士德理工学院 ，美国 01609） 摘要 针对计算机辅助夹具设计 (CAFD)中装夹规划问题，开发了自动装夹规划系统，并以工件装夹表面的影响性分析为核心，完成工件上装夹表面及点的优化选择 :讨论了几个用以评佑候选装夹表面的适合性的要点 ； 利用离散技术，基于所有表面离散点 影响性的评佑，建立了装夹表面影响性模型 ； 并给出了系统运行要 。 汽和分析设计实例 。 关键词 计算机辅助夹具设计 ； 装夹规划 ； 装夹表面 1. 简介 生产制造工艺活动在生产周期中很重要。计算机辅助夹具设计工艺已经发展起来并且成为 CAD/CAM 集成中的一部分。计算机辅助夹具设计的发展主要有助于减少机械加工的时间和产生最佳的机械制造方法，还有验证最佳的制造工艺设计。计算机辅助夹具设计在柔性制造系统中和计算机集成制造系统起着很重要的作用。设备的设计在机械加工系统中，计算出主程序数据主要包括下面三方面：客观的装备设计主要决定于零 件的需求，工件的定位分析，每个零件的表面粗糙度。夹具的设计主要取决于工件的定位方式，支撑方式，夹紧位置及工件的表面情况。夹具结构的设备选取和安装设计主要取决于工件的定位和夹紧方式。自动化的模块夹具结构设计系统已经发展到设备表面和位置能随工件的变化而变化。夹具单位自动生成和放到正确的位置和辅助夹具有很紧密的关系。在自动夹具设计发展中，夹具的表面和位置关于工件表面需要自动化。工程师必须考虑到好多影响夹具设计因素，如工件的表面几何形状，加工时候的震动等因素。准确关系和工件表面粗糙度的分析。夹具的稳定性和工件夹装方 式的简单可靠都必须验证。在这篇文章中，就是研究关于自动夹具装夹表面影响的。 很少能找到相应的文学报告。为了工件的装夹方式和验证夹具的设计，一种分析方法已经产生来验证工件安装在夹具上和工件在夹具上的拆卸的容易度。一种从不同角度的方式用来评价夹具设计装夹表面影响性。它限制于一个简单的表面影响性例子。 夹具可包括两方面：夹具屋面的易用性和工件装卸易用性。前者是一种在 混合的 设计中缓解了表面 性质 的评估程度，如下，在 表面 放置一个夹具元件 (定位或夹子 )是很容易的。 con- tact 表面是一 个 非常重要的 选择标 准的 固定准备表面。后者是用 来 判定 当夹 具被 设计 和建造好了，一 个 工件是否能容易得放 进 去。迄今 为 止，在文 献 中 还没 有找到一 种 广泛 应 用的 夹 具可分析方法。在 这 篇文章中， 提出了一 个 准确 评 估 夹 具可表面的方法。 这种 方法只能用在那些 经 常 应 用的 夹 具表面。第二部分， 讨论 了一些 夹 具表面的基本要求。第三部分，提出了一 个 广泛接受的 夹 具表面。一 个 离散化造型方法用 来 判定分析 夹 具表面 发 展的 实 施性 问题和分布特性。最后，一 个 分析的例子被提出 来 了。 2.对夹 具表面的基本要求 夹 具表面是在工件定位和 夹 具工件 时夹 具元件 (定位器的功能及 夹 子 )所接触 的 这 些表面。本 研 究 的焦点是分析 夹 具的 设计 ，卡具表面上的工件，首先，文章以工件表面被初步 评 定 为 “ 合格候 选 人 ” 的 夹 具的表面的基本要求 开 始的，主要定位的方向在 设 置的 规划 中决定了。这些被 评 定 为 “ 合格候 选 人 ” 的 夹 具表面的容易得到的性 质应该 用 来 作 为评 定 满 足 夹 具表面和点分布的最佳 选择 。在 这项研 究中，被接受的分析是在工件的 简 要 纯 几何信息基 础 上的 简 要考察和 从 CAD模型中被提取的表面。另外的信息例如表面的光 洁 和公差是不用考 虑 的，因 为它们 是作 为 精度分析 夹 具的 计划 要考 虑 在 内 的因素。 在一 个复杂 的工件，一些表面明 显 的不合格去成 为 工件表面候 选应 首先被去除。在我 们 最近的 研 究中，只有那些符合基本要求的表面能 够选 作最初的候 选 表面 :1、 未加工的表面 ； 2、 平面表面 ； 3、 接近正常方向的表面 ； 4、 足 够 大的表面。 识别这 些要求的主要目的，挑 选 出那有些明 显 的 资质 的工件表面 并 且假象所有的表面作 为 初步的候 选 人 夹 具的表面。 2.1 未加工的表面 在 真 正的 夹 具 设计 中，那些在装 备 中加工 过 的表面不能作 为夹 具表面是 众 所周知的，因此 绝对 无法 应 用于任何 夹 具元件。因此，一 个 合格的 夹 具表面必 须 是一 个 未加工的 夹 具表面。 2.2 平面表面 夹 具表面的模型 备 分 为 平面表面 类 型和 圆 柱体表面 类 型。然而， 研 究中接受的 类 型只有平面表面 类 型。不 过这种 放 发对 于 圆 柱体表面 类 型 还 是适用的。在一些方案中，平面表面 类 型 选 作 夹 具表面在 夹 具 设计 中 带 有高 优 越性 。 2.3 接近正常方向的表面 在 众 多的 设备 中， 主要定位表面 与 定位表面是相垂直的。定位表面是那些底和 边 被定位的定 义 。通常的 夹紧 形式是 顶夹紧 和 变夹紧 。 当 模 块 化的 夹 具被雇佣来 生 产时 ， 这 些 设 想是尤其正确的。 对 于那些 夹紧 和定位形式，一 个 人 约 束 条件是有效的， 边 定位和 边夹紧 的方向垂直于底定位的方向， 顶夹紧 的方向 与 底定位的方向是相 对 的。 国 家安全委 员会规 定所有的 归 一化向量 并 且能 够 作 为 一 种设备 的可接受的方向。一般的， NBL 总 是 设为 主要的定位方向。不像 弯 曲的表面，平面表面有 独 一无二的正 规 方向， 称为 nr。如果 nr 与 前面提到 的那些被 认 定接受的方向不相符和， 这 些表面就不能成 为 合格的候 选夹 具表面。 换 句 话说 ， 夹 具表面 应该 有一 个 被普遍接受的方向。 2.4 足 够 大的表面 那些太小的和形 状 太 细长 的表面是不能作 为夹 具表面候 选 是 夹 具 设计 的常识 。粗略估 计 一下 这个 表面是否足 够 大去做 为 一 个夹 具表面，一 个简单 的 规则 就出 来 了。 这个规则 表明了，表面的 边 框的 边缘长 度比入口 门槛 大那 它 在形 状 方面是合格的。 IT 的价 值 是建立在 夹 具 设计 的被使用者指定和修改的 夹 具零元件上的。根据前面的要求 过滤显 然无 资 格的表面之后， 这 些剩余的工件表面成 为 初步的合格的 夹 具表面。 并 且 它们 的那些容易得到的性 质 需要被 评 价。 3.表面影响性分析 自动夹具设计装夹表面影响性是一个模糊的理论 ,它与用于设计固定装置的固定装置组成成分紧密相关 。 要确定某普通固定装置 的组成成分是否可行 ,要看该备选零件的表面情况并计算出表面影响性值。以下几家公司设计情况如 下： A 是包含装置表面面积和形状方面信息