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实体造型及参数化实体造型(三维参数化建模)

发布时间:2009年06月10日 03:47 来源:建筑信息模型(BIM)网
摘要:

实体造型(Solidmodeling)的概念实体造型(Solidmodeling)是在计算机中对实体对象的三维表示,也就是表示该实体的具有计算机可读性的模型(可在进行计算机处理)。任何产品的形态,都可以看作是由三维几何形构成的组

实体造型(Solid modeling)的概念

实体造型(Solid modeling)是在计算机中对实体对象的三维表示,也就是表示该实体的具有计算机可读性的模型(可在进行计算机处理)。任何产品的形态,都可以看作是由三维几何形构成的组合体。用来描述产品的形状、尺寸大小、位置与结构关系等几何信息的模型称为几何模型。所以,实体造型技术也称为3D几何造型技术。

其他的造型方法包括曲面造型(在动画中广泛使用)和线框模型(所表示的立体有歧义)。

实体造型的主要用途是计算机辅助设计,工程分析,计算机图形学和动画,快速原型(rapid prototyping),医疗测试,和科学研究的可视化(visualization)。

 

相关基本概念

1. 扫掠(Sweeping) 
        把一个元素沿着一条路径"扫出"的一个立体特征叫做面特征。这些体积要么加到物体上("拉伸(extrusion)")要么切除材料("切割路径(cutter path)"). 
        也叫做'基于草图的造型(sketcher based modelling)'. 
        和各种制造技术类似,例如挤压(extrusion),(milling),车削(lathe)等等。

2. 边界表示(Boundary representation) 
        一个立体可以用其边界表面表达,然后填充成为实体. 
        也成为'曲面造型surfacing'。 
        和各种制造技术类似;注模(Injection moulding),铸造(casting),锻造(forging),热塑加工(thermoforming),等等。

3. 参数化体素(也称基元)实例化Parameterized primitive instancing. 
        从一个参数化的体素库中挑出并指定参数得到一个物体 
        例如,螺栓在库中有一个模型,通过修改它的参数集合这个模型可以用于所有螺栓的尺寸

4. 空间占领(Spatial occupancy,或空间枚举) 
        整个空间子分成规则块(cell,或细胞,胞腔),物体通过指定它占据了那些块来表示。 
        这样表示的物体可用于有限差分析 
        这通常是在模型完成之后作的,作为分析软件的预处理的一部分。

5. 分解Decomposition 
        和"空间占据"类似,但是块可以不规则,也不用"预编织". 
        这样表示的模型可以用于有限元分析 
        这通常是在模型完成之后作的,作为分析软件的预处理的一部分。

6. 构造实体几何(Constructive solid geometry). 
        用象并,差,交这样的布尔操作把简单的物体组合起来,通常有树形的等级结构(组合体可以再组合)。

7. 基于特征的造型(Feature based modelling) 
        物体和操作的复杂组合可以作为一个单元一起修改和复制 
        操作的顺序存储在一个树状结构(boolean tree or feature tree)中,参数的改变可以在树中传播(propagate)。

8. 参数化造型(Parameteric modelling) 
        特征的属性被参数化,并给予标签(变量名)而不仅是固定的数字尺寸,整个模型的参数间的关系也记录下来,使得参数值的改变变得更简单。 
        几乎总是和特征联合使用,称为基于特征的参数化造型系统

 

实际应用

1. Parametric Solid modeling CAD

Solid modelers have become commonplace in engineering departments in the last ten years due to faster PCs and competitive software pricing. They are the workhorse of machine designers.

Solid modeling software creates a virtual 3D representation of components for machine design and analysis. Interface with the human operator is highly optimized and includes programmable macros, keyboard shortcuts and dynamic model manipulation. The ability to dynamically re-orient the model, in real-time shaded 3-D, is emphasized and helps the designer maintain a mental 3-D image.

Design work on components is usually done within context of the whole product using assembly modeling methods.

A solid model generally consists of a group of features, added one at a time, until the model is complete. Engineering solid models are built mostly with sketcher-based features; 2-D sketches that are swept along a path to become 3-D. These may be cuts, or extrusions for example.

Another type of modeling technique is 'surfacing' (Freeform surface modeling). Here, surfaces are defined, trimmed and merged, and filled to make solid. The surfaces are usually defined with datum curves in space and a variety of complex commands. Surfacing is more difficult, but better applicable to some manufacturing techniques, like injection molding. Solid models for injection molded parts usually have both surfacing and sketcher based features.

Engineering drawings are created semi-automatically and reference the solid models.

The learning curve for these software packages is steep, but a fluent machine designer who can master these software packages is highly productive.

The modeling of solids is only the minimum requirement of a CAD system’s capabilities.

Parametric modeling uses parameters to define a model (dimensions, for example). The parameter may be modified later, and the model will update to reflect the modification. Typically, there is a relationship between parts, assemblies, and drawings. A part consists of multiple features, and an assembly consists of multiple parts. Drawings can be made from either parts or assemblies.

Example: A shaft is created by extruding a circle 100 mm. A hub is assembled to the end of the shaft. Later, the shaft is modified to be 200 mm long (click on the shaft, select the length dimension, modify to 200). When the model is updated the shaft will be 200 mm long, the hub will relocate to the end of the shaft to which it was assembled, and the engineering drawings and mass properties will reflect all changes automatically.

Examples of parameters are: dimensions used to create model features, material density, formulas to describe swept features, imported data (that describe a reference surface, for example).

Related to parameters, but slightly different are Constraints. Constraints are relationships between entities that make up a particular shape. For a window, the sides might be defined as being parallel, and of the same length.

Parametric modeling is obvious and intuitive. But for the first three decades of CAD this was not the case. Modification meant re-draw, or add a new cut or protrusion on top of old ones. Dimensions on engineering drawings were created, instead of shown.

Parametric modeling is very powerful, but requires more skill in model creation. A complicated model for an injection molded part may have a thousand features, and modifying an early feature may cause later features to fail. Skillfully created parametric models are easier to maintain and modify.

Parametric modeling also lends itself to data re-use. A whole family of capscrews can be contained in one model, for example.

三维CAD的一场革命始于1989年,当时第一个参数化造型系统T-FLEX在PC上发布了。Pro/ENGINEER也于同年发行。现在大部分实体造型系统都是参数化的。

参数化造型用参数来定义模型(例如尺寸)。参数可以以后修改,模型会更新以反映所作的改动。

例子:一个轴通过拉伸一个圆100mm产生。一个毂装配在其尾端。后来,轴改成200mm长(点击轴,选择长度尺寸,改为200mm)。当模型更新的时候轴变成200mm长,毂会重新定位到它所装配到的那个尾端,工程图纸和质量属性会自动反映所有的变化。

参数的例子有:用于创建模型特征的尺寸,材料密度,描述扫掠特征的公式,引入的数据(例如,描述参照曲面的数据)。

参数化造型和明显也很直观。但是CAD的头三十年却不是这样的。修改就意味着重化,或者在旧模型的上面增加新的切割或伸展。工程图纸上的尺寸不是根据模型显示,而是直接出来的。

参数化造型功能强大,但是模型建造过程需要相当的技巧。为一个注模零件制作的复杂模型可能包含上千个特征,修改早先建立的特征可能导致后续特征创建失败。有技巧的创建的参数化模型更易于维护和修改。

参数化模型易于数据重用。譬如一整族的带头螺钉可以从一个模型得到。

2. 医学实体造型

现代计算机断层摄影(俗称CT)和核磁共振扫描仪可以为体内特征构造实体模型

医学实体造型的用途:
        可视化 
        特殊组织的可视化(譬如血管和肿瘤) 
        为快速原型提供实体模型数据(例如,用以辅助外科医生准备困难的手术). 
        把医学实体模型和计算机辅助设计实体模型结合起来(例如,用于设计髋关节替代部件)

3. 娱乐电影

实体造型技术广泛应用于电影动画等娱乐产业。计算机产生的人物的动画是参数化造型的一个例子。佳佳(Jar Jar Binks,星战人物)用一组定位关键肢体位置的参数描述。模型从这些位置开始建造。对每一进行参数修改和模型重建就得到了动画

 

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