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Creation of the constructive concept of model in Rhino 3D
Rhino 3D – popular 3D-CAD for platform Windows from company McNeel. It use at least 150 thousand professionals worldwide which appreciate this CAD the developed possibilities of modelling of surfaces of the free form. The licence for one workplace costs less thousand euro though similar functionality can be found only in the software products standing in 20, and even in 50 times is more expensive. Rhino it is based on an open format of the data 3DM, freely accessible to any programmer for reading/record within the limits of the initiative openNURBS. As a result of such policy of Rajno has more than 120 connected plug-ins created by independent developers. The interface, a set of constructive elements and commands, this system is similar to others CAD that simplifies process of training and allows to solve challenges of 3D-designing easily. However at Rhino there is no one important feature: In this system it is impossible to set desirable behaviour at change of 3D-geometry.

The 3D-model created in majority CAD, it not simply set of 3D-forms; any model usually contains the important information named the constructive concept. This information is used for management of mutual relations between geometrical elements in a detail and between details in assemblage. The constructive concept defines behaviour of model at its change in the future. Usually change of the form or position of one geometrical element in model demands corresponding intellectual changes of its other parts. The constructive concept answers such questions, as “What happens, if diameter of this aperture is changed?” “what will occur at increase in length of this edge?” or “what reaction of the mechanism to moving of this link will be?”

It is known that in Rhino it is possible to create 3D-model, but it is impossible to set its constructive concept.

It is necessary to notice that program architecture Rhino – opened, is given to developers of appendices the developed interface of programming in languages C/C++.

To have possibility of creation of the constructive concept of model, the Russian company LEDAS has developed a tooling for Rhino, united by the general name Driving Dimensions (English the operating sizes). By means of these tools users Rhino can add the constructive concept to any 3D-model and use this information for intellectual editing of its geometry. The first tool in ruler Driving Dimensions for Rhino carries name Rhino Assembly. This connected module works in habitual environment Rhino and allows users of this system to use geometrical restrictions and the operating sizes for intellectual updating of the 3D-assemblages consisting of details with fixed geometry. After installation of plug-in Rhino Assembly users can easily collect difficult mechanisms and analyze their kinematics, considering from the different points of view work of moving parts. Simplicity of user interface Rhino Assembly hides difficult mathematical algorithms lying in its basis for the simultaneous permission of hundreds and thousand geometrical restrictions. Corresponding high technology innovative technologies are developed by company LEDAS since 2001 the Maturity of the given technologies proves to be true their use in numerous systems of classes CAD (the automated designing), CAE (the engineering analysis) and CAM (manufacture preparation), the majority from which are already presented in the market.

Let's consider the task of the constructive concept without a history tree.

Experienced users “mechanical” CAD (MCAD) will find system Rhino interface similar to the tools offered by other serial systems of this class. Following usual procedure, the designer creates volume model: at first draws a 2D-contour, then stretching transforms it into a prism, rounds sharp edges, “cuts out” apertures etc.

There is, however, an essential distinction between Rhino and other MCAD-systems, such as Pro/ENGINEER or SolidWorks. Distinction consists not in how 3D-forms are created, and in how they are edited. For example, in Rhino it is impossible to change diameter of an aperture simple click “mice”. Rhino does not allow users to change radius once the created rounding off. And if you change the 2D-contour underlying a 3D-body it will not affect in any way the form of the last. Users of serial MCAD-systems will be unpleasantly surprised by such behaviour, and, most likely, will refuse use Rhino.

The reason of such behaviour to explain simply: while the majority of MCAD-systems is based on history of construction of model, Rhino does not support such information. Rhino does not try to remember change which the user brings in model. These changes known as history of construction, underlie the task of the constructive concept of model. The construction history is similar to the medical or culinary recipe: when you wish to change something in the model, you at first modify the recipe of its construction, and then CAD-system automatically reconstructs your model according to the changed recipe. Usually such recipe looks like a tree of construction which is realised in the form of a superstructure over “mute” geometry, so-called boundary representation of a firm body (BRep, from English boundary representation).

The task of the constructive concept with the help of history of construction is a platitude almost all modern MCAD-systems though there is an alternative possibility of the specification of desirable behaviour of model in an environment which does not keep construction history. This alternative consists in application of the geometrical restrictions, the operating sizes, the engineering equations and other similar declarative specifications directly between BRep-model elements. (For statement simplification all similar specifications we will name further restrictions.)

Restrictions have not something in common with construction history. They can be added to model at any design stage: when you create your model or when modify it. It is important that restrictions can be added to any “mute” the geometrical model received from the most different sources: loaded of the public catalogue of the 3D-models, imported of other CAD-system, translated in one of neutral formats IGES/STEP, etc. The basic problem with construction history consists that she cannot be added to existing model (it is possible to guess only heuristically its any fragments) as the history is created at model designing “from zero”. Therefore in all situations set forth above the approach on a history basis cannot work is high-grade.

The device of restrictions is conceptually easier than a construction tree because the set of restrictions is disorder. All restrictions are equal among themselves and all of them are satisfied resolved) simultaneously. (In it basic difference from the construction tree which constructive elements are consistently reconstructed each time when you make changes to earlier constructed parts consists.) at any design stage you can remove any of created before restrictions without any influence on other restrictions since the order of the task of restrictions does not play any role. To manipulate the disorder list independent entity is much easier, than a construction tree: you can easily sort and filter the list of restrictions on any attribute: a name, type, arguments, values of parametres.

Restrictions with parametres are called also as operating sizes because they are similar for the usual sizes (lengths, distances, radiuses, corners). A word the managing director in their name specifies that value of parametre operates model geometry, rather the reverse – as in case of the usual sizes (which values are recalculated at geometry change). For example, when you edit value of the operating length, your model automatically updates the geometrical forms to correspond to new value. Parametres can be connected with the free variable and engineering equations that allows to set necessary interrelations between the sizes.

Thus, geometrical restrictions and the operating sizes completely replace with themselves constructive elements at geometry editing. Restrictions represent a powerful way of expression of the constructive concept in an environment without construction history. You can easily add, delete and modify them at any design stage. You can combine them with other means of the specification of engineering knowledge (formulas, rules, optimising algorithms and so forth) In the rest of article we will consider realisation of the given concept executed in company LEDAS within the limits of plug-in Rhino Assembly.

Let's consider designing of assemblages in Rhino with use of geometrical restrictions and the operating sizes.

Designing of assemblages – typical appendix of CAD in which geometrical and dimensional restrictions are used even in systems with construction history. Plug-in Rhino Assembly – the first appendix of ruler Driving Dimensions created by company LEDAS for Rhinoceros. It has been chosen as starting because corresponding functionality is familiar to users of MCAD-systems.

Well-known two approaches, used in MCAD for designing of assemblages: descending and ascending. At the descending approach you begin about mechanism designing “from zero” by turns creating geometry of its parts and placing it according to the constructive concept of your mechanism. Using such approach, you can design simple assemblage in Rhino, for example, model of an one-cylinder internal combustion engine.

But this model by definition will not contain the description of the constructive concept of the modelled mechanism – as well as any model created in Rhino. For example, if you move the piston in the cylinder, your model becomes incorrect while in systems with support of the constructive concept piston displacement conducts to rotation of the cranked shaft connected to the piston by means of a rod and a piston finger. How to achieve such behaviour from Rhino?

Let's present now that parts of your engine undertake from the standard catalogue of 3D-models (or that these details have been designed by you in advance). How to collect from isolated parts the engine (drawing see)? In this case the ascending approach to designing which consists in positioning of each detail concerning others is applied. In system Rhino this approach is extremely labour-consuming, since demands a considerable quantity of manual manipulations.

How to gather isolated parts of the engine?
How to gather isolated parts of the engine?


How to keep the constructive concept at moving of details of the mechanism? How to simplify and accelerate process of ascending designing? Plug-in Rhino Assembly gives definite answers on these questions. The 30-day fact-finding version of this plug-in on a site www is accessible. DrivingDimensions.com.

After installation of a plug-in and start Rhino you will see a new tool ruler in a familiar environment:

Tool ruler Rhino Assembly
Tool ruler Rhino Assembly


By means of this tool ruler you can easily add geometrical and dimensional restrictions between details in assemblage. You can impose restrictions both on already collected engine, and on isolated parts.

Let's consider kinematics modelling.

If you project mechanisms with moving details in Rhino or other package MCAD, you for certain wish to see them in operation. It is very important to analyse trajectories of moving details – still before your mechanism will be constructed in a reality! After all desirable and valid trajectories can not coincide; you can find out interosculation of a part of details each other, or unattainability of desirable limiting positions. In this case you should change mutual position of parts of the mechanism or even to return on level of the task of their geometrical forms and the sizes.

Plug-in Rhino Assembly allows to model kinematics of mechanisms dynamically.




Sources:
1. Ushakov D. Creating the concept of constructive in Rhino 3D//isicad. – 2009 [http://isicad.ru/ru/articles.php?article_num=13469]
2. Modeling tools for designers [www.rhino3d.com]

The author: Челябэнергопроект
Date: 03/23/2010

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