附录A外文翻译-------------原文部分
Research on Virtual Assembly System Based on WTK
Abstract: Assembly has been bottleneck of advanced manufacturing techno logy so that virtual as2sembly is the significant solution to that problem. The research of system structure and key technology of virtual assem -bly system p lays a decisive role in the research of virtual assembly. In this paper, the connotation and system structure of virtual assembly system is proposed at first, then the key problems and solutions are demonstrated in details, and finally a case about the virtual assembly of Double-Grade-Involute-Column-Gear2Retarder base d on WTK is cited and discussed.
Key words: virtual reality; virtual environment; virtual assembly;WTK
While the product design and manufacturing techniques develop fast, assembly processing has become the bottleneck of advanced manufacturing technology. In this text the contents and framework of virtual assembly system are introduced at first and then the process of realizing an assembly system is demonstrated through an example “the Virtual Assembly of Double Grade In volute Column Gear Retarder. .
1.The structure of virtual assembly system Traditionally
Virtual assembly system includes such contents as follows: the product assembly modeling subsystem , product assembly sequence planning, the product assembly path planning and real time dynamic collision detection. Furthermore, product assembly modeling subsystem can establish assembly relationship and create assembly models, such as solid models, surface models, framework models by using special model tools in the software or by inserting those existing models made by other modeling tools such as Pro／Engineer; Assembly sequence planning is used to solve the problem of product assembly process sequence and to create assembly process sequence list according to assembly approach or disassembly approach. Assembly movement planning includes assembly path planning and accurate path planning when the part contacts with others during the process of assembling.
Our virtual assembly system , as shown in Fig. 1, is composed by three subsystem saction detection and input subsystem , multi sense synthesize and output system and virtual assembly environment.
Among these three subsystems, action detection and input sub system attain a variety of information from a large number of sensors connected with virtual environment and the exterior real world and then transform the information into the interface of virtual environment. In the sub system , such information will be classified into several types according to its source as follows: input directly signature, environment signature and abnormality signature. In virtual assembly environment, these signatures will be disposed until accurate or fuzzy instructions can be sent in to the virtual world to control or participate intelligently in all kinds of affairs.
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Fig 1 The structure of virtual assembly system
The goal of output sub system is to export multifarious information through many kinds of virtual equipments. For example, sequence planning and path planning and the operation process can be shown dynamically and the active force can be outputted to external devices, such as data gloves with force feedback sensors, to export the force to the human beings who take part in the virtual environment. Virtual assembly environment sub system has the function as follows: receiving and analyzing the information from action detection and input sub system , deciding what to do by using the module of sequence planning and path planning and operation planning, finally sending the messages to output subsystem to instruct assembly machine to fulfill its assembly task automatically in proper order and in proper assembly path.
2.Key problems and solution strategies
In our virtual assembly system , many twisting knots as follows have been untied so that virtual assembly can be carried through resembling the real environment. In another word, virtual environment looks like true, the objects move and behave just like in the real world. 1) The virtual assembly environment modeling
A new technique of combining 2 dimensional picture and 3 dimensional graphics is Used here to build a virtual environment as shown in Fig. 2, therefore the models have two attributes concurrently real time at tribute and true attribute. All the pictures which constitute the environment come from the real world so that customers will feel true and all objects which must be moved in the environment, such as wheels ,are built based on 3 dimensional solid models or surface models so that the object s can be operated in virtual environment.
[2]
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Fig 2 The virtual assembly environment modeling
On the other hand, the reality attribute of operation should be considered. Based on the particularity of virtual assembly, a new management module is app lied, namely, a variety of information from act ion detect ion and input sub system such as the information that the part is grasped or released by the robots, are integrated and classified and a new sequence or path planning is produced and series of new orders is reproduced so that the virtual environment can be driven by physical action and real sense in hearing or feeling or taste about the virtual world can be obtained. 2) Dynamics modeling about assembly process
During the process of assembly, assembly force changes and even the state of objects varies frequently so that the contact state among those objects adjust s real time and then the constraint relationship and constraint force change continuously. During this process, immensenon－linear and transient contact state come to exist so that it is difficult to describe the state with a uniform geometry topology models .Therefore, minimum energy method and grid partition method are usually applied in our virtual environment.
3) Auto assembly sequence planning Considering special assembly objects, assembly sequence planning means seeking most optimal part assembly sequence series .In our virtual assembly, the abovementioned problems have been solved chiefly by present in dynamic flow and visualization vividly in the process of assembly and appending instructive knowledge and intelligent participation during searching. 4) Physically－based assembling
Physically based assembling means to append physical at tributes to the geometry model sand to simulate its state through numerical calculation. In other words , the behavior of objects is confirmed automatically by physics attributes. Physical attributes must be taken in to account as follows:
(1) physical at tribute of single part such as weight, mass, moment of inertia, coefficient of friction and Poisson’s ratio;
( 2 )constraints relationship among parts or sub assembly, such as mating between faces, aligning between line and face or between face and face , orienting a face; (3) flexible distortion of parts or subassembly. Generally speaking, the relation among physics attributes and action state of an object can be described as Newton dynamical expression. Based on the expression, the models of object can translate or rotate or even distort in the virtual world just like in the real world. 5) Interference verification
In order to improve the true and real time and accurate attribute during the process of virtual assembly, interference verification must betaken in to action before it is fixed in the assembly. Two steps of in terference
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verification have been taken in our system .Step 1 Bounding boxes In order to simplify the task of interference verification, a bounding box, bigger than the real object in volume, is used straight forwardly to replace the real object in virtual environment shape. And normally the type we have used is AABB (axis-aligned bounding boxes).Step 2 Accurate detection Some times, when two bounding boxes of two object s interfere but we do not know whether they contact each other or not, there are two accurate detection methods to apply in virtual environment.
1) Based on Boundary Representation Model (B2rep s) , space partition technique is beneficial to reduce the number of part s under checking;
2) Based on Constructive Solid Geometry(CSG) , such as O ct tree interference detection method.
3.Development platform and strategies
The sub subject “the virtual assembly of Double-Grade-In volute-Column-Gear-Retarder ”is accomplished under the support of Hubei province natural science fund subject“Distributed Virtual Design and Manufacturing ”(20011001).The system is developed based on World Tool Kit[ 3 ] , also called WTK, which is a commercial virtual reality tool of SENSE8 Company in United States and a function library constituted by more than 1000 C functions. By invoking these C function s and it s interface with software and hardware, the customers can construct a virtual world where all can be done just like in the real world in behavior and physics attributes. Furthermore, through such virtual reality devices as data gloves and space ball and soon, the interaction between the real world and the virtual world can come into true.
The system of“ virtual assembly of Double Grade in volute- Column-Gear-Retarder ” adopts five layers data structure as shown in Fig. 3.The first layer is hardware layer, including input and output devices such as data gloves, three dimension space balls, stereoscopic glasses, graph accelerate cards, etc. ; The second layer is used as interface for connecting hardware devices and it s main mission is to obtain data from data gloves or to set up parameter about the vision space of both eyes of stereoscopic glasses; The third layer is to build a variety of models—— solid models, framework models, surface models ,which can be established directly in the virtual environment or in other software, such as Auto-CAD, 3D MAX, MDT , Solid Works, Pro∕E , and so on. With the help of such models, we can establish a virtual world w here the behavior and physics attributes can also be embodied; The fourth layer is support tool layer, WTK here;
Fig 3 basic structure of virtual assembly system under W T K
The fifth layer is application layer w here an application is implemented to simulate a real assembly environment.
The application below is built by five layer structures as shown in Fig. 4: virtual assembly of
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Double-Grade- In volute - Column-Gear-Retarder, which shows the operation of closing the box of the gear retarder.
Fig. 4 virtual assembly of Double-Grade- In volute-Column-Gear-Retarder
References
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