Application and evaluation of VR-based CAPP systemQ.Peng *,F.R.Hall,P.M.ListerSchool of Engineering and the Built Environment,University of Wolverhampton,Wolverhampton WV11SB,UKAbstractThis paper proposes an approach to an integrated CAPP/VR system.A desktop VR environment is used to explore the machining process element of CAPP.An architecture of a virtual CAPP system is proposed based on an incorporated VR design interface,which can respond to the dynamic manufacturing process.The key problems under discussion are to ®nd an effective method to quickly capture the 3D information from the manufacturing environment,and to de®ne a uniform and well-organised structure of the program layout for achieving improved sharing of VR resources.Examples of the simulation for cutting process are supplied to demonstrate and verify the proposed CAPP/VR integration system.#2000Published by Elsevier Science B.V .Keywords:CAPP;Virtual reality;CAD/CAM;Engineering simulation1.IntroductionManufacturing competition has acted as a driving force for the application of new technologies in industries.Suc-cessful industrial companies must be able to adapt quickly to fast-changing conditions by the market and competitors within a shorter lead time at a low cost.Although great emphasis has been placed on concurrent engineering,it is still a challenge for engineers to bridge the gap created by the con¯ict between design demands and manufacturing requirements.As a bridge between computer-aided design (CAD)and computer-aided manufacturing (CAM),compu-ter-aided process planning (CAPP)plays an important role in an integrated CAD/CAM system.The aim of process planning is to de®ne in detail the process required for producing a product according to its speci®cations and available manufacturing resources.This can be a very complex process.The selection of a machine tool,cutter,or a material removal process is based on a wide variety of factors,therefore CAPP relies on a good interface between the engineer and the computer system being used.So far,research projects have led to the experimental systems that are strictly limited in their applications.These limitations can be summarised as follows:Failure to process dynamically .Since the manufacturing process is a dynamic system,classical programs are notable to satisfy the demands of current manufacture.Some-times,the new knowledge is required to ensure correct manufacturing,and often time does not allow for this knowledge base to be added to the CAPP system.Separate design and manufacturing process .A process planner receives a design and may complete the process plan separately and remotely from the design engineer.The plan is then sent to the shop floor for manufacturing.Direct communication and interaction between the design and manufacturing process is therefore not encouraged. Poor data collection .As process planning can be highly complex and involves the expertise and intricate relations of knowledge coming from various sources,it is a difficult and time consuming task to represent and formalise the manufacturing knowledge used by a CAPP system.Arti-ficially intelligent (AI)systems,fuzzy logic and neural network technologies have been employed to overcome this difficulty in the following areas [1]:the recognition and processing of machining features; part families,machining cells and group technologies; operation selection and planning;and operation sequence planning.These applications,however,currently work with rather restricted models.They cannot be used without a well formed model.The correct execution of these systems depends on the pre-de®ned knowledge and data provided by human experts.The AI systems readily fail with non-heuristic problems.To overcome these dif®culties,a potential method to assist in the generation of process plans is to supporttheJournal of Materials Processing Technology 107(2000)153±159*Corresponding author.Tel.: 44-1902-322276.E-mail address :pengQ@cc.Umanitoba.CA (Q.Peng).0924-0136/00/$±see front matter #2000Published by Elsevier Science B.V .PII:S 0924-0136(00)00677-4process planner by giving him direct access to the infor-mation about the products,manufacturing resources,and manufacturing methods.A virtual environment is a computer simulator in which the user feels that he or she is immersed within a realistic surroundings and is able to interact with the objects and operations.The virtual environment generated by computer is more affordable and interactive than a traditional simu-lator.Therefore,virtual reality(VR)offers the opportunity to visualise,explore,manipulate,and interact with objects within a computer generated environment,and is a useful tool to enhance CAPP involved in design,development, evaluation,and validation.It is expected that the application of this new technology will bridge the some of the gaps that currently exist.The following parts of this paper evaluate the applications of VR-based CAPP systems,and propose a cost-effective integrated CAPP/VR system aiming towards meeting the current requirement for competitive manufac-turing processes.2.Applications of CAPP and VRCAPP has been a very popular research area for the recent years.Many researches have been working towards the improvement of CAPP systems because of the shortage of experienced process planners and the need for integrated CAD/CAM systems.There are two major traditional approaches to CAPP system:variant and generative methods [2].The variant CAPP system uses an information retrieval procedure based on group technology(GT).All the parts involved in this system are grouped into component families according to their similarity of design and manufacture.This method may be useful for relatively well-established com-panies,since they have relatively stable products.The generative CAPP system uses manufacturing knowledge to generate process plans.This is advantageous for new product development,but it relies on the development of AI, fuzzy logic and neural network technologies as mentioned previously.A conventional CAPP procedure is shown in Fig.1.This process is completed generally by a`serial'approach from the input of part information to the output of the process plan.This procedure cannot meet the needs of concurrent engineering,and will not serve to develop new products within shortened lead times.VR technology has shown its advantages over conven-tional simulation processes where it can aid the understand-ing of sophisticated engineering problems.AVR system can provide not only basic computerised functions,but also an ef®cient way to utilise human sense and interaction to engineering applications in a virtual environment.A number of researchers have employed VR techniques to deal with various aspects and issues in design and manu-facturing.Mine[3]developed a meta-CAD system for virtual environments to take advantage of the natural and intuitive forms of interaction available in a virtual world. Dani and Gadh[4]provided an approach to concept shape designs via a virtual reality interface,whilst Schulz et al.[5] presented their research in developing a virtual environment for car-body engineering applications by creating VRML scenes using the Internet as a communication platform for analysing engineering simulation.The use of VR to support industrial applications is rapidly increasing[6,7].BMW uses VR to explore the visualisation of new car layouts to enhance communication andtheFig.1.An example of a conventional CAPP procedure.154Q.Peng et al./Journal of Materials Processing Technology107(2000)153±159simulation of processes,while Rolls Royce Aeroengines and Associates use VR technology in their early veri®cation of maintenance procedures,training maintenance engineers in these procedures,enhanced visualisation of complex envir-onments and rapid prototyping of plant control and instru-mentation.Several commercial software toolkits,such as dVise [8],Superscape [9],and the Deneb Robotics'package [10]are available to support the creation of virtual environment.These tools provide a VR environment to support existing CAD technology,such as modelling and database construc-tion.A user can generate a synthetic virtual environment which can make the user have the feeling of immersion through a wide variety of VR hardware.Deneb Robotics has developed several VR tools to support machining and robot environments [10].These tools have been used to support this research.Fig.2shows a virtual manufacturing workshop produced by the Deneb Robotics toolkit.Navigation around the VR environment can be performed by using the com-puter mouse and keyboard.A designer can design plan,test and redesign the object and the manufacturing process using VRs specialist input devices.When the design is satisfactory,the information may then be transferred into the CNC machines to generate a real prototype.The applications of VR in process planning can be justi®ed as follows:Process simulation can employ the virtual environment as a real-time,interactive graphical visualisation or inter-face.By using VR,it is hoped that process planners will be able to respond immediately to production demands. Easier user interface.Support via user groups or networks,a very powerful and cost-effective means of process planning will be obtained by combining VR technology and Internet technology.Desktop VR is seen as a tool which can provide ¯exible visualisation and simulation,and provide not only improved design and process planning but also training of engineers and the workforce.3.A virtual CAPP systemA virtual CAPP system can be de®ned as a computer system that can generate the same information about machining process planning as we can observe in a real workshop.A virtual CAPP system has been constructed by the authors based on the Deneb Robotics VR toolkit (see Fig.3).An example of 3D machining simulation of a component is shown in Fig.4.The ®rst step to run the system is to select the machine tool and workpiece blank from the database,and load the blank into the worktable of the machine tool.An interactive process allows the user to either select machine tools from a menu with the mouse button,or to type in or to select an input from data ®les.The position of the machine and blank can be moved or rotated so that the user's viewpoint could be positioned close to the workpiece.The next step is to load the machining process in the form of NC code that is used to machine the workpiece.The machining operations can then be run.The dimensions of the part can be inspected using the system control menu duringoperations.Fig.2.A virtual manufacturing environment produced by Deneb Robotics toolkit [9].Q.Peng et al./Journal of Materials Processing Technology 107(2000)153±159155This system allows the user to create a 3D model of components from an original design,simulate the machining process based on existing NC code,and pass the code to NC machines in the real world to create the real parts.The machining process of a part can be viewed from different directions in a 3D simulation process,such that the user can easily ®nd failures or weaknesses in the process planning.It is therefore a useful tool for either training or for trial run purposes in order to detect any errors that might exist in the process planning.This virtual CAPP system,however,is still a `series'process from top to bottom,as shown in Fig.3,with a VR environment providing a 3D simulator of the existing pro-cess planning.The models of machine tools and workpieces must exist in the database or library,and it cannot support a real-time respond for process planning.A virtual CAPP system ideally would integrate various activities of the manufacturing process,resources,and the virtual environment as shown in Fig.5.It should have the following capabilities:improve the response of real-time process for the CAPP system;enhance the application interface with existing software packages;ease the interaction of user with machines and equipment on the shop floor;flexible control of the shop floor;consider easily uncertainly factors during machining process from multiple viewpoints;bridge the gap between process planning and machining process;andovercome the difficulties of automatic generation of process plan from the component geometric model.Clearly,there are some dif®culties in developing user-based application using existing VR toolkits due to the following reasons:there are obvious differences in the performance of these VR toolkits between their demonstration and the practical applicability,because of the poor capability of integration with other CAD/CAM systems;it takes a time to construct a user-based application because of a relatively inefficient interface for data input and limited modelling capabilities;and it is difficult to support a real-time process.Most existing toolkits,for example,can only simulate pre-de®ned processes,and does not allow the user to interact with the virtual environment in real-time.Issues such as the immediate feedback for the modi®cation of a machining process,and real-time decision making cannot be addressed.Understandably,therefore,research efforts have been made to improve the current CAPP/VRsystem.Fig.3.The process of a virtual CAPP system based on Deneb Robotics VRtoolkit.Fig.4.An example of 3D machining simulation of acomponent.Fig.5.The resources of a virtual CAPP system.156Q.Peng et al./Journal of Materials Processing Technology 107(2000)153±1594.Capturing of user-based environments and modelling of objectsModelling is the most time consuming task in the con-struction of a VR system,and the creation of a virtual environment from separate objects can be daunting.To develop a cost-effective method in building a virtual envi-ronment will save considerable time and allow existing objects to be captured very quickly.Most current VR systems are designed to use CAD modelling tools outside the virtual environment.Feature-based methods are used by most of the current CAD systems.Starting with a simple shape,features are added or modi®ed to re®ne the model.This approach speeds up the model creation process because existing designs can be used as the basis for new models.However,modelling from other CAD tools requires translator programs to convert data from one CAD system to another,because there is not a universal database format available.It therefore takes a signi®cant amount of time to build a new VR environment if an existing CAD database is not available.The3D laser scanner is a popular and accurate tool to recover3D data from an object.However,its capturing process is slow,because it scans the surface line by line, and the high energy light source also has to be treated with care.An alternative tactile approach to3D shape capture measures the surface of an object by the touch of a mechani-cal arm.The relative coordinate positions are determined by the sensors in the joints of the arm,which can be supported by robotic devices.This method is accurate and reliable but the slow speed is a disadvantage.Furthermore,access to all surfaces may be dif®cult.In the electro-magnetic sensor system,a transmitter is driven by a pulsed DC signal.The sensor measures the transmitted magnetic-®eld pulse referenced to the magnetic transmitter.An electronics card controls the transmitting and receiving elements and converts the received signals into3D position and orientation measurements.This method sup-ports real-time measurement with high accuracy,but its disadvantages include the touch tracking process with the wire and sensor,limited captured points from sensors,and the limitation of the magnetic-®eld applied.An image is generally represented in a computer as a 2D array of pixels.Each pixel has a value which represents the image intensity sampled at a point,or a small rectangular picture cell.In a greyscale image,each pixel takes a number of different values,which allows256different intensity levels.Image-based methods are characterised by their non-contact measurement,low energy,and instant capturing of the overall surface data.Therefore,a new method has been used by the authors[11]which integrates the shape from shading and binocular stereo techniques to overcome the weaknesses of the above-mentioned methods.The emphasis of the methodology is to extract the geometric information from a3D object in the mini-mum time and transfer this information into the VR system.It can support the development of the manufacturing environments.5.Manufacturing knowledge representation and acquisitionManufacturing knowledge in process planning is mainly concerned with the machining process of a part,such as the part feature,set-up,tooling,®xture,process de®nition, machining parameter and NC program.It deals with the geometric representation of the parts,form-generating cap-ability of process,and constrained machining knowledge. The feature-based approach can be used to perform the manufacturing knowledge representation.The geometry and topology features of the part are linked by the different machining methods.The system consists of a parametric library of machining features.These features,for example, include:planar,cylinder,hole,slot,pocket,step,notch and ®llet.The hierarchy of the feature-based knowledge model is shown in Fig.6.Cutting parameters can be stored in a database,while the machining process knowledge can be stored in a knowledge base with either the form of the frame, or the rules.A user interface based on the VR environment enables the system to easily acquire,modify or update this knowledge base.The planner can easily construct the shape of the object,position the selected features,generate the operations and arrange the operation sequence.6.De®nition of a uniform and well-organised structure for the virtual environmentA uniform and well-organised structure for the virtual environment can create improved sharing of VR resources. Visualisation of both the product geometry and the physical arrangement of the process planning can help to generate acceptable,accurate and ef®cient process plans.It canalsoFig.6.Hierarchy of the feature-based knowledge model.Q.Peng et al./Journal of Materials Processing Technology107(2000)153±159157increase the degree of customer acceptance and improve the ®nished product.The architecture of an integrated CAPP/VR system is proposed by the authors in Fig.7.It can access directly various resources from a database,a knowledge base,and Internet data during the process planning.The design process of CAPP can be simulated visually and can interact with the designer in real-time.7.Example and discussionFig.8shows an example to simulate a machining process of a part in the system's user interactive ers can operate this window to complete the process plan,for data retrieval,machining simulation and feedback in real-time.The system has the capability to update process data,andtoFig.7.Proposed architecture for an integrated CAPP/VRsystem.Fig.8.An example for simulating a machining process of a part with the system's user interactive window.158Q.Peng et al./Journal of Materials Processing Technology 107(2000)153±159communicate information regarding emergency situations through the user interactive window.This system can overcome many conventional problems that affect the¯exibility and reactivity of the CAPP system. It has the following advantages:it provides the design for manufacturing practice at the early stages of process planning,with immediate feed-back from the integrated CAPP/VR system;dynamic on-line activity reduces the variety of shop floor manufacturing constraints,thereby improving the validity of the process plans;when re-planning,the VR-based tool makes quick deci-sion making possible;andsharing manufacturing resources through Intranet becomes feasible.However,there is clearly still much work required:An evaluation of the method,comparing the output with generalised models derived from industry.Compound features need to be defined in a feature library, including pattern holes,cross-slot,and multi-step.More compound features can be defined by combining primi-tive features through the user interface.Integration of the3D information capturing system and VR environment.Integration of process planning and supply chain issues, which can support order/purchase activities through the Internet.8.ConclusionsVR is becoming an important method to solve engineer-ing problems because of its special simulation features,such as immersion,real-time interaction,and3D graphics.Its further application will not only depend on the development of VR technology,but more on its capability to provide cost-effective solutions to real industrial problems.CAPP normally deals with the complex processes and multi-factors arising from a wide variety of manufacturing environments,and therefore,it relies on a good interface between the engineers and the computer system.The devel-opment and application of VR technology enhances the capability of the engineering simulation process.VR pro-vides a3D virtual environment and can be used in the design and validation of a CAPP system.The VR application of a particular CAPP system will have real potential in the manufacturing environment,however, currently it is dif®cult for user to build using existing VR toolkits,which identi®es the area for future improvements to be made.An integrated CAPP/VR model has been proposed by the authors to improve current CAPP applications. The development of an effective CAPP system will rely upon the application of VR technology and the integration of a wide variety of existing CAD,knowledge base,arti®cial intelligence,optimisation,CAM and internet/network technologies.References[1]L.Monostori,H.Van Brussel,E.Westkampfer,Machine learningapproaches to manufacturing,Ann.CIRP45(2)(1996)675±708.[2]C.McMahon,J.Browne,CAD/CAM Principle,Practice andManufacturing Management,2nd Edition,Addison-Wesley,Reading, MA,1998.[3]M.R.Mine,ISAAC:a meta-CAD system for virtual environments,Computer-Aided Des.29(8)(1997)547±553.[4]T.H.Dani,R.Gadh,Creation of concept shape designs via a virtualreality interface,Computer-Aided Des.29(8)(1997)555±563.[5]M.Schulz,T.Reuding,T.Ertl,Analysing engineering simulations ina virtual environment,IEEE Comput.Graphics Appl.18(6)(November/December1998)46±51.[6]J.R Wilson,S.Cobb,M.D'Cruz,R.Eastgate,Virtual Reality forIndustrial Application:Opportunities and Limitations,Nottingham University Press,1996.ISBN1-897676-573.[7]L.J.Rosenblum,M.R.Macedonia,Projects in VR,IEEE Comput.Graphics Appl.18(6)(November/December1998)6±9.[8]dVise User Manual,Version4.1,Division Ltd.,UK,1997.[9]Superscape web site:///.[10]Deneb Robotics,Inc.,Simulation Software Product,VNC,Quest,Reference Manuals,1998.[11]Q.Peng,M.Loftus,A fast3D modelling approach for virtual realityenvironments,advances in manufacturing technology,in:Proceed-ings of the XII National Conference on Manufacturing Research, Derby,UK,Professional Engineering Publishing Limited,1998, pp.309±314.ISBN1-86058-172-2.Q.Peng et al./Journal of Materials Processing Technology107(2000)153±159159。
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