FIXTURE DESIGN AND ANALYSIS 7.1 INTRODUCTION In a machining process there are different forces acting on the workpieces namely gravitational, inertial, machining and clamping forces. It is essential to find out deformation of workpiece under various loads is a challenging task. Proper positioning of fixture clamp and other fixture units leads to minimal workpiece deformation. This could be achieved by optimal fixture planning. Optimization of fixture assembly plan was explained in the previous chapter.
7.2 IMPORTANCE OF FIXTURE FORCE ANALYSIS The dimensional accuracy mainly depends upon the relative position of the workpiece and machine tool. Therefore it is essential to create new location scheme rather than traditional location scheme. In this research the design of fixture synthesis by means of different fixture location schemes was developed and also examines the work piece deformation based on different load applications. The FEM software package
‘Algor-Nastran’ is used to calculate the deformation of
workpiece under given clamping force specifications.
7.3 FIXTURE SYNTHESIS In view of model representation, the researchers mainly used rigid body model or workpiece-elastic model. It is assumed that the applied loads are concentrated. Few researchers considered elastic deformation of workpiece. In order to find the normal force acting on the workpiece during machining, it is essential to consider 3D Fixture layout. Melkote et al. (2001) presented the Fixture layout optimization problem for a prismatic workpiece. In his research 3-2-1 locating principle was used and Clamping force has been optimized. The optimum fixture layout deformation at initial load step is 0.14282mm. This research work is continued by other researchers and suggested that fixture stiffness can be increased by adding more fixture locators and varying the scheme. Therefore based on various literature reviews it is decided to change fixture scheme. At the same time there is no compromise with location accuracy and constraint satisfaction, fixture schemes have been changed. In addition to 3-2-1 method, other fixture location scheme have been coined namely 3-2-2, 3-3-2, 3-3-1 methods.
7.4 FIXTURE DESIGN PROCESS Fixture designer starts with reviewing earlier design and manufacturing plan which is very essential for computer aided fixture verification system. Determination of locating method and positioning other fixture elements are an integral part of the system. In order to locate the locators locating surface should be identified. Clamping force should be determined before applying against locators.
Figure 7.1 Basic flow chart of fixture design process
Fixture configuration is manly depending on the basis of geometrical shape of the workpiece and clamping device. Figure 7.1 shows the basic fixture design process.
7.5 FEM METHODOLOGY In this research workpiece is considered as homogeneous, isotropic, linear elastic and ductile material. Before performing analysis, it is important to specify the location where the clamp should be placed and define pre-processor requirements such as element type, material properties, and boundary conditions so on. After preprocessor definition, it is essential to give the type of analysis to be performed i.e. static or dynamic. Finally the post processor included the analysis of results.
7.6 FIXTURE ANALYSIS PROCEDURE Step 1: Import the solid model namely workpiece, locators clamps and base plates into FEA editor. Model should be in IGES format. Step 2: Identify the locating point for placing locators. Step 3: ‘Physical relationship’ describes the structural integrity between fixture elements such frictional contact or non-frictional contact. Step 4: ‘Load” includes the force which is acting on the workpiece namely clamping force, machining force and so on. Step 5: ‘Boundary conditions’ describes the constraints applied on the fixture components. Step 6: ‘Mesh’ describes the discretization of fixture structure. Step 7: ‘Solution’ describes the analysis type ie whether static or dynamic. Step 8: ‘Plot results’ describes the output of analysis in the form of stress or deformation. Fixture analysis procedure is depicted by the following flow chart 7.2. Components of fixture assembly are divided for individual analysis. Here each fixture units subjected to structural analysis and the results are a computer generated drawing of the part with the stresses plotted as contours.
Figure 7.2 Flow chart of FEA approach
7.7 POSITION OF CLAMP LOCATION Locating points should be identified before clamping. Locating error leads to manufacturing error. Therefore care should be taken while positioning the locators and other fixture elements. In 3-2-1 locating method, three perpendicular planes will be considered as datum planes in mutually perpendicular directions. An unrestricted object is free to move in any of the twelve directions. It consists of six translational movement and six rotational movements. By placing the workpiece on a three pin base five directions can be restricted namely 2,5,1,4 and 12. To restrict the motion of workpiece around ZZ axis and in direction 8, two more pin type locators are positioned. To restrict in direction 7, a single pin locator is used. The remaining directions are constrained by clamping device. Therefore in 3-2-1 locating method nine planes of movements constrained. The fixture for the part in figure 7.3 illustrates the principle of restricting movement.
Figure 7.3 Degrees of freedom and 3-2-1 locating principle
To locate the workpiece in a fixture without any movement, these movements of workpiece in any twelve degrees of freedom must be stopped. As shown in figure 7.4, consider workpiece is resting on three pins P, Q and R which are inserted through the base of the fixed body. The workpiece as it is resting on pins cannot rotate about XX and YY axes and also it cannot rotate about XX and YY axes and also it cannot move downwards. In this condition workpiece cannot rotate above Z axis and also cannot move in left direction. So by addition of two pins ‘S’ and ‘T’ three degrees of freedom are arrested. By inserting another pin ‘A’ in second vertical face of fixed body, degrees of freedom 9 can be arrested. Now only three degrees of freedom, 10, 11 and 12 are left. These can be arrested or restricted by inserting three more pins. But this will completely enclose the workpiece because of which it’s loading and unloading into the jig becomes impossible. So to avoid this, these three degrees of freedom can be arrested by clamping device. This method of locating workpiece is called “3-2-1” principle or “Six point location” principle.
Figure 7.4 Fixture location points
7.8 ASSUMPTIONS MADE IN FIXTURE DESIGN a) Deformations takes place during machining processes are elastic. b) Contact between locators and clamps should be surface-surface contact. c) High tightening loads in fixture components causes increase in stability and low contact deformation in static surfaces. d) Due to the stability of fixture components, it is necessary to keep eccentricity ratio of clamp should be below 40 percentage.
7.9 IDENTIFICATION OF VALID CLAMP LOCATION Clamp location is important in fixture synthesis. a) It is important to verify there is interference between the clamp and workpiece. b) Height of the clamp should be based on workpiece dimension. c) It should not create any twisting motion during machining process.
7.10.1 CLAMPING FORCE CALCULATION OF SCREW CLAMP Screw clamps are threaded parts with knurled collar, hand knob, tommy bar or spanner flats for rotating and tightening the screw. The clamping area of screw clamp can be increased by the provisions of a pad. The clamping pad is free to rotate on the pivot. This eliminates friction between work piece and pad. The clamping pad remains stationery on the workpiece while screw rotates and rubs on the top face of the pad. A swivel type clamping pad provides a spherical joint between clamping pad and clamping screw. This allows the clamping pad to swivel around the clamping screw. 𝐹𝑠
𝐹ℎ 𝐿 𝑅 𝑡𝑎𝑛(𝛼 + 𝜑) 100×100 = 6.6 𝑡𝑎𝑛(30+17)= 1412N =
Force on the screw clamp = 1412 N where
𝐹𝑠 = Force developed by screw 𝐹ℎ =Pull or push applied to spanner R =Pitch radius of screw thread 𝛼 =Helix angle of thread 𝜑 =Friction angle of thread 100
L =Length of spanner or lever 7.10.2 Clamping for calculations of Strap or plate clamp: These are made of rectangular plates and act like a levers. In a simplest form the clamp is tightened by rotating a hexagonal nut on a central screw. One end of clamp presses against the workpiece and other on the heel pin, thus, loading the clamp like a simply supported beam. 𝑊𝑖𝑑𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑙𝑎𝑚𝑝 𝑊
= 2.3𝑑 + 1.57 Thickness ‘t’ of the clamp for a bolt diameter ‘d’ is (3)
𝐴 𝑡 = √0.85𝑑𝐴 (1 − ) 𝐵 where
𝑑 = 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑜𝑓 𝑏𝑜𝑙𝑡 , 𝑚𝑚 A = Distance between pivot and bolt, mm B = Span, pivot to workpiece, mm W = Width of clamp, mm t = Thickness of clamp, mm
Load on bolt is a function of the toque on the bolt and the diameter of the bolt. Torque on the bolt 𝑇 = where
𝐹𝑜𝑟𝑐𝑒 𝑜𝑛 𝑏𝑜𝑙𝑡 5
𝑃𝑏 = 𝐿𝑜𝑎𝑑 𝑜𝑛 𝑏𝑜𝑙𝑡 , 𝑁 d = diameter of bolt, mm T = Torque on bolt, N-m. Stress on clamp 𝜎 =
M = moment on strap, N-m Z = Section modulus =
W = width of strap, mm C = width of slot, mm t = thickness of strap, mm 𝜎 =Stress on clamp, N/m2
d = 1.35√
Diameter of bolt where
𝜎 = Working stress, N/m2 d = Diameter of bolt, mm 𝑃𝑎 = 𝐴𝑥𝑖𝑎𝑙 𝑙𝑜𝑎𝑑, N
7.11 INTERACTIVE FIXTURE SCHEME EVALUATION Based on basic parameter of screw or strap clamp, clamping force will be calculated. Analysis data for different location scheme is stored in database. Thus the clamping force and the corresponding deformation on different schemes are generated by the software (Java) module. It is necessary to calculate the clamping force of fixture elements because of workpiece deformation caused by the clamps. The stress developed on contact area of cross section which is subjected to deformation is estimated by the following formula.
where F is the clamping force and A is the contact area between clamp and workpiece. If the stress developed on workpiece is larger than the lower yield point of the workpiece, the clamp may cause deformation to the workpiece. Interactive fixture clamping force calculation: The interactive software module was developed for the purpose of calculating deformation of workpiece. The database created from the finite element analysis results. This is integrated with the clamping force calculation. It is easy to determine the clamping force and the corresponding deformation. But the user has to enter the detail required for selecting clamping element and the software module will give the necessary results is show in figure 7.5.
Figure 7.5 Clamping force calculation
7.12 STRAP CLAMP CLAMPING CALCULATIONS As shown in figure 7.6, strap clamp is usually made to at least the same width as the washers under the head of the bolts used to tighten the clamp. The slots are made approximately 1.5748 wider than the diameter of the bolt. Table 7.1 gives the dimensions of strap clamp assembly.
Figure 7.6 Strap clamp FEA model Most strap clamps use the third- class leverage arrangement. The distance between the fastener (effort) and the workpiece should always be less than that between the fastener and the heel pin. This increases the mechanical advantage of the clamp and increases the holding force on the work piece. The chart in table 7.1 lists the
recommended clamping forces for the most commonly used strap clamps. Strap clamp can be operated by either manual devices or power driven devices. Table 7.1 Clamp dimensions
7.12.1. Checking with clamp height: Clamping surfaces must be rigid and capable of holding the part without bending. Bending can distort the machining operation. If the clamping surface can bend, it must be supported. In order to determine candidate clamp location to be valid, the height of the clamp location from the base plate should be within the minimum and maximum height of the clamp type in consideration. Although a candidate clamp location is valid when the part is supported by a base plate, it may become invalid when the part is raised by support or vice versa.
Figure 7.7 Fixture support plan 7.13 Fixture plans with different support: Fixture support also plays an important role in fixture design process. Fixture support is used to give stability to workpiece. It is verified by considering two cases namely fixture plan A (without support) and fixture plan B (with support). Fixture support plan is shown in figure 7.7. From the analysis the equivalent stress (Von-mises stress) and displacement were determined and are shown in figure 7.8 - 7.11.
Figure 7.8 Fixture clamp stress distribution with support
Figure 7.9 Fixture clamp stress distribution without support
Figure 7.10 Fixture clamp displacement without support
Figure 7.11 Fixture clamp displacement with support As shown in figure 7.13, it is inferred from the result of analysis that the base plate with support deform lesser than the base plate without support. To illustrate the effect of support height, base plate deformation versus support height is plotted as shown in figure 7.12.
Figure 7.12 Deformation Vs support height Based on fixture support analysis base plate deformation versus clamping forces is shown in figure 7.13.
Figure 7.13 Base plate Deformation Vs Clamping force
7.14. ESTIMATING CONTACT AREA BETWEEN CLAMP AND THE PART Depending on the contact area and clamping force, the part can be deformed at the clamping location if the stress exceeds the yield strength of the part material. The desirable contact area of the strap clamp and the swing clamp is shown in figure 7.6. The desirable contact area can be modeled as rectangle for fast estimation. While a strap clamp type has a large contact area, a swing clamp type may have small contact area. In actual fixture design, the first step is to analyze whether the clamps are able withstand the applied load or not. If it is not satisfactory it generates a moment tends to tilt or overturn the part. This will change the dimensional accuracy of the workpiece. Strap clamp assembly is constructed first to find deflection and locate the high stress areas. Once the critical stresses are found, a fine mesh model is constructed to get detailed analysis. Figure 7.11 shows the stresses on strap clamp assembly. From the analysis the displacement and equivalent stress (Von-mises stress) were determined and are shown in figure 7.14 and figure 7.15.
Figure 7.14 Strap clamp displacement results
Figure 7.15 Strap clamp stress distribution results
7.15 STRUCTURAL ANALYSIS OF STRAP CLAMP (TOP CLAMP) ASSEMBLY The aim of a structural analysis is to determine the behavior of the material for different loads. It gives us an insight into the various stages the material undergoes and gives us information regarding the distribution of the stress along with an understanding of the areas that are subjected to maximum and minimum stresses. By evaluating these results one can determine the areas for optimization in the design. Also other fixture elements are verified. This enables us to calculate maximum stress on a particular part which is experiencing stress concentration. Load (1500N) is applied at the clamping surface and restrained at the base of a clamp. From figure 7.16 it can be concluded that the pivotal portion of an assembly experiences more stress concentration than other regions. It is highlighted by an arrow mark in figure 7.16.
Figure 7.16 stress distribution of top clamp assembly Analysis of side clamp assembly: The next element that was analyzed was the side clamp assembly of fixture assembly. From the analysis deformations and von- misses stresses were determined and are shown in figure 7.17 (b) – 7.18. Load (10N) is applied at the clamping surface is shown in figure 7.17 (a) and restrained at the base of a clamp. It is inferred from the results of the analysis that no region of the side clamp assembly reaches the red color, which represents the ultimate level. The stress and deformations parameters are within the expected range to perform the task satisfactorily.
Figure 7.17 (a) Load image of side clamp assembly
Figure 7.17 (b) Deformation results of side clamp assembly (fc=10N)
Figure 7.18 Static structural stress analysis of side clamp assembly
7.16 Workpiece and other fixture element specification: The finite element analysis is carried out on 6061-T6 aluminium workpiece material. The element chosen for analysis is Solid 45- 4 node tetrahedral. Table 7.2 provides a list of material properties, workpiece and locator specification. Element type
Solid45 4-node tetrahedral
Workpiece material type
Locator material type
AISI 1144 steel
Young’s modulus, E
Workpiece material yield strength( σy)
Poisons ratio ( v)
Clamping force (Fcl 1)
Clamping force (Fcl 2)
Clamping force (Fcl 3)
Clamping force (Fcl 4)
Table 7.2 Specification of material Finite element analysis in Computer Aided fixture Design (CAFD) environment reduces un necessary trial and error experimentation. Before performing Finite element analysis, fixture unit assembly has to be converted in IGES file format. As shown in figure 7.19, Algor-Nastron has inbuilt material library. So the designer can specify material for FEA model. 7.16.1 Selection of workpiece and other fixture element material: While material selection arises at every stage in the design process, the opportunity for innovation in material selection occurs at the conceptual design stage. As shown in figure 7.19, the initial step of analysis is to specify material for workpiece and locator. The next step is to locate pin on specific locator points.
Figure 7.19 Selection of material properties from material library The first area the designer must consider is location. Locator must be positioned in reference to the part dimensions. Since the part is located by adjustable locators, establishing an accurate set block location is almost impossible. Therefore combination of different location schemes proposed in this research. The location arrangement shown in figure 7.20 (a) satisfies this requirement. The use of duplicate locators should always be avoided. By placing the part on a three-pin base, five directions of movements are restricted as shown in figure. Using pin or button type locators minimizes the chance of error by limiting the area of contact and raising the part above the chips.
Figure 7.20 (a) Selection of Locator positions 7.17 BOUNDARY CONDITION The 3-2-1 principle represents the minimum locator requirement for positioning a prismatic work piece. This method defines a part with three datum surfaces which are perpendicular to each other and constrain a part by: 3 positioning points on the primary datum surface, restricting 4 rotations and 1 translation. 2 positioning points on the secondary surface, restricting 2 rotations and 1 translation. 1 positioning points on the tertiary surface, restricting 1 translation. Figure 7.20(b) shows the surface to be fixed on workpiece and the corresponding boundary conditions.
Figure 7.20 (b) Selection of candidate surface to be fixed Analysis of rectilinear workpiece: Once all the pre-processor details have been selected, perform the structural analysis. From the analysis the equivalent stress (Von-mises stress), reaction forces were determined and are shown in figure 7.19 and figure 7.20.
Figure 7.21 Reaction forces of rectilinear workpiece
Figure 7.22 Stress distribution of rectilinear workpiece
Analysis of cylindrical workpiece: From the analysis the displacement and equivalent stress (Von-mises stress) were determined and are shown in figure 7.23 and 7.24.
Figure 7.23 Static structural deformation of cylindrical workpiece
Figure 7.24 Displacement model scaling 7.18. WORKPIECE DEFORMATION FOR DIFFERENT LOCATION SCHEMES Finite-element method is best suited for predicting an elastic deformation of the workpiece. So ALGOR Nastran software was used to calculate the workpiece elastic deformation. Cohen et al. (1992) described even a same fixturing forces in fixture assembly, force distribution may vary with different location of clamping sequences. The finite element analysis is carried out on on 6061-T6 aluminium workpiece. The loading conditions are assumed to be static. Based on various literature reviews it is decided to change fixture scheme. At the same time there is no compromise with location accuracy and constraint satisfaction, fixture schemes have been changed. It is inferred from the results of the analysis that 3-2-2 locating scheme is best suited for constraining the movement of workpiece and are shown in figure 7.25 (a) – 7.25 (c).
Figure 7.25 (a) 3-2-2 scheme:
Figure 7.25 (b) 3-3-1 scheme
Figure 7.25 (c) 3-3-2 scheme As shown in figure 7.26, it is observed that the 3-2-2 method gives lesser deformation when compared with other locator schemes.
0.0015 3-3-1 Method
3-2-2 Method 3-3-2 Method
Figure 7.26 Load vs deformation
Table 7.3 Fixture location schemes with deformation values Fixturing scheme
As shown in table 7.3, it can be seen that the increase of the fastening force will enhance the fixture unit stiffness and decrease the total deformation. However, large fastening forces may cause other problems such as the wear of fixture components, especially in the case of using modular fixtures. For the prismatic component in the end milling operation the 3-2-2 locating scheme is best suited for constraining the movement of workpiece while machining. It prevents the maximum elastic deformation causes by the clamping force acting on the workpiece. 7.19 CHAPTER SUMMARY
Fixture design is an iterative process which requires extensive knowledge. One of the general procedures of these systems is their ability to produce partial solutions, i.e. the locating and clamping elements for simple prismatic work pieces. Although this is not the only way to perform locating, the so far research has relied on the 3-2-1 locating method, as well as on a complete restraint of the work piece, in spite of the fact that this increases both costs of fixturing and the number of constituent fixture elements.
In this research different combination of fixture scheme is used and the corresponding deformation plotted in the graph as shown in figure 7.20.For the prismatic component in the end milling operation the 3-2-2 locating scheme is best suited for constraining the movement of workpiece while machining. It prevents the maximum elastic deformation causes by the clamping and machining force acting on the workpiece. This fixturing scheme
helps to maintaining the required accuracy and tolerance and surface finish of the finished products.
It can be seen that the increase of the fastening force will enhance the fixture unit stiffness and decrease the total deformation. However, large fastening forces may cause other problems such as the wear of fixture components, especially in the case of using modular fixtures. This fixturing scheme helps to maintaining the required accuracy and tolerance and surface finish of the finished products.
FIXTURE ASSEMBLY IN A VIRTUAL ENVIRONMENT 8.0. INTRODUCTION In a very simple term, Virtual Reality (VR) can be defined as a synthetic or virtual environment that gives a person the illusion of physical presence. For scientists and engineers involved in the computer graphics, VR is just another extension of computer graphics with advanced input and output devices. This definition would include any synthetic environment that gives a person a feeling of ‘being there’. The exposure is, most people have the concept of virtual reality is through reports in the media, science magazines, and science fiction. However the researchers involved in the actual science of virtual reality, the applications are much more mundane and the problems are much more real. This chapter describes the theory to perform a conversion from this twodimensional projection back to three-dimension for better understanding of the real world object. This idea may be applied to various facets of robotic applications varying from daily tasks such as perception and navigation to expert systems like medical image processing, satellite image analysis and so on. Also it describes the creation of such an environment for assembly planning and its integration with CAD methods. This system uses geometry and assembly information from a commercial CAD/CAM system and allows the user to plan and assemble using virtual reality technology.
ENVIRONMENT IN MODERN INDUSTRY The choice of the assembly sequence in which parts or subassemblies are put together in the mechanical assembly of a product can drastically affect the efficiency of the assembly process. Hence an efficient assembly plan, greatly determines lead-time, production cost, and, thus, potential product success. The labour cost for assembly varies between 50% and 75% of the total labour cost for manufacturing the product. Virtual assembly is an important branch of virtual manufacturing and one of the most challenging applications in the virtual reality field. Virtual assembly can help product manufacturing lessen their reliance on physical prototypes. It helps improve the quality and efficiency of assembly and decreases the cost and time of product development. With the help of virtual environment (VE) the virtual components can 122
be effectively and easily to simulate the assembly sequence. In VE designers can consider assembly problems during early stages of product design. In order to implement virtual flexible manufacturing cell, it is necessary to create virtual modeling and virtual factory. Hence virtual assembly plays a major role in modern manufacturing. It supports to create conceptual candidate design and provide accurate processing time, cycle time, costs and quality of products.
8.2 IMPORTANCE OF VIRTUAL FIXTURE IN DESIGN AND ASSEMBLY OPERATIONS Information that is created and maintained within VR systems must be sharable and capable of being applied and utilized by complementary systems such as CAE applications. In the case of assembly planning, this tight integration with other design and engineering systems (e.g., CAD and VR functionality with supporting input and display devices and data exchange) will enable manufacturing engineers to evaluate, determine and select more optimal component sequencing, generate assembly and disassembly plans, make better decisions on assembly methods (i.e., automated or manual assembly) and visualize the results.
8.3 NEED FOR VIRTUAL ASSEMBLY An important goal of designers and creators of computer- aided engineering (CAE) system is the complete integration of design and manufacturing tools. Achieving this type of integration will provide a means to envision, refine and develop products or processes with significant reduction in cost and time to market. Obtaining a true concurrent engineering effort requires a cohesive and comprehensive solution that supports both process and product views. With this in mind, the development of virtual reality techniques which accomplish these goals is highly desirable.
8.4 VIRTUAL PROTOTYPE SYSTEM In order to gain insight into the functioning of a complete virtual assembly design environment (VADE) implementation, two prototype systems were developed in the process of research. These initial prototypes built upon one another to extend the range of knowledge about the desired functionality of this type of application. In the case of these prototypes, the assembly models were generated in CATIAV5R16, as individual parts, subassemblies and subsequently assembled using assembly 123
constraint applied by the CATIAV5R16 interface. No constraint information was implemented except checking the final location and orientations of the part and gripping consist of attaching the part to the fingertip of a non – dexterous hand model. The main concern was the development of methods for transferring data between the solid modelling and the assembler.
8.5 DATA FORMAT FOR VIRTUAL FIXTURE ASSEMBLY Several formats of data transfer were investigated and it was determined that, for this implementation, steriolithograhy files would be the simplest to generate and convert. To perform the translation of fixture parts from its original position to final position the steriolithograhy file format is required. Fixture elements parts color can be define by VRML subroutine program. The final orientation could be performed using VRML transformation. This was done for each part file. Product data exchange standards include those technologies for the access, sharing, exchange, storage and retrieval of product information. The standards range from the simplest and most basic elements (e.g., x-y coordinates) to intelligent formats that define all aspects of product including orientation, appearance, properties, tolerances, materials, weight, cost and delivery information. VRML Languages: Authoring tools such as Worldviz software allows the developer to model the static scene (objects and the scene) at a level that is higher than the implementation level. Nevertheless, they assume that the developer has some knowledge on VR and some programming skills to program behaviors using scripting languages. Figure 8.1 illustrate VRML program interface. The scripting languages used by these authoring tools can change from one authoring tool to another. These object-oriented languages can be used in virtual reality design in conjunction with the following graphic support tools: a) VRML (virtual reality modeling language) b) OPEN GL (low-level graphics library) c) 3D Studio MAX and Other Graphic Design Tools d) True Vision 3D and other rendering engines.
Figure 8.1 Vizard VRML program interface
8.6 STEPS INVOLVED IN VIRTUAL ASSEMBLY Step1: Designer picks up a part from the CAD models library. Step2: Retrieve the assembly cases library to check if there are some similar parts or components that have been assembled in previous applications. If some previous cases are procured, and then skip to Step 6: or go to step 3. Step3: Move the part close to the solid body in the virtual environment. Step4: If a collision happens, check the interfering parts to confirm that an assembly relationship exists. Step5: After determining the assembly relationship between the interfering parts, the designer should adjust the position of the part and reinforce the union. Step6: Judge if the part meets the demands of accurate positioning. Step7: Once the positioning conditions are met, put on assembly forces. Add assembly restrictions to the assembly part, and finish the assembly work at last. As shown in figure 8.2, the various task performed in virtual assembly system includes real time collision detection, neutral assembly model creation and virtual training. 125
Figure 8.2 Flow chart of proposed virtual assembly system
8.7 STEREOPSIS OR STEREOVISION Use of one camera and knowledge of the co-ordinates of one image point allows us to determine a ray in space uniquely. If two cameras observe the same scene point X, its 3D
co-ordinates can be computed as the intersection of two such rays.
This is the basic principle of stereovision that typically consists of three steps: 1. Camera calibration, that is determining the intrinsic parameters of the camera 2. Establishing point correspondences between pairs of points from the left and the right images 3. Reconstruction of 3D co-ordinates of the points in the scene.
8.8 CAMERA CALIBRATION IN STEREOVISION Consider the case of one camera with a thin lens. The plane on the bottom is an image plane on which the object is projected, and the vertical dotted line is the optical axis. Camera calibration in stereovision is shown in figure 8.3.
Figure 8.3 Camera calibrations in Stereovision The lens is positioned perpendicularly to the optical axis at the focal point C (also called the optical center). The focal length f is a parameter of the lens. The projection is performed by an optical ray reflected from a scene point X. The optical ray passes through the optical center C and hits the image plane at the point U. Figure 8.4 shows the coordinate points in Euclidean co-ordinate system. There are four co-ordinate systems use in stereo vision:
World Euclidean co-ordinate system (subscript w)
Camera Euclidean co-ordinate system (subscript c)
Image Euclidean co-ordinate system (subscript i)
Image affine co-ordinate system (subscript a)
Figure 8.4 Coordinate systems
8.9 MODELING THE STATIC STRUCTURE IN VIRTUAL REALITY A concept represents an object type from the application domain that is relevant for the VR application. A concept can have a number of visual as well as non-visual properties, which can be given default values. A concept is graphical represented as a rectangle containing the name of the concept. The properties can be specified using the extended graphical notation.
graphics are very important in the field of VR, therefore it is necessary to allow describing how the objects should be visualized in the virtual world. Similarly for the conceptual specification step, this is done at two levels. In the domain mapping, the designer specifies how the concepts from the domain specification should be visualized by means of VR implementation concepts or existing 3D models is shown in figure 8.5.
Figure 8.5 Conceptual objects in virtual Environment
8.10 MODELING OF COMPLEX OBJECTS Usually, all components of an assembly should keep their own identity and it should be possible to manipulate them or let them behave individually as far as this should be allowed. Human avatar is an example of how the real human in a virtual world should be able to move his arm in the same way that the arm is limited to move for a human being. To model this, we use complex objects. Complex objects are defined using simple and/or other complex objects. They are composed by defining a connecting between two or more simple and/or complex objects. The connected objects are called components. In the virtual world, all components will keep their own identity and can be manipulated individually within the limits imposed by the connection. In VR, in general, different types of connections are possible. The type of connection used, has an impact on the possible motion of the components with respect to each other. Normally an object has six degrees of freedom, three translational degrees of freedom and three rotational degrees of freedom. The translational degrees of freedom are translations along the three axes of the coordinate system while the three rotational degrees of freedom are the rotations around these three axes. Different types of connections will restrict the degrees of freedom in different ways. Therefore it is important to be able to model different types of connections. This is done by means of connection relations. This can be easily understood from figure 8.6. 129
Figure 8.6 virtual Objects in VizardTM The z-coordinate and the depth of the image can be calculated by using stereovision. Thus, a three-dimensional picture of the object can be generated using two cameras. Such a technique, when implemented in robotic science, gives a much more detailed perception of the object and hence improving the quality of vision and intelligence of the robot. Also this research dealt with how conceptual modeling can be implemented by using Virtual Reality and Solid modeling in the VR environment is performed precisely in an intuitive manner through constraint-based manipulations, model modification and assembly modeling in the VR environment.
8.11 CREATION OF VIRTUAL ASSEMBLY Virtual environment has the potential to offer a more natural, powerful, economic, flexible platform than a traditional engineering environment to support assembly planning. This research examines the potential benefits of using VR environments to support assembly planning by comparing the assembly-planning performance of subjects in traditional and VR environments.
8.12 VIRTUAL FIXTURE ASSEMBLY The competition of manufacturing industry emphasizes many companies to reduce product development cost, improve product quality, and shorten the time to delivery of new products. VA technology, due to its development and application, 130
provides an innovative and effective tool to meet the requirement of modern industry. It utilizes VR technology and computer simulation to build a multimode virtual environment (VE) such as hearing, seeing and feeling. Through input and output devices such as data glove and helmet, designers can implement interactively assembly manipulation and process planning. Moreover, they can also verify and evaluate assembly performance to gain an economical, reasonable and practical process planning of a new product. Justify the capability to obtain feasible assembly sequences through an automatic approach based only on contact and interference information between components of a mechanical discrete product, independently of adopted virtual modeling techniques and human intervention. Analysis of assembly information available at early stages of design in virtual model of a product in order to identify reliable information to be used in a systematic methodology based on identification and evaluation of subassemblies. Virtual model of this approach allows obtaining automatically a lower finite number of assembly sequences than theoretical approaches with human intervention, in a faster way to be implemented at early stages of design using virtual model. Assembly process planning needs much experience and knowledge. Especially for large-scale products, we can only obtain the feasible assembly scheme, but not the optimized one. Some intelligent mechanisms should be provided to guide and optimize planning process. Considering the above factors the research work should be concentrated on the following aspects. a) CAD interface standardization. To realize the entire integration between VA systems and CAD systems, we should ensure a unified standard and regulation for data extraction and expression, information storage and management. b) Human activity during assembly process. Human-related factor is one of the most important factors during assembly process. Assembly process modeling considering machining and assembly factors. Most present VA systems based on ideal models do not consider the effect of actual machining and assembly environment on their shape precisions and size errors. So actual product may not be assembled, or assembly performance may not meet the requirements.
c) Tool and fixture design for VA. VA technology should be combined with tools and fixtures to realize the integration. According to requirement of DOF, fixture and precision of the parts, the basic structure of tools and fixtures are obtained for specific design. It can shorten fixture design cycle and improve product design.
8.13 VIRTUAL REALITY TRACKING SYSTEMS The tracking devices are the main components for the VR systems. They interact with the system’s processing unit. This relays to the system the orientation of the user’s point of view. In systems which let a user to roam around within a physical space, the locality of the person can be detected with the help of trackers, along with his direction and speed. The various types of systems used for tracking utilized in VR systems. These are as follows: a) A six degree of freedom can be detected (6-DOF) b) Orientation consists of a yaw of an object, roll and pitch. c) Position of the objects within the x-y-z coordinates of a space; however, it is also the orientation of the object. These however emphasizes that when a user wears a Head Mounted Display (HMD), as the user changes his view from right to left and up to down the view also shifts all tracking system consists of a device that is capable of generating a signal and the signal is detected by the sensor. It also controls the unit, which is involved in the process of the signal and sends information to the CPU. Some systems ask user to add the component of the sensor to the user (or the equipment of the user's). The Tracking devices have various merits and demerits:
Electromagnetic tracking systems – They calculate magnetic fields generated by bypassing an electric current simultaneously through 3 coiled wires. These wires are set up in mutually perpendicular manner to one another. The measurement shows the orientation and direction of the emitter. The responsiveness of an efficient electromagnetic tracking system is really good. They level of latency is quite low. The drawback is that whatever that can create a magnetic field, can come between the signals, which are sent to the sensors 132
Acoustic tracking systems
- This tracking system sense and produce
ultrasonic sound waves to identify the orientation and position of a target. They calculate the time taken for the ultrasonic sound to travel to a sensor. The sensors are usually kept stable in the environment. The efficiency of the system can be affected by the environment as the sound’s speed through air often changes depending on the humidity, temperature or the barometric pressure found in the environment.
Optical tracking devices - These devices use light to calculate a target's orientation along with position. The signal emitter typically includes a group of infrared LEDs. The sensors consist of nothing but only cameras. These cameras can understand the infrared light that has been emitted. The LEDs illuminates in a fashion known as sequential pulses. The pulsed signals are recorded by the camera and then the information is sent to the processing unit of the system. Data can be extrapolated by this unit. Infrared radiation or ambient light are also different ways that can make a system useless.
Mechanical tracking systems – This tracking system is dependent on a physical link between a fixed reference point and the target. One of the many examples is that mechanical tracking system located in the VR field, which is indeed a
Binocular - Omni Orientation Monitor (BOOM) display. A
BOOM display and HMD, is attached on the rear of a mechanical arm consisting 2 points of articulation. The detection of the orientation and position of the system is done through the arm. The rate of update is quite high with mechanical tracking systems, but the demerit is that they limit range of motion for a user.
8.14. VIRTUAL ASSEMBLY MODELING PROCEDURE The aim of virtual assembly is to simulate the assembly process in reality, and makes the virtual assembly process as close as possible to the real one. The whole assembly process is as follows: Initially, an assembly model is constructed inside a CAD system and then the assembly model is transmitted from the CAD system to the virtual environment for interactive assembly evaluation and planning. The imported assembly model in the virtual environment together with the default settings of the virtual environment
describes the initial state. Here, an interface is needed to import the assembly model into the virtual environment. Then the assembly of a part or subassembly inevitably requires some user interactions. Each user operation captured by the system is an input event that changes one or more states and may cause other actions to take place, which ultimately affect the visual, aural and force outputs.
8.15. WORKPIECE ORIENTATION IN VR ENVIRONMENT To obtain a stereoscopic projection, it is essential to obtain two views of a scene generated from a viewing direction corresponding to each eye (left and right). When we simultaneous look at the left view with the left eye and the right view with the right eye, the two views merge into a single image and we perceive a scene with depth.
Figure 8.7 Fixture configurations in a virtual environment Figure 8.7 shows the computer generated scene for stereoscopic projection. Tracking device compute the position and orientation of the HMD device and data glove relative to the object positions in the scene. With this system user can move through the scene and rearrange object positions with the data glove. The scene is then viewed through stereoscopic glasses.
Two coordinate systems are used to represent the position and orientation of the workpiece and the other fixturing towers on the base plate. There are two coordinate systems: the workpiece coordinate system Ow (Xw,Yw,Zw) which is associated with the workpiece and the global coordinate system Og (Xg,Yg,Zg) which is associated with the base plate.
8.16 FIXTURE ASSEMBLY CONSTRAINT A fixture clamp assembly was chosen assembly operation and was subsequently modelled in the CATIA CAD system. Firstly the fixture assembly was performed in the physical world and all of the constraint information used for assembly was noted. The fixture clamp assembly was the modelled in CATIA environment and a detailed description of CAD assembly requirements was created. A detailed analysis of the requirements indicated that comparable operations could be performed in each situation and that the virtual assembler could be used to achieve the desired result of a realistic sequence of assembly operation. Many of the constraints used in actual physical assembly of components are done automatically from our experience. For example, when putting a nut on a screw, the alignment of the axes and the matting of the appropriate planes are taken into account. This intuitive experience was to be of primary importance of a successful implementation of a virtual environment.
Figure 8.8 Fixture clamp assembly in a virtual environment
8.17 MANIPULATING PARTS USING VR To have an intuitive interface between the user and the Virtual Assembly Design Environment (VADE) system, it was desirable to simulate the human hand realistically within the virtual environment. This was accomplished by first employing a virtual technologies cyber glove to measure the bending and abduction of the fingers and the arch of the palm. To simulate, or abstract, a “skin” on the finger, a series of line segments, or “sensors” were attached to each finger of the modelled hand. Two different gripping methods were developed for use with Virtual Assembly. Two-point gripping consists of checking for angular deviance by determining the sign and magnitude of the scalar product of two sensors interacting with the part geometry. With this information, “skill levels” can be established by varying the range of acceptable values for the scalar product. Checking the minimum distance between two of the intersecting segments on different fingers provides a method to verify any tendency of the part to have actual physical rotation in real space. Three point gripping must also satisfy the direction and angular deviance metric of two point gripping. A minimum distance calculation between any two intersecting segments of the three contacting fingers becomes the equivalent of a “free-body” force diagram consisting of a system of forces (line segments) whose moment acting on the system should be near zero for gripping to occur. This will prevent gripping when a part in the real world would have simply rotated and not been gripped. The importance of including constraints and constrained motion in a virtual assembly system makes itself most apparent when assembling an object in a virtual environment without any restrictions. A user can move a part through other objects to their final destination and perform non intuitive assembly operations. Assembly constraints on a part or sub assembly serve no purpose unless the motion of that part or sub assembly is constrained during assembly to imitate the physical world operation.
8.18 OBJECT ORIENTED ANALYSIS OF VIRTUAL ASSEMBLY From the start of this research, it was decided that the VADE system should follow an object oriented design methodology to provide for the flexibility needed to have a robust, expandable system. Based on the object oriented design on the physical world allows the programmer to deal with physical world problems in the programming 136
environment. This lets the programmer use objects such as hands, gloves, helmets or head mounted glasses and assemblies in a logical and intuitive manner.
Figure 8.9 Fixture assembly manipulations If there is any error between matting parts or the model is not in design space, the following message will be highlighted. Here the model “table” could not be recognized. Because of the size of two models were different. After verification again redesign the table model and the export to VR.
Figure 8.10 Error notifications during virtual interpretation
8.19 DESIGN AND IMPLEMENTATION OF VIRTUAL FIXTURE ASSEMBLY An assembly plan is required to accomplish this task. Based on optimal sequence plan generated by using genetic algorithm the plan will be executed. The first task to be accomplished was the object oriented analysis and design of the system. This identified the different areas of investigation and the complete tasks to 137
be implemented. Creation of the virtual environment, including the implementation of stereo- viewing and head tracking, was the first task to be implemented. Once creation of the environment was completed, the graphical representation of the assembly and its parts were imported into the system. To allow the user to manipulate the objects within the virtual environment, gripping and releasing of objects will be performed next. This will not allow the user to perform further due to assembly operation need to be completed. To assemble the parts, the constraints and constraining the motion of the parts were then created. Along with the constraints on assembly, a tolerance for final part placement is needed to compensate the inherent “inaccuracies” of the hardware employed. Unless there is a clear position and tolerance the exact alignment of axes and planes is highly difficult to manipulate. The final step in the development of the system is the recording the trajectory information of the part travels through space to its final location and orientation. This provides the user to verify and evaluate the assembly plan precisely. 8.19.1 VRMLINSPECTOR VRML inspector is used to analyze individual parts features and their attributes such as color, texture, coordinates and so on. Scene graph of fixture elements and assembly are shown in figure 8.11 (a) and figure 8.11(c).
Figure 8.11 (a) VRML scene graph representation (fixture elements)
Figure 8.11 (b) VRML scene graph representation with lighting feature Figure 8.11(b) shows the anaglyphic image of the fixture assembly. We can construct the two views as computer generated scenes with different viewing positions, or we can use stereo camera pair to photograph some object or scene.
Figure 8.11 (c) VRML scene graph representation (fixture assembly)
Figure 8.12 VRML coding for virtual assembly To illustrate virtual assembly approach, it is necessary to write coding for virtual assembly. As shown in figure 8.12, VRML coding acts as a scripting language for execution of virtual assembly. 8.19.2 VRML coding: Import viz #Use the viz module's go #function to render a 3Dworld in a graphics window. viz.go() Use a for loop to . . . for i in range (5 ): #Use the viz module #to add a 3D model. #The viz.add method #will return a node3d object. Lapping Fixture = viz.add (‘strap.wrl’) #Use a node3D method #to place the object in the world. Fixture assembly setPosition (i*.2,1.8,3).
CONFIGURATION VR4 immersive display system was used exclusively, due to weight and usability considerations. Global position and orientation tracking was done by “Ascension of flock of birds” system with the addition of one receiver to track the location and orientation of the left hand for two-handed assembly. VIZARD
software toolkit used for the development of real time 3D graphics, visualization and simulation application.
8.21 CHAPTER SUMMARY
Over many years fixture design and assembly rely on conventional manual drawings and 2D blue prints. Nowadays all manufacturing companies shifted towards virtual manufacturing environment. So creating virtual manufacturing environment requires virtual reality technology.
This chapter describes the way to create virtual fixture assembly in a virtual environment. The efficiency and quality of interactive fixture assembly have been achieved by using virtual reality technology.
Animation and immersive environment allows user to understand the fixture assembly process effectively. This system will provide user with more design alternatives and solutions. However the main drawback of this system is high capital investment.