Chapter 7 FIXTURE DESIGN AND ANALYSIS -...
Transcript of Chapter 7 FIXTURE DESIGN AND ANALYSIS -...
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Chapter 7
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.
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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
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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 pre-
processor 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.
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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 Z-
Z 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
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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
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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 =
𝐹ℎ𝐿
𝑅 𝑡𝑎𝑛(𝛼 + 𝜑)
(1)
= 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
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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
(2)
Thickness ‘t’ of the clamp for a bolt diameter ‘d’ is
𝑡 = √0.85𝑑𝐴 (1 −𝐴
𝐵)
(3)
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 =𝐹𝑜𝑟𝑐𝑒 𝑜𝑛 𝑏𝑜𝑙𝑡
5 =
𝑃𝑏
5 (4)
where 𝑃𝑏 = 𝐿𝑜𝑎𝑑 𝑜𝑛 𝑏𝑜𝑙𝑡 , 𝑁
d = diameter of bolt, mm
T = Torque on bolt, N-m.
Stress on clamp 𝜎 =𝑀
𝑍 (5)
where M = moment on strap, N-m
Z = Section modulus = (𝑤−𝑐)𝑡2
6 (6)
where W = width of strap, mm
C = width of slot, mm
t = thickness of strap, mm
𝜎 =Stress on clamp, N/m2
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Diameter of bolt d = 1.35√
𝑃𝑎
𝜎
(7)
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.
σ =𝐹
𝐴
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.
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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
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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
Type Part
No
L1
mm
L2
mm
L3
mm
H
mm
W
mm
Min H
mm
Max H
mm
Force
N
Strap 1 0.5 1 3 1.5 0.5 1 12 200
Wing 2 0.3 0.6 1.5 2 0.5 2 10.5 300
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.
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Figure 7.8 Fixture clamp stress distribution with support
Figure 7.9 Fixture clamp stress distribution without support
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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.
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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
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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
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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.
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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
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Figure 7.17 (b) Deformation results of side clamp assembly (fc=10N)
Figure 7.18 Static structural stress analysis of side clamp assembly
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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 6061-T6 aluminum
Locator material type AISI 1144 steel
Young’s modulus, E 70 Gpa
Workpiece material yield strength( σy) 0.17 Gpa
Poisons ratio ( v) 0.35
Clamping force (Fcl 1) 100 N
Clamping force (Fcl 2) 100 N
Clamping force (Fcl 3) 100 N
Clamping force (Fcl 4) 100 N
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.
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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.
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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.
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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
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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
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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).
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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.
Figure 7.26 Load vs deformation
0
0.0005
0.001
0.0015
0.002
100 150 200
De
form
atio
n(m
m)
load (N)
3-3-1 Method
3-2-2 Method
3-3-2 Method
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Table 7.3 Fixture location schemes with deformation values
Fixturing scheme Deformation values(mm)
3-2-1 Method 0.14222
3-3-1 Method 0.00199
3-2-2 method 0.00031
3-3-2 Method 0.00125
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
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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.
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Chapter 8
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 two-
dimensional 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.
8.1 APPLICATIONS OF ASSEMBLY IN A VIRTUAL
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
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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
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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.
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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.
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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.
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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)
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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. Appealing visualizations and
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.
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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.
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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,
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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.
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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
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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
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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.
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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
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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
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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
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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)
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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)
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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).
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8.20.VR SYSTEM SOFTWARE AND HARDWARE
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 TM
is a
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.