International Journal on Mechanical Engineering and Robotics (IJMER)
________________________________________________________________________________________________
Analysis and Optimization of Robot Gun Support Structure for Welding of Light Weight Vehicle Door Frame
1JatinH. Varma, 2Onkar G. Sonare
1ME-CAD/CAM & Robotics Student, 2Associate Proffesor;
Department of Mechanical Engineering, Datta Meghe College of Engineering, Navi Mumbai Abstract : Robotic welding requires specialized supporting
structure to accurately hold the work piece during the welding operation. Although the large range of welding structure available today then also focus has shifted in building the welding arms more versatile, not the structure. To deal with this issue, we designed and try to analyze welding subsystem (robot gun support structure) with enhanced mobility and bending resistance by without affecting its functionality and application. The new subsystem design reduces complexity and operator mental fatigue while handling and also enhancing its resistance against bending which result longer life span of structure;which allows complex welding operations to be completed on simple two axis welding arms. In addition to that it also has less weight in order to optimize material cost for manufacturing of structure which helps to enhance its mobility while functioning and inducing less bending moment on structure.
Keywords: Robot gun support structure.
I. INTRODUCTION
Word robot was coined by a Czech novelist Karel Capek in 1920. The term robot derives from the Czech word robota, meaning forced work or compulsory service. A robot is reprogrammable, multifunctional manipulator designed to move material, parts, tools, or specialized devices through various programmed motions for the performance of a variety of tasks [1]. A simpler version it can be define as, an automatic device that performs functions normally ascribed to humans or a machine in the form of a human.
In this dissertation work FEA software is using for design and optimization of robot gun support structure.
The structure proposed in this work used for welding of automobile door hinges on large scale on two axis robotic welding arm. The existing structure in current date are having length up to 6500 mm, height 530 mm and width 400 mm which is made of Structural Steel (SA 516 grade70). Hence because of such bigger size it offer resistance to its mobility and responsible to inducing more bending so to enhance those things some modification made in existing structure with same dimension and in addition to that one alternate structure is also suggested in order to optimize cost of manufacturing material of it.
1.2 Problem Definition/ Description
Robot guns are being increasingly employed in automotive manufacturing to replace risky jobs and also to increase productivity. Using a single robot for a single operation proves to be expensive. Hence for cost optimization, multiple guns are mounted on a single robot and multiple operations are performed according to the specifications provided by the customer. But it is impossible to use number of end tool of robot which is moving independently to their work place or work point hence some structure are used on which tool for respective task is located at relatively to each other. So robot only needs to make a moment of that structure to their work place or work point to perform their task.
Hence that structure are supposed to be fail under the inertia forces while making moment against the gravitational force; just because of deflection of structure and induced stress.
1.3 Objectives
1.
To move to multiple locations quickly, the robot moves the structure rapidly causing force in tunes of 1.5 times of gravity. The objective of the work is to analyze the structure for such loads.2.
Optimization of Stress and deformations in the structure.3.
To make weight reduction if possible.1.4 Focus of Present Study
Using a single robot for a single operation proves to be expensive. Hence for cost optimization, multiple guns are mounted on a single robot and multiple operations are performed. Due to which we can
1) Optimize the stress in the structure 2) Reduce the Deformation
3) Reduce the Weight of Support Structure if possible.
4) Replace risky jobs 5) Increase productivity 6) Cost Saving
7) Time Saving 1.5 Limitation of Work
Robot Gun Support Structures are normally fails just because of their moment against the gravitational force.
Because it carrying maximum weight so under the action of inertia and gravitational forces maximum stress are induced in the member of it and those are responsible for the deflection of structure i.e. failure of it. Hence here work is only limited to optimize that stress and deflection value of robot gun supporting structure by making some modification in it with the help of analytical method. Here no any alternative material are going to be suggested for the manufacturing of robot gun structure so the weight of it reduces consequently inertia forces will also reduce.
II. METHODOLOGY
The structural analysis is used to study the stress &
Deformation in the structure. Also, as the focus was mainly on finding the stress & Deformation in the structure & to optimize the stress & deformation to minimum. Nonlinear analysis was performed for material by considering the effect of plasticity just after the stresses in the material crossed the yield point of the
material. The elastic region of the stress versus strain has a slope equal to the modulus of elasticity and the plastic region of the stress versus strain has a slope equal to the strain hardening modulus. Isotropic hardening model was used as it involves yielding the entire yield surface uniformly. The slope of plastic region was defined using tangent modulus which is the instantaneous rate of change of stress as a function of strain. The tangent modulus is useful in describing the behaviour of materials that have been stressed beyond the elastic region. The tangent modulus quantifies the softening of material that generally occurs when it begins to yield, although the material softens, it is still generally able to sustain more load before ultimate failure. All non-linearity’s are solved by applying the load slowly (dividing it into a number of small loads increments). The model is assumed to behave linearly for each load increment and the change in model shape is calculated at each increment. Stresses are updated from increment to increment, until the full applied load is reached. In a nonlinear analysis, initial conditions at the start of each increment are the state of the model at the end of the previous one. At each increment, the solver iterates for equilibrium using a numerical technique such as the Newton Raphson method.
Outline of Procedure Steps
Figure 1 Outlines of Procedure Steps The material used for robot gun support structure is
Structural Steel (SA 516 grade70) and the material properties of Structure under study are as shown in table 1
Table 1 Material Properties [15]
Sr.
No.
Property Value Units
1 Density 7850 Kg/m^3
2 Coefficient of thermal expansion
1.2E-0.5 C^-1 3 Reference
temperature
22 C
4 Young modulus 2E+11 Pa
5 Poisson’s ratio 0.3
6 Bulk modulus 1.6667E+11 Pa
7 Shear modulus 7.6923E+10 Pa 8 Strength coefficient 9.2E+0.8 Pa 9 Strength exponent -0.106
10 Ductility coefficient 0.213 11 Ductility exponent -0.47 12 Cyclic strength
coefficient
1E+09 Pa
13 Cyclic strain hardening exponent
0.2
14 Tensile yield strength 2.5E+0.8 Pa 15 Compressive yield
strength
2.5E+0.8 Pa 16 Tensile ultimate
strength
4.6E+0.8 Pa
Step1: Model a) Geometry
b) Coordinate system c) Connections d) Mesh
Step 2: Static structural a) Analysis setting
b) Acceleration c) Fix support
Step 3: Solution
a) Total Deformation
b) Equivalent Stress
a) Modeling
The modeling is done in ANSYS workbench. A Robot Gun Support Structure was modeled with Circular plates were the guns will be located.
Table 2 Robot Gun Support Structure (Basic Structure)
Parameter Dimensions
Total Length 6500 mm
Height 530 mm
Width 400 mm
Table 3 Modeling Parameters (Provided By the Company as Per Specification)
Cross Section al Areas
Type Lengt
h (mm)
Heig ht (mm)
Radi us (mm)
Thickne ss (mm) ALL
Bars
Hollow Rectangu lar
30 30 - 5
Weld Rectangu lar
38 38 - -
Circula r Plates
Circular - - 90 20
Arm Suppor t
Rectangu lar with Center hole
100 100 30 30
Three dimensional model of robot gun structure are shown in figure 2; which is built as per the specifcation provided by company. After that this structure are testd in four direction (i.e. posstive z and y , negative z and y) as acclerating body with 2 to 1.5 times of accelartion due to gravity aginst there inertia forces. During those analysis thickness of some critical member of structure are increases in order to reduce maximum deformation and induced stress in structure. Which al so help to make structure more rigid while moving from one place to other.
Figure 2 3D Model of Robot Gun Support Structure b)Analysis
1) Direction number: 1(+Y) (1g)
First Robot gun support structure will travel in positive Y direction with acceleration of 9810 mm/s2
Table 4 Base Run Results in positive y direction Equivalent (Von- misses) stress values (Mpa)
Total deformation (mm)
Basic Structure 282.21 15.30
Optimization:
A modification like increase in the thickness of particular critical tube is done, change in cross-section of critical member and finally addition of extra member at critical locations. Numbers of iterations have been carried out for optimizing the thickness and cross section of critical member. Following table shows the modifications made in basic structure.
Table 5 Optimized Structures in positive y direction Optimization
no.
Basic Structure
Optimized Structure
1 Basic
Structure
Add parallel bar at center
2 Basic
Structure
10 mm inward thickness for center long bar.
3 Basic
Structure
10 mm inward thickness for center long bar &
Short bar
4 Basic
Structure
10 mm inward thickness for center long bar , Short bar & one horizontal bar at –x-axis the length is decreased by -15 mm(i.e. increased the length of weld by 15mm =35 mm)
5 Basic
Structure
10 mm inward thickness for center long bar &
13.5 mm for short bar.length of weld 20 mm both sides
6 Basic
Structure
10mm inward thickness for long bar & 13.5 mm for short bar & 5 mm thickness for -x bottom horizontal bar (length of weld 20 mm both sides) mesh one weld size 2.5 mm
7 Basic
Structure
10 mm inward thickness for long bar & 14.5 mm for short bar & 5 mm thickness for -x bottom horizontal bar and one more parallel bar (offset 60 mm +ve x-dir).
(length of weld 20 mm both sides) mesh one weld size 2.5 mm
8 Basic Structure
10 mm inward thickness for long bar & 12.5 mm for short bar (two parallel short bars) &
5mm thickness for -x &
+x bottom horizontal bar and two more parallel bar (offset 60 mm +ve x-direction(length of weld 20 mm both sides) mesh one weld size 2.5 mm
9 Basic
Structure
10 mm inward thickness for long bar & 12.5 mm for short bar (two parallel short bars) &
5mm thickness for -x &
+x bottom horizontal bar and two more parallel bar offset 60mm +ve x- directionlength of weld 20 mm both sides) mesh one weld size 2.5 mm &
long base bar cross section 12.5 mm thickness.
Analysis results for above nine modifications are shown in table 6. From that it is noticeable modification number nine responsible to induces less bending stress in the structure and consequently less deformation of structure.
Table 6 Optimized Structures Results in positive y direction
Optimization no. Equivalent (Von-misses) stress values (Mpa)
Total deformation (mm)
1 360.37 14.82
2 235.99 14.98
3 231.09 14.61
4 233.59 14.17
5 211.11 13.62
6 332.73 13.76
7 183.46 11.56
8 167.17 10.02
9 185.67 9.47
2) Direction number: 2 (+Z) (1.5g)
Now Robot gun support structure will travel in positive Z direction with acceleration of 14715 mm/s2. Optimized Structure no.9 is used only direction and acceleration value changed. Results of analysis of model number nine are shown in table number 7.
Table 7 Stress & Deformation Results in positive Z- direction
Equivalent (Von-misses) stress values (Mpa)
Total deformation (mm)
142.91 8.4663
3)Direction number: 3 (-Z) (2g)
Now Robot gun support structure will travels in negative Z direction with acceleration of 19620 mm/s2. Suggested modification inthe above iteration for negative z direction is shown in table number 8.
Table 8 Optimized Structures in negative Z direction Optimization
no.
Basic Structure
Optimized Structure
1 Basic
Structure
Short bar 12.5mm thickness but consists of two parallel sets. Long bar =12.5mm thickness but consists of two parallel sets mesh method Multizone =2.5 mm body size) Base long bar=12.5mm thickness.Horizontal bar
=5mm thickness.(with two parallel sets offset 60mm)one weld body size
=2.5mmbase long bar mesh
= Multizone
Analysisresult of above modification is shown in table number 9 from that it is noticeable this modification responsible to induced low stress and deformation as occurs during above iteration.
Table 9 Stress & Deformation Results in Negative Z- direction
Equivalent (Von-misses) stress values (Mpa)
Total deformation (mm)
141.5 8.901
4)Direction number: 4 (-Y) (1.5g)
Robot gun support structure will travels in negative Y direction with acceleration of 14715 mm/s2.Suggested modification inthe above iteration for negative z direction is shown in table number 8.
Table 10 Optimized Structures in negative Y direction Optimization
no.
Basic Structure
Optimized Structure
1 Basic
Structure
short bar 12.5mm thickness but consists of two parallel sets ,long bar 12.5mm thickness but consists of two parallel sets(mesh method Multizone 2.5mm body size)base long bar 12.5mm thicknesshorizontal bar
5mm thickness (with two parallel sets offset 60mm)one weld body size 2.5mmbase long bar mesh Multizone
2 Basic
Structure
short bar 12.5mm thickness but consists of two parallel sets ,long bar 12.5mm thickness but consists of two parallel sets (mesh method Multizone 2.5mm body size)Base long bar 12.5mm thickness.Horizontal bar 5mm thickness. (with two parallel sets offset 60mm)one weld body size 2.5mmbase long bar mesh Multizonemiddle bars &
extra horizontal 8mm thickness
3 Basic
Structure
short bar 12.5mm thickness but consists of two parallel sets ,long bar 12.5mm thickness but consists of two parallel sets(mesh method Multizone 2.5mm body size)base long bar 12.5mm thicknesshorizontal bar 5mm thickness.(with two parallel sets offset 60mm)one weld body size 2.5mmbase long bar mesh Multizonemiddle bars 10mm thickness and distance from corner 120mm (i.e. from weld surface)and extra horizontal 8mm thickness Analysis result of above modification is shown in table number 11 from that it is noticeable this modification is not responsible to induced low stress and deformation as occurs during above iteration.
Table 11 Optimized Structures Results in negative Y direction
Optimization no.
Equivalent (Von- misses) stress values (Mpa)
Total deformation (mm)
1 235.08 14.45
2 203.2 12.85
3 201.16 12.18
In all above stage modification like increase in the thickness of particular critical tube is done in addition to that change in cross-section of critical member and finally addition of extra member at critical locations are performed. Under that numbers of iterations have been carried out for optimizing the deflection and induced
stress on structure but up to this point the weight of structure is not decreases considerably but its deflection and induced stress value get optimized. So to reduce the weight of modified structure the length of it can consider only 40 % of its original length and designing the structure such way which result minimum deflection and inducing minimum stress value in the member.
III. CONCLUSION
As the study shows, a Structure can be analyzed for such loads and induced Stress values can be optimize below endurance strength of the material and deformation is reduces up to minimum level. So that Robot gun support structure can move to multiple locations quickly even causing force in tunes of 1.5 times of gravity.
IV. FUTURE SCOPE
Using a single robot for a single operation proves to be expensive. Hence for cost optimization, multiple guns are mounted on a single robot and multiple operations are performed. But it results bending of structure under the inertia forces while moment of it against gravitational force; so to minimize that bending or deflection here suggested to reduce the weight of structure by reducing their length with take caring of induced stress as here length is not a bigger constrain. If length is bigger constrain for the structure then alternative material can alsobe used to reduce the weight of it to perform same activity; in order to achieve quick positioning on work location against the gravitational force. Which may also leads the following advantages.
1. Replace risky jobs.
2. Increase productivity.
3. Save cost.
4. Save time.
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http://www.ansys.com/Products/Workflow+Tech nology/ANSYS+Workbench+Patform/ANSYS+
Meshing/Features/Meshing+Methods:+Hexahedr al