Project Report

ANALYSIS OF
REPEATABILITY OF AN
INDUSTRIAL ROBOTIC
ARM
Mitul Khanchandani
Gaurab Kar
Mentor: Babak Hoseini
Adviser: Sanchoy Das
New Jersey Institute of Technology

A n a l y s i s o f R e p e a t a b i l i t y o f a n I n d u s t r i a l R o b o t i c A r m | 1
ABSTRACT
The Positional Repeatability of a Robotic Arm is a measure of its ability
to move back to the same position and orientation, and place its end point
on the same target point within the work envelope while doing a repetitive
task. One of the main obstacles of robotic applications in Industry is to
minimise positional errors under different working conditions. The
objective of this research is experimental analysis of how the repeatability
performance of a LabVolt 5200 Industrial Robotic Arm is affected by
varying the three factors:
i) Density of points
ii) Path sequence
iii) Speed of movement
Six experiments are designed in increasing order of complexity with
variations of the aforementioned factors. The Robot is programmed to
trace some predetermined points in a grid repetitively. After conducting
the experiment it is observed that the displacements of the traced points
from the target points are slightly different in almost each case. Statistical
analysis is done on the data by performing ANOVA(Analysis of
Variances) and plotting Box Charts to analyse which of the factors affect
positional repeatability.
Keywords: industrial robotic arm, repeatability, ANOVA.

INTRODUCTION
Article I. WHAT IS A ROBOTIC ARM
A robotic arm is a type of mechanical arm, usually
programmable, with similar functions to a human arm; the arm
may be the sum total of the mechanism or may be part of a
more complex robot. The links of such a manipulator are
connected by joints allowing either rotational motion (such as
in an articulated robot) or translational (linear) displacement.
The links of the manipulator can be considered to form a
kinematic chain. The terminus of the kinematic chain of the
manipulator is called the end effector and it is analogous to the
human hand.
Section 1.01 TYPES OF ROBOTIC ARMS
 Cartesian robot / Gantry robot: Used for pick and place
work, application of sealant, assembly operations,
handling machine tools and arc welding. It's a robot
whose arm has three prismatic joints, whose axes are
coincident with a Cartesian coordinator.
 Cylindrical robot: Used for assembly operations,
handling at machine tools, spot welding, and handling at

die casting machines. It's a robot whose axes form a
cylindrical coordinate system.
 Spherical robot / Polar robot (such as the Unimate): Used
for handling at machine tools, spot welding, die casting,
fettling machines, gas welding and arc welding. It's a
robot whose axes form a polar coordinate system.
 SCARA robot: Used for pick and place work, application
of sealant, assembly operations and handling machine
tools. This robot features two parallel rotary joints to
provide compliance in a plane.
 Articulated robot: Used for assembly operations, die
casting, fettling machines, gas welding, arc welding and
spray painting. It's a robot whose arm has at least three
rotary joints.
 Parallel robot: One use is a mobile platform handling
cockpit flight simulators. It's a robot whose arms have
concurrent prismatic or rotary joints.
 Anthropomorphic robot: Similar to the robotic hand
Luke Skywalker receives at the end of The Empire
Strikes Back. It is shaped in a way that resembles a
human hand, i.e. with independent fingers and thumbs.

Article II. REPEATABILITY
The performance of an Industrial Robotic Arm can be
determined by a few vital factors such as: i) Resolution, ii)
Repeatability, iii) Accuracy, iv) Operational speed,
v) Payload ability.
Repeatability of a Robotic Arm is a measure of its ability to
move back to the same position and orientation and place
its end point on the same target point within the work
envelope while doing a repetitive task. Basically,
Repeatability is the ability of the robot to return to a
programmed point. This is because everytime a Robot moves
back to the pre-defined point after a complete cycle, there will
be a miniscule of difference in position from the previous
position.
It may be that when told to go to a certain X-Y-Z position that
it gets only to within 1 mm of that position. This would be its
accuracy which may be improved by calibration. But if that
position is taught into controller memory and each time it is
sent there it returns to within 0.1mm of the taught position then
the repeatability will be within 0.1mm.

Repeatability is not the same as accuracy (Figure 1). Accuracy
can be improved to some extent by calibrating, but the
repeatability cannot be improved by calibration.
Figure 1: Difference between Repeatability and Accuracy.
One of the main obstacles of Robotic Applications in Industry is to
minimise positional errors under different working conditions. Zero
Overshoot becomes a necessity to avoid disastrous collisions with other
parts in the work cell.

Article III. PROJECT OBJECTIVE
The objective of this project is the experimental analysis of
how the repeatability performance of a LabVolt 5200
Articulated Industrial Robotic Arm is affected by varying the
three factors:
• Speed of movement (20%, 50%, 80% of the maximum
speed of the robot)
• Density of points (intra-point distance)
• Path sequence (complexity of the sequence in which the
robot traces the points)
Article IV. SPECIFICATIONS OF THE ROBOTIC ARM (Figure 2 and 3)
Model: LabVolt 5200
Type: Articulated Arm
No. axes of Rotational Freedom: 5 + Gripper
Maximum Load Capacity at arm extension: 4.5kg (10 lb)
Maximum Speed: 584 mm/s (23 in/s)
Motor: DC Servo Motor
Dimensions: 838 x 292 x 425 mm (33 x 11.5 x 16.75 in)
Net weight: 21.6 kg (47.6 lb)

Figure 2: Components of LabVolt 5200 Robotic Arm.
Figure 3: Rotational motion of the different components

Article V. APPROACH TO THE PROJECT
Preparing the Experimental Setup
Programming the Robotic Arm
Performing the Different Experiments
Data Collection
Statistical Analysis of the Data Obtained
Inference and Conclusions
1
2
3
4
5
6

PREPARING THE EXPERIMENTAL SETUP
Article VI. DESIGNING THE EXPERIMENTS
Six experiments are designed in increasing order of
complexity and carried out at different speeds to observe how
variation in the aforementioned factors leads to a change in the
repeatability of the robotic arm.
The six different grids are shown below (Figure 4) with the red
points indicating the points to be traced, and the blue arrows
showing the path sequence.
A B C
D E F
Figure 4: The 6 grids designed to observe the change in Repeatability
due to variation of mentioned factors.

Article VII. SETTING UP THE ROBOT
i. The robot is switched on by turning on the power switch
which is located in the left of the rear of the controller.
ii. The “Robotics” software on the connected computer is
opened and the robot is given some warm-up time.
iii. The robot is then “Hard Homed” (Figure 5). Now it is
ready to be used for experiments.
Figure 5: Hard Homed position of the Robotic Arm.

Article VIII. PROGRAMMING THE ROBOT
i. Using the open and close gripper arrows in the point
editor interface (Figure 6), the gripper is made to hold a
pen / marker with suitable tip.
ii. According to the experiment needed to be performed, the
required grid sheet is placed on the work surface.
iii. The speed of the robotic arm is set as required by the
operator. This can be changed at any time while using the
point editor interface. The range of possible speed values
is 1-99% of the maximum speed.
iv. The robotic arm is then moved manually to each point of
the grid using the arrows of the different components in
the point editor interface.
v. Once the tip of the pen held by the gripper hits the target
point, the point is saved using the save position point
button and is given a suitable name.
Note: For each target point on the grid, an intermediate point, vertically
above the target point is also saved. This is done so that the arm doesn’t
drag the pen along the grid sheet while going from one point to the next.
vi. Thus, using the point editor interface, all the target points
of the grid and their respective intermediate points are
saved.

vii. The points are then transferred to the white section of the
programming window using drag and drop operation and
then the program is written on the blue section using the
following commands (Figure 7):
1. HOME
2. SPEED <1-99>
3. MOVETO <point name>
4. RESTART
5. END
I1
OR1
I2
ORI2
I3
O3
I4
O4
I5
O5
I6
O6
I7
O7
I8
O8
I9
O9
I10
O10
I11
011
Figure 6: The Point Editor interface used to manually take the robot to
the required target and intermediate points.

Figure 7: Programming Window.
 In the screenshot given above OR1, ORI2, O3, O4, O5, O6…
represent the required target points for the experiment and I1, I2, I3,
I4, I5… represent the respective intermediate points.
Note: Only points present in the white section on the right can be used in the
program written in the blue section of the programming window.

KEY EXPERIMENTAL ACTIVITY
Article IX. EXECUTING THE PROGRAMS
i. The program written in the programming window is then
RUN (Figure 8), which instructs the robot to trace the
saved points according to the program sequence,
repetitively.
ii. The robot is made to trace the saved points (Figure 9) 30
times at different speeds for each experiment.
Figure 8: Run Task window-to Run, Pause or Stop the program
execution.
Figure 9: Pen held by the gripper tracing the grid points during program
execution.

DATA COLLECTION
Article X. MEASURING THE ERRORS
i. The center to center distance between the actual target point
of the grid and the point traced by the robot is measured
(Figure 10) for each point of the 30 trials of all experiments.
A dial caliper is used (Figure 11).
ii. The data is tabulated in an excel file, for further calculation
and analysis (Figure 12).
Figure 10: Measuring the error (X1i). Here the red point indicates the
target position and the blue point indicates the point traced by the
robotic arm.
Figure 11: Dial Caliper - used to measure the error.

Article XI. DATA CALCULATION
i. Repeatability is calculated using the formula:
 R.M.S.E =
where,
N= total no. of readings
l
i
= centre distance between the traced point and target point
ī= average of l
i
for N readings
ii. The R.M.S.E values are calculated row-wise, i.e. for all
points of one trial of an experiment. The same is repeated
for all thirty trials of all experiments.
Figure 12: Table for Experiment A - Speed 20, showing the calculated
RMSE values for each of the thirty trials of the experiment.

STATISTICAL ANALYSIS
Statistical Analysis is performed after Data Collection. Minitab software
is used to perform ANOVA (Analysis of Variances) and draw Box Plots
to find out how the three factors affect Repeatability (Figure 13).
Figure 13: Sequence of Analysis.
PLOTTING
•Box Plots are drawn to represent the data.
ANOVA
•Analysis of Variances is performed on
different data sets.
INFERENCE
•Conclusions are drawn based on ANOVA and
study of Box plots.

Article XII. VARIATION WITH SPEED
Using Minitab, the Box plots are drawn, taking the R.M.S.E
values along the vertical axis (Figure 14). The red box charts
represents data for Experiment A, the blue box represents data
for Experiment B, the green box represents data for
Experiment C and the yellow box represents data for
Experiment D all performed at all three speeds.
Figure 14: BOX PLOTS of Experiments A, B, C & D at 3 different speeds.
A80A50A20
0.10
0.09
0.08
0.07
0.06
0.05
0.04
RootMeanSquareError
Experiment A - Variation with Speed
B80B50B20
0.16
0.15
0.14
0.13
0.12
0.11
RootMeanSquareError
Experiment B - Variation with Speed
C80C50C20
0.185
0.180
0.175
0.170
0.165
0.160
0.155
RootMeanSquareError
Experiment C - Variation with Speed
D80D50D20
0.27
0.26
0.25
0.24
0.23
0.22
0.21
0.20
0.19
RootMeanSquareError
Experiment D - Variation with Speed

Figure 15: ANOVA Data for all A, B, C and D at all 3 speeds.
From the Box Plots and ANOVA (Analysis of Variance) data (Figure 15),
we can see that the R.M.S.E value changes with change in speed of
movement of the Robotic Arm.
The obtained P values are:
1. Experiment A: 0
2. Experiment B: 0
3. Experiment C: 0.022
4. Experiment D: 0
Since the ‘P’ value for all the 4 experiments is below 0.05, we can say that
SPEED OF MOTION is an important factor and significantly affects
the REPEATABILITY of the Robotic arm.

Article XIII. VARIATION WITH POINT DENSITY
To see the variation of the R.M.S.E values with the change of
point density, we make box plots of experiments A, B, C & D
at the different constant speeds. The data is then analyzed
using ANOVA (Figure 16, 17, 18).
Figure 16: Speed 20 - Experiments A, B, C & D Box plots and ANOVA.
D20C20B20A20
0.25
0.20
0.15
0.10
0.05
RootMeanSquareError
Speed 20% - Variation with Point Density

Figure 17: Speed 50 - Experiments A, B, C & D Box plots and ANOVA.
D50C50B50A50
0.25
0.20
0.15
0.10
0.05
RootMeanSquareError

Figure18: Speed 80 - Experiments A, B, C & D Box plots and ANOVA
From the Box Plots and ANOVA (Analysis of Variances) data we can say
that the RMSE value increases with increase in point density for all
constant-speed values. Thus, Repeatability decreases with increase in
point density.
D80C80B80A80
0.225
0.200
0.175
0.150
0.125
0.100
0.075
0.050
RootMeanSquareError

Article XIV. VARIATION WITH POINT DENSITY AND SPEED
To see how both the above factors affect the repeatability we
plot the R.M.S.E. values for Experiments A, B, C & D together
at all three speeds, i.e. at speeds 20, 50 and 80 (Figure 19).
Figure 19: Box Plot showing the variance of RMSE value with change in both
speed and point density for experiments A, B, C & D.
D80D50D20C80C50C20B80B50B20A80A50A20
0.30
0.25
0.20
0.15
0.10
0.05
RootMeanSquareError
Variation with Speed and Point Density

Article XV. VARIATION WITH PATH SEQUENCE
The Box plots are drawn, taking the R.M.S.E values along the
vertical axis.
 The yellow box represents data for Experiment D.
 The orange box represents data for Experiment E.
 The pink box represents data for Experiment F
All performed at 50% speed (Figure 20).
Figure 20: Box Plot showing Variation with Path Sequence at 50%
speed for experiments D, E and F.

Figure 21: ANOVA data for experiments D, E & F performed at 50% of maximum
speed.
From the Box Plot (Figure 20) and ANOVA data (Figure 21) we can say
that:
Number of turns and angle of turn in the path of motion affects the
R.M.S.E values. Greater number of turns and a larger angle of turn
lead to a decrease in error and in turn increases the positional
repeatability.
A quantification of the above conclusion can be done in the following
way:

No. of turns and angle of each turn for the three experiments are:
 Experiment D: 10 turns, 270° each turn.
 Experiment E: 10 turns, 348.7° each turn.
 Experiment F: 18 turns, 315° each turn.
If 90° = 1 turn unit,
 270° = 3 units
 348.7° = 3.87 units
 315° = 3.5 units
Thus the total turn units for the three experiments are:
 Experiment D: 30 turn units. (10 x 3)
 Experiment E: 38.7 turn units. (10 x 3.87)
 Experiment F: 63 turn units. (18 x 3.5)
As the total turn units increase, the error involved decreases. Thus, the
R.M.S.E values are in the given order:
D50>E50>F50
Thus, both, a greater number of turns and a larger angle of turn lead to a
decrease in the error produced by the robotic arm.

Article XVI. FUTURE SCOPE OF WORK
 Further experiments can be conducted at different speeds to
find out the exact nature of the relationship between
Repeatability and Variation in Speed.
 New experiments can be designed with different path
complexities to further analyse the relation of Repeatability
and Variation in Path Sequence.
 Experiments can be designed to find out how other factors
like distance from the base, load etc. affects Repeatability.
Article XVII. ACKNOWLEDGEMENT
 Dr. Sanchoy Das
 Babak Hoseini
 Dr. Durga Misra

Article XVIII. REFERENCE
 Ahmad Rasdan Ismail, Azmi Hassan, Syamimi Syamsuddin,
Mohd Zaki Nuawi, Shahrum Abdullah, Hairunnisa Mohamad
Ibrahim, “The Repeatability Analysis of Industrial Robot
under Loaded Conditions and Various Distances”, 8th
WSEAS Int. Conf. on Robotics, control and manufacturing
technology (rocom '08), Hangzhou, China, 2008, pages: 75-79
 Samuel Bouchard, “Robotic Gripper Repeatability Definition
andMeasurement”,URL:http://blog.robotiq.com/bid/36551/R
obotic-Gripper-Repeatability-Definition-and-Measurement
 Kevin L. Conrad, Panayiotis S. Shiakolas, T. C. Yih, “Robotic
Calibration Issues: Accuracy, Repeatability and Calibration”,
Proceedings of the 8th Mediterranean Conference on Control
& Automation (MED 2000), Rio, Patras, Greece, 2000
 NJIT Moodle, URL:http://njit2.mrooms.net/
 Definitions in Robotics. Resolution, Accuracy and
Repeatability
URL: http://www.fortunecity.com/campus/law/365/index-
pro-angl.htm
 Bob Williams, “An Introduction to Robotics”, URL:
http://www.ohio.edu/people/williar4/html/PDF/IntroRob.pdf
 LabVolt 5200 User Manual

Project Report

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (20)

Similaire à Project Report

Similaire à Project Report (20)

Project Report