Performance Analysis of SCORBOT ER 4u Robot

Arm

Prasad Vinayak Patil and Shantipal Suresh Ohol Mechanical Engineering Department, College of Engineering Pune, India

E-mail: prasadpatil2010@rediffmail.com

Abstract—Adaptive programming and flexible planning are

the key features of the Robotisation which is gaining a

popularity in the Industrial sector. Most important

parameter in the case of optimization of error in robotized

processes are testing and trials. It influences the working of

robot and demands to perform effectively at variable kinetic

conditions inside the work volume. The total focus of

presented work is to calibrate the operating parameters of

manipulator arm at various conditions. SCORBOT-ER 4u is

robust, easy to use and effective, which is used widely in

manufacturing engineering fields. In this paper, performance

analysis of robot arm mainly focuses on the performance

improvisation of a robot by using methods of reconditioning,

and programming. A robot performance can be measured by

the parameters such as, accuracy, repeatability, resolution,

workvolume, operational speed, positional error, etc.

SCORBOT ER 4u is operated for three different parameters

viz speed, load and distance. Robot arm is operated with

RoboCell software by giving different program and the

repeatability is checked by taking points on polar plot by

varying speed and load. And this difference in point is

checked by digital microscope.

Index Terms—error, repeatability, SCORBASE, SCORBOT

ER-4u

I. INTRODUCTION

Major industrial robot applications are pick and place,

welding, painting, assembly, packaging and palletizing -

de-palletizing, part inspection, testing, and data collection.

Theses all tasks are accomplished with high endurance,

speed, and precision. Production of it has reached up to

67%, compared to before the robotization in the

manufacturing industries.

Quality level of product is also found increased due to

the effective integration of industrial robots at various level

of automation. The total phase shift is only due to

improvised consistency and better repeatability. These

characters are not found in human being for repetitive,

monotonous and dangerous jobs, hence robots are used.

II. PERFORMANCE OF SCORBOT ER 4U

Now-a-days in any field more and more accuracy is

required for any application. And repetitive work is tedious

for human being, so robots are required which are having

Manuscript received December 25, 2013; revised February 27, 2014.

good repeatability [1]. So robot calibration especially

accuracy analysis plays very much important role in

robotics field [2] and [3]. Also it is very important to

increase the accuracy of robot, by analyzing parameters

which are affecting to the accuracy.

SCORBOT-ER 4u is a pick and place type robot. Hence

it is used for picking one object from one location and

placing that object at another location [3] and [4]. Also

these types of robots are used for welding purpose also.

Arc welding, spot welding, spraying, cutting material,

material handling, palletizing-depalletizing are the some

application of the robot. By increasing axis of the robot, it

can be utilized for different works.

III. PERFORMANCE ANALYSIS OF THE ROBOTIC

SYSTEM

It is important to carry-out performance analysis of the

robot for various practical reasons. Following are some

factors which shall be checked for the error-proof working.

Accuracy

Repeatability

Positional error

Operational speed

Payload capacity

Resolution

The repeatability and accuracy of the robot relies mostly

on to the resolution of programming instructions, hardware

accuracy & higher repeatability of the input itself.

Accuracy and repeatability is referred to as the ability of

each robot to place its end tip on pre-decided or desired

point within its work envelope and can be defined in the

resolution of space.

Resolution of the robot system is affected by the control

system of the robot. Resolution is defined as the smallest

possible increment of the movement, which can be done by

the robot. It is also one of the smallest sections of its work

envelope, which it may acknowledge inside the work

envelope. The resolution of the robot programming is the

smallest increment allowed by the robotic program of

instructions. Similarly a resolution control is the smallest

possible change in movement that can be detected by the

feedback device for servo controlling [4].

IV. ACCURACY ANALYSIS

International Journal of Materials Science and Engineering Vol. 2, No. 1 June 2014

©2014 Engineering and Technology Publishing 72doi: 10.12720/ijmse.2.1.72-75

The accuracy and repeatability performance of a robot

can be defined as half the value of the space resolution. It is

nothing but the movement of the probe or tool attached to

the robot’s arm. [4] Accuracy and repeatability of a robotic

system can also be defined as the ability of a robot to move

precisely in a three dimensional plane inside its work

envelop. The speed of movement and the payload capacity

are the major influencing factor on the accuracy and

repeatability of a robot. Change of speed always causes the

changes in the accuracy and repeatability level for all types

of robotic operations. The speed of robotic arm controlled

and bargained to avoid it malfunctioning and improper

positioning [5].

Repeatability represents an ability of robot arm to

exhibit a capacity towards consistent repeated action inside

the work volume. Every time a robot moves back to the

predefined point after a completed cycle, there will be

miniscule of difference in position. Hence repeatability

means how close the robot can move and reach that

particular point, again and again. In other words,

repeatability is the measurement of the robot’s ability to

move its tip of the tool / probe to a predefined point within

its work envelope [5] and [6].

V. EXPERIMENTAL SETUP

The complete setup for experimentation is shown in

below Fig. 1. After homing, pen is held by gripper.

Computer is placed in front of whole setup and connected

to the controller. Computer and USB-Controller are placed

outside the work envelope (610mm) of the robot. [7] Polar

plot is placed on the wooden drawing board for taking

three different points within work envelope. Pen is held by

gripper as shown.

Figure 1. Complete experimental setup

Some points are traced on the polar graph paper with the

help of pen and these points are repeated for number of

times for calculating the repeatability of the robot. The

points on polar paper are then kept on the digital

microscope for measuring the distance variation in each

point. Digital microscope having 50x or more zoom

capacity is used for measuring distance.

VI. STEPS OF ANALYSIS

The following steps should be followed for carrying out

repeatability analysis experiments conducted with

SCORBOT 4u [7]-[9].

After connecting all the cable as mentioned above,

switch on the power supply. Then check the

emergency switch (this switch should not be

pressed while doing experiment).

Then click ‘On-Line’ in options bar. (So that actual

power supply starts to all the motors of robot).

Then click on ‘Search Home - all axes’ in Run

command. (So that robot goes to its home position).

After coming to home position of robot, write a

program in Program window. And save the target

positions of robot by Teach-Position toolbar. Move

the robot by Robot movement toolbar as shown in

Fig. 2 and save this program.

Run this program continuously without any load

and at particular speed.

Figure 2. Robot movement toolbar

Then run this program for different speed and for

different load and check whether any change in

position is coming or not by taking points on paper.

If error is coming check how much error is coming?

If error is zero then robot is having repeatability

within the range as per provided by supplier or

manufacturer of robot.

Payload capacity of this robot is 1kg so now check

the performance of robot by increasing weight on

robot (holding in gripper) up to 1kg. And check

whether error is coming or not. And speed variation

available is from 1 to 9.

Error is checked by digital microscope. And

distance is measured under microscope with

zooming those two points on polar paper.

And the best set of experiment for working of robot

is selected by regression method.

VII. ACCURACY ANALYSIS BY DIGITAL MICROSCOPE

AND TAGUCHI METHOD

International Journal of Materials Science and Engineering Vol. 2, No. 1 June 2014

©2014 Engineering and Technology Publishing 73

For checking the distance, inverted digital microscope is

used with Axio-Vision LE software.

The Axio Vert 40 mat digital microscope is used. Axio

Vert. is a compact, inverted microscope that brings a big

insight into your objectives. It is having zooming capacity

from 100x to 1000x.

After taking points on the polar plot, the program is run

continuously for different conditions i.e. by varying speed

and by varying load of the robot. After number of

repetitions these points on polar plot may or may not come

to that position and orientation, so these points on polar

plots are taken for the measuring deviation on the digital

microscope.

Figure 3. Distance between two points of pen

These points are kept on the digital microscope; light is

focused on the points which are to be measured.

Axio-Vision LE software is used for taking images of

microscope on the computer screen as shown in Fig. 3.

With the help of this software images are taken on the

computer and distance between the points is measured. The

summaries of readings of experimentations are noted in the

Table I as given below.

TABLE I. ERROR TABLE WITH TAGUCHI METHOD (L9 ORTHOGONAL

ARRAY)

Exp Speed Load Dist Error

1 1 0.1 192 338.40

2 1 0.5 312 393.11

3 1 0.9 426 444.87

4 5 0.1 312 534.61

5 5 0.5 426 538.00

6 5 0.9 192 547.13

7 9 0.1 426 466.66

8 9 0.5 192 584.22

9 9 0.9 312 609.96

For processing the data of Table I for further analysis

Taguchi method is used for getting optimum solution in

terms of speed, load and distance. The equation obtained

by Taguchi method in Minitab for regression is as follows,

E = K + KnN + KlL + KdD (1)

After regression we get equation of regression is,

E = 501 + 76.9N + 49.9L+ 11.2D (2)

where N, L, D and E are the factors viz. speed, load,

distance and error. N, L and D are input parameters and E is

output parameter.

The equation of regression shows which the most

effective input parameter is and which less effective

parameter is, in comparison with all selected parameters.

The co-coefficient of each parameter shows which input

parameter greatly affects the performance of the robot

working [10] and [11].

Figure 4. Plot for SN ratios

Fig. 4 shows, the signal to noise ratio graph for each

input parameter i.e. speed, load and distance. The highest

point of each graph shoes that for that condition robot arm

gives best result. From the above charts shows for best

results robot should work at lowest speed, load and

distance. It is similar or closer observation for the other

Robots as well. [12] and [13] But distance parameter does

not affect greatly on the performance of the robot working.

VIII. CONCLUSION

Simulation helps for effective estimation of the actual

work cycle. [14] It has been observed that, the position

charts obtained by the RoboCell Software, shows robot is

working properly, and accurately. The repeatability is

found as 0.5mm, which is as per the manufacturer of the

robot arm. The position chart shows there is a run time

error occurs during the working of the robot arm.

We came to know after the microscopic measurements,

of distance between points is varying according to the

variation in speed, load and distance. But mean error is

495.01μm and which is within the range. The results

obtained by SN charts in Minitab software shows, the

following set of experiment gives best accuracy and

repeatability at Load=0.1kg, Speed=1 and

distance=192mm. Also we get the worst set up of

International Journal of Materials Science and Engineering Vol. 2, No. 1 June 2014

©2014 Engineering and Technology Publishing 74

experiment by SN charts is Load=0.9kg, speed=9 and

distance=426. An equation of regression is E = K + KnN +

KlL + KdD. And after regression equation is

E = 501 + 76.9N + 49.9L + 11.2D (3)

As the coefficient of speed is 76.9, which is greater than

any other coefficient. Hence speed is major factor affecting

the performance of the robot arm. And coefficient of the

distance is 11.2 is lowest than any other coefficient.

The distance is minor factor affecting the performance

of the robot arm. Hence performance analysis of

SCORBOT ER 4u is checked successfully and result is

repeatability is 495.01μm implies within a range.

REFERENCES

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[14] RoboCell Version 4.7 and higher for SCORBOT-ER 4u and

SCORBOT-ER-2u User Manual [Online]. Available:

http://www.intelitekdownloads.com/Manuals/Robots/100346-b-R

oboCell-usb-v47(0210).pdf

Prasad Patil, is from Pune India having birth

date 14 Aug 1989. He completed his

graduation in Mechanical Engineering

Department in 2010 from Shivaji University,

Kolhapur, India. He also completed post

graduation from College of Engineering Pune

from Mechanical Engineering Department in

2012. His major field of study is robotics and

automation.

Dr. Shantipal Ohol, is from Pune, India

having birth date 19 Apr 1971.. He completed

PhD in Mechanical Engineering in 2010 from

Pune University, India. Presently he is a faculty

member at Govt. College of Engineering, Pune

(COEP). His major field of study is robotics

and automation.

International Journal of Materials Science and Engineering Vol. 2, No. 1 June 2014

©2014 Engineering and Technology Publishing 75