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# Mathematical Approach for the Removal of Specular Reflection of Laser RangeFinder

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In order to make autonomous machines or robots it is very essential to measure the distance of neighboring objects. There are various types of errors which occurred while measuring the distance. In this paper we have focused on specular reflection error which occurs in the case of distance measurement using lasers. In this paper presents a mathematical approach for the removal of specular refraction of Hokuyo UBG-04LX-F01 Laser Rangefinder when a laser beam incident on a highly refractive material like mirror at an angle 90 0 .When a laser beam strike on a highly refractive material the laser rangefinder always shows a variation in the measurement i.e. wrong reading. The experiment result shows that the measurement error can be strongly removed by this approach.
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Page | 186
Volume 2, Issue 2, February 2012 ISSN: 2277 128X
International Journal of Advanced Research in
Computer Science and Software Engineering
Research Paper
Available online at: www.ijarcsse.com
Mathematical Approach for the Removal of Specular
Reflection of Laser RangeFinder
Ravinder Singh#1, Sachin Sharma#2, Gaurav Kumar#3, Raj Kumar#4
M.Tech (2nd), Dept. of Instrumentation & Control Engineering
Dr. B.R. Ambedkar National Institute of Technology
Jalandhar, Punjab (India)
Abstract In order to make autonomous machines or robots it is very essential to measure the distance of neighboring objects. There
are various types of errors which occurred while measuring the distance. In this paper we have focused on specular reflection error
which occurs in the case of distance measurement using lasers. In this paper presents a mathematical approach for the removal of
specular refraction of Hokuyo UBG-04LX-F01 Laser Rangefinder when a laser beam incident on a highly refractive material like
mirror at an angle 900 .When a laser beam strike on a highly refractive material the laser rangefinder always shows a variation in the
measurement i.e. wrong reading. The experiment result shows that the measurement error can be strongly removed by this approach.
Keywords-component; Hokuyo UBG-041LX-F01; Specular Reflection; Range Finder
I. INTRODUCTION
The distance measurement system in 2-D and 3-D
environment is essential to develop an autonomous mobile
robot [1-2, 4]. The laser rangefinder (LRF) is a good choice
for acquiring the accurate distance to an obstacle. The LRF
provides an accurate range with high angular resolution over a
long distance rather than infrared rangefinder investigated by
Alwan et al.[1].however the LRF considered a major
limitation that is Specular refraction. It’s generally occur when
a highly refractive material put in front of a laser beam due to
its refractive nature the laser beam diverts in another direction.
This can be removed by this approach very easily.
Hokuyo LRFs Since there is no research on the error
comes during the measured data by UBG for practical use.
Some LRFs as Hokuyo UBG-04LX-F01(UBG) and URG-
04LX(URG) shown in Fig. 1,use the phase shift measurement
principle to detect the distance to a target. The characteristics
and the calibration model of URG were proposed by Okubo et
al [3, 5].
UBG is the new and small LRF produced by Hokuyo
Automatic Co. Ltd. And it has improved specification
compared to the earlier Hokuyo LRFs. Since there is no
research on the error comes during the measured data by UBG
for practical use.
Fig. 1: Hokuyo UBG-04LX-F01
II. OVERVIEW OF THE HOKUYO UBG-041LX-F01
Firstly, the man specification of the UBG and URG are the
compared in Table 1.Some specifications of the URG and
UBG is the same, but the main difference between them is
scan, frequency, external dimensions, weight and power
source. Although UBG is slightly larger and heavier, it has fast
scan frequency to detect the shape of the target in a short time.
The distance measurement system with LRF [2] is the fast
and cost efficient solution when the accurate distance to a
target is required for a robot UBG consist of a laser diode, a
photo diode, a mirror, a lenses and an actuator. The actuator
rotates the mirror and the lens with the speed of 2140 rpm, due
to the rotation period of 28 ms and the photo diode measure
682 steps on the 240 per one rotation and therefore the angular
resolution is 0.3520 .Note that the minimum measured distance
is 20 mm and the data, less than 20, are error code resulting
from certain circumstances.
TABLE 1. Specifications of the Laser range finder
S.No
specification
URG-04LX
UBG-04Lx-F01
1
Measured distance
20-5600mm
2
Measurement
resolution
1mm
3
Measurement error
Up to 1m: +/- 10mm
4
Scanning angle
2400
5
Angle resolution
0.360
6
Scan frequency
100hz
36hz
7
External dimension
50x50x70mm
60x75x60mm
8
Power source
DC 5V
DC 12V
Volume 2, issue 2, February 2012 www.ijarcsse.com
Page | 187
III. SPECULAR REFLECTION
Specular reflection is the mirror-like reflection of light (or
of other kinds of wave) from a surface, in which light from a
single incoming direction is reflected into a single outgoing
direction, for this we placed a mirror in front of the laser
scanner at the distance of 2000mm and note down the few
reading at the same position and get the result on the graph as
shown in fig.2
Fig. 2: Error Measured
As shown in fig.2, the measured data consist of errors. The
actual distance is 2000mm but when we measured with the
LRF it shows the reading more than 3500mm, hence it shows
the error more than 1500mm. This error is due to specular
reflection. When laser ray falls on the plane mirror is gets
reflected and we get error in the measured readings. In this
paper we use a mathematical approach for removing this error.
Fig.3 shows how the laser ray reflected when it falls on the
reflected surface.
Fig. 3: Reflection of the laser Ray from Reflected surface
IV. EXPERIMENTAL SETUP
As shown in fig 4. The experiment consist of a LRF, 3
mirror, sliding arrangement for the sliding of two mirror at an
angle 450 .mirror 1 and 2 are fixed on a sliding arrangement to
move them at 450 w.r.t. LRF. Mirror 1 is the obstacle in front
of LRF .the distance between them is unknown; we have to
measure the distance between them. If we placed a non-
refractive obstacle in front of LFR it shows the actual distance
but if we placed a refractive material the LRF will show
variation in reading that is error. X is the actual distance that
has to be measured by using laser rangefinder.
Fig. 4: Experimental Setup for the range finder
A. Communication Interface
To connect the sensor with a host computer, both USB and
the RS-232 are provided. The maximum transfer of USB
interface is 9mbps and the maximum transfer rate of RS-232
varies from 115.2Kbps to 750 Kbps.
To transfer the measured data to a host computer, two
transfer protocols SCIP 1.1 and SCIP 2.0 are available. Since
SCIP 2.0 supports more functions to improve the performance
of the sensor, it is preferable .therefore SCIP 2.0 and the USB
is selected in this paper for measured distance to a target
B. Drift Effect of the Sensor
Drift effect is known characteristic of the LRF and it has a
different name as “warm up time”. In order to observe the drift
effect, the distance of a white colored target, which is at 2000
mm in front of the sensor, was performed for approximately
two hour and result is seen in the Fig. 5 as the time goes on,
the measured distance is decreasing during the first 40
minutes, then stabilizes, this phenomenon is called the drift
effect.
V. REMOVING OF SPECULAR REFLECTION
In this section, we will present the experiment result and the
approach for removing the error. First of all the mirror 3 is
placed in front of the laser rangefinder at an unknown distance
and the two mirror 1& 2 is at on the sliding arrangement at
Volume 2, issue 2, February 2012 www.ijarcsse.com
Page | 188
angle 450.Now selected the laser beam which incident on the
mirror 1 at an angle 450.
Fig. 5: Figure showing the drift effect of sensor
now slide the mirror 1 and 2 on the sliding arrangement
parallel .When the obstacle is not in the path of the laser beam
the LRF shown reading more than 4 meter that is wrong, now
when we slide the mirror 1 &2 ,then we reach a point when
the laser beam strike the mirror 3 at an angle 450 .As we know
that laser light has a property that is angle of incident is
always equal to angle of reflection.
This property is used in this mathematical approach so
when the laser beam strike the mirror 3 at angle 450 it also
reflect as the same angle that is 450 .when it strike at mirror 3
it again reflect at an angle 450 and strike on the mirror 2.
Mirror 2 is positioned in that way that it reflect the laser beam
at an angle 450 finally it reached the LRF hence it cover the
distance 4 time the distance between LRF and mirror 2.
Now divide the measured reading by 4 we get the distance
between mirror 2 and the LRF that is AB .now we have to find
the distance BC that is half of the distance between mirror 2 &
3 for this we have to multiply the distance between the mirror
2 and the LRF that is AB by 0.7 that is (AB*2) where 0.7 is a
constant, from here we get the distance between BC. As
shown in diagram a right angle triangle ABC is formed. We
have the value AB and BC from the previous measurements
and we have to find the distance between CA. This can be
calculated by using phythagorous theorem.
(Hypotenuse)2 = (base)2 + (perpendicular)2 ---- (1)
(AB)2 = (AC)2+ (BC)2
We have the value of AB and BC (as shown in the figure 2).
By using the above equation (1), we can find out the AC
.now multiply the AC with 2 that is (AC* 2) that will give the
actual distance X that is between the mirror 3 and the LRF
that is required.
VI. CONCLUSION
In this paper, the Hokuyo UBG-04LX-F01 is studied and
the specular reflection is removed by using a mathematical
approach. UBG can measure the distance up to 5600mm,
however if we placed a highly refractive material in front of it
will give error in the measurement but by using this approach
we can easily get the actual distance for an highly refracted
surface which is put in front of a LRF.
REFERENCES
[1] M.Alwan M.B Wagner, G. Wasson and P.Sheth,” characterization of the
infrared rangefinder PBS-03JN for 2D Mapping”, in.Proc. of the IEEE
Intl.Conf.on Robotics and Automation pp. 393-3945,2005
[2] J.AHancock, “Laser Intensity Based Obstacle Detection and Tracking”
PhD dissertation .Carnegie Mellon University, Pittsburgh
.Pennsylvinia.1990.
[3] Y.Okubo.C.Ye and J.Borestein ,”Characterization of Hokuyo URG-
04Lx laser range finder for mobile robot obstacle negotiation”,in
Unmanned System Technology X1,Orlando,FL,2009.
[4] H.Kwakernaal and R.Sivan,”An ultrasonic sensor for distance
measurement in automotive application”.IEEE Sensor Journal, p.p.143-
147.2001
[5] K.Kneip F.Tache,G.Caprari and R.siegwart,”Charcterisation of the
compact Hokuyo URG-04LX 2D laser range finder”. In Proc. Of IEEE
Intl.Conf. On Robotics and Automation pp.1447-1454.2009.
[6] C. Ye “Mixed pixel removal of the Laser range finder for mobile robot
3-D terrain mapping”. Intl. Conf. On Information and Automation
pp.1153-1158.2008.
Authors Ravinder Singh was born at Jalandhar,
Punjab (India) on 9th march 1987. Currently
he is pursuing his M.Tech degree in
Instrumentation and Control Engg. From
NIT Jalandhar. He did his B.Tech in
Electronics & Instrumentation Engg. From
C.T. Institute of Engineering &
Management, Jalandhar in 2010. His area of
research interest includes robotics, Machine
vision & sensor fusion.
Sachin Sharma was born at Dadri, Uttar
Pradesh, India on 5 July, 1987. Currently,
He is pursuing his M.tech degree in
Instrumentation and Control Engineering
from NIT Jalandhar. He did his B.tech in
Electronics and Communication
Engineering from GLA Institute of
Technology & Management, Mathura. He
has published several papers in
International conferences and International
journals on robotics and autonomous
systems. His area of research interest includes robotics, signal
processing, neural networks, embedded systems, system designing,
biomedical application and artificial intelligence.
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An ultrasonic sensor for distance measurement in automotive application
• H Kwakernaal
• R Sivan
H.Kwakernaal and R.Sivan,"An ultrasonic sensor for distance measurement in automotive application".IEEE Sensor Journal, p.p.143-147.2001
Punjab (India) on 9 th march 1987 Currently he is pursuing his M.Tech degree in Instrumentation and Control Engg His area of research interest includes robotics, Machine vision & sensor fusion
Authors Ravinder Singh was born at Jalandhar, Punjab (India) on 9 th march 1987. Currently he is pursuing his M.Tech degree in Instrumentation and Control Engg. From NIT Jalandhar. He did his B.Tech in Electronics & Instrumentation Engg. From C.T. Institute of Engineering & Management, Jalandhar in 2010. His area of research interest includes robotics, Machine vision & sensor fusion.
Characterization of Hokuyo URG-04Lx laser range finder for mobile robot obstacle negotiation
• Y C Okubo
• J Ye
• Borestein
Y.Okubo.C.Ye and J.Borestein,"Characterization of Hokuyo URG-04Lx laser range finder for mobile robot obstacle negotiation",in Unmanned System Technology X1,Orlando,FL,2009.