Design and development of a linear variable differential optical sensor for small range linear displacement measurement

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 02 Issue: 09 | Sep-2013, Available @ http://www.ijret.org 480
DESIGN AND DEVELOPMENT OF A LINEAR VARIABLE
DIFFERENTIAL OPTICAL SENSOR FOR SMALL RANGE LINEAR
DISPLACEMENT MEASUREMENT
Tuhin Subhra Sarkar1
, Subir Das2
, Badal Chakraborty3
1, 2
Assistant Professor, Department of Applied Electronics and Instrumentation Engineering,
Murshidabad College of Engineering and Technology, Berhampore, Murshidabad, West Bengal, India
3
Assistant professor, Faculty of Agriculture Engineering, B.C.K.V, Mohanpur, West Bengal India
1
sarkar.tuhinsubhra@gmail.com, 2
subir.mcet@gmail.com, 3
badal.chakraborti@gmail.com
Abstract
In this present paper, we have illustrated the development of a novel linear displacement sensor utilizing the intensity of light. This
sensor is based on temporal changes in the intensity of diffuse light beam for the movement of a solid metallic obstacle between the
source and detector instead of traditional reflector. An obstacle is moved over open surface of a hollow shaped channel, where light
beams impinged vertically into the channel. Two photo detectors has been used to sense the variation of diffuse light intensity at the
covered & uncovered area of channel surface due to the movement of obstacle over its open surface. This movement of an obstacle
represents the actual target displacement by differentiating the output transformation of two photo detectors; this phenomenon is
reported as linear variable differential optical sensor (LVDOS). The measured displacement recorded interns of voltage by the signal
processing circuit (SPC). Experimental results are shown a satisfactory performance of the sensor for small range displacement
measurement.
Index terms: Linear Variable Displacement Sensor, Intensity of diffuse light beam, solid metallic obstacle, Photo
detector
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1. INTRODUCTION
In modern industrial production processes the actual linear or
angular displacement of fast moving objects needs to be
detected and ideally done with or without mechanical contact
[1]. For this, there exist variety of suitable sensors that provide
an output signal (voltage or current) proportional to the
displacement of target and sensor. In industrial usage there are
exactly defined requirement for reliability, ruggedness,
measuring range etc [2]. Likewise, electrical parameters such
as supply voltage range, output signal and EMC requirements
are firmly defined in norms and standard [3]. Also the
displacement sensor used in various field of sensitive &
accurate systems such as a robot, biomedical measuring
device, rectilinear motor, controlling of servomotor piston
bump and so on. To find the best sensor for the displacement
purpose, the accuracy factor is very important to be
considered.
In this present work we have focused on small range of linear
displacement sensing methods. Various researchers have
developed varieties of linear displacement transducer in aspect
of range of displacement, sensitivity, linearity and accuracy.
Like Linear variable differential transformer (LVDT) [4],
capacitive transducer [5], potentiometeric transducer [6],
resistive transducer [7], etc. Each of these transducers has its
drawbacks and imperfections.
The drawback of LVDT is that it has larger body length,
affected by magnetic field and complicated signal
conditioning circuit. All of these tribulations are trying to
rectified by Dhiman et.al [8], implementing a strain gauge
based displacement sensor & A.K.Alia et.al [9], implementing
an Acoustic Displacement Transducer, which is suffering from
the environmental noise and self oscillating detector by means
of reflected sound energy. On other side strain gauge based
displacement sensor introduced mechanical error in terms of
ruggedness.
Where linear displacement measured in the range of
millimeter or less by applying very small force, capacitive
displacement sensors are useful for that operation. They have
a good frequency response but its nonlinearity behavior on
account of edge effects and high output impedance on account
of their small capacitance value; add to this, that the cable
which connects the transducer to measuring point is a source
of error. Moreover, the capacitance may be changed because

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of the presence of dust particles and moisture or because of
temperature change.
Potentiometric or resistive transducer operating range is
limited by the size of potentiometer and it is also affected by
the friction caused to obstruct the slider movement. Although
it is quite useful in some applications they have noticeable
friction and need physical coupling with the object. Wiper
friction and excitation voltage cause heating of potentiometer.
Optical sensors have advantages over other type of sensors in
that they can provide noncontact operation, greater sensitivity,
and better accuracy, freedom from electromagnetic
interference, wide frequency, and dynamic ranges. From these
advantages, optical techniques may be ideal for the
development of sensors for the measurement of small linear
displacement.
Optical incremental and absolute digital encoders are the most
common rotational position optical transducers. Absolute
encoders overcome some disadvantages of incremental type,
but are more expensive. Generally optical encoders find use in
relatively low reliability and low resolution applications [10].
Both types may suffer damage in harsh industrial environment
[11]. For sensing the linear displacement, linear encoder and
quadrature encoder are traditionally used. Historically, the
practical disadvantage of optical encoders have included
pattern inaccuracies, concentricity errors between disc and
shaft, susceptibility to electrical noise, vulnerability to shock
and lose of data in power failure. One of major problems with
optical encoders; ice crystal formation and resultant pattern
damage when the shaft turns has not really been completely
solved. Instead of optical encoder some of the researchers
work on the principle of variation of light intensity due to
reflection of optical signal from the target or measuring shaft
which is the source of displacement. Maiti.et.al [12] &
Mhdi.et.al [13] developed a linear variable differential
displacement sensor based on optical light intensity variation
method. The drawbacks in such systems are the reflected light
could not attain the same incident angle and reflection angle
between the source and detector after variation from steady
state position of the reflector, it was scattered from the
detector. So the expected variation of light intensity due to the
displacement makes an error. To overcome this problem S. J.
Lee.et.al [14] designed a Magneto-optic linear-displacement
sensor which is based on modulation of Faraday rotation due
to domain-wall motion in a magneto-optic sensor film under
the influence of ac and dc magnetic fields. It is largely
independent of the intensity of reflected light but its
displacement measurement process depends of time interval.
So this type of sensing arrangement is not suitable for fast
moving object displacement measurement. Without use of
traditional reflector and time independent sensor designed has
been proposed by S. Das and T.s.Sarkar [15] which introduced
better accuracy, linearity and sensitivity within a small space
range of 0 to 40mm.
In this present paper we have introduced a new Linear
Variable Differential Optical Sensor (LVDOS) which is
independent of time and conventional reflection scheme. It is
suitable for fast moving object and as well as measurement of
small linear displacement. The prototype sensor module works
on the principle of light intensity variation method by
establishment of an obstacle between the source and detector
instead of reflector, which is the source of displacement. To
sense the light intensity, a photo detector has been placed
horizontally at the ground surface plane of a one sided open
hollow channel. The open sided surface allows the incident
light beams vertically from the source and a solid piece of
obstruction was moved horizontally over the open sided
surface, initially which are kept at centre position of the
channel surface. In this connection, photo detectors
illuminated by the reflected light rays by the side walls of the
channel. As the solid metallic piece moves towards the end
position of the channel from the centre due to the
displacement of the target with the attachment of this, the open
sided surface being covered. Hence the ratio of covered area
and uncovered area of that surface changed accordingly. Due
to this phenomenon photo detector detects the variation of
reflected light intensity according to the rate of changes
impinging light incidents. The photo detector outputs recorded
in terms of voltage and difference between the intensity level
of covered and uncovered area exhibits the direction of
displacement & the magnitude of displacement. It is employed
by a signal processing circuit (SPC). We proved successfully
that this system works as a small linear-displacement sensor.
The introduction is followed by a representation of the
LVDOS working principle, constructional details, the
experimental investigation, and a series of discussion topics
based on experiments.
2. PHYSICAL BACKGROUND
As the light incident on the surface, it is reflected in all
direction due to internal scattering of light (i.e. the light is
absorbed and then re-emitted) and external scattering from the
rough surface of the object. This is called the diffuse reflection
as shown in the Fig.1
Fig-1: Diffuse reflection in all directions

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From fig.1 it is obvious that if a point of light source kept
perpendicularly over the surface then incident light rays are
not parallel like diameter of light source, it diverges at an
angel which is described by the Gaussian beam functions. If a
light beam of wavelength λ falls perpendicularly on a surface,
then it will be diverged at an angel of θ which is developed
between the incident light rays and the central axis of light
beam; expressed as
θ ≈
λ
πw0
(θ in radians)
Where, w0 is the radius of the spot near the source point. The
Gaussian beam functions of different diameter of light source
are shown in Fig.2.
Fig-2: The Gaussian beam divergence for different kind of
source point
From Fig.2 it is concluded that the point of light source
spreads largely on a surface rather than the others light source.
So, illuminating the more area by keeping a smallest distance
between source and object, a point of light source is very
useful.
The brightness or intensity of any point in the surface where
the light incidents, is depended only on how much light is
reflected or diffused from the surface. It can be calculated by
Lamberts cosine law which states that the intensity of diffuse
light rays depends on the cosine angle between the normal at
that point of the surface and the light source illuminating on it.
It is expressed as
I = Ip . Kd . cosθ
Where, Ip is the intensity of point light source and Kd is the
material's diffuse reflection coefficient; expressed between 0
and 1 which depending on material to material. Fig.3. shows
the diffuse lighting on the surface with different angles
between any points of a surface (i.e. normal on that point) and
the incidents light rays.
Fig-3: Diffuse light intensity at different angels
From Fig.3 we observed three cases of diffuse reflection on
the surface such as-
Case 1: The angle between the normal and the incident light
rays is large, so little light is reflected from the surface.
Case 2: More light is reflected than case 1 as the angle is
reduced between the normal and the incident light rays.
Case 3: A large amount of light is reflected from the surface
when the angle between the normal and the incident light rays
tends to zero.
So it is concluded that most of the light is reflected when the
angle is at 0 degree, none is reflected at 90 degree due to
cosine angel between the normal and the incident light rays.
For measuring the light intensity level if a photo detector has
been placed in case1 surface, it is observed that the detected
light intensity level at that surface has been varied due to
blocking the incident light rays by placing a non transparent
obstacle in light travelling path. Because no light incident or
reflected on that point directly and very small amount of
reflected light reached at that point from the nearest case2
surface. If this obstacle moved on serially from case1 to case2
and then case3 and so on then photo detector output changed
accordingly from smallest value to larger value; means case1
surface moves to much more darkness level. This phenomenon
has been employed in our proposed LVDOS system.
3. DESIGN OF LVDOS
In this present design, light source part & light receiving part
separated by a metallic obstacle. The receiving parts consist of
a light reflective container and two photo detectors. A one
sided open hollow shape aluminum channel is used as a light
reflective container instead of glass, which is commonly used
as a light reflective surface. From industrial aspect of
ruggedness glass is poor compared to any metal. Some
metallic surface oxidized naturally with the contact of air and
made surface layer less reflective whereas aluminum does not
exhibit this property. In this connection our prototype sensor’s
channel surface made with aluminum. The length of the
channel is 12 cm, height 1 cm, width 2.5 cm.

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At the centre position of the channel, two photo detectors were
placed back to back on a very thin insulated plate which is
separated the channel into two parts, each of having 6 cm
lengths. From the top view of the channel as shown in Fig.4, it
is observed that the width of the open sided surface is 1.5 cm
instead of 2.5 cm due to 0.5 cm (width) inner bending surface
of the two side walls of the channel. The purpose of that
bending surface is an obstacle of 6 cm long, 3 cm wide & 0.5
cm height cubic metal plate can moves over the open sided
surface horizontally with the displacement of the target body
by making an attachment of shaft at the both end of the
obstacle. The shaft designed & connected in such a way that it
could not behaves as an obstacle, so it is free from addition a
permanent error in intensity variation due to the original
movement of obstacle.
The light source part separated in two modules, each of length
is 3 cm. Each module is made with 8 no. of LEDs. Because
one LED is not enough to lighting up the whole length of a
channel part by keeping it small distance apart from the
surface as the statement of Gaussian divergence angel relation,
which is stated in physical background. Each module driven
by an array of current limiting resistors in series with +5 V
regulated power supply. The side view of the arrangement is
shown in Fig.5.
Fig-4: Top View of the Channel with Obstacle
Fig.5. Side View of LVDOS
4. WORKING PRINCIPLE OF LVDOS
At the initial position, obstacle is kept at the centre position
over the open surface of the channel i.e. half part of an
obstacle covered right portion & other half part of an obstacle
covered left portion of the channel. The two photo detector has
been placed just under the obstacle at that position as shown in
Fig.5. The light beam from the sources is impinged vertically
through the open surface of the channel where obstacle is not
present. As a result two photo detector sense some diffused
light intensity due to light reflection policy. Now, if we move
the obstacle towards right from the centre position of the
channel by a distance of 1 mm using a vernier scale
arrangement, the open surface of the right portion has been
covered and left portion has been uncovered by similar
distance. In this connection the initial detected light intensity
level of a right photo detector (RPD) is dropped due to
obstruction of incident light rays and left photo detector (LPD)
detected intensity level increased due to much more incident
light reflection as stated in physical background. This
movement of the obstacle is continued until it reached to
extreme right position of the channel and detected light
intensity level of RPD drops gradually towards zero i.e. right
portion of the channel is fully covered by obstacle and
detected light intensity level of LPD increase gradually
towards saturation point i.e. left portion of the channel is fully
uncovered. This same incident would be happened for obstacle
movement towards left side, but the detected light intensity
level variation of two photo detectors will be changed in
reverse order. The schematic diagram of complete sensor
module is shown in Fig.6.
To sense the light intensity variation we used two photo
diodes as detector. In reverse bias configuration of photo
diode, current will be changed in the circuitry according to the
light intensity level, which converted into voltage variation by
an op-amp based current to voltage converter circuit. Further
these two voltages of photo detectors are differentiate by a
signal processing circuit (SPC) & final output voltage (Vo) is
recorded with the displacement of the obstacle in the range of
+3cm to -3cm i.e. from centre position to extreme left and
centre position to extreme right position. Thus, measuring the
output voltage (V0) of the SPC, we can detect the linear
differential displacement/position of the obstacle.

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Fig-6: Schematic diagram of complete sensor system
5. RESULT ANALYSIS
The polynomial output voltage vs. displacement relationship
of two photo detectors are obtained with the help of MATLAB
curve fitting tools (considering up to 4th order only) , shown
in Fig.7 and can be written as
V01 (D) = - 0.006 D4 + 0.024 D3 + 0.076 D2 – 0.596 D +
0.980 (1)
Where V01 (D) is the output voltage of RPD for a movement
of obstacle with the displacement of D cm and
V02 (D) = -0.007 D4 - 0.024 D3 + 0.080 D2 + 0.597 D + 0.986
(2)
Where V02 (D) is the output voltage of LPD for a movement of
obstacle with the displacement of D cm similarly, the
polynomial relationship between the output voltage (V0) of the
SPC and the displacement of obstacle D is obtained by the
curve fitting tools (shown in Fig.8) using MATLAB and this is
expressed as
V0 (D) = P4 D4 + P3 D3 + P2 D2 + P1 D + P0 (3)
Where the above coefficient is calculated by the difference of
coefficient, which is provided in equations (1) & (2) Because,
the output voltage (V0) is the difference of two photo detector
output voltages V01 & V02. Therefore,
V0 = V01 – V02 (4)
So, polynomial coefficient of equation (3) is
P4 = 0.001 P3 = 0.048 P2 = -0.004
P1 = -1.193 P0 = -0.006
When the obstacle moves towards negative direction i.e.
towards the extreme right position of the channel, the darkness
level at the right channel will be increase rather than the
darkness level at the left channel. Hence, the output voltage of
the RPD (V01) will be smaller than the output voltage of the
LPD (V02). So the differential output voltage of the SPC (V0)
is (-) ve. Similarly, when the obstacle moves towards positive
direction i.e. towards the extreme left position of the channel,
the darkness level at the left channel will be increase rather
than the darkness level at the right channel. Hence, the output
voltage of the LPD (V02) will be smaller than the output
voltage of the RPD (V01) and the differential output voltage of
the SPC (V0) would be (+) ve. Thus, this technique provides
theoretically non-linear relationship between the differential
output voltage and linear displacement.
It is obvious from equations (1), (2) & (3) that when the
obstacle is in middle position i.e. when D = 0, all the output
voltages of two photo detectors and SPC are providing the
non-zero value i.e.V01 (0) ≠ 0, V02 (0) ≠ 0 and V0 (0) ≠ 0.
By acquiring the experimental data of two photo detector
output voltage (V01 & V02) and output voltage (V0) of SPC
for linear displacement (D) of an obstacle has been graphically
plotted in Figure 7 & 8 which supports the equations (1), (2) &
(3). The enlarge view of linear region of SPC output voltage
(V0) vs. linear displacement (D) curve has been shown in
Fig.9.
Fig-7: Plot of output voltages of two photo detectors against
linear displacement of an obstacle
Fig-8: Plot of output voltage (V0) of SPC vs. linear
dispalcement

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Fig-9: Enlarge view of linear range of output voltage (V0) of
SPC vs. linear dispalcement
The experimental result shows a linear variation of output
voltages with the variation of linear displacement in the range
of +1cm to -1cm instead of +0.5cm to -0.5cm displacement
range of a linear variable differential displacement sensor [12].
Eventually we have calculated the non linearity of that curve
utilizing best-fit straight line method and find the value of -
6.15% FSO (Full Scale Output) at the displacement of -8mm.
Also the accuracy factor of that measurement method is -2%
FSO at 4mm.
The proposed sensing method can measure the smallest
displacement of 1mm i.e; resolution of this method is 1mm.
So it exhibits better resolution than optical LVDT [12] and
Acoustic displacement transducer [9] which shown 10mm and
5mm respectively.
The repeatability of that sensing method has been evaluated as
0.04%FSO.
CONCLUSIONS
By utilizing the variation of light intensity of a LVDOS was
designed and tested successfully.
The designed sensor is efficient, in expensive, and
multipurpose. It could be integrated with other primary non
electrical sensors and order to get an electrical read out.
This LVDOS displacement sensor applicable in various
industrial fields such as robot, biomedical measuring device,
rectilinear motor & position feed back of a pneumatic or
hydraulic cylinder rod. In this case the sensor is not embedded
within the target, but externally tied through a yoke to the
target body
From Fig.9 the output voltage (V0) variation with target
displacement has been observed linear in the range of -1cm to
+1cm. But beyond this range curve shows non linearity in both
-ve & +ve range. So accurate measure of displacement in
present & extended range, computer or embedded based
programming can be used by interfacing this sensor with the
DAS card & PC, it is the future scope of work.
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Abuzalata, “Design of an Acoustic Displacement Transducer”,
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International Journal of Electronics and Electrical
Engineering, Vol. 1, No. 3, pp. 176-181, 2013
BIOGRAPHIES
Tuhin Subhra Sarkar was born in West
Bengal India in 1981. He received
Bachelor’s degree in electronics &
instrumentation engineering from
University of Kalyani, West Bengal, India
in 2004 and M.Tech degree in computer
science and engineering from University of
Kalyani, West Bengal, India in 2006.
He is currently an Assistant Professor in Applied Electronics
& Instrumentation Engineering at Murshidabad College of
Engineering & Technology, Berhampore, West Bengal, India.
He has been a visiting Lecturer in Sheikhpara A. R. M.
Polytechnic, West Bengal, India between 2006 and 2007. His
research interests include the design of sensors and
transducers, VLSI, network security and image processing. He
has authored or coauthored nearly 6 papers in the areas of the
sensors and transducers, network security and VLSI.
Subir Das was born in West Bengal, India
in 1984. He received Bachelor’s degree in
electronics & instrumentation engineering
from West Bengal University of
Technology, West Bengal, India in 2006
and M.Tech degree in instrumentation &
control engineering from University of
Calcutta, West Bengal, India in 2010.
He is currently an Assistant Professor in Applied Electronics
& Instrumentation Engineering at Murshidabad College of
Engineering & Technology, Berhampore, West Bengal, India.
He worked with Danieli Automation, West Bengal, India,
Core-Technologies, West Bengal, India and Stesalit India Ltd.
West Bengal, India between 2006 and 2008. His research
interests include the design of sensors and transducers,
robotics automation, industrial automation and image
processing. He has authored or coauthored nearly 8 papers in
the areas of the sensors and transducers, and design of
electronics measuring systems.
Badal Chakraborty received his
Bachelor’s degree in Electrical
Engineering from National Institute of
Technology; Agartala, India in 1998.He
obtained his Master degree in
Instrumentation and Control Engineering
and Ph.D. (Tech) in Instrumentation and
Measurement from University of Calcutta in 2000 and 2009
respectively. He completed his Post Doctoral work on
Biomedical Engineering from Indian Institute of Science;
Bangalore, India in 2010.He was working as a faculty member
in Murshidabad College of Engineering and Technology from
2000 to 2005. He is currently faculty member of Department
of Post Harvest Engineering, Bidhan Chandra Krishi
Viswavidayalaya, India. His research interest includes
Sensors, Measurement, Biomedical Instrumentation and
Application of electronics in agricultural fields.
Dr.Chakraborty published more than 30 research papers in
international and national journals. He is reviewer of so many
international journals

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Design and development of a linear variable differential optical sensor for small range linear displacement measurement