A liquid pendulum based optical tilt sensor

Accepted Manuscript Title: A Liquid Pendulum Based Optical Tilt Sensor Authors: Subir Das, Badal Chakraborty PII: DOI: Reference:

S0924-4247(18)30649-6 https://doi.org/10.1016/j.sna.2018.11.054 SNA 11149

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

15 April 2018 28 November 2018 29 November 2018

Please cite this article as: Das S, Badal C, A Liquid Pendulum Based Optical Tilt Sensor, Sensors and amp; Actuators: A. Physical (2018), https://doi.org/10.1016/j.sna.2018.11.054 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A Liquid Pendulum Based Optical Tilt Sensor Subir Das, Badal Chakraborty

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Murshidabad College of Engineering & Technology

ADDRESS: Cossimbazar Raj, Banjetia [email protected]

Highlights:

Proposed sensor has following advantage over the other liquid pendulum based design o Measuring range is high o Simple in design. o Free from toxic liquid



Its design is free from any conventional fiber optics based method. But provides better resolution and good accuracy.

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A novel approach to determine the tilt angle of an object surface is proposed in this paper. Its basic principle is based on tracking the liquid surface inclination over a pair of photo-detector and the sensing phenomenon is used a differential change in light intensity.

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The system has merits of high accuracy, resolution and sensitivity.

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Abstract— In this paper a simple, low-cost single axis optical tilt sensor is presented. The presented sensor body is a transparent, cube shaped container made of an acrylic sheet. Additionally, it is half filled with liquid (i.e. water) based on its inner volume. In order to sense the liquid column inclination during the tilting of the sensor body, two pairs of optical trans-receiver have been used and their unique orientation across the sensor body claims relatively high measuring range. The basic working principle of the sensor depends on Snell’s law of refraction and reflection. Experimentally, it is observed that the developed prototype is able to sense the tilt angle in the range of ±90°, but a significant linear range is found in ±20°. In this range, the sensor has a resolution of 0.02° and sensitivity of 225mV/°.

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Index Terms—Inclination; liquid pendulum; optical sensor; refraction of lights; Snell’s Law; tilt

I. INTRODUCTION

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Tilt sensor or inclinometer is used for measuring the horizontality of a surface or system such as robots, transportation vehicles, measuring the depth, rate of landslide movement, and other important fields. Typically, the sensor measures the angular tilt of the test surface from the pendulum displacement, because it responds to a change in the orientation of gravity. Mainly, three types of pendulum are used in the sensors: solid, gaseous and liquid. The choice of the pendulum is an important factor for the sensor performance. The solid pendulum structure is more complex and can be interfered as well by the external factors (i.e. vibration or mechanical shock) compared to the gaseous pendulum. But, the solid pendulum based sensor is insensitive to temperature variations. In contrast, the liquid pendulum based tilt sensor has many advantages, such as high sensitivity, corrosion, moisture resistance [1], high surface tension and low vapor pressure [8]. Several methods have been applied so far for measuring the liquid pendulum displacement like capacitive [1]-[4], magnetic [5]-[8], and optical [9]-[16] methods. In the capacitive sensor either a single or double layer different liquid medium acts as a dielectric element between the sensing electrodes and during inclination, the displacement of the liquid medium causes a change of capacitance. The sensitivity of the sensor is mostly depending on the sensor geometry and its measuring range varies ±20° [1] to 360° [3]. In the magnetic sensor, a magnetic fluid is used as a pendulum or guided medium around a permanent magnet. When it acts as a pendulum, its displacement due to the surface inclination is measured by optical interruption method [7]. In a system, a permanent magnet is placed inside a drop of magnetic fluid and its displacement creates a

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magnetic field variation near the sensors like Hall sensor [6] or inductive coil [5] which further made an electrical signal according to the magnetic field. Typically, their working ranges are ±5° [7] to ±15° [5], [6]. However, the sensors are required a well-sealed housing to protect the device from external electromagnetic interference. In Ref. [8], a core body inside the inductive coil act as a solid pendulum and its suspension inside the coil depends on magnetic bouncy, which is generated by two drops of magnetic fluid at the ends of the core body. On the other side, the optical type sensor is able to provide high sensitivity and accuracy even in the magnetic zone or hazards environment. In Ref. [9]-[16] it has been observed that due to high precision and accuracy the fiber-optic sensor is very reliable for the measurement of liquid pendulum displacement. The fiber-optic sensors mainly work on the principle of light reflection [13]-[15] or refraction [9]-[12]. Bajic.et.al proposed a method [9], [10], where the fiber-optic trans-receiver either placed horizontally or vertically across the air-liquid interface of a transparent container. So, a refraction or reflection of light is occurred due to the inclination of the interface surface and the propagation of light rays through the different refractive indices of the two-phase medium. Since the radius of the fiber ends is very small thus its measured range is ±5° or 30°. As the transmitted light rays are scattered in every direction hence the receiver fiber detects the weak intensity of light. Ozturk et.al [11] introduced an oil layer between these two-phase of interfaces as a light guided medium to reduce the transmission loss. On the other side, the reflection type sensor used toxic liquid (mercury) as a reflector [13] or used FBGs as a sensor [14], [15] to detect the drag or bouncy force of a floating container, which is exerted during the liquid column inclination. The sensors are providing high sensitivity and resolution, but their working range is limited to ±5° to ±27°. In contrast to the prior art, a liquid pendulum based inclinometer measures the tilt angle in the range of ±50° [16] by using the principle of light absorption in a liquid medium. In this research work, a simple, inexpensive and relatively high measuring range of tilt sensor is presented. As the sensor follows the basic principle of Snell’s law, though it is free from the fiber-optic element or toxic liquid and instead thereof used a dual pair of optical trans-receiver inside the air-liquid medium. The sensor is able to measure the tilt angle in the range of ±90°, but a significant linear range observed in ±20°. Moreover, it is very promising for those applications where high sensitivity and resolution is required.

II. IMPLEMENTATION OF THE TILT SENSOR

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A. Working Principle of The Sensor A transparent, cube-shaped container along with two-phase medium (i.e. air and water) act as a sensor body and the water inside the container behaves as a liquid pendulum. Thus, in order to measure the inclination of the sensor body, the change in current state of a liquid column that is caused by the orientation of gravity is sensed according to the light refraction and reflection principle. To illustrate the sensor working principle a graphical model is shown in Fig.1. Here, the container is partially filled with water and its height from the horizontal surface is H. Two separate near monochromatic light sources (L1 and L2) are placed at the inner bottom corners of the container by a separation gap of 2L and two photo-detectors (PD1 and PD2) are firmly attached at the outside walls of the container by maintaining a same height of the liquid column. The light sources are light emitting diodes (LED), which are able to focus their emitted light through its epoxy lens. Now, as per the principle of Gaussian light beam, it can be stated that if a point type light source focus on a flat surface, it will make a ring on the surface and its radius of divergence varies with the light traveling distance. According to the Gaussian distribution, the peak light intensity in the ring will be available only at the center position and decrease exponentially along the radius of the ring. Generally, the light intensity (I) profile within the ring area is expressed as

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Where r is the radius of a diverged Gaussian light beam and ω0 is a radius of the beam at which the light intensity has decreased (c) to 13.5% of the peak intensity (I0) [17]. So, as per the Gaussian distribution, the light intensity on the projected surface would be the maximum and minimum as well in the center and the edges of the ring respectively. In order to simplify the mathematical analysis, only the peak intensity of the emitted light beam has been considered in the Fig.1, but its intensity profile over the container wall is shown in Fig.1(b). As can be seen from the Fig.1(a), while the sensor body is kept parallel to the horizontal surface, the optical sources (L1 and L2) are completely immersed into the water but it may expose in the air medium if the sensor body inclined at a proper angle. Let the transmitting angle (α) of the L1 and L2 light beams (d) are α1 and α2, which are measured with respect to the surface normal of the two-phase medium. Assumed, the transmitting Fig.1 Theoretical model of the tilt sensor (a) Direction of transmitted and refracted light rays while the sensor body was rest horizontally and α1 = angle α1 and α2 of both the sources are 45° and the liquid height α2 = 45° (b) Gaussian distribution of the refracted light rays (c) The change (H) is maintained according to the half of separation gap distance in direction of refracted light rays for the L1 source only while the sensor between the optical sources. body inclined at 22.5° (d) For the L1 source only the change in direction of Let at the initial condition, the light beams are propagating refracted light rays after inclination of the sensor body. Dotted lines denotes through the water to air medium, but it will propagate through the current states. air to water medium while the sensor body inclined at a proper angle. As the refractive index of water (n1=1.33) is greater than the air (n2) so, the light will be refracted or reflected in different angles (β) according to the principle of Snell’s law. Assumed, the refraction angle of the L1 and L2 light beams with respect to the surface normal of the two-phase medium is β1and β2 (see Fig.1(a)) and can be determined as

n1 sin(1 )  n2 sin(1 )

; For L1 source only

(2)

n1 sin( 2 )  n2 sin( 2 ) ; For L2 source only

(3)

Before the critical angle, some part of the light rays will be refracted and some part will be reflected. Due to the incident angle variations, the Fresnel coefficients of reflection and transmission (i.e. refraction) also change. But after the critical angle, total internal reflection will take place. As a final effect, the light intensity distribution changes after refraction and total internal reflection. For the simplicity the Fresnel coefficients is not accounted in the mathematical modeling. A general expression for the refraction angle may be derived as

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  

  sin 1 1.33 sin tan 1 L H

(4)

  2l 2 I  I 0 exp 2  0

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TABLE I. THEORETICAL VALUES OF TRANSMITING AND PROPAGATING ANGLE OF LIGHT RAY AFTER TILTING THE SENSOR BODY

(5)

And l  L tan  1

(6)

  2L tan  1 2 Therefore, I  I 0 exp 02 

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In Fig.1(b), the gap distance between the refracted (i.e. only peak intensity of light beam) light beam and the photo-detector (PD) is assumed as l and according to the light intensity profile across the beam, the level of light intensity over the PD1 can be derived as

(7)

Tilt angle θ (°) 0

α1

3.8

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50

3

β2

L1 (°) 45

I (% of I0)4

L2 (°)

L1 (°)

L2 (°)

L1 (°)

L2 (°)

PD1

PD2

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77.17

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90

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25

89.12

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58.7

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26.3

37.5

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   

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  sin 1 1.33sin45    ; For L1 source only

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1.0

Variation due to refraction (air to water)

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0.4

(9)

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  sin 1 1.33sin45    ; L2 source only

Theoretical value of light intensity variation

I/I0

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Since the default transmitting angle for both the optical sources is 45° and their peak intense light rays intersection point on the interface of two medium is set at the middle position, therefore, the Eq.(8) may be written as

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(8)

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  sin 1 1.33 sin tan 1 L H  

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Here, 1 and 2 are the sensing angle of the PD1 and PD2 10 55 35 55 49.7 25 30.3 64.7 50.5 respectively and it is determined with respect to the horizontal axis between the PD1 and PD2. As can be seen from (6) and 15 60 30 60 41.6 15 33.4 86.7 41.9 (7), the photodetector will receive higher intense of lights when 22.5 67.5 22.5 67.5 30.6 0 37 100 32.12 the sensing angle will be very small and due to the different 45 90 0 90 0 45 45 13.5 13.5 transmitting angle of two light sources the output of PD1 will 50 85 5 48.5 6.65 8.5 46.6 95 10.68 be affected by the light sources L1 only and PD2 will be affected by the L2 only. 90 45 45 32.1 70.1 32.1 70.1 45 0 Now, assumed that the sensor body is inclined in a clockwise Change in transmittance angle after inclination of the sensor body, Change in propagation angle after inclination of the sensor body, Variation of Sensing angle after inclination of the sensor body. Change direction by the angle of θ and the resultant change in refracted of light intensity over the photodiodes; PD1 response for the L1 source and PD2 response for the L2 source. light beams direction across the surface normal of the twophase medium is shown in Fig.1(c) and Fig.1(d). From the figures, it is apparent that the transmitting angle of L1 has increased by the value of θ and decreased as well for the L2 source. So, the Eq.(4) is modified as

Variation due to reflection in water medium 0.2 0

For

(10)

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True Angle [degree]

Fig.2. Theoretical values of light intensity variation over the PD1 photodiode while the default transmitting angle of L1 is 50°.

The above equation (9) and (10) is true for θ ˂ 45° but at θ = 45° light travels in a straight path through the air-liquid interface and made the angle α = β = 90°. Afterward, if the sensor inclined (θ) above 45° range then the light will propagate through air to water medium and consequently, the refraction angle may be written as

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  sin 1 0.752 sin135    ; For L1 source only

(11)

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  sin 1 0.752 sin  45  ; For L2 source only

(12)

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Now, from the equations (9) to (12) the variation of light intensity over the photo-detectors may be determined. For the clockwise inclination of the sensor body in the range of 0° to 90° some theoretical values of transmitting and propagation angle of a peak intense light rays are shown in Table I. It is apparent from Table I that during the inclination range of 0° to 90° the projected light intensity over the PD2 will decrease uniformly, but a non-uniform variation can be seen on the PD1 photodiode. In the range of θ ˂ 45°, the transmitted light ray for the L1 source will be refracted and reflected as well in the range of 0° ≤ θ ˂ 3.8° and 3.8° ˂ θ ˂ 45° respectively. Consequently, for the refracted and reflected regions, two separate uniform variations may be observed. Interestingly, a continuous rising and falling slope of variation can be seen in the range of 5° ≤ θ ≤ 45° or 50° ≤ α ≤90°. In the range of 45° ˂ θ ≤ 90°, the light will propagate through air to water medium and refraction will occur accordingly. Similarly, another continuous rising and falling slope of variation can be seen in the range of 45° ≤ θ ≤ 90°. Again, all the condition will be true and vice versa if the sensor body inclined in the range of 0° ≤ θ ≤ -90°. Now, it can be concluded from the Table I that in the first linear slope, the tilt sensor is able to measure the surface inclination in the range of 0° to ±3.8° and it may be extended further if the transmitting angle (α) is set below the 45°. Secondly, to achieve a continuous variation of the light intensity over the two photodetectors and increase the measuring range, the default transmitting angle (α) should be increased above 45°. In order to reconcile this concept, the transmitting angle (α) is set at 50° by varying the liquid column height in a container and the Eq. (8) to (12) has been modified accordingly for the measuring range of 0° to 90°. Afterward, some theoretical values of change in light intensity over the PD1 detector have been plotted in Fig.2. Besides, the measurement range can be extended further if the distance between the photodiodes increases and keep the liquid height remain constant. Also, the linear range of output can be extended by increasing the detector area over the container wall. If assumed that the output of the photodiode is linear against the light intensity variation then the current state of the liquid pendulum can be determined from the difference of two photodiode outputs.

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B. Sensor Design Overview The cube shape container is made of acrylic sheet to avoid the internal reflection of lights from the container walls. Also, to minimize the effect of ambient lights it is encapsulated by an equal dimension of the opaque box, where the internal surface of the box is polished by anti-reflecting coating. To track the liquid pendulum position within the range of ±90°, dual optical transreceiver has been used. Here, two light emitting diodes (LED) having a wavelength of 640nm is used as near monochromatic light source and the dual photodiode is used as an optical receiver. The BPW34 photodiode is selected for this sensor due to the following reasons; (i) it provides an acceptable linear response for the wavelength range of 520-950nm, (ii) its dimension is suitable for mounting over the sensor body. According to the theoretical analysis, the sensor body is designed for the transmission angle of 50° and the other technical detail of each element is shown in Table II. TABLE II. DESCRIPTION OF THE SENSOR BODY ELEMENTS

Sensor Body Length:32mm Width:32mm Height:13mm Sheet thickness:3mm Liquid height: 13.4mm Detector height : 14.2mm

LED Diameter: 5mm Angle of half intensity: ±30° Wavelength (λ): 630< λ<640 Luminous Intensity: 15mcd

Photo-detector Length:5.4mm Width:4.3mm Height: 3.2mm Separation Gap between them:38mm

(b) Fig.3. Snapshot of the experimental setup. (a) Schematic diagram of the experimental setup, (b) Developed sensor module with SPC unit (1: Opaque sensor housing, 2:Liquid, 3:CVC, 4: Sensor body, 5:PCB, 6:LED driver unit, 7: Interfacing cable to DAQ card)

C. Experimental Setup In Fig.3 a block diagram of the experimental setup is presented. The prototype composed of a sensor body and a signal processing circuit (SPC). The SPC unit consists of dual current to voltage converter (CVC) circuit and LED driver unit. The CVC circuit is designed by a quad op-amp based single chip (LM324) and the LED driver unit consists of two PNP transistors (BC558). The output of CVC is directly connected with the 14-bit analog-to-digital converter (ADC) circuit, which is built in the NI-DAQ card (USB 6009). The SPC unit acquired the photodiode signal and transferred the same into PC through the DAQ card. Besides, to record the CVC output with respect to each light source the LED driver unit has been controlled using DAQ card and LabVIEW programming. Here, a manual control test rig is used to incline the sensor body with the angular interval of 5°. Thereafter, the inclination angle of the test rig is determined and displayed in PC based LabVIEW GUI platform by analyzing the SPC data.

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A. Result Analysis During the inclination of test rig in clockwise and anti-clockwise direction with the angular interval of 5°, the response of PD1 and PD2 is recorded using SPC for the individual optical source of L1 and L2 only. In Fig.4, the converted output of PD1 and PD2 is plotted within the tilt angle of ±90°. It is observed that the output response of PD1 and PD2 for the transmitting light of L1 and L2 is mostly identical in both of the tilt directions. It can be seen from Fig.4, the crosstalk effect of the photodiode output (i.e. PD1 vs. L2 and PD2 vs. L1) is very negligible compared to the direct response between the photodiode and the relative light source (i.e. PD1 vs. L1 and PD2 vs. L2). As the photodiode output has a linear relationship with the change of light intensity thus the PD response has followed the variation of reflected and refracted light intensity for α = 50° as the theoretical characteristics shown in Fig.2. Compared to the theoretical values, it is obvious that the first decreasing slope of PD1 is extended up to 50° instead of 40° and the magnitude of refracted light intensity in the range of 55° to 90° is too low because the Fresnel coefficients of reflection is not accounted in this range of theoretical analysis. Also, this error has been observed between the theoretical and experimental value due to the misalignment of photodiode and design imperfection. Now, from the difference of two photodiode output, a relationship between the sensor outputs and tilt angle has been determined and the resultant output is shown in Fig.4. It is observed from the resultant output that the sensor has five linear regions in the range of -90° to -55°, -50° to -25°, -20° to 20°, 25° to 50°, and 55° to 90° respectively. To select the appropriate linearity and sensitivity, a linear regression method has been applied in each region; is shown in Fig.5. From this analysis, only

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III. EXPERIMENTAL RESULTS AND DISCUSSIONS

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Output Voltage [V]

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1st Measurement data 2nd Measurement data 3rd Measurement data standard deviation

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Relative Error

PD1 Response for L2 PD1 Response for L1 PD2 Response for L2 PD2 Response forL1 Difference of PD1 and PD2 outputs for the L1 and L2 source respectively.

-2

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0.002 0.002

0.000 0.000 -0.002 -0.002 -0.004 -20

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Tilt Angle [Degree]

Fig.4. SPC output voltage variation with respect to the tilt angle

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Standard Deviation

Fig.5. ±(250.006Variation of sensor output in different range of tilting, (a) for0.006 50°), (b) for ±(55-90°), (c) for ±20°. 0.004

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Tilt Angle [degree]

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Output dfference of PD1 & PD2 Regression Curve

True Angle [degree]

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SPC Output Voltage [Volt]

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Y = 0.225*x-0.044 R2=0.99

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True Angle [degree]

Fig.6. Relative error between measured and ideal linear data for the inclination range of ±20° and a standard deviation for repeated measurement

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a range of ±20° has been considered for further measurement due to its high linearity of R2= 0.999 as shown in Fig.5(c) and from the slope of best-fit regression curve the sensitivity of the sensor is obtained as 225mv/°. In order to analyze the static characteristics of the presented sensor, the output of SPC unit has been observed for the three times of measurement. Firstly, the relative error between the measured and ideal linear curve has been plotted in Fig.6 and found the average error of ±0.025. As the output of SPC unit is mostly linear with the true scale of inclination hence, a calibration equation is obtained from the best fit linear regression model to determine the sensor inclination. Thereafter, the SPC output is recorded in PC using NI-DAQ card for each stepping angle of measurement and determined as well as display the sensor inclination in GUI platform. The calibrated output (i.e. measured inclination) vs. true inclination is shown in Fig.7 and observed the deviation of ±0.06% from the ideal linearity. To determine the precision of the device, the sensor repeatability can be used. The sensor output differences between the several repeated measurements are presented in Fig.8. As it can be noticed, the sensor achieved a good repeatability with maximum characteristic drift around ±0.04°. Also, using these three sets of data the standard deviation of measured inclination is drawn in Fig.9 and observed the maximum uncertainty of 0.025°. Based on the resolution of ADC, signal-to-noise ratio and the aforementioned sensitivity the resolution of the sensor is determined as 0.02°. The accuracy of the developed tilt sensor is ±0.035°, which is obtained from the deviation of true vs. experimental data of tilt angles as shown in Fig.10. During repeated measurement, it has been observed that the very small volume of water bubble is stick on the container wall and the air-water interface plane made a curve shaped due to the surface tension of water. Therefore, it may cause of error in several measurement readings which 0.03

Measured Value

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Fig.10. Measurement error of the proposed sensor

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is shown in Fig.6 to Fig.9. In order to compare the performance between the presented tilt sensor and prior art, a comparison TABLE III. COMPARISION BETWEEN PROPOSED SENSOR AND PRIOR ART Sensor type Liquid Capacitive [1]

Linear range ±50° for x-axis ±20° for y-axis

15°

Sensitivity 16.5mv/° for x-axis 57mV/° for y-axis 55.57mV/°

Resolution 0.6° for xaxis 0.1° for yaxis 0.139°

Ferrofluid Inclinometer [6] Magnetic Fluid [8] Fiber-optic[9] Fiber-optic[10] Opto-magnetic[7] Fiber-optic[13] Proposed sensor

±10° ±2.5° 30° ±5° 3.06° ±20°

NA 1V/° 84mV/° 0.56V/° 228mV/° 225mV/°

0.004° 0.02° 0.12° 0.25° NA* 0.02°

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statement is given in Table III and observed the sensor is more sensitive rather than the single trans-receiver [9].

*Not Available

Tilt Angle [degree] -30

-20

-10

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0.08

Y =4.444*x+0.1956 0.06

0.02 0

0.00 -0.02

-10 -0.04 1st Measurement data 2nd Measurement data 3rd Measurement data Linearity error curve

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Percentage Linearity Error

Measured Angle [degree]

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-0.08 -30

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Tilt Angle [degree]

Fig.7. Sensor output linearity against the true inclination range of ±20°.

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1st vs 2nd inclination measurement 2nd vs 3rd inclination measurement 3rd vs 1st inclination measurement

-0.06 -20

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True Angle [Degree]

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Fig.8. Error between several repeated measurements

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Standard Deviation

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Tilt Angle [Degree]

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Fig.9. Standard deviation of the sensor output.

IV. CONCLUSION In the present study, an optical inclination sensor is designed, developed and tested, which is free from any toxic material and used inexpensive sensing material. Compared to the prior liquid pendulum based fiber-optic inclination sensor, the proposed design is very simple and extended the linear measuring range up to ±20° with better static characteristics. In the prior art [9], [10], the diameter of the fiber-optic cable plays a critical role to achieve significant measuring range. In the proposed design this limitation has been overcome by the use of two photo-detectors and LED only. Here, the orientation of optical trans-receiver and volume of liquid inside the closed transparent container plays a significant role to set the sensor measuring range. Thus, the presented method is very eased to fabricate for a customized range of applications and this is the key contribution to the present investigation. This simple concept introduces a mechanical error-free sensor design. So, it is reliable and robust for hazard environmental condition.

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Also, the tilt sensor exhibits good sensitivity (225mv/°) and resolution (0.02°) compared to the prior art, especially in the tilt angle of ±20°. As the sensor module consists of BPW34 photo-diodes hence, its working temperature zone is -40° to 100°. In the experimental setup, it has been noticed that the error is correlated with the misalignment of the light source and photodiode. Also, the liquid sloshing in the container may disturb the stability of the sensor output. Although, it can be optimized by introducing oil instead of air medium but the air pressure inside the container can play a critical role for oil filling. If any bubble is present in the oil medium, the direction of light refraction path may vary. In order to protect the influence of ambient lights, opaque sensor housing is required. The unbalance air moisture level inside and outside of the sensor body may create a reflective surface of the vapor layer. So, in this case, the housing is very useful to protect this natural phenomenon. Moreover, measured range of the tilt sensor could be extended further by implementing a look-up table in other non-linear regions or by using an array of the photodetector in container side walls. Additionally, more experimental data and theoretical analysis are required to build-up the look-up table for the non-linear behavior of the sensor in the range of 25-90°. Also, it may be applicable for dual axis tilt measurement if the light source and photo-detectors are placed on another side of the wall. Therefore, these are the possible directions of our future implementation.

BIOGRAPHIES OF ALL AUTHORS

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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.

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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 more than 8 research papers in the areas of the sensors and transducers, and design of electronics measuring systems.

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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.

ACKNOWLEDGMENT The authors gratefully acknowledge the funding provided by the Science & Engineering Research Board of India under projects “Design and Development of low-cost novel absolute encoder for industrial applications”, number EMR/2017/005036.

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