Journal of Electromagnetic Waves and Applications
ISSN: 0920-5071 (Print) 1569-3937 (Online) Journal homepage: http://www.tandfonline.com/loi/tewa20
Step frequency continuous wave RADAR sensor for
level measurement of molten solids
Yugandhara R. Yadam, Balamurugan T. Sivaprakasam, Krishnamurthy C.
Venkata & Kavitha Arunachalam
To cite this article: Yugandhara R. Yadam, Balamurugan T. Sivaprakasam, Krishnamurthy C.
Venkata & Kavitha Arunachalam (2017): Step frequency continuous wave RADAR sensor for
level measurement of molten solids, Journal of Electromagnetic Waves and Applications, DOI:
10.1080/09205071.2017.1380540
To link to this article: http://dx.doi.org/10.1080/09205071.2017.1380540
Published online: 25 Sep 2017.
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Date: 26 September 2017, At: 00:22
Journal of Electromagnetic Waves and Applications, 2017
https://doi.org/10.1080/09205071.2017.1380540
Step frequency continuous wave RADAR sensor for level
measurement of molten solids
Yugandhara R. Yadama, Balamurugan T. Sivaprakasama, Krishnamurthy C. Venkatab
and Kavitha Arunachalama
Department of Engineering Design, Indian Institute of Technology Madras, Chennai, India; bDepartment of
Physics, Indian Institute of Technology Madras, Chennai, India
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a
ABSTRACT
Step frequency continuous wave (SFCW) RAdio Detection And
Ranging (RADAR) sensor is proposed for non-contact measurement
of the absolute level of molten solids in industrial furnaces. A conical
horn is optimized for operation over 10–16 GHz in mono-static
configuration for level measurement. The absolute distance of a
stationary target was measured with 3 mm accuracy using the SFCW
RADAR sensor in mono-static mode. Stable measurements were
recorded for 4.35 h in a furnace at 450 °C. Level measurements of
molten glass and aluminum in 1100 °C furnace indicate the feasibility
of industrial-level gauging.
ARTICLE HISTORY
Received 22 February 2017
Accepted 10 September 2017
KEYWORDS
Level measurement;
molten glass; molten metal;
microwaves; radar
1. Introduction
Detection of the absolute level of corrosive liquids and fluidized beds is an important requirement in petrochemical, pharmaceutical, glass, metal processing, and waste treatment industries, where the operating temperature and pressure conditions, and the interrogation
medium require remote and real-time inspection. Several techniques have been proposed
to interrogate such medium and they can be broadly classified into two groups namely,
contact and non-contact types. Contact-type methods include mechanical float [1], capacitive sensors [2], two-wire probe [3], fiber optics, and acoustic [4] techniques. The contact-type
sensors typically provide a small measurement range in centimeter and have limited life
time in hazardous and corrosive environment. Though fiber optic sensors can withstand
corrosive, conductive, and flammable conditions, it is only suited for liquids with refractive
index higher than that of the fiber cladding. Acoustic-level sensors are heavily influenced
by the variation in the operating temperature and pressure. Use of optics for non-contact
liquid-level measurement is challenging in the presence of cover gas due to poor visibility
and multiple scattering.
Narrow band RAdio Detection And Ranging (RADAR) sensors operating over 5.8–6.3 GHz
[5], 9.5–10.5 GHz [6], and 24 GHz [7] in pulsed [5], continuous wave frequency modulation
(CWFM) [7] or step frequency continuous wave (SFCW) [6] RADAR mode have been reported
CONTACT Kavitha Arunachalam
akavitha@iitm.ac.in
© 2017 Informa UK Limited, trading as Taylor & Francis Group
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Y. R. YADAM ET AL.
for non-contact liquid-level measurement using dielectric loaded horn, dielectric rod, or
waveguide [5,8–10] in mono-static antenna configuration. Unlike CW, FMCW, and pulsed
RADAR techniques, higher signal to noise ratio and range resolution can be achieved with
SFCW due to the large dwell time and wider band width (BW). A SFCW sensor based on
interferometry was proposed in X band (8–12 GHz) for monitoring the displacement in the
level of molten glass [10]. This technique is capable of measuring the displacement with
sub-millimeter accuracy and requires prior information on the absolute level. Though radarbased level measurement is not new, the limited literature available on radar-level gauging
in high temperature furnace indicates the scope for developing such a sensor. Thus, the
contribution of this paper is on the demonstration of a radar sensor for absolute level measurement in a furnace. In this work, we present a wideband SFCW RADAR sensor in monostatic mode to measure the absolute level of the liquid in furnaces and storage tanks. The
RADAR sensor consists of the optimized horn antenna operated in SFCW mode for real-time
level measurement. The organization of this paper is as follows: the methodology for sensor
design, SFCW data acquisition and processing for automated measurement of absolute level,
and sensor evaluation in furnace are presented in section 2. Results and discussion are presented in section 3 followed by conclusion.
2. Methodology
2.1. RADAR sensor design
A conical horn centered at 13 GHz with 6 GHz BW operated in mono-static mode was chosen
for wideband SFCW operation due to its directional radiation pattern, high gain, circular
polarization, and narrow beam width compared to wideband antennas such as Vivaldi and
spiral. The radiation characteristics of the horn are determined by the aperture diameter
(Da), horn length (L), and diameter of the circular waveguide (d) as indicated in Figure 1(a).
The circular wave guide exciting the conical horn was fed by rectangular to circular weguide
transition of length, Lc shown in Figure 1(b). The waveguide transition was used to convert
the TE10 mode in the rectangular waveguide to TE11 mode in the circular waveguide. The
generation of higher order modes inside the mode converter was minimized using third-order Spline for the transition. The rectangular waveguide dimensions (a × b) were chosen to
support TE10 mode over 10–16 GHz.
Figure 1. Sensor illustration. (a) Conical horn, (b) rectangular to circular waveguide transition.
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2.2. Numerical modeling
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Computer aided design (CAD) models of the antenna and waveguide transition developed
in finite element method (FEM)-based EM simulation software, ANSYS HFSS were used for
sensor optimization. The antenna body and waveguide transition feeding the horn were
assigned as perfect electric conductor (PEC). The antenna was surrounded by free space and
the outer boundary of the free space was terminated with perfectly matched layer (PML) to
absorb outward traveling waves. The computational model solves the 3D vector wave equation inside the computational domain. The EM field maintained by the antenna inside the
computational domain and in the far field was calculated for modal excitation.
2.3. Sensor optimization
The diameter of the circular waveguide, d was set as 19.05 mm for TE11 cut-off frequency, fc
of 9 GHz and 1.1fc of 10 GHz. The horn parameters, L and Da, were numerically determined
to satisfy the design criteria of 15 dB gain, 20° half power beam width (HPBW), and 10 dB
return loss over 10–16 GHz. A WR75 waveguide (a = 19.05 mm, b = 9.525 mm) was used for
the rectangular waveguide. The length of the waveguide transition (Lc) was determined such
that the insertion loss was less than 0.2 dB over 10–16 GHz with TE11 cut-off frequency of
9 GHz in the circular waveguide. To prevent sensor degradation at high temperature and
deposition of fumes, a 5-mm-thick quartz disk was fitted at the horn aperture. The thickness
of the quartz disk was chosen such that the return loss is at least 10 dB and voltage standing
wave ratio (VSWR) is below 2 over the operating BW for the horn with the quartz disk.
Numerical simulations of the conical horn with the mode converter and quartz disk were
carried out to determine the optimal design parameters of the RADAR antenna. The optimized antenna was fabricated in Aluminum using CNC machining with ±10 μm tolerance.
The radiation performance of the fabricated sensor was characterized using a vector network
analyzer (VNA).
2.4. SFCW RADAR data acquisition and processing
CW signal transmission and reception by the radar antenna in SFCW mode was implemented
using VNA for sensor evaluation in laboratory. The frequency of the CW signal was swept
from the lowest (fL) to the highest frequency (fH) in steps of Δf with 1.014 ms dwell time, td.
The sensor reflection measurements in mono-static mode were processed in real time for
automated level detection. The discrete complex measurements, V(k) over fL to fH frequency
were zero padded for alias free time domain signal. The padded complex SFCW reflection
data are given by,
⎧ 0;
0 ≤ k < k1
⎪
V1 (k) = ⎨ V (k); k1 ≤ k ≤ k2 ,
⎪ 0;
k2 < k ≤ ks
⎩
(1)
where ks ≥ 2k2, k1 and k2 are the indices of the low and high sweep frequencies, respectively,
and ks is the alias free sampling frequency. The data were filtered in frequency domain using
Kaiser window function, W1 (k) defined over k1 to k2 with roll factor, β = 6 and center frequency
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Y. R. YADAM ET AL.
same as V(k) to minimize ringing in the time domain RADAR signal. The filtered VNA data,
G(k) = V1 (k)W1 (k) were converted to time domain signal, g(l) using Inverse Fast Fourier
Transform (FFT). To enable automated target detection, the envelope of the time domain
signal, g(l) was extracted using Hilbert transform, g1(l). The signal envelope, h(l) = |g(l) + ig1(l)|
was processed to locate the echoes from the scatterers using the arrival time of the peaks.
Static reflections within the antenna and reflection from antenna–air interface were gated
in time domain for target-level detection. SFCW data acquisition using VNA and signal processing for real-time level monitoring were implemented in LabVIEW® (National Instruments,
USA).
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2.5. Sensor evaluation
2.5.1. Sensor characterization
The free space characteristics of the conical horn were measured in an anechoic chamber.
Reflection measurements of the sensor in mono-static RADAR configuration were recorded
for an 850-mm-long iron rod of 6 mm diameter located in free space at a distance d1 from
the sensor aperture. SFCW measurements were gathered over 10–16 GHz for −10 dBm power
and 3.75 MHz frequency stepping (f). Reflection measurements of the iron rod for varying
distance, d1 were used to determine the sensor measurement range and ability to measure
the target displacement, Δd from d1.
2.5.2. Measurements in furnace
Non-contact range measurements were carried out in a furnace. The furnace heating zone
is 600 × 600 × 900 mm with temperature gradient less than 10 °C. The sensor was fitted in
the furnace through an opening on the furnace wall as shown in Figure 2(a). A 3-mm-thick
quartz plate of 300 × 300 mm was positioned vertically inside the furnace at 835 mm distance
from the sensor aperture. The furnace was preheated to 150 °C and the temperature was
raised to 450 °C at the rate of 100 °C per hour. The temperature inside the furnace and on
Figure 2. Experimental setup for sensor assessment at high temperature. (a) Sensor fitted in furnace
for non-contact range measurement of 3-mm-thick quartz slab (300 × 300 mm); (b) non-contact level
measurement of aluminum and glass melts in 1100 °C furnace.
Note: Thermocouple distances in Figure 2(a) are in mm.
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the RADAR sensor surface was measured using K-type thermocouples. The sensor was air
cooled and SFCW RADAR measurements of the quartz plate were gathered during the heating interval to assess measurement stability at high temperature (100–450°C).
2.5.3. Level measurement of fluidized beds
Figure 2(b) shows the experimental setup to assess the feasibility of non-contact measurement of the absolute level of aluminum and glass melts. The sensor was placed in the line
of sight of the furnace at a distance of 330 mm from the furnace top surface. To avoid heat
loss, the lid was opened for level measurement only at steady-state temperature (1100 °C).
The change in the level of the molten mass was measured by adding raw material to the
crucible. After material addition, the lid was closed until the furnace reached steady-state
temperature. The SFCW RADAR measurements before and after material addition were used
to measure the absolute and change in the level of the molten mass.
3. Results and discussion
3.1. Sensor simulations
Figure 3 shows the simulation results of the conical horn. Figure 3(a) indicates that the
antenna broadside gain for 50 mm cone radius, Da/2 increases with horn length, L and is
greater than 15 dB for L > 70 mm. Figure 3(b) shows that the broadside gain for L = 100 mm
is maximum for 50 mm cone radius. On either sides of this optimal cone radius, the gain
decreased due to the increase in the phase error of the outward propagating spherical wave
front in the horn. Simulations indicated the shortest length of the mode converter, Lc satisfying the design criterion as 34 mm. The dispersion curves in Figure 3(c) for the optimized
horn (L = 100 mm, Da = 100 mm, Lc = 34 mm) show that the TE10 mode of the rectangular
waveguide is converted to TE11 and TM01 modes in the circular cross section of the mode
converter. Figure 3(d) shows the electric field propagation at 13 GHz inside the optimized
RADAR sensor fitted with the quartz disk. Due to the presence of the quartz disk, the outward
traveling wave at the sensor aperture is propagating toward the sensor broadside.
3.2. Fabricated sensor characteristics
The insertion loss of the waveguide mode coverter was measured to be less than 0.4 dB over
10 –16 GHz. Figure 4(a) shows the simulated and measured return loss of the conical horn
with and without the quartz disk. Figure 4(b) shows the VSWR of the radar sensor. From
Figure 4, it can be observed that the measurements satisfy the design criteria of at least
10 dB return loss and VSWR less than 2 with and without the quartz disk. Further, the addition
of the protective cover disk did not significantly affect the antenna characteristics in the
simulations and measurements. The mismatch between the simulation and measurement
is due to the finite VSWR (maximum of 1.3) of the WR75 adapter used in the fabricated
sensor.
Figure 5(a)–(c) shows the normalized antenna radiation patterns for co-polarized incident
plane wave (θ = 0, ϕ = 0 to 2π). Measurements were gathered with the quartz cover plate.
Figure 5(d) shows the sensor HPBW and gain in θ = 0 plane. Sensor radiation pattern measurements are in good agreement with simulations and the side lobe level (SLL) is 30 dB
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Y. R. YADAM ET AL.
Figure 3. Sensor optimization results. Broad side gain at center frequency, 13 GHz for varying (a) horn
length, L and (b) aperture radius, Da/2; (c) mode propagation in the mode converter for Lc = 34 mm; (d)
field propagation in the optimized sensor (L = 100 mm, Da = 100 mm, Lc = 34 mm) at 13 GHz fitted with
5-mm-thick quartz disk.
Figure 4. Comparison of sensor (a) return loss and (b) VSWR measurements with simulations.
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Figure 5. Sensor radiation characteristics with quartz plate. (a)–(c) Normalized radiation patterns and (b)
HPBW and broad side gain in free space.
below the main lobe. The sensor gain is more than 12.8 dB and nearly uniform across the
BW (10–16 GHz). The deviation in the antenna radiation characteristics is agreeably low
between measurements and simulations. The low HPBW of 14.6–24° indicates directional
radiation characteristics. The SLL (30 dB) and HPBW (14°) measurements at the center frequency of the optimized horn are better than the values reported at the center frequency
of the dielectric rod antennas (18.12 dB SLL, 21 ° HPBW) proposed for RADAR-level gauging
[8].
3.3. Sensor SFCW RADAR measurement
Figure 6(a) shows the processed time domain signal, g(l) and the signal envelope, h(l) for
sensor SFCW measurements in free space. The strong reflection in Figure 6(a) is due to the
standing waves in the WR75 coaxial to waveguide adapter. The rest are due to wave impedance mismatch between the mode converter and conical horn, and the horn and free space.
Figure 6(b) shows the processed time domain RADAR signal and its envelope for the 6-mm-diameter metal rod at 980 mm distance from the sensor. In Figure 6(b), the internal reflections
of the sensor shown in Figure 6(a) were time gated till the aperture, i.e. t = 0 in Figure 6(b),
corresponds to sensor aperture. The envelope peak indicated in time scale corresponds to
980 mm in space based on the time of flight calculation. The full width half maximum of the
RADAR signal for the thin wire scatterer is 0.35 ns.
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Y. R. YADAM ET AL.
Figure 6. SFCW RADAR time domain signal and envelope for (a) free space and (b) metal rod in free
space at 980 mm distance from the sensor. Sensor internal reflections in Figure 6(a) were time gated for
automated target detection in Figure 6(b).
Figure 7. Time gated SFCW radar signal envelope of 6 mm diameter and 850 mm long iron rod for (a)
varying distance, d1: (400, 2000) mm, and (b) displacement, ±Δd of 1 mm about d1 = 914 mm.
Figure 7(a) shows the time gated radar signal envelopes for line of sight measurements
of the 6-mm-diameter metal rod at varying distance, d1 in free space. In Figure 7, the time
scale of the RADAR echoes were converted to range using wave velocity in free space. The
peak corresponding to the target location in Figure 7(a) shifted to the right as the rod distance was increased from 490 to 1995 mm. The signal strength decreased with increase in
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distance due to wave attenuation with distance. Sensor range measurements were shown
only up to 1995 mm due to space constraints in the laboratory. Figure 7(b) shows the radar
signal envelope as the rod at 914 mm distance (d1) was moved toward the sensor in steps
of 1 mm (Δd). The rod location was measured as 911 mm (d̂ 1) by the algorithm with 3 mm
error and the peak shifted proportionately from 911 to 908 mm as shown in Figure 7(b). The
error d1 − d̂ 1 was calculated to be less than 3 mm for the 6-mm-diameter metal rod and can
be factored in the signal processing algorithm for reporting the target range. The measurement accuracy could be improved with an Ultra Wide Band (UWB) antenna such as the
Transverse EM (TEM) horn [13]. However, the directivity, gain, and HPBW of a UWB TEM horn
are poor compared to the conical horn. Furthermore, the aperture dimensions of the TEM
horn are larger than the conical horn; hence, it is not suited for closed environment such as
an industrial furnace and storage containers with limited angle of view and space constraints
on the sensor port. The measurement accuracy of the proposed sensor is better than the
pulsed RADAR sensor operating at 5.8/6.3 GHz [5] and the FMCW RADAR sensor [6]. The
dielectric loaded horn antenna was reported to measure up to 35 m with ±10 mm measurement accuracy [5]. Level measurement in a tank indicated measurement error within ±20 mm
for objects at a distance of 0.8 m from the antenna connected to the FMCW RADAR [6]. Better
measurement accuracy was achieved for the proposed RADAR sensor due to SFCW mode
of operation and the wide BW. The measurement accuracy of the proposed sensor can be
further improved with interferometry-based signal processing algorithms reported for SFCW
RADAR-level measurement [7,10].
3.4. Sensor evaluation at high temperature
3.4.1. Measurements in furnace
Figure 8(a) shows the temperature at the sensor aperture and the 3-mm-thick quartz target
measured by thermocouples T1 and T5, respectively, during the heating interval. The overlapping thermocouple measurements in Figure 8(a) clearly indicate nearly uniform temperature distribution inside the furnace shown in Figure 2(a). The outer surface of the air cooled
sensor was less than 100 °C during the measurements. Figure 8(b) shows the time gated
radar signal envelopes measured for the 300 × 300 × 3 mm3 quartz plate at 835 mm distance
from the sensor aperture during the heating interval shown in Figure 8(a). The low amplitude
peaks in Figure 8(b) are internal reflections from the alumina furnace walls. As the slab
thickness is 3 mm, the front and back wall reflections are merged in the measurements. The
location of the quartz slab is measured as 832.5 mm (d̂ 1) at all temperature. Due to the excellent thermal protection provided by the quartz disk fitted at the sensor aperture, stable
measurements were recorded by the RADAR sensor inside the furnace for a long duration
(4 h and 21 min). The softening point of quartz glass (1683 °C) is far greater than the operating
temperature of PTFE (150 °C) and alumina rods (400 °C) used in the pulsed radar sensor [5],
and the plastic-based dielectric rod antennas (PTFE, PP) proposed for industrial-level gauging
[8]. Furthermore, due to dielectric loading, the operating BW of these antennas is lesser
(≤2 GHz) than the BW of the proposed RADAR sensor (6 GHz). Due to larger BW and higher
number of frequency points (1601 points), the measurement accuracy of the SFCW sensor
is better than the pulsed and FMCW RADAR-level sensors [5,7].
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Figure 8. Sensor evaluation in the furnace with a 3-mm-thick quartz plate of 300 × 300 mm size. (a)
Thermocouple measurements near sensor aperture (T1) inside the furnace and near the sample (T5)
during the heating interval (150–450 °C); (b) time gated envelope of the processed radar signals for the
3-mm-thick quartz plate in the furnace at 835 mm distance from the sensor aperture.
Figure 9. High temperature level measurement of molten solids inside 1100 °C furnace. Time gated radar
signal envelopes of molten (a) aluminum and (b) glass. The vertical dashed lines indicate the shift in target
level after material addition. The reduction in the reflection amplitude for molten aluminum after material
addition is due to the merger of reflections from the crucible and aluminum.
3.4.2. Level measurement of molten solids
Figure 9(a) shows the time gated processed signal envelopes of molten aluminum measured
by the RADAR sensor with the furnace lid open. In Figure 9(a), the signal peaks at 395 and
332 mm are from the rim of the crucible containing molten aluminum and furnace top
surface, respectively. The prominent echo at 468 mm indicates the absolute level of the
molten metal in the crucible with respect to the sensor aperture. It should be noted that
wave reflection from the molten metal in Figure 9(a) is more than the reflection from the
interface between the sensor and free space. This is due to the complete reflection of the
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incident wave from the molten metal. The echo shifted toward the sensor with material
addition indicating the new level as 453 mm. Due to the smaller crucible size and the proximity of the aluminum melt from the crucible rim (58 mm), reflections from the crucible rim
and molten metal merged after material addition. This led to reduction in the signal strength
after material addition in Figure 9(a). As industrial furnaces and storage tanks are several
orders larger than the smelting furnace used in this work, such effects are less likely to happen. However, interference from static structures inside the furnace and large container
could potentially interfere with the target echo. Their effect could be handled with adaptive
signal processing [6] and prior knowledge of such features.
Figure 9(b) shows the time gated radar signal envelopes of molten borosilicate glass. The
reflection from the furnace top surface occurs at 332 mm as observed in Figure 9(a). The
subsequent reflections in Figure 9(b) are from the crucible rim and glass melt. The absolute
level of the molten glass from the sensor was measured as 512 mm. With the addition of
glass beads, the peak shifted to 506 mm indicating the change in the level. From Figure 9,
it can be observed that the reflected signal strength is higher for molten metal than the
glass melt as EM wave undergoes complete reflection at the metal interface. The lower
amplitude reflection measured for the molten glass indicates that only a part of the incident
EM wave is reflected at the glass boundary while the remaining is transmitted through the
glass melt. The 3 mm accuracy reported for a nearby target (450–512 mm) in this work is far
better than the measurement error of ±20 mm reported for objects at a distance of 0.8 m
from the FMCW RADAR sensor [7]. Though the dielectric constant of borosilicate glass is
lower, due to the losses associated with the impurities in the glass, the reflection from the
crucible bottom is not measurable in Figure 9(b).
The RADAR sensor measurements in Figures 8 and 9 clearly indicate the ability to measure
the absolute level of lossy and low loss materials maintained at elevated temperature (150–
1100 °C) and stable measurements for long hours (4.35 h) with the sensor fitted inside a
furnace. Unlike the proposed RADAR sensor, other RADAR-level sensors based on dielectric
antennas have limited operating temperature of about 200 °C due to the low melting point
of the dielectric material [8,9]. Furthermore, dielectric rod or tube antennas cannot be flush
mounted in all furnace port as EM wave propagation inside the dielectric will be influenced
by the refractory material surrounding the port. Dielectric antenna design using high dielectric constant material such alumina can yield better temperature stability (400 °C) as
reported in [5]. However, such antennas will have limited BW and higher SLL due to the
higher impedance mismatch with the free space. The results strongly support that the proposed RADAR sensor with SFCW operation and forced air cooling could be used for absolute
level measurement in industrial furnaces and storage tanks for longer duration with minimal
maintenance.
4. Conclusion
A non-contact wideband microwave sensor is presented for absolute level measurement of
low loss and lossy, solid, and fluidizied beds at high temperature (200–1100 °C). Due to the
high gain, low SLL, and narrow beam width of the RADAR sensor, encouraging results were
obtained for small size targets in laboratory furnaces. The 3 mm accuracy measured for a
static metal rod is on par with commercial non-contact ultrasound and optics-based level
sensors available for level monitoring in process vessels and storage tanks. The measurement
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Y. R. YADAM ET AL.
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accuracy can be further improved with phase-based interferrometric signal processing. The
additional advantage of the wideband SFCW RADAR sensor is the ability to operate at higher
temperature (>200 °C), variable pressure conditions, and in the presence of cover gas.
Furthermore, the time series RADAR signal can be processed to identify stationary and
non-stationary targets and monitor the change in the absolute level of the non-stationary
fluidized bed. The targets in industrial furnaces are relatively stationary unlike those often
encountered in surveillance and security applications. Thus, the time delay in data acquisition
and processing associated with SFCW RADAR (few ms) is less likely to impact industrial-level
gauging. Preliminary high temperature measurements indicate that the wideband SFCW
RADAR has potential for real-time level measurement in industrial furnaces.
Acknowledgments
The authors thank Dr. Jayesh Shah, Head Matrix Development Section, Bhabha Atomic Research Centre,
India for glass beads, Prof. Krishnan Balasubramanian, Department of Mechanical Engineering, IITM for
access to the VNA, and Mr. G. Balaganesan, Central Workshop, IITM for access to the smelting furnace.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work is funded by the Board of Research in Nuclear Sciences (BRNS), India [grant number BRNSIITM MoU].
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