Instrumentation & Measurement Magazine 24-5 - 20

Non-Contact Liquid Level
Measurement Using
Optical Interferometry and
Neural Networks
Juan Manuel Ramírez-Cortés, Ponciano Rodríguez-Montero,
Pilar Gómez-Gil, and Juan Carlos Sánchez-Díaz
M
easurement of liquid levels inside containers is
an important task in a number of industrial and
scientific processes. In applications such as fuel
monitoring, pharmaceutical industries, wastewater treatment,
and food processes, the knowledge of the liquid level is of primary
concern. Furthermore, the advent of new technologies
and a growing number of applications require exploration of
new sensors and techniques aiming at content level measurement.
Over the years, several methods have been proposed
to achieve this objective with pros and cons according to the
specific area. At first glance, those methods can be grouped
in mechanical [1], capacitive [2], [3], electromagnetic [4], [5],
acoustic [6], ultrasonic [7], optical fiber sensors [8], or self-mixing
interferometry [9], among others. A relevant condition of
these techniques is whether they are invasive or not; for instance,
transducers or sensors attached to the container or even
immersed in the liquid can be unacceptable in certain applications
in the pharmaceutical or food processing industries.
A brief description of the principle of operation of each technique
is as follows.
State-of-the-Art in Liquid
Measurements
Traditional mechanical techniques are based on the use of a
buoy-like floating device inside the container. The position of
the floating device can be then measured by radar, load cell, a
sliding resistance, or another method. Capacitive techniques
applied to the measurement of content level are based on several
principles of functioning, such as detecting differences
on the dielectric constant of the liquid and the air or using
differential pressure sensing. In the case of electromagnetic
techniques, the principle of operation relies on the use of electromagnetic
paired coils in which an excitation coil produces
eddy currents in the liquid with the consequent induction
of a secondary magnetic field. The eddy currents are closely
related to the liquid level, which then can be obtained by
measuring mutual coil inductances. Some variations of electromagnetic
techniques rely on the measurement of induced
fields by using arrangements of electrodes, transformers,
20
conventional electromagnetic flowmeters, or planar and
floating coils [10].
Ultrasonic echo signals are used for content level measurement
purposes under the principle of time-of-flight detection
and subsequent determination of distance between the sensor
and the product surface. Several approaches using single or
multiple sensors, according to the addressed problems, have
been recently reported. A recently proposed technique for
liquid level measurement makes use of a hollow coaxial cable
based on a Fabry-Perot resonator, and microwave signals,
and the liquid-level is derived from the resonant wavelength
shift of the resonator [11]. Most of the reported optical techniques
rely on fiber optic sensors in conjunction with a number
of light sources, which have been found very attractive as the
base of liquid level measurement techniques due to characteristics
such as small size, corrosion resistance and sensitivity.
The principle of operation in these techniques is based on the
changes that the refractive index of the surrounding medium
produces around the optical fiber due to changes in the liquid
level, as in a straight decladded optical fiber [12]. Some
recent approaches include the use of optical fibers with a grating
structure such as Bragg, long period, chirped gratings, and
others [13].
In acoustic resonance methods, which is the approach
used in this work, the container is excited using an impulsive
force, and the generated vibrations are detected using several
sensors, such as accelerometers [14], microphones [15], and
others. In those cases, the liquid level is obtained by the relationship
experimentally determined in certain ranges between
the natural frequencies of the container and the liquid level.
Proposed P-emf Sensor
In this paper, we propose a novel approach based on optical
interferometry for detecting the vibration patterns induced
in a container followed by data analysis using a neural network-based
technique. Fig. 1 shows a block diagram of the
proposed method. A brief explanation of each part is as follows.
The system makes use of a low power He-Ne laser and
an optical setup, aiming to sense the vibrations induced when
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
August 2021
Instrumentation & Measurement Magazine 24-5

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