Microwave sensor for molten glass level measurement
Introduction
The industrial glass furnaces are provided with level sensors and controlled by an electronic feedback system aimed to keep constant the level in the melting chamber.
These glass level sensors, in addition to the ability to work in extremely hostile environments, should meet a series of requirements in terms of measurement accuracy, long term stability, insensitivity to electromagnetic interference and reduced maintenance.
Here we review briefly the state of art of current level sensors for molten glass. Unfortunately, the scientific open literature is very poor in this specific field, probably due to the fact that it is a small industrial sector not very accustomed to share openly information and experiences. So what follows is not based on specific bibliographic references, but rather on the personal experience of authors, speeches with experts and web sites of companies operating in the field.
At the knowledge of authors existing sensors can be basically divided in four categories:
- (1)
Contact–proximity sensors: A metallic or ceramic tip, equipped with a platinum head, touches the molten glass and record the level with a position encoder. This technique suffers from problems of mechanical wear and thermal deformations due to temperature variations. Moreover, it is possible that a drop of glass remains attached on the tip of the sensor causing a significant measurement error.
- (2)
Optical systems: In the walls of the chamber there are two windows, a laser transmits a light beam through a window towards the glass surface, and the reflected beam is revealed by a sensor beyond the other window. Knowing the measurement geometry it is possible to determine the glass level by measuring the position of the reflected beam on the sensor. The laser emitter, however, requires frequent maintenance by specialized operators, and the revealing sensor has to be frequently cleaned due to the accumulation of volatile components on the lenses, and after these operations the system has to be newly calibrated. This results in a low availability of this kind of measurement systems.
- (3)
Radioactive systems: a small quantity of radioactive material is inserted at one end of the molten glass basin, and at the opposite end a receiver reveals the level of radioactivity. In this way it is possible to determine the mass of molten glass between emitter and receiver, thus the level of the glass. This system, however not widespread, suffers from safety problems that discourage its wide use. Furthermore, the radioactivity levels permitted by safety regulation are so low as to affect the measurement accuracy that is often not satisfactory.
- (4)
Electrical systems: two platinum electrodes are immersed in the molten glass, at a known distance. Measuring the variation of electrical resistance between the electrodes, it is possible to evaluate the change in the mass of glass, thus the glass level. The major drawback of this technology is due to the fact that glass resistivity is strongly related to its temperature, thus changes in measured resistance due to variations of temperature are often of the same order of magnitude of those due to the variation of the level. Consequently this method provides low measurement accuracy.
With the aim of overcoming some of the previous discussed drawbacks, the authors of this paper propose an interferometric radar as sensor for measuring molten glass level.
The use of radar as level gauge surely is not a novelty [1]. Many devices has been proposed operating in different bands 5.8 –6 GHz [2], 9.5–10.5 GHz [3], 24 GHz [4], with pulse modulation [2] or CWFM [4], using horn [2] or dielectric [5] antennas. This technology is rather consolidated, nevertheless it has never applied to glass furnaces. The reasons are probably the following:
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the required accuracy is at least 1 mm (possibly 0.1 mm), that is a very severe requirement for a radar system;
- (2)
the sensor has to operate in the hard environment inside a furnace
- (3)
vapour and flames over the glass surface produce turbulences that perturbs the propagation of the electromagnetic waves.
It is should be noted that although currently a company [6] commercialises a radar gauge for applications in iron furnaces, its accuracy (5 mm) and stand-off distance is not compatible with the requirements of glass industry.
Therefore, in order to meet the strict issues and requirements of the glass industry, the authors designed and tested a radar with specific features that characterizes it with respect to those in the scientific open literature:
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it operates a very large bandwidth (4 GHz);
- (2)
it applies radar interferometric techniques for retrieving the differential displacement;
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the perturbing effects of turbulences are minimized operating in close proximity of glass surface;
- (4)
its probe is cooled with water and can be inserted in the furnace through a side window.
Section snippets
Theory of operation
Radar interferometry is a consolidated technique used since many years in civil engineering for detecting with sub millimetric accuracy structural displacements of buildings [7], [8], bridges [9], dams [10], as well as for measuring deformations of volcanoes [11] and other natural scenarios [12].
For the application described in this paper a Continuous Wave Stepped Frequency (CW-SF) radar was designed and realized. It operates transmitting step by step a sweep of monochromatic waves. For each
The radar sensor
The radar system designed for this application is a CWSF sensor working in X band, arranged in a way to be used in a furnace in a very harsh environment.
A block diagram of the instrumentation is sketched in Fig. 4. A Vector Network Analyzer model HP 8720A generates a sweep of frequency tones over a band (B). Transmitted signal is then amplified through a power amplifier (PA) and it is applied to the furnace probe. PA is model Ciao Wireless CA812-204, gain 20 dB, NF 3 dB. This is a specially
Laboratory tests
First of all the system was tested in laboratory in order to assess its measurement performance. A special setup was prepared, as shown in Fig. 5. The probe was placed on a metal strut, downward, and the target was a steel Plate 50 × 50 cm wide, placed on a metal support.
This was a good approximation of the measurement conditions, as the molten glass is a good electrical conductor and the surface is a perfect plan. Microwave measurement was compared with the measurement made with a digital
Field tests
Following the satisfactory results of laboratory tests, the sensor was tested in a glass plant in real conditions, in order to verify the effectiveness of the proposed technique for measuring molten glass level.
The first test was carried out in a very small size furnace filled of molten glass. Picture in Fig. 9 shows the probe during this test. The microwave probe was inserted in the furnace through a lateral inspection hole, located in the channel between the melting chamber and the glass
Conclusion
A microwave level sensor for molten glass able to operate in an industrial furnace has been proposed and tested in operative conditions. Its performances has been fully satisfactory, in particular its accuracy that is significantly better then conventional mechanical sensors.
Nevertheless, it should be noted as this sensor is based on radar interferometry, the measurement is affected by an intrinsic ambiguity of half-a-wavelength, so the high accuracy of this sensor in detection of variations is
Massimiliano Pieraccini graduated in physics in 1994 (“Nello Carrara” degree prize) at the University of Florence, Italy, and received the Ph.D. degree in non-destructive testing in 1998. In 1995, he joined the Department of Electronics and Telecommunications of the University of Florence, where in 1997 he gained the permanent position of Assistant Professor. Since 2005, he has been an Associate Professor. His research interests has included optoelectronic sensors, ultrasound transducers, 3D
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Massimiliano Pieraccini graduated in physics in 1994 (“Nello Carrara” degree prize) at the University of Florence, Italy, and received the Ph.D. degree in non-destructive testing in 1998. In 1995, he joined the Department of Electronics and Telecommunications of the University of Florence, where in 1997 he gained the permanent position of Assistant Professor. Since 2005, he has been an Associate Professor. His research interests has included optoelectronic sensors, ultrasound transducers, 3D acquisition systems, interferometric radar, and ground penetrating radar.
Daniele Mecatti was born in Florence, Italy, in 1974. He received the degree in Electronic Engineering and the Ph.D. degree in electronic systems engineering from the University of Florence, Florence, Italy, in 2001 and 2006, respectively. He had a researcher position at the University of Florence from 2005 to 2012. Currently, he is dealing with design of industrial sensors.
Devis Dei was born in Castelfiorentino, Italy, in 1975. He graduated in Electronic Engineering at the University of Florence, Florence, Italy. In 2010 gained his Ph.D. graduation in Electronic Systems Engineering. Currently he holds a post doctoral position at the Department of Information of the University of Florence.
Filippo Parrini was born in Florence, Italy, in 1973. He received the degree in Electronic Engineering and the Ph.D. degree in Electronic Systems Engineering in 2003 and 2007 respectively. He was assistant professor at the University of Florence from 2011 to 2013. His research activities are mainly focused on design and development of radar and microwave systems for industrial and civil applications.
Giovanni Macaluso was born in Prato, Italy, in 1973. He received the his degree in Electronic Engineering and the Ph.D. degree in Electronics Systems Engineering at the University of Florence, Florence, Italy, in 2002 and 2007, respectively. He worked as a researcher at the University of Florence until 2013. Currently, he is dealing with design of industrial electronics and sensors.
Alessandro Spinetti was born in Pistoia in 1977. He is graduated in electronic engineering at the University of Florence, Italy, in 2003 and obtained his Ph.D. in Electronic System Engineering at the University of Florence, Italy, in 2007. He worked as a researcher at the University of Florence until 2013. His research interests has included 3D acquisition systems, radar systems, real-time data acquisition and control systems. Currently, he is dealing with design of industrial electronics and sensors.
Fulvio Puccioni was born in Castelfiorentino, Italy in 1964. He received the degree in Mechanical Engineering in Pisa University in 1990. He was the founder of the company Glass Service s.r.l. in 1994 active in the glass industry and specialized in glass melting furnace. Currently he is the Company President.
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