Microwave radiometry method and device for measuring the temperature of a moving, textile material

The temperature of a planar material (8) moving down a process path is measured by causing the material to pass through two openings (3,4) made in a waveguide (1) in such a way that the material effectively does not cut the electric field lines present on the walls of the guide (1) and also so the material (8) passes through the guide in a direction generally parallel to the electric field in the propagation mode of the waveguide and through a region of maximum field strength. The temperature of the planar material (8) is determined by measuring the thermal noise emitted by the material (8) as it passes through the slotted waveguide (1). The slots (3,4) for a rectangular waveguide operating in the TE.sub.10 mode are made along the centerline of the two broad sides of the guide.

This invention concerns the measurement of the temperature of a planar 
material, based upon said material's emittance of thermal noise in the 
microwave portion of the electromagnetic spectrum; specifially, it 
concerns the measurement of the temperature of a material in process, ie. 
moving down a production line in the course of its manufacture or 
processing, said material being for example one such as a textile, 
including a knitted fabric, or paper or a non-woven fabric. 
Temperature is very often a paramount parameter in manufacturing as well as 
in processing and finishing operations on a material. Moreover, when such 
operations are part of a continuous process, it is important that the 
temperature be kept constant for the duration of the process to ensure 
consistent characteristics for the final product. 
There are many known techniques for measuring the temperature of a planar 
material in process. Generally, these measure the temperature of the 
surrounding medium in which the material is moving, for example the air 
temperature in a tenter drying section, by means of thermometers placed in 
various locations within the section. However, this technique does not 
provide temperature data about the material itself, which depending upon 
its structure and composition may react in different ways to a given 
ambient temperature. Another temperature measurement technique makes use 
of the material's radiant emittance in the infrared frequency range; this 
technique effectively measures the temperature of the material itself, 
rather than of its environment, since what is measured is the incoherent 
radiation or thermal noise emitted by the material, to derive its 
temperature. Since the electromagnetic emissions of materials is greatest 
in the infrared range at temperatures close to room temperature, infrared 
sensors have come into widespread use. Yet the very principle of this type 
of measurement entails drawbacks which considerably restrict its 
industrial applications: IR radiation from deep within the material is 
attenuated by the material itself before reaching the receiving sensor. 
Besides, the radiation emitted in this range of wavelengths by a given 
material at a given temperature, varies according to the color of the 
material--an obvious liability in the textile industry. 
The prior art in the medical field otherwise teaches the use of microwave 
thermography, which is also based upon the thermal noise emitted by a 
body, albeit in this case in the microwave range. The use of these 
wavelengths enables measurement of the temperature of a volume of the body 
and not just of the surface alone, since microwaves, up to a certain 
depth, are not arrested by the body itself. In this case, the difficulty 
resides in the sensitivity of the sensors, since the emitted energy of the 
same body in the microwave frequencies is much weaker and is in fact on 
the order of 10.sup.8 times less than the radiant energy emitted in the 
infrared range. Besides this, the sensor or transducer must measure the 
thermal noise emitted by the material whose temperature one wishes to 
monitor and not the noise emanating from the material's environment. In 
the medical field, detection is accomplished by using a flat sensor that 
is applied to the outside surface of the volume under investigation: 
living tissues emit thermal noises which are picked up by the sensor and 
selected in a band of frequencies within the microwave range, then 
transmitted to a wideband receiver-amplifier tuned to a frequency within 
said band. The sensor, being applied directly to the skin, receives only 
the thermal noises emitted by the volume being investigated and not those 
from the surrounding medium. This technique is not practical for 
thermography in manufacturing or otherwise processing moving, planar 
materials. In fact the sensor's direct contact with the material could 
cause drastic defects. Besides, the radiant emittance of planar materials 
like textiles, being relatively small compared with that of living 
tissues, yields a very low-powered received signal. Furthermore, 
textile-type materials exhibit low dielectric losses (especially polyester 
and polyamide) and their thickness being substantially less than a 
microwave wavelength, the thermal noise emitted by the surrounding medium 
is transmitted to the receiver-amplifier via the sensor applied to the 
material, which no longer acts as a shield. These signals from the 
surrounding medium are often more powerful than those emitted by the 
material itself. 
We have discovered a method, which is the object of this invention, for 
measuring the temperature of a planar or flat material being conveyed in a 
process situation, based upon the radiant emittance of said material in 
the microwave region of the electromagnetic spectrum, which method does 
not have all of the above-mentioned drawbacks. This method consists in 
running the material through a waveguide, through two openings made in 
such a way that they effectively do not cut the electric field lines 
present on the walls and by the same token provide for the material in the 
waveguide to move in a general direction parallel to the electric field in 
the propagation mode of said guide and pass through a region of maximum 
field strength, and in measuring the thermal noise emitted by the material 
during its passage through the slotted waveguide. Thus, the material goes 
through the slots provided in the waveguide without making mechanical 
contact with the latter; the direction of conveyance of the material and 
the choice of slot location enable, on the one hand, a very satisfactory 
coupling to be obtained between the material and the waveguide and, on the 
other hand, a reduction of the parasitic radiation of the ambient medium 
to an acceptable level not masking the radiation emitted by the material 
inside the waveguide. In fact, since the material, in keeping with the 
method of the invention, passes through a region of maximum field and 
parallel to said field, the efficiency with which emitted energy is 
transformed into received energy will be close to 100%: all the thermal 
noise emitted by the material will be able to be transmitted to the 
receiver. In addition, since the openings or slots made in the waveguide 
are located such as to practically not cut the field lines present in the 
walls, there is little leakage one way or the other: the leakage affecting 
the thermal noise emitted by the material in the waveguide is low, and the 
leakage affecting the thermal noise emitted by the ambient medium outside 
the waveguide is low. 
The inventive method is particularly suitable for a process subjecting a 
material to a continuous heat treatment, in which one wants to control the 
heating means on the basis of the temperature actually exhibited by the 
material itself. In this case, the heat treatment means are regulated 
according to the radiant energy measurement obtained. According to a 
preferred control mode, the difference in radiant energy between the 
material, on the one hand, as it moves through the slotted waveguide, and 
a control sample of the same material, on the other hand, which has been 
heated to the desired temperature, is measured and the heat treatment 
means are controlled on the basis of the observed difference. This type of 
control is particularly worthwhile because it frees one from having to 
consider the dielectric properties of the material. Indeed, each material 
exhibits its own particular loss characteristic as a function of 
temperature. It is therefore normally necessary to be familiar with this 
loss-temperature relation in order to translate emissivity information 
into temperature information for a given material, and to draw up the 
corresponding scaling curves. The differential-emittance method according 
to the invention precludes the need for such scaling curves because the 
control of the heat treatment means is a null-seeking control, ie. it 
seeks merely to cancel any observed difference between the emitted 
radiation of the material in process and the control material at the 
desired, preset temperature. A positive deviation or negative deviation of 
the material-in-process radiant emittance will trigger action either on 
the heating means or the material feedrate to cancel this deviation. 
Preferably, the general direction followed by the material as it is 
conveyed through the process is not perpendicular to the direction of 
propagation of the wave in the waveguide. 
It is another object of the invention to provide a device specifically 
designed to implement the above-described measurement method. This device 
consists of: 
a waveguide provided with two slots designed to allow the material to pass 
therethrough in proceeding down the process path, said two slots being 
made such as to effectively not cut the field lines present on the walls 
of the waveguide and so that the plane passing through the two slots is in 
a region of maximum field strength, parallel to the electric field in the 
waveguide's propagation mode; 
and a receiver-amplifier connected to the waveguide and operable to measure 
the thermal noise emitted by the material as it passes through the 
waveguide. 
In the case of a rectangular waveguide operating in the TE.sub.10 mode, the 
two openings are made along the centerlines of the two broad sides. In 
this type of waveguide, which is well-known by microwave specialists, the 
electric field has a general orientation which is perpendicular to the two 
broad sides of the waveguide and a maximum component in the median plane 
of the waveguide (FIG. 1); also, looking at the configuration of the field 
lines on the waveguide walls (FIG. 2), it can be seen that none of these 
lines is cut by the plane passing through the center of the two broad 
sides of the guide. There will therefore theoretically not be any 
interferences with the wave's propagation, nor any conflict with the 
thermal noise emitted by the outside resulting from said two openings, 
whose length is not limited. In fact, the planar material whose 
temperature one wishes to measure having a certain thickness, it is 
necessary to provide openings allowing contact-free passage of the 
material and thus large enough to ensure that no contact will occur. To 
keep leakage to a minimum, it is nevertheless necessary for the width of 
each opening to be at most equal to a third of the length of the long 
dimension of the cross-section of the waveguide. 
In the case of a circular waveguide operating in the TE.sub.10 mode, the 
two openings must be made following the two diametrically opposite 
generatrices facing the regions of maximum field strength. 
It is within the competence of a person skilled in the art to position the 
openings for passage of the planar material so that the criteria defined 
in the foregoing are complied with.

The device according to the invention comprises the waveguide 1 and the 
receiver 2. The waveguide 1 has a rectangular cross section with a long 
dimension of length a and a short dimension of length b. It operates on 
the fundamental mode TE.sub.10 and the wavelength .lambda. corresponding 
to the receiver's tuned frequency is such that a&lt;.lambda.&lt;2a. For a 
frequency between 2 and 4 GHz, the dimensions of the rectangular 
cross-section of the waveguide 1 will for instance be: a=8 centimeters, 
b=4 centimeters. For a 10 GHz frequency, they will be respectively: a=3 
cm, b=1.5 cm. The waveguide 1 is slotted by two openings 3 and 4, made 
along the centerlines of the two broad sides of the guide, respectively 
labelled 5 and 6. The width of the two openings 3 and 4 for passage of the 
cloth 8 is 2 cm when a=8 cm and 0.75 cm when a=3 cm. The waveguide 1 is 
closed at one end by a moving short circuit 7 whose position within the 
guide 1 can be adjusted to match the impedance with the material present 
in the guide 1. The other end of the guide 1 is connected to the receiver 
2. The cloth 8 goes through the guide 1 by entering through opening 3 and 
leaving through opening 4. A compressed air supply, not shown, can be 
included to create a slight overpressure inside the waveguide 1 relative 
to the surrounding medium; this is especially worthwhile if the 
surrounding medium is polluted with dust or vapors and the like, to 
prevent fouling of the inside of the waveguide and the attendant 
disturbance of emissivity measurements. Also, if large openings 3 and 4 
are used, it can be worthwhile, for the purpose of preventing 
disturbances, to extend each opening with two outer lips 15 and 16 
arranged parallel to one another and to the general direction followed by 
the cloth 8 as it passes through the guide 1 (FIG. 3c). 
Operation of the device is as follows: 
The receiver 2 is tuned to a center frequency in the microwave frequency 
spectrum, from 0.5 to 10 GHz, specifically to 2.45 GHz for example. The 
waveguide 1 picks up the thermal noise signals emitted in its interior 
space, in the frequency band around the center frequency of the receiver 
2. In fact the thermal noise emitted inside the waveguide 1 comes 
essentially from the cloth 8 the temperature of which is to be measured, 
since the air contained in the guide radiates practically no thermal noise 
when compared with the noise emitted by the cloth, and the location 
selected for the two openings 3 and 4 avoids disturbances from the 
outside. The short circuit 7 is moved at will within the waveguide 1 to 
adapt the length of the openings 3 and 4 to the breadth of the cloth 8 
passing through the guide 1, after first calibrating the receiver 2 as 
required for the specific cloth 8 the temperature of which is to be 
measured, knowing that depending on the cloth's makeup, its loss factor 
will differently vary with temperature. 
If the cloth 8 is stationary in the waveguide 1, the thermal noise signals 
emitted by the cloth and transmitted by the waveguide to the receiver 2, 
and also translated to temperature, correspond to the mean temperature of 
the volume of cloth 8 held in the waveguide, between openings 3 and 4. 
When the cloth 8 moves along, the thermal noises emitted thereby are 
integrated by the receiver 2 at limited time intervals, enabling 
substantially the instantaneous and mean temperature of the entire strip 
of cloth 8 passing through the waveguide 1 to be known, and therefore 
enabling the deviations between said temperature and the desired 
temperature to be known. A better matching of impedance is obtained when 
the waveguide 1 is skewed with respect to the cloth's path, as illustrated 
in FIG. 3b, in other words when the general direction of cloth motion 
(arrow S.sub.1) is not perpendicular to the direction of propagation of 
the wave in the waveguide (arrow S.sub.2). 
The differential emittance-based temperature control installation shown in 
FIG. 4 comprises two waveguides 1 and 1' of the same type as those 
previously described. The first guide 1 is traversed by the cloth 8 as the 
latter is being thermally treated in enclosuer 9; said enclosure is 
equipped with heating means 10. After going through enclosure 9 the cloth 
8 is taken up on a spool 11, driven by a variablle speed motor 12. The 
second guide 1' is exactly like guide 1; it is located in an insulated 
enclosure 13 equipped with heating means not shown in the drawing making 
it possible to vary the temperature inside the enclosure 13 and to 
accurately maintain a given temperature in said enclosure. Enclosure 13 is 
provided with infeed means with which the operator can place a sample 8' 
of the cloth 8 into the waveguide 1', said being of the same width as the 
process clock 8. The two waveguides 1 and 1' are both connected to the 
same receiver 2, but due to the action of a bistable switch 14 the 
receiver 2 can only receive the thermal noise signals from one waveguide 
at a time. The receiver 2 is connected either to the heating means 10 or 
to the motor 12. 
Operation of the system or installation as a whole is as follows. The 
operator introduces into waveguide 1' a sample 8' of the cloth 8 to be 
treated. The operator adjusts the heating means of enclosure 13 so that 
the temperature inside said enclosure reaches the correct treatment 
temperature T for the cloth 8. When said temperature T has stabilized 
inside enclosure 13, the treatment installation can be started up. The 
cloth 8 traverses waveguide 1, located in enclosure 9. Thanks to the 
heating means 10, which may be for example hot air generators, the inside 
of the enclosure is heated to a temperature T.sub.1 greater than T, so 
that the cloth 8, which is at the outside ambient temperature when it 
enters enclosure 9, gradually heats to treatment temperature T and stays 
at this temperature. 
This temperature rise and temperature maintenance can be accomplished by 
operating different heating units 10 disposed throughout the enclosure 9. 
The waveguides 1 and 1' pick up the thermal noise signals emitted 
respectively by the process cloth 8 moving through waveguide 1 and by the 
cloth sample 8' held stationary in waveguide 1'. At regular time 
intervals, the receiver 2 is alternately connected to guide 1 and guide 1' 
and successively measures the emittance of one and the other. A correction 
element, operating according to the cloth 8 transport speed, makes 
possible to compare the measurement outputs and reveal the differences in 
emitted radiation. If the emittance of process cloth 8 is greater than 
that of the reference sample 8', this means that the temperature of the 
cloth 8 is higher than the treatment temperature T and the receiver acts 
either upon the heating means 10 or the motor 12, respectively to lower 
the temperature T.sub.1 in enclosure 9 or to speed up the cloth 8 
transport rate. Conversely, if the difference is reversed, receiver 2 acts 
upon said heating means 10 or motor 12 to increase the temperature T.sub.1 
in enclosure 9 or cut back the cloth transport speed. In practical term, 
receiver 2 is adjusted or set so that it will act on means 10 or motor 12 
only when the detected error, whether positive or negative, reaches a 
given value corresponding to the allowable temperature spread for the 
treatment. For instance, for the thermofixing of dyes after the printing 
of a polyamide fabric, the temperature T would be 125.degree. 
C..+-.2.degree. C. The controlling receiver 2 would then be set to act 
upon the heating means 10 or the motor 12 only if the detected deviation 
in thermal noise signals exceeds a value corresponding to 2.degree. C. 
In the embodiment described in the foregoing, the specific planar material 
being processed was a cloth. However, the inventive method, device and 
installation can be applied equally to any type of textile, knit or 
screen, or to nontextiles such as nonwoven fabric or paper.