Abstract:
The present invention relates to a device for the detection and measurement of the physical-chemical features of materials in the form of sheets comprising a microwave sensor coupled to a passive reflector on the other side of the material to be measured. The invention further comprises calculation methods and algorithms for processing output data, adapted to make the measurement immune from the typical environmental factors of industrial environments.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the technical field of the detection and measurement of the physical-chemical features of materials in the form of sheets, films, fabrics, layers deposited on a support or the like, such as, for example, paper and cardboard, plastic films, skins, layered and granular materials in the food-processing industry, fabrics, deposits of powered and granulated materials, such as ceramics and the like. 
         [0002]    Said detections and measurements are performed without the direct contact between the measuring device and the material to be measured. Said physical-chemical features comprise, for example, overall dielectric constant, surface density, consistency, i.e. the water-to-dry-part ratio, humidity or thickness. 
       BACKGROUND ART 
       [0003]    The detection and measurement of the physical-chemical features of materials, such as, for example, surface density, consistency, thickness and, more specifically, degree of humidity, are particularly important in the case of material production processes by means of deposition or processing of layers in which given features need to be monitored during the process itself. 
         [0004]    In most cases, technologies which allow a contact detection can be used, for example by means of the device described in Patent IT1367854 by the Applicant of the present patent application. In most other cases, however, the sensor cannot be placed in direct contact with the material to be measured because the material is delicate and fragile. This is the case, for example, of sheets of paper which may be only several tens of μm thick, or of granulated or powdered layers in the pharmaceutical, ceramic or food-processing industries. 
         [0005]    Moreover, in other cases, having sensors in contact with the material is not advisable for problems related to the contamination of the examined material, such as, for example, in the pharmaceutical and food-processing industries. 
         [0006]    In order to allow contactless measurements, optical sensors, typically based on the so-called NIR (Near Infra Red) technology, are used in the prior art. 
         [0007]    The advantages of the NIR technique are essentially related to the possibility of making a sensor which does not need to come into direct contact with the material and which works on only one side of the material without requiring receivers or reflectors placed on the opposite side, thus simplifying the overall installation of the device. 
         [0008]    However, the NIR technique has some major drawbacks, such as, for example, sensitivity to the surface layer only in case of materials which are not particularly thin, and low immunity to surrounding conditions, in the case of very thin materials, because most of the energy employed in the scan may pass beyond the material. Additionally, the use of devices based on the NIR technique requires to monitor also the direct illumination of the material being measured, which illumination may disturb the measurement itself. The color of the material may also affect the measurement performed with this technology. 
         [0009]    Using radio frequency for measuring chemical-physical parameters in general, and for measuring humidity in particular, is known. 
         [0010]    Water has a high dielectric constant which significantly interferes with the surrounding electromagnetic fields thus allowing to detect its presence, quality and features. For example, the use of microwave sensors to measure humidity in thin materials is the object of the following patents. 
         [0011]    U.S. Pat. No. 3,681,684 describes a number of possible solutions to the technical problem of measuring humidity in a sheet of paper. It tackles the problem of the stationary wave which occurs, in systems of this type, because of the interference between the transmitted wave and one or more reflected waves or between one or more mutually reflected waves. This causes the presence of maximum and minimum points in the electric field, and therefore the measurements may be strongly dependent on the mechanical configuration of the system and thus be extremely sensitive to possible changes of position. The device according to U.S. Pat. No. 3,681,684 solves the aforesaid problem with a broadband modulation or with a dielectric mechanical modulator. 
         [0012]    The device according to U.S. Pat. No. 3,681,684 comprises an antenna adapted to transmit a signal across a sheet the degree of humidity of which is intended to be determined, said signal crosses the sheet being measured, is reflected by a reflector, crosses the sheet being measured again and is received by the antenna itself. The reflected signal is thus detected by a directional coupler. 
         [0013]    U.S. Pat. No. 4,578,998 describes a device adapted to produce microwave fields which cross the sheet being measured in order to detect the humidity thereof. The receiver and the transmitter are on the two opposite sides of the suggested array, placed on the same side, and are used to detect the applied power and the reflected power. The system has two independent reading systems with different polarization, each of which comprises three receivers. 
         [0014]    As the measuring receiver is placed on the opposite side with respect to the transmitter, the device according to U.S. Pat. No. 4,578,998 has the problem that the displacement of the device along the sheet being measured is much more complicated and practically not feasible. Moreover, the structure of the device according to U.S. Pat. No. 4,578,998 tackles the problem of transmitter creep by using a high number of receivers thus making the structure complicated and large. 
         [0015]    U.S. Pat. No. 4,620,146 suggests a device which allows to measure the humidity in a sheet of paper using a transmitter and a receiver arranged on the same side of the sheet. The problem of the stationary wave and of the correct positioning of the sheet being measured is tackled by finding an optimal arrangement which makes the system not very flexible and not easy to be managed in small available spaces. 
         [0016]    It is an object of the present invention to suggest a device for the detection and measurement of the physical-chemical features of materials in the form of sheets, films, fabrics, skins or layers, which is an improvement with respect to the prior art, and which in particular allows to: be more insensitive to the positioning of the material being measured and to environmental conditions in which the measurement is carried out, so as to have a structure which allows flexible use and positioning and is simple to be constructed and implemented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a functional block chart of the device according to the invention. 
           [0018]      FIG. 2  shows the approximate positioning of the antennas and of the dihedral reflector in a preferred embodiment of the device according to the present invention with respect to the material the features of which are measured. 
           [0019]      FIG. 3  shows a reflector of the “multi-dihedral” type according to a second preferred embodiment of the present invention. 
           [0020]      FIG. 4  shows a reflector of the grid type with radio-absorbing material according to a third preferred embodiment of the present invention. 
           [0021]      FIG. 5  shows a reflector of the double-reflection grid type according to a fourth preferred embodiment of the present invention. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0022]    The present invention relates to a device for the detection and measurement of the physical-chemical features of materials in the form of sheets, fabrics, skins or layers, comprising an electromagnetic sensor, preferably a microwave sensor coupled to a passive reflector on the other side of the material to be measured. 
         [0023]    The invention further comprises methods and calculation algorithms for processing output data, adapted to make the measurement immune from the typical environmental factors of industrial environments. 
         [0024]    The device according to the present invention is characterized by a structure to be used to ensure flexibility of use and positioning and simplicity of construction, and is adapted to operate correctly with respect to any positioning of the sheet being measured, by virtue of the particular broadband processing method used. 
         [0025]    In a first preferred embodiment, the device according to the present invention uses a dihedral reflector, uniform in transversal direction with respect to the material being measured and fixed, and a sensor head adapted to be moved transversely with respect to the material being measured, which allows to measure the humidity in the whole section of the material being measured, for example. 
         [0026]    Moreover, the device according to the present invention arranges both the transmitter and the receiver on the same side as the material being measured, making possible, inter alia, to make a path with known attenuation for the measurement signal which may be used to compensate for possible creep of the electronic components used, thus making the measurement particularly accurate. In a preferred embodiment of the present invention, the reflector used is of the passive type, and therefore does not require electric connections, thus making the device of the present invention also adapted to difficult environments subject to high humidity, water and high temperatures. 
         [0027]    In further preferred embodiments, the device according to the present invention may further comprise self-cleaning means, heat-detecting and/or heat-adjusting means, adapted to keep the conditions of said device, and thus the measurements to be performed, constant and stable during use and over time. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    With reference to accompanying  FIG. 1 , the device according to the present invention comprises a sensor block  10 , adapted to be arranged in the vicinity of the material to be measured  11 , on one side of said material, and a reflector  12 , preferably a microwave reflector, arranged near the material to be measured  11 , on the opposite side of said material  11  with respect to said sensor block  10 . 
         [0029]    Said block sensor comprises: a broadband transmitter  13 , of the programmable frequency type, a broadband receiver, preferably of the programmable frequency type  14 , adapted to output a signal, for example digital data, proportional to the power received at a given frequency, a first radio frequency switch  15  associated with said transmitter  13 , a second radio frequency switch  16  associated with said receiver  14 , an attenuator  17  associated with said first  15  and second  16  switches, a transmitting antenna  18  associated with said first radio frequency switch  15 , a receiving antenna  19  associated with said second radio frequency switch  16 , a controller  20  associated with said transmitter  13  and said receiver  14  and adapted to drive and program said transmitter  13  and said, receiver  14  and to acquire and process the signal received from said receiver  14  and to calculate the measurement of at least one chemical-physical parameter of said material  11 . 
         [0030]    Said controller  20  may advantageously be provided with communication means, either wired or wireless, with which said measurement may be transferred to external electronic devices, for analyzing, processing or storing. 
         [0031]    In a preferred embodiment of the present invention, said receiving antenna  19  is in orthogonal polarization with respect to the transmission antenna  18 . For example, a HORN type antenna with a gain of 15 dB, which has excellent features in terms of bandwidth, directionality and polarization purity, may be used. Alternatively, other types of antennas may be used, such as for example planar antennas, which are less performing but smaller and cheaper than HORN antennas. 
         [0032]    Said receiving  19  and transmitting  18  antennas of the device according to the present invention are preferably provided with a radome consisting of a layer of non-conductive dielectric material. 
         [0033]    Said reflector  12  is of the passive type, free of power or electronics, and is shaped so that the polarization of the reflected electric field is rotated by 90 geometric degrees with respect to the incident electric field. 
         [0034]    Said first radio frequency switch  15  is made, for example, by means of a single-pole two-way device, in which the common terminal is connected to the transmitter  13 , while the other terminals are connected to said transmitting antenna  18  and to said attenuator  17 . 
         [0035]    Said second radio frequency switch  16  is made, for example, by means of a single-pole two-way device, in which the common terminal is connected to the transmitter  14 , while the other terminals are connected to said receiving antenna  19  and to said attenuator  17 . 
         [0036]    The operating principle of the present invention is based on the double crossing of the material being measured by the radio frequency electromagnetic field produced by said sensor  10 . 
         [0037]    The first crossing occurs along the trajectory from the sensor to the reflector, while the second occurs along the trajectory from the reflector to the sensor. The two crossings are parallel and have opposite directions. The electric fields along the two paths are preferably orthogonally polarized. Thereby, the received electric field will be orthogonally polarized with respect to the transmitted electric field. The spatial orthogonality of the receiving and transmitting polarizations allows to use an antenna which is orthogonally polarized with respect to the transmitted signal, and thus avoid the direct coupling between the transmitting and receiving antennas. Thereby, the sensor can receive only the desired signal, i.e. that intentionally produced by the reflector. This allows to have a completely passive reflector, which therefore does not require any connection to the sensor. Moreover, the fact that the transmitter and the receiver belonging to the same sensor are arranged on the same side of the material being measured allows a better management of the sensor itself, allows to perform a differential measurement, allows to perform a self-calibration procedure, allows not to have radio frequency connections outside the sensor block body itself, and finally to obtain smaller sizes and dimensions. 
         [0038]    The fields are attenuated/phase-shifted in relation to the chemical/physical properties of the material when they cross the material being measured and this variation is detected during the step of receiving by associating the measurement of the quantity to be measured with this variation. 
         [0039]    The electromagnetic fields used may have any frequency which may be varied according to needs, and in particular according to the dielectric permeability of the material being measured and the thickness thereof. Microwaves and the so-called millimetric waves are particularly effective. 
         [0040]    In a preferred embodiment of the present invention, all the frequencies in the 15 GHz-20 GHz band are used for detecting the amount of water in sheets from 30 to 400 g/m2. 
         [0041]    Different scanning frequencies are used to make the system as immune as possible from all factors which may change the stationary wave conditions (vibrations, offset of the sheet being measured, etc.), by mediating the measurements performed on a frequency band instead of on a single frequency. Moreover, in a preferred embodiment, the present invention uses a method of self-calibrating the measurements on the used frequency band which takes the frequency response of the entire electronics of the device into account by using a differential self-calibrated measurement. This self-calibration method also contributes to maintaining the conditions of said device, and thus of all the measurements performed therewith, constant and stable during use and over time, regardless of changes to external conditions. 
         [0042]    In detail, the sensor according to the present invention, in which the transmitter and the receiver are on the same side of the material being measured, allows to perform a periodical self-calibration of said transmitter and receiver during normal operation. The device according to the present invention operates as follows: by means of an appropriate setting of the controller, several measurements are performed during a first step, each measurement consisting of acquiring the attenuation encountered by the signal at the various selected single frequencies, along the path of the radio frequency wave between transmitter and receiver across said attenuator which attenuates the wave by a known, constant factor. This path is named “calibration path”. 
         [0043]    During a second step, other several measurements are performed consisting of acquiring the attenuation encountered by the signal at the various single selected frequencies, the path also comprising the antennas, the material being measured and the reflector. 
         [0044]    The measurements may be performed across the path comprising the antennas, the material being measured and the reflector, or across the calibration path comprising only the attenuator, as illustrated above. A power vector C, which is assumed as expressed in dBm, is obtained from the received power measurements according to the calibration path. The characteristic attenuation value of the path across the attenuator is known and is assumed equal to A dB: 
         [0000]    
       
         
           
             
               C 
                
               
                 [ 
                 dBm 
                 ] 
               
             
             = 
             
               ( 
               
                 
                   
                     
                       C 
                       1 
                     
                   
                 
                 
                   
                     
                       C 
                       2 
                     
                   
                 
                 
                   
                     
                         
                     
                   
                 
                 
                   
                     
                       C 
                       n 
                     
                   
                 
               
               ) 
             
           
         
       
       
         
           
             
               A 
                
               
                 [ 
                 dB 
                 ] 
               
             
             = 
             
               ( 
               
                 
                   
                     
                       A 
                       1 
                     
                   
                 
                 
                   
                     
                       A 
                       2 
                     
                   
                 
                 
                   
                     
                         
                     
                   
                 
                 
                   
                     
                       A 
                       n 
                     
                   
                 
               
               ) 
             
           
         
       
     
         [0045]    A power vector P, also expressed in dBm, is obtained from the power measurements performed across along the path comprising the antennas, the material being measured and the reflector, instead: 
         [0000]    
       
         
           
             P 
             = 
             
               ( 
               
                 
                   
                     
                       P 
                       1 
                     
                   
                 
                 
                   
                     
                       P 
                       2 
                     
                   
                 
                 
                   
                     
                         
                     
                   
                 
                 
                   
                     
                       P 
                       n 
                     
                   
                 
               
               ) 
             
           
         
       
     
         [0046]    The sought parameter is obtained, i.e. the attenuation value related to the measurement performed across the material, which will be indicated by M and expressed in dB, from the previous two measurements: 
         [0000]    
       
         
           
             M 
             = 
             
               
                 ( 
                 
                   
                     
                       
                         M 
                         1 
                       
                     
                   
                   
                     
                       
                         M 
                         2 
                       
                     
                   
                   
                     
                       
                           
                       
                     
                   
                   
                     
                       
                         M 
                         N 
                       
                     
                   
                 
                 ) 
               
               = 
               
                 P 
                 - 
                 C 
                 + 
                 A 
               
             
           
         
       
     
         [0047]    Thereby, the instrument can be periodically and automatically calibrated, making it immune from the inevitable creep of the electronics used, which creep could have consequences, such as fluctuations of transmitted power or reception gain. 
         [0048]    The device according to the present invention can thus simply obviate the gain variations that the system may display on the used frequency band by means of a self-calibration procedure. The self-calibration procedure may be implemented by periodically and automatically measuring the system gain at the same frequencies used for the other measurements, and thus obtain a system correction vector S, also formed by elements expressed in dB. At this point, the data related to power may be corrected in the value D: 
         [0000]    
       
      
       D=M+S  
      
     
         [0049]    The value D is used to calculate the root mean square (RMS) value: 
         [0000]    
       
         
           
             
               D 
               
                 rm 
                  
                 
                     
                 
                  
                 s 
               
             
             = 
             
               10 
                
               
                 log 
                 ( 
                 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       N 
                     
                      
                     
                       10 
                       
                         Di 
                         10 
                       
                     
                   
                   N 
                 
                 ) 
               
             
           
         
       
     
         [0050]    At this point, the value D is associated, by means of an appropriate function, with the value to be measured related to the physical parameter to be evaluated. In general, the quantity to be measured will be expressed as a function of the previously calculated value D: 
         [0000]        g =ƒ( D   rms )
 
         [0051]    This function f represents the mathematical model which describes the relationship between the D rms  data and the quantity to be measured, and is thus highly dependent on the type of measurement which must be performed. 
         [0052]    For example, in order to operate a calibration of the system of the present invention, an interpolation may be operated between a set of measurements of the D rms  value performed as illustrated above, and a set of values of the physical parameter to be measured, collected using different measurement techniques, for example, in the case of the measurement of the humidity parameter, by means of the gravimetric technique with drying. 
         [0053]    The ordinary least pairs rectilinear interpolation is to be preferred in the case of measurements of parameters displaying variations of only a few percentage points during the process in which the measurement is performed, because it is simple and effective, otherwise interpolation by means of spline functions may be used. 
         [0054]    A preferred embodiment of the present invention comprises means for cleaning the radome in order to keep the features of the device according to the present invention, and the measurements performed thereby, as stable and constant as possible. 
         [0055]    In a preferred embodiment of the present invention, said radome is kept clean by using jets of compressed air (or equivalent gas) emitted by appropriate means, appropriately associated with the device according to the present invention. Said appropriate means may be adapted, in particular, to emit laminar jets of air or conical-shaped jets of air, like those emitted by nozzles, or the like. 
         [0056]    Moreover, the device according to the present invention advantageously comprises temperature measuring means associated with said controller  20  and adapted to communicate the temperature value of said device and/or of at least one component of said device to said controller  20 , so that controller  20  can correct the measurement value performed by applying correct coefficients which are known on the basis of the current temperature. 
         [0057]    In a first preferred embodiment of the present invention, with reference to accompanying  FIG. 2  which shows the pair of antennas  18 ,  19  of sensor  10 , the material under test  20  and the reflector  12 , said reflector  12  may consist of a metal dihedral of simple construction and uniform geometry in the longitudinal direction, comprising two reflecting planes which are mutually inclined by an angle of approximately 90 degrees. 
         [0058]    For the dihedron to reflect the entire incident electromagnetic field and thus obtain a rotation of the polarization by 90 degrees, the incident electric field must be inclined by an angle of 45 degrees with respect to the longitudinal axis of the dihedron. 
         [0059]    This type of reflector, also named corner reflector, acts so that the incident electric field component with polarization parallel to the longitudinal axis of the dihedron is reflected by keeping the same direction, while the incident electric field component with polarization orthogonal to the longitudinal axis of the dihedron is reflected in opposite direction so as to obtain a reflected electric field orthogonal to the incident field in the case of incident electric field oriented by 45° with respect to the longitudinal axis of the dihedron. 
         [0060]    The dihedral reflector of the described type has many advantages: it is simple to be made; its electromagnetic behavior is simple by virtue of the fact that the dihedron can be approximated as a single reflector arranged on the conjunction axis between the two incident planes, and therefore does not give rise to multiple paths which could cause destructive or constructive interferences, which are harmful for the measurement; the cross-polarized reflection is total, so that the entire incident field on the dihedral reflector is reflected with polarization orthogonal with respect to the incident polarization in the above-described case in which the incident signal has polarization rotated by 45 degrees with respect to the longitudinal axis of the dihedron; development is uniform in longitudinal direction, which is particularly advantageous in the case of measurements on extended sheets, because it allows a single reflector to cover the whole section of the machine. The greatest drawback of the reflectors of this type is their size, particularly with regards to depth, which is equal to approximately half of their front opening. 
         [0061]    In a second preferred embodiment of the present invention, a reflector is formed by a plurality of dihedrons, as shown in accompanying  FIG. 3 . In this case, the depth of the reflector is considerably reduced, but some of the positive features of the single dihedron are lost. In detail, reflectors of the multi-dihedral type have the advantage of being smaller than the single dihedral reflector but have drawbacks related to the multiple reflection paths given by the presence of a plurality of reflectors which provide different width and shift phase fluctuations which are recomposed in a constructive or destructive manner in the receiver thus causing received signal width fluctuations. 
         [0062]    In a third preferred embodiment of the present invention, with reference to accompanying  FIG. 4 , said reflector is of the grid type with radio-absorbing material, i.e. formed by a plurality of thread-like parallel reflectors  40  and arranged to form a grid. A radio-absorbing material  41  is placed on the back of this grid. A reflector of the grid type, if reached by an incident wave inclined by 45 degrees with respect to the axis of the thread-like reflectors, only reflects the field component parallel to said thread-like reflectors. The reflected component may be broken down, in turn, into two components, one of which is polarized at 90 degrees with respect to the incident field and is received by the reception antenna. Thus, by using a grid reflector, only one part of the reflected field has a polarization rotated by 90 degrees in space. Therefore, the use of a grid reflector implies the reduction of the reflected power with respect to the case with dihedral reflector because the reflection is not total as the incident field component orthogonal to the reflectors of the grid is not reflected, but proceeds its path and is then absorbed by the absorbing material placed behind said grid. 
         [0063]    The reflectors of the grid type with radio-absorbing material have the advantage of being more compact in size with respect to the dihedral reflector but, like the multi-dihedral reflectors also, they have the described drawbacks related to the multiple reflection paths. However, if the grid is sufficiently extended and close-knit, the problem of the multiple paths can be ignored considering that the number of the aforesaid multiple reflection paths is sufficiently large to make the fluctuations with respect to the resulting mean effect negligible. 
         [0064]    In a further preferred embodiment of the present invention, said reflector is of the double-reflection grid type, as shown in accompanying  FIG. 5 . In this embodiment, the grid, which may be made by extrusion, is formed by a plurality of fins  50  which originate from a metal plane  51 . 
         [0065]    Said fins are made so as to cover said metal plane  51  completely. The height of said fins must be preferably equal to ¼ of the wavelength of the band center frequency of the sensor, i.e. with reference to accompanying  FIG. 5 , if h is the height of the fins and ƒ 0  is the band center frequency of the sensor system, the optimal operating condition is: 
         [0000]    
       
         
           
             h 
             = 
             
               
                 c 
                 
                   4 
                    
                   
                     f 
                     0 
                   
                 
               
               . 
             
           
         
       
     
         [0066]    The same considerations made for the grid reflector with radio-absorbing material apply to this type of reflectors although the amount of reflected, orthogonally polarized energy is higher. 
         [0067]    The present invention preferably uses the linear polarization antennas. In this case, the electric field must have a polarization inclined by 45 degrees with respect to the symmetry axis of the reflector for all types of reflector described with reference to the various embodiments of the present invention. 
         [0068]    The present invention, however, may also advantageously use circular polarization antennas. In this case, if reflectors with orthogonally polarized reflection are used, like those shown in accompanying  FIGS. 2, 3, 4 and 5 , both antennas used for transmitting and receiving must have the same electric field rotation direction. 
         [0069]    Again using circular polarization antennas, a uniform reflector can be used, for example consisting of a conducting material plane or surface which can be locally approximated as flat. In this case, the antennas used for transmitting and receiving must have mutually opposite electric field rotation directions. 
         [0070]    With respect to the case in which linear polarization antennas are used, circular polarization antennas have the advantage that the angle between sensor and reflector is not longer bound and that the reciprocal positioning can thus be arbitrary. 
         [0071]    On the contrary, circular polarization antennas have worse performance in terms of direct coupling and usable band.