Abstract:
The application relates to a microwave sensor for measuring a dielectric property, especially the density and/or moisture content of a product, having a microwave resonator, wherein a product introduced into the resonator interacts with a resonant microwave field generated in the resonator in order to determine suitable measured quantities, and is characterized in that at least two half-waves of the electric field are formed in the resonator in one direction, the product feed being effected in at least one region of high field intensity of one of the half-waves of the electric field.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from European Patent Application No. 04 003 110.6 dated Feb. 12, 2004 and from German Patent Application No. 10 2004 011 341.6 dated Mar. 5, 2004, the disclosure of each of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a microwave sensor for measuring a dielectric property, having a microwave resonator, wherein a product introduced into the resonator interacts with a resonant microwave field generated in the resonator in order to determine suitable measured quantities related to, for example, density and/or moisture content of a product. The invention further relates to use of the sensors in a spinning preparation machine. 
   With such sensors, the field energy is concentrated in a specific region of the resonator. The mass loading of the resonator in the region of high field energy is limited, since too great a mass loading leads to falsification of the measuring signal owing to losses. The use of several resonators, each one only for a portion of the product, is expensive and can lead to systematic errors owing to differences in the resonators and in the corresponding control and evaluating electronics. In the case of extended, especially web-form, products, a further problem comprises the sensing of the product over a relatively large dimension, for example, across the entire width of a product web. The enlargement of the region of high field energy, for example, by enlarging the wavelength, would lead to constructions of impractical dimensions. The use of several relatively small resonators across the expanse of the product is likewise disadvantageous for the above-mentioned reasons. 
   EP 0 889 321 A discloses a microwave sensor for measurement on an elongate sample, having a flat, dielectric-filled resonator that has a through-bore running through it for the sample. In the measuring region arranged centrally in the resonator there is an approximately homogeneous microwave field of relatively low intensity. 
   DE 102 04 328 A discloses a microwave sensor for determining the mass of a fibre sliver in a spinning preparation machine, the sensor comprising a resonator having two sample volumes for two fibre slivers. 
   WO 00/55606 A discloses a microwave sensor having a resonant strip transmission line for measuring the mass of a fibrous material. A half-wave of the electric field develops along the strip transmission line with intensity maxima at both ends, where there is a respective passage for a flow of product. The highly inhomogeneous fields at the open line ends are disadvantageous in terms of measurement techniques. Furthermore, it is necessary to shield the strip transmission line using a metallic enclosure in order to prevent falsification of the measurement through radiation loss. The enclosure has to be carefully adjusted in order to avoid resonance close to the measuring frequency. The construction of the sensor is therefore comparatively expensive. 
   It is an aim of the invention to provide an inexpensive microwave sensor having an improved measuring accuracy, and preferably to permit substantially exact measurement of a relatively large product quantity and/or an extensive product. 
   SUMMARY OF THE INVENTION 
   The invention provides a microwave sensor for measuring a dielectric property of a product comprising: 
   a microwave resonator chamber; 
   a microwave field generating device for generating a microwave field in the chamber; and 
   a pathway for receiving the product in the chamber such that the product can interact with the microwave field; 
   wherein the chamber is such that it permits at least two half-waves of the electric field to be formed in the resonator and the pathway is arranged to transport the product through at least one region of high field intensity of said half-waves of the electric field. 
   The above aim can thus be achieved according to the invention in particular in that at least two half-waves of the electric field are formed in the resonator in one direction, the product feed being effected in at least one region of high field intensity of one of the half-waves of the electric field. By using at least two half-waves of the electric field, the invention enables the mass loading of the resonator in relation to the total energy stored in the resonator to be reduced, which enables the measuring accuracy to be improved and at high mass loading can possibly even prevent collapse of the electric field. According to the invention, the sample feed is arranged in a region of high field energy of at least one half-wave around a corresponding intensity maximum in order to achieve a high measuring sensitivity. This is a region having at least 50%, preferably at least 70%, more preferably at least 90% of the intensity in the field maximum. This delimits the invention from a higher-mode resonator such as that from EP 0 889 321 A, where the sample feed takes place into a central resonator region of very low field intensity. 
   Preferably, a product feed takes place into at least two resonator sections each corresponding to a respective half-wave of the electric field. This enables the product to be distributed over several half-waves of the electric field. This reduces the mass loading per half-wave and per resonator section. In particular, by increasing the number of half-waves the mass loading per resonator section can be reduced to a reasonable level. A relatively large expanse of a product in one direction can be covered by distribution onto several half-waves, wherein the extent of an individual region of high field strength can be relatively small. Instead of one large region of high field strength, several smaller regions of high field intensity are therefore provided, thereby enabling the overall size of the resonator as a whole to be reduced. The regions of high field strength are here generated inside a resonator; the use of several relatively small resonators is not necessary. 
   The term “half-waves of the electric field” describes in the case of a specific measuring frequency a concavity of the cos form field, that is, starting from a zero point of the field via a field maximum or minimum to the next zero point. A half-wave has an extent corresponding to a half wavelength. The use of at least two half wavelengths means that the cavity resonator is operated in a higher mode. In the case of a plurality of measuring frequencies, the term “half-wave” can refer to a field region that extends from a zero point of the field via a field maximum or minimum to the next zero point. Alternatively, the term can relate to the measuring frequency with the highest intensity. 
   Around the intensity maximum of a half-wave of the electric field there exists a region of approximately homogeneous intensity, which enables the measuring accuracy to be improved when the product is arranged in the intensity maximum. In many cases, the product is therefore arranged in at least two maxima of the electric field corresponding to the at least two half-waves. If a balanced mean value of the measuring quantity is to be obtained for all parts of the product, then the product portions are arranged in each resonator section advantageously substantially in the respective maximum of the electric field. By selectively varying the arrangement of individual product portions from the maximum in the respective resonator sections, any desired weighting of the individual product portions can be set. 
   The resonator can be of different constructions. Preferably, it is a shielded resonator or cavity resonator, that is, a cavity defined by metallic walls, which is substantially closed apart from openings for the sample feed. In the case of a cavity resonator, a cavity resonance is induced internally. This construction is especially inexpensive, compared, for example, to the strip transmission line of WO 00/55606 A. A rectangular resonator is especially simple and therefore preferred. The resonator can alternatively be a resonator opened especially via chokes. Nevertheless, the invention relates just to those resonators with which the product is introduced into or guided through the resonator chamber for measurement. The subject matter of the application is therefore delimited with respect to those (stray field) resonators in which measurement is effected by means of an external electric stray field, so that a completely different kind of field-strength distribution is present. 
   The invention is especially useful for measuring a continuous stream of product guided through the resonator, since here the product mass or product expanse is pre-determined and cannot be matched to the resonator. This includes, in particular, strand-form and web-form products, for example, fibre products, tobacco, paper and the like. Also included is the measurement of individual production portions. In the case of a strand-form product, the at least two resonator sections are advantageously designed for passage of least one product strand each. It may be advantageous if two or even more product strands are guided through each resonator section, since if one fibre strand tears, it is carried along by the other strand. Preferably, however, not more than two product strands are guided through each resonator section, in order to keep the corresponding mass loading as low as possible. 
   The number and arrangement of the resonator sections can be freely matched to the particular requirements. The arrangement can be a one-dimensional, especially linear arrangement, for example, transversely across the width of a product web. An annular arrangement is also included, the at least two half-waves being arranged circumferentially. A two-dimensional arrangement is also possible, in which at least two half-waves of the electric field in the resonator are each provided in two directions. Even a three-dimensional arrangement is possible. The number of resonator sections amounts preferably to a least three, preferably at least four. Resonator sections that are not required can be allowed to run with no load. For example, a four-strand product can be measured with a resonator having four resonator sections, by guiding two strands through each of two resonator sections and running the two remaining resonator sections with no load. The same resonator construction (for example, a resonator with four resonator sections) can therefore be used for different applications (in the example mentioned, one to eight strands). Where appropriate, not all resonator sections have to have a product feed. 
   The resonator, if applicable the cavity resonator, can be filled with a solid dielectric having a dielectric constant greater than one, preferably at least two, more preferably at least five. Customary materials, especially ceramics, are known to the expert. The use of a dielectric filling enables field energy to be stored in a comparatively smaller space and therefore allows smaller forms of construction. Preferably, substantially the entire resonator is filled with dielectric, except for the space for receiving the sample. “Substantially” means apart from devices for coupling in microwaves, for guiding through the sample, and so on. The resonator can alternatively be filled substantially with air, however, which represents an especially simple and inexpensive construction. 
   The invention is essentially applicable both to a resonator operating on the basis of transmission measurement and to a resonator operating on the basis of reflection measurement. 
   The invention furthermore comprises an advantageous device for the inventive microwave sensor with a microwave resonator, especially for measuring the density of at least one sliver of textile fibres, for example of cotton, synthetic fibres or the like, in which the microwave sensor is used to control and/or to regulate a processing device for at least one textile fibre sliver. 
   The microwave sensor is advantageously arranged at the delivery end of a card. At least one microwave sensor is preferably arranged at the feed end and/or at the delivery end of the drawing system of a draw frame. The drawing system is advantageously a card drawing system at the delivery end of a card. The textile fibre sliver is preferably a card sliver. The textile fibre sliver is advantageously a draw frame sliver. The microwave sensor is preferably connected to an electronic control and regulating device, for example, a machine control and regulating device. The control and regulating device is advantageously connected to at least one actuator, for example, drive motor, for changing the density of the fibre sliver. An indicating device, for example, display screen, printer or the like, for displaying the density of or density changes in the fibre sliver is preferably connected to the control and regulating device. The microwave sensor is advantageously used to monitor the density of a sliver produced on a card or draw frame. 
   The invention further provides a microwave sensor for measuring a dielectric property, especially the density and/or moisture content of a product, having a microwave resonator wherein a product introduced into the resonator interacts with a resonant microwave field generated by a resonator in order to determine suitable measured quantities, especially for a spinning preparation machine, characterised in that at least two half-waves of the electric field are formed in the resonator in one direction, the product feed being effected in at least one region of high field intensity of one of the half-waves of the electric field. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a first embodiment of a sensor; 
       FIG. 2  is a view from the front of the sensor from  FIG. 1 ; 
       FIG. 3  is a view from the front of a second embodiment of a sensor; 
       FIG. 4  is a view from the front of a third embodiment of a sensor; 
       FIG. 5  is a plan view of a fourth embodiment of a sensor; 
       FIG. 6  is a view from the front of the sensor from  FIG. 5 ; 
       FIG. 7  is a plan view of a fifth embodiment of a sensor; 
       FIG. 8  is a diagrammatic sketch of a measuring arrangement having a sensor; 
       FIG. 9  is a plan view of a sixth embodiment of a sensor; 
       FIG. 10  shows the sensor from  FIG. 5 , the product in each resonator section being arranged substantially in the respective maximum of the electronic field; 
       FIG. 11  shows schematically in side view a card with the microwave sensor according to the invention; 
       FIG. 12  shows a can coiler with fibre sliver can with an autoleveller drawing system with the microwave sensor according to the invention; 
       FIG. 13  is a diagrammatic side view of an autoleveller draw frame having a respective microwave sensor according to the invention; 
       FIG. 14  shows an autoleveller draw frame with a closed control loop (regulation) and the measuring arrangement according to the invention; 
       FIG. 15  shows an autoleveller draw frame with an open control loop (control), and the measuring arrangement according to the invention; and 
       FIG. 16  shows an autoleveller draw frame with a combination of an open and a closed control loop (reference variable input) and two measuring arrangements according to the invention. 
       FIG. 17  shows as further embodiment of the invention in which a second, downstream, sensor is arranged in an offset position relative to a first, upstream, sensor; and 
       FIG. 18  shows an embodiment similar to that of  FIG. 17 , in which the second sensor is offset in the opposite direction. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   With reference to  FIG. 1 , a microwave sensor  10  comprises a cavity resonator  11 , in which a standing microwave field is generated. The coupling and decoupling of the microwave field is effected by means of coupling devices  12 . Microwaves are generated by means of a generator  14  controlled by a computer  13  and are relayed by means of a line to the coupling device  12   a . A microwave signal is decoupled from the resonator  11  via the coupling device  12   b  and fed by means of a line to an analyser  15 , the output signal of which can be processed by the computer  13 . Generator  14 , analyser  15  and computer  13  are expediently combined in one measuring unit  16  (cf.  FIG. 8 ). The arrows D, E, F and G indicate the direction of flow of the material. 
   The resonator  11  has a plurality of resonator sections  17   a ,  17   b ,  17   c , . . . . The division into different resonator sections is indicated in the Figures by broken lines. FIGS.  1 , 2 ,  5 ,  6 ,  7  and  9  each relate to elongate rectangular cavity resonators having, for example, four or two resonator sections. Elongate means that the extent along a longitudinal axis is longer by at least a factor of two, preferably by at least a factor of three, more preferably by a factor of at least five than the extent along the two directions perpendicular to the longitudinal axis. The resonator sections  17  are arranged in series along the longitudinal axis of the resonator  11 .  FIG. 3  shows a resonator in which the resonator sections are arranged in columns and rows, that is, in the form of a matrix. The number of columns, rows and generally the arrangement of the resonator sections can be adapted as desired to the particular requirements.  FIG. 4  relates to an annular resonator  11 , in which the resonator sections  17  are arranged in series along the annular axis of the resonator  11 . In this example, four resonator sections  17   b ,  17   d ,  17   f ,  17   h  run with no load. Generally, any desired number of resonator sections  17  can run with no load. 
   For sample feed, each resonator section has openings  21   a ,  21   b ,  21   c , . . . , which can be provided in particular in walls defining the resonator. In the case of measurements on a product flow, especially on a strand-form product flow ( FIGS. 1 to 4  and  7 ) or a web-form product flow ( FIGS. 5 ,  6 ), advantageously respective separate openings  21  are provided for entry and exit of the sample  19  into respectively from the resonator  11 . Preferably, the size of the openings  21  corresponds approximately to the cross-section of the product  19  to be measured. Preferably, the form of the openings  21  is matched to the cross-section of the product. For example, in the case of a strand-form product, the openings  21  are therefore preferably circular or oval. The resonator is preferably substantially closed except for the product feed openings  21 . “Closed” means impermeable to the microwaves used. The above-mentioned features contribute to the reduction in unfavourable radiation of microwave energy from the resonator. 
   To pass the product  19  through the resonator  11  or resonator sections  17 , sample guide devices  18   a ,  18   b ,  18   c , . . . can be provided, for example, small tubes  20   a ,  20   b ,  20   c , . . . , which can preferably consist of low-loss dielectric having a low temperature dependence, for example, quartz glass. 
   The internal dimensions of the resonator chamber and the frequency of the microwaves are such that a standing wave having a plurality of half-waves can develop in the resonator. In practice, the microwave frequency generated by the generator  14  is adapted so that a standing wave having at least two convex half-waves, each with a local intensity maximum, develops in the resonator  11 . In  FIGS. 1 ,  5  and  9 , electric field lines  22  for the case of a microwave of specific frequency are drawn in by way of example. In this case, a cos form half-wave of the electric field propagates per resonator section  17 . Any other field distributions having at least two half-waves, also of more than one frequency, in general of any frequency distribution, and/or several maxima per resonator section, are likewise possible. In  FIGS. 2 to 4 ,  6  and  7 , lines  23  of constant field intensity are drawn in. Inside the lines  23  there lies a region of high field intensity with a local intensity maximum, which lies approximately at the centre of the respective resonator section. Generally, this is not absolutely necessary. The field intensity is less outside the lines  23  than inside. 
   By distributing the entire production flow  19  over a plurality of resonator sections  17 , the mass loading per resonator section  17  is correspondingly reduced, in the example of  FIG. 1 , by a factor of four for example, in the example of  FIG. 3  by a factor of nine for example. The individual portions of the sample  19  are each guided through a region of the respective resonator section  17  arranged closely around the respective intensity maximum. As is apparent in  FIG. 1 , for example, in this region the field strength is approximately constant. Positional and orientation changes of the sample as well as spatial inhomogeneities within the product flow have therefore no or at most slight effect on the measuring signal, since all portions of the product flow enter the measuring signal with the same weighting—owing to the approximately constant field strength—as evident in the examples of  FIGS. 1 to 4 ,  7  and  9 . One opening  21  can be specifically arranged away from the respective intensity maximum in order to permit individual weightings of individual production portions. 
   The embodiment according to  FIGS. 5 and 6  relates to the measurement of a sheet-form product  31 , especially a panel-form or web-form product, for example, paper, fibre fleece or the like. The opening  21  is slot-form, the slot  30  comprising slot sections  21   a ,  21   b ,  21   c ,  21   d . The length of the slot  30  is matched to the width of the paper web  31 . 
   To measure a paper web  31  of twice as long, this same measuring unit  16  can be used at the same microwave frequency, by simply selecting a linear arrangement of eight instead of four resonator sections  17 , whereby the length of the resonator  11  would be approximately doubled. 
   In this way, the overall size of the resonator  11  needs to be changed only along the longitudinal axis, that is, in only one dimension, whilst the overall size of the resonator  11  can remain otherwise unchanged. The reference numeral H denotes the direction of flow of the material. 
   Generally, it is not necessary for the slot  30 , as in the example of  FIGS. 5 and 6 , to be closed. The resonator  11  can consist of two separate resonator halves, forming between them a sample feed slot. Preferably, the microwave coupling devices  12  are each arranged in different resonator halves. It is also possible to measure strand-form products using a slotted resonator. 
   The embodiment shown in  FIG. 7  illustrates that the walls of the resonator  11  are not completely closed, but are merely intended to be substantially impermeable to microwaves. In the case of  FIG. 7 , separating devices  32 ,  33 , for example, separating plates, are provided, which are arranged preferably at intervals from one another at a spacing that is less than the wavelength of the microwave field corresponding to the propagation limit frequency. The field can spill out a little between the separating plates  32 , but as a whole is concentrated substantially in the interior of the resonator  11 . 
   The embodiment shown in  FIG. 9  relates to a resonator  11  for measuring portions of a product  19 . The individual portions can be disposed in a sample container  33 , for example, a quartz glass tube. The resonator  11  has per resonator section  17   a ,  17   b  just one opening  21  in the resonator walls for introduction of the product  19 . This can be effected automatically, for example, by means of a robot. Guide means  20  are preferred but are not obligatory. Also in this example, at a relatively low mass loading per resonator section  17  a relatively large sample mass can be measured with just one resonator  11 . 
   Referring to  FIG. 10 , the fibre slivers  19   a ,  19   b ,  19   c  and  19   d  are arranged in each resonator section  17   a ,  17   b ,  17   c  and  17   d  respectively substantially in the respective maximum of the electric field. A half-wave of the electric field propagates per resonator section  17   a  to  17   d.    
     FIG. 11  shows a carding machine  54 , for example, a carding machine known as a TC 03 made by Trutzschler GmbH &amp; Co. KG of Mönchengladbach, Germany, with feed roller  55 , feed table  75 , licker-ins  56   1 ,  56   2 ,  56   3 , cylinder  57 , doffer  58 , stripping roller  59 , squeezing rollers  60 ,  61 , web-guide element  62 , web funnel  63 , take-off rollers  64 ,  65 , revolving card top  66 , can coiler  68  and can  67 . The directions of rotation of the rollers are shown by respective curved arrows. The take-off rollers  64 ,  65  draw off a card sliver  72 , which passes over guide rollers  69 ,  70  to the can coiler  68  and from there is laid in the can  67 . The microwave measuring arrangement  71  according to the invention (see  FIG. 1 ,  2 ) is arranged between the take-off rollers  64 ,  65  and the guide roller  69 . The microwave measuring arrangement  31  is connected to an electronic control and regulating device  73 , for example, a microcomputer, which alters the rotational speed of the feed roller  55  by way of a variable speed drive motor  74 . In this way, the density of the card sliver  72 , which can leave the take-off rollers  64 ,  65  at high speed, for example, 200 m/min or more, is adjusted. The letter A denotes the direction of working. The reference numeral  77  denotes a tuft feed device, for example, a DIRECTFEED tuft feed device made by Trützschler GmbH &amp; Co. KG, which provides a fibre tuft fleece to the feeding device of the card  54 . 
   Referring to  FIG. 12 , a drawing system  34 , which corresponds to the drawing system shown in  FIG. 13 , is arranged above the can coiler  68 ; reference will be made to the description of the drawing system of  FIG. 13 . At the feed end and delivery end of the drawing system  34  there is a respective microwave measuring arrangement  48 ,  49 , which are connected to the central computer unit  50  (e.g., electronic control and regulating device), which is furthermore connected to a drive motor  46  for the feed and middle pairs of rollers and a drive motor  47  of the delivery roller pair, take-off rollers and the can turntable  76 . 
   Referring to  FIG. 13 , a draw frame  55 , for example, a draw frame TD  03  made by Trützschler GmbH &amp; Co. KG, has a drawing system  34 , upstream of which is a drawing system feed  34   a  and downstream of which is a drawing system outlet  34   b . The fibre slivers  35  enter the sliver guide  36  from cans (not shown) and, drawn by the take-off rollers, are transported to the drawing system  34 . The drawing system  34  is designed as a 4-over-3 drawing system, that is, it consists of three bottom rollers I, II, III (I being the bottom delivery roller, II being the bottom middle roller and III being the bottom feed roller) and four top rollers  33 ,  38 ,  39 ,  40 . Drafting of the composite fibre sliver  35 ″, comprising several fibre slivers  35 , takes place in the drawing system  34 . The draft is made up from the preliminary draft and the main draft. The roller pairs  40 /III and  39 /II form the preliminary drafting zone and the roller pairs  39 /II and  38 ,  37 /I form the main drafting zone. The drawn fibre slivers  35 ′″ reach a web guide  41  at the outlet  34   b  of the drawing system and are drawn by means of the take-off rollers  42 ,  43  through a sliver funnel  44  in which they are condensed to a fibre sliver  45 , which is subsequently deposited in a can (not shown). The letter C denotes the working direction, and  35 ″ denotes the fibre slivers in the drawing system. The take-off rollers, the bottom feed roller III and the middle bottom roller II, which are mechanically linked, for example, by way of toothed belts, are driven by the variable speed motor  46 , it being possible to pre-set a desired value. (The associated top rollers  39  and  40  co-rotate). The bottom output roller I and the take-off rollers  42 ,  43  are driven by the main motor  47 . At the inlet  34   a  to the drawing system, a variable proportional to the density of the fed-in fibre slivers  35  is measured by the microwave sensor  48  (intake-side measuring device) according to the invention. At the outlet  34   b  of the drawing system  34 , the density of the fibre sliver is obtained by a microwave sensor  49  (delivery-side measuring device) according to the invention associated with the sliver funnel  44 . A central computer unit  50  (control and regulating device), e.g. a microcomputer with microprocessor, determines a setting of the regulated variable for the variable speed motor  46 . The measured variables of the two measuring devices  48  and  49  are sent during the drawing process to the central computer unit  50 . From the measured variables of the feed-side measuring device  48  and from the desired value for the density of the emerging fibre sliver  45 , the adjustment value for the variable speed motor  46  is determined in the central computer unit  50 . The measured variables of the delivery-side measuring device  49  are used to monitor the emerging fibre sliver  45  (monitoring of delivered sliver). 
   By means of this control system, fluctuations in the density of the fed-in fibre slivers  35  can be compensated by corresponding adjustments to the drafting process and the fibre slivers can be evened out. The reference numeral  51  denotes a display screen,  52  denotes an interface and  53  denotes an input means. 
     FIGS. 14 ,  15  and  16  show basic layouts of the drawing system of a draw frame with different constructions for the adjustment of the fibre sliver density.  FIG. 14  shows a closed control loop, in which the microwave measuring arrangement  49  is arranged at the delivery end of the drawing system. The fibre material leaving the drawing system passes through the measuring arrangement  49 , the output signal of which is compared in the central computer unit  50  with a desired value and is converted so that a corresponding control signal is supplied to an actuator (variable speed motor  46 , see  FIG. 13 ) for the roller II. The output signal corresponding to the density of the emerging fibre material thus influences the speed ratio of the drafting roller pairs  39 /II and  38 /I in the sense that the fibre material is evened out.  FIG. 15  shows an open control loop (control). Here, the microwave measuring arrangement  48  is located in the region in which the fibre material  35  approaches the drawing system, measures the density of the fibre material and the corresponding measuring signal is converted in the central computer unit  50  into a control signal which is supplied to an actuator (variable speed motor  46 , see  FIG. 13 ) for the roller II. Allowances are made electronically for the time taken by the fibre material  35  to run from the measuring arrangement  48  to the drawing system.  FIG. 16  shows a combination of an open and a closed control loop, in which the measuring signals of the measuring arrangement  49  are superimposed on the measuring signals of the measuring arrangement  48 . 
   “Resonator” relates to a spatial region in which a standing microwave field is able to propagate. The resonator can be a closed or a substantially closed cavity resonator. 
   A product of which the density is being measured is arranged in an area called a “product area”, which, when the sensor is operative, is in a fixed spatial relationship with the area of the resonator. The microwaves enter the product area in order to interact with the product. The microwave resonator is permeable to microwaves towards the product area. The product can be a consecutive and/or continuous stream of product, for example, a fibre sliver, a fibre web, fibre tufts or individual fibres in spinning preparation machines. 
     FIGS. 17 and 18  show two further embodiments of the invention, in which two microwave sensors  10 ,  10   a  are arranged perpendicular to the direction of flow H of the material offset by a quarter of a wavelength λ/4 with respect to one another. The sensors  10 ,  10   a  with a respective resonator  11 ,  11   a  are electrically coupled (not shown) with one another. The signals of the sensors  10 ,  10   a  are electrically totalled and evaluated (not shown). In this way, in the case of a flat product, for example, textile fibre fleece, the maxima of the electric fields  22  and  22   a  are better exploited. 
   Although the foregoing invention has been described in detail by way of illustration and example for purposes of understanding, it will be obvious that changes and modifications may be practised within the scope of the appended claims.