Abstract:
An ultrasonic transmitting and receiving device is provided for measuring the transmission and/or reflection of ultrasonic waves on a thin material sheet, in particular on a foil sheet. The device may include a plurality of ultrasonic transmitters, a plurality of ultrasonic receivers, wherein the number of the ultrasonic transmitters corresponds to the number of the ultrasonic receivers, and one receiver electronics respectively for each of the ultrasonic receivers or each a group receiver electronics respectively for a predetermined number of ultrasonic receivers. A method is provided for ultrasonic absorption and/or transmission measurement, in which signals are emitted by multiple ultrasonic transmitters at the same time or nearly at the same time which are received by ultrasonic receivers and in which the received signals are evaluated in parallel.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of German patent application number DE 10 2011 015 334.9 filed Mar. 28, 2011, the content of which is incorporated by reference herein in its entirely. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to an ultrasonic transmitting and receiving device for measuring the transmission and/or reflection of an ultrasonic signal on a material sheet (material foil). Based on the transmission and/or reflection measurement, the layer thickness and/or the grammage (mass per unit area) of the material sheet can be absolutely determined. 
       BACKGROUND 
       [0003]    From DE 42 36 436 A1, a measurement method for contact-less determination of the grammage of thin material sheets by means of ultrasonic sound is known. In the method, by means of an ultrasonic transmitter and an ultrasonic receiver, the transmission absorption of an ultrasonic beam upon passage through a material foil is determined in contact-less manner. Based on the absorption and a calibration factor, the grammage is calculated. 
         [0004]    From DE 201 09 119 U1, a further device for measuring the thickness of material sheets is known. There, the material sheet is pulled over a roller, wherein a sensor is arranged on a rolling cart moving back and forth over the roller in traversing manner for thickness measurement. 
         [0005]    Further ultrasonic sensor arrangements for grammage determination or for grammage comparison are known from DE 103 27 389 B3, DE 199 08 932 A1, DE 10 2005 037 086 A1, DE 203 12 388 U1 and DE 30 48 710 A1. 
         [0006]    There is a need for providing an ultrasonic transmitting and receiving device, an arrangement including the device as well as a method, in which the thickness measurement and/or grammage measurement on a material sheet are improved. 
       SUMMARY 
       [0007]    To address the foregoing problems, in whole or in part, and/or other problems that may have been observed by persons skilled in the art, the present disclosure provides methods, processes, systems, apparatus, instruments, and/or devices, as described by way of example in implementations set forth below. 
         [0008]    According to one implementation, an ultrasonic transmitting and receiving device is provided, by which the transmission and/or the reflection of the ultrasonic waves through or from a thin material sheet are measured by means of the ultrasonic waves. The thickness and/or the grammage (mass per unit area) of the material sheet may be determined based on the measured transmission and/or reflection. The device comprises a plurality of ultrasonic transmitters, wherein each of the ultrasonic transmitters is oriented such that an ultrasonic signal can be emitted towards a thin material sheet. The ultrasonic signal may be a signal pulse or a sequence of signal pulses (“signal burst”). The device unit may include a control unit, by means of which the absolute layer thickness and/or the grammage can be calculated using a calibration value or a calibration curve and the measured transmission and/or reflection and can be displayed and/or output as an absolute value. 
         [0009]    An ultrasonic receiver may be associated with each of the ultrasonic transmitters and oriented towards this such that the ultrasonic signal emitted by the associated ultrasonic transmitter can be received. For example, for determining the transmission, the receiver may be arranged on the side of the material sheet opposing the ultrasonic transmitter and oriented towards the ultrasonic transmitter. For example, for measuring the reflection, either a separate receiver may be arranged on the same side of the material sheet as the ultrasonic transmitter, or the ultrasonic transmitter may operate as a transceiver, by which the signal can be both radiated and received. 
         [0010]    Dedicated receiver electronics may be associated with each of the ultrasonic receivers, or group receiver electronics may each be associated with a group of ultrasonic receivers, such that parallel processing of the ultrasonic signals received and converted by the ultrasonic receivers can be executed with the receiver electronics or group receiver electronics. Within the group receiver electronics a dedicated electronics unit may be associated with each ultrasonic receiver in the group of the ultrasonic receivers which are associated with the group receiver electronics, such that parallel processing of the received voltage signal converted by the receiver can also be executed for each member of the group. 
         [0011]    By the plurality of ultrasonic transmitters, ultrasonic receivers and receiver electronics or group receiver electronics, it is provided to emit, to receive ultrasonic signals and to at least partially condition or process them by the electronics in time-parallel manner. Therefore, it is possible to substantially increase the spatial density and/or the repetition rate of the transmission and/or reflection measurements on the material sheet as compared to for example a reversing system with only one transmitting/receiving unit. Thereby, for example, the quality control by means of thickness and/or grammage determination on a material sheet can be substantially improved. For example, thereby, the fault detection in a material sheet such as a fuel cell membrane, battery membrane or the like increases such that the reject rate is considerably decreased related to the entire production process due to the fault detection of material defects. 
         [0012]    The ultrasonic transmitters may be arranged so as to be distributed on a supporting device such that they extend in transverse direction to the material sheet. Thereby, it is no longer required to provide a traversing device moving the ultrasonic transmitters back and forth across the width (transverse direction) of the material sheet. The ultrasonic transmitters may be fixedly arranged, i.e. they are moved neither in transverse direction nor in longitudinal direction to the material sheet, i.e. during the measurements, exclusively the material sheet moves in longitudinal direction. Thereby, the expenses for providing mechanically moving parts are reduced or are completely avoided. In an embodiment, it can be provided that the supporting device for the ultrasonic transmitters is moved over a small transverse stroke, for example a transverse stroke of less than 1/10, 1/15, 1/20 or 1/30 of the material sheet width. 
         [0013]    In an embodiment, the ultrasonic transmitters are arranged so as to be spatially distributed both over the transverse direction of the material sheet and over a predetermined depth in longitudinal direction of the material sheet. By the distribution of the ultrasonic transmitters in longitudinal direction, it becomes possible to achieve the coverage of the measurement points in longitudinal direction of the material sheet with temporal repetition in nearly continuous or even overlapping manner in a clocked, non-continuous measurement even at high sheet longitudinal velocities. Alternatively or additionally, it is possible to increase the resolution of the measurement locations in transverse direction with both transverse and longitudinal offset of the adjacent ultrasonic transmitters such that complete or nearly complete coverage with measurement points in transverse direction is achieved. 
         [0014]    Notably, multiple lines of ultrasonic transmitters extending in transverse direction of the material sheet may be provided, wherein all of the ultrasonic transmitters of a line are offset to each other in transverse direction and the lines are offset to each other in longitudinal direction. Therein, the dual offset is advantageous such that with projection of the ultrasonic transmitters in longitudinal direction, thus upon viewing the ultrasonic transmitters in the direction of the running direction of the material sheet, the projection of the ultrasonic transmitters is arranged equidistantly to each other. Therein, viewed in projection and in transverse direction, a uniform sampling density in transverse direction of the material sheet is achieved. In an alternative or additional embodiment, a sequence of the ultrasonic transmitters results in a projection in longitudinal direction such that a repeating permutation of the line number of the associated ultrasonic transmitter arises in projected sequence in transverse direction. (Example for illustration: if the lines and columns result in a skewed array or a skewed matrix A ij  of the size 3×3 with the column number i=1, 2, 3 and the line number j=1, 2, 3, then the permutation sequence a 11 , a 12 , a 13 , a 21 , a 22 , a 23 , a 31 , a 32 , a 33  results in longitudinal projection). With such an arrangement, the ultrasonic transmitters can be arranged next to each other on a small area in compact manner, while the coverage with ultrasonic transmitters (and the ultrasonic signals radiated by them to the material sheet, respectively) projected in longitudinal direction is provided in transverse direction to the material sheet in continuous or nearly continuous manner. 
         [0015]    The ultrasonic transmitters may be arranged such that only each one ultrasonic transmitter is located in longitudinal direction of the moving material sheet, i.e. all of the ultrasonic transmitters are offset to each other in transverse direction. However, this does not mean that the active transmitting surfaces of the ultrasonic transmitters do not overlap in longitudinal projection (see below). If for example two lines of ultrasonic transmitters with ultrasonic transmitters arranged as distributed over the transverse direction are provided and if the two lines are offset in longitudinal direction, thus, advantageously, the ultrasonic transmitters of the second line are arranged offset by half the distance of the ultrasonic transmitters of the first line in transverse direction with respect to the ultrasonic transmitters of the first line. Thereby, in longitudinal projection, each of the ultrasonic transmitters of the second line is located between two ultrasonic transmitters of the first line (except for the last ultrasonic transmitter of the second line). 
         [0016]    According to an embodiment, the device includes multiple lines of ultrasonic transmitters arranged to be offset to each other in longitudinal direction, wherein the ultrasonic transmitters of the same column are located on one line in the lines located one behind the other, which is under an angle with respect to the longitudinal direction of the material sheet. In some implementations, the angle of this line is in a range of 10° to 80° to the longitudinal direction, or in other implementations in a range of 20° to 70°, 30° to 60° or 40° to 50°. 
         [0017]    Each of the ultrasonic transmitters may have an active transmitting surface extending both in longitudinal direction and in transverse direction. In some implementations, the transmitting surface of the ultrasonic transmitters is round or elliptical. In an embodiment, the ultrasonic transmitters are arranged such that in projection of the ultrasonic transmitters in longitudinal direction of the material sheet, the transmitting surfaces of the ultrasonic transmitters lying next to each other in transverse direction overlap. Thereby, seamless coverage of the sampling of the material sheet in transverse direction in projection is achieved. In some implementations, the overlapping degree between each two ultrasonic transmitters adjacent in projected transverse direction is at least 10% in transverse direction of the projected transmitting surface, or in other implementations at least 15%, 20%, 30%, 35% or 40%. 
         [0018]    In some implementations, 100% of the sheet width is covered by means of the ultrasonic transmitters or the ultrasonic beam emitted by the ultrasonic transmitters, and in some implementations at least partially overlapping coverage in transverse direction of the material sheet is achieved. 
         [0019]    A sensor unit of the device is composed of an ultrasonic transmitter and one or two ultrasonic receivers associated with the ultrasonic transmitter (two receivers in case of the simultaneous measurement of transmission and reflection). The radiating direction of the ultrasonic signal from the ultrasonic transmitter towards the surface may be perpendicularly or approximately perpendicularly oriented to the material sheet. The receiver of the sensor unit therein may be oriented such that it is arranged opposing the ultrasonic transmitter with respect to the material sheet in transmission and receives the transmitted signal. Alternatively or additionally, the ultrasonic receiver measuring the reflection is arranged on the same side of the material sheet as the ultrasonic transmitter. The ultrasonic receiver may at the same time be the ultrasonic transmitter. 
         [0020]    If both transmission and reflection are measured, then in some implementations, one dedicated receiver electronics is provided for each of the reflection receivers and transmission receivers or one group electronics is provided each for a group of reflection receivers and transmission receivers. The described parallel processing of the received signals therein relates to both the transmission and the reflection signals. In the processing or calculation the quotients of the transmission and reflection signals may each be processed for each sensor unit (transmitter and associated transmission and reflection receivers) (quotient either T/R or R/T). 
         [0021]    In an embodiment, the ultrasonic receivers are arranged on a second supporting device paired to the arrangement of the ultrasonic transmitters on the first supporting device. In transmission measurement, the second supporting device may be arranged on the side of the material sheet opposing the first supporting device for the ultrasonic transmitters. By the arrangement of the receivers on a common, second supporting device, all of the receivers can be collectively oriented to the ultrasonic transmitters such that individual orientation of each receiver to its associated transmitter is not required. 
         [0022]    Each of the receiver electronics or group receiver electronics may include an amplifier, in which the amplification is adjustable. In particular, each of the amplifiers may be adjustable in multiple amplification stages and/or the amplification of each of the amplifiers is individually, but centrally adjustable under control of a main control unit via the control or programming of the main control unit. By means of the adjustable amplification, the basic amplification for the amplification of the received ultrasonic signal converted to a voltage signal can be calibrated for each of the ultrasonic receivers. Thereby, for example, an amplification of the received signal is adjusted such that the same signal (for example same signal amplitude and/or signal voltage) is present with all of the amplified signals on identical measurement conditions. Such a uniform or identical measurement condition for example exists if no material sheet is provided between transmitter and receiver or in reflection arrangement towards the radiated signal such that a basic signal in air (transmission signal/reflection signal against air) results in a uniform signal strength after amplification. In some implementations, even when using the group receiver electronics the amplification is also individually adjustable for each individual ultrasonic receiver of the group. Thereby, differences in the transmitting strength of the ultrasonic transmitters, the receiver sensitivities of the signal receivers or different attenuations on the signal paths can be normalized. 
         [0023]    The receiver electronics or the group receiver electronics may have a microcontroller, in particular a microcontroller, by which or in which the amplification of the amplifier of the receiver electronics is adjustable. This correspondingly applies to the group receiver electronics. Here too, the amplification again may be individually and independently adjustable for each of the ultrasonic receivers of the group. 
         [0024]    Alternatively or additionally, each of the receiver electronics or the group receiver electronics includes a signal processor for evaluating the signal or signals received by the associated ultrasonic receiver or the group of ultrasonic receivers. With the signal processor, it is possible to provide signal processing on the level of the receiver electronics or group receiver electronics such that parallel processing is implemented. The signal processor in the group receiver electronics may have a processing speed for processing the signals of the ultrasonic receivers of the group in parallel or sequentially. For example with clocked measurement the evaluation or calculation for each receiver of the group is terminated before a new measurement clock begins. Alternatively, a dedicated signal processor is provided or implemented for each of the receivers of the group in the group receiver electronics. 
         [0025]    A main control unit may be provided, by means of which a transmit signal can be generated, which is supplied to each of the ultrasonic transmitters via a bus line or parallel lines. In particular the signal generation and signal transmitting connection between the main control unit and the ultrasonic transmitters is such that the ultrasonic transmitters radiate the ultrasonic signal at the same time or with a slight temporal offset. If a temporal offset is present, then in clocked measurement operation the period of time between the earliest ultrasonic signal emitted by one of the ultrasonic transmitters to a time, at which the last ultrasonic transmitter emits the signal in the same measurement clock, is smaller than the propagation time of undesired sound reflections and/or propagation times between an ultrasonic transmitter and a signal receiver not associated with this ultrasonic transmitter (i.e. from an ultrasonic signal passing between different sensor units). 
         [0026]    Besides the ultrasonic transmitter and receiver pairs, a measuring group (in the following also referred to as a “sensor unit”) with at least one signal transmitter and a signal receiver may be associated with the device, by which the layer thickness and/or the grammage (mass per unit area) of the material sheet can be determined during a reversing movement or reciprocation of the measuring group transversely to the material sheet. In the sensor unit, a sensor for detecting transmission and/or reflection values of the material sheet is movable in transverse direction of the material sheet transported in longitudinal direction. The sensor unit can be moved back and forth in traversing manner along the device (for example between the outer longitudinal edges of the material sheet) by means of a drive unit. Thereby, for example, during a production process of the material sheet, the transverse distribution of the layer thickness and/or the grammage is monitored (e.g. along a zigzag-shaped measuring path along the material sheet). 
         [0027]    The device may include a sensor calibration position, in which the sensor unit is moved out of the material sheet measuring section. In the sensor calibration position, a supporting device with a calibration sample (as a calibration standard) is arranged. In the sensor calibration position, the sensor is moved relatively to the calibration sample and/or the calibration sample is moved relatively to the sensor by means of a drive. 
         [0028]    In an embodiment, a rotation and/or linear movement is effected by means of a rotation and/or linear drive, which moves the calibration measurement sample retained in the supporting device relatively to the sensor. Additionally or alternatively in the sensor calibration position the sensor can be moved over the calibration sample retained in the supporting device planar or at least in a linear direction, which in some implementations is preferably in the transverse direction of the material sheet. 
         [0029]    The sensor unit may include an ultrasonic sensor unit, in which the measurement signal for layer thickness and/or grammage determination is an ultrasonic pulse. Alternatively, an optical sensor is used, which for example uses a laser beam or a light emitting diode beam. Further alternatively, the sensor unit may have a ray sensor emitting and receiving gamma or beta rays. 
         [0030]    In an embodiment, the sensor unit is exclusively constructed as a transmission unit, in which only the absorption in transmission of the sensor signal through the material sheet is determined. Or the sensor unit is exclusively a reflection unit, in which the reflection of the measurement signal from the material sheet or from the backside of the material sheet is detected. Or alternatively, the sensor unit is a combined transmission and reflection measurement unit, in which both the attenuation of the sensor signal in reflection and in transmission are determined. In such a transmission and reflection measurement, one of the values can be used for plausibility check of the other value and/or for averaging in the layer thickness and/or grammage determination. 
         [0031]    The sensor unit may be used for calibration of the plurality of the ultrasonic transmitter and receiver pairs of the device and/or the sensor unit may be calibrated in the sensor calibration position with the calibration sample. 
         [0032]    According to another implementation, an arrangement includes a device according to one of the preceding embodiments and an extruder, in which an actuator of an extrusion die of the extruder is adjustable depending on the layer thickness and/or grammage measurement of the device. Thereby, control of the layer thickness or of the grammage in the manufacture of a material sheet, for example a foil sheet, is provided. 
         [0033]    According to another implementation, an ultrasonic transmitting and receiving device with a plurality of sensor units is used, wherein each sensor unit is formed of an ultrasonic transmitter and an ultrasonic receiver associated with it. The device may be formed as above described, i.e. the above described elements can also be used individually or in any combination with each other in the device for the method. 
         [0034]    In the method, a control signal is supplied to all of the ultrasonic transmitters at the same time or with a little temporal offset within a time window such that the ultrasonic transmitters emit their ultrasonic signal at the same time or within the time window. The time window is dimensioned such that a spurious reflection signal and/or a signal of another ultrasonic transmitter is not received by an ultrasonic receiver associated with the ultrasonic transmitter in a corresponding time window of the measurement of the ultrasonic signal. By the simultaneous or nearly simultaneous transmission of the ultrasonic signal, parallel measurement of all sensor units is provided, wherein at the same time cross-talk between the sensor units or an error signal by reflection within the measuring section of a sensor unit is excluded. 
         [0035]    Furthermore, in the method, a parallel or substantially parallel processing is provided, wherein one receiver electronics is associated with each of the ultrasonic receivers or one group receiver electronics is associated with a group of ultrasonic receivers, by which evaluation or pre-processing of the signals of the ultrasonic receivers can be executed in parallel or substantially in parallel. Using the group receiver electronics the processing may also be effected for all of the signals of the receivers associated with the group or by fast electronics such that in a clocked measurement the processing result for all of the receivers of the group is finished before the next measurement clock occurs. 
         [0036]    The evaluated or pre-processed signals may be supplied to the receiver electronics or the group receiver electronics of a main control device. Evaluation of the signal for determining the thickness and/or grammage determination may be effected either already on the level of the receiver electronics or group receiver electronics or in the main control device after forwarding the pre-processed signals to the main control device. 
         [0037]    Thereby, by means of the method, both parallel emission (multiple measurement positions on the material sheet at the same time) and parallel processing of the signals is provided such that an intensive detection of the thickness and/or the grammage of a material sheet is provided with high density and repetition rate. 
         [0038]    In an embodiment, the sensor units are calibrated by measuring a basic sensitivity based on a signal transmitted through the air (thus without material sheet). This value may be used in determining the sensitivity in order to adjust an amplification factor individual to the transmitting unit in the receiver electronics or group receiver electronics such that a receive signal is obtained after amplification with identical conditions on the measuring section, which has the same size or strength normalized for all transmitting units. 
         [0039]    Alternatively or additionally, a calibration and detection of a conversion factor between transmission value and/or reflection value and association with a thickness and/or a grammage of a material sheet is effected by using the calibration at a calibration sample or calibration standard, which is applied between the sensor units such that measurement on a material sheet is simulated. After removal of the calibration sample a corresponding material sheet to be measured can then be interposed between the sensor units, the measurements on the material sheet can be carried out and the thickness and/or the grammage can be assigned by means of the previously determined calibration value. 
         [0040]    Other devices, apparatus, systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    The invention can be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0042]      FIG. 1  is a schematic plan view of a measuring unit for measuring the grammage of a material sheet. 
           [0043]      FIG. 2  is a schematic side view of the ultrasonic transmitters and receivers of the measuring unit above and below the material sheet. 
           [0044]      FIG. 3  is a schematic side view of the measuring unit with the transmitting block arranged above the material sheet and the receiving block arranged below the material sheet. 
           [0045]      FIG. 4  is a block diagram of the transmitting, receiving and control unit of the measuring unit. 
           [0046]      FIG. 5  schematically shows a group of ultrasonic receivers and the group controller associated therewith. 
           [0047]      FIG. 6  is a time diagram with received signals and measurement time windows. 
           [0048]      FIG. 7  is a set of intensity profiles of the ultrasonic signals emitted by the ultrasonic transmitters in projection. 
           [0049]      FIG. 8  is a plot of the intensity of the ultrasonic signal in transmission and reflection depending on the thickness of the material sheet. 
           [0050]      FIG. 9  is a block diagram of the transmitting, receiving and control unit in further embodiment of the measuring unit. 
           [0051]      FIG. 10  is a schematic plan view a measuring unit according to further embodiment with a calibration station and a traversing sensor unit in an arrangement with a foil extruder. 
           [0052]      FIG. 11  is a schematic representation of the measuring and control arrangement when positioning the traversing sensor unit in the calibration station of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0053]      FIG. 1  shows in schematic plan view a measuring unit  2  for thickness measurement and/or grammage determination at a material sheet  100 . In the measuring unit, a plurality of ultrasonic transmitters  10  and an equal number of ultrasonic receivers  12  in the form of an array are arranged planar above the material sheet (in the illustrated example, the ultrasonic transmitters  10 ) and below the material sheet (in the illustrated example, the ultrasonic receivers  12 ). For simplicity, the transporting device for moving the material sheet  100  forward in the longitudinal direction x thereof is not represented. In the figures, the size ratios and distance ratios are not represented to scale, but are presented such that they serve for explaining the invention. 
         [0054]      FIG. 2  shows in schematic and partially perspective representation the distribution of the ultrasonic transmitters  10  and the ultrasonic receivers  12  arranged correspondingly opposite and coaxial to each of the ultrasonic transmitters. 
         [0055]      FIG. 3  shows in schematic lateral view a portal  4  of the measuring unit  2  with an upper transverse beam and a lower transverse beam as well as the lateral pillars, which together leave open a slot  7  extending in x direction through the portal  4 . The material sheet  100  is transported in x direction through the slot  7 . The portal and thus the array arrangement extends transversely, thus in y direction, to the material sheet  100 . 
         [0056]      FIG. 1  indicates by means of the circles, which represent the ultrasonic transmitters  10 , how the ultrasonic transmitters form three lines I, II and III, wherein each line is offset in x direction to the preceding one. The columns of the array are formed by the columns a, b, c and d. The ultrasonic transmitters  10  are not on a straight line in x direction, but are arranged offset to each other in an angle α to the x direction. An ultrasonic receiver  12  is oriented coaxially in z direction to each of the ultrasonic transmitters  10 . Thus each one of the ultrasonic sensors is formed by one of the ultrasonic transmitters and one paired ultrasonic receiver, which is arranged in the direction of the normal perpendicularly to the center of the sound emitting surface of the ultrasonic transmitter. I.e. in a sensor, the center of the receiving surface of an ultrasonic receiver  12  is oriented coaxially to the axis of a transmitter  10 . Thereby, a pairing of ultrasonic transmitter and ultrasonic receiver respectively results, wherein 4×3=12 ultrasonic transmitting/receiving pairs are provided in the example illustrated in the figures. 
         [0057]    Both the number of the lines I, II and III and the number of the columns a, b, c, d can be selected differently depending on the material sheet width and the desired overlap or the distance of the measurement regions (see below). The ultrasonic transmitters  10  are mechanically supported distributed in planar manner on a transmitting block  6 , wherein the transmitting block  6  in turn is mounted on the upper transverse beam of the portal  4 . Furthermore,  FIG. 3  shows a receiving block  8  below the material sheet  100  on which the ultrasonic receivers  12  are supported distributed in planar manner, wherein here the receiving block  8  is mounted on the lower transverse beam of the portal  4 . In an embodiment, it can be provided that both the transmitting block  6  and the receiving block  8  are displaced synchronously with each other in reversing manner in transverse direction (y direction), while the material thickness or the grammage is measured. By this optional transverse displacement it is effected that the non-uniform signal distribution of the ultrasonic signal is drawn over various “tracks” in x direction of the material sheet in a temporally varied manner. In a synchronous, harmonic transverse displacement of the blocks  6 ,  8 , thus, there results a sinusoidal extension of the measurement points in x direction of the material sheet. The reversing, synchronous displacement of the blocks  6 ,  8  is effected such that the coaxial orientation of the ultrasonic transmitters  10  to the ultrasonic receivers  12  is maintained. However, the blocks  6  and  8  are preferably rigidly supported on the portal  4  or on a frame of the measuring unit  2 . 
         [0058]    In the schematic side view of  FIG. 2  and the plan view of  FIG. 1 , the extension of the main intensity cone  14  of an ultrasonic signal distribution is illustrated. Therein, the signal lobe  14  illustrates the lateral divergence of an ultrasonic wave of the ultrasonic signal, which is radiated from an ultrasonic transmitter  10 . The diameter of the lobe  14  is greater than the diameter of the radiation surface of the ultrasonic transmitter  10  on the level of the receiving surface of the ultrasonic receivers  12 . As indicated by means of the dotted circles  14  in  FIG. 1 , the distance between the adjacent ultrasonic transmitters  10  is selected such that each ultrasonic receiver  12  substantially only receives the main intensity of the ultrasonic signal of the ultrasonic transmitter  10  associated with and opposing it. Thereby, interferences or cross-talk of a transmitter/receiver pair with the other transmitter/receiver pair (between the sensors) is minimized. 
         [0059]    The angular offset a of the transmitter/receiver pairs of the columns relative to the x direction allows performing the grammage determination in y direction in at least a partially overlapping manner such that in ideal case the thickness and/or grammage distribution of the material sheet  100  over the entire width of the material sheet is possible at each time. This arrangement thus substantially differs from that known from the above cited DE 201 09 119 U1, in which—due to the traversing operation of the ultrasonic transmitter—the detection of a material sheet defect (thickness and/or grammage deviation) rather is left to the statistical coincidence than a systematic measurement as it is allowed with the present invention. 
         [0060]      FIG. 4  shows in block diagram manner the construction of the measuring unit  2 . Here, only two of the ultrasonic transmitters  10  and ultrasonic receivers  12  are exemplarily illustrated. The ultrasonic signal  14  propagates from the transmitter  10  towards the material sheet  100 , passes it with attenuation of the intensity of the signal, which impinges on the receiving surface of the receiver  12  after exiting on the bottom of the material sheet. In each measurement interval (time window G in  FIG. 6 ), the signal illustrated as a signal sequence  24  in  FIG. 4  is supplied to each transmitter. The signal sequence  24  is generated in a main control unit  22  and supplied to the ultrasonic transmitters  10  in parallel or simultaneously via a transducer  18 . 
         [0061]    The ultrasonic signal  14  converted into an electrical signal by the ultrasonic transducer of the ultrasonic receivers  12  is supplied to a receiver controller  28 . A dedicated controller  28  is associated with each ultrasonic receiver  12 . The electrical signal is processed by means of a digital signal processor in the receiver controller  28  such that just on the level of the controllers  28  signal pre-processing is executed. The signal processing may be effected by means of corresponding calibration values such that the controllers  28  may output a signal value corresponding to the material thickness or the grammage to the control unit  22  via a receive signal line  32 . Thereby, the main control unit  22  is relieved from the individual signal conditioning or processing and only statistical and control tasks have to be executed by the control unit  22 . By the parallel processing in the controllers  28  respectively associated with a receiver  12 , a parallel processing is provided, which together with the planar distribution of the sensor units  10 ,  12  in the array allows a nearly complete “in situ” monitoring of the material quality of the material sheet  100 . Even in fast production processes for the material sheet, thereby, high-speed quality monitoring of high density is made available such that high-quality material sheets (such as electrolyte membranes, fuel cell membranes or high-performance battery isolator foils or membranes) can also be produced and the quality thereof can be monitored online. 
         [0062]      FIG. 5  shows a further embodiment, in which a group controller  29  is provided for each four receivers  12  instead of each one controller  28  per receiver  12 . In the 3×4 array of  FIG. 1 , then, three group controllers  29  are used instead of twelve individual controllers  28 . The group controllers have a dedicated amplifier component with individual amplification for each receiver  12  belonging to the group. By means of the signal processor DSP of the group controller  29 , the signals of the associated receivers  12  are evaluated sequentially one after the other, but with such a velocity that the computing result is available and output before the next measurement interval begins. With respect to  FIG. 6  this means that the result of the signal evaluation of all of the associated receivers  12  is transmitted to the main control unit before the next time window G starts (the evaluation period of time is thus shorter than the sum of the periods of time G and U). Programming of the amplification, the supply of the supply voltage(s) of the controller  29  and programming of the signal processor DSP of the controller  29  are effected via the control line  30  coming from the main control unit  22 . The evaluation signals of the controller  29  are supplied to the main control unit  22  via the line  32 . 
         [0063]    The time diagram of  FIG. 6  illustrates the voltage signal at the output of the receivers  12 . The measurement is performed with the repetition rate 1/(G+U), thus also with this rate, the signal sequence  24  is output to the transmitters  10 . Therein, G is the time window or the time gate, in which the received signal  24   a  is evaluated in order to determine from it the thickness and/or the grammage of the material sheet by means of the DSP. U is a time interval or a period of time, during which the ultrasonic signal is not received and processed. The duration of G is dimensioned such that the evaluation can be performed with an error as low as possible (multiple wave trains after excitation with the pulse burst  24 ). The duration of U is dimensioned such that undesired spurious signals are masked in this time. Here, exemplarily illustrated spurious signals are a reflection signal  24   b , which arises in that the signal radiated from the transmitter surface is partially reflected on the material sheet surface, returns to the transmitter surface, is again reflected there and again passes the material sheet and is recorded by the receiver  12  with corresponding propagation time delay. A further spurious signal  24   c  arises in that the transmit signal is radiated by a transmitter  10  laterally towards an adjacent receiver  12  and there in the adjacent receiver induces a signal  24   c  which is propagation time-delayed but weaker. 
         [0064]      FIG. 7  shows in x projection the ultrasonic signal intensity extending in y direction until it results by the array of the transmitting/receiving pairs across the width of the material sheet. In  FIG. 7 , for simplicity, only 3 of the overall 12 intensity curves P are illustrated. Therein, the intensity P is that of one transmitter  10 , respectively. Therein, the intensity curve is approximately Gaussian, and with respect to the z axis there is rotational symmetry of the intensity distribution because the ultrasonic transmitting surfaces are round and radiate in rotationally symmetric manner. 
         [0065]    As is apparent from  FIG. 7 , the maximum diameter ranges of the diameter D of the radiation surfaces of the ultrasonic transmitters  10  overlap in x projection such that an overlapping region O results in the transmitter  10  closest in y direction. Thereby, the measurement intensity P of all transmitters  10  never drops to 0 in the extension of the y direction, and in transverse direction (y direction) of the material sheet  100 , a width region does not arise, which is not covered by the measurement or the ultrasonic signal. Therefore, due to the array and the columns of the array arranged in an angular offset, a dead zone in transverse direction of the material sheet does not arise. 
         [0066]      FIG. 8  schematically shows the intensity curve of an ultrasonic signal in transmission T and in reflection R depending on the thickness d of the material sheet. Based on such calibration curves for an ideal material sample the thickness or the grammage can be determined based on the actually measured signal variation. In the illustrated arrangement, measurement is made in transmission such that the curve T is used in the thickness or grammage determination. 
         [0067]    For example, if a thicker or thinner material sheet would result in an intermediate region between the intensity maxima of the intensity curve P of two transmitters  10  adjacent in y direction in the material sheet to be measured due to a systematic production fault, thus, this systematic fault can be identified either by detecting systematically a deviating value in the two concerned transmitter/receiver pairs  10 / 12 . Or, alternatively, as described in connection with  FIG. 3 , the transmitting block and the receiving block  6 ,  8  are displaced synchronously with a short y stroke in y-direction such that the measurement sensitivity is shifted towards maximum of the intensity curve P with such a systematic fault. 
         [0068]      FIG. 9  shows a further configuration of the measuring unit  2  as a measuring unit  2   a . Identical, same or equivalently acting elements of the measuring unit  2   a  are labeled with the same reference characters or with reference characters supplemented with the addition “a” as the corresponding element of the measuring unit  2 . In the measuring unit  2   a , the reflection signal  15  reflected back from the material sheet  100  towards the ultrasonic transmitter  10  can also be evaluated or is evaluated with respect to the reflection R (cf.  FIG. 8 ). 
         [0069]    The unit  2   a  allows the following measurement modes: transmission measurement, reflection measurement, transmission and reflection measurement and additionally to these or alternatively to these a propagation time measurement for direct thickness calculation. 
         [0070]    With respect to the transmission measurement, the setup is as described above with respect to  FIGS. 1 to 8 . For reflection measurement, the ultrasonic transmitter  10   a  operates as a transceiver receiving the ultrasonic signal  15  reflected on the material sheet  100  and converting it into a voltage signal. First, the signal sequence  24  generated by the main control unit  22  is supplied to the receiving controllers  28   a  modified with respect to the receiving controllers  28 . During the transmit interval of the signal sequence  24 , a switch  31  provided at each receiving controller  28   a  connects the input of the line  20  to the input on the respective ultrasonic transmitter  10   a  such that it operates like the ultrasonic transmitter  10  in this phase and radiates an ultrasonic signal  14  towards the material sheet  100 . After the end of the signal sequence  24 , the switch  31  connects the ultrasonic transmitter  10   a  to the microcontroller μC+DSP at the receiving controller  28   a . Therein, the converted reflection signal  15  is supplied to the microcontroller and the signal processor thereof for evaluation. Concerning the evaluation and the temporal gating of the reflection receive signal, the above described in communication with the measuring unit  2  for evaluation of the transmission signal applies. 
         [0071]    Here, the adjustment of the amplification individually programmable for each receiving controller  28   a  and the calibration are correspondingly applicable, wherein it applies to the reflection signal instead of the transmission signal. If transmission and reflection are measured at the same time, the receiving controllers  28  and  28   a  operate in parallel as it was described above for the operation of the receiving controllers  28 . The results of the evaluated signals or the pre-processed signals are supplied to the main control unit  22  via the line  32   a . Programming, control and current supply of the receiving controllers  28   a  as well as switching of the switches  31  is effected from the main control unit  22  via the control line  30   a.    
         [0072]    In an embodiment, instead of individual receiving controllers  38   a , a group controller can be used, which is configured analogously to the group controller  29  of  FIG. 5 . The switch  31  can be a controllable switch, which connects the transmitter  10   a  to the microcontroller/DSP or to the signal line  20  according to control. Or the switch  31  can be a unidirectional direction gate passing signals  24  incoming from the line  20  towards transmitter  10   a  and supplying returning signals from the transmitter  10   a  to the microcontroller. 
         [0073]    In evaluation or processing of the signals for thickness and/or grammage determination, the quotient of the values of transmission and reflection (T/R) can be used in the calculation in the measuring unit  2   a . Via the quotient formation, measurement errors affecting proportionally or approximately proportionally both the transmission and the reflection are cancelled out. Examples of the cause of such measurement errors are the air temperature, air pressure or air humidity. 
         [0074]    If thick or highly absorbing materials are measured as the material sheet, the reflection measurement can provide the more accurate results. For example, in transmission, a thickness measurement is unfavorable if the material thickness approaches to λ/4 of the wavelength of the ultrasonic sound or exceeds this value. In the reflection signal, it is also possible to determine the thickness of the material sheet  100  directly from the propagation time difference of the reflection signal. The propagation time difference is the difference between the first reflected signal arising upon impinging of the signal  14  on the surface of the material sheet, and the last (without regard to multiple reflections) reflected signal occurring after passage of the signal  14  through the material sheet on the second surface or exit surface. With known sound velocity in the material, then, the material thickness can be directly determined via the propagation time difference determined by means of signal evaluation. 
         [0075]      FIG. 10  schematically shows in plan view a further configuration of a grammage measuring unit  2   b  arranged on a material sheet  100  advanced in longitudinal direction x. In the following, the same reference characters are used for the same or equally acting elements as for the above described embodiments of the measuring units  2 ,  2   a . Unless otherwise mentioned, the above explanations also apply to the measuring unit  2   b . As above, for simplicity, the transporting device for advancement of the material sheet  100  is not illustrated. The material sheet is directly measured during the manufacturing process with the measuring unit  2   b  (also  2  or  2   a ). As illustrated, the material sheet  100  is extruded as a foil through a flat extrusion die  102  of a foil extruder. In  FIG. 10  the further elements of the foil extruder are not illustrated—except for the control unit  101  of the extruder in  FIG. 11 . Instead of the flat extrusion die, a round extrusion die can also be employed for the foil extruder, wherein after extrusion the material sheet is stretched to a flat material sheet in the transport, before it enters the portal  4   b  of the measuring unit  2   b.    
         [0076]    As noted above, the grammage measuring unit  2   b  has a transverse portal  4   b  as the basic component, which extends above and below the material sheet  100  across the full width of the material sheet. In this case, the transverse portal  4   b  extends on one side considerably beyond the sheet width, since a standby and calibration station  116  for a traversing or reciprocating measuring group or sensor unit  108  is additionally arranged laterally to the material sheet. On the upper transverse beam of the transverse portal  4   b  (cf. arrangement of  FIG. 3 , the beam extending at the top in y direction), a carriage console  110  is supported on a carriage not visible in the figure, wherein the carriage is movable transversely (thus in y direction) to the material sheet  100  in reversing manner by means of a linear drive. 
         [0077]    The carriage console  110  supports a transmitting head  112 , which can be moved across the full width of the material sheet  100  by means of the carriage console  110 . The carriage console  110  supports the transmitting head  112  radiating ultrasonic pulses to the top of the material sheet for measuring. The ultrasonic signal propagates under attenuation through the material sheet  100  to the lower side thereof, where the attenuated ultrasonic signal exits and impinges on a receiving head  114  opposing the transmitting head  112 . The receiving head  114  is arranged on a carriage console not illustrated, which in turn is movable on the lower beam of the transverse portal  4   b  (cf.  FIG. 3 ). The lower carriage with the carriage console for the receiving head  114  is moved synchronously with the upper carriage with the console  110  such that the transmitting head  112  and receiving head  114  of the measuring group  108  oppose each other at any time in moving along the material sheet and in the standby and calibration station  116  (with the material sheet  100  or the calibration sample  122  in between). 
         [0078]    At the extrusion die  102 , the thickness of the material sheet is adjusted by means of the pivotable die lip  104 , wherein the adjustment is effected by means of actuators  106  arranged equidistantly to each other transversely to the sheet width. The actuators  106  are for example thermal expansion bolts, the longitudinal expansion of which is adjustable by temperature variation such that they adjust the work angle of the die lip  104  by the length variation. Therein, each actuator  106  acts locally in its width region such that the thickness of the material sheet in the corresponding y width range is substantially determined by the actuator  106  in this width range or in intermediate ranges between two adjacent actuators  106  by the cooperation of these two actuators. 
         [0079]    At the measuring unit  2   b , ultrasonic transmitters  10  and ultrasonic receivers  12  paired thereto are arranged over the width of the material sheet on the portal  4   b , the common measurement regions of which cover the full width of the material sheet. The arrangement is as described above, wherein here the transmitting block  6  with the transmitters  10  and the receiving block  8  with the receivers  12  are preferably stationary supported on the portal  4   b . In this implementation, two array lines I, II offset to each other in transverse direction (y) and six array columns a-f are provided. The above mentioned relating to the transmitters  10  and receivers  12 , the electronic control and evaluation thereof (cf.  FIGS. 4 and 9 ) as well as to the procedure of transmitting/receiving/evaluating correspondingly applies. 
         [0080]    As is apparent based on the dot-dashed lines in sheet longitudinal direction (x direction), within the measuring array of the measuring unit  2   b  one transmitting/receiving pair  10 ,  12  per actuator  106  is arranged in the longitudinal direction of the sheet  100 . The distance of the actuators is in the range of 20 to 40 mm or 25 to 35 mm, typically it is 25.4 or 30 mm. Correspondingly, the center distance of the pairs  10 / 12  in y direction is equal to the distance of the actuators  106 . Here, the association is exemplary from left to right, wherein the transmitting/receiving pair  10 / 12  in line I and column a is associated with the leftmost actuator  106  and the transmitting/receiving pair  10 / 12  in line II and column f is associated with the rightmost actuator  106 . By this association, it is provided that the measurement results of the layer thickness and/or grammage determination are transmitted from the main control unit  22  to the control unit  101  of the extrusion device as illustrated in  FIG. 11 . The adjustment of each individual one of the actuators  106  by means of the extruder control unit  101  can then be effected depending on which value of the thickness and/or of the grammage the associated transmitter/receiver pair  10 / 12  provides. The adjustment of the actuators  106  may be effected continuously and in the form of a feedback control arrangement. 
         [0081]    In the schematic cross-sectional view of  FIG. 11 , the arrangement of transmitting head  112  and receiving head  114  of the measuring group  108  is illustrated in lateral view, wherein the y direction is perpendicular to the drawing plane in  FIG. 10  in the lateral view. By  136  an ultrasonic transmission beam is graphically illustrated, which emanates from the transmitting head  112 . In the illustrated parking position  118  of the measuring group  108 , the transmission beam  136  passes through a calibration sample  122  and impinges on the receiving head  114 . 
         [0082]    The standby and calibration station  116  is laterally offset to the material sheet, thus offset in y direction or transverse direction to the material sheet  100 . The standby and calibration station  116  comprises the parking position  118 , in which both the transmitting head  112  and the opposing receiving head  114  are parked during measurement interruptions or for calibration of the measuring group  108  with the transmitting/receiving unit  112 ,  114 . 
         [0083]    In the standby and calibration station  116 , a clamping ring  120  is rotatably supported, which is set into rotation on its outside by means of a pinion  126 . As apparent from  FIG. 11 , the pinion or gear  126  is driven by a drive motor  124 . The pinion  126  engages with a ring gear formed at the outer side of the clamping ring  120  such that the rotating speed or angular position of the clamping ring  120  can be controlled by means of the motor  124 . The calibration sample  122  is clamped in the clamping ring  120 . The calibration sample  122  is a round blank of a standard material to be employed for calibration. The blank has an area of a square centimeter such that the grammage of the standard can be determined in simple manner by weighing the blank on precision weighing machine. The calibration standard in form of the calibration sample  122  represents a set value for the thickness and/or the grammage of the material sheet  100  and is employed for repeated calibration of the measuring group  108  including the transmitting and receiving heads  112 ,  114 . 
         [0084]      FIG. 11  shows the relative position of the calibration sample  122  besides the schematic lateral view of the transmitting and receiving head  112 ,  114 . Similarly, the control and monitoring electronics for the grammage measuring unit  2   b  relating to the measuring group  108  and calibration station  116  is illustrated in the form of a block diagram. The control of the measuring group  108  and the calibration station  116  is effected by the same main control unit  22 , which also provides the control, supply and read-out of the transmitting/receiving units  10 / 12  of the arrays of the measuring units  2  or  2   a . In the unit  2   b , the control unit  22  additionally provides the current supply, control, pulse triggering and signal read-out for the measuring group  108  and calibration station  116 . 
         [0085]    In the standby and calibration station  116 , by means of a position sensor  128 , it is detected whether the transmitting head  112  and the receiving head  114  have arrived at the correct parking position  118  in order to perform the calibration for example. The position sensor  128  reports its signal to the control unit  22  of the grammage measuring unit  2   b.    
         [0086]    The control unit  22  controls a transmitter controller  132  of the measuring group  108 . For example, the transmitting controller  132  obtains the supply voltage and an amplification adjusting signal for adjusting the signal amplification from the control unit  22 . With the preset signal amplification, a pulse signal also received from the control unit  22  for the transmitting head  112  is amplified. The transmitter controller  132  outputs the amplified pulse signal to the transmitting head  112 , which converts the voltage signal into the ultrasonic signal  136 . 
         [0087]    The ultrasonic signal received at the receiving head  114  is converted into an electric signal and supplied to a receiver controller  134 . The receiver controller executes signal conditioning and passes the conditioned receive signal to the control unit  22 . For example, the receiver controller  134  comprises a digital signal processor, which provides a signal processing algorithm by a corresponding programming via the control unit  22 , in order to perform the computationally intensive signal evaluation already on the level of the receiver controller  134 . 
         [0088]    In order to for example compensate for thermal drifts, ageing processes, contaminants on the transmitting/receiving path of the ultrasonic signal  136  and the like effects, the measurement of the grammage or of the layer thickness of the material sheet  100  is interrupted in presettable or predefined time intervals for calibration. To this, the transmitting and receiving head  112 ,  114  (group  108 ) moves laterally out of the measuring section or path (width of the material sheet  100 ) into the parking position  118 . If the correct position of the transmitting and receiving head  112 ,  114  is detected by means of the position sensor  128 , the control unit  22  controls the motor  124  such that the calibration sample  122  clamped in the clamping ring  120  is rotated between transmitting head and receiving head. The center of the transmitting/receiving surface of the transmitting/receiving head  112 ,  114  is offset radially to the center of the calibration sample  122  such that the center of the transmitting/receiving head is moved relatively to the calibration sample on a circular orbit. 
         [0089]    During rotation of the calibration sample  122 , ultrasonic transmit pulses are continuously transmitted by the transmitting head  112  and detected by the receiving head  114 . 
         [0090]    Thereby, the transmission values from the calibration sample  122  are measured at different positions distributed over the surface of the calibrations sample. The measured values are recorded by means of the control unit  22 . After one-time or multiple-time rotation of the calibration sample  122 , the control unit  22  forms an average from the measured transmission values and uses it to calibrate the calibration curve for the grammage determination or layer thickness determination by the traversing measuring group  108 . 
         [0091]      FIG. 8  shows exemplarily and schematically with the curve t a calibration curve for the intensity I of the transmission of the ultrasonic signal  136  (cf. signal  14  in  FIG. 2 ) depending on the thickness d (the same also applies to the grammage) of the material sheet  100 . If the averaging of the previously described determined transmission measurement results in a deviating calibration value for the layer thickness d or the grammage, thus, the calibration curve is correspondingly upwardly or downwardly corrected. 
         [0092]    Thereby, a newly calibrated calibration curve is available after the calibration and the layer thickness measurement by means of the measuring group  108  transversely to the material sheet  100  can be continued with the new calibration curve such that the grammage or layer thickness determinations can be performed with higher reliability. 
         [0093]    As illustrated in the configuration of the measuring unit  2   b  in  FIG. 10 , the measuring unit includes both the traversing measuring group  108  (i.e. reciprocating across the material sheet  100 ) as well as the stationary array with the (here twelve) transmitting/receiving pairs  10 / 12 . The measuring group  108  is absolutely calibrated in the calibration station  116  by means of the sample  122 . The calibrated measuring group  108  is then used to calibrate the transmitting/receiving pairs  10 / 12  as follows. The cooperation of material sheet  100  moved in longitudinal direction x and measuring group  108  reciprocating in transverse direction y results in the zigzag-shaped sampling path t illustrated dotted in  FIG. 10 . 
         [0094]    Along the sampling path t, the measuring group  108  detects measurement values of the layer thickness and/or the grammage with the repetition rate of the pulsed measurement (cf.  FIG. 6 ). Therein, the path t also sweeps the center of the measurement region of the transmitting/receiving pairs  10 / 12  extending in the longitudinal direction x, which is exemplarily illustrated dot-dashed with line s for the pair  10 / 12  in line I, column d (in the following referred to as “(I, d) pair  10 / 12 ”). For this example, the five measurement points m 1  to m 5  are the crossing points of the crossing measuring lines or paths s and t. For calibration of this (I, d) pair  10 / 12 , for example, the average of the five measurement points m 1 -m 5  for the thickness and/or the grammage each for the measurement by means of the measuring group  108  and the measurement by means of the (I, d) pair  10 / 12  is formed, and if they deviate from each other, the calibration value for the (I, d) pair  10 / 12  is newly set such that the averaged values for the measurement points m 1  to m 5  between measuring group  108  and (I, d) pair  10 / 12  coincide. At least, the new calibration value is used in the following measurements for the (I, d) pair  10 / 12  until it again has been calibrated against the measuring group  108 . In this manner, the other transmitting/receiving pairs  10 / 12  are calibrated with the corresponding measurement values on the crossing points of the sampling path t and the associated measuring line s of the transmitting/receiving pair  10 / 12  extending in x direction. This represents a relative calibration of the transmitting/receiving pairs  10 / 12  against the measuring group  108 . 
         [0095]    The feedback of the measurement value of the layer thickness or the grammage of the material sheet from the transmitting/receiving pairs  10 / 12  via the control unit  22  and the control unit  101  for adjusting the actuators  106  on the extrusion die  102  allows a substantially faster control as if for example only the reversing measuring group  108  would be used for measuring and controlling. Moreover, in the measurement at the material sheet  100  and when using only the reversing measuring group  108  it is not possible to identify whether a thickness variation in the longitudinal profile (direction x) or transverse profile (direction y) of the sheet  100  has occurred. A thickness variation in the longitudinal profile for example occurs if the sheet transport proceeds unstably or the foil material exits the die  102  non-uniformly (e.g. on the whole width) due to pressure variations in the extruder. A thickness variation in the transverse profile for example occurs if an individual actuator  106  operates non-uniformly or differently or the die  102  is locally occluded at one or more of the actuators. 
         [0096]    In an embodiment, in the measuring unit  2   b  too, the transmitting heads  112  and opposing the receiving heads  114  can be arranged on a transmitting block  6  and receiving block  8  y-displaceable synchronously with each other as in the measuring unit  2   a.    
         [0097]    It will be understood that various aspects or details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation—the invention being defined by the claims.