RELATED APPLICATIONS 
     This application claims the benefit of German patent application number DE 10 2011 014 518.4 filed Mar. 18, 2011, the content of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The invention relates to a method for calibrating a sensor which is used for measurement of thickness and/or surface weight (weight per unit area) of a material sheet. 
     BACKGROUND 
     From DE 42 36 436 A1 a measurement method is known for the contact-free determination of the surface weight of thin material sheets using ultrasound. In this method, the transmission absorption of an ultrasonic beam emitted through a sheet material is determined using an ultrasonic transmitter and ultrasonic receiver. The surface weight is calculated using the absorption and a calibration factor. 
     From DE 201 09 119 U1 another apparatus for thickness measurement of material sheets is known. There, the material sheet is drawn over a roller and the thickness measurement is executed by a sensor mounted on a dolly which traverses back and forth over the roller. 
     There is a need for providing a method for improved calibration of a sensor. Furthermore, there is a need for providing an improved method or an improved apparatus for measurement of layer thickness and/or surface weight of a material sheet. 
     SUMMARY 
     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. 
     According to one implementation, a method for calibrating a sensor is provided. The sensor is used for determining the surface weight and/or the material thickness of a material sheet or film material (in the following generally reference is made to material sheet which also includes film or foil). The measurement of the layer thickness or the surface weight can be implemented exclusively by transmission measurement, in which a transmission unit of the sensor sends a measurement signal through the material sheet to the opposite side where it is received by a receiving unit of the sensor. The layer thickness or surface weight can be determined from the attenuation of the signal at its passage (transmission) through the material sheet. Alternatively, the layer thickness is determined based on the signal reflected by the material sheet. In this case, the signal transmitter and the signal receiver of the sensor are arranged on the same side of the material sheet. As a further alternative, the sensor has a transmitter at the first side of the material sheet, a first receiver at the first side of the material sheet and a second receiver at the second side of the material sheet, wherein the transmission is measured by the second receiver and the reflection is measured by the first receiver. 
     By the calibration method, the sensor that is to be calibrated (which is used in the normal measurement mode for thickness measurement and/or surface weight measurement as mentioned previously) is calibrated using a two-dimensional or areal calibration sample. For this purpose, a plurality of transmission values and/or reflection values are detected, wherein the values are detected at different or distinct positions which are two-dimensionally or areally distributed across the calibration sample. 
     In some implementations, the calibration sample is a reference material with a known surface weight (for example a surface weight which has been previously determined with calibrated methods), which is then used as a reference standard for the ongoing control of the layer thickness and/or the surface weight measurement of the material sheet. It is also advantageous in case the calibration sample is a round punch piece having a surface area of one square decimetre, whose surface weight has been determined by weighing it in a precision weighing machine. 
     From the plurality of values detected at the different positions of the calibration sample, an average value of the transmission and/or reflection values is calculated (if applicable, transmission separately and reflection separately). The resulting average value is mathematically linked to the known surface weight of the calibration sample, so that this calibration value can be used for calibrating the thickness measurement and/or surface weight measurement of the sensor during material sheet measurement. A calibration curve of the signal transmission may be generated or calibrated (the course of the transmission intensity in dependency of the layer thickness and/or surface weight, if measurement is conducted in transmission or in combination of transmission and reflection). Alternatively, a calibration curve of the signal reflection may be generated or calibrated (the course of the reflection intensity in dependency of the thickness and/or the surface weight, if measurement is done in reflection or in combination reflection/transmission). 
     Since a transmission and/or reflection value is detected not only on a single position of the calibration sample but on different positions two-dimensionally or areally distributed over the sample, layer thickness deviations or surface weight deviations of the calibration sample are averaged, so that the error of sensor calibration is reduced corresponding to the averaging. Due to the higher precision of the sensor calibration, the layer thickness measurement and/or surface weight measurement of the material sheet is more exact. 
     In one embodiment, the positions two-dimensionally or areally distributed over the calibration sample are positionally approached or indexed by rotating the calibration sample relative to the sensor. This rotation can be implemented with a simple mechanical construction and, while the calibration sample is rotating, positions of the calibration sample within a circular path are positionally approached. 
     Alternatively or additionally, the positions at the calibration sensor are positionally approached by translatory movement of the sensor relative to the calibration sample and/or by translatory movement of the calibration sample relative to the sensor. A rotation of the calibration sample can, for example, be combined with a translator movement of the sensor. In the case of such a combination, spirally positioned locations or positions which are arranged on different circle diameters can be travelled to. 
     During the rotation of the calibration sample as well as during the shift via translation (translational movement), the movement or positional approach to the two-dimensional or areal distributed positions can be done one by one or can be done continuously. When the positions are positionally approached one by one, the position is positionally approached, the measurement is executed, and then the next position is positionally approached and the measurement is executes there. In continuous travel or while moving along the two-dimensionally or areally distributed positions, the measurement is executed while the rotation or translation is executed. 
     In some implementations, the measurement of the transmission values and/or reflection values is executed continuously or quasi-continuously, while the calibration sample is continuously moving relative to the sensor and/or while the sensor is continuously moved relative to the calibration sample. Quasi-continuously here means that the measurement value detection with the sensor is executed in measurement intervals, in the sequence signal impulse transmitting—signal receiving—signal processing as is for example determined by the digital processing rate for the received signal. A continuous detection is done, for example, by analog or digital integration of the continuously transmitted and received measurement signal. 
     In the method of layer thickness measurement and/or surface weight measurement of a material sheet, a calibration of the sensor is executed before, during and/or after determination of the layer thickness and/or the surface weight of the material sheet as described above. The measurement of the layer thickness and/or surface weight of the material sheet is preferably executed by moving the (calibrated or not yet calibrated) sensor transverse to the longitudinal direction in which the material sheet is moving. The transmission and/or reflection of the material sheet is detected during the transverse movement of the sensor. The layer thickness and/or the surface weight of the material sheet is determined by calculation based on the detected transmission and/or reflection values and by using a calibration value or as the case may be a calibration curve, either of which in turn was calibrated or will be calibrated by calibrating the sensor. 
     In one embodiment of the measurement method, the calibration of the sensor using the calibration sample is preferably executed in predetermined time intervals. However, a calibration can be triggered based on other incidents, for example, when a temperature drift in the production environment or measurement environment of the material sheet has been discovered, at a batch change of the starting material for the material sheet, or similar. 
     In some implementations, a sensor unit of a material sheet transportation device is assigned to an apparatus for determining layer thickness and/or surface weight of a material sheet. At the sensor unit a sensor for detecting transmission values and/or reflection values of the material sheet is moveable in a direction transverse to the material sheet which is transported longitudinally. The sensor can be traversed back and forth by and along the sensor unit, for example, between the outer longitudinal edges of the material sheet. Thereby for example the lateral distribution of the layer thickness and/or the material weight is monitored during a production process. 
     The apparatus may include a sensor calibration position in which the sensor is moved out of the material sheet measurement section and is positioned at a support device having a calibration sample, which is arranged at the sensor calibration position. In the sensor calibration position the sensor is moved relative to the calibration sample and/or the calibration sample is moved relative to the sensor by an actuator or drive. 
     In an embodiment, a rotational and/or linear movement is provided by a rotational and/or linear actuator or drive, which moves the calibration sample held in the support device relative to the sensor. Additionally or alternatively, when being in the sensor calibration position, the sensor can be moved two-dimensionally or areally over the calibration sample held by the support device—or at least in one linear direction, preferably in the transverse direction to the material sheet. 
     In some implementations, the sensor is an ultrasonic sensor whose measurement signal for layer thickness measurement and/or surface weight measurement is an ultrasonic impulse. Alternatively, an optical sensor is used which uses a laser beam or a light emitting diode beam. In a further alternative, the sensor is a radiation sensor which receives and transmits gamma radiation and/or beta radiation. 
     In the embodiment and as described above, the sensor can be constructed exclusively as a transmission unit, in which only the absorption of the transmission of the sensor signal by the material sheet is determined or detected. Alternatively, the sensor unit is exclusively a reflection unit, in which the reflection of the measurement signal from the material sheet or from the back side of the material sheet is detected or determined. Alternatively, the sensor unit is a combination of transmission measurement unit and reflection measurement unit, in which both the attenuation of the sensor signal in reflection as well as in transmission are determined. In the case of such a transmission and reflection measurement, one value may be used for a plausibility check of the other value or and/or may be used for averaging for the determination of layer thickness and/or surface weight. 
     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 
       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. 
         FIG. 1  is a schematic representation of a surface weight measurement unit with a calibration station, 
         FIG. 2  is a schematic representation of the measuring and control arrangement when the sensor is positioned in the calibration station, and 
         FIG. 3  is a curve for intensity versus surface weight or layer thickness and its recalibration. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic top view of a surface weight measurement unit  2 , which is arranged on a material sheet  100  being moved in a longitudinal direction x. For purposes of simplicity, the transport means for moving the material sheet  100  is not depicted. In the figures, the proportions and relative spacing are not represented to scale but instead are represented in such a manner as to illustrate the invention. 
     As a basic component, the surface weight measurement unit  2  has a transversal portal  4 , which extends above and below the material sheet  100  over the full width of the material sheet and beyond. The transversal portal  4  has a slit shaped opening through which the material sheet  100  is transported in longitudinal direction x. On a carriage (which is not visible in  FIG. 1 ) a carriage console  6  is supported on the upper cross beam of the transversal portal  4  (can be seen in  FIG. 1  from above as a bar running in y-direction). The carriage can be moved by a linear drive or actuator, reversing crosswise (in y-direction) to the material sheet  100 . 
     The carriage console  6  supports a transmission head  8 , which is movable over the entire width of the material sheet  100  by the carriage console  6 . The transmission head  8  emits an ultrasonic pulse onto the upper surface of the material sheet and the ultrasonic signal propagates, while being attenuated, through the material sheet  100  to its lower surface, where the attenuated signal exits and impinges on the receiver head  10  being arranged opposing to the transmission head  8 . The receiver head  10  is arranged on a carriage console (not shown) which is movable over the entire width of the material sheet  100  and up to the calibration position in the standby and calibration station  12 . The lower carriage (not shown) supporting the receiver head  10  is moved on by a linear drive or actuator supported at and moving along the lower cross beam of the transversal portal  4 . The movement of the lower carriage console is synchronized with the movement of the upper carriage console  6  such that the transmission head  8  and the receiver head  10  are always in an opposing arrangement during movement over the width of the material sheet  100  and in the calibration position as indicated in  FIG. 2 . 
     In the schematic cross-section view of  FIG. 2 , the arrangement of the transmission head  8  and receiving unit  10  are represented in a side view; in this side view the y-direction is vertical to the plane of projection in  FIG. 2 . The ultrasonic transmission beam originating from the transmission head  8  is depicted at  32 . In the parking position  14  of the transmission head  8 , the ultrasonic transmission beam  32  penetrates the calibration sample  18  and impinges on the receiver head  10 . 
     The receiver head  10  is also mounted on a carriage at the lower cross beam (not shown) of the transversal portal  4  and is moved synchronous with the transmission head  8  in y direction. At the same time, the synchronous movement of the transmission head  8  and the receiver head  10  are such, that these are positioned on an axis in z direction collinear to one another. 
       FIG. 1  further shows a standby and calibration station  12  of the surface weight measurement unit  2 . The standby and calibration station  12  is laterally offset to the material sheet, thus in y-direction or transversal direction to the material sheet  100 . The standby and calibration station  12  has a parking position  14  in which both the transmission head  8  as well as the receiver head  10  on the opposite side are parked during measurement interruptions or for calibrating of the transmission/receiver unit  8 ,  10 . 
     There is a pivot-mounted clamping ring  16  in the standby and calibration station  12 , which is rotated via a pinion gear  22  on its exterior. As can be seen in  FIG. 2 , the pinion gear or gear wheel  22  is driven by a drive motor  20 . The pinion gear  22  meshes with a ring gear formed at the exterior of the clamping ring  16  so that the rotational speed or the angular position of the clamping ring  16  can be controlled by the drive motor  20 . In the clamping ring  16  a calibration sample  18  is clamped or supported. The calibration sample  18  is a round punch of standard material suitable for the calibration. The round punch has a surface of one square decimetre, so that the surface weight of the punch can be determined simply by weighing it on a precision weighing machine. The calibration standard in form of the calibration sample  18  represents a reference value for the thickness and/or the surface weight of the material sheet  100  and is used for repeated calibration of the transmission/receiver unit comprised of transmission head  8  and receiver head  10 . 
       FIG. 2  shows, in addition to the schematic side view of the transmission head  8  and receiver head  10 , the relative position of the calibration sample  18 . Likewise, the control and monitoring electronics for the surface weight measurement unit  2  is represented in the form of a block diagram. Using position sensor  24 , it can be determined whether the transmission head  8  and the receiver head  10  have reached the correct park position  14  in order to, for example, execute the calibration. The position sensor  24  sends its signal to a control unit  26  of the surface weight measurement unit  2 . 
     The control unit  26  controls a transmission controller  28 . For example, the transmission controller  28  receives its supply voltage and gain setting signal for setting the signal amplification from the control unit  26 . Along with the specified signal amplification, an impulse signal, which is also received from the control unit  26  is amplified for the transmission head  8 . The transmission controller  28  sends the amplified signal to the transmission head  8 , which converts the voltage signal to the ultrasonic impulse  32 . 
     The ultrasonic impulse received at the receiver head  10  is converted into an electrical signal and is fed to a receiver controller  30 . The receiver controller implements signal processing and supplies the processed signal to the control unit  26 . The receiver controller  30 , for example, includes a digital signal processor, which, by proper programming via the control unit  26 , will provide a signal processing algorithm, in order to execute the computationally intensive signal processing at the level of the receiver controller  30 . 
       FIG. 2  also shows at  34  the transmission signal intensity distribution along the diameter of the active ultrasonic transmission surface of the transmission head  8 . There, the approximately Gaussian shape of the intensity distribution can be seen, the maximum intensity is in the center area. 
     In order to compensate for thermal drifts, ageing processes, contamination at the transmission and receiver path of the ultrasonic signal  32  and similar effects, the measurement of the surface weight or the layer thickness of the material sheet  100  is interrupted in predefined time intervals for calibration. For this purpose, the transmission head  8  and the receiver head  10  are moved sideways out of the measurement area (width of the material sheet  100 ) and into the parking position  14 . If the position sensor  24  detects position of the transmission head  8  and the receiver head  10  to be correct, the control unit  26  controls the motor  20  in such a manner so that the calibration sample  18  clamped in the clamping ring  16  is rotated between the transmission head and receiving head. The centre of the transmission/receiver area of the transmission head  8  and the receiver head  10  is radially offset to the centre of the calibration sample  18 , so that the centre of the transmission/receiver head is moved on a circular path relative to the calibration sample. 
     While the calibration sample  18  is being rotated, the transmission head  8  is continuously (and repeatedly) sending ultrasonic impulses which are received by the receiver head  10 . Thus, the transmission values from the calibration sample  18  can be measured at different positions two-dimensionally or areally distributed over the surface area. The measured vales are recorded with the control unit  26 . After a single rotation or a plurality of rotations of the calibration sample  18 , the control unit  26  calculates an average value from the measured transmission values and uses this average value for calibrating the calibration curve for the surface weight measurement or respectively for the layer thickness measurement. 
       FIG. 3  shows, in an exemplary manner and schematically, a calibration curve for the intensity I of the transmission T of the ultrasonic signal  32  in dependency of thickness d (the same is valid for the surface weight) of the material sheet  100 . The dependency of the intensity I of the sheet or layer thickness d using the previous calibration value can be seen clearly from the solid line. If the calculation of the average of the previously described transmission measurement results in a deviation of the calibration value for the layer thickness d or the surface weight, then the calibration curve will be corrected up or down, as indicated by the dashed curves. Thus, after the calibration has been made, a new calibration curve ( FIG. 3 ) is available and the layer thickness measurement transverse to the material sheet  100  can be continued using the new calibration curve, so that the surface weight measurement or layer thickness determination can be carried out with high precision. 
     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.
A method is provided for calibrating a sensor used for surface weight measurement or thickness measurement on a material sheet via transmission measurement and/or reflection measurement. The method may include: providing a two-dimensional calibration sample with a known surface weight; detection of the transmission values and/or the reflection values of the calibration sample via the sensor on a plurality of different positions two-dimensionally or areally distributed across the surface of the calibration sample; and the detection of the calibration value for the sensor via calculation of average from the plurality detected transmission values and/or reflection values and the composition of the average transmission and reflection values with the known surface weight of the calibration sample. Also provided is a method for measurement of layer thickness and/or surface weight of material sheets using the calibration method, as well as an apparatus for layer thickness measurements and/or surface weight measurement.