Abstract:
The present invention relates to a system for detecting characteristics of an elastic structure ( 13, 22, 23, 32, 33, 93, 1202, 1302, 1402 ), said structure being provided with at least one sensor ( 111, 112, 14, 211, 212, 311, 312, 911, 1211, 1311, 1411 ) and said system comprising at least one detector ( 16, 416, 516, 916, 1316, 1416 ). The sensor comprises a deformable member, which upon deformation generates a signal convertible to a signal representing said characteristic, said sensor further comprises a passive transmitter and said detector a receiver for receiving said signal representing said characteristic.

Description:
PRIORITY STATEMENT 
   This application is a PCT National Stage of Application of PCT/SE2004/000062 filed Jan. 19, 2004, which claims priority under on Swedish Patent Application No. SE 0300127-8 filed in Sweden on Jan. 17, 2003, and U.S. patent application No. 60/319,874 filed in United States on Jan. 17, 2003, the entire contents of each of which is hereby incorporated herein by reference. 

   TECHNICAL AREA 
   The present invention relates to a system and method for detecting characteristics of an elastic structure, which is provided with at least one sensor and the system comprises at least one detector 
   DESCRIPTION OF STATE OF THE ART 
   Cylinder shaped bodies, e.g. tyres and other structures, which are manufactured of rubber, synthetic or organic material or the like, are deformed because of the contact with another surface. Moreover, the characteristics and functions of different structures are deteriorated in course of time. 
   Besides cylinder shaped bodies, the characteristics of a flat body, such as a sheet material, e.g. paper, paper board, fabric or conveyer, such as wire felt belt or paper machine clothing are of special interest. 
   Different methods and devices have suggested for detection of deformation, e.g.: WO 00/02741 discloses a method and an apparatus for counting the revolutions of a pneumatic tyre utilizing a sensor, which responds to the periodic mechanical stresses when the tyre rotates under load on a load-bearing surface such as a roadway. The sensor can be constituted of a piezo-element, electrically connected to a revolution counting module. The piezo-element is suitably attached to or embedded within the inner wall of the tyre, under the tread or the side wall, in a way which causes it to flex with the tyre each time the circumferential sector of the tyre containing the piezo-element is compressed against the road or another vehicle supporting surface. 
   U.S. Pat. No. 6,105,423 discloses a sensor comprising a piezo-electric bar for detecting rotations of a pneumatic tyre on a vehicle. The piezo-electric bar is effectively fixed at its two ends, which ends are also attached to two spaced apart points on the inner surface of the tyre tread by means of adhesive. The centre of the plezo electric bar is attached to the centre of the base, and then to the inner surface of the tyre, at a point midway between said end points of the bar. Thus, when the tyre tread is flattened in the contact patch from its normal curved shape, the centre of the bar is radially moved inwards with respect to the fixed point at its ends. An electrical circuit block processes the signal so that an output pulse is produced only for radially inward movement of the contact point. 
   U.S. Pat. No. 4,862,486 discloses an apparatus comprising a piezoelectric polymer sensor, which senses a change in stress when a given section of the tyre is stressed with each revolution. The piezoelectric polymers are comprised of aligned dipoles acting to apply and release stresses on the piezoelectric polymer sensor when stressing the tyre. The apparatus is mounted to the inner sidewall of the tyre. 
   JP 04254730 A discloses a device for monitoring tyre air pressure, which allows the tyre pressure to be detected constantly while an ambient environment of the tyre is stable against fluctuation. The device comprises a piezo-electric element, of which the impedance changes according to an air pressure of a tyre, and a rim-side coil is changed by a fluctuation of an ambient environment of the tyre as well. 
   U.S. Pat. No. 5,546,070 discloses the use of a piezo ceramic element simultaneously as a sensor and also as an energy source for a capacitor to supply a transmission unit. 
   DE 197 45 734 A1 shows a sensor for detecting and registering the wear and the tread of a tyre. The tyre comprises single elements, however, which are not dispersed. In FR 2645799 a magnetic mass is arranged at a specified depth of the tyre, and consequently, can be supervised. 
   U.S. Pat. No. 5,559,437 relates to methods and apparatus for checking the condition of worn tyres, e.g. before recapping, for the non-destructive verification of the condition of a metallic reinforcement element of worn tyres. U.S. Pat. No. 6,005,388 relates to methods for detecting defects in tyres. 
   Common for prior art for detecting the characteristics of flat structures when passing one or several cylindrical bodies is arranging sensors in the cylindrical body itself, example of such a prior art includes: WO 02/066239, U.S. Pat. No. 5,562,027, US 2003/0144119, EP 0538 221, DE 199 20 133, WO 03/027623, U.S. Pat. No. 5,821,433 and U.S. Pat. No. 6,370,961. 
   To summarise, main difference between the present invention and prior art is that present invention uses capacitive and/or inductive transmission of signals from a sensor to a detector, which eliminates need for energizing elements for sensors and antennas. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an arrangement for determining the condition of a flat or substantially cylindrical body, being resilient (deformable), particularly a rotateble body or structure. The condition comprises mainly the deformation of the material. 
   Preferably, the substantially cylindrical resilient rotating body is a tyre, or a part thereof, and the arrangement is used for example, to determine the movement, e.g. revolution, which aids to determine velocity of a vehicle, the skid characteristics, the air pressure, etc. 
   Yet, another object of the present invention is to provide an arrangement for determining the condition of a substantially cylindrical rotating body in an apparatus and an associated structure, and the combination thereof as well. Preferably, in this case the cylindrical rotating body can be a roll or a cylinder for feeding a sheet material, such as paper. In this case, the arrangement is used for e.g. determining the movement and presence of the material, rotation speed, sliding characteristics. 
   Moreover, using sensors comprising a piezo-electric plastic or polymer material, according to one main aspect of the invention, gives certain advantages such as possibility for having larger detector elements. Moreover, the piezo-electric plastic material is less expensive than piezo-electric ceramic material. Furthermore, the piezo-electric plastic material can provide larger tensions and effects than the piezo-electric ceramic material as well. Yet, the two main advantages are the anisotropic characteristics of a piezo-elastic plastic material being utilized in the invention and the non-brittleness (plasticity). 
   According to one most preferred aspect of the invention, it is used as a detector for measuring the nip force, nip pressure and/or nip width of rolls in a nip roll press, for example for manufacturing a sheet material such as paper. 
   However, the invention can also be provided with additional types of sensors in form of a magnetic material being dispersed and disintegrated in the structure to be measured, or a magneto-elastic material in form of thin foils or the like. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention will be described with reference to the embodiments illustrated in enclosed drawings, in which: 
       FIG. 1   a  is a schematic cross-sectional view of a first application employing a device according to the invention, 
       FIG. 1   b  is a schematic frontal view of the first application employing a device according to the invention, 
       FIG. 2  is a cross-sectional view of a second application employing a device according to a second aspect of the invention, 
       FIG. 3  is a schematic cross-sectional view of a third application employing a device according to the third aspect of the invention, 
       FIG. 4   a  is a schematic side view of an arrangement comprising a device according to a fourth embodiment of the invention, 
       FIG. 4   b  is frontal view of the arrangement according to  FIG. 4   a,    
       FIG. 5   a  is a schematic side view of an arrangement using a device according to a fifth embodiment of the invention, and 
       FIG. 5   b  is a schematic side view of an arrangement according to a sixth embodiment of the invention. 
       FIGS. 6-8  illustrate equivalence circuit schemes, 
       FIG. 9  is a schematic view of seventh embodiment, 
       FIG. 10  illustrates an equivalence circuit scheme, 
       FIG. 11  is a cross-sectional view through a conveyer encapsulating an arrangement according to the invention, 
       FIG. 12  is a lateral view of a part of a conveyer according to  FIG. 11 , 
       FIG. 13  is a cross-sectional view through a conveyer encapsulating an arrangement according to another aspect of the invention, and 
       FIG. 14  is a cross sectional side view through a conveyer encapsulating an arrangement according to one embodiment of the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   To sum up the invention, all exemplary embodiments of the sensors described in the following, may be made of different materials, for example:
         Piezoelectric material, which generate charge upon deformation. The charge can be transferred into a signal (voltage and/or current). The signal can be detected by external means (electric antennae) or it may drive a low power amplifier, which enhances the signal. This amplifier can be placed within the body itself. An external device can detect the amplified signal.   Magnetic material (permanently magnetic), e.g. a strip or magnetic particles.   Magneto-elastic material, which changes its magnetization upon deformation. When the deformed material returns to its non-deformed state the magnetization returns to its initial state. A magnetic antenna, for example suitable coils, can detect the magnetic material.       

   In the first application, a substantially cylindrical rotating body  13  is schematically shown in  FIGS. 1   a  and  1   b . In this case, the body  13  comprises a wheel  10  of a vehicle  12 , comprising the tyre  13 . A number of sensors  111 , for example six, are arranged on/in the tyre  13  or embodied into the tyre material. The tyre  13  is of a conventional type and made of an organic, resilient, elastic, rubberised material. The sensors  111  can preferably be arranged inside the metallic reinforcement or on the surface of the tyre  13 . 
   In  FIG. 1   b , each sensor  111  is strip-shaped and arranged substantially in the axial direction of the tyre  13 , but may also extend in the radial direction ( FIG. 1   a ). However, in a second application according to  FIG. 2 , another type of sensor  112  is arranged in the longitudinal, peripheral direction of the tyre  13 . 
   The sensors  111  and  112  consist of a piezo-electrical polymer material or of a magneto elastic material. During the rotation of the tyre, as a result of the contact between the tyre  13  and the ground, the tyre  13  is locally elongated, particularly in the contact zone between the tyre  13  and the ground. Thus, an elongation of the tyre  13  results in an elongation of the sensor element in close proximity of the part of the tyre that has contact with the ground and generation of a signal. After a short while, the element relaxes back to its original shape and signal is zeroed. Then the next strip is deformed and so on. Typically, the speed of about 100 km/h, and a wheel with radius of about 0.5 m (at the place of the strip) and provided with six strips gives a frequency of about 1 kHz. 
   The signals can be generated when both sides of the elongated sensor  111  are short-circuited by means of an appropriate resistance (not shown). 
   A magnetic detection is also possible, e.g. by creating a magnetic path formed as an appropriate coil on the tyre, which can be used for generating a pulsed magnetic field, which e.g. can be detected by an antenna arranged in connection with the vehicle body. The signal receiver is an important part of such an antenna, which can be arranged as another coil being charged when a magnetic field is generated by the coil in the tyre  13 . Additionally, this arrangement can be arranged inside the metal reinforcement in the tyre  13  as well. 
   Another possibility is to use a magneto elastic material, which acts somewhat similar to the piezoelectric material. It changes its magnetization when deformed and returns to its initial magnetization when the deformation relaxes to equilibrium. Again, in this case an appropriate magnetic detection on the body of the vehicle may be used. 
   However, on the contrary, the sensor  112  in the second embodiment as shown in  FIG. 2 , elongates during the time the part of the tyre that contains the sensor has contact with the ground. During that time interval, an electric signal is created. The linear velocity of rotation can be obtained if the length of the strip is known simply by dividing this length by the duration of the voltage/current pulse. 
   Additional information is provided by the frequency at which the pulses occur due to the rotation of the vehicle. Both are related to each other and to the linear velocity of tyre motion. The difference in linear velocity of the tyre calculated from the pulse duration and from the frequency of tyre rotation (which is proportional to the angular speed of tyre rotation from which linear velocity of tyre motion can be calculated assuming certain tyre diameter, for example the diameter the tyre has at an appropriate air pressure) using appropriate software should be constant. When the difference is not constant but varies in time it signals that the skid did take place. 
   Yet another way to determine whether the skid occurred, or not, is to compare the readings (frequency or the time interval) obtained from the arrangements placed on other wheels to each other. The sensor readings give identical velocities of each of the wheels when the movement with no-skid takes place. When the skid takes place the readings obtained from the skidding wheel(s) will differ from the reading obtained from the non-skidding wheel(s). 
   In addition, small and slowly varying difference signals that the air pressure is not correct since this changes the tyre diameter and thus the difference in the linear velocity determined by each of the methods, respectively. 
   In the third embodiment of the invention, the sensor  14  is arranged inside the tyre material, see  FIG. 3 . In this case, the sensor  14  is made of a magnetic material, which is dispersed and disintegrated in the material of the tyre  13 , and preferably arranged, in a top layer of the tyre  13 . The magnetic sensors  14  are arranged spaced apart (distance  15 ) inside the material of the tyre  13 . As the tyre  13  wears out results are: (1) the amount of the magnetic material decreases, and (2) the distance between the tyre  13  and a preferred detector means, which detector means is to be explained in the following. 
   In all aforementioned embodiments, one or several detector means  16  for detecting the sensors  111 ,  112  and  14  (or a signal from the sensors) are arranged and generate a signal. Preferably, the detectors  16  are arranged inside a wheel housing in a conventional vehicle close to the tyre  13 . Advantageously, two detectors  16  are arranged on each side of the tyre  13  to be measured, and at a specified distance from each other. 
   Most preferably, the signal generated by the sensors is transmitted to the detector using capacitive and/or inductive transmission as disclosed in the embodiments of  FIGS. 6-14 . 
   The arrangements  10  according the first and the second embodiments of the invention are used for indicating movement. In addition, the atmospheric pressure can also be measured; since the sensor  112  is elongated the tyre  13  is deformed due to air pressure. 
   Note however that using only a radially placed strip as in  FIG. 1   a  or only a longitudinally placed strip as in  FIG. 2  or  3  does not allow one to determine a slow pressure reduction in the tyre during normal use since the piezoelectric materials are not sensitive to slow deformation rates. 
   In the second embodiment, the velocity is obtained by indicating the time intervals between the sensors  112 , contacting the ground and the length of the strip-shaped sensors  112 . 
   Furthermore, for obtaining the spin characteristics of the vehicle wheel, it is also possible to compare the axial frequency of the wheel to the frequency of the ground contact. If these frequencies are different, spinning may occur. 
   In the third embodiment, the material characteristics, e.g. the amount of the magnetic material in the tyre  13  is indicated. The signal intensity strength declines as the amount of the magnetic material  14  decreases and the distance between the tyre  13  and the detector means  16  increases, whereby the wear can be determined. 
   Clearly, in above-mentioned examples, the wheel can be substituted with any rotating structure. 
   In the following, the arrangements  20  and  30  according to additional embodiments of the invention for determining the condition and the characteristics of a material are described in conjugation with schematic  FIGS. 4-5   b , which relate to application of a sheet material production such as paper. In the embodiments, the condition and the characteristics mainly refer to the presence of material and the movement as well. 
   The arrangements  20  and  30  comprise at least one substantially cylindrical rotating body, which in the following denoted rolls  23  and  33 . Structures  22  and  32  are arranged to cooperate/interact with the rolls  23  and  33 . In this case, the structure  22  and  32  can be a wire, belt, paper sheet, banknotes, paper money or the like. Furthermore, the structures  22  and  32  are arranged adjacent to the rolls  23  and  33 ; however, in some cases the structures  22  and  32  can partially or completely be arranged at least partly in contact with the rolls  23  and  33 . 
   In  FIG. 4   a , a belt or a conveyer  22  extends between three rolls  23  in a paper-manufacturing machine, for example. Sensors  211  and  212  are arranged inside the material of the belt  22 , as shown in a lateral view in  FIG. 4   b . In this case, the sensors  211  and  212  are integrated into the belt  22 , either in the longitudinal direction of the belt  22 , and/or in the transverse direction of it. The sensors  211  and  212  may consist of wires of a piezo-electrical polymeric material or a magneto-elastic material (which may or may not be covered by a suitable polymer). 
   Moreover, at least one detector  416  can be arranged in connection with the sensors  211  and  212  and the paper-manufacturing machine. 
   During the operation of the paper-manufacturing machine, the sensors  211  and  211  are subjected to additional compression and elongation as the belt  22  passes the rolls  23 . If the sensors  211  are arranged in the transverse direction of the belt  22 , the frequency of the obtained pulses is measured when the sensors  211  is subjected to an additional tension. The frequency is proportional to the velocity, and the velocity of the roll  21  can easily be calculated by means of appropriate software. Alternatively, if the sensor  212  is arranged in the longitudinal direction, the absolute linear rotation speed of the rolls  23  is measured in substantially the same way as described above in connection with tyres, and subsequently converted to a frequency, which is then compared to the frequency of the belt  22 . 
   Thus, the speed of the belt  22 , which should be constant, can be measured for ensuring that it does not slide on the rolls  23 . The speed of the belt  22  can also be measured in relation to the rotation speed of the rolls  23 . In this way it is possible to detect if the belt  22  slides on the rolls  23 , which in turn indicates that the belt  22  is exhausted, probably due to its elongation, and should be replaced. 
   In this embodiment, it is also possible to use magnetic particles as an alternative to the piezo-electrical polymeric material. The magnetic particles are woven into the transverse or longitudinal threads in the same way as described above. Consequently, the primary signal is not an electrical pulse but a magnetic field pulse. Magnetic field pulses can be detected by means of, e.g., a hall-sensor or a coil. Furthermore, a pulsed magnetic field results in current pulses in the coil. It is also possible to use a substantially magneto-elastic material in the threads; whereby an elastic deformation of the threads will give rise to a magnetic field change. 
   Naturally, this embodiment can also be used in other applications involving stretching a material, such as a cloth or a fabric, wherein the movement can be controlled by means of rolls. 
   In a fifth embodiment, which is shown in  FIG. 5   a , two rolls  33  are preferably arranged in a printer application  20  for feeding paper sheets (or other information carrier). At least one sensor  331  is arranged on the surface of the printing paper, or substantially within the printer paper  32 . In the preferred embodiment, the sensor  331  comprises a foil integrated substantially inside the paper  32 . At least one detector  516  can be arranged to detect the presence of the sensors. 
   In this case, the movement is indicated in form of, e.g. Interruptions in the printer, which are possible to predict if the velocity of a paper moving through the printer rolls and the velocity of the printer rolls are known. 
   Yet another preferred embodiment in form of a banking paper application for the feeding of bank notes, paper money or the like is shown in  FIG. 5   b , which is principally designed as the printing application described above. The difference is that at least one sensor  312  is arranged as a strip. Preferably, several sensors  312  constitute a bar code in the banking paper or paper money  32 . 
   In the latter applications, the bank notes and paper money pass between the rolls  33 , which read the frequency of the pulses generated because of the sensors. Then, the frequency is determined by means of the distance between the sensors  312  and the speed with which the feeding arrangement, i.e. the rolls  33 , feed the banknote(s). If the bank notes or the paper money are forgeries, the sensors may not be present or the distances between the bar codes  312  will vary, which causes change of the frequencies. Thus, in this case a presence (existence/availability) of material in form of sensors  312  is indicated, and not the movement characteristics. 
   As mentioned, it is also possible to replace the piezo-electrical polymer material with magneto elastic material in all the embodiments mentioned above. Then the sensors should preferably be shaped as thin foils. An elongation or a compression of the foil causes a local variation in magnetization, which fades away when the material retains to its original shape. 
   Thus, in the first mentioned embodiment, the body  13  can be comprise of complete or parts of tyres arranged, rolls, rollers, cylinders, delivery bowls, rubber-covered cylinders, drums, hole cylinders, etcetera in other applications such as conventional rolls in all types of machines, e.g. paper making machines, printers, banking paper applications for bank notes and paper money including rolls, and arrangements of rubberised materials in general in other embodiments of the invention. 
   The sensors  111 ,  112 ,  14 ,  211 ,  212 ,  311  and  312  can comprise a strip-shaped band, a foil, a thread, a particle or the like. Furthermore, the detector means  16  is preferably constituted of coils, transponders or the like. Finally, it is also possible to put sensors  111 ,  112 ,  211 ,  212 ,  311  and  312  perpendicular to each other for obtaining an absolute velocity, i.e. independent or the radius of the substantially cylindrical body  13 ,  23 ,  33 . 
   The embodiments of  FIGS. 5   a  and  5   b  also can realise a nip formed by the rolls  33  and a conveyer shaped structure  32 , such as a shoe press, wire, felt or band. Sensors  311  are arranged inside the structure  32  providing a receiver with information about the pressure and pressure distribution in the nip between the rolls. A most preferred application is for example in the nip roll, roller press, Yankee machine, shoe press, smoothing coating machine of a paper-manufacturing machine or any other apparatus for applying a pressure to a structure passing there through. 
   Most preferably, the sensor can be made of a polymeric piezoelectric material, such as PVDF, polyvinylidenfluoride. The sensors may be shaped as films, cables, threads and filaments etc., depending on the application area. 
   The basic idea is that when a piezoelectric material having two electrodes is subjected to a pressure, a potential variation is obtained as an output signal between the electrodes. An equivalent circuit for a piezoelectric polymer sensor is illustrated in  FIG. 6 , which realises the piezoelectric polymer sensor as a high-pass filter with a characteristic cutoff frequency, f 0 : 
   
     
       
         
           
             
               
                 
                   f 
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                       RC 
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                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   When a load is applied to the piezoelectric sensor a charge displacement is obtained in the polymeric piezoelectric material, which gives rise to a voltage U that is directly  proportional to the force loading the sensor. The capacitance C f  is the result of the dielectric material between the two electrodes and its value depends on the sensor type (film, wire), its dimensions and the relative dielectric constant of the material used. R is the total resistance comprising the polymer material resistance and input resistance of a measuring unit. 
   When measuring the output signal V out  is measured but U, which provides the force on the sensor, is the interesting value. Having V out , U is obtained through: 
   
     
       
         
           
             
               
                 
                   
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   If the entire sensor is not loaded, the polymer capacitance is divided in a capacitance, which corresponds to the loaded area, C A , as illustrated in the circuit diagram of  FIG. 7 . C p  corresponds to the non-loaded part. 
   Due to the dielectric losses on the PVDF material, an apparently frequency dependent resistance and capacitance can be obtained, if the sensor is contemplated between the sensor electrodes is contemplated. This effect must be considered to be able to obtain a complete transfer function from the force load of the sensor to the output signal from the sensor system. The frequency dependent of the resistance and capacitance can be realised by inserting resistances R A  and R P  parallel to C A  and C P , as illustrated in the circuit diagram of  FIG. 8 . Wherein: 
               R   A     =     1     ω   ⁢           ⁢     C     0   ⁢   A       ⁢     ɛ   ″           ,       R   P     =     1     ω   ⁢           ⁢     C     0   ⁢   P       ⁢     ɛ   ″           ,       C   A     =         ɛ   ′     ⁢     C     0   ⁢   A       ⁢           ⁢   and   ⁢           ⁢     C   P       =       ɛ   ′     ⁢     C     0   ⁢           ⁢   P                   
wherein ε′ and ε″ are the real and the imaginary part of the relative permittivity for the dielectric material (such as PVDF), C 0A  and C 0P  are the capacitances of the sensor without dielectric material and ω is the frequency.
 
   This circuit will block signals at sufficient enough high frequencies, i.e. a band pass filter. 
   Having integrated the sensors in the conveyer, for example, the signals must be detected and transmitted to a processing unit. According to one preferred embodiment of the invention, it is achieved by contactless communication. 
     FIG. 9  illustrates an embodiment of the invention for detecting V out . A conveyer  92  extends between two (nip) rolls  93 . The conveyer is incorporated with a number of piezoelectric sensors  911  (only one illustrated for simplicity reason). Two conductors  913  are arranged as antennas connected to the outputs of the piezoelectric sensor. For receiving the transmissions from the antennas  913 , a receiver antenna  916  is arranged adjacent to the conveyer. Preferably, the receiver antenna  916  comprises two conductive plates. The conductive plates are then connected to a receiver  915 , comprising a differential amplifier and a analogue-digital converter (ADC), in which the received analogue signals from the antennas are converted to a digital signal by sampling with a predetermined rate and transmitted to a signal processing unit, for example in a computer  95 . Clearly, the receiver  915  can be integrated in the computer  95 . The result of the processed signal corresponds to the force, to which the piezoelectric sensors are subjected. 
   The contactless transmission can be compared to a capacitive coupling, i.e. a coupling capacitance C K  is obtained between the transmitting antenna and the receiver antenna.  FIG. 10  illustrates the equivalent circuit diagram for the entire system comprising the piezoelectric sensors, antennas, amplifier and the data collection. R I  is the input resistance of the amplifier, which can be a charge or voltage amplifier. The value of C K  will alter with respect to the relative position of the transmitting and receiving antennas. To compensate for the variation, the transmitter antennas can be made smaller than the receiver antennas, thus tracking the entire nip process. For obtaining a pressure value between the rolls, the signal from the sensor is traced from the moment the piezoelectric sensor is loaded until the sensor passes through the roller&#39;s nip. This requires that the extension of the receiver antennas is larger than the transmitter antennas. The transmitter antennas must be made small to allow incorporation into the conveyer material. 
     FIG. 11  is a cross sectional view through the conveyer  92  according to  FIG. 9 , Illustrating the sensors  911 , transmitting  913  and receiving antennas  916 . The sensor extends into the plane of the drawing. For obtaining a good capacitive coupling between the transmitting and receiving antennas, the transmitting antennas must be spaced apart in such a way that minimum or null electrical field lines are between the transmitting antennas. The aim is that main part of the electrical field lines extends between the transmitter and receiver antennas pairs. 
   The piezoelectric sensors  1211  can be distributed in the conveyer  1202  as illustrated in  FIG. 12. 1213  denote the transmitting antennas. The arrow shows the direction of motion of the conveyer. The overlapping arrangement of the sensors allows continues signal acquisition. However, the signal-processing unit must switch between the antennas to obtain relevant information. This embodiment allows measurement of pressure distribution in the width direction of the conveyer. 
   Having the length of each piezoelectric sensor, or the distances between the different sensors, start and end of each signal generated from each piezoelectric sensor allows computing the actual speed of the conveyer. Thus, it is also possible to detect if the belt or machine clothing has an askew motion, i.e. different speed at different parts. 
   Preferably, one receiving antenna is used for each transmitting antenna, however, if the signal level is strong enough and if it is possible to distinguish between the signals from different piezoelectric sensors, one antenna extending over the width of the conveyer will be enough. 
   Vibrations and flexures in the sensors may cause incorrect signal readouts; to reduce or eliminate the problem a differential signal between a loaded sensor, i.e. passing through the nip, and an unloaded sensor can be used. The technique implies that a differential signal between two adjacent sensors is generated. The same differential technique can be used to compensate for temperature variations. In this case two adjacent loaded and unloaded sensors can be used such that they have substantially same temperature. Thus, it is possible to measure temperature variations. 
   The capacitive transmission as described earlier can be substituted by an inductive transmission. In this case, as illustrated in  FIG. 13 , a coil  1313  is used as the transmitter instead of the antennas. The receiver antenna is substituted with another coil  1316 , e.g. having a magnetic core to obtain stronger signal from the transmitter coil  1313 . The receiver coil can be connected to an amplifier  1314  and a signal processor  1305 , in the same way as described earlier. 
   In yet another embodiment, as illustrated in  FIG. 14 , the piezoelectric sensor  1411  arranged inside the conveyer  1402  may be folded such that the piezoelectric sensor operates in a bending mod, which provides better signal level than thickness mod. The antennas  1413  connected at the ends of the sensor are thus arranged on one side of the sensor and in same level with respect to the receiver antenna  1416 . The arrow indicates the motion direction of the conveyer. 
   The above-described embodiments are not limited to piezoelectric sensors but magneto-elastic material may also be used. In this case the sensor and the antenna may be the same, and an inductive detection is used. 
   In yet another embodiment a magnetic material acting as a core in an inductor can be incorporated in the conveyer while the receiver is arranged as a coil for recovering inductance changes. 
   In a paper manufacturing machine, for example, the resulting signals from the sensors are used to control the distance between the rolls and thus the pressure or nip pressure of the rolls, e.g. In real time by means of the computer. 
   Moreover, it is appreciated that the term “conveyer” considers any type of a carrying arrangement for different types of material and in any application not being depending on the material it is made of. It may also consider the material itself passing through the rolls. 
   The invention is not limited to the shown embodiments but can be varied in a number of ways, e.g. through combination of two or more embodiments shown, without departing from the scope of the appended claims and the arrangement and the method can be implemented in various ways depending on application, functional units, needs and requirements, etc.