Patent Publication Number: US-6988398-B2

Title: Method and device for tracking the edge of a web

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This application claims priority on Finnish Application No. 20012528, filed Dec. 20, 2001, the disclosure of which is incorporated by reference herein. 
   STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a method and a device for tracking the position of the edge of a moving web, such as a fabric or a web in a paper machine. 
   SUMMARY OF THE INVENTION 
   Fabric loops running around rolls are used in paper and board machines, which fabric loops, without special guiding, may gradually drift to either edge of the rolls. A significant problem in the known systems used for fabric guiding has been the difficulty to reliably identify the position of the edge of the fabric. With increasing paper and board machine speeds, higher requirements, especially concerning the accuracy and rapidity of measurement, have been set for the fabric edge tracking systems. The provision of a reliable fabric tracking system has proven to be very difficult. The task becomes even more challenging, when the aim is to track the edge without touching it. To find a suitable location for the sensors monitoring the position of the fabric is in itself challenging. In addition, the costs often tend to become disproportionately high. 
   Traditionally, the fabric edge in the cross-direction of the paper machine has been tracked with a contacting guide plate. Problems associated with it are poor general performance as well as wear of the fabric edge and the plate. Detectors of the edge position based on optic, hydraulic, electric, pneumatic and acoustic operation are also known. Optic measurement based methods for contact-free tracking of the edge of a material web or fabric have been disclosed e.g. in FI patents 88828 and 94176. Disadvantages of the optic methods include that the optics tend to get dirty and that variations affecting the measurement result and the need for calibration of the measuring device occur in the colour and light transmission of the fabric. An additional risk is that, in case e.g. a shred of paper strays between the measuring head and the fabric, the measurement loses control of the situation. 
   An object of the invention is to provide a new and reliably operating method and device for tracking the edge of a moving web, with which arrangement it is possible to diminish problems commonly related to prior art. 
   In the method according to the invention, a pressure-sensitive sensor element is disposed in/onto the surface of a roll or the like, which sensor element reacts to the pressure applied to it by the moving web by producing an electric signal, whereby the exact position of the web edge on the roll can be determined based on the signals generated by the sensor element. The web being tracked may be a fabric loop, a material web or a combination of them in a paper or board machine. 
   A thin film- or band-type sensor is advantageously used as a sensor element, the resistance, capacitance, inductance, voltage or an optic quantity of said sensor changing as a function of pressure or force. An advantageous sensor material is described in U.S. Pat. No. 4,654,546. It is a thin and flexible electromechanical film, which is composed of a plurality of polymer layers separated from one another by air bubbles, which give the film its special characteristics. A change in the thickness of the film, generated by means of a force, creates in it a voltage proportional to the force. A permanently charged plastic film is created by charging the material electrically during the manufacturing process. Air voids inside the film make the film soft and elastic, which gives the material a very good electromechanical sensitivity. Thin plastic electrodes, laminated on both outer surfaces of the film, complete the structure of the electromechanical film. 
   Electromechanical film of the type described above is manufactured by EMFiTECH Ltd, and the product has been made known under the trademark EMFi™. The electromechanical film serves as a sensor when a dynamic pressure or force is exerted on it causing a local change in the thickness of the film. Since the polymer layers are stiffer than the air void layers, external pressure mainly changes the thickness of the air voids. The charges on the interfaces of the polymers and voids move relative to each other and as a consequence a mirror charge is created between the electrodes on the opposite surfaces of the film. The charge signal is thus proportional to the pore structure of the dielectric film but not to the piezoelectricity of the polymer material. The dielectric film is suitable only for dynamic force measurements due to its capacitive principle of operation. The sensitivity of the sensor can be increased by disposing several films on top of one another. 
   A band-like sensor element produced out of film material can be attached directly onto the surface of the roll or it can be embedded in the surface under a thin material layer. The electronics required by the measurement may be included in the film itself or they can be disposed in an electronics unit situated at the end of the roll, which unit is connected to the band-like sensor element by means of wires. This electronics unit analyses the signals coming from the sensor element. It also includes a transmitter for sending the data obtained from pressure measurement wirelessly to a stationary receiver situated near the end of the rotating roll, which receiver transmits the measurement data further to a data processing unit and to a process control device. In one embodiment there are two receivers, in which case they are placed such that, during the rotation of the roll, the first receiver receives a signal from the measurement element in a loaded state and the second receiver receives a signal from the measurement element in an unloaded state, whereby a reference value corresponding to zero loading is continuously obtained for determining the exact position of the web. 
   In addition to an electromechanical film, other sensor elements known in themselves and able to convert mechanical energy into electric energy, such as a capacitor band, resistance tape, parallel coupling elements, an ultrasonic film sensor or the like, may be used as a sensor element. Film sensors suitable for measuring nip pressure or nip width have been described, e.g. in publications FI 86771, U.S. Pat. No. 5,953,230 and WO 00/49379. 
   The sensor element band may comprise one single sensor or it may comprise several separate sensor strips placed one after another, each one of the strips giving a separate measurement signal. When the sensor element comprises several successive sensor strips spaced at a small distance from one another, the position of the web edge on the roll is determined by comparing the signals produced by the successive sensor strips with one another. When a single sensor strip placed axially in/onto the surface of the roll is used as a sensor element, the position of the web edge on the roll is determined by comparing the signals received from the loaded sensor element with the signals received from the unloaded sensor element. 
   In an embodiment of the invention, a sensor strip disposed spirally in/onto the surface of the roll functions as a sensor element. When the roll rotates, the sensor strip end closest to the centre of the roll always comes into contact with the web either first or last. The position of the web edge on the roll is determined based on the rotation speed of the roll, on the helix angle of the spirally disposed sensor element and on the signals produced by the sensor element. The rotation speed of the roll can be measured with the same sensor or with another device known as such. 
   The web edge tracking system according to the invention, in which system the pressure applied by the web to the sensor is monitored, is very reliable in operation. Placing the sensor element presents no problems, since it requires only little space. A sensor element on the surface level of the roll or embedded in the roll surface does not get dirty. The device comprises no moving parts. It withstands an unlimited number of loadings and is durable. The sensor element does not cause wear of the fabric or of the roll. The material or colour of the fabric does not affect the end result of the measurement. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the figures of the accompanying drawings, to the details of which the invention is, however, not intended to be narrowly confined. 
       FIG. 1  is a side view of a roll equipped with a tracking device according to the invention. 
       FIG. 2  is an end view of the roll of FIG.  1 . 
       FIG. 3  is a close-up view of part of the tracking device. 
       FIG. 4  is an illustration of principle of the electronics used in the tracking device. 
       FIG. 5  shows a tracking device in which a sensor element is disposed in the form of a spiral in the surface of the roll. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-3  show a device according to the invention for tracking the edge of a moving web, the device being placed in connection with a guide roll  10  guiding the run of a fabric  12 . The web tracked by means of the device may be a wire or a felt or another fabric of a paper machine but also an actual paper or board web or a combination of a fabric and a web. When the fabric  12  runs over the rotating roll  10  its cross-directional position in the axial direction of the roll  10  is monitored with the tracking device according to the invention. The data obtained from the tracking is used when guiding the run of the fabric  12  with fabric guiding devices (not shown in the figures) known in themselves. 
   A band-like sensor element  14  extending axially from an end  11  of the roll over at least part of the length of the roll  10  is disposed in the surface of the roll  10 . An electromechanical film able to convert the dynamic pressure to which the film is subjected into an electric quantity is advantageously used in the sensor element  14  as a pressure-detecting sensor. A film of this kind is known under the trademark EMFi™. An advantage of this type of film is that even very small pressures can be measured with it. Alternatively, e.g. a capacitor band, resistance tape, parallel coupling elements or an ultrasonic film sensor can be used as a sensor. The sensor element  14  can be attached directly to the surface of the roll  10  or it can be embedded in the surface of the roll  10 . 
   The electronics required by the measurement are disposed in an electronics unit  13  situated at the end  11  of the roll  10 , which unit comprises means for analysing the measurement signal coming from the sensor element  14  and means for sending a thus produced tracking signal s 1  wirelessly to a receiver  15  situated at a small distance from the end  111  of the roll. From the receiver  15  the tracking signal s 1  is passed further to an actuator (not shown in the figures) controlling the position of the fabric  12 . Advantageously, near the end  11  of the roll, there is also a second receiver  16  in such a position that the sensor element  14  is always in an unloaded state when passing the second receiver  16 . This means that the receiver  16  receives and transmits further a reference signal s 0  corresponding to zero loading. 
   In  FIGS. 1 and 2  the sensor element  14  is made up of one single measurement sensor strip placed in the axial direction of the roll  10 . The measurement data obtained from the sensor element  14  represents the pressure applied by the fabric  12  to the sensor element  14 , the magnitude of which pressure depends on how big a part of the length of the sensor element  14  is left underneath the fabric  12 , when the sensor element  14  and the fabric  12  meet each other. When the sensor element  14  is calibrated so as to take into account the tension and other factors of the fabric  12 , the electronics of the measurement device are able to calculate how big a part of the length of the sensor element is covered by the fabric  12 . 
   In the case of  FIG. 3  the sensor element  14  comprises several successive sensor strips  14   1 ,  14   2 ,  14   3 , . . . ,  14   n  spaced at fixed intervals along the length of the band-like sensor element  14 . Each sensor strip  14   i  generates a measurement signal representing the pressure to which the sensor element is subjected exactly at the point of the strip in question. Since the pressure changes radically at the edge of the fabric  12 , the exact position of the edge on the roll  10  can be determined by comparing the signals produced by the separate sensor strips  14   i  with one another. 
   The operating principle of the tracking system according to  FIG. 3  will now be explained with reference to  FIG. 4. A  band-like sensor element  14  is situated in the surface of a roll  10 , which sensor element comprises a plurality of sensor strips  14   i  made of electromechanical film, only the first three strips  14   1 ,  14   2 ,  14   3  being shown in the figures. The metallized upper and lower surface of each film-like sensor strip  14   i  is connected by means of a thin wire  30  to an electronics unit  13  at the end  11  of the roll. The sensor strips  14   1  . . .  14   3  react to the mechanical pressure applied to them by producing a voltage signal. These voltage signals are transmitted along the wires  30  via preamplifiers  18   1  . . .  18   3  to a multiplexer  19 , which is synchronized through a synchronizing circuit  23 . The multiplexed signal is passed further via an amplifier  20  to a transmitter  21 , which transmits a signal s 1  wirelessly to a stationary receiver  15  outside the rotating roll  10 . From the receiver  15  the signal s 1  is passed further to a data processing system  24  and from there further to process control. 
   Power transmission from outside the roll  10  to the electronics unit  13  rotating together with the roll is carried out wirelessly from a transmitter  27  of a power transmission unit  26  to a receiver  28  of a voltage regulator  29  and from there further via cables to the preamplifiers  18   i , via the synchronizing circuit  23  to the multiplexer  19 , to the amplifier  20  and to the transmitter  21 . 
     FIG. 5  shows an alternative way of carrying out the tracking of the position of the edge of the fabric  12  by using one or more sensor elements  14  placed spirally in/onto the surface of the roll  10  in the fabric loop  12  such that the inner end  14   a  of the sensor element  14  extends close to the centre of the roll  10  and the outer end  14   b  of the sensor element  14  extends to the end  11  of the roll. Alternatively, the spiral can also extend along the entire length of the roll  10 . As the roll  10  rotates in the direction depicted by the arrow, the sensor element end  14   a  close to the centre line of the roll comes first into contact with the fabric  12 . Depending on how big a part of the sensor element  14  is covered by the fabric  12  either a voltage increasing as a function of time or a constant voltage pulse monitoring the rotation of the roll  10  is obtained as a result of pressure measurement. The sensor element head  14   a  on the side of the centre line of the roll produces a pressure pulse at the moment when it enters the nip formed by the fabric  12  and the roll  10 . The voltage generated by the sensor element  14  and representing the pressure pulse ends at the moment when the part of the spiral sensor element band  14  on the side of the end  11  of the roll has rotated away from underneath the edge of the fabric  12 . The rotation speed of the roll  10  is measured with a device  31 , for example a pulse sensor, disposed in connection with the end  11  of the roll. Alternatively, the sensor element  14  can measure the rotation speed of the roll. The electronics associated with the measurement may be placed in the electronics unit  13  fastened to the end  11  of the roll or they may be included in the sensor element film  14  itself. 
   To determine the position of the edge of the fabric  12 , in the case of  FIG. 5 , data, obtained from the spiral sensor  14 , on the magnitude and duration of the voltage pulse as well as data on the rotation speed of the roll, this data being obtained from the sensor  31  measuring it, and data on the helix angle of the spiral  14  i.e. the distance it advances at a certain angle of rotation of the roll  10  are needed. The latter value is constant. 
   Thanks to the low cost of the sensor arrangement used in web edge tracking several sensor elements  14  can be fitted in the roll, as shown in FIG.  5 . This improves the reliability of the system and enables comparison between the sensors or an automatic sensor change in problem situations. 
   An advantage of the arrangement according to the invention is that the electronics are very simple and inexpensive, because a simple measurement sensor can be used in it. The device is reliable in operation and able, if needed, to measure very small forces. The device does not get dirty and it withstands an unlimited number of loadings. 
   Although, in the arrangements illustrated above, the sensor element is disposed in/onto a roll, it is also possible, according to the invention, to dispose the sensor element in some other paper or board machine element in contact with the moving web. 
   The claims will now be presented, and, within the inventive idea defined by the claims, the details of the invention may vary and differ from what is presented above as exemplary only.