APPARATUS FOR MEASURING PROPERTIES OF A MOVING PAPER WEB OR CARDBOARD WEB

Abstract: The invention relates to an apparatus for measuring properties of a moving paper web or cardboard web. The apparatus comprises a sensor (1) and a counterpart. The sensor (1) comprises at least one coil (2, 3). The sensor (1) is arranged on a first side of the moving web (7) and the counterpart is arranged on a second side of the moving web (7). The coil (2, 3) comprises a plurality of layers, each layer constituting a part of the coil, the layers being superimposed and coupled in series to constitute the coil.

Description: 1 APPARATUS FOR MEASURING PROPERTIES OF A MOVING PAPER WEB OR CARDBOARD WEB The invention relates to an apparatus for measuring properties of a moving paper web or cardboard web the apparatus comprising a sensor and a counterpart, the sensor being arranged on a first side of the web and the counterpart on a second side of the web, the sensor comprising at least one coil Measurements of properties of a moving paper web constitute the basis for controlling a paper machine Continuous monitoring of properties e g the caliper of paper, as the web moves, ensures that paper of the right quality is produced The problem in measuring the caliper of a moving web is that the web moves at an extremely high speed and that the web always comprises some discontinuities, such as holes and small bulges Measuring sensors located very near the surface of the web, or even in contact with it, should be able to give way to the discontinuities in order for the measurement to be reliable and the sensors not to cause additional hole formation Present devices are not capable of giving way sufficiently rapidly, and consequently formation of holes has been prevented e g by keeping sensors far away from the web This impairs measuring results US Patent 4,791 ,367 discloses a gauge for measuring the caliper of a web The gauge comprises an upper head having a U-shaped magnetic pole piece whose legs are provided with windings A lower head to be arranged on the opposite side of the paper web to be measured comprises a passive magnetic circuit which is a magnetic or fernte plate The magnetic flux established by the windings extends through the pole piece through the paper to the fernte plate The gap between the pole piece and the fernte plate varies as the caliper of the paper web varies The circuit inductance used to define the caliper of the paper web also vanes simultaneously US Patent 4,528,507 discloses a solution for measuring properties of a moving web by sensors arranged on different sides of the web One of the sensors is provided with a coil, the sensor on the opposite side of the web being provided with a counterplate As the gap between the coil and the coun- terplate varies, the density of the magnetic flux established by the coil varies and the caliper of the web is measured by detecting the variation in the den- sity German Offenlegungsschπft 28 29 264 discloses a measuring ap- paratus in which a sensor including a coil is arranged on both sides of the web The caliper of the web is measured by defining the amplitude of the magnetic field established by the coils In all the above solutions the sensor coils, typically made from copper wire, are heavy making the sensors heavy and difficult to move flexibly along formations on the surface of paper Furthermore, because the construction is heavy, the sensors are also very poorly adapted to float by air bearing, completely clear of the surface of paper It is an object of the present invention to provide an apparatus for avoiding the above drawbacks The apparatus of the invention is characterized in that the coil comprises a plurality of layers, each layer constituting a part of the coil, the layers being superimposed and coupled in series to constitute the coil It is an essential idea of the invention that the apparatus for meas- unng properties of a paper web comprises at least one coil comprising a plurality of coil parts arranged as superimposed layers and coupled in series at their ends to form an integral coil It is the idea of a preferred embodiment that the injection frequency used in the solution exceeds 50 kHz It is an advantage of the invention that the sensors of the measur- ing apparatus of the invention can be made lightweight, whereby the sensors are able to give way to irregularities on the surface of the paper more flexibly than before This decreases formation of holes in the paper The lightweight construction of the invention also allows the sensors to be made such that they float by means of air bearing The construction can be made lightweight, since, owing to the high injection frequency, the measuring apparatus requires no heavy ferromagnetic materials, such as iron, fernte or a large conductor coil made from copper wire In the solution of the invention the number of wire turns of the coils can be low, resulting in a small and lightweight measuring apparatus The invention will be described in greater detail in the attached drawing, wherein Figure 1 schematically shows an apparatus of the invention, Figure 2 schematically shows one layer of a coil structure to be used in the apparatus of Figure 1 , and Figure 3 schematically shows a coil structure to be used in the apparatus of Figure 1 Figure 1 shows a measuring sensor 1 comprising a feed coil 2, a differential coil 3 comprising two coils and a reference plate 4 The measuring apparatus further comprises a control device 5 for feeding control current to the feed coil 2 The injection frequency of the feed coil is advantageously high i e above 50 kHz preferably above 200 kHz Typically the injection frequency is about 250 kHz but may be even 500 kHz or more If the injection frequency is high, the number of wire turns of the coils 2 and 3 can be low, resulting in a small and lightweight measuring sensor The feed coil 2 induces to the differential coil circuit a voltage which can be measured by the control device 5 by e g lock-in type of detection The reference plate 4 is arranged on one side of the differential coil The gap between the differential coil 3 and the reference plate 4 is substantially constant at all times A sensor plate 6 is arranged on the side of the differential coil 3 which is opposite with regard to the reference plate 4 The structures of the reference plate 4 and the sensor plate 6 are substantially identical The plates are made from a material which efficiently conducts electricity, e g copper The plate can also comprise merely a conductive metal surface produced by vaporizing When the material effectively conducts electricity, a thin conductive layer is sufficient A thin conductive layer is sufficient because the higher the injection frequency the smaller is the current penetration depth inside the conducting body Furthermore, owing to a high injection frequency, the plates 4 and 6 can be made thin and consequently their masses can be kept small The measuring sensor 1 is placed on a first side of a web 7 and the sensor plate 6 to a second side of the web 7, i e the opposite side with regard to the measuring sensor 1 The web 7 is a moving web and can be e g a paper or cardboard web, whose caliper is to be measured The measuring sensor 1 and the sensor plate 6 are arranged either in contact with the web 7 or close to the surface of the web 7 e g by means of air bearing As the caliper of the web 7 varies, the distance between the sensor plate 6 and the measur- ing sensor 1 and naturally the differential coil 3 and the reference plate 4, varies This variation in distance caused by the caliper of the web 7 affects the voltage of the differential coil 3 to be detected by means of the control device 5 Accordingly, the control device 5 produces a voltage U x , which is proportional to the distance between the differential coil 3 and the sensor plate 6, i e the caliper of the web 7 The coils 2 and 3 comprise a plurality of coil parts arranged as su- peπmposed layers Figure 2 shows one such layer The layer of Figure 2 has been formed e g by exposing a copper-coated circuit board 8, except for the location of the conductor 9 and by then corroding the removed copper in such a way that only a spiral copper conductor 9 remains on top of the circuit board 8 The conductor spiral 9 begins by an inner opening 10a and ends in an outer opening 11 a The end of the conductor 9 is connected via the inner opening 10a to the conductor of the layer underneath, the latter conductor providing the layer underneath with a spiral of substantially the same shape but unwinding in a different direction the conductors 9 of the different layers however winding in the same direction The conductor 9 end which ends in the outer opening 1 1 a is connected to the end of the conductor 9 of the upper layer by through coppering The conductor spiral 9 in the upper layer also winds in an opposite direction with respect to the spiral in Figure 2 whereby the directions of winding of the conductors remain the same The circuit board of Figure 2 also comprises other inner openings 10b and 10c and outer openings 1 1 b and 11 c, which can be utilized in combining the conductors of layers on the lower or upper side of a circuit board according to Figure 2 The circuit board 8 is further provided with connectors 12 for connecting a coil construction comprising superimposed circuit boards to a meas- unng apparatus The circuit board 8 is further provided substantially in the middle with an axis opening 13 by means of which the circuit boards 8 to be superimposed can be aligned The pattern formed by the conductor 9 is most preferably as round and symmetric as possible If required, the symmetric construction can be slightly deviated from according to the disposition of the inner openings 10a to 10c and the outer openings 11 a to 11 c required in the different layers, as is the case in the lower left edge of the conductor spiral 9 in Figure 2 Figure 3 shows a coil construction with circuit boards 8 according to Figure 2 being superimposed to form a coil About 30 layers, for example can be superimposed and yet the resulting coil construction is only a few millimetres high A conductor 9 substantially according to Figure 2 can be provided both below and above the circuit board 8, an insulating layer being arranged between the circuit boards 8 to be superimposed In the coil construction of Figure 3 the direction of winding of the conductor 9 can be changed e g in the middle part of the construction in order to achieve a differential coil 3 according to Figure 1 Naturally the point where the direction of winding changes can easily be combined with a connector 12 The coils can be manufactured mechanically in series production, resulting in the significant advantage that the coils are identical irrespective of production lot The drawing and the related description are only intended to illus- trate the inventive idea The details of the invention may vary within the scope of the claims Consequently, the one layer forming a part of the coil can be made e g by the printed circuit technique or by arousing the conductor elec- trolytically or by vaporization or by some other corresponding manner The feed coil is not necessary, but electric current may also be fed directly to a part of the differential coil 3 In this case said part of the differential coil 3 would serve as the feed coil 2 Furthermore the structure of the differential coil 3 may comprise more than two coils The differential coil 3 is most preferably symmetric, whereby the voltage U x produced by the control device 5 is close to zero at least m e g caliper measurements involving thin- ner qualities

Claims: CLAIMS 1 An apparatus for measuring properties of a moving paper web or cardboard web the apparatus comprising a sensor (1) and a counterpart the sensor (1 ) being arranged on a first side of the web (7) and the counterpart on a second side of the web (7) the sensor (1 ) comprising at least one coil (2,3) characterized in that the coil (2 3) comprises a plurality of layers each layer constituting a part of the coil the layers being superimposed and coupled in series to constitute the coil 2 An apparatus as claimed in claim ^characterized in that the layer constituting a part of the coil is formed on a circuit board (8) by providing the circuit board (8) with a spiral-shaped conductor (9) 3 An apparatus as claimed in claim 1 or2 characterized in that the injection frequency of the apparatus exceeds 50 kHz 4 An apparatus as claimed in any one of the preceding claims, characterized in that the sensor (1 ) comprises a differential coil comprising at least two coils, and a reference plate (4), that the counterpart is a sensor plate (6) and that the sensor plate (6) is substantially similar to the reference plate (4) 5 An apparatus as claimed in claim 4, characterized in that the sensor (1) comprises a feed coil (2) for feeding electric current to the differential coil (3) 6 An apparatus as claimed in any one of the preceding claims characterized by measuring at least the caliper of a paper web or a cardboard web

Inventors: MOISIO HANNU, MOISIO, HANNU
Applicants: MOISIO HANNU, VALMET AUTOMATION INC, MOISIO, HANNU, VALMET AUTOMATION INC.
Application year: 1999
Main classification: G01B7
IPC codes: G01N27, G01N33