BACKGROUND [0002] A sheet, such as paper or board, is often measured using a test device wherein a sample of the sheet is placed between two pressing sur- faces such that the plane-like surfaces of the sample come into contact with the pressing surfaces, and pressing the sample to be measured between the pressing surfaces at a desired pressure. It is thus an object of the measure- ment to find out e. g. the thickness of the sample to be measured.

[0003] A problem with such pressing measurement is that since it is difficult to make the pressing surfaces completely parallel, the plates of a press exert a different pressure on the sample at different points within the area to be measured. Sheets being thin, even a slightest deviation from an even pressure within the area to be measured may cause a significant error in the measure- ment.

BRIEF DESCRIPTION [0004] An object of the invention is to provide an improved ar- rangement for producing pressure so as to enable a compressing pressure to be provided evenly over an area to be measured. This is achieved by an ar- rangement for measuring a sheet, the arrangement comprising two pressing parts having matching pressing surfaces, the sheet being arranged to be posi- tioned therebetween during measurement such that the pressing surfaces come into contact with opposite surfaces of the sheet. The arrangement further comprises supporting elements, each comprising a fastening part for fastening the pressing parts to the supporting elements; and at least one of the fastening parts comprises a tilting structure by means of which the normals of the press- ing surfaces are arranged to tilt with respect to each other in the direction of two different dimensions.

[0005] Preferred embodiments of the invention are disclosed in the dependent claims.

[0006] The idea underlying the invention is that the pressing parts are, to a limited extent, allowed to swing with respect to each other in the direc-

tion of two different dimensions. Either both pressing parts then swing in at least two directions of different dimensions, or at least one pressing part swings in the direction of two different dimensions.

[0007] The method and system of the invention provide several ad- vantages. They enable the pressing force to be directed evenly over the entire pressing area, which also contributes to improving the measurement being carried out.

LIST OF DRAWINGS [0008] The invention is now described in closer detail in connection with the preferred embodiments and with reference to the accompanying draw- ings, in which [0009] Figure 1 shows pressing of a sheet, [0010] Figure 2A is a side view showing a pressing part and a sup- porting element, [0011] Figure 2B is an exploded view showing the pressing part and the supporting element, [0012] Figure 3 shows a use of an elevation, and [0013] Figure 4 shows a measuring device measuring compressibil- ity.

DESCRIPTION OF EMBODIMENTS [0014] In the present application, a sheet refers to a sheet of paper and a sheet of board of a desired size. Furthermore, in the solution of the pre- sent application, a sheet may also be a corresponding thin sheet made of an- other material, such as metal, glass, plastic, rubber, etc.

[0015] Let us now examine a pressing structure shown in Figure 1.

A sheet 100 resides between pressing parts 102 and 104 such that pressing surfaces 1020 and 1040 of the pressing parts come into contact with opposite surfaces 1000 and 1002 of the sheet 100. The pressing part 102 is fastened to a supporting element 106 by a fastening part 120 while the pressing part 104 is fastened to a supporting element 108 by a fastening part 122. The support- ing part 106 may be fixedly installed. The supporting element 108 is fastened to a supporting arm 110, which is a part of a pressure cylinder 112. The pres- sure cylinder 112 moves the supporting arm 110 in the vertical direction, mak- ing the supporting element 108 and the pressing part 104 move as well. The supporting structure 106 is provided with an opening 114 for measuring the

sheet. When the sheet 100 enters the pressing structure to reside on top of the pressing part 104, the pressure cylinder 112 pushes the sheet 100 towards the pressing part 102 until the surface 1002 of the sheet comes into contact with the pressing surface 1020 of the pressing part 102. The pressure cylinder 112 includes a pressure sensor 116 for measuring the pressure in the pressure cylinder. Applying a simple method known per se, this pressure can be utilized for determining the force directed at the pressing part 104. Since the surface area of the pressing part 104 is also known (can be measured), it is possible to determine the compressing pressure and force directed at the sheet 100.

[0016] Since the sheet 100 is to be pressed as evenly as possible at the measuring point, it is important that the pressing surfaces 1020 and 1040 of the pressing parts 102 and 104 press evenly against the surfaces 1000 and 1002 of the sheet and that the distance between the pressing surfaces 1020 and 1040 at different points is determined only according to the thickness of the sheet 100. The sheet 100 is usually made of a material of uniform section, which is why the normals of the different surfaces 1000 and 1002 of the sheet 100 are parallel. If the pressing parts 102 and 104 are fixedly installed in the supporting elements 106 and 108, it is difficult to provide a pressing structure wherein the normals of the pressing surfaces 1020 and 1040 are parallel with the normals of the sheet 100 at different points of the pressing surfaces. In such a case, the distance between the pressing surfaces 1020 and 1040 is not the same within the entire area of the pressing surfaces 1020 and 1040, al- though it should stay constant, so an angle of tilt exists between the pressing surfaces 1020 and 1040, which is not due to the sheet 100. This, again, results in uneven pressure distribution within the measurement area in the sheet 100.

The disclosed solution enables this problem to be efficiently avoided when the pressing parts 102 and 104 are self-steering. This, again, is achieved such that the fastening part 120 residing in at least one of the supporting elements used for fastening the pressing part to the supporting element comprises a tilting structure by means of which the normals of the pressing surfaces 1020 and 1040 are arranged to tilt with respect to each other in the direction of two dif- ferent dimensions in order to provide an even pressure on both sides of the sheet 100.

[0017] Let us now examine the pressing part 104 and the support- ing element 108 in closer detail by means of Figures 2A and 2B. The pressing part 104 may be made e. g. of quartz. The surface area of the pressing part is

e. g. some square centimetres. The pressing part 104 is fastened to the sup- porting element 108 by the fastening part 122. The fastening part 122 com- prises a supporting base 202 and a collar structure 204. The pressing part 104 is fastened onto the supporting base 202 e. g. by gluing. The supporting base 202 comprises a border 2020 extending outside the edges of the pressing part 104. The supporting base 202, again, is fastened to the supporting part 108 by means of the collar structure 204 such that the pressing part 104 resides in an opening 2040 in the collar structure 204 while the pressing surface of the pressing part 104 remains above a collar 2042 of the collar structure 204. In the solution of Figure 2A, the supporting element 108 and the collar structure 204 are provided with matching threads 208 to fasten the supporting element 108 and the collar structure 204 together. The supporting element 108 and the collar structure 204 can also be glued together. In such a case, since the open- ing 2040 provided by the collar 2042 is smaller than the collar 2020 of the sup- porting base 202, the supporting base 202, including its pressing parts 104, stays in place between the collar structure 204 and the supporting part 108. An elevation 210 is provided between the supporting base 202 and the supporting element 108, in the middle of the pressing part 104 and the supporting base 202. The elevation 210 supports the pressing part 104 and the supporting base 202 from the middle. The elevation 210 may be a part of the supporting base 202 or a part of the supporting element 108. The elevation 210 may also be a separate part to be fastened either to the supporting base 202 or to the supporting element 108. Furthermore, a gap 212 is provided between the col- lar structure 204 and the supporting base 202. The elevation 210 and the gap 212 constitute a tilting structure 2120 which, during measurement, enables the normal N of the pressing surface of the pressing part 104 to tilt in a self- steering manner according to the surface of the sheet. When the elevation 210 resembles a pin, thus in no direction extending even close to the edges of the supporting base 202, the normal N of the pressing surface is allowed to tilt at a solid angle co, which means tilting in the direction of two different dimensions.

Excluding the pressing part 104, the parts are made of metal. The pressing part 102 may be fastened to the supporting element 106 in a similar manner.

For optical measuring, the supporting structure 106 then comprises an opening 114 designated by a broken line in Figure 2A, which is not necessarily needed for the pressing part 104. When the pin-like elevation 210 disclosed in Figures 2A and 2B and the gap 212 are used to enable the pressing surface to tilt, it

will suffice that such a tilting structure 2120 is provided either in connection with the pressing surface 104 or the pressing surface 102. On the other hand, it is also possible that a tilting structure is provided in connection with both pressing surfaces 102 and 104 to enable tilting in the direction of two different dimensions. Figure 2B shows a corresponding structure with that of Figure 2A, only as an exploded view.

[0018] Since in the disclosed solution it is desirable to make the tilt- ing between the normals of the pressing surfaces occur in the direction of two dimensions, the tilting can be divided among different pressing parts such that a first pressing part tilts in the direction of one dimension while a second press- ing part tilts in the direction of another dimension (i. e. one pressing part only tilts at a plane angle). Such a solution is disclosed in Figure 3. A bar-like eleva- tion 300 is then provided between the supporting base 202 and the supporting element 108, relating to the pressing part 104 and having a length substantially covering the entire supporting base. Similarly, a bar-like elevation 304 is pro- vided between the supporting base 302 of the pressing part 102 and the sup- porting element (the supporting element is not shown in closer detail in Figure 3 since the structure is similar to that used in connection with the pressing part 104). The elevation 300 supports the pressing part from the middle. The bar- like elevation 300 enables the normal N of the pressing surface to tilt only at plane angles a, ß but not at a solid angle. When the plane angles a and ß dif- fer from each other, the normals of the pressing surfaces may tilt with respect to each other at a solid angle. The plane angles a and P are, however, pref- erably at a right angle with respect to each other.

[0019] It is possible in the disclosed solution to provide the following tilting alternatives for the pressing parts: the normal of a first pressing part tilts at a solid angle while a second pressing part is fixedly installed (cannot tilt at all), the normal of the first pressing part tilts at a solid angle while the normal of the second pressing part may only tilt at a plane angle, or the normals of both pressing parts tilt at a plane angle, only in different directions.

[0020] By means of Figure 4, let us now examine in closer detail a measuring device for which the disclosed solution is suitable. As a sample, the measuring arrangement comprises a sheet 100, an optical power source 400 for emitting optical radiation 450 to a surface 1002 of the sheet 100, and a col- limating optical block 402 for collimating the optical radiation 450 and a semi- transparent mirror 404 for directing the collimated radiation 450 to the surface

of the sheet 100. Optical radiation 452 being reflected from the sheet 100 in- cludes specularly reflected optical radiation and some of the scattered optical radiation. The reflected radiation 452 is collected by an optical block 406. An aperture 408 limits the reception angle of the radiation emitted to a camera 410, i. e. limits the access of the scattered radiation in particular to the detector surface of the camera 410. The size of the opening 406 determines depth resolution. The measuring arrangement preferably also comprises a computer 412 for a computer-based image analysis. The scanning optical block 406 is designed to enable details of the order of micrometres to be distinguished in an image produced on the detector surface of the camera 410 showing the sur- face 1002 of the sheet. The camera 410 is e. g. a CCD (Charge Coupled De- vice) camera known per se whose detector surface is made up of a matrix consisting of pixels. This measuring arrangement enables the optical charac- teristics, such as gloss, reflection and e. g. roughness, of the sheet 100 to be measured. The sheet 100, which is e. g. paper or board, is then illuminated by collimated radiation parallel with the normal of the surface of the sheet. The illuminated surface is focused on the pixels of the detector surface of the cam- era 410. The reflected optical radiation 452 is controlled by the aperture 408 of a desired size prior to the camera 410. Using the reflected optical radiation 452, an image is produced of the surface of the sheet for the pixels of the de- tector surface of the camera 410, each pixel scanning an area of the order of micrometres of the surface of the sheet. Finally, the roughness or gloss of the sheet 100 is measured on the basis of the intensities of the pixels of the detec- tor surface of the camera 410. This solution is described in closer detail in PCT application PCT/FI00/00412, by means of which the surface characteristics to be measured by applying the disclosed solution.

[0021] The pressing part 102, which operates as the measurement window of the device, is made of the material permeable to optical radiation used in the measurement, e. g. sapphire. Furthermore, the pressing part 102 has to be mechanically durable. The pressing part 104 is made e. g. of matt- black quartz. The colour of the pressing part 104 can, however, be freely cho- sen; the disclosed solution is by no means restricted to the colour of the press- ing part 104. The measuring device also comprises a thickness gauge 420 for measuring the thickness of the sheet 100 while the sheet is being pressed.

The thickness gauge 420 is an eddy-current sensor known per se. A high- frequency alternating current is fed through a coil in the eddy-current sensor,

which produces a high-frequency alternating magnetic field. The alternating magnetic field, in turn, produces vortexes in electrically conductive materials, such as the material e. g. of the supporting element 108. These vortexes, in turn, generate a magnetic field of their own which, through induction, affects the inductive reactance of the coil, which can be seen in the impedance of the coil. The impedance of the coil is then proportional to the distance between the electrically conductive material and the coil, i. e. the distance between the sen- sor 420 and the supporting element 108 can be measured. Since the distance between the sensor 420 and the supporting element 108 depends on the thickness of the sheet 100, the thickness of the sheet 100 can be measured by the measuring device. The computer 412 determines the thickness of the sheet 100 by means of a signal of the thickness sensor 420. Furthermore, the computer 412 determines the pressure or force directed at the sheet 100 by means of a pressure signal measured by the pressure sensor of the pressure cylinder 112. When measuring paper, the maximum force is e. g. 2500 N, the surface of the pressing part then being e. g. 2 cm2. The computer 412 also con- trols the pressure of the pressure cylinder 112 during measurement.

[0022] The disclosed solution can be used for measuring the com- pressibility of a sheet. The compressibility of paper or board can be measured indirectly e. g. by measuring a desired surface character from the sheet at two different pressures, the change in the measured characters thus representing compressibility. A measured characteristic may be e. g. roughness or smooth- ness. Smoothness s is function s = f (r) known per se of roughness r. Com- pressibility c can thus be determined in the following manner: c = Ar/AP, wherein Ar is the change in roughness and AP is the change in pressing force or pressure.

Compressibility c can also be determined in the following manner: c = As/AP, wherein As is the change in smoothness and AP is the change in pressing force or pressure. In the disclosed solution, compressibility can be measured directly by means of changes At in the thickness of the sheet. In such a case, compressibility can be determined e. g. in the form: c = At/AP, wherein At is the change in thickness and AP is the change in pressing force or pressure. Compressibility thus means the change in thickness with respect to

the pressing force or pressure. Measurement of a surface character can also be utilized.

[0023] Although the invention has been described above with refer- ence to the example of the accompanying drawings, it is obvious that the in- vention is not restricted thereto but can be modified in many ways within the scope of the inventive idea disclosed in the attached claims.