SHEET COATING THICKNESS MEASURING APPARATUS

Abstract: A thickness measuring apparatus of a sheet-like product, wherein the sheet (1) to be measured is supported by partly surrounding a shaft (2) while a light shading plate (3) is posi­ tioned so as to form a slit (30) between the shaft (2) and the shaft side edge of the light shading plate (3), is able to mea­ sure the sheet thickness directly by scanning the slit width with a light beam or indirectly by comparing the scanned results of the light beam of this slit with that of a reference slit (31) provided at the light shading plate (3) so that, for instance, the thickness of magnetic material coated on a magnetic tape can be measured continuously in the production line thereof.

Description: This invention relates to an apparatus suitable for measuring the thickness of sheet-like products such as magnetic tapes, particularly the thickness of coatings on sheet-like products. An apparatus utilizing electric vortex current as shown in Fig. 1 of the accompanying drawings is already known for measuring the thickness of sheet-like products such as audio tapes, video tapes, etc., particularly, the thickness of coatings of magnetic materials on the tape-like supports which are made of polyester or other plastics. In Fig. 1 reference numeral 1 designates a magnetic tape, in which tape-like support layer 1a made of transparent polyester or other plastics, is provided with a coating layer 1b of magnetic materials coated on a predetermined surface thereof, while 14 is a detector comprising an iron core 14a with one open end, on which is wound an exciting coil 14b connected with an oscillator. The functions of the apparatus will now be described. Referring to Fig. 1, when the oscillator 15 is excited by the exciting coil 14b, a magnetic field due to the exciting coil 14b is generated at the open end of the iron core 14a. The exciting impedance fluctuates when a coating layer 1b of magnetic material takes an adjacent position as shown. Since fluctuation of the exciting impedance depends upon characteristics, sizes (width, thickness), position, etc. of the magnetic material, the coating thickness of the magnetic material, i.e. the coating layer 1b, can be measured by detecting the impedance fluctuation under predetermined conditions of the characteristics of the magnetic width and the position thereof. It is difficult for the conventional apparatus with the construction mentioned above to measure reliably the thickness of the coating layer if the distance between the coating layer and the detector cannot be kept constant and narrow. Therefore, the conventional apparatus has a disadvantage in that it is, in practice, difficult to install in production lines. On the other hand, an optical micrometer is also known, wherein light beams, such as, laser beams, are utilized with the body thereof positioned relatively far from the object whose thickness is to be measured. By using such apparatus, it is possible to measure the thickness after coating the magnetic material and then determine the coating thickness, if the tape-like support layer thereof is controlled at a constant thickness. Examples of such measuring apparatus are disclosed in GB-A-2072840 and U.S. Patent No. 4,082,463 as well as Japanese Patent Laid Open 57-161, 608 (1982) and 57-104, 807 (1982), wherein the construction shown in Fig. 2 is adopted. Referring to Fig. 2, reference numeral 6 is a scanner such as polygonal mirror and a light beam from a light source 4 is scanned upward and downward onto a first lens 52, which irradiates a parallel beam onto a second lens 55, while the parallel beam reaches the second lens 55, through a gate 53 comprising an iris or restrictors 53a, 53b and collected by a receiving surface of a photoelectric converter 7. Therefore, when the light beam is scanned at a constant speed from the scanner 6 onto the first lens 52, the photoelectric converter 7 receives the light beam travelling up and down in the span of the gate 53. An object 54 to be measured is positioned between the first and second lenses 52, 55. Therefore, the photoelectric converter 7 does not receive any of the light beam whose path is covered by the vertical dimension of the object 54 to be measured. The vertical direction (a distance between the iris or the restrictor 53a and another restrictor 53b) thereof being known, it is possible to measure the vertical (in Fig. 2) dimension of the object 54 if the time interval from initial incidence of the light beam onto the photoelectric converter 7 to the interruption thereof, the length of time of interruption and the time interval from reincidence of the light beam to the reinterruption thereof can be detected. In such apparatus, as shown in Fig. 3, the object 54 to be measured is substituted by a shaft, which supports a magnetic tape which partly surrounds it, so that a clearance between the upper iris or the restrictor 53a and the top surface of the magnetic tape 1 supported by the shaft 2 can be measured. However, such an apparatus has the following disadvantage for some configurations and reflectances of the object to be measured (the magnetic tape 1 in this case). For instance, if the magnetic tape 1 (the object to be measured) is cylindrical and has a high surface reflectance and the light beam is projected upon a part (as identified by "d") near to the top cylinder-shaped part of the magnetic tape, the reflection 59 thereof is projected upon the light receiver 7. Therefore, in spite of the actual presence of the magnetic tape 1 as the object to be measured, the light receiver 7 gives the detection result as if the magnetic tape 1 is not present. Moreover, the aforementioned influence of the reflected light beam depend upon the configuration of the object to be measured (the curvature difference of the magnetic tape 1 due to the diameter difference of the shaft 2), the surface reflectance thereof etc resulting in difficult error compensation. This invention aimed to solve the aforementioned difficulties. US-A-4182259 discloses and apparatus for measuring amounts of a coating material on an applicator roll. A light beams is projected so that a portion of it is intercepted by the coating meaterial on the applicator roll. At least a fraction of the remaining light passes through an aperture defined by a slotted shield member and the coated roll surface, the fraction of light passing varying with the thickness of the coating material on the roll surface. Detector means are provided to measure this variable fraction of light. The main object of this invention is to provide an apparatus enabling continuous measurement without contact and with high precision the thickness of a sheet-like product such as magnetic tape and a coating thereof during the production process thereof. Accordingly the invention provides an apparatus for measuring the thickness of a sheet or of a coating thereon comprising: a light beam source; a shaft for supporting the sheet partly surrounding said shaft; a light shading plate positioned in a plane including the central axis of said shaft and passing through the contact area between the surface of said shaft and the sheet, the light shading plate forming a slit of a predetermined width between the surface of said shaft and the edge thereof; characterised in that there is provided: a scanning apparatus comprising scanner means for scanning, at a constant speed, a light beam irradiated from said light beam source towards said slit in a plane which crosses the sheet supported by said shaft within said slit, and a photoelectric conversion device which receives the light beam through said slit, converts it into electric signals and outputs them; a light receiving slit at the light receiving surface of said photoelectric conversion device, of an appropriate width in the longitudinal direction perpendicular to the scanning direction of the light beam, such that said scanner means is positioned at the front focal point of a first lens, said slit is positioned at the back focal point of said first lens and also at the front focal point of a second lens, and said light receiving slit is positioned at the back focal point of said second lens, so that the light beam is projected through said light receiving slit onto the photoelectric conversion device; a timer for measuring the signal output duration of said photoelectric conversion device; and an arithmetic means which determines the thickness or coating thickness of the sheet on the basis of the measuring result of said timer. The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings. Figure 1 shows schematically a conventional, electric vortex current type sheet thickness measuring apparatus which is used to measure the thickness of magnetic material coated on a magnetic tape; Figure 2 shows schematically a conventional optical micrometer applicable also as a sheet thickness measuring apparatus; Figure 3 is an operational illustration thereof; Figure 4 is a block diagram showing a sheet/coating thickness measuring apparatus for illustrating in combination with Figures 10 and 11, an embodiment of the invention; Figures 5, 6 are operational illustrations thereof; Figure 7 is a block diagram showing another sheet thickness measuring apparatus for illustrating in combination with Figures 10 and 11, a further embodiment of the invention, Figure 8 is an operational illustration thereof; Figure 9 is a diagram showing the received light amount of the photoelectric converter; Figure 10 shows schematically a construction of the scanning apparatus of a sheet/coating thickness measuring apparatus according to the invention; and Figure 11 is an operational illustration thereof. Embodiments of the apparatus of the invention will be described in detail with reference to the drawings attached. Figure 4 shows the block diagram of a sheet/coating thickness measuring apparatus, wherein reference numeral 1 is a sheet such as magnetic tape, whose thickness is to be measured. This sheet 1 is kept under tension with an appropriate force which tends to push the sheet 1 upward from the shaft 2. A light shading plate 3 is positioned on a plane (identified by reference numeral 20 in Fig. 4) which includes the central axis of the shaft 2 and passes through the contact area of the shaft 2 with the sheet 1. The edge of the light shading plate 3 facing the shaft 2 is positioned at a predetermined distance in parallel with the surface of the shaft 2, with the result that a slit 30 of predetermined width C is formed between them. A laser beam irradiated from the laser source 4 is condensed by the condenser 5 and transmitted as a light beam to the scanner 6, which scans the laser beam so that it traverses across the aforementioned slit 30 from the light shading plate 3 to the shaft 2 (or vice versa), by revolving a single plane or polygonal mirror. The photoelectric converter 7 for converting the received laser beam into an electric signal output is positioned facing the scanner 6 through the slit 30. The output signal of the photoelectric converter 7 is transmitted to the comparator 8 and the threshold circuit 11, which calculates the maximum output (available when the laser beam from the scanner 6 passes perfectly through the slit 30 and is then received by the photoelectric converter 7) and the minimum value (available when no laser beam from the scanner 6 is received by the photoelectric converter 7) therefrom and transmits the average value as a threshold value to the comparator 8. According to the aforementioned threshold value from the threshold circuit 11, the comparator 8 converts the output signal of the photoelectric converter 7 into a rectangular waveform, which is outputted to a timer 9. The timer 9 counts the pulse width of the rectangular waveform outputted from the comparator 8 and acts as a timer which measures the outputting duration of the rectangular waveform, in other words, the receiving time duration of laser beam by the photoelectric converter 7. The output of the timer 9 is transmitted to the arithemetic circuit 10, which converts the output of the converter 9 according to a predetermined mathematical relationship or a conversion table. The thickness of the sheet 1 as the result of subtracting the converted value by the arithmetic circuit 10 from the width C is displayed at a display unit 12. The sheet 1 is moved at a predetermined constant speed while the shaft 2 is driven at the synchronized speed. Now the functions of the apparatus above mentioned will be described. The laser beam irradiated from the laser source 4 is condensed and focused at the slit 30 and scanned by the scanner 6 so that the laser traverses across the slit 30 from the light shading plate 3 to the shaft 2. Therefore, the photoelectric converter 7 receives the laser beam only when the laser beam from the scanner 6 is scanning within the slit 30. The scanning time duration can be detected from the length of time output signals from the timer 9 are present while the scanning speed of the laser beam and the distance between the scanner 6 and the photoelectric converter 7 are predetermined, and the counting result corresponding to the width C of the slit 30 is outputted to the arithmetic circuit 10 from the timer 9. However, the sheet 1 is actually positioned on the surface of the shaft 2 within the slit 30, therefore, the time counting value transmitted to the arithmetic circuit 10 corresponds to the value B (see Fig. 5) equal to the width C of the slit 30 less the thickness of the sheet 1, which is converted by the arithmetic circuit 10 and subtracted from the width C of the slit 30 so that the thickness of the sheet 1 is finally obtained. The thickness of the sheet 1 is displayed by the display unit 12. If the coating thickness of magnetic material coated on the magnetic tape should be measured, it is enough simply to subtract the thickness of the support material of the magnetic tape 1 from the value determined and displayed by the arithmetic circuit 10 and display unit 12. Any measuring error due to eccentricity of the shaft 2 can be avoided by using a measuring method shown in Figure 5, wherein two laser beams are scanned separately a part 2 ○ with the sheet 1 being present and another part 1 ○ with the sheet 1 being absent, in order to determine the difference and consequently obtain the thickness of the sheet 1. Therefore, the apparatus is so constructed that the laser beam is divided by a diffraction grating or a prism at an arbitrary position up to the scanner 6 and projected onto the same scanner 6. As an alternative, the system parts following the photoelectric converter 7 are positioned in two circuits and the output signals of two timers 9a, 9b, are processed by the arithmetic circuit 10. When the surface of the sheet 1, whose thickness is to be measured, is not coated completely but only partly and the supported layer 1b is exposed, two laser beams respectively scan the clearance B of a part 2 ○ of a coating layer 1a and the clearance D of a part 3 ○ where the support layer 1b is exposed, as shown in Fig. 6. Then a thickness of coating layer 1a of sheet 1 is measured directly by the same process as mentioned above. Another embodiment of a sheet thickness measuring apparatus will now be described with reference to the block diagram in Fig. 7 and the schematic drawing in Fig. 8. The reference numerals 1, 2, 3, 4, 5, 6, 10, 12 and 30 designate the same parts as shown in Fig. 4, while the reference numerals 7a, 7b; 8a, 8b; 9a, 9b and 11a, 11b in Figs. 6 and 7 correspond to the reference numerals 7, 8, 9 and 11 of the embodiment shown in Fig. 4. In this embodiment, the light shading plate 3 is provided with a slit (hereinafter called a reference slit) of a predetermined width A, which extends parallel with the surface (centreline) of the shaft 2. There are two circuits similar to that of the embodiment of Fig. 4, consisting of the photoelectric converters 7a, 7b; the threshold circuits 11a, 11b; the comparators 8a, 8b and the timers 9a, 9b, while the output signals of the timers 9a, 9b are transmitted to the arithmetic circuit 10. The functions of this embodiment will now be described, wherein the distance between the light shading plate 3 and the shaft 2, i.e. the width of the slit 30, is defined as C. In this embodiment the laser beams through the reference slit 31 and the slit 30 are received by the photoelectric converter 7a and 7b respectively. Fig 9 is a graph showing the received light amounts of the both photoelectric converters 7a and 7b. When the laser beam begins to enter the reference slit 31 (or the slit 30) from the upper edge thereof, the received light amount of the photoelectric converter 7a (or 7b) increases from the initial zero value, retains the maximum value during the time duration when the whole laser beam is passing through, then decreases slowly when the laser beam comes to the lower edge of the reference slit 31 (or the top part of the sheet 1) and finally becomes zero when the laser beam is entirely out of the reference slit 31 (or the slit 30). TA and TB in Fig. 9 show the times measured by the timers 9a and 9b with respect to the threshold value as reference determined by the threshold circuits 11a and 11b respectively. If it is supposed the traversing speed of the light beam on the light shading plate 3 is V, TA = A V TB = B V Therefore, from the above equations (1) and (2), TB TA = B A and finally, B = A · TB TA where A, the width of the reference slit 31 is known, so B can be determined independently from the traversing speed V of the laser beam. Moreover, the width C of the slit 30 is known, so the thickness X of the sheet 1 is obtained from the following equation (4). X = C - B The above calculation is processed by the arithmetic circuit 10 and the calculated thickness X of the sheet 1 is displayed on the display unit 12. The scanning apparatus consisting of the scanner 6 and the photoelectric converter 7 (7a, 7b) will be described next. Fig. 10 shows schematically a construction of the scanning apparatus for measuring coating/sheet thickness according to the invention, wherein reference numeral 4 is the light source (or the condenser), 6 is the scanner (revolving mirror, etc.) 52 is a first lens, 54 is an object to be measured, 55 is a second lens, 7 is the light receiving device (or the photoelectric converter) and 70 is the light receiving slit provided at the light receiving surface of the photoelectric converter 7 and designed as a narrow rectangular form perpendicular to the traversing direction of the laser beam. The light receiving surface of the photoelectric converter 7 receives only the laser beam passing through the light receiving slit 70. In Fig. 10 f₁ and f₂ are the focal distances of the first and second lenses 52 and 55 respectively. As shown in Fig. 10, the centre of the scanner 6 is positioned at the front focal point of the first lens 52 and the object 54 to be measured (strictly speaking the point thereof to be measured) is positioned at the back focal point of the first lens 52 and the front focal point of the second lens 55, while the light receiving slit 70 is positioned at the back focal point of the second lens 55. The operations of the apparatus will be described next. The parallel laser beam from the laser source 4 is angularly scanned by the scanner 6 and projected onto the first lens 52. The reflection surface centre of the scanner 6 being positioned at the front focal point of the first lens 52, the scanned light beam as condensed through the first lens 52 is scanned in parallel with the light axis thereof. The received light amount rapidly changes by traversing across the object 54 to be measured, resulting in the improved detecting precision because the object 54 to be measured is positioned at the back focal point of the first lens 52 and the beam diameter is condensed to a minimum. The light beam is projected onto the second lens 55 after scanning the object 54 to be measured. The front focal point of the second lens 55 is positioned at the back focal point of the first lens 52 and the light beam passed through the second lens 55 becomes again a parallel beam. The light receiving slit 70 is positioned at the back focal point of the second lens 55, so that the parallel beam passes through with restriction of the width in the axial direction parallel to the traversing direction thereof and is then received by the photoelectric converter 7. The function of this light receiving slit 70 will be described with reference to Fig. 11, wherein the left side shows the position of the object 54 to be measured. As described previously, the light beam scanning the object 54 to be measured is reflected from the surface at the end part thereof, producing the beam component with an angle ϑ larger than the original divergence angle of the beam. The light beam reflected from the top and bottom part of the object 54 to be measured follow the beam trajectories 60 and 61 respectively. These beams 60 and 61 are collimated to be in parallel with the centre light 62 through the second lens 55, but take positions deviated upward and downward from the light axis at the back focal point of the second lens 55. If the light receiving slit 70 is provided at the back focal point of the second lens 55 so as to make only the light in the range of a minute width "a" near to the light axis pass through, the light reflected from the object surface is interrupted almost perfectly and is unable to reach the light receiving surface of the photoelectric converter 7. As this phenomenon takes place at any position when the object is perpendicular to the light axis, the reflected light in the whole scanning range is interrupted by the light receiving slit 70. Therefore almost all reflected light harmful to the detection of peripheries of the object to be measured is removed in this scanning apparatus and only the necessary centre light is received by the light receiving surface of the photoelectric converter 7, resulting in exact measurement. According to this invention as described above, the thickness of any sheet-like product can be measured with high precision with a non-contact process. It is also possible to measure the thickness of magnetic material coated on magnetic tapes, continuously in the production lines thereof.

Claims: 1. An apparatus for measuring the thickness of a sheet or of a coating thereon comprising: a light beam source (4); a shaft (2) for supporting the sheet (1) partly surrounding said shaft; a light shading plate (3) positioned in a plane including the central axis of said shaft (2) and passing through the contact area between the surface of said shaft and the sheet (1), the light shading plate forming a slit (30) of a predetermined width between the surface of said shaft and the edge thereof; characterised in that there is provided: a scanning apparatus comprising scanner means (6) for scanning, at a constant speed, a light beam irradiated from said light beam source (4) towards said slit (30) in a plane which crosses the sheet supported by said shaft within said slit (30), and a photoelectric conversion device (7) which receives the light beam through said slit (30), converts it into electric signals and outputs them; a light receiving slit (70) at the light receiving surface of said photoelectric conversion device (7), of an appropriate width in the longitudinal direction perpendicular to the scanning direction of the light beam, such that said scanner means (6) is positioned at the front focal point of a first lens (52), said slit (30) is positioned at the back focal point of said first lens and also at the front focal point of a second lens (55), and said light receiving slit (70) is positioned at the back focal point of said second lens (55), so that the light beam is projected through said light receiving slit (70) onto the photoelectric conversion device (7); a timer (9) for measuring the signal output duration of said photoelectric conversion device (7); and an arithmetic means (10) which determines the thickness or coating thickness of the sheet on the basis of the measuring result of said timer (9). 2. An apparatus according to claim 1 for measuring the sheet thickness, wherein the arithmetic means determines the clearance between the surface of the sheet (1) supported by said shaft (2) and the edge of said light shading plate (3), on the basis of the measuring result of said timer (9) and the scanning speed of the light beam by said scanning apparatus and determines the sheet thickness by subtracting the detected result from said slit (30) width. 3. An apparatus according to claim 1 wherein the light beam source (4) irradiates a further light beam; the scanner means (6) is also provided for scanning at constant speed said further light beam towards said slit (30) on another plane which crosses said shaft within said slit at a portion which does not support the sheet to be measured, and the arithmetic means (10) detects the clearance between the surface of the sheet (1) supported by said shaft (2) and the edge of said light shading plate (3) and the clearance between the surface of said shaft (2) and the edge of said light shading plate (3) on the basis of the measuring result of said timer (9) and the scanning speed of the light beam by said scanning apparatus (6) and determines the sheet thickness (1) by calculating the difference between said clearances. 4. An apparatus according to claim 1 for measuring a coating thickness of the sheet, wherein said light beam is irradiated from said light beam source (4) towards said slit (30) in a plane which crosses the sheet at a coated portion thereof; the light beam source (4) irradiates a further light beam; the scanner means (6) is also provided for scanning, at a constant speed, the further light beam towards said slit (30) in another plane which crosses the sheet (1) at a non-coating portion within said slit, and the arithmetic means (10) detects the clearance between the surface of the coating portion of the sheet supported by said shaft (2) and the edge of said light shading plate (3) and the clearance between the surface of the non-coating portion of the sheet (1) and the edge of said light shading plate (3) on the basis of the measuring result of said timer (9) and the scanning speed of the light beam by said scanning apparatus (6) and determines the coating thickness by calculating the difference between said clearances. 5. An apparatus according to claims 3 or 4 wherein said light beam source (4) irradiates two light beams by branching one light beam. 6. An apparatus according to claim 1, wherein the light shading plate has a reference slit (31) of reference width extending parallel to the axial direction of said shaft, positioned in a plane including the central axis of said shaft and passing through the contact area between the surface of said shaft and the sheet; the scanner means are provided for scanning at a constant speed the light beam towards both said slit (30) and said reference slit (31) in a plane which crosses said reference slit (31) and the sheet supported by said shaft within said slit (30); two photoelectric conversion devices (7a, 7b) are provided which, respectively, receive the light beams through said slit (30) and said reference slit (31), convert them into electric signals and output them; two timers (9a, 9b) are provided respectively for measuring the signal output duration of each of said photoelectric conversion devices (7a, 7b) and the arithmetic means (10) determines the sheet thickness on the basis of the ratio of a measured result of said timers (9a, 9b) and said reference slit (31) width. 7. An apparatus according to any preceding claim, wherein the or each timer (9) measures the outputs of the or each photoelectric conversion device (7) which are converteed into rectangular waveforms by using the mean values of the maximum and minimum outputs thereof as a threshold value. 8. An apparatus according to any preceding claim, wherein the shaft (2) is rotated at a speed controlled corresponding to the transfer speed of the sheet (1). 9. An apparatus according to any preceding claim, wherein said light beam is a coherent light. 10. An apparatus according to claim 9, wherein said coherent light is a laser beam.

Inventors: IDA, YOSHIAKI ITAMI SEISAKUSHO MITSUBISHI DENKI, NAGAO, TOSHISHIGE ITAMI SEISAKUSHO MITSUB. DENKI
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1986
Main classification: G01B11
IPC codes: G01B11