Non-contact thickness gauge for non-metallic materials in the form of film, foil, tape and the like

Abstract: The invention relates to a thickness gauge (1) for non metallic materials (4) in the form of foil, film, tape or the like which comprises first non contact measuring means (3), designed to measure the distance of the faced material surface, and second non contact measuring means (5), designed to measure the distance of a metal reference surface (6), said first and second measuring means (3, 5) being installed on one side of a space for receiving the material (4) and operating in two coaxial spatial regions, so that their measurements refer to a same measuring axis (8), wherein the first non contact measuring means (3) is a backscattering fluid sensor.

Description: This invention relates to a thickness gauge for non-metallic materials in the form of foil, film, tape or the like. The thickness of such materials can be measured indirectly by measuring the absorption of a radiation source, or directly by measuring and comparing the distance of the two faces or surfaces of the film. Gauges which directly measure the distance of the film surfaces can perform the measurement either with or without contact. Document GB 2,217,835A describes a non-contact thickness gauge usable to measure the thickness of foil, film and tape made of non-metallic materials. In particular, it illustrates the combination of a first optical sensor which measures the distance of the upper surface of the film, and a second inductive sensor which measures the distance of a metal reference surface. The inductive sensor is hollow to allow the housing of the optical sensor, and the two sensors are installed coaxially so that they perform their measurements along the same measuring axis. Document US-A- 3 884 076 describes a device similar to the previous, which optical sensor has been substituted by a fluid sensor. While the measurements of the first sensor are performed directly on the measuring axis, those of the second sensor naturally take place on an area surrounding that measuring axis (typically an annulus), so that in practice the second sensor estimates the distance of the metal reference surface along that measuring axis. In a first form of construction, the metal reference surface comes into contact with the lower surface of the film. Since the film moves, the said reference surface is preferably constituted by a roller in order to minimise friction. The said document also describes a second form of construction in which the metal reference surface is positioned at a distance from the lower surface of the film, such distance being known as a result of the use of a third, optical sensor. The above-mentioned thickness gauge presents some drawbacks, however. Depending on the composition of the material to be examined and its surface characteristics, the ratio between the regular reflection factor and the diffuse reflection factor can vary considerably, as can the diffusion indicating curve. The optical sensors used in thickness gauges designed in accordance with the known technique must therefore be specially constructed on the basis of the optical characteristics of the materials to be measured. Equally, the optical characteristics of the materials to be measured often present intentional or unintentional dissimilarities which can prejudice the regulation system of the sensor. In this respect, it should be noted that the speed of the material to be measured relative to the thickness gauge can reach values of around 10 m/sec., while the time constant of the regulator of an optical sensor is measured in tenths of a second, with the result that sudden variations in the optical characteristics of the material overload the regulation circuit of the optical sensor, making the measurements performed unreliable because of the adjustement time required by the optical sensor regulator (typically a few tenths of a second). Moreover, the large size of currently known optical sensors means that inductive sensors with a very large diameter have to be used, thus reducing the reliability of the measurements taken. In fact, the larger the size of the annulus on which the measurement of the inductive sensor is performed, the less precise is the estimated distance of the metal reference surface along the measuring axis. It should be remembered that optical distance sensors have a maximum accuracy of around ± 10 um, which is not always satisfactory, especially when the average thickness of materials measured is under 1 mm. Moreover a fluid sensor according to US-A-3 884 076 is not able to directly measure the distance between said sensor and the upper surface of the film. In general the non contact thickness gauges currently known for non metallic materials present reliability problems. In the light of these drawbacks, this invention offers an improved thickness gauge for non-metallic materials in the form of film, foil, tape and the like. According to one aspect of this invention, a gauge for measuring the thickness of foil, film, tape and similar materials has been designed as specified in independent claim 1. The invention also relates to a method of managing the gauge in question, as specified in claim 6. Finally the invention also relates to a method for calibrating the gauge in question, as specified in claim 7. A form of construction of this invention is described below by way of example, by reference to the annexed drawings in which: Figure 1 is a front elevation in partial cross-section of a thickness gauge in accordance with a possible form of construction; Figure 2 is a side elevation in partial cross-section of the same device as shown in figure 1; Figure 3 is the same view as in figure 2, showing the thickness gauge mechanically de-activated; Figure 4 is a cross-section view of a detail of the thickness gauge in accordance with a possible form of construction; Figure 4a is an enlarged cross section view similar to figure 4. Figure 5 is the same view as in figure 4, showing an alternative form of construction of the invention; Figure 6 is a diagram showing the curves of the sensor implemented in the gauge in question; Figure 7 is a diagram showing the curve of a pressure differential amplifier implemented in an embodiment of the gauge in question. With reference to the annexed drawings, number 1 (figures 4 and 5) indicates the thickness gauge. The said thickness gauge 1 basically comprises first non-contact measuring means 3, designed to measure the distance of a first or upper face 2 of a non-metallic material 4 in the form of film, foil, tape or the like, and second non-contact measuring means 5, designed to measure the distance of an upper metal reference surface 6. First and second means 3 and 5 are both positioned above the upper face 2 of material 4, and operate in coaxial space regions so that the measurements relate to the same measuring axis 8. In accordance with a first form of construction (figure 4), film 4 moves in contact with metal reference surface 6, ie. metal reference surface 6 coincides with the second or lower face 18 of film 4. To reduce friction between film 4 and the said surface 6, surface 6 can be a rotating surface. A characteristic of the thickness gauge illustrated is that first means 3 consist of a backscattering fluid sensor. The principle of functionning of a backscattering fluid sensor is to detect the pressure induced inside a detecting duct 36 due to an air flow coming from the outlet 35 and reflexed from the surface 2 to be measured. In fact the pressure induced in the detecting duct 36 is a function of the distance between the outlet duct 35 and the surface to be measured. According to a preferred embodiment the air flow coming from the outlet duct 35 has a pressure of about 250mBAR, while the pressure induced in the detecting duct 36 can be up to 70 mBAR. The said fluid sensors, normally used to detect the presence of objects, are known, and are therefore not further described. The use of backscattering fluid sensors allows the construction of a gauge designed to measure upper surface 2 of material 4 which is totally insensitive to the optical characteristics of material 4. Backscattering fluid sensors present very low radial dimensions which are smaller than those of an optical sensor (approximately 5 mm in diameter), thus allowing the use of second measuring systems 5 with lower radial dimensions than those normally used, which therefore provide more precise measurements. Backscattering fluid sensors also present a higher level of accuracy than optical sensor, typically around 1,5 µm/mbar. It should be noted that the backscattering fluid sensor presents a relatively large measurement spot (typically a circle of about 5 mm in diameter), quite larger than the spot a laser distance gauge (typically an ellipse of about 0,3 mm x 1 mm). This characteristic can be advantageously used when the sheet 4 presents surface profile extremely irregular (e.g. in the case of an embossed surface) as the measurement obtained represents an average value which is after all the most important information. When materials 4 are measured on exit from a production line, their temperature is often much higher than ambient temperature. This heat emission can affect the permeability of the inductance in inductive sensor 5, thus producing measuring errors. For this purpose, a preferred embodiment includes means 7, designed to maintain inductive sensor 5 at a constant temperature. In accordance with a form of construction, the said means 7 comprise an electrical resistor controlled by an electronic temperature regulation device. In an alternative form of construction, the said means 7 are constituted by a thermistor. The reliability of the readings of first means 3 depends (inter alia) on the quality of the power source. Any variations in input pressure, even of very short duration and relatively modest value, can make the readings of fluid reference sensor 3 unreliable. For these reasons, means 9, designed to differentiate between the part of the global signal emitted by first means 3 which is due to variations in the profile of surface 2, and the part due to disturbances in the power source of first means 3, could be installed. In a possible form of construction, the said means 9 comprises a second backscattering fluid sensor or fluid reference sensor 9 (figure 2), powered by the same source as first backscattering fluid sensor 3, which measures a constant preset distance, e.g. a distance equal (or roughly equal) to the mean value of the measuring range in which first fluid sensor 3 operates. As reference sensor 9 measures a constant distance, any variation detected by it is obviously attributable to disturbances in the power source. If the difference between the signals of sensor 3 and reference sensor 9 is calculated, e.g. by means of a pressure differential amplifier (figure 7) a signal practically free of such errors is obtained until the signals are substantially of the same values, that is till the error components have roughtly the same values. According to a different embodiment means 9 comprises two reference backscattering fluid sensors powered by the same source powering the backscattering fluid sensor 3. In this case the two reference sensor 9 measure two distinct constant values, for example two intermediante distances in the sensor measuring rauge. Again since the distances measured by said two reference sensors 9 are constant, any variation detected by them is obviously attributable to disturbances in the power source. A big advantage of using two reference sensor (9) concerns the possibilty to achieve a good error suppression through all the measuring range. While an inductive sensor presents a relatively wide measuring range (in practice from approx. 0 to 8 mm), backscattering fluid sensors present a far narrower measuring range (in practice from approx. 0.1 to 1 mm) (fig. 6 line with little circles). In order to increase the range of use of the gauge in question up to the limits allowed by inductive sensor 5, fluid sensor 3 can be made to move along measuring axis 8. In the form of construction illustrated, sensors 3 and 5 are both mobile, and for this purpose are both coupled to a slide 10 which runs on straight tracks 21 (figure 3), along measuring axis 8. Slide 10 can be driven by suitable motor systems 20, such as a direct-current motor or a step motor, which transmit motion via a suitable mechanical drive such as a recirculating-ball screw. The possibility of adjusting the position of first and second means 3 and 5, allows the use of the gauge in object with a wide range of thicknesses. A position detector or transducer 11 is provided for knowing the position of gauge 1. According to the preferred embodiment, the presence of the means for moving the first and second measuring means 3 and 5 can be suitably used for performing the calibration of sensor 3 and 5 even when they are fitted on the machine. In other words the sensor 3 and 5 are moved along the measurement axis 8 and their measurements are compared with the measurement of detector 11. The measuring surface can be for example the metal reference surface 6. It should be noted that the calibration method does not require means for moving the measured surface. Since the precision of the calibration thus effected depends primarily on the precision of detector or transducer 11, it is necessary to select the said detector or transducer 11 with particular care. In accordance with a preferred form of construction, transducer 11 is an absolute position transducer, such as the type comprising an electric transformer with axially mobile core which supplies output voltage proportional to the position of the core, commonly known as LVDT's (Linear Variable-Differential Transformers). In order to eliminate the presence of moving mechanical parts which can be a source of malfunctions in the long run, an optical device can be used to make detector 11. According to an alternative form of construction, transducer 11 can be an incremental position transducer, such as an electronic revolution counter (also known as an encoder) which counts the revolutions of motor 20. In this case, the current vertical position of thickness gauge 1 is the algebraic sum of the movements previously performed. It is obviously possible to use any other position transducer which provides an acceptable degree of precision. In accordance with the annexed drawings, thickness gauge 1 oscillates around an axis 23 which is basically parallel to the plane of the material to be measured 4, and perpendicular to the direction of movement of material 4. This latter system is preferable, as it enables thickness gauge 1 to be retracted rapidly from material 4 in the case of slipping or contact between material 4 and gauge 1, thus preventing possible damage to gauge 1. For this purpose, thickness gauge 1 can be connected to slide 10 via a first hinge 13, and to a linear actuator 15 via a second hinge (not shown). In a preferred form of use, thickness gauge 1 is immobile along axis 8 during measurement of the material. If the material presents a thickness profile with high variations, basically of the same magnitude as the measuring range of fluid sensor 3, it is preferable (and may even be necessary) for thickness gauge 1 to move in the direction of axis 8, continually compensating for variations in the thickness of the material, so that the distance of gauge 1 from upper surface 2 of the material remains constant. For this purpose, a suitable electronic circuit of known type is installed to compensate for variations in distance between thickness gauge 1 and upper surface 2 of material 4. This circuit detects the variation in distance (or error) between thickness gauge 1 and upper surface 2 of material 4 at first preset intervals, and controls electronic motor 20 at second preset intervals to compensate for variations in distance between gauge 1 and upper surface 2. The position regulation of thickness gauge 1 can be the proportional (P), proportional-plus-derivative (PD) or proportional-plus-derivative-plus-integral (PID) type. Figure 5 shows an alternative form of construction of the invention which is preferable when, in view of the low flexibility or adherence of material 4, lower surface 18 is not in constant contact with metal reference surface 6. In this case, film 4 is maintained at a distance from reference surface 6, and third non-contact measuring systems 25, integral with the said reference surface 6, are installed to measure the distance between lower surface 18 and reference surface 6. For the reasons explained above, the said third systems 25 are constituted by a backscattering fluid sensor. In a possible form of construction, metal reference surface 6 and third sensor 25 are designed to move along measuring axis 8, driven by motor systems for example, like the motor systems used to move sensor 3 and 5. A second position transducer or detector could also be installed (not shown) associated with the said third sensor 25. The measurements taken by fluid sensor 25 can be corrected by comparing them with the measurements taken by one or more fluid reference sensors as already explained. For calibrating the gauge 1 a device (not shown) for allocating a reference metallic surface in a known position along the measurement axis 8 could be provided.

Claims: Gauge (1), designed to measure the thickness of non-metallic materials (4) in the form of film, foil, tape or the like, of the type comprising: first non-contact measuring means (3) designed to measure the distance of the faced or upper face (2) of material (4), and second non-contact measuring means (5) designed to measure the distance of a metal reference surface (6), said first and second means (3 and 5) being both installed on one side of a space for receiving said material (4) and operating in two coaxial spatial regions so that their measurements refer to a same measuring axis (8), and said metal reference surface (6) being installed on the opposite side of said space for receiving said material (4), the said gauge (1) being characterised by the fact that the said first means (3) are constituted by a first backscattering fluid sensor, so that the gauge operation is not affected by the optical properties of the material (4) to be gauged. Thickness gauge (1) in accordance with claim 1, characterised by the fact that it includes third non-contact measuring means (25) installed on the opposite side of the space for receiving said material (4) and integral with the said metal reference surface (6), which are designed to measure the distance between reference surface (6) and the lower face (18) of material (4) along the said measuring axis (8), the said third means (25) being of the same nature as the said first means (3). Thickness gauge (1) in accordance with one of claims 1 or 2, characterised by the fact that it includes means (11) designed to detect the position of the said first and second (and third, if any) means (3, 5, 25) on measuring axis (8), so that they can be calibrated while fitted on the machine. Thickness gauge (1) in accordance with claim 3, characterised by the fact that the said systems (11) comprise an absolute position transducer. Thickness gauge (1) in accordance with one of claims 3 or 4, characterised by the fact that it includes means designed to compensate for variations in distance between upper surface (2) of material (4) and first and second non-contact measuring means (3 and 5), and between lower surface (18) of material (4) and third non-contact measuring means (25) (if fitted). Procedure for measuring the thickness of non metallic materials (4) in the form of film, foil, tape and the like by comparing the measurements of first (3) non-contact measuring means designated to measure the distance of the faced surface or upper surface (2) of the film 4 and second non contact measuring means (5) designated to measure the distance of a metallic reference surface (6), said first and second means (3, 5) being both installed on one side of a space for receiving said material (4) and operating in two coaxial spatial regions so that their measurements refer to a same measuring axis (8), and said metal reference surface (6) being istalled on the opposite side of said space for receiving said material (4), characterised by the fact that said first means (3) are constituted by a first backscattering fluid sensor (3), so that the gauge operation is not affected by the optical properties of the material (4) to be gauged. Procedure for calibrating a non-contact thickness gauge device (1) for non-metallic materials (4) in the form of film, foil, tape or the like, the gauge device (1) comprising a backscattering fluid sensor as first non-contact measuring means (3) designated to measure the distance of a surface (2), second non-contact measuring means (5) designated to measure the distance of a reference metallic surface (6), wherein said first and second means (3, 5) are both installed on one side of a space for receiving said for receiving said material (4) and operate in two coaxial regions so that their measurement refer to a same measuring axis (8) characterized by the fact that said first and second means (3, 5) are moved along said measuring axis (8), the reference surface (6) being stationary, to allow a measurement of the distance between said reference surface (6)and said second non contact measuring means (5), the measurement of said first and second means (3, 5) being in correspondence with the measurements of a linear position transducer (11), located on board of the machine in a position apt to take the position of said first and second means (3, 5) along the measurement axis (8).

Inventors: MASOTTI ALESSANDRO, MASOTTI, ALESSANDRO
Applicants: ELECTRONIC SYSTEMS SPA, ELECTRONIC SYSTEMS S.P.A.
Application year: 1997
Main classification: G01B13
IPC codes: G01B13