AN ARRANGEMENT AND A METHOD FOR GAUGING A DISTANCE BETWEEN SURFACES OF AN OBJECT

Abstract: The present invention relates to a method and an arrangement for gauging a distance between at least two surfaces of an object (11), the arrangement including illumination means (15, 16, 20) arranged to illuminate each of said at least two surfaces, at least one optical element (17, 29, 30) arranged to project reflections from each surface onto a sensor (18). The optical element (17, 29, 30) is arranged in a section in front of said sensor to direct scattered illumination rays (37), directly incident from said at least two surfaces onto said sensor and generate a representation of the distance between said at least two surfaces of the object (11).

Description: AN ARRANGEMENT AND A METHOD FOR GAUGING A DISTANCE BETWEEN SURFACES OF AN OBJECT TECHNICAL FIELD The present invention relates to a method and arrangement for gauging a distance between at least two surfaces of an object, the arrangement including illumination means, at least one optical element and a sensor. BACKGROUND OF THE INVENTION AND PRIOR ART Thickness of a rolled sheet of metal, such as rolled steel for example in a rolling mill, is generally essential for the end product features. When rolling the material, it is important that the thickness variations are kept within narrow margins. Presently, sheets of steel are rolled to about 10 μm and measurement accuracy of approximately 1 μm is demanded. In known apparatuses which measure through direct contact with the sheet material, the parts in contact with the sheet material include two diamond points provided on associated holders arranged on each side of the sheet material. Each holder is connected to a transducer, which generates a signal corresponding to the thickness variation of the sheet material. The difference between the output-signals from the transducers represents the thickness of the sheet. Generally, this type of arrangement has limited precision, it is temperature sensitive and the maintenance cost due to wear is high. Another problem with this type of gauging arrangement is that irregularities and interrupts in the gauged sheet material, usually running between two rolls at a high speed, can effect the function of the apparatus and even break down the apparatus. Other non-contacting arrangements are also known. US 4,037,968, for example, teaches a method of measuring width of an object between two of its edges by directing a narrow beam of parallel light through a rotating prism so as to scan the aperture and object therein. The light passing through an aperture is then detected and electrical signals generated proportionally to the time the object interrupts the light are summed with correction signals to provide a composite signal representative of width of the object. In US 5,359,532 a film thickness controller capable of obtaining the rapidity of predetermined thickness control always even if the operation conditions are changed is disclosed. The film thickness controller includes a die having a mechanism for adjusting and operating a discharge amount along the width direction of film, a thickness gauge for detecting variation of thickness of the film moved with a lag time from a position of the die to a position of the thickness gauge, and adapters added to an operational calculator, a state prediction device, a state shifter and an operation amount commander constituting the film thickness controller, respectively, to thereby automatically modify a matrix of a control operation equation to be coincident with the operation conditions with respect to variation of the lag time due to movement of the film and automatically modify a gain matrix of the operational calculator, a gain matrix of a regulator and a matrix in the control operation equation to be coincident with the operation-condition with respect to variation of film thickness sensitivity to heater input variation by change of the operation conditions. An apparatus and method for measuring thickness of sheet material, particularly in a rolling mill, and controlling a roll stand is known through US 4,088,886. An X-ray fluorescence analyser for determining the components of the sheet material, a radiation thickness gauge with a calibration system for gauging material of varying composition, and a computer for calculating a calibration signal for the thickness gauge based on the output of the analyser, with the thickness gauge output providing a measure of the thickness of the sheet material at the gauge and a control signal for controlling the roll stand. Other apparatuses are also known employing ultrasonic, nuclear radiation absorption etc. However, the performances of these apparatuses depend mainly on the composition of the measured material and other surrounding conditions, which may require frequent re-calibrations of the apparatus. Most relevant apparatus is disclosed in GB 1 340 741 , which shows a gauging arrangement for gauging the distance between two opposite surface areas of a moving rolled product. The arrangement uses a pair of pulsed lamps to form light impulses and a slit diagram to form fine lights directed onto said surface areas. The light beams are then reflected by each surface and through an optical system comprising of objectives, deflecting mirrors and further objectives are focussed onto a screen of an electron-optical storage element fitted with a focus and deflection system. Clearly, the arrangement is complicated and to obtain an accurate gauging the optical system must be arranged with great precision. Moreover, the system is sensitive to the environmental influences such as dust, soil on the optical system and temperature depending on the working site and mechanical influences such as vibrations. An apparatus for measuring the distance to an object from a measurement surface is known through US 4,911,511, issued to the inventor. The apparatus contains an illumination unit, an optical image reproduction system with which a light ray projected onto the object by the illumination unit is after reflection projected on an optical sensor, which consists of a two- dimensional charge-coupled semiconductor sensor, which has a pulse train as output. The pulse train represents the illuminated and not illuminated parts of the sensor. To evaluate the position of a light spot point on the sensor all pulse-trains from the sensor are evaluated. In this way a high resolution is achieved. This device is intended for measurement of variations of the distance to a surface of an object, and by itself does not give the thickness of the object. THE OBJECTS OF THE INVENTION The main object of the present invention is to provide an arrangement for high precision, non- contact gauging, preferably in real time, of the thickness of an object. Another object of the present invention is to provide an apparatus less sensitive for, e.g. ambient temperature changes or other influencing conditions and thus less in need of frequent calibration procedures. Above-mentioned objects are achieved by means of an arrangement, according to the preamble, in which the optical element is arranged in a section in front of said sensor to direct scattered illumination rays, directly incident from said at least two surfaces onto said sensor and generate a representation of the distance between said at least two surfaces of the object. In a preferred embodiment only one sensor is arranged and said sensor is a photo-detector, e.g. a charged-coupled device, a diode array, a CMOS-based detector etc. Said illumination means may consist of a light source for each surface but preferably one light source is arranged and mirrors and/or prisms are arranged for directing light beams onto each surface. To project the light onto the sensor, the optical element consists of lenses, prisms, lens systems etc. In one embodiment, the centre axis of the optical element is substantially in line with the centre axis of the sensor. However, it is possible to arrange the optical elements laterally displaced in front of the sensor. Advantageously, the illumination means illuminates said surfaces substantially perpendicularly. In a preferred embodiment, the arrangement includes a processing unit, an A/D converter, shift registers, a multiplier, a storing unit and a position-computing unit. For correct operation of the arrangement, each light beam directed onto each surface, incides onto said each surface in same angles and at positions that has correspondence at each said surface. Advantageously, the projection onto the sensor is an enlargement of the distance between the surfaces. The invention also concerns an apparatus for optical and non-contact gauging of the thickness of an object. The apparatus comprises: a housing having a main section and supporting parts; illuminating means, arranged on said supporting parts to illuminate surfaces of said object when it is placed between said supporting parts; an optical sensor; optical elements arranged to collect and project the reflections from said surfaces, and means for processing a signal from said optical sensor for generating a signal representing the distance between said surfaces. The method according to the invention includes the steps of: illuminating each surface by means of an illumination device, collecting reflections of said illuminations from each surface, projecting said reflections onto a sensor unit for generation of a signal indicating the positions of the projected reflections on the sensor, and processing the signal from said sensor unit for calculating the distance between the surfaces. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in a non-limiting way under reference to the accompanying drawings, in which: Fig. 1 schematically illustrates a cross-section through one embodiment of a first apparatus for thickness gauging according to the invention. Fig. 2 is a cross-sectional view of a second schematic embodiment, showing an alternative arrangement of a light source. Fig. 3 is a schematic light projection representation obtained from a sensor unit. Fig. 4 is a cross-section through a third schematic embodiment according to the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS Fig. 1 is a schematic cross-sectional view of an apparatus 10 for gauging the thickness of an object 11, for example a sheet material. The apparatus 10 may preferably be arranged in a rolling mill between two rolls, at least one of which is provided, e.g. to control the thickness of the material. The sheet material passes by the apparatus 10, e.g. from a feeding cylinder to a collecting cylinder or a rolling station (not shown.) The apparatus 10 consists of a housing 12 having a main section and two shanks 13 and 14, light-emitting devices 15 and 16, an optical element 17, at least one sensor 18 and a processing unit 19. The shanks 13, 14 are so arranged that the sheet material 11 runs between them in a certain distance, preferably in an equal distance from both shanks. The light-emitting devices 15 and 16 are arranged inside or outside the shanks and direct light beams, illustrated by dashed lines 23, onto both sides of the sheet material 11. The light beams 23 incide substantially perpendicularly onto the surfaces of the sheet material. For the correct operation of the apparatus, it is important that the light beams from each light-emitting device 15, 16 coincide (exactly) at same point onto each side of the sheet material and that they have same angle of incidence (coaxial light beams in case of perpendicular illumination). The arrows show the direction of the light beams. The incident light rays are reflected and scattered by the surface of the sheet material. A part of the reflected light rays is collected by the optical element 17. The optical element 17 may consist of one or several lenses, a lens system including different types of lenses, prisms, mirrors etc., and are mainly arranged to collect and project the light reflections from the surface of the sheet material onto the surface of the sensor 18. In this embodiment, the centre axis of the optical element 17 is substantially in line with the centre axis of the sensor. The sensor is preferably a light-detector, and more preferably a Charge-Coupled Device (CCD) adapted to detect the type of the emitted light from the light emitters. The CCD consists of lines of charged coupled sensors arranged in a matrix, i.e. arranged in an X and Y coordinate system for one or several colours. By electronically scanning each line of sensors, it converts the light projected on it to electrical signals and produces a video signal. The video signals can later be analysed in different ways to detect the position of the light projection on it. The CCD 18 is connected to the processing unit 19, which is arranged to convert the signals received from the CCD to a suitable output, e.g. a digital signal, for calculating the position of the light projections. The light-emitting devices 15 and 16 can be of any well-known types of light sources, such as a laser (diode), infrared or ultraviolet illuminators etc. However, a laser source having a well- defined radiation is preferred. Another embodiment of a light-emitting arrangement is shown in fig. 2, in which only one light source 20 is provided. The light source 20 is arranged in a section outside the shanks, e.g. the main section, and the light beams 25 are directed onto the sheet material, for example by means of prisms 26 and/or mirrors 27. In the disclosed embodiments, perpendicular light beams are illustrated only as an example. For a person skilled in the art, it is obvious that light beams can be directed in any suitable angle, as long as they have the same angle of incidence. In the following, the operation of the embodiment according to fig. 1 will be described with reference to a practical application. The sheet material 11, e.g. a steel sheet, is arranged to move between two rollers (not shown) in a roller bed. The rollers are arranged, for example to roll down the sheet to a predetermined thickness. Additional supporting cylinders 28 are arranged to provide better stability at the measuring point(s). The cylinders are so arranged that they do not interfere with the illumination and reflection of the light. One or several apparatuses according to the invention could be arranged between the rolls. The light emitters 15 and 16 illuminate both sides of the sheet 1 1 with light pulses (or continuous light rays). The surfaces partly reflect and partly scatter the incident light in several directions. The scattered light 37 forms a lobe of diffused reflections on the material surface, substantially radiating in 180°. The optical element 17 collects and projects the diffused reflection beams (point and dashed lines 24) from each side of the sheet material onto the CCD 18. To prevent projections from interfering reflections, the optical element may at least partly be masked. The distance between the light projections on the CCD represents the distance between the two surfaces of the sheet material, i.e. the thickness of the sheet material. The CCD is periodically (synchronised with the periodical illumination pulses) or continually scanned, as described above, and the signals are converted to digital signals (if needed) to be processed by a position computing unit. The signal representing the light projections on the CCD is schematically illustrated in a V/S curve, i.e. voltage per surface unit of the charged couple sensor, of fig. 3. The peaks 21 , 22 of the graph represent the most intense points of the light projections, i.e. the centre of the projections and the thickness d is the distance between the peaks 21 and 22. The signal representing the distance d may then be processed and converted to a real distance (e.g. in a metric unit) for presentation or it is transmitted to the roller controllers to adjust the roller gap to further mill the sheet material. Obviously, the light projection does not need to be concentrated (point) and can be linear (incident into longitudinal direction of the sheet material) without affecting the computation of the distance d. The signal from the CCD may be processed analogously or digitally. The analogous signal processing involves conversion of the signal (according to fig. 3) to pulses partly representing characteristics with respect to a threshold value, then determining a centre for each pulse width and calculating the difference between the centres representing the distance d. Depending on the type of the optical element, it is also possible to obtain a magnification of the projected distance between the surfaces on the sensor surface (which must be compensated when calculating the actual distance.) In the embodiment shown in fig. 4, the optical elements 29 and 30 are arranged in front of the sensor, but laterally displaced in the path of the scattered light reflection 37 and 24. Each optical element 29, 30, e.g. consisting of lenses and/or prisms etc., refracts the light beams and projects them onto the CCD 18. In this embodiment, the processing unit 19 includes an A/D converter 31, shift registers 32, 34, multiplier 33, a storing unit 35 and position-computing unit 36. The output from the CCD is delivered to the A/D converter 31 and to the storing unit 35. In the storing unit 35, the data for light spots of a specified form is stored. The digital data from the A/D converter 31 is passed to the shift register 32 and data from storing unit 35 is passed to the shift register 34. The multiplier 33, multiplies data from the shift registers and a cross- correlated output is obtained. The output is delivered to the position-computing unit 36, which calculates the position of the light projections and thus the distance d. Obviously, other conventional methods, such as interpolation of the signal provided from CCD or determination of the centre of gravity for each projection representing signal, can be used to compute the position of the projections on the sensor and thereby determine the distance d. In a preferred embodiment the signal from the CCD is processed as described in US 4,911 ,511 , incorporated herein as a reference. The invention is not limited to the shown embodiments and can be varied in a number of ways without departing from the scope of the appended claims. The arrangements and the method can be implemented in various ways depending on application, functional units, needs and requirements etc. For example, more than one CCD or other optical-sensors, such as diode arrays or CMOS-based detectors for generating (video) signals may be employed, the lenses may be substituted and/or combined by other optical elements, such as diffraction optical elements (reliefs), the shanks can be arranged movable and the signal processing may be carried out externally in a computer unit etc. Moreover, the light sources may be arranged externally without need for shanks. The apparatus according to the invention may also be used for determining the distance between two points on an object other than sheet material. Moreover, sweeping light sources can be arranged to at least cover the entire surface of the sheet material. In this case, optical elements and the senor unit must be arranged in a suitable way to collect, project and detect the entire reflected light and the sweep motion of the light sources must be synchronized. It is also possible to combine one or several parts of the shown and described embodiments. REFERENCE SIGNS 10 Gauging arrangement 11 Sheet material 12 Housing 13 .Shank 14 Shank 15 Light source 16 Light source 17 Optical element 18 Sensor 19 Processing unit 20 Light source 21, 22 Peak 23-25 Light beam 26 Prism 27 Mirror 28 Cylinder 29, 30 Optical elements 31 A/D converter 32, 33 Registers 34 Multiplier 35 Storing unit 36 Computing unit 37 Scattered light beam

Claims: CLAIMS 1. An arrangement for gauging a distance between at least two surfaces of an object (11), the arrangement including illumination means (15, 16, 20) arranged to illuminate each of said at least two surfaces, at least one optical element (17, 29, 30) arranged to project reflections from each surface onto a sensor (18), characterised in, that said optical element (17, 29, 30) is arranged in a section in front of said sensor to direct scattered illumination rays (37), directly incident from said at least two surfaces onto said sensor and generate a representation of the distance between said at least two surfaces of the object (11). 2. An arrangement according to claim 1, characterised in, that only one sensor (18) is arranged and that said sensor is a photo-detector. 3. An arrangement according to claim 1, characterised in, that the sensor is at least one of a charged-coupled device, a diode array or a CMOS-based detector. 4. An arrangement according to claim 1, characterised in, that said illumination means (15, 16, 20) consists of a light source for each surface. 5. An arrangement according to any of claims 1-4, characterised in, that one light source is arranged and that mirrors (27) and/or prisms (28) are arranged for directing light beams onto each surface. 6. An arrangement according to any one of claims 1-5, characterised in, that the optical element (17, 29, 30) consists of one or several of lenses, prisms or lens systems. 7. An arrangement according to any one of claims 1-6 characterised in, that the centre axis of said optical element (17) is substantially in line with the centre axis of the sensor. 8. An arrangement according to any one of claims 1-6, characterised in, that the optical elements (29, 30) are arranged laterally displaced in front of the sensor. 9. An arrangement according to any of preceding claims, characterised in, that the light source illuminates said surfaces substantially perpendicularly. 10. An arrangement according to any of preceding claims, characterised in, that said arrangement includes a processing unit (19), an A/D (Analog/Digital) converter (31), shift registers (32, 34), multiplier (33), a storing unit (35) and position computing unit (36). 11. An arrangement according to claim 1 , characterised in, that the arrangement includes a Charged Coupled Device (CCD) and that an output from the Charged Coupled Device is delivered to the A/D converter (31) and to the storing unit (35), storing data for light spots of a specified form, that the digital data from the A/D converter (31) is passed to the shift register (32) and data from storing unit (35) is passed to the shift register (34), said multiplier (33) being arranged to multiply data from the shift registers whereby a cross correlated output is obtained and that the output is delivered to the position computing unit (36), which calculates the position of the light projections and thus the thickness. 12. An arrangement according to any of preceding claims, characterised in, that each light beam directed onto each surface, incide onto said each surface in same angles and at positions that has correspondence at each said surface. 13. An arrangement according to any of preceding claims, characterised in, that the projection on the sensor is an enlargement of the distance between the surfaces. 14. An arrangement according to any of preceding claims, characterised in, that said illumination is pulsed. 15. An arrangement according to any of claims 1-13, characterised in, that said illumination is continuous. 16. An apparatus (10) for optical and non-contact gauging of the thickness of an object, wherein said apparatus (10) comprises: - a housing (12) having a main section and supporting parts (13, 14), - illuminating means (15, 16, 20), arranged on said supporting parts to illuminate surfaces of said object when it is placed between said supporting parts, - an optical sensor (18), - an optical element (17, 29, 30) arranged in a section in front of said sensor to direct scattered illumination rays (37), directly incident from surfaces of the object onto said optical sensor and generate a representation of the distance between said surfaces of the object (11), and - means for processing a signal from said optical sensor (18) for generating a signal representing the distance between said surfaces. 17. A method for measuring the distance between at least two surfaces of an object, characterized in, - illuminating each surface by means of an illumination device, - arranging an optical element (17, 29, 30) in a section in front of said sensor to direct scattered illumination rays (37), directly incident from said at least two surfaces onto a sensor device for generating a representation of the distance between said at least two surfaces of the object (1 1), and - processing the signal from said sensor unit for calculating the distance between the surfaces.

Inventors: MORANDER KARL-ERIK, MORANDER, KARL-ERIK
Applicants: MORANDER KARL ERIK, PRECIMETER AB, MORANDER, KARL-ERIK
Application year: 1999
Main classification: G01B11
IPC codes: G01B11