Method and system for determining the straightness of a surface of an article

Abstract

A straightness of a surface (120, 122) of an article (118), is determined by conveying the article (118) along a conveying direction (x), projecting onto a first surface (120) of the conveyed article

Description

Field The present invention relates to a method and system for determining the straightness of a surface of an article. Background to the invention Generally, thermomechanical processing of steel is used to form sections (also known as profiles) from semi-finished casting products. Thermomechanical processing typically combines mechanical and / or plastic deformation processes, for example rolling, forging, extrusion, drawing and / or rotary piercing with thermal processes, for example heat-treatment, water quenching, controlled heating and / or cooling. Hot rolling is an example of thermomechanical processing and is concerned with forming shapes and / or geometries of the sections and / or modifying microstructural properties of the steel, for example. During hot rolling in rolling mills, a semi-finished casting product is typically heated above its recrystallization temperature and plastically deformed between one or more sets of rolls. Hot rolling permits large deformations to be achieved with a low number of rolling cycles. Hot rolling typically defines cross-sectional dimensions of the sections while subsequent cold forming, such as straightening, may be used to improve axial parameters of the sections. Cross-sectional parameters may include, for example, thicknesses, radii, squareness and / or concavity / convexity and axial dimensions may include, for example, straightness, unevenness and / or twist. Typically, the rolling mills are adaptable to process tens or even hundreds of sections having different cross-sectional parameters, for example by changing rolls, roll gaps and / or roll stands, to output the sections at rates of up to 10 ms1, in some cases higher. Articles produced by such processes, in particular steel sections, are typically made to a predetermined specification. Articles having parameters falling outside the tolerances of the predetermined specification may be deemed unfit for purpose and scrapped. Such articles are thus subject to checks before further processing, to ensure that they comply with the predetermined specification. Dimensional tolerances of the sections may be controlled, for example by standards, with respect to axial dimensions, cross-sectional dimensions, and / or defects, such as indentations. Typically, compliance with cross-sectional dimensional tolerances may be inspected manually using gauges, for example at a particular frequency along a length of a section, while visual inspection may be used to assess the presence of defects in or on a proportion of accessible surfaces. The use of laser gauges to assess cross-sectional dimensions at intervals along the length of the section is also known but provides limited information and may require subsequent manual confirmation. Particularly, lengths of sections are typically arranged statically on racks for inspection by human inspectors. The inspection is typically performed after completion of processing, prior to despatch, at a stage when quality control rejection of sections is costly. Furthermore, since such inspection is intermittent, parts of given length of a section may still be outside of permitted tolerances, resulting in later quality control rejection by a customer. In some applications, an article such as a steel section is required to have a sufficiently straight surface, for example to ensure adequate fit within a larger structure, to reduce wear or to provide an acceptable aesthetic quality. The inspection of sections by human inspectors to determine the straightness of their surfaces can be slow and unreliable. Furthermore, as such an inspection occurs after the production of a batch of sections is complete, the detection of an error at this stage can result in the whole batch being scrapped. Additionally, the detection of such an error across a whole batch may indicate a need for halting the operation of all or part of a manufacturing unit to enable checks, maintenance or repair to be carried out. It is beneficial that such an error be detected as early as possible to avoid waste. Hence, there is a need to improve the determination of the straightness of a surface of an article, such as a hot rolled steel section and to ensure that articles are sufficiently straight and / or flat before being passed on to a customer. Summary of the Invention It is one aim of the present invention, amongst others, to provide a method of determining a straightness of a surface of an article which at least partially obviates or mitigates at least some of the disadvantages of the prior art, whether identified herein or elsewhere. For instance, it is an aim of embodiments of the invention to provide a method of determining a straightness of a surface of an article that is insensitive to position, angle and spatial movements of the article being measured. It is a further aim of embodiments of the invention to provide a method of determining a straightness of a surface of an article that can be easily automated and carried out at high speed. Yet another aim of embodiments of the invention is to provide a method of determining a straightness of a surface of an article that can be carried out continuously in situ in the course of processing an article such as a section or plate. The invention is defined by the features of the independent claims. Preferred embodiments of the invention are set out in the dependent claims. In a first aspect, the invention relates to a method of determining a straightness of a surface (120, 122) of an article (118), the method implemented at least in part by a computer comprising a processor (130) and a memory, the method comprising: conveying the article (118) along a conveying direction (x); projecting onto a first surface (120) of the conveyed article (118) a first series of light beams (112) to generate at least 3 optical markers (102) spatially separated along the conveying direction (x), the first series of light beams (112) being projected onto the first surface (120) of the conveyed article (118) obliquely to the conveying direction (x), whereby each light beam of the first series of light beams (112) extends along a predetermined path; determining positions of the first series of optical markers (102) by: detecting positions of the first series of optical markers (102) along the conveying direction (x); and calculating positions of the first series of optical markers (102) along an elevation direction (z), the elevation direction (z) being orthogonal to the conveying direction (x), using the detected positions of the first series of optical markers (102) along the conveying direction (x) and the predetermined paths of the first series of light beams (112); and determining the straightness of the first surface (120) of the article (118) along the conveying direction (x) based on a spatial distribution of the positions of the first series of optical markers (102). The method of the invention advantageously permits a rapid, continuous monitoring of the straightness of a surface with a high degree of automation whilst avoiding contact between any measuring equipment and the article surface. The method can thus advantageously be carried out in situ within or at the outlet of a manufacturing unit, providing a swift alert to an operator or a control system if the straightness of the surface is found to be outside of predetermined tolerances. It may also be quickly determined that the manufacturing unit is not operating correctly, reducing the production of scrap. In particular, conveying the article along the conveying direction allows the article to be transported between stations of a manufacturing facility and optionally processed further while the straightness of the surface is determined. The determination of surface straightness may thus be carried out before a full batch of the articles has been manufactured and stacked for manual inspection. Errors in surface straightness can be detected immediately after manufacture of an article. The responsible manufacturing unit may be stopped or its operating parameters adjusted to prevent the repetition of the error in further articles. The amount of scrap produced can thus be minimised. Additionally, the method of determining the straightness of the surface can be carried out while the article is deformable and / or too hot for the use of conventional measurement instruments and / or for too hot for visual inspection. The provision of an optical marker on a first surface by projecting a light beam obliquely to the conveying direction (x) allows the elevation of the first surface to be tracked. This is because, fora non-orthogonal angle of incidence of the light beam with respect to the conveying direction (x), the position at which the light beam intersects the first surface (120) along the conveying direction (x) depends on the local elevation of the surface. In other words, the position of the optical marker along the conveying direction (x) depends on and provides information about the elevation of the first surface (120) of the article at said position. By using multiple optical markers projected onto the first surface, the method allows a variation in the local elevation of the first surface along the conveying direction (x) to be tracked. Advantageously, this does not rely solely on measurement of the absolute position of each optical marker in space. Instead, the straightness along the conveying direction (x) is determined based on the spatial distribution of positions of the first series of optical markers at a given instant in time. As such, the method can reliably determine the straightness of the first surface despite movement (e.g. vibration) of the article and a corresponding variation of the absolute positions of the optical markers between different instants in time. At least three optical markers (i.e. three local elevations of the first surface of the article in a single plane) spatially separated along the conveying direction (x) may be colinear such as to fall on a single straight line. The at least three coplanar optical markers are however not colinear by necessity. It is possible for the spatial distribution of the at least three coplanar optical markers to deviate from a single straight line, in particular a straight reference line. The spatial distribution of the at least three optical markers is susceptible to geometric analysis such as to determine whether a surface on which the at least three optical markers lie at a given instant is straight and / or falls within a straightness tolerance, e.g. by determining whether the positions of the at least three optical markers form a straight line or are sufficiently close to a straight reference line such as to be within a straightness tolerance. In a preferred embodiment of the invention, the number of optical markers projected onto the first surface (120) is at least 3, preferably at least 4, more preferably at least 5. By using a greater number of optical markers (102), the inventive method is capable of determining both long and short pitch deviations of the surface of the article from an ideal reference surface. A precise analysis of the geometric properties of the surface is made possible. Each light beam of the first series of light beams preferably forms an acute angle of incidence (Ai - Am) to the conveying direction (x) when viewed from a conveying-elevation plane (x-z) defined by the conveying direction (x) and the elevation direction (z). Each light beam of the first series of light beams need not, however, be confined to this conveying-elevation plane (x-z) and may optionally have a lateral component (y). Preferably, for each light beam of the first series of light beams, a path is predetermined within at least one projection plane. That is, a linear path for the light beam is preferably known for at least one plane within which the light beam extends. The at least one projection plane is preferably a conveying-elevation plane (x-z) defined by the conveying direction (x) and the elevation direction (z). The determination of the positions of the first series of optical markers (x) preferably comprises determining the positions of the first series of optical markers (x) in the conveying-elevation plane (x-z) using the predetermined paths of the light beams of the first series of light beams in said at least one projection plane. It is also preferable that determining the straightness of the first surface of the article along the conveying direction (x) based on a spatial distribution of the positions of the first series of optical markers comprises determining the straightness of the first surface of the article along the conveying direction (x) based on a spatial distribution of the positions of the first series of optical markers in said conveying-elevation plane (x-z). In a preferred embodiment ofthe invention, determining the straightness ofthe first surface (120) along the conveying direction (x) comprises determining a collinearity metric ofthe positions of the first series of optical markers (102). The collinearity metric may be a measure ofthe deviation of the spatial distribution of the first series of optical markers (102) from a linear spatial distribution. For example, the determined collinearity metric may be compared with a reference collinearity metric to determine whether the straightness ofthe first surface ofthe article along the conveying direction (x) falls within a predetermined tolerance. In a preferred embodiment ofthe invention, determining the straightness ofthe first surface (120) along the conveying direction (x) comprises calculating a first surface contour using the positions ofthe first series of optical markers. Determining the straightness ofthe first surface (120) along the conveying direction (x) preferably further comprises determining whether the first surface contour lies between a pair of tolerance lines. The tolerance lines may be parallel. In some examples, the tolerance may correspond to a set of predetermined values. The fit of the positions of the first series of optical markers against these values may be calculated to determine the straightness ofthe first surface. The first surface contour preferably extends in the conveying-elevation plane (x-z). The first surface contour is preferably calculated using the positions of the first series of optical markers (102) in the at least one projection plane. For example, the first surface contour may be a line or curve joining each position of the first series of optical markers in the conveying-elevation (x-z). The pair of tolerance lines may also extend in the same conveying-elevation plane (x-z). In a preferred embodiment ofthe invention, determining the straightness ofthe first surface (120) along the conveying direction (x) comprises calculating a first surface contour using the positions ofthe first series of optical markers (102). The first surface contour is preferably calculated using the positions ofthe first series of optical markers (102) in the at least one projection plane. For example, the first surface contour may be a line or curve joining each position ofthe first series of optical markers in the at least one projection plane which may be a conveying-elevation plane (x-z). Determining the straightness of the first surface (120) preferably further comprises calculating a curvature ofthe first surface contour and determining, based on the curvature of the first surface contour, whether the first surface (120) ofthe conveyed article (118) comprises and / or is a straight surface, a convex surface and / or a concave surface along the conveying direction (x). Where a straight first surface is required but the first surface is found to be convex or concave, it can be determined which type of correction must be made at an upstream manufacturing process in order to ensure subsequent production achieves the desired, e.g. straight and / or flat, form. For example, the alignment and / or pressure between hot or cold rollers of a steel section manufacturing facility may need to be adjusted. In a preferred embodiment ofthe invention, the article (118) comprises and / or is a section, for example a rolled steel section, such as a hot rolled steel section. Where the article is or comprises a section, the method may be carried out substantially continuously in tandem with the conveying ofthe section along its x-axis, i.e. longitudinally, for example after the manufacture ofthe section. Where the section is manufactured continuously, the method may be carried out without interruption. As the section typically has a substantially uniform cross-section, the first surface may be an upper surface which is intended to be substantially flat and extends longitudinally along the section. The use of the inventive method in determining the straightness of a surface of a steel section is especially advantageous due to the challenges in correcting surface errors of finished steel section and the high economic cost of scrap. It is particularly advantageous to perform the method on hot rolled steel section as the method is substantially insensitive to the temperature of the steel section and may be made insensitive to its colour. For example, where the method is performed on a hot rolled steel section that is still red, the optical markers may be configured to have a contrasting colour such as green or blue. The optical markers may thus still be visible. Additionally, the surface of the hot rolled section which is to be analysed by the method need not be brought into contact with any equipment. The straightness or flatness of the surface is thus unaffected by the inventive method. It is a further advantage that the method may be carried out before quenching and / or further processing of the hot rolled section. As a result, defects may be detected early in the manufacturing process. In a preferred embodiment of the invention, the method of determining the straightness of a surface of an article comprises rolling the article (118) before conveying the article (118) along the conveying direction (x), for example hot rolling a hot rolled steel section before conveying the hot rolled steel section along the conveying direction (x). The rolling step may be configured to define cross-sectional and / or axial dimensions of the article, in particular straightness, flatness, concavity or convexity of a surface. The rolling step may also be configured to provide a surface quality to the first surface of the article, in particular to provide a straight surface quality. The method of the invention may thus be useful for determining whetherthe rolling step has been carried out correctly. Errors in the rolling step may thus be identified and corrected, for example in a feedback loop. In a preferred embodiment of the invention, conveying the article (118) along the conveying direction (x) occurs with a speed in a range from 0.1 ms1 to 10 ms1, more preferably 0.5 ms1 to 5 ms1. The method may thus be carried out rapidly and continuously, without the need to slow the article’s movement along the conveying direction (x), far exceeding the limitations of manual inspection. The method may thus be carried out in-line in the course of a manufacturing process. It may be preferred that the article is conveyed along the conveying direction (x) at a lower speed, e.g. « 0.001 ms1 or at a higher speed, e.g. »10 ms1 than the range set out above. The method may thus only be limited by a measurement speed of a detection apparatus for detecting positions of the optical markers along the conveying direction (x), e.g. the frame speed of a camera. In a preferred embodiment of the invention, the first series of light beams are projected onto the first surface (120) of the conveyed article (118) with acute angles of incidence (Ai - Am) to the conveying direction (x), wherein the acute angles of incidence (Ai - Am) of the first series of light beams (112) to the conveying direction (x) are in a range from 5° to 45°, preferably 20 - 30°. In a preferred embodiment of the invention, the acute angles of incidence (Ai - Am) of the first series of light beams (112) to the conveying direction (x) are angles between the first series of light beams (112) and the conveying direction (x) in the conveying-elevation plane (x-z). Preferably, the acute angles of incidence (Ai - Am) are not equal such that the first series of light beams (112) are projected with a corresponding series of acute angles (Ai - Am), wherein M is the number of light beams in the first series of light beams (112). By projecting the first series of light beams at acute angles of incidence to the conveying direction (x) of less than 45°, the intercept of the position of each light beam at the first surface in the elevation direction (z) is amplified along the conveying direction (x). In other words, the position of each optical marker along the conveying direction (x) varies depending on the orientation of the respective region of the first surface to the light source when the light beam is projected at an angle of less than 45°, whereby the variation in the position of each optical marker along the conveying direction (x) is greater than the variation in the elevation. This allows nonstraightness in the first surface to be detected and optionally quantified in an especially reliable manner. The magnitude of the angle at which a light beam is projected onto the first surface affects the position at which a corresponding optical marker is produced on the first surface. A reference position (e.g. along the conveying direction (x)) of an optical marker on the first surface when the first surface is straight thus depends on the acute angle of incidence of the light beam according to known algebraic relationships. The actual position of the optical marker on the first surface when the first surface is not straight will also depend on the magnitude of the acute angle of incidence. The actual position will deviate further from the reference position of the optical marker on the same surface when the acute angle is less than 45° compared to when the acute angle of incidence is between 45 and 90°. By setting the acute angles of incidence of the first series of light beams upon the first surface to be at least 5°, the method can be realised in a compact manner without interference between the light source, detection apparatus and any other equipment which may be in use during a manufacturing process or inspection of the article. The light source may be provided at a sufficient elevation above the first surface. The first series of optical markers may more reliably fall within the field of view of the detection apparatus. The processor preferably determines or has access to information on the angles of incidence (Ai - Am) of each light beam of the first series of light beams (112) to the conveying direction (x), optionally in or viewed from a conveying-elevation plane (x-z). Such information may be used to determine the paths of the first series of light beams (112), in particular when the projection plane for which the paths of the light beams is known is the conveying-elevation plane (x-z). The processor may also determine or have access to information on the positions of the light sources, optionally within the projection plane, particularly with reference to the conveying direction (x). In a preferred embodiment of the invention, the corresponding first series of light beams (112) are projected upon the first surface (120) of the conveyed article (118) using a set of lasers (108), including a first laser. Due to the collimated nature of light from the set of lasers, the optical markers produced may have an especially well-defined form when scattered from the first surface. This allows for reliable detection of the location of the lasers by easily available means, such as a camera. Furthermore, the optical markers may have sufficient intensity to be visible and detectable on a wide variety of surfaces, including red hot surfaces. In a preferred embodiment of the invention, the corresponding first series of light beams (112) are projected upon the first surface (120) of the conveyed article (118) using an optical element, for example a grating. In a further preferred embodiment of the invention, the first series of optical markers extends along both the conveying direction (x) and a lateral direction (y) on the first surface of the article. The lateral direction (y) is orthogonal to the conveying direction (x) and the elevation direction (z). This preferably means that the first series of optical markers as a whole produces an optical pattern which is at least two-dimensional. The first series of optical markers may for example be configured as an array extending along the conveying direction (x) and along the lateral direction (y), the array comprising dots, crosses, lines or otherwise formed optical markers. In a preferred embodiment of the invention, the first series of optical markers (102) comprises and / or are linear optical markers each extending transversally to and spatially separated along the conveying direction (x). Linear optical markers extending transversally to the conveying direction (x) have an extension in a lateral direction (y) and form an angle to the conveying direction (x). Preferably, the linear optical markers extend substantially parallel to the lateral direction (y), wherein the lateral direction (y) is orthogonal to the conveying direction (x) and the elevation direction (z). A series of linear optical markers may advantageously highlight the contour of the first surface in at least two dimensions, in particular by having varying positions along the conveying direction (x) in different projection planes, the different projection planes preferably being distributed along the lateral direction (y). As a result, linear optical markers may have varying spatial separations along the conveying direction (x) when measured from different positions along the lateral direction (y). This may indicate the exact location of a region of non-flatness on the first surface, allowing more efficient identification and correction of the error. In addition, the use of linear optical markers allows simple classification of the analysed region of the first surface as flat, convex or concave by means of known digital image processing techniques such as de-warping. Moreover, the exact curvature and / or deviation from a reference plane can be calculated digitally by the processor. The method can thus accurately determine whether the straightness of a surface falls within the tolerances of a given specification and can additionally be used to identify a correction needed in the parameters of a manufacturing process. This may advantageously be carried out in a rapid, continuous manner, such that unacceptable surface qualities are quickly identified, and the production of the relevant article stopped or corrected, such as to reduce scrap. In a preferred embodiment of the invention, detection of the positions of the first series of optical markers (102) along the conveying direction (x) occurs by means of a detection apparatus (106), the detection apparatus (106) having a field of view extending at least along the conveying direction (x) and a lateral direction (y), wherein the lateral direction (y) is orthogonal to the conveying direction (x) and the elevation direction (z). The field of view may extend additionally in the elevation direction (z), e.g. where the first surface (120) is tilted with respect to the conveying plane (x-y). The projection of the field of view is preferably a three-dimensional volume which extends from the detection apparatus (106) and encompasses a portion of the relevant surface of the article (118) comprising the first series of optical markers (102). In a preferred embodiment of the invention, detection of the positions of the first series of optical markers (102) along the conveying direction (x) occurs by means of a detection apparatus (106), wherein the detection apparatus (106) is an imaging device, for example a camera such as a video camera. The detection apparatus (106) is preferably provided at a higher elevation along the elevation direction (z) than the relevant surface of the article (118), the straightness of which is being determined. In other words, a first distance along the elevation direction (z) between the detection apparatus (106) and the conveying plane (x-y) is preferably greater than a second distance along the elevation direction between the first surface (120) of the article (118) and the conveying plane (x-y), wherein the conveying plane (x-y) is defined by the conveying direction (x) and the lateral direction (y). The second distance may include any likely deviation of the first surface (120) from a reference plane of the first surface (120). Preferably, said first distance is at least twice, preferably at least three times, more preferably at least four times, even more preferably at least five times the magnitude of said second distance. This improves the linearity of measurements made by the detection apparatus (106) and reduces lens distortion. In a preferred embodiment of the invention, the detection apparatus (106) is oriented at a different angle to the conveying direction (x) than the first series of light beams (112). For example, a viewing angle formed between the detection apparatus (106) and the conveying direction (x) is preferably between 60° - 120°, more preferably between 80° - 100°. It is especially preferred that the detection apparatus (106) be oriented orthogonally to the conveying direction (x), in particular in the conveying-elevation plane (x-z). In a preferred embodiment of the invention, the first series of optical markers (102) extends transversally to the conveying direction (x). The method preferably further comprises: determining positions of the first series of optical markers (102) in three dimensions by: selecting or determining at least three positions of the first series of optical markers (102) along a lateral direction (y), the lateral direction (y) being orthogonal to the conveying direction (x) and the elevation direction (z); detecting positions of the first series of optical markers (102) along the conveying direction (x) which correspond to the at least three selected or determined positions of the first series of optical markers (102) along the lateral direction (y); and calculating corresponding positions of the first series of optical markers (102) along the elevation direction (z) using the positions of the first series of optical markers along the conveying direction (x) and the lateral direction (y) and the predetermined paths of the first series of light beams (112); and determining the straightness of the first surface (120) of the article (118) along the lateral direction (y) based on a spatial distribution of the positions of the first series of optical markers (102) in a three-dimensional space defined by the conveying direction (x), the lateral direction (y) and the elevation direction (z). Preferably, the light beams are light curtains arranged transversally to the conveying direction (x). Preferably, the predetermined paths of the same light beam of the first series of light beams (112) extend in a first series of at least three projection planes. The method of determining the straightness of the surface of the article (118) preferably comprises determining positions of the first series of optical markers (102) in each projection plane of the first series of projection planes. This preferably includes detecting positions of the first series of optical markers (102) along the conveying direction (x) at a selected or predetermined position along the lateral direction (y) corresponding to each projection plane of the first series of projection planes; and calculating positions of the first series of optical markers (102) along the elevation direction (z) using the detected positions of the first series of optical markers (102) along the conveying direction (x) and the predetermined paths of the first series of light beams (112) in the plurality of projection planes. In a preferred embodiment of the invention, the method of determining the straightness of the surface of the article includes calibrating the process of determining the positions of the first series of optical markers (102), preferably at least in two dimensions, e.g. in at least one projection plane which may be a conveying-elevation plane (x-z). Said calibration preferably comprises experimentally determining the predetermined paths of the first series of light beams (112). In a preferred embodiment of the invention, experimentally determining the predetermined paths of the first series of light beams (112) comprises providing a reference article having a first surface (120) at a first elevation (Z1) in the elevation direction (z) from a conveying plane (x-y), the conveying plane (x-y) being orthogonal to the elevation direction (z); projecting onto the first surface (120) of the reference article a first series of light beams (112) to generate at least 3 optical markers (102) spatially separated along the conveying direction (x), the first series of light beams (112) being projected onto the first surface (120) of the reference article obliquely to the conveying direction (x); detecting a first position (X1,i -m) of each optical marker of the first series of optical markers (102) along the conveying direction (x) while the first surface (120) is at the first elevation (Z1); placing the first surface (120) of the reference article at a second elevation (Z2) in the elevation direction (z) from the conveying plane (x-y), the difference (Az) between the first elevation (Z1) and the second elevation (Z2) being known; detecting a change (Ax) in the position of each optical marker of the first series of optical markers (102) along the conveying direction (x) and / or determining a second position (X2i. m) of each optical marker of the first series of optical markers (102) along the conveying direction (x) while the first surface (120) is at the second elevation (Z2); and calculating a path through determined positions ((X1i. m, Z1), (X2i . m, Z2)) of each optical marker of the first series of optical markers (102). In this embodiment, M preferably denotes the relevant optical marker of the first series of optical markers (102). In a preferred embodiment of the invention, the reference article is static during the steps of detecting the positions of each optical marker of the first series of optical markers (102) along the conveying direction (x). In a preferred embodiment of the invention, the step of placing the first surface (120) of the reference article at the second elevation (Z2) in the elevation direction (z) from the conveying plane (x-y) occurs by the addition of an offset of a known elevation to the reference article. Preferably, the offset has an elevation of at least 1 mm, more preferably at least 2 mm, even more preferably at least 5 mm. A large offset is preferable for reducing measurement and rounding errors. In a preferred embodiment of the invention, the step of placing the first surface (120) of the reference article at the second elevation (Z2) in the elevation direction (z) from the conveying plane (x-y) occurs by displacement of the reference article along the elevation direction (z). In a preferred embodiment of the invention, the first series of optical markers (102) are equally spaced along the conveying direction (x). In an alternative preferred embodiment of the invention, the first series of optical markers (102) are mutually unequally spaced along the conveying direction (x). In other words, it is preferred that a spatial separation of a first optical marker to a second optical marker along the conveying direction (x) is not equal to the spatial separation of the second optical marker to a third optical marker along the conveying direction (x), wherein the first, second and third optical markers are arranged consecutively. Where the first series of optical markers (102) are mutually unequally spaced along the conveying direction (x), a spacing of a first pair of optical markers along the conveying direction (x) may be compared with a spacing of a second pair of optical markers along the conveying direction (x). This may for example produce a ratio. Such a ratio can provide information on the straightness of the surface. The mutually unequal spacing of the optical markers (102) along the conveying direction (x) may optionally provide straightness information at different pitches along the article (118). For example, straightness may be assessed over a small pitch, e.g. to detect surface texture, and over a large pitch, e.g. to detect form. The mutually unequal spacing also reduces the small risk that a cyclical change in the flatness of the first surface (120) goes undetected, in particular where the cycle has an identical pitch to the spacing of the optical markers (102). In a preferred embodiment of the invention, the first series of light beams (112) comprises light beams having a wavelength or wavelengths in a range from 380 nm to 750 nm, preferably in a range from 495 to 570 nm. Preferably the wavelength(s) of the light beams are selected to provide a visual contrast with the colour of the surface of the article, the straightness of which is being determined. The method of the invention may comprise calculating the straightness or curvature of the first surface based not only on the spatial distribution of the positions of the first series of optical markers (102) resulting from such first series of light beams (112), but also on the absolute positions of the first series of optical markers (102) along the conveying direction (x). A reference plane for the surface of the article may be set or calculated, preferably dynamically calculated, and used to determine reference positions of the optical markers along the conveying direction (x). For example, where the position of a reference plane of the first surface of the article is known, e.g. by means of a proximity detection apparatus or by detecting that the optical markers have become visible by interacting with the article surface, the processor may calculate ideal reference positions for the first series of optical markers (102) along the conveying direction (x), based on known geometric principles. Such reference positions may be compared with actual positions of the optical markers (102) along the conveying direction (x) to determine the straightness of the first surface. In a further preferred embodiment of the invention, the method further comprises the dynamic determination of a reference plane for the first surface (120) of the article (118). The dynamic determination of the reference plane preferably comprises the determination of an actual distance between the first surface (120) and a detection apparatus (106) and the determination of a plane which would be occupied by the first surface (120) at said actual distance if the first surface had an ideal form, e.g. an ideal flatness, e.g. corresponding to a predetermined specification. In other words, the reference plane preferably corresponds to a plane which would be occupied by the first surface of the article according to a predetermined specification. Preferably, the method further comprises the determination of reference positions of the first series of optical markers (102) and / or ideal respective spatial separations between the first series of optical markers in the reference plane of the first surface of the article along the conveying direction (x). The detected actual positions of the first series of optical markers (102) along the conveying direction (x) and / or measured spatial separations of the first series of optical markers along the conveying direction (x) may be compared with the reference positions and / or ideal respective spatial separations to determine the straightness of the first surface. This comparison may be carried out by the processor, e.g. using a suitable algorithm. In a preferred embodiment of the invention, the method of determining a straightness of a surface of an article comprises a step of calculating a ratio between at least two respective spatial separations of the first series of optical markers along the conveying direction (x) and determining the straightness of the first surface of the article along the conveying direction (x) using the calculated ratio of spatial separations. For example, the calculated actual ratio may be compared with an ideal ratio. In a preferred embodiment of the invention, the method of determining the straightness of the surface of an article comprises determining the straightness of a first surface (120) and a second surface (122) of the article. Features of the invention disclosed with respect to the first surface (120) may apply to the second surface (122) and vice versa. In a preferred embodiment of the invention, the method of determining the straightness of the surface of an article comprises: projecting onto a second surface (122) of the conveyed article (118) a second series of light beams (112) to generate at least 3 optical markers (102) spatially separated along the conveying direction (x), the second series of light beams (112) being projected onto the second surface (122) of the conveyed article (118) obliquely to the conveying direction (x), whereby the second series of light beams (112) extend along a predetermined path; determining positions of the second series of optical markers (102) by detecting positions of the second series of optical markers (102) along the conveying direction (x); and calculating positions of the second series of optical markers (102) along a lateral direction (y) using the detected positions of the second series of optical markers (102) along the conveying direction (x) and the respective predetermined paths of the second series of light beams (112), wherein the lateral direction (y) is orthogonal to the conveying direction (x) and the elevation direction (z); and determining the straightness of the second surface (122) of the article (118) along the conveying direction (x) based on a spatial distribution of the positions of the second series of optical markers (102). In a preferred embodiment of the invention, the number of optical markers projected onto the second surface (122) is at least 3, preferably at least 4, more preferably at least 5. In a preferred embodiment of the invention, the first surface (120) of the conveyed article (118) and the second surface (122) of the conveyed article are mutually transverse. The second surface (122) is transverse to the first surface (120) by forming an angle thereto, the angle optionally but not necessarily being a right angle. For example, a mutually transverse first surface (120) and second surface (122) may be mutually orthogonal. Multiple surfaces of the same conveyed article may thus simultaneously be analysed by the method. A rotation of the conveyed article and repeated conveying past a light source and / or detection apparatus may thus be omitted. The method may therefore be carried out rapidly without delaying the processing or reducing the throughput of the conveyed article. At the same time, a required quality standard may be maintained, and scrap may be reduced. In a further aspect, the invention relates to a computer comprising a processor (130) and a memory configured to implement a method of determining the straightness of a surface of an article as described herein, a computer program comprising instructions which, when executed by a computer comprising a processor (130) and a memory, cause the computer to perform a method of determining the straightness of a surface on article as described herein or a nontransient computer-readable storage medium comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform a method of determining the straightness of a surface on article as described herein. In a further aspect, the invention relates to a system for determining a straightness of a surface (120, 122) of an article (118), the system (100) comprising: a light source (108) configured to project onto a first surface (120) of the article (118) a first series of light beams (112) to generate at least 3 optical markers (102) spatially separated along a conveying direction (x) in which the article (118) is to be conveyed, the first series of light beams (112) being projected onto the first surface (120) of the conveyed article (118) obliquely to the conveying direction (x), whereby each light beam of the first series of light beams (112) extends along a predetermined path; a detection apparatus (106) configured to detect positions of the first series of optical markers (102) along the conveying direction (x); and a computer, comprising a processor (130) and a memory, configured to: calculate positions of the first series of optical markers (102) along an elevation direction (z), the elevation direction (z) being orthogonal to the conveying direction (x), using the detected positions of the first series of optical markers (102) along the conveying direction (x) and the predetermined paths of the first series of light beams (112); and determine the straightness of the first surface (120) of the article (118) along the conveying direction (x) based on a spatial distribution of the positions of the first series of optical markers (102). In a preferred embodiment of the invention, the system (100) further comprises a conveyor (110) configured to convey the article (118) along the conveying direction (x). The system (100) is preferably configured for carrying out the method of the invention. The system (100) thus allows the surface quality of an article to be automatically and continuously monitored in line with the processing of said article. This improves the quality control and reduces scrap. A person skilled in the art recognises that technical features and advantages that have been disclosed in relation to the method described herein, equally apply to the computer and system for determining a straightness of a surface of an article, and vice versa. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention, as set out herein are also applicable to all other aspects or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or exemplary embodiment of the invention as interchangeable and combinable between different aspects and exemplary embodiments. Brief Description of the Drawings For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which: Figure 1 schematically depicts a side perspective view of a system for determining a straightness of a surface of an article according to a preferred embodiment; Figure 2 schematically depicts a perspective view of a system for determining a straightness of a surface of the article according to Figure 1; Figure 3 schematically depicts a top perspective view of a system for determining a straightness of a surface of an article according to a further preferred embodiment; Figure 4 schematically depicts a perspective view of a system for determining a straightness of a surface of the article according to Figure 3; Figure 5 is a schematic representation of a system for determining a straightness of a surface of the article according to a preferred embodiment, the system viewed from a conveying-elevation plane x-z; Figure 6 is a schematic representation of a surface of the article of Figure 5 viewed from above; Figure 7 depicts a frontal perspective view of a system for determining a straightness of a surface of an article according to a further preferred embodiment; and Figure 8 depicts a side view of a system for determining a straightness of a surface of an article according to a further preferred embodiment. Detailed Description of the Preferred Embodiments As shown in Figure 1, a system 100 according to a preferred embodiment of the invention comprises a conveyor 110 for conveying an article 118 along a conveying direction x. The conveyor 110 may define a conveying plane x-y. In this case, the conveying plane x-y is substantially parallel to a first surface 120 of the article 118. The system 100 further comprises a light source 108, in this case a set of three green lasers, and a detection apparatus 106, in this case a video camera. The video camera 106 and the set of lasers 108 are provided on a beam 104, whereby the beam 104 is configured at a distance from the article 118 along an elevation direction z such as to avoid contact therewith. The video camera 106 may have a field of view which captures a portion of the first surface 120 of the article 118. A field of view projection of the video camera 106 may be a three-dimensional volume in space. The video camera 106 may be positioned such that a portion of the article 118 intersects said three-dimensional volume. This is shown schematically as a cone in Fig. 1, the cone having its apex at the video camera 106 and its base below the first surface 120 of the article 118. The field of view of the video camera 106 notionally spans a series of lateral planes y-z, each extending along a lateral direction y and the elevation direction z. The lateral direction y is orthogonal to the elevation direction z and the conveying direction x. The series of lateral planes y-z are distributed along the conveying direction x and may e.g. correspond to rows of pixels of the video camera 106 when the video camera 106 is precisely aligned with respect to the conveying plane x-y. The video camera 106 may detect the first surface 120 of the article 118, in particular detecting optical markers 102 on the first surface 120 of the article 118, when the relevant portion of the article 118 intersects a lateral plane y-z within the field of view of video camera 106. The video camera 106 may identify a position of the detected portion of the article 118 along the conveying direction x, in particular a detected optical marker 102, based on the lateral plane y-z within which it is located at a given point in time, e.g. based on the row of pixels containing the detected optical marker 102 within a single frame captured by the video camera 106. The skilled person will appreciate that the video camera 106 may identify the position of the detected portion of the article 118 along the conveying direction x by a number of alternative principles. For example, where the pixel array of the camera is not aligned with the x and y axes, the processor may identify the conveying direction x by comparing camera frames to identify the direction of movement of the article 118. Additionally or alternatively, the processor may account for any skew in the position of the video camera 106 with respect to the elevation direction z. The set of lasers 108 is configured to illuminate a first surface 120 of the article 118 with a first series of three optical markers 102 which are spatially separated along the conveying direction x. For example, the optical markers 102 of the first series are green lines extending along the lateral direction y. The green lines may be spatially separated along the conveying direction x. The green lines may be well-defined due to the collimated nature of the light emitted from the lasers of the set of lasers 108 and may advantageously provide a good visual contrast to the surface of the article 118 when the article 118 is made of a variety of materials and is at different temperatures (e.g. red-hot or cool). Each optical marker 102 of the first series of optical markers is provided at an intersection between a light beam 112 and the first surface 120. Both the light beams 112 and the first surface 120 intersect the field of view of the video camera 106 to enable contemporaneous detection of the optical markers by the same video camera 106. Each light beam 112 may extend from a set of lasers 108 to the first surface 120. For ease of illustration, the extension of each light beam 112 in a projection plane which coincides with the conveying-elevation plane x-z defined by the conveying direction x and the elevation direction z will be considered in examples set out herein. The skilled person will however appreciate that each light beam 112 may extend along any plane (e.g. wherein ax + by + cz = d) as long as the light beam 112 is directed towards the first surface 120 and forms an oblique angle to the conveying direction x. Where the set of lasers 108 produce expanding light curtains, it will be appreciated that the light beams 112 may extend along various planes which diverge away from one another. The various planes may include the conveying-elevation plane x-z. Each light beam 112 may form an acute angle of incidence Ai - A3 to the conveying direction x in the conveying-elevation plane x-z. As a result, each light beam 112 may also fall onto the first surface 120 with an acute angle in the conveying-elevation x-z. That is, each light beam 112 may fall onto the first surface 120 at an angle smaller than 90°. In the example of Fig. 1, each light beam 112 has the form of a light curtain which extends in a straight line along a single direction in at least one projection plane (the conveying-elevation plane x-z) and expands along the lateral direction y from the location of its respective laser. The light curtains are schematically shown as triangles having their apex at the location of their respective laser. The article 118 may be a metallic article, in particular a steel article. The article 118 may be elongate along an x-axis and may have a substantially uniform cross section. The article 118 may be conveyed along a conveying direction x parallel to its x-axis. The article 118 may have two or more surfaces which are orthogonal to one another. The article 118 of the figures 1 and 2 is a T-beam, whereby the first surface 120 is a flanged surface. The skilled person will appreciate that the invention would be carried out in an analogous manner if the article 118 were any othertype of beam or section, e.g. an I-Beam, a H-Beam, Z-Beams, hollow structural section (HSS), pipes, angled section, flanged section, structural channel, C-beam, railway rail, asymmetrical or custom steel section. The T-beam 118 may have emerged from a manufacturing process, in particular a rolling process, while the method of determining a straightness of the flanged surface is carried out. While the T-beam shown in the figures is of a limited length along the conveying direction, the T-beam may alternatively be of any length, including being of an uninterrupted continuous length upstream of the system 100. A cutting process may be provided downstream ofthe system 100, where the T-beam may be cut into predetermined lengths of beam. The T-beam 118 may be red hot while detection of positions ofthe first series of optical markers 102 along the conveying direction x is carried out, e.g. due to emerging from a hot rolling process. The green optical markers 102 thus provide a good optical contrast to the first surface 120, allowing the video camera 106 to precisely identify positions ofthe optical markers 102. While the video camera 106 is in this case oriented downwards substantially orthogonally to the first surface 120, the video camera 106 may be tilted at any angle to the first surface 120, whereby the tilt may be accounted for in the method of determining the straightness ofthe first surface 120, e.g. by known image processing means such as de-warping. A processor may calculate positions ofthe first series of optical markers 102 along the elevation direction z using the detected positions of the first series of optical markers 102 along the conveying direction x. This calculation is based on predetermined paths ofthe first series of light beams 112, preferably in at least one projection plane. The predetermined paths ofthe first series of light beams 112 may be stored in the memory, e.g. as the formula of a line in the projection plane, before the method for determining the straightness of surface 120 is carried out. Such predetermined paths ofthe first series of light beams 112 may be determined by the processor based on known information on the angles of incidence Ai - As to the conveying direction x, preferably in the conveying-elevation plane x-z. The predetermined paths ofthe first series of light beams 112 may additionally or alternatively be determined experimentally, as will be explained further herein. As the predetermined path of each light beam of the first series of light beams 112 is known to the processor, by detecting the position of an optical marker 102 along the conveying direction x, the position of the same optical marker 102 along the elevation direction z (the “z-position”) corresponds to the elevation at which the path of the respective light beam reaches the detected position along the conveying direction x (the “x-position”). In turn, this indicates the instantaneous position of the first surface 120 in the at least one projection plane. Positions of multiple optical markers 102 are calculated, giving an instantaneous “snapshot” of a portion of the first surface 120. As multiple optical marker positions are calculated for a single moment in time, the spatial distribution of the optical marker positions is unaffected by movement of the first surface 120. The spatial distribution may be used to determine whether the first surface 120 is sufficiently straight along the conveying direction, for example by calculating a surface contour and / or determining whether the spatial distribution lies within a predetermined tolerance. As the calculated position of each optical marker 102 corresponds to an instantaneous position on the first surface, a contour for a portion of the first surface 120 falling within the field of view of the video camera 106 may be calculated. For example, the contour may be a line or curve passing through each of the positions of the optical markers 102 in the conveying-elevation plane x-z. The straightness of the first surface 120 along the conveying direction x may be determined based on said contour. For example, a straight contour may indicate a straight surface. The contour may however indicate a surface texture, concavity or convexity. The straightness of a surface along a given direction may be determined by determining whether a contour of the surface lies between a pair of tolerance lines in a plane substantially orthogonal to the surface. The pair of tolerance lines are preferably parallel and may have a given spacing between them corresponding to a maximum straightness tolerance. In a preferred embodiment of the invention, calculated positions of the optical markers 102 in the conveying-elevation plane x-z may be compared with a pair of tolerance lines in the same conveying-elevation plane x-z, the pair of tolerance lines preferably corresponding to a straightness tolerance along the conveying direction x. For example, the pair of tolerance lines may be parallel to the conveying direction x and separated along the elevation direction z by a predetermined distance. If at least one of the calculated positions of the optical markers 102 in the conveying-elevation plane x-z does not lie between the pair of tolerance lines in the conveying-elevation plane x-z, the first surface 120 may not be considered straight along the conveying direction x. In a preferred embodiment of the invention, the system 100 is used to carry out a calibration before being used to determine the straightness of the article 118. During calibration, the increments Az of the position of the first surface 120 along the elevation direction z may be predetermined, while the corresponding changes Ax in the position of the optical markers 102 along the conveying direction x may be determined, e.g. detected by the detection apparatus 106. Based on the predetermined increments in elevation Az and the corresponding changes Ax in position of the optical markers 102 along the conveying direction x, a relationship between the position of the optical markers 102 along the conveying direction x and the local elevation z of the first surface 120 may be determined. Optionally, during calibration, changes Ay in the position of the optical markers 102 along the lateral direction y which correspond to the increments Az of the position of the first surface 120 along the elevation direction z may also be determined, e.g. detected by the detection apparatus 106. Based on the predetermined increments in elevation Az and the corresponding changes Ax and Ay in position of the optical markers 102, a relationship between the positions of the optical markers 102 in the conveying direction x lateral direction y and the local elevation z of the first surface 120 may be determined. This may be particularly advantageous where the paths of the first series light beams 112 have a lateral component. A reference article may be provided for the calibration. The reference article may for example be a T-beam conforming to a predetermined specification. The reference article may have a first surface 120 in a reference plane substantially parallel to the conveying plane x-y. The set of lasers 108 may be used to illuminate the first surface 120 of the reference article with a first series of optical markers 102. The set of lasers 108 preferably emits a first series of light beams 112 having the same angles of incidence to the conveying direction x, preferably in the conveying-elevation plane x-z, as will be used to determine the straightness of articles 118. The first surface 120 of the reference article may be disposed at a series of displacements relative to the first series of light beams 112. The series of displacements may be spaced apart along the elevation direction z, for example, increasing or decreasing in elevation by known increments Az. The detection apparatus 106 may have the same position and orientation towards the conveying plane x-y during calibration as will be used to determine the straightness of articles. The displacement of the first surface 120 of the reference article along the elevation direction z may be carried out by the addition (or removal) of an offset from the first surface 120 or from between the reference article and the conveying plane x-y. In an exemplary embodiment of the calibration process, the position of the respective optical marker of the series of optical markers 102 on the first surface 120 of the reference article may be detected at the series of displacements along the elevation direction z. For a known first position of the respective optical marker 102 along the elevation direction z (Z1), a first position of the respective optical marker 102 along the conveying direction x (X1 ,i-m) may be determined, in particular by means of the detection apparatus 106. For a known second position of the respective optical marker 102 along the elevation direction z (Z2), a second position of the respective optical marker 102 along the conveying direction x (X2,i-m) may be determined, in particular by means of the detection apparatus 106. This process may be repeated at a suitable number of known positions of the respective optical marker 102 along the elevation direction z, e.g. at a third known position, a fourth known position, a fifth known position etc. Using the determined x-z-positions of the respective optical marker 102, a formula for a line passing through the determined x-z-positions may be determined using known geometric and / or statistical processes. The line may represent the predetermined path of the respective light beam of the first series of light beams 112 in a projection plane, preferably in the conveying-elevation plane x-z. This process may be repeated for each light beam 112 of the first series of light beams 112. This process may also be repeated for any number of projection planes and / or for determining the paths of light beams within a three-dimensional space. In a further embodiment of the invention, the system shown in Figures 1 and 2 may be used to determine the straightness of the first surface 120 of article 118 along a direction transverse to the conveying direction, preferably along the lateral direction y. This may be used to calculate a three-dimensional topography of the first surface 120. The first series of light beams 112 may follow multiple predetermined paths distributed along the lateral direction. This is due to each light curtain emanating from its respective laser having an extension along the lateral direction y and / or the light beams being distributed along the lateral direction y. The method of determining the straightness ofthe first surface 120 along the conveying direction x may include steps of determining the straightness of the first surface 120 along the lateral direction y. Such steps may comprise selecting or determining a first series of positions along the lateral direction y and detecting the position of each optical marker 102 along the conveying direction x at the selected or determined position along the lateral direction y. A set of at least three selected positions along the lateral direction y may be provided to the processor (e.g. stored in the memory) or selected by the processor. The video camera 106 may be used to detect the positions ofthe optical markers 102 along the conveying direction x for each position along the lateral direction y. Each selected position of the optical markers 102 along the lateral direction y may correspond to a light beam or a portion of a light beam 112 extending in a projection plane (e.g. having a formula aX + bY + cZ = d). The detected positions of the optical markers 102 along the conveying direction x may be used, e.g. by the processor, to calculate positions of the first series of optical markers along the elevation direction z. Such calculation may make use of predetermined paths of the first series of light beams 112 in the relevant projection plane, the relevant projection plane preferably corresponding to the selected position along the lateral direction y. Based on this, three-dimensional co-ordinate positions of the first series of optical markers in each of the conveying direction x, the lateral direction y and the elevation direction z may be identified. A contour of the first surface 120 along the lateral direction y may be calculated for one or more of the optical markers 102. Additionally or alternatively, a three-dimensional topography of the first surface 120 may be calculated. Figures 3 and 4 show schematically the use of a system 100 for analysing the straightness of multiple surfaces of an article 118. Figure 3 shows schematically a top perspective view of a system 100 for determining a straightness of a first surface 120 of an article 118 according to a further preferred embodiment. In this example, the article 118 represents a universal beam or column and the first surface 120 is located between flanges of the beam. The beam is conveyed on a conveyor 110 in a conveying direction x. The set of lasers 108 may project a first series of 3 light beams 112 onto the first surface 120, the light beams 112 providing 3 optical markings 102 which are spatially separated along the conveying direction x. A video camera 106 may continuously detect the positions of the optical markings 102 along the conveying direction x, e.g. at a predetermined frame rate. Each set of positions of the optical markings 102 along the conveying direction x may be associated with a single frame of the video camera 106 and therefore with a single instant in time. The video camera 106 may be in communication with a processor 130. The processor 130 may calculate the positions of the optical markings 102 in an elevation direction z based on the detected positions of the optical markings 102 along the conveying direction x and based on known predetermined paths of the first series of light beams 112. The processor 130 may thus provide a set of two-dimensional positions (e.g. x-z co-ordinates) of the optical markings 102 for the respective instant in time. This may be provided continuously or at intervals, e.g. for lengths or batches of the article 118. It is however preferred that the processor 130 continuously provide sets of two-dimensional positions of the optical markings 102 along the conveying direction x and the elevation direction z. These sets of two-dimensional positions of the optical markings 102 may be associated, e.g. by virtue of a timestamp, with known article identities or with known portions of the article 118. The processor 130 may calculate, for a respective instant in time, a contour of the first surface 120 along the conveying direction x using a corresponding set of two-dimensional positions of the optical markings 102, preferably all lying within the same projection plane. The processor 130 may additionally determine whether the contour of the first surface 120 along the conveying direction x meets a straightness requirement, e.g. by being a straight line passing through only colinear positions. Figure 4 shows a perspective view of a system for determining a straightness of a surface of the article according to Figure 3. The article 118 is shown rotated about an x-axis thereof, such that a second surface 122 faces the video camera 106. The video camera 106, beam 104 and set of lasers 108 may remain in the same position with respect to the conveying system 110. The set of lasers 108 may project the first series of light beams 112 onto the second surface 122 of the article 118 which may continue to be conveyed along the conveying direction x. The light beams 112 may produce a first series of optical markers 102 on the second surface 122. The video camera 106 may detect positions of the first series of optical markers 102 along the conveying direction x as set out above for the first surface 120. The processor may then calculate the positions of the first series of optical markers 102 along the elevation direction z as set out above for the first surface 120. This may be used to determine the straightness of the second surface 122 along the conveying direction x. While the embodiment shown in Figure 4 shows a single stationary setup of the light source (set of lasers 108) and detection apparatus (video camera 106), it is conceivable that these components can be moveable and / or be multiplied. As such, multiple surfaces of the article 118 may be analysed without rotation of the article 118 with respect to the light source (set of lasers 108) and detection apparatus (video camera 106). With this arrangement, multiple surfaces of the article 118 can be analysed simultaneously. For example, a first, second and / or third video camera 106 and first, second and / or third set of lasers 108 may be directed towards a first, second, third or further surface of the article 118. Figures 5 and 6 show schematically the positions of a set of three optical markers 102 according to an embodiment of the invention in a three-dimensional space. Figure 5 is a schematic side elevation of a system 100 according to an embodiment of the invention, an article 118 and a conveyor 110. The figure schematically illustrates an example of a calibration process. The article 118 is viewed from a conveying-elevation x-z. The article 118 has a first surface 120 which may be straight along the conveying direction x and may function as a reference article. A light source 108 may project a first series of three light beams 112 onto the first surface 120. The three light beams 112 form acute angles of incidence Ai, A2 and As to the conveying direction x, preferably in the conveying-elevation plane x-z. Where each of the three light beams 112 notionally intersects the first surface 120, an optical marker 102 of a first series of optical markers is formed on the first surface. Each optical marker is shown schematically as a cross. A detection apparatus 106 may be arranged at an elevation Z4 above the x-y plane which is formed by the conveyor system 110. The detection apparatus 106 may be oriented to have a viewing angle which is orthogonal to the x-y plane and to a reference plane formed by the first surface 120. The first surface 120 may be at a first elevation Z1 above the x-y plane of the conveyor system 110. Afield of view (FOV) of the detection apparatus 106 may capture all three optical markers 102 of the first series of optical markers. The detection apparatus 106 may detect positions (X11, XI2, XI3) of each optical marker 102 along the conveying direction x. These positions correspond to the first elevation Z1 of the first surface 120. This detection step may be carried out for a selected position along the lateral direction y which runs orthogonal to the conveying-elevation plane x-z. When the paths of the first series of light beams 112 are known, it may be determined that each position (X11, XI2, XI3) of each optical marker 102 corresponds to the same position along the elevation direction (Z), the same position being Z1. It may also be determined that the optical markers are colinear along the conveying direction x and that the first surface 120 is therefore straight. In other words, the contour of the first surface 120 along the conveying direction x in the selected projection plane is a straight line, which in this example is parallel to the conveying direction x. To determine the paths of the first series of light beams 112 in a projection plane experimentally, e.g. to calibrate the method of determining the straightness of the first surface 120 of article 118, the first surface 120 may be displaced to a series of elevations including Z2 and Z3. The displaced first surface 120 is shown using a dashed horizontal line. Positions of each optical marker 102 (e.g. X1i, XI2, XI3, X2i, X22, X2s and X3i, X32, X3s) along the conveying direction x at each elevation (Z1, Z2, Z3) may be detected and used to formulate the line corresponding to the respective light beam 112 in the respective projection plane. While this process is most easily illustrated for the case where the first surface 120 is parallel to the conveying plane x-y, it is conceivable that the calibration be carried out for a first surface 120 which is not parallel to the conveying plane x-y and / or comprises different elevations along the conveying direction x. Similarly, while the whole first surface 120 may be displaced by equal elevations in the elevation direction z, the skilled person will appreciate that the calibration process may be adapted to the case where the first surface 120 is unequally displaced along the elevation direction z. Figure 6 shows schematically a method of determining the straightness of the first surface 120 of the article 118 in a lateral direction y, the lateral direction y being orthogonal to the conveying-elevation plane x-z. The first surface 120 is viewed from the position of the detection apparatus 106. The first surface 120 may extend substantially parallel to the x-y plane of the conveyor system 110. The light source 108 may illuminate the first surface 120 with a first series of linear optical markers 102 extending along the lateral direction y. In an easily illustrated embodiment, the first series of linear optical markers 102 may be produced by collimated light curtains such that all light beams of the light curtains extend along parallel projection planes x-z. The skilled person is nevertheless aware of how the method of determining the straightness of the first surface of the article 118 along the lateral direction y may be adapted to the case where the light beams extend along non-parallel projection planes, e.g. where the light beams diverge, or along projection planes which are oblique to the conveying-elevation plane x-z. The linear optical markers 102 are preferably mutually unequally separated from one another along the conveying direction x. By spacing the linear optical markers 102 unequally along the conveying direction x, a cyclical change in the flatness of the first surface 120 may be detected, even if the change in flatness has the same pitch as the spacing between a pair of consecutive linear optical markers 102. A set of three positions (Y1, Y2 and Y3) along the lateral direction y may be selected. Each selected position along the lateral direction y defines, along with the elevation direction z, a respective projection plane ((X-Z)i, (X-Z)2 and (X-Z)s). The positions along the conveying direction x at which the linear optical markers 102 intersect these projection planes are detected by the detection apparatus 106. These positions are each marked with an x. The respective elevation z of each optical marker of the linear optical markers 102 at the selected positions Y1, Y2 and Y3) along the lateral direction y is then calculated. The calculation preferably uses the predetermined paths of the first series of light beams 112 in each respective projection plane to determine the elevation of the respective optical marker 102 along the elevation direction z at the selected position along the lateral direction y and at the detected position along the conveying direction x. It may thus be determined that the first surface 120 is not flat (e.g. concave or convex) in the lateral direction y but straight along the conveying direction x. A three-dimensional topography of the first surface 120 may be calculated. Figure 7 shows an exemplary system 100 for determining the straightness of a surface of an article. For simplicity, the article 118 and the conveyor 110 are not shown. Avideo camera 106 may be provided on a beam 104 extending substantially parallel to a direction along which the article 118 would be conveyed. The video camera may be oriented substantially orthogonally downwards towards an area where a first surface 120 of the article 118 would be located during conveying along the conveying direction x. A set of lasers 108 may be provided and oriented acutely towards a table onto which the conveyor 110 and article 118 would be brought. Each laser of the set of lasers 108 may have a different angle to the plane of the table, in particular an x-y plane. A first series of optical markers 102 can be seen projected onto the table, whereby the spacing between consecutive optical markers 102 is unequal along the direction of the x-axis. The position and / or orientation of the video camera 106 may be calibrated such that the first series of light beams 112 intersect lateral planes y-z and the optical markers 102 fall within the field of view of the video camera 106 when in use, in particular when an article 118 is provided below the video camera 106. Alternatively or additionally, the position and / or orientation of the set of lasers 108 may be calibrated. Figure 8 depicts a side view of a system for determining a straightness of a surface 120 of an article 118 according to a further exemplary embodiment. A first series of optical markings in the form of three (green) lines extending along the lateral direction y is visible. The three lines may each be formed by a green laser and may thus have sufficient sharpness and intensity as well as a good contrast to the colour of the article 118, so that positions of the lines along the conveying direction x may be precisely measured. As can be seen in the figure, a first longitudinal spacing 114 between a first and second optical marking 102 is smaller than a second longitudinal spacing 116 between a second and third optical marking 102 of the first series of optical markings. This allows a reference ratio between the longitudinal spacings 114, 116 to be unequal to one. The actual ratio of the longitudinal spacings 114 and 116 may be compared to the reference ratio to further ensure straightness of the relevant surface along the conveying direction x and to estimate a concavity and / or convexity of the relevant surface along the conveying direction x. The article 118 is conveyed along the conveying direction x by means of rollers of a conveyor 110. The article 118 may vibrate or bounce up and down along a conveying direction x whilst being conveyed. By detecting the instantaneous positions of at least 3 optical markers created by light beams of predetermined paths, the method and system of the invention are insensitive to such vibration, movement or angular misalignment of the article 118. By determining the straightness of the relevant surface at a respective instant in time, the temporal development of the surface straightness and / or surface contour along the conveying direction x may also be monitored. Although preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above. Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method of determining a straightness of a surface (120, 122) of an article (118), the method implemented at least in part by a computer comprising a processor (130) and a memory, the5 method comprising:conveying the article (118) along a conveying direction (x);projecting onto a first surface (120) of the conveyed article (118) a first series of light beams (112) to generate at least 3 optical markers (102) spatially separated along the conveying direction (x), the first series of light beams (112) being projected onto the first surface (120) of10 the conveyed article (118) obliquely to the conveying direction (x),wherein the first series of optical markers (102) are mutually unequally spaced along the conveying direction (x); andwhereby each light beam of the first series of light beams (112) extends along a predetermined path;15 determining positions of the first series of optical markers (102) by means of a detectionapparatus (106), wherein the detection apparatus (106) is orientated orthogonally to the conveying direction (x):detecting positions of the first series of optical markers (102) along the conveying direction (x); andcalculating positions of the first series of optical markers (102) along an elevation direction20 (z), the elevation direction (z) being orthogonal to the conveying direction (x), using thedetected positions of the first series of optical markers (102) along the conveying direction (x) and the predetermined paths of the first series of light beams (112); anddetermining the straightness of the first surface (120) of the article (118) along the conveying direction (x) based on a spatial distribution of the positions of the first series of optical markers 25 (102).

2. The method according to claim 1,wherein determining the straightness of the first surface (120) along the conveying direction (x) comprises determining a collinearity metric of the positions of the first series of optical markers 30 (102).

3. The method according to any previous claim,wherein determining the straightness of the first surface (120) along the conveying direction (x) comprises calculating a first surface contour using the positions of the first series of optical 35 markers (102); anddetermining whether the first surface contour lies between a pair of tolerance lines.

4. The method according to any previous claim,18 08 25wherein determining the straightness of the first surface (120) along the conveying direction (x) comprises calculating a first surface contour using the positions of the first series of optical markers (102);calculating a curvature of the first surface contour; and5 determining, based on the curvature of the first surface contour, whether the first surface (120) of the conveyed article (118) comprises and / or is a straight surface, a convex surface and / or a concave surface along the conveying direction (x).

5. The method according to any previous claim, wherein the article (118) comprises and / or is a 10 section, for example a rolled steel section, such as a hot rolled steel section.

6. The method according to any previous claim, comprising rolling the article (118) before conveying the article (118) along the conveying direction (x), for example hot rolling a hot rolled steel section before conveying the hot rolled steel section along the conveying direction (x).

157. The method according to any previous claim, wherein conveying the article (118) along the conveying direction (x) occurs with a speed in a range from 0.1 ms1 to 10 ms1.

8. The method according to any previous claim, wherein the first series of light beams are 20 projected onto the first surface (120) of the conveyed article (118) with respective acute angles of incidence (Ai - Am) to the conveying direction (x), wherein the acute angles of incidence (Ai -Am) of the first series of light beams (112) to the conveying direction (x) are in a range from 5° to 45°, preferably 20° - 30°.25 9. The method according to any previous claim, wherein the corresponding first series of lightbeams (112) are projected onto the first surface (120) of the conveyed article (118) using a set of lasers (108), including a first laser.

10. The method according to any previous claim, wherein the corresponding first series of light 30 beams (112) are projected onto the first surface (120) of the conveyed article (118) using an optical element, for example a grating.

11. The method according to any previous claim, wherein the first series of optical markers (102) comprises and / or are linear optical markers each extending transversally to and spatially 35 separated along the conveying direction (x).

12. The method according to any previous claim, wherein detection of the positions of the first series of optical markers (102) along the conveying direction (x) occurs by means of the detection apparatus (106), the detection apparatus (106) having a field of view extending along18 08 25the conveying direction (x) and a lateral direction (y), wherein the lateral direction (y) is orthogonal to the conveying direction (x) and the elevation direction (z).

13. The method according to any previous claim, wherein detection of the positions of the first 5 series of optical markers (102) along the conveying direction (x) occurs by means of a detection apparatus (106), wherein the detection apparatus (106) is an imaging device, for example a camera such as a video camera.

14. The method according to any previous claim,10 wherein the first series of optical markers (102) extends transversally to the conveying direction (x); andthe method further comprises:determining positions of the first series of optical markers (102) in three dimensions by:selecting or determining at least three positions of the first series of optical markers (102) 15 along a lateral direction (y), the lateral direction (y) being orthogonal to the conveyingdirection (x) and the elevation direction (z);detecting positions of the first series of optical markers (102) along the conveying direction (x) which correspond to the at least three selected or determined positions of the first series of optical markers (102) along the lateral direction (y); and20 calculating corresponding positions of the first series of optical markers (102) along theelevation direction (z) using the positions of the first series of optical markers (102) along the conveying direction (x) and the lateral direction (y) and the predetermined paths of the first series of light beams (112); anddetermining the straightness of the first surface (120) of the article (118) along the lateral 25 direction (y) based on a spatial distribution of the positions of the first series of optical markers (102) in a three-dimensional space defined by the conveying direction (x), the lateral direction (y) and the elevation direction (z).

15. The method according to any previous claim, comprising:30 calibrating the determination of the positions of the first series of optical markers (102) by experimentally determining the predetermined paths of the first series of light beams (112), whereby experimentally determining the predetermined paths of the first series of light beams (112) comprises:providing a reference article having a first surface (120) at a first elevation (Z1) in the elevation 35 direction (z) from a conveying plane (x-y), the conveying plane (x-y) being orthogonal to the elevation direction (z);projecting onto a first surface (120) of the conveyed article (118) a first series of light beams (112) to generate at least 3 optical markers (102) spatially separated along the conveying18 08 25direction (x), the first series of light beams (112) being projected onto the first surface (120) of the conveyed article (118) obliquely to the conveying direction (x);detecting a first position (X1i-m) of each optical marker of the first series of optical markers (102) along the conveying direction (x) while the first surface (120) is at the first elevation (Z1);5 placing the first surface (120) of the reference article at a second elevation (Z2) in the elevation direction (z) from the conveying plane (x-y), the difference (Az) between the first elevation (Z1) and the second elevation (Z2) being known;detecting a change (Ax) in the position of each optical marker of the first series of optical markers (102) along the conveying direction (x) and / or determining a second position (X2i-m) of each10 optical marker of the first series of optical markers (102) along the conveying direction (x) while the first surface (120) is at the second elevation (Z2); andcalculating a path through determined positions ((X1i-m, Z1), (X2i-m, Z2)) of each optical marker of the first series of optical markers (102).15 16. The method according to the previous claim,wherein the reference article is static during the steps of detecting the positions of each optical marker of the first series of optical markers (102) along the conveying direction (x).

17. The method according to any of claims 15 or 16,20 wherein the step of placing the first surface (120) of the reference article at the second elevation (Z2) in the elevation direction (z) from the conveying plane (x-y) occurs by the addition of an offset of a known elevation (Az) to the reference article.25 18. The method according to any previous claim, wherein the first series of light beams (112)comprises light beams having a wavelength or wavelengths in a range from 380 nm to 750 nm, preferably in a range from 495 to 570 nm.

19. The method according to any previous claim, comprising:30 projecting onto a second surface (122) of the conveyed article (118) a second series of light beams (112) to generate at least 3 optical markers (102) spatially separated along the conveying direction (x), the second series of light beams (112) being projected onto the second surface (122) of the conveyed article (118) obliquely to the conveying direction (x),whereby the second series of light beams (112) extend along a predetermined path;35 determining positions of the second series of optical markers (102) bydetecting positions of the second series of optical markers (102) along the conveying direction (x); andcalculating positions of the second series of optical markers (102) along a lateral direction (y) using the detected positions of the second series of optical markers (102) along the18 08 25conveying direction (x) and the respective predetermined paths of the second series of light beams (112), wherein the lateral direction (y) is orthogonal to the conveying direction (x) and the elevation direction (z); anddetermining the straightness of the second surface (122) of the article (118) along the conveying 5 direction (x) based on a spatial distribution of the positions of the second series of optical markers (102).

20. The method according to the previous claim,wherein the first surface (120) of the conveyed article (118) and the second surface (122) of the 10 conveyed article are mutually transverse, for example mutually orthogonal.

21. A computer comprising a processor (130) and a memory configured to implement a method according to any of claims 1 to 20, a computer program comprising instructions which, when executed by a computer comprising a processor (130) and a memory, cause the computer to 15 perform a method according to any of claims 1 to 21 or a non-transient computer-readable storage medium comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform a method according to any of claims 1 to 21.20 22. A system (100) for determining a straightness of a surface (120, 122) of an article (118), thesystem (100) comprising:a light source (108) configured to project onto a first surface (120) of the article (118) a first series of light beams (112) to generate at least 3 optical markers (102) spatially separated along a conveying direction (x) in which the article (118) is to be conveyed, the first series of light 25 beams (112) being projected onto the first surface (120) of the conveyed article (118) obliquely to the conveying direction (x),wherein the first series of optical markers (102) are mutually unequally spaced along the conveying direction (x); andwhereby each light beam of the first series of light beams (112) extends along a 30 predetermined path;a detection apparatus (106) configured to detect positions of the first series of optical markers (102) along the conveying direction (x), wherein the detection apparatus (106) is orientated orthogonally to the conveying direction (x); anda computer, comprising a processor (130) and a memory, configured to:35 calculate positions of the first series of optical markers (102) along an elevation direction (z), the elevation direction (z) being orthogonal to the conveying direction (x), using the detected positions of the first series of optical markers (102) along the conveying direction (x) and the predetermined paths of the first series of light beams (112); anddetermine the straightness of the first surface (120) of the article (118) along the conveying direction (x) based on a spatial distribution of the positions of the first series of optical markers (102).5 23. The system (100) for determining a straightness of a surface (120, 122) of an article (118)according to the previous claim,the system further comprising a conveyor (110) configured to convey the article (118) along the conveying direction (x).1018 08 25