DESCRIPTION

The present invention relates to a method and an apparatus for controlling tyres or related semi-finished products in a tyre production line, in particular a method and an apparatus for controlling the possible presence of irregularities and/or foreign bodies on the (or in proximity to the) surface of a tyre or of a semi-finished product thereof.

The present invention also relates to a process and a tyre production line comprising respectively the aforesaid method and the aforesaid apparatus for controlling tyres or related semi-finished products.

By "tyre" it is intended the vulcanised tyre, i.e. after vulcanisation.

By "semi-finished product" it is intended both a precursor of the tyre in one step of the production process, such as the green tyre, i.e. the semifinished product at the end of the different building steps and before vulcanisation, and elements used during the process for producing the tyre which contribute to forming the tyre in any step of the production process, and which typically are formed by means of elongated elements, strips and/or rubber-covered plies (possibly reinforced with textile or metallic material) arranged and/or to be arranged on one or more building supports.

In the scope of the processes for producing and building tyres for vehicle wheels, the need is perceived to carry out quality controls on the products obtained, with the object of preventing defective tyres from being sold and/or of progressively adjusting the apparatuses and the machinery employed, in a manner so as to improve and optimise the execution of the operations performed in the production process.

Such quality controls include, for example, those carried out by human operators who dedicate a pre-established time period, e.g. comprised between 30 s and 60 s, to a visual and tactile examination of the tyre; if, in light of one's experience and sensitivity, the operator should suspect that the tyre does not respect certain quality standards, the tyre itself is subjected to further controls, by means of suitable equipment, for the purpose of evaluating possible structural and/or quality deficiencies.

Methods are known for the optical inspection of the external shape of a tyre.

For example, the patent document JP201 1 -196741 describes a method for the optical inspection of the external shape of a tyre comprising irradiating polarised light on the surface of the tyre, receiving reflected light of the polarised light from the surface of the tyre by using a polarisation video camera, and detecting the non-uniformities on the surface of the tyre from the analysis of the change in the polarisation angle of the reflected light. For such purpose, a polarisation camera is used comprising a polariser over at least three or more different directions.

The patent document JP2010261724 describes an apparatus for the inspection of the internal surface of a tyre which includes means for the irradiation of polarised light on the two opposite sides of the internal surface of a tyre and a camera provided with means for the polarisation of the light reflected by the internal surface of the tyre in order to prevent the formation of rings.

The Applicant has addressed the problem of providing that the quality controls in the production of tyres can be at least partly performed by means of reliable optical inspection methods, so as to be able to provide a technical aid to human operators, and/or to automate the recognition of at least some flaws.

The Applicant has observed that in order for the quality control to be industrially employed in a tyre production line, it is necessary that the control itself be carried out in limited times, less than or corresponding to the normal times used by the conventional methods for controlling quality. The Applicant has realised that the known methods and apparatuses for the optical inspection require a step of acquisition of the light reflected by the surface of the tyre, which involves considerable time consumption. For example, the Applicant has realised that the optical inspection method described in the abovementioned document JP201 1 -196741 necessarily provides for a minimum of three acquisitions of the light reflected by each portion of tyre on respectively three different states of linear polarisation. Since each single acquisition on a specific polarisation state requires considerable time consumption (in the context of the process and the tyre production line), especially if the control is conducted on each single tyre or semi-finished product, there follows a disadvantageous lengthening of the production times.

The Applicant has also realised that the known methods and apparatuses (such as that described in the abovementioned document JP2010261724) do not ensure the desired accuracy and/or sensitivity in detecting possible irregularities in the surface shape of a tyre and/or non-uniformities of the structure of the surface material of a tyre and/or presence of foreign bodies on the surface of a tyre.

The Applicant has found that using one or more components of a sub- selection of components of the Stokes vector of the light reflected by the surface of a tyre or a semi-finished product, it is possible to develop a method and an apparatus for controlling the tyre that can be implemented and respectively inserted in a tyre production line, without substantially altering the productivity of the production line itself.

In a first aspect, the invention relates to a method for controlling tyres or semi-finished products in a tyre production line.

In accordance with the present invention it is provided to:

- light with a light radiation at least one portion of the surface of a tyre or semi-finished product to be inspected;

- detect the optical intensity of the light radiation reflected by said surface portion on a number N of pairs of polarisation states belonging to a group of three pairs of polarisation states corresponding to three pairs of points on Poincare sphere, the three pairs of points being located at the opposite ends of three respective diameters of the Poincare sphere orthogonal to each other, said number N being equal to 1 or 2 or 3;

- calculate, on said surface portion, a number M of values representative of an equivalent number of components of the Stokes vector belonging to a subgroup of three components of the Stokes vector comprising a second component, Q, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the first pair of polarisation states, a third component, U, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the second pair of polarisation states and a fourth component, V, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the third pair of polarisation states, wherein said number M is equal to 1 or 2 or 3;

- generate, for said surface portion, a respective control signal representative of each of said values representative of said components of the Stokes vector;

- analyse said respective control signal on said surface portion for detecting the possible presence of surface flaws (e.g. irregularities and/or foreign bodies on the, or in proximity to the surface portion).

In a second aspect, the invention relates to an apparatus for controlling tyres or semi-finished products in a tyre production line.

In accordance with the present invention the apparatus comprises:

- a member for moving a tyre or a semi-finished product having a support plane, preferably horizontal and preferably adapted to rotate around an axis perpendicular to itself;

- a source adapted to emit a light radiation for lighting a surface portion of the tyre or semi-finished product, when lying on the support plane, and, at a distance from said source, a detection system adapted to detect the optical intensity of the light radiation reflected by said surface portion on a number N of pairs of polarisation states belonging to a group of three pairs of polarisation states corresponding to three pairs of points on Poincare sphere, the three pairs of points being located at the opposite ends of three respective diameters of the Poincare sphere orthogonal to each other, said number N being equal to 1 or 2 or 3, and to generate a respective detection signal; and

- a processing unit adapted to receive from the detection system said detection signal and configured for the following functions:

- calculating on said surface portion a number M of values representative of an equivalent number of components of the Stokes vector belonging to a subgroup of three components of the Stokes vector comprising a second component, Q, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the first pair of polarisation states, a third component, U, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the second pair of polarisation states and a fourth component, V, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the third pair of polarisation states, wherein said number M is equal to 1 or 2 or 3;

- generating, for said surface portion, a respective control signal representative of each of said M values representative of said components of the Stokes vector and

- analysing said respective control signal on said surface portion for detecting the possible presence of surface flaws.

In a third aspect, the invention relates to a process for producing tyres comprising at least one operation for building green tyres, an operation for vulcanising green tyres and an operation for controlling tyres or semifinished products in accordance with the method of the first aspect of the present invention.

In a fourth aspect, the invention relates to a tyre production line comprising at least one work station, i.e. an area structured and configured for carrying out at least one operation for building green tyres, at least one vulcanisation station, i.e. an area structured and configured for carrying out a green tyres vulcanisation operation, and at least one control station comprising an apparatus for controlling tyres or semi-finished products according to the apparatus of the second aspect of the present invention. The Applicant has found that for the purpose of detecting flaws on the surface of a tyre or a semi-finished product, the Stokes vector of such surface has proven particularly significant, in at least one, or more, of its components selected within the aforesaid subgroup of three components. The Applicant observes that generally the components of the Stokes vector (in brief "the Stokes components") define exactly one state of polarisation, even for light not completely polarised, i.e. one polarisation state is perfectly well-known when all the Stokes components are known. The method for testing described in JP201 1 -196741 , which also detects the radiation reflected on at least three different states of linear polarisation (e.g. 0°, 90° and 135°), nevertheless does not use any Stokes component, but rather is limited to determining the (linear) polarisation angle of the reflected light starting from a polarised incident radiation (linearly, for example at 45°).

The Applicant has found that the use of the Stokes vector in one or more of its three aforesaid components, Q, U and V, allows obtaining significant information with the least number of acquisitions possible, at the very least with only two acquisitions.

In addition, the use of the Stokes vector in one or more of the three aforesaid components, Q, U and V, allows the use of incident light, even

(and rather preferably) not polarised, unlike the known methods which require polarised incident light.

With reference to at least one of the aforesaid aspects, the following preferred solutions are provided.

The terms 'optical', 'light' and the like make reference to a spectrum of the electromagnetic radiation which has at least one portion falling within an enlarged range of the optical band, and not necessarily falling strictly within the optical band (i.e. 400-700 nm), e.g. such enlarged range of the optical band can range from the ultraviolet to the infrared (e.g. wavelength from 350 nm to 900 nm).

Preferably the light radiation has a spectrum centred in the optical band, e.g. centred around 550 nm. Preferably the spectrum has narrow band, e.g. with width at half height equal to 20 nm (advantageously in such a manner one limits the dependence of the polarisation of the wavelength of the radiation). Alternatively, the light radiation can be broadband (e.g. white light).

The expression "detecting the optical intensity of the light radiation on a polarisation state" indicates detecting the optical intensity of the radiation projected on the polarisation state.

Advantageously the calculation of each value representative of a Stokes component requires a small consumption of computational resources, unlike for example JP201 1 -196741 , where the intensities detected are interpolated with a predefined and parameterised curve of the intensity as a function of the linear polarisation angle and where the maximum point of such interpolated curve is calculated.

Preferably the three pairs of polarisation states are respectively: the horizontal and vertical linear polarisation for the first pair, the linear polarisation at +45° and -45° for the second pair and the right and left circular polarisation for the third pair. Conventionally, such polarisation states will be termed "main polarisation states". It is observed that the reference system selected for defining the aforesaid main polarisation states is arbitrary (in other words, the method is unchanging for any rigid rotation of the six polarisation states on Poincare sphere). The optical intensity of the light radiation detected on the horizontal and vertical linear polarisation will be indicated with the symbols E1 and E2, respectively, while that detected on the linear polarisation at +45° and -45° and on the right and left circular polarisation will be respectively indicated with the symbols E3, E4, E5 and E6.

In general the Stokes vector is a vector with four components (indicated with the symbols I, Q, U, V) where the first, l=E1 +E2, is equal (or proportional) to the sum of the two optical intensities of the light radiation reflected by the surface portion on the first pair of polarisation states, the second, Q=E1 -E2, is equal to the difference of the two optical intensities of the light radiation reflected by the surface portion on the first pair of polarisation states, the third, U=E3-E4, is equal to the difference of the two optical intensities of the light radiation reflected by the surface portion on the second pair of polarisation states and the fourth, V=E5-E6, is equal to the difference of the two optical intensities of the light radiation reflected by the surface portion on the third pair of polarisation states. It is observed that the present invention optionally also considers the calculation, in addition to that stated above, of a value representative of the first component of the Stokes vector, I, as discussed as an example hereinbelow.

Preferably said number M of values representative of an equivalent number of Stokes components comprises the value representative of the second component, Q (in which case, E1 and E2 must be detected).

Preferably it is provided to detect the optical intensity of the light radiation reflected by said surface portion on the first pair of polarisation states by passing the reflected light radiation through a linear polariser aligned along the horizontal axis and through a linear polariser aligned along the vertical axis, respectively. The two polarisers used respectively for each pair of main polarisation states can be used in time succession (preferably) or simultaneously upon spatial division of the reflected light radiation.

Preferably it is provided to detect the optical intensity of the light radiation reflected by said surface portion on the second pair of polarisation states by passing the reflected light radiation through a linear polariser aligned at +45° with respect to the horizontal and through a linear polariser aligned at -45° with respect to the horizontal, respectively.

Preferably it is provided to detect the optical intensity of the light radiation reflected by said surface portion on the third pair of polarisation states by passing the reflected light radiation through (as regards the right circular polarisation) a quarter-wave plate with the fast axis thereof ("fast axis") positioned along the horizontal axis followed, downstream of the plate with respect to the direction of propagation of the light radiation (which thus first crosses the plate and then the polariser), by a linear polariser aligned at +45° with respect to the horizontal axis and, respectively (as regards the left circular polarisation), through a quarter-wave plate with the fast axis thereof positioned along the horizontal axis followed by a linear polariser aligned at -45°with respect to the horizontal axis.

Preferably said number N is equal to 1 and said number M is equal to 1 , more preferably the pair of polarisation states on which the optical intensity of the reflected light radiation is detected is the first pair (E1 , E2) and said value representative of the Stokes components is representative of the second Stokes component, Q. According to the Applicant, in such a manner with only two acquisitions (e.g. as opposed to three of the patent JP201 1 -196741 ), very significant information is obtained.

In an alternative embodiment, said number N of pairs of polarisation states is equal to 2 or 3 and said number M is equal to 2 or 3, wherein said M values representative of the Stokes components comprise the representative value of at least one between the third and the fourth Stokes component, U and V, optionally of both.

Preferably it is further provided to calculate, on said surface portion, a further value representative of a first component of the Stokes vector, I, equal to the, or a function of the, sum of the two optical intensities of the light radiation reflected by the surface portion on the first pair of polarisation states. More preferably said M values representative of the components of the Stokes vector further comprise the value representative of the second component Q (i.e. I, Q and U or I, Q and V or I, Q, U and V). According to the Applicant, advantageously in such a manner one obtains high sensitivity and/or accuracy in the detection of surface flaws, when subjected to a number of acquisitions greater than two.

Preferably it is provided to calculate on said surface portion, as a function of said M (with M equal to 2 or 3), values representative of the Stokes components and said further value representative of the first Stokes component, at least one first polarisation parameter belonging to a group of polarisation parameters consisting of a degree of linear polarisation, DOLP=sqrt(Q ² +U ² )/l, of a polarisation angle, PSI=0.5 ^* atan(U/Q) and of a degree of circular polarisation, DOCP= V/l. Optionally, the method comprises calculating both DOLP and PSI.

In one embodiment said number N is equal to 3 and said number M is equal to 3. Preferably the method comprises calculating on said surface portion, as a function of said M values representative of the Stokes components and said further value representative of the first Stokes component, PSI, DOCP, DOLP and also a polarisation degree, DOP=sqrt(Q ² +U ² +V ² )/l. According to the Applicant, one or more of the aforesaid polarisation parameters constitute highly significant parameters for identifying surface flaws.

Preferably the light radiation which lights or for lighting at least one surface portion of the tyre or the semi-finished product is not polarised or it is at least partly not polarised.

Preferably in the operation for lighting a portion of the surface the angle formed between the direction of propagation of the light radiation which lights the surface and the normal to the plane locally tangent to the envelope of the surface portion is greater than or equal to 30°, preferably greater than or equal to 45° (e.g. equal to 90°). Advantageously, in such a manner the surface portion produces sensitive effects of polarisation on the light radiation. Preferably it is provided to detect the optical intensity of the reflected light radiation by means of at least one digital camera (also of linear type) having a digital optical sensor adapted to detect said reflected light radiation and an optical objective with an axis thereof (e.g. the barycentre axis of the objective lenses), where more preferably the angle formed between such axis and the normal to the plane locally tangent to the envelope of the surface portion is smaller than or equal to 50°, preferably smaller than or equal to 30° (e.g. equal to 0°).

Preferably the angle formed by the direction of propagation of the light radiation which lights the portion of the surface and said axis of the objective of the camera is greater than or equal to 40°, preferably greater than or equal to 60° (e.g. equal to 90°).

Preferably said detection system comprises said at least one digital camera and a number 2N of polarisers, each structured for projecting the reflected light radiation on a respective state belonging to said N pairs of polarisation states.

In one embodiment, some of or all the 2N polarisers can be positioned upstream of the camera with respect to the direction of propagation of the reflected light radiation, in a mutually exclusive manner. For example, the detection system can comprise one and only one camera and, placed upstream before the same, a member on which all the 2N polarisers are mounted, such member configured for positioning only one polariser at a time in front of the camera. Preferably in such case said source comprises one and only one light source.

In an alternative and preferred embodiment, the detection system comprises a number 2N of digital cameras, each with a respective polariser different from the other polarisers. The 2N cameras can be angularly uniformly distributed around the circumference of the tyre (also in the green state), and in such case said source comprises a number 2N of light sources, each associated with a respective camera. Alternatively, the 2N cameras are positioned in a single angular position (e.g. one next to the other) and use the same single light source.

Preferably the apparatus comprises (at least) one robotised arm, at whose free end said detection system and said source are mounted.

Preferably it is provided to discretise said surface portion in a finite number of points. In this context, by "point" it is naturally intended a small subsurface portion having finite size, e.g. comparable to a pixel of a digital image of the surface itself, and not a mathematical point having zero size. Preferably it is provided to detect and/or calculate and/or generate a respective control signal and/or analysing the respective control signal, for each point of the surface portion.

Preferably it is provided to create, for each of said M values representative of the Stokes components and/or for each of said polarisation parameters, an image of said surface portion in which each point of the surface is displayed as a function of the respective value of said representative value and/or, respectively, of said polarisation parameter. More preferably each point of the surface is displayed with a colour, or a grey scale, or white or black, as a function of the respective value of said representative value and/or polarisation parameter.

Preferably said processing unit is also configured for driving and controlling said apparatus .

Preferably the station for controlling tyres comprises a pair of the aforesaid apparatuses for controlling tyres in a production line and, interposed between the two apparatuses with reference to the production flow, a machine for tilting the tyre around an axis orthogonal to the cylindrical symmetry axis of the tyre.

Preferably the operation for controlling tyres or semi-finished products comprises controlling a plurality n of tyres or semi-finished products in a predefined time range.

Preferably it is provided, in the same predefined time range, to introduce k tyres or semi-finished products into a production flow, with k=<n, subsequently to the control, and forward n-k tyres or semi-finished products, subsequently to the control, outside said production flow, as a function of the aforesaid respective control signal.

The Applicant deems that in this manner it is possible to obtain, in acceptable times and in a precise and reliable manner, a control of the surface of the tyres or semi-finished products that can be advantageously used for controlling the quality of the tyres or semi-finished products in a production process.

Preferably said control station is provided downstream of said vulcanisation station with respect to the production flow, or said control of tyres in a production process is performed after the vulcanisation of said tyre. In this manner, it is possible to verify the presence of possible surface flaws of the tyre which could have occurred also during or following a vulcanisation operation.

Preferably each of said n-k tyres or semi-finished products forwarded outside the production flow is sent to a verification station for carrying out at least one further analysis on the same tyre or semi-finished product. In this manner, it is possible to determine with greater accuracy the problems that each tyre or semi-finished product has and it is thus possible to decide, in a more reliable manner, if the same tyre must be definitively discarded.

Preferably said predefined time for controlling the plurality n of tyres or semi-finished products is comprised between about n/5 times and about 5n times a cycle time (defined as the maximum time employed by a semi- finished product to move from a work station). In this manner, the control method according to the invention can be advantageously applied in a line and in a process of production without significantly reducing the productivity of the line itself. Preferably said predefined time corresponds with about n times a cycle time.

Further characteristics and advantages will be clearer from the detailed description of several exemplifying but not exclusive embodiments of a method and an apparatus for controlling tyres or related semi-finished products in a tyre production line, as well as a related process and production line, in accordance with the present invention. Such description will be set forth hereinbelow with reference to the set of drawings, provided only for exemplifying and hence non-limiting purposes, in which:

- figure 1 schematically shows a production line according to the present invention;

- figure 2 shows a schematic diagram, in terms of functional blocks, of an apparatus for controlling tyres in accordance with the present invention; - figure 3 shows a partially schematic perspective view of an apparatus for controlling tyres in accordance with the present invention;

- figure 4 shows a schematic diagram, in terms of functional blocks, of a control station in accordance with the present invention;

- figure 5 is a visible-light photograph in grey tones, in negative, and without polariser filters of a surface portion of a tyre on which a metallic foreign body was suitably included;

- figures 6, 7 and 8 show a black-and-white image of a surface portion substantially coinciding with that of figure 5 obtained by associating each discrete point of the surface with a value representative respectively of the second, third and fourth Stokes component, Q, U and V, and after thresholding on grey tones;

- figures 9-12 show a black-and-white image of a surface portion substantially coinciding with that of figure 5 obtained by associating, with each discrete point of the surface, a value representative respectively of the degree of linear polarisation, DOLP, of the degree of polarisation,

DOP, of the degree of circular polarisation, DOCP, and of the polarisation angle, PSI and after thresholding on the grey tones;

- figure 13 shows a flow diagram of a tyre production process in accordance with the present invention.

With reference to the figures, reference number 1 generally indicates a production line in which an inspection is performed by means of the method and/or the apparatus according to the present invention. In general the same reference number is used for similar elements, even in the modified embodiments thereof.

The production line 1 comprises at least one work station 10, at least one control station 20 and at least one vulcanisation station 30. The production line 1 can also comprise further stations, not illustrated in detail herein. In the embodiment of figure 1 , the control station 20 is provided downstream of the vulcanisation station 30 (with reference to the production flow indicated by the arrows), in order to control the tyres after vulcanisation. Optionally (or alternatively, not shown), a further control station 20 (shown in dashed line) is operatively situated downstream of the work station 10 and upstream of the vulcanisation station 30 (in order, for example, to control the green tyres or the semi-finished products).

A flow diagram of a process 200 for producing tyres in accordance with the present invention is schematically illustrated in figure 13.

The process 200 comprises at least one operation 210 for building green tyres (typically performed in the aforesaid work station 10), an operation 230 for vulcanising green tyres (typically performed in the aforesaid work station 30 subsequently to the building operation 210) and an operation 220 for controlling tyres or semi-finished products (typically performed in the aforesaid work station 20 subsequently at least to the building operation 210).

The operation 220 for controlling tyres or semi-finished products provides for controlling n tyres or semi-finished products in a predefined time range, for example equal to about n times a cycle time. For example, in the predefined time range, a flow of n tyres or semi-finished products is fed into the inlet of the control station 20. By way of example, the cycle time can be comprised between about 20 s and about 120 s, more preferably between about 60 s and about 90 s.

The following description will make reference to a single tyre or semifinished product. That described can be applied to each of the n tyres or semi-finished products constituting the entering flow.

Preferably the n tyres or semi-finished products will be sequentially processed, one after the other, by the same devices present in the control station 20.

The control operation provides for lighting with a light radiation at least one portion of the surface of the tyre or semi-finished product; detecting the optical intensity of the light radiation reflected by the surface portion on a number N (with N equal to 1 , preferably, or 2 or 3) of pairs of polarisation states belonging to a group of three pairs of polarisation states corresponding to three pairs of points on Poincare sphere, the three pairs of points being located at the opposite ends of three respective diameters of the Poincare sphere orthogonal to each other; calculating, on the surface portion, a number M (with M equal to 1 , preferably, or 2 or 3) of values representative of an equivalent number of components of the Stokes vector belonging to a subgroup of three components of the Stokes vector comprising a second component, Q, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the first pair of polarisation states, a third component, U, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the second pair of polarisation states and a fourth component, V, equal to the, or a function of the, difference of the two optical intensities of the light radiation reflected by the surface portion on the third pair of polarisation states; generating, for the surface portion, a respective control signal representative of each value representative of the components of the

Stokes vector and analysing such respective control signal on the surface portion for detecting the possible presence of surface flaws.

As a function of the control signal generated for each tyre or semi-finished product, the control station 20 establishes how to direct the n tyres or semi-finished products received in inlet:

- k tyres or semi-finished products (with k<=n) are re-introduced in the production flow (e.g. they are sent to further test and/or verification stations or subjected to labelling, storing operations etc.), since they are deemed suitable;

- n-k tyres or semi-finished products are instead forwarded outside the production line, since at least preliminarily they were deemed unsuitable. Preferably the re-introduction of the k tyres or semi-finished products and the forwarding of the n-k tyres or semi-finished products is carried out in a time equal to the abovementioned predefined time range.

Preferably the n-k tyres forwarded outside the production flow are sent to a verification station 40 (outside the production line), where more in-depth analyses can be carried out to verify if the tyres must actually be definitively discarded. The production process consequently provides for a further verification operation 240 (carried out in the verification station 40) subsequent to the control operation only carried out on the n-k tyres or semi-finished products at least preliminarily deemed to be unsuitable.

The control station 20 comprises at least one apparatus 100 for controlling tyres in a production line according to the method of the present invention. The apparatus 100 comprises a member 1 10 for moving a tyre 101 (shown in a dashed line in figure 2) having a horizontal support plane 120, such member adapted to rotate around an axis perpendicular to itself (in fig.2 perpendicular to the plane of the sheet), such axis coinciding with the cylindrical symmetry axis of the tyre.

The apparatus 100 further comprises a source 130 adapted to emit a light radiation for lighting a surface portion of the tyre or semi-finished product supported on the support plane, and, at a distance from the source, a detection system 140 adapted to detect the optical intensity of the light radiation reflected by the surface portion on a number N, with N ranging from 1 to 3, of pairs of polarisation states belonging to a group of three pairs of polarisation states corresponding to three pairs of points on Poincare sphere, the three pairs of points being located at the opposite ends of three respective diameters of the Poincare sphere orthogonal to each other.

In the example schematically shown in figure 2, corresponding to a number N equal to 1 , the detection system comprises two digital cameras 141 , each having a digital sensor adapted to detect the reflected light radiation, and each camera associated with a polariser 142 according to the present invention arranged upstream of the respective camera. The source 130 comprises, for example, two light sources 131 each associated with a respective camera. The two cameras are for example diametrically opposed with respect to the tyre. For N equal to 2, a possible solution (not shown) provides for two further cameras, each with the respective polariser and the respective light source, diametrically opposite each other along a diameter (i.e. in figure 2 vertically arranged) orthogonal to that (horizontal in figure 2) of the two cameras 141 shown in figure 2. For N equal to 3 (not shown), the six overall cameras can be distributed around the circumference of the tyre in a manner such that the angle subtended at the centre between the two adjacent cameras is equal to 60°. Typically each group formed by a camera with the respective polariser and the respective light source can be moved in space in a manner so as to be able to sequentially control both the surface of the tread and that of the accessible sidewalk

In the example schematically shown in figure 3, the apparatus 100 comprises a robotised arm 102 at whose free end the source 130 and the detection system 140 (represented only schematically) are rigidly mounted. For example, assuming N equal to 1 , when subjected to one and only light source 131 the detection system 140 can comprise two cameras mounted side-by-side on the robotised arm with their respective polariser, or only one camera and, placed upstream before the same, a member (not shown) on which the two polarisers are mounted, such member configured for positioning only one polariser at a time in front of the camera. The apparatus 100 further comprises a processing unit 150 adapted to receive a detection signal from the detection system, and configured for carrying out the abovementioned operations.

In operation, a tyre 101 is supported on the support plane 120 and subjected to a first complete rotation around the symmetry axis thereof in order to control the entire perimeter extension of the surface of the tread, and (possibly upon suitable repositioning of the light sources and the detection system) subjected to a second complete rotation around the symmetry axis thereof in order to control the entire perimeter extension of the surface of the accessible sidewall (the sidewall turned upward).

As shown in figure 4, the control station 20 for the tyres can comprise a pair of the aforesaid apparatuses 100 for controlling tyres in a production line and, interposed between the two apparatuses with reference to the production flow, a machine 50 for tilting the tyre around an axis orthogonal to the cylindrical symmetry axis of the tyre. In such a manner, it is possible to control the two sidewalls of the tyre respectively in the two apparatuses. It is observed that in a single control station 20, multiple apparatuses can also be present in accordance with the present invention and/or multiple pairs of apparatuses with a tilting machine interposed (which can be shared by multiple pairs of apparatuses). The flow of tyres or semi-finished products to be inspected can then be subdivided between the different control apparatuses.

Figures 5 to 12 show an example of implementation of the method in accordance with the present invention. Figure 5 shows a normal visible- light photograph in grey tones and without any polarisation filter of a surface portion of a tyre, on which a foreign metallic body D has been intentionally included. The surface portion is lit with white light. The axis of the objective of the camera is normal to the plane tangent to the envelope of the surface of the tread at the centre of the tread. The angle between the axis of the objective and the direction of propagation of the radiation incident on the surface portion is equal to about 90°. Figures 6, 7 and 8 show the respective image in black and white of the surface portion substantially coinciding with that of figure 5 obtained by associating each discrete point of the surface with a value representative respectively of the second, third and fourth Stokes component, Q, U and V, and after thresholding on the grey tones. The reference system for the main polarisation states is oriented with the horizontal and vertical linear polarisation axes, respectively parallel to the horizontal axis and vertical axis of the figures 6-12.

Figures 9-12 show the respective image in black and white of the same surface portion obtained by associating each discrete point of the surface with a value representative respectively of the degree of linear polarisation, DOLP, of the degree of polarisation, DOP, of the degree of circular polarisation, DOCP and of the polarisation angle, PSI, and after suitable thresholding on the grey tones.

As is seen, the flaw D clearly stands out in all the images, in particular in those of figures 6, 9, 10 and 12.

It is observed that the images, and/or for even more reason the respective detection signals, can be subjected to suitable processing algorithms (such as the algorithms that actuate a median filter or those known as "blob detection", which are able to recognise and isolate parts of the image with high pixel densities) in order to distinguish the flaw from the rest of the image.