WO2001025761A1

WO2001025761A1 - Improved opto-electronic apparatus for the automatic inspection of preforms

Info

Publication number: WO2001025761A1
Application number: PCT/EP2000/008738
Authority: WO
Inventors: Corrado Marchetti; Stefano Rinaldin
Original assignee: Sipa S.P.A.
Priority date: 1999-10-05
Filing date: 2000-09-07
Publication date: 2001-04-12
Also published as: IT1311712B1; ITPN990076A1; ITPN990076A0

Abstract

Apparatus for the optical inspection of transparent preforms, and comprising illuminating means, means for detecting luminous images and generating electric signals corresponding to such images, as well as means for processing and comparing these electric signals, in which said means for detecting luminous images are constituted by a single image pick-up means and, as it moves between said illuminating means and said image pick-up means, each one of said transparent products is picked up, ie. shot in a plurality of distinct orientations by said single image pick-up means. To each one of said distinct orientations corresponds a distinct different position of the respective preform between said image pick-up means and said illuminating means.

Description

IMPROVED OPTO ELECTRONIC APPARATUS FOR THE

AUTOMATIC INSPECTION OF PREFORMS

DESCRIPTION

The present invention refers to an improved apparatus for inspecting, during the utilization thereof on a large-scale, semi-finished transparent or opaque products, known in the art as preforms or parisons, made of thermoplastic resin such as in particular polyethylene terephtalate (PET) and polypropylene (PP), which are usually intended for subsequent blow- moulding into the final product, for a possible presence of defects of various nature and kind.

In particular, the present invention can be used in a most advantageous manner when the described apparatus is associated to or preferably integrated in a plant for manufacturing plastic containers, especially bottles, which is possibly equipped with own means for producing the respective preforms of a transparent type, known in the art also as single-stage plant.

Since the preforms are intended to be eventually blow moulded and, therefore, expanded at a temperature of approx. 100°C and more, almost any defect that is introduced in their body during the production thereof, such as in particular inclusions, blisters and pockets (even very small moulding into the cause of corresponding defects that are of course considerably amplified owing to the high stretching ratio which the part is subject to during the process, and that usually make the final container as good as practically useless, so that it must be rejected with resulting economic damages.

This state of things is largely known to all those skilled in the art and is reminded here only in view of more clearly and effectively explaining the actual reasons behind the present invention.

A few optical methods adapted to check each single preform, and detect the possible presence of the afore cited defects, have therefore been devised and adopted in the art.

One of the less recent, but nevertheless quite effective methods in this connection is the one described in the patent publication JP 60049920, granted to TOPPAN PRINTING. According to such a disclosure, each preform is illuminated by a laser beam, which is reflected and picked up by an appropriate device that analyses it and, according to the characteristics thereof, generates a signal adapted to cause the same preform to be either accepted or rejected.

Such a method, however, has the twofold drawback of a considerable complexity of the detecting and analysing method and the fact that, although being particularly effective in detecting the presence of foreign matters and materials, such as for instance inclusions of charred particles, it fails to ensure a same extent of effectiveness and reliability in detecting other kinds of defects, such as for instance micro-blisters, localized anomalous stretchings of the material, and other defects that are typical of continuous plastication, ie. melting processes. an apparatus for inspecting plastic bottles by means of a television camera. But this apparatus is namely adapted to check the bottles after, and not before, final blow moulding, so that possible defective preforms, from which said bottles have been so produced, are not checked and sorted out in advance. The result is that defective bottles are ultimately produced, which, even if checked and removed from subsequent production and utilization cycles, have anyway brought about, ie. generated the related blow moulding costs, so that the circumstance of having so failed to preliminarily check and sort out the preforms is the cause of additional cost burdens and poorer manufacturing efficiencies.

In view of checking the preforms themselves, the practice is largely known of making use of apparatuses and methods that call for the use of several television cameras that are adapted to pick up, ie. shoot the body of the preform to be checked from different angles, and to compare the corresponding image, or practically the related electric signal, with the electric signal of a virtual image representing the acceptable standard for that type of preform being viewed from the viewing point of the respective television camera.

Such a technique has proven very effective in the detection of a wide range of defects, owing also to the same technique being exalted in its performance capability by the fact that, thanks to the transparency of the preform, the latter can be situated between a preferably uniform, LED- based light source and the television camera itself. In this manner, the television camera is capable of receiving the light as emitted by said light source and appropriately filtered by the transparent body of the preform, in such a manner as to ensure that any possible, even slight irregularity is exalted, ie. magnified to a considerable extent, so as to be able to be most easily identified and detected. television camera is far from proving sufficient to the purpose, since only the front and rear portions of the body of the preform can actually be examined effectively with such a single camera, whereas the side portions remain practically excluded, owing to the fact that the light falling upon them is in practice obscured by the greater thickness thereof, as the latter is of course seen by the television camera itself.

In view of doing away with such a drawback, the solution has therefore developed into a common practice consisting in inspecting the body of the preform from a number of different, distinct angles, in such a manner as to ensure that the whole body of the preform is practically shot, ie. picked up by the television camera in distinct portions thereof, and examined separately.

This method, however, gives rise to two kinds of problems: the first one derives from the fact that the installation of a plurality of television cameras in a radial arrangement, each one of them associated to a respective light source, demands a lot of space that is not always available in complex production plants of this kind, considering also the fact that, owing to easily understandable reasons, these apparatuses should be as compact as possible.

The problems of the second kind arise on the contrary from the fact that the presence of illumination sources arranged in a circle-like manner, ie. radially around the preform gives rise to undesired optical effects, such as in particular refraction zones, on the television camera that is not arranged exactly in front of said sources. Now, these effects may disguise or conceal really existing defects, while mere optical effects that are neither predictable nor eliminable can on the other hand be detected and erroneously recognized as actual defects existing in the preform. the present invention to provide an apparatus for the optical inspection of preforms in a continuous production line, which is free of the two kinds of problems described above, is adapted to operate in a substantially continuous and automatic manner in the plants in which it is used, without any interposition of non-automatic phases or phases that are such as to interrupt the continuity of the flow of preforms. Furthermore. This apparatus must be capable of being easily implemented using readily available, existing manufacturing techniques and, therefore, is reasonably low-cost.

Such an main of the present invention, along with further features thereof, is reached in an apparatus that is made and operates as recited in the appended claims.

The present invention may take the form of a preferred, although not sole embodiment such as the one that is described in detail below by way of non-limiting example with reference to the accompanying drawings, in which:

- Figure 1 is an overall perspective side view of an apparatus adapted to make use of the present invention;

- Figure 2 is a top view of the same apparatus;

- Figure 3 is an enlarged view of the central portion of Figure 2;

- Figure 4 is the same view as the one illustrated in Figure 3, but with the apparatus shown in a different operating condition;

- Figure 5 is a vertical plane view of a portion of a preform conveying line according to the present invention; line taken from the A- A section of Figure 5.

Reference is therefore made to the above listed Figures that represent an apparatus for the optical inspection of at least partially transparent products, such as in particular preforms of plastic material, and comprising:

- illuminating means 1 for illuminating said preforms 2,

- pick-up means 3 for picking up luminous images, such as television cameras or the like, arranged on the opposite side of said illuminating means 1 with respect to said preforms, adapted to generate respective electric signals corresponding to said luminous images,

- conveying means 4 capable of ensuring the transfer and passage in an orderly flow of a sequence of said preforms between said illuminating means and said pick-up means for picking up luminous images,

and further comprising, although not shown in the Figures,

- electronic memory means adapted to be loaded with and store data corresponding to electronic images of said products,

- means for processing said electric signals, adapted to further carry out a comparison between said signals and respective signals picked from said previously loaded electronic storage or memory means,

- means for selectively separating said products whenever the result of said comparison fails to fall within pre-determined values.

The present invention is based on the contrivance consisting in replacing: means and a therewith associated image pick-up means (which may in particular be constituted by a television camera), arranged on the opposite sides of the preform to be inspected

- with a single light source and related television camera, in which the preform is caused to move forward with continuous motion into a plurality of distinct successive positions x, a, b, c.

With reference to Figure 3, the same can be noticed to represent four distinct positions taken in respective successive moments by the same generally illustrated preform. Anyway, since the same preforms are aligned regularly along said conveying means 4, Figure 3 itself also illustrates, at the same time, the arrangement of four different successive preforms relative to the apparatus according to the present invention.

The same can be said as far as Figure 4 is concerned. In particular, when comparatively looking at said two Figures 3 and 4 it can be noticed that:

- the preform 2, which occupied the position x in Figure 3, occupies the position a in Figure 4;

- the preform 12, which occupied the position a in Figure 3, occupies the position b in Figure 4;

- the preform 22, which occupied the position b in Figure 3, occupies the position c in Figure 4;

- the preform 32, which occupied the position c in Figure 3, is no longer represented in Figure 4 since in the meantime it has been moved outside the circle of the same Figure. viewing the preform in said plurality of positions, and said illuminating means 1 is provided with a width dimension "V that is sufficiently large as to enable the generally represented preform 2 to practically continue to stay, in all of these positions, within the cone defined between the viewing angle "p" of the television camera and said illuminating means, so that the same television camera is able to receive the light transmitted by said source and filtered through the preform.

Furthermore, and this is an essential feature of the present invention, it is necessary for the preform, as this is symbolically illustrated in Figures 3 and 4, to exhibit its front and rear portions H and K to the television camera, which are then preferentially examined and compared by said processing means and said electronic storage memory means.

In fact, it is just and solely in these portions H, K, which are arranged in an almost orthogonal manner with respect to the axis s joining the light source with the television camera (see Figure 4), that two particularly favourable conditions for optical inspection purposes take place, ie.:

- the light is filtered to a reduced extent, and

- the light is refracted to a reduced extent.

As a result, if an optically detectable defect happens to lie or be situated in such portions, the optimum condition to detect it consists in examining it in the direction of the light/ camera axis.

As a matter of fact, should a defect on the contrary happen to be situated in the side portions R, T, it clearly emerges, without any need for particular explanations to be given in this connection, that the visibility of such a defect would be greatly reduced owing to the much greater total thickness of said cylindrical-wall side portions R, T of the preform viewed picked up by the television camera.

In a few words, according to the present invention the preform is therefore examined essentially by just a single pair of devices, ie. light source and television camera. However, these means are adapted to examine said preform a plurality of times, in which such examination or inspection occurs each time according to a different orientation of the preform with respect to the light source/ television camera axis.

In this manner, at each such inspection the television camera is able to pick up respective front and rear portions H, K whose appropriately combined images are then able to form an "overall" transparency virtual image of the body of the preform.

Based on the examination of such an "overall" image, and therefore the examination of practically all images that are so picked-up by the camera, it is then possible for an optical inspection of the entire cylindrical body of the preform to be carried out by distinct portions thereof.

However, during the either single-stage or two-stage bottle manufacturing process, the related preforms are usually introduced in and aligned on the conveying means 4 which then displaces them with a continuous movement with respect to the illuminating means/ television camera assembly. As a consequence, owing to the need for the fact to be taken into due account that the preforms actually constitute a moving target, for the television camera to be able to shoot, ie. pick up each preform at different angles it will be necessary for each such preform to be shot in different positions thereof, and therefore at successive moments owing to the same preforms being a moving target, with respect to said illuminating means/ television camera assembly, in such a manner as to enable the time interval between a shooting moment and the next one, and therefore between a position of the preform and the other one, to be above described shots of the television camera to occur from several different angles.

In substance, the whole process can summarily be highlighted as being based on the fact that:

- the preforms are aligned and moving forward along a conveying means in a continuous, uniform motion;

- in a specific zone, which is situated between the illumination source and the television camera, the preforms are caused to rotate so as to take a plurality of distinct orientations; and

- in each one of such plurality of distinct orientations thereof, each preform is picked up, ie. shot by the television camera, wherein the same preform offers respectively different sides to the view of the television camera shooting it.

The images that are so picked up at each shot of the television camera generate respective electric signals which are then compared with the data relating to corresponding virtual images stored in said electronic storage memory means. Should at least one of all said comparisons end with a positive outcome, ie. brings to light deviations in excess of pre-determined levels or extents, a cut-off signal is released which controls appropriate actuation members adapted to shut out, ie. exclude the respective preforms from said preform conveying means.

Anyway, both the signal storage, comparison and processing techniques and the means used to remove the non-complying preforms from the conveying means are generally known in the art and are not a part of the present invention.

in inspection of the substantially transparent preforms without any need for redundant and, therefore, excessive examinations to be carried out, it proves sufficient for three pick-up shots to be made of the same preform as rotated by 60° and 120° with respect to the axis t joining the television camera with the axis O of the preform in the initial shooting or pick-up position thereof (Figure 3).

It can quite readily be appreciated that such a rotation of the preform into the two subsequent shooting or pick-up positions shall not necessarily be of precisely 60° and 120°, respectively, since it must be duly taken into account that the displacement of the preform from a shooting position to the next one is such as to inherently lead to an autonomous different orientation with respect to the television camera and, as a result, such a displacement also brings a respective rotation with respect to the television camera with it. However, those skilled in the art are fully capable of easily taking into account such a displacement- induced rotation in its different consequences from both a mechanical and electronic-processing point of view.

As far as other types of preforms are concerned, such as the opaque or substantially opaque ones, so that it is not possible for the image thereof to be examined in transparency even in the case that an intense light source were used, it is anyway possible for images of an adequate number of respective portions of the outer surface to be picked up, in such a manner as to be able to anyway compare the various images so obtained with respective virtual images stored in said electronic storage memory means.

In this particular case it has been found and confirmed experimentally that, for an optimum inspection of the substantially opaque preforms without any need for an excessive number of different images to be picked up, it proves sufficient for three pick-up shots to be made of the same television camera with the axis O of the preform in the initial shooting or pick-up position thereof (Figure 3).

As far as the means are concerned which are needed to cause the preform to rotate by the desired angle along the path of the conveyor, various technical solutions are available. The preferred solution is anyway the one represented in the Figures, which illustrate how appropriate portions F of the preforms, preferably situated near the respective neck ring thereof, are placed and arranged on said transfer or conveying means 4 in such a manner as to be touched by appropriate contact members 16, 17 that are firmly joined with the television camera. Since the preforms are supported on said transfer and conveying means 4 in such a manner as to be able to rotate about their own axis, the contact and resulting friction between said portions F of each preform and said appropriately sized and arranged contact members cause practically the preform to rotate, at two successive moments, by two respective angles, preferably of 60°, as duly corrected to take the afore explained displacement-induced rotation effect into account. The preform is therefore picked up by the same television camera in three successive orientations thereof, ie. in a first reference orientation, eg. in the position indicated at "a" in Figure 4, and then in the successive positions "b" and "c", in which it offers to the view of the television camera sides that are rotated by 60° and 120° altogether.

Claims

1. Apparatus for the optical inspection of at least partially transparent products (2), such as in particular preforms of plastic material, and comprising:

- illuminating means for said transparent products, adapted to illuminate respective ones of said products,

- pick-up means for picking up luminous images, such as television cameras or the like, arranged on the opposite side of said illuminating means with respect to said respective products, and adapted to generate respective electric signals corresponding to said luminous images,

- electronic storage memory means adapted to be loaded with and store data corresponding to electronic images of said products,

- means for processing said electric signals, and adapted to further carry out the comparison between said signals and respective signals picked from said previously loaded electronic storage memory means,

- means for selectively separating said products whenever the outcome of said comparison fails to fall within pre-determined values.

- conveying means capable of ensuring the transfer and passage in an orderly flow of a sequence of said products between said illuminating means and said pick-up means for picking up luminous images, in which transparent products move individually between said illuminating means and said pick-up means for picking up luminous images, characterized in that - said means for picking up images are constituted by a single image pick-up means (3),

- during the passage between said illuminating means (1) and said image pick-up means (3), each one of said transparent products (2) is individually shot in transparency, in a plurality of distinct orientations thereof, by said single image pick-up means (3).

2. Apparatus according to the preamble of claim 1, in which said products (2) are on the contrary opaque, characterized in that:

. - said means for picking up images are constituted by a single image pick-up means (3),

- during the passage between said illuminating means ( 1) and said image pick-up means (3), each one of said transparent products (2) is individually shot in a plurality of distinct orientations thereof by said single image pick-up means (3).

3. Apparatus according to claim 1 or 2, characterized in that to each one of said distinct shooting or pick-up orientations of the product there corresponds a distinct different position (x, a, b. c) of the respective product (2) between said image pick-up means (3) and said illuminating means.

4. Apparatus according to claim 3, characterized in that there are provided means (4) adapted to conveying said products (2) in said respective (pick-up) positions thereof with a continuous, uniform motion.

5. Apparatus according to claim 3 or 4, characterized in that there are provided further contact means ( 15, 16) adapted to cause said each single product (2) to rotate by a pre-determined angle as it is conveyed each one of said products is shot individually, and successively in said distinct positions thereof, after it having been caused to rotate by predetermined angles.

6. Apparatus according to any of the preceding claims 1, 3, 4 or 5, characterized in that said distinct different (pick-up) positions are in the number of three, and to each one of these positions there correspond three respective orientations that are mutually displaced by an angle of 60° between said image pick-up means and the axis of the preform.

7. Apparatus according to any of the preceding claims 2, 3, 4 or 5, characterized in that said distinct different (pick-up) positions are in the number of three, and to each one of these positions there correspond three respective orientations that are mutually displaced by an angle of 120° between said image pick-up means and the axis of the preform.

8. Apparatus according to any of the preceding claims, characterized in that said contact means (15, 16) are firmly joined with said image pick- up means and are adapted to successively come into contact with appropriate portions (F) of the outer surface of said respective products (2) in such a manner as to induce a rotation by rolling friction on the products themselves.

9. Apparatus according to any of the preceding claims, characterized in that said processing means for said electric signals are adapted to compare the electric signals relating to the images picked up in said distinct positions with respective images that are stored in said electronic storage memory means.