EP4786389A1

EP4786389A1 - Method for monitoring the quantity of packaging material in a buffer device of a packaging machine for producing sealed packages containing a pourable product and packaging machine

Info

Publication number: EP4786389A1
Application number: EP26152987.9A
Authority: EP
Inventors: Fabio GIOVANARDI; Davide SANTINI; Angelo Verardi; Armando STAGLIANO
Original assignee: Tetra Laval Holdings and Finance SA
Current assignee: Tetra Laval Holdings and Finance SA
Priority date: 2025-01-30
Filing date: 2026-01-20
Publication date: 2026-08-05
Also published as: WO2026162334A1

Abstract

There is described a method for monitoring the quantity of packaging material accumulating in a buffer device (10) of a packaging material processing unit, the method comprises the steps of:
- detecting a distance between a predetermined section (S) of a web (4) of packaging material loop (11) in the buffer device (10) during the accumulation therein and a fixed reference point of the buffer device (10);
- receiving a first sensor signal (S1) indicative of said distance;
- calculating the quantity of packaging material accumulated in the buffer device (10) as a function of the first sensor signal (S1).

Description

TECHNICAL FIELD

The present invention relates to a method for monitoring the quantity of packaging material accumulating in a buffer device of a packaging machine for producing sealed packages containing a pourable product.
The present invention also relates to a packaging machine for producing sealed packages containing a pourable product.

BACKGROUND ART

As it is generally known, many pourable food products, such as fruit juice, UHT (ultra-high temperature-treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example is the parallelepiped-shaped package for pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing a laminated web of packaging material.
The packaging material has a multilayer structure comprising a base layer, e.g. made of paper, covered on both sides with layers of heat-sealable material, e.g. polyethylene.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material, e.g. an aluminum foil, which is superimposed on a layer of heat-sealable material, and is in turn covered with another layer of heat-sealable material forming the inner face of the package eventually contacting the food product.
Packages of this sort are normally produced on fully automatic packaging machines, which form and fill the packages starting from a multilayer web of packaging material.
In particular, according to a non-limiting example of a packaging machine, a continuous tube is formed from the web of packaging material which is initially wound in a reel and fed through a plurality of unwinding rollers along an advancing path.
The web of packaging material is typically sterilized, e.g. by applying a chemical sterilizing agent, such as hydrogen peroxide solution, which, once the sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating.
The sterilized web is then maintained in a closed, sterile environment, and, while advanced by the aforementioned unwinding rollers along the advancing path, is folded to form the tube by means of a known web folding unit and then sealed longitudinally.
In order to form the tube, the sterilized web of packaging material is vertically fed through a number of folding devices, which are part of said web folding unit and which sequentially interact with the packaging material to fold it gradually from a sheet-like configuration into a tube shape.
More specifically, the folding devices are arranged in succession, along a tube-forming path, and comprise respective folding members, in particular rollers, arranged to define a series of passages for the packaging material varying gradually in section from a C-shape to a substantially circular shape.
In order to perform the package forming operations, the tube is continuously fed along a straight vertical direction, is filled with the sterilized food product from above and is formed, sealed and subsequently cut along equally spaced transversal cross-sections extending along a direction orthogonal to the vertical direction, according to a manner known and not described in detail.
So-called pillow packs are obtained thereby, which have a longitudinal sealing band, a top transversal sealing band and a bottom transversal sealing band. The pillow packs are then cut at the cross-sections to be separated from one another and directed to a folding unit of the packaging machine for the final folding thereof.
The finished packages are thereby obtained.
To open the sealed packages described above, i.e. to allow access to the pourable product therein, various configurations of opening devices have been proposed.
According to a known solution, the web is initially provided with holes, or prelaminated holes (i.e. holes formed through the base layer only and covered by the aforementioned lamination layers and/or barrier layer and defining a pierceable or removable portion of the packaging material), and opening devices which are applied, at an application station, at such holes or prelaminated holes, respectively.
In greater detail, each hole or prelaminated hole defines a predetermined area of the web at which an opening device must be applied.
Packaging machines of the above-described type typically comprise units which operate continuously, i.e. through which the packaging material is advanced in a continuous manner, and units which operate intermittently, i.e. through which the packaging material is advanced in an intermittent manner or stepwise (that is, at substantially regular intervals of advancement followed and preceded by intervals of stoppage).
For example, one of such units that operate intermittently is defined by an injection molding station at which each opening device is applied at the respective predetermined area of the web, by injecting plastic material thereat.
In use, the web is unwound from the aforementioned reel. Subsequently, the web is stepwise fed to the injection molding station before the packaging material is folded to form the tube. In particular, the web is fed towards the injection molding station along an advancing path (or advancing direction).
EP-A-2357138 , in the name of the same Applicant, discloses an application unit configured for applying opening devices onto respective pre-laminated holes at the injection molding station.
Typically, an application unit of the aforementioned type includes an infeed buffer device and an outfeed buffer device.
Each buffer device is configured to accumulate (and temporarily store) a certain quantity of packaging material in a respective loop.
In this way, a correct operative interaction between the units operating intermittently (such as the application unit) and the units operating continuously is ensured.
It is therefore of the utmost importance that the buffer devices contain the desired amount of packaging material, to ensure continuity of the packaging process.
A proposed solution for monitoring the quantity of packaging material within each buffer device entails the use of photocells: a pair of photocells is arranged at respective heights within the buffer device so that each photocell is located at a predetermined height level of the loop accumulating within the respective buffer device.
The quantity of packaging material in the loop is then estimated as a function of the signal received by the photocells.
Although working satisfyingly well, the Applicant has observed that the above packaging machines are still open to further improvement. In particular, a desire is felt for improving the precision and effectiveness of the control and monitoring of the quantity of packaging material accumulating in the buffer devices.

DISCLOSURE OF INVENTION

It is therefore an object of the present invention to provide a method for monitoring the quantity of packaging material accumulating in a buffer device which is designed to meet the above-mentioned need in a straightforward and low-cost manner.
This object is achieved by a method for monitoring the quantity of packaging material accumulating in a buffer device as claimed in the appended independent claim 1.
It is a further object of the present invention to provide a computer program product configured, when executed, to carry out the steps of the method for monitoring the quantity of packaging material accumulating in a buffer device. This object is achieved by a computer program product as claimed in the appended independent claim 8.
It is a further object of the present invention to provide a packaging machine for producing sealed packages which is designed to meet the above-mentioned need in a straightforward and low-cost manner.
This object is achieved by a packaging machine as claimed in the appended independent claim 9.
Preferred embodiments of the present invention are laid down in the appended dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a schematic perspective view, with parts removed for clarity, of a packaging machine configured to produce sealed packages containing a pourable product;
Figures 2a and 2b are larger-scale, schematic lateral views, with parts removed for clarity, of a buffer device of the packaging machine of Figure 1, during two distinct operative conditions.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to Figure 1, number 1 indicates as a whole a non-limiting example of a packaging machine configured to produce sealed packages 2 containing a pourable product, preferably a pourable food product such as pasteurized or UHT milk, water, fruit juice, wine, peas, beans, etc.
Packaging machine 1 is configured to form, seal and fold packages 2 starting from a web 4 of packaging material, which is initially wound in a reel 4a, and then folded into a tube 3 of packaging material.
Preferably, the packaging material has a multilayer structure (not shown), and comprises a layer of fibrous material, e.g. paper, covered on both sides with respective layers of heat-sealable material, e.g. polyethylene.
In the case of aseptic packages 2 for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas-and-light barrier material, e.g. aluminum foil or ethylene vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-sealable material, and is in turn covered with another layer of heat-sealable material, the latter forming the inner face of package 2 eventually contacting the pourable product.
Packaging machine 1 comprises a conveyor configured to advance web 4 along an advancing path P.
In one embodiment, the conveyor comprises a plurality of unwinding rollers, as schematically shown in Figure 1.
Packaging machine 1 further comprises:

a folding device 5 configured to progressively fold web 4 into a tube 3, in a manner known and not described in detail;
a sealing element 40 for longitudinally sealing tube 3, in a manner known and not described in detail;
a forming and sealing unit 6 configured to form tube 3 and to seal tube 3 along successive transversal cross-sections thereof in order to obtain a sequence of pillow packs 2a from which packages 2 are subsequently obtained;
a filling device 50 for filling tube 3 (from above) with the pourable product.

In detail, the forming and sealing unit 6 is configured to sequentially impart a predetermined external shape, corresponding to a precursor of the shape of package 2, to successive longitudinal portions of tube 3, and to transversally seal tube 3 at equally spaced cross-sections.
Conveniently, packaging machine 1 further comprises:

an isolation chamber (known per se and not shown) internally defining an environment containing a controlled atmosphere, in particular sterile and/or aseptic gas, preferably sterile and/or aseptic air, and housing at least the forming and sealing unit 6, the tube folding device 5, the sealing element and at least part of the filling device 50;
a folding unit (not shown) configured to sequentially fold packs 2a in order to obtain a plurality of sealed packages 2.

Preferably, forming and sealing unit 6 has an axis X along which tube 3 is fed, in use.
Axis X is parallel to a straight direction, which preferably is a straight vertical direction.
Hence, in use, tube 3 is fed along axis X, downwards, and while being filled from above is formed and sealed by forming and sealing unit 6.
More specifically, tube 3 is drawn (downwards) along axis X by forming and sealing unit 6 in a known manner.
In this way, a plurality of pillow packs 2a are obtained.
In light of the above, packaging machine 1 is configured to form and seal a plurality of pillow packs 2a containing the pourable product starting from web 4 and tube 3 and then to fold the pillow packs 2a for obtaining the aforementioned formed, sealed and folded packages 2 containing the pourable product.
In light of the above, packaging machine 1 comprises a plurality of processing stations (the tube folding device 5, the forming and sealing unit 6, etc.) which are configured to carry out a specific operative process on the web 4 of packaging material.
The processing stations described above are configured to process the packaging material in a continuous manner. That is, the packaging material is fed, in use, through such processing stations in a continuous manner.
Packaging machine 1 further comprises an application unit 7 configured for applying opening devices onto the web 4, at respective predetermined areas thereof.
In particular, the web 4 is initially provided with holes, or prelaminated holes (i.e. holes formed through the base layer only and covered by the aforementioned lamination layers and/or barrier layer and defining a pierceable or removable portion of the packaging material), each defining one of such predetermined areas.
Application unit 7 is configured to apply opening devices to each of such prelaminated holes.
More precisely, application unit 7 is configured to apply a pouring neck portion of the opening devices, whilst a lid or cap portion of the opening devices is preferably applied after the packages 2 are fully formed, filled and folded, according to a manner known and not described in detail.
To this end, application unit 7 comprises a series of injection molding devices 8 each configured to inject plastic material directly at the respective prelaminated hole, sequentially.
Expediently, application unit 7 is arranged along path P upstream of the tube folding device 5.
In use, the web 4 is unwound from the aforementioned reel 4a. Subsequently, the web 4 is stepwise fed to application unit 7 before the packaging material is folded to form the tube 3.
Thus, the application of opening devices on the web 4 requires that the predetermined areas are temporarily stopped in respective nominal positions with respect to the injection molding devices 8, for a correct application of each opening device.
Hence, application unit 7 defines a processing station of packaging machine 1 which is configured to operate in an intermittent manner. That is, the web 4 of packaging material is fed, in use, through the application unit 7 stepwise, i.e. in an intermittent manner.
An application unit 7 of the above type is known from EP-A-2357138 , in the name of the same Applicant.
In order to ensure a smooth operation between first processing stations which operate in a continuous manner and second processing stations which operate in an intermittent manner, packaging machine 1 further comprises at least one buffer device 10 operatively interposed between a first processing station and a second processing station.
For example, as shown in Figure 1, an inlet buffer device 10 is arranged upstream of the application unit 7 and an outlet buffer device 10 is arranged downstream of the application unit 7.
For the sake of simplicity, reference will be made in the following to a single buffer device 10. However, the functional and structural features described in connection with such buffer device 10 are equally applicable to all buffer devices 10 which may be present in packaging machine 1.
Buffer device 10 is configured to accumulate packaging material so as to define a loop 11 of packaging material therein. In particular, with the term "loop" it is intended a section of the web of packaging material that is designed to create slack or surplus material, e.g. immediately upstream and/or downstream of the processing station.
It is stated that with the wording "accumulate" or "accumulating" it is intended, in the present description and in the appended claims, that a quantity of packaging material is temporarily stored within the buffer device 10. In particular, such wording refers both to the increase and the decrease of packaging material in the buffer device 10, i.e. to both an increase and a decrease of the size or length of the loop 11.
This is schematically shown in Figures 2a and 2b.
Packaging machine 1 comprises a control unit 20, as schematized in Figures 2a and 2b.
Buffer device 10 comprises a first sensor 12 arranged at a fixed frame 13 of the packaging machine 1.
In particular, first sensor 12 is mounted on fixed frame 13 at a position defining a fixed reference point.
More precisely, the position of the first sensor 12 itself defines a fixed reference point of the buffer device 10. The control unit 20 may be configured to receive and store a value indicative of the position the fixed refence point.
First sensor 12 is configured to detect a distance between the fixed reference point, i.e. the position at which the first sensor 12 is arranged, and a predetermined section S of web 4. Such distance is schematically shown in Figures 2a and 2b. In one embodiment, the section S of the web 4 may exhibit a curved shape, in particular a substantially U-shape. The first sensor 12 may be configured to detect a furthest point of the section S, e.g. a bottom of the U-shaped section S.
In practice, such predetermined section S is defined by the bottom loop of the loop 11, i.e. by the curved bottom bend of the loop 11 (Figures 2a, 2b).
More precisely, the distance is measured between the fixed reference point and the inner surface of the web 4 at said curved bottom bend.
In other words, a height of the loop 11 is measured.
First sensor 12 is further configured to generate a first sensor signal S1 indicative of the measured distance.
According to an aspect of the present invention, the control unit 20 is configured to calculate a quantity of packaging material accumulated, in use, in the buffer device 10 as a function of the first sensor signal S1.
Advantageously, the first sensor 12 includes a laser device configured for emitting a laser beam and for pointing the laser beam onto the loop, i.e. the curved bottom bend, of the web defining said predetermined section S.
Advantageously, buffer device 10 comprises a second sensor 14 configured to detect an advancement of the web 4 through the buffer device 10 itself and to generate a second sensor signal S2 indicative of the measured advancement.
In particular, packaging machine 1 comprises at least one actuator 15 for advancing web 4 through the buffer device 10.
For example, the actuator 15 may be an electric motor operatively coupled to one of the unwinding rollers.
In the embodiment shown in Figures 2a-2b, the actuator 15 is coupled to an unwinding roller located inside buffer device 10, and is configured to actuate such roller in rotation about its own axis.
Packaging machine 1 further comprises the second sensor 14 associated with the actuator 15.
The second sensor 14 may comprise an encoder, a resolver and/or any type of position transducer.
According to a further aspect of the present invention, the control unit 20 is configured to calculate the quantity of packaging material accumulated, in use, in the buffer device 10 as a function of the first sensor signal S1 and of the second sensor signal S2.
In particular, the control unit 20 is configured to receive the first sensor signal S1 and the second sensor signal S2 and to perform a merging of the two signals, so as to output a resulting first sensor signal S1 which is "cleaned" or "smoothened" by the second sensor signal S2.
In practice, the control unit 20 is configured to filter the first sensor signal S1 as a function of the second sensor signal S2.
Advantageously, the present solution provides a more accurate monitoring during transients, e.g. when the buffer device 10 is rapidly filled or discharged. The Applicant has observed that in this way the reading and detection output by the first sensor 12 (i.e. the first sensor signal S1) is significantly improved when combined (filtered, smoothened, cleaned) with the second sensor signal S2.
In detail, the resulting signal is substantially cleaned of any signal imperfection that may occur in the first sensor signal S1 due to the stepwise or intermittent feeding of the web through application unit 7: in fact, the sudden advancement of the web 4 after a period of stoppage may cause waves that propagate through the web 4, thereby propagating along the loop 11 and causing imperfections in the reading of the first sensor 12.
The above-mentioned filtering performed by the control unit highly limits such imperfections.
In this way, a more precise estimate of the quantity of packaging material within the buffer device 10 can be obtained, thereby improving the precision and effectiveness of such control and monitoring of the packaging material accumulating in the buffer device 10.
As visible in Figures 2a and 2b, buffer device 10 comprises an accumulation chamber 16 which is delimited at least partially by lateral walls 16a and by a bottom wall 17.
Advantageously, buffer device 10 further comprises a third sensor 18, or presence sensor, configured to detect the presence of predetermined section S of web 4 at bottom wall 17.
Furthermore, third sensor 18 is configured to generate a third sensor signal S3, or presence sensor signal, indicative of the presence or absence of the loop.
According to a preferred embodiment of the invention, buffer device 10 comprises a photocell arranged at bottom wall 17 and defining the third sensor 18.
More precisely, photocell 18 is arranged (slightly) above bottom wall 17, in such a way that if an extremity of the loop 11 defining the predetermined section S reaches substantially the bottom wall 17, during the accumulation of packaging material within buffer device 10, its presence is detected by photocell 18. The extremity of the loop 11 may be defined by the bottom curved portion of the loop 11.
According to another aspect of the present invention, the control unit 20 is configured to calculate the quantity of packaging material accumulated, in use, in the buffer device 10 also as a function of the third sensor signal S3, or presence sensor signal.
In use, if the aforementioned bottom or extremity of the loop 11 reaches the bottom wall 17 of accumulation chamber 16, wrinkles in the web 4 may form that can interfere with the reading of the first sensor 12. In fact, web 4 continuing to accumulate in the accumulation chamber 16 may cause the loop 11 to rise with respect to bottom wall 17, which would in turn cause a false detection of decreasing of the quantity of packaging material present in the accumulation chamber (because the detected distance between the fixed reference point and the predetermined section S would decrease).
To this end, the control unit 20 is advantageously configured to:

calculate the quantity of packaging material accumulated in the buffer device 10 as a function of both the first sensor signal S1 and the second sensor signal S2, if the third sensor signal S3 corresponds to no presence of the predetermined section S of the web 4 of packaging material at the bottom wall 17; and
calculate the quantity of packaging material accumulated in the buffer device 10 as a function of only the second sensor signal S2, if the third sensor signal S3 corresponds to a presence of the predetermined section S of the web 4 of packaging material at the bottom wall 17.

Hence, by merging the third sensor signal S3 with the first sensor signal, and possibly also with the second sensor signal S2, the precision, reliability and effectiveness of the measure of the quantity of packaging material accumulating within buffer device 10 are further improved.
From the foregoing, it becomes clear that the packaging machine 1 according to the present invention allows to implement a method for monitoring the quantity of packaging material accumulating in a buffer device 10 of a packaging material processing station, the method comprising the steps of:

detecting a distance between a predetermined section S of a web 4 of packaging material loop in the buffer device 10 during the accumulation therein and a fixed reference point of the buffer device 10;
receiving a first sensor signal S1 indicative of said distance;
calculating the quantity of packaging material accumulated in the buffer device 10 as a function of the first sensor signal S1.

Preferably, the method further comprises the steps of:

detecting the advancement of the web 4 of packaging material through the processing station during its accumulation in the buffer device 10;
receiving a second sensor signal S2 indicative of said advancement;
calculating the quantity of packaging material accumulated in the buffer device 10 as a function of the first sensor signal S1 and of the second sensor signal S2.

Preferably, the method further comprises the step of:

filtering the first sensor signal S1 as a function of the second sensor signal S2, for smoothing a noise in the first sensor signal S1 with the second sensor signal S2.

Preferably, the step of detecting said distance is carried out by means of a laser device 12 and by pointing a laser beam onto a loop of the web 4 of packaging material defining said predetermined section S.
Preferably, the step of detecting said advancement is carried out by means of an encoder 14.
Conveniently, the method comprises the steps of:

detecting the presence of said predetermined section S of the web 4 of packaging material at the aforementioned bottom wall 17;
receiving a third sensor signal S3 indicative of said presence;
calculating the quantity of packaging material accumulated in the buffer device 10 also as a function of the third sensor signal S3.

Advantageously, the method comprises the steps of:

calculating the quantity of packaging material accumulated in the buffer device 10 as a function of both the first sensor signal S1 and the second sensor signal S2, if the third sensor signal S3 corresponds to no presence of the predetermined section S of the web 4 of packaging material at the bottom wall 17;
calculating the quantity of packaging material accumulated in the buffer device 10 as a function of only the second sensor signal S2, if the third sensor signal S3 corresponds to a presence of the predetermined section S of the web 4 of packaging material at the bottom wall 17.

Preferably, the step of detecting said presence is carried out by means of a photocell 18 arranged at said bottom wall 17.
It is further clear that the packaging machine 1 according to the present invention allows for implementing a method for producing sealed packages 2 containing a pourable product starting from a web 4 of packaging material, the method comprising the steps of:

advancing the web 4 along a path P, along which a buffer device 10 is arranged;
feeding the web 4 of packaging material to the buffer device 10 for accumulating a quantity of packaging material within the buffer device 10, so as to define a loop 11 of packaging material;
preferably, withdrawing the packaging material from the buffer device 10;
monitoring the quantity of packaging material accumulated in the loop 11 within the buffer device 10 by carrying out a method as previously described.

It is further clear that the packaging machine 1 according to the present invention, and particularly its control unit 20, allows for implementing a computer program product loadable in one or more memories of at least one control unit and comprising software code portions comprising instructions which, when executed, cause the at least one control unit to perform the steps of the method for producing sealed packages or of the method of controlling and monitoring the quantity of packaging material accumulating in a buffer device as described above.
The advantages of the method and of the packaging machine 1 according to the present invention will be clear from the foregoing description.
In particular, thanks to the above configuration, the reading and detection (i.e. the first sensor signal S1) output by the first sensor 12 is significantly improved when combined (filtered, smoothened, cleaned) by the second sensor signal S2.
In detail, the resulting signal is cleaned of any signal imperfection that may occur in the first sensor signal S1 due to the stepwise or intermittent feeding of the web through application unit 7: in fact, the sudden advancement of the web 4 after a period of stoppage may cause waves that propagate through the web, thereby propagating along the loop 11 and causing imperfections in the reading of the first sensor 12.
The above-mentioned filtering performed by the control unit 20 highly limits such imperfections.
In this way, a more precise estimate of the quantity of packaging material within the buffer device 10 can be obtained, thereby improving the precision and effectiveness of such control and monitoring of the packaging material accumulating in the buffer device 10.
Clearly, changes may be made to packaging machine 1 as described herein without, however, departing from the scope of protection as defined in the accompanying claims.
In particular, buffer device 10 may be any buffer device within packaging machine 1, that is filled and emptied with packaging material, and not necessarily a buffer device for the application unit 7.
Furthermore, second sensor 14 may include more than one encoder, for example it may comprise another encoder associated with another respective unwinding roller.

Claims

Method for monitoring the quantity of a web of packaging material accumulating in a buffer device (10) of a packaging material processing station (7), the method comprising the steps of:
- detecting a distance between a predetermined section (S) of a web (4) of packaging material loop (11) in the buffer device (10) during the accumulation therein and a fixed reference point of the buffer device (10);

- receiving a first sensor signal (S1) indicative of said distance;

- calculating the quantity of packaging material accumulated in the buffer device (10) as a function of the first sensor signal (S1).
Method as claimed in claim 1, and further comprising the steps of:
- detecting, preferably by means of an encoder associated with a conveying device configured for advancing the web (4), the advancement of the web (4) of packaging material through the processing station (7);

- receiving a second sensor signal (S2) indicative of said advancement;

- calculating the quantity of packaging material accumulated in the buffer device (10) as a function of the first sensor signal (S1) and of the second sensor signal (S2).
Method as claimed in claim 2, and further comprising the step of:
- filtering the first sensor signal (S1) as a function of the second sensor signal (S2), for smoothing a noise in the first sensor signal (S1) being a result of the advancement of the web (4).
Method as claimed in any of the previous claims, wherein the step of detecting said distance is carried out by means of a laser device (12) and by pointing a laser beam towards a loop (11) of the web (4) of packaging material defining said predetermined section (S).
Method as claimed in any one of the preceding claims, wherein the packaging material is accumulated within an accumulation chamber (16) of the buffer device (10), the accumulation chamber (16) being at least partially delimited by lateral walls (16a) and by a bottom wall (17);
and wherein the method comprises the steps of:
- detecting, preferably by means of a photocell (18), the presence of said predetermined section (S) of the web (4) of packaging material at said bottom wall (17);

- receiving a third sensor signal (S3) indicative of said presence;

- calculating the quantity of packaging material accumulated in the buffer device (10) also as a function of the third sensor signal (S3).
Method as claimed in claims 2 and 5, and comprising the steps of:
- calculating the quantity of packaging material accumulated in the buffer device (10) as a function of both the first sensor signal (S1) and the second sensor signal (S2), if the third sensor signal (S3) corresponds to no presence of the predetermined section (S) of the web (4) of packaging material at the bottom wall (17);

- calculating the quantity of packaging material accumulated in the buffer device (10) as a function of only the second sensor signal (S2), if the third sensor signal (S3) corresponds to a presence of the predetermined section (S) of the web (4) of packaging material at the bottom wall (17).
Method for producing sealed packages (2) containing a pourable product starting from a web (4) of packaging material, the method comprising the steps of:
- advancing the web (4) along a path (P), along which a buffer device (10) is arranged;

- feeding the web (4) of packaging material to the buffer device (10) for accumulating a quantity of packaging material within the buffer device (10);

- monitoring the quantity of packaging material accumulated in the buffer device (10) by carrying out a method as claimed in any one of the preceding claims.
Computer program product loadable in one or more memories of at least one control unit and comprising software code portions comprising instructions which, when executed, cause the at least one control unit to perform the steps of the method as claimed in any one of the preceding claims.
Packaging machine (1) configured to produce sealed packages (2) containing a pourable product starting from a web (4) of packaging material, the packaging machine (1) comprising:
- a buffer device (10) configured to accumulate packaging material so as to define a loop (11) of packaging material therein;

- a control unit (20);
wherein the buffer device (10) comprises a first sensor (12) arranged at a fixed frame (13) of the packaging machine (1), at a position defining a fixed reference point of the buffer device (10), and configured to detect a distance between a predetermined section (S) of said web (4) of packaging material and the fixed reference point and to generate a first sensor signal (S1) indicative of said distance;
and wherein the control unit (20) is configured to calculate a quantity of packaging material accumulated, in use, in the buffer device (10) as a function of the first sensor signal (S1).
Packaging machine as claimed in claim 9, wherein the first sensor (12) includes a contactless sensor, preferably an optical or ultrasound sensor, even more preferably a laser device configured for emitting a laser beam and for pointing the laser beam onto a loop (11) of the web (4) of packaging material defining said predetermined section (S).
Packaging machine as claimed in claims 9 or 10, wherein the buffer device (10) comprises a second sensor (14) configured to detect an advancement of the web (4) of packaging material through the buffer device (10) and to generate a second sensor signal (S2) indicative of said advancement;
and wherein the control unit (20) is configured to calculate the quantity of packaging material accumulated, in use, in the buffer device (10) as a function of the first sensor signal (S1) and of the second sensor signal (S2) .
Packaging machine as claimed in claim 11, and comprising at least an actuator (15) for advancing the web (4) of packaging material through the buffer device (10), the second sensor (14) being associated with the actuator (15).
Packaging machine as claimed in any one of the claims 9 to 12, wherein the buffer device (10) comprises:
- an accumulation chamber (16) delimited at least partially by lateral walls (16a) and by a bottom wall (17);

- a presence sensor (18) configured to detect the presence of said predetermined section (S) of the web (4) of packaging material at said bottom wall (17) and to generate a presence sensor signal (S3) indicative of said presence;
and wherein the control unit (20) is configured to calculate the quantity of packaging material accumulated, in use, in the buffer device (10) also as a function of the presence sensor signal (S3).
Packaging machine as claimed in the previous claim, wherein the buffer device (10) comprises a photocell (18) arranged at said bottom wall (17) and defining said presence sensor.
Packaging machine as claimed in any one of the claims 9 to 14, and comprising at least a first processing station (5, 6) which is configured to process the packaging material in a continuous manner and through which the packaging material is fed, in use, in a continuous manner, and a second processing station (7) which is configured to operate in an intermittent manner and through which the packaging material is fed, in use, stepwise;
wherein the buffer device (10) is operatively interposed between the first processing station (5, 6) and the second processing station (7).