WO1994010530A1 | 1994-05-11 |
US4271699A | 1981-06-09 | |||
EP1816468A1 | 2007-08-08 | |||
GB2167179A | 1986-05-21 |
Claims 1. A device for measuring the thickness of a moving web material, comprising in combination: a supporting structure, a web material feed path ; a detection unit, movable with respect to the supporting structure in a direction transverse to the feed path of the web material and arranged on a first side of the feed path of the web material; a contrasting element comprising a motorized roller rotating about a rotation axis transverse to the feed path of the web material cooperating with the detection unit and positioned on a second side of the feed path of the web material, the motorized roller and the detection unit being configured and arranged to be in contact with respective opposite surfaces of the web material in the feed path; a sensor carried by the detection unit, said sensor being configured to measure the distance between the sensor and the contrasting element, said distance being indicative of the thickness of the web material. 2. The device as claimed in claim 1, wherein said supporting structure comprises at least one guide transverse to the feed path of the web material, on which the detection unit is arranged and guided, an actuator being provided to control a reciprocating movement of the detection unit along said transverse guide. 3. The device as claimed in claim 1 or 2, wherein said rotation axis is stationary with respect to the supporting structure. 4. The device as claimed in claim 1, 2 or 3, wherein the motorized roller is controlled so as to rotate at a peripheral speed substantially equal to a feed speed of the web material along said path. 5. The device as claimed in one or more of the preceding claims, wherein the sensor is configured and arranged to perform a direct measurement of the distance between said sensor and the motorized roller. 6. The device as claimed in one or more of the preceding claims, wherein the sensor is a proximity sensor. 7. The device as claimed in one or more of the preceding claims, wherein the sensor is selected from the group comprising: a capacitive sensor; an eddy current sensor. 8. The device as claimed in one or more of the preceding claims, wherein said detection unit comprises a measuring head pivoted to a slide around at least a first axis substantially parallel to the feed path of the web material, said slide being movable in a direction transverse to the feed path. 9. The device as claimed in claim 8, wherein said measuring head is constrained to the slide by means of a pivoting arm, said pivoting arm being constrained to the slide around said first axis and said measuring head being pivoted to the pivoting arm around a second axis parallel to the first axis. 10. The device as claimed in claim 8 or 9, wherein said measuring head is elastically biased toward a position as close as possible to the contrasting element. 11. The device as claimed in claim 8, 9 or 10, comprising a limiting member for limiting the movement of the measuring head with respect to the slide, so that said measuring head does not come into contact with the contrasting element when no web material is present in the feed path. 12. The device as claimed in claim 11, wherein said limiting member comprises a shaped slot rigidly constrained to the slide and in which a feeler rigidly connected to the measuring head engages. 13. The device as claimed in claim 12, wherein said slot is shaped so as to increase or reduce the permitted pivoting movement of said measuring head as a function of the distance of the measuring head from the contrasting element. 14. The device as claimed in one or more of claims 8 to 13, wherein said measuring head comprises at least two web material contact members, said contact members being spaced apart from one another and the sensor being arranged between said contact members, and wherein the contrasting element comprises at least two projections, spaced apart from one another so that by arranging the measuring head at said projections when there is no web material between the detection unit and the projections, the contact members rest on the projections with the measurement sensor positioned between said projections. 15. The device as claimed in one or more of claims 8 to 14, wherein said measuring head comprises two sliding shoes, configured and arranged to slide in contact on the web material, said two shoes being aligned along the direction of the reciprocating movement of the detection unit and said sensor being arranged between said two shoes. 16. The device as claimed in one or more of claims 8 to 15, wherein said measuring head comprises at least a first wheel and a second wheel mounted idle on rotation axes parallel to the direction of the reciprocating movement of the detection unit. 17. The device as claimed in one or more of claims 8 to 16, wherein said measuring head comprises a first group of wheels mounted idle around rotation axes mutually staggered along the direction of feed of the web material along said feed path, and a second group of wheels mounted idle around rotation axes mutually staggered along the direction of feed of the web material along said feed path, the sensor being arranged in an intermediate position between the first group of wheels and the second group of wheels, the idle wheels of each group defining a plurality of contact points with the web material, said contact points being mutually staggered in the direction of feed of the web material. 18. The device as claimed in one or more of claims 8 to 17, wherein lateral deflectors are associated with said measuring head, aligned according to the direction of reciprocating movement of the detection unit, arranged and configured to accompany the web material under the detection unit. 19. A method for measuring the thickness of a web material, comprising the steps of: arranging a feed path of the web material; arranging a detection unit on a first side of the feed path; arranging a sensor on said detection unit; arranging a contrasting element, comprising a motorized roller, cooperating with the detection unit, on a second side of the feed path; rotating the motorized roller at a peripheral speed substantially equal to the feed speed of the web material; feeding the web material along the feed path with a first face contacting the detection unit and a second face contacting the contrasting element; translating with a reciprocating movement the detection unit in a direction transverse to the direction of feed of the web material while the web material is being fed along said feed path; measuring the thickness of the web material by means of said sensor during the transverse movement of the detection unit. 20. The method as claimed in claim 19, wherein the sensor generates a signal that is a direct function of the distance between the sensor and the surface of the motorized roller toward which the sensor faces, said sensor preferably being a proximity sensor, and even more preferably a capacitive sensor or an eddy current sensor. |
WEB MATERIAL DESCRIPTION
Technical Field
The invention relates to a device for measuring the thickness of a web material, for example corrugated cardboard. More in particular, the invention relates to a device for measuring the thickness of a web material that advances continuously along a feed path, for example along a web material production or converting line. State of the art
In the production or processing of web materials fed continuously along a production or converting line, it is often necessary to monitor the thickness of the material in order to correct the production parameters, discard portions of web material with a thickness that does not correspond to the product specifications, or for other purposes.
In particular, in the production of corrugated cardboard it is necessary to monitor the thickness of the cardboard, as errors in the thickness can lead to defects in the finished product and, in particular, insufficient strength of the cardboard, which compromises the use thereof, for example in the production of packaging, boxes or the like.
Many devices and systems have been developed for continuously checking the thickness of the web material, in particular corrugated cardboard, while the web material is continuously advancing along a feed path. Prior art devices are normally based on optical systems and are particularly complex and costly, also due to the need to ensure high precision of the mechanical structure that supports them. Examples of prior art devices for measuring the thickness of corrugated cardboard or of other web material are disclosed in US 5210593, US 6836331, US 6264793, US 4031752, US 3671726 and US 20090091761.
In some of these systems, for example US 3671726, several measurement units are distributed along a direction transverse to the feed path of the web material, so as to measure the thickness in different points of the width of the web material being fed along the path. Each measurement unit comprises an upper measuring head and a lower measuring head. The upper and lower measuring heads are positioned on opposite sides of the cardboard feed path and direct laser beams to the two opposed faces of the web material, to measure the thickness thereof.
In other prior art devices (US 5210593, US 6836331, US 6264793) only two measuring heads are used, placed one above and the other underneath the feed path of the web material. Each head has an optical measurement system that detects the position of the respective surface of the web material being fed along the path between the two heads. The two heads travel in a direction transverse to the feed path of the web material to measure the whole of the width thereof.
In these prior art devices, as measurement of the thickness is based on the combination of measurement of the distance between the upper head and the lower head of the web material and between the lower head and the lower surface of the web material, in order to obtain a sufficiently accurate measurement very precise parallelism is required of the guides along which the measuring heads travel. This, together with the complexity of the optical measurement instruments used, means that this type of device is extremely expensive.
A further system for measuring the characteristics of a web material formed of corrugated cardboard and using two opposed laser measuring heads is described in US 5581353.
The systems described in the aforesaid patents are contactless system, in other words systems in which the measuring instrument does not come into physical contact with the web material. In other prior art devices, such as US 5,074,050, the thickness of the web material is measured by means of members that are in mechanical contact with the web material. The device described in US 5,074,050 comprises a proximity sensor housed inside a measuring head located under the feed path of the web material and over which the web material passes. Located above the measuring head is a shoe, elastically stressed against the web material and in contact with the surface thereof opposite the surface in contact with the measuring head. As the web material is fed it rubs against the measuring head and against the opposed shoe, which maintains a slight pressure on the web material. The proximity sensor measures the distance between the shoe and the sensor. This distance is assumed as measurement of the thickness of the web material. This prior art device has some serious problems that make it unsuitable for measuring the thickness of web materials of high transverse dimension. Moreover, it can cause damage to the web material during measurement or a modification of the feed movement thereof, resulting in difficulty in guiding the material.
US 4,271,699 discloses a system for monitoring the thickness of a sheet. The system comprises a lower idle roller and an upper contact roller, defining a nip where through the sheet passes. The upper roller is constrained to a rod forming a moving ferromagnetic core that moves coaxially to and inside a primary winding and a secondary winding of a transformer. Magnetic coupling between the two windings varies as a function of the position of the ferromagnetic core. In this way the transformer provides a signal that is a function of the position of the ferromagnetic core and, therefore, indirectly, of the thickness of the sheet that passes between the lower idle roller and the lower contact roller, constrained to the moving ferromagnetic core. This system is affected by considerable drawbacks, which make it somewhat inaccurate. Firstly, the configuration of the measuring system is subject to errors due to thermal expansion, as the extent to which the ferromagnetic core extends inside the volume surrounded by the windings is not only a function of the position of the contact roller, but also of the temperature, which can lengthen or shorten the material between the contact point of the contact roller with the sheet and the free end of the ferromagnetic core. Moreover, the sheet is not positively controlled and can swerve.
Therefore, there is a need for a measuring system that is simple and inexpensive, but also reliable, to measure the thickness of a web material, such as a continuous sheet of cardboard, for example a corrugated cardboard.
Summary of the invention
In order to remove or alleviate one or more of the drawbacks of prior art devices, according to the invention there is provided a device for measuring the thickness of a moving web material, comprising in combination: a supporting structure; a feed path of the web material; a detection unit, movable with respect to the supporting structure in a direction transverse to the feed path of the web material and arranged on a first side of the feed path of the web material. The device also advantageously comprises a contrasting element, cooperating with the detection unit and positioned on a second side of the feed path of the web material. The contrasting element and the detection unit are advantageously configured and arranged to contact respective opposite surfaces of the web material in the feed path. Advantageously, there is also provided a sensor carried by the detection unit. The sensor is advantageously configured to measure the distance between the sensor and the contrasting element, said distance being indicative of the thickness of the web material.
In advantageous embodiments the detection unit is positioned above the feed path of the web material and the contrasting element is positioned below the feed path of the web material.
The supporting structure can comprise at least one guide transverse to the feed path of the web material, on which the detection unit is arranged and guided. An actuator can be provided to control a reciprocating movement of the detection unit along said transverse guide.
Advantageously, the contrasting element comprises a motorized roller, the peripheral speed whereof can be controlled to be approximately equal to the feed speed of the web material. In this way more regular operation of the measuring device is achieved.
The rotation axis of the roller can be stationary with respect to the supporting structure.
The roller can be the same width as the whole of the useful width of the device, i.e. in substance preferably at least the same width as the maximum width of the web material that can be fed along the feed path. In this case, the roller can remain fixed with respect to the direction parallel to its axis and is therefore stationary (except for rotation about the axis) with respect to the supporting structure.
However, in other embodiments the contrasting element can have a width that is less than the width of the web material or in any case less than the width on which the measurement is to be performed. In this case, the contrasting element can move transversely, together with the detection unit. For example, an autonomous actuator can be provided to move the contrasting element transversely, in synchronism with the movement of the detection unit. It would also be possible to use the same actuator to control both the movement of the detection unit and of the contrasting element.
In some embodiments the contrasting element can be formed of a roller, which is translated along a shaft extending transversely with respect to the direction of feed of the web material. The roller can be supported idle on the shaft or can be coupled torsionally to the shaft to be made to rotate.
In general, the direction of translation of the detection unit, and optionally of the contrasting element, can be orthogonal to the direction of feed of the web material. However, this is not strictly essential. Translation can also take place along a direction that forms an angle different from 90° with respect to the direction of feed of the web material. The term "transverse" referring to the direction of motion of the detection unit is used to identify a direction not parallel to the direction of feed of the web material.
In some embodiments, other elements, units, instruments or accessories can be positioned on the detection unit and/or on the contrasting element (especially if it too moves in transverse direction). For example, there can be provided systems for detecting the position of cutting, scoring or printing lines, or the like, which can be generated longitudinally along the web material.
Advantageously, systems can be provided for detecting the transverse position of the detection unit, for example an encoder on an electric control motor, so as to associate any thickness defects or other characteristics to be detected with their transverse position along the width of the web material. The device can be interfaced with a control unit that can be provided with an interface configured to signal defects or the like, indicating their position if necessary.
In some embodiments, the detection unit can comprise a measuring head on which the sensor is mounted, which moves with respect to a slide to adapt to the position of the web material and be pressed in contact on a face or surface thereof, opposite the face or surface in contact with the contrasting element. To obtain effective positioning and continuous contact of the measuring head with the web material, a system can be provided for connecting the measuring head to the slide that permits a movement around two pivot axes, preferably parallel to each other and preferably parallel to the direction of feed of the web material through the space between the contrasting element and the measuring head.
In some embodiments the measuring head can be connected to the slide by means of a pivoting arm that acts as a connecting rod, pivoted on one side to the slide and on the opposite side to the measuring head. In this way the measuring head is free to rotate around two axes parallel to each other and orthogonal to the direction of translation of the slide.
In some embodiments the measuring head can comprise two sliding shoes, configured and arranged to slide in contact on the web material and aligned, i.e. placed side by side, along the direction of the reciprocating movement of the detection unit. Advantageously, the sensor can be arranged between the two shoes. The two shoes can be formed, for example, as ribs or projections formed on a plate inside which a seat for the sensor is provided.
Advantageously, the detection unit can comprise lateral deflectors, aligned according to the direction of reciprocating movement of the detection unit, arranged and configured to accompany the web material under the detection unit. The deflectors can advantageously be fixed to the measuring head.
According to a further aspect, the invention relates to a method for measuring the thickness of a web material, for example and in particular (but not exclusively) corrugated cardboard, comprising the steps of:
providing a feed path of the web material;
arranging a detection unit on a first side of the feed path;
arranging a sensor on said detection unit;
arranging a contrasting element, in particular a motorized roller, co-acting with the detection unit, on a second side of the feed path;
feeding the web material along the feed path with a first face contacting the detection unit and a second face contacting the contrasting element;
translating with a reciprocating movement the detection unit in a direction transverse to the direction of feed of the web material while the web material is being fed along said feed path;
measuring the thickness of the web material by means of said sensor during the transverse movement of the detection unit.
The method can advantageously be performed with a device as described and as defined in the appended claims.
Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims. In this respect, before explaining several embodiments of the invention in details, it is understood that the various embodiments of the invention are not limited in their application to the details of the construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception, upon which the disclosure is based, may readily be utilized as a basis for designing other structures, methods, and/or systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood by following the description and accompanying drawing, which shows practical non-limiting embodiments of the invention. More specifically, in the drawing:
Fig. 1 shows a front view of a device according to an embodiment of the invention;
Fig.lA shows an enlargement of the detection unit of Fig.1; Fig. 2 shows a section according to II-II of Fig. 1;
Fig. 3 shows a front view of a measuring head in a further embodiment of the measuring head; Fig. 4 shows a bottom view according to IV -IV of Fig. 3;
Fig. 5 shows a side view according to V-V of Fig. 3;
Fig.6 schematically shows the operation of the measuring head of Figs.
3 to 5.
Detailed description of embodiments of the invention
The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.
Reference throughout the specification to "one embodiment" or "an embodiment" or "some embodiments" means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase "in one embodiment" or "in an embodiment" or "in some embodiments" in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. Figs. 1 and 2 show a first embodiment of a device for measuring the thickness of a web material, particularly suitable for measuring the thickness of a continuous sheet of corrugated cardboard. The measuring device, indicated as a whole with 1, comprises, or is carried by, a stationary supporting structure, indicated as a whole with 3. The supporting structure 3 can be arranged in a processing line, along which the web material N is fed, which can for example be a corrugated cardboard consisting of at least two sheets of smooth cardboard between which there is placed a sheet of cardboard with a series of flutes, the crests whereof are glued to the two smooth sheets.
In some embodiments, the supporting structure 3 comprises two opposite side members 5 joined to each other by cross members 7, 9. A feed path P of the web material N extends between the two side members 5. A contrasting element 11 is arranged between the two side members 5, on which contrasting element the web material N rests during feeding according to the arrow F (Fig. 2) along the feed path, schematically indicated with P.
In advantageous embodiments the contrasting element 11 can comprise, or be formed of, a motorized roller 13 supported between the two side members 5 with suitable bearings, not shown. Advantageously, the roller 13 of the contrasting element 11 can be made to rotate around an axis A- A (see arrow fl3), by a motor 15, by means of a pulley 17 mounted on a shaft 13A of the roller 13. A belt 19 is entrained around the pulley 17 and is driven into motion by a drive pulley 21 mounted on the shaft of the motor 15.
A detection unit 23 cooperates with the contrasting element 11 ; in the manner described below, a sensor 24is mounted on the detection unit . The sensor 24 can be a proximity sensor, for example a capacitive sensor, or an eddy current sensor, configured to detect the distance of the contrasting element 11 , and more in particular of the cylindrical surface of the roller 13, from the sensor 24. The sensor 24 can be interfaced with a control unit 26. Advantageously, the sensor is configured to perform a direct measurement of the distance between sensor and motorized roller 13. By direct measurement it is intended that the sensor generates a signal that is a direct function of the distance between sensor and surface of the roller 13, and not, as in other prior art systems, an indirect signal, for example a signal that is a function of the position of a member connected to a feeler. Using a sensor, especially a proximity sensor, that generates a signal that is a direct function of the distance between sensor and opposite surface of the opposing roller 13, toward which the sensor faces, an accurate signal can be obtained, which is not affected by errors caused, for example, by thermal expansion.
In advantageous embodiments, the detection unit 23 comprises a slide 25 provided with shoes 27 that engage in transverse guides 29, advantageously supported by the cross member 7.
The slide 25 can move with a reciprocating motion according to the double arrow f25 along the guides 29. For this purpose, a motor 31 can be provided, for example carried by one of the side members 5. The motor 31 can drive a drive pulley 33 around which a belt 35 or other flexible member is guided, which extends transversely between the side members 5 and on the opposite side with respect to the drive pulley 33, is guided around an idle pulley 37, carried by the side member 5 opposite the side member carrying the motor 31. The flexible member 35 can be fastened in 35 A to the slide 25, so that rotation of the motor 31 in one or other direction causes the slide 25 to translate with reciprocating motion according to f25.
A measuring head 41 can be mounted on the slide 25 of the detection unit 23. In advantageous embodiments, the measuring head 41 is connected to the slide 25 with a mechanism with a double pivot axis, which permits a movement of the measuring head 41 with respect to the slide 25.
In some embodiments, the measuring head 41 is connected to the slide 25 by means of a pivoting arm 43. The pivoting arm 43 can be pivoted to the slide 25 by means of a first pivot pin 45, the axis whereof is oriented orthogonally to the direction of movement of the slide 25 and therefore generally parallel to the direction of feed F of the web material along the path P. The arm 43 can thus pivot according to the double arrow f43 about the axis of the pivot pin 45.
The arm 43 is also pivoted, by means of a second pin 47, to the measuring head 41. The second pivot pin 47 is substantially parallel to the first pivot pin 45. In this way the measuring head 41 can pivot about the axis of the pin 47 and about the axis of the pin 45.
In advantageous embodiments the measuring head 41 is biased towards the contrasting element 11 so as to be pressed with the lower surface thereof against the upper surface S 1 of the web material N and so as to press the lower surface S2 of said web material N against the contrasting element 11 below.
In some embodiments an elastic member that stresses the measuring head 41 toward the contrasting element 11 is provided. In the embodiment illustrated, for this purpose a piston-cylinder system 49, advantageously of pneumatic type, is provided, which acts as a pneumatic spring acting between the slide 25 and the measuring head 41. The pneumatic spring formed by the piston-cylinder 49 can be constrained on one side at the pivot pin 47 of the measuring head 41 and on the other at a pivot pin 51 to the slide 25, for example to a bracket 53. The pneumatic spring 49 can thus follow the movements of the pin 47 and of the measuring head 41. In other embodiments different elastic biasing systems can be provided. In advantageous embodiments, to limit the downward movement of the measuring head 41, a limiting member 55 can be provided. The limiting member 55 can comprise a shaped slot 57 rigidly fixed to the slide 25 and a feeler 58, engaged in the slot 57 and rigidly constrained to the measuring head 41. As shown in particular in Fig.l, the shape of the slot 57 is such that the closer the measuring head 41 moves toward the contrasting element 11, the more the movement of the measuring head 41 around the pins 45 and 47 is limited. The shaped slot 57 is configured so that in the lowest position of the measuring head 41 this latter is maintained at a distance from the contrasting element 11 sufficient to prevent mutual contact between measuring head 41 and contrasting element 11, but less than the minimum thickness of the web material N. In this way mechanical contact between the measuring head 41 and the contrasting element 11 is prevented even when there is no web material N in the feed path P.
In advantageous embodiments the shaped slot 57 can be formed of two components 57A and 57B, the distance whereof can be adjusted so as to adapt the position and the dimension of the shaped slot 57 with respect to the slide 25.
In advantageous embodiments the measurement sensor 24 is positioned between two contact members 59, which can be formed, for example, in the form of shoes applied below the measuring head 41. The shoes 59 are arranged side by side with each other along the direction f25 of movement of the measuring head 41 and of the slide 25 and on opposite sides with respect to the sensor 24. It would also be possible to provide a larger number of contact shoes 59.
Advantageously, the measurement sensor 24 can be positioned so as to be distanced from the lower surface of the shoes 59. In this way, when the shoes 59 slide on the surface S 1 of the web material N, the sensor 24 can be located a slight distance therefrom, thus preventing wear of the sensor caused by rubbing on the web material.
In advantageous embodiments, deflectors 61 can be associated with the measuring head 41, positioned symmetrically on the two sides of the measuring head 41 parallel to the direction of feed F of the web material N. The deflectors 61 are advantageously oriented sloping from the bottom toward the top, forming sloping surfaces facing the feed path P with the purpose of deflecting any impediments that may be present above the web material N, such as debris, trimmings of web material, or the like, and of flattening any bumps or other deformations that could be present on the web material N and that could impede the transverse movement of the measuring head 41.
In some advantageous embodiments the device 1 can have a system for calibration of the sensor 24. For this purpose, for example, two annular projections 63 can be provided on the roller 13. When the contrasting element 11 is formed by a component different than a rotating roller, the projections can instead be provided on the surface of the contrasting element 11 facing the measuring head 41. In the embodiment illustrated, the annular projections 63 are positioned at a distance from each other coherent with the distance between the shoes 59. The outer diameter of the projections 63 is a few tenths of millimeter greater than the outer diameter of the roller 13.
Calibration of the sensor 24 is carried out by positioning the measuring head 41 with the shoes 59 in direct contact with the outer surfaces of the projections 63. The measurement sensor 24 detects the distance with respect to the surface of the roller 13 between the two projections 63. As the thickness of the projections 63 is known, the sensor 24 can be calibrated as a function of this measurement.
The device 1 described above operates as follows. The web material N is fed with a movement according to the arrow F along the feed path P with a speed that can be constant or variable, as a function of the requirements of the processing line in which the device 1 is installed. While the web material N is fed along the feed path P, it is in contact with the roller 13, the rotation whereof, controlled by the motor 15, facilitates feed of the web material N, preventing negative effects caused by friction with other components of the measuring device 1. As the roller 13 extends for the whole width of the feed path P, the thrusting or drawing action of the motorized roller 13 on the web material N is uniform on the width thereof.
The measuring head 41 is pressed by the pneumatic spring 49 against the upper surface SI of the web material holding this latter in contact both with the measuring head 41 at the sliding shoes 59, and with the surface S2 against the roller 13 of the contrasting element 11.
While the web material N is fed according to the arrow F, the detection unit 23 translates with reciprocating motion according to the double arrow f25 controlled by the motor 31 continuously reading the thickness of the web material N on the whole of the width of this latter according to a zigzag measurement line. The density of the measurement, i.e. the proximity of the consecutive lengths of sloping path along the surface of the web material N depends on the ratio between the speed of feed of the web material N and the translation speed of the detection unit 23 according to the double arrow f25. This ratio can be varied if necessary as a function of the measurement accuracy requirements.
The embodiment illustrated allows friction on the web material N to be limited to the contact area between the surface SI of the web material N and the sliding shoes 59, while rotation of the motorized roller 13 facilitates feed of the web material N along the feed path P.
To further reduce the effect of friction exerted by the measuring device 1 on the web material N during feeding thereof, different embodiments of the contact members between the measuring head 41 and the surface SI of the web material N can be provided.
Figs. 3 to 5 show a modified embodiment of the measuring head 41, wherein the shoes 59 are replaced, on each side of the measuring head 41 here shown isolated from the other members of the detection unit 23, by a group of wheels indicated as a whole with 71. Each group of wheels 71 can, for example, comprise three wheels, indicated with 73, 75 and 77 respectively. The number of wheels 73-77 shown in Figs. 3 to 5 is provided purely by way of example and it must be understood that this number may differ from three, for example there can be provided only two wheels or more than three wheels. Preferably, the number of wheels is in any case greater than one for the purposes explained below.
In the embodiment illustrated, each wheel 73-77 is in the form of a ball bearing, as shown in the partial sections of Figs. 3 and 4. It must also be understood that in other embodiments the wheels 73-77 can be configured differently.
As can be seen in particular in the view and partial section of Fig. 4, the three wheels 73, 75 and 77 of each group 71 of wheels are eccentric to one another. More in particular, the rotation axes of the wheels 73, 75 and 77 are staggered in the direction of movement of the web material N, direction that is indicated with arrow F in Fig. 4. The rotation axes of the wheels 73, 75 and 77 are coplanar, in the sense that they lie on a common plane parallel to the lower surface of the measuring head 41 and therefore parallel to the feed path of the web material N. This can be seen in particular in Fig. 3.
The wheels 73, 75, 77 thus arranged define contact lines coplanar with the web material N, so that each wheel 73, 75, 77 touches the web material N pressing against the contrasting element below 11. The contact lines between each wheel 73, 75 and 77 and the web material N are staggered in the direction F with respect to the contact line of the other wheels of the same group 71. With this arrangement the contact lines are staggered in the direction of alignment of the flutes of the corrugated cardboard forming the web material N whose thickness is measured by the device 1.
This can be more clearly understood referring to Fig. 6, which schematically shows a portion of a web material N formed by a corrugated cardboard comprising an inner sheet Fl with a plurality of flutes O, the crests whereof are glued to two opposed smooth outer sheets F2 and F3 forming the liners of the corrugated cardboard. Fig. 6 also schematically illustrates the wheels 73, 75 and 77 of one of the groups of wheels 71. The references L73, L75 and L77 indicate the contact lines of the wheels 73, 75 and 77 with the upper surface SI of the web material N formed by the smooth upper sheet F3. It can be seen that the contact lines L73, L75 and L77 are staggered with respect to the crests of the flutes formed by the inner sheet Fl . This makes it possible to prevent errors in the measurement performed by the sensor 24 caused by the slight corrugation that can form on the sheet F3 of the corrugated cardboard N due to the presence of the corrugated inner sheet Fl .
The embodiments described above and illustrated in the drawings have been discussed in detail as examples of embodiment of the invention. Those skilled in the art will understand that many modifications, variants, additions and omissions are possible, without departing from the principles, concepts and teachings of the present invention as defined in the appended claims. Therefore, the scope of the invention must be determined purely on the basis of the broadest interpretation of the appended claims, comprising these modifications, variants, additions and omissions therein. The term "comprise" and derivatives thereof do not exclude the presence of further elements or steps besides those specifically indicated in a given claim. The term "a" or "an" preceding an element, means or characteristic of a claim does not exclude the presence of a plurality of these elements, means or characteristics. When a device claim lists a plurality of "means", some or all of these "means" can be implemented by a single component, member or structure. The stating of given elements, characteristics or means in distinct dependent claims does not exclude the possibility of said elements, characteristics or means being combined with one another. When a method claim lists a sequence of steps, the sequence in which these steps are listed is not binding, and can be modified, if the particular sequence is not indicated as binding. Any reference numbers in the appended claims are provided to facilitate reading of the claims with reference to the description and to the drawing, and do not limit the scope of protection represented by the claims.