DESCRIPTION “FILLING DEVICE WITH A VALVE COMPRISING A MAGNETIC SHUTTER” TECHNICAL SECTOR The invention relates to a filling device to fill containers, in particular containers made of plastic or glass, for example bottles, or of a metal material, for example cans, with a pourable product, preferably with a food-related product, such as for example water, milk, fruit juices, sparkling or still beverages or the like. STATE OF THE ART Filling machines are known, which are typically used to fill containers with a pourable product and basically comprise a carousel rotating around a vertical axis, a tank containing the pourable product and a plurality of filling valve devices, which are carried by the carousel in the area of a radially peripheral portion thereof, are connected to the tank by means of respective circuits or ducts and are moved by the carousel along a substantially circular path. These machines also comprise an input device, typically a star-shaped input drum, which is configured to sequentially feed empty containers to the carousel, and an output device, typically a star-shaped output drum, which is configured to sequentially receive the containers filled with pourable product from the carousel. In particular, the carousel receives a succession of empty containers from the star-shaped input drum and directs the full containers towards the star-shaped output-drum. The carousel generally comprises a plurality of support elements, each designed to receive and hold in a vertical position, under each filling device, a respective container to be filled. Each filling device is designed to feed, during its movement along the aforesaid circular path by means of the rotary movement generated by the carousel, a predetermined volume of pourable product to the relative container. Typically, known filling devices basically comprises: - a hollow tubular body fixed to a peripheral portion of the carousel and internally defining a flow channel to feed the pourable product to a respective container to be filled located under the tubular body itself; - a shutter, which engages the tubular body in a sliding manner and is movable inside the flow channel so as to selectively allow or prevent the pourable product to flow or from flowing towards the respective container; and - an actuator configured to mover the shutter inside the flow channel defined by the tubular body. The tubular body generally has a longitudinal axis parallel to the axis of the carousel and ends, at a lower end if its, with an axial outlet opening fluidly communicating, in use, with an end opening defined by an upper edge of the respective container to be filled. The flow channel extends coaxially to the axis of the tubular body and comprises a usually cylindrical segment with a constant cross section and at least one segment with a variable cross section, usually having the shape of a truncated cone, located upstream of the outlet opening and narrowing in the direction of the latter, up to a section with a minimum diameter or narrow section. The shutter typically engages the flow channel in a coaxial manner and is axially movable inside the flow channel between: - a closing position, in which the shutter closes the variable cross-section segment of the channel in a sealing manner, so as to interrupt the flow of pourable product through the outlet opening; and - an opening position, in which the shutter axially delimits, together with the variable cross-section segment, an annular passage fluidly communicating with the outlet opening, so as to allow the pourable product to flow towards the end opening of the respective container. In detail, the shutter comprises a main body, usually with a substantially cylindrical shape, and an interaction portion, for example shaped like a piston, axially protruding from the main body in the direction of the outlet opening and configured to strike against the variable cross-section segment of the flow channel when the shutter is in the closing position. More in detail, the interaction portion is designed to close the aforesaid section with a minimum diameter in a fluid-tight manner, when the shutter is in the closing position. Pneumatic, hydraulic or oil-operated actuators are known. However, actuators of this type are scarcely suited for some applications, for they require a complicated separation between “dirty” actuation components (which absolutely must remain out of the flow channel) and “clean” components (for example, the shutter) inside the flow channel. In order to overcome this drawback, magnetic or electromagnetic actuators were developed: they comprise a driving member outside the flow channel and a driven member integral to the shutter and, hence, inside the flow channel. The driven member is usually defined by a ferromagnetic portion of the shutter or by the shutter being entirely made of a ferromagnetic material. The driving member can be defined by: - permanent magnets; in this case, an axial movement of the driving member corresponds to an axial movement of the driven member thanks to the magnetic interaction between the two members; or by - an electrically powerable solenoid; in this case, depending on the power supplied to the solenoid, an axial movement of the driven member - and, hence, of the shutter - is generated in one direction or in the other thanks to the electromagnetic interaction between the two members. In this way, the movement of the shutter is obtained without jeopardizing the sterility and/or the aseptic character of the filling. Even though they are functionally and structurally effective, the Applicant found out that filling devices of the type described above can further be improved, in particular in terms of precision, efficacy and reliability of the electromagnetic control of the shutter, in terms of minimization of wear and in terms of handling of pourable products with pieces, namely consisting of a mix of liquid and suspended parts. EP3543205A1 discloses a filling device with an electromagnetic actuator having a driven magnet which is integral with the shutter and a solenoid which is located outside the flow channel. EP3323773A1 discloses a filling device having driven magnets which are connected to the shutter and solenoids which are located outside of flow channel. SUBJECT-MATTER AND SUMMARY OF THE INVENTION The object of the invention is to provide a filling device, which is highly reliable and has a limited cost as well as solves at least some of the drawback discussed above relating to known filling devices. According to the invention, this object is reached by a filling device as claimed in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be best understood upon perusal of the following description of a preferred non-limiting embodiment, which is provided by mere way of example, with reference to the accompanying drawings, wherein: - figure 1 is a schematic top view, with parts removed for greater clarity, of a filling machine comprising a plurality of filling devices according to the invention; and - figures 2 and 3 are schematic side views, with partial cross sections, on a larger scale and with parts removed for greater clarity, of a filling device during two distinct operating conditions. DETAILED DESCRIPTION With reference to the accompanying figures, number 1 indicates, as a whole, a filling machine configured to fill containers 2 with a pourable product, preferably a food-related product, such as for example water, fruit juices, milk, sparkling or still beverages, wine, beer or the like. The containers 2 can be defined, for example, by bottles made of plastic or glass or by cans made of a metal material (aluminium). The machine 1 comprises: - a carousel 3 rotatable around a (preferably vertical) axis X and having a plurality of peripheral receiving seats 3a to hold respective containers 2; - a tank (not shown) to contain the pourable product; - a star-shaped input drum 4 to feed containers 2 to be filled to the carousel 3; - a star-shaped output drum 5 to retrieve the full containers 2 from the carousel 3; and - a plurality of filling valve devices 6 peripherally carried by the carousel 3 above the seats 3a, each fluidly connected to the tank by means of a duct (not shown) and each configured to measure out a predetermined quantity of pourable product for a respective container 2 carried by the seat 3a located underneath. In particular, each filling device 6 can selectively be activated so as to control the flow of pourable product towards a respective container 2 carried by the carousel 3 and vertically arranged under the device 6. For the sake of simplicity, hereinafter reference will be made to one single filling device 6. However the structural and functional features described for said filling device 6 can equally be applied to each filling device 6 of the machine 1. With reference to figures 2 and 3, the filling device 6 comprises: - a tubular body 7 having a longitudinal axis A, internally defining a flow channel 8 for the pourable product, in particular comprising an inner longitudinal wall 8a delimiting the channel 8, and ending at one end with an outlet opening 10 for feeding the pourable product towards a respective container 2; - a valve member or shutter 11 engaging the flow channel 8 and axially movable therein between a maximum opening position (figure 3), in which the shutter 11 allows the pourable product to flow through the outlet opening 10, and a closing position (figure 2), in which the shutter 11 prevents the pourable product from flowing through the outlet opening 10; and - an electromagnetic actuator 12 configured to control the axial movement of the shutter 11 inside the channel 8. Hereinafter, “axially” means along a direction parallel to the axis A and “radially” means a radial direction relative to the axis A. Hereinafter, opening position indicates the maximum opening position. In detail, the tubular body 7 substantially is cylindrical and the axis A preferably is vertical and parallel to the axis X of the carousel 3. The tubular body 7 further has an inlet opening 13 arranged on an axially opposite side relative to the outlet opening 10, through which the channel 8 receives the pourable product from the tank. In the case shown herein, both the inlet opening 13 and the outlet opening 10 are coaxial. The channel 8 conveniently extends coaxially to the axis A and comprises: - a substantially cylindrical segment with a constant cross-section and provided with the inlet opening 13; and - a segment with a variable cross-section 8b substantially frustoconical and tapered towards the outlet opening 10, up to a section with a minimum diameter corresponding to the outlet opening 10 itself. Hence, the variable cross-section segment 8b is arranged downstream of the constant cross-section segment relative to a direction of the pourable product flow inside the channel 8. Preferably, the shutter 11 is arranged coaxially to the axis A inside the channel 8 and comprises a main body 11a, specifically an axial stem 11a having a substantially cylindrical shape, and an interaction portion 11b, for example shaped like a cap, axially extending from the main body 11a in the direction of the outlet opening 10 and configured to strike against the variable cross- section portion 8b and to close the latter in a fluid- tight manner. To this aim, the shutter 11 is axially movable inside the channel 8 between: - the opening position, in which the interaction portion 11b delimits, together with the variable cross- section segment 8b, an annular passage fluidly communicating with the outlet opening 10, so as to allow the pourable product to flow towards the container 2; and - the closing position, in which the interaction portion 11b cooperates with the variable cross-section segment 8b in a fluid-tight manner, so as to interrupt the flow of pourable product through the outlet opening The shutter 12 has a driving member 15, which is located on the outside of the channel 8, and a driven member 16, which is integral to the shutter 11. The driving member 15 is configured to electromagnetically interact with the driven member 16 in order to move the shutter 11. The driven member 16 comprises at least one permanent magnet, which is arranged so that the magnetic poles N-S are axially oriented, meaning that the magnetic pole with the first sign N and the magnetic pole with the second sign S are axially arranged one after the other. In the case shown herein, the magnetic pole with the first sign N is defined by the magnetic North pole and the magnetic pole with the second sign S is defined by the magnetic South pole. The driving member 15 includes at least one solenoid, which is arranged so as to radially surround part of the driven member 16 and is electrically powerable in order to define an electromagnetic field. According to an important aspect of the invention, each solenoid 18 is positioned in such a way that, both when the valve member 11 is in the opening position and when the valve member 11 is in the closing position, said solenoid radially faces magnetic poles only of the same respective sign, said respective sign being associated with the respective solenoid. Thanks to this configuration, the movement of the shutter 11 from the opening position to the closing position can be obtained more precisely and/or more quickly. Furthermore, the handling of these movements is very simple, since, in order to shift from the opening movement to the closing movement and vice versa, a sole change in the current direction in said at least one solenoid 18a is needed. The driven member 16 includes a plurality of permanent magnets 17, each arranged so that the respective magnetic poles N, S are axially oriented. The driving member 15 comprises a plurality of solenoids 18. Each one of the solenoids is arranged so as to radially surround part of the driven member 16 and is electrically powerable in order to define an electromagnetic field. The permanent magnets are axially distributed, meaning that they are arranged one after the other along a direction parallel to the axis A. The solenoids are axially distributed, meaning that they are arranged one after the other along a direction parallel to the axis A. In this way, attractive/repulsive magnetic forces are determined between the driving member 15 and the driven member 16, so that a given powering of the driving member 15 corresponds to an axial movement of the driven member 16 and, hence, of the shutter 11 inside the channel 8, thus determining the selection between opening position and closing position. The magnets are arranged so that, for each pair of axially consecutive permanent magnets, magnetic poles with the same sign belonging to a magnet and to the other magnet, respectively, of the pair axially face one another. For instance, in the example of the figures there are a first permanent magnet 17a, a second permanent magnet 17b, which is axially arranged after the first permanent magnet 17a, and a third permanent magnet 17c, which is arranged so that the second permanent magnet 17b is axially interposed between the first permanent magnet 17a and the third permanent magnet 17c. The first permanent magnet 17a and the second permanent magnet 17b define a first pair of axially consecutive magnets and are arranged so that the pole with the second sign S of the first magnet 17a axially faces the pole with the second sign S of the second magnet 17b. The second permanent magnet 17b and the third permanent magnet 17c define a second pair of axially consecutive magnets and are arranged so that the pole with the first sign N of the second magnet 17b axially faces the pole with the first sign N of the third magnet 17c. This arrangement of the magnetic poles of the consecutive permanent magnets increases the electromagnetic interaction between the driving member 15 and the driven member 16, so as to obtain a more efficient filling device. Furthermore, by so doing, a larger space can be left between the driven member 16 and the inner wall 8a, thus increasing the maximum dimension of the pieces possibly contained in the product for which the device can be used. The driven member comprises at least one ferromagnetic sector, which is axially interposed between at least two axially consecutive permanent magnets. The driven member comprises a plurality of ferromagnetic sectors. The permanent magnets are axially alternated with said plurality of ferromagnetic sectors. Each sector is axially interposed between two respective axially consecutive permanent magnets. In the example of the figures there are a ferromagnetic sector 20, which is axially interposed between the first magnet 17a and the second magnet 17b, and another ferromagnetic sector 21, which is axially interposed between the second magnet 17b and the third magnet 17c. Each ferromagnetic sector increases the field lines of the magnetic field and/or channels the field lines of the magnetic field so that they locally are more ordered and/or so that they locally are more radially oriented, hence optimizing the developed force and further increasing the efficiency of the device. The signs associated with the respective solenoids are axially alternated. In the example of the figures there are a first solenoid 18a, a second solenoid 18b, which is axially arranged after the first solenoid 18a, a third solenoid 18c, which is arranged so that the second solenoid 18b is axially interposed between the first solenoid 18a and the third solenoid 18c, and a fourth solenoid 18b, which is arranged so that the third solenoid 18c is axially interposed between the second solenoid 18b and the fourth solenoid 18d. The sign associated with the first solenoid 18a is the first sign N, the sign associated with the second solenoid 18b is the second sign S, the sign associated with the third solenoid 18c is the first sign N and the sign associated with the fourth solenoid 18d is the second sign S. Therefore, in this last example, both in the closing position and in the opening position, the first solenoid 18a radially face only magnetic poles with the first sign N, the second solenoid 18b only magnetic poles with the second sign S, the third solenoid 18c only magnetic poles with the first sign N and the fourth solenoid 18d only magnetic poles with the second sign S. The device is configured so that the movement from the closing position to the opening position or from the opening position to the closing position can be obtained by powering the solenoids with currents reverse to one another in an axially alternating manner. This means that, taking into account - for example - the four solenoids of the figures, in order to obtain the movement from the opening position to the closing position, the first solenoid 18a has to be powered with a current in a first direction, the second solenoid 18b with a current in a second direction, which is contrary to the first direction, the third solenoid 18c with a current in the first direction and the fourth solenoid 18d with a current in the second direction. In order to obtain the movement from the closing position to the opening position, on the other hand, the first solenoid 18a has to be powered with a current in the second direction, the second solenoid 18b with a current in the first direction, the third solenoid 18c with a current in the second direction and the fourth solenoid 18d with a current in the first direction. In this way, in order to shift from the generation of the movement from the opening position to the closing position to the generation of the movement from the closing position to the opening position, the direction of the respective current of each solenoid has to be changed. This leads to a simplification in the generation of the movements of the shutter and, at the same time, to an increase in the speed and/or the precision with which said movements are obtained. More precisely: - the driven member 16 includes at least one first permanent magnet 17a, which is arranged so that its magnetic poles N, S are axially oriented; - the driving member 15 comprises at least one first solenoid 18a, which is arranged so as to radially surround part of the first permanent magnet 17a and is configured to electromagnetically interact with the first permanent magnet 17a; and - according to the invention, the first solenoid 18a is arranged so as to always radially face a magnetic pole with the first sign N of the first permanent magnet 17a, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. Therefore, both when the valve member 11 is in the opening position and when the valve member 11 is in the closing position, the first solenoid 18a radially faces magnetic poles only with the first sign N, in particular at least one magnetic pole with the first sign N of the first permanent magnet 17a In other words, the first solenoid 18a is arranged so as to never radially face the magnetic pole with the second sign S of the first permanent magnet 17a, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. Thanks to this configuration, an electromagnetic interaction between the magnetic field generated by the driven member 16 and the electromagnetic field generated by the driving member 15 can be obtained in order to control the movement of the shutter 16 with an improved precision compared to known solutions. Advantageously, the driven member includes a first ferromagnetic sector 19 axially adjacent to the magnetic pole with the first sign N of the first permanent magnet 17a and the first solenoid 18a is arranged so as to always radially face said first ferromagnetic sector 19, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. The Applicant found out, through extensive experiments, that the first ferromagnetic sector 19 determines a “channelling” of the electromagnetic and magnetic field lines, which get curved in a radial direction and better interact with the first solenoid 18a. This, together with the peculiar positioning of the first solenoid 18a, results in a more effective control of the shutter 11 and in a more intense magnetic force (Lorentz force) compared to known cases. Advantageously: - the driven member 16 comprises a second permanent magnet 17b arranged so that its magnetic poles N, S are axially oriented and so that the magnetic pole with the second sign S of the first permanent magnet 17a axially faces the magnetic pole with the second sign S of the second permanent magnet 17b; - the driving member 15 comprises a second solenoid 18b, which is arranged so as to radially surround part of the first permanent magnet 17a and of the second permanent magnet 17b and is configured to electromagnetically interact with the first permanent magnet 17a and with the second permanent magnet 17b; - the second permanent magnet 17b is axially arranged after the first permanent magnet 17a; - the second solenoid 18b is axially arranged after the first solenoid 18a; and - according to the invention, the second solenoid 18b is arranged so that, both when the valve member 11 is in the opening position and when the valve member 11 is in the closing position, the second solenoid 18a radially faces magnetic poles only with the second sign S and, in particular, at least one magnetic pole with the second sign S of the first permanent magnet 17a and one magnetic pole with the second sign S of the second permanent magnet 17b. In other words, the second solenoid 18b is arranged so as to never radially face magnetic poles with the first sign N of the first permanent magnet 17a and of the second permanent magnet 17b, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. Thanks to this configuration, the effectiveness and the precision of the control of the movement of the shutter 11 are further improved compared to the preceding case. Conveniently, the driven member 16 includes a second ferromagnetic sector 20 axially interposed between the magnetic poles with the second sign S of the first permanent magnet 17a and of the second permanent magnet 17b and the second solenoid 18b is arranged so as to always radially face the second ferromagnetic sector 20, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. In this way, an even better “channelling” of the electromagnetic and magnetic field lines is obtained, thus determining a magnetic force with an even greater intensity and an improved effectiveness in the control of the position of the shutter 11. Advantageously: - the driven member 16 comprises a third permanent magnet 17c arranged so that its magnetic poles N, S are axially oriented and so that the magnetic pole with the first sign N of the second permanent magnet 17b axially faces the magnetic pole with the first sign N of the second third magnet 17c; - the driving member 15 comprises a third solenoid 18c, which is arranged so as to radially surround part of the second permanent magnet 17b and of the third permanent magnet 17c and is configured to electromagnetically interact with the second permanent magnet 17b and with the third permanent magnet 17c; - the second permanent magnet 17b is axially arranged between the first permanent magnet 17a and the third permanent magnet 17c; wherein the second solenoid 18b is axially arranged between the first solenoid 18a and the third solenoid 18c; and - according to the invention, the third solenoid 18c is arranged so that, both when the valve member 11 is in the opening position and when the valve member 11 is in the closing position, the third solenoid 18c radially faces magnetic poles only with the first sign N and, in particular, at least one magnetic pole with the first sign N of the second permanent magnet 17b and one magnetic pole with the first sign N of the third permanent magnet 17c. Therefore, both when the valve member 11 is in the opening position and when the valve member 11 is in the closing position, the third solenoid 18c radially faces magnetic poles of the second permanent magnet 17b and of the third permanent magnets 17c only with the first sign N and, hence, magnetic poles only with the first sign N. In other words, the third solenoid 18c is arranged so as to never radially face magnetic poles with the second sign S of the second permanent magnet 17b and of the third permanent magnet 17C, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. Thanks to this configuration, the effectiveness and the precision of the control of the movement of the shutter 11 are further improved compared to the preceding case. Conveniently, the driven member 16 includes a third ferromagnetic sector 21 axially interposed between the magnetic poles with the first sign N of the second permanent magnet 17b and of the third permanent magnet 17c and the third solenoid 18c is arranged so as to always radially face the third ferromagnetic sector 21, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. By so doing, the aforesaid “channelling” of the electromagnetic and magnetic field lines is further improved, this determining a magnetic force with an even greater intensity. Advantageously: - the driving member 15 comprises a fourth solenoid 18d, which is arranged so as to radially surround part of the third permanent magnet 17c and is configured to electromagnetically interact with the third permanent magnet 17c; - the third solenoid 18b is axially arranged between the second solenoid 18b and the fourth solenoid 18d; - according to the invention, the fourth solenoid 18d is arranged so that, both when the valve member 11 is in the opening position and when the valve member 11 is in the closing position, the fourth solenoid 18d radially faces magnetic poles only with the second sign S, in particular at least one magnetic pole with the second sign S of the third permanent magnet 17c. In other words, the fourth solenoid 18d is arranged so as to never radially face the magnetic pole with the first sign N of the third permanent magnet 17c, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. Thanks to this configuration, the effectiveness and the precision of the control of the movement of the shutter 11 are further improved compared to the preceding case. Conveniently, the driven member 16 includes a fourth ferromagnetic sector 22 axially adjacent to the magnetic pole with the second sign S of the third permanent magnet 17c. In particular, the first ferromagnetic sector 19 and the fourth ferromagnetic sector 22 are arranged at the respective axial ends of the driven member 16. According to the invention, the fourth solenoid 18d is arranged so as to always radially face the fourth ferromagnetic sector 22, both when the shutter 11 is in the opening position and when the shutter 11 is in the closing position. By so doing, the aforesaid “channelling” of the electromagnetic and magnetic field lines is further improved, this determining a magnetic force with an even greater intensity. Advantageously, the driving member 15 includes a ferromagnetic element 23 arranged in a radially more external position than each solenoid 18 and so as to radially surround each solenoid 18 relative to the longitudinal axis A. In detail, the ferromagnetic element 23 is defined by a tubular element or cover fixed to each solenoid 18 so as to radially surround each solenoid 18. The Applicant found out, through extensive experiments, that the presence of the ferromagnetic element 23 leads to a concentration and channelling of the electromagnetic and magnetic field lines, thus supporting the closing thereof within each permanent magnet 17. Hence, the precision in the control of the movement of the shutter 11 and the intensity of the magnetic force are increased. Furthermore, the ferromagnetic element 23 produces a screening effect, which helps reduce the electromagnetic or magnetic coupling between filling devices used in the same filling machine, in particular if they are close to one another. Each solenoid 18 preferably comprises a voice coil moving magnet actuator. The Applicant found out that this type of actuator is the ideal configuration to obtain the advantages discussed above (and below). It is stated that each solenoid 18 is fixed relative to the axis A, whereas is permanent magnet 17 is movable through interaction with the respective solenoid 18, thus determining the axial movement of the shutter 11. In particular, in use, when the shutter 11 is in the closing position, in order to obtain the movement thereof to the opening position, the solenoids 18 have to be powered with currents with alternated directions, due to the particular arrangement of the permanent magnets 17. Similarly, when the shutter 11 is in the opening position, in order to obtain the movement thereof to the closing position, the solenoids 18 have to be powered with currents with alternated and reverse directions compared to the previous case, due to the particular arrangement of the permanent magnets 17. According to figures 2 and 3, the main body 11a of the shutter 11 is preferably defined by the aforesaid axial stem, to which the permanent magnets 17 are radially fixed and to which the interaction portion 11b is axially fixed. In practice, the interaction portion 11b constitutes an axial end of the shutter 11. According to a further aspect of the invention, the narrow section 8b and the interaction portion 11b are made of a ceramic material, preferably of the same ceramic material. In this way, the wear and the need to provide elastomer sealing elements, which generally have s shorter life than ceramic components, are significantly reduced. According to a further aspect of the invention, the stem 11a is provided with recesses and/or indentations 24 on its own outer longitudinal surface, preferably arranged between the interaction portion 11b and the driven member 16. Said recesses 24 decrease the rigidity of the stem 11a and increase the flexibility thereof, thus “absorbing” possible centring errors of the interaction portion 11b in the engagement of the narrow portion 8b. Preferably, with reference to figures 2 and 3, the filling device 6 further comprises a pair of linear magnetic bearings 25 configured to axially guide and radially support the shutter 11 in levitation with respect to the inner wall 8a of the channel 8 and/or to centre the shutter 11 within the channel 8 during the axial movement between the opening position and the closing position. In particular, the filling device 6 comprises a first bearing 25 proximal to the inlet opening 13 and a second bearing 25 proximal to the outlet opening 10. In detail, each linear magnetic bearing 25 comprises: - a first magnetic element 25a fixed to the tubular body 7 and arranged on the outside of the flow channel 8; and - a second magnetic element 25b, which is fixed to the shutter 11 in an integral manner, is arranged inside the channel 8 and magnetically interacts with the first magnetic element 25a so as to cause a magnetic levitation of the shutter 11 with respect to the inner wall 8a. Furthermore, preferably: - the first magnetic element 25a and the second magnetic element 25b are arranged so as to radially face one another with respective magnetic poles with the same sign, so as to determine a magnetic repulsion force and cause said magnetic levitation; and/or - the first magnetic element 25a and the second magnetic element 25b are each defined by an annular permanent magnet around the longitudinal axis A and radially polarized to have a first radially inner magnetic pole and a second radially outer magnetic pole with a sign contrary to the first magnetic pole, the first magnetic pole of the first magnetic element 25a having the same sign as the second magnetic pole of the second magnetic element 25b; and/or - the shutter 11 is movable between the opening position and the closing position by an axial stroke with a predetermined length, and wherein the first magnetic element 25a has an axial extension at least equal to said stroke; and/or - the second magnetic element 25b has a smaller axial extension than the axial extension of the first magnetic element 25a, so as to completely remain radially surrounded by the first magnetic element 25a for the entire length of said stroke; and/or - the filling device 6 comprises a first annular ferromagnetic ring 26 around the longitudinal axis A, which is fixed to the tubular body 7, is preferably arranged on the outside of the channel 8 and is axially interposed between the outlet opening 10 and the first magnetic element 25a of the first linear magnetic bearing 25 to magnetically interact with the second magnetic element 25b of the first linear magnetic bearing 25 and attract it to itself in order to bias a displacement of the valve shutter 11 towards the closing position; and/or - the filling device 6 comprises a second annular ferromagnetic ring 27 around the longitudinal axis A, which is fixed to the tubular body 7, is preferably arranged on the outside of the channel 8 and is axially interposed between the inlet opening 13 and the first magnetic element 25a of the second linear magnetic bearing 25 to magnetically interact with the second magnetic element 25b of the second linear magnetic bearing 25 and attract it to itself so as to bias a displacement of the shutter 11 towards the opening position; and/or - the filling device 6 comprises a Hall effect position sensor 28, which is arranged in the area of one of the linear magnetic bearings 25, in a position radially proximal to the first magnetic element 25a, and is configured to detect an axial position of the shutter 11 within the channel 8 through the magnetic interaction of the second magnetic element 25b with the first magnetic element 25a during the movement of the shutter 11 between the opening position and the closing position. With a correct sizing of the magnetic bearings it is possible to obtain the same effects of the ferromagnetic rings 26 and 27, thus leaving the ferromagnetic rings 26 and 27 out. The features of the filling device 6 according to the invention lead to evident advantages that can be obtained. In particular, thanks to the special configuration of the permanent magnets 17 and of the solenoids 18 as well as thanks to the presence of the ferromagnetic sectors 19, 20, 21, 22 and of the ferromagnetic element 23, there is an increase in the precision of the control of the movement of the shutter 11, in the effectiveness and the reliability of the electromagnetic interaction between driving member 15 and driven member 16 and in the magnetic force generated through the interaction between them, compared to known configurations. More in particular, this last aspect increases the radial distance between the shutter 11 and the inner wall 8a, thus improving the outflow of pourable product and the handling of pourable products with pieces, namely consisting of a mix of liquid and suspended parts. Furthermore, thanks to the presence of the bearings 25 and to the fact that the interaction portion 11b and the narrow section 8b are made of a ceramic material, the wear of the filling device 6 and the consequent need for periodic maintenance thereof are minimized. Finally, the presence of recesses 24 on the stem 11a allows for an absorption of centring tolerances of the interaction portion 11b relative to the narrow portion 8b, thus resulting in a further decrease in the overall wear. The perfect control of the position of the shutter, which has ho friction with the product pipe, the dynamics made possible by the particular configuration of the “Voice Coil Moving Magnet” actuators and the new configuration of magnetic sensors enable a large “real- time” control of the filling flow rate, permitting the setting of infinite recipes with flow rate profiles suited for all filling needs. Furthermore, the filling device can be made intelligent so that it adjusts to external dynamic corrections. Finally, the filling device 6 described and shown herein can be subjected to changes and variants, without for this reason going beyond the scope of protection set forth in the appended claims.