Abstract:
A method and device for imparting a given law of motion to an object traveling along a path, whereby at least one timing body is fed along the path with the same given law of motion, and generates a magnetic field which interacts with at least one portion of the object to connect the object magnetically to the timing body.

Description:
The present invention relates to a method for dynamic control of an object along a path. 
     In particular, the present invention relates to a method of imparting a given law of motion to an object traveling along a path. 
     The present invention may be used to advantage on continuous packing machines, to which the following description refers purely by way of example. 
     BACKGROUND OF THE INVENTION 
     To feed articles along a wrapping path, certain known continuous packing machines (for example of the type disclosed in EP-A1-496046) employ a conveyor having a series of carriages (or trains of carriages) fed along a fixed guide by an actuating device. Along one or more portions of the path, each carriage must be fed according to a given law of motion to time the carriage with respect to, and enable the carriage to cooperate with, other movable members along the path, and so prevent improper timing resulting in damage to the packing machine. 
     On known packing machines of the type described above, each carriage is timed along a portion of the path by engaging the carriage mechanically by means of a locating member, which advances according to the desired law of motion and normally engages a seat formed in the bottom of the carriage. This solution poses several drawbacks by the locating members engaging and releasing the respective seats cyclically, thus resulting, especially during engagement, in a relatively high noise level caused by the impact of each locating member against the walls of the respective seat, and, above all, in severe wear of the locating member-seat connection. 
     EP-A2-276409 or US-A1-5476035 discloses a magnetic conveyor having a plurality of carriages, each of which is fed along a given path by connecting such carriage to a traveling body using a magnetic field generated by the body or the carriage. The magnetic conveyors disclosed in both EP-A2-276409 and US-A1-5476035 may be advantageously used for feeding a carriage along a linear path with a relatively low speed and precision in position, but are not able to give the relatively high performances of conveyance (i.e. high speed and/or high precision in position) requested by a timing device of a modern packaging machine. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for dynamic control of an object along a path, designed to eliminate the aforementioned drawbacks, and which, in particular, is straightforward and cheap to implement. 
     According to the present invention, there is provided a method for dynamic control of an object along a path as recited in claim  1 . 
     The present invention also relates to a device for timing an object along a path. 
     According to the present invention, there is also provided a device for dynamic control of an object along a path as recited in claim  6 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 shows a partial, partly sectioned, schematic side view of a preferred embodiment of the device according to the present invention; 
     FIG. 2 shows a section along line II—II in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Number  1  in FIG. 1 indicates as a whole a conveyor (shown partly) for feeding a train  2  of carriages  3  along a path P defined by two rails  4  (only one shown in FIG. 1) and comprising a horizontal, straight forward portion P 1  and a horizontal, straight return portion P 2  (both shown partly in FIG. 1) parallel to each other and connected to each other by a semicircular portion P 3  extending about a horizontal central axis  5  perpendicular to the FIG. 1 plane. 
     Train  2  comprises three identical carriages  3  connected mechanically to one another by respective connecting arms  6 , each of which is hinged to the ends of two respective adjacent carriages  3  to enable carriages  3  to rotate with respect to each other along the curved portions of path P. 
     Along straight portions P 1  and P 2 , carriages  3  are run along rails  4  by a known actuating device (not shown), which, in one embodiment not shown, comprises a linear electric motor, and, in an alternative embodiment not shown, comprises a releasable connecting member for mechanically connecting each carriage  3  to a chain traveling parallel to rails  4 . 
     Conveyor  1  comprises a timing device  7  for feeding train  2  of carriages  3  along portion P 3  with a given law of motion and in time with known external operating means (not shown) cooperating with carriages  3  along portion P 3 . 
     As shown in FIGS. 1 and 2, each carriage  3  comprises a frame  8  made of nonmagnetic material (e.g. aluminium or plastic) and supporting an upper box  9  for housing an article  10  (defined in the FIG. 1 example by a group of cigarettes) to be conveyed along path P; two horizontal pins  11 , each end of each of which extends outwards of frame  8  and defines a slide engaging in transversely sliding manner an axial seat  12  in a respective rail  4 ; two permanent magnets  13  forming part of timing device  7  and located a given distance D apart; and a bottom pin  14  extending downwards from and integral with frame  8 . 
     As shown in FIG. 2, in addition to permanent magnets  13  of each carriage  3 , timing device  7  also comprises a powered shaft  15  coaxial with axis  5  and supported by a hollow fixed frame  16  via the interposition of two bearings  17 . Along portion P 3 , frame  16  supports rails  4  by means of annular bodies  18  connected to rails  4  and to frame  16  by means of respective screws. 
     Timing device  7  also comprises two disks  19  fitted to shaft  15  and each supporting a number of permanent magnets  20 , each facing a permanent magnet  20  on the other disk  19 . Magnets  20  of each disk  19  are divided into pairs  21  equally spaced along disk  19 , with the magnets  20  in each pair  21  separated by a distance equal to distance D. Disks  19  have respective axes  22  lying in plane II—II, at an angle to each other, and oppositely inclined to define, between disks  19 , a variable-section, semicircular passage  23  through which travels the bottom portion of each carriage  3  supporting permanent magnets  13 . Disks  19  being oppositely inclined with respect to shaft  15 , passage  23  is relatively narrow at an initial portion of portion P 3 , and relatively wide at an end portion of portion P 3 . 
     A fixed semiannular body  24  is housed in the gap between the two disks  19  and radially inwards of passage  23 , and has, along the outer periphery, a semicircular groove  25  coaxial with shaft  15  and engaged in transversely sliding manner by bottom pins  14  of carriages  3 . Shielding members  26  made of ferromagnetic material (preferably magnetic steel) are fitted in fixed positions to body  24  and to rails  4  so as to be interposed between permanent magnets  20  of disks  19  and permanent magnets  13  of carriages  3  at an initial portion and end portion of portion P 3 ; and the thickness of shielding members  26  varies from a maximum at the ends facing portions P 1  and P 2 , to a minimum of substantially zero at the ends facing the central portion of portion P 3 . 
     Thrust bearings  27  are interposed between frame  16  and intermediate portions of disks  19  to enable the axial thrust produced by in-service electromagnetic interaction of permanent magnets  13  and  20  to be transmitted by disks  19  to frame  16 . 
     Operation of timing device  7  will now be described with reference to FIGS. 1 and 2 and to a train  2  comprising three carriages  3  as in the example shown. 
     Train  2  is fed along portion PI by said known actuating device (not shown)—preferably a linear electric motor—until the first carriage  3  in train  2  engages portion P 3 , where the bottom portion of carriage  3  engages passage  23 , and the interacting magnetic forces established between the two permanent magnets  13  of carriage  3  and two corresponding pairs  21  of permanent magnets  20  on the two disks  19  move and lock carriage  3  into a given fixed position with respect to disks  19 . From this moment on, train  2  is fed along portion P 3  in perfect time with the rotation of disks  19 , by each pair  21  of magnets  20  acting as a timing member to time carriages  3  with disks  19  along portion P 3 . 
     For timing device  7  to work properly, the polarities of a permanent magnet  13  and of the two respective permanent magnets  20  facing magnet  13  in use must obviously be opposite at all times, so that the forces between magnets  13  and  20  are always of attraction and never of repulsion. This condition is met substantially at all times, by the positioning error of train  2  of carriages  3  with respect to disks  19  being relatively small (about a few millimeters) at all times, and such as never to result in a relative position in which opposite polarities are positioned facing. 
     Magnets  13  are maintained in a substantially fixed position with respect to respective magnets  20  along portion P 3 , but change position with respect to magnets  20  along the portions connecting portion P 3  to portions P 1 , P 2 ; which change in position is opposed or assisted by the forces of magnetic attraction generated along portion P 3 , thus resulting in severe mechanical stress of both carriages  3  and disks  19 . The purpose of shielding members  26  is to considerably reduce the intensity of such stress, which is done by interposing a member of ferromagnetic material between a magnet  13  and respective magnet  20 , so that the magnetic field lines of each magnet  13 ,  20  tend to close in the iron without affecting the other magnet  20 ,  13 , thus greatly reducing the forces of electromagnetic attraction between the two magnets  13  and  20 . 
     The tilt of disks  19  with respect to shaft  15  provides for obtaining a passage  23  varying in width along portion P 3 , and, hence, a force of electromagnetic attraction between magnets  13  and  20  which also varies along portion P 3  by being closely dependent on the distance between magnets  13  and  20 . For effective timing, while at the same time reducing mechanical stress, disks  19  are so tilted as to obtain a relatively narrow passage  23  where more accurate timing is required, and a relatively wide passage  23  where magnets  13  and  20  move away from one another. In the example shown, the choice made is for extremely precise position control at an input portion of portion P 3 . 
     In an alternative embodiment not shown, permanent magnets  13  are replaced by members made of ferromagnetic material and which, in use, interact with the magnetic field generated by permanent magnets  20 . 
     In yet a further embodiment not shown, permanent magnets  20  are replaced by electromagnets. 
     Timing device  7  as described is particularly advantage for use in conjunction with said known actuating device (not shown) comprising a linear electric motor, by permanent magnets  13  of carriages  3  being used as part of both timing device  7  and the linear electric motor.