WO2004086596A1

WO2004086596A1 - Linear electric motor with an arrangement for reducing vibrations

Info

Publication number: WO2004086596A1
Application number: PCT/IB2004/000999
Authority: WO
Inventors: Fabrizio Tale'; Mario Spatafora
Original assignee: G.D S.P.A.
Priority date: 2003-03-25
Filing date: 2004-03-22
Publication date: 2004-10-07
Also published as: ITBO20030163A1

Abstract

A linear electric motor is composed of a tubular stator element (2) referable to a respective longitudinal axis (A), equipped with a predetermined number of ferromagnetic elements (3) and a set of windings (4) centred on the axis (A), also a rod (5) inserted through the stator element (2) and presenting a given number of permanent magnets (6) designed to establish a state of equilibrium in which the rod (5) remains motionless, and at least one state of non-equilibrium in which the rod (5) shifts along the longitudinal axis (A). The motor (1) also includes an auxiliary ferromagnetic element (10), positioned at a first end (5a) of the rod (5) projecting from the tubular stator element (2), such as will counteract the magnetic interaction between the permanent magnets (6) and the ferromagnetic elements (3).

Description

LINEAR ELECTRIC MOTOR WITH AN ARRANGEMENT FOR REDUCING VIBRATIONS

Technical Field

The present invention relates to a linear electric motor.

In particular, the invention relates to tubular linear electric motors, known also as cylindrical linear electric motors, employed advantageously in automatic machines such as packers or cartoners and machines for forming groups of cigarettes .

It should be stated, in order to allow a fuller comprehension of the following specification, that reference is made here exclusively to linear electric motors of tubular design utilized in machines for forming groups of cigarettes. Nonetheless, it will be appreciated that such tubular linear electric motors find application to advantage in a wide range of automatic machines or robotic devices.

Background Art

In particular, the prior art embraces machines for forming groups of cigarettes that consist in a hopper through which a continuous flow of cigarettes is directed, following a downward vertical path, also a pocket conveyor generally including an intermittently driven belt by which pockets are positioned in succession alongside an outlet opening afforded by the hopper. Such machines further comprise a tubular linear motor located on the opposite side of the hopper from the conveyor, presenting a push rod that serves to transfer a group of cigarettes from the hopper into the respective pocket . More exactly, one end of the rod is insertable into an inlet opening on the side of the hopper opposite from the outlet opening, in such a way as to engage the cigarettes and direct them into the pocket .

The rod is invested with reciprocating motion by the tubular linear motor, which will be synchronized with the conveyor in such a manner as to direct the rod into the hopper and cause it to eject a group of cigarettes each time a pocket is positioned alongside the outlet opening.

More particularly, the linear motor comprises a stationary member or stator of tubular geometry, and a moving member (provided by the aforementioned rod) , slidably accommodated within the stator.

The stator consists in a plurality of annular core laminations ordered along the length of the selfsame stator and centred on a common axis. The laminations are arranged in sets distanced one from the next and alternated with toroidal coils ordered likewise along the length of the tubular stator and centred on the same common axis .

The rod consists in a set of permanent magnets, spaced apart one from the next and ordered with poles in alternating sequence along the length of the rod. The rod is influenced by a magnetic field with lines of force looped in such a way as to establish a magnetic circuit of which the reluctance varies with the sliding movement of the rod within the stator. During the operation of the motor, accordingly, with stator currents acting on the rod and causing its displacement, the rod is affected by a disturbance force linked to the aforementioned variation in reluctance. The force in question is substantially sinusoidal, correlated to the pitch at which the permanent magnets are set, and gives rise to cogging, an effect whereby the rod tends to move in discrete steps generating appreciable vibrations that spread throughout the entire automatic machine. At high operating speeds of the linear motor these vibrations become more and more problematic, often causing the automatic machine to malfunction, as well as damaging certain structural parts of the motor itself, and of the machine. Another drawback is that of the excessive noise made by the rod when cogging, which is intermittent in nature and tends to be particularly annoying and intense especially at high operating speeds .

The object of the present invention is to provide a linear electric motor unaffected by the drawbacks mentioned above.

In particular, it is an object of the invention to set forth a tubular linear electric motor in which the travel of the rod is characterized by a more uniform motion. Disclosure of the Invention

The stated objects are realized according to the invention in a linear electric motor of the type comprising a tubular stator element referable to a respective longitudinal axis, furnished with a predetermined number of annular ferromagnetic elements centred on the longitudinal axis and spaced apart one from the next; also a rod inserted through the stator element, aligned on the longitudinal axis and presenting a predetermined number of permanent magnets ordered with poles in alternating sequence along the length of the selfsame rod, characterized in that it further comprises at least one auxiliary ferromagnetic element located at a first end of the rod projecting from the tubular stator element, centred on the longitudinal axis and movably associated with a first end of the stator element.

The stated objects are realized likewise in a machine for forming groups of cigarettes comprising: a feed hopper in receipt of a continuous flow of cigarettes, presenting a first opening, and a second opening on the side opposite to the first; a pocket conveyor advancing intermittently alongside the first opening of the hopper; and a reciprocating linear actuator with a pusher element insertable through the second opening of the hopper in such a way as to direct a group of cigarettes into a respective pocket positioned in alignment with the first opening; characterized in that the actuator comprises a linear motor as recited in claims 1 to 8 appended. The invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which:

-figure 1 is a perspective view of a machine for forming groups of cigarettes, utilizing a linear electric motor according to the present invention; -figure 2 is a longitudinal section through the linear motor illustrated in figure 1; -figure 2a is an enlarged detail of the motor in figure 2, shown in a first operating state;

-figure 2b shows the detail of figure 2a in a second operating state;

-figure 3 is a cross sectional view taken on III-III in figure 2. With reference to the drawings, 1 denotes a linear electric motor, in its entirety.

In detail, as illustrated to advantage in figure 2, the linear electric motor comprises a tubular stator element 2 extending along a respective longitudinal axis A. The stator element 2 presents two opposite ends, a first end 2a and a second end 2b, delimiting a cylindrical housing 2c designed to accommodate a predetermined number of ferromagnetic elements 3 spaced apart one from the next, each encircling and centred on the longitudinal axis A. More precisely, the single ferromagnetic element 3 is substantially annular in appearance and consists in a set of core laminations, that is to say a plurality of annular laminations 3a offered in frontal contact one to the next and centred on a common axis. The ferromagnetic elements 3 are spaced apart one from the next and alternated with a predetermined number of windings 4 , these too centred on the longitudinal axis A. Each winding 4 is thus interposed between two respective ferromagnetic elements 3 and presents a substantially toroidal appearance. More exactly, and in a manner familiar to a person skilled in the art, the windings 4 consist in coils coupled to an electrical power source not illustrated in the drawings, in such a way as to generate an electromagnetic field aligned on the longitudinal axis A.

In practice, neither the number of electromagnetic elements 3 nor the number of windings 4 is a fixed number, being variable according to how the linear electric motor 1 is sized.

The tubular stator element 2 houses a rod 5 of substantially elongated appearance aligned on the aforementioned longitudinal axis A. The rod 5 consists in a predetermined number of permanent magnets 6, advantageously spaced apart one from the next and designed to generate a magnetic field such as will interact with the ferromagnetic elements 3. In particular, the permanent magnets 6 are distributed along the entire length of the rod 5, with respective north and south poles 6a ordered in alternation. Likewise in this instance, the number and size of the permanent magnets 6 will depend strictly on the sizing of the motor 1, also on the positioning and size of the ferromagnetic elements 3 and the windings 4.

As mentioned previously, and as will emerge more clearly in describing the operation of the electric motor 1, the permanent magnets 6 generate a magnetic field interacting with the ferromagnetic elements 3 to establish a state of equilibrium and of minimum reluctance in which the rod 5 is motionless. Whenever the rod 5 is set in motion internally of the stator element 2, the magnetic interaction between the permanent magnets 6 and the ferromagnetic elements 3 will generate a state of non-equilibrium and higher reluctance, in which forces tending to displace the rod 5 are brought into play.

The first end 2a and the second end 2b of the stator element 2 each present a sleeve 7 of substantially annular geometry aligned coaxially with the longitudinal axis A and combining to create the aforementioned cylindrical housing 2c.

As illustrated to advantage in the cross section of figure 3, the sleeve 7 preferably incorporates three internal supports 8 issuing from the inside surface of the sleeve wall, equispaced at 120° around the longitudinal axis A.

Each support 8 is equipped with a wheel 9 mounted rotatably on a pivot 8a. Accordingly, the wheels 9 are rotatable about respective axes lying orthogonal to the longitudinal axis A and able thus to roll on the outer surface of the rod 5.

The linear electric motor 1 further comprises an auxiliary ferromagnetic element 10 associated preferably with a first end 5a of the rod 5 projecting from the stator element 2.

It will be observed, as illustrated and described purely by way of example, hence with no limitation implied, that the auxiliary ferromagnetic element 10 and the first end 5a of the rod 5 are associated with the first end 2a of the stator element 2 (figure 2) . Nonetheless, the auxiliary ferromagnetic element 10 might equally well be located at a second end 5b of the rod 5 associated with the second end 2b of the stator element 2.

More particularly, the auxiliary ferromagnetic element 10 appears as an annular portion 10a centred on the longitudinal axis A and associated movably with the first end 2a of the stator element 2, such as will induce a magnetic interaction between the permanent magnets 6 and the selfsame auxiliary ferromagnetic element 10, tending to counteract the equilibrium and non-equilibrium state of the rod 5. In a preferred solution, the motor will be equipped with a plurality of annular portions 10a associated in frontal contact one with the next and held together as a pack (not illustrated in the drawings) . Thus, by adding or subtracting portions 10a to or from the pack, it becomes possible to embody an auxiliary ferromagnetic element 10 of variable dimensions .

The auxiliary ferromagnetic element 10 further comprises respective connection means 11 by which the selfsame auxiliary ferromagnetic element 10 is associated with the sleeve 7 at the first end 2a of the tubular stator element 2.

In a preferred embodiment, these connection means take the form of a worm screw 11 extending parallel to the longitudinal axis A, anchored rotatably to the sleeve 7 and engaging a threaded hole afforded by the auxiliary ferromagnetic element 10.

To advantage, the connection means 11 might be provided by any other component capable of shifting the auxiliary ferromagnetic element 10 along the longitudinal axis A toward or away from the sleeve 7, and therefore equivalent in terms of the art .

The linear electric motor 1 described above finds application to particular advantage in a machine 12 for forming groups of cigarettes 13 , of the type illustrated in figure 1.

In detail, such a machine 12 comprises a hopper 14 through which a continuous flow of cigarettes 13 is caused to descend vertically. The hopper 14 presents a first opening denoted 14a, and a second opening 14b on the side opposite to the first.

The machine further comprises a conveyor 15 with pockets 16 extending alongside the first opening 14a of the hopper 14 and driven intermittently in such a manner as to bring the pockets into alignment with the selfsame first opening 14a. In particular, and to advantage, the conveyor 15 consists in a belt carrying a plurality of pockets 16, each presenting a mouth directed toward the first opening 14a. Also forming part of the machine 12 is a reciprocating linear actuator embodied advantageously as a linear electric motor 1 according to the present invention.

In greater detail, the motor 1 presents a pusher element 17 insertable into the second opening 14b of the hopper 14 in such a way as to direct a group of cigarettes 13 toward a relative pocket 16 stationed in alignment with the first opening 14a.

As illustrated in figure 1, the pusher element 17 is composed of an arm aligned on the longitudinal axis A of the motor 1, presenting a first end 17a associated with one end of the rod 5, and a second end 17b opposite to the first end 17a. The shape of the second end 17b substantially matches that of the second opening 14b presented by the hopper 14 , so that the pusher element 17 is able to enter the hopper and displace the cigarettes 13.

Following a description concerned thus far solely with structural aspects, the operation of the linear electric motor 1 will now be explained.

The starting condition, by way purely of example, is one of equilibrium and minimum reluctance, that is to say, a state in which the magnetic field created by all the permanent magnets 6 interacts with the respective ferromagnetic elements 3 in such a way that no force is generated and therefore no movement of the rod 5 induced. As illustrated by the detail of figure 2a in particular, with the rod 5 in a state of equilibrium, the poles 6a of at least one of the permanent magnets 6 are positioned in alignment with two respective adjacent ferromagnetic elements 3, thereby generating a magnetic field able to loop within the selfsame ferromagnetic elements 3.

Neither, in this situation, will there be any force resulting from the magnetic interaction between the auxiliary ferromagnetic element 10 and the permanent magnet 6 nearest the selfsame element 10.

The rod 5 is now caused to shift, assuming a state of non-equilibrium. The movement in question is due to the magnetic field generated by the windings 4, which forces the rod 5 along the longitudinal axis A.

In this situation, the poles 6a of each permanent magnet 6 are positioned between two respective adjacent ferromagnetic elements 3 (figure 2b) in such a way as to generate a magnetic field that attracts or repels, depending on the nature of the pole 6a, in conjunction with the ferromagnetic elements 3. Thus, responding to the force of attraction/repulsion generated by the magnets, the rod 5 will tend to shift toward a successive state of equilibrium and minimum reluctance. At the same time, one pole 6a of a given permanent magnet 6 will draw into alignment with the auxiliary ferromagnetic element 10, generating an attractive or repulsive magnetic field and a force equal and opposite to the resultant of the forces generated by the interaction between the magnets 6 and the elements 3. In other words, the auxiliary element 10 is positioned relative to the rod 5 in such a way that when forces are generated within the stator element 2 (between the permanent magnets 6 and the ferromagnetic elements 3) , these will be cancelled out by a force deriving from the magnetic field interacting between the magnet 6 and the auxiliary ferromagnetic element 10. Accordingly, the field generated by the windings 4 can move the rod 5 along the longitudinal axis A without any interruptions being caused by magnetic forces, which nullify one another. To advantage, the reciprocating movement induced in the rod 5 by the windings 4 occurs substantially without vibration. The movement can be governed by way of a dedicated electronic control system neither described further nor illustrated in the drawings, being conventional in embodiment . Installed in a machine 12 for forming groups of cigarettes 13, advantageously, the linear electric motor 1 serves to direct successive groups of cigarettes 13 into the pockets 16.

In particular, the pusher element 17 is directed toward the hopper 14 by the rod 5, advancing to the point at which the respective second end 17b penetrates the second opening 14b. Once inside the hopper 14, the second end 17b of the element enters into contact with a group of cigarettes 13 and thereupon pushes it through the first opening 14a into a waiting pocket 16 positioned appropriately by the conveyor 15 (see figure 1) .

With the group of cigarettes 13 thus formed inside the pocket 16, the movement of the pusher element 17 is inverted so as to withdraw the second end 17b from the hopper 14 and allow the flow of cigarettes to drop into the space between the first opening 14a and the second opening 14b.

The invention affords significant advantages. In effect, it will be seen that the stated problem of cogging is overcome completely by eliminating discontinuous movement from the rod 5, and therefore from the pusher element 17 of the machine 12.

More exactly, the auxiliary ferromagnetic element is able to counteract the magnetic forces interacting between the permanent magnets 6 and the ferromagnetic elements 3. In this way, the travel of the rod occurs at uniform speed and is completely vibrationless . Another advantage of the linear motor is its simple construction, which brings benefits in terms of cost and manufacture. This advantage is due to the structural simplicity of the auxiliary ferromagnetic element, which consists in a plain annular component of ferromagnetic material . Yet another advantages is the versatile nature of the auxiliary ferromagnetic element 10. It will be seen, in effect, that the auxiliary element 10 can be proportioned so as to allow its integration into any linear electric motor of tubular design by including a simple worm screw mechanism that can be used to impose a condition of phase opposition between the disturbance force and the equilibrating force, equal in magnitude to the disturbance force. The proportioning step in question consists in compacting together a predetermined number of annular portions 10a.

Claims

1) A linear electric motor of the type comprising: a tubular stator element (2) referable to a respective longitudinal axis (A) , furnished with a predetermined number of ferromagnetic elements (3) aligned each on the longitudinal axis (A) and spaced apart one from the next ;

- a predetermined number of windings (4) centred on the longitudinal axis (A) , each interposed between two respective ferromagnetic elements (3);

- a rod (5) inserted through the stator element (2) , aligned on the longitudinal axis (A) and presenting a predetermined number of permanent magnets (6) designed to generate a magnetic field interacting with the predetermined number of ferromagnetic elements (3) and to establish at least a state of equilibrium between the rod (5) and the stator element (2) characterized in that a first end (5a) of the rod (5) projecting from the tubular stator element (2) is associated with at least one auxiliary ferromagnetic element (10) , centred on the longitudinal axis (A) and movably associated with a first end .(2a) of the stator element (2) , of which the function is to counteract the magnetic interaction occurring between the permanent magnets (6) and the ferromagnetic elements (3) . 2) A motor as in claim 1, wherein each ferromagnetic element (3) comprises a plurality of core laminations (3a) offered in frontal contact one to another and centred on a common axis.

3) A motor as in preceding claims, wherein the single windings (4) are of substantially toroidal geometry.

4) A motor as in claim 4, wherein each winding (4) consists in a coil that can be coupled electrically in such a way as generate an electromagnetic field aligned on the longitudinal axis (A) .

5) A motor as in preceding claims, wherein the permanent magnets (6) are spaced apart one from the next .

6) A motor as in preceding claims, wherein the auxiliary ferromagnetic element (10) comprises an annular portion (10a) centred on the longitudinal axis (A) .

7) A motor as in preceding claims, wherein the auxiliary ferromagnetic element (10) comprises connection means (11) through the agency of which the selfsame auxiliary element (10) is associated with the first end (2a) of the stator element (2) and positionable along the longitudinal axis (A) . 8) A motor as in claim 7, wherein the connection means (11) comprise at least one worm screw (12) extending parallel to the longitudinal axis (A) , anchored rotatably to the first end (2a) of the stator element (2) and engaging a threaded hole afforded by the auxiliary ferromagnetic element (10) .

9) A machine for forming groups of cigarettes comprising: a hopper (14) by means of which to feed a continuous flow of cigarettes (13) , presenting a first opening (14a) , and a second opening (14b) on the side opposite to the first opening,- a conveyor (15) with pockets (16) , advancing intermittently alongside the first opening (14a) of the hopper (14) ; and,

- a reciprocating linear actuator equipped with a pusher element (17) insertable through the second opening (14b) of the hopper (14) in such a way as to direct a group of cigarettes (13) into a respective pocket (16) positioned in alignment with the first opening (14a) ; characterized in that the actuator comprises a linear motor (1) as in claims 1 to 8.

10) A machine as in claim 9, wherein the pusher element (17) presents an arm having a first end (17a) associated with one end of the rod (5) of the linear motor, also a second end (17b) opposite to the first end (17a) , shaped substantially to match the second opening (14b) of the hopper (14) and insertable thus into the hopper.