NEEDLE PRINTING HEAD

Abstract: Needle printing head of the permanent magnet type where the armatures (8, 15) of a plurality of printing actuators are attracted, in rest position, against the poles of a permanent magnet (1) and released in printing position by the "cancellation" of the magnetic flux caused by control windings (5) and where the impact of the armatures against the permanent magnet poles, when they return to the rest position is damped by a multilayer element interposed between armatures and magnetic poles and consisting in two thin sheets of magnetic steel (18, 19) between which a thin film (20) of resilient material, such as polyester is interposed.

Description: The present invention relates to a needle printing head of the permanent magnet type, where the armatures of a plurality of printing actuators are attracted, in rest position, against the poles of a permanent magnet and released in a printing position by the "cancellation" of the magnetic flux caused by control windings. A basic problem encountered in such printing heads, which enable to obtain high operating speed, resides in the impact occuring between armatures and magnetic poles when the armatures return in the rest position. This inpact occurs when the kinetic energy of the armatures is maximum and causes bouncing and consequent armature oscillations. wearing of the contacting surfaces and very high internal tensions in the armature. These internal tensions , which due to the repeatitive operation of the armature are of periodical nature, cause fatigue failure of the armatures and limit their useful life. Several systems for impact damping nave been proposed. The most common one is the use of a thin resilient sheet in polyester or equivalent material, interposed between armatures and magnetic poles. This element, which in a certain sense acts as a residual sheet, must be very thin in order not to cause relevant magnetizing force drop in the air gap so formed. At the same time it must be capable of absorbing the mechanical stresses imparted by the movable armatures. The experience shows that this arrangement, however effective, has a very limited life, as to the desired operation life of the printing head. The resilient sheet, subjected to repeated compression where pressure is not distributed in a unifor way or in a controllable way, is weared and torn, ceasing to perform the requested damping. In order to overcome this problem, US-A-4674896 proposes to have a film, one for each armature and related pole, of non magnetic material, such as stainless steel or polymide resin, fixed to a thin plate of magnetic material wich is cantilever mounted, the same way as the armature, and attracted against the related pole. A leaf spring supports each armature and is provided with a reinforcement magnetic plate which is attracted, together with the related armature, by the related pole, so that the non magnetic film results interposed between the reinforcement plate and the thin plate supporting the non magnetic film. This arrangement is suggested as preventing movement of the film relative to the pole, thus reducing its wear. However a direct impact of the reinforcement magnetic plate on the film still occurs and wear and tearing of the film cannot be completely avoided. Moreover, each magnetic pole must have its own shock adsorbing film mounted as an independent element, so that assembling of the print head is complicate and expensive. These disadvantages are overcome by the permanent magnet needle printing head of the invention. In the printing head of the invention damping of the impact between the movable armatures and the cores of the permanent magnet is obtained by adopting a sandwich composite laminar structure comprising two thin metal sheets, preferably of magnetic steel, among which a thin poliester sheet is interposed. This structure extends beyonds the limited contacting surfaces between armatures and magnetic poles and forms a unitary assembly which can be easily mounted and which encompasses the whole assembly of armatures and magnetic poles. It has been found that by adopting such structure the wearing and tearing phoenomena of the damping element are substantially eliminated and it becomes possible to achieve a useful print head life in the order of hundreds of millions of printed characters. In addition, the metal sheets which protect the resilient sheet perform a magnetic shunting action which increases the release speed of the armature, without affecting the return phase in negative way. The features and the advantages of the invention will appear more clearly from the following description of a preferred form of embodiment and from the enclosed drawings where: Figure 1 shows partially in section view, a preferred embodiment of print head according to the invention Figures 2 and 3 show in top view, as related to figure 1, the elements forming a sandwich damping structure for the print head of figure 1. Figure 4 shows in top view a variant of the element of figure 3. Figure 1 shows partially in section view, a preferred embodiment of print head according to the invention. The print head comprises a base circular plate made of magnetic material, a magnetic toroidal ring 2 which is axially magnetized, a toroidal ring 3 made of magnetic material, a plurality of cylindrical magnetic cores radially arranged around the central axis of the print head. A coil is inserted on each of the magnetic cores. One of the cores and the corresponding coil are shown in figure 1 where they are referenced by numerals 4 and 5 respectively. The printing head further comprises a thin circular ring 6 of magnetic material and a further circular ring 7 in material which is both magnetic and elastic, such as steel. Ring 7 is provided with a plurality of elastic leaves which extend inwardly towards the central axis of the print head. The number of leaves is equal to that of the cylindrical cores and each leaf is radially aligned with a corresponding cylindrical core. Figure 1 shows one of such leaves, referenced by numeral 8. The print head further comprises a circular ring 9 for retaining ring 7 and a closing lid 10 having a nose on the end of which a needly guiding ruby 12 is mounted. The various elements 1,2,3,6,7,8,9 are rigidly packed one over the other so as to form a unitary assembly held together by screws inserted in suitable seats. Two screws 13,14 are shown with dashed lines in figure 1. Each of the elastic leaves 8 is provided with a cylindrical post 15, made of magnetic material and axially aligned with a corresponding cylindrical core 4. Each leaf further has an actuation arm 16 with a free end to which a printing needle is mounted. The various cores 4, plate 1 and rings 2,3,6,7 form a plurality of magnetic circuits, each closed by a movable armature formed by an elastic leaf 8 and by a post 15. In the absence of magnetic forces, the elastic leaves 8 extends perpendicularly to the print head axis and an air gap, in the order of 0,3 mm, is present between the lower face of the posts 15 and the upper face of the magnetic cores 4. However, due to the magnetic field developed by permanent magnet 3, the elastic leaves 8 are normally bent and attracted towards the cores 4 and the posts 15 are in contact with the cores 4. The selective energization of the several coils 5, acts to selectively demagnetize the various magnetic circuits and to selectively release the various leaves 8, so that the corresponding printing needles are actuated to perform the printing operation. When the various coils as 5 are deenergized and the magnetic field is reestablished, the various armatures consisting in leaves as 8 and posts as 15 ire attracted towards the magnetic cores the posts impacting agains cores 4. This inpact causes remarkable mechanical stresses in the actuation arms as 16, which tend to bend owing to their inertia and ultimately fail by fatigue. Further a rotation occurs of posts 15 around a contact point on cores 4, with consequent wear. These phoenomena are further complicated by the occurrence of bouncings. According to the invention the above described structure is improved by adding a sandwich multiple layer damping element interposed between the top of the magnetic poles as 4 and the upper side of toroidal ring 2 on one side and the lower face of ring 3 and the lower face of posts 15 on the other side. As clearly shown by Fig. 2, the multiple layer element comprises two flat thin steel sheets 18,19 between which a thin resilient sheet, preferably in polyester resin, is interposed. Both steel sheets are in permanent contact with the resilient sheet. In a preferred embodiment the three sheets have a thickness in the order of 0,03 mm and the two outer sheets are in magnetic steel having a good hardness, such as the one provided by AISI R51 steel. A multilayer element of the type described has prooved to be particularly effective for damping the impact of the armatures (and more properly of the posts as 15) against the head of the magnetic cores. As a consequence stresses in the actuation arms and bouncing are greatly reduced whilst the wear and tearing of the polyester film is avoided thanks to the protection assured by outer steel sheets. In addition the release speed of the armature from the magnetic core is increased, whilst the return is not substantially affected. This phoenomena is due not only to the magnetic force drop caused in the air gap formed by the polyester film but to factors which results from the following. The thin steel sheets 18,19, which protect the polyester film and which extends from each magnetic core 4 to the cores 4 contiguous thereto on one side, so as to be interposed between the permanent magnet 2 and the toroidal ring 3 on the other, form a magnetic shunt for the flux generated by the permanent magnet. This magnetic shunt has a very small useful section as a path for the magnetic flux and therefore is saturated and diverts a minimum amount of the magnetic flux generated by the permanent magnet. It therefore has a negligible effect on the attractive force exerted on the armature by the magnetic flux. However, when the winding associated with a magnetic core is energized to cancel the magnetic flux, the energization causes a relevant reduction only in the magnetic flux, not a full neutralization. The residual flux which, however small, opposes the elastical return force of the armature, hence its release, is contained to a certain extent in the shunt path holding the path in saturation and only in part exerts a braking action on the movable armature. While figure 1 shows the multilayer element in section view, figures 2 and 3 show in top view (with reference to figure 1) the intermediate polyester film 20 and the two protecting metal sheets 18,19 respectively. The polyester film has the shape of a disc or circular corona having an outer diameter equal to the print head diameter and an inner radius lesser than the radial distance of the cores as 4 from the print head axis. The dotted lines indicate the zones of the film which are juxtaposed to the magnetic cores as 4. Three openings 22,23,24 are provided for insertion of adjusting pins which aid the mounting of the film in the print head. Other openings 25,26,27 are provided for insertion of clamping screws, as the screws 13,14 of figure 1. It can be seen from figure 3 that the protecting metal sheets 18 and 19 have the same shape of the polyester film and corresponding openings for the adjustment pins and the clamping screws. In addition the metal sheets 18 and 19 may have openings 28,29,30 which are circumferentially elongated as shown in figure 3 or distributed on differing diameter or radiuses (as shown in figure 4) to increase the reluctance of the magnetic shunt and to adapt it to specific requirements, for instance in order to obtain a better magnetical decoupling among adjacent magnetic circuits, still assuring the unity of each of the two elements 18,19.

Claims: 1. Needle printing head of the permanent magnet type where the armatures (8,15) of a plurality of printing actuators are each attracted towards a corresponding pole (4) of a plurality of attraction poles magnetized by a permanent magnet (2), characterized by that it comprises a unitary multilayer damping element (18,19,20) interposed between all said armatures (15) and all said attraction poles (4) and consisting in two metal sheets (18,19) having high hardness and in a resilient film (20) interposed between said two metal sheets (18,19), said element (18,19,20) being in permanent contact with said poles. 2. Printing head as in Claim 1 wherein the metal of said two sheets is a magnetic material 3. Printing head as in Claim 2 wherein said two sheets (18,19) have slots for magnetically decoupling each of said poles from the contiguous ones. 4. Printing head as in Claim 2 wherein said two sheets (18,19) extend up to a reclosing element (2) for the magnetic flux of said poles. 5. Printing head as in claim 4 wherein said two sheets (18,19) have slots (28,29,30) for magnetically decoupling said poles (4) from said reclosing element (2).

Inventors: CATTANEO, SERGIO
Applicants: BULL HN INFORMATION SYSTEMS ITALIA S.P.A.
Application year: 1989
Main classification: B41J2
IPC codes: B41J2