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(11) | EP 0 041 126 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Mosaic printing head with cross-talk prevention means |
| (57) A mosaic printing head with cross talk prevention means comprises a plurality of
electromagnetic structure formed by magnetic circuits (39), each coupled to a coil
(41) and closed by a movable armature (43), which at rest leave an air gap in the
magnetic circuit. The magnetic circuits are radially distributed on a supporting member, very closely together at least with a certain portion. The flux leakage which occurs in these portions, due to the proximity of such portions, is avoided by using conductive non magnetic shields (42) partially surrounding such portions. The shields dinamically limit flux leakage transients, i.e. during the attraction of the armature, dueto the eddy currents induced by flux leakage. The shields are designed in order to be coupled with flux leakages and not with the main flux in the magnetic circuit. |
[0001] The present invention relates to a mosaic printing head with cross-talk preventing.means in the electromagnetic structures of the printing head, the cross-talk limiting the printing speed.
[0002] A mosaic printing head generally comprises a needle guide assembly and a plurality of electromagnetic structures coupled to the guide as sembly.
[0003] The electromagnetic structures are mounted on an electromagnetic support and each structure includes a magnetic circuit formed by two po le pieces connected by a yoke and by a movable armature which comple tes the magnetic circuit. It further includes at least a coil wound around one of the two pole pieces.
[0004] Each of the electromagnetic structures acts as an activator for one of the printing needles. To this purpose, each movable armature is provided with at least an arm which protudes externally to the electro magnetic structure and operates as an actuating arm for the associated needle.
[0005] Generally the electromagnetic structures are radially arranged on the electromagnetic support preferably constituted by a ring member.
[0006] One of the two pole pieces of each magnetic circuit is arranged proximate to the internal periphery of the support ring, whilst the other is arranged in proximity to the external periphery of it (from now on the pole pieces will be named internal or external pole piece).
[0007] The radial arrangement of the magnetic circuits involves that the internal pole pieces are closer together than the external pole pieces. Besides, the internal pole piece is generally that one which attracts the free end of the armature: therefore an air gap exists at rest between such pole piece and the armature.
[0008] When an electromagnet is excited, the magnetic field produced in the magnetic circuit has a leakage in correspondence of the initially open air gap: the more the ratio between the open air gap length and the adjacent pole piece distance increases, the more the flux lines tend to link with adjacent internal pole pieces.
[0009] As a consequence of such leakage, the magnetic circuits adjacent to an excited magnetic circuit steal to this last one magnetic energy in the air gap zone.
[0010] The armature of the excited electromagnet is attracted as soon as the attraction force exerted on it by the magnetic field exceedes the va lue of the resisting force produced by a spring which determines at rest the opening of the magnetic circuit.
[0011] As the attraction force on the movable armature is directly proportio nal to the value of the magnetic energy in the air gap, owing to the stealing of magnetic energy in the air gap from the adjacent magnetic circuits, the armature of the excited magnetic circuit is attracted with a certain delay compared with the ideal case when there is no leakage or stolen flux.
[0012] It is to be noted that, if the two magnetic circuits adjacent to the one under consideration are excited at the same time, their magnetic fields oppose to that of the intermediate one and tend to compensate its leakage near the air gap.
[0013] In such case the magnetic energy stolen to the magnetic examination circuit tends to become null, hence the delay in the armature attrac tion tends to decrease.
[0014] As a consequence, in a mosaic printing head of such type, the actuation time of a printing needle changes according to whether the elec tromagnets near to the one associated to the needle are excited or not.
[0015] This problem limits the printing speed of the head as the upper limit of such speed is imposed by the maximum flying time of the needles. Besides that it causes misalignment in the printing operations due to the fact that such operations do not occur in steady printing head conditions but when the head is moving along the printing line.
[0016] In fact, as the excitation command is given to the electromagnets when the printing head is in predetermined positions, the effective actuation of the several needles simultaneously excited occurs with a variable delay and therefore the resulting impression is variably off set as to the predetermined position, owing to such cross-talk.
[0017] A solution to reduce the above mentioned inconvenients could be the one of increasing the diameter of the circular support of the electro magnetic structures, so that the open air gap width is much lesser than the distance between the internal pole pieces of two adjacentma gnetic circuits.
[0020] A solution used for line printers, described in the U.S. Patent N. 2.940.385, involves the use. of magnetic shields for the actuator elec tromagnets of the printing hammers, in order to avoid differences in hammer flying time.
[0021] Magnetic shields could be used for mosaic printing heads to, but such solution would however involve leakage fluxes and performance degradation.
[0023] Such inconvenientsare avoided by the cross-talk prevention means of the present invention, which provide a "dynamic" shielding against the leakage fluxes instead of the static shielding achievable by means of the magnetic shields.
[0024] These means, consisting in shields of non magnetic conductor material (for instance copper), partially shield the magnetic circuit of an electromagnetic structure in the portion of the magnetic circuit whe re the path of the magnetic field is more critical.
[0025] The critical portion of the magnetic circuit in a matrix printing head is, as already mentioned, the pole piece arranged in proximity to the internal periphery of the electromagnetic support, because of the air gap existing at rest between such pole piece and the armature. If such pole piece is shielded by conductive material, the magnetic flux of the magnetic circuit including such pole piece is prevented from linking to the internal pole pieces of the adjacent magnetic cir cuits. Therefore the conductor material shield confines the flux with in the appropriate magnetic circuit.
[0027] In fact the variable magnetic flux tending to cross the metallic shield during the electromagnet initial excitation phase induces eddy .currents within it. Such eddy currents generate a magnetic field opposing the inducing one.
[0028] The induced magnetic field causes a flux concentration inside the air gap and a leakage flux reduction.
[0029] By means of such conductive shield a simple and unexpensive reduction of the cross-talk is obtained without causing a substantial reduction of the magnetic flux acting on the armature.
[0030] These and other features will appear clearer from the following de- scription of a preferred embodiment of the invention and from the at
[0031] tached drawings where:
- Figure 1 shows in side view a simplified printing head already known in the prior art;
- Figure 2 shows in perspective exploded view some elements forming the electromagnetic assembly of a printing head according to thepre sent invention;
[0033] Similar and more detailed structures of such printing heads are described in numerous patents/applications, as for instance British Pa tent N. 1.477.661, U.S. Patent N. 4.051.941 and Italian Patent Appli cation N. 27496 A/78 filed by the same applicant.
[0034] The printing head of fig. 1 comprises a needle guide assembly 2 and an electromagnetic assembly 3.
[0035] The needle guide assembly comprises a frame 4 having a bracket 6, fixed to the frame by a screw 5, which supports a guide for the printing ends of the needles generally formed by a pair of suitably-shaped ruby plates 7.
[0036] The needle guide assembly 2 is fixed to electromagnetic assembly 3by means of two screws (in fig. 1 the head 8 of one of such screws is visible) which lock two brackets formed on the sides of frame 4 against the electromagnetic assembly 3 (in fig. 1 one of such two brackets, evidenced by 9, is visible).
[0037] The electromagnetic assembly 3 comprises a supporting disk 10, to which a suitable number of electromagnets, such as 12, 13, 14, ...18, are fixed, an internal bushing (11, Fig. 2) fixed by calking or shrin kage in a central opening of disk 10 (in alternative the bushing and the disk may form a unitary, sintered, casted, or molded body), an ar mature retainer 19 and a central cap 20.
[0038] The armature retainer 19 is fixed to the bushing (reference 11 in fig. 2) by means of the central cap 20.
[0039] Central cap is provided in proximity of its external periphery with openings, each one receiving a screw (in fig. 1 only two screws21, 22 are visible).
[0040] Such screws insert into corresponding openings present in the armatu re retainer 19 and engage in corresponding threaded holes (references 23, 24, 25 in fig. 2) of the bushing.
[0041] For purposes of completeness, fig. 1 also shows that the external sur face of the ruby plates 7 is placed at a certain distance from a pla ten 26 of the printer.
[0042] A paper printing support 27 and an inked ribbon-28 are interposed bet ween platen 26 and ruby plates 7.
[0043] Even though it is not shown in fig. 1, it is clear that such a printing head is intended to be mounted on a printing carriage of a printer.
[0045] The supporting disk 10, in form of annular element having a central opening where the bushing 11 is engaged, has a suitable number of rec tangular openings (in fig. 2 nine openings 29, 30, ....37 are visible) intended to restrain an extension which each magnetic circuit is pro vided with.
[0046] These openings are radially arranged around the disk center and uniformly distributed near the disk periphery.
[0047] For purposes of drawing clearness, fig. 2 shows a single electromagne tic structure as example of the plurality of electromagnetic structu res generally included in the electromagnetic assembly 3. Electromagnet 38 comprises a magnetic core 39 formed by a pack of U shaped magnetic sheets provided in correspondence of the yoke with an extension 40 for insertion in one of the rectangular openings'of disk 10, for instance opening 30.
[0048] A coil 41 is wound around one of the two pole pieces of the core 39, and precisely around the one arranged in proximity of the external periphery of disk 10 (the external pole piece).
[0049] The internal pole piece of the magnetic core 39 is partially surroun ded by a conductive, non magnetic shield 42 (for instance copper) having C cross section.
[0051] The armature 43 has an extended arm 44 which acts as an activator arm on the head 45 of a needle 46 associated to such armature.
[0052] At rest, the magnetic circuit formed by core 39 and armature 43 has an air gap in correspondence of the internal pole piece, owing to the action of a spring 47 on arm 44 of armature 43 through head 45. The armature retainer 19 (fig. 1) keeps the armature 43 in contact with the external pole piece of magnetic core 39.
[0053] A detailed description of armature retainer 19, as well as of central cap 20, is omitted, since it is not required for the purpose of the present invention.
[0054] However examples of such elements are described in the already cited Italian Patent Application N. 20717 A/80, same applicant.
[0055] As already said, the characteristic object of the present invention is the conductive non magnetic shield 42 which partially surrounds the internal pole piece of the magnetic core 39.
[0056] A similar arrangement is provided for all the other magnetic circuits of the electromagnetic assembly.
[0057] It is to be noted that shield 42 does not cover the pole piece face leaning against bushing 11: the shielding effect on such face may be carried out by the same bushing, which can be made in aluminium.
[0058] It is also to be noted that the shield 42 must not be in contact with bushing 11, if such last is of conductor material.
[0059] In fact, the open turn or C section of shield 42 is essential to avoid that such shield be linked with the main path of the magnetic flux, as it would occur in case of a closed ring.
[0060] In such way the shield is only crossed by possible leakage fluxes and its action only concerns such fluxes in variable or dinamic condition. It has been found by testing that the conductive non magnetic shielding of the internal pole pieces of the electromagnets allows to mini mize the difference among the flying times of the several printing needles without causing any appreciable delays in comparison with an identical electromagnet where the leakage fluxes are minimized becau se of the absence of high permeability elements arranged in the vici nity.
1. An impact mosaic printing head with cross talk prevention means of the type including
a needle guiding assembly having.a plurality of needles and an electromagnet assembly,
said electromagnet assembly' comprising a plurality of electromagnetic structures radially distributed each one
adjacent to the other on a supporting ring mem ber, each of said plurality of electromagnetic
structures including a magnetic core formed by an inner pole piece, an outer pole
piece and a yoke, and an armature, said armature having a lever arm acting on one
of said plurality of needles to drive said nee dle towards a platen, characterized
by that each of said plurali ty of electromagnetic structures is magnetically decoupled
from the adjacent electromagnetic structures by means of a non magnetic conductive
shield partially surrounding each of said inner pole piece.
2. An impact mosaic printing head as claimed in claim 1 wherein each of said shields
has an open turn or C section embracing one of said inner pole pieces.