[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 assembly.
[0003] The electromagnetic structures are mounted on an electromagnetic support and each
structure includes a magnetic circuit formed by two pole pieces connected by a yoke
and by a movable armature which completes 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 one arm
which protrudes externally to the electromagnetic 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. 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 pieces).
[0006] 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.
[0007] 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.
[0008] 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.
[0009] The armature of the excited electromagnet is attracted as soon as the attraction
force exerted on it by the magnetic field exceeds the value of the resisting force
produced by a spring which determines at rest the opening of the magnetic circuit.
[0010] As the attraction force on the movable armature is directly proportional 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.
[0011] 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.
[0012] In such case the magnetic energy stolen to the magnetic examination circuits tends
to become null, hence the delay in the armature attraction tends to decrease.
[0013] As a consequence, in a mosaic printing head of such type, the actuation time of a
printing needle changes according to whether the electromagnets near to the one associated
to the needle are excited or not.
[0014] 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. 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 when a variable delay and therefore the resulting impression is variably
offset as to the predetermined position, owing to such cross-talk. A solution to reduce
the above mentioned inconvenients could be the one of decreasing the diameter of the
circular support of the electromagnetic structures, so that the open air gap width
is much lesser than the distance between the internal pole pieces of two adjacent
magnetic circuits.
[0015] Such solution, however, requires an unacceptable increase in the printing head size.
[0016] The cross-talk phenomenon among electromagnetic structures occurs in line printers
too.
[0017] A solution used for line printers, described in the U.S. Patent N. 2.940.385, involves
the use of magnetic shields partially surrounding the excitation coils for the actuator
electromagnets of the printing hammers, in order to avoid differences in hammer flying
time.
[0018] Magnetic shields could be used for mosaic printing heads too, but such solution would
however involve leakage fluxes and performance degradation.
[0019] Besides it would still require an unacceptable size increase.
[0020] Such inconvenients are 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.
[0021] 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 where the path of the magnetic field is more critical.
[0022] 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 circuits. Therefore the conductor material shield confines
the flux within the appropriate magnetic circuit.
[0023] The shield effect is accomplished by the well known eddy current phenomenon.
[0024] 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 as is e.g. described
in the prior publication "Hand- buch für Hochfrequenz- und Elektrotechniker", 1965,
Vol. 1, p. 406 and 407. However, here again, the non magnetic conducting shield surrounds
the coil to be shielded.
[0025] By means of such conductive shield which, in accordance with the invention as defined
in claim 1, partially surrounds the said inner pole piece not provided with a coil,
a simple and unexpensive reduction of the cross-talk is obtained without causing a
substantial reduction of the magnetic flux acting on the armature. The induced magnetic
field causes a flux concentration inside the air gap and a leakage flux reduction.
[0026] These and other features will appear clearer from the following description of a
preferred embodiment of the invention and from the attached 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 the present invention;
[0027] Referring to Fig. 1, it shows in side view an example of mosaic printing head structure.
[0028] Similar and more detailed structures of such printing heads are described in numerous
patents/applications, as for instance British Patent N. 1.477.661, U.S. Patent N.
4.051.941 and Italian Patent Application N. 27496 A/78 filed by the same applicant.
[0029] The printing head of Fig. 1 comprises a needle guide assembly 2 and an electromagnetic
assembly 3.
[0030] 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.
[0031] The needle guide assembly 2 is fixed to electromagnetic assembly 3 by means of two
screws (in Fig. 1 the head 8 of one of such screws is visible) which locks 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).
[0032] 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 shrinkage in a central opening of disk 10 (in alternative
the bushing and the disk may form a unitary, sintered, casted, or molded body), an
armature retainer 19 and a central cap 20.
[0033] The armature retainer 19 is fixed to the bushing (reference 11 in Fig. 2) by means
of the central cap 20.
[0034] Central cap is provided in proximity of its external periphery with openings, each
one receiving a screw (in Fig. 1 only two screws 21, 22 are visible).
[0035] Such screws insert into corresponding openings present in the armature retainer 19
and engage in corresponding threaded holes (references 23, 24, 25 in Fig. 2) of the
bushing.
[0036] For purposes of completeness, Fig. 1 also shows that the external surface of the
ruby plates 7 is placed at a certain distance from a platen 26 of the printer.
[0037] A paper printing support 27 and an inked ribbon 28 are interposed between platen
26 and ruby plates 7.
[0038] 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. Fig. 2 shows in exploded view some
of the elements forming the electromagnetic assembly 3.
[0039] The supporting disk 10, in form of annular element having a central opening where
the bushing 11 is engaged, has a suitable number of rectangular openings (in Fig.
2 nine openings 29, 30, ..., 37 are visible) intended to restrain an extension which
each magnetic circuit is provided with.
[0040] These openings are radially arranged around the disk center and uniformly distributed
near the disk periphery.
[0041] For purposes of drawing clearness, Fig. 2 shows a single electromagnetic structure
as example of the plurality of electromagnetic structures generally included in the
electromagnetic assembly 3.
[0042] Electromagnet 48 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.
[0043] 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).
[0044] The internal pole piece of the magnetic core 39 not provided with an excitation coil,
is partially surrounded by a conductive, non magnetic shield 42 (for instance copper)
having C cross section.
[0045] The electromagnet 38 is completed by a movable armature 43 closing the magnetic circuit.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] However examples of such elements are described in the already cited Italian Patent
Application N. 20717 A/80, same applicant.
[0050] 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.
[0051] A similar arrangement is provided for all the other magnetic circuits of the electromagnetic
assembly.
[0052] 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. 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.
[0053] 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.
[0054] In such way the shield is only crossed by possible leakage fluxes and its action
only concerns such fluxes in variable or dynamic condition. It has been found by testing
that the conductive non magnetic shielding of the internal pole pieces of the electromagnets
allows to minimize 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 because of the absence of high permeability
elements arranged in the vicinity.
[0055] The conductive non magnetic shields of the magnetic circuits in the electromagnetic
structures of a mosaic printing head may clearly have shapes other than the described
one, without departing from the scope of the invention.