EP0401881A2 - Needle printhead with driving and return flap armature electromagnets - Google Patents

Abstract

A needle print head with armature electromagnets (3, 4, 5) which are arranged in an annular arrangement in a metal body (1). The armature (5) is mounted so as to be swivellable in a spacer (70, 71, 71A) and electromagnets (3, 4; 3A, 4A) are arranged in mirror image on both sides of the armature (5), of which electromagnets one serves in each case as return electromagnet. An actuation switch for the attraction and return electromagnets (3, 4; 3A, 4A) is disclosed. High print accuracy and speed are achieved. <IMAGE>

Description

The invention relates to an electromagnetic drive for the printing needle of a needle print head, with a folding armature magnet, the folding armature of which acts with its free end on the printing needle and can be moved into its rest position by means of a reset arrangement.

From DE-AS 18 17 848 a wire magnet with electromagnetic attraction and retraction is known, for which, however, two completely separate magnet arrangements are provided, which have their own armature. This double aneker mass brings with it a slow operation. In addition, a spring is also provided for specifying the anchor resting position, which must be actuated when the suit is tightened, which requires more energy during the tightening time and reduces the tightening speed.

It is known from DE-OS 21 10 410 a needle print head with such a folding armature magnet arrangement, in which the individual folding armature magnets with the anchors, storage and restoring means are mounted on a base plate. This arrangement has the disadvantage that with it narrow air gaps the armature to the hinged armature magnets of a few tenths of a millimeter in width, as required for high-speed heads, can only be adjusted by individual adjustment of each armature with appropriate adjustment means, or that very high expenditure for the dimensional accuracy of all components , namely the anchor and yokes and the stop parts, must be driven during manufacture. In addition, the power consumption of the magnetic coils must be limited due to the small heat transfer cross section to the magnetic head housing, which determines or limits the size and operating speed of the magnets.

From DE 22 01 049 B2 a needle printhead, the armature of which is made from a ferromagnetic part by turning and milling, is known. The anchor ends lie in one plane; however, these are only leveled by turning and not by lapping, since a protruding retaining edge is provided with a bearing groove for the anchor, which does not permit any other processing. Since the armature rests on the middle yoke, the air gap results from the distance between the cover contact surface and the armature end face, the thickness of the stop and the thickness of the armature. He is u. a. dependent on the rotational accuracy of the end face and the support surface.

Furthermore, DE 31 49 300 A1 discloses a magnetic system of a wire dot print head, in which the pole faces are flat and an armature holder surface and an armature surface without an air gap are arranged in the same plane. An air gap is therefore not available as an intermediate energy store; the pressure energy comes from a spring, which entails considerable tolerances. In the absence of a defined air gap, the retraction speed is completely undefined.

Furthermore, from DE-GM 19 23 036 a line printer folding armature magnet with an electromagnetic return magnet is known, the armature of which is designed as an angle lever, one leg of which carries the print hammer and the other leg of which is the effective armature. This is widened at the end, so that the pole faces of the magnets assigned on both sides are at an angle to one another, which leads to high assembly costs. The anchor mass is a multiple of one of the magnetically effective areas, so that the working speed is reduced compared to a simple magnet.

It is an object of the invention to disclose a simple, relatively small folding armature magnet arrangement of high working speed for a needle printhead.

The solution to the problem is that the reset arrangement is a second hinged armature magnet, the hinged armature of which is identical to that of the first and the magnetic yoke is arranged on the side of the hinged armature facing away from the magnetic yoke of the first hinged armature magnet.

Advantageous refinements are specified in the subclaims.

The spacer is made from stamped and layered sheets, preferably from three sheets. In this case, enlarged cutouts are preferably punched into the middle of the layered sheets, which serve to receive a pivot bearing pin of the armature. The hinged armature magnets are expediently mounted on a base plate with recesses, and the windings are then pushed on and soldered. The light metal body is turned out to accommodate the coils and the base plate.

After the magnets have been installed, the sealing compound is introduced and the end and pole surfaces are ground together, so that a defined reference surface is provided for the spacer assembly. The sheets of the spacer can be easily manufactured as stamped parts with a closely tolerated sheet thickness. The anchors of a needle head are grasped and ground as a complete set before the bearing pins are inserted, so that only one grinding process, namely the one relevant to the anchor thickness, results in all air gap widths, which thus have practically no difference between them.

When grinding the armature together, it is advantageously provided that the armature pole faces are radially tapered with respect to the pivot bearing, so that a flat contact is achieved both on the pole faces and on the stop face for the purpose of high damping and a minimization of the remaining gap. The end face of the stop body facing the anchor is also ground so that the air gap and thus the armature stroke is determined by the thickness of the spacer or the total thickness of its sheets less the armature thickness. This ensures the same anchor flight times until the printing needles hit the print material, which in each case produces a typeface that corresponds exactly to a specification, since there is practically no shift in the point of impact of the needles on the paper in the direction of movement of the head relative to its desired position in the case of the usual flying impression. This temporal imprint accuracy is all the more important, the faster the character string and thus the head feed is guaranteed, which, for example, with high-speed print heads. B. is 200 characters per second, which corresponds to a feed rate of 50 cm / sec. Because of the exact temporal flight behavior of the needles and the resulting good typeface at high speed, there is the advantage that a high-resolution typeface with z. B. 24 or 36 needles with so-called "near letter quality" can be created at high speed with a corresponding number of anchors and needles.

Furthermore, it is advantageously possible to additionally arrange a spring on the reset side of the armature, which forms a reset or holding aid.

A particularly advantageous embodiment enables a high working speed with a simple construction and simple manufacture, with the electromagnets with yokes and windings, the windings of which are fastened on the spacer in mirror image of the pull-up magnet as the restoring means and as the stop body on the same armature can be connected to the control device, which energizes these reset electromagnets only when the mirror electromagnet is not energized.

In order to enable a simple construction with a tightly tolerated air gap, the spacer is made of three stamped and layered sheets. In this case, enlarged cutouts are punched into the middle of the layered sheets, which serve to receive a pivot bearing pin of the armature. The hinged armature magnets are expediently mounted on a base plate with recesses, and the windings are then pushed on and soldered. The light metal body is turned out to accommodate the coils and the base plate. After the magnets have been installed, the sealing compound is introduced and the end and pole faces are ground together so that a defined reference surface is provided for the spacer assembly. The sheets of the spacer can be easily manufactured as stamped parts with a closely tolerated sheet thickness.

The anchors of a needle head are grasped and ground as a complete set before the bearing pins are inserted, so that only one grinding process, namely the one relevant to the anchor thickness, results in all air gap widths, which thus have practically no difference between them.

When grinding the armature together, it is advantageously provided that the armature pole faces are radially tapered with respect to the pivot bearing, so that a flat contact is achieved both on the pole faces and on the stop face for the purpose of high damping and a minimization of the remaining gap. The end face of the stop body directed towards the anchor is also ground so that the air gap and thus the anchor stroke is determined by the thickness of the spacer or the total thickness of its sheets less the anchor thickness. This ensures the same anchor flight times until the printing needles hit the print material, which in each case produces a typeface that corresponds exactly to a specification, since there is practically no shift in the point of impact of the needles on the paper in the direction of movement of the head relative to its desired position in the case of the usual flying impression. This temporal imprint accuracy is all the more important, the faster the character string and thus the head feed is selected, which, for example, with high-speed print heads. B. is 200 characters per second; what one Corresponds to a feed rate of 50 cm / sec. Because of the exact temporal flight behavior of the needles and the resulting good typeface at high speed, there is the advantage that a high-resolution typeface with z. B. 24 or 36 needles with so-called "near letter quality" can be created at high speed with a corresponding number of anchors and needles.

The high-speed writing that can be achieved with narrow air gap tolerances and with spring return of the armature is increased in that a reset electromagnet is assigned to each pull-in magnet of the hinged armature instead of a spring as the restoring means. The working magnet only needs to set the armature and the needle in motion when the return magnet is deenergized and bring in the energy required for printing; there is no tensioning of the return spring, so that with the same dimensioning of the components and otherwise corresponding operating conditions, a printing speed which is about 30% higher is achieved.

The arrangement of the return magnets is preferably a mirror image of the pull-in magnets, and their manufacture is correspondingly simple. The pole faces of the reset magnets serve as a stop for the armature in the idle state. Since less power is required for the reset; because the impact energy of the needles, which is not consumed during the printing process, causes the needles to rebound; the return magnets can also be provided with shorter legs and smaller coils. To hold the returned armature, only a relatively small flow of about 20% of the suit flow is required, since the remaining gap is very narrow, so that only very small losses occur in the windings when holding, which are known to be square-dependent on the flow, i.e. approximately 0.5 per mille.

Furthermore, it is advantageously possible to additionally have a spring or a spring on the drive or reset side of the anekr Arrange permanent magnet that forms a reset or holding aid or a drive aid. The non-linear polar force characteristic of a restoring magnet can be fully exploited without special effort in that the pole faces of the permanent magnet are machined together with the pole faces of the electromagnets during the grinding process and thus a flat contact of the armature is achieved. A strong shear of the permanent magnet force due to a large air gap remaining on the rear side to the reflux yoke, which is formed by the return magnet, avoids a noticeable effect of temperature changes on the magnetic force.

A practically stress-free and torsion-free mounting of the armature in relation to the magnetic poles and the pressure needle is advantageously achieved in that the needles and / or the pivot bearing bolts are welded to the armature in situ, for which purpose preferably laser or electron beam welding is used. In order to achieve an exact centering of the armatures with respect to the magnetic field, these electromagnets are each advantageously excited by pulsating current before the welding and are subjected to a continuous current during the welding.

• Fig. 1 zeigt vergrößert einen axialen Ausschnitt durch einen Nadelkopf mit Rückholmagneten;
• Fig. 2 zeigt eine Ansteuerschaltung für einen Anzugs- und einen Rückholmagneten;
• Fig. 3 zeigt einen Ausschnsitt eines Abstands- und Lagerblocks;
• Fig. 4 zeigt einen Ausschnitt eines Abstandsbleches;
• Fig. 5 zeigt einen Anker in Aufsicht im Maßstab gem. Fig. 1.

1 to 5 advantageous configurations are shown.

• Fig. 1 shows an enlarged axial section through a needle head with return magnets;
• 2 shows a control circuit for a pull-in magnet and a return magnet;
• Fig. 3 shows a section of a spacer and bearing block;
• Fig. 4 shows a section of a spacer plate;
• Fig. 5 shows an anchor in supervision according to the scale. Fig. 1.

Fig. 1 shows etrwa 5 times enlarged a cross section through a needle printhead from the central axis (M) in the radial direction with a light metal body (1) in which a magnet yoke (3) carrying a winding (4) is cast. The magnetic yoke (3) is inserted into a recess in the base plate (2). The base plate (2), which is used for the complete assembly of the tightening magnets with the windings (4) and the electrical connections, is held in one turn (12) centrally in the metal block (1). The segment-shaped recesses (42) in the light metal body (1) are filled with casting compound, via which the heat from the windings (4) of the magnet is dissipated. For good heat dissipation, cooling ribs (17) are integrally formed on the outside of the light metal body (1), and the webs between the magnets serve to absorb heat. The casting compound is selected with high heat conduction, for which purpose. B. serve as filler metal particles. The end face (S1) of the light metal body (1) and the pole faces (S2) of the magnet yokes (3) are ground together. Spacers (70, 71, 71A) are arranged on the end face (S1), in which the hinged armature (5) is pivotably mounted, at the end of which extends to the center of the head, a pressure needle (51) is attached, which is arranged in web-shaped needle guides (61) are slidably mounted towards the pressure mouthpiece, not shown. The needle guides (61) are held in a known manner in a housing (6) which is fastened to the light metal body (1) with screws (62) in cylindrical grooves (18).

The pivoting mobility of the armature (5) is limited by the stop surface, which is just ground with the support surface (S1A)) of the stop body on the spacer (71A, 70, 71). The air gap (SP) of the hinged armature magnet thus results from the difference between the total thickness (D) of the spacer and the armature thickness.

A reset electromagnet (3A, 4A) is arranged on the armature (5) as a reset means for the armature (5). In addition, a spring (15) and / or a permanent magnet (15M) can be inserted into cylindrical openings in the light metal body (1, 1A). The return electromagnets (3A, 4A) are each arranged symmetrically with respect to the Anekr (5) in mirror image to the tightening electromagnets (3, 4), whereby they are also fixed in a base plate (2A) and cast in a light metal body (1A) . The pole faces of the return magnets (3A) form the stop faces (S2A). On the outside, the two base plates (2, 2A) are closed off by cover plates (41, 41A).

Fig. 2 shows a circuit arrangement for controlling the windings of a pull-in magnet and a return magnet (4, 4A). The operating voltage (U) is fed to a controllable current source (1Q), which expediently contains a pulse pause control (PP) and a free-wheeling circuit (FD), the output (I) of which via controllable switches (RS, AS) to the winding (4A) of the reset magnet or the winding (4) of the control magnet is switched. A central pressure control (ZS) sends a control signal (A) to the switch (AS) for a given triggering time, which is determined depending on the desired stroke strength and the available pressure, and the pressure control (ZS) determines the pressure at the same time Current strength of the current source (IQ) via the current control signal (IS). At the same time, the switch (RS) is opened via a correspondingly polarized signal (R) and the reset and holding magnet (4A) is released. At the end of the actuation time, for example when the needle hits, the control signal (A) is switched off and the reset magnet current is switched on with the control signal (R). For a return time, the current strength is advantageously chosen to be approximately the same size as during propulsion, so that approximately the same initial magnetic field strength is built up in the air gap, which causes the direction of the anekr to be reversed quickly. Thereafter, a much lower current is specified by changing the current control signal (IS), whereby the Stop the Anekrs not a strong rebound but a hold, so that the needle can be operated again immediately or at any predetermined time without waiting.

In an advantageous embodiment of the formwork, the energy of a winding (4, 4A) that has just been switched off is transferred to the winding that is switched on at the same time and actuates the same armature, thereby significantly accelerating the current build-up or breakdown. To transfer the current between the windings (4, 4A), transfer diodes (D3, D4) mutually form a series connection of these windings, the blocking diodes (D1, D2) effecting their mutual decoupling.

For the fastest possible armature actuation and, on the other hand, for a large degree of independence from the saturation property of the magnetic material, in particular from its temperature dependency, it is advisable to select the flooding during the pull-in time according to an approximately 70% saturation magnetization of the armature. Limiting the saturation also keeps crosstalk between the magnets and neighboring magnets within permissible limits. In a performance-saving version, the flooding during the reset time is expediently about 1: 3 of the suit flooding.

The flow is then reduced to a holding flow of approximately 2% of the value of the suit flow.

A particularly rapid changeover of the armature between the two magnets acting alternately on it takes place if the magnetic fields of both magnets run in the same direction due to the corresponding polarity of the windings in the armature. There are no switching eddy current losses and field build-up delays in the armature.

An advantageous energy-saving energization of the restoring magnets (3A, 4A) results in each case by utilizing the rebound energy of the printing needles (51) and the armature (5), in that after switching off the energization, which occurs, for example, in the event of a needle impact, there is a waiting time without energization, which lasts until for complete anchor reversal, e.g. B. 10 to 20 microseconds, after which the energization of the restoring magnets (3A, 4A) takes place with 1/3 to 1/10 times the suit flow and until the armature (5) strikes the stop and has given off its rebound energy there, which takes about 3/2 to 1 times the tightening time. The current is then switched down to the holding current of approx. 2% of the tightening current. The specified working areas relate to printing up to five uses and more than five uses. The appropriate values, depending on the application, are specified. Preferably, the default values can be changed between two operating states in a switchable manner. If there are more than five uses, the maximum suit flow is used, and if there are fewer than five uses, the suit flow is reduced from 3/4 of this maximum value.

3 and 4 show the spacer plates (70, 71) which have segment-shaped extensions, between which the anchors are arranged in cutouts (75, 72). The laterally widened cutouts (76) serve to accommodate the pivot pin. The bores (73, 74) serve for pinning with the metal bodies.

Fig. 5 shows an anchor (5), which is composed of sheets (53), of which the middle sheet (53M) is extended to the needle (51). The pivot bearing journal (52) is fastened in a groove (54) on the rear anchor end. The thickness of the armature (5) is tapered in accordance with the swivel angle, so that it has a minimal final air gap over its length in both swivel directions, which results in a good use of energy and a low holding flow.

Claims (7)

1. Electromagnetic drive for the printing needle (51) of a needle printing head, with a folding armature magnet (3, 5), the folding armature (5) of which acts with its free end on the printing needle (51) and can be moved into its rest position by means of a reset arrangement,
characterized in that the resetting arrangement is a second hinged armature magnet (3A, 5), the hinged armature (5) of which is identical to that of the first and the magnet yoke (3A) on the side facing away from the magnet yoke (3) of the first hinged armature magnet (3, 5) the hinged anchor (5) is arranged. 2. Electromagnetic drive according to claim 1, characterized in that the excitation windings (4,4A) of the two hinged armature magnets (3,3A) are alternately energized by a control device. 3. Electromagnetic drive according to claim 1 or 2, characterized in that the pole faces (S2, S2A) of the two magnet yokes (3.3A) are aligned. 4. Electromagnetic drive according to claim 3, characterized in that the two magnet yokes (3,3A) are of identical design. 5. Electromagnetic actuator according to one of the preceding claims, characterized in that the longitudinal center plane of the hinged armature is a mirror symmetry plane for both hinged armature magnets (4,4A). 6. Needle printhead with a plurality of electromagnetic drives arranged surrounding a printing needle bundle according to one of the preceding claims, characterized in that the drives between two parallel base plates (2.2 A) are arranged, the distance between which is predetermined by spacing elements (1.1A, 70.71.71A). 7. Needle printhead according to claim 6 with drives according to claims 3 and 4, characterized in that the two base plates (2.2 A) are of identical design.

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