PRINT HEAD INCORPORATING A ONE PIECE ARMATURE

Abstract: A wire dot matrix print head suitable for a low cost computer printer is described. The print head incorporates a one piece armature (68) constructed of thin sheet metal. The armature (68) has a plurality of radial arms (204) radiating inwardly from a flexible periphery (200). Each of the arms (204) has affixed to it a moving core (70) and a pair of flanges (206) for adding rigidity. Each of the radial arms (204) is connected to the periphery (200) at an arm base (202). Each of the arm bases (202) is provided with a slight bend (208). The combination of torsional action of the arm base (202) at the bend (208) and a fixed clamping action of a cap (78) at the flexible periphery (200) provides a biasing force to each of the radial arms (204).

Description: Wire dot matrix print heads have been in use for many years and are noted for their versatility and low cost. In the prior art, wire dot matrix print heads are known to include a circular arrangement of electromagnets which are selectively energized to attract a moving core mounted on an armature. The attraction of the moving core urges the armature against a print wire. The print wire is driven against a ribbon which leaves a single dot on a record medium such as paper. The accumulation of dots produces printed characters. Examples of dot matrix print heads are disclosed in United States Patent Nos. 3,770,092; 3,892,175; 4,244,658 and 4,569,605. In dot matrix print heads of the prior art, individual armatures or actuating levers are provided. The individual armatures are radially aligned about a center point with each armature extending inwardly to a tip. The tip of the armature engages the end of a print wire. Electromagnetic coils are positioned relative to the armatures so that energization current passing through the coils creates a magnetic field attracting the armature toward the coil. The armature, moving under the attractive influence of the magnetic field, drives the print wire toward a print position. Each of the armatures of the prior art functions as a cantilever in that the armature is supported at the base, along the periphery of the print head, by a leaf spring. The objectives in the design of a print head are to combine high speed with ease of manufacture. One way to achieve higher print speeds is to minimize both the armature mass and the gap between the core and armature. As the armature mass and core/armature gap decrease, there is a concomitant increase in the difficulty of manufacturing the print head. From EP 0104260 there is known a print head for a dot matrix printer with a plurality of print wires, a frame and a stator assembly seated against said frame and including a plurality of fixed posts constructed of magnetically permeable material and a plurality of coils, each of said fixed posts surrounded by one of said coils, each of said fixed post and coil pairs defining a cavity, and a one piece armature having a periphery with a plurality of radial arms radiating inwardly, each radial arm having affixed thereto a movable core for driving print wires against a record medium. The present invention is characterised by said cores extending into said cavities to define a gap between said core and said fixed post when said armature is in said rest position, said radial arms being connected at said periphery by an associated arm base, each of the latter having a slight bend for biasing said radial arm in a direction opposite to that travelled by said driven print wire in printing a character, each of said radial arms having a pair of flanges extending from said arm base for rigidity, said one piece armature being constructed of thin sheet metal, each of said moving cores being shaped at least in part as a frustum of a cone and a flux plate covering the coils having openings of a shape complementary to the shape of said moving cores. With the print head according to the invention numerous goals and objectives have been obtained that combine in a synergistic way to provide a simple, easily manufactured and fast print head. A feature of a one piece armature design is a reduction in the number of parts that require critical alignment. In the prior art designs using multiple armatures, the individual armatures had to be aligned with each other as well as the electromagnetic coils. With a one piece armature, the relative positions of the armatures may be accounted for in the manufacturing process thus eliminating an entire set of constraints and potential adjustments. Another feature of a one piece armature may be a substantially reduced armature mass with a proportionate rise in print speed. A lower mass is achieved in the present invention with the use of thin sheet metal, for example Carpenter Steel Co., No. 455. An additional advantage of low mass is that less heat is generated in the print head with a resulting increase in print head life and improved performance. The print head of an embodiment this invention has a frame assembly that includes a main frame, a wire guide and a nose. In the frame assembly is mounted a print wire assembly. The print wire assembly includes a plurality of wire rods; each of the wire rods has one end for driving a ribbon against a record medium and a second end that includes a head and a spring. The spring is compressed between the head and the main frame spring seat. A stator assembly includes a plurality of coil assemblies, a focus plate and a residual or insulating means. Each of the coil assemblies has a coil surrounding a fixed post defining a cavity. The fixed post and the focus plate are constructed of a magnetically permeable material. A signal means is mounted between the frame assembly and the stator assembly and is provided for selectively activating and deactivating each of the coil assemblies. Mounted adjacent to the stator assembly is the one piece armature comprised of a flexible periphery with a plurality of rigid arms connected at the periphery and radiating inwardly therefrom for driving the print wire assemblies in a forward direction. Each of the radial arms has a pair of flanges for rigidity and is associated with an individual arm base. Each of the arm bases has a slight bend for biasing the associated radial arm in a rearward direction against a backstop. The rearward biasing of said radial arm is provided by the combination of torsional action in the flexible periphery and the fixed clamping action of a pad. When activated, each of the coil assemblies drives the associated radial arm against one of said print wire heads. When the coil is deactivated, the slight bend in the arm base, in combination with the print wire spring, provides a biasing action for returning the radial arm to a rest position. Brief Description of the Drawings Figure 1 shows a print head without heat sink and retention means made according to the present invention. Figure 2 is an exploded view of the print head of Figure 1 including a heat sink and retention means. Figure 3 is a one piece armature including entirely cylindrical cores. Figure 4 is a side view of the one piece armature in Figure 3 taken along the line A-A. Figure 5 shows the underside of the nose 4 depicted in Figure 2 with the print wire guides exposed. Figure 6 is an enlarged sectional view of a partially disassembled print head of Figure 1 (without ribbon guide) which includes an armature having entirely cylindrical cores. Figure 7 is a side view of a print wire useful in the present invention. Figure 8 is an enlargement of the underside portion of the end cap depicted in Figure 2. Figure 9 is a manufacturing jig used to align the coils accurately in a metallic cup. Figure 10 is an embodiment of the print head made according to this invention where the magnetic gap between the moving core and focus plate has the shape of the lateral surface of a frustum of a cone. The print head for a serial dot matrix printer made according to the present invention is illustrated in Figure 1 and comprises frame assembly 2, including nose 4, with a carriage mounting bracket 6. Affixed to nose 4 is ribbon guide 8 for guiding the ribbon (not shown) during the printing operation. Flexible cable 10 electronically connects the print head to suitable print control means in a printer as is well known by those skilled in the art. Referring now to Figure 2, the individual components of the print head will be described. Frame assembly 2 is comprised of nose 4, main frame 12 and wire guide 14. In the preferred embodiment, frame assembly 2 is constructed of molded plastic. Mounting bracket 6 comprises part of frame assembly 2 in the preferred embodiment and is disposed for mounting the entire print head on a carriage assembly (not shown) of the printer (not shown). A ribbon guide 8 is mounted on nose 4 by means of mounting brackets 18. The underside of nose 4 is depicted in Figure 5 and shows wire separator 5 and nose guide 7 exposed. Referring again to Figure 2, wire guide 14 has a plurality of wire receiving means 22 for supporting each of the wire assemblies 24. In the preferred embodiment there are nine wire receiving means for each of the nine wire assemblies. Each wire assembly 24 (one is shown) includes a wire rod 26, which has one end 28 for driving an inked ribbon against a print medium and a second end 30 for receiving a driving force. Second end 30 includes head 32 for compressing biasing spring 34 against spring seat 36 of wire receiving means 22. Referring to Figure 7, a cut away view of second end 30 is shown where head 32, constructed of a molded plastic material, for example Delrin, (trademark) surrounds hook 33. Again referring to Figure 2, a magnetic flux conducting or constraining yoke, in the form of metallic cup 38, abuts frame assembly 2 separated by signal means 60. Metallic cup 38 has central bore 40 and opening 42 for receiving and surrounding wire receiving means 22. An annular seat 44, surrounding bore 40, has a plurality of posts 46. Each of posts 46 has mounted thereon a coil assembly 48, only one of which is shown. Coil assembly 48 has a bobbin 50, about which is wound coil 52, with a cavity 54 for receiving post 46. Bobbin 50 also has pins 56 which have soldered to them the ends of coil 52. Pins 56 extend through holes 58 in annular seat 44 and are connected to the signal means 60. Bobbin 50 includes locating pin 62 for orienting the position of flux plate 64, residual 66, one piece armature 68 and cap 78. Referring also to Figure 9, there is shown a jig 100 used to align precisely the coil assemblies 48 on posts 46 of metallic cup 38. The individual coil assemblies 48 are mounted on the mounting posts 102 so that locating pin 62 fits in holes 104. When all of coil assemblies 48 are mounted on jig 100, the posts 46 of metallic cup 38 are aligned with cavity 54 of bobbins 50 and pressed together. Jig 100 is then removed and the individual coil assemblies 48 are precisely mounted in metallic cup 38. A potting compound is then applied to hold the assemblies 48 in place. Focus plate 64, also known as a flux plate, abuts metallic cup 38 and has openings 65 for receiving moving posts 70. Holes 67, of focus plate 64, are for receiving locating pin 62 of coil assembly 48. Focus plate 64 is constructed of a highly permeable magnetic material such as Hiperco 50A (trademark). Electrically separating the focus plate 64 from the one piece armature 68 is a residual 66 constructed of an insulating material, e.g. Dupont Nomex (trademark) aramid paper type 410. Residual 66 has openings 69 and holes 71 that align with openings 65 and holes 67 of focus plate 64. Moving core 70, constructed of Hiperco 50B, of one piece armature 68 is seated inside opening 69 of residual 66 and opening 65 of focus plate 64 and fits within cavity 54 of coil assembly 48 to define a gap 47 between moving core 70 and post 46. Damper 72, comprised of skin 74 and rubber 76, is provided with fingers 73 that align with arms 204 of one piece armature 68 to absorb recoil energy produced by the return of wire assembly 24. In an alternative embodiment, damper 72 is a single piece of urethane constructed of 100 parts Vibrathane 821 (trademark) from Uniroyal to 12. 7 parts curative A931 also available from Uniroyal. Referring now also to Figures 3 and 4, one piece armature 68 has periphery 200 in the shape of a polygon, however, a circular or other shape for periphery 200 is an acceptable alternative. Affixed to periphery 200, at arm base 202, is radial arm 204. Each of radial arms 204 has a pair of flanges 206 for rigidity. Each radial arm 204 also has a moving core 70 affixed by either riveting (Figures 4 & 10) or welding (Figure 6). A slight bend 208, of about 3 degrees in the preferred embodiment, is made along the arm base 202 to bias the radial arms 204, when armature 68 is in a rest position, in a rearward direction opposite to that traveled by wire assembly 24 when it is activated. The biasing action is produced from the combination of the bend 208 in periphery 200 and the clamping action produced by cap 78 (Figure 8). This is shown clearly in Figure 4, where under normal conditions radial arms 204 are out of alignment with plane 210 which is formed by the sides of periphery 200. Holes 214 are provided in one piece armature 68 for receiving locating pins 62 of coil assembly 48 (Figure 2). Now referring to Figure 2, cap 78 clamps, at 18 to 27 pounds (80 to 120 newtons), periphery 200 of one piece armature 68, residual 66 and focus plate 64 to metallic cup 38 by the action of spring 80 and clamping ring 82. The individual fingers 84 of spring 80 fit in the ledges 86 of cap 78. The end positions 88 of clamping ring 82 press against up-fingers 90 of spring 80 and when said clamping ring 82 is positioned in groove 17 of heat sink 16, the entire print head assembly is held in place. Figure 8 shows the back of cap 78, which abuts the one piece armature 68. Cap 78 has holes 300 for receiving locating pins 62 of coil assembly 48 and platform 302 for positioning damper 72. Clamping arm 304 of cap 78 contacts and loads clamping pad 212 of one piece armature 68 against the stator assembly comprised of cup 38, coil assemblies 48, flux plate 64 and residual 66. Cap 78 can be constructed of any long wearing non-conductive material. In the preferred embodiment, cap 78 is constructed out of nylon. Now referring to Figure 6, which shows the partially assembled print head, the operation of the print head will be explained. A voltage is placed across pins 56 by signal means 60 causing current to flow in coil 52. The current in coil 52 produces a magnetic field having field lines 51 that are concentrated in metallic cup 38. The magnetic field follows lines 51 along outer edge 39 of metallic cup 38, up through post 46, crossing gap 47, through moving core 70 to focus plate 64 and back to metallic cup 38. The magnetic field produced by the current flowing in coil 52 causes moving core 70 to be attracted to post 46. The action of moving core 70 drives radial arm 204 of one piece armature 68 in a forward direction 400. The movement of radial arm 204 imparts a driving force to head 32 that compresses biasing spring 34 and drives end 28 against an ink ribbon and the ribbon against a record medium, such as paper, leaving a dot on the paper. When the voltage is removed from pins 56, current stops flowing in coil 52 eliminating the magnetic field and ending the attraction between moving core 70 and fixed post 46. Without a force applied in direction 400, radial arm 204 returns to its normal rest position under the action of both compressed biasing spring 34 and torsion in the periphery 200, through the bend 208 in one piece armature 68. Damper 72 absorbs any excess recoil energy in rigid arm 204 and print wire assembly 24. By selectively energizing the individual coils, the individual print wires are selected and dots are placed on the record medium in such a way that characters are formed. Figure 10 illustrates an alternative embodiment of one piece armature 68 and focus plate 64. In this alternative embodiment, moving core 500 is comprised of head 502 and base 504. Head 502 is formed in the shape of a frustum of a cone while base 504 is formed in the shape of a cylinder. Modified hole 506 in focus plate 64 is configured in a complementary shape to that of head 502. The configuration of head 502 and modified hole 506 enhances the magnetic attraction acting on moving core 500 and thereby improves the performance of the print head. It is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. While the invention has been particularly shown and described with reference to the preferred embodiment, it will be understood by those skilled in the art that changes in form and detail may be made without departing from the scope of the invention as defined by the claim.

Claims: A print head for a dot matrix printer with a plurality of print wires (24), a frame (2) and a stator assembly (38) seated against said frame (2) and including a plurality of fixed posts (46) constructed of magnetically permeable material and a plurality of coils (52), each of said fixed posts surrounded by one of said coils, each of said fixed post and coil pairs defining a cavity (54), and a one piece armature (68) having a periphery with a plurality of radial arms (204) radiating inwardly, each radial arm having affixed thereto a movable core (70,500) for driving print wires against a record medium, characterised by said cores (70,500) extending into said cavities (54) to define a gap (47) between said core and said fixed post (46) when said armature is in said rest position, said radial arms being connected at said periphery by an associated arm base (202), each of the latter having a slight bend (208) for biasing said radial arm in a direction opposite to that travelled by said driven print wire in printing a character, each of said radial arms (204) having a pair of flanges (206) extending from said arm base (202) for rigidity, said one piece armature (68) being constructed of thin sheet metal, each of said moving cores (500) being shaped at least in part as a frustum of a cone (502) and a flux plate (64) covering the coils (52) having openings of a shape (506) complementary to the shape of said moving cores.

Inventors: SMITH, NORMAND C.
Applicants: LEXMARK INTERNATIONAL, INC.
Application year: 1987
Main classification: B41J2
IPC codes: B41J2