ELECTROMAGNETIC PRINT ELEMENT ACTUATOR

Abstract: The present invention relates to an electromagnetic print element actuator which comprises a stator member (10) formed with two spaced apart pole pieces defining a gap and a coil (12) surrounding the stator member between the pole pieces, an elongated armature member (16) fixed at one end (18) and extending adjacent to the gap, and a print element (22) attached to the other end of the armature member. Energisation of the coil (12) causes attraction of the armature member (16) towards the gap and actuation of the print element (22). The print element actuator of the invention is characterised in that the armature member (16) extends substantially perpendicular to the flux path across the gap so that the flux path extends transversely through the width of the armature member (16). The invention also relates to a bank of electromagnetic print element actuators comprising a plurality of print element actuators of the above type in which the stator members of the actuators are formed into an integral stator unit (30) with the armature members extending parallel to each other.

Description: The present invention relates to electromagnetic print element actuators for use in the printing mechanism of printers, and to a bank of print element actuators. The art is replete with electromagnetic print element actuator devices. Such devices seek to achieve high speeds and greater print density using a variety of actuator configurations. Wire matrix printers, in particular, seek to increase print density by decreasing the distance between adjacent actuator wires. Consequently, a standing requirement in this field is to reduce overall actuator size. United States Patent Specification No. A-3,138,427 describes a facsimile system utilising a transducer assembly comprising an armature, coil and a core comprising leg elements. A marking member is clamped to one leg. The amount of pressure exerted by the forward longitudinal edge of the marking element is a function of the energisation of the winding from the source. A moving coil assembly, as illustrated in United States Patent Specification No. A-3,780,650, employs a coil with pole pieces positioned between pole plates. The magnetic reluctance is reduced by having the pole pieces arranged with the air gaps parallel to each other. IBM Technical Disclosure Bulletin, Volume 21, Number 11, pp. 4452-4453 (April 1979) discloses a print hammer assembly employing a bank of print hammers individually supported on a base by means of a cantilever arrangement. Armature poles have coils wound in series on bobbins placed over the armature poles. The flux path is minimised due to the series winding of the coils and is disposed in a longitude direction aligned with the direction of movement of the spring hammer elements. A variation of this mechanism is shown in IBM Technical Disclosure Bulletin, Volume 25, Number 9, pp. 4901-4902 (February 1983). The actuator disclosed therein employs a print hammer cantilever-mounted on a magnet yoke carrying an energising coil, a spherical stop and a rest stop. The rest stop includes a permanent magnet for biasing the print hammer into a rest position. Upon energisation of the coil, the armature flexes, deflecting the hammer element about the spherical stop which acts as a fulcrum. Another example of a print hammer mechanism employing a pivoting print finger is illustrated in IBM Technical Disclosure Bulletin, Volume 22, Number 8B, pp. 3536-3537 (January 1980). The actuator therein employs a holding magnet and a separate coil for purposes of releasing the print finger from its retaining structure. A somewhat different arrangement is illustrated in IBM Technical Disclosure Bulletin, Volume 23, Number 5, pp.1765-1766 (October 1980). Each print wire is driven by a piston and is held in a home position by means of a magnetic circuit including housings and a permanent magnet. A coil bobbin assembly having magnetic return elements is offset relative to the travel of the print wire. The magnetic flux path acts in a direction aligned with the travel of the print wire. Other art considered, but deemed less germane to the present invention, is disclosed in IBM Technical Disclosure Bulletin, Volume 22, Number 8A, pp. 3171-3172 (January 1980) and Volume 22, Number 8B, pp. 3672 (January 1980). These disclosures relate to electronic techniques for flight time control of print hammers. Also considered, solely for purposes of the magnetic circuit, is the United States Patent Specification No. A-2,202,729, which discloses a coil, armature and pole pieces. The relay disclosed in that patent specification is not considered pertinent to a print hammer assembly. US-A-3 973 661 describes an electromagnetic print element actuator which comprises a stator member formed with two spaced apart poles pieces defining a gap and a coil surrounding the stator member between the pole pieces. An elongated armature member is fixed at one end and extends adjacent to the gap in the stator member. The armature member also extends substantially perpendicular to the flux path across the gap so that the flux path extends transversely through the width of the armature member. A print element is attached to the other end of the armature member. Energisation of the coil causes attraction of the armature member towards the gap and actuation of the print element. The object of the present invention is to provide an improved electromagnetic print element actuator and an improved bank of electromagnetic print element actuators. The present invention relates to an electromagnetic print element actuator which comprises a stator member formed with two spaced apart pole pieces defining a gap and a coil surrounding the stator member between the pole pieces, an elongated armature member fixed at one end and extending adjacent to the gap, and substantially perpendicular to the flux path across the gap so that the flux path extends transversely through the width of the armature member. The print element of the invention is characterised in that the pole pieces extend along substantially the full length of the armature member. The invention also relates to a bank of electromagnetic print element actuators comprising a plurality of print element actuators of the above type. The bank of actuators of the invention is characterised in that the stator members of the actuators are formed into an integral stator unit with the armature members extending parallel to each other. Since the flux path for each armature member extends transversely across the width of the armature member, adjacent armature members in the bank of actuators can share a common flux path when the adjacent print elements are selected for printing. Alternatively, the coils of adjacent stator members may have an opposite polarity to eliminate magnetic interaction. In order that the invention may be more readily understood an embodiment will now be described with reference to the accompanying drawings, in which: FIGURES 1-5 illustrate a simplified general embodiment of the invention, FIGURE 1 is an end view, partly in section, of a print element actuator in accordance with the invention, FIGURE 2 is a side view of the actuator illustrated in Figure 1 sectioned on the line 2-2, FIGURE 3 is a semischematic sectional view of a one-piece stator unit having a plurality of independent coils wrapped thereon forming a multi-actuator assembly using the same polarity in adjacent coils, FIGURE 4 is a semischematic sectional view of a one piece stator unit employing opposite polarity in adjacent coils, and FIGURE 5 is a semischematic elevation view of a multi-actuator armature member plate, FIGURES 6-8 illustrate a more detailed practical embodiment of the invention, FIGURE 6 is a perspective view of a laminated stator unit, FIGURE 7 is a plan view of a multi-actuator armature member plate, and FIGURE 8 is an end view of the plate illustrated in Figure 7 sectioned along the line 8-8, showing construction details and illustrating the stator unit profiled for optimum dynamics. FIGURES 9 and 10 are perspective views illustrating alternative techniques of coil termination using printed circuit boards integral with the stator unit. Figures 1-5 illustrate the invention in a simplified general embodiment. Referring now to Figures 1 and 2, a single print element actuator stator member 10 is shown in partial section. The stator member 10 serves as a winding bobbin for a coil 12. The stator member has a generally H-shape core wherein the vertical walls serve as pole pieces with a gap between their upper ends and confine the coil 12 in a defined bobbin winding area. This technique provides an effective heat transfer path from the coil through the core of the stator member and into ambient air. It thereby allows higher input power and higher duty cycles for the actuator without damaging the coil. Cost is reduced by eliminating the requirement of a separate coil bobbin and its subsequent assembly onto the actuator. The stator member 10 includes a non-magnetic stator section 11 and a magnetic stator section 13. The magnetic stator section 13 is limited to the areas of desired magnetic flux; the non-magnetic stator section 11 completes the physical package for coil support, and serves as a heat transfer medium. Materials may be varied, but iron (Fe) for the magnetic section and aluminium (Al) for the non-magnetic section are shown. Pole plates 14 are disposed on the stator 10 on the ends of the pole pieces and effectively reduce the size of the gap between the ends of the pole pieces. An armature member 16 is supported above the pole plates 14. The pole plates 14 permit a narrow armature member 16 to be employed in conjunction with a wide coil 12. This improves actuator efficiency since resistance losses in the coil are inversely proportional to the cross sectional area of the coil. Alternatively, the pole plates 14 can be eliminated and the armature member 16 made slightly wider than the coil 12 to rest on the ends of the pole pieces of the stator member 10. The stator member 10 may alternatively be manufactured utilising sintered iron to have a shape approximately like the combination of the core and the pole plates. The coil 12 is illustrated in Figure 1 as a conventional circular wire. It is possible to employ a thin ribbon wire having a width W and wound on itself as a continuous tape around the stator member 10. In this case, a complete bobbin is not necessary since the flat ribbon wire will not spread. Magnetic components are ferromagnetic; non-magnetic components are diamagnetic or weakly ferromagnetic, as is known in the art. Magnetic components, that is the stator member 10, the pole plates 14, and the armature member 16, may be machined from iron, magnetic steel, silicon iron or the like, or, alternatively, may be formed using sintered iron and standard powder-metallurgy techniques. Figure 1 illustrates the magnetic flux path through the actuator. The magnetic flux path through the armature member 16 is in the transverse direction. That is, the flux flows into the armature member along one edge (left side), passes across the width of the armature member, and then returns to the stator member core through the opposite edge (right side). The lower portions of the core serve only to contain the coil. As illustrated in Figure 1, the desired flux does not flow through the lower portion of the core, especially with a non-magnetic material such as the aluminium heat-sink of Figures 1 and 2. The transverse flux path permits the armature member 16 to be very thin, and accordingly very light, without saturation, and yet providing a large total air-gap area. The large air-gap area produces large magnetic forces on a low-mass armature member. This results in high acceleration and fast response. As illustrated in Figure 2, the armature member 16 is fixed about a point 18, that is, clamped between the pole plates 14 and a back-up plate 20. The armature member can be relatively rigid and pivot about point 18, but, for low-energy applications such as wire matrix printing, the preferred armature member is a thin flexing cantilever beam rather than a rigid pivoting body. The back-up plate 20 serves to limit the travel of the free end of the armature member 16. Given the large contact area between the armature member 16 and the back-up plate 20, an improvement in the settle-out characteristics of the armature member is achieved. The back-up plate 20, not forming a part of the magnetic circuit, is preferably moulded of an energy-absorbent polymer, and has a profile chosen to provide optimum armature member settle-out (e.g. the static deflection profile of the end-loaded cantilever beam 16). Armature member attachment by other techniques may be employed. For example, the armature member 16 and the stator member 10 can each be mounted to a third member forming a mounting structure supporting both the stator member 10 and armature member 16. Figure 2 also illustrates schematically the remaining components of the print element actuator, including a print wire 22 suspended near the free end of the cantilever armature member 16, and a compression return spring 24. The return spring has one end fixed to a rigid plate 26 with the other end coupled to the armature member 16. Consequently, the armature member 16 is normally biased and flexed upward by the compression spring 24, placing the print wire 22 in the at-rest position shown. Actuation by the energisation of the coil 12 causes the armature member 16 to be electromagnetically driven in a direction downward toward the stator member 10, thereby overcoming the bias provided by the compression spring 24 and placing the wire 22 in a print contact position. Figure 2 illustrates one mode of operation where the actuator pushes the print wire 22 towards a ribbon and paper (not illustrated) whenever the coil is energized. An alternative mode is to hold magnetically the armature member and print wire cocked against a spring force whenever printing is not required, and then to release the armature member 16 when a dot is required to be printed. In this so-called "pick-and-hold" operation, the armature member itself can provide the spring force through bending. The stored energy is employed to accelerate the wire into the ribbon and the paper. The coil may be energised with a relatively low current to hold the armature member back. The armature member can then be released by temporarily stopping the coil current. The armature member can then be restored to the "hold" position with a short burst of high current in the coil. This mode of operation allows for simplification of the actuator structure, eliminates the cost and space requirements of the permanent magnets used in other stored-energy designs, and allows for more compact packaging. It does, however, incur a penalty in power requirements since power is being dissipated in the printhead even when it is not printing. This power problem can be controlled if the printhead or the platen can be retracted under electronic control which allows the armature member to be released without marking the paper when the printer is not receiving any date. A typical movement for the armature member 16 is in the range of 0.35 mm with an actuation time of approximately 250-300 microseconds. In some cases the print wire may have some overtravel, or "ballistic" flight, associated with its motion following stoppage of the armature member. Cycle times in the range of 500 microseconds or less can be achieved. Referring now to Figure 3, a further modification of an actuator according to the invention is illustrated, using a common stator unit for several print positions. While the overall number of parts in the actuator assembly of Figure 1 is small, a further decrease can be achieved by packaging groups of actuators as illustrated in Figure 3. A one-piece stator unit 30 has a plurality of coils 31-35 wrapped upon it. The armature elements 36, 37, 38, 39 and 40 are disposed on the stator unit 30. Consequently, Figure 3 illustrates five effective actuators disposed on a single stator core. As long as the vertical core segments of the stator unit 30 do not severely saturate, any combination of coils may be actuated without firing any of the actuators whose coils are not actuated. Figure 3 illustrates the dominant magnetic flux path for the gang of five actuators wherein actuator coils 1, 2 and 4 have been energized. The flux path through adjacent stator unit sections 1 and 2 is such that the sections are energised with the same polarity so that the magnetic flux bypasses the vertical core segment separating the two coils. Instead, the flux path  circulates around both coils, passing through armature members 36 and 37 in the process. Both the magnetomotive force driving this flux path and the reluctance of the path are twice that of a single actuator. Thus, the armature members 36 and 37 experience the same total flux flow, irrespective of whether a neighbouring actuator has been energised. Actuator 4 has a flux path ', consistent with that shown in Figure 1. As illustrated in Figure 3, for actuator 3, not energised, there is no significant flux flowing through its armature member because the unsaturated vertical core segments isolate the actuator from the flux flowing in the neighbouring core segments. Figure 4 illustrates the same structure as Figure 3. However, the coils 31-35 have an alternating polarity along the length of the stator 30. This technique of having coils 31, 33 and 35 with a flux path  and coils 32, 34 with a flux path '' eliminates the possibility of an unacceptably large quantity of flux flowing through an inactive actuator if a large number of actuators are fired on each side of an actuator that is to remain inactive. It is understood that other possible driving arrangements exist, including the use of bipolar drivers to set the polarity of adjacent coils as a function of the pattern being printed. Thus, an actuator in accordance with the invention is not limited to a particular arrangement of coil polarities. A further reduction in the total number of parts for a multi-actuator assembly can be achieved if cantilever armature members are combined in a comb-plate configuration as illustrated in Figure 5. An armature plate 42 is a unitary structure comprising a base portion 44 and five comb-like fingers 46, 48, 50, 52 and 54. This individual plate would replace the five individual armature members 36, 37, 38, 39 and 40 in Figure 3. Because the armature member fingers are thin, the amount of flux which can bypass the air gaps by travelling through the continuous base portion 44 of the comb-plate will be small. This will not substantially affect the magnetic force on the armature members. In accordance with this arrangement, it is possible to construct a single stator unit having wound thereon N separate coils. A single comb-plate having N fingers can be constructed as the armature assembly for this device. A single back-up plate, not illustrated in Figures 3 and 4, but similar to that shown as element 20 in Figure 2, can be employed. These three elements can be clamped or glued together. Figure 6 illustrates a preferred stator unit 10' employing laminated thin ferrous laminations bonded together. The use of such a laminated assembly reduces the effects of eddy currents during high speed operation by electrically isolating each of the thin laminates. A further reduction in cost is achieved since the laminations may be stamped and simply bonded together. Figure 6 also illustrates a linear arrangement wherein a common linear stator unit has multiple winding sections. To assist in defining the shape of the bobbin for coil windings, the non-magnetic stator unit section is an aluminium slotted bar 11', back-to-back against the magnetic stator unit section 13'. It is noted that the same function could be obtained with a stator unit bar having coil slots on top and bottom. However, flux leakage through the inactive slot would tend to degrade the performance of the actuators. By making the slots 15 relatively deep vis-a-vis coil depth, the aluminium bar 11' acts as a finned heat-sink to dissipate the heat generated in the coil to the ambient air. The intimate coupling of the coils to the aluminium fins provides a very efficient thermal transfer path, thus reducing the peak printhead temperatures associated with a particular level of power dissipation. The heat dissipating fins may take many forms, as is shown in phantom in Figure 6. Figures 7 and 8 illustrate a common armature plate 56 for a serial printhead operated in a "pick and hold" mode as described herein. The armature plate 56 is used in conjunction with two stator unit bars (not shown) of appropriate length. Print wires are replaced with a series of small protrusions 62 integrally formed on the armature plate 56. Slots 64 are used to define armature members 60 and to isolate these armature members from each other. The armature plate 56 is mounted along its periphery 58. The armature plate 56 is manufactured by first embossing protrusions 62 on a flat plate. The slot pattern 64 is then etched through the plates to define the beam elements 60 forming the armature members. It is to be understood that alternative techniques of manufacture may be employed. Figure 8 shows a twin-stator assembly for use with the common armature plate of Figure 7 (or its assembled equivalent). Common armature plate 56 is mounted on a structural support 66, which is shown as if divided to show unobscured the print protrusion 62 on each armature 60. Note that coils 12 and stator units 10' also are slightly interleaved. Support 66 may be opened to form windows for cooling fins 15 of Figure 6, or may be otherwise complementary to enhance heat dissipation. Stator units 10' have profiled top surfaces achieved by grinding them after assembly. The grinding operation may simply be a smoothing grinding operation to eliminate rough edges, but preferably is a profiling grinding operation. Each profiled surface 68 of each stator unit is matched to the profile of the first free-vibrating bending mode for the canitlever beam of the associated armature member, for best results in the "pick and hold" mode of operation. It is understood that other profiles can be used to modify the armature dynamics as required. Referring to Figures 9 and 10, two preferred techniques of terminating the coil windings are illustrated. Figure 9 shows a stator unit 10' having laminations as illustrated in Figure 6. A pair of thin printed circuit boards 70, 72 are bonded onto each end of the ferromagnetic laminations forming the stator unit 10'. Each printed circuit board has a series of copper pads 74. Before winding the coils (not shown), one end of each coil wire is connected (typically soldered) onto the printed circuit board 72 at one end of the stack, such as at point 76. Following winding of the coil, the free end of each coil is connected to another pad 74 on the printed circuit board 70 at the other end of the stack, such as at point 78. Each printed circuit board has appropriate wiring patterns to provide electrical isolation of the coils and also provide convenient solder pads for the final connection of the coils to the drivers. Alternatively, as illustrated in Figure 10, a single printed circuit board 70 is placed on the bottom of the stator unit 10'. The copper pads 74 from beneath the stack protrude outward. The protrusions 80 would also serve as wire restraints during winding of the coils. While illustrated in the Figures as being aligned in a straight line, a bank of actuators may be curved about various axes to accommodate situations where the print wires are required to converge to form a densely-packed linear cluster. As an example, the bank of actuators may be curved so that the armature members are arranged radially in a conventional wire-matrix print head configuration. The comb-plate 42 would then take the form of a circle with the armature members extending radially inward.

Claims: An electromagnetic print element actuator comprising a stator member (10) formed with two spaced apart pole pieces defining a gap and a coil (12) surrounding said stator member between said pole pieces, an elongated armature member (16) fixed at one end (18) and extending adjacent to said gap, and substantially perpendicular to the flux path across said gap so that said flux path extends transversely through the width of said armature member, and a print element (22) attached to the other end of said armature member, whereby energisation of said coil (12) causes attraction of said armature member (16) towards said gap and actuation of said print element (22), characterised in that said pole pieces extend along substantially the full length of said armature member (16). A print element actuator as claimed in claim 1 characterised in that said stator member includes two plates (14) attached to said pole pieces so as to reduce the size of said gap. A print element actuator as claimed in either of the preceding claims characterised in that said stator member comprises a back-up plate (20) for said armature member having a profile of the first free-vibrating bending mode of said armature member about its fixed end. A print element actuator as claimed in any one of the preceding claims characterised in that said stator member comprises a portion (11) made from a non-magnetisable material. A print element actuator as claimed in any of the preceding claims characterised in that said stator member comprises portions constraining said coil into the required position. A print element actuator as claimed in any of the preceding claims characterised in that said stator member comprises a portion (11) forming a heat sink. A bank of electromagnetic print element actuators comprising a plurality of print element actuators as claimed in any of the preceding claims, characterised in that the stator members (14) of said actuators are formed into an integral stator unit (30) with the armature members (16) extending parallel to each other. A bank of actuators as claimed in Claim 7 characterised in that said armature members are combined into a unitary structure (42) having a plurality of fingers (46-54) forming said armature members extending from a common base portion (44). A bank of actuators as claimed in Claim 7 or Claim 8 characterised in that said integral stator unit (30) is curved so as to converge the print elements (22) on the ends of said armature members into a cluster.

Inventors: KARIDIS, JOHN PETER
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: B41J2
IPC codes: B41J2, H01F7