GB2039418A - Electromagnetic matrix printing cell and head assembly - Google Patents

Abstract

In an electromagnet assembly (12) for driving a printing wire (14) in a printing head for dot matrix printers, a pair of coils (56,58) generally radial to the printing wire axis drive an armature (18) coaxial with the printing wire and to which it is attached. The armature which is of low mass is mounted on a pair of springs (20,22). The ramp angle of the respective pole faces 52,60 and 54,62 lie in the range 7 DEG -26 DEG and preferably 16 DEG -20 DEG . A damping arrangement may be provided on a rear support member 34. <IMAGE>

Description

SPECIFICATION Matrix printing cell and head assembly The present invention relates generally to printing heads for dot matrix printers and, more particularly it relates to electromagnet designs for use therein.

In dot matrix printers, printing is accomplished by driving selected printing wires in an array of printing wires against a printing surface, typically an inked ribbon adjacent a paper-bearing platen. The individual printing wires are energized by means of solenoids or electromagnets, and during each printing stroke a spring element is tensioned, which pulls the printing wire back to its rest position at the completion thereof.

The printing head is a housing having an aperture adjacent the printing face where the printing wires are arranged in the desired array. Within the housing, means are provided to guide the wires, from the respective solenoids or electromagnets to the aperture. The housing also serves as a mount for these driving elements.

Because the aperture is small, being the size of a printed character or a fraction therof, and the driving elements are relatively large, printing heads tend to be cone-shaped, with the drive elements in the base of the cone at an angle to the printing axis. Thus, the printing wires must curve through that angle to arrive at the aperture and printing face in the printing axis. Since this means that the wires will be rubbing against the guiding means, generating heat and wear and reducing power delivered as printing stroke, it is desireable to keep this angle as small as possible. Further, in prior printing heads there has been an inherent change in the wire angle as the stroke proceeds, which can set up undesired oscillations and introduce axial stresses.

The drive elements always include a core for a winding or coil, with pole pieces for confining and concentrating the magnetic field, and an armature that moves in response to the magnetic field, thus driving the print wire.

A first embodiment of the present invention (FIGS.

1-5) comprises a rugged and reliable electromagnet assembly and print head which is adapted for operation at moderate speeds, 60-90 characters-persecond (cps), or about 480 Hz (cycles per second), with a long and powerful stroke that is capable of producing 4-10 copies on varying print stock. Each electromagnet assembly includes a U-shaped core having two sloped pole faces at the top, the degree of slope being called the "ramp" angle, and being in the range of 7 to 26 degrees of incline to the axis of the print-wire extending thru the armature. Each core carries a coil wound thereon. The armature is mounted above the cores by two flexure elements, e.g. flat springs, screwed into the base of the core and at the ends of the armature. The print-wire is welded inside the forward screw on the armature.

The latter has a pair of pole faces substantially mating with the core pole faces and defining a closeable gap. A saddle on the rear of the core and separately mounted in assembly with the core into a head includes an adjustment screw for pretensioning the armature-spring assembly. The screw deflects an intermediate pierlike member which is in line contact with the deflected flat spring and armature components and acts as a rigid brace for the returned spring-armature assembly upon the completion (return) of its forward deflection. A desired number of these assemblies are mounted in a parabolic array in a generally conical head casting, which has a suitable nose bearing receiving the printing ends of the print wires.

In operation, movement of the armature-spring assembly is described as a "collapsing parallelogram", and it is significant that while there is a slight vertical displacement of the armature-print wire during movement (less than the wire radius), there is no angular change; this distinguishes this type of assembly from previously-known clapper-type print cells, (wherein an electromagnet is not located co-linear with the axis of wire motion, e.g., any departure from the standard cylindrical linear solenoid which has a coil and center armature). Such "clapper" cells revert somewhat to the buzzer or clapper-bell design wherein a cantilevered armature is located and acted upon by a coil pole at one end.

The angular pivoting of the armature, by having a print-wire affixed to one end, permits movement of the wire atwhateverfrequencythe armature can be made to move in and out from the coil. If the print-wire is attached to the armature which is pivoting about a fulcrum, the wire end attached to the armature is "S-flexured" to the same angle of rotation as the armature. This absorbs energy required to print and reduces print-excursion frequency.

The second embodiment of the present invention (FIGS. 6-10) follows the broad structural outline of the above-described cell and head, but is improved in substantial and significant ways which bring about dramatically improved operation.

A general object of the present invention is to provide an improved printing head for a dot matrix printer.

Still another object of the present invention is to provide a printing head for dot matrix printer wherein each printing stroke is carried out without angular displacement.

A still further object of the present invention it to provide a dot matrix printing head wherein print wire mounting problems are eliminated, and adjustment is smple and positive.

Yet another object of the present invention is to provide a novel electromagnet for use in a dot matrix printing head capable of achieving the foregoing objects.

Another object of the present invention is to provide a printing cell of substantially lower mass and higher printing speed than previous cells.

A further object of the present invention is to provide a printing cell capable of quieter operation at higherfrequenciesthan previous cells.

Various other object and advantages will become clear from the following description of embodiments, and the novel features will be particularly pointed out in connection with the appended claims.

Reference will hereinafter be made to the accom panying drawings, where: Figure 1 is an elevation view, partly in section, of an electromagnet assembly in accordance with one embodiment of the invention; Figure 1A is an enlarged section of a core pole face of the Figure 1 assembly; Figure 2 is an end view of the Figure 1 assembly, taken along line ll-ll of Figure 1.

Figure 3 is a cross-sectional view of the printing head assembly, including plural electromagnets; Figure 4 is a top view of the printing head of the same general type as Figure 3; Figure 5is an elevation view, partly in section of an alternative embodiment of the electromagnet assembly of the invention; Figure 6 is an elevation view, partly in section, of an electromagnet assembly in accordance with a second embodiment of the invention and including the head casting in which it is mounted; Figures 7 and 8 are elevation and plan views, respectively, of the preset element used in the Figure 6 embodiment;; Figure 9 is a schematic layout illustrating the positioning of seven or nine printing cells in a parabolic array with additional space for more cells around the interior of the casting shown in Figure 6, and Figure 10, is a schematic side view showing the wire path of a single electromagnet assembly representing any assembly in the head array.

With reference to FIGURE 1, the printing head of the present invention comprises a housing or mount 10 supporting a plurality of electromagnet assemblies 12, each of which drives a printing wire 14.

Broadly, assemblies 12 each comprise a U-shaped magnetic pole piece 16, an armature 18 secured by machine screws 24, 26, 28,30 and an L-shaped aluminum saddle 32 secured to mount 10 with screw 28, and including a brace 34 supporting a tension and stroke adjusting screw 36. and a downwardlyextending keel 38 which cooperates with a slot 40 (FIGURE 2) in housing 10. Saddle 32 also includes an integral, upstanding and flexible tab 42 which bears against flexure 22 on its forward side and is adjusted for position by screw 36 bearing on the opposed surface of same. Assembly 12 is secured to housing 10 with a further machine screw 44.

Electromagnet assemblies 12 will now be considered in more detail, with general reference to FIGURES 1-3. Pole piece 16 comprises a narrow, rectangular base 46 and two integral, upstanding winding cores 48, 50 having parallel but inclined pole faces 52, 54. The pole piece 16 is constructed of a suitable magnetic material, either monolithic or laminated. The dimensions of base portion 46 are sufficient only to support cores 48, 50 and the screws. The winding cores 48, 50 provide a constant magnetic path cross-section and preferably have rectangular cross-sections with rounded corners, adapted to receive pre-wound windings 56, 58 (shown in phantom), as winding coils directly onto such cores would be difficult and expensive.

Armature 18 has a pair of pole faces 60,62, of similar dimension and ramp angle as faces 52, 54 of cores 48, 50, which define therebetween a pair of air gaps 64,66. Armature 18 is, of course, also constructed of magnetic material, and must have a sufficient mass to provide desired printing stroke power, and which should have the center of mass in the same axis as print wire 14, attached thereto with machine screw 26 in a manner hereinafter described.

Armature 18 is held in the position shown in FIGURE 1, which is the rest or unenergized position, by flexure springs 20, 22, which are secured thereto by machine screws 26, 30. At their opposite or base ends, flexures 20, 22 are secured to the base 46 with machine screws 24, 28.

While the base portion 46 is secured to housing 10 by means of machine screw 44, certain important mounting and adjustment functions are carried out by the saddle 32. As shown in FIGURES 1 and 2, saddle 32 is L-shaped and is secured against the rear face of base portion 46 by machine screw 28, with flexure 22 clamped therebetween. It will be appreciated that saddle 32 will be preferred in some embodiments in a general U-shape, with the respective leg portions secured to base 46 at both ends (i.e., with machine screws 24 and 28). Saddle 32 is fabricated from a suitable non-magnetic material such as aluminum or magnesium.

On the "long" side of saddle 32, which is the side parallel with and adjacent to the long side of base portion 46, a keel section 38 extends downwardly into a closely-fitting slot 40 machined into housing 10. With machine screw 44 loosened, this permits sliding adjustment of electromagnet assembly 12 for rapid and precise alignment of the printing face (not shown) or print wire 14.

Saddle 32 also includes an integral, upstanding brace 34 having, at its upper end, a tension and stroke adjust screw 36, and a flexible tab 42. Tab 42 may be integral with saddle 32 or it may be retained in a slot (not shown) machined in the forward place therof and retained by screw 28, as is flexure spring 22. By itself, tab 42 is parallel and adjacent one surface of flexure spring 22, and acts as a damper for armature 18 on the return (unenergized) stroke to the rest position. In combination with set screw 36, which bears against the opposite or rear side of the tab 42, it can be emplayed to impart a slight pre-tension to flexures 20 and 22. This is important for reliable operation and "fine tuning" of the head, and also mitigates against any fluttering of the flexures. A further important feature is that tab 42, which acts as a damper, lies on the vertical plane of and be adjacent the vertical rest position of flexure 22 or, as noted above, through adjustment of set screw 36, apply a slight pre-tension thereto.

In operation, the magnetic fields generated by coils 56, 58 in cores 48, 50 will generate a force on armature 18 toward the left, to close air gaps 64,66.

On one hand, the force will be greatest at the moment of energization, when the air gap is greatest, and will decrease and the respective faces come into registration. However, at the same time, the flux lines of force die normal to the pole faces, which are inclined with respect to the armature axis, and the component thereof in the armature axis (which is the same as the print axis) increases as the air gap decreases, adding power to the printing stroke. These two forces, acting concurrently and additively during a printing stroke, result in a more constant-powered stroke at no sacrifice in printing speed.

Theoretical and practical considerations, in optimum balance, indicate that the ramp angle should be in the range of 70 to 260, for most efficient use of the coil energy.

Attention is directed at FIGURE 1A, which is an enlarged view of the top of a core 50 showing a pole face 54 and armature face 62 in greater detail. It will first be noted that there is a small 450 bevel 53 at the top of the ramp. This is believed to act as a flux-leakage break. It does not effect the initial force of the pole face on the armature, but were bevel 53 not present, that portion of the ramp it replaces would have a very high flux density at about the time the pulse ends and the field collapses. Bevel 53 curves the flux field out into space, reducing the force on the armature at this critical time.

The second feature shown in FIGURE 1A is that the pole face 54 is in fact not a plane surface, but is slightly convex. More particularly, from edge of bevel 53 back, it is machined to a slight curve (i.e., a large radius r). This also is believed to open up the flux field slightly, but is further felt to (1) increase power at the beginning of the stroke, and (2) facilitate a very rapid collapse of the field at the end of the power pulse; Housing 10 is best illustrated in FIGURES 3 and 4, and attention is directed thereto. Again, nonmagnetic materials are necessary, and aluminum and magnesium are preferred. On the other hand, if cost is more important than weight, a precision zinc die casting would be perfectly satisfactory.

Housing 10 is, roughly, one-half of a cone, with the print wire aperture 68 at its apex an intermediate mounting shoulder 70 eliminating waste space (and unnecessary mass), and having individual electromagnet assemblies 12 mounted around its periphery at the base. A removable cover 72 is provided for access to the interior.

By placing each electromagnet 12 in a parabolic array around the print axis 74, (see FIGURE 9, discussed below) each printing wire is at precisely the same angle to the print axis 74. While this arrangement is preferred it will be appreciated that in larger or smaller units, other designs could be employed. Print wire 14 is joined to machine screw 26. Screw 26, typically manufactured of an alloy steel, is provided with a bore 76 dimensioned for an interference or shrink fit with printing wire 14. When wire 14 is inserted into bore 76, an ideal arrangement for electrical resistance welding is presented: electrodes are clamped to the two workpieces, resulting in the weldzone 78. Depending on specific compositions of the work-pieces, an inert atmsphere should be provided during welding.Testing to destruction of welds made in the foregoing manner indicated that the weld 78 was stronger than wire 14.

In FIGURES, pole piece 16 may be of laminated construction. Keel 38 is integral with base 46 of pole piece 16, an expedient effected by stamping certain of the laminae in a die adapted for same; keel 38 and slot 40 need only be thick enough to provide guidance while adjusting the assembly, support therefore being provided by shoulder 80. In this embodiment, saddle 32 is entirely eliminated, and machine screw 28 retains only flexure 22 and damping tab 42, which may be any material of choice. Appropriate laminae are also stamped to provide an aperture 82 into which a steel threaded insert 84 is inserted upon assembly, and the latter includes a locking ring 86. A threaded bolt 88 is used to secure the assembly to base 10.In this embodiment, the adjust screw 36 is mounted on an extension 90 of base 10, so that it is effectively in the same position as in the FIGURE 3 and 4 embodiment.

Those skilled in the art will also appreciate that while the present invention has been described with reference to dot matrix printing, the electromagnets 12 have application in other types of printing. More particularly, and referring again to FIGURE 5, the print wire 14 and bolt 26 can be replaced with a hardened steel, truncated cone-headed bolt 92, with the result that the device could be employed as a hammer or character printer.

Referring now to the embodiments in FIGURES 6-10, several improvements are apparent. The flat spring flexure elements of FIGURES 1-5 are replaced by cylindrical wire springs, which, at their respective ends, are welded or sintered into holes in the core and armature at a point which removes these components from an impact-distorting relationship with other components of the structure. By itsef, this improvement eliminates four screws and allows the mass of the armature to be significantly reduced as well as the total weight of the cell assembly.

In another aspect, FIGURES 6-10 introduce a new element in matrix impact print cells that is called a "preset". The preset performs several other functions as well as armature-damping, which contributes to the ability of the device to achieve impactfrequen- cies of over 2,500 Hz, and to do so with a remarkably quiet action. Like a damping element, the preset has some resiliency, and is contacted by the rear face of the armature.

Additionally, as the name implies, the nonmagnetic preset is initially adjusted to set the required air-gap between the armature-core pole faces with a minimum tension in the armaturespring assembly, by applying pressure exclusively to the armature. Unlike the earlier saddle element, the preset does not "preform" the spring, which adds to the pre-load of the operating electromagnet. Still further, the preset encloses or nests the rear-spring element on three sides to prevent off-axis wandering of the armature-spring assembly, but does not contact the spring component in the rest position.

On the return stroke during operation, the preset "nests" the spring element, which then touches the 'floor' of the nest, curved to receive the spring element. An integral element not shown in the drawing is a resilient adhesive which is added to the assembly of the preset and its mount while permitting a setting-adjustment of the preset in initial assembly, and allowing some deflection of the preset during impacting of the armature-spring assembly during operation. The adhesive is cured during post-assembly and becomes attached to both the preset and the core 'tower' section combined with the core element.

With reference to FIGURE 6, the electromagnet assembly 110 is mounted in a print 'head' casting 112 by means of a single screw 114. Electromagnet assembly 110 has eight parts. The core 117 is generally W-shaped, and includes base portion 118, two parallel cores 120,122 terminating in sloped pole faces 124,126. The third 'leg' of the "W" is 'tower' portion 128 which supports preset 130, described hereinbelow. Base 118 has a central, threaded hole 132 to receive mounting screw 114, hole 134 at the forward end to receive columnar spring 136, and, between core 120 and tower portion 128, hole 138 to receive rear spring 140 in assembly.

A slot 116 in casting 112 which accepts screw 114 allows assembly 110 to be adjusted in the axial (print-wire) direction so as to not introduce a bias flexure into the print-wire. As an alternate the screw can be replaced with a male stud (not shown) extending from the core base. Such an alternative would reduce the cross-section of the base and further reduce core weight as additional crosssection is added in the presene embodiment to compensate for the tapped hole 132. However, studs so applied have a tendency to fracture, and additional means adding to costs have to be provided for the alternate embodiment.

In the present invention, the ramp angle of pole face 124, 126 has been refined, and falls in a range of 16 to 20 degrees of angle (the angle being measured from the axis of the print-wire). An angular difference is introduced between ramp interfaces of the armature faces 158, 160 and the core faces 124, 126, to avoid 'locking' of the operating components in operation. Under certain conditions in a machine without paper (the printer equipment using the invention), the armature are core pole faces may be caused to contact. To avoid 'wedge-locking' contact in this event, the armature is designed with a tolerance toward a negative value, resulting in an angle of approximately one quarter of a degree acute from the specified ramp angle of the core.This intentional difference of ramp-mating angles between the core and armature does not introduce a discernable change in operation at the frequencies and power level ranges at which the present embodiment has been operated. A consideration that must be taken for satisfactory operation with this angular difference, is the axial dimension (length) of the ramps involved vs. the ramp angle ratio. An approximate ratio of ramp length to angular 'error' must also include a variable for the desired stroke length and frequency, but I have generally specified a minimum ratio of 50 1.

Rather than bevel the forward edge of the core pole face, equal or better performance is achieved if this edge is milled flat 142, 144. In either case the desired end is to avoid flux leakage to the closely set armature in the neutral position.

Tower 128, the function of which is to support preset 130, has a transverse threaded hole 146 therethrough near the top to receive a set screw 147.

The set screw is of the commercially available type having a nylon insert at its tip, or nose, which is used as a means of dispersing the high-density forces occuring at the contact point between the screw and the inner preset surface, a pressure point on the preset used to overcome the armature-spring resistance to a pre-load setting of the armature gap.

The height of tower 128 is sufficient for its function in the location of a pressure point herein described, and operating in conjunction with a fulcrum point 180, but not so high as to interfere with the magnetic circuit (e.g., establish a flux path to the armature).

Magnetic cores 120, 122 carry a pair of coils 148, 150 wound on plastic bobbins 152, 154, as in FIGURE 1. It is essential in a higher frequency application that the coils be wired in parallel, as low coil inductance contributes measurably to the operating speed of the cell while adding ampere-turns. Bobbins 52, 54 should have a clearance fit on cores 20, 22 and be made of a material of suitable resiliency and other physical properties such as absorption, resistance, strength, heat resistance etc. The selected material in one embodiment has been nylon.

Springs 136,140, being of circular cross-section, are not susceptible to edge fracture in the manufacturing process, which precipitates spring failure, e.g.

metal fatigue and separation. The circular (columnar) spring offers greater strength with less mass carried by the armature, (than if the spring were non-circular), and contributes significantly to the overall electrical and geometrical properties necessary to achieve demonstrated current response time and velocities. As a spring material a commercially available beryllium copper is preferred.

A vertical hole(s) 162 near the rear end (and front, not shown) further reduces mass without affecting structure. The rear face 164 of armature 156 contacts the preset 130, and the front face 166 has the print-wire secured therein, by either welding or sintering.

Preset 130 is shown in more detail in FIGURE 7 and 8, and attention is directed to this component. 2/31t should be manufactured from a material having the appropriate physical properties of strength, absorption-resistance, heat resistance and surface (friction) co-efficient including a mild resiliency for the immediate contact with the armature, and intermittent sliding relationship with the rear columnar spring.

Glass filled Delrin (TM) and G.E. Valox (TM) and nylon have been selected as acceptable. In elevation, preset 130 is generally rectangular, but has a cut-out step section including tread surface 170 and (contact) riser surface 172 at its upper end. Centrally on the long axis a cavity 174 is provided, which is somewhat larger than tower 128 and more so in the axial direction of the preset and armature adjustment with only slight clearance in the width dimension to permit movement without constraint. A vertical aerating hole 176 is provided between cavity 174 and the top of the preset for providing expansion and curing means of the aforementioned resilient adhesive. A horizontal hole 178 at the rear surface matches thread hold 146 in tower 128 with additional clearance for the aforementioned set screw 147 and angular desplacement of the preset when the screw is advanced thru the preset hole and engaged in the tower thread 146. Inside the cavity 174 in the lower rear edge of same is a ridge portion 180 acting as the fulcrum for tilting the preset in setting adjustment (of the armature gap, as described). As seen most clearly in FIGURE 8, preset 130 also includes a vertical slot 182 centrally disposed in the front face thereof. Generally, the size of the slot 182 will be such that there will be from one to a few thousandths clearance all round spring-wire 140; in operation wire 140 never leaves slot 182.It is important to note that, in the rest position, wire 140 does not touch any surface of slot 182; the only contact between preset 130 and the armature assembly is at surfaces 164 and 172. It is preferred that when in contact that armature surface 164 and preset surface 172 be parallel, or nearly so to minimize edge contact and possibility of wear.

Therefore, a negative incline (with respect to the vertical axis of preset 130) is introduced into riser surface 172 such that when the armature-gap setting is achieved, then the tilted preset riser surface 172 will be in a more-nearly parallel relationship with armature contact surface 162. It should be noted again, that the armature does not rotate throughout its excursion and therefore the angle considerations are for the preset component only. Additionally, preset 130 is provided with vertical die-parting slots 184 in the side walls to facilitate removal after molding.

An important function of the preset 130 is the efficiency with which it aborts, re., absorbs, or dissipates forces developing by the returning armature-wire assembly. Extending as it does above the tower 128 and being additionally resilientlymounted, the mass of the preset does make a significant contribution to the rapid distribution of the forces involved. Corrections have been introduced in the final shaping of the preset by removal of material at radius 179.

Electromagnet assemblies 110 are mounted at the rear end of casting 112, with print-wires 168 passing to a nose bearing 186 through a curve, with constraining tabs, or guides at 188. Because of the size and shape of the print-cell invention, the print wires can be positioned with exact replication of each cell print-wire and with an in-line (planar) curve which permits each wire to avoid any change in torque loads, or pressure points, individually or one to another by comparison, in the head assembly. In the present embodiment, the narrow width of the print cell permits placement of the print cells such that each print-wire has the identical curve, and without displacement regardless of its origination.

Figure 9 illustrates the placement of cells 110 in casting 112 in schematic fashion. To insure that each print-wire has exactly the same length and path of travel, which is essential for quality printing and particularly high speed printing, cells 110 are disposed on the vertical axis by and at constant distance from the print-wire bearing hole, a dimen sion I referto as value OA (FIGURE 10), with each cell "pointed" such that the center-line of the cell is coincident with the print-wire path and the bearing hole. Wire bearing holes are typically contiguous in a vertical column and with the constant distance OA wire-path from each cell a paraboloid cell mounting array pattern is evident.However, wire bearing holes may be aligned differently as in a double row which can be accomodated with the "planar" shape of the invention and positioning of the cells in a head is not necessarily confined to either an annular or parabolic-like array. Further, the actual size of the mounting parabola, and theangle 0 atwhichthe cells are mounted with respect to the axis of the head casting (FIGURE 10), are selected for (1) the desired print-wire curvature and (2) suficient inter-cell spacing to prevent heating and cross talk problems, for the number of cells selected. FIGURE 9 shows that a 7,9 or 11 wire head can be built on a single casting 12.It can be shown that increasing the diameter slightly (OA - of the mounting circle) will permit a larger number of cells to be mounted with no sacrifice to the planar-wire embodiment. It should also be noted that the print-cell to nose-bearing distance S, (FIGURE 10) is the same for all print-cells in an array, and has been established at minimum values depending upon OA values selected. In practice, the angle 8 will fall in the range of 100 to 330.

Although the flat spring of my FIGURE 1 design prevented any spring flexure at ninety degrees to the plane of motion, which is desireable and particularly adaptable to heavier armatures, a structurally suitable flat spring must be quite thin at the desired spring-rate for higher frequencies, and is therefore structurally unsuitable at high frequencies. In the FIGURE 6 embodiment, the constraint in sidewise movement is no higher than the spring rate of the columnar structure.As a result some slewing or skewing or the print-wire-armature (spring assembly) might be expected, particularly on the return stroke when at least in most matrix impact printers, the moving elements flex on the impact with the printing medium and literally bounce irregularly therefrom, also relieving some radial spring tension that has accumulated during the forward (printing) stroke which can not be transmitted into the printed media. In some designs employing a free 'ballistic' components this energy resonates as undesireable sound. In the present embodiment, several factors preventthisfrom happening.One important factor occurs in the mounting of the print-cell in the head in that the curvature heretofore described in print-wire 168 tensions the moving system (armature, printwire and spring) away from the core pole faces, and upon energization resists collapse of the "collapsing parallelogram". This function uses a small portion of the print energy but the effects are more than considerable which favor both the characteristics of noise and high speed function.

On the return stroke, armature surface 148 strikes preset surface 164 initially, compressing it but also 'rocking' preset about the fulcrum 180. This movement also affects slot 182 and, in a few millionths of a second, "nests" spring 140 as it bottoms therein against the back wall of slot 182. It is intended that the spring (which is an integral component of the armature-spring assembly), create a counterforce to the rotational or rocking force created by the initial impact, with the net result that the entire moving system is brought into a total "rest" position in the neighborhood of about 50 ll sec.While not wishing to be bound buy a particulartheory of operation, it is believed that it is these two counteracting forces combine to, at least in part, enable the cell to operate at exceptionally high frequencies, without either undesired resonances or forces that would derogate from the high frequency performance observed.

The adjustment of preset 30 affects the ultimate frequency of the operating unit by virtue of the gap-displacement of the armature-spring assembly.

Since the spring rate is relatively unchanged by a slight shortening of the gap by vertical displacement of the armature-spring assembly (as a corollary to advancing the armature assembly by tilting the preset), the same end conditions is achieved.

It should be noted that in FIGURES 1-5 the print-wire axis corresponded with the center of mass of the armature. In the FIGURE 5 embodiment, while the print-wire is at the center of the armature, this is not the precise center of mass. Because of the greatly reduced mass and higher force ratio acting on the armature with attendant armature alignment control, this change is not deemed alone significant.

Also, the mass of the moving system should include a part of the spring components, and this has been a consideration.

Various changed in the details, steps, materials and arrangements of parts. which have been herein described and illustrated to explain the nature of this invention, may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims. For example, it is noted that with proper clearance between the armature and print wire (.0015"), Eastman 910 cement can be used in place of welding or sintering. Also, it will be appreciated that when slower, more powerful printing strokes are desired, no extra holes would be used in the armature. Also, as should be apparent, coils are wired in parallel but with reverse polarity, so that both coils inducing a flux path in the same direction.

Claims (21)

1. An electromagnet assembly for a dot matrix print head comprising: flexible support means secured to and supporting an armature for movement thereof from a rest position to a printing position and back without angular change during the movement; a single, generally U-shaped magnetizable core member having a base and a pair of legs extending toward said armature, the ends of said legs forming a first pair of pole faces; the support means being also secured to the respective ends of the base; said pole faces being adjacent said armature and each forming an identical acute angle in the range of 7 to 26 degrees with the axis thereof; a second pair of magnetizable pole faces on said armature at the same angle to said axis as said first pair of pole faces and defining therebetween a pair of closeable air gaps;; coil means on each leg of said core member, energizing of said coil means creating a magnetic field between said pairs of pole faces and moving said armature to close said gaps; and print wire means secured to an end of said armature so as to be pushed thereby to a printing position on energising of said coils.

2. An electromagnet assembly as claimed in claim 1 wherein the centre of mass of said armature lies on the axis of said print wire.

3. An electromagnet assembly as claimed in claim 1 or 2 wherein said flexible support means comprises a pair of flat, parallel spring flexure elements; each element extending from an end of said armature to the corresponding base side of said core member.

4. An electromagnet assembly as claimed in claim 3 comprising stop means adjoining one said spring element in the rest position and arranged to dampen return motion of the armature.

5. An electromagnet assembly as claimed in claim 1 or 2 wherein said flexible support means comprise front and rear columnar wire spring secured near the ends of said armature.

6. An electromagnet assembly as claimed in claim 5 having a third leg at the rear of the base forming a tower and slightly resilient preset means secured on the tower, the preset means comprising a vertical surface pressing against the rear end of said armature in the rest position and a vertical slot closely surrounding but not touching the rear spring in the rest position.

7. An electromagnet assembly as claimed in claim 6 having set screw means in the tower whereby the preset may be adjusted with respect to said armature.

8. An electromagnet assembly as claimed in claim 6 wherein the preset means has a vertical cavity fitting loosely over the tower and is secured thereto with a resilient cement.

9. An electromagnet assembly as claimed in claim 5,6,7 or 8 wherein the springs are secured in the armature and base by welding or sintering.

10. An electromagnet assembly as claimed in any preceding claim wherein the flexible support means are sized so that vertical motion of said print wire, upon moving horizontally from said rest position to said printing position, is less than the radius of said wire.

11. An electromagnet assembly as claimed in any preceding claim wherein said first pair of pole faces is shaped to be slightly convex.

12. An electromagnet assembly as claimed in any preceding claim wherein the upper edge of the first pair of pole faces is shaped to have a small bevel.

13. An electromagnet assembly as claimed in any one of claims 1 to 10 wherein the forward edges of the first pair of pole faces are flattened.

14. An electomagnet assembly as claimed in any preceding claim having a screw member threaded into an end of the armature, the end of the print wire being secured in a bore in the screw member, and the screw member securing one of the flexure means against the armature.

15. An electromagnetic assembly as claimed in any preceding claim having means on the core member for securing the core in a print head.

16. An electromagnet assembly as claimed in any preceding claim wherein said coil means are wound on bobbins having a slight resiliency, said bobbins fitted over said pair of legs with a clearance fit.

17. The electromagnetic assembly as claimed in any preceding claim wherein said coil means are connected in parallel.

18. An electromagnet assembly as claimed in any preceding claim wherein said angle is 16 to 200 to the axis of said armature.

19. An electromagnet assembly substantially as herein described with reference Figures 1, 1A and 2 to 4, FigureS, or Figures 6 to 10 of the accompanying drawings.

20. A print head for a dot matrix printer comprising: a housing having a nose bearing at one end and generally conical walls; a plurality of electromagnet assemblies each as claimed in any preceding claim mounted in a near paraboloid array interiorly around the conical periphery of said housing. with the print wire of each assembly extending from the armature into said nose bearing on a curved path, and said curve acting to tension said springs; and all said print wires being of equal length and having the same path of travel upon energisation of a respective electromagnet assembly.

21. A print head as claimed in claim 19 wherein the angle of said conical surface with the print wire axis at said nose bearing is in the range of 100 to 330,