GB2035219A - Method of coil winding and magnet arrangement for dot matrix print head - Google Patents

Abstract

The print head includes a number of hammer assemblies, each composed of a plurality of extremely thin, substantially planar hammers 50 suspended in closely packed side-by-side relationship, all of which are situated between a single pair of "U"-shaped primary magnets and having a printing needle 90. Two sets of field shaping magnets are utilized to concentrate the magnetic flux between the primary magnets to produce a more uniform field. Each hammer 50 consists of an aluminium frame 69 to which a coil 80 is mounted. To maximize the number of coil windings within the very limited space, the wire is wound with the middle portion adjacent the inner periphery thereof, forming an inner turn, and the ends wound sequentially in different directions, to form a coil where the leads extend from the outer turns in the plane of the windings. <IMAGE>

Description

SPECIFICATION Method of coil winding and magnet arrangement for dot matrix print head The present invention relates to a print head for a dot matrix printer and, more particularly, to a method of winding the coils mounted on the hammers thereof and to a magnet arrangement which permits a plurality of hammers to be situated in side-by-side relationship between a single pair of primary magnets so as to reduce the size, complexity and cost of the head.

The dot matrix printer is an apparatus which causes a plurality of closely spaced dots to be printed at high speed in selected locations on a paper strip to form letters, numerals or other intelligible symbols thereon. The dots are formed by causing contact between the paper and the ink impregnated surface at the desired locations by selectively electromagnetically displacing elongated print wires mounted within the print head.

In order to imprint dots in the desired locations on the paper, a plurality of selectively displaceable print wires are required. To provide the necessary print quality, the dots must be imprinted on the paper in close proximity to each other. To achieve this, the ends of the print wires must be situated on a very closely packed matrix array.

In most conventional dot matrix printers, the print wires located in the head are displaced by selectively electrically energizing solenoids, from which the print wires extend, by momentarily connecting the solenoid to a power source. The impact ends of the print wires are retained in position with respect to the paper, and each other, by a wire bearing having a plurality of openings therein arranged in a matrix array. The impact end of each wire is received in one of the openings and is movable through the opening to cause the end thereof to protrude beyond the surface of the bearing when the solenoid is actuated to cause contact between the paper and an ink impregnated surface.It is desirable to have the print wires extend between the solenoid and the opening in the wire bearing through which the impact end of the same passes in a direction co-linear with the axis of movement of the solenoid, such that all of the forces developed by the actuation of the solenoid are utilized effectively. Thus, it is an important design consideration to provide print wires which extend along lines which are as straight as possible throughout the lengths of the wires.

Both linear and "clapper" type solenoids have been utilized as actuators. However, such solenoids are large and bulky and, therefore, require a great deal more space than the distance permitted between the impact ends of the print wires, if characters of the required quality are to be obtained. In order to design a print head having the required number of actuators and still have print wires extend in a direction co-linear with solenoid movement as much as possible, the lengths of the print wires have been extended and the solenoid actuators have been staggered in different planes and/or arranged in a variety of different arcuate, circular, or flared configurations.However, even with such configurations, it is impossible to situate all the solenoid actuators in positions which are co-linear with the direction of movement of the print wires connected thereto, because of the size of the solenoids. At least some of the wires, therefore, must be situated along a curved path between the solenoids and the wire bearing. In one method used to guide the print wires through the curved path, each of the print wires is surrounded by a tubular sheeth composed of plastic or beryllium-copper alloy, which extends along the length thereof, and acts to retain the wire in the proper position, when the solenoid is actuated, and to direct the displacement forces in a plane perpendicularto the paper. Other methods include guiding the wires through guide holes in parallelly situated discs or through guide grooves around a conical center piece.

Regardless of the guide method utilized, substantial friction is created between the print wires and the guide, as the print wires are displaced along the curved path. This friction significantly reduces the speed and efficiency of the printing operation, creates unwanted heat, and causes the wearing out of the parts, reducing the life of the print head. In addition, a print head comprising a large number of solenoids arranged in a flared, staggered or circular configuration is heavy, bulky and expensive to produce, repair and maintain.

In order to overcome these problems, actuators of different configurations have been investigated.

Instead of the conventional linear or "clapper" type solenoid, an actuator, commonly referred to as a hammer or flag, has been proposed. Such a hammer is formed of a thin planar frame having an opening into which a flat coil is mounted. The hammer is suspended from a support between a pair of primary magnets. The frame has a print wire extending outwardly from one side thereof. When the coil is energized by connecting the ends thereof to a power source, the coil, and thus the frame and print wire mounted thereon, are abruptly displaced relative to the primary magnets due to the electromagnetic forces created. This causes the print wire to cause contact between the paper and an ink impregnated surface so as to imprint the dot.

Such hammers constitute a significant advance over the conventional linear or "clapper" solenoid actuators, because same significantly reduce the weight, bulk and cost of the print head. Further, it is possible, by appropriately staggering the hammers and by utilizing extremely thin primary magnets at either side thereof, to reduce the curvature of print wires and, therefore, to reduce the problem of friction created by the movement of the wire through a curved path. However, since a pair of primary magnets is required for each hammer in the assembly, it is still not possible with conventional hammers of this type to locate the hammers sufficiently close together to permit all of the hammers to be co-linear with the movement of the print wire ends.Thus, since a pair of magnets is required between each of the hammers, this configuration still did not completely solve the friction problems or reduce the bulk of the head to a minimum.

In order to completely eliminate the friction prob lem caused by curved print wires and to reduce the bulk of the head to a minimum, a new method of winding the coil on the hammer and a new magnet assembly have been devised which permit all of the hammers to be situated between a single pair of primary magnets. Thus, the magnets normally required between the hammers are eliminated such that all of the hammers and the attached print wires move in substantially the same plane.

In order to achieve an apparatus which can operate efficiently at high speeds, the hammers must be quite thin, substantially planar, and be mounted in closely packed, side-by-side relationship, and the magnetic flux density developed by the magnet assembly must be relatively high and uniform across the space within which the hammers are mounted.

These results are achieved through a unique method of winding the coil on the hammer, and through the use of field-shaping magnets which, in conjunction with the primary magnets, act to concentrate the magnetic flux across the hammers.

In accordance with one aspect of the present invention, each hammer comprises a very thin substantially planar frame member. A recess, defined within the frame, is adapted to at least partially receive a flat wire coil. Preferably, the coil is wound around a bobbin. The coil is wound to permit the maximum number of turns possible without extending beyond the plane of the frame. The coil has first and second end sections adapted to be operably connected to the energizing means. The coil is wound such that each of the end sections extends from a different one of the outer turns of the coil, remote from the bobbin surface, within the plane of the frame. An intermediate section, remote from said end sections, forms a portion of an inner turn of the coil.

Normally, a coil is wound starting with one end of the wire adjacent to the bobbin such that the other end of the wire extends from the coil in the plane of the outermost turn. With this winding configuration, in order to connect the inner end of the wire of the coil to an external source, it is necessary that this inner end be brought out of the inner turn of the coil in a plane substantially perpendiculartothe plane of the coil and thereafter be extended along the side of the coil until it reaches the periphery of the coil where it may again be situated within the plane of the coil. Thus, the inner end of wire of the coil requires as much space as an entire radially extending column of the turns. This is usually not serious when the dimensions of the coil are not critical.

However, in a situation where the hammer assembly must be extremely thin and the coil have as many turns thereon as is possible, winding in the conventional manner cuts down the possible width of the coil by the width of an entire radially extending column of windings. In this case, where only five radially extending, side-by-side columns of windings are possible because of the size restrictions, the elimination of a column of windings reduces the number of windings by one-fifth, substantially reducing the efficiency of the hammer.

In order to overcome this problem, in a method in accordance with the present invention, the coil is wound in a unique manner, with the middle ofthe wire adjacent the bobbin, such that the ends of the wire each extend from a different outside turn of the coil and, thus, are directed entirely within the plane of the frame and, therefore, do not reduce the width of the coil.

To develop the necessary impact force, the coil mounted on the hammer must be situated in a field of sufficient magnetic flux density. The magnetic flux density at a given point in space between the magnets is a function of the distance from that point to the magnetic poles. Thus, as the magnets are situated farther and farther apart to permit the insertion therebetween of an increasing number of hammers, the magnetic flux density falls off near the center of the area where the hammers are mounted. This is because more of the flux lines are directed between the opposite poles of the same magnet instead of across the area between the magnets.

In order to create a more uniform magnetic flux density pattern across the area where the hammers are situated, field shaping magnets are utilized to converge the magnetic flux. A first set of field shaping magnets is provided with one of the field shaping magnets, situated between the opposite poles of each primary magnet, respectively, with the field thereof oriented in a direction opposite to the direction of the field of the primary magnet. In this manner, those lines of magnetic flux which are normally directed between the opposite poles of the same magnet, are redirected towards the other primary magnet of the pair. In addition, a second set of field shaping magnets, situated at the exterior of the primary magnets, are provided to further converge the lines of flux across the area where the hammers are situated.

The first set of field shaping magnets are situated between the poles of each primary magnet, respectively, and extend in a line perpendicular to the plane of the hammers and, thus, would normally intersect the path of movement of the hammers. However, in order to prevent these field shaping magnets from interfering with the movement of the hammers, the coils, if no bobbins are used, orthe bobbins, if same are present, are designed with a recess therein into which the field shaping magnets are at least partially received. With this configuration, no additional space is required within the head in order to accommodate the field shaping magnets.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, wherein like numerals referto like parts and in which: Fig. 1 is a plan view of a dot matrix printer of which a head in accordance with the present invention forms a part; Fig. 2 is a side view of the head of the dot matrix printer of Fig. 1; Fig. 3 is a rear of the head of the dot matrix printer ofFig.1; Fig. 4 is a view of the head of the dot matrix printer taken along line 4-4 of Fig. 3; Fig. 5 is an enlarged cross-sectional view of a hammer of the dot matrix printer; Fig. 6 is a bottom view of the wire bearing of the dot matrix printer; Fig. 7 is a plan view of a first preferred embodi mentofa magnet assembly;; Fig. 8 is a plan view of a second preferred embod iment of the magnet assembly; Figs. 9A, 9B, 9C and 9D are views illustrating the preferred method of coil winding; and, Figs. 10A, 10B and 10C are graphical representations of the magnetic flux density between the prim ary magnets.

Certain aspects of the dot matrix printer of which the present invention forms a part are described in detail in U.S. application Serial No.963171, filed 22nd November, 1978, in the name of Thomas P.

Sapitowicz, entitled "Print Head For Dot Matrix Printer" and corresponding to U.K. Application No.

The reader is referred to that application for a complete description thereof. However, the printer is described herein in general to enhance the reader's understanding of the present invention.

As seen in Fig. 1, the dot matrix printer incorporating a print head in accordance with the present invention includes a paper tray 10 comprising a circular bottom portion 12 having an upstanding cylindrical peripheral wall 14 and a center dereeler mechanism 16 of known configuration. Papertray 10 is rotatable about a point 18 at the center thereof. A roll of paper to be imprinted (not shown) is situated within tray 10 between the dereeler 16 and outer peripheral wall 14 such that the bottom thereof rests on surface 12. Tray 10 revolves about pivot point 18, such that a paper strip is continuously removed from the inside thereof and fed through a print head, generally designated 20.

The paper strip, as it is removed from the roll, passes from a point beneath head 20 vertically through head 20 towards the viewer, as seen in Fig. 1. As the paper strip travels through head 20 it passes between an ink impregnated surface in the form of a roller 22 freely rotatably mounted within an opening in the head housing 24 or a ribbon situtated in a cassette (not shown), and a wire bearing 26. Located behind wire bearing 26 are situated a plurality of hammers or flags. Preferably, twenty-eight hammers are provided. The hammers are situated in a magnetic field created by a magnet assembly, generally designated 28, which is mounted between a top magnet bracket 29 and a bottom magnet bracket 31.Each of the hammers are selectively actuatable by a conventional energization circuit in order to cause a print wire connected thereto to be displaced towards the ink impregnated surface, so as to press a section of the paper against the ink roller to imprint a dot thereon. A plurality of dots in closely spaced relationship are imprinted on the paper so as to form letters, numerals, or other intelligible symbols as the paper passes between the ink roller and the wire bearing.

Also located nearthe rear of the head are a pair of linear solenoids 30 mounted on brackets 32 con necked to head housing 24. Solenoids 30 are of conventional design and, when actuated, serve to imprint a bar code on either side of the space on the paper where the dots are imprinted. The imprinted bar code contains machine readable information or the like.

The details of print head 20 can best be observed from Fig. 2. The paper strip 34 after it is unwound from a paper reel (not shown) passes around a dancer roller arm (not shown) to a spool guide 36, in the form of a roller rotatably mounted on bracket 37 which forms a portion of head housing 24, and thereafter along a paper guide 38. The paper strip 34 then passes between a carborundum roller 40 and a powered pinch roller 42, both of which are rotatably mounted on head housing 24. Thereafter, the paper strip 34 passes between ink roller 22 and wire bearing 26 at which point the dots and bars are imprinted thereon. After passing wire bearing 26, the paper strip is directed between a pair of knife blades 44 and 46 which are actuatable by a conventional solenoid mechanism (not shown) to cut the paper at the desired point, so as to form a ticket or the like.

Located behind the wire bearing 26 is a hammer housing 48 in which a plurality of hammers 50 are mounted. As can best be seen in Fig. 3, the hammers 50 are situated in four groups or banks of hammers 52, 54, 56 and 58, each comprised of seven hammers. Each of the hammers is suspended from a pivot shaft 60 situated behind hammer housing 48.

Between each of the hammer assemblies 52, 54, 56 and 58 is a spacer member 62 also mounted on pivot shaft 60. The leads from the hammers (not shown) are connected to a printed circuit board 64, of conventional design, which contains the circuitry required for the high speed actuation of the individual hammers 50 and the bar code solenoids 30.

Also visible in this figure is the stepping motor housing 66 which rotates pinch roller 42 in order to advance the paper strip 34 through head 20. As is best seen from Fig. 4, each of the hammers 50 comprises a substantially planar frame 68, preferably manufactured by stamping an aluminum sheet.

Frame 68 includes an elongated flexible support member 70 having a groove or recess 72 along its length. Nearthe rear of suspension member 70 is a bifurcated part 74 having an opening therein which is adapted to be received over pivot shaft 60. Bifu rcated part 74 extends downwardly towards the bottom of hammer housing 48 and the parts thereof are located between a pair of protrusions 76 extending parallel to pivot shaft 60. In this manner, flag 50 is supported in cantilever fashion from hammer hous ing 48.

Within the body of frame 68, which is generally rectangular in configuration, is situated a coil 80. Coil 80 is preferably wound about a bobbin 78. The periphery of the coil adheres to the inside of the body of frame 68 by a potting compound 81 situated therebetween. Coil 80 has a pair of leads 82, 84, one or both of which extends along groove 72 in suspension member 70 in order to connect coil 80 with printed circuit board 64.

Each of the coils 80 are situated within a magnetic field supplied by magnet assembly 23, described in detail below. By passing an electrical current through leads 82, 84 and, thus, coil 80, a force is developed such that the hammer 50 abruptly moves a short distance towards paper strip 34, in a slight arc about the axis of pivot shaft 60. When the current ceases to flow through coil 80, the force developed by the magnetic field terminates and the hammer 50 returns to its original position through the flexing of support member 70 and from energy returned by impact force.

Located on the forward end of hammer 50 near the top corner thereof is a wire guide 86 which passes through an opening 88 in the top surface of hammer housing 24 so as to guide the movement of hammer 50. Located on the forward portion of hammer 50 near the bottom corner thereof is a print wire 90, preferably formed of tungsten in order to eliminate variations in the length thereof due to wear.

The tungsten print wire 90 is affixed to aluminum frame 68 by soldering same.

Extending from the bottom of head housing 24 towards hammers 50 is a movement limiting member 92 which serves to limit the rebound movement of the hammer after the current through the coil therein has been terminated. Print wire 90 extends along a channel 94 formed in the bottom portion of the hammer assembly 48 and terminates in wire bearing 26 situated at the lower end thereof. Wire bearing 26, as can best be seen from Fig. 6, has a plurality of groups of openings 26a,26b,26c,26d,27a and 27b therein. The openings are arranged in four groups 26a, 26b, 26c, 26d of seven circular openings each, each group being spaced from the adjacent groups, and a pair of bar openings 27a, 27b, located at each end of the wire bearing to accommodate the impact ends of the bar code solenoids 30.Each of the circular openings in each group of openings 26 is designed to accommodate a single print wire 90.

One of the unique features of this embodiment of the present invention is that all of the hammer assemblies 52, 54, 56, 58, comprising seven hammers each, are situated in the magnetic field formed between a single pair of primary magnets, in contradistinction to prior art configurations wherein a pair of magnets is required for each of the hammers.

Thus, the size, bulk and complexity of the head is greatly reduced. In order to achieve this unique result, two characteristics are required: 1) each hammer must be extremely thin and retain its substantially planar configuration relative to adjacent hammers and 2) the field created by the primary magnets must be substantially uniform and strong enough in order to create sufficient force on each hammer to displace same when the coil therein is energized. The manner in which these structural requirements are achieved is best understood by reference to Figs. 5, 7, 8, 9 and 10.

Figure 5 is a greatly enlarged cross-sectional view taken through hammer 50 along line 5-5 of Fig. 4. At the top of the figure is shown in cross section aluminum frame 68 which is stamped from an aluminum sheet and is approximately .016 inch in width. Aluminum is chosen for this component because it will retain its substantially planar shape and because of its strength, lightness and flexibility.

Bobbin 78, one wall of which is shown in the figure, is hollow, made of anodized aluminum and has a width of approximately .020 inch. Bobbin 78 is purposely designed to be wider than the width of frame 68 such that it extends outwardly on either side of the plane of the frame so as to form bearing surfaces 78a and 78b on its peripheral edges.

The hammers 50 within each group of hammers are mounted in side-by-side relationship in close proximity such that a minimum of space is required between the two primary magnets, thereby enhancing the magnetic flux density therebetween. Therefore, even the slightest misalignment of one of the hammers 50 will cause same to rub against the adjacent hammers. Such rubbing between adjacent hammers could prevent the free displacement thereof or result in the wearing of parts, eventually destroying the hammers. In order to avoid this, each of the bobbins 78 is made slightly widerthan frame 68 such that if the hammers are slightly misaligned, it will be the bobbins of adjacent hammers, and particularlythe peripheral anodized bearing surfaces 78a and 78b thereof, which rub together during displacement of the hammers.Thus, any wear on the hammers is confined to the anodized bearing surfaces, which are situated in face-to-face relationship, such that the planar configuration of each of the hammers is maintained and any wearing of the hammer is confined to an area wherein it is not detrimental to the operation of the hammer.

It is therefore clearthatthe entire coil 80 must be confined within a width of less than .016 inch such that adjacent coils will not rub against each other during displacement. This limitation must be offset against the fact that coil 80 must have as many turns as possible thereon such that sufficient ampere turns will be present to provide the necessary displace mentforce. In order to achieve this result, the coil 80 must be wound in a unique manner.

Conventional coils are wound such that one end of the wire is affixed to the surface of the bobbin. An arm holding the wire is rotated about the bobbin such that the wire is wound to form a layer of sideby-side turns on the surface of the bobbin. The layer is as wide as the desired width of the coil. Successive layers of side-by-side turns are wound until a coil of the desired dimensions and number of turns is obtained. With this structure, the outer end of the wire extends from a turn on the outermost layer and, therefore, can be used to form a lead without requiring any additional widthwise space.

However, the inner end of the wire, which extends from one of the innermost turns adjacent the bobbin's surface, must also form a lead such that a complete circuit is achieved. This wire end must extend for a short distance from the inside turn of the coil in a direction parallel to the axis of the bobbin, and, therefore, perpendicular to the plane of the turns.

This wire may be bent in a direction parallel to the plane of the turns, immediately adjacent to the place where it extends from the inside of the coil and then situated alongside the coil surface to the periphery of the coil. Thus, this portion of the wire will require a space alongside the coil at least equal to the diameter of the wire.

In most situations, this additional amount of space is available and, therefore, the conventional configuration creates no problems. In the present situation, however, the width of the coil must be confined to an area less than .016 of an inch and, preferably, to an area of .015 of an inch. For practical purposes, the wire utilized to wind a coil must be a minimum of .003 inch in diameter. Thus, if all of the space permitted for the coil is used effectively, five side-by-side radially extending columns ofturns can be placed within the permissible .015 inch maximum width.

If the coil is wound in the conventional manner, starting with one end adjacent the bobbin surface and winding outwardly to form successive layers of side-by-side turns, the lead extending from the inner turn will require a space of .003 inch (the diameter of the wire) such that it can pass along the side surface of the coil to the periphery thereof. Thus, the space which could be used for an entire radially extending column of turns is eliminated, if the coil is to be placed within a confined area.Since the wire of .003 inch diameter would permit, at best, five side-byside radially extending columns of turns within a .015 inch area, if the coil is wound in the conventional manner, such that a lead from the inner turn had to run alongside the coil, only four side-by-side radially extending columns of turns would be possible, the space where the other radially extending column of turns normally would be situated being taken up by the wire lead.

In order to eliminate this problem and, therefore, permit as many turns as possible, the coil is wound outwardly from the middle of the wire, such that both ends of the wire extend from different turns on the outer layer in the respective planes of the radially extending column of turns in which those turns are situated. Thus, no additional widthwise space is required for the leads. If the space between the outer bobbin surface and the inner wall of the frame recess is approximately .10 inch, this enables the coil to have five side-by-side radial columns of 35 turns each, resulting in a total of 175 turns, within the required .015 inch width.

This configuration is achieved by initially placing an intermediate portion 95a of the wire 95 adjacent the middle support surface (shown as bobbin 68), as illustrated in Fig. 9A. One section of the wire 95b is held by a first flying coil winding arm 97 and the other section 95c is held by a second flying coil winding arm 99. Arm 97 is caused to revolve about the axis of the bobbin in a first direction (clockwise, as seen in Fig. 9A) wrapping wire 95b around the bobbin surface to form half the coil, as seen in Fig. 9B.

Therefter, arm 99 is caused to revolve in the opposite direction (counterclockwise, as seen in Fig. 9B) to cause section 95cto be wrapped around the bobbin surface to form the remainder of the coil, as seen in Fig. 9C. Viewed endwise, as in Fig. 9D, it can be seen that the leads 82, 84 are entirely within the plane of the coil and, thus, require no additional space.

It is to be understood that the coil may be removed from the support (bobbin) after it is wound and, thereafter, mounted within the recess in the hammer, such that the hammer contains a bobbinless coil. However, it is preferable to use a bobbin as a support and to mount the entire assembly, including the coil and bobbin, to the hammer.

With the winding method described above, the maximum number of radially extending columns of turns, and thus the maximum number of turns, may be situated within the permitted widthwise space.

This winding configuration results in the maximum number of windings within the limited widthwise space, and permits an increase in the number of turns by 20% over conventional configurations. This results in a substantial increase in the field created by the current flowing through the coil and, thus, contributes substantially to the amount of force developed for displacement.

It should be noted that the width permitted for the support member 70 of the hammers 50 is also limited. However, leads 82,84 from coil 80 must be connected to printed circuit board 64 such that the coil can be energized. Normally, if leads 82, 84 were placed alongside member 70, they would take up a minimum of .003 inch space in addition to the .016 inch width of member 70 and, thus, create the possibility of rubbing against adjacent suspension members, eventually resulting in damage to the wires. In order to prevent this situation, member 70 is provided with a slot or groove 72 along its length into which one or both of the leads 82, 84 are received. In this manner, the running of the leads from coil 80 to the printed circuit board 64 requires no additional widthwise space.

It is necessary to have the magnetic flux density created by the two primary magnets located on either side of the hammer assemblies be as high and as uniform as possible along the space within which the hammers are situated. In order to achieve this result, field shaping magnets, in addition to the primary magnets, are utilized.

Fig. 7 illustrates a first preferred embodiment of magnet assembly. The magnet assembly comprises fourbarmagnets 100,102, 104 and 106. Magnets 100 and 102 are placed in side-by-side spaced relationship in opposite orientations, that is, poles of opposite polarity aligned with each other. A steel shunt 108 is situated in abutting relationship with the outer end of each of the magnets 100 and 102 so as to form a first "U"-shaped primary magnet. Similarly, magnets 104 and 106 are situated in side-by-side relationship in opposite orientation such that poles of opposite polarity are aligned. A steel shunt 110 abuts against the outer end of each of the magnets 104, 106 to form a second "U"-shaped primary magnet.

The ends of magnets 100 and 102 on one hand, and the ends of magnets 104 and 106 on the other hand, are spaced from each other by a space 112 within which the hammer assemblies are situated. A magnetic field is created across space 112 between the "U"-shaped primary magnets. Most of the lines of magnetic force cut across space 112 in a substantially horizontal direction, as viewed in Fig. 7. However, the magnetic flux density of space 112 is not uniform but is a function of the distance from the nearest pole. Thus, the magnetic flux density is highest adjacent the pole and falls off as one reaches the center of space 112 (this is illustrated graphically in Fig. 10A). From the center of space 112tothe opposite pole of the magnet on the other side of space 112, the magnetic density increases to the maximum. This variation in the flux density results because some of the lines of the magnetic flux, which optimally would traverse area 112 between the magnets in a direction perpendiculartothe hammers, are diverted back to the opposite pole of the same "U"-shaped primary magnet, instead of the opposite pole of the other "U"-shaped primary magnet, that is, between the north pole of primary magnet 100 and the south pole of primary magnet 102, on the one hand, and between the south pole of primary magnet 104 and the north pole of primary magnet 106, on the other hand, and, thus, do not reach across the area where the hammers are situated.

Since this diverted magnetic flux does not cut across the area 112 where the hammers are situated, it is wasted, decreasing the amount of force developed to displace the hammers. This result is particularly acute when the opposite poles of each set of magnets 100, 102 and 104, 106, respectively, are spaced close together, such that the north pole of magnet 100 is in close proximity to the south pole of magnet 102 and the south pole of magnet 104 is in close proximity to the north pole of magnet 106.

However, in the print heads of the type hereunder discussion, the close proximity of the opposite poles of the magnets in each set cannot be avoided due to the lack of space. The effect of the reduction in the magnetic flux density in the middle of the space 112 isthatthe hammers situated near the middle of space 112 are in a field of less density than the hammers at the peripheries of space 112, thus resulting in less force being applied to the hammers in the central portion of space 112 when energized than the hammers on the periphery of space 112.

This problem is eliminated through the use of field shaping magnets. Two types of field shaping magnets are utilized. One set 114, 116 are situated to extend within space 112 and a second set 118, 120, 122 and 124 are situated outside of space 112. Field shaping magnets 114 and 116 each have two parts, 114a, 114band 116a, 116b,respectively. Part114a is situated between the north pole of magnet 100 and the south pole of magnet 102. Part 1 14b extends from part 1 14a into space 112. Similarly, part 1 16a of field shaping magnet 116 is situated between the south pole of magnet 104 and the north pole of magnet 106. Part 1 16b offield shaping magnet 116 extends from part 11 6a into space 112.The orientation of field shaping magnet 114 is such that the north pole thereof is adjacent to the north pole of magnet 100 and the south pole thereof is adjacentto the south pole of magnet 102, such that the lines of magnetic flux density which would normally be directed between the north pole of magnet 100 and the south pole of magnet 102 are redirected, such that they extend across space 112 to the south pole of magnet 104 and the north pole of magnet 106, respectively.

In a similar manner, field shaping magnet 116 is oriented such that the south pole thereof is adjacent the south pole of magnet 104 and the north pole thereof is adjacent the north pole of magnet 106 such that the lines of magnetic force which would normally be directed between the south pole of magnet 104 and the north pole of magnet 106 are redirected in a plane across space 112 to the north pole of magnet 100 and the south pole of magnet 102, respectively. In this manner, some of the lines of magnetic force which normally would not cut across the plane of the hammers, are redirected to do so.

Outside field shaping magnets 118, 120, 122 and 124 have the same effect, accomplished by redirecting lines of magnetic force, which would normally be directed between the north pole of magnet 100 and the south pole of magnet 104, on the one hand, and between the south pole of magnet 102 and the north pole of magnet 106, on the other hand, but would be curved around the outside of space 112, such that same travel across the space 112. This is accomplished by placing magnets 118, 120, 122 and 124 outside the primary magnets oriented such that poles of similar polarity are adjacent the poles of the primary magnets.Specifically, the north pole of field shaping magnet 118 is adjacent the north pole of magnet 100, the south pole of field shaping magnet 120 is adjacent the south pole of magnet 104, the south pole of field shaping magnet 122 is adjacent the south pole of magnet 102, and the north pole of field shaping magnet 124 is adjacent the north pole of magnet 106.

The result of the use of the field shaping magnets is to shape the magnetic field within space 112 such that the magnetic flux density across each of the hammers situated therein is more nearly uniform.

This is illustrated in Fig. 1 or, which is an idealized graphic representation of the magnetic flux density across space 112 when the field shaping magnets are utilized.

In an alternative embodiment, as illustrated in Fig.

8, an even more uniform field is created by replacing inner field shaping magnets 114, 116 with four spaced magnets 115,117,119 and 121, oriented in the same direction as magnets 114, 116. Magnets 115 and 121 are mounted between magnets 100,102 and 104,106 in the identical manner as magnets 114, 116, respectively. Magnets 117 and 119 are respectively mounted on connecting members 123, 125, extending from magnets 115, 117, respectively. The resulting field is graphically illustrated in idealized fashion in Fig. 1 Oc. The field resulting from the use of magnets 115, 117,119 and 121 is somewhat more uniform than the field resulting from the use of only two field shaping magnets.It will now be appreciated that while the magnetic field across space 112 is still not uniform at every pointtherealong, the magnetic field density is more uniform and will have a more equal effect on each of the hammers situated within space 112 than would be possible in the situation where no field shaping magnets are utilized.

From Figs. 7 and 8, it is evident that inner field shapingmagnets114and116,or115,117,119and 121, extend across area 112 and thus would normally interfere with the movement of the hammers situated therein. In orderto eliminate this interference, bobbins 78 of the hammers are hollow so as to form a recess into which the field shaping magnets are at least partially received. The recesses are large enough, as compared with the magnets, to permit displacement of the hammers without interference from the magnets.

It will be appreciated that the head for a dot matrix printer described above generally comprises a plur ality of hammers situated in closely spaced side-byside relationship between a single pair of primary magnets. This configuration is possible because of the unique design of the hammers, including the novel method of forming the coil mounted thereon and the configuration of the magnet assembly, which induces a relatively high, substantially uniform field across the area in which the hammers are mounted.

Claims (37)

1. A print head for use in a dot matrix printer or the like, said head comprising a support (48) and a pair of magnets (100,102 and 104,106) mounted in spaced relationship on said support (48), characterized by at least two hammers (50) mounted inside by-side relationship on said support (48) between said magnets (100, 102 and 104, 106) for displacement relative to said support (48) in substantially parallel planes, each of said hammers (50) comprising a substantially planar frame (68) having a recess therein; a coil (80) mounted in said recess, said coil (80) having a side surface situated in closely spaced, face-to-face relationship with the corresponding side surface of the coil (80) mounted on an adjacent hammer (50), said coil (80) comprising a wire having first (95b) and second (95c) end portions and an intermediate portion (95a) relatively remote from said end portions (95b, 95c), said intermediate portion (95a) forming a portion of an inner turn of said coil, and said first (95b) and second (95c) end sections extending from different outer turns of said coil (80); a print wire (90) mounted on and extending from said frame (68), a power source and means (64) for energizing said coil (80) to displace said hammer (50) by operably connecting said coil (80) to said power source.

2. The head of Claim 1, characterized in that at least a portion of each of said first (95b) and second (95c) end sections are situated in the respective planes of said outer turns from which same extend.

3. The head of Claim 1, characterized in that said coil (80) is situated entirely within the plane of said frame (68).

4. The head of Claim 2, characterized in that said coil (80) is situated entirely within the plane of said frame (68).

5. The head of Claim 3, characterized in that said plane of said frame (68) is approximately .016 inch in width.

6. The head of Claim 5, characterized in that said coil (80) is formed of wire approximately .003 inch in diameter.

7. The head of Claim 6, characterized in that said coil (80) comprises at least five side-by-side radially extending columns of turns.

8. The head of Claim 4, characterized in that said wire is approximately .003 inch in diameter.

9. The head of Claim 8, characterized in that said plane of said frame (68) is approximately .016 inch in width.

10. The head of Claim 9, characterized in that said coil (80) comprises at least five side-by-side radially extending columns of wire.

11. The head of Claim 1, characterized by first means (114, 116) for concentrating the magnetic flux in the area (112) between said magnets (100,102 and 104, 106).

12. The head of Claim 11, characterized in that said first concentrating means (114,116) is afield shaping magnet(114,116).

13. The head of Claim 11, characterized by second means (118, 120, 122, 124) for concentrating the magnetic flux in the area (112) between said magnets (100,102 and 104,106).

14. The head of Claim 12, characterized by second means (118, 120, 122, 124) for concentrating the magnetic flux in the area (112) between said mag nets (100, 102 and 104, 106).

15. The head of Claim 14, characterized in that said second concentrating means (118, 120, 122, 124) comprises a second field shaping magnet (118, 120,122,124).

16. The head of Claim 2, characterized by first means (114, 116) for concentrating the magnetic flux in the area (112) between said magnets (100, 102 and 104,106).

17. The head of Claim 3, characterized by first means (114, 116) for concentrating the magnetic flux in the area (112) between said magnets (100, 102 and 104,106).

18. The head of Claim 16, characterized by second means (118, 120, 122, 124) for concentrating the magnetic flux in the area (112) between said mag nets (100, 102 and 104,106).

19. The head of Claim 17, characterized by second means (118,120,122,124) for concentrating the magnetic flux in the area (112) between said mag nets (100,102 and and 104, 106).

20. The head of Claim 17, characterized in that said first concentrating means (114, 116) is a field shaping magnet(114, 116).

21. The head of Claim 20, characterized by second means (118, 120, 122, 124) for concentrating the magnetic flux in the area (112) between said magnets (100, 102 and 104, 106).

22. A print head assembly for use in a dot matrix printer or the like, said assembly comprising a support (48) and a pair of primary magnets (100, 102 and 104, 106) mounted in spaced relationship on said support (48), characterized by at least two hammers (50) mounted in closely spaced, side-by-side relationship between said magnets (100, 102 and 104, 106) for displacement relative to said support (48) in substantially parallel planes, each of said hammers (50) comprising a substantially planar frame member (68) having a recess therein, a coil (80) mounted within said recess; a power source, means (46) for operably connecting said coil (80) to said power source, and wherein each of said primary magnets (100, 102 and 104, 106) comprises a first (100, 104) and second (102, 106) pole, said primary magnets (100, 102 and 104, 106) being oppositely oriented such that poles of opposite polarity are aligned with each otherto create a magnetic field therebetween, and means (114, 116) for concentrating said magnetic field within the area (112) between said primary magnets (100, 102 and 104,106).

23. The head of Claim 22, characterized in that said field is substantially perpendicular to the planes of displacement of said hammers (50).

24. The head of Claim 22, characterized in that said field concentrating means (114,116,118,120, 122, 124) comprises a first field shaping magnet (114,116).

25. The head of Claim 24, characterized in that said first field shaping magnet(114, 116) is situated between the opposite poles of one of the primary magnets (100, 102 and 104, 106) in said pair of primary magnets (100, 102 and 104, 106).

26. The head of Claim 25, characterized in that said first field shaping magnet (114, 116) has poles of opposite polarity and wherein said poles are situated adjacent to poles of similar polarity on said one of said primary magnets (100, 102 and 104, 106) in said pair.

27. The assembly of Claim 24, characterized in that said field concentrating means (114, 116, 118, 120, 122, 124) further comprises a second field shaping magnet (118, 120, 122, 124) situated outside the area (112) between said primary magnets (100, 102 and 104, 106).

28. The head of Claim 27, characterized in that said second field shaping magnet (118,120,122, 124) has poles of opposite polarity and wherein said second field shaping magnet (118,120,122,124) is oriented in a direction so as to converge said field toward the area (112) between said primary magnets (100,102and104,126).

29. The assembly of Claim 26, characterized in that said field concentrating means (114, 116, 118, 120, 122, 124) further comprises a second field shaping magnet (118,120,122,124) situated outside the area (112) between said primary magnets (100, 102 and 104,106).

30. The head of Claim 29, characterized in that said second field shaping magnet (118,120,122, 124) has poles of opposite polarity and wherein said second field shaping magnet(118, 120,122,124) is oriented in a direction so as to converge said field toward the area (112) between said primary magnets (100, 102 and 104,106).

31. A method for winding a wire on a support (98) to form a coil (80), wherein the wire ends (82,84) each extend from a different one of the outer turns of the coil (80), with a coil winding machine having first (97) and second (99) revolvable coil winding arms, the method being characterized by the steps of: (a) holding the wire with the first (97) and second (99) coil winding arms such that an intermediate portion (95a) of said wire, remote from the end portions (95b, 95c) thereof, is adjacent a surface of the support (78); (b) causing the first arm (97) to revolve about the support (78), in a first direction, so as to wind a first part of the wire, comprising a segment between the intermediate portions (95a) and one end portion (95b), around the support (78) to form a first half of the coil (80); and (c) causing the second arm (99) to revolve about the support (78), in a second direction, so as to wind a second part of the wire, comprising a segment between the intermediate portion (95a) and the other end portion (95c), around the support to form a sec ond half of the coil (80).

32. The method of Claim 31, characterized in that said first and second directions are opposite to each other.

33. A coil for use in a hammer in a dot matrix printer or the like comprising a central opening, first (95b) and second (95c) end portions and an intermediate portion (95a) relatively remote from said end portions (95b, 95c) said intermediate portion (95a) being situated adjacent said opening and forming an inner turn of said coil (80), and said first (95b) and second (95c) end sections extending from outer turns of said coil (80).

34. The head of Claim 33, characterized in that said first (95b) and second (95c) end sections are situated in the respective planes of said outer turns from which same extend.

35. The head of Claim 34, characterized in that said coil (80) is less than .016 inch wide.

36. The head of Claim 35, characterized in that said coil (80) is formed of wire approximately .003 inch in diameter.

37. The head of Claim 36, characterized in that said coil (80) comprises at least five side-by-side radially extending columns of turns.