GB2035220A - Print head for dot matrix printer - Google Patents

Abstract

The print head includes a plurality of substantially planar hammers 50 suspended in closely packed on side-by-side relationship, all of which are situated between a single pair of "U"-shaped primary magnets. Each hammer 50 has an aluminium frame 68 a flat coil 80 and tungsten print wire 94. Preferably, the coil 80 is wound on a bobbin 78 whose sides extend beyond the plane of the frame 68 to form bearing surfaces for the corresponding surfaces of adjacent hammers 50 to maintain the spacial relationship therebetween. The hammer is suspended from a support 48 by an elongated member 70 having a recess within which the ends of the coil are received for electrical connection with the hammer selector. The coil 80 is sandwiched between two foil heat sinks 98 in order to dissipate the generated heat. <IMAGE>

Description

SPECIFICATION Print head for dot matrix printer The present invention relates to a print head for a dot matrix printer and, more particularly, to a print head wherein a plurality of hammers are situated in side-by-side relationship between a single pair of primary magnets so as to reduce the size, complex ity 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 impregnatedsurface. Itis desirable to have the printwires extend between the solenoid and the opening in the wire bearing through which the impact end of 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 orderto 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, and some means must be provided to guide these wires through their respective curved paths.

One method which has been used to guide the print wires through the curved path is by surrounding each of the print wires with a tubular sheeth made of plastic or beryllium-copper alloy. The tubular sheeths extends along the length of the wire and act to retain the wire in the proper position, when the solenoid is actuated, and to direct the displacement forces in a plane perpendicular to the paper. Other methods used to achieve this result include guiding the print wires through guide holes in an array of parallel discs or along grooves situated around a conical center piece.

However, regardless of the particular configu ration of the print wire guide means, substantial friction is developed between the print wires and the surrounding 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 expen size 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 results in the print wire causing engagement between the paper and the 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 togetherto permit all of the hammers to be colinear with the movement of the print wire ends.Because of the necessity for providing magnets 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 problem caused by curved print wires and to reduce the bulk of the head to a minimum, a new coil carrying 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 design of the hammer, including a novel method of winding the coil and through the use of field-shaping magnets which, in conjunction with the primary magnets, actto concentrate the magnetic flux across the hammers. Moreover, because a substantial amount of heat is generated by current passing through coils as the closely spaced hammers are repeatedly actuated, a method of heat dissipation has been devised such that significant heat build-up is prevented during operation.

In accordance with 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 mounted on a bobbin. The coil is wound to permit the maximum number of turns possible without extending beyond the plane of the frame.

The manner in which the coil is wound to achieve this result forms no portion of the present invention.

For detailed information concerning this feature of the print head, the reader is referred to U.S. application Serial No. 962908 filed 22nd November 1978, entitled "Method of Coil Winding and Magnet Arrangement in Dot Matrix Print Head" in the name of John Mark Tewksbury corresponding to U.K.

Application No.

The coil has first and second end sections adapted to be operably connected to the energizing means.

An elongated member suspends the hammer from the support in cantilever fashion. This elongated member has a double function in that it acts as a mounting member and also retains wire ends for connection with the energizing means. Since it is necessary that the mounting member be no wider than the width of the frame, a recess or groove along the elongated member acts as a passageway for the wire ends, such that same can extend through the elongated member without requiring any additional space In order to operate in the extremely confined area permitted for each of the hammers, it is necessary that the hammer frames be substantially planar and be accurately positioned with respect to each other so as to prevent interference between adjacent hammers when one or the other is actuated.In order to achieve this result, the bobbin, which is mounted to the frame, is designed to be wider than the plane of the frame, such that it extends beyond the side surfaces of the frame. The edges of the bobbin are preferably anodized to form bearing surfaces therealong. With this configuration, all contact between adjacent hammers will be along the bearing surfaces on the opposing edges of the bobbins which are in face-to-face relationship. In this manner, the spacing between the hammers is maintained and any wear caused by rubbing between the adjacent hammers is confined to the anodized bearing surfaces at the edges of the bobbins, where such wear is not harmful to the assembly.

Actuation of the hammers is achieved by passing a current through the coil which is situated in a magnetic field. The passage of current through the coil results in the generation of a certain amount of heat. When the hammers are situated in a closely confined area, the rapid successive actuation of the hammers can cause a heat build-up which will eventually damage the head. It is therefore necessary that some heat dissipating device be utilized in conjunction with the hammers. Such a heat dissipating device must, however, not add substantially to the mass or the size of the hammer. It is necessary to keep the mass of the hammer as low as possible to reduce the reaction time. A heat dissipating device which added substantially to the mass of the hammer would substantially reduce the efficiency of the hammer.Moreover, the hammers must be quite thin in order to permit a plurality of the hammers to be situated between a single pair of primary magnets. A heat dissipating device which would add substantially to the width of the hammer would therefore reduce the number of hammers which could be situated between the magnets.

In orderto provide the necessary heat dissipating function, without substantially adding to the mass or the size of the hammer, a thin foil of heat conductive material is situated in thermal communication with the coil at either side thereof such that the coil is sandwiched therebetween. The foil is so thin so as not to substantially add to the mass or the thickness of the hammer. However, the foil is capable of dissipating a sufficient amount of heat such that heat build-up in the coil is considerably reduced.

Because the frame must be extremely thin and planar and must retain its planar configuration during operation, it has been found that it is preferable to stamp the frame out of a sheet of aluminum. The print wire, which extends outwardly from the frame in the direction of the paper, is subjected to a great amount of wear at the impact end. Further, the length of the print wire is critical because wear at the end thereof will cause an improperly formed dot to be imprinted and eventually, if the wear is severe enough, will result in a dot not being printed at all.

Thus, it is necessary that the print wire be composed of tungsten. Tungsten is not only highly wear resistant, it also has a high Young's Modulus, as com pared to steel, for instance. This permits a long free length, minimizing curvature, without attendant buckling of the wire column.

However, aluminum and tungsten are two metals which are extremely difficult to join together by pre sently practiced industrial techniques. In order to overcome this problem, the aluminum frame is provided with an opening in the body thereof adjacent to the edge from which the tungsten print wire extends. The print wire is provided with a curved end portion. The curved end portion of the print wire is received within the opening in the frame such that the end portion may be reliably soldered within the opening and, thus, the print wire is mounted to the frame by a relatively strong rigid bond.

In order to develop the necessary impact force, 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. As the distance between the magnets increases, more of the flux lines are directed between the opposite poles of the same magnet instead of across the area between the opposite poles of the opposing magnet of the pair.

In orderto 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. The use and configuration of the field shaping magnet assembly forms no portion of the present invention. For detailed information concerning this feature of the print head, the reader is referred to the aforementioned co-pending application entitled "Method of Coil Winding And Magnetic Arrangement in Dot Matrix Print Head", in the name of John MarkTewksbury.

It is important to understand, however, that certain of the field shaping magnets in the assembly are situated in a position which would normally intersect the path of movement of the hammers. In order to prevent these field shaping magnets from interfering with the movement of the hammers, the coils, if bobbins are not used, or the bobbins, if same are used, 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.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings, wherein like numerals refer to like parts, and in which: Fig. 1 is a plan view of a dot matrix printer incorporating a print head in accordance with the present invention; Fig. 2 is a side view of the head of the dot matrix printer; Fig. 3 is a rear view of the head of the dot matrix printer; 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 print head; Fig. 6 is a bottom view of a wire bearing; Fig. 7 is a plan view of the magnet assembly which forms a portion of the dot matrix print head; and Fig. 8 is an enlarged view of a section of the hammer frame.

As seen in Fig. 1, the dot matrix printer 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 of the roll 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, either in the form of a roller 22 freely rotatably mounted within an opening in the head housing 24 or an ink ribbon, preferably mounted in a cassette (not shown), and a wire bearing 26. In a similar embodiment, the paper strip may pass between a print platen (not shown) and the ink ribbon, with the wire bearing located behind the ribbon. Located behind wire bearing 26 are situated a plurality of hammers or flags. In the preferred embodiment, 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. Or press a section of the ink ribbon against the paperto similarly imprint a dot. 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 impregnated surface and the wire bearing.

Also located near the rear of the head are a pair of linear solenoids 30 mounted on brackets 32 connected 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 roll (not shown) passes around a dancer 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 and along a paper guide 38. The paper strip 34 then passes between a powered carborundum roller 40 and a 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 carborundum roller 40 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. Near the rear of suspension member 70 is a bifurcated part 74 having an opening therein which is adapted to be received over pivot shaft 60. Bifurcated 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 wrapped around a bobbin 78. However, coil 80 may be mounted without a bobbin, if required. 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 extend 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 28, 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 throngh the flexing of support member 70 and the 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 cornerthereof is a print wire 90, preferably formed of tungsten in order to eliminate variations in the length thereof due to wear. The method in which the tungsten print wire 90 is affixed to aluminum frame 68 is disclosed in detail below.

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 grouped in four groups 26a, 26b, 26c, 26d of seven round openings each, each group being spaced from the adjacent groups. A pair of bar openings 27a, 27b, located at each end of the wire bearing are provided to accommodate the impact ends of the bar code solenoids 30. Each of the round openings in each group of openings is designed to accommodate a single print wire 90.

One of the unique features of the head 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 orderto 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 permanent magnets must be substantially uniform and strong enough to create sufficient force on each hammer to displace same when the coil therein is energized.

The manner in which the first of these structural requirements is achieved is best understood by referring to Figs. 4 and 5. The manner in which the second of these requirements is achieved is illustrated generally in Fig. 7 and described in detail in the co-pending application referred to above.

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 ofwhich is shown in the figure, is hollow, made of anodized aluminum and has a width of approximately .020 inch. Bobbin 78 is pur poselydesignedto be wider than the width of frame 68 such that it extends outwardly on either side of the plane ofthe bobbin so as to form anodized 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 wider than frame 68 such that it is the bobbins of adjacent hammers, and particularly the peripheral 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 clear that the entire coil 80 must be confined within the plane of the frame 68, 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 in order to provide the necessary displacement force. In order to achieve this result, the coil 80 may be wound in the unique manner described in the aforementioned copending application.

When this coil winding method is used, the maximum number of turns, preferably 175 in total of a wire having a diameter of approximately .003 inch, may be situated in five side-by-side layers within the permitted width-wise space. This winding configuration results in the maximum number of windings within the limited width-wise space, permitting 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 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.

As mentioned above, a small amount of heat is generated by current passing through the coil when same is energized. Because of the closely packed relationship of the side-by-side hammers in each assembly, sufficient heat may build up within this confined space, after numerous successive actuations of the hammers, to eventually destroy the hammers. Thus, the use of a heat dissipating device is required. However, the heat dissipating device must be such that it does not contribute substantially to the width of the coil, which is confined within a very limited space and, in addition, does not contribute substantially to the mass of the hammer, which must be maintained as low as possible in order to overcome inertia and maintain a small response time.

In orderto accomplish this result, the coil of the hammers of the present invention are sandwiched between a pair of heat sinks 96,98, as illustrated in Fig. 5. Sinks 96, 98 are formed of thermally conductive foil, preferably composed of aluminum, of approximately .001 inch thickness. Each of the foils 96, 98 are bonded to the surface of the coil 80 by means of a very thin layer of adhesive. This configuration allows approximately three times as much heat to be dissipated as would a comparable coil without heat sinks affixed thereto. It should be appreciated that this enhanced heat dissipation is achieved without substantially increasing the mass or width of the coil.

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 orderto achieve this result, field shaping magnets, in addition to the primary magnets, are utilized, as is described in detail in the aforementioned co-pending application.

Fig. 7 illustrates a magnet assembly utilized for the hammer assemblies. The magnet assembly com prises four bar magnets 100,102, 104 and 106. Mag- nets 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 inside-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. From the center of space 112 to the 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 magnetic flux, which optimally would traverse area 112 between the magnets in a direction perpendiculartothe hammers are diverted back to the same "U"-shaped primary magnet, instead of the opposing "U"shaped primary magnet, that is, from the north pole of primary magnet 100 towards the south pole of primary magnet 102 and from the south pole of primary magnet 104 towards the north pole of prim ary magnet 106, and, thus, do not reach across the area 112 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 lessening of the magnetic flux density in the middle of the space 112 is that the 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 may be substantially reduced 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, 114b and 116a, 116b, respectively. Part 1 14a 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 116a offield shaping magnet 116 is situated between the south pole of magnet 104 and the north pole of magnet 106.Part 1 16b of field 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 adjacent to the south pole of magnet 102, such that the lines of magnetic field density which would normally curve around between the north pole of magnet 100 and the south pole of magnet 102 are redirected to extend in a direction 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 curve around between the south pole of magnet 104 and th2 north pole of magnet 106 are redirected 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 pass between the north pole of magnet 100 and the south pole of magnet 104, and between the south pole of magnet 102 and the north pole of magnet 106, but would be curved around the outside of space 112, such that same travel across space 112.

This is accomplished by orientating magnets 118, 120, 122 and 124 such that poles of similar polarity are adjacent the poles of the primary magnets. Spec ifically,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 field horizontally extending across space 112 such that the magnetic flux density cutting across each of the hammers situated therein is substantially uniform.

From Fig. 7, it is evident that inner field shaping magnets 114, 116 extend across area 112 and thus would interfere with the hammer assemblies situ atedtherein. In orderto eliminate this interference, bobbins 78 or coils 80, if bobbins 78 are not used, are hollow so as to form a recess 125 into which the magnets 114, 116 are at least partially received. The recesses 125 are large enough, as compared with the magnets, to permit displacement of the hammers without interference from the magnets.

Another aspect of this head relates to the manner in which print wire 90 is fixedly mounted on frame 68. As indicated previously, frame 68 is manufactured from aluminum because of the strength, light weight and flexibility of aluminum and, also because aluminum will retain its planar structure. However, print wire 90 must be made of tungsten in order to prevent wear at the tip and minimize curvature.

Clearly, wear at the tip of the print wire will cause a dot to be imperfectly imprinted and, if wear is severe enough, no dot to be imprinted at all. However, it is impossible, by means of known practical production welding techniques, to make a connection between aluminum and tungsten of sufficient strength to maintain the print wire on the frame. In order to overcome this problem, the end of the print wire which is to be affixed to frame 68 is bent to form a curve 126, as best illustrated in Fig. 8, which is an enlarged view of the end of the print wire and the portion of the frame to which same is affixed. An opening 128 is formed in frame 68. The curved portion 126 of print wire 90 is thereafter soldered within opening 128 of frame 68 so as to form the necessary bond. The configuration of the bent portion 126 of print wire 90 and the opening 128 in frame 68 permits the solder to hold print wire 90 to frame 68 with sufficient strength and rigidity, such that the hammer can function as intended.

Claims (55)

1. A print head for use in a dot matrix printer or the like, said head comprising a support (48), 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 in sideby-side relationship on said support (48) between said magnets (28) 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 substantially within said recess, said coil (80) having a side surface situated in closely spaced, face-to-face relationship, with the corresponding coil (80) surface on the adjacent hammer (50), 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 connected said coil (80) to said power source.

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

3. The head of Claim 2, characterized in that said plane of said frame (68) is approximately .016 inch in width.

4. The head of Claim 3, characterized in that said coil (80) is formed of wire approximately .003 inch in diameter.

5. The head of Claim 1, characterized by a bobbin (78) about which said coil (80) is wound, and wherein said bobbin (78) extends outwardly of the plane of said frame (68) in the direction of said adjacent hammer (50).

6. The head of Claim 5, characterized in that the width of said bobbin (78) is approximately .020 inch.

7. The head of Claim 6, characterized in that said plane of said frame (68) is approximately .016 inch.

8. The head of Claim 5, characterized in that said bobbin (78) has a peripheral surface (78a, 78b) situated in closely spaced, face-to-face relationship, with the corresponding bobbin surface (78a, 78b) on the adjacent hammer (50).

9. The head of Claim 8, characterized in that said bobbin surface (78a, 78b) is a bearing surface.

10. The head of Claim 9, characterized in that said bearing surface (78a, 78b) is anodized.

11. The head of Claim 2, characterized by a bobbin (78) about which said coil is wound, and wherein said bobbin (78) extends outwardly of the plane of said frame (68) in the direction of said adjacent hammer (50).

12. The head of Claim 11, characterized in that the width of said bobbin (78) is approximately .020 inch.

13. The head of Claim 12, characterized in that said plane of said frame (68) is approximately .016 inch.

14. The head of Claim 11, characterized in that said bobbin (78) has a peripheral surface (78a, 78b) situated in closely spaced, face-to-face relationship, with the corresponding bobbin surface (78a, 78b) on the adjacent hammer (50).

15. The head of Claim 14, characterized in that said bobbin surface (78a, 78b) is a bearing surface.

16. The head of Claim 15, characterized in that said bearing surface (78a, 78b) is anodized.

17. The head of Claim 1, characterized in that said coil (80) comprises a wire with end sections (82, 84) and further comprising an elongated member (70) extending from said frame and adapted to be affixed to said support (48) so as to mount said frame (68) thereto, said elongated member (70) having a recess (72) therein, one of said end sections (82,84) of said wire being at least partially received within said recess (72).

18. The head of Claim 17, characterized in that said elongated member (70) is substantially situated within the plane of said frame (68).

19. The head of Claim 5, characterized in that said coil (80) comprises a wire with end sections (82, 84) and further comprising an elongated member (70) extending from said frame (68) and adapted to be affixed to said support (48) so as to mount said frame (68) thereto, said elongated member (70) having a recess (72) therein, one of said end sections (82,84) of said wire being at least partially received within said recess (72).

20. The head of Claim 19, characterized in that said elongated member (70) is substantially situated within the plane of said frame (68).

21. The head of Claim 1, characterized by a heat sink (96,98) mounted on said frame (68) and in thermal communication with said coil (80).

22. The head of Claim 5, characterized by a heat sink (96, 98) mounted on said frame (68) and in thermal communication with said coil (80).

23. The head of Claim 22, characterized in that said sink (96, 98) is situated within the plane of said bobbin (98).

24. The head of Claim 11, characterized by a heat sink (96, 98) mounted on said frame (68) and in thermal communication with said coil (80).

25. The head of Claim 16, characterized by a heat sink (96, 98) mounted on said frame (68) and in thermal communication with said coil (80).

26. The head of Claim 21, characterized in that said heat sink (96, 98) comprises a thermally conductive foil.

27. The head of Claim 22, characterized in that said heat sink (96, 98) comprises a thermally conductive foil.

28. The head of Claim 24 characterized in that said heat sink (96, 98) comprises a thermally conductive foil.

29. The head of Claim 25, characterized in that said heat sink (96, 98) comprises a thermally conductive foil.

30. The head of Claim 26, characterized in that said foil is bonded to said coil (80).

31. The heat of Claim 1, characterized in that said frame (68) has an opening (128) therein and said print wire (90) comprises a curved end portion (126), said end portion (126) being received within said opening (128) such that said end portion (126) may be soldered within said opening (128).

32. The head of Claim 31, characterized in that said frame (68) is composed of aluminum and said print wire (90) is composed of tungsten.

33. The head of Claim 5, characterized in that said frame (68) has an opening (128) therein and said print wire (90) comprises curved end portion (126), said end portion (126) being received within said opening (128) such that said end portion (126) may be soldered within said opening (128).

34. The head of Claim 11, characterized in that said frame (68) has an opening (128) therein and said print wire (90) comprises a curved end portion (126), said end portion (126) being received within said opening (128) such that said end portion (126) may be soldered within said opening (128).

35. The head of Claim 19, characterized in that said frame (68) has an opening (128) therein and said print wire (90) comprises a curved end portion (126), said end portion (126) being received within said opening (128) such that said end portion (126) may be soldered within said opening (128).

36. The head of Claim 22, characterized in that said frame (68) has an opening (128) therein and said print wire (90) comprises a curved end portion (126), said end portion (126) being received within said opening (128) such that said end portion (126) may be soldered within said opening (128).

37. The head of Claim 1, characterized by a field shaping magnet (114, 116) and wherein said coil (80) has a recess therein and said field shaping magnet (114, 116) is at least partially received within said coil recess.

38. A print head for use in a dot matrix printer or the like, said assembly 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 on said support (48) between said magnets (28) for displacement relative to said support (48) in substantially parallel planes, each of said hammers (50) comprising a substantially planar frame (68) and a wire coil (80) mounted on said frame, a power source, means (64) for actuating said hammer by operably connecting said coil to said power source and a heat sink (96, 98) mounted on said frame (68), in thermal communication with said coil (80), so as to dissipate the heat generated by said coil (80) when said coil (80) is connected by said actuation means (64) to said power source.

39. The head of Claim 38, characterized in that said heat sink (96, 98) comprises a thermally conductive foil bonded to said coil (80).

40. The head of Claim 38, characterized in that said heat sink (96, 98) comprises a pair of thermally conductive foils bonded to different side surfaces of said coil (80).

41. The head of Claim 40, characterized in that said coil (80) is sandwiched between said foils.

42. The head of Claim 39, characterized in that said foil (80) is substantially situated in the plane of said frame (68).

43. The head of Claim 39, characterized in that said foil (80) is approximately .001 inch thick.

44. The head of Claim 38, characterized by a bobbin (78) around which said coil (80) is wound, and wherein said bobbin (78) extends outwardly of the plane of said frame (68) in the direction of an adjacent hammer (50).

45. The head of Claim 44, characterized in that said bobbin (78) has a bearing surface (78a, 78b) adapted to be situated in face-to-face opposing relationship with the corresponding bearing surface (78a, 78b) of the bobbin (78) mounted on an adjacent hammer (50), such that all contact between adjacent hammers (50) takes place along said bearing surface (78a, 78b).

46. The head of Claim 38, characterized in that said coil (80) has a wire with a pair of end sections (82,84) adapted to be connected to said actuation means (64) and wherein said frame (68) further comprises an elongated member (70) extending from said frame (68) and adapted to be affixed to said support (48) so as to mount said frame (68) thereto, said elongated member (70) having a recess (72) therein, one of said end sections (82,84) being at least partially received within said recess (72).

47. The head of Claim 38, characterized in that said frame (68) has a print wire (90) extending therefrom and wherein said frame (68) has an opening (128) therein, said print wire (90) comprising a curved end portion (126), said end portion (126) being received within said opening (128) such that said end portion (126) may be soldered within said opening (128).

48. The head of Claim 38, characterized by a field shaping magnet (114, 116), and wherein said coil (80) has a recess therein and said field shaping mag net (114, 116) is at least partially received within said recess.

49. A print head for use in a dot maxtix printer or the like, said assembly 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 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), a bobbin (78) and a coil (80) of wire wound around said bobbin (78); a power source, means (46) for operably connecting said coil to said cource, said bobbin (78) extending beyond the plane of said frame (68) in the direction of an adjacent hammer (50) and having a bearing surface (78a, 78b) in face-to-face opposing relationship with the corresponding bearing surface (78a, 78b) on the bobbin (78) of said adjacent hammer (50), such that all contact between said adjacent hammers (50) takes place between said respective surfaces (78a, 78b).

50. The assembly of Claim 49, characterized in that said bobbin (78) is wider than said plane of said frame (68).

51. The assembly of Claim 50, characterized in that said bobbin (78) extends beyond said plane of said frame (68) on both sides thereof.

52. The assembly of Claim 49, characterized in that said wire coil (80) has a pair of end sections (82, 84) adapted to be connected to said connecting means (64) and wherein said frame (68) further comprises an elongated member (70) extending from said frame (68) and adapted to be affixed to said support (48) so as to mount said frame (68) thereto, said elongated member (70) having a recess (72) therein, one of said end section (82,84) being at least partially received within said recess (72).

53. A print head for use in a dot matrix printer or the like, said assembly 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 on said support 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 coil (80) mounted thereon and an elongated print wire (90) extending outwardly from said frame (68) in the direction of movement of said frame (68), said frame (68) having an opening (128) therein, said print wire (90) having a curved end portion (126), said end portion (126) being received within said opening (128) such that said end portion (126) may be soldered within said opening (128).

54. The assembly of Claim 53, characterized in that said frame (68) is composed of aluminum and said print wire (90) is composed of tungsten.

55. A print head for use in a dot matrix printer or the like, said assembly 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 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 means (70) for mounting said frame to said support (48), a coil (80) mounted on said frame (68), and having at least one lead (82, 84) extending therefrom, a power source, means (64) for operably connecting said lead (82,84) to said source, said mounting means (70) comprising an elongated member (70) extending from said frame and adapted to be mounted on said support (48), said member (70) having a recess (72) extending along at least a portion of the length of said member (70), said recess (72) being adapted to receive at least a portion of said lead (82,84) therein.