PRINTER-HEAD DRIVING MECHANISM

Abstract: A printer head driving mechanism having a first shaft (40) for carrying the printer head (1), a linear motor actuator (20) for driving the printer head back and forth, a second shaft (30) parallel to the first shaft (40), a counterbalance (2, 3) moveable along or with the second shaft, and a counter transmission mechanism (U) having a rotatable member arrangement (5; 12a, 12b; 50) around which two flexible members, belts or wires for example (6a, 7a; 11a, 11b; 51, 52) oppositely, each flexible member being connected at one end to the printer head (1) at the other end to the counterbalance (2, 3) so that printer head and counterbalance move in opposite directions.

Description: The present invention relates to a printer-head driving mechanism. Today, high-speed operation is increasingly required of printers. A multihead printer, which has plurality of printer heads, can be used for high-speed printing. Ideally, the driving mechanism of a high-speed printer should be such that it does not generate noise or shock when it performs a reciprocal (reciprocating) motion. Ideally, also, a printer should be small, and such that repairs can be effected easily and quickly. Fig. 1 is a schematic plan view of a previously proposed printer-head driving mechanism. In the Figure, printer-head 1 is coupled with a linear motor 2 by wires 3a and 3b wound around pulleys 5a to 5d. The linear motor 2 drives the printer head 1 reciprocally left and right along a guide shaft 7. 4a and 4b are frames and the guide shaft 7 is fixed to the frames 4a and 4b. 6 is a guide roller, and 8 is a shaft fixed to the yoke of the linear motor 2. In such a printer (also called a shuttle printer), vibration and noise are generated because the printer-head 1 is driven reciprocally (back and forth) at high speed. In an effort to prevent this there has been proposed a printer-head drive mechanism having a counterbalance 3 as illustrated in Fig. 2(A), with the counterbalance being driven in a direction opposite to the direction of motion of the printer head 1, by means of a link mechanism 4. A structure which couples printer head 1 and counterbalance 3 with wires or tape 3a, 3b via pulleys in a manner as illustrated in Fig. 2(B) or in IBM TECHNICAL DISCLOSURE BULLETIN vol. 25, no. 9, February 1983, pages 4675-4676 has been proposed. In the first case, the driving motor 2 may be used as a counterbalance. Counter transmission mechanisms using a link mechanism and wire or tape have thus been proposed. However, a structure employing a link mechanism inevitably generates noise. For smooth operation of the mechanism as shown in Fig. 2(A), it is necessary to precisely align the guide shafts of the printer head and the counterbalance. For this purpose, a self-aligning shaft may be used, but this results in an increase of cost. In a structure as illustrated in Fig. 2(B), a space for expanding the wire or tape must be secured. Moreover, as shown in Fig. 1, the pulleys 5a to 5d are positioned on both sides of the apparatus, so, the size of the printer becomes relatively large. There is also a problem in that wire lengthens as a result of heat generated by high-speed operation of the printer head. If the wire slackens, printing operations are degraded. Further, if the wire is broken, it is troublesome to replace the wire, which is in a confined space, and replacement of the wire is a lengthy task. A guide mechanism for guiding reciprocal (reciprocating) movement of a printer head is illustrated in Fig. 3. The printer head 1 is mounted on a mount 11. A guide shaft 7 extends through one side of the mount 11 and guides reciprocal motion thereof. On the other side of the mount 11 there is provided a roller 6, which rolls on a rail 31. The roller 6 is held between the rail 31 and a rectangular C-shaped stopper 32 which prevents the roller 6 from being released from the rail 31. In Fig. 3, 20 is a platen. In order to secure smooth rotation of the roller 6, there must be a small gap between the stopper 32 and the roller 6. The presence of this gap can, however, allow release of the roller 6 from the rail 31, and can allow a small degree of rotation of the mount 11 around the guide shaft 7, causing print misalignment. With such a mechanism, therefore, precise adjustment is necessary to attain high print quality. An embodiment of the present invention can provide a small size or compact printer head driving mechanism which does not suffer, or suffers to a lesser degree, disadvantages of the previously proposed mechanisms, and which provides smooth operation of the printer head. Embodiments of the present invention may include the features of a counter transmission mechanism which couples a printer head and a counterbalance; reduced total mass of moving parts achieved by utilizing a driving linear actuator as counterbalance; and the use of a spring for protecting lift-up of a guide roller from its guide rail, and reducing the energy of return motion of the printer head. An embodiment of the present invention can provide a printer head driving mechanism for scanning a print head left and right for printing, capable of absorbing shock, vibration and noise caused by such movement. Reference is made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a schematic plan view of a previously proposed printer head driving mechanism; Fig. 2 illustrates schematically previously proposed arrangements for driving a printer head and a counterbalance in opposite directions, wherein Fig. 2(A) is a schematic perspective view illustrating a counterdriving arrangement using a link or lever, and Fig. 2(B) is a schematic plan view illustrating a counterdriving arrangement using a wire or belt; Fig. 3 is a schematic perspective view illustrating a guide mechanism for guiding movement of a printer head; Fig. 4 illustrates schematically a counterdriving mechanism of an embodiment of the present invention; Fig. 5 is a schematic perspective illustration of a printer apparatus to which an embodiment of the present invention is applied; Fig. 6 is a plan view schematically illustrating a counterdriving unit of an embodiment of the present invention. Fig. 7 is a schematic disassembled perspective view of the counterdriving unit of Fig. 6; Fig. 8 is a perspective view schematically illustrating coupling of parts of a printer apparatus which utilizes a counterdriving unit in accordance with an embodiment of the present invention; Fig. 9 schematically illustrates, in views (A), (B) and (C), operation of an arrangement in which, in effect, the pulley used in Fig. 4 is replaced by a cam; Fig. 10 schematically illustrates a perspective view of a guide mechanism for guiding movement of a printer head in accordance with an embodiment of the present invention; and Fig. 11 schematically illustrates a modification of the guide mechanism of Fig. 10, for use in a case in which the scan length of the printer head is longer. A counter transmission mechanism as mentioned above is explained in outline with reference to Fig. 4. In the Figure, a counterbalance 3 is connected to a first shaft 30 used for driving the counterbalance left and right, and a second shaft 40 is provided in parallel with the first shaft 30 for guiding motion of printer-head 1. A pulley 5 is provided between the first shaft 30 and the second shaft 40, and a pair of belts 6a (unhatched) and 7a (hatched) are wound for respective half-turns, in opposed directions to one another, around the circumference of the pulley 5. One end of each of belts 6a, 7a is fixed to the first shaft 30, while the other end of each belt is fixed to the second shaft 40. When the print-head 1 moves to the right as indicated by the adjacent arrow-mark directed from A to B in Fig. 4, the pulley 5 rotates in a direction as indicated by a curved arrow directed from A to B, so the counterbalance 3 is driven in a direction from A to B, that is in the opposite direction to the printer head 1. When the printer-head 1 moves to the left as shown by the adjacent broken-line arrow-mark directed from B to A, the pulley 5 and the counterbalance 3 move respectively in directions as shown by broken-line arrow-marks. By such a counter transmission mechanism, reciprocal (reciprocating) movement of the first shaft is transmitted to the second shaft 40 as counterreciprocal movement. The pulley 5 and the belts 6a and 7a are not in an embodiment of the present invention, required to be provided on both sides of the device (compare Fig. 1) and therefore the apparatus can be reduced in size. Repairs in the event of a fault can also be effected relatively easily, only one side of the apparatus needing to be opened. In accordance with an embodiment of the present invention a movable part of a linear motor can be utilized for a counterbalance, as indicated at 3 in Fig. 4. Fig. 5 illustrates an example of a printer apparatus to which an embodiment of the present invention is applied. Printer-head 1 is guided by a guide shaft 7 and a guide roller 6, and it is swept to right and left by a linear motor 2 via a counterdriving unit U. The linear motor 2 is provided under platen 20 which supports a sheet P of printing paper. 10 is a coupling plate for coupling the printer-head 1 and the counterdriving unit (reverse coupling unit) U. 21 is a tape (ribbon). Although not shown in the Figure, counterbalance 3 is provided at the lower part of (below) the platen 20. Printer-head 1 is on one side of (in front of) printing paper P and driving motor 2 and counterbalance 3 (not shown) are on the other side of (behind) the printing paper P. However, this arrangement may be modified, and head, motor and counterbalance can be arranged all to one side (for example, in front) of the paper P. Since the counter driving unit U is provided to one side of the apparatus (the right side in Fig. 5), it will be understood that the width of the printer apparatus can be reduced as compared with that of the previously proposed apparatus, and repair of the counterdriving unit can be effected from one side of the apparatus. Fig. 6 is a schematic plan view of a counter driving unit U of an embodiment of the present invention and Fig. 7 is an exploded perspective view of such a counterdriving unit U. In this case, a pulley 5 as seen in Fig. 4 is replaced by a pair of pulleys 12a and 12b which are used for widening the distance between the printer head 1 and the counterbalance 3, for convenience of assembling components of the printer apparatus. Belts 6a, 7a in Fig. 4 are respectively replaced by steel wires 11a (unhatched) and 11b (hatched). These steel wires are spun (strung) between the pulleys 12a and 12b which are rotatable around shafts 14a and 14b fixed to a mounting plate 13 (Fig. 7). In the embodiment of Figs. 6 and 7, a first coupling plate 9 is screwed to the counterbalance 3, while another coupling plate 10 (second coupling plate) is screwed to the printer head 1, using screws 19a and 19b respectively. The ends of each of the wires 11a and 11b are fixed respectively to the first and second coupling plates 9 and 10, and each wire is wound a half-turn around the pulleys 12a and 12b. The wires are wound in opposite directions to each other, in a form of a letter x. Each wire is wound approximately a quarter turn on each pulley, the two wires being wound on opposite sides of each pulley. As can be seen in Figs. 6 and 7, the first coupling plate 9 is composed of two plate sheets 9a and 9b. The first sheet 9a is provided with a first pin 15a and a first projection 17a, while the second plate 9b is provided with a second pin 15b and a second projection 17b. The first and second plates 9a and 9b are coupled so that they can slide in parallel to each other along a guide hole 18 and a slit 18′, which allows the root of the second pin 15b to pass, guided by a holding screw 21. A tension spring 16 is provided between the first and second projections 17a and 17b, urging the first and second pins 15a and 15b away from each other, applying tension to wires hung around (wrapped around) the pins 15a and 15b. In Figs. 6 and 7, one end A of the wire 11a is fixed (although this is particularly illustrated) to the coupling plate 10 with a fixing means. The wire 11a passes around one side of each of the pulleys 12a and 12b making a 1/4 turn around each of them, turns around the second pin 15b, turns back again making a 1/4 turn around the same side of each of the pulleys 12a and 12b, and is fixed at its other end B to the same fixing means as the end A. Similarly, the one end C of the wire 11b is fixed to the second coupling plate 10 by another fixing means (not shown), passes around the other side of the pulleys 12a and 12b making a 1/4 turn around each pulley, on the opposite side to wire 11a, turns around the first pin 15a, turns back again making a 1/4 around each pulley, and its other end D is fixed to the same fixing means to that of end C. The pulleys 12a, 12b and pins 15a, 15b are provided with grooves to prevent dislocation of the wires. As mentioned above, slackening of the wires is prevented by the tension spring 16 spun (stretched) between the projections 17a and 17b. The mechanism indicated above for preventing slackening of the wires is but an example, various modifications are possible. The fixing means for fixing the ends of the wires 11a, 11b may also be of any suitable kind, a bolt and nut or other fastening may be used. A simple fastening involves merely hooking a knot in the wire on a properly shaped hole or notch. Referring to Fig. 6, when the counterbalance 3 is driven in a direction as indicated by a solid line arrow-mark on the counterbalance in Figs. 6, the pair of wires 11a, 11b rotate the pair of pulleys 12a, 12b as indicated by solid line arrow-marks on the pulleys in Fig. 2, causing the printer head 1 to move in a direction as indicated by a solid line arrow-mark thereon in Fig. 6. If the linear motor drives the counterbalance 3 in an opposite direction, as indicated by a broken-line arrow-mark thereon, the other components move as indicated also by broken-line arrow-marks. In this case, the tension spring 16 serves to absorb shock of reverse movements of the components, and to help prevent positional misalignment of the printer head. If a linear driving motor is positioned coaxially with the counterbalance, moving parts of the linear motor can be included in (counted as part of) the weight of the counterbalance, so, the total weight of moving parts can be saved. This is effective for reducing the driving power required of the linear motor. Since the counter transmission mechanism of Fig. 6 uses two folded wires 11a and 11b, the wire strength is twice as much as that of a single wire, offering a longer operation life. Moreover, the counter transmission mechanism is formed as a unit, and the replacement of wires 11a, 11b can be effected by replacing the unit. This requires only a short period of time because it can be achieved merely by fixing or removing the coupling plates 9 and 10 to/from the printer head 1 and the linear motor 2 with the screws 19a and 19b. This will be more apparent from Fig. 8. In this embodiment, steel wires are used (as belts) but steel wire can be replaced by any suitable alternative of material which is flexible and does not elongate. A steel belt, for example, may be used. Fig. 8 is a perspective view illustrating a part of a printer apparatus to which a driving mechanism embodying the present invention is applied. This Figure also illustrates couplings of various parts of the apparatus as viewed from a rear side of the apparatus (Fig. 5 is a view of printing apparatus from a front side). Frame 4b is provided with apertures through which coupling plates 9, 10 are extended. These plates are screwed respectively to the printer head 1 and linear motor 2. Mounting plate 13 of reverse coupling unit U is also screwed to the frame 4b. As indicated above, it will be understood that such structure is very convenient for replacement of the reverse coupling unit U. Although not illustrated, a core of the linear motor 2 is supported in relation to the frames by proper means. In the apparatus illustrated in Fig. 8, the yoke of the linear motor moves in the directions of shaft 30 which is supported slidably along its axis by proper means, such as bearings for example. The movement of the yoke is transmitted to the reverse coupling unit U by shaft 30 and coupling plate 9. As illustrated in Fig. 8, the yoke of the linear motor and the shaft 30 mainly become equivalent to the counterbalance of the printer-head 1. Since the printer head is generally heavy, so the shaft 30 is formed thick and in some cases it is provided with a weight (not shown). The embodiment illustrated in Fig. 8 employs a multihead-type printer-head 1, which is provided with fourteen dot printer heads 1′ arranged in parallel to increase printing speed. A print-head driving mechanism embodying the present invention is especially useful for such high-speed printers, whose heavy printer head moves left and right quickly. However, the present invention may be applied to printer heads of any kind, for example, a single printer head, a thermal printer head or ink jet printer head. A variety of types of linear motor are available on the market. The present invention places no restriction on the type of linear motor to be used, except insofar as the motor should have the necessary driving force and stroke length. Moreover, a linear actuator such as a pulse motor provided with gears may be used in place of a linear motor. The counter transmission mechanisms described above employ a circular pulley arrangement. In an embodiment of the present invention a non-circular pulley or cam arrangement may be used. This is especially suitable for a driving mechanism for a multihead-type printer head whose stroke from left to right is not so large. An example of an embodiment of the present invention using a modification of a circular pulley shape is illustrated in Fig. 9. In this example, a cam 50 having a form (in plan) of a circle cut away at both its sides (cut along parallel chords) is used. The configuration of Fig. 9 provides that unnecessary parts are cut away and unwanted weight and inertia eliminated. Such a cam configuration is particularly effective when the distance between the first and second shafts is large. In Fig. 9, the cam 50 has a periphery which conforms in part to a circle of equivalent diameter D, which includes the thickness of belts 51 and 52, equal to the distance between shafts 30 and 40. The centre of the cam 50 is supported by an axle 50a around which the cam is rotatable. The belts 51, 52 are formed of metal ribbon or wire, wound around the cam and fixed to the shafts 30 and 40 in like or similar manner to that described above. If the diameter of the cam is large, it will be apparent, from Figs. 9(A), (B) and (C), that a sufficient stroke of the printer head can be attained by only a small rotation of the cam. As mentioned with reference to Fig. 3, some looseness of the guide roller 6 is inevitable in the previously proposed printer head guide mechanism. This looseness can cause print misalignment and decrease print quality. Fig. 10 illustrates schematically a printer-head guide mechanism of an embodiment of the present invention. The printer head 1 is mounted on a mount 11. The mount 11 is supported at one side by a first shaft 7, and is slidable along the shaft 7. At another side of the mount 11 a guide roller 6 is provided which rolls along a rail 31. One end of a spring 33 is engaged with the shaft 35 of the roller 6 and the other end of the spring 33 is engaged with a pin 34 fixed on a side of the rail 31. So, the guide roller 6 is pulled down toward the rail 7. Compared to the structure of Fig. 3, the mechanism of Fig. 10 utilizes tension of spring 33 to prevent lifting up of the roller 6 from its guide rail 31. The roller 6 is pulled down toward the rail 31, and it moves smoothly along the rail 31 without releasing from the rail. So deterioration of printing quality can be avoided to the extent that looseness of the guide roller 6, present in the previously proposed mechanism, is eliminated. A guide mechanism as illustrated in Fig. 10 requires virtually no adjustment to keep the roller tightly on the rail 31. Moreover, since one end of the spring 33 is fixed to the rail 31 by the pin 34, the spring 33 is pulled and expanded when mount 11 moves left and right along the shaft 7, resulting in the provision of a strong recovery force. Therefore, for reverse movements, at turn-round points in the motion of the printer head, where maximum energy is required for driving the head, required driving force is reduced. Particularly when high-speed printing is carried out using a multihead-type printer, the time required for reciprocal movement of the printer head is short, and the load on the linear motor for driving the head back and forth is large. By using a spring such as spring 33 the driving energy required of the linear motor is reduced, and a smaller motor can be used. Alternatively, a higher operational speed can be attained if a linear motor of unreduced size is used. The structure illustrated in Fig. 10 does not give rise to any problems when the scan length of the printer head is relatively short, but when the scan length is long small changes in scanning speed (along the scan length) may occur. This is because the component of tensile force exerted by spring 33 in the direction of movement of the printer head is reversed at the centre point of the scanning motion of the printer head. To avoid this, an arrangement as seen in Fig. 11 may be employed. A slide groove 35 is provided in parallel with rail 31, and spring fixing pin 34 is fixed on a slider 36 which is slidable along the slide groove 35. Such a sliding mechanism may employ a pulley or any kind of slider. It should be pointed out that looseness between slider 36 and slide groove 35 is also absorbed by the spring 33. The length of the slide groove 35 is determined to be a little shorter than the scanning length of the printer head. So, for most of a scanning movement, the slider 36 moves together with the mount 11, and the tensile force of the spring 33 is directed to pull the roller 30 toward the rail 31, and no longitudinal force tending to pull back the mount 11 appears (no force component in the direction of movement of the printer head). But when the printer head approaches its turn-round points E or E′, that is the end points of the scanning motion, the slider 36 is stopped by the end point of the sliding groove 35, and the spring 33 is elongated to apply a force to pull back the mount 11. In this way variation of scanning speed is avoided. Although a coil spring 33 is illustrated in Figs. 10 and 11, the coil spring may be replaced with any type of spring, a plate spring for example may be applicable. The guiding rail 31 may be provided above the roller 30, the roller 6 being pulled up by a spring towards the rail. In a printer apparatus having a printer head driving mechanism embodying the present invention a printer head is connected to its counterbalance by a counterdriving mechanism. Shock and noise arising when the printer head is driven left and right by a printer head drive motor (e.g. a linear motor) are reduced. The counter driving mechanism comprises, for instance, a pulley arrangement and a pair of belts. The belts or wires are wound each approximately a half-turn around the pulley arrangement, in opposite directions on opposite sides of the pulley arrangement. One end of each belt or wire is connected to the counterbalance, and the other end of each belt or wire is connected to the printer head. So, the counterbalance is moved in directions opposite to the directions of motion of the printer head. A moving part of the linear motor is connected coaxially to the counterbalance, so the weight of the moving part of the linear motor is included in the total weight of the counterbalance. The printer head is mounted on a mount which slides along a shaft. The mount is provided with a roller away from the shaft. The roller rolls along a rail provided in parallel to the shaft. A spring is connected between the axis of the roller and the rails, and pulls the roller toward the rail to prevent the roller and rail from disengaging. The energy necessary to reverse the motion of the printer head at turn-round points is reduced by a spring action. An embodiment of the present invention provides a driving mechanism of printer head which scans on a sheet, said driving mechanism comprising: a first shaft for guiding the scanning motion of said printer head; a linear actuator for driving left and right said printer head; a counterbalance which shifts along a second shaft provided in parallel to said first shaft in the opposite direction to that of the motion of said printer head, said counterbalance having substantially same weight to the total weight of said printer head and means moving in the same direction together with said printer head; and a counter transmission mechanism for transmitting the shift motion of said printer head to said counterbalance or vice versa, said counter transmission mechanism comprises: a pulley positioned between said first and second shafts, and being rotatable around its centre axis; and a pair of belts or wires wound approximately half round in opposite directions to each other respectively on each opposite side of said cam, one end of the respective belt or wire being connected to said counterbalance, and another end of the respective belt or wire being connected to said printer head. The pulley may be constituted by a single pulley, or two pulleys rotatable around parallel axes, or a cam body having a profile in the form of a circle from which two equal and opposite segments are cut off, in each case the centre axis of rotation being perpendicular to the plane containing the longitudinal axes of the first and second shafts.

Claims: A printer-head driving mechanism, including:- a first shaft, for carrying the printer head; a linear motion actuator for driving the printer head back and forth, along or with the first shaft, to provide scanning motion of the printer head; a second shaft, parallel to the first shaft; a counterbalance, moveable along or with the second shaft; and a counter transmission mechanism, operable to couple motion of the printer head to motion of the counter balance, or vice versa, so that the printer head and counterbalance move in opposite directions, the counter transmission mechanism comprising:- a rotatable member arrangement, positioned between and towards one end of said first and second shafts; two flexible members each passing around the rotatable member arrangement, the respective members passing around opposite sides of the rotatable member arrangement, each turning a half-turn around the arrangement, each flexible member connected at one end to the printer head and at the other end to the counterbalance. A mechanism as claimed in claim 1, wherein each flexible member is a wire or belt. A mechanism as claimed in claim 1 or 2, further comprising:- first connecting means for mechanically connecting the counterbalance to the counter transmission mechanism; and second connecting means for mechanically connecting the printer head to the counter transmission mechanism; said one end of each flexible member being connected to the counterbalance via the first connecting means and said other end of each flexible member being connected to the printer head via the second connecting means. A mechanism as claimed in claim 1, 2 or 3, further comprising tensioning means for tensioning the flexible members by applying a tensioning force to one end of each flexible member. A mechanism as claimed in claim 4 when read as appended to claim 3, wherein the tensioning means are comprised in either the first or the second connecting means. A mechanism as claimed in any preceding claim, wherein the rotatable member arrangement comprises a circular pulley having a diameter substantially equal to the distance between the first and second connecting means. A mechanism as claimed in any one of claims 1 to 5, wherein the rotatable member arrangement comprises first and second pulleys. A mechanism as claimed in any one of claims 1 to 6, wherein the rotatable member arrangement comprises a cam having a profile in the form of a circle with two segments removed. A mechanism as claimed in any preceding claim, wherein the linear motion actuator is a linear motor arranged coaxially with the counterbalance so that the direction of motion of a moving part of the motor is opposite to the direction of motion of the printer head, whereby the weight of the moving part is counted as part of the weight of the counterbalance. A driving mechanism as claimed in any preceding claim, further comprising a guide mechanism for guiding the scanning motion of said printer head, said guide mechanism comprising:- a mount, mounting the printer head, slidable along the first shaft; a rail provided in parallel to the first shaft; a roller provided on the mount and rolling on the rail; and a spring with one end engaged to the axis of the roller and with the other end engaged to a pin provided on the rails, urging the said roller toward the rail. A mechanism as claimed in claim 9, wherein the pin is movably located in a guide slot extending in parallel with the rail.

Inventors: INAGAKI, MASAHISA FUJITSU LTD. PATENT DEPT., TAKEKOSHI, MAKOTO FUJITSU LTD. PATENT DEPT., TOKUNAGA, HARUHIKO FUJITSU LTD. PATENT DEPT.
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: B41J19
IPC codes: B41J25