Shuttle apparatus for a printer

Abstract: A printer shuttle apparatus of the kind having a reciprocating print shuttle unit and a counter-reciprocating balance shuttle unit. The apparatus further comprising means (17), for detecting the position of the print shuttle unit, a linear motor (15, 16) for driving the print shuttle unit, a further linear motor (25, 26) for driving the balance shuttle unit, a drive circuit (5) for driving the linear motors and a controller (6) for controlling the drive circuit (5), in response to a signal from the print shuttle unit position detecting means (17), to drive synchronously the print shuttle unit and the balance shuttle unit. There is also means (27) for detecting the position of the balance shuttle unit, signals from which are supplied to the controller (6) to allow monitoring of overrun of the balance shuttle unit by the controller.

Description: The present invention relates to a shuttle apparatus for a printer, for example a line printer. In line printers or other similar printers, a print shuttle unit equipped with a print head needs to reciprocate at high speed, and a linear motor is used as a device for driving the print shuttle unit. In one example, the linear motor comprises a row of electromagnetic coils attached to a print shuttle unit, movable along a stay shaft with a print head mounted thereon, and a row of permanent magnets secured to a base frame so as to face the electromagnetic coils. In operation, as current is passed through the electromagnetic coils attached to the print shuttle unit, thrust is induced in the electromagnetic coils according to Fleming's left-hand rule. By appropriate control of the current supplied to the electromagnetic coils, the direction of the thrust can be changed to cause the print shuttle unit to reciprocate. Since the electromagnetic coils are mounted on the print shuttle unit, if the volumetric capacity of the electromagnetic coils is increased in order to raise the output of the linear motor, the print shuttle unit becomes heavier, resulting in an increase in the load. Accordingly, the speed of the reciprocating motion cannot be raised as high as expected for the increased output. A line printer also includes lead wires for connecting the electromagnetic coils to a power supply. The lead wires are secured at one end for connection to the power supply to the base frame and at the other end to the print shuttle unit so that the lead wires at this end reciprocate together with the print shuttle unit. It is therefore possible that the lead wires can be damaged or disconnected by the repeated reciprocating motion. In addition, with a print shuttle unit of larger weight, the high-speed reciprocating motion can cause increased vibration of the whole printer. To suppress vibration generation, a balance shuttle unit is generally employed. Moreover, when the print shuttle unit and the balance shuttle unit move parallel to each other in opposite directions, rotational moment can be induced by the motions of the two shuttle units. As a result, rotational vibration is generated in the whole printer, causing the print quality to be degraded, for example, by undesired movement of printing paper. In one arrangement, a balance shuttle unit, having approximately the same weight as that of the print shuttle unit, is driven in linked relation to the reciprocating motion of the print shuttle unit so that the two shuttle units move parallel to each other in opposite directions, thereby cancelling reaction force generated in the base frame of the printer by the reciprocating motion of the print shuttle unit. US-A-4941405 discloses a shuttle printer having a print shuttle unit and a balance shuttle unit, the print and balance shuttle units being driven collectively by a single electric motor. In use, the electric motor oscillates the print shuttle unit back and forth by resonantly vibrating the print shuttle unit at a natural frequency thereof. The balance shuttle unit moves in a counter-reciprocating manner relative to the print shuttle unit by virtue of mechanical coupling between the print and balance shuttle units provided by springs held in tension. A further shuttle printer having a print shuttle unit and a balance shuttle unit is disclosed in GB-A-2063579. The balance and print shuttle units are elongate plate-like elements arranged on opposite sides of a pair of rollers so that rotation of the rollers causes the shuttle units to move in opposite directions by virtue of the mechanical coupling between the respective undersides of the shuttle units and portions of the surfaces of the rollers. The shuttle units are held in contact with the rollers by magnetic force provided by electromagnetic coils disposed on the shuttle units and permanent magnets fixed to non-reciprocating parts of the printer, the magnets and electromagnetic coils being energised so as to also generate lateral force for reciprocating and counter-reciprocating the print and balance shuttle units. According to the present invention, there is provided a shuttle apparatus for a printer, the apparatus comprising a print shuttle unit, which has a print shuttle for carrying a print head, a balance shuttle unit, means for detecting the position of the print shuttle unit and means for driving the print shuttle unit to reciprocate and the balance shuttle unit to counter-reciprocate so as to generate a force to counterbalance the momentum of the reciprocating print shuttle unit, characterised in that the driving means comprises: a linear electric motor for driving the print shuttle unit; a further linear electric motor for driving the balance shuttle unit; drive circuit means for driving said two linear electric motors; and control means for controlling the drive circuit means, in response to a signal from the print shuttle unit position detecting means, to drive synchronously the print shuttle unit and the balance shuttle unit. With preferred embodiments of the invention it is possible to provide a shuttle apparatus with which, in operation, vibration generation is reduced, since the load on the motors is lower than in a shuttle apparatus in which the balance and print shuttle units are coupled with a mechanical linkage mechanism. With embodiments of the present invention it is possible to provide a print shuttle unit which is operable with a more rapid reciprocating motion and which is relatively free from the problem of disconnecting lead wires of electromagnetic coils. With embodiments of the present invention it is also possible to provide a shuttle apparatus for a printer which is designed so that vibration of the printer is reduced considerably by suppressing rotational moment induced by the motion of the reciprocating print shuttle unit and a balance shuttle unit which moves in a direction reverse to the direction of the reciprocating motion of the print shuttle unit, thereby enabling an improvement in print quality to be obtained. Preferred embodiments of the invention are recited in the dependent claims. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a perspective view of a first embodiment of the present invention, showing a print shuttle unit and a balance shuttle unit; Fig. 2 is a plan view of the first embodiment of the present invention, showing the print shuttle unit and the balance shuttle unit; Fig. 3 is a sectional side view of the first embodiment of the present invention, showing the print shuttle unit and the balance shuttle unit; Fig. 4 is a plan view of permanent magnets in the first embodiment of the present invention; Fig. 5 is a plan view of electromagnetic coils in the first embodiment of the present invention; Fig. 6 is a schematic plan view of the first embodiment of the present invention, showing the print shuttle unit and the balance shuttle unit; Fig. 7 shows schematically a circuit configuration of the first embodiment of the present invention; Fig. 8 is a perspective view of a second embodiment of the present invention; Fig. 9 is a perspective view of a third embodiment of the present invention; Fig. 10 is a fragmentary front view of the third embodiment of the present invention; Fig. 11 is a fragmentary perspective view of a fourth embodiment of the present invention; Fig. 12 is a fragmentary perspective view of a fifth embodiment of the present invention; Fig. 13 is a perspective view of a sixth embodiment of the present invention; Fig. 14 is a schematic view of a seventh embodiment of the present invention; Fig. 15 is a schematic view of an eighth embodiment of the present invention; Fig. 16 is a schematic view of a ninth embodiment of the present invention; Fig. 17 is a schematic view of a tenth embodiment of the present invention; and Fig. 18 is a schematic view of an eleventh embodiment of the present invention. Figs. 1 to 3 show a first embodiment of the present invention, as applied to a line printer. Fig. 1 is a perspective view of part of the line printer which includes a print shuttle unit and a balance shuttle unit. Figs. 2 and 3 are respectively a plan view and a sectional side view of the same part of the line printer. A base frame 1 is secured to a casing 50. A pair of parallel stay shafts 2 and 3 extend horizontally and are each secured at both ends thereof to the base frame 1. In Fig. 1 illustration of the casing 50 and the base frame 1 is omitted, and in Fig. 2 illustration of the casing 50 is omitted. A print shuttle 12 is slidably fitted on the first stay shaft 2, which is disposed in the central portion of the base frame 1. The print shuttle 12 is equipped with a print head 11 comprising a plurality of print pins arranged in a row. The print shuttle 12 is supported by the first stay shaft 2 and a roller 13 capable of travelling on the base frame 1. The print head 11 is of the electromagnetic release type in this example. The print head 11 comprises a row of 12 (for example) print head assemblies 11a of the 24-pin type arranged horizontally. Each print head assembly 11a is formed from 4 sets of 6 print elements which are respectively arranged in front upper, front lower, rear upper and rear lower stages in such a manner that the two sets of print elements in the front and rear upper stages are disposed symmetrically with respect to those in the front and rear lower stages. The print elements are for printing in units of dots by means of print pins. When the print head 11 is driven, the distal ends of the print pins project in the direction of the arrow A, shown in Fig. 3, thereby striking printing paper, which is fed in the direction of the arrow B through a paper feed passage 4, through an ink ribbon (not shown). Thus, impact dot printing is carried out. A yoke 14, which is a planar iron plate, is attached to the bottom of the print shuttle 12. A plurality of rectangular plate-shaped permanent magnets 15 are disposed in a row on the lower surface of the yoke 14 in a line extending parallel to the axis of the first stay shaft 2. The permanent magnets 15 are each magnetized in the direction of the thickness thereof. That is, each permanent magnet 15 has two magnetic poles at the upper and lower end faces thereof. The permanent magnets 15 are formed by using rare-earth magnets, which have a strong magnetic property. Samarium-cobalt magnets can for example be used. Accordingly, the permanent magnets 15 are thin and light in weight in comparison to ferrite magnets or other magnets (e.g., the thickness and weight are each 1/5 of that in the case of the latter). Each permanent magnet 15 has a slightly larger width than that of each print.head assembly 11. As shown in Fig. 4, a series of 11 permanent magnets 15 are disposed so that N and S poles alternate with each other. Among the 11 permanent magnets 15, a row of 9 permanent magnets are disposed contiguously, and one permanent magnet is disposed at each end of the row of permanent magnets with a spacing provided between the same and the end of the row. Thus, the print shuttle 12, and the print head 11, the yoke 14 and the permanent magnets 15, which are attached to the print shuttle 12, form a print shuttle unit 10 which is movable along the first stay shaft 2. A row of electromagnetic coils 16 are secured to a coil base 18, which is formed from an iron plate secured to the base frame 1, so that the electromagnetic coils 16 face the permanent magnets 15 of the print shuttle unit 10 across a slight gap. Thus, the permanent magnets 15 and the electromagnetic coils 16 form a linear motor (first linear motor) for driving the print shuttle unit 10. Lead wires 19 are used to feed electric power to the electromagnetic coils 16. Each electromagnetic coil 16 is spirally coiled so as to have a width double that of each permanent magnet 15. As schematically shown in Fig. 5, a row of six electromagnetic coils 16 are disposed contiguously. It should be noted that the outer edges of each pair of adjacent electromagnetic coils 16 are in contact with each other, although they are schematically shown as being separate from each other in Fig. 5. Of the six electromagnetic coils 16, the two end ones of them, designated with the reference numeral 16a, (i.e. those two disposed at the ends of the row of electromagnetic coils 16) are used to reverse the operation of the first linear motor. The electromagnetic coils 16a are connected in series to the same lead wires. The four further electromagnetic coils 16b (i.e. those disposed in between the electromagnetic coils 16a) are used to drive the first linear motor at a constant speed. These electromagnetic coils 16b are connected in series to lead wires in pairs, which are different from those for the end electromagnetic coils 16a. In the first linear motor, arranged as described above, as current is passed through the electromagnetic coils 16, which are placed in the magnetic fields produced by the permanent magnets 15, thrust is induced in the electromagnetic coils 16 on the basis of Fleming's left-hand rule. However, since the electromagnetic coils 16 are immovably fixed to the base frame 1, the reaction force to the thrust acts on the permanent magnets 15. As a result, the print shuttle unit 10 moves along the first stay shaft 2. By appropriate control of the current supplied to the electromagnetic coils 16, the print shuttle unit 10 can be rectilinearly reciprocated at high speed along the first stay shaft 2. In addition, a position detecting sensor 17 is provided, as shown in Fig. 2. The position detecting sensor 17 comprises slits formed in the yoke 14 of the print shuttle unit 10, and a transmissive photosensor that is attached to the base frame 1. In Figs. 1 and 3, illustration of the position detecting sensor 17 is omitted. A balance shuttle 22, which is formed in the same way as the print shuttle 12, is slidably fitted on the second stay shaft 3, which is disposed parallel to the first stay shaft 2. A counterweight 21 is mounted on the balance shuttle 22, and a yoke 24 is attached to the bottom of the balance shuttle 22. A row of permanent magnets 25, which are similar to the permanent magnets 15 of the print shuttle unit 10, are attached to the lower surface of the yoke 24. A roller 23 is rotatably attached to the balance shuttle 22 so that the balance shuttle 22 travels on the base frame 1. The balance shuttle 22 is supported by the roller 23 and the second stay shaft 3. The balance shuttle 22 further has a pair of arms 22a which are connected thereto so as to project from both lateral ends, respectively, of the base frame 1. The arms 22a are bent to extend beyond the position of the print shuttle unit 10 as far as the other end of the base frame 1, and a counterweight unit 21a is attached to the distal ends of the arms 22a. Thus, a balance shuttle unit 20 is formed from the balance shuttle 22 and the counterweight 21, the yoke 24, the permanent magnets 25 and the arms 22a, which are attached to the balance shuttle 22, together with the counterweight unit 21a attached to the distal ends of the arms 22a. The constituent elements of the balance shuttle unit 20 can move as one unit in parallel to the print shuttle unit 10. Rollers 31 are rotatably attached to the counterweight unit 21a so that the counterweight unit 21a travels on the base frame 1. The balance shuttle unit 20 is formed so that the overall weight thereof is approximately equal to that of the print shuttle unit 10. Distribution of weight in the balance shuttle unit 20 is made so that, as shown in Fig. 6, the line C of travel of the centre of gravity of the whole balance shuttle unit 20 during the movement along the second stay shaft 3 is approximately coincident with the line D of travel of the centre of gravity of the print shuttle unit 10 during the movement along the first stay shaft 2 ('e' equal or approximately equal to zero). Referring back to Figs. 1 to 3, a coil base 28 is secured to the base frame 1, and a row of electromagnetic coils 26, which are similar to the electromagnetic coils 16 shown in Fig. 5, are secured to the coil base 28 so as to face the row of permanent magnets 25 disposed on the balance shuttle 22 across a slight gap. Thus, the permanent magnets 25 and the electromagnetic coils 26 form a linear motor (second linear motor) for driving the balance shuttle unit 20. Lead wires 29 are used to supply electric power to the electromagnetic coils 26. By appropriate control of the current passed through the electromagnetic coils 26, the balance shuttle unit 20 can be rectilinearly reciprocated at high speed along the second stay shaft 3. Fig. 7 schematically shows a circuit configuration for the first and second linear motors. The electromagnetic coils 16 and 26 are supplied with the same driving current from a single driver circuit 5 so that the print shuttle unit 10 and the balance shuttle unit 20 move relative to each other in opposite directions at the same speed to perform high-speed reciprocating motion. For this purpose, the print shuttle unit 10 and the balance shuttle unit 20 are arranged in reverse relation to each other in terms of either the polarities of the permanent magnets 15 and 25 or the winding direction of the electromagnetic coils 16 and 26. A controller 6 for controlling the operation of the driver circuit 5 is fed with signals for reversing and constant-speed travel from the position detecting sensor 17 of the print shuttle unit 10 to effect feedback control for the reciprocating motion. The controller 6 is further fed with a signal from a position detecting sensor 27 provided on the balance shuttle unit 20 to monitor for the occurrence of overrun or other trouble of the balance shuttle unit 20. In the first embodiment of the printer shuttle apparatus, arranged as described above, the electromagnetic coils 16 and 26 of both the first and second linear motors are secured to the base frame 1, and the permanent magnets 15 and 25 are attached to the print shuttle 12 and the balance shuttle 22, respectively, which are movable members. Accordingly, even if the volumetric capacities of the electromagnetic coils 16 and 26 are increased in order to increase the output of the linear motors, there is no increase in the weight of the movable members. Therefore, the reciprocating motion of the print shuttle unit 10 and that of the balance shuttle unit 20 can be sped up with ease. Further, since the lead wires 19 and 29 for the electromagnetic coils 16 and 26 are not connected to the movable members, there is no likelihood of disconnection of the lead wires 19 and 29 by the repeated reciprocating motion. Since the permanent magnets 15 and 25 can be made thin and light in weight by forming them using rare-earth magnets having a strong magnetic property, it is possible to reduce the overall weights of the shuttle units 10 and 20 and to narrow the gaps between the yokes 14 and 24 on the one hand and the coil bases 18 and 28 on the other so as to raise the magnetic flux density. Thus, it is possible to realize an increase in the output of the linear motors and also an increase in the speed thereof. Although in this embodiment the permanent magnets 15 are attached to the shuttle 12 (i.e. to a moving part) and the electromagnetic coils 16 to the coil base 18 (i.e. to a stationary part) both for the print shuttle unit and the balance shuttle unit, it may be advantageous to employ this arrangement alone for the print shuttle unit of a printer that is not provided with a balance shuttle unit 20. In the printer shuttle apparatus of this embodiment, as the print shuttle unit 10 reciprocates along the first stay shaft 2, the balance shuttle unit 20, which is approximately equal in weight to the print shuttle unit 10, moves along the second stay shaft 3 in a direction reverse to the direction of travel of the print shuttle unit 10 at the same speed as that of the print shuttle unit 10 in linked relation to it. Accordingly, reaction force that is induced in the base frame 1 by the reciprocating motion of the print shuttle unit 10 is cancelled by the reciprocating motion of the balance shuttle unit 20. In addition, during the reciprocating motion, the centre of gravity of the balance shuttle unit 20 moves on a line approximately the same as the travel line of the centre of gravity of the print shuttle unit 10. Accordingly, no rotational moment is induced by the reciprocating motions of the two shuttle units 10 and 20. The balance shuttle unit 20 may be arranged such that the balance shuttle 22 and the counterweight unit 21a are connected by using ropes or belts in place of the arms 22a so that the counterweight unit 21a moves in the same direction and at the same speed as the balance shuttle 22. Fig. 8 shows a second embodiment in which the balance shuttle unit 20 is provided with permanent magnets 125 of relatively low magnetic property comprising, for example, ferrite magnets, which show a magnetic property weaker than that of the permanent magnets 15 of the print shuttle unit 10, which are rare-earth magnets. The permanent magnets 125 need a considerably large volumetric capacity in order to obtain the same magnetic flux density as that of rare-earth magnets. Accordingly, the weight thereof increases. As a result, it becomes unnecessary to mount a counterweight, such as the counterweight arrangement of the first embodiment comprising counterweight units 21 and 21a, on the balance shuttle 22, as shown in Fig. 8, or it is only necessary to mount a small counterweight thereon. Therefore, an efficient structure is realized. Further, since ferrite magnets are less costly than rare-earth magnets, the cost of the apparatus can be lowered. Fig. 9 shows a third embodiment of the present invention, in which both end portions 14a of the yoke 14, which is attached to the print shuttle 12, are bent so as to face the respective outer sides of the two end permanent magnets 15 across a slight gap. By virtue of the above-described arrangement magnetic flux that leaks sidewardly from the end permanent magnets 15a is effectively transmitted to the yoke 14 through the end portions 14a of the yoke 14 without leaking to the outside. Accordingly, it is possible to eliminate such an adverse effect of leakage magnetic flux on the surroundings that the leakage magnetic flux is vibrated by the reciprocating motion of the print shuttle unit 10, causing fluctuations in the surrounding magnetic field, which would, for example, make the screens of various displays unstable. Figs. 11 and 12 show fourth and fifth embodiments, respectively, in which a yoke that is provided on the print shuttle 12 for mounting the permanent magnets 15 has a structure which forms a closed magnetic circuit. Even the flat plate-shaped yoke 14 as shown in the first embodiment can prevent magnetic flux from leaking from the reverse side of the yoke 14, that is, the side thereof which is reverse to the side where the permanent magnets 15 are attached, provided that the thickness of the yoke 14 can be sufficiently increased. However, the thickness of the yoke 14 cannot always be increased satisfactorily because it is necessary to minimize the load on the linear motor. Further, it is impossible in many cases, to eliminate completely leakage of magnetic flux from the reverse side of the yoke 14 due to holes provided in the yoke 14 for securing it to the print shuttle 12. Accordingly, in the fourth embodiment shown in Fig. 11, a yoke 114, to which the permanent magnets 15 are attached, is formed in an annular structure so that a closed magnetic circuit is formed. In the fifth embodiment shown in Fig. 12, an auxiliary yoke 214 having a multiplicity of legs in the shape of the teeth of a comb is laid on the outer side of the flat plate-shaped yoke 14 having the permanent magnets 15 attached thereto, thereby obtaining a yoke structure which forms a closed magnetic circuit. With such a structure, leakage magnetic flux, which may occur due to some reason, can be confined within the yoke so as not to leak out. Accordingly, no fluctuation is caused in the surrounding magnetic field. Although in the third to fifth embodiments, shown in Figs. 9 to 12, description of the yoke has been made with reference to the print shuttle unit 10, it will be understood that, if a balance shuttle unit is also provided, a similar yoke structure may also be adopted for the yoke of the balance shuttle unit. Fig. 13 shows a sixth embodiment in which the whole print shuttle unit 10 is covered with a magnetic shield cover 30, for example as cover formed of iron plate. Reference numeral 50 denotes a printer casing. In general, printer equipment has various covers. Therefore, as long as the cover material is a magnetic material and the covers are not saturated with leakage magnetic flux, a sufficient magnetic shield effect can be obtained by connecting together as many covers as possible to construct a magnetic shield cover which forms a closed magnetic circuit. In a case where the covers are formed from a plastic material or in a case where saturation of magnetic flux cannot be prevented by the covers alone and hence the magnetic shield effect is not perfect, it is necessary to form a closed magnetic circuit inside the covers by a magnetic shield cover 30 made of a magnetic material. The above-described structure, in which magnetic flux is prevented from leaking out by the magnetic shield cover 3C, can be constructed by using the covers of the printer, which in practice are generally provided, and will not increase the load on the linear motor. Therefore, it is extremely efficient. In a printer that is provided with the balance shuttle unit 20, it is preferable to cover also the balance shuttle unit 20 with a magnetic shield cover 30 which may be the same as or different from that for the print shuttle unit 10. Fig. 14 schematically shows a seventh embodiment in which a pair of balance shuttle units 120 are provided and arranged to face each other across the print shuttle unit 10, each balance shuttle unit 120 being the same as the balance shuttle unit 20 in the first embodiment except that it is not provided with the arms 22a and the counterweight unit 21a. Reference numeral 32 denotes a stay shaft. In this case, the total weight of the two balance shuttle units 120 is made approximately equal to the weight of the print shuttle unit 10, and the two balance shuttle units 120 are disposed so that the travel line of the centre of gravity of the balance shuttle units 120 is approximately coincident with the travel line of the centre of gravity of the print shuttle unit 10. The two balance shuttle units 120 are driven to move in the same direction and at the same speed. It should be noted that three or more balance shuttle units 120 may be provided. It is also possible to combine a plurality of balance shuttle units with a counterweight unit, for example, which is connected thereto through arms or the like. Fig. 15 shows schematically an eighth embodiment which differs from the first embodiment in that a balance shuttle unit 220 is not provided with the arms 22a and the counterweight unit 21a, which is connected to the distal ends of the arms 22a in the first embodiment, and the weight of the balance shuttle unit 220 is made approximately equal to the weight of the print shuttle unit 10. Accordingly, rotational moment is induced by the reciprocating motion of the print shuttle unit 10 and that of the balance shuttle unit 220. Therefore, in this embodiment a motor 35, which generates torque that is approximately equal in magnitude but opposite in direction to the rotational moment induced by the motions of the two shuttle units 10 and 220, is disposed in between the casing 50 and the base frame 1 with the axis thereof made coincident with that of the rotational moment induced by the two shuttle units 10 and 220. With the above-described arrangement, the rctational moment induced by the two shuttle units 10 and 220 is cancelled by the torque generated by the motor 35. Accordingly, no rotational vibration is generated. It should be noted that the motor 35 may be replaced by a rotary solenoid, for example. Fig. 16 shows schematically a ninth embodiment in which the balance shuttle unit 220 is arranged in the same way as in the above-described eighth embodiment, and the base frame 1 is provided to be rotatable relative to the casing 50 in coaxial relation to the rotational moment. In addition, an extremely heavy counterweight 36 is attached to the base frame 1. The counterweight 36 exerts moment of inertia which is equivalent to or larger than the rotational moment induced by the two shuttle units 10 and 220 and has the centre of gravity lying on the axis of the rotational moment. Reference numeral 37 denotes bearings. With the above-described arrangement, it is possible to minimize the effect of the rotational moment, which is induced by the two shuttle units 10 and 220, on the base frame 1 and to thereby suppress generation of rotational vibration. Fig. 17 schematically shows a tenth embodiment in which the counterweight 36 in the ninth embodiment is formed by the casing 50. In this embodiment, a fixed support 51 for rotatably supporting the casing 50 is disposed coaxially with the centre of rotation of the two shuttle units 10 and 220 so that the rotational moment induced by the two shuttle units 10 and 220 is cancelled by the moment of inertia of the casing 50. In general, the moment of inertia of the casing 50 is sufficiently larger than the rotational moment induced by the two shuttle units 10 and 220, and the period of rotational vibration is sufficiently short. Therefore, there is no possibility that the casing 50 will rotate. Fig. 18 shows an eleventh embodiment in which rubber vibration isolators 41 are installed to fasten the base frame 1 to the casing 50, thereby absorbing rotational vibration by the elasticity of the rubber vibration isolators 41. In addition, this embodiment also makes use of the fact that the period of rotational vibration is short. Thus, rotational vibration caused by the two shuttle units 10 and 220 is prevented from being transmitted to the casing 50. In general, it is only necessary to provide rubber vibration isolators 41a for fastening the base frame 1 to the casing 50. However, if necessary, rubber vibration isolators 41b for regulating rotational vibration may also be mounted. As described above, by having an arrangement in which magnets are attached to a print shuttle, which is a movable member, and the electromagnetic coils are provided on a fixed member, the volumetric capacity of the electromagnetic coils can be increased without increasing the weight of the movable member. Accordingly, it is possible to increase the output of the linear motor according to need, and hence possible to readily speed up the reciprocating motion of the print shuttle. Further, since no lead wires for the electromagnetic coils are connected to the movable member, there is no possibility of disconnection of the lead wires due to the repeated reciprocating motion. Thus, superior durability and reliability can be obtained. If the permanent magnets are formed by using rare-earth magnets, which show a high magnetic property, it is possible to reduce the weight of the print shuttle unit and to increase the magnetic flux density. Thus, the reciprocating motion of the print shuttle unit can be readily sped up. It is also possible to prevent magnetic flux from leaking out and to thereby avoid adverse effect on the surroundings by effectively bending both ends of the yoke, or forming the yoke in a closed magnetic circuit structure, or providing a magnetic shield cover. If permanent magnets of relatively low magnetic property are used as permanent magnets provided on the balance shuttle, it is possible to reduce the weight of the counterweight attached to the balance shuttle and to thereby attain an efficient structure. As described above, an arrangement may be chosen in which the balance shuttle unit has approximately the same weight as that of the print shuttle unit and moves parallel to the print shuttle unit in a direction reverse to the direction of travel of the latter. The reaction force that is generated in the base frame by the motion of the print shuttle unit can thus be cancelled by the balance shuttle unit and generation of vibration suppressed. Further, the centre of gravity of the balance shuttle unit can be arranged to travel on the same, or approximately the same line as the line of travel of the centre of gravity of the print shuttle unit, so that no rotational moment is induced by the motions of the two shuttle units thus serving to further reduce vibration of the printer. Excellent print quality is thus obtainable. Noise generation is also reduced. If the base frame, which supports both the print shuttle unit and the balance shuttle unit, is given torque, which is approximately equal in magnitude but opposite in direction to rotational moment induced by the motions of the two shuttle units, by a torque generating device, the rotational moment induced by the two shuttle units is cancelled by the torque. Similarly, rotational vibration caused by the rotational moment of the two shuttle units is suppressed by providing a counterweight exerting moment of inertia which is equivalent to or larger than the rotational moment induced by the two shuttle units in place of the torque generating device. If vibration absorbing members are interposed between the base frame and the casing, rotational vibration caused by the rotational moment is prevented from being transmitted to the casing. Thus, vibration that is generated in the printer is minimized, and excellent print quality is obtained.

Claims: A shuttle apparatus for a printer, the apparatus comprising a print shuttle unit (10), which has a print shuttle (12) for carrying a print head (11), a balance shuttle unit (20), means (17) for detecting the position of the print shuttle unit (10) and means (15, 16, 25, 26) for driving the print shuttle unit (10) to reciprocate and the balance shuttle unit (20) to counter-reciprocate so as to generate a force to counterbalance the momentum of the reciprocating print shuttle unit (10), characterised in that the driving means comprises: a linear electric motor (15, 16) for driving the print shuttle unit (10); a further linear electric motor (25, 26) for driving the balance shuttle unit (20); drive circuit means (5) for driving said two linear electric motors (15, 16, 25, 26); and control means (6) for controlling the drive circuit means (5), in response to a signal from the print shuttle unit position detecting means (17), to drive synchronously the print shuttle unit (10) and the balance shuttle unit (20). A shuttle apparatus according to claim 1, wherein the print shuttle unit (10) has a plurality of print heads provided thereon in a horizontal row (12). A shuttle apparatus according to claims 1 or 2, wherein the linear motors comprise respective electromagnetic coils (16, 26). A shuttle apparatus according to claim 3, wherein said drive circuit means (5) is operable to control current passed through the electromagnetic coils (16, 26) of the two linear motors. A shuttle apparatus according to claim 3 or 4, wherein the electromagnetic coils of the first-mentioned electric motor and the further electric motor are arranged in reverse relation to each other in terms of winding direction. A shuttle apparatus according to any of the preceding claims, wherein the linear motors comprise respective permanent magnets (15, 25). A shuttle apparatus according to claim 6, wherein the permanent magnets of the first-mentioned electric motor and of the further electric motor are arranged in reverse relation to each other in terms of polarity. A shuttle apparatus according to claim 6 or 7 when appended to claims 3, 4 or 5, wherein said permanent magnets (15) are arranged in a row and attached to said print shuttle (12) and said electromagnetic coils (16) are arranged in a row and secured to a stationary member so as to face said permanent magnets (15) across a gap. A shuttle apparatus according to any one of the preceding claims, wherein the print shuttle unit position detecting means (17) comprises a slit formed in the print shuttle unit (10) and a photosensor provided on a stationary member so as to face said slit. A shuttle apparatus according to any one of the preceding claims and comprising means (27) for detecting the position of the balance shuttle unit (20). A shuttle apparatus according to claim 10, wherein the control means (6) is arranged to receive a signal from the balance shuttle unit position detecting means (27) for monitoring overrun of the balance shuttle unit (20). A shuttle apparatus according to any one of the preceding claims and comprising guide means (2) along which the print shuttle (12) is movable. A shuttle apparatus according to any one of claims 6, 7, or 8, and claims 9 to 12 when appended to claim 6, wherein the permanent magnets (15) of said first-mentioned linear motor are rare-earth magnets. A shuttle apparatus according to any one of claims 6, 7 or 8, and claims 9 to 12 when appended to claim 6, wherein the permanent magnets (15) of said first-mentioned linear motor are rare-earth magnets and the permanent magnets (25) of said further linear motor are formed from a magnetic material weaker in magnetic property than said rare-earth magnets (15). A shuttle apparatus according to claim 8, or one of claims 9 to 12 when appended to claim 8, wherein the permanent magnets (15) of said first-mentioned linear motor are attached to a yoke (14) provided on said print shuttle (12), said yoke (14) having both end portions (14a) thereof bent so as to face respective outer sides (15a) of the permanent magnets (15) of said first-mentioned linear motor at both ends of said row of permanent magnets (15) across a gap. A shuttle apparatus according to any one of claims 6 to 8, and claims 9 to 12 when appended to claim 6, wherein the permanent magnets (15) of said first-mentioned linear motor are attached to a yoke (14; 114; 214) provided on said print shuttle (12), said yoke forming a closed magnetic circuit. A shuttle apparatus according to claim 16, wherein said yoke (114) has an annular configuration. A shuttle apparatus according to claim 16, wherein said yoke comprises a flat plate-shaped yoke (114) and an auxiliary yoke (214) having a multiplicity of legs in the shape of the teeth of a comb, said auxiliary yoke being laid on top of said flat plate-shaped yoke. A shuttle apparatus according to any one of the preceding claims and comprising a magnetic shield cover (30) surrounding said print shuttle (12).

Inventors: KOBAYASHI ATSUHISA, MORIYAMA HITOSHI, TOKUNAGA HARUHIKO, KOBAYASHI, ATSUHISA, MORIYAMA, HITOSHI, TOKUNAGA, HARUHIKO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1993
Main classification: B41J25
IPC codes: B41J25