Drive mechanism for use in a printer

Abstract: Drive mechanism for use in a printer comprising ink sheet feeding means (9-12); printing sheet feeding means (2,3,6,7); printing head driving means (18) for moving a printing head (5) between a printing position (Figure 4A) and a non-printing position (Figure 3A); and drive means for driving the ink sheet feeding means (9-12), the printing sheet feeding means (2,3,6,7) and the printing head driving means (18) characterised in that inertia control means (22,24) are provided for controlling inertia of the printing head driving means (18).

Description: The present invention relates to a drive mechanism for use in a printer such, for example, as a thermal colour printer. In a known thermal colour printer, four separate driving motors are used one of which is employed for driving an ink sheet feed mechanism, two of which are employed for driving a printing paper feed mechanism, and another of which is employed as part of a printing head drive mechanism for lifting and lowering the printing head relative to a platen facing the printing head. A colour printer of this type basically has a printing paper feed mechanism which comprises a paper feed roller, a printing head, a platen facing the printing head, and a capstan; and an ink sheet feed mechanism which feeds an ink sheet sequentially coated in the printing direction with the primary colours. In a first forward pass of the said known thermal colour printer, a printing paper superposed on an ink sheet passes between the platen and the printing head under pressure to thermally print a first colour ink thereon. Then the printing paper which has been so printed in the first colour ink is fed backwards for a second forward pass so as to effect printing of the second colour ink thereon. When a third forward pass, during which the third colour is printed, is finished, one printing operation of a full colour print is completed. During each backward feeding operation, the ink sheet is reeled for the print of the subsequent colour, and the print head is temporarily separated from the platen. As indicated above, two out of the four separate driving motors are used for the printing paper feed mechanism, and the other two separate driving motors are used for the ink sheet feed mechanism and the printing head drive mechanism respectively. Four separate driving motors, however, take up considerable space in a limited installation area within a thermal colour printer. Consequently, such an arrangement imposes a limitation on the miniaturization of thermal colour printers. In order to reduce this problem and lower the cost of a colour thermal printer of this type, an improved thermal colour printer is known to the Applicants in which only one reversible motor is used for driving both the paper feed mechanism and the printing head drive mechanism. In this improved thermal colour printer, the reversible motor when driven in one angular direction effects the required up/down movement of the printer head relative to the platen, and when driven in the reverse angular direction effects driving of the ink sheet feed mechanism, and vice versa. When the motor is driven in one direction to transmit power either to the ink sheet feed mechanism or to the printing head drive mechanism, power transmission to the other is cut by means of a one-way clutch or a one-way power transfer mechanism installed between the motor and the respective ink sheet feed mechanism and printing head drive mechanism. Precise control of the printing head drive mechanism is, however, required in order to stop and start the operation of lifting and lowering the printing head relative to the platen. In particular, the printing head drive mechanism has to be stopped precisely in order to provide a proper contact of the printing head with the adjacent surface of the platen for optimum printing. However, the said one-way clutch or one-way power transfer mechanism has a substantially large inertia. This inertia is very difficult to control, and without control of the inertia, the printing head drive mechanism overruns due to the inertia of the one-way clutch. Further, the control of the operation of the printing head drive mechanism, such as the switching on and off of the motor, has to be decided on the basis of experiments which can render the control unreliable. In EP-A-0,274,266 there is disclosed a serial printer comprising ink sheet feeding means; printing sheet feeding means; a platen; a printing head; printing head driving means for moving a printing head between a printing position and a non-printing position; and drive means for driving the ink sheet feeding means, the printing sheet feeding means and the printing head driving means. According, however, to the present invention, there is provided a drive mechanism for use in a printer comprising ink sheet feeding means; printing sheet feeding means; a platen; a printing head; printing head driving means for moving the printing head between a printing position and a non-printing position; and drive means for driving the ink sheet feeding means, the printing sheet feeding means and the printing head driving means characterised in that the printing head is carried by a pivotally mounted support member which is pivotably movable towards and away from the platen to effect printing, the printing head driving means being provided with at least one braking cam means which is moveable by the printing head driving means into and out of braking contact with cam stopper means in dependence upon the position of the printing head driving means, the braking cam means and the cam stopper means collectively forming inertia control means for controlling inertia of the printing head driving means when the latter is in the printing position. The drive means preferably comprises a reversible motor which is drivingly connected to the printing head driving means by way of a part of a one-way power transmission means. The one-way power transmission means may comprise two clutches one of which is provided in a drive transmission between the reversible motor and the printing head driving means and the other of which is provided in a drive transmission between the reversible motor and the ink sheet feeding means. The printing head driving means preferably comprises a printing head drive shaft, there being provided a pivotally mounted support member which carries a printing head, pivotal movement of the support member moving the printing head towards and away from a platen, the printing head drive shaft having means engaging or engageable with a support member or with means secured thereto for effecting the said pivotal movement of the support member. The printing head drive shaft may be provided with at least one cam which is engageable with means secured to the support member. Detection means are preferably provided for detecting the angular position of the printing head drive shaft. The invention also comprises a printer, e.g. a colour hard copy printer such as a thermal colour printer, having a drive mechanism as set forth above. The invention is illustrated, merely by way of example, in which:- Figure 1 is a front view of a first embodiment of a drive mechanism according to the present invention; Figure 2 is a schematic side elevation of a thermal colour printer provided with the drive mechanism shown in Figure 1; and Figures 3(A) - 3(C) and Figures 4(A) - 4(C) schematically show the operation of the drive mechanism shown in Figure 1. In Figure 2 there is shown a thermal colour printer having a stack 30 of sheets 1 of printing paper or other record material and a sheet transfer roller 31. The printer has a paper feed mechanism for feeding a sheet 1 of printing paper from an upstream position constituted by the stack 30 to a downstream position (not shown), the paper feed mechanism comprising a paper feed roller 2, a pinch roller 3, a capstan 6 and a pinch roller 7. The paper feed mechanism 2, 3, 6, 7 feeds the sheet 1 of printing paper between a platen 4 and a printing head 5. An ink sheet feeding means for feeding an ink sheet 8 between the platen 4 and printing head 5 comprises a rewinding bobbin 9, an unwinding bobbin 10, and guide rollers 11 and 12. The rollers 11, 12 are driven by a reversible motor (not shown) which may also be arranged (by means not shown) to drive the sheet transfer roller 31, the paper feed roller 2, and the capstan 6. As described in greater detail below, the reversible motor is arranged to bring the printing head 5 into contact via the sheets 1, 8 with the platen 4 under pressure when the sheet 1 is fed forwards, whereby to effect thermal printing of an image on the paper. The printing head 5 is thereafter separated from the platen 4, during which time the paper sheet 1 is fed backwards and the ink sheet 8 is forwarded for the following pass. As shown in Figure 1, the printing head 5 is arranged in a position facing the platen 4. The printing head 5 is supported by a support member 13 so that it is vertically movable in a direction perpendicular to a shaft 14 of the platen 4. The support member 13 is pivotally mounted on a shaft 15 which is disposed parallel to the platen shaft 14 as shown in Figures 3 and 4. The support member 13 is urged by a pair of springs or other resilient members 16 towards the platen shaft 14. A printing head drive shaft 18 is rotatably mounted at its opposite ends in a printer frame 17 and is drivingly connected to the said reversible motor (not shown) via a one-way clutch 19 and a drive gear 20. Secured to the drive shaft 18 are a pair of driving cams 21 such as eccentric cams. The cams 21 are disposed above the support member 13 and are movable into and out of contact with a pair of square or L-shaped flanges 13A of the support member 13. The drive shaft 18 has secured thereto at each of its opposite ends a braking cam 22 such as an eccentric cam. The latter may have the same cross sectional shape and the same angular disposition as the driving cams 21. Below each of the cams 22 is a stopper 24 supported by a spring 23, each cam 22 being movable by the drive shaft 18 into and out of contact with its stopper 24. The driving shaft 18 is also provided with a semi-circular plate 25 for detecting the angular disposition of the drive shaft 18 and thus of the driving cams 21. The angular disposition of the semi-circular plate 25 is optically detected by a light sensor 26. Figures 3 and 4 explain the operation of the drive mechanism described above. Figure 3 shows one stage in the printing operation in which the printing head 5 is being held at a lifted position while spaced at a predetermined separation relative to the platen 4. Figure 4 shows another stage in the printing operation in which the printing head 5 is held at a lowered position while brought into contact with the platen 4. In Figure 3(A), the driving cams 21 have been rotated into the angular disposition shown in the Figure by the one-way rotation force (see the arrow) transmitted via the one-way clutch 19 from the reversible motor. The driving cams 21, in opposition to the resilient members 16, press against the flanges 13A extending from the supporting member 13 so that the printing head 5 moves to the lifted or non-printing position. During this printing head lifting stroke, the cams 21 are brought to a stopping position immediately before reaching upper dead centre to allow the printing head 5 to be accurately positioned. As shown in Figure 3(B), the angular position immediately before the upper dead centre is detected by the combination of the semi-circular plate 25 and the optical sensor 26, and it is at this moment that the motor stops rotating. As shown in Figure 3(C), the braking cams 22 are arranged in antiphase relative to the driving cams 21 in the sense that they are not yet engaged with the stoppers 24 at this moment, whereby no braking force is applied to the drive shaft 18. Figure 4(A) shows the driving cams 21 in their flange releasing stroke in which they are released from the flanges 13A. When the same one-way rotation force (see the arrow) of the reversible motor is transferred via the one-way clutch 19 to the driving cams 21 which have been in a position immmediately prior to reaching the upper dead centre, the driving cams 21 rotate into a predetermined angular position in which they are disengaged from the flanges 13A. As a result, the support member 13 is caused to rotate around the shaft 15 by the force exerted by the resilient members 16, and the printing head 5 is brought into contact with the platen 4 under pressure. The angular disposition of the driving cams 21 when the printing head 5 is in contact with the platen 4 is detected by the combination of the semi-circular plate 25 and the optical sensor 26 as shown in Figure 4(B), and it is at this moment that the motor stops rotating. As shown in Figure 4(C), the braking cams 22 are then in contact with the stoppers 24 to prevent the drive shaft 18 from rotating due to inertia. If no braking were provided, the drive shaft 18 would overrun because it is connected to the one-way clutch 19 and this would result in the driving cams 21 being set at the wrong angular disposition. In the embodiment shown in the drawings, a drive mechanism for a thermal colour hard copy machine is provided in which only one reversible motor is used for driving both the paper feeding means 2, 3, 6, 7 and the printing head driving means constituted by the drive shaft 18, whereby the printing head 5 is lifted and lowered relative to the platen 4, the inertia of the printing head driving means being precisely controlled by the inertia control device 22, 24, or 27. The inertia control device 22, 24 or 27 controls the inertia of a one-way power transmission device 19 for precisely stopping the printing head 5 at a predetermined position relative to the platen 4.

Claims: A drive mechanism for use in a printer comprising ink sheet feeding means (9-12); printing sheet feeding means (2, 3, 6, 7); a platen (4); a printing head (5); printing head driving means (18) for moving the printing head (5) between a printing position and a non-printing position; and drive means for driving the ink sheet feeding means (9-12), the printing sheet feeding means (2, 3, 6, 7) and the printing head driving means (18), characterised in that the printing head (5) is carried by a pivotally mounted support member (13) which is pivotably movable towards and away from the platen (4) to effect printing, the printing head driving means (18) being provided with at least one braking cam means (22) which is movable by the printing head driving means (18) into and out of braking contact with cam stopper means (24) in dependence upon the position of the printing head driving means (18), the braking cam means (22) and the cam stopper means (24) collectively forming inertia control means (22, 24) for controlling inertia of the printing head driving means (18) when the latter is in the printing position. Drive mechanism as claimed in claim 1 characterised in that the drive means comprises a reversible motor which is drivingly connected to the printing head driving means by way of a part of a one-way power transmission means. Drive mechanism as claimed in claim 2 characterised in that the one-way power transmission means comprises two clutches (19) one of which is provided in a drive transmission between the reversible motor and the ink sheet feeding means (9-12). Drive mechanism as claimed in any preceding claim characterised in that the printing head driving means comprises a printing head drive shaft (18), the printing head drive shaft (18) having means (21) engaging or engageable with the support member (13) or with means (13A) secured thereto for effecting the said pivotal movement of the support member (13). Drive mechanism as claimed in claim 4 characterised in that the printing head drive shaft (18) is provided with at least one cam (21) which is engageable with means (13A) secured to the support member (13). Drive mechanism as claimed in claim 4 or 5 characterised by detection means (25, 26) for detecting the angular position of the printing head drive shaft (18). A printer characterised by having a drive mechanism as claimed in any preceding claim. A printer as claimed in claim 7 characterised in that the printer is a colour hard copy printer.

Inventors: IMAI SABURO C O SEIKO INSTRUME, IMAI, SABURO C/O SEIKO INSTRUMENTS INC.
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1989
Main classification: B41J2
IPC codes: B41J25, B41J23, B41J2