Print head movement control

Abstract: A printing machine wherein printing on to a substrate is effected by a first movement of a print head (1) downwards into contact with the substrate and then by a second movement over the surface of the substrate while thermal printing elements in the print head (1) are selectively energised. The first movement is controlled by downward movement of a pair of rollers (6,7) operated by synchronized angular movement of a pair of eccentric shafts (8,9) one of which is freely mounted within each roller (6,7). The first and second movements are driven by a first and second stepper motor respectively. The rollers may be coupled by drive belts (4,5).

Description: This invention is related to a printing machine and more particularly to a flat bed thermal printing machine using a matrix of dots to form letters and/or symbols and devices. Thermal printers have become popular during the last ten years or so and, broadly speaking, two types of thermal printer are now available. A first type uses a thermally sensitive substrate on which to print and a second type uses a conventional substrate with a thermally sensitive ribbon between a print head and the substrate. A machine in accordance with this invention may be used with either type of thermal printing but we prefer to use a thermally sensitive ribbon in combination with a conventional substrate. It is an essential feature of the invention that the machine is a flat bed printer so that the substrate is in a substantially flat position during the printing operation. One advantage of flat bed printing as compared with printers in which a print head operates against a platen in the form of a roller is that a flat bed printer may more easily be used to print on to a relatively inflexible substrate which is difficult to bend around a platen in the form of a roller. On the other hand there is a problem with flat bed printing in that printing may be carried out over a relatively large flat area and in order to achieve a good printing result over the whole arra it is important to make sure, as far as it is possible to do so, that the print head applies a predetermined required pressure to the substrate over the whole printing area while the substrate is sandwiched between the print head and the flat bed or platen of the printer. It is an object of the present invention to provide improved means for moving the print head during printing in order to produce a thermal printer which can operate at very high speed and which can effect printing of high quality and uniform density and which is less expensive to produce than other printers. It should here be mentioned that DE-A-4023411 discloses a matrix printer in which printing is effected while a substrate is applied to a cylindrical platen. In the prior machine a print-head is attached to a carriage which is axially displaceable on a shaft which lies parallel to the cylindrical platen. A stepper motor is used to drive an eccentric shaft which in operation moves the print head towards and away from the carriage. The main thrust of the prior specification is to provide a device for setting the distance between the print head and the surface of the substrate. The said device comprises (a) a sensor movable between a reference position and a measuring position, (b) a computing device for calculating the thickness of the substrate and (c) a control device which displaces the print head in dependence on the measured thickness of the substrate. According to the present invention there is provided a matrix printing machine adapted to print information on to an elongate substrate at a printing area within the machine, the said substrate being movable through the machine in a step by step movement with a stop for printing between steps, wherein the machine comprises a carriage mounted print head normally disposed in an inoperative position out of contact with the substrate and provided with a multiplicity of individually energisable dot type thermal elements, means selectively to energise the thermal elements, first moving means including a stepper motor and an eccentric shaft to move the print head from its normal position towards and into contact with the substrate for printing and to move the print head back again away from the substrate to its normal inoperative position after printing, second moving means to move the print head along the surface of the substrate while the thermal elements are selectively energised to effect printing with the substrate stationery at the printing area between stepwise movements and to move the print head back after printing characterised in that the machine is a flat bed printer so that in operation the substrate is disposed in a substantially flat position at the printing area during printing, and that the first moving means includes a pair of synchronised eccentric shafts each rotatable within a roller, a pair of print head driving belts interconnecting the rollers and that the first stepper motor in operation rotates the eccentric shafts relatively to the rollers so that as the eccentric shafts rotate within the rollers the print head is pushed towards and away from the substrate by the rollers and that the second moving means includes a second stepper motor arranged positively to drive one of the rollers around its eccentric shaft to move the print head along the surface of the substrate, the other roller being driven by the print head driving belts, the arrangement being such that both the movements of the print head towards and away from the substrate and the backwards and forwards movements of the print head along the substantially flat substrate are effected by the movement of the rollers. It is important that the driving belts do not slip and for that reason we prefer to use toothed belts to engage with the toothed rollers. In this specification we shall assume that the machine is in an upright position and so from time to time we shall refer to the movement of the print head towards and away from the substrate as being an up and down movement. It will be understood from the above than an underlying idea of the invention is to control carefully the up and down movement and the side to side movement of the print head. The print head is attached to a carriage which in turn may be attached to two bottom guide members and to a pair of toothed belts which mesh with a pair of toothed rollers. As the rollers rotate the print head is moved from side to side, ie. is pushed backwards and forwards across the substrate. The carriage is also preferably attached to two top guide members similar to the bottom members but the top guide members are not connected to the belts so that the belts run freely relative to the top guide members which simply rub against the belts as the carriage moves. The print head is moved up and down relative to the substrate e.g. is raised and lowered by rotation of the eccentric shafts inside the rollers which are free for angular displacement relative to the shafts. The eccentric shafts are arranged in pairs linked together by an arrangement of belts and pulleys to ensure that the print head is moved substantially parallel to the flat bed or platen by keeping the eccentric shafts in phase. The movement of the print head towards and away from the flat bed or platen is achieved by the use of the first stepper motor which directly drives one of the eccentric shafts and as a consequence of the toothed belts drives both eccentric shaft. In order that the invention may be more clearly understood reference is now directed to the accompanying drawings, given by way of example, which show various views of a printing machine in accordance with the invention using a thermally sensitive printing ribbon. In the drawings:- Figure 1 is a pictorial view of mechanism to raise and lower the print head of the printing machine, Figure 2 is a detail view of one or the eccentric shafts inside a toothed roller, Figure 3 is a detail end view of one of the eccentric shafts showing in full line the raised position and in dash lines the lowered position, Figure 4 shows part of the mechanism provided to keep the rotation of the eccentric shafts in phase. Figure 5 is an end view of the eccentric shafts and toothed rollers with top and bottom guides in position, Figure 6 shows part of the drive mechanism for the print head, Figure 7 is a rear elevation of a machine in accordance with the invention, Figure 8 is a front elevation of the machine shown in Figure 7, Figure 9 is a side elevation of the machine shown in Figure 7, and Figure 10 is a top plan view of the machine shown in Figure 7 The printing machine shown in the drawings is designed to print on to a substrate disposed upon a flat bed. Printing is effected by transferring ink from a ribbon on to the substrate using a thermal print head which employs a multiplicity of individual heating elements adapted to be selectively energised to produce a fine deposition of ink from the ink carrying ribbon on to the substrate. In the embodiment illustrated the print head is positioned above the ribbon and substrate and when not printing the head is spaced apart from the ribbon in its normal or inoperative position and so the head, when in its normal or inoperative position, applies no pressure to the ribbon or to the substrate both the ribbon and the substrate being free to move relative to each other and to the print head. The flat bed or platen, which may be coated with rubber or the like is disposed below the substrate and remains in a substantially static position during the operation of the machine though, if desired the flat bed may be spring mounted so as to be self-adjusting. For printing purposes the ribbon and substrate must be sandwiched between the print head and the flat bed and the print head must be moved down into contact with the ribbon so as to apply a predetermined pressure to the ribbon and substrate. During printing the required pressure or load must be maintained to grip the ribbon and substrate and to ensure good print quality. Up and down movement of the print head is effected by the provision of a stepper motor not shown in Figures 1 to 6. Up and down movement of carriage and print head Referring now particularly to Figure 1, the print head is connected to a print head carriage, the head and carriage being indicated generally in Figure 1 by the reference 1. In turn the carriage is connected to a print head moving mechanism adapted to move the print head up and down substantially in a vertical plane. The carriage is connected to two bottom guides 2 and 3 and to two toothed print head driving belts 4 and 5 riding on toothed rollers 6, 7, mounted on eccentric shafts 8 and 9 so that in operation, as the eccentric shafts rotate, the head is pushed upwards and downwards. Top guides 10 and 11 are also provided and are connected to the carriage but the top guides 10, 11 are not connected to the print head driving belts 4, 5 so that the top guides 10, 11 simply rub against the belts 4, 5 as the print head and carriage 1 move. It will be understood from the above that the print head and carriage 1 are raised and lowered by rotation of the eccentric shafts 8, 9 inside the rollers 6, 7, which are free to rotate around the shafts 8, 9. The eccentric shafts 8, 9 are linked together using a toothed belt 12 and toothed pulleys 13, 14 (see e.g. Figure 4). To ensure that the print head is moved in a plane substantially parallel to the plane of the flat bed the eccentric shafts 8, 9 are kept in phase during their movements. The print head 1 is driven up and down by a stepper motor, not shown, which drives the eccentric shaft 9 and by means of pulleys 13 and 14 and belt 12, see Figure 4, drives the other eccentric shaft 8. Backwards and forwards movement of the print head and carriage. The print head 1 is driven forwards and backwards using a second stepper motor 15 (Figure 6) which drives one of the toothed rollers 7 via a toothed pulley 16 and a toothed belt 16¹. An idler pulley 17 is also provided to maintain tension on the belt 16¹. Only one of the rollers i.e. roller 7 is driven directly by the stepper motor 15, the second roller 6 being driven from the first roller 7 by the toothed belts 4, 5. The idler pulley 17 is positioned to minimise any change in length of the path of the drive belt 16¹ as the eccentric shafts 8 and 9 rotate to raise and lower the print head. When the machine is operating the print head and carriage 1 are moved up and down and backwards and forwards using the two stepper motors as described above. The movements of the print head are synchronised with the energisation of the thermal elements and with the movement of the ribbon and substrate which are moved into printing position over the flat bed and are then momentarily stopped while the print head sweeps over the surface of the substrate and ribbon with selectively energised thermal elements to effect printing. Details of the electronic circuitry to move the various parts of the machine and to energise the thermal elements are not included because the circuitry is now conventional and will be well understood by someone skilled in the art. The ribbon used for printing is supplied on a reel, one end of which may be shaped to fit a drive dog connected to a third stepper motor, the operation of which is synchronised with that of the other two stepper motors. Figures 7, 8, 9 and 10 show various views of a machine constructed in accordance with the invention using mechanism illustrated in Figures 1 to 6.

Claims: A matrix printing machine adapted to print information on to an elongate substrate at a printing area within the machine, the said substrate being movable through the machine in a step by step movement with a stop for printing between steps, wherein the machine comprises a carriage mounted print head (1) normally disposed in an inoperative position out of contact with the substrate and provided with a multiplicity of individually energisable dot type thermal elements, means selectively to energise the thermal elements, first moving means including a stepper motor and an eccentric shaft to move the print head (1) from its normal position towards and into contact with the substrate for printing and to move the print head (1) back again away from the substrate to its normal inoperative position after printing, second moving means to move the print head (1) along the surface of the substrate while the thermal elements are selectively energised to effect printing with the substrate stationery at the printing area between stepwise movements and to move the print head (1) back after printing characterised in that the machine is a flat bed printer so that in operation the substrate is disposed in a substantially flat position at the printing area during printing, and that the first moving means includes a pair of synchronised eccentric shafts (8,9) each rotatable within a roller (6, 7), a pair of print head driving belts (4, 5) interconnecting the rollers (6, 7) and that the first stepper motor in operation rotates the eccentric shafts (8, 9) relatively to the rollers (6, 7) so that as the eccentric shafts (8, 9) rotate within the rollers (6, 7) the print head (1) is pushed towards and away from the substrate by the rollers (6, 7) and that the second moving means includes a second stepper motor (15) arranged positively to drive one of the rollers (7) around its eccentric shaft (9) to move the print head (1) along the surface of the substrate, the other roller (6) being driven by the print head driving belts (4, 5), the arrangement being such that both the movements of the print head (1) towards and away from the substrate and the backwards and forwards movements of the print head (1) along the substantially flat substrate are effected by the movement of the rollers (6, 7). A printing machine according to claim 1, characterised in that the print head (1) mounted on its carriage is connected both to two bottom guides (2, 3) and also to the print head driving belts (4,5) which are toothed belts for cooperating with the rollers (6, 7) which are also toothed, and further characterised by the provision of two top guides (10, 11) connected to the carriage but not connected to the print-head driving belts (4, 5) which simply rub against the top guides (10, 11).

Inventors: SLATER DAVID GARY, SLATER, DAVID GARY
Applicants: ITW LTD, ITW LIMITED
Application year: 1993
Main classification: B41J25
IPC codes: B41J3, B41J25, B41J19