GB2301559A - A printer wih a movable print head - Google Patents

Abstract

A thermal printer has a multiple element print head (29) which is movable relative to a platen (48) in a printing stroke to transfer ink from a ribbon to a substrate film sandwiched between the print head (29) and the platen (48). The print head (29) is mounted so as to be movable across the platen (48) in the printing stroke and in a return stroke, and towards and away from the platen (48) at the beginning and end of the printing stroke respectively. The print head (29) is driven by a print head drive mechanism including an electric motor (22), and the motor is so coupled to the print head (29) as to cause it to execute each of the above-described print head movements.

Description

A PRINTER WITH A MOVABLE PRINT HEAD This invention relates to a thermal printer with a multiple element print head which is movable across a platen in a printing stroke.

In our published British Specification No. 0556066 we disclose a thermal printer in which a thermal print head is driven in a longitudinal printing stroke across a substrate material supported by a stationary platen. A ribbon coated with thermally transferable ink passes between the print head and the substrate so that when the print head passes across the ribbon with its elements energised and in contact with the ribbon, ink is transferred from the ribbon to the substrate material. At the end of the printing stroke, the head is lifted away from the platen and driven back in a return stroke, whereupon it is lowered again towards the platen to begin another printing stroke.The substrate material is moved intermittently in that it is held stationary during the printing stroke and then advanced during the reverse movement of the print head to bring a new printing area into registry with the platen and the print head. The combination of print head movements described above is achieved by mounting the head in a print head carrier assembly having a looped belt running between rotatable sleeves on two respective eccentrically-mounted shafts so that rotation of the sleeves in alternate directions causes the print head, which is secured to one of the belt runs between the sleeves, to move in a direction parallel to the platen in the printing stroke and the return stroke.Rotation of the eccentrically-mounted shafts causes the sleeves and looped belt to be lowered and raised to move the print head towards and away from, respectively, the platen at the beginning and end of the printing stroke.

It is an object of the invention to provide a simpler printer of less expensive construction.

According to this invention a thermal printer comprises a pattern for supporting a substrate on which information or a platen is to be printed, a multiple element print head mounted so as to be movable across the platen in a printing stroke and a return stroke and towards and away from the platen at the beginning and end of the printing stroke respectively, and a print head drive mechanism including an electric motor, wherein the motor is coupled to the print head to cause it to execute each of the said movements. Use of a single electric motor to drive the print head in both the longitudinal direction and towards and away from the platen allows the printer to be simpler and less expensive to manufacture than the prior printer described above.

Preferably, the print head is mounted on a print head carrier assembly forming part of the print head drive mechanism, the carrier assembly being movable towards and away from the platen in response to operation of the motor.

The print head is preferably driven in its printing stroke and return stroke by a print head drive belt extending between shafts forming part of the carrier assembly, the axes of the shafts running substantially perpendicularly to the direction of movement of the print head in the printing stroke. The print head may be mounted in a print head carriage which is able to slide in the longitudinal direction in the print head carrier assembly, or is otherwise mounted for executing a linear reciprocating motion in the longitudinal direction.

The shafts may be in the form of a single pair of parallel spaced apart shafts which are fixed with rotatable sleeves or themselves rotate, in either case the print head drive belt being formed into a loop around the shafts with two parallel belt runs, both substantially parallel to the platen. The print head carriage is secured to one of the belt runs, preferably that nearest the platen, and the print head mechanism has means for transferring rotary motion from the above-mentioned single motor to the print head drive belt. Such transferring means may comprise a transfer drive belt wrapped around the output shaft of the motor and one of the shafts of the carrier assembly.

The print head carrier assembly may be so mounted that the axes of its shafts are reciprocable in unison along respective paths of travel which are inclined with respect to the direction of movement of the print head in the printing stroke thereby to allow the print head to be moved towards and away from the platen. Such reciprocating motion may be brought about by mounting the shafts on two pairs of cranks, the cranks of each pair being pivotable about a respective crank axis running parallel to the axis of the shafts, with the crank axes and shaft axes thereby defining the vertices of a parallelogram so that as the cranks are pivoted about the crank axes, the shaft axes follow arcuate paths to alter the spacing between the print head and the platen.The angle of the cranks with respect to the direction of movement of the print head during the printing stroke varies typically between angular limits which lie within the range of from 450 to 900 with respect to the platen surface, the actual angular limits being set by stops formed, for example, by the ends of slots which receive end portions of the shafts.

The transferring means are arranged such that when the motor is driven first in one direction and then in the opposite direction, the transfer drive belt causes the cranks to swing to one limit at the beginning of the printing stroke and in the opposite direction to the other limit at the end of the printing stroke whilst driving the print head from the vicinity of one shaft to the vicinity of the other shaft between the two angular movements of the crank. This may be achieved by wrapping the transfer drive belt around one of the shafts of the carrier assembly, the degree of wrap being such that in the vicinity of the shaft, the transfer drive belt forms a "V,, with an included angle of greater than 900.The crank axes are preferably located between the arms of the "V,,, and each "V,, arm makes an angle of less than 900 with a line normal to the path of travel of the axis of the shaft at the mid-point of the path. Typically, when the cranks supporting the shaft are midway between their two angular limits of rotation, the longitudinal axis of one of the cranks bisects the included angle of the belt "V", with the angle of that longitudinal axis with respect to each arm of the "V" being greater than or equal to 45 , and typically in the region of 600.

The cranks may be formed by two pairs of arms, the ends of the arms of one pair being connected to the ends of the arms of the other pair by longitudinal connecting members to maintain the print head drive belt in a stretched condition.

The V-shape of the transfer drive belt adjacent the print head carrier assembly shaft around which it is wrapped may be defined by idler rollers with axes of rotation parallel to the axis of the shaft, the rollers being located on opposite sides of the shaft.

To avoid slipping, both the transfer drive belt and print head drive belt are toothed, with the shafts of the print head carrier and the motor being correspondingly splined.

The preferred printer has just two electric motors, one being the print head drive motor described above, the other being a ribbon drive motor coupled to a take-up spool for taking up used ribbon. This spool and a ribbon supply spool are preferably mounted directly above and on the opposite side of the print head carrier assembly from the platen, with both motors arranged side-by-side on the opposite side of the ribbon spools from the print head carrier assembly, for instance with both motors being located on the same side of the transfer drive belt as the print head and directly above the ribbon spools in the interests of minimum printer depth in a direction perpendicular to the direction of movement of the print head in its printing stroke, i.e.

parallel to the motor axes.

The invention will now be described by way of example with reference to the drawings in which: Figure 1 is a diagrammatic front view of a printer in accordance with the invention with the print head raised, and Figure 2 is a side view, partly sectioned along the line A-A in Figure 1, with the print head lowered.

The printer shown in the drawings is designed to print onto a substrate disposed upon a platen. Printing is effected by transferring ink from a ribbon onto 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 ribbon on the substrate.

In the embodiment illustrated, the print head is positioned above the ribbon and substrate, and when not printing, the head is spaced from the ribbon and applies no pressure to the ribbon or to the substrate so that both ribbon and substrate are free to move relative to each other and to the print head. The platen, which may be coated with rubber or the like, is disposed below the substrate and remains in a substantially static position during operation of the printer. For printing purposes, the ribbon and substrate are sandwiched between the printer head and the platen and the print head is moved down into contact with the ribbon so as to apply a predetermined pressure to the ribbon and the substrate.During a printing stroke in which the print head is moved longitudinally across the ribbon, the required pressure or load is maintained to grip the ribbon and substrate and to ensure good print quality. In accordance with the invention both the up and down movement and the movements in the longitudinal direction are effected by a single electric motor, preferably a stepper motor.

Referring to Figure 1 in combination with Figure 2, the main components of the printer are mounted on a vertical back plate 10 to which are attached side plates 12,14, a bottom plate 16, and a top plate 18. The space between the side plates 12 and 14 to the front of the back plate 10 is divided into two compartments 17,19 by a partition 20.

Above the partition 20 is a motor compartment 17 housing two stepper motors 22,24, while below the partition 20 there is a cantilevered support frame 26 attached to the back plate 10, the support frame 26 mounting a movable print head 29 carrier assembly 28 to which a print head carriage 30 is attached. Carriage 30 houses a multiple-element print head 29 which has a series of electrically energisable thermal printing elements extending laterally, i.e. in a line perpendicular to the back plate 10. The print head 29 is fixed in the carriage 30 and the elements are exposed to the underside.

The lower compartment 19 also has space above the support frame 26 for ribbon supply and take-up spools which are not shown, these being part of a front plate assembly (also not shown) which is removable from the side plates 12,14.

Behind the back plate 10 is a third compartment 31, closed off by a rear cover 32. The output shaft of motor 24 projects into this compartment 31 and has a drive pulley 34 around which is wrapped a ribbon drive belt 36 for transmitting drive from the motor 24 to a ribbon spindle 38 via a pulley 40, the spindle 38 being mounted in a bush 42 housed in the back plate 10. When the front plate assembly mentioned above is fitted to the side plates 12,14, the ribbon take-up spool, which is mounted on a bearing on the front plate, engages with spigots 44 projecting from the ribbon drive spindle 38.

The ribbon supply spool (not shown), also attached to the front plate, is located alongside the take-up spool, and when the front plate is mounted to the side plates 12,14, is centred on the axis 46 shown in Figure 1.

It will be understood, then, that the ribbon, when located in the lower compartment between the side plates 12,14, extends from the supply spool around bars (not shown) attached to the front plate, and thence beneath the print head carriage 30, around one or more further deflecting bars (not shown), and is collected at the take-up spool in the right-hand side of the lower compartment 19.

The motor compartment 17 and the rear compartment 31 are substantially sealed to prevent the ingress of dirt. The electronics controlling the motors are preferably located in an external unit.

A flat platen 48 is fixed beneath the print head carrier assembly 28 to act as a support for the substrate material on which information or patterns are to be printed. By allowing the print head carriage 30 to pivot about a longitudinal axis 49 (see Figure 2), misalignment of the platen 48 can be obviated.

The mechanism which drives the print head carriage 30 both across the platen 48 and up and down with respect to the platen 48 will now be described in more detail.

Referring to both Figure 1 and Figure 2, the print head carrier assembly comprises two parallel shafts 50,52 with axes perpendicular to the back plate 10, front and rear connecting members 54,56 (see Figure 2) linking the front ends and the rear ends respectively of the shafts 50,52, and front and rear looped drive belts 58,60 which are wrapped around the shafts 50,52, and have parallel upper and lower belt runs parallel to the platen 48.

Each shaft 50,52 has a rotatable outer sleeve 50S,52S mounted on the shaft core by bearings (see 52B in Figure 2), the sleeves 50S,52S being splined to receive the toothed inner faces of the print head drive belts 58,60. Each shaft core has a pin (see 52E in Figure 2) at each end which not only receives one of the connecting members 54,56, but also is housed in the end of a respective crank arm 62,64,66 or 68. Two of these crank arms 62,64 are pivotally mounted on studs 69 fixed to the front edge of the cantilevered support frame 26, while the other two, the rear two 66,68, are mounted on studs 70 projecting from the rear of the back plate 10 (see Figure 2). Thus, each shaft 50,52 hangs on a front crank arm 62,64 and a rear crank arm 66,68, the front and rear crank arms supporting each shaft having a common pivot axis.These pivot axes are spaced apart by a distance equal to the spacing of the axes of the shafts 50,52 so that the arms form the sides of a parallelogram, as seen in Figure 1. It will be noted that each shaft 50,52, passes through a respective slot 72,74 in the back plate 10, the ends of which slots act as stops to limit the degree of angular rotation of the arms. Where the shafts 50,52 pass through the slots 72,74 they have rubber cushioning sleeves 76.

The print head carriage 30 includes two pairs of guide rails 77,78 which lie respectively above and below the print head drive belts 58,60. The lower guide rails 78 are fixed to the belts 58,60 in their lower belt run between shafts 50,52 with the upper guide rail 77 free to slide over the back of the belts 58,60 along the upper belt run.

It will be appreciated from the foregoing that the print head carrier assembly, comprising shafts 50,52 and the print head drive belts 58,60, is movable towards and away from the path of the substrate material as the arms swing in unison, as well as in the printing and return stroke directions, thereby causing the print head 29 alternately to press down against and withdraw from the ribbon and substrate material.

The manner in which this approaching and withdrawing movement is brought about will now be described.

Referring to Figure 2, immediately in front of the back plate 10 is a third belt 80 which transfers drive from the print head drive motor 22 to the print head carrier assembly 28 by virtue of the belt also being wrapped around the sleeve 50S of one of the shafts 50 of the carrier assembly.

In fact, as can be seen from Figure 1, this transfer drive belt 80, which is also toothed like the print head drive belts 58,60, wraps around the sleeve 50S over an angle of less than 900 at the axis of shaft 50 so that on each side of shaft 50 it forms a shallow "V" defined by the positions of idler rollers 82,84 mounted on the back plate 10. When the shaft 50 is at the mid-point of its arcuate path of travel, the line joining its axis with the pivot axis of the suspending crank arms 62,66 approximately bisects the angle made by the "V" of the transfer drive belt 80 between the idler rollers 82,84. Each arm of the "V" makes an angle of greater than 500 with the bisecting line, and preferably 600.

In Figure 1, the print head carriage 30 is shown in its raised position and approximately at the mid-point of its return stroke. It will be noted that the shafts 50,52 are at the upper ends of the slots 72,74. At this point of the printing cycle, the motor 22 is driven in a clockwise direction so that the shafts 50,52 are driven in an anticlockwise direction and the print head carriage is being driven to the right.

Due to the frictional resistance against rotation of the shafts 50,52, rotation of the transfer drive belt 80 in the anti-clockwise direction keeps the shafts 50,52 at the ends of the slots 72,74, thereby keeping the print head carriage 30 in the raised or retracted position. When the carriage 30 has reached the end of its return stroke, the print head drive motor 22 is switched to drive in the opposite direction. As a result, transfer drive belt 80 is driven clockwise and arms 62,66,64,68 are immediately swung clockwise so that the shafts 50,52 move to the lower ends of slots 72,74, bringing the print head 29 downwards to an active position so as to apply pressure against the ribbon and substrate material (not shown) lying over the platen 48.

Once the shafts 50,52 have reached the limit of their movement along slots 72,74, both are driven clockwise and the print head 29 is caused to move across the platen 30 in its printing stroke with the force applied to the arms 62,66,64,68 by the motor 22 and belt 80 resulting in the print head 29 being pressed against the ribbon and substrate which lie over the platen 48.

At the end of the printing stroke, the print head drive motor 22 is reversed once again to cause transfer drive belt 80 to move anti-clockwise, thereby moving shafts 50,52 to the right again and lifting the print head 29. When the shafts 50,52 have reached their upper limit of movement, they are rotated anti-clockwise by the belt 80 to execute another return stroke.

During the printing stroke, the substrate material on which information or patterns are to be printed, and the ribbon are maintained stationary on the platen 48. During the return stroke, both ribbon and substrate material are fed across the platen 48 towards the right-hand side of the printer as seen in Figure 1 in readiness for the next printing stroke.

The print head drive motor 22 is a stepper motor which is capable of rapid acceleration. The relatively short periods of acceleration take place largely during the movement of the shafts 50,52 along their respective slots 72,74 so that during the printing stroke the print head drive belts 58,60 and the print carriage 30 are moving with substantially constant velocity.

Claims (24)

1. A thermal printer comprising a platen for supporting a substrate on which information or a pattern is to be printed, a multiple element print head mounted so as to be movable across the platen in a printing stroke and a return stroke and towards and away from the platen at the beginning and end of the printing stroke respectively, and a print head drive mechanism including an electric motor, wherein the motor is coupled to the print head to cause it to execute each of the said movements.

2. A printer according to claim 1, including a print head drive belt extending between shafts the axes of which run substantially perpendicularly to the direction of movement of the print head in the printing stroke with the belt extending in a direction generally parallel to the platen and being connected to the print head to move the head in its printing and return strokes.

3. A printer according to claim 1 or 2, wherein the print head is mounted on a print head carrier assembly forming part of the print head drive mechanism and movable towards and away from the platen in response to operation of the motor.

4. A printer according to claim 3, wherein the print head drive mechanism includes a single transfer drive belt extending between the motor and the carrier assembly for transmitting drive to the assembly so as to cause the print head to execute each of the said movements.

5. A printer according to claim 3, wherein the print head is mounted on a print head carriage which is arranged to execute a linear reciprocating motion in the carrier assembly in a direction substantially parallel to the platen.

6. A printer according to claim 5, wherein the carrier assembly comprises a pair of parallel spaced apart shafts and the print head drive belt which is formed into a loop around the shafts, the print head carriage being secured to one of the belt runs between the shafts, and wherein the print head drive mechanism further comprises means for transferring rotary motion from the said motor to the print head drive belt.

7. A printer according to claim 6, wherein the print head carrier assembly is so mounted that the axes of the shafts are reciprocable in unison along respective paths of travel which are inclined with respect to the platen thereby to cause the print head to move towards and away from the platen.

8. A printer according to claim 7, wherein the carrier assembly is mounted on spaced apart cranks pivotable about respective crank axes running parallel to the axes of the said shafts, with each crank pivotally connected to the carrier assembly at pivot axes which, with the crank axes, define the vertices of a parallelogram, whereby the paths of travel of the said shaft axes are arcuate.

9. A printer according to claim 8, wherein the said pivot axes are also the axes of the said shafts.

10. A printer according to any of claims 7 to 9, wherein the reciprocating movement of the shafts is limited by stops.

11. A printer according to claim 10 in combination with claim 8 or claim 9, wherein the stops are positioned such that the cranks are constrained to pivot between angular limits which lie within the range of from 450 to 900 with respect to the platen.

12. A printer according to any of claims 6 to 11, wherein the transferring means comprises a transfer drive belt, and wherein the motor is arranged to be driven alternately in opposite directions in order to drive the print head in its print and return strokes respectively.

13. A printer according to claim 12, wherein the print head is secured to the print head drive belt in the belt run which is furthest from the motor.

14. A printer according to claim 12 or claim 13, wherein the transfer drive belt is wrapped around one of the said shafts of the carrier assembly thereby to cause rotation of the shaft and the print head drive belt.

15. A printer according to claim 14, wherein the transfer drive belt is wrapped around the said shaft to an extent such that it forms a "V" with an included angle of greater than 900, and wherein, on the same side of the transfer drive belt as the shaft, each arm of the "V" makes an angle of less than 900 with a line normal to the path of travel of the axis of the shaft at the mid-point in the said path.

16. A printer according to claim 15, wherein each of the two angles is greater than or equal to 500.

17. A printer according to claim 9 and any of claims 14 to 16, wherein the cranks are formed by two pairs of interconnected arms, each pair of crank arms carrying one of the shafts with an arm at each end of the shaft, and wherein the axis of the print head carrier assembly shaft carrying the transfer drive belt lies between the respective crank pivot axis and the transfer drive belt where it wraps around the shaft.

18. A printer according to claim 15 or claim 16, having idler rollers with axes of rotation parallel to the axis of the shaft carrying the transfer drive belt, the rollers being located on opposite sides of the shaft to define the "V" form of the belt.

19. A printer according to any of claims 14 to 18, wherein the transfer drive belt and the print head drive belt are toothed belts.

20. A printer according to claim 19, wherein the transfer drive belt and the print head drive belt engaged common splines formed on the shaft carrying the transfer drive belt.

21. A printer according to any of claims 12 to 20, wherein the motor and the print head are both located to one and the same side of the transfer drive belt in the lateral direction with respect to the printing stroke direction.

22. A printer according to claim 21, including a ribbon drive motor having an axis of rotation parallel to the axes of the print head carrier assembly shafts, and mounted alongside the motor of the print head drive mechanism.

23. A printer according to claim 22, further comprising ribbon supply and take-up spools mounted between the said motors and the print head carrier assembly, the ribbon drive motor being coupled to the ribbon take-up spool.

24. A printer constructed and arranged substantially as herein described and shown in the drawings.