GB2404896A - Ribbon transport mechanism having tensioning means - Google Patents

Abstract

A printing apparatus 10 includes a ribbon transport mechanism including a path for a ribbon 14 containing marking medium between a ribbon take-up spool 11 onto which spent ribbon 14 is wound and a ribbon pay-out spool 12 on which a supply of fresh ribbon is stored, each of the take-up and pay-out spools 11, 12 being driveable simultaneously by respective first and second motors 21, 22 to effect ribbon transport, and there being a controller 30 to control the first and second motors 21, 22 to maintain the ribbon tension within boundary limits and characterised in that the first motor 21 is a DC motor and the second motor 22 is a stepper motor.

Description

Title: Printing Apparatus.

Description of Invention

This invention relates to a printing apparatus and to a ribbon transport mechanism for use in a printing apparatus.

Printing apparatus are known which include a take-up spool onto which ribbon is fed during or after use in a printing operation, and a pay-out spool on Historically, particularly for a printing apparatus of the socalled thermal kind, in which a print head has an array of heating elements which when selectively energised, are operative to remove marking medium from a ribbon.

Such an apparatus provides for differential movement between the print head and the ribbon (and substrate onto which a print is to be performed) during printing either by moving the ribbon and substrate past the print head whilst maintaining the print head stationary or at least moving at a speed different to the ribbon and substrate, or by moving the print head relative to a stationary ribbon and substrate. The take-up spool has been driven by a stepper motor, with the pay-out spool being operative to maintain a tension in the ribbon, via a slipping or drag clutch.

However new faster operating print heads being developed have led to a requirement for faster printing speeds which in order to avoid frequent ribbon breakages, requires more reliable and positive ribbon transport It is known to drive the take-up and pay-out spools simultaneously to achieve ribbon transport either during a printing operation so that for example the ribbon is traversed past a stationary print head, or in- between printing operations in a printing apparatus of the kind in which the ribbon is generally stationary during printing and the print head moves relative to the ribbon. For example, in WO-A-03/0290 13 an arrangement is disclosed in which the take-up spool is driven by a stepper motor and the pay-out spool is driven by a DC motor.

It is important that the tension of the ribbon along a ribbon path between the spools, is maintained between certain limits. If the ribbon is over tensioned, for example by rotating the take-up spool at a rate which exceeds the rate at which ribbon is paid out from the pay-out spool, ribbon breakage can occur, particularly where the ribbon is thin or inelastic. If the ribbon tension is not lO great enough though, during a printing operation, this can lead to poor quality printing and moreover in an apparatus in which the ribbon tension is not controlled, fluctuations in tension between the slack and taut can lead to ribbon breakage.

The arrangement of WO-A-03/029013 suffers from certain disadvantages. In this arrangement, to control ribbon tension, the DC pay-out spool driving motor provides a braking effect depending on the current which is supplied to it. The amount of ribbon actually transported is controlled by operation of the stepper motor. To determine the appropriate current to supply to the DC pay-out spool motor to maintain a constant tension in the ribbon, an estimation of the amount of ribbon on each of the pay-out and take-up spools is made, in the example, by an optical determination method. This determination may take account of other factors which will effect the ribbon tension, such as the ribbon width and type.

Inevitably with such an arrangement the take-up spool stepper motor operates in opposition to the DC pay-out spool motor when stepping, which may lead to inaccurate ribbon feeding or ribbon breakage.

According to a first aspect of the invention we provide a printing apparatus including a ribbon transport mechanism including a path for a ribbon containing marking medium between a ribbon take-up spool onto which spent ribbon is wound and a ribbon pay-out spool on which a supply of fresh ribbon is stored, each of the take-up and pay-out spools being driveable simultaneously by respective first and second motors to effect ribbon transport, and there being a controller to control the first and second motors to maintain the ribbon tension within boundary limits and characterized in that the first motor is a DC motor and the second motor is a stepper motor.

Thus unlike the prior arrangements described above in which a stepper motor is employed to drive the take-up spool and either a clutch or DC motor provides drag on the ribbon in an effort to control ribbon tension around the ribbon feed path, in the arrangement of the invention, the DC motor driving the take-up spool controls the ribbon tension, whilst accurate feeding of the ribbon is still achieved by the stepper motor, but driving the pay-out spool.

Thus although the take-up spool and pay-out spool driving motors will still operate in opposition in order to maintain a tension in the ribbon, accurate ribbon feeding is less dependent upon the torque on the ribbon provided by the DC motor driving the take-up spool.

Thus in accordance with the invention, the rate of pay-out of the ribbon from the pay-out spool may be achieved incrementally by rotating the payout spool by the stepper motor, in discrete steps, whilst the take-up spool may be driven during ribbon transport, continuously by a DC motor, the controller varying the current supplied to the DC motor to vary the ribbon tension along the ribbon feed path to maintain the ribbon tension within boundary limits.

It is well known to entrain the ribbon around a roller of known external diameter of the ribbon feed path, and to monitor rotation of the roller with a sensor, to determine amount of ribbon used, and from which the controller may make an estimation of the amount of ribbon on the take-up spool. Such an arrangement is taught in prior specification GB-A-23 10405, which also teaches a method of calibrating the apparatus. The present invention enables such well Referring to figure 1 of the drawings, a printing apparatus 10 is provided which includes a ribbon transport mechanism including a ribbon take-up spool 11, onto which spent ribbon is wound, and a ribbon pay-out spool 12 on which a store of ribbon is provided, the ribbon 14 being fed in use from the pay-out S spool 12 along a ribbon feed path onto the take-up spool 11, the ribbon 14 passing around a plurality of rollers 15 and 16. The take- up spool 11 is driven by a DC motor 21 whilst the pay-out spool 12 is driven by a stepper motor 22, the DC motor 21 and the stepper motor 22 together being driven to transport the ribbon 14 past the print head 18 during printing.

In this particular embodiment, the print head 18 is stationary during printing, whilst the ribbon 14 and a subskrate 19 onto which marking medium is to be transferred from the ribbon 14 by selectively operating heating elements of an array of the print head 18 during a printing operation, are traversed together past the print head 18.

IS However, the invention may be applied to other printing apparatus for example a printing apparatus in which the print head 18 too moves during printing, along the ribbon 14 and substrate 19 but at a differential speed, or to a printing apparatus in which the ribbon 14 and substrate 19 are stationary during printing and the print head 18 is traversed along the ribbon 14 and substrate 19 in the general direction of arrow A, to effect printing, or to a printing apparatus in which during printing relative movement occurs between each of the ribbon 14, print head 18 and substrate 19.

In the embodiment shown in figure 1, each of the stepper and DC motors 21 and 22 is driven under the control of a controller 30 during ribbon transport.

The controller 30 may also control the operation of the print head 18 as indicated, although a separate print head 18 controller may be provided as desired.

In use, the controller 30 not only controls the amount of ribbon 14 paid out from the pay-out spool 12 e.g. appropriate for printing, by controlling the stepper motor 27, but also the controller 30 controls the tension of the ribbon 14 around the ribbon feed path during ribbon transport, by adjusting the current fed to the DCmotor21. In order to determine an appropriate current for feeding to the DC motor 21 to maintain the ribbon tension within acceptable boundary limits, the controller 30 estimates the diameters andlor the relative diameters of the ribbon on each of the take-up and pay-out spools 1 1, 12.

This is achieved in the embodiment of figure 1 by monitoring the use of ribbon 14 by entraining the ribbon 14 around an idler roller 35 of known external diameter, and monitoring the number of rotations of the idler roller 24 as the ribbon 14 is fed. The controller 30 may be calibrated initially, by determining a relationship between the rotation of at least one of the take-up 11 and pay-out 12 spools and the idler roller 35. By knowing the thickness of the ribbon 14, for a tape tension maintained within boundary limits, the diameter of the take-up spool 11 at least, may be determined by the controller 30 at any time as rotations of the idler roller 35 are monitored.

The controller 30 may thus determine from the relative diameters of the pay-out 12 and take-up spools 11, or at least from the diameter of the take-up spool 12, a suitable current to control the DC motor 21 tot provide the appropriate tension of ribbon 14 around the ribbon transport path.

The speed of transport of the ribbon 14 around the ribbon feed path can thus be determined by rotating the pay-out spool 12 at a rate, incrementally, by indexing the stepper motor 22. However, the current applied to the DC motor 21, driving the take-up spool 11, may be varied by the controller 30 in accordance with the spool diameters or at least the take-up spool 11 diameter to maintain the tension of the ribbon 11 around the ribbon feed path within predetermined boundary limits.

Various modifications may be made without departing from the scope of the invention. For example, instead of or in addition to the idler roller 35 for determining the amount of ribbon used, and hence the spool 11, 12 diameters, an optical scanning system may be provided to determine the diameter of each spool 11, 12 at any given time. Data from the optical scanning system may be used by the controller 30 to adjust the current supplied to the DC motor 21 to maintain the ribbon tension within boundary limits.

As illustrated in figure 2, instead of determining the changing diameters of the spools 11, 12, or at least the take-up spool 11, in order to provide a parameter from which the controller 30 may derive an appropriate current for DC motor 21 to maintain the ribbon 14 tension within the boundary limits, instead, ribbon tension may actively be monitored at least during ribbon feed, in order to provide an input to the controller 30 and from which the controller 30 may determine an appropriate current for the DC motor 21.

Referring to figure 2, in which like parts to those indicated in figure 1 are referenced by the same references, a resiliently biased member including a mechanical lever 36 is biased into engagement with the ribbon 14. The member 36 is pivotal about an axis B and is biased by a spring 37, e.g. a coil spring 37 in the illustration, towards the ribbon 14. The lesser the tension in the ribbon 14, the greater movement of the member 36 is permitted in response to the force of the spring 37 about the axis B. As tension in the ribbon 14 increases, the member 36 will be moved back about its pivot axis B against the force of the spring 37.

Thus the amount of angular rotation of the member 36 will give a direct indication of the tension of the ribbon 14 in the ribbon transport path. A sensor 38 determines the angular position of the member 36 and provides a signal to the controller 30 which responds by adjusting the current provided to the DC motor driving the take-up spool 11 to maintain the ribbon 14 tension within predetermined boundary limits.

In the examples described, the motors 21, 22 are both actuated by the controller 30 actually during printing and so the ribbon is transported past the print head 18 in the direction of arrow A during printing solely by the motors 21, 22. The invention does not preclude winding the ribbon 14 past the print head 18 in an opposite direction to the arrow A, for example to enable ribbon features for example as described in our previous patent GB- A-2289441, which describes a method in which pixels of marking medium not removed from the ribbon 14 in a first pass of the print head 18 may be removed in a second pass of the print head 18.

The invention does not preclude an arrangement in which the ribbon 14 is transported during printing, by a shuttle around which the ribbon 14 is entrained, with the ribbon 14 being advanced in-between actuations of the print head 18, onto the take-up spool 12 whilst ribbon 14 is paid out from the pay-out spool 12 into the ribbon feed path to prime the shuttle with ribbon 14 to move past the print head 18 during a subsequent print head 18 actuation.

The invention does not preclude maintaining the ribbon 14 and substrate 19 stationary during printing, whilst the print head 18 is relatively moveable, the ribbon 14 being transported in between print head actuations, to wind spent ribbon onto the take-up spool 11.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (8)

1. A printing apparatus including a ribbon transport mechanism including a path for a ribbon containing marking medium between a ribbon take-up spool onto which spent ribbon is wound and a ribbon pay-out spool on which a supply of fresh ribbon is stored, each of the take-up and pay-out spools being driveable simultaneously by respective first and second motors to effect ribbon transport, and there being a controller to conko1 the first and second motors to maintain the ribbon tension within boundary limits and characterized in that the first motor is a DC motor and the second motor is a stepper motor.

2. An apparatus according to claim 1 wherein the rate of pay-out of the ribbon from the pay-out spool is achieved incrementally by rotating the pay-out spool by the stepper motor, in discrete steps, whilst the take-up spool is driven during ribbon transport, continuously by a DC motor, the controller varying the current supplied to the DC motor to vary the ribbon tension along the ribbon feed path to maintain the ribbon tension within predetermined boundary limits.

3. An apparatus according to claim 1 or claim 2 wherein the ribbon is entrained in the ribbon feed path, around a roller of known external diameter, and rotation of the roller is monitored by a sensor to determine the amount of ribbon used and from which the controller makes an estimation of the amount of ribbon at least the take-up spool.

4. An apparatus according to claim 3 wherein the controller relates the amount of ribbon used to the ribbon thickness to estimate the diameter on at , least the take up spool. lo

5. An apparatus according to claim 1 or claim 2 wherein a resiliently biased member is provided which is biased towards or away from the ribbon along the ribbon feed path, the amount of movement of the member permitted by the ribbon depending upon ribbon tension in the ribbon feed path, there being a sensor to sense the position of the member and provide an input to the controller which adjusts the current supplied to the DC take-up spool motor in response to maintain the ribbon tension within boundary limits.

6. A printing apparatus substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.

7. A ribbon transport mechanism for use in the printing apparatus of any one of the preceding claims.

8. Any novel feature or novel combination of features disclosed herein and/or as shown in the accompanying drawing.