WO2024100163A1

WO2024100163A1 - Printing apparatus and printing method

Info

Publication number: WO2024100163A1
Application number: PCT/EP2023/081224
Authority: WO
Inventors: Samuel ACHAMFUO-YEBOAH
Original assignee: Dover Europe Sarl
Priority date: 2022-11-08
Filing date: 2023-11-08
Publication date: 2024-05-16
Also published as: GB202216646D0

Abstract

A printing apparatus (10) and a method of operation of a printing apparatus (10) including a printhead (24) and a monitoring device (40) positioned adjacent the printhead (24), the monitoring device (40) including an emitter (42), a receiver (44), and a processor (46); the emitter (42) being operable to emit a first output (SO) and a second output (SO) towards a curved target surface (T), and the receiver (44) being operable to receive first data corresponding to a reflected portion (SR) of the first output (SO) and data corresponding to a reflected portion (SR) of the second output (SO), each reflected portion (SR) having been reflected by the target surface (T), wherein each output (SO) emitted by the emitter (42) is electromagnetic radiation, and wherein the processor (46) is operable to compare the data corresponding to the reflected portion (SR) of the first output (SO) and the data corresponding to the reflected portion (SR) of the second output (SO) to monitor a parameter of the curved target surface (T).

Description

PRINTING APPARATUS AND PRINTING METHOD

The invention relates to a printing apparatus, in particular, but not exclusively, to a transfer printing apparatus, and a method of operation of the printing apparatus.

Transfer printing is well known in the art of commercial printing. An ink carrying tape or ribbon is transferred along a ribbon path from a supply spool, to a take up spool, past a printhead which is operable to transfer ink from the ribbon to a substrate. The printhead may include thermally energisable printing elements or pixels, which are operable to warm the ink on the ribbon such that it can be removed from the ribbon and transferred to the substrate to form an image, for example data, a pattern, a bar code, QR code, etc.. One or both of the supply spool and the take up spool are typically driven in order to transfer the ribbon between the spools. The ribbon can typically be moved in two directions between the spools, i.e. , in a forward direction and reverse direction. The substrate on to which ink is to be transferred during a printing operation is typically moveable relative to the printhead. There are two principal operating modes - intermittent printing and continuous printing. In intermittent printing, the substrate is advanced to a printing position near to the printhead and then stops, the printhead is then moved relative to the substrate to transfer ink from the ribbon to the substrate to create an image. The substrate then advances to a next printing position. This printing operation process is typically repeated until all required printing operations have been completed, and/or the inked ribbon is depleted and/or the substrate is depleted. In continuous printing, the substrate moves substantially continuously relative to the printhead, whilst the printhead remains in a substantially fixed lateral position relative to a substrate support. In this instance, “lateral” movement of the printhead (or the absence thereof) is intended to mean movement substantially parallel with the direction of movement of the substrate. In both intermittent and continuous printing modes, the printhead typically moves in a longitudinal direction, towards and away from the ribbon and the substrate, between a non-printing position, in which the printhead is not in contact with the ribbon (or at least not sufficiently close to be able to transfer ink from the ribbon) and a printing position, in which the printhead is in contact with the ribbon (or at least is close enough to transfer ink from the ribbon). The nonprinting position may include a plurality of positions, including a “ready to print position” and a retracted position, which may be required, for example, for maintenance purposes.

It is important to be able to accurately control the speed of the ribbon relative to the speed of the substrate, so as to optimise print quality. In some circumstances, it is desirable to accurately control the ribbon speed to correspond with the speed of the substrate, for example to prevent smearing of ink, and/or to ensure that ink is present on the portion of ribbon adjacent the printhead, to avoid failed or poor quality prints. The ribbon speed may be different from the speed of the substrate, but may be linked to the substrate speed. The ribbon speed may be slower than the speed of the substrate. Therefore, monitoring the speed of the ribbon and/or the substrate is desirable.

It is known to provide a rotary encoder externally of the printing apparatus to provide information relating to the speed of the substrate. Such a rotary encoder is often rotated by a wheel placed on the substrate, the wheel rotating as the substrate moves, or the encoder may be connected to a roller in the substrate path, for example to the shaft of such a roller. The encoder typically produces one or two separate square waves, on respective digital outputs. Each digital output may indicate one of speed and direction of movement of the substrate. One complete revolution of the encoder will generate a known number of pulses in the outputs. The outputs may be used to determine the speed and the direction of movement of the object causing the encoder to rotate, e.g., the substrate in the present example. A linear substrate speed can be determined from the rotational speed and the circumference of the wheel or roller. Encoders have several disadvantages, including relatively high cost; potential inaccuracy - for example if the substrate or ribbon slips on the roller/wheel; the encoder outputs need to be processed by a controller (e.g., the time period between square wave edges must be monitored/measured and converted to a speed - as the rotational speed increases the frequency of the square wave increases, placing a high computational load on the processor/controller computing the speed); the encoder is external to the printing apparatus, mounted in the production line, typically with a cable to connect it to the printing apparatus; contact encoders must be mounted close to, or at the point where the printing operations occur (to ensure that the image is printed at approximately the same speed as the substrate is moving at the printhead, to avoid stretching or compression of the image - this can be difficult owing to space constraints around the printing apparatus); and maintenance of contact encoders may include replacing worn parts, cleaning and, in some cases, sterilisation, (for example where the substrate that the encoder contacts is food packaging).

It is desirable to be able to determine the position of the printhead, for example relative to the ribbon and/or the substrate, accurately. This is such that a printhead drive apparatus which is responsible for positioning the printhead relative to the ribbon and/or the substrate does not cause too much or too little pressure to be applied during a printing operation, for example, and may also ensure that the printhead does not contact other parts of the printing apparatus and/or contact the ribbon and/or the substrate and/or a substrate support in the wrong place. If the printhead moves out of alignment, for example in a lateral direction along the ribbon path, or in a direction perpendicular to the ribbon path, but in approximately the same plane as the ribbon path, the image produced may be incorrectly positioned on the substrate or may be incomplete. Inaccurate positioning of the printhead may lead to damage to the printhead, and/or damage to other parts of the printing apparatus and/or inadequate print quality, for example. It is known to determine the pressure applied by the printhead on the ribbon/substrate by the use of a load cell. The load cell determines that the printhead has contacted the ribbon/substrate when the load cell detects that a predetermined pressure has been reached or exceeded.

The invention is defined by the claims.

There is provided a printing apparatus including a printhead and a monitoring device, the monitoring device including an emitter, a receiver, and a processor; the emitter being operable to emit a first output and a second output towards a curved target surface, and the receiver being operable to receive first data corresponding to a reflected portion of the first output and data corresponding to a reflected portion of the second output, each reflected portion having been reflected by the target surface, wherein each output emitted by the emitter is electromagnetic radiation, and wherein the processor is operable to compare the data corresponding to the reflected portion of the first output and the data corresponding to the reflected portion of the second output to determine a parameter of the curved target surface. The printing apparatus may include a controller operable to use data indicative of the parameter of the curved target surface, to control one or more operations of the printing apparatus.

The operation of the printing apparatus that is controlled may be movement of the printhead.

The printing apparatus may be a transfer printing apparatus including supports for carrying spools of ribbon, wherein the operation of the printing apparatus that is controlled may be the rotation of one or more of the spool supports so as to enable the movement of ribbon from one spool towards the other.

The parameter of the target surface may be one or more of position, speed, acceleration or a size of the target surface.

The curved target surface may be one of a ribbon, a substrate and a substrate support.

The parameter may be a diameter of a platen roller.

The monitoring device may include a laser scatter sensor.

The printhead may be moveable with the printhead relative to the target surface.

There is also provided a method of operation of a printing apparatus printhead and a monitoring device positioned adjacent the printhead, the monitoring device including an emitter, a receiver, and a processor; the emitter being operable to emit a first output and a second output towards a curved target surface, and the receiver being operable to receive first data corresponding to a reflected portion of the first output and data corresponding to a reflected portion of the second output, each reflected portion having been reflected by the target surface, wherein each output emitted by the emitter is electromagnetic radiation, and wherein the processor is operable to compare the data corresponding to the reflected portion of the first output and the data corresponding to the reflected portion of the second output to monitor a parameter of the curved target surface.

The parameter of the curved target surface may be obtained during use of the printing apparatus to carry out printing operations.

The method may include determining the parameter of the curved target surface whilst the curved target surface is rotating and/or moving linearly.

The target surface may be a curved substrate support, and the method may include determining a distance between the printhead and a centre or curvature of the substrate support.

The method may include determining a radius of curvature of the target surface.

The method may include determining a top dead centre position of the curved target surface.

The method may include moving the sensor relative to the substrate support, at a substantially constant speed, in a substantially straight line, whilst maintaining the target surface substantially stationary, and obtaining a plurality of data.

The method may include moving the monitoring device relative to the target surface, with the printhead.

The parameter may be the position of a part of the target surface relative to the printhead.

The method may include controlling at least one operation of the printing apparatus using data indicative of a parameter of the curved target surface.

BRIEF DESCRIPTION OF THE FIGURES

In order that the present disclosure may be more readily understood, preferable features thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is an illustrative cut away view of a printing apparatus;

FIGURE 2 is an enlarged, illustrative view of a monitoring device of the printing apparatus of Figure 1 and a target surface;

FIGURE 3 is an illustrative perspective view of a printhead adjacent a substrate to be printed, to illustrate possible directions of displacement of the printhead and/or a target surface; and

FIGURE 4 is a perspective view of a part of the printing apparatus of Figure 1 . FIGURE 5 is a representation of a monitoring device and a target surface;

FIGURE 6 is a representation of a pair of monitoring devices and a target surface;

FIGURE 7 illustrates a change in signal as the printhead moves relative to a target surface; and

FIGURE 8 depicts a change in signal caused as a monitoring device moves relative to a target surface.

DETAILED DESCRIPTION OF THE DISCLOSURE

Referring to the figures, there is shown a printing apparatus 10, having a body 11 configured to house a first spool 12, and a second spool 14 of inked ribbon or tape 16. Each of the spools 12, 14 is mounted on a respective spindle 18, 20. Each of the first spindle 18 and the second spindle 20 may be driven by a ribbon drive apparatus 22, which may include one or more motors. Rotation of each spindle 18, 20 causes rotation of the respective spool 12, 14, and may be controlled so as to transfer ribbon from one of the first spool 12 and the second spool 14, to the other of the first spool 12 and the second spool 14. Ribbon 16 may be transferred in two directions, i.e. a forward direction and a reverse direction, between the spools 12, 14. The rotation of the or each spindle 18, 20 and/or spool 12, 14 may be controlled and/or monitored in accordance with any appropriate method, for example by a single motor, or a respective motor for each spindle 18, 20, etc.. The printing apparatus 10 may be a print and apply printing apparatus, e.g. operable to print labels for application to another object, or may be operable to print directly on to product packaging, e.g., “overprinting”. The printing apparatus may be a thermal transfer printer. The ribbon drive apparatus 22 is operable to transfer ribbon between one of the first and second spools 12, 14 and the other of the first and second spools 12, 14, past a printhead 24. The printhead 24 may include a plurality of energisable printing elements, for example thermally energisable printing elements, which may be selectively operable to transfer ink from the ribbon 16 to a substrate 26. The substrate 26 may be a roll of material, for example paper, film, e.g., biaxially oriented polypropylene, labels, etc.. The substrate 26 may be provided as discrete items to be printed, e.g., labels, as in “print and apply” applications.

The substrate 26 is supported adjacent the printhead 24 by a substrate support 28. The substrate support 28 may be a platen roller, or a substantially planar platen, for example. The substrate support 28 may be driven by a substrate drive apparatus 29, for example a motor, to advance the substrate 26. It will be appreciated that the substrate drive apparatus 29 need not be positioned in the substrate support 28 and/or need not drive the substrate support 28 directly. The substrate support 28 may be rotated to move the substrate 26 relative to other parts of the printing apparatus 10, for example the printhead 24. The substrate drive apparatus 29 may be operable to advance the substrate 26 into a printing position adjacent the printhead 24. Whilst the substrate drive apparatus 29 is primarily provided to move the substrate in a lateral direction x (see Figure 3), the substrate drive apparatus 29 (and/or additional/alternative substrate drive apparatus, not shown) may be operable to move the substrate 26 in one or more directions and/or along each of the axes x, y, z, as shown in Figure 3.

It will be understood that the substrate support 28 may include a curved support surface - for example, the part of the substrate support 28 which brings the substrate into a printing position relative to the printhead 24 and/or the ribbon 16 may be curved. A platen roller as shown in Figure 1 is substantially cylindrical, and therefore has a curved outer surface which supports the substrate 26.

The printing apparatus 10 may include a controller 30. The controller 30 may be operable to control one or more functions of the printing apparatus 10. For example, the controller 30 may be operable to control the movement of the ribbon 16, for example by controlling the ribbon drive apparatus 22, movement of the printhead 24, and/or the energization of printing elements of the printhead 24. The controller 30 may be operable to monitor parameters and/or functions of the printing apparatus 10. For example, the controller 30 may be operable to receive signals which are indicative of functions and/or parameters of the printing apparatus 10. For example, the controller 30 may be operable to determine and/or monitor the diameters of each spool 12, 14, and/or to determine and/or monitor tension in the ribbon 16 for example. It will be understood that these examples are not intended to be limiting, and any combination of functions of the controller 30 to enable the printing apparatus 10 to carry out printing operations, may be possible.

The printing apparatus 10 may include a monitoring device 40. The monitoring device 40 is shown in illustrative form in Figure 2.

The monitoring device 40 may be operable to monitor one or more parameters or characteristics of one or more parts of the printing apparatus 10, for example the printhead 24 and/or the ribbon 16, and/or an item co-operating with the printing apparatus 10, for example the substrate 26. The monitoring device 40 may be operable to determine and/or monitor one or more characteristics or parameter of a target surface T.

The monitoring device 40 may include a non-contact sensor. The monitoring device 40 may include an emitter 42 and a receiver 44. The monitoring device 40 may include a processor 46, for example a microprocessor. The emitter 42, receiver 44 and the processor 46 may be integrated in a single device, for example a single integrated circuit or chip.

The emitter 42 may be operable to emit an output signal So. The output So may be or include electromagnetic radiation. The output So of the emitter 42 may include coherent light, e.g. the emitter 42 may be or include a laser. The output of the emitter 42 may be in the infra-red range of the electromagnetic spectrum. The wavelength of the output of the emitter 42 may be between approximately 800 and 1000nm, for example 850nm. The monitoring device 40 may include a vertical-cavity surface-emitting laser (VCSEL). A series of output signals may be emitted. The output signal So may be a substantially continuous signal. The output signal So may be a pulsed signal. There may be a time interval t between each output signal So being emitted. The output signal So may be emitted at a substantially constant frequency, with the interval t between each output being substantially the same. The time interval t between successive output signals So may be nominally substantially constant, but it will be appreciated that it may be necessary to identify and account for jitter.

The output signal So of the emitter 42 may be directed towards a target surface T, a parameter or characteristic of which is to be determined and/or monitored. An angle of incidence 0i of the signal So at the target surface T may be in the range 80 to 100°, for example. The angle of incidence 0i of the signal at the target surface T may be approximately 90°.

The target surface T may be a part of the printing apparatus 10, an item which cooperates with the printing apparatus 10 or an item separate from the printing apparatus 10. The target surface T may be the substrate 26, for example. The target surface may be the substrate support 28, for example.

The receiver 44 of the monitoring device 40 may be configured to receive all or part of the output signal So from the emitter 42. The receiver 44 may be configured to receive a reflected portion SR of the signal output S_o from the emitter 42. The receiver 44 may be a CCD or CMOS device, for example. The emitter 42 may be positioned adjacent the receiver 44. The receiver 44 may partially or completely surround the emitter 42. The receiver may be configured to receive a speckle pattern created by the reflection of the output signal So by the target surface T.

The monitoring device 40 may be a miniature chip. The monitoring device 40 may include integrated digital image process circuits. The monitoring device 40 may be a surface mountable component. The monitoring device 40 may be motion detector. The monitoring device 40 may be a laser scatter motion detector. The monitoring device 40 may be of a kind which does not require a lens. The monitoring device 40 may be selected to have an operating region, e.g. depth of field (DOF) appropriate for positioning the monitoring device inside a printing apparatus. The operating region or depth of field of the monitoring device 40 may be selected to be in the range 5mm to 60mm, for example.

A plurality of monitoring devices 40 may be associated with the printing apparatus 10. Each monitoring device 40 may be provided to monitor a particular characteristic or parameter. A plurality of parameters and/or characteristics may be monitored by one or more monitoring devices 40. Each parameter or characteristic to be monitored may be monitored by one monitoring device 40 or a plurality of monitoring devices 40. The number of monitoring devices 40 required to monitor a particular parameter/characteristic may depend on the parameter/characteristic to be monitored and/or the target surface(s) T, and may be selected accordingly. The position of the or each monitoring device 40 relative to other parts of the printing apparatus 10 may be selected in accordance with a primary purpose or function of the monitoring device 40. The monitoring device 40 may be positioned in, on or near the printhead 24. The monitoring device 40 may be moveable relative to other components of the printing apparatus 10 and/or components that co-operate with the printing apparatus 10, for example relative to the substrate support 28. The monitoring device 40 may be mounted on a carriage 25 configured to move the printhead relative to other parts of the printing apparatus 10 and/or co-operating components. The monitoring device 40 may be positioned such that the emitter is operable to emit an output signal So towards the substrate support 28. The monitoring device 40 (or a part of the monitoring device) may be positioned such that the emitter is operable to emit a signal output So towards a top dead centre position of the substrate support 28. This position may or may not be covered by the substrate 26. The monitoring device 40 (or a part of the monitoring device) may be positioned such that the emitter 42 is operable to emit an output signal So towards a position offset from the top dead centre position of the substrate support.

The or each monitoring device 40 may be communicable with the controller 30. A single monitoring device 40 is generally referred to herein, for the sake of simplicity, but it is intended that such references may encompass the use of a plurality of monitoring devices 40, where appropriate.

The monitoring device 40 may be operable to measure changes in the position of the target surface T relative to the monitoring device 40, i.e., the monitoring device 40 may be operable to monitor displacement of the target surface T. For example, the monitoring device 40 may be operable to monitor displacement of the substrate 26 and/or the ribbon 16, relative to the monitoring apparatus 40, and/or relative to another part of the printing apparatus 10. The monitoring device 40 may be operable to determine the speed of the target surface T relative to the monitoring device 40. The monitoring device 40 may move and the target surface may remain stationary and vice versa.

The speed of the target surface T may be an absolute speed, or may be a speed relative to another part of the printing apparatus 10, for example the printhead 24 and/or the monitoring device 40.

The monitoring device 40 may be operable to determine changes (or perceived changes) in position of the target surface T. The target surface T has a reflectivity RT. The reflectivity RT may not be constant, in other words different areas of the target surface may have different reflectivity values. The reflectivity of the surface may also vary with time, for example, the reflectivity of the ribbon 16 may depend on the quantity of ink on the ribbon, and therefore the reflectivity RT of a portion of the ribbon may have one value before ink has been removed during a printing operation, and a different reflectivity RT after ink has been removed. The texture or roughness of the target surface T may affect the reflectivity RT, and/or the signal or pattern reflected by the target surface T.

The target surface T may have a roughness RA. The roughness RA may not be constant, in other words different areas of the target surface may have differing roughness. The roughness RA of the surface may also vary with time, for example, the roughness of the ribbon 16 may depend on the quantity of ink on the ribbon, and therefore the roughness RA of a portion of the ribbon may have one value before ink has been removed during a printing operation, and a different roughness RA after ink has been removed.

The roughness RA may have an order of magnitude which corresponds with a wavelength of the output signal So.

In use, the monitoring device 40 may be operable to acquire sequential surface images (frames) of the target surface T, and by comparing two or more frames with one another, determining one or more of the speed, direction and magnitude of motion of the target surface T relative to the monitoring device 40. This method will be described in more detail below. The monitoring device 40 may be operable to carry out a series of monitoring operations to generate displacement data. During a first monitoring operation, the emitter 42 is configured to emit a first output signal So towards the target surface T, and the receiver 44 is configured to receive the reflected portion SR of the output signal So which is reflected by the target surface T. Data representative of the reflected portion SR, for example a pattern, e.g a speckle pattern created by the reflected portion SR of the output may be stored by the processor 46. A series of monitoring operations may be carried out. Monitoring operations may be carried out multiple times per second, for example several thousand times per second. The monitoring device 40 may emit output signals substantially continuously. In each monitoring operation, the emitter 42 is configured to emit an output signal So towards the target surface T and the receiver 44 is configured to receive a respective reflected portion SR corresponding with each output signal So. The reflected portion SR is reflected by the target surface T, and is characteristic of the portion of the target T surface that the output was directed towards and reflected from. The pattern created by each reflected portion SR, or data that is characteristic of each reflected portion SR may be stored, for example as an image or “frame”, or in any appropriate manner. The pattern or data stored may be stored in memory of the processor 46. The processor 46 may be operable to compare data representative of two or more reflected portions SR, to determine a change or changes in the patterns created, over time. The changes may be caused by the movement of reflective features on the target surface T, since the reflectivity RT of the target surface T is not uniform, and if the target surface T moves (and/or the monitoring device 40 moves) during the interval between the first monitoring operation and the second monitoring operation, the portion of the target surface T struck by the first output signal So, and which reflects the reflected portion SR of the first output signal S_o back to the receiver 44 will be a different portion from the portion which is struck by the second or subsequent output signals in a series. Each reflected portion SR may be an interference pattern, e.g. a speckle pattern. Comparing the interference patterns enables a parameter of the target surface to be determined and/or monitored.

The monitoring device 40 may be operable to report the movement of the target surface T during a monitoring period, as a change in position, or displacement, in one or more dimensions x, y, z since a previous monitoring operation/reading. The monitoring period may be one or more time intervals t, in other words the monitoring period may include a plurality of monitoring operations. Change in position data, i.e. , displacement, of the target surface T relative to the monitoring device 40 may be provided to a controller, e.g., the controller 30. Change in position data determined by the monitoring device 40 and passed to the controller 30 may be read by the controller 30. The monitoring device 40 may pass data to the controller 30 automatically, for example in accordance with a duty cycle and/or in response to a trigger event. Additionally or alternatively, the controller 30 may interrogate the monitoring device for data relating to a change in parameter. Interrogation of the monitoring device 40 by the controller 30 may be automatic, for example in accordance with a duty cycle and/or in response to a trigger event. The term “reading” is intended to mean the receipt of data by the controller 30 from the monitoring device 40, regardless of whether the controller 30 has interrogated the monitoring device 40, or whether the monitoring device 40 has passed the data automatically, for example in accordance with a predetermined schedule, or in response to a trigger event.

The monitoring device 40 may be operable to determine that a change in parameter has been zero since the previous monitoring operation or during a monitoring period. The monitoring device 40 may be operable to indicate to the controller 30 that the change in parameter has been zero since the previous monitoring operation or during a monitoring period. No change in a parameter since the previous monitoring operation or during a monitoring period may be a trigger event. A change in parameter following a period of no change in parameter may be a trigger event. Therefore, monitoring periods may be approximately the same duration, or may be of varying duration. The controller 30 and/or the monitoring device 40 may determine or set the duration of each monitoring period.

Change in parameter data, e.g., displacement, of the target surface T relative to the monitoring device 40, may be summed by the processor 46, and the summed values may be passed to the controller 30. Change in position data in one, two or three dimensions may be provided. The change in one dimension may be zero, whilst the change in position in another dimension may be non-zero during the same monitoring period or time interval t. The monitoring device 40 may be operable to indicate that it has detected a change in parameter, e.g. when it has detected movement (displacement) of the target surface T in at least one dimension. If, for example, the target surface T is stationary for a period of time, the monitoring device 40 may carry out a number of monitoring operations during that period of time, and no displacement will be detected. The monitoring device 40 may not pass the data relating to the time during which the target surface is stationary to the controller 30. If at the end of the period of time during which the target surface was stationary, the monitoring device 40 detects or infers that the target surface T has moved in at least one dimension, this change may trigger the monitoring device 40 to pass the data relating to the change in position (displacement) to the controller 30. The controller 30 may be operable to use a single datum indicative of a change, for example a datum which indicates that the target surface T has moved by a distance d relative to the monitoring device 40. in at least one of the dimensions x, y, z during the most recent time interval. The time interval may be set by the monitoring device 40.

By carrying out monitoring operations and/or readings at regular intervals, the speed of the target surface T can be calculated, since the frequency of the monitoring operations and/or the frequency at which the data provided by the monitoring device 40 is sampled (read) by the controller 30 is known. The controller 30 receiving the change in position data may process the data and use the change in parameter (e.g., position) data to control aspects of the printing process, and the operation of the printing apparatus. The time interval between each monitoring operation by the monitoring device and/or between each sample (reading) by the controller 30 may or may not be the same. A clock of the controller 30 may be synchronized with a clock of the monitoring device 40, in order to be able to carry out accurate calculations, for example to determine the speed and/or acceleration of the target surface T relative to the monitoring device 40.

Displacement data obtained by the monitoring device 40 may be sent to the controller 30, and used by the controller 30 in calculations of other parameters/characteristics of the printing apparatus 10 and/or an associated item, e.g. the substrate 26, and/or may be used to determine control functions of the printing apparatus 10, e.g. movement of the ribbon 16, to enable printing operations to be carried out. As explained briefly above, a parameter or characteristic may be monitored by a plurality of monitoring devices 40. A monitoring array may include a plurality of monitoring devices 40. Each monitoring device 40 in the monitoring array may be spaced from one or more adjacent monitoring devices 40 by a distance less than 10mm, for example 6mm. The plurality of monitoring devices 40 may be arranged in any appropriate manner. The arrangement of the plurality of monitoring devices 40 may be a line, a rectangular array, a substantially circular array, etc.. The arrangement and/or type of the monitoring devices 40 in the monitoring array may depend upon the target surface T being monitored and/or the parameter of the target surface T being monitored.

A plurality of monitoring arrays may be provided. The plurality of monitoring arrays may be operable to monitor a plurality of target surfaces and/or a plurality of parameters, and/or to provide data indicative which may be used to determine one or more further parameters. Where a monitoring device 40 is referred to herein, the skilled person will understand that a single monitoring device 40 may be used or a monitoring array may be used. The most appropriate arrangement of monitoring devices 40 may be selected dependent upon the space available, and/or the parameter/characteristic to be monitored, and/or the accuracy of the monitoring required, and/or the target surface(s) T to be monitored. For example, a matte target surface may be optimally monitored using a monitoring array, to provide sufficiently accurate data indicating a parameter of the surface (it will be understood that this simply a suggestion of a possibility, rather than an indication of an essential feature).

Where a monitoring array is provided, the controller 30 may be operable to distinguish between data provided by each monitoring device 40 of the monitoring array. The controller 30 may be operable to combine data from some or all of the monitoring devices 40 of the monitoring array to determine further parameters of the target surface and/or a component of the printing apparatus associated with the target surface, and/or determine control functions of the printing apparatus 10, to enable printing operations to be carried out. The further parameter may be a radius of curvature, speed or acceleration for example. The associated component may be the support surface 28, for example. The target surface T towards which the signal of the monitoring device 40 is directed may be the substrate 26. The monitoring device 40 may be operable to determine a parameter/characteristic of the substate 26, for example the displacement and/or the speed and/or the acceleration of the substrate 26. The target surface T may be a surface which is able to give signals or readings which are indicative of a parameter/characteristic of the substrate 26. For example, the target surface T may be a part of the substrate support 28.

In order to determine a parameter/characteristic of the substrate 26 and/or the substrate support 28, the monitoring device 40 may be positioned inside the body 11 of the printing apparatus 10. A part of the monitoring device 40 may be positioned inside the body 11 of the printing apparatus 10. Alternatively, the monitoring device 40 or a part thereof may be positioned externally of the body 11 of the printing apparatus 10. The monitoring device 40 may be positioned on a carriage 25 upon which the printhead 24 is mounted. The carriage 25 may be moveable relative to the body 11 and/or the substrate support 28. The monitoring device 40 may be spaced a predetermined distance longitudinally from the target surface T. The longitudinal distance between the monitoring device 40 and the target surface may be substantially constant. The longitudinal distance between the monitoring device 40 and a centre (or origin O) of the substrate support 28 may be substantially constant. A distance between the monitoring device 40 (e.g. a distance between the emitter 42 and/or the receiver 44 and the printhead 24, e.g. the centre line of the printhead 24, may be known. The monitoring device 40 may be spaced between approximately 10mm and approximately 40mm from the target surface T, e.g. the substrate 26 or the substrate support 28. The monitoring device 40 may be positioned such that the angle of incidence 0i of the output signal emitted by the monitoring device 40 at the target surface T is approximately 90°. The monitoring device 40 may be mounted adjacent the printhead 24. The monitoring device 40 or a part thereof may be mounted on or in the printhead 24. The monitoring device 40 may be positioned at or near a centre line of the printhead 24. The monitoring device 40 may be positioned such that the or each output signal So of the monitoring device 40 is incident on the target surface T at a position which is substantially aligned with one or more printing elements of the printhead 24. In a printing position, the printhead 24 may be aligned with the ribbon 16 and the substrate 26, so as to enable printing operations to be carried out. In the printing position, the monitoring device 40 may be positioned such that the output signal may be emitted towards a part of the substrate 26 and/or a part of the substrate support 28, that are not covered by the ribbon 16. It will be understood that since a plurality of monitoring devices 40 may be provided, a monitoring device 40 may be configured to monitor a parameter of the ribbon 16 and a separate monitoring device 40 may be configured to monitor a parameter of the substrate 26 (and/or a parameter that is indicative of a parameter of the substrate 26. The method of determining a characteristic, e.g displacement, speed, acceleration, of the substrate 26 may be as described above. The monitoring device 40 may be positioned such that the output signal So is incident on the substrate 26, but not on the ribbon 16, despite the ribbon 16 being positioned adjacent (e.g. overlying) the substrate 26 at a position near the printhead 24, and therefore near the monitoring device 40. The monitoring device 40 may be positioned such that the output signal So is incident on the substrate support 28 but not on the ribbon 16 or on the substrate 26, despite the substrate 26 and/or the ribbon overlying part of the substrate support 28. The position of the or each monitoring device 40 must be selected carefully to ensure appropriate data collection.

The positioning of the monitoring device 40 may be adjustable. The position of the monitoring device 40 may be automatically and/or manually adjustable. The position of the monitoring device 40 may be adjustable in one or more of the x, y and Z directions. The position of the monitoring device 40 may be adjustable such that the angle of incidence 0i of the signal emitted by the monitoring device 40 at the target surface T is adjustable.

The target surface T may be the ribbon 16. The monitoring device 40 may determine and/or monitor the displacement and/or speed of the ribbon 16 directly. The monitoring device 40 may be mounted inside the body 11 of the printing apparatus 10. The monitoring device 40 may be positioned such that the angle of incidence 0i of the signal emitted by the monitoring device 40 at the ribbon 16 is approximately 90°. The monitoring device 40 may be mounted on or near the printhead 24, for example on the printhead carriage 25, to determine a parameter of the ribbon 16 at or near the printhead 24. The method of determining a characteristic, e.g displacement, speed and/or acceleration of the ribbon 16 may be as described above generally, and in relation to the substrate 26.

The monitoring device 40 may be positioned adjacent one of the first and second spools 12, 14. A monitoring device 40 may be associated with each of the first and second spools 12, 14. The or each monitoring device 40 may provide an indication of displacement (i.e. rotation) of the associated spool 12, 14, and/or the speed of rotation of its associated spool 12, 14. This may include determining the displacement and/or speed of rotation of an outer surface of the ribbon 16. A linear displacement of ribbon 16 near to the or each spool 12, 14 may be monitored. Such linear displacement may be indicative of the amount of ribbon 16 wound on to or off the associated spool 12, 14. The angle of rotation of the spool 12, 14 may be known, therefore the diameter of the spools may be calculated from the monitored linear displacement of ribbon 16 near the spool. The monitoring device 40 may be operable to determine a rotational displacement of the outer surface of one or both spools 12, 14 in order to determine the circumference of the or each spool 12, 14. The determination of the speed of rotation and/or the diameter of the or each spool 12, 14 may be carried out as a calibration step. Determination of the speed of rotation and/or the diameter of the or each spool 12, 14 may take place in a very short space of time, which may be the time taken for a few millimeters, for example less than 10mm, of ribbon to be wound on to and/or unwound from one of the spools 12, 14. This enables virtually instantaneous determination of the diameters of the spools 12, 14. This reduces the time spent carrying out the calibration process, and removes the need to transfer the ribbon in both directions. The ability of the monitoring device 40 to directly monitor characteristics/parameters such as displacement, means that determination of a characteristic of the or each spool 12, 14 may take place during operation of the printing apparatus 10, rather than during a calibration step. Monitoring of one or more parameters/characteristics may take place continually (i.e. repeatedly) or substantially continuously during use of the printing apparatus 10. Such monitoring during use of the printing apparatus 10 may be instead of or in addition to a calibration step or steps.

The ability of the or each monitoring device 40 to directly determine characteristics/parameters of the or each spool 12, 14 means that ancillary parts, for example a monitoring roller and/or a Hall effect sensor are not required to determine the relative speeds of the spools 12, 14, for example. The speed of one of or each of the spools 12, 14 (and/or the relative speed of the spools 12, 14, and/or a signal indicative of the speed of each spool 12, 14) may be communicated to the controller 30. The controller 30 may use this information relating to the speed of the or each spool 12, 14 to determine control of the ribbon drive, for example to determine the control of a motor drive circuit of the ribbon drive apparatus 22.

The monitoring device 40 may be operable to determine a characteristic, e.g. the position, displacement and/or speed of the printhead 24. The monitoring device 40 may be operable to determine displacement of the printhead 24 relative to another part of the printing apparatus 10 and/or a component that co-operates with the printing apparatus 10, for example. In this case, the monitoring device 40 may be in a fixed position relative to the printhead 24, e.g. attached to the printhead 24 or the printhead carriage 25, and the target surface T may be another part of the printing apparatus 10, for example a part of the body 11 or a component that co-operates with or is associated with the printing apparatus 10. Alternatively, the monitoring device 40 may be mounted on a (preferably stationary) part of the printing apparatus 10, for example the body 11 , and the target surface T may be a part of the printhead 24. This is useful in determining whether the printhead 24 is in the correct position relative to the ribbon 16, and/or the substrate 26, and/or one or more other parts of the printing apparatus 10, e.g. the substrate support 28.

The monitoring device 40 may be used to determine the position and/or displacement of the printhead 24 into and out of the printing and non-printing positions. The monitoring device 40 and/or the controller 30 may be operable to determine the speed and/or acceleration of the printhead relative to the substrate support 28, for example. As mentioned above, it is desirable to be able to determine the position of the printhead 24accurately. The monitoring device 40 may be operable to determine an absolute position of the printhead 24, and/or be able to determine the position of the printhead 24 relative to another object, for example another part of the printing apparatus 10. The monitoring device 40 may be operable to determine the position of the printhead relative to the ribbon 16 and/or the substrate 26 and/or the substrate support 28, for example. The monitoring device 40 may be operable to determine a distance of the printhead 24 from another part of the printing apparatus, for example one or more of the ribbon 16, the substrate 26 and the substrate support 28. The monitoring device 40 may be operable to determine a lateral position and/or lateral movement of the printhead 24 along a first axis x, which may be substantially parallel to the direction of travel of the ribbon 16 and/or the substrate 26; along a second axis Y which may be substantially perpendicular to the first axis x; and/or along a third axis z, which may be substantially parallel to each of the first and second axes x, y, i.e. towards and away from the ribbon 16 and/or the substrate 26, as depicted in Figure 3.

The monitoring device 40 may additionally or alternatively be able to determine an angle of rotation of the printhead 24, and/or an angle of the printhead 24 relative to one or more of the ribbon 16, the substrate 26 and the substrate support 28. Since the monitoring device 40 may be operable to directly determine the displacement of the printhead 24, the position of the printhead 24 may be more accurately determined. The monitoring device 40 may enable the displacement and/or position of the printhead 24 to be determined directly and accurately without the need for load sensors, and without reliance upon measuring or determining parameters which are indicative of the position of the printhead 24.

This is particularly relevant to ensure that printing operations are carried out accurately and in the correct place. Determining the position, displacement or speed of the printhead 24 along any or all of the axes x, y, z may be beneficial in ensuring that the correct printing position and hence printing pressure is achieved during a printing operation, such that the quality of the print is optimised.

As described above, the monitoring device 40 determines or infers movement of the target surface T by comparing multiple readings of reflected portions SR of output signals So directed towards the target surface. The monitoring device 40 may operate on the principle of decorrelation of a moving speckle pattern. The movement of the target surface T determined by the monitoring device 40 is typically assumed to be linear. However, where the target surface T is curved, or includes a curved portion, for example where the target surface T is supported by a curved support, e.g the substrate 26 is supported by a curved support surface 28 such as a platen roller, or where the target surface T is formed by a material wound around a curved core, for instance ribbon 16 wound around a substantially cylindrical core, determining the movement of the target surface T, e.g. the distance actually moved and/or speed of the target surface T is not a trivial calculation. The radius of curvature of the target surface T may be taken into account in order to improve the accuracy of the determined movement.

For a given angular rotation 0 of the target surface T (for example caused by rotation of the support surface 28) the monitoring device 40 determines a movement s which is a curve. Where the target surface T describes a curve, the angular rotation may be 0.

Determining the angular movement of the target surface (for example by monitoring the rotation of the support surface 28 and/or the substrate support drive apparatus 29), may enable the arcuate distance s moved to be determined, since s = Qn where H is the radius of curvature of the target surface T at the observed point, i.e. the position on the target surface T where the output signal So is incident.

The radius of curvature H of the target surface T may be substantially constant, and is the distance from an origin 0 to the target surface T. The origin 0 may be the centre of a circle, for example where the target surface T is substantially circular, or includes a substantially circular portion. There may be a gap r_g between the monitoring device 40 and the target surface T. The gap r_g may be a print gap - e.g. the distance between a printing position and a ready to print position, e.g. when the printhead 24 is in contact with the printing surface (i.e. the substrate support 28), and when the printhead 24 is spaced from the printing surface to allow relative (nonprinting) movement between the printhead 24 and the ribbon 16 and/or the substrate 26, respectively. The gap r_g may be measured substantially perpendicularly from a tangent of the target surface T struck at the point on the target surface T where the output signal So is incident on the target surface T. A distance r_s between the monitoring device 40 and the origin 0 may be the sum of the gap r_g and the distance from the origin 0 to the target surface T, i.e.: r_s = ri + r_g The output signal S_o is directed towards the origin 0. When the output signal reaches the target surface T, the reflected portion SR is reflected back towards the monitoring device 40, such that it appears that the features appear to have come from the origin 0. If the target surface T is rotating at co radians per second, the linear speed v across the sensor may be expressed as follows:

V = (jJKs

A pair of monitoring devices 40a, 40b may be used to determine the true circumferential movement of the target surface T, as shown in Figure 6.

The determined circumferential movement s can be expressed as: s = rd

A first monitoring device 40a is positioned a distance L from the origin 0. A second monitoring device 40b is position a distance d + L from the origin 0, where d is a “radial” distance between the first and second monitoring devices 40a, 40b. The first and second monitoring devices 40a, 40b may be spaced apart laterally by a distance e. The distance e may be a small distance compared with the distance L, for example. The distance e may be as small as physically possible given the configuration of the monitoring devices 40a, 40b.

The radius r of the target surface T may be unknown. The radius r may be the radius of the substrate support 28, or the outer surface of one of the spools 12, 14 of ribbon 16 for example. The distance L may be unknown. The distance d may be known. The distance e may be known.

The target surface T may be moved, during which time the first and second monitoring devices 40a, 40b monitor movement corresponding to an angular rotation 9. The first monitoring device 40a may record a movement si and the second monitoring device 40b may record a movement S2. The determined displacements may be expressed as: si = QL and s₂ = Q(L+d)

Solving for rotation 0,

Q > ^S1 _ ^s2

~ L ~ (L+d)

From which it is possible to show that:

Si cl L = 7 - ¹ - 7

0 2 - 51)

Monitoring operations may be repeated above for statistical confidence and/or noise reduction, for example.

Once the distance L to the origin 0 has been determined as shown, the radius r of the target surface T may be derived either by:

A) an operator entering the measured radius of the support surface 28 or

B) using movement of the printhead 24 to determine the distance r = L - r_g.

Since the printhead 24 may be mechanically coupled to the monitoring devices 40a, 40b by known offsets, L may be derived as explained above, then the printhead 24 may be moved until it is in contact with the support surface (although the substrate 26 and ribbon 16 will be between the printhead 24 and the support surface 28, their thickness is negligible), and subtracting the distance moved from the distance L gives the radius of curvature of the target surface T, which may be the radius of the substrate support 28. Method B) has the advantage that real time flex and deformation in the support surface 28 changing the distance between the printhead 24 and the target surface T can be tracked and accounted for during printing operations.

The distance L (or d + L) between the monitoring device 40 and a centre of curvature (origin, 0) of the target surface T may be determined as follows. The distance between the monitoring device 40 and the centre of curvature 0 may be determined in a calibration step.

The monitoring device 40 may be moved in a first direction A, relative to the target surface T. The monitoring device 40 may be moved substantially linearly relative to the target surface T. Where the target surface T is the substrate 26, or the substrate support 28, the monitoring device 40 may be mounted on or adjacent the printhead 24. For example, the monitoring device 40 may be mounted on the printhead carriage 25 which is arranged to move the printhead 24 relative to the substrate support 28 (as well as the substrate 26 and the ribbon 16), for example before, during and/or after a printing operation. During a determination period, the monitoring device may be swept across the target surface T, and may be operable to carry out successive measurement operations or readings during the determination period. Referring to Figure 7, it can be understood that as the monitoring device 40 passes a curved target surface T, for example in the direction of the arrow A, it carries out a series of measurement operations, emitting output signals So in a direction that is generally perpendicular to the direction of travel of the monitoring device 40. The reflected portion SR of each output signal So will be received by the receiver 44. The speed at which the monitoring device moves relative to the target surface in the first direction (i.e. the direction indicated by arrow A) is known or may be assumed. The speed at which the monitoring device 40 moves should ideally be substantially constant. It must also be assumed that the target surface T is stationary during the determination period, for example not rotating.

When the monitoring device 40 is not aligned with the target surface T, the signal strength of the reflected signal(s) SR will be relatively weak, or there may be no reflected signal SR received by the receiver 44. When the monitoring device 40 is aligned with the target surface T, the signal strength of the reflected portion(s) SR will be relatively strong. The signal strength of the reflected portion(s) SR may be strongest when the monitoring device 40 is closest to the target surface T. The monitoring device 40 may be closest to the target surface T when the monitoring device 40 is substantially aligned with a top dead centre position of the target surface T.

The monitoring device 40 receives a plurality of readings, and determines the change in distance between the target surface T and the monitoring device 40. The target surface 40 may appear as though it is moving relative to the monitoring device 40 in a longitudinal direction, i.e. in a direction substantially parallel to the signal So. The processor 46 may use edge detection to determine a change in signal strength, which corresponds with the monitoring device passing from a region where it is not aligned with the target surface T to a region where it is aligned with the target surface T, and vice versa (see Figure 7). The monitoring device 40 may be moved relative to the target surface (e.g the substrate support 28) at a substantially constant speed, for example between 6mm/s and 8mm/s.

Example readings are shown in Figure 8. Each trace illustrates a “pass” of the monitoring device 40 relative to the target surface T. The monitoring device was moved at a speed of approximately 7mm/s relative to the target surface T. For the first approximately 0.4s, the monitoring device 40 receives no reflected signal SR from the target surface T. Reflected signals SR are then detected between approximately 0.45s and 0.85s, and then the reflected signal strength returns abruptly to zero (or substantially zero). Since the travel speed of the monitoring device 40 is known, and the time during which reflected signals are received can be calculated, the length (e.g. diameter) of the target surface T may be calculated as follows:

Where the target surface is curved, for example substantially circular, as the substrate support 28 may be, the length (L) may be the diameter of the target surface, for example the diameter of the substrate support 28. The movement of the monitoring device 40 relative to the target surface T may be repeated. Figure 8 shows a plurality of traces. The first set of traces (between times 0.4 and 1s are signals received whilst the monitoring device 40 moved past the target surface T in a first direction and the second set of traces (between times 1 .2 and 2s are signals received whilst the monitoring device 40 moved past the target surface T in a second direction. For example, the printhead carriage 25 may be moved past the substrate support 28 in a first direction until reflected signals are no longer received, and then moved back past the substrate support 28 in the opposite direction. Repeated results may improve accuracy. Repeated results may be averages or analysed in any appropriate way to obtain a value for the length L of the target surface T.

The length L may be halved to give a value for the radius r of the target surface, e.g. the substrate support 28 or the spools 12, 14.

This method may be used to determine a top dead centre position of the substrate support 28. The top dead centre position will be half way across the determined diameter of the substrate support 28, in the direction of substrate (and ribbon) movement.

Once the distances L and r have been determined, the second monitoring device 40b may be disabled to reduce risk of interference with the first monitoring device 40a, or may remain operable to provide redundancy, for example.

The ability of the or each monitoring device 40 to directly determine characteristics/parameters of the or each spool 12, 14 means that ancillary parts, for example a monitoring roller and/or a Hall effect sensor are not required to determine the relative speeds of the spools 12, 14, for example. The speed of one of or each of the spools 12, 14 (and/or the relative speed of the spools 12, 14, and/or a signal indicative of the speed of each spool 12, 14) may be communicated to the controller 30. The controller 30 may use this information relating to the speed of the or each spool 12, 14 to determine control of the ribbon drive, for example to determine the control of a motor drive circuit of the ribbon drive apparatus 22. The monitoring device 40 may be operable to determine a characteristic, e.g. the position, displacement and/or speed of the printhead 24. The monitoring device 40 may be operable to determine displacement of the printhead 24 relative to another part of the printing apparatus 10, for example. In this case, the monitoring device 40 may be in a fixed position relative to the printhead 24, e.g. attached to the printhead 24, and the target surface T may be another part of the printing apparatus 10, for example a part of the body 11. Alternatively, the monitoring device 40 may be mounted on a (preferably stationary) part of the printing apparatus 10, for example the body 11 , and the target surface T may be a part of the printhead 24. This is useful in determining whether the printhead 24 is in the correct position relative to the ribbon 16, and/or the substrate 26, and/or one or more other parts of the printing apparatus 10, e.g. the substrate support 28.

The monitoring device 40 may be used to determine the position and/or displacement of the printhead 24 into and out of the printing and non-printing positions. The monitoring device 40 and/or the controller 30 may be operable to determine the speed and/or acceleration of the printhead relative to the substrate support 28, for example. As mentioned above, it is desirable to be able to determine the position 24 of the printhead accurately. The monitoring device 40 may be operable to determine an absolute position of the printhead 24, and/or be able to determine the position of the printhead 24 relative to another object, for example another part of the printing apparatus 10. The monitoring device 40 may be operable to determine the position of the printhead relative to the ribbon 16 and/or the substrate 26 and/or the substrate support 28, for example. The monitoring device may be able to determine a size of a part of the printing apparatus 10 and/or a part that co-operates with the the printing apparatus, for example one or more spools and/or a substrate support. The monitoring device 40 may be operable to determine a top dead centre position of the printing surface (e.g. the substrate support 28). The monitoring device 40 may be operable to determine the position of the printhead relative to the target surface, for example a printing surface. The monitoring device 40 may be operable to determine and/or monitor the position of the printhead 24 relative to the top dead centre position of the printing surface.

The monitoring device 40 may be operable to determine a distance of the printhead 24 from another part of the printing apparatus, for example one or more of the ribbon 16, the substrate 26 and the substrate support 28. The monitoring device 40 may be operable to determine a lateral position and/or lateral movement of the printhead 24 along a first axis x, which may be substantially parallel to the direction of travel of the ribbon 16 and/or the substrate 26; along a second axis Y which may be substantially perpendicular to the first axis x; and/or along a third axis z, which may be substantially parallel to each of the first and second axes x, y, i.e. towards and away from the ribbon 16 and/or the substrate 26, as depicted in Figure 3.

The monitoring device 40 may additionally or alternatively be able to determine an angle of rotation of the printhead 24, and/or an angle of the printhead 24 relative to one or more of the ribbon 16, the substrate 25 and the substrate support 28. Since the monitoring device 40 is operable to directly determine the displacement of the printhead 24, the position of the printhead 24 may be more accurately determined. The monitoring device 40 enables the displacement and/or position of the printhead 24 to be determined directly and accurately without the need for load sensors, and without reliance upon measuring or determining parameters which are indicative of the position of the printhead 24.

The ability to monitor characteristics, e.g. position, displacement, speed and acceleration during use of the printing apparatus 10, means that the operation of the printing apparatus 10 may be more accurate. It is not necessary to rely on assumptions or approximations, or even calibration data, since directly monitored characteristics can be used in real time, or close to real time.

Any combination of characteristics and/or parameters described above may be monitored by one or more monitoring devices 40. The controller 30 may use data obtained by the or each monitoring device to determine and/or monitor one or more parameters and/or characteristics and/or one or more further parameters/characteristics. Data relating to any or all of the characteristics/parameters determined/monitored by the or each monitoring device 40 may be passed to the controller 30 of the printing apparatus 10 and may be stored and/or processed and/or utilized in determining operational functions of parts of the printing apparatus 10, e.g. control of the ribbon drive apparatus 22, energization of the printing elements of the printhead 24, control of the substrate drive apparatus 29, etc..

An advantage of the monitoring device 40 being a non-contact sensor device is that the problem of slippage of the material (e.g., ribbon or substrate) around the roller/wheel which can occur with encoders is removed. There is no contact between the material (e.g. ribbon 16 or substrate 26) with a wheel or roller, and therefore the likelihood of damaging the material is reduced or eliminated. There are fewer moving and/or contacting parts, which reduces the need for maintenance and repair of the printing apparatus 10. This leads to reduced “down-time” and increased efficiency of the production line in which the printing apparatus 10 is installed.

The monitoring device may be surface mountable, such that a risk of disturbance, misalignment or damage during or after installation is reduced as compared with other types of monitoring device, such as an encoder.

Positioning the monitoring device 40 inside the printing apparatus 10, i.e. , integrating the monitoring device 40 with the printing apparatus 10 is advantageous since the monitoring device 40 does not have to be installed, connected or configured separately from the printing apparatus 10, by an installation engineer. The printing apparatus 10 is able to carry out printing operations without requiring data from an external component, e.g. an encoder. There is no wiring external to the printing apparatus, meaning that the printing apparatus 10 is “plug and play”, for example with regard to its ability to monitor parameters relating to the substrate and/or ribbon. The monitoring device 40 is also protected by the body 11 of the printing apparatus 10 during installation and/or use of the printing apparatus 10.

It will be understood that the target surface T may be inside or outside of the body 11 , and the target surface T need not be a part of the printing apparatus per se. The substrate 26, for example, may be considered to co-operate with or be associated with the printing apparatus 10, or may be considered to be a part of the printing apparatus 10. The ribbon 16, is an essential feature of a thermal transfer printing operation, but is a consumable item which may be provided separately from the printing apparatus 10, and therefore may be considered to co-operate or be associated with the printing apparatus 10, rather than being considered a part of the printing apparatus 10, per se. It will be appreciated that the ribbon 16 may be considered a part of the printing apparatus 10.

Using the monitoring device 40 to carry out monitoring operations at a nominally consistent rate means that there is a substantially constant processor load, regardless of the parameter/characteristic being measured, e.g. regardless of the substrate and/or ribbon speed. This is an advantage compared with using an encoder.

Using a plurality of monitoring devices 40 in a monitoring array may allow for redundancy, and may also increase accuracy, by being able to account for/correct for noise and/or other errors in data output by each monitoring device 40. Signal processing may be used to account for/correct for noise and/or other errors. This may increase the accuracy of the data received by the controller 30, and therefore lead to improved performance of the printing apparatus.

An advantage of the emitter 44 being a coherent light source, for example a laser, is that a plurality of monitoring devices 40 may be positioned adjacent one another, relatively close to one another, for example less than 10mm apart, for example 6mm apart. A further advantage is that a lens is not required to form part of the or each monitoring device. The monitoring devices 40 may be arranged in an array as explained above. To the extent that it may be necessary to shield each monitoring device 40 from other nearby monitoring devices 40 or other monitoring arrays, it may be possible to use light guides to direct the output(s) of each emitter 42 towards the corresponding receiver 44.

An advantage of the emitter 42 being a vertical-cavity surface-emitting laser is that such emitters are highly reliable, and not susceptible to damage of an exit aperture of the emitter 42.

The or each monitoring device 40 outputting coherent electromagnetic radiation, e.g. a laser, enables the monitoring device to be operable through a substantially transparent shield. The monitoring device 40 does not require a lens, and therefore may be placed behind or inside a shield device without the need for other components (e.g., a lens) to also be placed behind or inside the shield. The or each monitoring device 40 may be covered by or housed in a shield apparatus. The shield apparatus may protect more than one monitoring device 40. The shield apparatus may be substantially transparent, or include a substantially transparent portion, through which outputs So and reflected portions SR may pass. The shield may be a box. The shield may include a portion of substantially transparent plastics material. The shield apparatus may be hermetically sealed. Providing a shield apparatus enables the or each monitoring device 40 to be positioned inside the printing apparatus 10 whilst the risk of contamination of the monitoring device 40 is reduced. Even if the shield apparatus becomes contaminated, the shield apparatus may be cleaned and the monitoring device 40 will not be damaged by the original contaminant (e.g., ink) or the cleaning method/materials. The shield may be removeable and/or replaceable.

The monitoring device 40 saves space since it can be placed inside the printing apparatus 10, and therefore takes up less space in a production line (no external components/cables). Whilst some space is taken up by the monitoring device 40, the use of a laser emitter means that no lens is required, such that the space taken up by the monitoring device 40 is small compared with other types of sensors, e.g., optical sensors. The monitoring device 40 is very small compared with other types of monitoring device, for example an encoder.

The applicants have determined that despite the type of monitoring device described herein being typically used in applications with highly reflective surfaces, it is possible to use a sensor of this type with target surfaces having a typically lower reflectivity, for example inked ribbon and matte substrates. The maximum speed of the target surface T may be selected and/or set in accordance with the reflectivity of the target surface T. For example a glossy target surface may have a higher reflectivity than, say, matte paper. The selection of an appropriate monitoring device may be dependent on the maximum speed of the target surface relative to the monitoring device. The monitoring device 40 may be operable to monitor a target surface T moving at speeds in the range 0.5ms’¹ to 2.3ms’¹ relative to the monitoring device 40. An alternative monitoring device may be provided, instead of, or in addition to one or more monitoring devices 40. In applications where it may not be appropriate to include a monitoring device 40 (as described above) inside the printing apparatus 10, an optical monitoring device may be provided, for example in combination with a lens, to focus the output emitted and/or the reflected portion of the output. The emitter may be an LED, for example. Such an optical monitoring device being a non-contact sensor device still provides the advantage that the problem of slippage of the material (e.g., ribbon or substrate) around the roller/wheel which can occur with encoders is removed. There is no contact between the material (e.g. ribbon 16 or substrate 26) with a wheel or roller, and therefore the likelihood of damaging the material is reduced or eliminated. There are fewer moving and/or contacting parts, which reduces the need for maintenance and repair of the printing apparatus 10. This leads to reduced “down-time” and increased efficiency of the production line in which the printing apparatus 10 is installed.

Use of such an optical sensor may be appropriate to replace existing apparatus, e.g a reference roller and encoder/proximity sensor, for determining displacement and/or speed of the substrate and/or linear displacement/speed of the ribbon.

Speed of calibration may be increased using such an optical sensor, since it is possible to determine parameters substantially in real time. Monitoring of param eters/characteristics may take place during use of the printing apparatus 10 for printing operations. It may be possible to dispense with “traditional" calibration methods entirely, by monitoring parameters/characteristics substantially in real time.

The ability to monitor characteristics, e.g. position, displacement, speed and acceleration during use of the printing apparatus 10, for example during printing operations and/or in the time between printing operations, means that the operation of the printing apparatus 10 may be more accurate. It is not necessary to rely on assumptions or approximations, or even calibration data, since directly monitored characteristics can be used in real time, or close to real time.

Positioning the monitoring device 40 inside the printing apparatus 10, i.e. , integrating the monitoring device 40 with the printing apparatus 10 is advantageous since the monitoring device 40 does not have to be installed, connected or configured separately from the printing apparatus 10, by an installation engineer. The printing apparatus 10 is able to carry out printing operations without requiring data from an external component, e.g. an encoder. There is no wiring external to the printing apparatus 10, meaning that the printing apparatus 10 is “plug and play”, for example with regard to its ability to monitor parameters relating to the substrate 26 and/or the ribbon 16. The monitoring device 40 is also protected by the body 11 of the printing apparatus 10 during installation and/or use of the printing apparatus 10.

It will be understood that the target surface T may be inside or outside of the body

11 , and the target surface T need not be a part of the printing apparatus per se. The substrate 26, for example, may be considered to co-operate with or be associated with the printing apparatus 10, or may be considered to be a part of the printing apparatus 10. The ribbon 16, is an essential feature of a thermal transfer printing operation, but is a consumable item which may be provided separately from the printing apparatus 10, and therefore may be considered to co-operate or be associated with the printing apparatus 10, rather than being considered a part of the printing apparatus 10, per se. It will be appreciated that the ribbon 16 may be considered a part of the printing apparatus 10.

Using a plurality of monitoring devices 40 in a monitoring array may allow for redundancy, and may also increase accuracy, by being able to account for/correct for noise and/or other errors in data output by each monitoring device 40. Signal processing may be used to account for/correct for noise and/or other errors. This may increase the accuracy of the data received by the controller 30, and therefore lead to improved performance of the printing apparatus 10.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The invention may also broadly consist in the parts, elements, steps, examples and/or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and/or features. In particular, one or more features in any of the examples described herein may be combined with one or more features from any other examples described herein. Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure.

Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and/or to encompass equivalents.

Claims

1. A printing apparatus including a printhead and a monitoring device, the monitoring device including an emitter, a receiver, and a processor; the emitter being operable to emit a first output and a second output towards a curved target surface, and the receiver being operable to receive first data corresponding to a reflected portion of the first output and data corresponding to a reflected portion of the second output, each reflected portion having been reflected by the target surface, wherein each output emitted by the emitter is electromagnetic radiation, and wherein the processor is operable to compare the data corresponding to the reflected portion of the first output and the data corresponding to the reflected portion of the second output to determine a parameter of the curved target surface.

2. A printing apparatus according to claim 1 including a controller operable to use data indicative of the parameter of the curved target surface, to control one or more operations of the printing apparatus.

3. A printing apparatus according to claim 2 wherein the operation of the printing apparatus that is controlled is movement of the printhead.

4. A printing apparatus according to claim 2 or 3, wherein the printing apparatus is a transfer printing apparatus including supports for carrying spools of ribbon, wherein the operation of the printing apparatus that is controlled is the rotation of one or more of the spool supports so as to enable the movement of ribbon from one spool towards the other.

5. A printing apparatus according to any of the previous claims, wherein the parameter of the target surface is one or more of position, speed, acceleration or a size of the target surface.

6. A printing apparatus according to any of the preceding claims wherein the curved target surface is one of a ribbon, a substrate and a substrate support. A printing apparatus according to claim 6 wherein the parameter is a diameter of a platen roller. A printing apparatus according to any of the preceding claims wherein the monitoring device includes a laser scatter sensor. A printing apparatus according to any of the preceding claims wherein the printhead is moveable with the printhead relative to the target surface. A method of operation of a printing apparatus including a printhead and a monitoring device positioned adjacent the printhead, the monitoring device including an emitter, a receiver, and a processor; the emitter being operable to emit a first output and a second output towards a curved target surface, and the receiver being operable to receive first data corresponding to a reflected portion of the first output and data corresponding to a reflected portion of the second output, each reflected portion having been reflected by the target surface, wherein each output emitted by the emitter is electromagnetic radiation, and wherein the processor is operable to compare the data corresponding to the reflected portion of the first output and the data corresponding to the reflected portion of the second output to monitor a parameter of the curved target surface. A method according to claim 10 wherein the parameter of the curved target surface is obtained during use of the printing apparatus to carry out printing operations. A method according to claim 10 or 11 including determining the parameter of the curved target surface whilst the curved target surface is rotating and/or moving linearly. A method according to claim 10 or 11 wherein the target surface is a curved substrate support, and the method includes determining a distance between the printhead and a centre or curvature of the substrate support. A method according to any of claims 10 to 13 wherein the method includes determining a radius of curvature of the target surface, and/or a top dead centre position of the curved target surface. A method according to claim 14 including moving the sensor relative to the substrate support, at a substantially constant speed, in a substantially straight line, whilst maintaining the target surface substantially stationary, and obtaining a plurality of data. A method according to any of claims 10 to 15 including moving the monitoring device relative to the target surface, with the printhead. A method according to any of claims 10 to 16 wherein the parameter is the position of a part of the target surface relative to the printhead. A method according to any of claims 10 to 17 including controlling at least one operation of the printing apparatus using data indicative of a parameter of the curved target surface.