A printing apparatus

Abstract

A thermal printing apparatus 10 includes the printhead 12; a printhead actuation system 20 attached to the printhead, and a controller 14, which is operable to control the printhead actuation system t

Description

FIELD Embodiments of the present invention relate to a printing apparatus suitable for thermal printing. More specifically the present invention relates to thermal transfer printing or thermal label printing. BACKGROUND Thermal printing is well known and includes both thermal transfer printing and thermal label printing. In thermal transfer printing, a print head including an array of heating elements is moved towards and away from an inked ribbon and a substrate during printing. The heating elements are selectively heated and melt the ink from the ribbon, onto the substrate to print the desired image. In thermal label printing (also known as direct thermal printing), a similar process is employed except there is no separate inked ribbon. A heat-sensitive media is used as the substrate that darkens in response to the heat from the heating elements on the print head. Thus, as the substrate passes under the print head the heating elements are controlled in the same way as for transfer printing and a desired image is formed directly in the substrate. BRIEF DESCRIPTION OF THE INVENTION According to a first aspect of the invention we provide a printing apparatus for thermal printing in which a print head is moved towards / away from a substrate, including: a print head, and a print head actuation system attached to the printhead, a controller, which is operable to control the print head actuation system to move the print head between a non-printing position and a printing position, wherein the print head actuation system includes: a solid state actuator which is connected to the print head to actuate movement of the print head to the printing position. The print head may include a control module and at least one heating element. In such a print head the solid state actuator may be connected to the control module. The solid state actuator and the at least one heating element may be controlled by the control module. Optionally, the solid state actuator may also be powered by the control module. The solid state actuator may be positioned between the control module and a housing member. The solid state actuator (or another solid state actuator if there is more than one) may be positioned between the control module and the at least one heating element. The solid state actuator (or each of them if there is more than one solid state actuator) may include a plurality of solid state actuator modules. The solid state actuator modules may be positioned in an offset position with respect to a central axis of the print head. Such positioning may allow uneven movement of the heating elements to be actuated. The solid state actuator modules may be connected to the control module and in an offset position with respect to a central axis of the print head. There may be a plurality of heating elements and each solid state actuator module may be aligned with a heating element. The print head actuation system may include a second actuator. The second actuator may be a motor or solenoid or pneumatic actuator. The second actuator may be connected to the print head to provide additional movement (i.e. both the solid state actuator and the second actuator may work in cooperation to move the print head as desired). The second actuator may be external to the print head. The solid state actuator may be integrated within the print head. The second actuator may provide coarse movement of the print head. The solid state actuator may control movement of the print head to the printing position and / or may control application of a printing pressure of the print head on the substrate (i.e. the solid state actuator may be in control of fine print head / heating element movement for the printing operation). The controller may be operable to implement a closed-loop control system. The closed-loop system may operate such that the pressure applied by the print head during printing is monitored and / or optimised. The printing apparatus may include one or more sensor(s). The sensor(s) may be operable to directly or indirectly monitor the pressure applied by the print head during printing (i.e. the one or more sensors may be used to provide the closed loop system). The sensor may be an optical sensor, which may monitor a printed image. The sensor (or another sensor if there is more than one) may be a vibration sensor, which may monitor the vibration of the print head. The sensor (or another sensor if there is more than one) may be a pressure sensor, which may directly monitor the pressure between the print head and the solid state actuator and / or between the print head and the substrate. The sensor (or another sensor if there is more than one) may be incorporated into the solid state actuator (i.e. the solid state actuator may be a self-sensing solid state actuator). The closed-loop control system may use a sensor output to update one or more control parameters for a future printing operation. The solid state actuator may be one of a piezoelectric, magnetostrictive, electrostrictive, or aerographene actuator. The solid state actuator may include at least one of: a unidirectional shape memory alloy-, a piezoelectric ceramic-, a ferroelectric polymer-, a dielectric elastomer-, an ionic polymer metal composite- and a conducting polymer-based actuators. The solid state actuator may be grounded to a housing member. The solid state actuator may include multiple solid state actuator modules connected across the print head. The closed loop system may result in an individual control signal for each solid state actuator module (or for groups of solid state actuator modules). According to a second aspect of the invention we provide a method of operating a printing apparatus for thermal printing in which a print head is moved towards / away from a substrate including: performing a printing operation by moving a print head into contact with a substrate using a solid state actuator; monitoring performance data received from a sensor relating to an image printed in the printing operation or a physical characteristic of the print head; determining, based on the performance data, a pressure profile correction to be applied to the solid state actuator; and applying the pressure profile correction to the solid state actuator. Monitoring the performance data may include: using an optical sensor to provide an output relating to the image printed; identifying one or more areas of the printed image that include a defect, artifact, and / or disturbance; and wherein determining the pressure profile correction is based on the one or more identified areas. Monitoring the performance data may include: using an inertial sensor to provide an output relating to print head movement / vibration; identifying an unwanted component of the print head movement; and determining the pressure profile correction to be applied based on the identified unwanted component of the print head movement. Any of the optional features of the first aspect of the invention may be combined with the method of operating the printing apparatus in the second aspect of the invention. Embodiments of the present invention require less power to generate the required pressure to print optimal images on a substrate because they reduce the heat generation from the system. Thus, resulting in less power overall being consumed. BRIEF DESCRIPTION OF THE FIGURES In orderthatthe present disclosure may be more readily understood, preferable embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 a is a schematic view of a printing apparatus, embodying the present disclosure; FIGURE 1b is a focused schematic view of part of the printing apparatus in FIGURE 1a; FIGURE 2 is a focused schematic view of a part of a printing apparatus; FIGURE 3 is a focused schematic view of a part of a printing apparatus; FIGURE 4 is a system overview diagram; FIGURE 5 is a system overview diagram; FIGURE 6 is an overview of a method of operating the printing apparatus; FIGURE 7a and 7b are overviews of a method providing a closed-loop operation, and FIGURE 8 illustrates a second embodiment. DETAILED DESCRIPTION OF THE DISCLOSURE A printing apparatus 10 is illustrated in figures 1a, 4, and 5. The printing apparatus 10 is suitable for thermal printing - particularly thermal transfer printing and thermal direct printing, in which a print head is moved towards / away from a substrate. The printing apparatus 10 includes a print head 12, a print head actuation system, and a controller 14. The print head actuation system is attached to the print head 12. The controller 14 is operable to control the print head actuation system to the move the print head 12 between a non-printing position and a printing position. The print head 12 is movable towards and away from a substrate 32 - this is performed by the print head actuation system, which is controlled by the controller 14. The print head actuation system includes a solid state actuator 20. The solid state actuator 20 is connected to the print head 12 to actuate movement of the print head 12 to the printing position and / or to a printing pressure. A solid state actuator is an actuator that produces motion from an internal molecular action (i.e. within a single solid body, without relative interaction with other bodies). In other words, it is a material that deforms in an electric field (and generates charge when mechanically stressed). In embodiments, the solid state actuator 20 includes at least one of a piezoelectric, magnetostrictive, electrostrictive, or aerographene actuator. In some embodiments, the actuator 20 may include at least one of: a unidirectional shape memory alloy-, a piezoelectric ceramic-, a ferroelectric polymer-, a dielectric elastomer-, an ionic polymer metal composite-, and a conducting polymer-based actuators. Technologies such as piezoelectric and magnetostrictive materials have useful properties when compared to conventional actuation technologies like motors or compressed air. A solid state actuator can achieve a high pressure (strain) - so in the printing environment it may be easier to achieve the print head 12 pressure on the substrate 32 for optimal print quality. A solid state actuator offers high frequency operation - so the print head 12 can be moved quickly. Further, solid state actuators are compact. This enables multiple of them to be integrated into small volume. However, they are limited to low travel distance (stroke) - so while the movement of the print head 12 provided is fast, it can only move a short distance. Newer technologies such as aerographene actuators are also suitable for this application (and share the advantages / disadvantages above). An aerographene actuator uses joule heating of the graphene-infused foams to rapidly heat up and expand the air within, which generates a pressure wave. An example system is illustrated in figure 5. In some embodiments, the print head 12 is connected to a housing member 24 (which may be further connected to a second actuator 30 - discussed in more detail further below). The print head 12 may be connected pivotably to the housing member 24, so that the print head 12 angle, relative to the substrate 32 (and relative to the housing member 24), is changeable. In embodiments, the print head 12 includes one or more heating element(s) 22 (in the present example, the print head 12 includes an array of heating elements 22). For a printing operation, the heating elements 22 are selectively heated so that when the print head 12 is in the printing position, portions of ink on an inked ribbon 34 are transferred to the substrate 32 (or the portions of heat sensitive media are heated) to result in the desired image. In the present example, the print head 12 includes a control module 26 which controls the heating and cooling of the individual heating elements 22 (in other words, the heating elements 22 are connected to the control module 26). The control module 26 may be a printed circuit board (PCB) or similar and receives and transmits the required signals to the heating elements 22 (i.e. through voltage / current signals in the electronic system). In embodiments, each heating element 22 includes a resistor, so that when a current flows through the heating element 22, heat is generated from the resistance through the resistor. The solid state actuator 20 is connected within the print head 12 such that it can actuate movement of the printing elements (i.e. the heating elements 22) towards the substrate 32. In some embodiments, the solid state actuator 20 may be responsible for ensuring the correct pressure is formed on the substrate 32. In other words, the actuator 20 “movement” may increase the pressure to a printing pressure. In the illustrated example of figure 5, the solid state actuator 20 is connected to the control module 26. In other words, the control module 26 (which controls the operation of the heating element(s) 22) also controls the operation of the solid state actuator 20. The solid state actuator 20 may be positioned / connected such that it draws power from the control module 26 (i.e. the control module powers both the heating elements 22 and the solid state actuator 20). In other words, the solid state actuator 20 has an electrical connection to the control module 26. It should be appreciated that the controller 14 (which controls the control module 26 on the print head 12) may also directly control the solid state actuator 20. This is illustrated in figure 4 with dashed communication connections - the controller 14 either communicates directly with the solid state actuator 20 or the control module 26 (which in turn communicate with the solid state actuator 20). In such an embodiment, the solid state actuator 20 is positioned between the control module 26 and the heating elements 22. In other words, the control module 26 may have a casing (or a planar member between it and the actuator 20) and the heating elements are mounted to a planar member or have casing. The solid state actuator 20 is positioned between the casings / members, so that it can actuate movement and / or increase in pressure of the heating elements 22 in the direction A (shown in figure 5). In short, the heating elements 22 are attached to the solid state actuator 20, so that they are movable and when the actuator 20 is actuated the internal action of the material will push the heating elements towards / away from the control module 26. In some embodiments, a solid state actuator 20 is provided between the control module 26 and the housing member 24 (shown in figure 1 b). A similar principle applies to this configuration as above in that the solid state actuator 20 acts against solid planar members / casings to provide the movement desired. It will be appreciated that the configuration illustrated in figure 1 b includes a first and a second solid state actuator 20 which are positioned both between the housing member 24 and the control module 26 and between the control module 26 and the heating elements 22. In such an arrangement, the solid state actuators 20 are effectively mounted on opposite faces of the control module 26. In such a configuration, the “sides” may be controlled to apply forces in opposite directions (i.e., up / down) with respect to the substrate 32. In such an embodiment where the solid state actuator 20 is connected to a housing or casing, the connection may also provide a grounded connection (i.e. which can be used to sink return current). This is illustrated in figure 2. As can be seen, the solid state actuator 20 includes a power connection to the control module 26 (illustrated by “V+”) and, further, it also includes a ground connection to the housing member 24 (illustrated by “GND”). In some embodiments, the solid state actuator 20 is mounted directly to the control module 26, and the other side against the control module casing. In other words, the solid state actuator 20 is positioned inside the housing of the control module 26. In some embodiments, the solid state actuator 20 includes an array of solid state actuator modules 20a. This is illustrated in figure 1 b and 3, where it can be seen that there are multiple modules provided in the solid state actuator 20 ‘layer’. In such an example, the position of each solid state actuator module 20a is known (i.e. by the controller 14). The location of each solid state actuator module 20a may be stored as a position or coordinates relative to a central axis of the control module 26 on the print head 12. This information can be used to apply spatially resolved forces from individual solid state actuator modules 20a. Thus, the solid state actuator modules 20a are mounted in such a way that they enable spatial pressure control across a printing area (i.e. they provide control of the pressure applied by the print head 12 across the area of the print head 12 / heating elements 22). In some embodiments, the solid state actuator modules 20a are positioned and connected to the print head 12 in an offset position with respect to a central axis. This allows uneven movement of the print head 12. In other words, individual modules 20a are mounted on the same face of the control module 26 but offset with respect to a central axis. In such an example, tilt and roll may be achieved by actuating the modules 20a to different lengths. For example, by being offset relative to a central axis, actuating one solid state actuator module 20a to a greater distance than another solid state actuator module 20a causes a rotation about this axis. Thus, the pressure across the print head 12 area can be altered / adjusted as desired. In some embodiments, the solid state actuator modules 20a are aligned with heating elements 22 provided on the print head 12 (or, the may be aligned with areas of heating elements 22). This allows the control of the pressure applied by individual heating elements 22 (or specific areas / groups of heating elements 22). This is shown in figure 3 - the axes are illustrated and the rotation about the axes is shown by “X” and “Y”. In embodiments (see for example, figures 1a, 4 and 5), the print head actuation system includes two actuators - one of the actuators is the solid state actuator 20 discussed in detail above, and the second actuator 30 is a motor or solenoid or pneumatic actuator. Thus, the solid state actuator 20 assists the second actuator 30 in moving the print head 12 (and thus, the heating elements 22) towards and away from the substrate 32 (i.e. the print position). Further, the solid state actuator 20 assists with applying the correct or ideal pressure to the substrate or inked ribbon 34 for optimal printing. In other words, the second actuator 30 is responsible for coarse movement of the print head 12 and the solid state actuator 20 is responsible for fine movement of the print head 12 and / or printing pressure of the print head 12 on the substrate 32 / ribbon 34. It should be appreciated that in some embodiments, the solid state actuator 20 replaces the second actuator 30 in terms of applying pressure. In other words, the second actuator 30 is responsible for movement of the print head 12 only and the solid state actuator 20 controls the pressure of the heating elements 22 / print head 12 on the substrate 32 or ribbon 34. In some embodiments, the printing apparatus includes one or more sensor(s) 40 which are operable to directly or indirectly monitor the pressure applied by the print head 12 during printing. Examples of an indirect sensor 40 are optical or vibration sensors and examples of a direct sensor are pressure or inertia sensors. In some embodiments, the sensor 40 is an optical sensor. The optical sensor monitors a printed image. In other words, the optical sensor is configured to evaluate the substrate 32 after printing has occurred and provide data so that the quality of the printed image can be assessed. For example, the optical sensor detects where insufficient pressure was applied by the print head 12 / heating elements 22. The sensor provides this data by imaging the substrate 32 and areas with a reduced quantity of ink are determined. Machine vision systems such as cameras could be suitable. In some embodiments, the sensor 40 is a vibration sensor. The vibration sensor monitors the vibration of the print head 12. An excess of vibration in the print head 12 may result in a lower print quality because the pressure applied to the inked ribbon 34 / substrate 32 is not maintained at a constant level. For example, the vibration sensor detects an incoming vibration to the print head 12, and outputs a signal that allows the solid-state actuator 20 to compensate for the change in pressure that this vibration causes. In some embodiments, the sensor 40 is an inertial sensor, such as an accelerometer. The accelerometer directly detects vibrations and / or movement of the print head 12. In some embodiments, the sensor 40 is a pressure sensor. The pressure sensor directly monitors the pressure between the print head 12 and the solid state actuator 20 and / or between the print head 12 and the substrate 32. In addition to or alternatively, the sensor could be incorporated into the solid state actuator 20 itself in the form of a self-sensing solid state actuator. Such an actuator is able to sense the pressure exerted through them. For example, a piezoelectric material can “self-sense” because a force applied on the material is translated as voltage, which can be detected by the system. It should be appreciated that while the system may only have one sensor 40 (and it could be any one of the examples, discussed here), it may also have more than one sensor. For example, the system could include an ‘indirect’ sensor and a ‘direct’ sensor. Hence, any of the sensors 40 discussed here could be combined with any other sensor if desired. The methods of using the printing apparatus 10 discussed above will now be described in detail. In embodiments, there are two related processes in which the printing apparatus 10 can be used (in both the print head actuation system is controlled to achieve different modes of operation). The first is so called “printing assistance” in which the presence of the solid state actuator 20 enhances the pressure and / or speed capability of the second actuator 30. The second is “closed-loop calibration” where the solid state actuator 20 is used to correct errors and / or misalignments in the printing apparatus 10 / the print head 12. Each of these processes is discussed in more detail below. Printing Assistance The controller 14 is operable to implement one of two procedures (illustrated at step 100 in figure 6) - either the solid state actuator 20 is used to enhance the pressure (step 102) or speed capability of the second actuator 30 (step 104). An advantage of this mode enables the second actuator 30 to be specified with lower capabilities. In other words, the second actuator 30 (which may be a motor or solenoid, etc.) may have one or more of: a lower maximum pressure level, a lower pressure control precision, and a lower maximum speed than an actuator in a printing apparatus in which a solid state actuator is not present. This is enabled by the solid state actuator 20, which allows provision of desirable printing performance even though the second actuator 20 has one or more aspects of lower performance. In both procedures a printing operation occurs by the following method. The substrate 32 is moved past the print head 12 at a desired speed. The substrate 32 is moved using motors or the like. In the case of thermal transfer printing, the inked ribbon 34 is moved between a supply spool and a take-up spool along a ribbon / tape path. The ribbon path passes past the print head 12 and between the print head 12 and the substrate 32. In some embodiments, the print head 12 has a parked / stop position that is a distance away from the printing position. The print head 12 may be moved from the parked position to a “ready” position - essentially so that the print head 12 is ready to start printing when required. Then when desired the controller 14 begins operation to perform printing operations. During a printing operation, the print head 12 is moved to the printing position - this involves a position (contacting the substrate 32 and / or ribbon 34) and a pressure exerted on the substrate 32 and / or the ribbon 34. Pressure on the substrate for printing In embodiments, the print head 12 is moved to the printing position by the second actuator 30. In this example, the second actuator 30 is engages fully and applies the maximum pressure of which it is capable (step 106). In some embodiments, the solid state actuator 20 is also engaged. The solid state actuator 20 acts against the part of the print head 12 to which it is connected (for example, it may act against the heating elements 22 or the control module 26). The solid state actuator 20 is used either simultaneously with the second actuator 30 or subsequently (i.e. after the second actuator 30 has moved the print head 12 to the further extent it can). In other words, the solid state actuator 20 applies additional pressure to bring the print head 12 to a printing pressure (step 108). In some embodiments, the second actuator 30 is held stationary while the solid state actuator 20 provides the pressure onto the substrate 32. In such an example, the maximum pressure that the print head 12 / the heating elements 22 can exert on the substrate 32 is dictated to some extent by the ability to hold the second actuator 30 in the desired stationary position (i.e. the ability of the second actuator 30 to resist ‘back-driving’). The controller 14 / control module 26 operate to ensure that the maximum resistance to the backwards force that the second actuator 30 possesses is not exceeded (which avoids damaging the second actuator 30). This system is beneficial since it reduces the likelihood of print artifacts emerging due to insufficient pressure during the printing process (because the solid state actuator 20 can ensure sufficient pressure is exerted on the substrate 32). Further, the solid state actuator 20 may compensate for a smaller / lower-power second actuator 30 than would otherwise be required. Extra speed for fast printing When the printing apparatus is in a high-speed mode - i.e. the rate of printing required is high - the actuators 20, 30 cooperate to deliver the printing rate required. In some embodiments, second actuator 30 almost fully engages. In other words, the print head 12 is moved by the second actuator 30 to bring the print head 12 and substrate 32 into close proximity. However, the second actuator 30 does not apply the necessary printing pressure. This is at step 110. The solid state actuator 20 is actuated to engage / disengage at a higher frequency and / or speed (step 112). Thus, the print head 12 is moved into the printing position by the solid state actuator 20 at high speed and / or with a high extension / retraction rate. The second actuator 30 is held stationary and the solid state actuator 20 controls the printing action (which is faster than the second actuator 30 is capable of driving). Depending on the type of printing required and the type of printed image required, this may have several benefits. In the case of intermittent printing (i.e. where the substrate is moved intermittently and the print head 12 must be withdrawn from the printing position regularly), this method reduces the time needed for the printhead to arrive in the printing position (i.e. the time to arrive at a printing pressure is reduced). This reduces the time the substrate 32 is held stationary, which allows for a higher number of prints per second (i.e. a higher print rate). In the case of continuous printing (i.e. where the substrate is moved continuously past the print head 12), this method reduces the time needed for the print head 12 to engage / disengage with the substrate 32. This allows for the substrate 32 to be actuated faster, hence allowing a higher number of prints per second. In other words, the increased speed provided by the solid state actuator allows a faster print process (and, thus, a faster print rate)(step 114) In some embodiments, where the solid state actuator modules 20a are arranged alongside specific heating elements 22 or groups of heating elements 22, the solid state actuator 20 removes, reduces or at least alleviates the need to wait for the heating elements 22 to cool down. The solid state actuator modules 20a are actuated to bring specific areas of the print head area into a printing condition (i.e. providing pressure to print on the substrate 32). Thus, the used (i.e. hot) heating elements 22 can be removed from contact with the ribbon 34 and / or substrate 32 in areas not requiring ink transfer / printing. In other words, the alignment between the solid state actuator modules 20a and a heating element 22 / group of heating elements 22 allows control of the area of the print head 12 actually exerting pressure on the ribbon 34 and / or the substrate 32. It also allows the retraction of a single heating element 22 / group of heating elements 22 after they have been used (i.e. heated) from a printing condition (i.e. position and pressure for printing) and the heating element 22 / group of heating elements 22 are able to cool away from the printing condition. This is shown at step 116. In some cases, this reduces the time interval between printing different portions of a 2D pattern / image (i.e. because the heating elements 22 that have been used for a first part of the image are retracted and other heating elements 22 that are required for a subsequent portion of the image can be extended). Thus, the solid state actuator modules 20a provide an increase the printing speed. Closed-loop calibration In addition to the function of assisting the main actuator in its function of disengaging / engaging the print head 12 in order to increase speed of operation, the solid state actuator 20 can be used to enable new closed-loop calibration functions. For example, using feedback from an optical sensor or similar, the solid state actuator(s) 20 can be actuated rapidly while printing to dynamically correct for observed errors arising from print head misalignment, vibrations, or other sources. In short, the solid state actuator 20 is used to correct errors or misalignments in the printing apparatus 10 or print head 12, by using feedback from the sensor 40. An advantage of this method is that the solid state actuator is able to correct different types of errors to a prior art system. For example, because of the inherent higher speed of a solid state actuator 20 and / or because multiple solid state actuator modules 20a can be mounted close to the print head 12. The solid state actuator 20 (i.e. the individual solid state actuator modules 20a in the solid state actuator) can be used to provide immediate corrective action or future corrective action. In other words, the controller 14 is operable to correct the identified issues immediately while printing is still occurring. Further, the controller 14 is also operable to assess and implement changes after a printing operation has occurred in order to improve the printing quality for future printing operations (i.e. the past printing operation provides a calibration for future printing operations). The controller 14 is operable to receive signals from the one or more sensors 40. The output from the sensor(s) 40 is assessed to identify corrections that should be implemented. In other words, the method of operating the printing apparatus 10 includes: a) performing a printing operation by moving a print head into contact with a substrate using a solid state actuator; b) monitoring performance data received from a sensor relating to an image printed in the printing operation or a physical characteristic of the print head; c) determining, based on the performance data, a pressure profile correction to be applied to the solid state actuator; and d) applying the pressure profile correction to the solid state actuator. In some embodiments, the controller 14 is operable to process and provide immediate corrective actions and receive / assess information for the calibration to provide corrective action in future printing operations. In other words, the controller 14 implements a corrective loop where the controller 14 assesses the outputted printed images continuously (or at a predetermined time interval) to identify if further corrective action is necessary for future printing operations. Alternatively, the controller 14 may implement a “calibration printing operation” first, to assess the quality of the printed image. This allows the controller 14 to implement any identified corrective actions for subsequent “proper” printing operations. Calibration for future printing Before a true printing operation, a calibration process can be performed. In some embodiments, the controller 14 applies inconsistent pressure across the printing area deliberately. In other words, the solid state actuators modules 20a are controlled to apply inconsistent pressure to the substrate 32 and / or ribbon 34. This process allows the controller 14 to determine what features of the inconsistent pressure does not disrupt the printed image. In other words, it generates calibration features within the print that do not disrupt the function of the print. For example, the controller 14 controls the solid state actuator modules 20a to apply a gradient of pressure across the width of the print head 12. In some examples, this gradient is applied during a small section of printing (e.g. across a few pixels). The controller 14 is operable to choose the section of the image to perform this process on. In other words, the size and / or position of this “gradient” section may be chosen so as not to be noticeable or barely noticeable in the final print. In the present example, the resulting quality of the print can then be checked by the controller 14 (i.e. the controller 14 assesses the feedback signal from the optical sensor within this test region as a function of position along this gradient). A region can be identified as the “best quality” by controller 14 from the output from the optical sensor. Further, once this is established the controller 14 determines what pressure gradient was applied to which region. In other words, the controller 14 can determine what pressure gradient results in acceptable print quality (and / or what gradient results in the best quality region) In some embodiments, the controller 14 is operable to apply the determined pressure level / gradient to upcoming printing operations across the printing area. In other words, the solid state actuator 20 / solid state actuator modules 20a and the second actuator 30 are operated to apply the pressure gradient. In some embodiments, the controller 14 stores data relating to what constitutes an ideal print quality, which can be used in an ongoing iterative process (as outlined below). Using the calibration The process of using the calibration allows correction to be applied in a subsequent printing operation once the controller 14 establishes what action is required (this is illustrated in figure 7a). The process steps contained in the dashed outline are those that occur during a printing process. In this process, a disturbance, defect and / or misalignment is present in the printing apparatus 10 which may affect the printing operation and result in defects or suboptimal print quality (step 200). For example, a vibration or misalignment between the print head 12 and the substrate 32 may lead to inconsistent pressure across the printing area. Another example that could affect the quality of the print image is a non-homogeneous substrate material, which could also lead to inconsistent pressure across the printing area. Essentially, the print head 12 applies inconsistent pressure across the area being printed (step 202). The inconsistent pressure leads to a printing image that is suboptimal and / or contains artifacts / defects (step 204). Defects or suboptimal print quality is detected in the sensor feedback. For example, the optical sensor provides feedback on the printed image and, as such, visible artifacts in the printed image can be identified. Thus, the controller 14 identifies one or more issues from the signals provided by the sensor (step 206). In some embodiments, the controller 14 has an example of an ideal / good print quality stored (i.e. the controller 14 has data that is identified as constituting a good print quality). In some embodiments, the controller 14 is operable to compare the ideal printed image data to the output from the sensor (i.e. from step 206 where the sensor outputs the signals about the current printed image). In some embodiments, the controller 14 determines a pressure profile correction (step 208). The pressure profile correction is determined to mitigate / remove the artifacts in a subsequent printed image. The pressure profile correction includes instructions for the solid state actuator modules 20a, so that the inconsistent pressure across the printing area can be corrected by altering one or more the of the action of the solid state actuator modules 20a. In other words, the pressure profile correction is applied to future prints by modifying the control signals to the solid state actuator modules 20a during the printing process (step 210). In some embodiments, steps 200 to step 210 may be repeated for every print in order to continuously optimise the printing pressure. Immediate correction The process of implementing an immediate correction from the controller 14 is illustrated in figure 7b. The process steps contained in the dashed outline are those that are steps that occur during a printing process. During a printing operation, a disturbance, defect and / or misalignment in the thermal printing system may affect the printing process. This may be caused by inconsistent / inhomogeneous pressure applied by the print head 12 to the printing area. This is at step 220 in figure 7b. In some embodiments, the disturbance, defect and / or misalignment is detected by the assessment of output from a sensor 40 before that disturbance, etc. has time to significantly affect the final print quality (step 222). For example, an inertial sensor (i.e. an accelerometer or the like) may output a signal that indicates vibrations or misalignment in the print head 12 / heating elements 22. In some cases, an output from the optical sensor may indicate more major disturbances. Further, if the solid state actuator 20 is a self-sensing solid state actuator, then additional forces (e.g. vibrations) on the actuator 20 may be detected / output as a signal. The controller 14 receives these signals and determines that one or more unacceptable behaviour is occurring. In some embodiments, the controller 14 determines a pressure profile correction (step 224). In the present example, the controller 14 uses the pressure profile correction to alter the operation of the one or more of the solid state actuator modules 20a (step 226). For example, the pressure profile correction alters the actuation pattern or signal to the solid state actuator modules 20a so that the resulting pressure applied by the print head 12 on the substrate 32 / ribbon 34 is corrected. The controller 14 does this assessment / determination dynamically to mitigate the impact of the disturbance, defect and / or misalignment. In other words, the controller 14 implements a correction in real time before the unacceptable behaviour affects print head pressure levels to the extent that the print quality is affected (or, at least, corrects it before the print quality becomes unacceptable) (step 228). The controller 14 is operable to apply the pressure profile correction at the appropriate times or locations. For example, if a vibration is detected, the solid state actuator 20 may be operated to apply an equal and opposite vibration to damp its effect on the print head 12 / heating elements 22. In other words the controller 14 implements a damping vibration to correct for the unwanted vibration happening. In such an example, monitoring the performance date includes: a) using an inertial sensor to provide an output relating to print head movement / vibration; b) identifying an unwanted component of the print head movement; and c) determining the pressure profile correction to be applied based on the identified unwanted component of the print head movement. In the case of a misalignment (i.e. the print head 12 / heating elements 22 are not parallel to the substrate 32 / ribbon 34 during a printing operation), the controller 14 may operate the solid state actuator 20 / solid state actuator modules 20a to apply an equal and opposite rotation and / or pressure gradient across the control module 26 or heating elements 22 to counteract the measured misalignment. In this example (i.e. using an optical sensor to establish an issue), monitoring the performance date includes: a) using an optical sensor to provide an output relating to the image printed; b) identifying one or more areas of the printed image that include a defect, artifact, and / or disturbance; and c) wherein determining the pressure profile correction is based on the one or more identified areas. These real-time corrections or calibration and future correction processes have multiple advantages for the operation of the printing apparatus 10. Further, a printing apparatus 10 that incorporates the solid state actuator 20 may have multiple benefits / advantages over existing printing apparatus. For example, the printing apparatus may be capable of applying higher pressures, may have greater control, and / or may have greater response speed than would otherwise be achievable if only the second actuator 30 was used. This allows the resulting printing quality to be improved. It follows that it may also allow for a smaller / cheaper second actuator 30 to be used, or allows the improvement in operation for a printing apparatus using a high specification second actuator 30. Further, it may be possible to retrofit a solid state actuator 20 to an existing printing apparatus. For example, if the solid state actuator 20 is mounted to the control module 26 and actuating against the casing, then it is possible to fit a new control module 26 with a solid state actuator 20 fitted without having to replace the second actuator 30. Further, it is possible to fit a new print head 12 (i.e. control module 20 and heating elements 22 and associated housing parts with the additional of the solid state actuator 20 / solid state actuator modules 20a) without replacing the remainder of the printing apparatus. The high-speed operation capability of the solid state actuator 20 enables new calibration or correction functions which will also ultimately lead to improved printing outcomes. Further, because the solid state actuator 20 is located close to the point of contact between the control module 26 / heating elements 22 and the substrate 32 / ribbon 34, the improvement seen from the calibration and / or corrections may have greater effect. In embodiments with solid state actuator module 20a spatially-resolved forces can be applied to the print head 12 (i.e. to the heating elements 22 and the pressure they apply during a printing operation). This further enhances the calibration or correction functions. Solid-state actuator for print head movement Figure 8 illustrates another embodiment that could be used / implemented in isolation from the above fine control using a solid state actuator or could be used in combination with the actuator 20. The references for features in figure 8 that overlap with features already described are provided with the same number proceeded by a “3” - for example, the controller 14 will become the controller 314. The printing apparatus 310 includes a print head 312, a print head actuation system, and a controller 314. The print head actuation system is attached to the print head 312. The controller 314 is operable to control the print head actuation system to the move the print head 312 between a non-printing position and a printing position. In this illustrated example, the print head 312 is movable towards and away from the substrate 332 and ribbon 334, which is controlled and actuated by the controller 314 and actuator 350, respectively. Similarly, to the earlier described examples, the print head actuation system includes a solid state actuator 350. In this example, there is no requirement for an additional actuator to move the print head 312, as the solid state actuator 350 is capable of handling all up / down movement required by the print head 312 (i.e. the actuator 350 is a long travel piezo actuator). In other words, the actuator 350 is connected to the print head 312 to actuate movement of the print head 312 to the printing position and / or to a printing pressure. In some embodiments, the print head 312 is pivotally mounted, such that the actuator 350 can drive movement of the print head 312 to the printing position. Alternatively, the print head 312 may be slidably mounted on a rail (with bearings), so that the actuator 350 can drive movement of the print head 312 along the rail. In some embodiments, the actuator 350 is a piezo motor. Preferably, a stepping piezo motor (also known as a walking piezo, a stepper piezo, or a walk-drive motor) or an ultrasonic motor is used. In the case of a stepping action motor, multiple moving legs including solid state actuator material / piezo material are used to drive an actuator rod in a desired direction. The stepping action motor operates as follows. At least two legs are provided, which are configured to contact an actuator rod / slider. The first leg is held in a stationary position and the second leg is connected to a third leg that provides the second leg with translational movement (i.e. along / parallel to the actuator rod). During motor actuation, initially, the first leg is in its contracted state (i.e. out of contact with the actuator rod) and the second leg is in its expanded state (i.e. in contact with the actuator rod). The third leg moves to its expanded state, which pushes the second leg to its second position. This generates movement of the actuator rod (that the second leg is in contact with) in the same direction that the third leg moves. Next, the first leg moves to its expanded state to contact the actuator rod and the second leg moves to its retracted state. Then the third leg also moves to its retracted state (thus, pulling the second leg back to its first position). The first leg holds the actuator rod stationary while the second leg returns to its first position. Thus, the system is ready to move the actuator rod again. The first leg returns to its retracted state and the second leg moves to its expanded state, so the third leg can push the second leg again. Through these repeated cycles the actuator rod is moved in one direction. It should be appreciated that the cycle can be reversed, so that the third leg pulls the second leg (while in contact with the actuator rod) towards its first position - thus resulting in movement of the actuator rod in the opposite direction. It should be appreciated that additional legs or leg units can be incorporated into the actuator system. In general, the more legs that are provided the more force that can be generated. An ultrasonic motor operates by using the ultrasonic vibration of a component placed against another component. In other words the vibration of the stator against a rotor or slider (depending on whether rotation or linear translation is desired). Essentially, a contact is caused to oscillate, which movement is passed to the actuator part (rotor or slider). There are two types of ultrasonic motor - standing wave and travelling wave. A travelling wave ultrasonic motor pushes the actuator rod forward as the contact vibrates. A standing wave motor relies on having multiple contacts, all of which vibrate (with an elliptical pattern) against the actuator rod, which results in movement of the actuator rod. The elliptical pattern (which is the result of using two resonance frequencies) allows the contact to have lower impact on the surface of the actuator rod. An advantage of the travelling wave ultrasonic motor is that they often have a longer lifetime than a standing wave ultrasonic motor. An ultrasonic motor typically offers both increased force and high speed movement. In particular, the “XLA” range (an ultrasonic motor technology option) of miniature linear actuators from Xeryon are considered to be particularly suitable for use as the actuator for the print head 312. Other options considered suitable are the “Piezo Positioner PP-18” range (another ultrasonic option) from MICRONIX USA and the “PiezoWalk Piezo Motors” range (a stepping action motor option) from PI. 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 embodiments described herein may be combined with one or more features from any other embodiment(s) 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 for thermal printing in which a print head is moved towards / away from a substrate, including:a print head, and a print head actuation system attached to the printhead,a controller, which is operable to control the print head actuation system to move the print head between a non-printing position and a printing position, whereinthe print head actuation system includes:a solid state actuator which is connected to the print head to actuate movement of the print head to the printing position.

2. A printing apparatus according to the preceding claim wherein the print head includes a control module and at least one heating element, and the solid state actuator is connected to the control module.

3. A printing apparatus according to claim 2 wherein the solid state actuator and the at least one heating element is controlled by the control module, and optionally the solid state actuator is powered by the control module.

4. A printing apparatus according to claim 3 wherein the solid state actuator is positioned between the control module and a housing member and / or the or another solid state actuator is positioned between the control module and the at least one heating element.

5. A printing apparatus according to any one of the preceding claims wherein the solid state actuator includes a plurality of solid state actuator modules.

6. A printing apparatus according to claim 5 wherein the solid state actuator modules are positioned and connected in an offset position with respect to a central axis of the print head, such that uneven movement can be actuated.

7. A printing apparatus according to any one of claims 5 or 6 wherein there are a plurality of heating elements and the solid state actuator modules are aligned with heating elements.

8. A printing apparatus according to wherein the print head actuation system includes a second actuator, optionally a motor or solenoid or pneumatic actuator, which is connected to the print head to provide additional movement.

9. A printing apparatus according to claim 8 wherein the second actuator is external to the print head and the solid state actuator is integrated within the print head.

10. A printing apparatus according to claim 8 or 9 wherein the second actuator provides coarse movement of the print head and the solid state actuator controls movement of the print headto the printing position and / or controls application of a printing pressure of the print head on the substrate.

11. A printing apparatus according to any one of the preceding claims wherein the controller is operable to implement a closed-loop control system such that the pressure applied by the print head during printing is monitored and / or optimised.

12. A printing apparatus according to claim 11 wherein the printing apparatus includes one or more sensor(s) which is operable to directly or indirectly monitor the pressure applied by the print head during printing.

13. A printing apparatus according to claim 12 wherein the sensor is an optical sensor, which monitors a printed image.

14. A printing apparatus according to claim 12 or 13 wherein the or another sensor is a vibration sensor, which monitors the vibration of the print head.

15. A printing apparatus according to any one of claims 12 to 14 wherein the or another sensor is a pressure sensor, which directly monitors the pressure between the print head and the solid state actuator and / or between the print head and the substrate.

16. A printing apparatus according to any one of claims 12 to 15 wherein the or another sensor is incorporated into the solid state actuator to form a self-sensing solid state actuator.

17. A printing apparatus according to any one of claims 12 to 16 wherein the closed-loop control system uses a sensor output to update one or more control parameters for a future printing operation.

18. A printing apparatus according to any one of the preceding claims wherein the solid state actuator is one of a piezoelectric, magnetostrictive, electrostrictive, or aerographene actuator.

19. A printing apparatus according to any one of the preceding claims wherein the solid state actuator includes at least one of: a unidirectional shape memory alloy-, a piezoelectric ceramic-, a ferroelectric polymer-, a dielectric elastomer-, an ionic polymer metal composite-and a conducting polymer-based actuators.

20. A printing apparatus according to any one of the preceding claims wherein the solid state actuator is grounded to a housing member.

21. A printing apparatus according to any one of the preceding claims wherein the solid state actuator includes multiple solid state actuator modules connected across the print head, sothat each solid state actuator module is actuatable on the basis of its own individual control signal.

22. A method of operating a printing apparatus for thermal printing in which a print head is moved towards / away from a substrate including:performing a printing operation by moving a print head into contact with a substrate using a solid state actuator;monitoring performance data received from a sensor relating to an image printed in the printing operation or a physical characteristic of the print head;determining, based on the performance data, a pressure profile correction to be applied to the solid state actuator; andapplying the pressure profile correction to the solid state actuator.

23. A method of operating a printing apparatus according to claim 21 wherein monitoring the performance date includes:using an optical sensor to provide an output relating to the image printed;identifying one or more areas of the printed image that include a defect, artifact, and I or disturbance; andwherein determining the pressure profile correction is based on the one or more identified areas.

24. A method of operating a printing apparatus according to claim 21 or 22 wherein monitoring the performance date includes:using an inertial sensor to provide an output relating to print head movement / vibration;identifying an unwanted component of the print head movement; anddetermining the pressure profile correction to be applied based on the identified unwanted component of the print head movement.

25. A method of operating a printing apparatus according to any one of claims 21 to 23 using a printing apparatus according to any one claims 1 to 20.23A

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