WO2020188259A1

WO2020188259A1 - Method and apparatus for printing

Info

Publication number: WO2020188259A1
Application number: PCT/GB2020/050661
Authority: WO
Inventors: Christian Tamblyn; Andrew HEADLAND
Original assignee: Magicard Ltd
Priority date: 2019-03-15
Filing date: 2020-03-16
Publication date: 2020-09-24
Also published as: GB2582179B; GB201903560D0; GB2582179A

Abstract

We describe a method and a printer for printing on thermochromic re-writable surfaces, for example on cards such as Identity Cards. Data relating to an image to be printed onto the surface (which may already have an image printed thereon) is received and processed to identify dark and light pixels in the image. From this processing, a sequence of drive signals are produced to drive a thermal print head to produce the image on the re-writable surface. The thermal print head is driven to print the identified dark pixels in the image as dark pixels. For pixels of the image identified as being light pixels, the thermal print head is driven to erase those corresponding pixels on the surface

Description

Method And Apparatus For Printing

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for printing. More specifically, a method and apparatus for printing data and/or images on a re-writable surface.

BACKGROUND OF THE INVENTION

Direct to Card (DTC) printers are capable of printing bi-level images onto rewritable surfaces, for example card such as ID cards. These are substrates having thermochromic surfaces (or portions thereof), each part of which can be caused to change state from a light colour (e.g. white) to a darker colour (e.g. black or blue) through the judicious application of heat. In the description below, these colours are simply referred to as“white” and“black”, for simplicity.

Methods for printing on, and erasing from, such re-writable thermochromic surface are known. Known methods drive the thermal print head in much the same way as those used with thermal printing with monochrome resin film, that is a precisely chosen temperature is applied by a thermal print head to each small surface element of the card, causing that element’s colour to change to black. A lower temperature, too low to introduce any visible state change, is applied to the pixels that should remain white. By this means the desired bi-level image is produced pixel by pixel.

During the erase operation, the printer applies another chosen temperature to each element of the card that should be erased, causing its colour to change to white, while card areas that should not be altered are subjected to a lower temperature which introduces no visible state change.

In all previously existing implementations, if the user wishes to erase a card before printing a new image onto it, the erase operation is performed in its entirety before the print operation begins; i.e. the operations take place in separate passes over the card. This introduces a limit to the throughput which can be achieved when the user wishes to erase a card prior to printing a new image onto it, arising from the sum of the erase and print times. As such, we have identified a need for an improved card erasing and printing technique that addresses the time that is taken to erase and print a card.

SUMMARY OF THE INVENTION

The present invention comprises a method for printing on a thermochromic re-writable surface, the re-writable surface being configured to change from one colour to another on application of heat, the method comprising: receiving data representing an image to be written on to the surface; processing the image to produce a sequence of drive signals for driving a thermal print head, the processing comprising identifying dark pixels in the image and identifying light pixels in the image; and driving a thermal print heat using the sequence of drive signals to print the image onto the re-writeable surface, wherein the print head is driven to print dark pixels for the identified dark pixels in the image, and the print head is driven to erase pixels for the identified light pixels in the image.

Through this method, a surface can be erased and printed in a single pass, which greatly improves the speed at which surfaces may be printed, and also reduces wear and tear on the thermal print head of the printer.

Processing the image may comprise, for each identified pixel in the image, selecting a drive signal from a plurality of drive signals to produce the sequence of drive signals for driving the thermal print head. The drive signal to print a dark pixel may correspond to a temperature to which the thermal print head is driven for applying to a desired area of the surface such that a dark pixel is printed on the surface. The drive signal to erase a pixel may correspond to a temperature to which the thermal print head is driven for applying to a desired area of the surface such that a pixel is erased from the surface.

Data representing the image may only cover a portion of the available surface. In this case, printing only occurs on the desired area, and portions of the surface outside of the desired area are not printed on. That is, a driving signal is chosen not to influence the surface under the print head in that area. The step of processing may further comprise compensating for a temperature of the print head. This may comprise: measuring a temperature of the print head; and adjusting the selected drive signal based on the temperature of the print head to compensate for the temperature of the print head.

Compensating for a temperature of the print head may also comprise, for each respective drive signal, adjusting the respective drive signal based on a thermal model of the print head, the thermal model comprising data corresponding with thermal characteristics of the print head. The thermal characteristics may comprise data corresponding with at least one of thermal accumulation, thermal dissipation and energy throughput of the print head during the present printing operation.

Processing may further comprise compensating for a voltage drop across the print head, comprising: for each row of pixels in the image to be printed, processing the data representing an image to determine how many print head elements from a plurality of print head elements comprising the print head will be used to print the row of pixels; and adjusting the drive signal based on the number of print head elements being used to print the row of pixels. This attempts to alleviate issues resulting from voltage drops due to the number of printer head elements being driven, which would otherwise adversely affect the image being printed.

Processing may also further comprise adjusting the selected drive signal based on the surface to which the image is to be printed. This enables various surfaces to be used with the method described.

The present invention also provides a printer for printing on a thermochromic re writable surface, the re-writable surface being configured to change from one colour to another on application of heat, the printer comprising: an input for receiving data representing an image to be written on to the surface; a thermal print head for applying heat to print pixels on a surface; and a processor for processing the image to produce a sequence of drive signals for driving the thermal print head, the processor is configured to: process the image to produce a sequence of drive signals for driving the thermal print head, the processing comprising identifying dark pixels in the image and identifying light pixels in the image; and drive the thermal print head using the sequence of drive signals to print the image onto the re-writeable surface, wherein the processor drives the print head to print dark pixels for the identified dark pixels in the image, and the print head is driven to erase pixels for the identified light pixels in the image.

Using such a printer, a surface can be erased and printed in a single pass, which greatly improves the speed at which surfaces may be printed, and also reduces wear and tear on the thermal print head of the printer.

The processor may be configured to process the image by, for each identified pixel in the image, selecting a drive signal from a plurality of drive signals to produce the sequence of drive signals for driving the thermal print head. The drive signal to print a dark pixel may correspond with a temperature to which the thermal print head is driven for applying to a desired area of the surface such that a dark pixel is printed on the surface. The drive signal to erase a pixel may correspond with a temperature to which the thermal print head is driven for applying to a desired area of the surface such that a pixel is erased from the surface.

The data representing the image may only cover a portion of the available surface. For the area of surface outside of the image, the processor may drive the print head such that no pixels are printed.

The processor may be further configured to compensate for a temperature of the print head. In such a scenario, it is envisaged that the printer may comprise a temperature sensor to sense the temperature of the print head, and wherein the processor is configured to compensate for a temperature of the print head by: measuring a temperature of the print head using the temperature sensor; and adjusting the selected drive signal based on the temperature of the print head to compensate for the temperature of the print head.

The processor may be configured to compensate for a temperature of the print head by, for each respective drive signal, adjusting the respective drive signal based on a thermal model of the print head, the thermal model comprising data corresponding with thermal characteristics of the print head. The thermal characteristics may comprise data corresponding with at least one of thermal accumulation, thermal dissipation and energy throughput of the print head during the present printing operation. The processor may further be configured to compensate for a voltage drop across the print head by: for each row of pixels in the image to be printed, processing the data representing an image to determine how many print head elements from a plurality of print head elements comprising the print head will be used to print the row of pixels; and adjusting the drive signal based on the number of print heads being used to print the row of pixels.

The processor may further be configured to adjust the selected drive signal based on the surface to which the image is to be printed.

LIST OF FIGURES

The present invention will be described, by way of an example only, and with reference to the accompanying figures, in which:

Figure 1 represents an image to be printed on a re-writable surface;

Figure 2 represents a re-writable surface already having an image printed thereon;

Figure 3 represents the image to be printed overlayed with the re-writeable surface (10) already having an image (30) printed thereon;

Figure 4 represents a re-writeable surface to be printed where the area to be printed thereon is smaller than the whole rewritable surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In brief, the present invention provides a method and a printer for printing on thermochromic re-writable surfaces, for example on cards such as Identity Cards. Data relating to an image to be printed onto the surface (which may already have an image printed thereon) is received and processed to identify dark and light pixels in the image. From this processing, a sequence of drive signals are produced to drive a thermal print head to produce the image on the re-writable surface. The thermal print head is driven to print the identified dark pixels in the image as dark pixels. For pixels of the image identified as being light pixels, the thermal print head is driven to erase those corresponding pixels on the surface. Through this method, a surface can be erased and printed in a single pass, which greatly improves the speed at which surfaces may be printed.

As discussed above, known methods for printing on, and erasing from, re-writable thermochromic surfaces drive the thermal print head at a precisely chosen temperature to each small surface element of the card depending on the image to be printed. The temperature causes that element’s colour to change to black. A lower temperature, too low to introduce any visible state change, is applied to the pixels that should remain white. By this means the desired bi-level image is produced pixel by pixel.

During an erase operation, which is carried out separately and before the print operation, the printer applies another chosen temperature to each element of the card that should be erased, causing its colour to change to white, while card areas that should not be altered are subjected to a lower temperature which introduces no visible state change.

For example, our Rio Pro 360 driver, when a bi-level monochrome job is sent to the printer, the background (white) pixels are assigned the value 0 and the foreground (black) pixels the value 255. Each of these values are associated with a printer head temperature. Setting the print head power level appropriately and making use of these two values together with a carefully chosen third, intermediate pixel value the driver causes the printer to apply heat to areas of the card in such a way as to either print (pixel value = 255), erase (pixel is the intermediate value), or leave unchanged (pixel value = 0) each element of the card surface independently. The erase value is dependent on the re-writable surface being written to.

The method of printing according to the present invention is summarised below, and with reference to figures 1 to 3, where figure 1 represents the image (20) to be printed on a re-writable surface (10), figure 2 represents a re-writable surface (10) already having an image (30) printed thereon, figure 3 represents the image (20) to be printed overlayed with the re-writeable surface (10) already having an image (30) printed thereon. Firstly data representing an image (20) to be printed onto a re-writeable surface (10) is received. This could be received at the printer itself, or at a different device, for example a PC or other processing device.

The received image (20) is processed so that a sequence of appropriate drive signals can be generated that are used to drive the thermal print head to reproduce the image on the rewritable surface (10). Again, the processing can occur in the printer itself, or could be done remotely from the printer, for example in a remote PC.

The purpose of the processing is to identify dark and light pixels that comprise the image. From the identification of the dark and light pixels, appropriate drive signals are selected in order to drive the thermal print head to reproduce those dark and light pixels on the rewritable surface.

In order to solve the problem of extended print times when having to first erase and then separately print the image on the rewritable surface, the present invention is able to erase and print in the same pass through the choice of appropriate drive signals.

For pixels that are identified as being dark, a drive signal suitable for driving the thermal print head to reproduce a dark pixel is selected. However, for pixels in the image that are identified as being light, a drive signal suitable for driving the thermal print head to erase that pixel is chosen.

In that way, pixels that are dark over-write any image (30) that was previously present on the re-writable surface (10). For pixels that are identified as being light, the erase drive signal retains any light pixels already present on the rewritable surface, or erases any dark pixels already present on the rewritable surface, that is, turns the dark pixels into light pixels.

Once the appropriate drive signals are selected for the image to be printed, the thermal print head is driven using the drive signals in order to print the image onto the rewritable surface in a single pass. Since dark pixels are written, and light pixels are erased, the image can therefore be printed on an already used rewritable surface in a single pass, which greatly increases the speed at which surfaces can be printed compared to the two-pass process of known printers.

Using the method described above, we are able to erase a previous image and print a new one onto a rewritable card during a single pass over a card, greatly increasing throughput which is now limited by the greater of the erase and print times, rather than by their sum. For example, while the Magicard Rio Pro can erase and print in approximately 13 seconds (discounting the time taken to download the data from the PC driver), other known products can achieve no better than 8 seconds for the same task. However, a printer adapted to print using the above-described method can erase and print a card in under 5 seconds.

In addition to the increase in throughput, the new method has the advantage that since half as many passes over the card are being made for jobs requiring both erasing and printing, there is a reduction in mechanical wear on the print engine, in particular the print head.

With regards to the image to be printed (20), it does not need to cover the whole of the re-writeable surface. That is, there may be scenarios where portions of the re-writable surface may not be written to.

In this example, the data received regarding the image to be printed (20) relates only to a portion (40) of the re-writeable surface (10) to be written to. As such, for pixels outside of the area defined as outside of the print area (40) are not printed, and only the portions inside the area (40) are printed thereon using the method described above.

In the above methods, the sequence of drive signals is generated based on the light and dark pixels identified during processing of the data representing the image. The values, that is the drive signals required to drive the thermal print head to achieve a desired temperature for a desired pixel (dark, light or erase), may be selected from a plurality of values in a look up table or some other data store.

Once the pixel is identified and the values for each pixel selected, the sequence of drive signals is generated and used to drive the thermal printer head.

The method may also provide suitable processing of the image data in order to ensure that pixel values from the driver translate properly into applied temperatures at the print head elements. This processing includes adjustments to compensate for the print head temperature, model-based adjustments to compensate for the accumulation and dissipation of heat energy throughout the print head while printing, and adjustments to compensate for various other unwanted image artefacts introduced by the printing process.

For example, if a constant level of power is applied to a printing element of the print head, its temperature will not remain constant over time due to heat building up in the element in question, and being transferred to/from material in contact with that element (such as neighbouring elements, the substrate the elements are mounted on, the card we're printing onto etc.). The printer may use a mathematical model of the heat flow between the print head's printing elements, the re-writeable surface, the substrate, heat sinks etc. in order to determine how to drive that element in order to achieve the desired temperature in order to achieve the desired pixel.

The resulting power applied to an element therefore depends on factors such as the print head temperature, which may be sensed using a temperature sensor such as a thermistor (or other types of temperature sensors) mounted onto the print head, and the image content in those parts of the image already printed (i.e. the recent history of all of the elements' drive signals, particularly those near to the element in question).

Furthermore, due to the nature of the print head's design, there is a small drop in the voltage applied to the elements due to internal resistances in the print head's driving/control circuitry. The greater the number of print head elements activated at any given time, the greater this voltage drop. The method also compensates for this by analysing the image data and adjusting the drive signals appropriately. The type of surface being written to also affects how the image will be printed. As such, the method can adjust the drive signals based on the surface being printed on. For example, the overall amount of energy applied to the card may be scaled, in order to account for differences in surface and bulk properties of print media. This may be via a user selection entered by a user of the type of surface being written to, or there may be some form of identification mechanism, whereby the printer detects the type of surface being used, and adjusts the power levels of the drive signals accordingly.

As discussed above, whilst in present invention the image processing is carried out by the printer’s processor, it would also be possible to perform some or all of this processing in, for example, a component of the PC driver or in hardware such as a programmable logic device or GPU. Such an implementation choice may be advantageous in situations where the computational power of the printer is limited, for example.

Although the present invention has been described hereinabove with reference to specific embodiments, the present invention is not limited to the specific embodiments and modifications will be apparent to a skilled person in the art which lie within the scope of the present invention. Any of the embodiments described hereinabove can be used in any combination.

Claims

CLAIMS:

1. A method for printing on a thermochromic re-writable surface, the re-writable surface being configured to change from one colour to another on application of heat, the method comprising:

receiving data representing an image to be written on to the surface;

processing the image to produce a sequence of drive signals for driving a thermal print head, the processing comprising identifying dark pixels in the image and identifying light pixels in the image; and

driving a thermal print heat using the sequence of drive signals to print the image onto the re-writeable surface,

wherein the print head is driven to print dark pixels for the identified dark pixels in the image, and the print head is driven to erase pixels for the identified light pixels in the image.

2. A method according to claim 1 , wherein processing the image comprises, for each identified pixel in the image, selecting a drive signal from a plurality of drive signals to produce the sequence of drive signals for driving the thermal print head.

3. A method according to claim 1 or 2, wherein the drive signal to print a dark pixel corresponds with a temperature to which the thermal print head is driven for applying to a desired area of the surface such that a dark pixel is printed on the surface.

4. A method according to claim 1 , 2 or 3, wherein the drive signal to erase a pixel corresponds with a temperature to which the thermal print head is driven for applying to a desired area of the surface such that a pixel is erased from the surface.

5. A method according to any preceding claim, wherein the data representing the image covers only a portion of the available surface.

6. A method according to claim 5, wherein for the area of surface outside of the image, the print head is driven such that no pixels are printed.

7. A method according to any preceding claim, wherein processing further comprises compensating for a temperature of the print head.

8. A method according to claim 7, wherein compensating for a temperature of the print head comprises:

measuring a temperature of the print head; and

adjusting the selected drive signal based on the temperature of the print head to compensate for the temperature of the print head.

9. A method according to claim 7 or 8, wherein compensating for a temperature of the print head comprises, for each respective drive signal, adjusting the respective drive signal based on a thermal model of the print head, the thermal model comprising data corresponding with thermal characteristics of the print head.

10. A method according to claim 9, wherein the thermal characteristics comprise data corresponding with at least one of thermal accumulation, thermal dissipation and energy throughput of the print head during the present printing operation.

11. A method according to any preceding claim, wherein processing further comprises compensating for a voltage drop across the print head, comprising:

for each row of pixels in the image to be printed, processing the data representing an image to determine how many print head elements from a plurality of print head elements comprising the print head will be used to print the row of pixels; and

adjusting the drive signal based on the number of print head elements being used to print the row of pixels.

12. A method according to any preceding claim, wherein processing further comprises adjusting the selected drive signal based on the surface to which the image is to be printed.

13. A printer for printing on a thermochromic re-writable surface, the re-writable surface being configured to change from one colour to another on application of heat, the printer comprising:

an input for receiving data representing an image to be written on to the surface;

a thermal print head for applying heat to print pixels on a surface; and a processor for processing the image to produce a sequence of drive signals for driving the thermal print head, the processor is configured to:

process the image to produce a sequence of drive signals for driving the thermal print head, the processing comprising identifying dark pixels in the image and identifying light pixels in the image; and

drive the thermal print heat using the sequence of drive signals to print the image onto the re-writeable surface,

wherein the processor drives the print head to print dark pixels for the identified dark pixels in the image, and the print head is driven to erase pixels for the identified light pixels in the image.

14. A printer according to claim 13, wherein the processor is configured to process the image by, for each identified pixel in the image, selecting a drive signal from a plurality of drive signals to produce the sequence of drive signals for driving the thermal print head.

15. A printer according to claim 13 or 14, wherein the drive signal to print a dark pixel corresponds with a temperature to which the thermal print head is driven for applying to a desired area of the surface such that a dark pixel is printed on the surface.

16. A printer according to claim 13, 14 or 15, wherein the drive signal to erase a pixel corresponds with a temperature to which the thermal print head is driven for applying to a desired area of the surface such that a pixel is erased from the surface.

17. A printer according to any one of claims 13 to 16, wherein the data representing the image covers only a portion of the available surface.

18. A printer according to claim 17, wherein for the area of surface outside of the image, the processor drives the print head such that no pixels are printed.

19. A printer according to any one of claims 13 to 18, wherein the processor is further configured to compensate for a temperature of the print head.

20. A printer according to claim 19, comprising a temperature sensor to sense the temperature of the print head, and wherein the processor is configured to compensate for a temperature of the print head by:

measuring a temperature of the print head using the temperature sensor; and adjusting the selected drive signal based on the temperature of the print head to compensate for the temperature of the print head.

21. A printer according to claim 19 or 20, wherein the processor is configured to compensate for a temperature of the print head by, for each respective drive signal, adjusting the respective drive signal based on a thermal model of the print head, the thermal model comprising data corresponding with thermal characteristics of the print head.

22. A printer according to claim 21 , wherein the thermal characteristics comprise data corresponding with at least one of thermal accumulation, thermal dissipation and energy throughput of the print head during the present printing operation.

23. A printer according to any one of claims 13 to 22, wherein the processor is further configured to compensate for a voltage drop across the print head by:

adjusting the drive signal based on the number of print heads being used to print the row of pixels.

24. A printer according to any one of claims 13 to 23, wherein the processor is further configured to adjust the selected drive signal based on the surface to which the image is to be printed.