WO2012010879A1

WO2012010879A1 - Method of testing the health of a heating element of a thermal print head

Info

Publication number: WO2012010879A1
Application number: PCT/GB2011/051351
Authority: WO
Inventors: Simon Edwards
Original assignee: Markem-Imaje Limited
Priority date: 2010-07-20
Filing date: 2011-07-19
Publication date: 2012-01-26
Also published as: GB201012166D0; GB2482139B; GB2482139A

Abstract

A method of testing the health of a heating element (30) of a print head (14) is described in which the print head (14) is of the kind having a plurality of heating elements (30) which are each selectively energiseable by a controller (20) to effect printing of a desired image on a substrate (17), wherein each heating element (30) is arranged in a circuit in parallel with acapacitance (37), and the method includes during a testing period, simultaneously applying an electrical current to the heating element (30) under test and the capacitance (37), and monitoring the voltage across the capacitance (37).

Description

Title: Method of Testing the Health of a Heating Element of a Thermal Print Head

Description of Invention

This invention relates to a method of testing the health of a heating element of a thermal print head. More particularly the invention relates to the testing of a heating element of a thermal print head which is of the kind having a plurality of heating elements which in normal use, are selectively energisable by a controller to effect printing of a dot of a desired image on a substrate.

In one example, the invention may be applied to a print head of a thermal transfer printer in which each heating element when selectively energised heats a pixel of ink of an ink carrier so that the pixel is removable from the carrier for transfer on to the substrate.

In another example, the invention may be applied to a print head of a thermal printer in which the substrate may be heat sensitive.

In each case, such print heads are known as thermal print heads, and typically an array, usually a linear array, is provided across the print head which may include a very large number of heating elements wh ich are very small . Typically a print head may have 300 heating elements per inch of the array in order to be able to print a high resolution image. By relatively moving the print head and the substrate and ink carrier, and selectively energising heating elements in each of a plurality of positions along the substrate and ink carrier, a desired image can be built up from printed dots.

When such a print head is manufactured, and periodically in use, it is known to test the print head to determine whether all of the heating elements are operating when energy is applied. Not all of the heating elements need be operable for the print head to remain functional, but when too many of the heating elements are inoperable, or if one or more heating elements in a crucial region of the array fails to operate, the print head may require replacement.

It is usual for the print head to have an associated capacitance in parallel with the heating elements, which capacitance assists in maintaining the voltage across the heating elements when it is desired to energise the heating elements.

Methods are known for testing individual heating elements of the array.

It is known to check the resistance of the heating element by creating a potential divider using a known resistance and the heating element. By knowing the input voltage to the divider, the resistance of the heating element can be determined . If the heating element exhibits some resistance this indicates that the heating element is healthy i.e. operative; if the there is an infinite resistance (open circuit) this indicates that the heating element is not healthy and is inoperative.

This method is quick and reliable but only so far as there is no capacitance in parallel with the heating element. If there is a capacitance, this will slow down the testing procedure as it is necessary to wait until the charge on the capacitance has levelled before reliable resistance determinations can be made. Typically this may be a period of five times the RC constant of the heating element and the capacitance, a period wh ich m ig ht be several seconds. Thus to test every heating element of an array of very many heating elements where a capacitance is present, takes too long to be commercially viable. One option is to remove the capacitance by isolating the capacitance from the heating element during heating element testing. However this would require a switching device for the capacitance which would considerably increase the cost of the print head if the switches were provided in the print head, or the printer, and in any event, the provision of such switches would considerably complicate the electronic configuration of the printer in which the print head is installed. In each case providing such extra components inevitably leads to a reduction in reliability and an increase in cost. According to a first aspect of the invention we provide a method of testing the health of a heating element of a print head which is of the kind having a plurality of heating elements which are selectively energisable by a controller to effect printing of a desired image on a substrate, and wherein each heating element is arranged in a circuit in parallel with a capacitance, and the method includes during a testing period, simultaneously applying an electrical current to the heating element under test and the capacitance, and monitoring the voltage across the capacitance.

The present invention enables the health of a heating element to be tested without having to isolate the capacitance from the heating element.

Desirably the method includes, at or towards the beginning of a testing period, applying the electrical current to the capacitance only and isolating the heating element under test, and when the capacitance is partially charged, connecting the heating element in parallel with the capacitance.

In the event that the heating element is healthy, in monitoring the voltage across the capacitor, it can be determined whether the charging rate of the capacitor will change e.g. whether the capacitance will discharge, or at least the rate of charging will slow, as current passes through the heating element; however if the heating element under test is inoperative (open circuit), the capacitance will continue to charge as before the heating element is connected. Thus by monitoring the voltage across the capacitance, it can readily be determined whether the heating element under test is healthy or inoperative. The present invention involves monitoring the voltage across the capacitance, which will change with time, as opposed to having to wait for a steady state voltage condition as with the prior art method outlined above.

In a print head or exteriorly to the print head, isolating switches to enable each individual heating element to be selectively energised and de-energised in use, are already provided, and there is no need for performing the method of the invention, for any switch for the capacitance. Hence the method of the invention can be performed without making at least major modifications to the construction of the print head.

The method may include, during the testing period, providing a signal indicative of the voltage across the capacitance to a first input of a differential amplifier, and providing a reference voltage to a second input of the amplifier, and monitoring an output from the amplifier which will depend on the voltage of across the capacitance. In this way the voltage across the capacitance is indirectly monitored.

Desirably the differential amplifier has a gain sufficient so that, the voltage across the capacitance during a small part of the normal charging curve can be monitored more easily by a suitably programmed processor device, such as the controller, to identify whether the changing voltage across the capacitance indicates that the heating element under test is healthy or not.

The method may including providing an analogue to digital converter between the output from the differential amplifier and the processor device. Desirably the reference voltage applied to the second input of the differential amplifier is adjustable during calibration, to enable the voltage across the capacitance during an optimum small part of the normal charging curve to be selected for monitoring, irrespective of any component tolerances.

According to a second aspect of the invention we provide for a printer which includes a print head having a plurality of heating elements which are selectively energiseable by a controller to effect printing of a desired image on a substrate, each heating element being arranged in a circuit in parallel with a capacitance, there being a voltage monitoring device for monitoring the voltage across the capacitance during performance of the testing method of the first aspect of the invention. The printer may include a differential amplifier, having a first input for a signal indicative of the voltage across the capacitance, and a second input for an adjustable reference voltage, and an output from the amplifier, and wherein the circuit of the printer includes a known resistance arranged in parallel with the capacitance, and there being a calibration switch which is closable so that during calibration, when no electrical current is provided to any heating element, simultaneously electrical current is provided to the calibration resistance and the capacitance whilst the voltage across the capacitance may be monitored. Embodiments of the invention will now be described with reference to the accompanying drawings in which:-

FIGURE 1 is an illustrative view of a printer having a print head with heating elements the health of which may be tested by the method of the invention; FIGURE 2 is a an enlarged perspective view of the print head of the printer of figure 1 ;

FIGURE 3 is a diagrammatic view of part of a circuit of the print head and printer of figure 1 .

Referring to figures 1 and 2, a printer 14 has a print head 10 which includes a body 1 1 with an array 12 of heating elements at an edge of the body 1 1 . The array 12 may include a substantial number of heating elements, for example up to 300 heating elements per linear inch. The print head 10 is mounted in the printer 14 at a printing station 15.

In use, in this example, during printing, the print head 10 is stationary at the printing station 15 whilst substrate 17, such as a film of paper or plastic, is moved past the print head 10 together with an ink carrier 18.

A controller 20 which is a suitably programmed processor device, selectively energises and de-energises the individual heating elements of the array 12, e.g. many times each second, but depending on the speed at which the substrate 17 passes the print head 10. The energised heating elements of the array 12 thus become heated, and each energised heating element melts an adjacent pixel of ink on the ink carrier 1 8 so that the pixels of ink may be transferred from the ink carrier 18 to the substrate 17, e.g. with the assistance of a peeling roller 19 which is positioned adjacent the print head 12. Thus at each of a plurality of positions along the substrate 17, dots are printed by the heating elements, the dots together making up a desired image.

The heating elements of the array 12 are selected and energised and de- energised under the control of the controller 20 which receives and/or stores data relating to an image to be printed, and selects heating elements of the array 12 to be energised and de-energised as the substrate and ink carrier 18 move past the print head 10, and when the heating elements are thus energised, they print the dots of the image. The controller 20 typically will receive information relating to substrate 17 speed in order to print rows of dots at appropriate positions on the substrate 17.

The ink carrier 18 is transported from a storage spool 22, around an ink carrier path, around guide rollers 23, 24 (and the peel roller 19), to a take-up spool 25. The rotation of the spools 22, 24 is achieved by respective electrical motors (not seen) which may be rotated under the control of the controller 20, to feed the ink carrier 18 as and when required, and to maintain a desired tension in the ink carrier 18 around the ink carrier path.

Referring now also to figure 2, part of a control circuit 29 for the print head 10 is shown.

As mentioned above, there may be a very large number of heating elements in the array 12 of the print head 10, and each of these needs to be individually addressable by the controller 20, so that dots can be printed in desired positions on the substrate 17 to form the desired image.

For the purposes of explaining the method of the invention, a single heating element 30 is illustratively depicted as a resistance. In parallel with the heating element is a capacitance 37. The heating element 30 and the capacitance 37 are each connected to a common rail 40 and to a zero volt rail 35, but there is an isolating switch 38 between the heating element 30 and the common rail 40, which isolating switch 38 is openable and elosable under the control of the controller 20.

There is provided a 24 volt electrical supply in the example, indicated at 36. During a printing cycle, a charging switch 33 which is between the 24 volt supply 36 and the common rail 40, is closed by the controller 20, so that electrical current flows from the 24 volt supply 36, through the closed charging switch 33 to the common rail 40, and hence to the capacitance 37, to charge the capacitance 37.

In normal use, if it is desired to energise the heating element 30 to print a dot on the substrate 17, the controller 20 closes the isolating switch 38 between the common rail 40 and the heating element 30. Electrical current can then flow through the heating element 30 from, in the example, the 24 volt supply 36 to the zero volt rail 35 through the heating element 30 to energise the heating element 30. The capacitance 37 will maintain a stable print head voltage Each heating element of the array 12 will be connected to the common rail 40 and the zero volt rail 35, and will have its own isolating switch 38. The switches 38 may typically be provided within the body 1 1 of the print head 10, but may be provided within the printer externally of the print head 30. The capacitance 37 may be a combination of specific components within the print head power supply 36, and capacitances provided by the construction of the print head 10.

It will be appreciated that with such a large number of heating elements 30 produced to such a small scale that there may be a number of heating elements 30 which in manufacture or subsequent use, fail. Upon such failure, a heating element 30 typically will cease to allow any current flow when the isolating switch 38 is closed, but will exhibit an infinite resistance, or open circuit. In accordance with the present invention the health of an individual heating element 30 is tested in situ, by the controller 20 disconnecting the print head power supply 36 by opening the charging switch 33. Instead of the 24 volt supply 36, a lower voltage supply e.g. a 5 volt supply 34, is connected to the common rail 40 by closing the test switch 32. Electrical current then flows from the lower 5 volt supply 34, through a test resistance 31 to the capacitance 37, which will again be charged, by the lower voltage supply 34. By monitoring the voltage across the capacitance 37 with a voltage monitoring device 39 the rate of charge of the capacitance 37 can be determined.

The lower voltage power supply 34 is used for testing purposes so as not to expose the heating elements 30 under test to the normal higher operating voltage 36, thus to protect the heating elements 30 during the testing procedure.

More particularly, the method includes, towards the beginning of a testing period for the individual heating element 30, applying the electrical current from the lower voltage supply 34, to the capacitance 37 only and isolating the heating element 30 under test by opening or maintaining open its associated isolating switch 38. When the capacitance 37 is partially charged, the heating element 30 is connected in parallel with the capacitance 37 by closing its isolating switch 38.

When the isolating switch 38 is closed, if the heating element 30 is healthy, the charging rate of the capacitance 37 will change. Typically the capacitance 37 will discharge through the heating element 30 when the isolating switch 38 is closed, or at the least, the charging rate of the capacitance 37 will slow. If the heating element 30 is inoperative and thus showing an infinite resistance or open circuit, the capacitance 37 will continue to charge at the same rate as in the period before the isolating switch 38 was closed. In accordance with the invention, the charging rate of the capacitance 37 is monitored, by monitoring the voltage across the capacitance 37 at a position between the test switch 32 and the isolating switch 38 i.e. by monitoring the voltage on the common rail 40. Whereas any desired device for monitoring the voltage across the capacitance 37 during the testing period may be used, preferably as shown, the common rail 40 is connected to one input 42 of a differential amplifier 45. Another input 43 to the differential amplifier 45 is provided by a reference input voltage 47. The differential amplifier 45 in the present example has a gain sufficient so that the voltage across the capacitance 37 during a small part of the normal charging curve can be monitored more easily and quickly by a suitably programmed processor device, such as the controller 20.

An output 50 from the differential amplifier 45 is provided to the controller 20 via a suitable analogue to digital converter.

The controller 20 thus indirectly monitors the voltage across the capacitance 37. To protect the analogue to digital converter, the output 50 from the differential amplifier 45 is limited by a resistance 49 between the second input 43 and the zero voltage rail 35. At least below the linnited output 50, the output 50 from the differential amplifier 45 will be indicative of the voltage difference between line 40 and the reference voltage input 47. By using a high gain differential amplifier 45, this enables the controller 20 to monitor the voltage across the capacitance 37 during a small part of the normal charging curve of the capacitance 37, and such monitored part will thus appear linear to simplify monitoring. Also the reference voltage input 47 is preferably adjustable so as to compensate for tolerances in circuit components.

As desired, the testing method may be calibrated for consistency as each heating element 30 is tested. To achieve this, in parallel with the heating element 30 there is provided a calibration resistor 52 of known resistance. Instead of closing the isolating switch part 38 for a heating element 30 under test, as during a testing period, a calibration switch 53 which is in series with the test switch 32, is closed.

As the resistance of the calibration resistor 52 is known and is selected to be of about the same resistance as a healthy heating element 30, the reference voltage input 47 providing the second input 43 to the differential amplifier 45 can be adjusted to provide an output 50 from which the controller 20 can readily identify as indicating that a heating element 30 under test, is or is not healthy.

All of the heating elements of the array 12 of the print head 10 may be tested sequentially, the voltage across the capacitance 37 being monitored during each heating element 30 test by providing a first input 42 to the differential amplifier 45 during an individual testing period for each individual heating element 30. The controller 20 may coordinate the testing of each of the heating elements 30 of the array 12 sequentially, by addressing each heating element 30 during a respective testing period. As the testing method of the invention monitors the voltage across the capacitance 37 rather than the voltage across a potential divider in which the heating element 30 is located, there is no need to wait for the capacitance 37 to reach a steady charge state during each heating element 30 test and so the method of the invention enables the testing period for each heating element 30 to be significantly shorter than in prior art, potential divider testing methods.

Various other modifications to those described already may be made without departing from the scope of the invention. The kind of thermal printer 14 described above by way of example is a so- called continuous thermal transfer printer, as the substrate 17 at least may move continuously past the print head 10 during printing, and pixels of ink from the ink carrier 18 are transferred from the ink carrier 18 to the substrate 17 to print dots which make up the image.

The invention may be applied to other kinds of thermal printers. For example the invention may be applied to a thermal transfer printer in which the print head is moved at the print station to effect relative movement between the substrate 17 and ink carrier 18 and the print head 14. Further alternatively the invention may be applied to a thermal printer of the kind in which there is no ink carrier 18 but the substrate 17 is heat sensitive so that dots of the substrate discolour in response to heat generated by energised heating elements. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1 . A method of testing the health of a heating element (30) of a print head (10) which is of the kind having a plurality of heating elements (30) which are each selectively energisable by a controller (20) to effect printing of a desired image on a substrate (17), wherein each heating element (30) is arranged in a circuit (29) in parallel with a capacitance (37), and the method includes during a testing period, simultaneously applying an electrical current to the heating element (30) under test and the capacitance (37), and monitoring the voltage across the capacitance (37).

2. A method according to claim 1 wherein the method includes, at or towards the beginning of a testing period, applying the electrical current to the capacitance (37) only and isolating the heating element (30) under test, and when the capacitance (37) is partially charged, connecting the heating element (30) in parallel with the capacitance (37).

3. A method according to claim 1 or claim 2 wherein the method includes determining the charging rate of the capacitance (37) by monitoring the voltage across the capacitance (37).

4. A method according to any one of the preceding claims wherein the method includes, during the testing period, providing a signal indicative of the voltage across the capacitance (37) to a first input (42) of a differential amplifier (45), and providing a reference voltage (47) to a second input (43) of the differential amplifier (45), and monitoring an output (50) from the differential amplifier (45).

5. A method according to claim 4 wherein the method includes monitoring the voltage across the capacitance (37) during a small part of the normal charging curve of the capacitance (47), using a suitably programmed processor device (20) via the differential amplifier (45).

6. A method according to claim 5 wherein the method includes providing an analogue to digital converter (51 ) between the output (50) from the differential amplifier (45) and the processor device (20).

7. A method according to any one of claims 4 to 6 wherein the reference voltage (47) applied to the second input (43) of the differential amplifier (45) is adjustable during calibration, to enable the voltage across the capacitance (37) during an optimum small part of the normal charging curve to be selected, for monitoring.

8. A method of testing the health of a heating element (30) substantially as hereinbefore described with reference to the accompanying drawings.

9. A printer (14) which includes a print head (10) having a plurality of heating elements (30) which are selectively energiseable by a controller (20) to effect printing of a desired image on a substrate (17), each heating element (30) being arranged in a circuit (29) in parallel with a capacitance (37), there being a voltage monitoring device (39) for monitoring the voltage across the capacitance (37) during performance of the testing method according to any one of the preceding claims.

10. A printer (14) according to claim 9 which includes a differential amplifier (45), having a first input (42) for a signal indicative of the voltage across the capacitance (37), and a second input (43) for an adjustable reference voltage, and an output (50) from the differential amplifier (45), and wherein the circuit (29) of the printer (14) includes a known calibration resistance (52) arranged in parallel with the capacitance (37), and there being a calibration switch (53) which is closable so that during calibration when no electrical current is provided to any heating element (30), simultaneously electrical current is provided to the calibration resistance (52) and the capacitance (37) whilst the voltage across the capacitance (37) may be monitored.

1 1 . A printer (14) substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.

12. Any novel feature or combination of features described herein and/or as shown in the accompanying drawings.