GB2249996A - Selection routine for energising print elements of a thermal printhead - Google Patents

Abstract

Thermal printing apparatus is disclosed in which the thermal printing elements are energised in two groups in order to reduce the peak power required to energise the elements. Each group of elements is energised while the other group is un-energised. The disclosure relates to use of the thermal printing apparatus in a franking machine. The elements of the two groups may be arranged alternately for printing a franking impression or where the machine is capable of printing a franking impression and a destination address on a mail item, the two groups may be on two print heads one group being used to print the franking impression and the other to print the destination address. <IMAGE>

Description

THERMAL PRINTING APPARATUS This invention relates to thermal printers and in particular to reducing the peak power required to operate thermal print heads.

Thermal printers comprise a print head having a plurality of resistive elements which are selectively heated to cause heating of areas of a thermally sensitive medium.

Printing may be effected to heating of a heat sensitised paper whereby areas of the paper which are heated change colour to form an image contrasting with the unheated portions of the paper. In an alternative method of thermal printing, ink is transferred selectively from a thermal transfer ink ribbon to a paper sheet. The thermal transfer ribbon comprises a backing layer or substrate carrying a layer of ink. The ribbon is located with the ink layer in contact with the paper sheet and the elements of the print head are located in heat transfer engagement with the backing layer of the ribbon. When a thermal element of the print head is heated, the ink layer immediately adjacent the heated element is heated to an extent such that the bond between the backing layer and the ink is reduced to enable the ink to be transferred to and bond with the surface of the paper.The thermal transfer ink ribbon may have a single layer of ink or may have multiple layers of ink which are transferred successively in successive printing cycles.

The resistive elements of the thermal print head are heated by passing an electric current through the elements. In order to heat the ink layer of the ribbon to a sufficiently high temperature to effect transfer of ink from the ribbon to the paper, or to heat the sensitised paper to a sufficiently high temperature to cause a colour change, it is necessary to input sufficient electrical power to heat the element.

Usually the thermal elements of the print head are disposed closely together in a line and the print receiving medium, together with the thermal transfer ink ribbon, is moved in a direction transverse to the line of elements. The elements are selectively energised by a pulse of electric current to print dots in required positions along the line and the elements are energised successively and selectively as the print receiving medium is moved past the elements to build up line by line a required pattern or character on the print receiving medium.

In printing a required pattern or character which has a bar extending in the direction of the line of print elements a large number of the print elements will be required to be energised simultaneously and hence during the printing of this bar the total electrical power input required to heat this large number of thermal elements will be high. However printing of portions of a pattern or character which have small extent in the direction of the line of elements only a small number of elements will be heated and the electrical power requirement will be correspondingly low.It will be appreciated that the power requirement for printing a part of a character or pattern having a large extent in the direction of the line of thermal elements will have a peak value considerably in excess of the average power requirement for heating of the elements during the printing of a complete pattern or character. Accordingly although the average power requirement may be relatively low, a power supply for supplying electrical power to the print head needs to be capable of meeting the demand for the much higher peak power requirement during some periods of printing.

It has been proposed to use thermal transfer printing for the printing of franking impression on mail items. The form of such franking impressions is determined by the relevant postal authority. The franking impression usually comprises a pattern which is easily recognisable as a franking and, within the pattern, postage data such as postal charge, the date and the license number of the franking machine is printed. The pattern of the franking impression, at least in some countries, includes long bars extending in both horizontal and vertical directions. A bar extending transverse to the line of thermal print elements is printed by a succession of energisations of a small number of elements during feeding of the mail item past the print head.However a bar which extends in the direction of the line of thermal print elements will be printed by simultaneous energisation of substantially all the thermal print elements and this will require a very high peak electrical power input to the print head.

The need to provide a power supply capable of meeting this very high peak power demand results in the power supply being larger and more costly than would be required if the power supply needed only to be capable of meeting a lower demand closer to the average demand. Electrical power supplies tend to be bulky and occupy a large amount of space in electronic equipment. Accordingly it is desirable to reduce the bulk and cost of power supplies in order that the overall size and cost of the electronic equipment can be reduced.

According to one aspect of the invention a method of reducing the peak power requirement for selective electrical energisation of a plurality of resistive thermal printing elements comprises the steps of energising selected ones of a first group of said thermal printing elements in a sequence of first printing cycles and energising selected ones of a second group of said thermal printing elements in a sequence of second printing cycles, said second printing cycles being timed to occur in first intervals between successive first printing cycles and said first printing cycles being timed to occur in second intervals between successive second printing cycles, said elements of the first group being unenergised during said first intervals and elements of the second group being un-energised during said second intervals.

According to a second aspect of the invention thermal printing apparatus includes a plurality of thermal print elements; means operable to energise selected ones of a first group of the thermal printing elements in a succession of first printing cycles; means operable to energise selected ones of a second group of the thermal printing elements in a succession of second printing cycles; timing means to determine the timing of the first printing cycles to occur in second intervals between successive second printing cycles and the timing of the second printing cycles to occur in first intervals between successive first printing cycles.

Embodiments of the invention will now be described with reference to the drawings in which: Figure 1 is a block diagram of a franking machine with two thermal print heads in which the thermal printing elements are energised in accordance with the invention, Figure 2 illustrates the relative timing of strobe pulse trains for timing the energisation of the thermal print elements, Figure 3 is a block diagram of circuitry for controlling energisation of thermal printing elements of a single print head, and Figure 4 illustrates the printed impression obtained with the print head of Figure 3.

Referring first to Figure 1, a franking machine includes a microprocessor 10, or other electronic circuitry, to carry out control and accounting functions. A keyboard 11 connected to the microprocessor enables input by a user of the machine of control signals to control operation of the machine and input of data such as postage charges to determine the value of a franking applied to a mail item.

A display device 12 is provided to display data and other information to a user to assist the user in the operation of the franking machine. A read only memory (ROM) 13 for the storage of fixed data and program information and a working store comprising a random access memory (RAM) 14 for the storage of information and data in the course of operation of the machine are connected to the microprocessor 10.

As is well known in the franking machine art, non-volatile memories 15, 16 are provided for the storage of accounting data relating to the use of the franking machine in franking mail items. The accounting data usually comprises a value of credit available for franking and stored in a descending register, a tote value of accumulated postage charges already used in franking and stored in an ascending register, an items count comprising the number of items franked and a high items count comprising the number of items franked with a postage charge in excess of a predetermined value.In the course of operation of the franking machine, each time a mail item is franked, the microprocessor carries out, under the control of a program routine stored in the ROM 13, a franking routine in which a determination is made as to whether there is sufficient credit available for a required- value of franking input on the keyboard 11, the credit value in the descending register is decremented by the required postage value, the tote value in the ascending register is incremented by the required postage value and the items count is incremented by unity. If the postage charge is in excess of the predetermined value, the high items count also is incremented.In order to enable integrity of the accounting data to be maintained in the event of fault arising, each of the non-volatile memories provides duplicate registers to store all of the accounting data so that each item of accounting data is stored twice in each of the memories. Periodically, for example at the start of every franking operation, the accounting data is checked to determine if there are any errors. In the event of detection of an error in the accounting data further operation of the franking machine is inhibited until it has been checked by a service engineer.

The printing of a franking impression on a mail item is effected by means of thermal print head 17 comprising a row of thermal print elements 18 carried on a substrate (not shown). Print data defining the pattern of the franking impression together with any variable data required to be printed within the franking impression is output by the microprocessor 10 by means of an interface 19. The print data is loaded into a print data buffer 20 which has a plurality of storage locations corresponding to the plurality of thermal print elements 18. The storage locations of the print data buffer are connected respectively to one input of gates 21 and the output of the gates are connected respectively to the thermal print elements. A print strobe pulse is output by the microprocessor via the interface to a line 22 connected to a second input of the gates.Upon occurrence of the print strobe pulse, any gate receiving a data bit input from the print data buffer representing a dot to be printed causes a current from a power supply 23 to pass through the associated thermal print element and thereby effects printing of dots at positions determined by the print data in the buffer 20. The line of thermal print elements 18 of the head 17 spans a distance at least as great as the height of the required franking impression so that the franking impression is printed column by column in a single pass of the mail item past the print head. It will be appreciated that consecutive energisations of the thermal print elements are spaced in time to permit a thermal element energised in one print cycle to cool sufficiently that it does not cause printing in the next print cycle unless it is re-energised in that subsequent cycle.

The franking machine, in addition to including a print head to print the franking impression, is provided with a second print head 24 to print a destination address on the mail item. The second print head 24 is similar to the print head 18 and has a line of thermal print elements, carried on a substrate (not shown) which span a distance sufficient to print a number of lines of the destination address in parallel. The thermal print elements of print head 24 are controlled by gates 25 in dependence upon destination print data loaded into a destination print buffer 26. Print data defining the destination address to be printed is output by the microprocessor 10 via the interface 19 to the buffer 26.Destination address data may be input by means of the keyboard, if the keyboard is provided with a set of alphabetic keys, or destination address data may be input from a peripheral device such as a personal computer connected to the franking machine by a line 27. In the latter arrangement the destination address data input on line 27 is input to the microprocessor 10 via the interface 19. The destination address is stored temporarily in RAM 14. Timing of energisation of the thermal print elements of the print head 24 is determined by a strobe pulse on line 28 connected to a strobe input of the gates 26.

It will be appreciated that heating of the thermal printing elements may effect printing by heating of thermally sensitive paper or by thermal transfer of ink from a thermal transfer ink ribbon. Generally it is preferred to use a thermal transfer printing process because printing can then be effected on any mail item fed through the franking machine. The thermal transfer ribbon may have a single layer of ink of which substantially the entire layer is transferred at any area subjected to heating by an energised thermal printing element. The ribbon is used only once. Alternatively the thermal transfer ink ribbon may have multiple layers of ink and be capable of being used a number of times. On each pass of the ribbon, the ink in the outermost layer of any area subjected to heating by an energised thermal printing element is transferred.

As has been explained hereinbefore, if a large number of dots are required to be printed in a column, the peak power demand to energise and heat the print elements to print those dots is very high as compared with the average power demand. The franking impression often includes a pattern which contains solid vertical bars which require at least the majority of the print elements 18 of the print head 17 to be operated simultaneously in a print cycle and the thickness of the bars may require similar energisation of the majority of the print elements in a succession of print cycles.

Many alphabetic characters contain vertical bars and hence it is possible for such vertical bars to be required to be printed in corresponding positions in the different lines of the destination address. This can result in a high peak current demand in printing cycles in which these bars are printed and it is possible that peak current demands arising from printing of vertical bars of characters of the destination address may coincide with peak current demands for printing vertical bars of the franking impression by the first printing head 17. As a result not only are the current demands of the two print heads subject to peak demand but it is possible for these peak demands to coincide and the resultant peak demand to be even higher.

In order to reduce the instantaneous peak power requirement of the print heads, the strobe pulses on lines 22 and 28 are timed to occur at different times, as shown in Figure 2, such that the strobe pulse 22 occurs during an interval between consecutive strobe pulses 28 and similarly the strobe pulses 28 occur in intervals between consecutive pulses 22.. Accordingly any instant of time in which the thermal elements of print head 17 are energised due to the strobe pulse 22 occur during a time interval between pulses 28 during which the print head 24 is quiescent and not energised. Similarly any instant of time in which the thermal elements of print head 24 are energised due to the strobe pulse 28 occur during a time interval between pulses 22 during which the print head 17 is quiescent and not energised.Therefore the peak demand of current to energise the two print heads is never greater than the peak demand of current for either print head alone. As a result it is possible to operate two print heads from a power supply of the same capacity as that required to operate one of the print heads. The offset in timing of the two strobe pulse trains causes the impressions printed by the two the respective print heads to be slightly offset relative to one another in the direction of feed of the mail item. The form of the franking impression is dissimilar to that of the destination address formed of strings of characters so that even relatively large offsets are not noticeable.

However the density of the printed dots in the direction of offset is sufficiently high that some offset of printed impressions of similar form is acceptable. Accordingly if desired the thermal elements of a single head may be energised at different strobe timings to reduce the peak power demand for that head. Contiguous print elements used for printing the different lines of the destination address may be strobed as groups of elements. For example contiguous elements used for printing a first line may be strobed by a first strobe pulse train, contiguous elements used for printing a second line may be strobed by a second strobe pulse train, and contiguous elements used for printing third and fourth lines may be strobed by third and fourth trains respectively.If a smaller reduction in peak power requirement is acceptable the same pulse train may be used to strobe two groups of elements.

A block diagram of a circuit in which the thermal print elements of a single print head are strobed in two groups is shown in Figure 3. A thermal print head 30 includes a line of thermal print elements 311 ... 31n Print data from the microprocessor is loaded into a print data buffer 32 having storage locations 331 33n corresponding to the thermal print elements. The storage locations are connected to print data inputs of a plurality of gates 341 ... 34n and the outputs of the gates are connected to the associated thermal print elements 31. The gates 341 ...34n have second inputs driven by strobe signals so that, when a strobe pulse is input to the gates, the thermal printing elements are energised in dependence upon the binary values of the print data in the corresponding storage locations of the buffer 32.The second inputs of the gates are connected in two or groups and differently timed strobe pulse trains are applied to the second inputs of each group. The thermal printing elements may be grouped as desired and may be arranged in groups of contiguous elements or the elements of two or more groups may be interposed with one another. For example, if the elements are grouped in two groups, the elements in odd numbered positions may form one group and the elements in even numbered positions form the second group. With this latter arrangement, the second inputs of odd numbered gates 341 343, 345 etc are connected to a first strobe input line 35 and the second inputs of even numbered gates 342 344 346 etc are connected to a second strobe input line 36.The strobe pulse trains are timed as shown in Figure 2 so that a strobe pulse from one train causes power from a power supply 37 to be applied to selected ones of odd numbered thermal printing elements 311, 313, 315 etc while the even numbered elements remain unenergised and after an interval a strobe pulse from the other train causes power to be applied to selected ones of even numbered thermal printing elements. Accordingly at any instant of time only one half of the thermal printing elements can be energised. The offset in timing of the two groups of elements will cause some degradation of the printed impression so that the impression tends to have a checkered pattern as shown in Figure 4. However for printing of a franking impression the degradation would be sufficiently slight as to be acceptable. If desired the thermal print elements may be arranged to be energised in a larger number of groups than two as hereinbefore described and where a fixed pattern is to be printed, as when printing franking impression, the grouping of the may be arranged in dependence to at least some extent upon the form of pattern to be printed whereby it may be possible to offset the timing of energisation of thermal printing elements which would have the least effect on the visual appearance of the printed impression.

Claims (11)

1. A method of reducing the peak power requirement for selective electrical energisation of a plurality of resistive thermal printing elements comprising the steps of energising selected ones of a first group of said thermal printing elements in a sequence of first printing cycles and energising selected ones of a second group of said thermal printing elements in a sequence of second printing cycles, said second printing cycles being timed to occur in first intervals between successive first printing cycles and said first printing cycles being timed to occur in second intervals between successive second printing cycles, said elements of the first group being un-energised during said first intervals and elements of the second group being un-energised during said second intervals.

2. Thermal printing apparatus including a plurality of thermal print elements; means operable to energise selected ones of a first group of the thermal printing elements in a succession of first printing cycles; means operable to energise selected ones of a second group of the thermal printing elements in a succession of second printing cycles; timing means to determine the timing of the first printing cycles to occur in second intervals between successive second printing cycles and the timing of the second printing cycles to occur in first intervals between successive first printing cycles.

3. Thermal printing apparatus as claimed in claim 2 including a first group of buffer storage locations for storing first print data for determining those thermal printing elements of the first group selected to be energised in a first printing cycle; a second group of buffer storage locations for storing second print data for determining those thermal printing element of the second group selected to be energised in a second print cycle; first gate means connecting storage locations of the first group to corresponding thermal printing elements of the first group; second gate means connecting storage locations of the second group to corresponding thermal printing elements of the second group; means to apply a first strobe pulse to all said gate means of the first group to cause energisation of the selected thermal elements of the first group and after termination of said first pulse to apply a second strobe pulse to all the gate means of the second group to cause energisation of the selected thermal elements of the second group.

4. Thermal printing apparatus as claimed in claim 2 or 3 wherein the thermal printing elements of the first group are contiguous and wherein the thermal printing elements of the second group are contiguous.

5. Thermal printing apparatus as claimed in claim 4 wherein the thermal printing elements of the first group are carried by a first substrate and the thermal printing elements of the second group are carried by a second substrate.

6. Thermal printing apparatus as claimed in claim 2 or 3 wherein the thermal printing elements of the first and second groups are interposed and are carried by a single substrate.

7. Thermal printing apparatus as claimed in any one of claims 2 to 6 wherein the thermal printing elements of the first group are disposed in a first line and wherein the thermal printing elements of the second group are disposed in a second line.

8. Thermal printing apparatus as claimed in claim 7 wherein the first and second lines are co-linear.

9. A method of reducing the peak power requirement for selective electrical energisation of a plurality of resistive thermal printing elements substantially as hereinbefore described with reference to the drawings.

10. Thermal printing apparatus constructed and arranged to operate substantially as hereinbefore described with reference to Figures 1 and 2 or as modified by Figures 3 and 4.

11. A franking machine incorporating thermal printing apparatus as claimed in any one of claims 2 to 8 or 10.