Printing method and apparatus

Abstract

 Digital printing apparatus is provided with a sensor (24, 25) to sense the roughness of a surface being printed on and the output of the sensor (24, 25) is utilised to provide a display to enable manual adjustment of power input to a print head (12) or to automatically control the power input to the print head (12) in dependence upon the surface roughness so as to produce a quality of printing independent of the surface roughness. This is of particular interest in relation to thermal printing and may be used in mailing machines to ensure a good print quality for franking impressions.

Description

[0001] This invention relates to printing and in particular to digital printing in which print elements are operated selectively to print a desired impression on a print receiving medium.

[0002] Digital printing apparatus is well known and includes a digital print head having a plurality of printing elements selectively operable to print a plurality of dots which together form a required printed impression. Commonly, the printing elements are located in a line and as the print receiving medium is moved past the print head, or the print head is moved relative to the print receiving medium, in a direction transverse to the line of printing elements the printing elements are operated selectively in each of a plurality of print cycles such as to print dots in selected positions of the line in each cycle so that a required printed impression is formed in a line-by-line manner. By appropriate selective operation of the printing elements alpha-numeric characters or other patterns may be printed on the medium.

[0003] Known digital printing heads include impact print heads in which print wires are propelled toward the print receiving medium, thermal print heads in which print elements are selectively heated by passage of pulses of electric current and ink jet print heads in which droplets of ink are ejected from the head to the print receiving medium. When using impact print heads or thermal print heads, an inked ribbon is fed between the printing elements of the print head and the print receiving medium. When the ink ribbon is struck by a propelled print wire ink is transferred by the impact from the ribbon to the print receiving medium. When using a thermal print head, a thermal transfer ink ribbon is utilised. The print receiving medium is pressed toward the thermal printing elements of the print head so that, in the region of the thermal printing elements, the print receiving medium is in engagement with a layer of ink carried by the ribbon and the rear of the ribbon is in engagement with the thermal printing elements. Heating of a thermal printing element causes the ink layer adjacent the heated thermal printing element to be heated and to adhere to the print receiving medium. After passing the thermal print elements the ribbon is peeled from engagement with the print receiving medium and those parts of the ink layer which have been heated remain adhered to the print receiving medium to form a required printed impression.

[0004] The quality of the printed impression when printed by thermal printing elements varies to a substantial extent depending upon characteristics of the surface of the print receiving medium and in particular upon the roughness of this surface. Generally, to obtain a given quality of printed impression, when printing on a rough surface it is necessary to energise the thermal printing elements with a higher energy level than that required for printing on a relatively smooth surface.

[0005] It has been proposed that thermal printing techniques be used in postage franking machines to print impressions on envelopes and other mail items to indicate that a postage charge has been made and accounted for in respect of each mail item. Franking machines must be capable of printing on a relatively wide range of paper material of which envelopes are manufactured. In order to ensure that printing of a satisfactory print impression meeting standards determined by postal authorities is achieved on the range of paper materials which may be used for envelopes it is necessary to set the energy level at which the thermal printing elements are energised to a sufficiently high level that satisfactory print impressions are obtained on the poorest quality paper material likely to used. That is to say, the energy level of energisation of the thermal printing elements is chosen to meet the requirements in respect of a worst case print receiving medium so that a minimum quality of print impression is achieved. As a result when printing on relatively high quality paper material, the energy supplied to the thermal printing elements is substantially higher than is necessary to achieve the required quality of printed impression. When printing on relatively smooth print receiving media, the excess level of energy input to the printing elements may result in degradation of the printed impression from the optimum which could be achieved and furthermore the higher level of energisation of the thermal printing elements tends to reduce the working life of the printing elements.

[0006] According to one aspect of the invention printing apparatus including printing elements selectively operable during relative motion between a print receiving medium and the thermal printing elements; a source of power to energise said thermal printing elements; is characterised by sensing means responsive to surface roughness of said print receiving medium to generate and display a roughness signal representing said roughness; and control means operable to set energy input from said source of power to said printing elements at a level corresponding to said displayed roughness signal.

[0007] According to a second aspect of the invention printing apparatus including printing elements selectively operable during relative motion between a print receiving medium and the thermal printing elements; a source of power to energise said thermal printing elements; is characterised by sensing means responsive to surface roughness of said print receiving medium to generate a roughness signal representing said roughness; and control means operative to control energy input from said source of power to said printing elements in dependence upon said roughness signal.

[0008] The printing apparatus may be incorporated in a franking machine for printing a value impression on an item indicating that a postage or other value charge has been made in respect of the item.

[0009] According to a third aspect of the invention a method of printing by selective energisation of thermal printing elements while a print receiving medium is fed past said thermal printing elements and energisation of the thermal printing elements is effective to transfer ink from a thermal transfer ink ribbon to the print receiving medium to form a required printed impression on the print receiving medium is characterised by the steps of sensing surface roughness of the print receiving medium and varying energy level of energisation of the thermal printing elements in dependence upon the sensed surface roughness.

[0010] An embodiment of the invention will be described hereinafter by way of example with reference to the drawings in which:-

Figure 1 shows a franking machine embodying the present invention, and

Figure 2 is a block circuit diagram of the franking machine.

[0011] Referring first to Figure 1, a franking machine includes a feed bed 10 extending over a base 11 of the machine. A thermal print head 12 is mounted over the feed bed in spaced relationship relative to the feed bed. The spacing of the thermal print head from the feed bed is sufficient to permit the passage of mail items 13, such as envelopes containing inserts, between the thermal print head 12 and the feed bed 10. Mail items are fed onto the feed bed by means of a pair of input feed rollers 14, 15 and thence to the thermal print head. An impression roller 16 is mounted on a cradle (not shown) in the base 11 of the machine such that the impression roller is opposed to a line of printing elements of the print head 12 disposed along a line extending transversely to the direction of feeding of mail items along the feed bed. The cradle is pivotable so as to move the impression roller between an operative position in which the impression roller is resiliently urged toward the thermal printing elements of the thermal print head and an inoperative position in which the periphery of the impression roller is spaced from the printing elements and lies in the plane of, or below, the feed bed. A thermal transfer ink ribbon 17 passes from a supply spool 18 between the thermal print head and the feed bed to a take-up spool 19. A rear face of the ribbon is in engagement with the thermal printing elements and a front face of the ribbon carries a layer of ink. When a leading edge of the mail item enters between the thermal print head and the feed bed, the cradle carrying the impression roller is pivoted to move the impression roller into the operative position such that the impression roller resiliently engages the mail item and urges the mail item into engagement with the ink layer and the rear face of the ribbon into engagement with the thermal printing elements. The impression roller is rotatively driven by a drive motor (not shown) to feed the mail item past the thermal printing elements of the print head. Due to the engagement of the mail item with the ink layer, the ink ribbon is drawn from the supply spool past the printing elements by the feeding of the mail item. After passing the printing elements, the ink ribbon is wound onto the take up spool, the take-up spool being driven so as to maintain the used ribbon under slight tension and thereby peel the used ribbon away from the mail item. The feed path of the ink ribbon may be defined by guide rollers 20, 21 located respectively upstream and downstream of the print head. During passage of the mail past the thermal printing elements, the thermal printing elements are energised selectively with pulses of electric current to cause heating of the selected elements and thereby transfer ink in from the ribbon to selected locations of the mail item. The selective energisation of the thermal printing elements is effected in a plurality of successive printing cycles such that in each cycle dots are printed in required locations along the line of the thermal printing elements and energisation of printing elements in successive cycles as the mail item is moved past the line of elements builds up a required printed impression in line-by-line manner on the mail item.

[0012] After passing through the nip between the impression roller and the thermal printing head, the mail item passes through a nip between a pair of ejection rollers 22, 23, one of which rollers is driven to eject the mail item from the franking machine.

[0013] The energy level with which the thermal printing elements are energised is determined by the magnitude and/or duration of the pulses of electric current applied to the thermal printing elements. The energy level is chosen such that the heating of the thermal printing elements is sufficient to effect reliable transfer of ink from the ink layer of the ribbon to the mail item but is not so great as to result in a smeared printed impression.

[0014] For a given energy level the quality of printing is dependent upon characteristics of the print receiving surface of the mail item and in particular upon the surface roughness of the print receiving medium. The quality of the impression is degraded with increase of surface roughness of the mail item. Previously, in order to achieve a printed impression of required quality on mail items formed from a range of materials having a range of different roughness, the energy level supplied to the printing elements is set sufficiently high as to ensure a printed impression under worst case conditions, namely when printing on materials having the greatest roughness of the range. However the relatively high energy level supplied results in printed impressions on materials having the least roughness of the range being less than an optimum quality which could be achieved with a lower level of energy supplied. Furthermore, high energy level supply to the thermal printing elements results in a shorter working life of the thermal printing elements.

[0015] A sensor 24 is mounted above the feed bed 10 upstream of the thermal print head between the print head and the input rollers 14, 15. The sensor 24 has a stylus 25 engaging the surface of the mail item as the mail item is fed along the feed bed. The stylus is free to move in a direction perpendicular to the plane of the fed bed and is resiliently biased toward the feed bed. Roughness of the surface of the mail item causes the surface of the mail item to have peaks and indentations. Hence, as the peaks and indentations of the surface of the mail item surface pass the stylus, the stylus as it rides over the surface is caused to move respectively away from and toward the feed bed. The motion of the stylus is dependent upon the number of peaks and indentations per unit area of the surface of the mail item. A smooth surface will have a relatively large number of peaks and indentations of relatively small magnitude per unit area whereas a rough surface will have a smaller number of peaks and indentations per unit area. Accordingly the frequency at which the stylus is moved and the amplitude of such movement due to engagement of the stylus with the surface of the mail item correspond to the roughness of the surface of the mail item. The sensor 24 generates an electrical signal corresponding to the movement of the stylus 25. In effect the sensor 'listens' to the surface of the mail item with which the stylus is engaged and generates an electrical signal due to sensing of the roughness of the surface.

[0016] The sensor may comprise a moving coil transducer. A coil is suspended for motion in an air gap of a magnetic structure. A permanent magnet generates magnetic flux in the air gap and the coil is electromagnetically coupled with the magnetic flux in the air gap. The coil is mechanically connected to the stylus such that movement of the stylus is transmitted to the coil. Resultant movement of the coil in the air gap induces an electrical current in the coil corresponding to movement of the coil and hence of the stylus. Alternatively the sensor may comprise a moving magnet transducer in which a permanent magnet is mechanically connected to the stylus and moves relative to a coil. The coil and magnet are electromagnetically coupled and an electrical current is induced in the coil as a result of movement of the stylus. Similarly the sensor may comprise other forms of transducer, for example piezoelectric.

[0017] Referring now to Figure 2, operation of the franking machine is effected by means of a micro-processor 30 operating under program routines stored in a read only memory (ROM) 31. As is well known in electronic franking machines, a keyboard 32 is provided for input of data by a user and a display 33 is provided to enable display of information to the user. A random access memory (RAM) 34 is provided for use as a working store for storage of temporary data during operation of the franking machine. Non-volatile duplicated memories 35, 36 are provided for the storage of data which is required to be retained even when the franking machine is not powered. Accounting data relating to use of the franking machine for printing frankings representing postage charges for mail items and any other critical data to be retained is stored in the non-volatile memories 35, 36. A motor controller 37 is controlled by the microprocessor to control operation of motors for driving the input drive rollers, the impression roller, the impression roller cradle, ejection rollers and ink ribbon take-up spool. Sensors 38 are provided to sense and monitor feeding of the mail item and of the ink ribbon. The sensors provide signals to the microprocessor to enable the microprocessor to control operation of the machine. For example a sensor is provided to indicate the speed of feeding of the mail item along the feed bed to enable the microprocessor to control speed of drive of a motor driving the impression roller such that the feed speed is maintained substantially constant. As the mail item is fed past the thermal printing elements of the print head, the microprocessor outputs, on line 39, to the print head in each of a plurality of printing cycles signals selecting those ones of the printing elements which are to be energised in the respective cycle. A pulse of electrical power is supplied to the selected thermal printing elements from a power source 40 when a strobe signal, on line 41, is supplied by the microprocessor.

[0018] The energy level supplied to the thermal printing elements by a pulse of electrical power from the source 40 is controlled and varied by an energy level controller 42. The energy level controller 42 may be operated by the microprocessor in dependence upon an input by the user of the franking machine on the keyboard selecting an energy level in corresponding to a display of a signal derived from sensing the roughness of the print receiving medium. However it is preferred that the energy level controller 42 is operated by the microprocessor directly in dependence upon signals received from the sensor 24 indicating surface roughness of the mail item.

[0019] It will be appreciated that as the mail item is fed past the sensor 24 with the stylus 25 thereof engaging the surface of the mail item the sensor outputs an electrical signal. Due to the sensing of peaks and indentations of the surface of the mail item the stylus will be caused to have an oscillatory movement and the electrical output of the sensor will comprise a generally sinusoidal waveform, the frequency components and amplitude of the waveform being related to the surface roughness of the mail item. The electrical signal output from the sensor is processed to remove components which are not significant, for example components having a frequency outside a range of frequencies significant to detection of roughness of the surface of the mail item, and then is processed to provide a roughness signal indicative of the surface roughness of the mail item. The removal of unwanted signal components from the output of the sensor 24 may be carried out by a passive filter or by the microprocessor. The roughness signal may be an analogue or digital signal.

[0020] The microprocessor controls the energy level controller 42 in dependence upon the roughness signal such that the energy level supplied to the thermal printing elements is relatively high when a rough surface is sensed and is relatively low when a smooth surface is sensed. The energy level may be varied in a continuous manner between the limits of the range of roughness or the energy level may be controlled stepwise at a selected one of a number of levels.

[0021] Generally the roughness of the surface of a mail item will be substantially uniform over the surface of the mail item and hence it is necessary only to sense a relatively small portion of the surface. However if desired, the sensing of the surface may be effected continuously throughout passage of the mail item past the sensor and if desired a delay may be provided to the electrical signals from the sensor to the microprocessor in order that the sensing signals relate to the area of the mail item currently adjacent the thermal printing elements.

[0022] It is to be understood that Figure 1 of the drawings shows only those elements required to explain the operation of the invention and does not show other elements which would be present in a franking machine. The construction of electronic franking machines is well known in the art and hence it is believed to be unnecessary to provide a detailed explanation of the operation of the franking machine. Figure 2 shows logical connections between the microprocessor and other electronic elements and in practice these connections may be provided by one or more busses.

Claims

1. Printing apparatus including printing elements (12) selectively operable during relative motion between a print receiving medium (13) and the thermal printing elements (12); a source of power (40) to energise said thermal printing elements (12); characterised by sensing means (24, 25) responsive to surface roughness of said print receiving medium (13) to generate and display a roughness signal representing said roughness; and control means (42) operable to set energy input from said source of power (40) to said printing elements (12) at a level corresponding to said displayed roughness signal.

2. Printing apparatus including printing elements (12) selectively operable during relative motion between a print receiving medium (13) and the thermal printing elements (12); a source of power (40) to energise said thermal printing elements (12); characterised by sensing means (24, 25) responsive to surface roughness of said print receiving medium (13) to generate a roughness signal representing said roughness; and control means (42) operative to control energy input from said source (40) of power to said printing elements (12) in dependence upon said roughness signal.

3. Printing apparatus as claimed in claim 1 or 2 wherein the printing elements are thermal printing elements energisable by electric current pulses to cause heating of said thermal printing elements (12) and including a thermal ink transfer ribbon (17) heatable by the energisation of said thermal printing elements to deposit ink in selected locations on the print receiving medium (13).

4. Printing apparatus as claimed in claim 1, 2 or 3 wherein the print receiving medium (13) is fed past the thermal printing elements (12) and the sensing means (24, 25) at substantially constant speed.

5. Printing apparatus as claimed in any preceding claim wherein the sensing means (24, 25) generates a waveform signal corresponding to the sensed surface of the print receiving medium (13) and including signal processing means (24) to process said waveform signal to generate the roughness signal.

6. Printing apparatus as claimed in claim 5 wherein the waveform signal includes signal components which are not significant to determination of roughness of the surface of the print receiving medium and wherein the signal processing means (24) includes means to remove said signal components from the waveform signal.

7. Printing apparatus as claimed in claim 5 or 6 wherein the waveform signal generated by the signal processing means (24) has a characteristic varying in proportion to the roughness of the surface of the print receiving medium (13).

8. Printing apparatus as claimed in claim 7 wherein the waveform signal has an amplitude varying in dependence upon the roughness of the surface of the print receiving medium (13).

9. Printing apparatus as claimed in claim 7 or 8 wherein the waveform signal has at least one frequency component varying in dependence upon the roughness of the surface of the print receiving medium (13).

10. Printing apparatus as claimed in any one of claims 5 to 9 wherein the roughness signal generated by the processing means (24) is an analogue signal having a magnitude representing the roughness of the surface of the print receiving medium (13).

11. Printing apparatus as claimed in any one of claims 5 to 9 wherein the roughness signal generated by the processing means (24) is a digital signal representing a value indicative of the roughness of the surface of the print receiving medium (13).

12. Printing apparatus as claimed in any preceding claim wherein the sensing means (24, 25) comprises a transducer having a stylus (25) engaging the surface of the print receiving medium (13).

13. Printing apparatus as claimed in claim 12 wherein the transducer is a moving coil or moving magnet electromagnetic device.

14. Printing apparatus as claimed in any preceding claim and incorporated in and constituting a part of a postal franking machine.

15. A method of printing by selective energisation of thermal printing elements (12) while a print receiving (13) medium is fed past said thermal printing elements (12) and energisation of the thermal printing elements (12) is effective to transfer ink from a thermal transfer ink ribbon (17) to the print receiving medium (13) to form a required printed impression on the print receiving medium (13) characterised by the steps of sensing surface roughness of the print receiving medium and varying energy level of energisation of the thermal printing elements (12) in dependence upon the sensed surface roughness.

Drawing