EP0761454A1 - Tape printing apparatus - Google Patents

Abstract

The printer (1) has a print head (16) with a number of selectively operated print head elements arranged along a longitudinal axis of the print head. A receiving image tape (4) is moved relative to the print head to enable printing of an image onto the tape in the form of a number of adjacent pixel rows from a thermal transfer tape (12). The print head is controlled to produce a number of print cycles, in each of which selected print elements are activated to print a line on the tape. Each pixel in a printed row is printed over a number of successive cycles so that the same print elements are activated corresp. often at adjacent points on the tape.

Description

The present invention relates to tape printing devices according to the preambles of claims 1 and 9.

Known tape printing devices of the type with which the present invention is concerned are disclosed in EP-A-0 322 918 and EP-A-0 322 919 (Brother KK) and EP-A-0 267 890 (Varitonic). The printers include a printing device which has a cassette accommodating space for accommodating a cassette or a tape holder case. In EP-A-0 267 890 the ribbon holder housing contains an ink ribbon and a substrate ribbon, the latter comprising an upper image-receiving layer which is attached to a backing layer by adhesive. In EP-A-0 322 918 and EP-A-0 322 919 the tape holder housing contains an ink ribbon, a transparent image receiving tape and a double-sided adhesive tape which is applied with one of its sticky surfaces to the image-receiving tape after printing and which has a peelable backing paper on the opposite surface. In all of these devices, the image transfer medium (ink ribbon) and an image reception tape (substrate) are contained in the same cassette.

The applicant has developed another type of tape printing device which e.g. in EP-A-0 578 372, the content of which is incorporated herein by reference. In this printing device, the substrate ribbon is similar to that described in EP-A-0 267 890, but is contained in its own housing, while the ink ribbon is arranged analogously in an associated housing.

In all of these cases, the image-receiving tape is passed overlapping the ink ribbon through a printing zone consisting of a fixed print head and a platen roller against which the print head can be pressed to transfer an image from the ink ribbon to the image-receiving tape. This can be done in a number of ways, including dry lettering or dry film printing, but currently the most common is the thermal printing process, in which the printhead is heated and the heat causes ink to transfer from the ribbon to the image receiving ribbon. Alternatively, the printhead can be in direct contact with a thermally sensitive image receiving tape, an image being formed on the image receiving tape by heating the printhead.

The print head of such a printing device generally comprises a large number of printing elements which are activated, that is to say heated, individually. The activated print elements of the print head heat up, so that the ink is transferred from the parts of the ink ribbon that are in contact with the heated print elements to the image receiving tape. Alternatively, the heated printing elements can directly contact a thermally sensitive image receiving tape, so that an image is also formed on it. These known print heads generally comprise a series of printing elements which have a height which generally corresponds to the width of the image receiving belt used. All pressure elements are arranged in such a way that they can be activated simultaneously if required.

During operation, the ribbon is moved past the printhead and the printhead is activated in cycles to produce the desired image on the image receiving ribbon. A typical cycle takes 10 milliseconds. The printing elements to be activated (in this cycle) are activated (heated) for 2 milliseconds in this cycle. No print elements of the printhead are activated for 8 milliseconds of the cycle. This allows the power supply to recover and also allows the printhead to cool down. In order to be able to activate all the pressure elements in the 2 millisecond part of the cycle, the device is designed to be able to deliver a relatively large peak current. This is disadvantageous since relatively high current peaks reduce the battery life, which is disadvantageous for battery-operated devices.

According to a first aspect of the present invention, a tape printing device according to the teaching of claim 1 is proposed.

As discussed above, the peak current is proportional to the area of the printhead that can be activated at any given time. By reducing the area of each print element of the printhead so that activation of the same print element in at least two consecutive cycles is necessary to generate a pixel in the printed image, it is possible to reduce the peak current required. This has been found to extend the life of the battery. This is particularly advantageous if the power supply is implemented in the form of one or more batteries. The peak current required thus becomes smaller and is preferably less than three times the average current over the pressure cycle. A smoother, averaged current is thus obtained, which means that a lower voltage power supply can be used which reduces the cost of the tape printing device as a whole. This also extends the life of the batteries. In general, the greater the peak current, the shorter the lifespan of the battery with the same energy consumption. Lowering the peak current extends the life of the battery.

The means for establishing the relative movement between the image receiving tape and the printhead preferably perform a continuous movement, ie they are controlled so that there is essentially no change in the relative speed occurs between the printhead and the image receiving tape during a print cycle. This allows the use of a DC motor instead of a stepper motor, which can be less expensive and smaller.

Preferably, each pixel is printed by creating two consecutive print cycles so that the same print elements are activated twice at adjacent locations on the ribbon. In other embodiments, it may prove necessary to activate the same printing elements three or four times in successive cycles to print a pixel.

The printing elements can be arranged in at least two groups, which can be activated individually at different times in a printing cycle. This further helps reduce the peak current, which in turn results in the advantages discussed in connection with the first aspect of the invention.

Each of the printing elements can be substantially rectangular and each pixel of the printed image can be substantially square.

In the embodiments having a plurality of groups of print elements that are activated at different times within a print cycle, the image printed on the image receiving tape can have an offset effect caused by the print elements of the print head that occur at successive times during one Print cycle can be activated. This can occur in the embodiments in which the image receiving tape is moved continuously along the printhead, for example with a DC motor.

It may therefore prove necessary to compensate for the offset along the longitudinal axis of the printhead caused by the continuous movement of the image receiving tape. In particular, the offset of the print head is preferably the reverse of that of an image that would be obtained if an exactly linear print head with a plurality of groups of print elements that were activated at different times were to be produced in connection with an image receiving belt that was continuously moved relative to the print head . The activation of the offset groups of print elements of the printhead can be controlled so that little or no offset is visible in the printed image. However, it should be noted that this difficulty can be avoided by using a stepper motor that allows the belt to stand still during the printing cycle and move it between cycles. In other embodiments of the invention, the offset effect can have no significant influence on the resulting image or can be within tolerable limits.

Each group of print elements preferably extends along the longitudinal axis of the print head. The centers of the staggered groups preferably lie along a line which defines an acute angle with the longitudinal axis of the printhead. Through these embodiments, a complete Compensation of the offset effect can be achieved. However, there may be disadvantages associated with this arrangement in the case of printheads in which the printing elements are arranged on semicircular 'glaze bumps' (glaze bump). A printhead is generally made from a ceramic substrate on which resistive elements are applied. These resistance elements are the print elements of the print head. In order to improve the contact between the image receiving tape and the printing elements, the resistance elements are arranged on the top of a protective glaze. This glaze has a substantially semicircular profile and extends approximately in the direction of the longitudinal axis of the printhead so as to define a 'glaze bump'. If the groups of printing elements are offset relative to one another in the manner discussed above, the center of each group of printing elements can lie at different points relative to the longitudinal axis of the glaze bump. In certain embodiments, this can affect the quality of the print. However, it has been found that in certain embodiments of the invention this problem can be solved by increasing the radius of the glaze bump.

Alternatively, it is proposed that each group of printing elements be arranged at an acute angle in relation to the longitudinal axis of the print head. A center of print elements is preferably along a line that extends substantially along the longitudinal axis of the print head. This embodiment is advantageous because the reduction in print quality that occurs in print heads with print elements arranged on a semicircular glaze hump can be compensated for by the fact that the center of each group of printing elements will have the same relationship to the longitudinal axis of the glaze bump. The offset of the groups in this embodiment will reduce the eye-disrupting offset in the printed image, although in general no full compensation will be achieved.

The acute angle is preferably in the range of 0.1 ° to 1 ° and preferably about 0.38 °.

It should be noted that the different embodiments with offset print heads can each be used in different situations.

Embodiments of the present invention preferably use a low voltage linear regulator to provide a constant voltage. Since the present invention reduces the peak current required, the voltage of the power supply used can be reduced and a low-voltage linear regulator can be used accordingly. It has been found that a low voltage linear regulator works satisfactorily, so that a high voltage switching regulator generally used in the prior art is unnecessary. Because low voltage linear regulators are generally cheaper than high voltage switching regulators, the cost of embodiments of the invention can be reduced.

By dividing the print head into at least two groups and activating these groups at different times the peak current with which the printing elements are acted on can be reduced in the printing cycle. This is because the current required is proportional to the area of the pressure element. For example, if the printhead is split into two groups that are activated at different times during the print cycle, the peak current required is halved accordingly. In addition, it has been found that a uniform distribution of the activation periods of the individual groups over the pressure cycle extends the life of the battery. This is particularly advantageous if the power supply is implemented in the form of one or more batteries. The peak current required can thus come closer to the average current and is preferably less than three times greater than the average current averaged over the pressure cycle. A smoother, averaged current is thus obtained, which means that a lower voltage power supply can be used which reduces the cost of the tape printing device as a whole. It also extends the life of the batteries. In general, the greater the peak current, the shorter the battery life, with the same amount of energy. By reducing the peak current, the battery life is extended.

According to a second aspect of the present invention, a tape printing device according to the teaching of claim 9 is provided.

DE-A-4 438 600 discloses a thermal printing device with approximately 2,560 printing elements which are arranged in four groups and controlled by corresponding strobe signals.

In the known arrangements, such as DE-A-4438600, a switch is generally provided for each print element of the print head. In embodiments of the present invention, the same switching means can be used to control the printing elements in each of the group. The number of switches required can therefore be reduced, so that the print head and thus the device is less expensive.

Preferably, each switch of the common set of switches is arranged to control one push element in each group. Thus the number of switches required in the common set of switches is equal to the number of printing elements in each group.

The group selection means preferably includes a plurality of switches corresponding to the number of groups of printing elements. This means that each group is controlled by its own switch. In the various embodiments of the invention, more than one group of pixels can be activated at a given time. Under these circumstances, more than one of the switches of the group dialing means mentioned would be switched on.

A diode is preferably connected in series with each pressure element in order to avoid inadvertent activation. Parasitic currents can arise which are the result of the activation of selected pressure elements which are prevented by the diodes.

The number of printing elements, the number of groups of printing elements and / or the properties of the printing elements can be selected such that the current flowing through the printing elements that are not to be activated remains at a level at which the printing element concerned is not printed. In this way a diode connected in series with the pressure element can be avoided. In the context of the present disclosure, activation of a printing element is to be understood as an application of a current which is sufficient to generate an image on the image receiving medium.

A resistor can be provided for each group of printing elements, each resistor being arranged in parallel with the printing elements of the corresponding group, so that the resistor draws current from the printing elements which cannot be activated in order to inadvertently activate the printing elements which are not to be activated, to be prevented. The resistance of each group can be controlled by an associated switch. The group selection means can control each of the groups of printing elements and the associated resistors. It should be noted that the latter arrangement is particularly suitable for the embodiments having three or more groups of pressure elements. The resistors can e.g. can only be activated if the number of activated printing elements in a group is below a specified value.

It has been found that parasitic current problems are most severe when a certain number of printing elements in a group are not activated. The selective Switching on the resistors allows the resistors to reduce the problem, so that parasitic activation of a pressure element can be avoided.

Note that the tape printing device described in connection with the second aspect can incorporate features of the first aspect and vice versa.

The printhead is preferably a thermal printhead. The thermal print head is preferably only one printing element wide at any given point in the direction of movement of the image receiving tape and has a height corresponding to the maximum width of the tape that can be used in the corresponding tape printing device. In practice, the printhead height would be slightly less than the width of the ribbon because edges are provided above and below the print area on the ribbon.

However, in certain embodiments of the invention, the height of the printhead can be much smaller than the width of the ribbon.

• Figur 1: Eine Ansicht eines ersten Banddruckgerätes, das die vorliegende Erfindung unter Verwendung eines Zwei-Kassetten-Systemes verkörpert;
• Figur 2: Die Ansicht eines zweiten Banddruckgerätes, in dem die vorliegende Erfindung unter Verwendung eines Ein-Kassetten-Systemes benutzt wird;
• Figur 3: Ein Diagramm, in dem die Steuerungsschaltung des Druckgerätes von Figur 1 oder Figur 2 dargestellt ist;
• Figur 4a: Eine schematische Ansicht eines herkömmlichen Druckkopfes;
• Figur 4b: Eine schematische Ansicht eines ersten Druckkopfes, in dem die vorliegende Erfindung verkörpert wird;
• Figur 4c: Eine schematische Ansicht eines zweiten Druckkopfes, in dem die vorliegende Erfindung verwendet wird;
• Figur 4d: Eine schematische Ansicht eines dritten Druckkopfes, in dem die vorliegende Erfindung verwendet wird;
• Figur 4e: Ein mit dem Druckkopf der Figur 4b gedrucktes Bild;
• Figur 4f: Eine Querschnittsansicht durch den Druckkopf der Figur 4c;
• Figur 5a: Das Verhältnis zwischen Strom und Zeit für den Druckkopf für Figur 4a;
• Figur 5b: Das Verhältnis zwischen Strom und Zeit für den Druckkopf für Figur 4b, wenn er in einer ersten Art betrieben wird;
• Figur 5c: Das Verhältnis zwischen Strom und Zeit für den Druckkopf der Figur 4b, wenn er in einer zweiten Art betrieben wird;
• Figur 6: Ein Schaltungsdiagramm der Steuerung des in Figuren 4d, 4c oder 4e dargestellten Druckkopfes;
• Figur 7: Eine veränderte Version des in Figur 6 wiedergegebenen Schaltungsdiagrammes zur Steuerung des in Figur 4b, 4c oder 4d wiedergegebenen Druckkopfes.

For a better understanding of the present invention and to show how it can be implemented, reference is made below to the drawings, in which embodiments of the invention are shown. Show it:

• Figure 1 is a view of a first tape printing device embodying the present invention using a two-cassette system;
• Figure 2: The view of a second tape printing device in which the present invention is used using a one-cassette system;
• Figure 3: A diagram showing the control circuit of the printing device of Figure 1 or Figure 2;
• Figure 4a: A schematic view of a conventional printhead;
• Figure 4b: A schematic view of a first printhead in which the present invention is embodied;
• Figure 4c: A schematic view of a second printhead in which the present invention is used;
• Figure 4d: A schematic view of a third printhead in which the present invention is used;
• Figure 4e: An image printed with the printhead of Figure 4b;
• Figure 4f: A cross-sectional view through the print head of Figure 4c;
• Figure 5a: The relationship between current and time for the printhead for Figure 4a;
• Figure 5b: the relationship between current and time for the printhead for Figure 4b when operated in a first manner;
• Figure 5c: the relationship between current and time for the printhead of Figure 4b when operated in a second manner;
• Figure 6 is a circuit diagram of the control of the printhead shown in Figures 4d, 4c or 4e;
• Figure 7: A modified version of the circuit diagram shown in Figure 6 for controlling the print head shown in Figure 4b, 4c or 4d.

Figure 1 shows the view of the first tape printing device 1, in which the present invention is used and which contains two cassettes arranged therein. This tape printing device 1 is typically a hand-held or a small table device. The upper cassette 2 is arranged in a first cassette receiving section 26 and contains a supply of image receiving tape 4 which is guided through the printing zone 3 of the tape printing device 1 to an outlet 5 of the tape printing device 1. The image receiving tape 4 includes an upper layer for receiving a printed image on one of its surfaces while its other surface is coated with a sticky layer to which a peelable backing layer is applied. The upper cassette 2 has an incision 6 for receiving a pressure roller 8 of the tape printing device 1, and guide sections 22, 24 for guiding the tape 4 through the printing zone 3. The printing roller 8 is rotatably arranged within a cast cage 10. Alternatively, the pressure roller 8 can be rotatably arranged on a pin.

The lower cassette 11 is disposed within a second cassette receiving section 28 and contains a thermal transfer ribbon 12 which extends from a supply spool 30 to a take-up spool 32 within the cassette 11. The thermal transfer ribbon 12 extends through the printing zone 3 in overlap with the image receiving ribbon 4. The cassette 11 has an incision 14 for receiving a printhead 16 of the ribbon printing device 1 and guide sections 34, 36 for guiding the thermal transfer ribbon 12 through the printing zone 3. The printhead 16 is between an operating position, which is shown in Figure 1, in which he is in contact with the platen roller 8 and holds the thermal transfer ribbon 12 and the image receiving ribbon 4 in overlap between the print head 16 and the platen roller 8, and an inoperative position in which it is moved away from the platen roller 8 to release the thermal transfer ribbon 12 and image receiving ribbon 4 . In the operating position, the platen roller 8 is rotatably driven to transport the image receiving tape 4 along the print head 16, while the print head 16 is driven to print an image on the image receiving tape 4 by thermal transfer of the ink of the ink ribbon 12 onto the image receiving tape 4. Printhead 16 will be described in more detail later, but generally includes a thermal printhead with a number of print elements that can be thermally activated in accordance with the desired image to be printed.

The tape printing device 1 has a lid, which is not shown, but is pivotally attached to the rear of the cassette receiving sections 26 and 28 and which covers both cassettes in the closed state.

A direct current motor 7 (see FIG. 3) drives the pressure roller 8 continuously. The platen roller 8 is arranged so that it continuously conveys the image receiving belt 4 through the printing zone 3 by the application of its torque. Here, reference is made to our European patent application 94308084.6, which describes the control of the DC motor and whose content is incorporated by reference.

The image is printed by the print head 16 line by line onto the image receiving belt 4, the lines being adjacent to one another in the direction of movement of the belt 4. The DC motor 7 contains an encoder disk arranged on the shaft for monitoring the rotational speed of the motor 7. The sequential printing of the rows of pixels (picture elements) by the print head 16 is controlled as a function of the monitored rotational speed of the motor 7. Control of the speed of motor 7 is accomplished by microprocessor chip 100, which is discussed below in connection with FIG. 3. The encoder disc generates pulses, the frequency of which depends on the speed of the motor 7, the pulses causing the microprocessor chip 100 to generate strobe signals, each of which causes the print head 16 to print a series of pixel data.

FIG. 2 shows the view of the cassette receiving space of a second tape printing device 1 which embodies the present invention using a single-cassette system. The same reference numerals are used for the parts which are also shown in FIG. 1. The cassette accommodating space is represented by the dashed line 40. The cassette receiving space 40 comprises a thermal print head 16 and a printing roller 8, which cooperate to define a printing zone 3. The print head 16 is pivotable about a pivot point 42 so that it can be brought into contact with the printing roller 8 for printing and can be moved away from the printing roller 8 to enable removal and replacement of a cassette as in the first embodiment. One in the cassette recording room 40 inserted cassette is generally represented by reference numeral 44. The cassette 44 contains a supply roll 46 of the image receiving belt 4. The image receiving belt 4 is guided through a cassette (not shown) through the cassette 44 and then out of the cassette 44 through an outlet along the printing zone 3 to a cutting location C. The same cassette 44 also contains an ink ribbon supply spool 48 and an ink ribbon take-up spool 50. The ink ribbon 12 is guided by the ink ribbon supply spool 48 through the printing zone 3 and wound on the ink ribbon take-up spool 50. As in the first embodiment, the image receiving tape 4 passes through the printing zone 3 in an overlap with the ink ribbon 12, its image receiving layer being in contact with the ink ribbon 12.

The platen roller 8 of this second embodiment is also driven by a DC motor 7 (see Figure 3) so that it rotates to continuously drive the image receiving belt 4 through the printing zone 3 during printing. In this way, an image is printed on the tape 4 and the latter is moved from the printing zone 3 to the cutting location C, which is provided at a location on a portion of the wall of the cassette 44 which is close to the printing zone 3. The portion of the wall of the cassette 44 where the cutting location C is defined is represented by the reference numeral 52. A slot 54 is defined in the wall section 52 and the image receiving belt 4 is transported through the printing zone 3 to the cutting location C, where it is supported by opposite wall sections on both sides of the slot 54.

The second tape printing device 1 comprises a cutting mechanism 56 with a cutting support element 58 which carries a blade 60. The blade 60 cuts the image receiving tape 4 and then enters the slot 54.

The basic circuit for controlling the tape printing device 1 of Figure 1 or the tape printing device 1 'of Figure 2 is shown in Figure 3. It contains a microprocessor chip 100 with a read-only memory (ROM) 102, a microprocessor 101 and freely accessible memory capacity, which is represented by the RAM 104. The microprocessor chip 100 is connected to a data input device, such as a keyboard 106, for receiving input data. The microprocessor chip 100 outputs data to a display 108 via a display driver chip 109 in order to display a label (or a part thereof) to be printed and / or a message to the user. The display driver 109 may alternatively be part of the microprocessor chip 100. In addition, the microprocessor chip 100 also outputs data for operating the print head 16 so that the label data is printed on the image receiving tape 4 to form a label. Finally, the microprocessor chip 100 also controls the DC motor 7 for driving the platen roller 8. The microprocessor chip 100 can also control the cutting mechanism 56 of FIG. 2 or a cutting mechanism of FIG.

The print head 16 shown in FIGS. 1 and 2 is described in more detail below with reference to FIGS. 4 and 5. The type of printhead 16 with which embodiments are concerned of the present invention generally includes a plurality of printing elements 120 that are selectively heated to enable thermal printing. This thermal printing can be carried out directly on thermally sensitive ribbon receiving tape 4 or by means of an ink ribbon 12, as shown in the embodiments of FIGS. 1 and 2. As discussed in connection with these embodiments, the ink ribbon 12 is disposed between the print head 16 and the image receiving tape 4. The effect of heat on the ink ribbon 12 by selected printing elements 120 of the print head 16 causes the transfer of an image to the image receiving tape 4.

Before various printheads 16 in which the present invention is implemented are explained in more detail, the general structure of a known printhead 16a is described with reference to FIGS. 4a and 5a. The known printhead 16a includes a plurality of printing elements 120a. In the schematic representation shown there are 12 pressure elements. Real print heads generally have considerably more print elements, for example 128. The print head 16a has a height H which is insignificantly smaller than the width of the image receiving tape 4 for use in the tape printing device 1. If more than one width of the tape is to be used in the tape printing device 1, the printhead 16a will generally have a height H which corresponds to the width of the largest (widest) image receiving tape 4 for use in the tape printing device 1. In general, the width W of the print head 16a is equal to the width of a print element 120a to obtain a row-shaped print head 16a. Each pressure element 120a is essentially square, to print a square pixel on the image receiving tape 4.

Each pressure element 120a is a resistance element, which heats up when electrical current flows through it. The printing elements 120a are selectively heated to enable an image to be printed on the image receiving tape 4 as it passes through the print head 16a. The image printed on the image receiving tape 4 is defined by a plurality of adjacent rows of pixels. The image printed on the image receiving tape 4 thus depends on which printing elements 120a are activated or heated and when. The image receiving belt 4 generally moves in the direction of arrow A, i.e. in the longitudinal direction of the image receiving tape 4 and perpendicular to the longitudinal axis L of the print head 16a.

Reference is made below to FIG. 5a, which shows the relationship between current and time for the print head 16a in FIG. 4a. As can be seen, the printhead 16a has a cycle 122 which comprises two parts. In the first part 124 of the cycle 122, only the pressure elements 120a are energized, which are to be activated in this cycle 122. It is possible that at the same time in the first part 124 of the cycle 122 all elements of the printhead 16a are switched on or activated. For a 12 volt power supply, the peak current will be 1.6 amperes, for example. A typical value for the duration of the first part 124 of the cycle 122 is two milliseconds. The second part 126 of cycle 122 is typically 18 milliseconds in duration, so that a total of 20 milliseconds results for the entire cycle 122. Of the second part 126 of cycle 122 allows the power supply to recover between activations of printhead elements 120a. The current averaged over cycle 122 is approximately 0.16 amps. It is obvious that the peak current is much larger than the average current.

A first printhead 16b embodying the present invention will now be described with reference to Figures 4b and 5b. The printhead shown in Figure 4b is similar to that shown in Figure 4a. However, there is a difference in the shape and size of the printing elements 120b. The print elements 120b are essentially rectangular and not square like the print elements 120a of the known print head 16a. The same printhead elements 120b form a square pixel (picture element) on the image receiving tape 4 in the case of two successive activations. In other words, each printing element 120b defines half a pixel of the printed image and has the shape of a halved square. Since the print elements 120b are resistors, the current required to heat the print head elements 120b shown in FIG. 4b to the desired temperature is proportional to the area of the print head element 120b. Correspondingly, by halving the area of the printhead element 120b, the current required for heating the printhead element 120b is halved while the voltage remains the same. It is apparent that all printhead elements of printhead 16b can be activated at the same time.

Reference is now made to FIG. 5b, which shows the relationship between current and time for the printhead 16b. For For a given cycle 122, also 22 milliseconds, printhead elements 120b of printhead 16b would be energized twice in a given cycle 122 to print a series of pixels on the image receiving tape. Each application of current would take approximately 2 milliseconds and each have a peak current of 0.8 amperes. An average current of 0.16 amperes would be obtained. In comparison to the embodiment shown in FIG. 4a, the peak current is halved, but the average current remains constant.

The printhead elements 120b can also be in the form of a third or even a quarter of a square that defines a square pixel in the printed image when the same printhead element 120b is activated three or four times in succession.

As has already been discussed, the image receiving tape 4 moves in the direction of arrow A along the print head. In order to improve the appearance of the image printed on the image receiving tape, the speed of the image receiving tape 4 can be reduced in comparison with the prior art. This would reduce the average current over a print cycle 122 since the length of the print cycle was effectively increased. The image receiving tape 4 can typically be guided past the print head 16b at a speed of 7 mm per second. However, it is possible to pass the image receiving tape 4 faster or slower past the print head 16b.

The embodiment shown in Figure 4b can be modified to divide the printhead 16b into three sections 130, 132 and 134. For purposes of illustration, each section 130, 132 and 134 has only four printhead elements 120b. In a preferred embodiment, each section 130, 132 and 134 has thirteen printhead elements. The three sections 130, 132 and 134 are arranged such that they can be activated or strobed one after the other. In other words, the selected printhead elements 120b of the first section 130 are activated first. The selected printhead elements 120b of section 132 are then activated and finally the selected printhead elements 120b of third section 134 are activated. Thus, a maximum of one third of the total number of printhead elements 120b of printhead 16b are activated at any time. The relationship between current and time for this modified embodiment of FIG. 4b is shown in FIG. 5c. As can be seen from FIG. 5c, the three sections 130, 132 and 134 of the printhead 16b are activated or strobed at equally spaced intervals during the cycle 120. There are thus six periods 138 in which current acts on the individual sections 130, 132 and 134 of the printhead elements 120b. The peak current for printhead elements 120b would be one sixth of that shown in Figure 5a because printhead elements 120b are half the size of those in Figure 4a and only a third of printhead elements 120b can be energized or activated at any given time. The average current will be the same as in Figure 4b since printhead 16b is activated twice during each print cycle 122 in its entirety. If the Printhead elements are 'full size' as in the prior art and need only be activated once to print a pixel on tape 4, each group of printhead elements need only be activated once in a cycle 122.

Since the image receiving tape 4 moves continuously along the print head 16b in the direction of arrow A, the use of a print head 16b as shown in Figure 4b with the three sequentially activated sections 130, 132 and 134 results in that on the Image receiving band 4 an offset image is formed, as shown in Figure 4e. For some embodiments, this offset may not have a significant impact on print quality. In those developments in which an improvement in the print quality is intended, the print head 16c shown in FIG. 4c can be used. Printhead 16c has three sections 130c, 132c and 134c that are staggered relative to one another in a direction opposite to the direction of the image that would be produced by printhead 16b (as shown in Figure 4e). Each section 130c, 132c and 134c is formed by four printhead elements 120c similar to those shown in Figure 4b. As with printhead 16b in Figure 4b, printhead elements 120c in section 130c are activated first, followed by printhead elements 120c in section 132c, followed by printhead elements 120c in section 134c. Since the image receiving tape 4 moves in the direction of arrow A, the offset in the image printed with printhead 16b of FIG. 4b can be corrected. The print head 16c is especially designed to take into account the speed of the image receiving tape 4, so that the second section 132c, when activated, prints an image directly below and in alignment with the image printed by section 130c of the printhead 16c. Likewise, third section 134c is arranged to print an image directly below and in alignment with the images printed by first and second sections 130c and 132c. In certain embodiments, the offset between two adjacent sections 130c, 132c, and 134c may be equal to one third of the offset between adjacent print positions. This can be slightly less than a third of the width of each printhead element. Printhead elements 120c can be the same as those shown in Figure 4b. Looking at the print head 16c in cross-section, as shown in FIG. 4f, it can be seen that the print head elements 120c of the print head 16 lie along the top 140 of a 'glaze bump' 142. A printhead is essentially made of a ceramic substrate, on which resistance elements are applied. These resistive elements are the printhead elements of the printhead. However, in order to improve the contact between the medium to be printed and the printhead elements, the resistance elements are applied to the top of a glaze. This glaze has a substantially semicircular profile and extends substantially in the direction of the longitudinal axis L of the printhead 16 so as to define a 'glaze bump'. In the case of an offset print head 16c, as shown in FIG. 4c, the center of the print head elements 120c of the individual sections 130c, 132c and 134c will be at different locations on the glaze bump 142, for example at the locations 144a, 144b and 144c. The printhead elements of each section 130c, 132c and 134c become corresponding have a different position with respect to the platen 8 with which the printhead 16c interacts. This can affect print quality, although it is negligible in some embodiments of the invention. In some embodiments of the invention, the radius of the glaze bump 142 can be increased, which can improve print quality.

The printhead shown in Figure 4d has been proposed to solve this problem. In this embodiment, each of the sections 130d, 132d and 134d is arranged at an angle with respect to the longitudinal axis L of the printhead 16d, the angle being between 0.1 ° and 1 ° and preferably around 0.38 °. In addition, all centers 133a, 133b and 133c of sections 130d, 132d and 134d lie on the longitudinal axis L of printhead 16d. While full compensation for the offset is not achieved with the print head 16d, the appearance of the offset to the eye in the printed image is reduced in comparison with the print head 16b from FIG. 4b. With the print head 16d, it is additionally possible to avoid a potential reduction in the print quality which can result with the print head 16c of FIG. 4c. It should be noted, however, that there are embodiments of the present invention in which printheads 16b, 16c and 16d, as shown in Figures 4b, 4c and 4d, each have particular advantages.

The relationship between current and time for the printheads shown in Figures 4c and 4d is the same as that shown in Figure 5c.

For comparison purposes, it was assumed in the discussion above that all printheads 16a, 16b, 16c and 16d in Figure 4 have the same supply voltage and cycle times. Table 1 shows current values of various parameters for embodiments of the present invention and the prior art. Table 1 Printhead Supply voltage Peak current / duration Medium current Cycle length 16a 12V 1.6A / 2ms 0.32A 10ms (State of the art - Figure 4a) 16b 12V 0.8A / 2ms 0.32A 10ms (Rectangles - Figure 4b) 16b / 16c / 16d 5V 0.67A / 2ms 0.38A 21ms (3 sections + rectangles - Figures 4b, 4c, 4d)

As can be seen from Table 1, embodiments of the present invention are capable of applying smaller peak currents to printheads 16b, 16c, and 16d compared to the prior art embodiment. Alternatively, if the peak current remains comparable to prior art arrangements, the voltage requirements can be reduced. The embodiments of the figures 4b-4d are thus able to reduce the ratio of peak current to average current compared to the prior art, as shown for example in Figure 4a, so as to obtain a smoother, averaged current over the pressure cycle. In other words, the peak current is closer to the average current and preferably less than three times larger.

Reference is now made to FIG. 6, which shows the control of the print head elements 120 of the print head 16 in FIGS. 4b, 4c or 4d.

As discussed above, each printhead element 120b generally includes a resistance element. Accordingly, printhead elements 120b are represented in FIG. 6 by resistors R1-R12. Printhead elements 120 or resistors R1-R12 are grouped into three groups of four. These three groups reflect sections 130, 132 and 134 of printhead 16b, 16c and 16d. Each section 130, 132 and 134 is connected to the printhead supply voltage V via corresponding switches S1, S2 and S3 which define group selection switches. A switch S1, S2 and S3 is thus provided for each section 130, 132 and 134 of the print head 16. These switches can take any suitable form and are preferably either bipolar transistors or FETs.

The first resistors, R1, R5 and R9 of each section 130, 132 and 134 are all connected together. Likewise, the second resistors, R2, R6 and R10, are just like the third Resistors, R3, R7 and R11 of each section 130, 132 and 134 connected together. Finally, the fourth resistors, R4, R8 and R12 of each section 130, 132 and 134 are also connected together.

The first resistors R1, R5 and R9 of each section 130, 132 and 134 are connected to switch T1, the second resistors, R2, R6 and R10 of each section 130, 132 and 134 are connected to switch T2, the third resistors R3, R7 and R11 are connected to the switch T3 and finally the fourth resistors R4, R8 and R12 of each section 130, 132 and 134 are connected to the switch T4. Just like switches S1-S3, switches T1-T4 are preferably bipolar transistors or FETs. However, any other suitable switch can also be used. Switches T1-T4 define a common set of switches arranged to control the selective activation of printhead elements 120 in each group 130, 132 and 134.

The switches T1-T4 are all arranged parallel to each other, as are the resistors R1-R12. The switches T1-T4 are all connected to earth. The switches T1-T4, S2 and S3 are all controlled by a controller 154, which may be the microprocessor 100. Alternatively, a separate control can be provided, which forms part of the print head 16. If necessary, means can also be provided to convert the microprocessor's serial output to a parallel output. Alternatively, a parallel output from microprocessor 100 can be provided. The arrangement shown in Figure 6 allows multiplexing of printhead 16, which can simplify control of printhead elements 120. Particular embodiments of the invention can dispense with printhead control, as is present on printheads in the prior art, which reduces the cost of printhead 16. The control can, as described above, be carried out simply by the microprocessor.

The printhead power supply V comprises a battery supply, which typically consists of six 1.5 V batteries, which give a total supply voltage of 9 volts. The voltage supply V is connected to a low-voltage linear regulator, which provides a constant voltage of approximately 5 V. The low voltage linear regulator 152 regulates the voltage down to the required 5 volt level. A 9 volt supply is required so that it can be ensured that there is always a 5 volt supply because the linear regulator 152 requires a certain voltage loss to operate. In addition, the battery voltage will drop during the printing process.

The operation of the circuit in Figure 6 will now be described. The switches S1, S2 and S3 are energized or strobed, ie activated by successive strobe pulses provided by the controller 154. In this way, printhead sections 130, 132 and 134 are activated. Since only one of these switches S1, S2, or S3 is closed (ie, turned on) at any given time, only a portion 130, 132, and 134 of the printhead is activated at a time. The switches T1 - T4 are closed selectively, ie switched on, depending on which of the printhead elements 120 of the activated section 130, 132 and 134 is to be activated. Thus, all of switches T1-T4 may be off, all switches T1-T4 may be on, or only some of switches T1-T4 may be on. In other words, switches T1-T4 determine which printhead elements 120 are activated in each section 130, 132 and 134 of the printhead when each of these sections 130, 132 and 134 is activated sequentially. For example, if section 130 is energized by closing switch S1 and printhead element R1 is to be activated, switch S1 is closed. On the other hand, if printhead element R1 is not to be activated, switch T1 will remain open, ie off. In this way, the individual printhead elements 120 can be controlled by the switches T1-T4.

It is apparent that the total number of switches required corresponds to N / M + M, where N is the number of printhead elements 120 and M is the number of sections 130, 132 and 134 into which the printhead 16 is divided. It is apparent that the number of switches required is thus reduced compared to the prior art, in which there is a switch for each individual printhead element.

The following situation is now considered. Switch S1 is on, switches S2 and S3 are off. Switches T1 and Switches T2, T3 and T4 are all off. The arrows in Figure 6 show the direction of the various currents that occur in the circuit. Current E1 is the activation current that is the printhead element R1 activated. E2, E3 and E4 are parasitic currents. These parasitic currents can cause unintended activation of printhead elements 120. These printhead elements can be in sections 132 and 134 which is inactive or in section 130 which is active.

It has been determined that a printhead element 120 will not print if the applied energy level, i.e. a current flowing through printhead element 120 is below a given level. Experimentally, it has been found that in certain embodiments of the invention, printing does not occur when the energy level applied to printhead element 120 is less than 40% of the normal printing energy level required. If the circuitry in Figure 6 is designed so that the parasitic currents such as E2, E3 and E4 do not exceed a level which activates the corresponding printhead element 120 - i.e. an energy level which is greater than 40% of the normal printing energy - the unintentional activation of Avoid printhead element 120 that should not be activated. In these embodiments, the presence of diodes is not necessary to avoid parasitic currents. Sizes that affect the current level in inactive printhead elements can include one or more of the following: the number of printhead elements; the number of groups of printhead elements; and / or the characteristics (such as resistance or the like) of the printhead elements.

It has been found that this last embodiment is particularly advantageous when M is about 3 and N is one Number does not exceed about 24. Such embodiments of the present invention, in which the printhead is divided into three sections, have been found to be advantageous because the amount of energy lost in parasitic currents is minimized, parasitic currents do not exceed the 40% energy threshold, and a reduction in the peak current can also be achieved .

Reference is now made to FIG. 7, which represents a modification of the circuit shown in FIG. 6. The circuit in Figure 7 is the same as in Figure 6 with three more resistors RX, RY and RZ added. Rx is connected to switch S1 and connected in parallel with resistors R1 - R4. RY is connected to switch S2 and connected in parallel with resistors R5 - R8. Finally, RZ is connected to switch S3 and connected in parallel with resistors R9 - R12. The resistors RX, RY and RZ are connected in parallel and connected to a further switch S4, the other end of which is connected to earth. Switch S4 is also controlled by controller 154.

The embodiment shown in Figure 7 is particularly advantageous when the parasitic currents are so large that they can cause unintended activation of printhead elements 120, ie the parasitic currents are so large that 40% of the normal printing energy is exceeded. The levels of parasitic currents that could cause problems typically arise when only a few of the printhead elements in a selected portion 130, 132, and 134 of printhead 16 are activated. As discussed above, can unintended activation of a printhead element 120 occur when they are activated with an energy level that is greater than 40% of the normal printing energy. Resistors RX, RY and RZ are provided to derive current from printhead elements 120 that could parasitically print. In particular, switch 4 is turned on when there is a risk of parasitic printing, as discussed in the above circumstances. The current then preferably flows through RX, RY, and RZ and away from printhead elements 120, which could print parasitically. The values of the resistors RX, RY and RZ are generally lower than those of the printhead elements R1-R12. The optimal value of RX, RY and RZ can be determined experimentally. With switch S4 on, the presence of resistors RX, RY, and RZ can keep any parasitic current through printhead elements 120 that are not to be activated below the 40% threshold during a given cycle. In some embodiments, the switch S4 can be omitted, the resistors RX, RY and RZ always providing an alternative route and thus reducing the risks of parasitic printing. However, this can be an unnecessary waste of energy, so preferred embodiments use switch S4 to restrict the use of the resistance paths defined by resistors RX, RY and RZ to situations where there is a real risk of parasitic printing.

The embodiment shown in FIG. 7 is particularly suitable for those embodiments which typically have more than 24 printhead elements.

Obviously there are various possible variations to the described embodiments. For example, the DC motor can be replaced by a stepper motor. In these circumstances, the belt would move gradually. In addition, the offset print head in FIGS. 4c and 4d can be dispensed with when using the stepping motor. The activation of different sections of the printhead at different times is also advantageous in the embodiments in which a stepper motor is used.

In the illustrated embodiments, the printhead is divided into three sections that are activated one after the other. However, the printhead can also be divided into any other suitable number of sections. Furthermore, the printhead elements need not be adjacent to each other in each section. For example, alternating printhead elements can be selectively activated and define the corresponding sections. With these embodiments, the effects created by offset may be less obvious.

Claims (10)

Tape printing device (1) comprising: a printhead (16) comprising a set of selectively activatable printhead elements (120) arranged substantially along a longitudinal axis of the printhead (16); Means (7) for establishing relative movement between an image receiving tape (4) and the printhead (16) for printing an image on the image receiving tape (4) in the form of a plurality of adjacent rows of pixels; and a control circuit for controlling the print head (16) to generate a plurality of print cycles (122), selected print elements (120) being activated in each print cycle (122) in order to print a line on the image receiving tape (4), characterized in that that each pixel in a printed row is printed by generating a plurality of successive print cycles (122) so that the same printhead elements (120) are activated correspondingly often at adjacent locations on the image receiving tape (4). Tape printing device according to claim 1, characterized in that the means (7) for producing the relative movement between the image receiving tape (4) and the print head (16) produce a continuous relative movement. Tape printing device according to claim 1 or 2, characterized in that each pixel is printed by generating two successive printing cycles, so that the same printing elements (120) are activated at adjacent locations on the tape (4). Tape printing device according to one of claims 1 to 3, characterized in that each printhead element (120) is substantially rectangular and each pixel of the printed row is substantially square. Tape printing device according to one of claims 1 to 4, characterized in that the printhead elements (120) are arranged in at least two groups which can be activated separately at different times in a printing cycle (122). Tape printing device according to claim 5, characterized in that the groups of print head elements (120) are arranged offset from one another, the offset of the groups of the print head (16) being designed such that when the image receiving tape (4) moves relative to the print head (16) essentially a compensation of the activation periods of the groups of printhead elements (120) takes place. Tape printing device according to claim 6, characterized in that each group of printing elements (120) extends substantially along a longitudinal axis of the print head (16) or lies at an acute angle with respect to a longitudinal axis of the print head (16). Tape printing device according to one of claims 5 to 7, characterized in that the activations of the groups of printing elements (120) take place at different times in the printing cycle (122) and are distributed essentially equally over the printing cycle (122). Tape printing device (1) with a print head (16), wherein the print head elements (120) can be activated individually to produce an image on an image receiving tape (4), characterized in that the print head comprises at least two groups of print head elements (120), each of which can be activated at different times during a printing cycle (122) and are provided with control means, the control means comprising a common set of switches (T1-T4) which are arranged in each group to control the selective activation of print head elements (120) and group selection means ( S1 - S4) to select between the groups so that only one group of printhead elements (120) is activated at a given time. Tape printing device according to one of claims 1 to 9, characterized in that the print head is a thermal print head.

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