Abstract:
Ink jet printhead comprising a driving and encoding circuit ( 20 ) having a grid-like structure, including a plurality of inputs ( 24 ), a plurality of selecting elements ( 12 ), a plurality of actuating elements (I1) and at least one identifying element ( 21 ) of said printhead. Each identifying element ( 21 ) is associated with a corresponding selecting element ( 12 ) and corresponds to nodes arranged along a row or column of said grid-like structure, and is scanned, together with said actuating elements ( 11 ), during a preliminary checking step.

Description:
TECHNICAL AREA OF THE INVENTION  
       [0001]     This invention relates to a thermal type ink jet printhead, in which a plurality of actuating elements are activated selectively by an external control circuit, to cause the ejection of ink droplets through nozzles placed in correspondence with the actuating elements themselves.  
         [0002]     In particular, this invention relates to an integrated printhead, comprising in the integrated circuit elements identifying the head itself, in accordance with the description provided in the main claim.  
       TECHNICAL BACKGROUND  
       [0003]     The constitution and general mode of operation of a thermal type ink jet printhead, and in particular of the type known as “top shooter”, i.e. that emits the ink droplets in a direction perpendicular to the actuating assembly, are already widely known in the art, and, therefore will is not be described in detail here, while only some characteristics of relevance for the purposes of the understanding of this invention will be described more particularly.  
         [0004]     With reference to  FIG. 1 , an integrated printhead (head)  10 , according to the known art, is made of an integrated circuit, for instance NMOS or bipolar type and comprises a plurality of nozzles  31 , positioned on the head  10  according to a predefined order and suitable for ejecting ink on a medium, generally of paper, and a plurality of inputs or contacts  23 ; provided for connecting the head  10  to an external control circuit, suitable for commanding the selective actuation of the various nozzles  31 .  
         [0005]     The known head  10  ( FIG. 2 ) is made of a grid-like driving circuit (M×N) comprising a plurality, M of actuating assemblies  14 . Each actuating assembly  14  in turn comprises a plurality N of selecting-elements or transistors  12  and an equivalent number of actuating elements or resistors  11  which are provided for causing, in a known way, ejection of the ink from the nozzles  31 .  
         [0006]     Also described by the U.S. Pat. No. 5,363,134 is an integrated printhead that comprises an encoding circuit, in which, by burning programmable fuses, information can be stored about the general characteristics of the head, such as for instance: colour head, its resolution, number of nozzles.  
         [0007]     The encoding circuit is integrated on the same substrate as the selecting and activating circuit of the resistors and is composed of a row of programmable fuses, each of which is connected in series with a transistor.  
         [0008]     Besides, each couple comprising a fuse and a transistor of the encoding circuit, is permanently connected to an Address Line and has the advantage of permitting reading of the coding stored without increasing the number of head connections.  
         [0009]     It is in fact advantageous to have the possibility of identifying, through the printer&#39;s control circuit, a set of head characteristics, stored during the production of the head, that can cause a non-uniformity of operation between the various nozzles, considerably worsening the print quality.  
         [0010]     Some of the characteristics that may be stored are, for instance, misalignments or differences of shape, diameter or centre distance between the nozzles.  
         [0011]     In possession of this information, the control circuit can compensate, by varying the energy supplied to the resistors, any differences in volume of the ink droplets or of speeds which are caused by these non-uniformities.  
         [0012]     This solution has the disadvantage, however, of adding an encoding circuit, formed by N couples of fuses and transistors, where N is the number of addresses in the grid, to those already existing on the head, with a relative increase in the surface area of the integrated circuit and greater costs and manufacturing difficulties. In addition, the encoding circuit can contain at most N fuses.  
       SUMMARY OF THE INVENTION  
       [0013]     The object of this invention is to produce an encoding of the head by using free positions of the grid-like circuit of the head, without having to add a new circuit for the encoding.  
         [0014]     A second object is that of using circuits already existing in the driver of the head for reading of the encoding, without having to build a specific circuit, modifying only the software.  
         [0015]     A third object is that of producing an encoding of the head using the already present circuits mentioned above, to which only a line of fuses for the encoding is added.  
         [0016]     A fourth object is to produce an encoding with a number of fuses greater than the number of addresses.  
         [0017]     These objects are attained by the integrated printhead of the invention, according to the characteristic parts of the main claims.  
         [0018]     These and other characteristics of this invention will become clear from the following description, provided by way of non-restrictive example, with the aid of the accompanying drawings. 
     
    
     LIST OF FIGURES  
       [0019]      FIG. 1  represents a schematic assembly view of an integrated printhead according to the known art;  
         [0020]      FIG. 2  represents a wiring diagram of the circuit elements of an integrated printhead according to the known art;  
         [0021]      FIG. 3  represents a,block diagram of the control circuit of the head according to the invention;  
         [0022]      FIG. 4  represents a wiring diagram of the circuit elements of an integrated head according to the invention;  
         [0023]      FIG. 5  represents a wiring diagram of the circuit elements of a second embodiment of an integrated head according to the invention;  
         [0024]      FIG. 6  represents a block diagram of a second embodiment of the control circuit of the head according to the invention. 
     
    
     DESCRIPTION  
     First Embodiment  
       [0025]     With reference to  FIG. 4 , an integrated printhead (head), according to the invention, is made up of an integrated circuit, for example NMOS or bipolar type, and comprises a driving and encoding circuit  20 , consisting of a plurality of actuating assemblies  14 , of known type, and a plurality of actuating assemblies with encoding or encoding assemblies  24 .  
         [0026]     Each actuating assembly  14 , of known type, comprises a plurality of actuating elements or resistors  11 , which are provided for causing ejection of the ink droplets from the nozzles  31 , and corresponding selecting elements or transistors  12 .  
         [0027]     In each actuating assembly  14 , each transistor  12 , of known type, has its drain terminal connected to one of the two terminals of the resistor  11 , the source terminal connected in common to the sources of the transistors  12  belonging to the same actuating assembly  14 , and the gate terminal connected to the inputs or contacts  23  corresponding to address selection lines (Address Line Select or addresses) A I=1−N .  
         [0028]     The resistors  11  belonging to an actuating assembly  14  have the second terminal connected in common and to an input or contact  23  corresponding to a primitive feeding line (Primitive Select or primitive) P J=1−M .  
         [0029]     Each encoding assembly  24  comprises, in addition to the actuating elements, (resistors)  11  of known type, a plurality of identifying elements (resistors or fuses)  21 , placed in correspondence with the nozzles of the grid not used for printing, and also comprises a plurality of selecting elements (transistors)  12 , in correspondence with the resistors or fuses  21  and the resistors  11 .  
         [0030]     The identifying elements  21  are therefore in correspondence with empty is positions of the grid (M×N).  
         [0031]     In each encoding assembly  24 , each transistor  12 , of known type, has the drain terminal that can be connected to one of the two terminals of the resistor  11  or of the fuse  21 , the source terminal connected in common to the sources of the transistors  12  belonging to the same encoding assembly  24  and the gate  20  terminal connected to the contacts  23  corresponding to address selection lines (Address Line Select or addresses) A I=1−N .  
         [0032]     The resistors  11  and the fuses  21  belonging to an encoding assembly  24  have the second terminal connected in common and to a contact  23  corresponding to a primitive feeding line (Primitive Select or primitive) P J=1−M .  
         [0033]     In particular, each actuating assembly  14  and each encoding assembly  24  are activated by means of contacts or primitives P J=1+M  and each transistor  12  is selected by means of contacts or addresses A I=1+N .  
         [0034]     The head according to the invention therefore comprises a driving and encoding circuit ( 20 ) having a grid-like structure, formed by the actuating  11 , identifying  21  and selecting  12  elements, having M rows and N columns in which:  
         [0035]     N is the number of Selectable addresses A I , and is equal to the sum of the number of resistors  11  and fuses  21 ; and  
         [0036]     M is the number of primitives P J  suitable for activation.  
         [0037]     Generally, in the known heads, not all the available positions of the M×N grid are used for printing.  
         [0038]     For instance, there are polychromatic heads in which the nozzles are divided into three colour groups, separated by gaps. The nozzles, of each group are used for printing with ink of one of the three basic colours, and the gap is greater than the centre distance between two nozzles. The nozzles located in the gaps are not therefore used for printing, and the positions of the M×N grid corresponding to these nozzles are therefore free. In the head according to the invention, the circuitry part which is usually occupied by the resistors  11 , placed in correspondence with the unused nozzles, may therefore be used to accommodate the identifying elements or fuses  21 , that form the encoding circuit.  
         [0039]     The fuses  21  are burnt, for instance at the time of production of the head, to store the desired characteristics.  
         [0040]     In this way, a circuit is obtained for encoding of the head, using positions of the grid that would otherwise be unused, without adding any circuits and without using any extra area.  
         [0041]     Take for example a colour printhead according to the invention, comprising  192  nozzles ( 31 ) and having  16  (M) primitives P J=1−M  and  13  (N) addresses A I=1+N  suitable for use.  
         [0042]     This gives  208  (16*13=208) positions available for driving the nozzles, of which only  192  are actually used.  
         [0043]     Therefore  16  (208−192=16) positions remain free, and these are used, in accordance with a characteristic element of this invention, to accommodate, instead of resistors  11 , the fuses  21  that form the encoding circuit.  
         [0044]     As is known, the actuating elements  11 I and selecting elements  12  are selected and commanded by a control circuit  40 , external to the head, which comprises a “controller”  41 , and a “head driver”  42 , and is connected to the head by means of flexible circuits  44  ( FIG. 3 ).  
         [0045]     The controller  41  sends, through buses  43 , the signals containing the print code (DATA), decoding of the 4-bit addresses A I=1+N  (CODE) and the: timings (CONTROL LINE) of the nozzles to be selected, to the head driver  42 , which in  
         [0046]     turn converts them into current pulses suitable for activating ejection of the ink from the corresponding nozzles.  
         [0047]     The control circuit  40  is connected to the head by means of flexible circuits  44  and in particular by means of the contacts P J=1+M  and A I=1+N  ( FIG. 3 ).  
         [0048]     In the known heads, the control circuit  40  activates the ejection of ink by the nozles of the head, according to the following order:  
         [0049]     activates for a given time, a first address A I ;  
         [0050]     electrically powers with predetermined current pulses and through the contacts P J , a predefined configuration of primitives;  
         [0051]     activates in sequence a second address;  
         [0052]     electrically powers, with predetermined current pulses and through the contacts P J , a second predefined configuration of primitives; and so on in successive steps until activation of the N addresses is complete.  
         [0053]     Through logic signals and in a known way the control circuit  40  of the printer, therefore, is suitable for activating in sequence the addresses A 1−N  and, consequently, the N gates of the transistors  12  of the actuating assemblies  14  and of the encoding assemblies  24 .  
         [0054]     By means of the contacts P J , the control circuit is suitable, in a known way, for electrically powering, upon variation of the addresses A I , predefined configurations of primitives so as to activate the ejection of ink by the nozzles corresponding to the active address A I  and to the configuration of primitives powered.  
         [0055]     In the head according to the invention the encoding of the head is physically produced by activating, through the contacts A I , the addresses corresponding to the fuses  21  that have to be burnt, and by powering the corresponding primitives with pulses having a voltage and a duration suitable for burning the fuses (for example, voltage of 10 V and pulses of 3÷4 μs).  
         [0056]     In this way, codes can be stored on each head corresponding to data relating to characteristics of the head, such as for instance droplet diameter and speed, threshold energy.  
         [0057]     The maximum pulse voltage +V applied to the contacts P J  in the encoding step must be less than, with a safety margin, the breakdown voltage (drain/substrate) of all the selecting elements  12  (MOS) connected to P J  to avoid damaging the head. For instance, in heads with droplets of a few pl,  
         [0058]     where the threshold energy is 1-2 μJ, the currents are less than 100 mA and the voltage applied in the encoding step is approx. 5-6 V, while the breakdown voltage of the transistors is approx. 15 V. In this way, there is a delta V of safety between the voltage needed to blow the fuses  21  and the breakdown voltage of the transistors  12 .  
         [0059]     According to a characteristic of this invention, to read the encoding, a known type of circuit called “nozzle check”, which is part of the head driver  42 , may be used without having to build a specific circuit or modify the driver, using the known control circuit  40 , simply by modifying the head management software.  
         [0060]     At the start of each printing step, before activating the ejection of ink, the control circuit  40  checks integrity of the resistors, by means of the function called “nozzles check” incorporated in the head driver  42 .  
         [0061]     During the nozzle check, the entire grid is scanned by sending to the primitives P J  a current called “check current” to verify integrity of the resistors.  
         [0062]     The check current is very low (10 mA) and is not sufficient to activate the ejection of ink by the nozzles, but simply verifies efficiency of the resistors.  
         [0063]     At the same time, through the check current, the fuses  21  can be checked to see whether they are open or integral, and thus the encoding stored in the head is read.  
       Second Embodiment  
       [0064]     The second embodiment refers to the case of heads that have the grid (M×N) full, i.e. that use all the M×N nozzles  31 , activated by the corresponding resistors  11 .  
         [0065]     A printhead according to the invention comprises a driving and encoding circuit  20   a,  a grid, made up of a plurality of actuating assemblies  14 , of known type, to which are added one or more addresses A I=N+K , a fuse  21  and a corresponding transistor  12  are also made, connected to each of the addresses A I=N+K  added to the grid circuit ( FIG. 5 ).  
         [0066]     Take by way of example a head comprising  208  nozzles  31  and having 16 (M) primitives P J=1-M  and 13 (N) addresses A I=1+N  suitable for use.  
         [0067]     There are therefore  208  (16*13=208) positions available for driving the nozzles, all occupied by the resistors  11 , and no free position remains to be used for the encoding circuit, that could accommodate the fuses  21 .  
         [0068]     In this case, unable to make use of empty positions of the grid M×N to accommodate the encoding circuit, one or more addresses A I=N+K  are added to the grid circuit, to which a fuse  21  and a corresponding transistor  12  are connected.  
         [0069]     In the example cited, of a head with 16 (M) primitives P J=1−M  and 13 (N) addresses A I=1+N , for each address A I=N+K  added, there are 16 (M) positions available for accommodating a fuse  21  and a transistor  12 , and therefore 16 encoding bits available for each new address.  
         [0070]     The encoding circuits (fuses  21  and transistors  12 ) cannot be included in the grid, and are therefore placed in the area available between the contacts  23 .  
         [0071]     In the second embodiment also, reading of the encoding takes place in the nozzle check phase; in this case, in the nozzle check the driver  42  scans all the N×M positions occupied by the resistors  11  activating the nozzles, and K connections  44  more are needed between the controller  41  and the addresses A I=N+K  added for the encoding, as depicted in  FIG. 6 .  
         [0072]     The K addresses added are therefore only activated in the step of reading the encoding and in that of physically producing the encoding, which takes place in the same way as described in the first embodiment.  
         [0073]     The printhead, according to the invention, offers numerous advantages in comparison with the known art. In fact, the encoding circuit uses parts of the existing driving circuit and contacts, without changing them or with a limited increase in the surface area occupied by this circuit. Furthermore, for reading of the encoding of the head, the nozzle check step already present in operation of the head is used, without slowing down the printing preliminaries.  
         [0074]     Naturally, without prejudice to the principle of the invention, the embodiments and construction details may be amply varied with respect to what has been described and illustrated purely by, way of non-restrictive example, without departing from the scope of this invention.