Patent Publication Number: US-6709084-B1

Title: Measuring pen-to-paper spacing

Description:
BACKGROUND 
     This invention relates generally to inkjet printing mechanisms, and in particular to techniques for measuring a pen-to-paper spacing. 
     Inkjet printing mechanisms such as thermal inkjet printers and piezoelectric printers use printheads to eject drops of liquid colorant, referred to generally herein as “ink,” onto a media sheet. Each printhead is formed with very small nozzles through which the ink drops are fired. 
     As shown in FIG. 2, in this application, a pen-to-paper spacing (PPS)  205  is defined as the spacing between the printhead  110  of an inkjet printing mechanism and an upper surface  203  of a media sheet  201  in a print zone  106 . It is understood that the large variations in part manufacturing and the dependency of print quality on PPS make it a critical parameter to be adjusted at the printer manufacturing stage. 
     Present day methods use expensive tools such as laser-based linear variable differential transducers (LVDTs) or high precision mechanical LVDTs to measure the PPS. This process includes multiple measurements across the print zone on a special calibrated media sheet being fed through the printing mechanism. 
     However, the use of these additional tools adds extra costs to the printer manufacturing process, apart from making PPS measurement a very manual and time-consuming procedure, and requires constant operator involvement. All these may not be desirable to the manufacturer. 
     Therefore, there is a need for a more convenient and less expensive way of measuring the PPS in an inkjet printing mechanism. 
     SUMMARY 
     According to an aspect of the invention, a method for measuring a pen-to-paper spacing in an inkjet printing mechanism is provided. Firstly, a first nozzle of a printhead in the printing mechanism ejects a first ink drop onto a medium at a first ejection velocity when the printhead moves at a first moving velocity along a scanning axis and when the printhead reaches a first position along the scanning axis. Subsequently, a second nozzle of the printhead ejects a second ink drop onto the medium at a second ejection velocity when the printhead moves at a second moving velocity along the scanning axis and when the printhead reaches a second position along the scanning axis. The spacing between the two nozzles along the scanning axis and the spacing between the two positions along the scanning axis are predetermined. Then the spacing between the first and second ink drops on the medium along the scanning axis is measured. The pen-to-paper spacing can be determined from the first and second moving velocities, the first and second ejection velocities, the spacing between the first and second nozzles along the scanning axis, the spacing between the first and second positions along the scanning axis and the spacing between the first and second ink drops on the medium along the scanning axis. 
    
    
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmented, partially schematic, perspective view of one form of an inkjet printer in which an exemplary embodiment of the invention can be implemented; 
     FIG. 2 is a side view illustrating the printhead printing on a media sheet having an upper surface to define PPS in the printer of FIG. 1; 
     FIG. 3 is a perspective view of an orifice plate with nozzles thereon; and 
     FIG. 4 is a flow chart illustrating an exemplary process of measuring PPS. 
    
    
     DETAILED DESCRIPTION 
     For convenience, the concepts of the present invention are illustrated in the environment of an inkjet printer  100 , while it is understood that the present invention as illustrated by the exemplary embodiment can also be used in other inkjet printing mechanisms such as facsimile machines and copiers. 
     While it is apparent that the printer components may vary from model to model, the typical inkjet printer  100  includes a chassis  104  surrounded by a housing or casing enclosure  102 . 
     The printer  100  also has a printer controller, illustrated schematically as a microprocessor  118 , which receives instructions from a host device, typically a computer, such as a personal computer (not shown), and manages different operations of different components of the printer  100 . 
     In a print zone  106 , a media sheet (not shown in FIG. 1) receives ink drops from an inkjet cartridge  108  on the carriage. The cartridge  108  is also often called a “pen” by those in the art. The illustrated pen  108  includes a reservoir (not shown) for storing a supply of ink. The pen  108  also has a printhead  110 , which has an orifice plate  300  (see FIG. 3) with a plurality of nozzles  302  (see FIG. 3) formed therethrough in a manner well known to those skilled in the art. The illustrated printhead  110  is a thermal inkjet printhead, although other types of printheads may be used, such as piezoelectric printheads. The printhead  110  typically includes a substrate layer having a plurality of resistors (not shown), which are associated with the nozzles  302 . Upon energizing a selected resistor, a bubble of gas is formed to eject a droplet of ink from the nozzle at a certain velocity and onto the media sheet in the print zone  106 . The printhead resistors are selectively energized in response to enabling or firing command control signals. The control signals may be delivered by a conventional multi-conductor strip (not shown) from the controller  118  to the printhead carriage  112 , and through conventional interconnects (not shown) between the carriage and the pen  108  to the printhead  110 . Furthermore, the controller  118  maps the firing information to the appropriate nozzles and coordinates the nozzles firing to pre-determined locations. 
     A carriage guide rod  114  is supported by the chassis  104  to slidably support the inkjet carriage  112  for travel back and forth across the print zone  106  along a scanning axis  116  defined by the guide rod  114 . To provide carriage positional feedback information to the printer controller  118 , an optical encoder reader (not shown) may be mounted to the carriage  112  to read an encoder strip extending along the path of carriage travel. Furthermore, an optical sensor (not shown) can be attached to the carriage  112  for the purpose of, for example, pen alignment. 
     As shown in FIG. 2, for the purpose of this application, a pen-to-paper spacing (PPS)  205  is defined as the spacing between the printhead  110  and an upper surface  203  of the media sheet  201  in the print zone  106 . 
     FIG. 3 illustrates the orifice plate  300  of the printhead with nozzles  302  formed therethrough. 
     FIG. 4 shows a flow chart illustrating an exemplary process of measuring PPS. In the initial step  401 , both the carriage and the printhead remain stationary at a pre-selected position along the scanning axis, and the controller of the printer selects a first nozzle to eject a first ink drop onto the media sheet in the print zone at a predetermined ejection velocity V 1 . Subsequent to this ejection, the carriage, as well as the printhead, moves at a predetermined moving velocity U 1  along the scanning axis across the print zone. In a second step  403 , when the carriage reaches the pre-selected position along the scanning axis, the controller controls the first nozzle, which ejected the first ink drop, to eject a second ink drop onto the media sheet at the predetermined ejection velocity V 1  while the carriage, as well as the printhead supported by the carriage, is moving at the moving velocity U 1 . After these two ejection steps, in a third step  405 , the controller controls to measure the spacing D along the scanning axis between the first and second ink drops on the media sheet by, for example, moving the optical sensor attached to the carriage over the media sheet for measurement. After D is ascertained, in a final step  407 , the controller calculates the PPS according to the following formula: 
     
       
           PPS=D*V   1 / U   1 . 
       
     
     In this way, PPS is automatically measured by using elements, which are already available in the inkjet printer, without introducing new parts to the printer. 
     Alternatives can be made to the above exemplary process. For example, if the ejection velocity of the ink drops can be adjusted, in the first ejection step, the printhead, along with the carriage, may move at a first moving velocity U 1  and shoot the first ink drop at a first ejection speed V 1  when the printhead or the carriage reaches a first ejection position along the scanning axis. In the second ejection step, however, the printhead moves at a second moving velocity U 2  and shoots the second ink drop at a second ejection speed V 2  when the printhead or the carriage reaches a second ejection position along the scanning axis. The spacing X between the first and second ejection positions along the scanning axis is predetermined, and the spacing D between the two ink drops on the media sheet along the scanning axis is also ascertained. Furthermore, if the printhead moves in a same direction along the scanning axis during these two ejection steps, the ratio of V 1  to U 1  should be different from the ratio of V 2  to U 2 , and in that case, the PPS can be calculated according to the following formula: PPS=(D−X)(V 1 *V 2 )/(U 2 *V 1 −U 1 *V 2 ). If the printhead moves in different directions along the scanning axis during the two ejection steps, the PPS can be calculated in a similar way. It is noted that the velocities U 1 , U 2 , V 1  and V 2  are vectors and hence each takes a positive or negative value depending upon the sign convention being followed. It is further noted that the scanning axis along which the printhead travels is presumably parallel to the media sheet in the print zone and that the printhead plane defined by the orifice plate of the printhead is also presumably parallel to the media sheet in the print zone. 
     Also, the controller may control two different nozzles to eject the first and second ink drops during the first and second ejection steps respectively. In that case, the spacing along the scanning axis between these two nozzles, which can also be pre-determined relatively easily, needs to be considered when the PPS is calculated. 
     In addition, instead of ejecting a single ink drop by a single nozzle, a group of nozzles can be used to print a predetermined pattern onto the media sheet during each ejection step. In this case, the ejection velocities of each of the nozzles within the group need to be equivalent.