Abstract:
To accurately position printheads (C,M,Y,K) of an inkjet printer  100  in the capping area of their service station ( 130 ), the position of service station insert members (e.g.  130 K) is determined by an optical sensor ( 117 ) detecting a through hole ( 132 K) in the cartridge lid ( 131 ), and the separation (j) of the optical sensor and the printheads (e.g. K) is precisely determined. The separation (j) is determined by using the printhead nozzles to print lines ( 144,146 ) in the directions of the scan and media axes, scanning the lines with the optical sensor ( 117 ), and calculating from the results the separation (j). The results of locating the four reference holes ( 132 C,  132 M,  132 Y,  132 K) of the service station insert members may be averaged.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to determining the relative location of components of a printing apparatus, for example a large format printer. In particular, in inkjet printers having a printhead service station, there is a need to accurately position the inkjet printheads relative to their respective service station cartridges. 
     2. Description of the Related Art 
     The service station is an essential subsystem in printers based on thermal inkjet technology (TIT). Its main purpose is to maintain optimal print quality and to do so, it can act on the printhead or pen in different ways (such as wiping, spitting . . . ). These actions are termed “servicing primitives”. They are arranged in complex sequences (servicing algorithms), that are executed in response to various triggers. 
     The functions a service station provides depend on the architecture, but as the printhead complexity increases, more and more sophisticated features need to be implemented. For instance, in the present printing apparatus there may be up to eight different “primitives”, namely: capping, spitting, wiping, PEG (i.e. the application of polyethylene glycol liquid), scraping, snout wiping, priming and drop detection. Since the volume occupied by the service station is much the same as in previous products and because more functionality has been added, there is less space available for each “primitive”. 
     The servicing functions can work correctly with a maximum placement error of 0.8 millimeters for the scan axis. The service station axis substantially perpendicular to the scan axis is somewhat more tolerant accepting up to 1.5 millimeters. Unfortunately, the mechanical tolerances alone, while meeting the goal in the service station axis, are unacceptable for the scan axis (the worst-case placement error is 1.4 millimeters). 
     Therefore, a method of reducing tolerance problems along the scan axis is required. One possible way would involve an expensive production process with better tolerances. An alternative is to calibrate the error out of the system. This determines the physical position of the components and adjusts each function based on this measurement. 
     The printers in the series HP Designjet 2000 and 2500, first marketed in March 1997, adopted the solution of placing an optical reference mark on the service station housing which is scanned by an optical sensor mounted on the printer carriage. This did not allow for any tolerances between the housing and the position of the service station cartridges within the housing. Moreover, no measures were taken to allow for possible variations in the relative positions on the printer carriage of the optical sensor and the printhead cartridges. Such prior art printers are disclosed in U.S. patent application Ser. No. 09/031,115. 
     EP-A-0863009 discloses an optical encoding arrangement incorporating a marker for orientation on a service station carriage and a label for carrying information related to service station functions. The service station module may incorporate holes for storing and conveying information. 
     The present invention seeks to overcome or reduce one or more of the above problems. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of determining the relative location of a component of a printer apparatus with respect to a printhead carriage of the printer apparatus, said component comprising one or more sub-units co-operating with a corresponding number of printheads on the carriage, wherein a sensor is used to determine the position of a reference location on said component relative to the sensor, characterised in that the sensor is also used to determine the position of at least one printhead relative to the sensor. 
     According to a second aspect of the present invention, there is provided a method of relatively locating at least one component of a printer apparatus to a printhead mounted on a printer carriage, wherein a sensor mounted on the carriage is used to scan a reference location on the component, characterised in that there is also determined the relative location of the optical sensor to the printhead on the printer carriage. 
     The sensor is preferably an optical sensor and, to determine the relative location of the optical sensor to the printhead (K), the printhead is caused to print during a carriage movement a reference mark extending substantially in the direction of the media axis, and the optical sensor subsequently scans the reference mark along the scan axis and there is determined therefrom the sensor to printhead distance (“j”) in the direction of the scan axis. The reference mark is preferably a straight line. 
     In a preferred method the step of determining the sensor to printhead distance (j) is preceded by causing the printhead (K) to print a reference mark in the direction of the scan axis, then using the optical sensor to scan the reference mark along the media axis of the printing apparatus, and then determining the sensor to printhead distance (r) in the direction of the media axis and taking into account its effect on the determination of the sensor to printhead distance (j) in the direction of the scan axis. The reference mark extending in the direction of the scan axis is also preferably a straight line. 
     According to a third aspect of the present invention, there is provided a method of determining the relative location of at least one printhead mounted on a printer carriage with respect to one or more service station insert members mounted in a service station, the method using an optical sensor mounted on the printer carriage. characterised in that the sensor detects a reference location provided on at least one of said service station insert members. The insert members are preferably cartridges which perform various servicing functions and may each be associated with a respective printhead. 
     According to a fourth aspect of the present invention, there is provided an inkjet printing apparatus comprising one or more printheads mounted on a printer carriage and means for determining the position of at least one other component of the printing apparatus relative to the or each printhead, said determining means comprising a sensor mounted on the printer carriage, characterised in that calibration means are provided which determine the relative positions of the sensor and the or each printhead. The sensor is preferably an optical sensor. 
     According to a fifth aspect of the present invention, there is provided an inkjet printing apparatus having means for locating one or more printheads mounted on a printer carriage with respect to one or more respective service station insert members mounted in a service station, the apparatus comprising an optical sensor mounted on the printer carriage and means defining one or more reference locations within the service station area, the reference locations being optically detectable by the optical sensor, characterised in that the or each reference location is provided on a respective service station insert member. 
     Preferably each reference location is constituted by a through a hole in a part of the respective service station insert member, e.g. in its handle or lid. 
     Preferably, each service station insert member has a respective reference location. These locations may be averaged to reduce any residual errors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
     FIG. 1 is a partial perspective view of part of the optical sensor and a reference mark of a prior art printer; 
     FIG. 2 is a schematic front sectional view of the parts of a printing apparatus used for determining the relative location, the view being taken in a plane which bisects the printer components in the direction of the paper axis; 
     FIG. 3 is a top plan view of the printing apparatus of FIG. 2; 
     FIG. 4 is a front view of the printing apparatus with its printheads capped; 
     FIGS. 5 a  and  5   b  are diagrams illustrating the distance calibration along the media advance axis; 
     FIGS. 6 a  and  6   b  are diagrams illustrating the distance calibration along the scan axis; 
     FIGS. 7 a  and  7   b  are optical sensor waveforms obtained with clean and dirty service station cartridges respectively; and 
     FIG. 8 shows the printouts produced on the print medium during calibration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings. FIG. 1 shows a prior art arrangement illustrating the type of location system used in Hewlett-Packard Designjet printers in the 2000 and 2500 series. FIG. 1 corresponds to FIG. 13 of the previously-mentioned pending U.S. patent application Ser. No. 09/031,115. An optical sensor  17  including a light emitting diode and a photocell (not shown) is mounted on a printer carriage  10 . A service station housing  24  is of black plastics material and has a mount section  71  for an insert section  70  which is of white material for the purposes of contrast. Section  70  has a white top surface  75  which defines a rectangular slot  76  to constitute a reference mark which is traversed by the sensor  17  to locate its position relative to the printer carriage  10 . 
     FIGS. 2 and 3 show front and top plan views of a printing apparatus  100  in accordance with the present invention. Mounted on printer carriage  110  are a black printhead or pen K, three colour printheads C, M and Y, e.g. cyan, magenta and yellow, and a line sensor  117  incorporating a light-emitting diode and a photocell. The end  111  of carriage defines its position along the scan axis  120 , which has a point of origin at a right bump position  122 . Also illustrated in FIG. 2 are a “LED to K” distance “j” and a “carriage edge to K” distance “q” along the scan axis. 
     A service station  130  is located adjacent to one end of the carriage scan axis and comprises four respective service station insert members. The insert members are manufactured independently and subsequently inserted in the service station housing. The insert members are preferably service station cartridges incorporating four printhead cleaners  130 C,  130 M,  130 Y,  130 K, corresponding to the four colour printheads. As shown in FIG. 3, each cleaner lid has a handle or grip portion  131  which includes a through hole or slot  132  adjacent to and at a predetermined distance from one end of portion  131 . At a central position of each portion  131  there is attached a label  133  which is originally blank but is subsequently inked to represent use of the printing apparatus. The label may incorporate information in written and/or coded form indicating the appropriate type of ink. 
     Because the nozzles of each printhead are spaced transversely to the scan axis, and because the printer carriage  110  moves continuously during printing, it is known to angle the printheads C, M, Y, K by a small angle of 1.79° relative to the true perpendicular direction, and this is shown in exaggerated fashion in the top plan view of FIG.  3 . Of course, the printhead cleaners are also slanted at the same angle so that the paper movement axis  140  is at 1.79° to the so-called service station axis  142 . 
     FIG. 3 also indicates a “LED to K” distance “r” (distance between centers) along the paper axis. Both FIGS. 2 and 3 also indicate a reference hole  132  to printhead cleaner center distance “s”. 
     FIG. 4 is a view corresponding to FIG. 2 but with the printheads C, M, Y and K in their capped disposition, with carriage edge  111  in its “capping position”. As will now be described, the purpose of the calibration process is to ensure that each printhead is aligned as accurately as possible with the center of its corresponding cleaner. 
     The calibration process comprises the following steps: 
     a) LED Calibration 
     To have an optimal signal-to-noise ratio when using the line sensor  117 , the printer needs to ascertain the optimum gain and LED type for the particular printing medium type which is loaded. This is accomplished by LED calibration by printing a black box  150 , FIG. 8 on an inserted paper sheet  155 , and then scanning over the black box and the adjacent white area. In practice, box  150  is printed solidly in black ink. The line sensor control block then modifies its internal parameters to work optimally in this range. 
     b) Drop detector calibration for the K printhead 
     The following step c) of the process involves the printing of a line in the direction of the scan axis. Since such a pattern is very sensitive to “nozzles out” (a nozzle out at the boundary produces an error of one dot), step b) involves facilitating the avoidance of such an error. To achieve independence from the K printhead status, the drop detector is calibrated and a drop detection process is then performed to look for a group of  32  consecutive working nozzles. This information is used subsequently when drawing the line in the direction of the scan axis. 
     c) Line sensor to K pen distance “s” calibration for the media axis 
     A reference mark in the form of a black line pattern  144 , FIGS. 5 a  and  8 , of 32-nozzle height “w”, is drawn in the direction of the scan axis, in this case horizontally. Healthy nozzles are used, as a result of step b). Sensor  117  then scans (with bottom-up scanning) along the paper axis  140 , to produce the sample shown in FIG. 5 b , and the distance “r” from the line sensor to the K printhead is then determined, taking into account which 32 nozzles were used. Knowing the distance “h” of the first nozzle reached from the center of the printhead, one obtains the result: 
     
       
           r=p−p′−h−w /2 
       
     
     where p represents the position of the center of the printed line and p′ represents the position as read by the sensor. 
     d) Line sensor to K pen distance “j” calibration for the scan axis 
     A reference mark in the form of a black line  146 , FIGS. 6 a  and  8 , is drawn, by firing the K printhead during a carriage movement. The line  146  extends substantially in the direction of the media axis, in this case substantially vertically. As explained above, this pattern will actually have a slant of 1.79°. Sensor  117  then scans (with left to right scanning) along the scan axis  120 , to produce the sample shown in FIG. 6 b , and the distance “j” from the line sensor to the K printhead is then determined. Knowing the distance “r” relating to the paper axis, one obtains the result: 
     
       
           j=p−p″−r  tan 1.79° 
       
     
     where p represents the position of the center of the printed line and p″ represents the position as read by the sensor. 
     In practice, lines  144  and  146  are printed solidly in black ink. 
     e) Detection of printhead cleaners to determine the capping position 
     The line sensor  117  is then scanned over the portions  131  of the printhead cleaners, typically obtaining a waveform as shown in FIG. 7 a  for new printhead cleaners. The small “bumps” at the extreme left and right of the waveform represent the edges of the service station housing. The waveform in between represents features of the service station cartridges, and in particular, from the left, the vertical lines represent respectively: 
     left end of cartridge portion  131   
     left side of hole  132   
     right side of hole  132   
     left side of label  133   
     right side of label  133   
     right end of cartridge portion  131   
     left end of portion  131  of next cartridge etc. 
     The scanning process includes the following steps: 
     (i) Signal treatment 
     This involves signal normalization, non-linear morphological filter and derivative calculation. This normalizes the signal and eliminates noise peaks and valleys which could adversely affect the following steps. 
     (ii) Cross-shape recognition 
     This enables determination of the approximate position of each printhead cleaner cartridge. The samples are then divided and analyzed separately. 
     (iii) Feature extraction 
     For each printhead cleaner the various edges, holes and marks are identified. The position of hole  132  is selected as the most accurate indicator. The positions of the holes for the four service station cartridges are indicated at  132 C,  132 M,  132 Y and  132 K in FIG.  7 . FIG. 7 b  indicates a waveform corresponding to FIG. 7 a  but of a dirty printhead cleaner, e.g. after a prolonged period of use. Despite general deterioration of the waveform because of ink covering portions  131 , the locations of the through holes  132  are still distinct because they provide a sharper contrast. 
     (iv) Determination of center of service station cartridges 
     This is simply achieved by adding the distance “s” to the hole positions. 
     (v) Determination of “K printhead to K service station cartridge” distance 
     Once the location of the center of the cartridge  130 K relative to the line sensor is known from step (iv), the position relative to the K printhead can be calculated simply by adding the LED to K printhead distance “j”. In practice, the center positions of the other three cartridges  130 C,  130 M and  130 Y are also taken into account by means of a suitable averaging process. In this way, the effect of any misalignment of these other cartridges is minimized, and a satisfactory capping of all printheads can occur. Accuracy of positioning with less than ±0.7 mm of error is obtained. 
     The above-described calibration process typically occurs only once during the lifetime of a printer. If the printheads or the service station cartridges are replaced, they usually remain within satisfactory tolerances. However, should there be a loss of memory, or should the entire service station housing need replacing, for example, then the control system is configured so that a service engineer may use the process to re-calibrate the positions of the printer components. 
     The above-described arrangement has the advantage of reducing tolerance problems in two ways, namely calibrating the position of the service station cartridges  130  themselves, rather than the housing, and also taking into account the actual “LED to K” distance “j”. Using holes  132  in the cartridges provides accurate location thereof, and taking into account the location of all four cartridges  130 , and then averaging their displacements from a nominal position, reduces any residual errors in positioning for the capping process. The capping function of the service station is the one requiring the tightest tolerance and so the capping region is the best region to locate the calibration holes  132 . The thus-determined capping position serves as a reference for the rest of the servicing “primitives”. 
     Various modifications may be made to the above-described arrangement. For example, in addition to averaging over the four printhead cleaner cartridges, one could also average over the four printhead positions to obtain a modified value for “j”. Also, or in addition, to shorten and simplify the process, only the distance to the hole  132 K of the black printhead cleaner  130 K may be determined. 
     The calibrated part of the service station cartridge may be other than the capping position. 
     If desired, a hole or other reference marking on the service station housing could be used, as in the prior art. This still gives an improved positioning accuracy, since the distance “j” is more accurately determined than allowed previously by the tolerance of the mechanical housing. Indeed, the reference location may be on a component of the printer other than the service station. For example, the precise determination of “j” can be used in the printhead alignment process to accurately place the line sensor  117  over narrow patterns. It can also be used to allow the calculation of the real position of the media margins, which is crucial for the media loader. 
     Alternatively, the value of “j” may be taken as a preset value, in which case the improved calibration results from the determination of the actual positions of one or more of the printhead cleaner cartridges. 
     In other printers it may be arranged that the scan axis and media axis have other orientations, e.g. vertical and horizontal respectively.