An inkjet printer includes one or more ink-filled print cartridges that are mounted to a carriage in the printer body. Normally, the carriage is scanned across the width of the printer as paper or other print media is advanced through the printer. Each ink-filled cartridge includes a printhead that is driven to expel droplets of ink through an array of nozzles in the printhead toward the paper in the printer. The timing and nominal trajectory of the droplets are controlled to generate the desired text or image output and its associated quality.
Hereafter, the term "paper" will be used to collectively refer to any type of print media (cut-sheet paper of any weight, including photo-grade paper, transparencies, envelopes, etc.) that may be used as media in modern inkjet printers.
The scanning-type printers expel ink while the carriage is reciprocated across the width of the paper. Thus, a swath of ink is printed with each scan, and the paper is advanced to a new location between printing swaths.
Throughput, which is normally measured in printed pages per minute, is an important design consideration in connection with printers of all types. The goal is to maximize throughput without deleterious effects on print quality.
One way to increase throughput in inkjet printers is to combine inkjet cartridges in a printer so that the swath width is enlarged but while maintaining the printheads and paper parallelism. To this end, the paper may be carried on a rotating drum or similar carrier member and advanced through the printer. Sets of cartridges, each set having the same color of ink, are carried near the drum. The cartridges are arranged in a carriage such that the swath of one cartridge combines with the swath of the other cartridge of the set. As a result, the width of the printed swath for a given color is the sum of the individual cartridge swath widths.
A carriage may be provided for carrying the cartridges in the just mentioned arrangement for combining the swath widths of the individual cartridges. The components of the carriage may be configured such that two cartridges of the same color ink are precisely positioned relative to each other, and so that multiple sets of relatively small cartridges are carried near the drum so that a full range of colors can be printed.
In a rotating-carrier type of printer just mentioned the printable surface of the paper (which can be considered as the entire surface of the paper between the leading and trailing edges of the paper) is repeatedly directed to be adjacent to the printheads of the cartridges that are carried by the carriage. A swath of ink is printed while the printable surface is adjacent to the printheads. After the trailing edge of the paper passes from adjacent to the printheads, and before the leading edge of the paper is again directed to be adjacent to the carriage, the carriage is shifted by an incremental amount corresponding to one swath width.
Put another way, the space on the paper carrier that is between the trailing and leading edges of the paper can be considered as a gap. This gap has a length dimension. Moreover, the gap has a time dimension that relates to the size and rotation speed of the carrier, as well as the size of the paper. Accordingly, the gap on a rotating carrier is adjacent to the carriage for a specific time period (hereafter referred to as the "gap period") depending on these factors.
To maximize throughput, the printer should be operated at a high rotation speed so that the required swaths can be printed as quickly as possible. This means that the size of the gap should be minimized. The gap size must be large enough, however, to enable the carriage to shift into position while the gap is adjacent to the carriage, so that the carriage is in position to print the next swath before the leading edge of the paper is again directed to be adjacent to the printheads of the carriage.
The overall mass of a carriage, especially a carriage that carries a plurality of print cartridges, is an important consideration in determining how quickly and precisely the carriage can be moved between printing swaths. The time required for precise carriage movement may be a limiting factor in sizing the gap and gap period. In short, the gap period must be long enough to allow the complete and precise incremental movement of the carriage.
The present invention is directed to a method and apparatus for minimizing the time required to accomplish the incremental carriage movement, hence minimizing the gap size to increase printer throughput.
In a preferred embodiment there is provided a movable indexing stop that is advanced relative to the carriage and in the path that the carriage travels. The stop is advanced while the printheads are printing one, "first" swath (that is, while the carriage is stationary). The stop is advanced to a position corresponding to the location where the carriage is to be moved to facilitate printing of the next, "second" swath. Once the first swath is printed, the carriage is controlled to move toward and abut the stop, thus arriving in the location for printing the second swath.
The use of the stop simplifies the carriage movement in at least two respects. Firstly, the stop position is changed during the time period that the carriage is stationary for printing. This time period (hereafter referred to as the print period) corresponds to the amount of time that the paper is adjacent to the carriage. In a preferred embodiment, the print period is considerably longer than the gap period. As a result, there is a relatively long time period (as compared to the gap period) for precisely advancing stop to the position for stopping movement of the carriage to print the next swath. Secondly, the controlled, incremental movement of the carriage is simplified to quickly moving the carriage against the pre-positioned stop. This obviates the need for more precise movement control that would be required in the absence of a stop, especially for a carriage having a large overall mass.
As a result of the use of the stop, and the attendant, quick incremental movement of the carriage, the gap and/or gap period can be minimized and thereby increase printer throughput.
Other advantages and features of the present invention will become clear upon study of the following portion of this specification and the drawings .