Laser printers form an image on paper similar to that of a photocopy device. A photocopy device forms an image on a print drum by exposing chemicals on the drum to a light source. The chemicals are then washed off, except in the regions of exposure. The exposed regions will pick up ink and transfer the ink to paper which is pressed against the drum.
A laser printer exposes an electrically charged drum which is permanently coated with a photosensitive insulator. The exposed regions electrically attract particles of ink or "toner". This toner is then transferred to paper by applying a charge to the paper causing the toner to move from the drum to the paper.
Typically, laser printers are designed around a print engine manufactured by a third party. The basic components of the print engine are the laser, a drum, and a rotating mirror which directs the laser beam in a straight line across the drum (the "scanline"). In order to generate the image, the laser beam is pulsed on and off as desired while the mirror rotates to expose dots on the drum.
Importantly, each print engine is adjusted at the factory to ensure that the first dot on each scanline is precisely aligned. In most cases, the laser scans across the drum from left to right; however, the alignment problem exists regardless of the direction of the scanning. In order to align the first dots on each scanline, a sensor placed to the left of the drum detects the laser beam prior to the beginning of the scan. Once the laser beam is detected by the sensor, the beam is disabled for a predetermined number of clock cycles. When the predetermined number of clock cycles have expired, the laser is enabled to begin pulsing to form the image across the scanline. The distance between the sensor and the start of printing is referred to as the "margin". It should be noted that the number of clock cycles between sensing the laser beam and enabling the laser will vary depending upon the paper size. Accordingly, a number of predetermined margins are supported by the engine to accommodate various standard paper sizes.
Each print engine is tuned at the factory, by adjusting the number of clock cycles of delay after sensing the beam, such that the left margin will typically be accurate within 1/4 of a dot using the print engine standard frequency. Thus, for a 300-dot per inch print engine, the left margin will be accurate within 1/1200th of an inch.
Many print engines also allow direct control of the modulation frequency of the laser beam, thereby determining the resolution of the printer. For example, for a 300-dot per inch laser print engine, doubling the modulation frequency of the laser would result in a scanline resolution of 600-dot per inch. However, alignment of the left margin is accurate only within 1/4 dot at the standard frequency. Thus, for a 300-dot per inch engine, the accuracy of the left margin is within 1/4 of a 300 dpi dot, or 1/2 of a 600 per inch dot. Consequently, while the print engine is controlled externally at a higher frequency, the relative accuracy of the margin control will suffer.
A current method of externally controlling the left margin requires that the engine communicate, via a serial communications channel, the number of dots for the margin. This value is typically stored in non-volatile memory inside the print engine. This data is determined for each engine at the time of manufacture. The engine, when requested, transmits this information to the controller. The controller then replicates the margin at the desired resolution ensuring precise margin placement. This process, however, requires that the controller microprocessor and the engine microprocessor communicate this information for each page. This consumes hardware resources and slows the controller microprocessor in its main task of generating the image in memory.
Thus, a need has arisen in the industry to provide an accurate, automatic method and device for determining an accurate margin in a laser printer.