Many electrophotographic (EP) printers and copiers utilize laser printheads that operate by discharging small areas of a photoreceptor, after which toner is applied to the photoreceptor in a manner such that some toner is attracted to the areas of the photoreceptor that have been discharged by the laser light. Once that has occurred, a sheet of print media is applied to the photoreceptor in a manner that the toner adheres to the sheet of print media in the same pattern to which the toner was attracted to the photoreceptor.
EP printers and copiers usually provide toner through a developer section that uses a doctor blade to uniformly spread the toner particles along the entire width of the developer section so that toner is available across the imaging width of the photoreceptor. In many laser printers, the photoreceptor is cylindrical in shape and is commonly referred to as a photoconductive (PC) drum, which is charged to a relatively high voltage by a charge roller. The areas of the PC drum that maintain this large voltage magnitude tend to repel toner particles, while the discharged areas of the PC drum tend to attract toner particles.
The electrical characteristics of the main components of a laser print engine will typically degrade somewhat during usage of the components. For example, the charge voltage on a PC drum will tend to decrease over its usage life because the voltage supplied by the charge roller will encounter a higher resistance as the surfaces of the major components become somewhat contaminated. The PC drum itself can have its capacitive characteristics change over time as a cleaner blade scrubs away toner particles that did not adhere to print media during a printing operation.
As the components of a laser print engine tend to degrade somewhat during the life span of the print engine, the printer output darkness will gradually become either lighter or darker. In many EP printers, the printer output darkness gradually turns darker over the life of a removable EP process cartridge, and without any type of correction, this gradual darkness increase can become noticeable to the human user. By the end of the life span of some EP process cartridges, the printer output darkness may have shifted by as much as 15 L* units (of CIELAB L*a*b* color space). Consequently, when a new process cartridge is installed, the printer output darkness will be noticeably lighter to many human users.
There have been some attempts in the past to adjust certain operating parameters to account for certain changes in properties of EP printers and copiers. For example, U.S. Pat. No. 4,136,945 (by Stephens) discloses an illumination control system for EP copiers that can compensate for changes in the electro-photosensitive properties of the reusable photoconductive insulator member during its useful life. The Stevens system senses and stores usage information about the photoconductive member, and over the life of the photoconductor can increase the exposure in accordance with a non-linear schedule that corresponds to photochemical, sensitometric changes that occur with increasing cumulative usage.
U.S. Pat. No. 3,575,505 (by Parmigiani) discloses an EP printer or copier that compensates for the gradual deterioration of performance of the electrophotosensitive member by gradually raising the bias voltage on the development electrode. The Parmigiani system counts the number of cycles of operation of the EP printer or copier and changes the bias voltage applied to the magnetic brush in response to a predetermined number of cycles in the machine. In this manner, the bias voltage is automatically raised, based upon the number of copies produced by the machine.
U.S. Pat. No. 5,272,503 (by LeSueur) discloses an EP printer or copier that stores a value that varies as a function of usage of a replaceable sub-assembly, and a controller automatically adjusts a selected operating parameter in accordance with a stored value to maintain the printing quality of the printer/copier. The operating parameters that may be adjusted include the photoreceptor charge level, exposure level, developer bias level, and the response level of the automatic density control system. The memory of a replaceable cartridge or cassette that holds toner can store the print count, number of revolutions of the photoconductive belt, number of charge cycles, and the difference between an intended and the actual charge on the photoreceptor. One of the problems that LeSueur notes is when the age of the cassette increases, the characteristics of the developer device may change, thereby leading to deterioration of the line development performance of the printer, even though the solid area development capability may remain constant. This may be corrected by adjusting the automatic density control setpoint to restore the line development performance. Another effect noted by LeSueur that may occur as the cassette ages is that concentration of toner in the sump of the developer device may drop even though the print quality parameters are satisfied. Use of the cassette at such reduced toner concentration levels may cause a shortening of the life of the developer material, and thus of the cassette. The LeSueur system adjusts the parameters to maintain the toner concentration at its desired level over the age of the cassette, by increasing the toner concentration to prevent a shortening of the life of developer material, thus prolonging the useable life of the cassette.
The patents discussed above may increase the life of a replaceable cassette or cartridge, or they may keep certain operating parameters to a more nominal value over the life of the EP printer, but they do not specifically compensate for a gradual increase or decrease in printer output darkness of an EP printer or copier.