Method and apparatus for controlling transfer belt velocity of a color printer

#The present invention relates generally to image forming equipment and is particularly directed to color laser printers of the type which have transfer belts that receive latent images from multiple photoconductive members. The invention is specifically disclosed as a motion control system that maintains a substantially constant belt velocity under varying environmental conditions and for various styles of drive motors and variations in individual belt physical parameters.

#In color printers a plurality of color planes are sequentially aligned and deposited onto a transfer media such as a transfer belt. The transfer belt is then used to transfer the accumulated color planes to a piece of paper or other media. A problem associated with this process is misregistration or misalignment of one or more of the color planes. Alignment of the color planes is crucial in achieving a high quality image. Due to the fact that each individual color plane is transferred onto the belt or paper at different locations along the travel path of the transfer belt, the belt position within the travel path must be controlled with a high degree of precision. The motion of the drive motor that drives the belt must be accurately controlled to insure that there is little or no misregistration of the color planes on the belt such that the resulting image is of good quality.

#There are many instances where motion inaccuracy can develop and cause a concomitant degradation in the resulting image. Factors such as variations in the thickness of the belt, variations in the belt tension, and variations in the drive motor system itself are examples of factors that lead to motion inaccuracy.

#The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description and claims serve to explain the principles of the invention. In the drawings:

#FIG. 1 is a diagrammatic view illustrating the apparatus of the present invention;

#FIG. 2 is a schematic block diagram illustrating DC velocity control in accordance with the invention;

#Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings, wherein like numerals indicate the same elements throughout the views.

#Referring now to the drawings, FIG. 1 shows a transfer belt subassembly 10 and color stations 12, 14, 16 and 18 each associated with a respective color plane. In the illustrated embodiment station 12 is utilized for providing a black color plane, and stations 14, 16, and 18 respectively provide the primary color planes cyan, magenta and yellow. Each of the color stations includes a print head 20 and a PC drum 22. The print head forms a latent image on the associated PC drum and toner is supplied to the PC drum and the developer assembly produces a developed toned image, also known as a color plane, from the latent image on the PC drum. Each color station may be realized through any one of a plurality of known configurations.

#The transfer belt subassembly 10 contains a transfer belt 30, a first home position sensor 32, a second home position sensor 34 (in some embodiments), a thermistor 54 and a memory 36. The sensors are typically optical sensors which are cooperative with one or more reference holes or indicia in the belt, as will be described. A reference indicia in the illustrated embodiment is provided by a hole in the transfer belt which is sensed by electro-optical sensors 32 and 34. The indicia can be of other types such as magnetic or electrostatic marks, or reflective surfaces on the belt sensed by appropriate sensors (e.g., magnetic, electrical charge, or optical sensors). The memory 36 is preferably a semiconductor memory such as a non-volatile memory.