In media handling assemblies, particularly in printing systems, accurate and reliable registration of the substrate media as it is transferred in a process direction is desirable. In particular, accurate registration of the substrate media, such as a sheet of paper, as it is delivered at a target time to an image transfer zone will improve the overall printing process. The substrate media is generally conveyed within the system in a process direction. However, often the position and/or timing of the substrate media can deviate from that which is intended or desired. The sheet might be ahead or behind in its process direction position, or the sheet can shift in a cross-process direction (lateral to the process direction) or even acquire an angular orientation (referred herein as “skew”) such that the opposed linear edges are no longer parallel to the process direction. Thus, there are three degrees of freedom in which the substrate media can move, which need to be controlled in order to achieve accurate delivery thereof. A slight skew, lateral misalignment or error in the arrival time of the substrate media through a critical processing phase can lead to errors, such as image and/or color registration errors relating to arrival at an image transfer zone. Also, as the substrate media is transferred between sections of the media handling assembly, the amount of registration error can increase or accumulate. A substantial skew and/or registration error can cause pushing, pulling or shearing forces to be generated, which can wrinkle, buckle or even tear the sheet.
Contemporary systems transport a sheet and deliver it at a target time to a “datum,” based on positional measurements from the sheet. That datum, also referred to herein as a delivery registration datum, can be a particular point in a transfer zone, a hand-off point to a downstream nip assembly or any other target location within the media handling assembly. Typically, the time and orientation of the sheet arriving in a sheet registration system is measured by sensors located near the input of the registration system. A controller then computes a sheet velocity command profile designed to deliver the sheet at a target time that delivery registration datum. A sheet velocity actuator commanded by the controller then executes a command profile in order to timely and accurately deliver the sheet. Examples of typical sheet registration and deskewing systems are disclosed in U.S. Pat. Nos. 5,094,442, 6,533,268, 6,575,458 and 7,422,211, commonly assigned to the assignee of record herein, namely Xerox Corporation, the disclosures of which are each incorporated herein by reference. While these systems particularly relate to printing systems, similar paper handling techniques apply to other media handling assemblies.
Such contemporary systems attempt to achieve position registration of sheets by separately varying the speeds of laterally spaced apart drive rollers in registration nip assemblies to correct for skew mispositioning of the sheet, which is also referred to as differentially driven drive or nip assemblies, such as that disclosed in U.S. Pat. No. 7,422,211. By imparting specific differential drive velocity profiles to the two drive nips over a small period of time, skew, process direction and/or lateral position of the sheet can also be corrected. Separate drive motors and/or belt assemblies are often included in differential drive systems, for imparting an angular velocity to the driven wheels. While each motor may be connected directly to the driven wheels, belts (also referred to as timing belts) are often employed. Also, the motors may be stepper motors or DC servo motors with encoder feedback from an encoder mounted on the motor shaft, a driven wheel shaft or the idler shaft. Such registration nip assemblies also generally includes sheet sensors, which are used to detect the arrival of a sheet, its lateral position, skew and other characteristics. Temporarily driving the laterally spaced nips at slightly different rotational speeds will produce a slight difference in the total rotation or relative pitch position of each drive roll while the sheet is held in the two nips. In this way, one side of the sheet moves ahead of the other to induce skew (small partial rotation) in the sheet, in order to eliminate and/or correct for detected skew or positional errors in the lateral or process directions.
Alternatively, contemporary systems include a translating carriage on which the registration nip assemblies are mounted, such as that disclosed in U.S. Pat. No. 5,094,442. As shown in FIG. 6, a nip assembly 2 includes a driven wheel 6 (also referred to as a drive roll) and an idler wheel 8, (also referred to as an idler roll) which together engage opposed sides of the sheet S and conveying it within the printing system in a process direction P. The system includes two laterally spaced apart nip assemblies 2 that are together mounted on a carriage 40. The carriage 40 is able to translate laterally with the use of a separate motor 42 and screw drive shaft 44, as well as a carriage guide collars 46 slideable along a carriage guide shaft 48. The motor 42 turns the screw drive shaft 44, which then translates the carriage 40 laterally, along with the nip assembly 2. In this way, as the carriage 40 with the nip assembly 2 translates laterally, so does the sheet S.
Further sheet registration systems are disclosed in U.S. Pat. Nos. 5,697,608 and 6,866,260, commonly assigned to the assignee of record herein, namely Xerox Corporation, the disclosures of which are each incorporated herein by reference. Such systems use a pair of sheet edge sensors, located on one side of the sheet path, to measure the position of a sheet upon arrival in the sheet registration nip assembly. One of the two edge sensors is generally located laterally adjacent or just upstream of the registration nip assembly, with the other edge sensor disposed further upstream. In this way, when the sheet arrives at the registration nip assembly, the differential measurements from the two edge sensors can be used to calculate lateral position and skew of the sheet. This information is then fed to a controller, which in turn signals the registration nip assembly in order properly register the sheet position laterally and in skew. Typically, the controller calculates the correction of the sheet lateral and skew position to be completed prior to each sheet's arrival at the downstream delivery registration datum. That earlier point for completion of the registration correction is a virtual registration datum that lies somewhere between the registration nip assembly and the delivery registration datum. However, often a sheet can arrive at the virtual registration datum with its registration not fully corrected. Also, further registration errors can occur as the sheet travels from the virtual registration datum to the delivery registration datum.
Accordingly, it would be desirable to provide a method and apparatus capable of more accurately registering a sheet in a media handling assembly, which overcomes the shortcoming of the prior art.