1. Field of the Invention
The present invention relates to a sheet media position sensing device for an image producing apparatus, such as a laser printer. It is more specifically directed to a sheet media position sensor for registering the printed image with the sheet media.
2. Discussion of the Background
In an image forming apparatus, such as a laser printer, a sheet media feeder is provided for transporting the individual sheets into substantial alignment with a xerographic printing device.
In most printing apparatus of this type a sheet media storage tray is mechanically positioned to register the sheet media as it is fed into the mechanism in proper alignment with a transport path to the image producing device. Thus, mechanically the sheet media is properly aligned by the storage tray with respect to both the transport path and the print producing apparatus. A suitable mechanism, such as a sheet feeder roller, is employed in relation to the storage tray to feed a single sheet of the media from the tray into nipped rollers provided along the transport path of the sheet. The feed roller and nipped rollers move the sheet media as accurately as possible, but in many cases the sheet becomes skewed or shifted with respect to the center line of the transport path, which places the sheet media out of proper position with respect to the image producing portion of the printing apparatus.
Up to now, the primary way of attempting to correct this condition is to provide some kind of mechanical apparatus along the transport path to reposition the sheet media into an accurate position required for the image transfer. In the past, individual rollers set laterally with respect to the transport path or individual positioning rollers along the path which can be arranged perpendicular to the longitudinal axis of the path or skewed at an angle with respect to the path have been provided. Various types of sensors, such as photoelectric detectors have been provided in the transport path to determine the position of the sheet media. It is necessary to not only measure the position of the sheet media laterally with respect to the center line of the transport path but also with respect to whether it is skewed with respect to that axis. Once this measurement has been taken and the position calculated the mechanical correction systems, such as the lateral rollers or individual rollers, are rotated at different velocities in an attempt to return the sheet media to the proper position prior to being fed into the image producing device.
Thus, this method produces a sheet alignment mechanism for correcting the positional shift in the sheet media during transportation which is usually caused by rollers which are misapplied or worn within the sheet transporting mechanism. These types of correction devices usually include one of two different types of registration systems. One system is a xe2x80x9cleadxe2x80x9d registration reference in which the posture or position of the sheet media is realigned with respect to the leading edge of the sheet, and the second registration system is a xe2x80x9csidexe2x80x9d registration reference in which the position of the sheet is realigned with respect to the side edge of the sheet media. With the first system, a lateral gate is usually positioned across the transport path so that the sheet media as it moves along the transport path will be stopped momentarily by the lateral gate to reposition the sheet properly with respect to the image producing device. The second registration system causes the sheet to move laterally so as to register the side of the sheet against a fixed elongated member which aligns the sheet with respect to the proper transport path and the image producing device.
The problems with these systems are that with the leading edge registration the sheet is caused to abut and temporarily stop to finalize the correct position of the sheet media. The side registration does not always function properly in that the side of the sheet may buckle preventing the sheet from becoming properly aligned with the respect to the imaging device. This is especially true when the thickness of the sheet media is quite thin and provides little strength or rigidity for the alignment process.
In other prior art, small individual rollers are used at various locations along the transport path and once the position of the sheet is determined, various mechanisms, such as stepper motors or motor driven belts, are used to rotate the rollers at various speeds in order to move the sheet into proper alignment with the transport path. Various slippage between the sheet media and the rollers and the inevitable wear of the mechanical devices driving the rollers as well as the rollers themselves cause variations in the rotational speed of the placement rollers which, in turn, affect the actual correction or realignment of the sheet media with respect to the image producing device.
One of the most critical elements of this type of position alignment and correction is a system for measuring the actual location of the sheet media with respect to the transport path. In one configuration, an LED light bar is used in conjunction with a photosensitive bar. These two elements are placed one above and one below the transport path for the sheet media so that the sheet will pass between. Thus, the LED light bar and photosensitive bar are set perpendicular to the transport path. At predetermined time intervals, readings of the position of the sheet media are recorded which Identifies the actual position of the sheet media and whether it is skewed in relation to the center line of the transport path. This arrangement is quite expensive due to the costs of producing the LED light bar as well as the sensing bar. In fact, it is so expensive that the use of this type of position sensor has been substantially ignored.
The present invention addresses the limitations and problems which exist with the present type of sheet media alignment registration and de-skewing devices. The arrangement provided in the present invention and disclosed in this application is considerably cheaper and more reliable than those prior art devices. No attempt is made to actually move the sheet media, thus, eliminating the considerable number of electrical and mechanical devices which have been utilized in the past. As a result, the reliability and the accuracy of the present invention will considerably improve any printing process using the present correcting arrangement.
The following statement is provided to comply with the applicant""s acknowledged duty to inform the Patent and Trademark Office of any pertinent information of which he is aware. The following information refers to the most pertinent patents of which the applicant has knowledge with respect to the subject matter of the present invention. There is no intent to show that a comprehensive patent ability search has been performed on this subject.
In the Castelli, et al. patent (U.S. Pat. No. 5,887,996) a de-skewing and registering device for an electro-photographic printing machine is provided. A single sensor determines the actual position and the skew of a sheet in a paper path and generates a signal indicative of this position. A pair of independently driven nip rollers forwards the sheet to a registration position at a proper time based on signals from a controller which generates the motor control signals for applying the necessary corrections to reposition the sheet. An additional single photoelectric sensor can be used to provide feedback for updating the control signals for positioning the sheet.
The Uedo, et al. patent (U.S. Pat. No. 5,857,130) shows an image forming apparatus which has the capabilities of correcting a slant of the image which is recorded after the scanning of a document. The image signal is electrically processed so as to correct the slant of the image to be printed in a xerographic printing process.
The Fujikura, et al. patent (U.S. Pat. No. 6,273,418) discloses a sheet registration device which includes a sheet positioning member which is disposed along a side of the sheet transport path and parallel with the transport direction. Lateral moving rollers are provided for moving the sheet into contact with the side positioning member to register the sheet properly with respect to the image transferring device. Sensors are provided for identifying the position of the sheet and moving it accordingly into contact with the side positioning member. This arrangement properly aligns the sheet with respect to the transport path whereby the text from the imaging device will be positioned properly on the sheet.
The Nakamura, et al. patent (U.S. Pat. No. 5,458,324) allows detection of multiple sheets of paper being conveyed in an image reading or forming operation. In addition, a facility is provided for recovery to eliminate double or triple sheet feeding in the process. A pressure sensitive electrical conductive rubber roller is included which identifies when changes exist in the length and thickness of the sheet being conveyed. Electrical signals are produced so that multiple sheet conveying can be detected. This patent has nothing to do with the positioning of the sheet or identifying the position of the sheet in the transport path.
The Tanaka, et al. patent (U.S. Pat. No. 5,676,477) discloses a sheet carrying apparatus which is arranged for reducing the skew of a sheet in the transport path. Electrical and magnetic holding devices are disclosed in contact with the sheet as part of the holding device for positioning the sheet and correcting the skew. Position detecting devices are disclosed which primarily include an optical light sensing device on each side of the sheet transport path. Multiple readings as the sheet is transported provide output signals which are used to calculate the skew of the sheet. From this information, course determinations are produced for moving the sheet in proper position to eliminate the skew.
The Brookner patent (U.S. Pat. No. 6,234,694) is a device for printing postage or other images onto media of variable size. A detection device determines the size of the media in at least one dimension prior to or during the printing of the image. From the information received, the image is adjusted accordingly. As a result, the image printing facility is positioned to apply the image to the media in a proper location.
The Haselby patent (U.S. Pat. No. 6,241,334) discloses a technique for aligning the ink jet print head cartridge of an ink jet printer. The print cartridge moves along a horizontal carriage which is transverse to the transport path of the media. Optical sensors are supported on the movable carriage which read the horizontal positions of vertical test lines which are imaged on a detector in conjunction with horizontal alignment corrections of the media. In this way, alignment of the printed image is made possible.
The Kimura patent (U.S. Pat. No. 6,160,608) discloses an improved image recording apparatus which includes a recording light modulated in accordance with the image which is allowed to be incident in a specified recording position. Light sensitive material is transported in an auxiliary scanning direction as it is held in the recording position. The transport has a first nip transport device and a second nip transport device. The supply device for transporting the light sensitive material is supplied in an appropriate posture into the first nip transport device; any skew which is detected can be corrected in the position of the light sensitive materials upstream of the recording position without affecting the exposure thereby ensuring consistent production of photographic prints properly positioned on the light sensitive material.
The present invention is directed to a novel arrangement for sensing and measuring the position of sheet media as it is transported in a recording or printing operation. It is understood that this sensing device can be utilized with any type of printing operation and is not specifically limited to any one type.
As is well known, paper and other types of sheet media have a relatively high resistance with respect to the passage of an electrical current. The present sensing device utilizes this characteristic in determining the position and skew of the sheet media as it is being transported.
A pair of nipped rollers is positioned transverse to the longitudinal direction of the sheet media transport path. As is common in most devices of this type, the sheet media travels along the transport path at a known constant velocity. The rotating nipped rollers, according to the present invention, can include a solid metal roller which is usually positioned below the transport path with the corresponding parallel upper roller formed from a generally metallic support shaft having a thickness of electrically conductive rubber evenly applied thereto. Thus, it is possible to have a relatively thin diameter center shaft with a relatively thick coating of conductive rubber as the outer surface or a relatively large diameter metallic roller provided with a relatively thin coating of electrically conductive rubber forming the outer surface. In most cases, the thin coating of conductive rubber is desirable since it produces a lower resistance in the electrical circuit making up the sensing device. The mid-point of the conductive roller has an insulator which separates the roller into two substantially equal halves. The centered insulator position is aligned with the center line of the transport path of the printing device. The contact area for the pair of nipped rollers is a contact line or nip along the entire length of the rollers. A predetermined constant DC biasing voltage is applied to the metal roller and an electrical lead is connected to each half of the rubber conductive roller to provide output signals of the current passing through the conductive halves of the roller. Thus, without having any sheet material positioned in the nip or contact area of the rollers, the output current flowing through the electrical conductive halves will be equal. These output circuits can also be balanced for calibration purposes to maintain the output current in the idle condition as being equal. The output current from each half of the rubber conductive roller is fed to a position calculating circuit which can be part of a microprocessor contained within the printer device.
As stated earlier, paper and other types of sheet media have a relatively high electrical resistance. As the paper or sheet media is moved along the transport path, it is introduced into the electrically conductive rubber nipped roller and metal roller. The paper causes a change in the current flowing in the two halves of the nipped roller. These two output currents are measured and compared and an output signal is generated indicative of the proportion of the current flowing in each half of the nipped roller. This output signal indicates the exact position of the sheet material with respect to the center of the divided roller and, thus, the center line of the transport path. Multiple readings can be taken at predetermined time intervals as the sheet passes between the rollers with variations in the output signals being used to calculate the actual skew or lateral shift of the sheet media. The skew calculations, in the form of the output signals, are then passed directly to the processor for realignment of the image on the photosensitive drum or print head. Thus, the output signals precisely indicate the position of the sheet media and the skew or shift of the sheet media, if present. In this way, exact registration of the print image can be provided for each page of the sheet media.
As is well known, the processor within the printer device, such as a laser printer, receives the incoming digital data signals and scans a photosensitive drum which electrostatically transfers an image producing media, such as toner, to the sheet media as it passes along the transport path. The toner is transferred electrostatically to the sheet media where it is fixed by heat and pressure applied by a pair of fuser rollers.
The microprocessor in a laser scanning circuit applies the image directly to the photosensitive drum. Additional aligning circuitry can be provided in the processor for adjusting the position of the scanned image on the photosensitive drum in accordance with the skew or shift that has been identified for the individual sheet media upon which the image is to be placed. Thus, the position of the image is adjusted in accordance with the output signals received from the sheet media sensing device rather than the sheet media itself. In this way, the image as it is applied directly to the sheet media is registered properly with respect to the outer dimensions of the sheet. Through this very inexpensive and novel arrangement, image registration on the sheet media can be provided to produce a high quality printed page.
As stated above, it is possible to use this sheet media positioning sensor in any type of transport where a sheet media, such as paper, labels, or transparencies, can be properly printed with aligned text or images. This is true with any printing operation wherein the image transfer device is alignable with respect to the transport path of the media. This is not only true with a xerographic type of printer, such as a laser printer, but could be used with an ink bubble jet printer or a dot matrix printer, so long as the print head can be realigned with the determined position of the sheet media.