Patent Application
20100247116
Error correction within high precision positioning systems can compensate for imperfections within the system and produce more precise results. For example, printers use a number of high precision positioning devices to precisely place ink on a sheet of print media. To precisely place ink on the sheet of print media, it is desirable that the relative position of the ink delivery device and the sheet of print media be accurately controlled. For example, a duplexing printer first applies an image to the first side of a sheet of print media, then flips the sheet over and prints an image on the opposite side of the sheet. A measure of the quality of the duplex printing process is the accurate registration of the back image with respect to the front image. Accurate registration is needed so that books and folders containing a picture that is divided on two pages connect in such a way that the image appears well aligned to the reader. For this reason, it is desirable that front (simplex side) to back (duplex side) registration should be very precise.
The accompanying drawings illustrate various embodiments of the principles described herein and are a part of the specification. The illustrated embodiments are merely examples and do not limit the scope of the claims.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.
Printers use a number of high precision positioning devices to precisely place ink on a sheet of print media. To precisely place ink on the sheet, it is desirable that the relative position between the ink delivery device and the sheet be accurately controlled. For example, the motion of a print carriage over a sheet during the printing process should be accurate and repeatable so that the desired image is formed on the sheet of print media.
In another example, a printer first applies an image to the first side of a sheet of print media, then inverts the sheet and prints an image on the opposite side of the sheet. This process is generally referred to as duplex printing. A measure of the quality of the duplex printing process is the accuracy of the registration of the back image with respect to the front image. Accurate registration is needed so that books and folders containing a picture that is divided on two pages connect in such a way that it doesn't disturb the reader. For this reason front (simplex side) to back (duplex side) registration should be very tight.
Accordingly, the present application describes systems and methods in which the position of a perfector arm that is used to transport a sheet of print media between printing a first side and printing a second side is detected relative to the sheet of print media so that any difference from an expected positional relationship between the perfecter arm and print media can be compensated as the sheet is feed to the print engine for printing on the second side. A homing sensor is used to detect the presence of the perfecter arm as the perfector arm engages the print media.
In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods. It will be apparent, however, to one skilled in the art that the present apparatus, systems and methods may be practiced without these specific details. Reference in the specification to “an embodiment,” “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least that one embodiment, but not necessarily in other embodiments. The various instances of the phrase “in one embodiment” or similar phrases in various places in the specification are not necessarily all referring to the same embodiment.
The sheet of print media enters the printing system (100) from the right, passes over the feed tray (125), and is wrapped onto the impression cylinder (130). The blanket cylinder (120) transfers the ink pattern to the sheet as the sheet passes between the blanket cylinder (120) and the impression cylinder (130). To form a single color image (such as a black and white image), one pass through the impression cylinder (130) and blanket cylinder (120) completes the desired image. For a multiple color image, the sheet is retained on the impression cylinder and makes multiple contacts with the blanket cylinder (120). At each contact, an additional color is placed on the sheet of print media. For example, to generate a four color image, the photo charging unit (110) forms a second pattern on the photo imaging cylinder (105) which receives the second ink color from a second binary ink developer. As described above, this second ink pattern is transferred to the blanket cylinder (120) and impressed onto the sheet as it continues to rotate with the impression cylinder (130). This continues until the desired image is formed on the sheet of print media.
After the desired image is formed on a single sided print, the impression cylinder (130) passes the printed sheet to the perfecter (135) which moves the sheet to the exit guide (145). For double-sided prints, the perfecter (135) and duplex conveyor (140) perform the more complex task of reversing the sheet and reintroducing the sheet to the impression cylinder so that the blank surface of the sheet is on the outside of the impression cylinder (130) to receive the second image. Inaccuracies in performing the duplex processing result in registration errors between the images on the front and back sides of the sheet. For example, when the perfecter feeds the sheet onto the drum imprecisely, the second image is incorrectly placed on the back side of the sheet. When significant errors occur, a visible discontinuity in image placement between facing pages in a book or folder can be disturbing to the reader. For example, when a picture is divided across two pages, image displacements can be particularly noticeable.
When the image on the front surface of the sheet of print media (205) has been formed, the sheet is removed from the impression cylinder (130). As shown in
To pick up the sheet (205) from off the impression cylinder (130), the drive motor (220) is rotated such that the sprocket (230) and attached perfecter arm (215) rotate to bring a suction surface on the end of the perfecter arm (215) into contact with the front surface of the sheet (205). The suction surface on the end of the perfecter arm (215) lifts the sheet (205) from the impression cylinder (130). Ideally, the perfecter arm (215) repeatably and precisely picks up the sheet from the impression cylinder. However, there may be some amount of error in the pickup process, either because of an error in positioning of the perfecter arm, an error in positioning of the paper, or a combination of both. For example, various sheets may interact differently with the suction cup because of variations in surface quality. Additionally, various tolerances and limitations of the system, such as limitations in encoder resolution, speeds, diameters, positional errors of within the control system, undesirable positioning of the sheet of print media on the impression cylinder, and other factors can result in pickup errors. Pickup errors can result in image registration errors because pickup errors can result in the sheet being incorrectly positioned on the duplex conveyor and impression cylinder.
If the sheet (205) is only being used as single sided print, the perfector arm (215) will continue to rotate in a clockwise direction and place the sheet on the exit guide (145,
As discussed above, the belt (225) may introduce an undesirable degree of error in the position of the perfecter arm (215) which results in registration errors between the front and back of a duplex print. These errors may be related to a number of characteristics of the belt (225). For example, the belt (225) necessarily has a length that is greater than the circumference of the sprocket. Consequently, the belt may be in any one of a number of orientations during the operation of the perfector. Variations in the belt (225) over its length may then introduce repeatability and accuracy errors which adversely affect the registration precision. Because of these variations, the encoder which measures rotations of the motor does not precisely correspond to the actual position of the perfecter arm.
By creating a system where one complete rotation of the belt (225) produces an integer number of rotations of the driven sprocket (230), errors produced by variation in the belt (225) may occur over shorter and repeatable cycles. According to one illustrative embodiment, the length of the belt (230) may be substantially equal to the circumference of the sprocket times an integer number. For example, the belt length may be two times the circumference of the sprocket (230). Consequently, one complete rotation of the belt (225) results in two rotations of the sprocket (230) about its axis. Various events in the duplex process (as illustrated in
The differences in the performance of the belt (225) at the various positions can result from a number of factors. By way of example and not limitation, these factors may include variations in stiffness of the belt (225) along its length, variations in the geometric dimensions of the belt (225) or its teeth (300), variations over time, etc.
Additionally, the belt (225) is flexible so that it can conform to the diameters of the sprocket (230) and drive motor (220). In some embodiments, the flexibility is provided by molding the belt (225) out of a polymer, plastic or rubber material.
As shown in
The horizontal axis shows the rotation of the sprocket (230,
According to one illustrative embodiment, the perfecter arm then continues its motion through a second revolution to pick up a second sheet and follows the same process described above with respect to the first sheet. As illustrated in
In many print systems, there is a total error budget which specifies the maximum allowable error in duplex registration for all sources. For example, the total error budget may be 0.6 millimeters. To stay within this budget, all of the errors, from whatever source, must result in a shift in the image from the front to the back side of a sheet of no more than 0.6 millimeters. A variety of factors can contribute to this error, of which the belt drive mechanism is only one. For example, differences in paper size, paper thickness, encoder accuracy, drum dimensions, velocity errors, temperature differences, and other factors must all be accounted for within the 0.6 millimeter budget.
These irregularities can be sensed using the encoder on the motor and a homing sensor which detects the motion of the perfecter arm. For each rotation or cycle, the change in encoder counts between homing sensor pulses can be used to quantify the error or deviation. Using this information, the motor position can be corrected to produce the desired perfector arm position.
The first characteristic of the perfecter mechanism that may contribute to registration errors is imperfections in the belt (225). These imperfections can be partially corrected by using the following homing sequence. During the homing sequence, the control system (222) uses the first index of the homing sensor (235) to set the absolute position of the arm (215) at a first encoder position. The arm (215) is then rotated around one revolution and the homing sensor (235) again senses the arm (215) as it passes. The actual number of encoder counts required for the perfecter arm (215) to make one full revolution is then recorded. The actual encoder counts are differenced with the expected number of encoder counts to create a position error. The control system (222) then accounts for this position error during the motion of the perfector arm. This can improve the accuracy of the arm (215) position during the duplexing operation.
A similar calibration can be performed during the second rotation of the perfector arm which corresponds to the second portion of the belt. As discussed above, the errors on the second side of the belt can be significantly different than the errors generated by the first side of the belt. Consequently, separate calibrations for the two rotations of the perfector arm can be generated and the control system (222) can be configured to apply desired calibration during the corresponding rotation of the perfector arm.
Additionally, this calibration and monitoring of the perfector arm can be useful to correct for errors in real time. According to one illustrative embodiment, this calibration routine can be performed during each of the rotations of the perfecter arm during the duplex process. This can correct for changes in the belt or other time dependent factors. For example, belt characteristics can change over time as a result of thermal changes within the system, wear, stretch, etc. A sudden change in the encoder count difference or the encoder counts exceeding a limit can point to a faulty belt or undesirable belt tension.
A second characteristic of the perfector mechanism that may contribute to registration errors is the pickup error. As discussed above with respect to
The calculation of the angle αSC is an independent measurement of the paper position with respect to the perfector arm (215) which is decoupled from all previous actions. The actual suction cup margin can be compared to the desired suction cup margin and corrective action can be taken to compensate for errors between the actual and desired suction cup margins. Consequently, as the perfecter arm (215) reverses its motion, feeds the sheet (205) into the duplex conveyor (140) and releases the sheet (205), the accumulated errors can be corrected. According to one illustrative embodiment, the perfector arm (215) releases the paper shortly after encountering the homing sensor (235) for a second time. This provides a second confirmation of the position of the perfecter arm (215) just before the release of the sheet (205).
The second calibration routine incorporates the paper sensor (240). For example, the actual suction cup margin may be calculated in encoder counts. The desired number of encoder counts can be differenced from the actual suction cup margin. Deviations of the suction cup margin from the optimum are, in fact, pickup errors of the system. This error is then fed into the control system (222), which corrects for the error. In this way, the pickup error can be corrected on a sheet-by-sheet basis.
In some printing systems, there may be two independent perfecter arm mechanisms which cooperate to improve the throughput of the printing system. According to one illustrative embodiment, each of the perfector arm mechanisms use separate motors/encoders, belts, sensors, and sprockets, which allows for independent motion of each arm. If a first perfecter arm A and a second perfector arm B are used, arm A picks up the to-be-duplexed sheet and feeds it again while arm B picks up the next sheet. While arm B feeds the sheet back, arm A picks up the duplexed sheet and exits it. Arm A then picks up the next to-be-duplexed sheet and arm B exits the already duplexed sheet. By working cooperatively, the efficiency of the printing system is improved. However, in printing systems with multiple perfecter arms, it can become increasingly important to compensate for registration errors so that differences between the sheets duplexed by arm A do not have a significantly different registration from those duplexed by arm B.
According to one illustrative embodiment, the initial calibration of the perfecter arm motion (process 800) may include a first step of detecting actual motion of the perfector arm at multiple positions produced during one rotation of the belt (step 810). This may be accomplished using a homing sensor which is strategically placed in travel of the perfector arm to increase the accuracy of calibration at locations where the perfecter arm performs an action, such as the pickup point or the feed point. Next, differencing the actual encoder counts required to produce the motion of the perfecter mechanism with the expected encoder counts produces a measure of the error in the perfecter arm position (step 815). By way of example and not limitation, the control system could expect that it would require 10,000 encoder counts of the motor encoder to produce a first revolution of the perfecter arm. However, due to belt variations or other inaccuracies, the first revolution of the perfector arm may require 10,030 encoder counts to complete a first revolution past the homing sensor. This produces an error of 30 encoder counts. For example, the belt may have stretched slightly during the motion. In the second revolution, the actual encoder counts may be 9,950, producing an error of −50 encoder counts. These belt position dependent errors are fed into the control system so that it can compensate for the errors and produce more accurate perfecter arm motion (step 820).
Following the calibration of the perfecter arm motion through one rotation of the belt, a process for compensating for suction cup margin error (process 805) can be performed. A first step may include making a first measurement of a position of the perfector arm (step 825). Then a second measurement can be made of the sheet location which respect to the base structure using a paper sensor (step 830). Differencing the first measurement and second measurement produces the actual suction cup margin (step 835). The actual suction cup margin may be expressed in a variety of ways including an angle, a distance, or encoder counts. Comparing the actual suction cup margin with the expected suction cup margin produces an error measurement (step 840). This error measurement is input into the control system which alters the action of the motor or other actuators to compensate for the error (step 845). This process can be repeated for each duplexed sheet (step 850).
In sum, moving the homing sensor to a more optimum location and incorporating the calibration routines described above allows for the correction of errors within a high precision positioning device. Further, this implementation can be very low cost when existing hardware is simply reconfigured to make better use of sensors. Additionally, this method continuously calibrates and corrects component motion to correct for variation in the characteristics of the belt or system over time. This improves the performance of the system and could reduce maintenance costs.
The preceding description has been presented only to illustrate and describe embodiments and examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. For example, these principles could be applied to a number of high precision systems which incorporate belt-driven mechanisms, such as belt-driven print heads or paper feeding mechanisms.
