Abstract:
An encoding system is described for a media handling system including an endless belt. The encoding system includes encoding indicia associated with the endless belt. A first encoder transducer is responsive to the encoding indicia to provide a first encoder signal. A second encoder transducer is responsive to the encoding indicia to provide a second encoder signal, the second encoder transducer spatially separated from the first encoder transducer. An encoder processor is responsive to the first encoder signal and the second encoder signal to determine position data regarding the endless belt. Interruption of indicia due to a belt seam is accommodated by transferring belt position calculation from one transducer to the other as the seam passes between the two transducers. The processor is adapted to compensate for variations in the encoder indicia pitch caused by belt temperature differentials.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to printers, and more particularly to encoding systems for media advance systems. 
     BACKGROUND OF THE INVENTION 
     Printers, such as ink-jet printers, can typically employ a media drive system to advance the print media along a media path through a print zone. The print zone in a typical ink-jet printer is located beneath the nozzles of the ink-jet print cartridges. For swath-type printers, the print cartridges are typically mounted on a carriage which moves along a carriage scan axis transverse to the media path, and ink droplets are ejected in a controlled fashion as the carriage is driven. 
     Accurate control of the carriage drive and the media advance is needed to achieve good print quality. If the media and the print elements cannot be accurately positioned while the ink droplets are ejected onto the media, the image quality will degrade. To achieve accurate control of the carriage drive, an encoder system is typically used to monitor carriage motion and position in response to commanded motor-driven movements. 
     This invention addresses problems associated with the accurate media positioning with a belt drive. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, an encoding system is described for a media handling system including an endless belt. The encoding system includes encoding indicia associated with the endless belt. A first encoder transducer is responsive to the encoding indicia to provide a first encoder signal. A second encoder transducer is responsive to the encoding indicia to provide a second encoder signal, the second encoder transducer spatially separated from the first encoder transducer. An encoder processor is responsive to the first encoder signal and the second encoder signal to determine position data regarding the endless belt. 
     According to another aspect of the invention, the processor is adapted to compensate for variations in the encoder indicia pitch caused by belt temperature differentials. 
     According to a further aspect of the invention, a method is described for providing belt position information for a media handling system including an endless belt having encoder indicia associated with the belt. The method includes: 
     reading a first encoder signal from a first encoder transducer responsive to the encoding indicia; 
     reading a second encoder signal from a second encoder transducer responsive to the encoding indicia, the second encoder transducer spatially separated from the first encoder transducer; 
     processing the first encoder signal and the second encoder signal to determine position data regarding the endless belt. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic side view of elements of an ink-jet printer embodying aspects of this invention. 
     FIG. 2 is a schematic block diagram of elements of the ink-jet printer of FIG.  1 . 
     FIG. 3 is a diagrammatic view illustrating encoding indicia on the belt. 
     FIG. 4 is a graphical illustration of a technique for passing control from one belt encoder transducer to the other in accordance with the invention. 
     FIG. 5 is a graphical illustration of the encoder outputs from an analog quadrature encoder. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates aspects of an ink-jet printer  50  using an encoded endless belt to provide the media positioning drive. A motor driven pick roller  52  is activated to pick a sheet of the print media from an input source  54 , and pass it into the nip between drive roller set  56 . The print media may be any type of suitable material, such as paper, card-stock, transparencies, photographic paper, fabric, mylar, metalized media, and the like, but for convenience, the illustrated embodiment is described using paper as the print medium. The invention is also applicable to roll-fed media as well. 
     The print medium is advanced onto an endless perforated belt or web transport  58 , mounted for rotation on belt pulleys  60 ,  62 . One or both of the pulleys  60 ,  62  can be selectively motor-driven in either a clockwise or counter-clockwise direction which allows the belt to move in either direction, to position the sheet relative to the print zone  65  under the printing element system or print engine, represented here by ink-jet pen  66 . The print engine, for the example of an ink-jet printer, can comprise a single pen for emitting a single monochrome ink, a plurality of pens emitting ink droplets of different colored inks, a pen with multiple ink nozzle arrays for emitting a plurality of inks of different colors, by way of example. The belt or web  58  is in fluid communication with vacuum platen  68  by, for example a plurality of apertures (not shown) formed through the belt or web. In this manner, a print medium can be held against the belt  58  for the span of the length of the platen and can be moved to and from the print zone  65 . The exiting sheet is passed through the nip formed by output roller set  64  to an output tray (not shown). 
     The invention is not limited to use in an ink-jet printer, and has utility for other types of printers and machines employing media handling systems, including, for example, facsimile machines, scanners, and multi-function machines which combine two or more functions, e.g. print, scan, copy or facsimile. 
     The printer  50  also includes a belt encoder system comprising a first encoder transducer  70  and a second encoder transducer  72 . The belt encoder system is employed by the printer controller to precisely move the belt to position the print media, e.g. by a precise media advance distance between print swaths. Each transducer is adapted to sense encoder indicia  58 B (FIG. 3) associated with the belt  58 , e.g. by imprinting the indicia on the belt surface, embedding the indicia within the belt, forming an opening pattern defining the indicia, e.g.,by etching the belt material, to list a few exemplary indicia associating techniques. The transducer can be an optical sensor for optically sensing the indicia, or a magnetic transducer for sensing the indicia, by way of example. One preferred technique is to apply or etch a pattern of encoder markings along the interior belt surface in a position such that the markings pass through the field of view of optical sensors serving as the transducers  70 , 72 . Alternative encoding techniques can also be employed, including magnetoresistive and capacitive encoding and sensing techniques. Moreover, for an optical encoder system, either transmissive or reflective systems can be employed. By way of example only, the HEDS-9700 series of optical encoder module, marketed by Agilent Technologies, Palo Alto, Calif., can be used or adapted for use in this application. 
     FIG. 2 is a schematic block diagram of the control system for the printer of FIG. 1. A controller  100  such as a microcomputer or ASIC receives print job commands and data from a print job source  110 , which can be a personal computer, digital camera or other known source of print jobs. The controller acts on the received commands to activate the pick roller motor  82  to pick a sheet from the input tray  54 , advance the sheet to the nip between the drive roller and pinch roller set  56 , and activate the drive motor system  76  to advance the sheet onto the belt  58 , and drive the pulley  60  to move the belt along a media path to advance the sheet to the print zone. The carriage drive  78  is driven by the controller to position the carriage holding the printing elements  66  for commencement of a print job, and to scan the carriage in a direction transverse to the media path. As this is done firing pulses are sent to the printhead(s) comprising the pen(s)  66 . The controller receives encoder signals from the carriage encoder  80  to provide position data for the carriage. The controller is programmed to advance incrementally the sheet using the media drive motor  76  and the belt  58  to position the sheet for successive swaths, and to eject the completed sheet into the output tray. 
     The belt in this example is provided with encoder markings or indicia spaced at an encoder pitch. If the encoded belt  58  is produced as a flat sheet and then attached end-to-end at a seam  58 A (FIG. 3) to make an endless loop, it will be difficult to align the encoder markings at the seam. As a result, there can be a loss of position as the belt seam passes the encoder transducer. Moreover, if the belt encoding markings are manufactured at a different temperature than the operating temperature in the printer, the pitch of the encoding marks will change. Also, the belt temperature can vary in use in the printer, depending on the print mode and media type. The temperature differentials can result in differences in the pitch of the encoder markings, and therefore affect the accuracy of the encoder during operation. 
     In accordance with the invention, the controller further receives encoder signals from the two separated belt encoder transducers  70  and  72 . The encoder signal from one encoder transducer, say the second encoder transducer  72 , is used until the belt seam  58 A is between the first and the second encoder transducers  70 ,  72 . Then control is passed off to the first encoder transducer  70 , i.e. position of the belt  58  is determined by the first transducer now “in control.” When the seam passes the second encoder transducer, control is handed back to the second encoder transducer. 
     The condition that the seam is between the two transducers can be detected in various ways. For example, the passage of the seam past the first sensor can be detected when the first transducer output indicates no belt movement, while the second transducer output indicates belt motion. Another technique is to add another indicia, an index indicia  58 C, to the belt  58  and near the seam  58 A. The index indicia  58 C can provide a clear signal of the arrival of the belt seam. Typically, optical encoder transducers are provided with two channels, which provide quadrature outputs, and an index channel for detecting an index mark. In this event, the index channels of transducer  70  and/or  72  can be used to detect the index indicia  58 C. Another alternative is to use a third sensor  74  (FIGS. 3 and 4) to sense the index indicia  58 C. 
     In order to perform the handoff from one transducer to the other, there are several options. With one exemplary option, as the belt is moving, the position is read using both encoder transducers with an insignificant delay between the two position reads. At an exemplary belt movement rate of 20 inches per second, and an error of 0.000050 inch, the time delay would need to be no larger than the ratio of the error and the movement rate, or 2.5 μseconds in this example. 
     FIG. 4 graphically illustrates the first option for handing off control from one transducer to the other transducer. Position data from the first transducer  70  is indicated as A_, and position data from the second transducer  72  is indicated as B_. Initially, at time T 0 , transducer  70  reads position A 1  and transducer  72  reads position B 1 . The second transducer  72  is used for providing the belt position during time T 0 -T 3 . When the belt seam reaches the first transducer  70  at time T 1 , the position A 1  stops incrementing, even though the belt is still moving, as illustrated by the position data B from the second transducer  72 . The seam passes the first transducer during the time interval between T 1  and T 2 , at which time the transducer  70  output starts incrementing again. Control is passed from transducer  72  to transducer  70  at time T 3 . The seam is passing between the two transducers between times T 3  and T 4 . At time T 4 , the seam reaches the second transducer  72 , and its output stops incrementing between T 4 -T 5 . At T 5 , the seam has passed the second transducer  72 , and its output again starts incrementing. Control is handed back to transducer  72  from transducer  70  at time T 6 . 
     At the handoff time T 3  (FIG.  4 ), the position A 2  read by the first transducer  70  will need to be corrected to A 2 ′=B 2 −(B 1 −A 1 ), where B 2  is the second encoder position at T 3 , and B 1  and A 1  are the respective transducer positions at T 0 . A similar correction process will be performed at the handoff from transducer  70  to transducer  72  at time T 6 . 
     A second exemplary option is to stop the belt with the seam between the two encoders and memorize the offset between the two encoder transducers. Handoff of position control from one encoder transducer to the other then occurs as described above, taking into account the offset between the two transducers. For this option, the time between encoder transducer reads is not important. 
     This invention can also be used to address problems associated with the operating temperature of the belt being different from its manufacturing temperature. The belt material has associated with it a corresponding temperature coefficient of expansion (TCE). The effective pitch of the encoder indicia printed on or embedded in the belt can be estimated from the TCE and the belt temperature to provide some temperature compensation. Due to tolerances on these parameters, the compensation will have errors. A more robust technique is to use the two encoder transducers with a known separation and monitor the phase difference between the signals generated by the two encoder transducers. This technique is illustrated by the following example. 
     If the belt TCE is 20 parts per million per °C. (ppm/°C.), the distance between the encoders is 1 inch and the pitch of the encoder is {fraction (1/150)} inch, a 10° Celsius change in the belt temperature will correspond to a belt length change of (20 ppm/°C.)×(10° C.)×(1 inch)=0.2 mils. This corresponds to about 3% of the encoder pitch or 10.8 electrical degrees. 
     FIG. 5 illustrates an exemplary quadrature output analog encoder transducer waveform, wherein output A and output B are provided in quadrature. FIG. 5 shows the relationship between the phase of the transducer signal and the encoder indicia pitch, in this example {fraction (1/150)} inch. Each cycle is 360° in electrical phase. To calibrate the belt for hot temperature operation, the following process can be employed. After powering up the printer, record the phase difference between encoder transducers over time as the belt warms up to its operating temperature. The phase difference Δθ C  is the phase difference measurement between encoder transducers  70  and  72  while the belt is cold. The phase difference Δθ H  between encoder transducers  70  and  72  is the phase difference when the belt is at the hot operating temperature. Since the phase difference between the two transducers could vary by more than 360° over time, the phase difference is monitored as the machine warms up to keep track of the number of cycles of the phase difference. Now the “hot” encoder indicia pitch is calculated as (encoder spacing)/(((encoder spacing)/(cold encoder pitch))−((Δθ C −Δθ H −)/360)). The cold encoder pitch is known, and is the nominal encoder pitch for the belt. 
     In calculating the position of the belt during normal printing operations, the “hot” encoder indicia pitch is used by the controller at operating temperature, instead of the cold temperature pitch value. This provides a temperature compensation for differences in the indicia pitch. 
     An alternate technique for determining the encoder pitch at any belt temperature is to utilize the index indicia  58 C and the index channels on encoder transducers  70 ,  72 . As the index indicia  58 C passes the first encoder transducer  70 , the control is handed off from encoder transducer  72  to transducer  70 . Encoder transducer  70  is then used to count the number of electrical cycles until the index indicia  58 C passes the second encoder transducer  72 . Since the distance between the two transducers is known, the pitch of the encoder marks  58 B can be determined by the following relationship: (encoder spacing)/((total phase difference between indicia  58 C passing transducers  70  and  72 )/360°). This technique allows re-calibration as often as once per belt revolution. If additional index indicia  58 C are added around the belt, the re-calibration can be done more than once per belt revolution. In practice, the re-calibration of the indicia pitch for belt temperature change should only be necessary during warm up until the belt temperature is stabilized. This technique will also compensate for variations in the encoder indicia pitch caused by factors such as belt stretching. 
     It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.