Abstract:
In a web printer, tension on the moving web is controlled by monitoring the tension on the web between two rollers and selectively operating an actuator driving the second roller to restore the tension to an acceptable range. The operation of the actuator includes modulating the speed at which the second roller is driven.

Description:
TECHNICAL FIELD 
     This disclosure relates generally to methods for controlling a tension applied to a web member moving through a device, and more particularly to methods for maintaining tension applied to media webs in printers. 
     BACKGROUND 
     In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink may be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase when heated to a melting temperature. The printhead unit ejects melted ink supplied to the unit onto media or an imaging member. Once the ink is ejected onto media, the ink droplets quickly solidify. 
     The media used in both direct and offset printers may be in web form. In a web printer, a continuous supply of media, typically provided in a media roll, is entrained onto rollers that are driven by motors. The motors and rollers pull the web from the supply roller through the printer to a take-up roll. The rollers are arranged along a linear media path, and the media web moves through the printer along the media path. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web. 
     Moving the web through the media path in a controlled manner presents challenges to web printing systems. As the media web moves through various portions of the media path, one or more of the rollers apply tension to the web. An appropriate level of tension between the media web enables the media web to engage the rollers in the media path via friction without slipping. During operation, however, one or more rollers that contact the media web may slip relative to the motion of the web, resulting in a drop in the tension of the media web. Various causes of the drop in tension include excess slack introduced by a media web winder or rewinder, variations in the rotational rates of two or more drive rollers that are positioned on the media path, and a loss of friction between the media web and a roller due to oil or other substance on the web that reduces the coefficient of friction between the media web and the roller. 
     If the web slips when engaged with one or more rollers in the media path, the position of the media web with respect to the printheads is affected and errors in images formed on the media web may occur. Media slippage may produce positional errors based on calculations of web velocity because the actual velocity of the web and the web velocity identified with respect to the angular velocity of the rollers differ. These positional errors may adversely impact the effectiveness of registration techniques that coordinate the operation of multiple printheads to form ink images on the media web. When a media web slips over rollers in the media path, one solution known to the art increases a normal force between one or more rollers and the media web to reduce or eliminate the slip and restore tension to the web. The increased normal force applied to the media web may be great enough to distort or break the media web. Thus, improvements in operating continuous web printing systems to reduce slip on the media web while maintaining the normal force that is applied to the media web within an operating range would be beneficial. 
     SUMMARY 
     In one embodiment, a method of adjusting operation of a printing apparatus has been developed. The method includes moving a web over a first roller and along a media path in a process direction at a web linear velocity, operating an actuator to rotate a second roller that is in contact with the web, the second roller being at a location on the media path that is offset from the first roller in the process direction, identifying at least one of a first velocity difference and a second velocity difference between a linear velocity of the second roller and at least one of a linear velocity of the first roller and the web linear velocity, respectively, the at least one of the first and second velocity differences being positive when the linear velocity of the second roller is greater than the at least one of the linear velocity of the first roller and the web linear velocity, respectively, and adjusting operation of the actuator to modulate the linear velocity of the second roller between a first linear velocity and a second linear velocity in response to the identified at least one of the first and second velocity differences being positive and above at least one of a first velocity differential threshold and a second velocity differential threshold, respectively, the first linear velocity being greater than the second linear velocity and the first and second linear velocities being greater than the at least one of the linear velocity of the first roller and the web linear velocity. 
     A printing apparatus that is configured to adjust tension on a media web has been developed. The apparatus includes a plurality of rollers configured to move a media web along a media path in a process direction, a first roller in the plurality of rollers rotates to move the web in the process direction at a web linear velocity, and a second roller in the plurality of rollers is positioned along the media path at a location that is offset from the first roller in the process direction, an actuator operatively connected to the second roller, the actuator being configured to rotate the second roller to move the media web past the second roller in the process direction, and a controller operatively connected to the actuator, the controller being configured to operate the actuator to adjust a rotation of the second roller to move the media web past the second roller, identify at least one of a first velocity difference and a second velocity difference between a linear velocity of the second roller and at least one of a linear velocity of the first roller and the web linear velocity, respectively, the at least one of the first and second velocity differences being positive when the linear velocity of the second roller is greater than the at least one of the linear velocity of the first roller and the web linear velocity, respectively, and adjust operation of the actuator to modulate a linear velocity of the second roller between a first linear velocity and a second linear velocity in response to the identified at least one of the first and second velocity differences being positive and above at least one of a first velocity differential threshold and a second velocity differential threshold, respectively, the first linear velocity being greater than the second linear velocity and the first and second linear velocities being greater than the at least one of the linear velocity of the first roller and the web linear velocity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a prior art printer modified to operate a web tension control method as disclosed herein. 
         FIG. 2  is a flow diagram of one embodiment of a process for controlling web tension in a printer. 
         FIG. 3  is a flow diagram of a process for operating the components of a printer according to one embodiment of the web tension control method. 
         FIG. 4  is a graph of a velocity difference between a second roller and a first roller during operation of one embodiment of the web tension control method. 
         FIG. 5  is a graph of a signal provided to an actuator during operation of the web tension control method in  FIG. 4 . 
         FIG. 6  is a block diagram of a prior art inkjet printing system in which the web tension control method disclosed herein may be used. 
         FIG. 7  is an enlarged view of a printhead assembly included within the inkjet printing system showing an arrangement of a series of printhead modules used to eject ink of different colors. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, the drawings are referenced throughout this document. In the drawings, like reference numerals designate like elements. As used herein the term “printer” refers to any device that is configured to eject a marking agent upon an image receiving member and includes photocopiers, facsimile machines, multifunction devices, as well as direct and indirect inkjet printers and any imaging device that is configured to form images on a print medium. As used herein, the term “process direction” refers to a direction of travel of an image receiving member, such as an imaging drum or print medium, and the term “cross-process direction” is a direction that is perpendicular to the process direction along the surface of the image receiving member. As used herein, the terms “web,” “media web,” and “continuous media web” refer to an elongated print medium that is longer than the length of a media path that the web traverses through a printer during the printing process. Examples of media webs include rollers of paper or polymeric materials used in printing. The media web has two sides forming surfaces that may each receive images during printing. Each surface of the media web may be viewed as a grid-like pattern of potential drop locations, sometimes referred to as pixels. 
     As used herein, the term “capstan roll” refers to a cylindrical member that is configured to have continuous contact with media web moving over a curved portion of the member and to rotate in accordance with a linear motion of the continuous media web. As used herein, the term “angular velocity” refers to the angular movement of a rotating member for a given time period, sometimes measured in rotations per second or rotations per minute. The term “linear velocity” refers to the velocity of a member, such as a media web, moving in a straight line. When used with reference to a rotating member, the linear velocity represents the tangential velocity at the circumference of the rotating member. The linear velocity ν for circular members may be represented as: ν=2πrω where r is the radius of the member and ω is the rotational or angular velocity of the member. A media web that is in contact with a roller slips when the tension differential across the roller is greater than what the capstan friction e μθ  can support to enable traction. In identifying capstan friction, μ represents the coefficient of friction of the capstan roller, and θ represents the angle of the surface of the capstan roller that contacts the media web. Media web slip generates velocity errors between the media web that is in contact with the roller and the surface of the roller. 
       FIG. 6  depicts a prior art inkjet printer  10  having elements pertinent to the present disclosure. In the embodiment shown, the printer  10  implements a solid ink print process for printing onto a continuous media web. Although the media web tension control method and apparatus are described below with reference to the printer  10  depicted in  FIG. 6 , the subject method and apparatus disclosed herein may be used in any printer, such as a cartridge inkjet printer, which uses serially arranged printheads to eject ink onto a web image substrate. 
     The printer  10  includes a web supply and handling system  60 , a printhead assembly  14 , a web heating system  100 , and a fixing assembly  50 . The web supply and handling system  60  includes one media supply roll  38  for supplying a media web  20  to the printer  10 . The supply and handling system  60  is configured to feed the media web  20  in a known manner along a media pathway in the printer  10  through a print zone  18  located adjacent to the printhead assembly  14 , past the web heating system  100 , and through the fixing assembly  50 . To this end, the supply and handling system  60  includes any suitable device  64 , such as drive rollers, idler rollers, tensioning bars, etc., for moving the media web  20  through the printer  10 . The printer  10  includes a take-up roll (not shown) for receiving the media web  20  after printing operations have been performed. Alternatively, the media web  20  can be fed to a cutting device (not shown) as is known in the art for cutting the media web into discrete sheets. The printhead assembly  14  is appropriately supported to eject drops of ink directly onto the media web  20  as the web moves through the print zone  18 . In other printers in which the media web tension control method and apparatus is used, the printhead assembly  14  is configured to eject drops onto an intermediate transfer member (not shown), such as a rotating drum or belt, for subsequent transfer to a media web or media sheets. 
     Referring to  FIG. 7 , the printhead assembly  14  includes a series of printhead modules  21 A,  21 B,  21 C, and  21 D with each printhead module effectively extending across the width of the media web  20 . As is generally familiar, each of the printhead modules  21 A,  21 B,  21 C, and  21 D selectively ejects a single color of ink. In some embodiments, each module ejects one color of ink typically used in color printing; namely, the primary colors cyan, magenta, yellow, and black (CMYK). The printhead module for each primary color typically includes two or more serially arranged printheads with the multiple printheads formed into a multiple row array. Although the embodiment shown discloses a series of printhead modules, a printer may include as few as one printhead module for printing images using only one color, such as black. A plurality of inkjets is arranged in a row and column fashion on each printhead. Each of the inkjets is coupled to a source of liquid ink and each one ejects ink through an inkjet nozzle in response to a firing signal being received by an inkjet actuator, such as a piezoelectric actuator, in the inkjet. 
     Referring again to  FIG. 6 , the printer  10  uses “phase-change ink,” by which is meant ink that is substantially solid at room temperature and that transitions to a liquid phase when heated to a phase change ink melting temperature. The melted ink is provided to the printheads for jetting onto the image receiving surface. The phase change ink melting temperature is any temperature that is capable of melting solid phase change ink into liquid or molten form. In one embodiment, the phase change ink melting temperature is approximately 70° C. to 140° C. In alternative embodiments, the ink utilized in the printer is UV curable gel ink. Gel ink is heated to change the viscosity of the ink from a gel to a liquid phase before the ink is ejected by the inkjet ejectors of the printhead. As used herein, liquid ink refers to melted solid ink, heated gel ink, or other known forms of ink, such as aqueous inks, ink emulsions, ink suspensions, ink solutions, or the like. 
     Ink is supplied to the printhead assembly from a solid ink supply  24 . In aqueous or emulsion ink systems, which use the media web tension control method and apparatus disclosed herein, the liquid ink may be stored in one or more volumetric containers installed in the printer. Since the printer  10  of  FIG. 6  is a phase change ink multicolor device, the ink supply  24  includes four sources of solid phase change ink, including a cyan source  28 , a yellow source  30 , a magenta source  32 , and a black source  34 . The imaging device  10  also includes a solid phase change ink melting and control assembly or apparatus (not shown) for melting the solid form of the phase change ink into a liquid form, and then supplying the liquid ink to the printhead assembly  14 . Each color of ink is supplied to one of the series of printhead modules  21 A,  21 B,  21 C, and  21 D within the printhead assembly  14 . The differently colored inks are supplied through separate conduits. A single line connects the ink supply  24  with the printhead assembly  14  in  FIG. 6  to simplify the representation depicted in the figure. 
     Referring still to  FIG. 6 , operation and control of the various subsystems, components, and functions of the printer  10  are performed with the aid of a controller  40 . In some embodiments, the controller  40  is implemented with general or specialized programmable processors that execute programmed instructions. The instructions and data required to perform the programmed functions are stored in memory associated with the processors or controllers. The processors, their memories, and interface circuitry configure the controllers and/or print engine to perform the printer functions described above. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in VLSI circuits. Also, the circuits described herein can be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits. 
     In order to form an image with the ink ejected by the printhead assembly  14 , image data received by the printer  10  are converted into firing signals that selectively actuate the inkjets in the printheads to eject ink onto the web  20  as the web  20  moves past the printhead assembly  14 . The controller  40  receives velocity data from encoders mounted proximately to rollers positioned on either side of the portion of the path opposite the four printhead modules  21 A,  21 B,  21 C, and  21 D to compute the position of the web  20  as the web moves past the printhead modules  21 A,  21 B,  21 C, and  21 D. The controller  40  uses this velocity data to generate timing signals that are delivered to printhead controllers in the printhead modules that enable the printhead controllers to generate firing signals that actuate selected inkjet ejectors in the printheads. The inkjet ejectors actuated by the firing signals correspond to image data processed by the controller  40 . 
     Referring still to  FIG. 6 , after drops of ink are ejected onto the moving web  20  within the print zone  18  to form an image, the web  20  continues along the media path so that the image passes through a fixing assembly  50 , which fixes the ink drops to the web  20 . In the embodiment shown, the fixing assembly  50  comprises at least one pair of fixing rollers  54  that are positioned in relation to each other to form a nip through which the media web  20  is fed. The ink drops on the media web  20  are pressed into and spread out on the web  20  by the pressure formed by the nip. Although the fixing assembly  50  is depicted as a pair of fixing rollers  54 , the fixing assembly can be any suitable type of device or apparatus, as is known in the art, which is capable of fixing, drying, or curing an ink image onto the media. 
     Referring now to  FIG. 1 , the prior art printer system modified to operate the web tension control method disclosed herein is shown. The printer  10  includes a web system for delivering a continuous supply of media web  20  to the media path of the printer. The web system includes an in-feed section  88  for storing and delivering the media web  20 , a printing section or print zone  18  for printing images on the web  20 , and a take-up section  90  for rewinding and storing the web  20  after printing. The configuration of the different sections  18 ,  88 ,  90  as shown in  FIG. 1  enables the media web  20  to be moved from left to right. The media path of the printer  10  includes both a first roller  70  located upstream of the print zone  18  and a second roller  72  located downstream of the print zone  18 . In the embodiment shown, the first and second rollers  70 ,  72  are circular, each being rotable about an axis and having an outer circumference about the axis. The media path is configured to provide sufficient wrap of the media web  20  about the first and second rollers  70 ,  72  to enable a tension force to be generated along the web between the rollers  70 ,  72 . In the disclosed embodiment, the media path provides sufficient wrap about the first and second rollers  70 ,  72 , by being configured to place at least one half of the outer circumferences of the first and second rollers  70 ,  72  in contact with the media web. The normal force and the coefficient of friction of the interface between the media web  20  and the rollers  70 ,  72  determine the maximum force generated along the web  20 . The tension of the media web  20  is measured by a tension sensor  76  located along the media path between the first and second rollers  70 ,  72 . 
     The second roller  72  is configured to adjust the tension of the media web  20  in response to the measured tension being below a predetermined tension threshold. As used herein, the term “predetermined tension threshold” is a tension value or a range of tension values that enable the media web  20  to engage the first and second rollers  70 ,  72  by means of static, or non-slip, friction and that do not distort or break the media web  20 . When the web tension is at or within the predetermined tension threshold, web tension adjustment is achieved by operating an actuator  74  to increase or decrease a rotation of the second roller  72  to respectively increase or decrease the web tension. However, when the web tension is below the predetermined tension threshold, such as when transient conditions of low friction or insufficient normal force exist between the rollers  70 ,  72  and the media web  20 , the second roller  72  is unable to impose sufficient force along the web  20  by increasing the rotation of the second roller  72  alone. 
     The web tension control method disclosed herein varies operation of the actuator  74  to increase the tension of the media web  20  when the measured web tension is below the predetermined tension threshold. In one embodiment, proper tensioning is achieved by operating the actuator  74  to modulate a linear velocity of the second roller  72  between different linear velocities. In another embodiment, proper tensioning is achieved by deactivating the actuator  74  to enable the second roller  72  to rotate at a linear velocity of the web  20 . In yet another embodiment, proper tensioning is achieved by selectively activating and deactivating the actuator  74  to enable the second roller  72  to rotate at the linear velocity of the web. In yet another embodiment, proper tensioning is achieved by identifying the linear velocity of the web and operating the actuator  74  to rotate the second roller  72  with the identified linear velocity. 
     The web tension control method can also operate additional actuators in association with the actuator  74  to increase the tension of the media web  20  when the measured web tension is below the predetermined tension threshold. In one embodiment, proper tensioning is achieved by rotating the first roller  70  with a second actuator  80  and operating the actuator  74  to modulate the linear velocity of the second roller  72  between different linear velocities. In another embodiment, proper tensioning is achieved by operating the second actuator  80  to rotate the first roller  70  to move the web  20  a predetermined distance along the media path and operating the actuator  74  to rotate the second roller  72  after the web  20  has moved the predetermined distance. In yet another embodiment, proper tensioning is achieved by operating a third actuator  82  to engage a third roller  78  with the second roller  72  to form a nip with the second roller  72 . Additional embodiments of printers employing the web tension control method disclosed herein are configured by combining individual elements of the above-referenced embodiments; thus, this listing of embodiments is not exhaustive. 
     Referring still to  FIG. 1 , the actuator  74  is operatively connected to the second roller  72  and configured to rotate the second roller  72  to move the media web  20  past the second roller  72  in the process direction. In one embodiment, the second actuator  80  is operatively connected to the first roller  70  and configured to rotate the first roller  70  to advance the media web  20  along the media path. In an alternative embodiment, the third actuator  82  is operatively connected to the third roller and configured to selectively move the third roller  78  into and out of engagement with the media web  20  to form the nip with the second roller  72  and move the media web through the nip and past the second roller  72 . Although the second and third actuators  80 ,  82  have been shown as alternative embodiments, a printer system using the web tension control method disclosed herein can employ both the second and third actuators  80 ,  82  in a single embodiment to control the first and third rollers  70 ,  78 , respectively. 
     The printer  10  includes a controller  40  that is operatively connected to the tension sensor  76  and the actuator  74 . In an alternative embodiment, the controller  40  is operatively connected to one or both of the second and third actuators  80 ,  82 . The controller  40  is configured to identify the tension of the media web  20  with reference to signals generated by the tension sensor  76 . The controller  40  is also configured to operate the actuator  74  to adjust the rotation of the second roller  72 . In one embodiment, the controller  40  is similarly configured to operate the second actuator  80  to adjust the rotation of the first roller  70 . In an alternative embodiment, the controller  40  is configured to activate the third actuator  82  to move the third roller  78  into engagement with the web  20  to form the nip with the second roller  72 . Although the controller  40  has been shown with alternative configurations to operate the second and third actuators  80 ,  82 , a printer operating the web tension control method disclosed herein can employ these configurations in a single embodiment to operate both of the second and third actuators  80 ,  82 . 
     Referring still to  FIG. 1 , one embodiment of the printer  10  includes a first rotational speed sensor  84  configured to generate a signal corresponding to a rotational speed of the first roller  70  and a second rotational speed sensor  86  configured to generate a signal corresponding to a rotational speed of the second roller  72 . The printer controller  40  is operatively connected to the first and second rotational speed sensors  84 ,  86 . In this embodiment, the controller  40  is configured to identify the rotational velocities of the first and second rollers  70 ,  72  with reference to the signals generated by the first and second rotational speed sensors  86 ,  84 , respectively. The controller  40  is further configured to identify the web linear velocity with reference to the identified rotational velocities of the first and second rollers  70 ,  72 . 
     Although the disclosed embodiment is shown using rotational speed sensors  84 ,  86  to generate the rotational speed signals that are used by the controller to identify the web linear velocity, a web velocity sensor  87  can be configured to monitor the web directly and generate a signal corresponding to a linear velocity of the web. In this embodiment, the printer controller  40  is operatively connected to the web velocity sensor  87  and configured to receive the signal from sensor  87  that corresponds to the linear velocity of the web. The web velocity sensor  87  can be located anywhere along the media path, but is preferably located near the second roller  72 . The sensors  84 ,  86 ,  87  are known sensors that operate with known techniques to generate their respective signals. Such techniques include mechanical, optical, radio, or laser techniques to measure a physical parameter and generate an electrical signal corresponding to the measurement. 
     A flow diagram of a process  200  that controls the tension of the media web in a printer is shown in  FIG. 2 . The controller configured to execute the programmed instructions to implement the process  200  begins by computing a velocity difference between a velocity of the second roller and a velocity of the first roller (block  202 ). As the web is moved along the media path, the first and second rotational speed sensors generate respective signals corresponding to the rotational speeds of the first and second rollers. The controller implementing the process  200  transforms the signals into standardized quantities, such as linear velocities, and computes the difference between the second roller velocity and the first roller velocity. This velocity difference is positive if the difference between the second roller velocity and the first roller velocity is greater than zero. A positive velocity difference indicates the second roller is rotating faster than the first roller. 
     After the first velocity difference is computed (block  202 ), the controller configured to execute the programmed instructions to implement the process  200  determines whether the computed velocity difference is greater than a first velocity differential threshold (block  204 ). As use herein, the term “first velocity differential threshold” is a velocity differential or a range of velocity differentials that indicate the media web is engaged in static, or non-slip, friction with the first and second rollers. The controller implementing the process  200  compares the velocity difference to the first velocity differential threshold to determine whether the first velocity difference is greater than the first velocity differential threshold. Computing a positive velocity difference that is greater than the first velocity differential threshold can be indicative of the media web being slack or broken or of slipping contact between the media web and the second roller. 
     If the velocity difference is not greater than the first velocity threshold (block  204 ), the controller implementing the process  200  computes a velocity difference between the velocity of the second roller and the velocity of the web (block  206 ). As the web is moved along the media path, the second rotational speed sensor and the web velocity sensor generate respective signals corresponding to the rotational speed of the second roller and the linear velocity of the web. The controller implementing the process  200  transforms the signals into standardized quantities, such as linear velocities, and computes the difference between the second roller velocity and the web velocity. This roller/web velocity difference is positive if the difference of the second roller velocity to the web velocity is greater than zero. A positive roller/web velocity difference indicates that the second roller is rotating faster than the web is moving over the second roller. 
     After the velocity difference is computed (block  206 ), the controller implementing the process  200  determines whether the computed roller/web velocity difference is greater than a second velocity differential threshold (block  208 ). As used herein, the term “second velocity differential threshold” is a velocity differential or a range of velocity differentials that indicate the media web is engaged in static, or non-slip, friction with the second roller. The controller implementing the process  200  compares the velocity difference to the second velocity differential threshold to determine whether the velocity difference is greater than the second velocity differential threshold. Computing a positive roller/web velocity difference that is greater than the second velocity differential threshold can be indicative of the media web being slack or broken or of slipping contact between the media web and the second roller. 
     In one embodiment of the process  200 , the first and second velocity differential thresholds are equal. In an alternative embodiment, the first and second velocity differential thresholds are different. Selecting the first and second velocity differential thresholds can depend on a number of operational parameters, such as the process speed of the printer, the weight and thickness of the web, and the like. In addition, in one embodiment of the process  200 , only one of the velocity differences is computed. Similarly, and depending on which one of the velocity differences is computed, only one of the velocity differences is compared to the respective first or second velocity differential thresholds. Selecting which one of the velocity differences is computed can similarly depend on a number of operational parameters, such as those listed above. 
     If the computed velocity differences are not greater than the respective first and second velocity differential thresholds (blocks  204 ,  208 ), the controller implementing the process  200  operates the actuator with a tension controller, such as a servo or PID controller, to adjust the web tension (block  210 ). Under this type of actuator control, the tension controller receives signals from the tension sensor, transforms those signals into a standardized quantity, such as force or tension, and compares that measured tension to a desired tension, or a tension set. The tension controller then implements an algorithm to generate a varying command to operate the actuator to keep the web tension constant, or near constant, at the tension set. 
     The tension sensor is preferably located within or near the print zone of the media path; however, the tension measurement can be made anywhere along the media path between the first and second rollers. The tension sensor can employ any method to obtain the tension measurement, such as a load cell positioned at one of the idler rollers located between the first and second rollers. The tension set can be any tension value or range of tension values at or within the predetermined tension threshold. The tension set can be contained in the programmed instructions stored in memory and associated with the controller or can be entered by a user through an I/O device  41  as shown in  FIG. 1 . The I/O device  41  can include a user interface, graphical user interface, keyboards, pointing devices, displays, and other devices that allow externally generated information to be provided to the printer, and that allow internal information of the printer to be communicated externally. 
     Referring again to  FIG. 2 , if one or both of the computed velocity differences are greater than the respective first and second velocity differential thresholds (blocks  204 ,  208 ), the controller implementing the process  200  generates and applies a zero command to operate the actuator (block  212 ). In some embodiments, the zero command can also be used to operate one or both of the second and third actuators as is discussed in more detail below. The controller implementing the process  200  applies the zero or near zero command by sending unique command signals to the actuator and, optionally, to the second and third actuators. These unique command signals enable the printer to achieve tension in a web system when transient conditions of low friction or low normal force exist between the web and the first and second rollers. Thus, the controller implementing the process  200  determines whether to generate and apply the zero command or to generate and apply a varying command to operate the actuator based on the existence and magnitude of a computed velocity difference indicating web slip. 
       FIG. 3  depicts a flow diagram of a process  300  for operating the actuator, the second actuator, and the third actuator according to the zero command. The process  300  for operating the actuators is performed in block  212  of  FIG. 2 . The controller implementing the process  300  begins by selecting one or more control methods to operate the actuator and, optionally, the second and third actuators (block  312 ). In one control method, the actuator is operated to modulate the linear velocity of the second roller between a first linear velocity and a second linear velocity (block  314 ). Referring to  FIG. 4 , a graph of the velocity difference  42  between the second roller and the first roller in an example velocity modulation is shown. The linear velocity of the first roller is constant and in this graphical representation is located along a horizontal line passing through zero on the y-axis. Although the linear velocity of the first roller is represented as zero on the graph, the actual linear velocity of the first roller may be any velocity that moves the web along the media path. 
     The velocity difference  42  between the second roller and the first roller generally increases from zero at 49.5 on the x-axis and decreases to zero at approximately 57.5 on the x-axis during operation of the control method in this example. The increasing magnitude of the velocity difference starting at about 49.5 represents the second roller losing non-slip contact with the web while the decreasing magnitude of the velocity difference ending at about 57.5 represents the second roller regaining non-slip contact with the web. 
     Referring still to  FIG. 4 , the velocity modulation in this example is initiated when the velocity difference between the second roller and the first roller exceeds a velocity difference maximum  44 . When the velocity difference maximum  44  is exceeded, the controller operates the actuator to modulate the linear velocity of the second roller between the first and second linear velocities. The first linear velocity is represented by the velocity difference maximum  44  and the second linear velocity is represented by lowest velocity difference value  46  immediately following the velocity difference maximum  44 . The modulation of the linear velocity of the second roller is repeated until neither of the computed velocity differences are greater than the respective first and second velocity differential thresholds (block  210 ). 
     Referring to  FIG. 5 , a graph of the signal  48  provided to the actuator during the velocity modulation in this example is shown. The measured web tension  52  extends across the entirety of the graph from approximately 49 on the x-axis to 59 on the x-axis. The measured tension  52  is approximately zero until about 57.5 on the x-axis where the measured tension exceeds the tension set point  54 . Referring to  FIGS. 4 and 5 , to perform the velocity modulation of the second roller between the first and second velocities  44 ,  46 , the signal  48  to the actuator is modulated between a signal maximum  56  and a signal minimum  58 . The signal maximum  56  is any signal having an amplitude that causes the second roller to rotate at the first linear velocity  44 . The signal minimum  58  is any signal having an amplitude that causes the second roller to rotate at the second linear velocity  46 . The amplitude of the signal  48  can be reduced to zero to cause the second roller to rotate at the second linear velocity  46 . However, the signal  48  can also be reduced to an amplitude lower than the signal maximum  56  and greater than zero to cause the second roller to rotate at the second linear velocity  46 . 
     Referring again to  FIG. 3 , in another control method the controller implementing the process  300  deactivates the actuator to enable the second roller to rotate at a linear velocity (block  328 ). The actuator can be deactivated by any means, such as by reducing or suspending the signal to the actuator. Deactivation of the actuator in this method quickly reduces the linear velocity of the second roller. In yet another control method, the actuator is selectively activated and deactivated to rotate the second roller at a linear velocity (block  316 ). The actuator can be selectively activated by any means, such as by providing the signal to the actuator for a discrete duration or by providing the signal to the actuator until the actuator rotates at a predetermined linear velocity. The actuator can be selectively deactivated in a similar manner to the selective activation of the actuator. The selective activation and deactivation of the actuator in this method provides a more controlled deceleration of the second roller to match the linear velocity of the web. 
     In yet another control method, the controller implementing the process  300  operates the third actuator to move the third roller into engagement with the second roller to form a nip with the second roller (block  318 ). The formation of the nip between the second and third rollers urges the web into contact with the second roller. Operation of the second roller after formation of the nip advances the web past the second roller to enable the printer to achieve tension when conditions of low friction or low normal force exist between the web and the first and second rollers. 
     In yet another control method, the controller implementing the process  300  identifies the linear velocity of the web (block  320 ) and then operates the actuator to rotate the second roller with the identified linear velocity (block  322 ). In one embodiment, the linear velocity of the web is identified by first measuring the rotational velocities of the first and second rollers and then by calculating the linear velocity of the web with reference to the measured linear velocities of the first and second rollers. In an alternative embodiment, the web velocity can be measured directly. In another embodiment, the measured velocities of the first roller, the second roller, and the web can be used to identify the linear velocity of the web. 
     In yet another control method, the controller implementing the process  300  operates the second actuator to rotate the first roller with the third linear velocity (block  324 ) and then actuates the actuator to modulate the linear velocity of the second roller between the first and third linear velocities (block  326 ). The velocity of the second roller is modulated in the same manner as the velocity modulation control method described in block  314 . In yet another control method, the controller implementing the process  300  deactivates the actuator of the second roller (block  328 ), operates the second actuator to rotate the first roller to move the web a predetermined distance (block  330 ), and activates the actuator to rotate the second roller after the web has moved the predetermined distance (block  332 ). In this control method, deactivation of the actuator (block  328 ) and operation of the second actuator to rotate the first roller (block  330 ) advance the media web along media path and past the second roller. Subsequent activation of the actuator to rotate the second roller after the web has moved the predetermined distance allows the second roller to engage a new portion of the web. After implementation of one or more of the control methods disclosed in blocks  314 - 332  of process  300 , the controller implementing the process  200  computes one or both of the velocity differences (blocks  202 ,  206 ) and assesses whether either of the computed velocity differences are greater than the respective first and second velocity differential thresholds (blocks  204 ,  208 ). 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the following claims.