Patent Application
20130284846
Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. ______ (Docket K001036), entitled: “Automatically-adjusting web media tensioning mechanism”, by Turner et al., which is incorporated herein by reference.
This invention generally relates to a digital printing system for web media, and more particularly to a method for automatically adjusting a level of tension responsive to changes in characteristics of the web media.
Continuous web printing allows economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web of paper or other print media material is fed past one or more printing subsystems that form images by applying one or more colorants onto the print media surface. With this type of printing system, finely controlled dots of ink are rapidly and accurately propelled from the printhead onto the surface of a moving print media, with the web of print media often coursing past the printhead at speeds measured in hundreds of feet per minute. During printing, variable amounts of ink may be applied to different portions of the rapidly moving print media web, with drying mechanisms typically employed after each printhead or bank of printheads. Variability in ink or other liquid amounts and types or variability in drying times can cause print media stiffness and tension characteristics to vary dynamically for different types of print media, contributing to the overall complexity of print media handling and print media dot registration.
In some prior art web printing systems, such as the KODAK VERSAMARK VT3000 Printing System, the web media is slack when it enters the printing system and an “S-wrap” tensioning mechanism is used to add tension to the web media in preparation for feeding the web media into the rest of the system. S-wrap tensioning mechanisms provide an S-shaped media path where the web media is pulled across curved surfaces of tensioning shoes. Friction between the web media and the tensioning shoes introduce a tension into the web media.
The amount of tension introduced into the web media by an S-wrap tensioning mechanism will be a function of the coefficient of friction between the web media and the tensioning shoes. As a result, the amount of tension provided in a particular configuration can vary widely depending on the factors such as characteristics of the web media, operating speed and environmental conditions. Therefore, it is commonly necessary to manually adjust the geometry of the S-wrap tensioning mechanism (for example, by adjusting a wrap angle) to tune the system performance in accordance with the variation in these factors. Such manual adjustments can be time-consuming, and can be prone to operator error.
U.S. Patent Application Publication 2009/0101686 to Lane, entitled “Web processing apparatus,” discloses a web tensioning assembly configured to balance the tension across the width of a web. With this arrangement, the tension in the web media before and after the tensioning assembly will be the same. Therefore it is incompatible with applications where tension needs to be added to a slack web media.
U.S. Patent Application Publication 2011/0077115 to Dunn, entitled “System and method for belt tensioning,” discloses a method for adding tension to a belt which involves using a spring to apply a force to a tensioning roller. This configuration provides a controlled amount of tension throughout a closed belt, but cannot be used to add tension to a slack web media.
There remains a need for a tensioning mechanism for adding tension to a slack web that provides a consistent level of tension independent of varying media and environmental characteristics.
The present invention represents a method for automatically adjusting a level of tension in web media being transported in a roll-fed web media transport system, the web media having a width, comprising:
receiving web media into an automatically-adjusting tensioning mechanism from a source roller, the automatically-adjusting tensioning mechanism including:
feeding the web media through an S-shaped media path in the automatically-adjusting tensioning mechanism where the web media is wrapped around the first curved surface of the first tensioning shoe and is wrapped around the second curved surface of the second tensioning shoe such that a frictional drag resulting from friction between the web media and the first and second tensioning shoes provides a tension in the web media as it exits the automatically-adjusting tensioning mechanism, the web media being in contact with the first curved surface for a first contact distance and being in contact with the second curved surface for a second contact distance; wherein the pivot angle of the bracket assembly automatically adjusts in response to differences in a coefficient of friction between the web media and the first and second tensioning shoes such that the tension in the web media as it exits the automatically-adjusting tensioning mechanism has a reduced level of variability as a function of the coefficient of friction relative to configurations where the bracket assembly is held in a fixed position; and
pulling the web media through the automatically-adjusting tensioning mechanism using a feed mechanism provided downstream of the automatically-adjusting tensioning mechanism.
This invention has the advantage that it provides adequate pre-tensioning of the web media independent of the frictional characteristics of the web media without the need for manual reconfiguration.
It has the additional advantage that the tensioning mechanism automatically and passively adjusts to correct for variations in the friction coefficient in real time during a printing process.
It has the further advantage that the tensioning mechanism is more robust and less prone to human errors that may be introduced with prior art tensioning mechanisms that require manual reconfiguration.
In the detailed description of the example embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale.
The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense.
The apparatus and method of the present invention are well suited for roll-fed web media transport systems. In a preferred embodiment, the roll-fed web media transport system is part of a roll-fed printing system that applies colorant (e.g., ink) to a web of continuously moving print media. In some embodiments, the printing system is a non-contact printing system that provide for the application of ink or other colorant onto web media. In such systems a printhead selectively moistens at least some portion of the media as it moves through the printing system, but without the need to make contact with the print media. While the present invention will be described within the context of a roll-fed printing system, it will be obvious to one skilled in the art that it could also be used for other types of systems that include a roll-fed web media transport system. For example, the present invention can be used in a roll-fed coating system that coats one or more layers of material onto a web of continuously moving substrate.
In the context of the present invention, the terms “web media” or “continuous web of media” are interchangeable and relate to a media (e.g., a print media) that is in the form of a continuous strip of media as it passes through the web media transport system from an entrance to an exit thereof. The continuous web media serves as the receiving medium to which one or more colorants (e.g., inks or tonors), or other coating liquids are applied. This is distinguished from various types of “continuous webs” or “belts” that are actually transport system components (as compared to the print receiving media) which are typically used to transport a cut sheet medium in an electrophotographic or other printing system. The terms “upstream” and “downstream” are terms of art referring to relative positions along the transport path of a moving web; points on the web move from upstream to downstream.
Additionally, as described herein, the example embodiments of the present invention provide a printing system or printing system components typically used in inkjet printing systems. However, many other applications are emerging which use inkjet printheads to emit liquids (other than inks) that need to be finely metered and deposited with high spatial precision. As such, as described herein, the terms “liquid,” “ink,” “print,” and “printing” refer to any material that can be ejected by the liquid ejector, the liquid ejection system, or the liquid ejection system components described below.
Kinematic web handling is provided not only within each module of the system described below, but also at the interconnections between modules, as the continuously moving web medium passes from one module to another. Unlike a number of conventional continuous web imaging systems, the apparatus described below does not require a slack loop between modules, but typically uses a slack loop only for media that has been just removed from the supply roll at the input end. Removing the need for a slack loop between modules or within a module allows the addition of a module at any position along the continuously moving web, taking advantage of the automatically-adjusting and self-correcting design of media path components. As part of this adaptation, techniques have been developed to enable the moving web media to maintain proper tension in a “passive” manner.
Referring to the schematic side view of
The second module 40, positioned downstream from the first module 20 along the path of the web media 60, also has a support structure 48, similar to the support structure 28 for module 20. Affixed to one or both of the support structures 28 and 48 is a kinematic connection mechanism that maintains the kinematic dynamics of the continuous web of web media 60 in traveling from the module 20 into the module 40. Also affixed to one or both of the support structures 28 and 48 are one or more angular constraint structures 26 for setting an angular trajectory of the web media 60.
Printing system 10 optionally includes a turnover mechanism 30 that is configured to turn the media 60 over, flipping it backside-up in order to allow printing on the reverse side as the web media 60 as it travels through module 40. When printing is complete, the web media 60 leaves the digital printing system 10 and travels to a media receiving unit, in this case a take-up roller 18. A roll of printed media is then formed, rewound from the printed web media 60. The printing system 10 can include a number of other components, including, for example, dryers 14 and additional print heads (e.g., for different colored inks), as will be described in more detail below. Other examples of digital printing system components include web cleaners, web tension sensors, or quality control sensors.
Referring to the schematic side view of
Table 1 identifies the lettered components used for web media transport and shown in
The first angular constraint is provided by in-feed drive roller B. This is a fixed roller that cooperates with a drive roller in the turnover section TB and with out-feed drive roller N in module 40 in order to move the web media 60 through the printing system with suitable tension in the direction of movement or travel in the web media 60 (generally from left to right as shown in
The media transport system of the example embodiment shown in
In one example embodiment of the present invention, cross track position of the print media is center justified as it enters the media operating zone. This is done at transport element E either by a passive centering web guide (for example, by a web guide such as is described in commonly-assigned U.S. Pat. No. 5,360,152 entitled “Web guidance mechanism for automatically centering a web during movement of the web along a curved path” by Matoushek, the disclosure of which is incorporated by reference herein in its entirety) or by an active centering web guide (for example, by a servo-caster with gimbaled roller (i.e., a steered angular constraint with hinge), as is described in commonly-assigned U.S. patent application Ser. No. 13/292,117, the disclosure of which is incorporated by reference herein in its entirety). Fixed rollers F and L precede printhead(s) 16 in the first module 20 and the second module 40, respectively, providing the desired angular constraint to the web in each print zone 54. These rollers provide a suitable location for mounting an encoder for monitoring the motion of the web media 60 through the printing system 10. Under printheads 16, the web media 60 is supported by fixed non-rotating supports 32, for example, brush bars. Alternatively, fixed rollers can support the paper under the printheads, if the print media has minimal wrap around the rollers. Supports 32 provide minimal constraint to the web.
Printhead 16 prints in response to supplied print data on the web media 60 in the span between roller F and G, which includes the media operation zone. Water-based inks add moisture to the print media, which can cause the print media to expand, especially in the cross-track direction. The added moisture also lowers the stiffness of the print media. Dryer 14 following the printhead 16 dries the ink, typically by a directing heat and a flow of air at the print media. The dryer drives moisture out of the print media, causing the print media to shrink and its stiffness to change. These changes to the print media in the media operation zone can cause the print media to drift in the cross-track direction as it passes through the media operation zone. The width of the print media as it leaves the media operation zone can also differ from the width of the print media as it entered the media operation zone. To accommodate these effects, one example embodiment of the present invention includes a servo-caster with gimbaled roller G (i.e., a steered angular constraint with hinge) to center justify the print media as it leaves the media operation zone. Because of the relative length to width ratio of the web media 60 in the segment between rollers F and G, the continuous web media 60 in that segment is considered to be non-stiff, showing some degree of compliance in the cross-track direction. As a result, the additional constraint provided by the steered angular constraint can be included without over constraining that web segment.
A similar configuration is used in the second module 40. Accordingly, in one example embodiment of the present invention servo-caster with gimbaled roller M (a steered angular constraint with hinge) is included to center justify the web media 60 as it leaves the media operation zone. Roller K includes either a passive web centering guide (for example, the centering guide of U.S. Pat. No. 5,360,152) or an active mechanism such as a servo-caster with gimbaled roller (a steered angular constraint with hinge) to center justify the print media as it enters the media operation zone.
The angular orientation of the web media 60 in the print zone containing one or more printheads and possibly one or more dryers is controlled by a roller placed immediately before or immediately after the print zone. This is critical for ensuring registration of the images printed from multiple printheads 16. It is also critical that the web not be over constrained in the print zones 54. As a result of the transit time of the ink drops from the printhead 16 to the web media 60 that can result from variations in spacing of the printhead to the web media 60 from one side of the printhead to the other, it is desirable to orient the printheads 16 parallel to the web media 60. To maintain the uniformity of the spacing between the printheads 16 and the web media 60, constraint relieving rollers placed at one end of the print zones 54 are preferably not free to pivot in a manner that will alter the spacing between printheads 16 and the web media 60. Therefore, the castered roller following the print zone should preferably not include a gimbal pivot. However, the use of non-rotating supports 32 under the media 60 in the print zone as shown in
Another example embodiment of a printing system 10 shown schematically in
For the embodiments shown in
Load cells are provided in order to sense web tension at one or more points in the system. In the embodiments shown in
The configuration shown in
An entrance module 70 is the first module in sequence, following the media supply roll, as was shown earlier with reference to
An end feed module 74 provides an angular constraint to the incoming web media 60 from printhead module 72 by means of gimbaled roller H. Turnover module TB accepts the incoming media 60 from end feed module 74 and provides an angular constraint with its drive roller, as described above. Optionally, digital printing system 10 can also include other components within any of the modules described above. Examples of these types of system components include components for inspection of the print media, for example, components to monitor and control print quality.
A forward feed module 76 provides a web span corresponding to each of its gimbaled rollers J and K. These rollers again provide angular constraint only. The lateral constraint for web spans in module 76 is obtained from the edge of the incoming web media 60 itself Roller K includes either a lateral constraint (for example, an additional edge guide like the one included at roller A) or a servo-caster with gimbaled roller (i.e., a steered angular constraint with hinge) in order to maintain the cross-track position of the web media 60.
A second printhead module 78 accepts the web media 60 from forward feed module 76, with the given edge constraint, and applies an angular constraint with fixed roller L. A series of stationary fixed non-rotating supports 32, for example, brush bars or, optionally, minimum-wrap rollers then feed the web along past a second series of printheads 16 with their supporting dryers and other components, while providing little or no lateral constraint on the print media. In one example embodiment of the present invention, roller M is a servo-caster with gimbaled roller (i.e., a steered angular constraint with hinge) to center justify the web media 60 as it leaves the media operation zone that is located between rollers L and M. Here again, because of considerable web length in the web segment (that is, extending the distance between rollers L and M), that segment can exhibit flexibility in the cross track direction which is an additional degree of freedom enabling the use of the steered angular constraint without over constraining the print media in that span.
An out-feed module 80 provides an out-feed drive roller N that serves as angular constraint for the incoming web and cooperates with other drive rollers and sensors along the web media path that maintain the desired web speed and tension. Optional rollers O and P (not shown in
Each module in this sequence provides a support structure and an input and an output interface for kinematic connection with upstream or downstream modules. With the exception of the first module in sequence, which provides the edge guide at A, each module utilizes one edge of the incoming web media 60 as its “given” lateral constraint. The module then provides the needed angular constraint for the incoming media 60 in order to provide the needed exact constraint or kinematic connection of the web media transport. It can be seen from this example that a number of modules can be linked together using the apparatus and methods of the present invention. For example, an additional module could alternately be added between any other of these modules in order to provide a useful function for the printing process.
When multiple modules are used, as was described with reference to the embodiment shown in
As noted earlier, slack loops are not required between or within the modules described with reference to
The bracket assembly (i.e. bracket pates 106A and 106B), is mounted to a frame 100, and is adapted to freely pivot around a pivot axis 108 through a range of pivot angles. The pivot axis 108 is oriented in a direction across the width of the web media 60 (not shown in
In some embodiments, the tensioning mechanism 24 can also include other optional components such as an upper brush bar 110 and a lower brush bar 111 as shown in
In the
In the
In accordance with the present invention, the pivot angle of the bracket assembly is allowed to freely adjust to provide a passive and automatic adjustment of the tension in the web media 60. As will be discussed in more detail later, the result is that the tension in the web media as it exits the automatically-adjusting tensioning mechanism has a reduced level of variability as a function of the coefficient of friction between the web media and the tensioning shoes 102 and 104 relative to configurations where the bracket assembly is held in a fixed position.
In the embodiment illustrated in
In accordance with the embodiments of
The counter-clockwise torque component τ1 will be given by:
τ1=W1×R1 (1)
where W1 is the weight of the left-side components (i.e., the first tensioning shoe 102 and the portion of the bracket assembly to the left of the pivot axis), and R1 is the radius to the center of mass for the left-side components. Similarly, the clockwise torque component τ2 will be given by:
τ2=W2×R2 (2)
where W2 is the weight of the right-side components (i.e., the second tensioning shoe 104 and the portion of the bracket assembly to the right of the pivot axis), and R2 is the radius to the center of mass for the right-side components.
The torque imbalance Δτ will be given by the difference between the counter-clockwise torque component τ1 and the clockwise torque component τ2:
Δτ=τ1−τ2=(W1×R1)−(W2×R2). (3)
From this equation it can be seen that there are several different ways that the components can be arranged to provide the torque imbalance. In some embodiments the pivot axis 108 can be position off center relative to the bracket plate 106A so that R1>R2. This will cause τ1>τ2 so that Δτ>0. In other embodiments, additional weight can be added to the left-side components so that W1>W2. Once again, this will cause τ1>τ2 so that Δτ>0. In some embodiments, both the weights and the radiuses can be non-equal so that both effects combine to provide the torque imbalance.
There are a number of ways that additional weight can be added to the left-side components to provide the desired torque imbalance. In a preferred embodiment, a weight of the first tensioning shoe 102 is adjusted to be larger than a weight of the second tensioning shoe 104. One way to accomplish this is illustrated in
In some embodiments, it can be beneficial to form a series of fine grooves 138 (e.g., 40 grooves/inch) into the surface of the tensioning shoes 102 and 104 as illustrated in the inset 136 in
The total amount of torque imbalance that is provided in the tensioning mechanism 24 will determine the amount of tension that is introduced into the web media 60. In an application where the tensioning mechanism is used in the printing system 10 with 20 inch wide web media 60, it has been found that providing a total tension in the range of 20-40 lb is desirable. In other applications, the preferred tension may be higher or lower.
There are a number of factors which should be considered when determining the preferred method to provide the torque imbalance. The use of an off-center pivot axis 108 has the advantage that less weight is required to create the same torque imbalance. However, it has the disadvantage that it requires a larger space for the tensioning mechanism 24 to accommodate the larger swing radius. Therefore, for applications where there is a tight space constraint, it is preferable to use a centered pivot axis 108, and to provide the torque imbalance by the addition of weight to the first tensioning shoe 102.
In some embodiments, the torque imbalance can be provided (or supplemented) using other means. For example, an external weight can be attached to the bracket assembly using a cable, or a spring can be connected between the bracket assembly and the frame 100 that provides a torque on the bracket assembly in a direction that opposes the torque applied by the tension in the web media 60. An example embodiment where the torque imbalance is provided by an external weight or spring force will be discussed later with respect to
Friction between the web media 60 and the tensioning shoes 102 and 104 as the media is pulled through the tensioning mechanism 24 produces a drag force and consequently provides a tension in the web media 60. The magnitude of the drag force will be a function of the coefficient of friction between the web media 60 and the tensioning shoes 102 and 104. There are a variety of different factors that will affect the coefficient of friction including the physical characteristics of the web media 60 (e.g., width, thickness, stiffness, glossiness, texture and chemical composition) and the physical tensioning shoes 102 and 104 (e.g., glossiness, texture, chemical composition of the tensioning shoes 102 and 104, temperature, as well as any coatings that are applied intentionally or contamination that is picked up over time as the web media 60 rubs on the tensioning shoes). It will also be affected by other factors such as the speed that the web media 60 is being pulled through the tensioning mechanism 24 and the environmental characteristics (e.g., temperature and humidity). In some embodiments, the web media 60 may be treated by applying a chemical substance to the surface of the web media 60 before it enters the tensioning mechanism 24 (e.g., a conditioning pre-treatment, or ink applied at an earlier point in a printing process), which can also affect the coefficient of friction. Some of these factors can change gradually over time even if the same type of web media 60 is being used (e.g., environmental characteristics, changes in the physical characteristics of the tensioning shoes 102 and 104 due to wear, heating, burnishing or contamination that build up on the surface). Others of these factors may change when operating conditions (e.g., web speed) are changed, a pre-treatment process is initiated, or a new type of web media 60 is loaded into the roll-fed web media transport system.
Let us assume that the tensioning mechanism 24 in
If the coefficient of friction between the web media 60 and the tensioning shoes 102 and 104 now increases for some reason (e.g., changing environmental characteristics, different type of web media 60, or different web speed), this will increase the drag force and thereby will increase the tension in the web media 60. As a result, the clockwise torque on the tensioning mechanism 24 will increase, and the torques will no longer be balanced, thereby disturbing the steady state condition. This will cause the tensioning mechanism 24 to rotate in a clockwise direction. As the tensioning mechanism 24 rotates, the contact distance between the web media 60 and the tensioning shoes 102 and 104 will decrease, this will cause the drag force to be reduced, and will consequently reduce the clockwise torque. (The counter-clockwise torque will also change to some degree due to the change in lever arm resulting from the change in the angle between the gravitational force and the orientation of the tensioning mechanism.) The tensioning mechanism 24 will continue to rotate until it reaches a new steady state condition where the torques are once again balanced.
With the conventional fixed S-wrap tensioning mechanism, the tension produced in the web media 60 varies over the range of 10-198 lbs. Web media #2 has the highest coefficient of friction, and therefore produces the highest tension. (This tension was so high that it actually resulted in the media breaking during the test.) Web media #4 has the lowest coefficient of friction, and accordingly produces the lowest tension. The application of the pre-treatment coating to web media #3 significantly lowers the coefficient of friction, and consequently lowers the tension provided by the conventional S-wrap tensioning mechanism. This range of tensions would be too large to provide acceptable system performance for many of the media types (the tension should preferably be in the range of 15-40 lbs). A costly and time consuming manual reconfiguration of the S-wrap tensioning mechanism would therefore be required to determine an acceptable operating position for the S-wrap tensioning mechanism each time the web media is changed.
In accordance with the present invention, the pivot angle of the automatically-adjusting tensioning mechanism 24 automatically adjusts to the characteristics of the different web media types in
It has been found that when the printing system 10 is initially started up, it typically takes some initial period of time until the system reaches a steady state condition. During this initial period of time, the tension in the web media 60 can vary significantly when using a conventional S-wrap tensioning mechanism as the various the characteristics of the various system components change (e.g., due to heating). This can significantly complicate the process of manually adjusting the configuration of the conventional S-wrap tensioning mechanism, and can sometimes result in significant frustration for the system operators. However, in accordance with the present invention, the automatically-adjusting tensioning mechanism 24 will continuously and passively adjust to account for the changing system characteristics without the need for any manual operator interaction.
In a preferred embodiment, the position of the pulley 164 will be symmetric with the position of the roller B relative to the axis of symmetry 166, which passes vertically through the pivot axis 108. This arrangement has the advantage that as the tensioning mechanism 24 rotates around the pivot axis 108 (e.g., to the position in
As shown in
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
