BACKGROUND

[0001] In some printing systems, a print medium is advanced though a print zone in an advance direction. A printing process maybe performed when the print medium is positioned in the print zone. The printing process may comprise depositing a print fluid onto the print medium.

BRIEF DESCRIPTION OF DRAWINGS

In the following, a detailed description of various examples is given with reference to the drawings. These examples are illustrative and help the understanding of the invention. However, the invention is not limited to the individual examples as described in the following in connection with the drawings. It is appreciated that aspects of different examples that are disclosed in the following and in the drawings may be combined with one another within the scope of the present invention.

FIG. 1 is a schematic view of a printing system according to an example.

FIG. 2 shows example functions for different print media.

FIG. 3 is a schematic view of a printing system according to a further example.

FIG. 4 is a cross-sectional view of a printing system according to another example.

FIG. 5 is a cross-sectional view of a printing system according to yet another example.

FIG. 6 is a perspective view of a backtension device according to an example.

FIG. 7 is a flow diagram of a method according to an example.

FIG. 8 shows schematic perspective views of a specific example of the printing system as shown in FIG. 1.

DETAILED DESCRIPTION

[0002] In the following detailed description, same reference numerals and letters are used to refer to the same, similar or related features in different examples. Wherever the same reference numeral or letter is used, the respective structural or functional features of the other example also apply to the currently described example, unless otherwise indicated.

[0003] FIG. 1 schematically shows an example of a printing system too. The printing system too maybe part of a printing device. As such, the example shown in FIG. 1 may also be referred to as a printing device too. Additionally or alternatively, the printing system too may be implemented in a distributed printing system. The printing system too may be operable according to the method that is disclosed herein and described below. Additionally or alternatively, the printing system too may be capable of carrying out the method as disclosed herein and described below.

[0004] The printing system too may be capable of executing a printing process, in which a print fluid is deposited on a print medium P. In particular, the print fluid may be deposited on the print medium P in a print zone Z of the printing system too.

[0005] The print medium P may be supplied in any suitable form, for example, as sheets or as a continuous roll. The print medium P may be or comprise any suitable material of any suitable rigidity and flexibility. For example, the print medium P maybe or comprise any type of paper (e.g., regular paper and photo paper), cardboard, textile, leather, woven material, synthetic material or any combination thereof. A specific example of the material of the print medium P includes so-called Backlit material.

[0006] The printing system too may comprise a medium support member 102. The medium support member 102 may be arranged to support a print medium P. The medium support member 102 may be arranged below or in the print zone Z. In particular, the medium support member 102 maybe arranged such to support the print medium P during the printing process.

[0007] In some examples, the medium support member 102 may be or comprise a plate like member to support the print medium P. In a specific example, the medium support member 102 maybe or comprise a platen to support the print medium P.

[0008] The printing system too may comprise an advancing device 104. The advancing device 104 may (be operable to) advance the print medium P to the medium support member 102 in an advance direction A. The advance direction A as used herein may refer to a direction along which the print medium P is advanced to the medium support member 102. Additionally or alternatively, the advance direction A may correspond to an input direction of the print medium P along which the print medium P is input into the print zone Z. In the following, for the sake of simplicity, the expressions upstream and downstream refer to the advance direction A of the print medium P, unless indicated otherwise.

[0009] Further in the drawings, an arrow M implies an advancing force M (i.e., a movement or transport) of the print medium P to the medium support member 102. The advancing force M is directed in (i.e., parallel to) the advance direction A. The advancing force M may be exerted in whole or in part by the advancing device 104. For the sake of a simple description, the advancing force M refers hereinafter to the force that is applied by the advancing device 104 to advance the print medium P to the medium support member 102.

[0010] The advancing device 104 may (be operable to) advance the print medium P also over and beyond the medium support member 102. In such examples, the advance direction A may be extended accordingly. Additionally or alternatively, the advancing device 104 may (be operable to) advance the print medium P from a print medium supply such as a roll of print medium (not explicitly shown in FIG. 1). In such examples, the advance direction A may correspond to a direction along which the print medium P is conveyed from a medium supply to (towards) the medium support member 102. The print medium supply may comprise or be combined with a backtension device 106 as described below.

[0011] The advancing device 104 may be or comprise an advance roller (not explicitly shown in FIG. 1) to advance the print medium P. Additionally of alternatively, the advancing device 104 may comprise a conveying unit, such as a belt, to advance the print medium P.

[0012] The printing system too may comprise a backtension device 106 to apply a backtension force B to the print medium P in a direction that is opposite to the advance direction A. The term backtension force as used herein may refer to any force exerted on the print medium P that is directed in a direction opposite to the advance direction A. The backtension force B may be such to resist the advancing force M exerted by the advancing device 104 on the print medium P. Generally, the backtension force B may increase the tension of the print medium P along the advance direction A, which can be exploited to increase the precision of the advancing operation of the print medium P.

[0013] The backtension device 106 may be located in an upstream position relative to the medium support member 102, wherein the upstream position refers to the advance direction A of the print medium P as mentioned above. The backtension device 106 may (be operable to) apply the backtension force B to the print medium P. As a result, the backtension device 106 may (be operable to) pull the print medium P in a direction opposite to the advance direction A.

[0014] In some examples, the backtension device 106 may be part of or combined with a print medium supply (not explicitly shown in FIG. 1). The backtension device 106 may comprise a mechanical unit, such as a roller, a pincher or the like, to apply the backtension force B to the print medium P. In specific examples, the backtension device 106 may include an input roller to supply the print medium P, wherein the print medium P is wrapped around the input roller in a continuous manner. Additionally or alternatively, the backtension device 106 may comprise a friction unit, independently from and in addition to a friction device 108 as discussed below, to generate additional friction against the print medium P, thereby exerting or increasing the backtension force B.

[0015] The printing system too may comprise a friction device 108 to increase a friction force F on the print medium P against the medium support member 102. The friction force F as used herein may be related to the advancing operation of the print medium P and effective in a direction opposite to the advance direction A. When the print medium P is supported by the medium support member 102, i.e., there is a physical contact between the print medium P and the medium support member 102, a (non-zero) friction is present therebetween. The friction device 108 may be capable of increasing the friction between the print medium P and the medium support member 102, and thus the friction force F resulting therefrom, by increasing the physical contact (i.e., an adhesion strength of the print medium P to the medium support member 102) therebetween.

[0016] The terms friction and friction force as used herein refer to the friction that occurs when the print medium P is advanced in the advance direction A. Accordingly, the friction and friction force as used herein may be limited to those occurring in the direction opposite to the advance direction A, unless indicated otherwise. For the sake of simplicity, the terms friction and friction force may be used interchangeably herein, unless indicated otherwise. Nonetheless, it is noted that, generally, the term friction may refer to the physical phenomenon of interfacial attraction between two surfaces, and the term friction force may refer to the physical quantity resulting from the friction. Furthermore, the friction force F as used herein may cover specific operation situations in which any of a static component and a dynamic component of friction is zero.

[0017] The friction device 108 may (be operable to) increase the friction force F between the print medium P and the medium support member 102 on and within a main surface 102s of the medium support member 102. The main surface 102a of the medium support member 102 may be a flat (or at least partially curved) surface to support the print medium P upon. For example, the main surface 102s of the medium support member 102 may refer to an entire upper surface of the medium support member 102 or a portion thereof. Alternatively, the main surface of the medium support member 102 may be refer to an area in which the friction force F between the print medium P and the medium support member 102 is increased by the friction device 108. Hereinafter, the medium support member 102 and its main surface 102s may be used interchangeably regarding the contact with, and friction against, the print medium P.

[0018] In some examples, the friction device 108 may be a vacuum device to generate a pressure difference across the medium support member 102 such to increase the friction force F on the print medium P against the medium support member 102. In specific examples, the friction device 108 may be capable of generating a low-pressure environment. The friction device 108 may be or comprise a vacuum generator, a blower, a fan or any combination thereof for this purpose. The friction device 108 may (be operable to) generate the low-pressure environment such that the print medium P is pressed, pulled, attracted or otherwise accelerated against the medium support member 102. Accordingly, the friction device 108 may increase (or cause) the physical contact between the print medium P and the medium support member 102 and thus increase (or give rise to) the friction force F.

[0019] In specific examples, the friction device 108 may be arranged on a side of the medium support member 102 that is opposite to the side on which the print medium P is supported. The medium support member 102 may comprise an array of through holes, a perforated plate or the like, to allow a gas flow through the medium support member 102 towards the low-pressure environment generated by the friction device 108. Such a gas flow may increase (or cause) the physical contact between the print medium P and the medium support member 102, and thereby increase (or cause) the friction therebetween. This may result in increasing (or giving rise to) the friction force F between the print medium P and the medium support member 102.

[0020] Additionally or alternatively, the friction device 108 may comprise a friction unit (not explicitly shown in the drawings) that may otherwise increase the friction between the print medium P and the medium support member 102. In particular, the print medium P may be pulled, pressed, attracted or otherwise accelerated towards the medium support member 102 by any suitable means, which may include a blower or a fan to generate a gas flow towards the medium support member 102. In further examples, the friction device 108 may include an electrostatic unit (not explicitly shown in the drawings) to cause the print medium P to be accelerated towards the medium support member 102 by means of attractive electrostatic force.

[0021] The printing system too may comprise a controller 110. The controller 110 may (be operable to) control at least one of the backtension device 106 and the friction device 108 such that a sum of the backtension force B and the friction force F equals a predefined target value. The controller 110 may be or comprise any suitable processing unit and may further comprise a memory to store data.

[0022] The predetermined target value may be a constant value independent of a change in position of the print medium P relative to the medium support member 102. In other words, the constant value as used herein may indicate that the predetermined target value is invariant while the print medium P advances to the medium support member 102. Accordingly, the sum of the friction force F and the backtension force B may be maintained constant as the print medium P moves to the medium support member 102.

[0023] The predetermined target value may be a value according to a function of a position of the print medium P relative to a specific part of the printing system too, in particular relative to the medium support member 102 or relative to the print zone Z. The function of the position of the print medium P may refer to any suitable mathematical function that includes, for example, a linear term, a polynomial term, a logarithmic term, an exponential term, trigonometrical term or any combination thereof with the position of the print medium P being the respective variable. Accordingly, the sum of the friction force F and the backtension force B may be adapted according to the function of the position of the print medium P. In addition, the function may comprise a constant term that is invariant with the position of the print medium P.

[0024] Additionally or alternatively, the predetermined target value may be determined according to the advancing force M that is applied to the print medium P by the advancing device 104. For example, the advancing force M may vary during the advancing operation of the print medium P, and the predetermined target value may be adapted to this variation of the advancing force M.

[0025] In some examples, the controller 110 may control the backtension device 106 and the friction device 108 by controlling (varying, changing, adjusting) a respective driving power. The controller 110 may (be operable to), for example, control a driving current or a driving voltage in order to control the driving power of at least one of the backtension device 106 and the friction device 108. Accordingly, the backtension force B may be controlled by controlling at least one of the driving power, driving current or driving voltage of the backtension device 106 by the controller 110. The friction force F may be controlled by controlling at least one of the driving power, driving current or driving voltage of the friction device 108 by the controller 110. The driving power, driving current or driving voltage of the backtension device 106 and the resulting backtension force B maybe linearly or non-linearly correlated. The driving power, driving current or driving voltage of the friction device 108 and the resulting friction force F may be linearly or non-linearly correlated.

[0026] The controller 110 may control the backtension device 106 and the friction device 108 individually and independently from each other. In some examples, the controller 110 may control a single one of the backtension device 106 and the friction device 108 such that the sum of the backtension force B and the friction force F equals the predetermined value. Hereinafter, the controller 110 controlling any of the backtension device 106 and the friction device 108 may refer to controlling any of a driving power, driving current, driving voltage, or any combination thereof.

[0027] Specifically, the controller 110 may control the backtension device 106 according to a backtension function of the position X of the print medium P relative to the medium support member 102. In particular, the backtension function maybe a linear backtension function with a first slope applying between a start position XoB and an end position XeB. Furthermore, the backtension function may comprise multiple mathematical functions that are applied one after the other or at least in parts simultaneously.

[0028] For example, the start position XoB may be defined as a downstream boundary of the advancing device 104. For example, the reference position R may be defined as the end position XeB. It is noted that these positions are examples, and any of the start position XoB and the end position XeB may be varied depending on the print medium P and the actual implementation of the printing system too. Additionally or alternatively, the backtension function may comprise a term of any other suitable mathematical function type as mentioned above.

[0029] In the example shown in FIG. 1, the position X may be determined as a distance of a leading edge E of the print medium P from a reference position R in the medium support member 102. In other examples, the position X may be determined with respect any other suitable position of the printing system too, such as a specific position in the medium support member 102 or the print zone Z. [0030] Additionally or alternatively, the controller 110 may control the friction device 108 according to a friction function of the position X of the print medium P relative to the medium support member 102. The backtension function and the friction function may be independent from each other. In particular, the friction function may be a linear friction function with a second slope applying between a start position XoF and an end position XeF. The first slope and the second slope may be independent from each other, and may be equal or different. Additionally or alternatively, the friction function may comprise a term of any other suitable mathematical function type as mentioned above. Furthermore, the friction function may comprise multiple mathematical functions that are applied one after the other or at least in parts simultaneously.

[0031] For example, the start position XoF may be defined to be a halfway between the downstream boundary of the advancing device 104 and an upstream boundary of the medium support member 102. For example, the end position XeF may be slightly upstream of the end position XeB of the backtension function. It is noted that these positions are examples, and any of the start position XoF and the end position XeF may be varied depending on the print medium P and the actual implementation of the printing system too. In the example of FIG. 1, the start positions XoB, XoF and the end positions XeB, XeF are different from one another. In other examples, the start positions XoB, XoF and the end positions XeB, XeF may coincide with another one.

[0032] In specific examples, the backtension function may be such that it maintains an initial value until X reaches the starting point XoB, i.e., when X is below XoB. When X moves from the starting point XoB to the end point XeB, the value of the backtension function may change from the initial value to the nominal value of the backtension force B (or the backtension device 106) as mentioned above. In some examples, the initial value of the backtension function may be larger than the nominal value, in order to compensate for the increase of the friction force F as discussed above. The change of the backtension function value from the initial value to the nominal value may be continuous. In examples where the backtension function is linear, the backtension function value may change linearly according to the slope of the backtension function. Accordingly, the controller 110 may control the backtension device 106 particularly between the start position XoB and the end position XeB of the print medium P relative to the medium support member 102.

[0033] In more specific examples, the backtension function may output a numeral factor by which a nominal value for controlling the backtension device 106 is to be multiplied. The nominal value may be at least one of the driving power, driving current or driving voltage. The backtension function Fb X ) maybe expressed as follows:

FbO, if X £ startPositionB

Fb(X) = FbO + Gb(X), if startPositionB < X < endPositionB vl, if X ³ endPositionB where Gb(X) is a linear function with a slope Sb, such as:

Gb{X ) = Sb · X .

Furthermore, the nominal value may be defined as a lower boundary value of the backtension function, such as:

Fb{X ) = 1, if Fb{X) < 1.

Accordingly, a ramp function may be provided, according to which the controller no controls the backtension device 106. The ramp function may be configured such that the sum of the vacuum friction force F and the backtension force B is the predetermined target value. Accordingly, the ramp function may compensate for the change (transient, increase) in the vacuum friction force F as the print medium P advances over the medium support member 102. In total, four parameters are to be determined in the ramp function, namely the initial value (Fbo), the start position (startPositionB), the end position (endPositionB) and the slope (Sb). The parameters may depend on the material of the print medium P as discussed in detail below.

[0034] Additionally or alternatively, the backtension function may be such that it maintains an initial value until X reaches the starting point XoF, i.e., when X is below XoF. When X moves from the starting point XoF to the end point XeF, the value of the friction function may change from the initial value to the nominal value of the friction force F (or the friction device 108) as mentioned above. In some examples, the initial value of the friction function may be larger than the nominal value. The change of the friction function value from the initial value to the nominal value maybe continuous. In examples where the friction function is linear, the friction function value may change linearly according to the slope of the friction function. Accordingly, the controller 110 may control the friction device 108 particularly between the start position XoF and the end position XeF of the print medium P relative to the medium support member

102.

[0035] In specific examples, the friction function may output a numeral factor by which a nominal value for controlling the friction device 108 is to be multiplied. The nominal value may be at least one of the driving power, driving current or driving voltage. The friction function Ff(X) maybe expressed as follows: where Gf(X ) is a linear function with a slope Sf, such as:

GfQ = Sf X .

Furthermore, the nominal value may be defined as a lower boundary value of the friction function, such as:

Ff(X ) = 1, if Ff(X ) < 1.

Accordingly, a ramp function may be provided, according to which the controller no controls the friction device 108. The ramp function may be configured such that the sum of the vacuum friction force F and the backtension force B is the predetermined target value. Accordingly, the ramp function may compensate for the change (transient, increase) in the vacuum friction force F as the print medium P advances over the medium support member 102. In total, four parameters are to be determined in the ramp function, namely the initial value (Ffo), the start position (startPositionF), the end position (endPositionF) and the slope (Sf). The parameters may depend on the material of the print medium P.

[0036] The parameters used in the backtension function or the friction function may be are adjusted to each print medium material. The parameters are affected by at least one of a thickness, a stiffness, a coefficient of friction of the print medium material. Further, the parameters may be affected by the material that is used in the printing system too. Furthermore, a concrete configuration of the friction device 108, such as the mechanism and areal distribution of the suction force F, or a nipping force at the advancing device 104 may also affect the parameters.

[0037] In some further examples, the advancing device 104 may also be controlled, for example by the controller 110, according to an advance function. The advance function maybe configured in a similar manner to the backtension function or the friction function. For example, the advance function may be configured such that the advance force M at least partially compensates for the increase of the friction force F as described above.

[0038] The parameters may be measured, for example, by determining an advance error of the print medium P of a certain material and adjusting the parameters until advance error is minimized. Additionally or alternatively, empirical values may be used for determining the parameters and that comes in contact with the print medium P.

[0039] FIG. 2 shows example functions 210, 220, 230 for different print media plotted against the position X of the print medium P relative to the medium support member 102. The functions shown in FIG. 2 may be used as the backtension function or the friction function.

[0040] A first function 210 may be associated with the print medium P being made of a first material, for example the Backlit material. A second function 220 may be associated with the print medium P being made of a second material, for example a photo paper. A third function 230 may be associated with the print medium P being made of a third material, for example a coated paper. As shown herein, at least one of the backtension function and the friction function may alter depending on the material of the print medium P.

[0041] Specifically, the first function 210 maintains a constant initial value 212 when X is below a start position 214. When X increases from the start position 214 to an end position 250, the first function 210 linearly decreases from the start value 212 to a nominal value 240 according to a slope. After reaching the end position 250, the print medium P is in a constant friction area, where the nominal value may be maintained as discussed above. The first function 210 may applicable for any of the backtension function and the friction function. Accordingly, the initial value 212 may correspond to any of the initial values XoB, XoF as discussed above. The slope as shown may correspond to any of the slopes Sb, Sf. The start position 214 may correspond to any of the start positions startPositionB, startPositionF. The end position 250 may correspond to any of the end positions endPositionB, endPositionF.

[0042] The second function 220 maintains an initial value 222 when X is below a start position 224. The start position 224 of the second function 220 is closer to the medium support member 102 than the start position 214 of the first function 210. When X increases from the start position 224 to the end position 250, the second function 220 linearly decreases from the start value 222 to the nominal value 240 according to a slope. After reaching the end position 250, the print medium P is in the constant friction area, where the nominal value may be maintained as discussed above. The second function 220 may applicable for any of the backtension function and the friction function as mentioned above with respect to the first function 210.

[0043] The third function 230 maintains a constant initial value 232 when X is below the start position 214 which coincides with the start position 214 of the first function 210. When X is between the start position 214 and an intermediate position 234, the third function 230 linearly decreases from the start value 232 to an intermediate value 236 according to a first slope. Then, when X increases from the intermediate position 234 to the end position 238, the third function 230 linearly decreases from the intermediate value 236 to the nominal value 250 according to a second slope. After reaching the end position 238, the print medium P is in the constant friction area, where the nominal value maybe maintained as discussed above. The third function 230 may applicable for any of the backtension function and the friction function as discussed above with respect to the first and second functions 210, 220.

[0044] The functions described in connection with FIG. 2 are illustrative examples and the backtension function or the friction function are not limited thereto. As discussed above, any of the functions 210, 220, 230 may be altered to any suitable mathematical function type, or added by a term of any suitable mathematical function type.

[0045] Accordingly, the compensation for the change of the friction force between the print medium P and the medium support member 102 as disclosed herein may be optimized to any material of the print medium P. The optimization may be performed by determining and adjusting the parameters including the start position, the end position, the initial value and the slope for the respective material of the print medium P.

[0046] A specific example of the printing system too as described above is shown in FIG. 8. Accordingly, the printing system too as shown in FIG. 8 may comprise the components and features as described above with reference to FIG. 1 and 2. In particular, the backtension device 106 and the friction device 108 in the example of FIG. 8 may be controlled as described above with reference to FIG. 1 and 2. In FIG. 8, schematic perspective views 180, 181 illustrate different stages of a printing process in connection with an advancing operation as described above, according to a specific and detailed example.

[0047] The medium support member 102 may have a planar main surface 102s to support the print medium P. For example, the medium support member 102 comprises a structure or element that is plate-shaped, e.g., a platen as described above, and an upper surface of the same may correspond to the main surface 102s. In the example of FIG. 8, the medium support member 102 is illustrated as a plate-shaped structure having a rectangular main surface 102s elongated along a direction perpendicular to the advance direction A. This a simplified example for illustration. In further examples not explicitly shown in the drawings, the medium support member 102 may have a larger or smaller width (i.e., extension perpendicular to the advance direction A) and a larger or smaller length (i.e., extension parallel to the advance direction A). Further, the main surface 102s is not limited to a rectangular shape but may have any other suitable geometrical shape. The medium support member 102 has a non-zero thickness.

[0048] In FIG. 8, the print zone Z has a single-stepped edge on the upstream side. This is an example. In other examples that are not explicitly shown in the drawings, the edge of the print zone Z on the upstream side may have multiple edges or no edges. In the example of FIG. 8, the medium support member 102 in FIG. 8 extends in the advance direction A such to partially cover (from below; overlap) the print zone Z. In other examples, e.g., as shown in FIG. 1, the medium support member 102 may extend in the advance direction A such to cover the entire print zone Z.

[0049] In the example of FIG. 8, the advancing device 104 may be or comprise an advance roller to advance the print medium P in the advance direction A. The advancing device 104 may advance the print medium P by means of friction. In some examples, the advancing device 104 comprises a counterpart (not shown in FIG. 8) that is pressed against the advance roller to support the transfer of the advancing force M to the print medium P.

[0050] In the example of FIG. 8, the print medium P maybe supplied as a roll wound around a shaft of the backtension device 106. Additionally or alternatively, the print medium P maybe supplied in any other suitable manner and conveyed towards the print zone Z such that the print medium P rests on the backtension device 106 and is partially wraps around the backtension device 106. In any of the examples, the backtension device 106 may apply the backtension force B to the print medium P in the above described manner. For example, the backtension device 106 may be or comprise a roller to apply the backtension force B in the direction opposite to the advance direction A.

[0051] In the example of FIG. 8, the friction device 108 is arranged below the medium support member 102, as implied by arrow 108 in FIG. 8. In the example of FIG. 8, the friction device may comprise a vacuum device that decreases pressure below the medium support member 102, i.e., generate a low-pressure environment, thereby causing a suction force that pulls (attracts) the print medium P towards the medium support member 102. For this purpose, the medium support member 102 may comprise an array of through holes (of any shape) distributed over its main surface 102s or a perforated plate to allow a gas flow through the medium support member 102.

[0052] In the first perspective view 180, the print medium P is advanced from the backtension member 106 to the medium support member 102 such that the leading edge E of the print medium P has reached the upstream side of the print zone Z. As described above, the advancing device 104 applies the advancing force M to the print medium P to advance the print medium P in the advance direction A. The backtension device 106 applies the backtension force B to the print medium P in the direction opposite to the advance direction A. The friction device 108 increases the friction force F between the print medium P and the medium support member 102.

[0053] The area of the medium support member 102 (i.e., its main surface 102s) covered by the print medium P increases as the print medium P arrives at an upstream edge of the medium support member 102 and advances in the advance direction A, thereby changing, in particular increasing, the friction between the print medium P and the medium support member 102 as described above. This change may be further amplified due to a particular configuration of the medium support member 102 and the friction device 108, where a total vacuum suction force on the print medium P increases in a non-linear manner as the number of through holes covered by the print medium P increases.

[0054] In order to compensate for such change in friction, the controller 110 (not shown in FIG. 8) controls the backtension device 106 and the friction device 108 during the advancing operation in the manner described herein. In particular, either or both of the backtension device 106 and the friction device 108 may be controlled such that either or both of the backtension force B and the friction force F is first increased to a respective initial value and then decreased to a respective final value.

[0055] The final value may correspond to a respective nominal value. The nominal value may correspond to a driving power, driving current, driving voltage or the like for driving the backtension device 106 or the friction device 108 in an operating state in which the print medium P fully covers the main surface 102s of the medium support member 102. In other words, the nominal value may correspond to an operating state in which the friction remains unchanged while the print medium P advances in the advancing direction. An example of such an operating state is shown in the perspective view 181 in FIG. 8, when the print medium P has been advanced in the advance direction A such that the leading edge E is past a downstream boundary of the print zone Z. In such an operating state, the print medium P covers the main surface 102s such that the friction does not change when the print medium P further advances in the advance direction A. In some examples, the main surface 102s of the medium support member 102 may have a dimension that is larger than the corresponding dimension of the print medium P. Further, as mentioned above, at least one of the medium support member 102 and the friction device 108 may extend further along the advance direction A than illustrated in FIG. 8.

[0056] Referring back to FIG. 1, according to a specific example, the printing system too may comprise the medium support member 102 to support the print medium P; the advancing device 104 to advance the print medium P to the medium support member 102 in the advance direction A, the backtension device 106 to apply the backtension force B to the print medium P in the direction opposite to the advance direction A, the friction device 108 to increase the friction force F between the print medium P against the medium support member 102, and the controller 110 to control at least one of the backtension device 106 and the friction device 108 such that the sum of the backtension force B and the friction force F equals the predefined target value.

[0057] The sum of the backtension force B and the friction force F may represent an entirety of forces that are opposite to the advancing force M and thus opposite to the advance direction A (inertia forces of the print medium P may be negligeable). By controlling the sum of the backtension force B and the friction force F to the predetermined value, the forces acting on the print medium P during the advancing operation may be determined and controlled. Thus, an equilibrium between the advancing force M (which may also be referred to as traction force) and the sum of the backward forces, i.e., the backtension force B and the friction force F, may be obtained and maintained.

[0058] Accordingly, the printing system too provides for an accurate control of the advancing operation of the print medium P. The accurate control may be used to reduce defects in print products caused by misarrangement of the print medium P in the print zone Z. Thus, the printing system too provides for an increase of image quality of print products.

[0059] More generally, the printing system as disclosed herein may adjust at least one of the backtension force or the friction force acting on the print medium to compensate for the variation (transient) of the friction that occurs as the print medium advances over the medium support member. In particular, the printing system as disclosed herein may compensate for an increase in friction force between the print medium and the medium support member in a phase when the print medium is introduced into the print zone of the printing system. [0060] By means of control and compensation of the variation of the friction force during the advancing operation of the print medium, the advancing of the print medium can be controlled in a precise manner. The increase of the accuracy reduces the occurrence of defects, such as dark (black) line banding, white (light) line banding, grains or the like that may result from inaccurate advancing operations. Accordingly, the image quality of the print products may be increased.

[0061] The advancing operation of the print medium P using the printing system too is described in more detail by means of a specific example. A coverage of the main surface 102s of the medium support member 102 by the print medium P increases, as the print medium P advances over the main surface 102s of the medium support member 102. In examples where the friction device 108 increases the friction force F within the main surface 102s, an increasing coverage by the print medium P results in an increase of an effective friction force between the print medium P and the medium support member 102, until the print medium P fully covers the main surface 102s of the medium support member 102.

[0062] Once the print medium P fully covers the main surface 102s of the medium support member 102, a continuation of the advancing operation of the print medium P does not change the effective friction force between the print medium P and the medium support member 102. Hereinafter, a nominal value of the friction force F refers to the effective friction force that acts between the print medium P and the medium support member 102 when the print medium P fully covers the main surface 102s of the medium support member 102. A nominal value of the backtension force B as used herein may refer to a reference backtension force B acting on the print medium P.

[0063] In specific examples, the predetermined target value may be a constant value, and the controller 110 may decrease the backtension force B as the friction force F increases with an increase of the coverage of the main surface 102s of the medium support member 102 by the print medium P. Additionally or alternatively, the controller 110 may control the friction device 108 such that the increase of the friction force F is at least partially compensated for, e.g., by initially applying a high driving power to the friction device 108 and then gradually decreasing it. As mentioned above, the backtension device 106 and the friction device 108 may be controlled independently from each other. In some examples, the controller 110 may control a single one of the backtension device 106 and the friction device 108 while maintaining a steady control of the respective other one.

[0064] FIG. 3 shows a schematic view of a printing system too according to a further example. Similar or same features are labeled with the same reference signs as in FIG. 1 or 8, and the respective description is omitted for the sake of brevity. The features of the printing system too as described above with reference to FIG. 1 also apply to the printing system too of FIG. 3, unless explicitly indicated otherwise. Accordingly, the printing system too of FIG. 3 may also be considered as a printing device too.

[0065] In contrast to the printing system too as shown in FIG. 1, the printing system too of FIG. 3 comprises advance rollers 104a, 104b, which replace the advancing device 140 of FIG. 1. Further, the printing device too of FIG. 3 additionally comprises a media advance sensor 120. Although not explicitly shown, the features of any of the advance rollers 104a, 104b and the media advance sensor 120 as described below may also be implemented in the printing device too as shown in FIG. 1.

[0066] The advance rollers 104a, 104b may be an embodying example of the advancing device 104 as described above. The advance rollers 104a, 104b may (be operable to) advance the print medium P to the medium support member 102. While two rollers 104a, 104b are depicted in FIG. 3, other examples may employ a single roller 104a or 104b in combination with a counter-roller or a suitable counterpart. Further, one of the rollers 104a, 104b may be actively driven to advance the print medium P, while the other one of the rollers 104a, 104b is used as a counter-roller.

[0067] The media advance sensor 120 may be at least partially inserted or embedded in the medium support member 102, which may comprise a platen as discussed above. The media advance sensor 120 may comprise a roller to roll as the print medium P advances along the main surface 102s of the medium support member 102, which may be used to determine the position of the print medium P relative to the medium support member 102. Additionally or alternatively, the media advance sensor 120 may comprise at least one of an optical sensor, an electromagnetic sensor, a haptic sensor or any suitable type of sensor.

[0068] While the media advance sensor 120 is depicted to be within the print zone Z in FIG. 3, the media advance sensor according to other examples may be arranged upstream or downstream of the print zone Z with respect to the media advance direction A. In specific examples, the media advance sensor 120 may be arranged such that a distance between the advance rollers 104a, 104b (or the advancing device 104) and a sensitive area of the media advance sensor 120 is 0.2 to 20 inches, or 1 to 10 inches, or 2 to 5 inches.

[0069] The use of the media advance sensor 120 may further increase the accuracy of the advancing operation of the print medium P by the printing system too. The media advance sensor may contribute to decreasing defects caused by an inaccurate advance operation. Accordingly, the use of media advance sensor 120 may increase the image quality of print products of the printing system too. In particular, the media advance sensor 120 maybe used in addition or parallel to the controlling the backtension device 106 and the friction device 108 as described above, thereby further increasing the accuracy of the advancing operation.

[0070] In specific examples, the controlling of at least one of the backtension device 106 and the friction device 108 as described above may be executed in a phase during the advancing operation in which the leading edge E of the print medium P is located between the advance rollers 104a, 104b (or the advancing device 104 of FIG. 1) and the media advance sensor 120 such that the leading edge E cannot be detected by the media advance sensor 120 yet. In particular, any of the backtension function and the friction function as described above may be applied until the leading edge E of the print medium P arrives a sensitive area of the media advance sensor 120.

[0071] In some examples, angular rotation (e.g., number of rotation), or an angular position, of any of the advance rollers 104a, 104b, or both of them, may be determined additionally in order to further determine the position of the leading edge E of the print medium P. For example, an encoder located at a shaft of any of the advance rollers 104a, 104b may be used.

[0072] In particular, a mismatch between the angular rotation of the advance roller 104a, 104b and the (true) position of the leading edge E of the print medium P may be observed due to the variation in friction force, among other forces, as discussed above. From this mismatch, the above-indicated parameters to be used in either of the backtension function and the friction function may be computed.

[0073] FIG. 4 is a cross-sectional view of a printing system too according to a further example. Similar or same features are labeled with the same reference signs as in FIG. 1, 3 and 8, and the respective description is omitted for the sake of brevity. The features of the printing system too as described above with reference to FIG. 1, 2, 3 or 8 also apply to the printing system too of FIG. 4, unless explicitly indicated otherwise. The printing system too of FIG. 4 may be part of a printing device.

[0074] In the example of FIG. 4, the print medium P is supplied as a continuous roll. The backtension device 106 of the printing system too of FIG. 4 includes an input roller (which may be also labeled with 106 in FIG. 4) to supply the print medium P, which is wound around the input roller in a continuous manner. The print medium P may be guided from the input roller 106 along a path defined by guiding baffles 142 to the advancing device 104 and eventually to the print zone Z and beyond. The advancing device 104 may comprises an advance roller (labeled with 104 in FIG. 4).

[0075] Further shown in the example of FIG. 4 is a carriage 144 on which a print fluid ejector (not labeled in FIG. 4) may be mounted. The advancing device 104 may advance the print medium P by a fixed advance distance at a time, i.e., per single advancing operation. After each advancing operation, the print fluid ejector may deposit a print fluid onto the print medium P, while the carriage 144 moves over and across the print medium P in a direction perpendicular to the advance direction A.

[0076] The friction device 108 in the example of FIG. 4 may comprise a vacuum unit to provide a low-pressure environment below the medium support member 102, thereby sucking the print medium P towards the medium support member 102 and thus increasing the friction force F between the print member P and the medium support member 102 in the above described manner. The accuracy of the advancing operation may be increased by controlling the backtension device 106 and the friction device 108 as discussed above.

[0077] FIG. 5 is a cross-sectional view of a printing system too according to a further example. Similar or same features are labeled with the same reference signs as in FIG. 1, 3, 4 and 8, and the respective description is omitted for the sake of brevity. The features of the printing system too as described above with reference to FIG. 1, 2, 3, 4 or 8 also apply to the printing system too of FIG. 5, unless explicitly indicated otherwise. The printing system too of FIG. 5 may be part of a printing device.

[0078] The examples shown in FIG. 4 and FIG. 5 are similar to each other. In contrast to the example of FIG. 4, the printing system too of FIG. 5 in addition comprises the media advance sensor 120. The media advance sensor 120 of FIG. 5 maybe implemented as described above with reference to FIG. 3.

[0079] Further in the example of FIG. 5, an input roller 152 is explicitly shown. The print medium P is provided as a continuous roll and wound around the input roller 152. As described above, the backtension force B maybe varied by applying a torque to the input roller 152, or its shaft, that resists the advancing force M applied by the advance roller 104. For this purpose, an electromotor 154 may be provided and coupled with the input roller 152 to generate and apply torque to the input roller 152, as shown in detail in FIG. 6.

[0080] FIG. 6 is a perspective view of a backtension device 106 according to an example. In particular, the example shown in FIG. 6 may present a partial explosion view of the example of FIG. 5. Similar or same features are labeled with the same reference signs as in FIG. 5, and the respective description is omitted for the sake of brevity.

[0081] The electromotor 152 is coupled to the input roller 152 via a transmission gear 156 and a transport gear 158. The transport gear 158 is fixedlyjoint with the input roller 152. The transmission gear may be operable to transmit the torque from the electromotor 154 with a translated rotational speed.

[0082] According to the printing system as disclosed herein, a uniform friction force may be provided to the print medium P by dynamically controlling at least one of the friction device 108 and the backtension device 106 as a function of the position of the print medium. Accordingly, a transient of friction force may be avoided.

[0083] Such a dynamic control during the advancing operation of the print medium may further allow for reducing print medium waste due to reduced margins by a precise control. Further, the backtension device or the friction device may be controlled according to parametric profiles, which allow for customizing profiles to each of print medium material. Furthermore, an additional sensor for determining the position of the print medium may be omitted, thereby reducing material, cost and time for the manufacturing. In addition, the determination of the print medium position is not dependent on an additional sensor.

[0084] According to another specific example, a printing device may be provided. The printing device may be implemented in the printing system as disclosed herein. Alternatively, the printing device may include the printing system as disclosed herein. The printing device may comprise a print zone Z and a platen 102 arranged in the print zone Z to support the print medium P. The printing device may comprise a print medium supply to store the print medium P and to supply the print medium to the print zone Z. The print medium supply may apply the backtension force B to the print medium P opposite to the advance direction A of the print medium P. The printing device may comprise an advance roller 104a, 104b to advance the print medium P to the print zone Z in the advance direction A. The printing device too may comprise a vacuum device 108 to generate a pressure difference across the platen 102 such to increase the friction force F on the print medium P against the platen 102. The printing device too may comprise a controller 110 to control at least one of the print medium supply and the platen such that a sum of the backtension force and the friction force is constant regardless of a position of the print medium relative to the print zone

[0085] FIG. 7 is a flow diagram of a method 170 according to an example. The method 170 may be applicable to any of the printing systems too as described above with reference to FIG. 1 to 6. The printing systems too as described above with reference to FIG. 1 to 6 and 8 may be operable to execute the method 170 as described below with reference to FIG. 7. Accordingly, any of the features and aspects described above in connection with the printing system too may be also applicable to the method 170.

[0086] At 172, a print medium may be advanced in an advance direction towards a medium support member. At 174, a backtension force maybe applied to the print medium in a direction opposite to the advance direction. At 176, a friction force on the print medium against the medium support member may be increased. At 178, at least one of the backtension force and the friction force may be controlled according to a respective function of a position of the print medium relative to the medium support member.

[0087] In some examples, at least one of the backtension force and the friction force is controlled such that a sum of the backtension force and the friction force equals a constant value regardless of a position of the print medium relative to the medium support member

[0088] Alternatively or additionally, at least one of the backtension force and the friction force is controlled such that a sum of the backtension force and the friction force equals a value according to a function of the position of the print medium relative to the medium support member.

[0089] In some examples, the backtension force may be controlled according to a linear backtension function of a position of the print medium relative to the medium support member with a slope.

[0090] In some examples, the friction force is controlled according to a linear friction function of the position of the print medium relative to the medium support member with a slope.

[0091] In some examples, at least one of the backtension force and the friction force is controlled such that the respective force decreases linearly from an initial value at a start position to a nominal value an end position of the print medium relative to the medium support member.