Patent Publication Number: US-11027537-B2

Title: Measured sensor distance correction

Description:
BACKGROUND 
     A web printing press may apply tension to a web-fed print substrate. The web tension may be adjusted by changing the relative velocity of rollers or nip pressure of the feeding mechanism of the web printing press. The web tension and variations in tension may change the scaling of images printed on the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various examples will be described below with reference to the following figures. 
         FIG. 1  is a block diagram of an example apparatus that includes a scaling measurement correction module according to an implementation. 
         FIG. 2  is a schematic diagram of an example apparatus for correcting a scaling measurement according to an implementation. 
         FIG. 3A  is a side view of an example apparatus for correcting a scaling measurement according to an implementation. 
         FIG. 3B  is a side view of an example apparatus for correcting a scaling measurement according to an implementation. 
         FIG. 4  is an example method for determining a correction factor according to an implementation. 
         FIG. 5  is an example method for correcting a scaling measurement according to an implementation. 
         FIG. 6  is a block diagram showing a non-transitory, machine-readable medium encoded with example instructions to determine a correction factor. 
         FIG. 7  is a block diagram showing a non-transitory, machine-readable medium encoded with example instructions to determine a corrected scaling measurement. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, a web printing press may apply tension to a web-fed print substrate. This web tension may be adjusted by changing the relative velocity of rollers or nip pressure of the feeding mechanism of the web printing press. The web tension and variations in web tension may cause images printed on the substrate to be distorted. This distortion may also be known as “scaling error”. Calibration marks may be printed on the substrate at intended distances between marks, and automated control systems may attempt to detect the scaling error based on those marks. The control systems may also adjust the tension to correct the scaling error. However, measurements of the distance between marks as printed may be sensitive to tension variations and substrate thickness, and such control systems may inaccurately adjust the tension. Scaling error may also be manually detected by cutting a portion of the substrate from the web, measuring the distance between marks, and adjusting the web tension accordingly. However, such manual processes may be inefficient. 
     Referring now to the figures,  FIG. 1  is a block diagram of an example apparatus  100  for correcting a scaling measurement according to an implementation. In some implementations, the apparatus  100  may be included in a web printing press. The apparatus  100  may include a first sensor  102 , a second sensor  104 , an encoder  106 , and a scaling measurement correction module  108 . The term “module” as used herein may include a series of instructions encoded on a machine-readable storage medium and executable by a processor. Additionally or alternatively, a module may include one or more hardware devices including electronic circuitry for implementing functionality described herein. In some implementations, the first sensor  102  and the second sensor  104  may be mounted in the web printing press and may be separated by a calibrated sensor distance. The first sensor  102  and the second sensor  104  may be to detect marks on a substrate as the substrate advances through the press. The encoder  106  may be to detect advancement of the substrate. The scaling measurement correction module  108  may be to determine a measured sensor distance based on an amount of substrate advancement, as detected by the encoder  106 , between detection of a mark by the first sensor  102  and detection of the mark by the second sensor  104 . The scaling measurement correction module  108  may also be to determine a correction factor to convert the measured sensor distance to the calibrated sensor distance. 
       FIG. 2  is a schematic diagram of an example apparatus  200  for correcting a scaling measurement according to an implementation. In some instances, some aspects of the apparatus  200  may be example implementations of analogous aspects of the apparatus  100  of  FIG. 1 . In some implementations, the apparatus  200  may be included in a web printing press  202  (also referred to herein as a “press”) for printing on a web-fed substrate  204 , such as paper, fabric, plastic, or other suitable printing material. For example, in some implementations, the press  202  may print on the substrate  204  (e.g., using an offset printing technique) as the substrate  204  is advanced in a web feed direction  206  (e.g., a linear direction) at least in part by nip rollers, such as a nip roller  208 . In some implementations, the web printing press  202  may include a web tension controller  210  to adjust operating parameters of rollers of the press  202  (which may or may not include nip roller  208 ), such parameters including, for example, velocity of the rollers, nip pressure, and the like, in order to control a web tension applied to the substrate  204  as it is advanced through the press  202 . In some implementations, the web tension controller  210  may include a series of instructions encoded on a machine-readable storage medium and executable by a processor, and additionally or alternatively, may include one or more hardware devices including electronic circuitry for implementing functionality described herein. 
     The press  202  may print a mark (e.g.,  226 - 1 ; which may also be referred to as a calibration mark) on the substrate  204 , and more particularly, may print a plurality of such marks (e.g.,  226 - 1  through  226 -N) along an edge of the substrate  204 . Additionally, the web printing press  202  may be instructed to print the plurality of marks  226 - 1  through  226 -N with a particular intended inter-mark distance (i.e., the distance between marks), but owing to the web tension applied to the substrate  204  and variability in the web tension, an actual inter-mark distance  228  as printed may differ from the intended inter-mark distance. Thus, it may be useful for the apparatus  200  to determine an accurate measurement of the inter-mark distance  228  as printed to be used as a control input by the web tension controller  210  to adjust the web tension on the substrate  204  and the scaling error of the press  202 . 
     The apparatus  200  may include a first sensor  212 , a second sensor  214 , an encoder  216 , and a scaling measurement correction module  218 . In some implementations, the apparatus  200  also may include a temperature sensor  220  placed between the first sensor  212  and the second sensor  214 . The functionality of the foregoing features of the apparatus  200  and interactions thereof will be described in turn. 
     The first sensor  212  and the second sensor  214  may be mounted in the web printing press  200  and may be separated by a calibrated sensor distance  222 . More particularly, the first sensor  212  and the second sensor  214  may be separated by the calibrated sensor distance  222  along the web feed direction  206 . The calibrated sensor distance  222  may be highly accurate (e.g., to at least approximately ±7 μm) owing to tight engineering and manufacturing tolerances, periodic maintenance and calibration, and/or other suitable mechanisms for achieving high dimensional tolerance. The first sensor  212  and the second sensor  214  may detect the aforementioned mark or marks ( 226 - 1  through  226 -N) printed on the substrate  204  as the substrate  204  advances through the press  202 . In some implementations, the first sensor  212  and the second sensor  214  may be optical reflectance or transmittance sensors that, for example, can detect light-dark transitions related to the printed marks. For example, the marks  226 - 1  through  226 -N may contrast with the substrate  204  (e.g., black marks on a white substrate  204 ), or if the substrate  204  is transparent, a stationary background that contrasts with the marks (e.g., a white or neutral background for black marks) may be placed underneath the substrate  204  and may be mounted to the press  202 . 
     The encoder  216  may be to detect advancement of the substrate  204  (i.e., along web feed direction  206 ). In some implementations, the encoder  216  may be a rotary encoder coupled to the nip roller  208  of the web printing press  202  (e.g., coupled by a zero backlash coupling) to detect an angular displacement resulting from and corresponding to advancement of the substrate  204  over the nip roller  208 , which may have a particular radius (R roller ). For example, a rotary encoder may output a number of counts corresponding to the detected angular displacement. The number of counts may be converted back into the detected angular displacement (e.g., θ in radians, degrees, etc.) based on, for example, the resolution of the rotary encoder (e.g., an 8-bit encoder may have 256 counts for a full rotation of the rotary encoder, that is, 2π radians). A corresponding linear displacement (ΔX) may then be calculated as the product of the detected angular displacement (θ) and a radius (R), as shown in equation (1) below, where R roller  may be used as an approximation of radius R:
 
Δ X=R*θ   (1)
 
     The scaling measurement correction module  218  may communicate with, and more particularly, receive output signals from the first sensor  212 , the second sensor  214 , the encoder  216 , and the temperature sensor  220 . For example, the scaling measurement correction module  218  may receive from the temperature sensor  220  a temperature measurement from between the first sensor  212  and the second sensor  214 . In some implementations, the scaling measurement correction module  218  may receive optical detection signals from the first sensor  212  and the second sensor  214 , such as, for example, detection signals that indicate (e.g., upon analysis by the scaling measurement correction module  218 ) when a mark (e.g.,  226 - 1 ) crosses the sensor. As another example, the scaling measurement correction module  218  may receive from the encoder  216  a number of counts representative of an angular displacement reading, as described above, and the scaling measurement correction module  218  may convert the number of counts back to an angular displacement (e.g., in radians, degrees, etc.). 
     In some implementations, a signal from either one of the first sensor  212  or the second sensor  214  may trigger a reading from the encoder  216 . For example, in some implementations, as the substrate  204  advances, the first sensor  212  may detect the crossing of the mark  226 - 1  followed by the crossing of a subsequent mark  226 - 2 , and may trigger reading(s) from the encoder  216  (e.g., a number of counts) in response to the crossings. In some implementations, the scaling measurement correlation module  218  may convert the number of encoder counts between the detection of mark  226 - 1  and the detection of mark  226 - 2  into a linear displacement that may correlate to (or may be a measurement of) the distance between the marks  226 - 1  and  226 - 2  (inter-mark distance  228 ), in the manner described above with respect to equation (1). This calculated linear displacement of the inter-mark distance  228  may be referred to as a scaling measurement. However, in some instances, the scaling measurement may be a less than accurate measurement of the inter-mark distance  228 , if R roller  is used as an approximation for R in equation (1) owing at least in part to variations in thickness of the substrate  204  and variations in web tension applied to the substrate  204 , as will be explained further with reference to  FIGS. 3A and 3B . 
       FIGS. 3A and 3B  are side views of the web printing press  202  and the apparatus  200  of  FIG. 2 .  FIG. 3A  and  FIG. 3B  respectively illustrate a substrate  304  with a thickness  306  and a substrate  310  with a thickness  312 , both substrates being advanced through the press  202 , over the nip roller  208  (which has a radius  302  of R roller ) coupled to the encoder  216 . In the present illustrations of  FIGS. 3A and 3B , thickness  306  is greater than thickness  312 , which may be by design (e.g., different substrate materials, dimensions, etc.) or may be due to different web tension (e.g., more or less tension may stretch a same substrate to different thicknesses). The marks  226 - 1  and  226 - 2  are printed on the surface of the substrates  304  and  310 . As the substrate  304  passes over the nip roller  208 , an effective radius  308  may be formed by the sum of the roller radius  302  and the substrate thickness  306 . Similarly, for the substrate  310 , an effective radius  314  may be formed by the sum of the roller radius  302  and the substrate thickness  312 . The effective radius  308  is greater than the effective radius  314 , owing to the different substrate thicknesses. Thus, measurement of the inter-mark distance  228  (i.e., the scaling measurement) using equation (1) may be more accurate if the effective radii  308 ,  314  are used for the radius R in equation (1) rather than R roller . However, the effective radius may not be available for such calculation. 
     Referring again to  FIG. 2 , the scaling measurement correction module  218  may be to determine a correction factor to compensate for less than accurate measurements of linear displacements, including the scaling measurement, as described above. The scaling measurement correction module  218  may determine a measured sensor distance based on an amount of advancement of substrate  204  as detected by the encoder  216  (e.g., a number of counts representing an angular displacement detected by a rotary encoder  216  and converted to a linear displacement), between detection of a mark  226 - 1  by the first sensor  212  and detection of the mark  226 - 1  by the second sensor  214 . For example, the measured sensor distance may be calculated by the scaling measurement correction module  218  as the product of the radius of the nip roller  208  (R roller ) and the angular displacement of the encoder  216  between two trigger roller, points: detection of the mark  226 - 1  by the first sensor  212  and detection of the mark  226 - 1  by the second sensor  214  (θ sensor 1−sensor 2 , in radians for example). In some implementations, the measured sensor distance may be expressed as the following equation (2):
 
Measured Sensor Distance= R   roller *θ sensor 1−sensor 2   (2)
 
     The scaling measurement correction module  218  may then determine a correction factor to convert the measured sensor distance to the calibrated sensor distance  222 . In some implementations, the calibrated sensor distance  222  may be stored as a programmable constant in a machine-readable medium included in or accessible by the scaling measurement correction module  218 . For example, in some implementations, the correction factor (C correction ) may be determined by dividing the measured sensor distance by the calibrated sensor distance  222  (that is, a ratio of the measured sensor distance to the calibrated sensor distance), as expressed in the following equation (3):
 
 C   correction =Measured Sensor Distance/Calibrated Sensor Distance  (3)
 
In some implementations, the correction factor may be determined in other suitable ways, such as, for example, by subtracting the calibrated sensor distance from the measured sensor distance.
 
     Using the correction factor (C correction ), the scaling measurement correction module  218  may determine a more accurate scaling measurement, which the web tension controller may compare with the intended inter-mark distance to adjust the web tension on the substrate  204 . For example, in some implementations, the scaling measurement correction module  218  may determine a scaling measurement based on an amount of advancement of substrate  204 , as detected by the encoder  216  (e.g., as a number of counts representing an angular displacement detected by the rotary encoder  216  and converted to a linear displacement), between detection of the mark  226 - 1  by the first sensor  212  and detection of a subsequent mark  226 - 2  by the first sensor  212 . For example, the scaling measurement may be calculated by the scaling measurement correction module  218  as the product of the radius of the nip roller  208  (R roller ) and the angular displacement of the encoder  216  between two trigger points: detection of the mark  226 - 1  by the first sensor  212  and detection of the mark  226 - 2  again by the first sensor  212  (θ mark 1−mark 2 , in radians for example). Owing to variations in the substrate  204  thickness and web tension, the scaling measurement may be deemed an estimate of the inter-mark distance  228  (e.g., as measured by a calibrated ruler). It should be noted that, in the some implementations, the second sensor  214  may be used instead of the first sensor  212  to detect both the mark  226 - 1  and the subsequent mark  226 - 2  in the foregoing example. In some implementations, the scaling measurement may be expressed as the following equation (4):
 
Scaling Measurement= R   roller *θ sensor 1−sensor 2   (4)
 
     The scaling measurement correction module  218  may then convert the scaling measurement to a corrected scaling measurement using the correction factor (C correction ). For example, the corrected scaling measurement may be calculated by the scaling measurement correction module  218  as the product of the scaling measurement and the inverse of the correction factor (C correction ), as expressed by the following equation (5):
 
Corrected Scaling Measurement=Scaling Measurement* C   correction   −1   (5)
 
In some implementations, the scaling measurement may be converted to the corrected scaling measurement using the correction factor in other suitable ways, depending at least in part on how the correction factor was determined. For example, the correction factor may be added to or subtracted from the scaling measurement to calculate the corrected scaling measurement, particularly if the correction factor is the difference between the calibrated sensor distance and the measured sensor distance. In some implementations, the scaling measurement correction module  218  may transmit the corrected scaling measurement to the web tension controller  210  of the press  202 . As described above, the web tension controller  210  may compare the corrected scaling measurement to the intended inter-mark distance and adjust the web tension to minimize the difference between those values.
 
     In some cases, temperature changes may result in thermal expansion or contraction of parts of the press  202 , and more particularly, temperature changes near the first sensor  212  and the second sensor  214  may cause the calibrated sensor distance  222  to change. As described above, in some implementations, the apparatus  200  may include a temperature sensor  220  that may output a temperature measurement from between the first sensor  212  and the second sensor  214  to the scaling measurement correction module  218 . The scaling measurement correction module  218  may adjust the correction factor based on the temperature measured by the temperature sensor  220 . For example, the scaling measurement correction module  218  may adjust the calibrated sensor distance value used in equation (3) based on a known relationship between the temperature and the calibrated sensor distance  222 . For example, an increased temperature may be known to correlate to an increased calibrated sensor distance  222 . 
     In some implementations, the scaling measurement correction module  218  may update the correction factor (e.g., as calculated by equation (3)) for each mark (or at least some of the marks) of the plurality of marks  226 - 1  through  226 -N, as each mark passes the first sensor  212  and the second sensor  214 . Additionally, the scaling measurement correction module may determine a corrected scaling measurement using an updated correction factor for each pair of consecutive marks (e.g., marks  226 - 1  and  226 - 2 ). Accordingly, the apparatus  200  may provide frequent and accurate input to the web tension controller  210 . 
       FIG. 4  is a flowchart of an example method  400  for determining a correction factor according to an implementation. Method  400  may be described below as being executed or performed by an apparatus, such as apparatus  100  of  FIG. 1 . Various other suitable systems may be used as well, such as, for example, apparatus  200  of  FIG. 2 . Method  400  may be implemented in the form of executable instructions stored on a machine-readable storage medium and executed by at least one processor of the apparatus  100 , and/or in the form of electronic circuitry. In some implementations of the present disclosure, one or more blocks of method  400  may be executed substantially concurrently or in a different order than shown in  FIG. 4 . In some implementations of the present disclosure, method  400  may include more or less blocks than are shown in  FIG. 4 . In some implementations, one or more of the blocks of method  400  may, at certain times, be ongoing and/or may repeat. 
     The method  400  may begin at block  402 , and continue to block  404 , where the apparatus  100  may detect a mark at a first sensor (e.g.,  102 ), the mark being on a substrate fed through a web printing press. At block  406 , the apparatus  100  may detect the mark at a second sensor (e.g.,  104 ), the first sensor and the second sensor being separated by a calibrated sensor distance. For example, the first sensor and the second sensor may be mounted at different locations within the apparatus  100  (or more generally, mounted in the web printing press in some implementations), separated by the calibrated sensor distance. At block  408 , the apparatus  100  may determine a measured sensor distance between the first sensor and the second sensor based on the detecting the mark at the first sensor (e.g., at block  404 ) and the detecting the mark at the second sensor (e.g., at block  406 ). In some implementations, the apparatus  100  determines the measured sensor distance at least in part by a rotary encoder (e.g., encoder  106 ) detecting advancement of the substrate between the detecting the mark at the first sensor (e.g., at block  404 ) and the detecting the mark at the second sensor (e.g., at block  406 ). At block  408 , the apparatus  100  may determine a correction factor to convert the measured sensor distance to the calibrated sensor distance. For example, the correction factor may be the measured sensor distance determined at block  406  divided by the calibrated sensor distance. In some implementations, the apparatus  100  may perform block  406  using a scaling measurement correction module (e.g.,  108 ). In some implementations, the mark is a plurality of marks on the substrate, and the measured sensor distance and the correction factor may be determined for each mark of the plurality of marks. In other words, the method  400  (and more particularly, blocks  404 ,  406 ,  408 ,  410 ) may be repeated for a plurality of marks as the substrate is advanced or fed through the web printing press. At block  412 , the method  400  may end. 
       FIG. 5  is a flowchart of an example method  500  for correcting a scaling measurement according to an implementation. Method  500  may be described below as being executed or performed by an apparatus, such as apparatus  200  of  FIG. 2 . Various other suitable systems may be used as well to perform at least part of method  500 , such as, for example, apparatus  100  of  FIG. 1 . Method  500  may be implemented in the form of executable instructions stored on a machine-readable storage medium and executed by at least one processor of the apparatus  200 , and/or in the form of electronic circuitry. In some implementations of the present disclosure, one or more blocks of method  500  may be executed substantially concurrently or in a different order than shown in  FIG. 5 . In some implementations of the present disclosure, method  500  may include more or less blocks than are shown in  FIG. 5 . In some implementations, one or more of the blocks of method  500  may, at certain times, be ongoing and/or may repeat. In some implementations, prior to beginning the method  500 , the apparatus  200  may determine a correction factor by performing method  400 . 
     The method  500  may begin at block  502 , and continue to block  504  where the apparatus  200  (or a scaling measurement correction module, e.g.,  218 ) may adjust a correction factor based on a temperature measured between a first sensor (e.g.,  212 ) and a second sensor (e.g.,  214 ), by a temperature sensor (e.g.,  220 ) for example. The correction factor may be, for example, the correction factor determined according to block  410  of method  400 . At block  506 , the apparatus  200  may detect a mark (e.g.,  226 - 1 ) at the first sensor, the mark being on a substrate (e.g.,  204 ) fed through a web printing press (e.g.,  202 ). At block  508 , the apparatus  200  may detect, at the first sensor, another mark (e.g.,  226 - 2 ) on the substrate. At block  510 , the apparatus  200  (or the scaling measurement correction module) may determine a scaling measurement between the mark and the another mark based on the detecting the mark at the first sensor (e.g., at block  506 ) and the detecting the another mark at the first sensor (e.g., at block  508 ). For example, in some implementations, the apparatus  200  may determine the scaling measurement in part by a rotary encoder (e.g., encoder  216 ) detecting advancement of the substrate  204  between the detecting the mark at the first sensor (that is, a first trigger event) and the detecting the another mark at the first sensor (that is, a second trigger event). At block  512 , the apparatus  200  (or the scaling measurement correction module) may convert the scaling measurement determined at block  510  to a corrected scaling measurement using the correction factor. At block  514 , the apparatus  200  (or the scaling measurement correction module) may transmit the corrected scaling measurement determined at block  512  to a web tension controller (e.g.,  210 ). In some implementations, the mark  226 - 1  and the another mark may be among a plurality of marks on the substrate, and the method  500  (and more particularly, blocks  506 ,  508 ,  510 ,  512 ,  514 ) may be repeated for pairs of consecutive or adjacent marks. The method  500  may end at block  516 . 
       FIG. 6  is a block diagram illustrating a processor-based system  600  that includes a machine-readable medium encoded with example instructions to determine a correction factor according to an example implementation. In some example implementations, the system  600  may be or may form part of a printing device, such as a web printing press. In some implementations, the system  600  is a processor-based system and may include a processor  602  coupled to a machine-readable medium  604 . The processor  602  may include a single-core processor, a multi-core processor, an application-specific integrated circuit, a field programmable gate array, and/or other hardware device suitable for retrieval and/or execution of instructions from the machine-readable medium  604  (e.g., instructions  606 ,  608 ) to perform functions related to various examples. Additionally or alternatively, the processor  602  may include electronic circuitry for performing the functionality described herein, including the functionality of instructions  606  and/or  608 . With respect to the executable instructions represented as boxes in  FIG. 6 , it should be understood that part or all of the executable instructions and/or electronic circuits included within one box may, in alternate implementations, be included in a different box shown in the figures or in a different box not shown. 
     The machine-readable medium  604  may be any medium suitable for storing executable instructions, such as random access memory (RAM), electrically erasable programmable read-only memory (EEPROM), flash memory, hard disk drives, optical discs, and the like. In some example implementations, the machine-readable medium  604  may be a tangible, non-transitory medium, where the term “non-transitory” does not encompass transitory propagating signals. The machine-readable medium  604  may be disposed within system  600 , as shown in  FIG. 6 , in which case the executable instructions may be deemed “installed” on the system  600 . Alternatively, the machine-readable medium  604  may be a portable (e.g., external) storage medium, for example, that allows system  600  to remotely execute the instructions or download the instructions from the storage medium. In this case, the executable instructions may be part of an “installation package.” As described further herein below, the machine-readable medium  604  may be encoded with a set of executable instructions  606 ,  608 . 
     Instructions  606 , when executed by the processor  602 , may determine a measured sensor distance based on a number of counts from a rotary encoder coupled to a roller of a web printing press, the number of counts corresponding to a distance a substrate advances through the web printing press between detection of a mark on the substrate by a first sensor and detection of the mark by a second sensor, the first sensor and the second sensor being separated by a calibrated sensor distance. Instructions  608 , when executed by the processor  602 , may determine a correction factor based on a ratio of the measured sensor distance to the calibrated sensor distance. 
       FIG. 7  is a block diagram illustrating a processor-based system  700  that includes a machine-readable medium encoded with example instructions to determine a correction factor according to an example implementation. In some example implementations, the system  700  may be or may form part of a printing device, such as a web printing press. In some implementations, the system  700  is a processor-based system and may include a processor  702  coupled to a machine-readable medium  704 . The processor  702  may include a single-core processor, a multi-core processor, an application-specific integrated circuit, a field programmable gate array, and/or other hardware device suitable for retrieval and/or execution of instructions from the machine-readable medium  704  (e.g., instructions  706 ,  708 ,  710 ) to perform functions related to various examples. Additionally or alternatively, the processor  702  may include electronic circuitry for performing the functionality described herein, including the functionality of instructions  706 ,  708 , and/or  710 . With respect to the executable instructions represented as boxes in  FIG. 7 , it should be understood that part or all of the executable instructions and/or electronic circuits included within one box may, in alternate implementations, be included in a different box shown in the figures or in a different box not shown. 
     The machine-readable medium  704  may be any medium suitable for storing executable instructions, such as random access memory (RAM), electrically erasable programmable read-only memory (EEPROM) flash memory, hard disk drives, optical discs, and the like. In some example implementations, the machine-readable medium  704  may be a tangible, non-transitory medium, where the term “non-transitory” does not encompass transitory propagating signals. The machine-readable medium  704  may be disposed within system  700 , as shown in  FIG. 7 , in which case the executable instructions may be deemed “installed” on the system  700 . Alternatively, the machine-readable medium  704  may be a portable (e.g., external) storage medium, for example, that allows system  700  to remotely execute the instructions or download the instructions from the storage medium. In this case, the executable instructions may be part of an “installation package.” As described further herein below, the machine-readable medium  704  may be encoded with a set of executable instructions  706 ,  708 ,  710 . 
     Instructions  706 , when executed by the processor  702 , may determine a scaling measurement based on a number of counts from the rotary encoder corresponding to a distance the substrate advances through the web printing press between detection of the mark by the first sensor and detection of a subsequent mark by the first sensor. Instructions  708 , when executed by the processor  702 , may convert the scaling measurement to a corrected scaling measurement using the correction factor. Instructions  710 , when executed by the processor  702 , may transmit the corrected scaling measurement to a web tension controller of the web printing press. 
     In view of the foregoing description, it can be appreciated that error and inaccuracy in a scaling measurement, determined by an apparatus that detects calibration marks on a web-fed substrate, may be reduced, corrected, or compensated by calibrating apparatus measurements against a calibrated sensor distance between sensors of the apparatus. Moreover, by virtue of improving the accuracy of the scaling measurement, a web tension controller may control the tension applied to the web-fed substrate in a web printing press with greater accuracy. 
     In the foregoing description, numerous details are set forth to provide an understanding of the subject matter disclosed herein. However, implementation may be practiced without some or all of these details. Other implementations may include modifications and variations from the details discussed above. It is intended that the following claims cover such modifications and variations.