Patent Publication Number: US-2023158813-A1

Title: Adjusting distance between print media and printhead

Description:
BACKGROUND 
     A printing system may include a pen or a printhead with a plurality of nozzles that deliver print agent onto a print medium so as to print an image. In printing processes, a distance between the printhead and the print medium, known as the printhead-to-print medium spacing (also known as pen-to-paper spacing, PPS), may influence print quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various example features will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, wherein: 
         FIG.  1    illustrates a side view of a printing system according to an example of the present disclosure and a zoom-in view schematically representing a non-transitory machine-readable storage medium according to an example of the present disclosure. 
         FIG.  2    illustrates an isometric view of an adjusting system according to an example of the present disclosure. 
         FIG.  3    illustrates a zoom-in view of a portion of the adjusting system of  FIG.  2   . 
         FIG.  4    illustrates a driving assembly and a sensor assembly according to an example of the present disclosure. 
         FIG.  5    illustrates a side view of a driving system of a driving assembly according to an example of the present disclosure. 
         FIG.  6    schematically represents a motion of an outer shaft and an eccentric pin according to an example of the present disclosure. 
         FIG.  7    schematically represents a motion of an eccentric pin and a print medium input beam according to an example of the present disclosure. 
         FIG.  8    schematically represents a sensor assembly according to an example of the present disclosure. 
         FIG.  9    is a block diagram of an example of a method to adjust a distance between a print medium support and a printhead of a printing system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates a side view of a printing system according to an example of the present disclosure. The printing system  100  comprises a printhead  120  to deliver print agent on a print medium  110 , a print medium support  140  to support the print medium  100  advancing in a print medium advance direction  111 . The printing system  100  comprises an adjusting system  10  to adjust a distance  123  between the print medium support  140  and the printhead  120 . 
     The printhead  120  may be provided with a plurality of nozzles to deliver print agent, e.g. ink, onto the print medium  110  so as to print an image. During printing, dots of print agent may be precisely delivered onto the print medium  110  at a specific printhead-to-print medium spacing or distance  121 . In this disclosure, delivering print agent on a print medium includes firing, ejecting, spitting or otherwise depositing print agent onto the print medium. The printhead may comprise a print agent chamber containing print agent to be delivered onto the print medium. 
     In some examples, a heating element may cause a rapid vaporization of print agent in a print agent chamber, increasing an internal pressure inside this print agent chamber. This increase in pressure makes a drop of print agent exit from the print agent chamber to the print medium through a nozzle. These printing systems may be called as thermal inkjet printing systems. 
     In some examples, a piezo electric may be used to force a drop of print agent to be delivered from a print agent chamber onto the print medium through a nozzle. A voltage may be applied to the piezo electric, which may change its shape. This change of shape may force a drop of print agent to exit through the nozzle. These printing systems may be called as piezo electric printing systems. 
     In some examples, the printhead may be static. The printhead or a plurality of printheads may extend along a width of a print medium, i.e. in a print medium width direction. A printhead may be mounted in a print bar spanning a width of the print medium. The plurality of nozzles may be distributed within the printhead or a plurality of printheads along the width of the print medium. The width of the print medium extends in a print medium width direction. The print medium width direction may be substantially perpendicular to the print medium advance direction. Such an arrangement may allow most of the width of the print medium to be printed simultaneously. These printing systems may be called as page-wide array (PWA) printing systems. 
     In some examples, the printhead may travel repeatedly across a scan axis for delivering print agent onto a print medium which may advance along a print medium advance direction. The scan axis may be substantially perpendicular to the print medium advance direction. The scan axis may be substantially parallel to print medium width direction. The printhead may be mounted on a carriage for moving across the scan axis. In some examples, several printheads may be mounted on a carriage. In some examples, four printheads may be mounted on a single carriage. In some examples, eight printheads may be mounted on a single carriage. 
     The print medium support  140  supports the print medium  110  to receive the print agent delivered by the printhead  120 . The printhead  120  is above the print medium support  140  and a print zone may be defined therebetween. The print medium support may guide and support the print medium in the print zone during printing. A lower side of the print medium may lie on the print medium support. 
     The print medium is a material capable of receiving print agent, e.g. ink. The print medium may comprise paper, cardboard, cardstock, textile material or plastic material. The print medium may be a sheet, e.g. a sheet of paper or a sheet of cardboard. 
     The print medium support may comprise a hold down system to apply a holding force on the print medium to hold down the print medium on the print medium support  140 . The hold down system may thus help to flatten the print medium when passes over the print zone. In some examples, hold down system may comprise a vacuum assembly to apply vacuum in the print medium support for flattening the print medium onto the print medium support. The print medium support may be permeable, so as to allow the vacuum through an upper side of the print medium support to pull the print medium against the print medium support. For example, the print medium support may comprise an upper plate having a plurality of through-holes in fluid communication with a vacuum source. The vacuum assembly may suck the print medium towards the print medium support. 
     In some examples, the printing system may comprise a print medium feed mechanism for feeding print medium to a print zone. The print medium feed mechanism may make the print medium advance in the print medium advance direction. 
     The printing system of  FIG.  1    comprises an adjusting system  10  to adjust a distance  123  between the print medium support  140  and the printhead  120 . The adjusting system  10  comprises a support structure  20  and a print medium input beam  31  and print medium output beam  32  to support the print medium support  140 . The print medium input beam  31  and the print medium output beam  32  are movable coupled to the support structure  20 . 
     In addition, the adjusting system  10  comprises an input beam driving assembly  40  and an output beam driving assembly  50  to respectively move the print medium input beam  31  and print medium output beam  32  relative to the support structure  20  between an upper and a lower end including a home position. 
     The adjusting system  10  of  FIG.  1    comprises an input beam sensor assembly  50  and an output beam sensor assembly  60 . The sensor assemblies  50  and  60  comprise a reference sensor  53  and  63  to respectively detect if the print medium input beam  31  and the print medium output beam  32  are at the home position and a relative sensor  54  and  64  to respectively determine a distance between the print medium input beam  31  and the print medium output beam  32  and the home position. 
     The adjusting system  10  of  FIG.  1    may accurately adjust the distance  123  between the print medium support  140  and the printhead  120  by moving the print medium support  140  relative to the printhead  120  in a Z-direction  112 . The printhead  120  may thus be maintained at the same position in the Z-direction  112 . Adjusting the distance between the printhead and the print medium support may be simplified. The distance  123  may thus be adapted to different print medium thicknesses by moving the print medium support. A printhead-to-print medium spacing  121  may be set for a given print medium thickness. The distance  123  may also be adapted as a function of a desired print image quality. For example, the print medium support may be adjusted as a function of a print medium composition and/or of a print image category, e.g. photo, graph, poster, CAD (computer aided design) or GIS (image of geographical information). Versatility of the printing system may thus be increased. Providing a sensor assembly and a driving assembly for each of the beams may increase the precision and the flatness of the adjusting system. 
     Movement of the print medium input beam  31  and of the print medium support beam  32  relative to the support structure  20  is independently driven the input beam driving assembly  40  and the output beam driving assembly  50 . The input beam driving assembly  40  may lift and lower the print medium input beam  31  between an upper and a lower end. The output beam driving assembly  50  may lift and lower the print medium output beam  32  between and upper and a lower end. Accordingly, an up and down movement of print medium support may be limited by the motion of the driving assemblies. Mechanical stoppers or bumps to stop the movement of the print medium support when hits a mechanical stopper may be avoided. Therefore, crashes between printing system components may be reduced and the operational life of the adjusting system components may consequently be extended. This may also allow using driving assemblies with motors with high torque and low speed, which may increase the accuracy of the position of the print medium support relative to the printhead, i.e. a distance between the printhead and the print medium support. Consequently, image quality may be enhanced for different types of print media, e.g. different print medium thicknesses, and/or a print image category. 
     The reference sensor  53  may detect if the print medium input beam  31  is at the home position and the relative sensor  54  may determine a distance travelled by the print medium beam input from the home position. A home position may thus be detected avoiding mechanical stoppers or bumpers. Using a reference sensor to determine a distance travelled by the print medium input beam from the home position may increase the precision of the measurement. In addition, reliability and robustness of the system may be improved and sensor costs may be reduced. 
     In some examples, the reference sensor of the input beam sensor assembly may comprise an optical sensor at one of the print medium input beam and the support structure and a reference line at the other of the print medium input beam and the support structure. The optical sensor may detect the reference line. Detecting the reference line may indicate that the print medium input beam is at the home position. In some examples, the optical sensor may be coupled to or at the print medium input beam and the reference line coupled to or at the support structure. In some examples, the optical sensor may be coupled to or at the support structure and the reference line coupled to or at the print medium input beam. 
     In some examples, the reference sensor of the output beam sensor assembly may be according to any of the examples of reference sensors of the input beam sensor assembly. 
     In some examples, the relative sensor of the input beam sensor assembly comprises a plurality of sensor strips at one of the print medium input beam and the support structure and an optical sensor at the other of the print medium input beam and the support structure. The optical sensor may thus determine the distance travelled by the print medium input beam from the home position by identifying the number of sensor strips crossed when the print medium input beam is lowered. In some examples, the optical sensor may be coupled to or at the print medium input beam and the plurality of sensor strips coupled to or at the support structure. In some examples, the optical sensor may be coupled to or at the support structure and the plurality of sensor strips coupled to or at the print medium input beam. 
     In some examples, the relative sensor of the output beam sensor assembly may be according to any of the examples of relative sensors of the input beam sensor assembly herein disclosed. 
     The print medium input beam may extend in an input beam direction, e.g. between a first end portion to a second end portion. The input beam direction may be perpendicular to the Z-direction and to the print medium advance direction. Similarly, the print medium output beam may extend in an output beam direction parallel to the input beam direction. 
     In some examples, the input beam driving assembly may comprise a first driving system engaging a first end portion of the print medium input beam and a second driving system engaging a second end portion of the print medium input beam. The print medium input beam may thus be lifted and lowered by actuating the first driving system and the second driving system. The first and the second driving system may be independently driven. The print medium input beam may be precisely positioned along the Z-direction. 
     Similar to the input beam driving assembly, the output beam driving assembly may comprise a first driving system engaging a first end portion of the print medium output beam and a second driving system engaging a second end portion of the print medium output beam. 
     In some examples, each of the input beam driving assembly and the output beam driving assembly may comprise a first and a second driving system. This may increase flatness and stability of the print medium support. In some examples, the first and the second driving system of each of the input and output beam driving assembly may comprise a drive motor. Less powerful drive motors may thus be used. Accordingly, the adjusting system may be more compact. 
     In some examples, the input beam sensor assembly may comprise a plurality of reference sensors and a plurality of relative sensors. In some examples, the input beam driving assembly may comprise a first driving system and a second driving system at opposite end portions of the print medium input beam. A first reference sensor and a first relative sensor may be associated with the first driving system. A reference sensor and a relative sensor may form a sensor system. A second reference sensor and a second relative sensor may be associated with the second driving system. Accordingly, a detection of the movement provided by each of driving system may be enhanced. 
     In some examples, the output beam sensor assembly may be according to any of the examples of input beam sensor assemblies herein disclosed. For example, a first reference sensor and a first relative sensor, i.e. a first sensor system, may be associated with a first driving system of the output beam sensor assembly by sensing a movement provided by the first driving system to the print medium output beam. A second reference sensor and a second relative sensor, i.e. a second sensor system, may sense a movement provided by a second driving system to the print medium output beam. Position of opposite ends of the print medium output in the Z-direction be may thus be precisely set. Flatness of the print medium support may thus be improved. 
     The printing system  10  of  FIG.  1    further comprises a controller  130  to control the operation of the adjusting system  10 . In some examples, the controller may further control the operation of the printing system. 
     In  FIG.  1    the controller  130  includes a processor  131  and a non-transitory machine-readable storage medium  132 . The non-transitory machine-readable storage medium  132  is coupled to the processor  131 . 
     The processor  131  performs operations on data. In an example, the processor is an application specific processor, for example a processor dedicated to control the adjusting system. The processor  131  may also be a central processing unit. 
     The non-transitory machine-readable storage medium  132  may include any electronic, magnetic, optical, or other physical storage device that stores executable instructions. The non-transitory machine-readable storage medium  132  may be, for example, Random Access Memory (RAM), an Electrically Erasable Programmable Read-Only Memory (EEPROM), a storage drive, an optical disk, and the like. 
       FIG.  1    additionally comprises a zoom-in view schematically representing an example of a non-transitory machine-readable storage medium  132  according to one example of the present disclosure. The non-transitory machine-readable storage medium is encoded with instructions which, when executed by the processor  131 , cause the processor  131  to lower a print medium input beam  31  and a print medium output beam  32  supporting the print medium support  140  from a safety distance between the print medium support  140  and a printhead  120  as represented at block  710 , determine if the print medium input beam  31  and the print medium output beam  32  reaches a respective home position as represented at block  720 , stop lowering the print medium input beam  31  and the print medium output beam  32  when the respective home positions are detected as represented at block  730 , lower the print medium input beam  31  and the print medium output beam  32  from the respective home positions as represented at block  740 , monitor a distance between the print medium input beam  31  and the print medium output beam  32  from the respective home positions when the print medium input beam  31  and the print medium output beam are lowered as represented at block  750  and stop lowering the print medium input beam  31  and the print medium output beam  32  when a respective printing position is detected for each of the print medium input beam  31  and the print medium output beam  32  as represented at block  760 . 
     At block  710 , the print medium input beam may be lowered by actuating an input beam driving assembly. In some examples, actuating an input beam driving assembly may comprise actuating a pair of driving systems at opposite ends of the print medium input beam. In some examples, the safety distance between the print medium support and the printhead may be an upper end limited by the actuation of the input beam driving assembly. Crashes against the printhead or other components of the printing systems may thus be prevented. The print medium output beam may be lowered in a similar way. 
     In some examples, the print medium input beam and the print medium output beam may be lifted from an initial position to a safety position at which the print medium support is at a safety distance relative to the printhead. 
     At block  720 , a sensor may detect if the print medium input beam and the print medium output beam are at a home position. For example, a reference sensor of an input beam sensor assembly, i.e. an input beam reference sensor, according to any of the examples herein disclosed may determine if the print media input beam is at the home position. 
     In some examples, determining if the print medium input beam and the print medium output beam reaches a respective home position may comprise receiving data from an input beam reference sensor and from an output beam reference sensor respectively indicating if the print medium input beam and the print medium output beam are at the home position. 
     For example, the input beam reference sensor may comprise an optical sensor and a reference line. The optical sensor may detect the reference line when the print medium input beam is lowered from the safety line. Detecting the reference line may thus indicate that the print medium input beam is at the home position. In some examples, a plurality of input beam reference sensors may indicate if several portions of the print medium input beam are at the home position. The home position of the print medium output beam may be determined in a similar way. 
     At block  730 , the print medium input beam and the print medium output beam may be stopped at the home position. The processor may receive data from reference sensors associated with each of print medium input and output beams. This data may indicate that the print medium input and output beams are at their respective home positions. Then, the processor may actuate the respective driving assemblies to stop the movement of the print medium input beam and the print medium output beam at their respective home positions. The print medium input beam and the print medium output beam may be maintained at the home positions during a predetermined period of time to enhance the flatness of the print medium support. A self-locking transmission of the driving assemblies may prevent the print medium input and output beams from lowering. 
     After ensuring the home position for the print medium input beam and the print medium output beam, the processor  132  may lower the print medium input and output beam as represented at block  740 . For example, where input beam reference sensor or sensors are to determine if the input beam are at the home position, the print medium input beam may start lowering the print medium input beam after each of the input beam reference sensors indicate that the print medium input beam is at the home position. Home position for the print medium input beam may thus be reliably determined. 
     The print medium input beam and the print medium output beam may be lowered by actuating a respective input and output beam driving assemblies. The print medium input and output beam may be lowered according to any of the examples herein disclosed, for example, as described with respect to block  710 . 
     At block  750  a distance travelled by each of the print medium input and output beams may be monitored when are lowered from their respective home positions is represented. The position of the print medium input and output beams relative to their home positions may thus be precisely monitored. A distance increase may be monitored by a relative sensor according to any of the examples herein disclosed. 
     In some examples, monitoring a distance between the print medium input beam and the print medium output beam from the respective home positions may comprise receiving data from an input beam relative sensor and from an output beam relative sensor respectively counting the number of sensor strips detected by each of the relative sensors. Counting the number of sensor strips detected by a relative sensor may indicate a distance travelled by a print medium input and/or output beam from the home position. In some examples, a plurality of input beam relative sensors may monitor a distance between the print medium input beam from the home position, e.g. a distance between portions of the print medium input beam. Similarly, a plurality of output beam relative sensors may be used for monitoring a distance between the print medium output beam and the home position. 
     Block  760  may represent positioning the print medium input beam and the print medium output beam at a respective printing position. As the processor monitors a distance between the print medium input and output beams, the print medium input and output beams may be stopped at a respective printing position. A printing position of a print medium input or output beam correspond to the position of these beams at which the printhead and the print medium support are at a distance to ensure a predetermined printhead-to-print medium spacing. Such a predetermined printhead-to-print medium spacing may be set for a given print medium thickness and/or for a given print medium composition and/or for a given print image category. 
     In some examples, the non-transitory machine-readable storage medium  132  may further cause the processor  131  to obtain a print medium thickness and determine the printing position of the print medium input beam and of the print medium output beam based on the obtained print medium thickness. A dedicated sensor may be measured the print medium thickness before reaching a print zone. In some examples, a print medium thickness may be provided by a user via a user interface device coupled to the processor. In some examples, the non-transitory machine-readable storage medium may cause the processor to obtain a print medium composition and determine the printing position of the print medium input beam and of the print medium output beam based on the obtained print medium composition. In some examples, the non-transitory machine-readable storage medium may cause the processor to obtain a print image category and determine the printing position of the print medium input beam and of the print medium output beam based on the obtained print image category. 
     The instructions encoded in the non-transitory machine-readable storage medium for the processor represented at blocks  710 ,  720 ,  730 ,  740 ,  750  and  760  may participate in adjusting a distance between a printhead and print medium support of a printing system. 
       FIG.  2    illustrates an isometric view of an adjusting system according to an example of the present disclosure. The adjusting system  10  comprises a support structure and a print medium input beam  31  and a print medium output beam  32  to support the print medium support  140 . The print medium input beam  31  and the print medium output beam  32  are movably coupled to the support structure. In this figure, the support structure comprises an input support structure  21  and an output support structure  22 . The print medium input beam  31  may be movably coupled to the input support structure  21  and the print medium output beam  32  may be movably coupled to the output support structure  22 . 
     The print medium input beam  31  and the print medium output beam  32  may be moved in a Z-direction  112 . The print medium support  140  of this figure is coupled to the print medium input beam  31  and to the print medium output beam  32 . Accordingly, the print medium support  140  may be moved in the Z-direction  112 . 
     In this example, the print medium input beam  31  extends from a first end portion  311  to a second end portion  312  in an input beam direction  313 . The print medium output beam  32  extends between a first end portion  321  to a second end portion in an output beam direction  323 . The input beam direction  313  may be parallel to the output beam direction  323 . 
     The adjusting system of  FIG.  2    further comprises an input beam driving assembly and an output beam driving assembly to respectively move the print medium input beam  31  and the print medium output beam  32  relative to the support structure, e.g. relative to the input support structure  21  and to the output support structure  22 , between an upper and a lower end including a home position. In this example, the input beam driving assembly comprises a first driving system  41  engaging a first end portion  311  of the print medium input beam  31  and a second driving system  42  engaging a second end portion  312  of the print medium input beam  31 . In this figure, the output beam driving assembly comprises a first driving system  53  engaging a first end portion  321  of the print medium output beam  32  and a second driving system (not shown in  FIG.  2   ) engaging a second end portion of the print medium output beam  32 . 
     Furthermore, the adjusting system of  FIG.  2    comprises an input beam sensor assembly and an output beam sensor assembly. In this figure, the input beam sensor assembly comprises a first sensor system  61  and a second first sensor  62 . In this example, the first sensor system  61  is associated with the first driving system  41  and the second sensor system  62  is associated with the second driving system  42 . 
     In  FIG.  2   , the first sensor system  61  and the second sensor system  62  comprise a reference sensor to detect if the print medium input beam  31  is at the home position. For example, the reference sensor of the first sensor system  61  may detect if the first end  311  of the print medium input beam  31  is at the home position. In this figure, the first sensor system  61  and the second sensor system  62  comprise a relative sensor to determine a distance between the print medium input beam  31  and the home position. 
     The output sensor assembly may comprise a first sensor system and a second sensor system. The first and the second sensor system may comprise a reference sensor to detect if the print medium output beam  32  is at the home position and a relative sensor to determine a distance between the print medium output beam  32  and the home position. The reference sensors and the relative sensors of the output beam sensor assembly may be according to any of the examples of reference sensors and relative sensors herein disclosed. 
     In this figure, a pair of driving systems move the print medium input beam and a pair of driving systems move the print medium output beam. The print medium input and output beams may thus be precisely supported and moved. Accordingly, a distance between the print medium support and a printhead may be accurately adjusted. Power of drive motors actuating the driving systems may be reduced and reliability of the system may be improved. A reference sensor and a relative sensor may be associated to each of the driving systems. Movement provided by each of the driving system may be measured. The driving systems may thus be independently controlled to ensure a flatness of the print medium support. 
     In  FIG.  2   , the print medium support  140  may support print medium advancing in a print medium advance direction  111 . In this example, the print medium support  140  comprises a print medium input roller  141  and a print medium output roller  142 . The print medium input roller  141  may be at an opposite side of the print medium support  140  along a print medium advance direction  111 . In some examples, the print medium input roller may be rotatably coupled to the print medium input beam and the print medium output roller may be rotatably coupled to the print medium output beam. 
     The print medium input roller  141  may rotate about an axis parallel to an input beam direction  313  and the print medium output roller  142  may rotate about an axis parallel to the output beam direction  323 . The input beam direction  313  may be parallel to the output beam direction  323 . 
     A belt or a plurality of belts  143  may engage the print medium input roller  141  and the print medium output roller  142 . In this example, the print medium output roller  142  may be rotated to cause a displacement of the belts  143 . Supporting plates  144  may be between belts to contact the print medium. Print medium may be supported by the supporting plates and by the belts. A displacement of the belts  142  may cause a displacement of the print medium. The print medium may thus advance in the print medium advance direction by the displacement of the belts. In some examples, the supporting plates and/or the belts may comprise a plurality of through-holes in fluid communication with a vacuum source to hold down a print medium towards the supporting plates. 
     In  FIG.  2   , the adjusting system comprises a connecting structure  145  connecting the print medium input beam  31  to the print medium output beam  32 . The connecting structure  145  may comprise a plurality of connecting beams extending in a direction parallel to the print medium advance direction  111 . The connecting beams may be flexibly connected to the print medium input beam and to the print medium output beam. This may increase the flexibility of the print medium support. For example, the flexible connection between the adjusting system and the print medium beams may compensate deformations or misalignments caused by a delay between the driving systems moving the print medium beams. 
     In some examples, a column or a plurality of columns may be connected the support structure to guide the up and down movement of the print medium input beam and of the print medium output beam. A bushing assembly may be between the column and the print medium input and output beams. The bushing assembly may absorb misalignments of the print medium input and output beam. For example, a pair of columns may guide the up and down movement of the print medium input beam and a bushing assembly may between each of the columns and the print medium input beam. These bushing assemblies may absorb inclinations and/or deformations of the print medium input beam. 
       FIG.  3    illustrates a zoom-in view of a portion of the adjusting system of  FIG.  2   .  FIG.  3    shows a first driving system  41  of the input driving assembly and first driving system  51  of the output driving assembly. Other driving system of the adjusting system may be according to the first driving system  41  described herein. 
     The first driving system  41  of this figure is connected to the input support structure  21  and may induce an up and down movement of the print medium input beam  31  relative to the input support structure  21 . Similarly, the first driving system  51  of the output beam driving assembly may be connected to the output support structure  22  to cause an up and down movement of the print medium output beam  32  relative to the output support structure  22 . 
     In  FIG.  3   , the first driving system  41  comprises a drive motor  81  and a transmission  82  to transmit a driving force from the drive motor  81  to the print medium input beam  31 . The transmission  82  of this figure includes an outer shaft  81  rotatably about an outer shaft direction  183 . The driving force may cause the rotation of the outer shaft  83 . The outer shaft  83  may engage the print medium input beam  31  to cause the up and down movement. In this example, the outer shaft direction  183  is perpendicular to the rotational axis of the drive motor  81 . The rotational axis of the drive motor of the first driving system  41  is parallel to the input beam direction and the outer shaft direction  183  is parallel to the print medium advance direction. 
     In some examples, the transmission may include a self-locking transmission to lock the position of the print medium input beam. Accordingly, a position of the print medium input beam may be maintained in absence of a driving force provided by the input beam driving assembly. Furthermore, external retention or braking systems to hold the print medium input beam in a predetermined position, i.e. at a predetermined height, may be avoided. 
     Similar to the first driving system  41  of the input beam driving assembly, the first driving system  51  of the output driving assembly of this figure comprises a drive motor  81  and a transmission  82  to transmit a driving force from the drive motor  81  to the print medium output beam  32 . The transmission  82  includes an outer shaft  81  rotatably about an outer shaft direction  183  perpendicular to rotational axis of the drive motor  81 . However, the rotational axis of the drive motor  51  of the first driving system  51  of the print medium outer beam is parallel to Z-direction  112 . 
     In some examples, the driving systems may comprise a gearbox to reduce the rotational speed provided by the drive motor and increase the torque. Accordingly, higher torque may be provided which may increase the capacity the lift heavier loads, e.g. heavier print medium supports. Lower speeds may increase the accuracy of the movements of the print medium support 
       FIG.  4    illustrates a driving system of a driving assembly and a sensor system of a sensor assembly according to an example of the present disclosure. The driving assembly illustrated in this figure is an input beam driving assembly and the sensor assembly is an input beam sensor assembly. However, an output beam driving assembly and an output beam sensor assembly may be according to any of the examples described with respect to  FIG.  4   . 
     The input beam driving assembly  40  of this figure comprises an input beam driving system  41  comprising a drive motor  81  and a transmission  82  to transmit a driving force from the drive motor  81  to the print medium input beam  31 . The transmission  82  may transform a rotational movement provided by the drive motor  81  to a linear movement to lift and lower the print medium input beam  31  relative to the support structure  20 . 
     In  FIG.  4   , the transmission  82  comprises a worm drive mechanism having a worm screw  84  driven by the drive motor  81  and a worm wheel  85  coupled to an outer shaft  83 . The worm screw  84  meshes the worm wheel  85  so as to transmit a driving force from the drive motor  81  to the outer shaft  83 . A worm drive mechanism may reduce rotational speed and transmit higher torque. The worm screw  84  may rotate about a rotational axis  181  of the drive motor and the worm wheel  85  about an outer shaft direction  183 . In this figure, the rotational axis  181  of the drive motor is perpendicular to the outer shaft direction  183 . Motion may thus be transferred in 90 degrees. The driving assembly may thus be more compact. 
     A worm drive mechanism may be an example of a self-locking transmission as only rotation may be transmitted from the worm screw  84  driven to the worm wheel  85 . 
     In some examples, the driving system may comprise an eccentric pin protruding from the outer shaft in a direction parallel to the outer shaft direction. The worm wheel may be coupled at one end of the outer shaft and the eccentric pin may protrude from the opposite end. The eccentric pin may engage the print medium input beam to transform a rotational motion to a linear motion. 
     In some examples, the input beam driving assembly may comprise a plurality of driving systems according to any of the examples herein disclosed. The output beam driving assembly may be according to any of the examples of input beam driving assemblies herein disclosed. 
     The input beam sensor assembly  60  of this figure comprises a sensor system  61  including a reference sensor  62  and a relative sensor  63 . 
     In this example, the reference sensor  62  comprises an optical sensor  621  coupled to the print medium input beam  31  and a reference line  622  coupled to the support structure  20 . 
     In  FIG.  4   , the relative sensor  63  comprises an optical sensor  631  coupled to the print medium input beam  31  and a plurality of sensor strips  634  coupled to the support structure  20 . 
       FIG.  5    illustrates a side view of a driving system of a driving assembly, e.g. of an input beam driving assembly and/or of an output beam driving assembly, according to an example of the present disclosure. This figure illustrates a side of a driving system facing print medium input or output beam. 
     The driving system of this figure comprises a drive motor  81  and a transmission  82  to transmit a driving force from the drive motor  81  to the print medium input or output beam. The transmission comprises an outer shaft  83  rotatably about an outer shaft direction  183 . In this figure, the outer shaft direction  183  is perpendicular to the paper. In this figure, the rotational axis  181  of the drive motor is perpendicular to the outer shaft direction  183 . 
     In this figure, an eccentric pin  86  protrudes from the outer shaft in a direction parallel to the outer shaft direction (perpendicular to the paper). The eccentric pin may engage a print medium input or output beam to transform a rotational motion to a linear motion. The eccentric pin and a slot comprised in the print medium input or output beam may form a Scotch yoke (also known as slotted link mechanism). A Scotch yoke is a reciprocating motion mechanism that converts a rotational motion to a linear motion. 
     The center of the eccentric pin of this figure is separated from the outer shaft direction. In this example the eccentric pin comprises a cylindrical shape. 
       FIG.  6    schematically illustrates a motion of an outer shaft and an eccentric pin according to an example of the present disclosure. The outer shaft  83  rotates about an outer shaft direction  83 . An eccentric pin  86  protrudes from the outer shaft in a direction parallel to the outer shaft direction  183 . In this example, the eccentric pin rotates together with the outer shaft  83  and may adopt different positions during rotation. 
     In this figure, the eccentric pin  86  is at the home position  187 , the eccentric pin  86   a  is at a top dead center position  185  and the eccentric pin  86   b  at a bottom dead center position  186 . Accordingly, the eccentric pin may be moved between a top dead center position  185  and a bottom dead center position  186  in a Z-direction  112 . The eccentric pin at the home position is between the top dead center position  185  and the bottom dead center position  186 . Movements of the eccentric pin in the Z-direction may thus be limited between the top dead center position and the bottom dead center position. 
     In some examples, a distance between the top dead center position  185  and the bottom dead center position  186  may be between 5 mm and 30 mm. In some examples, a distance between the top dead center position  185  and the bottom dead center position  186  may be between 8 mm and 20 mm. 
     In some examples, a distance between the top dead center position  185  and the home position  187  in a Z-direction  112  may be between 0.5 mm and 5 mm. In some examples, a distance between the top dead center position  185  and the home position  187  in a Z-direction  112  may be between 0.8 mm and 3 mm. 
       FIG.  7    schematically represents a motion of an eccentric pin and a portion of a print medium input beam according to an example of the present disclosure. An eccentric pin  86  having a rotational motion engages with a print medium input beam  31 . The eccentric pin extends in a direction parallel to the outer shaft direction (see for example  FIG.  6   ). In some examples, the eccentric pin  86  may engage with a print medium output beam rather than with the print medium input beam. 
     The print medium input beam  31  of this figure extends in an input beam direction  313 . The input beam direction is perpendicular  313  to the outer shaft direction. In this figure, the print medium input beam  31  comprises a slot  318  extending in a direction parallel to the input beam direction  313  to receive the eccentric pin  86  to transmit a driving force from the outer shaft to the print medium input beam  31 . 
     In this figure, the eccentric pin  86  rotates together with an outer shaft (not shown in this figure). In  FIG.  7   , the eccentric pin  86  is at a top dead center position  185  and the eccentric pin  86   b  at the bottom dead center position  186 . The eccentric pin  86  may thus rotate between the top dead center position  185  and bottom dead center position  186 . 
     In  FIG.  7   , the eccentric pin is inserted into the slot  318 . The eccentric pin  86  and the slot  318  may form a Scotch yoke mechanism. The eccentric pin may slide inside the slot in a direction parallel to the input beam direction  313  but may cause an up and down movement of the print medium input beam  31  in the Z-direction  112 . When the eccentric pin rotates, the pin may contact an upper and/or a lower surface of the slot to transform a rotational motion to a linear motion. 
     Accordingly, the eccentric pin  86  may cause an upwards and downwards movement of the print medium input beam between an upper end  315  and a lower end  316 . 
     In this figure, the print medium input beam  31  is at the upper end  315 . When the eccentric pin  86  is at the top dead center position  185 , the print medium input beam  31  is at the upper end  315 . This figure also shows a print medium input beam  31   b  at a lower end  316  when the eccentric pin  86   b  is at the bottom dead center position  186 . 
     Accordingly, the rotational movement of the eccentric pin may induce a linear movement of the print medium input beam in a Z-direction  112 . The upwards and downwards movements of the print medium input beam may thus be constrained between the upper end and the lower end. The print medium input beam  31  may thus be moved between an upper end  315  and a lower end  316  to adjust a distance between the print medium support and a printhead. 
     In some examples, the distance between top dead center position  185  and the bottom dead center position  186  may be substantially the same than between the upper end  315  and the lower end  316  of the print medium input beam  31 . 
       FIG.  8    schematically represents a sensor assembly according to an example of the present disclosure. The sensor assembly illustrated in this figure is an input beam sensor assembly. The input beam sensor assembly  60  comprises a reference sensor  62  and a relative sensor  63  forming a sensor system. In some examples, the input beam sensor assembly may comprise a plurality of sensor systems having a reference sensor and a relative sensor. 
     In this example, the reference sensor  62  comprises an optical sensor  621  coupled to the print medium input beam  31  and a reference line  622  coupled to the support structure  20 . The reference line  622  may be integrated in a plate  64 . A bracket  25  may be used for coupling the reference line  622  to the support structure  20 , e.g. the bracket  25  support the plate  64  comprising the reference line  622 . 
     In some examples, the optical sensor of the reference sensor may be at or coupled to the support structure and the reference line may at or coupled to the print medium beam, e.g. to the print medium input or output beam. 
     The optical sensor  621  of the reference sensor  62  may detect the reference line  622 . When the optical sensor  621  reads the reference line  622 , the print medium input beam is at the home position. The print medium input beam may thus be stopped when the optical sensor of the reference sensor when detects the reference line, i.e. at the home position. 
     In  FIG.  8   , the print medium input beam  31  is at the home position. The print medium input beam  31  may be moved between an upper end  315  and a lower end  316  to adjust a distance between the print medium support and a printhead. 
     In some examples, the input beam sensor assembly may comprise a quadrature encoder sensor to detect a direction of the movement of the print medium input beam relative to the support structure. In some examples, a quadrature encoder sensor may be integrated with the reference sensor. In some examples, a quadrature encoder sensor may be integrated with the relative sensor. In some examples, a quadrature encoder sensor may be independent from the reference sensor and from the relative sensor. 
     In  FIG.  8   , the relative sensor  63  comprises an optical sensor  631  coupled to the print medium input beam  31  and a plurality of sensor strips  632  at the support structure  20 . The plurality of sensors strips  632  may be in the plate  64 . The plate  64  may be supported by the bracket  25 . 
     The relative sensor may monitor a distance travelled by the print medium input beam from the home position. 
     The optical sensor  631  of the relative sensor may comprise a linear incremental encoder. The linear incremental encoder may report position changes of the print medium input beam. Number of sensor strips crossed may thus be counted. The linear incremental encoder may comprise a quadrature encoder sensor to indicate the detection of a sensor strip and the direction of the movement. A linear incremental encoder comprising a quadrature encoder sensor may be called as a quadrature linear incremental encoder. A relative distance and a direction of the movement may thus be determined. Accordingly, the quadrature linear incremental encoder may detect if the print medium input beam is lifted or lowered. 
     Using a quadrature linear incremental encoder may increase the precision of the detection. 
     Information provided by the quadrature linear incremental encoder of the relative sensor may be used in the detection of the reference line. The input beam sensor assembly may thus distinguish detecting the reference line when the print medium input beam moves in an upwards direction from when it moves in a downwards direction. For example, this may allow stopping the print medium input beam when the reference line is detected at lowering the print medium input beam but not when the reference line is detected at lifting the print medium input beam. 
     In this example, the quadrature lineal incremental encoder is integrated with the optical sensor  631  of the relative sensor. In some examples, a quadrature lineal incremental encoder may be independent from the optical sensor  631 . For example, a quadrature linear incremental encoder may be comprised in the optical sensor  621  of the reference sensor  62 . 
     In some examples, the plurality of sensor strips  632  may comprise a resolution of 150 LPI (150 lines per inch). Distance between sensor strips may thus be about 170 μm (170 micrometers). If the optical sensor  631  of the relative sensor comprises a quadrature linear incremental encoder the measurement resolution may be about 42 μm (42 micrometers). This measurement resolution may provide a precise information of the position of the print medium input beam. 
       FIG.  9    is a block diagram of an example of a method to adjust a distance between a print medium support and a printhead of a printing system. The method  800  comprises actuating a plurality of driving assemblies to lift the print medium support to a minimum predetermined distance between the print medium support and the printhead as represented at block  810 , actuating the plurality of driving assemblies to lower the print medium support as represented at block  820 , stopping the plurality of driving assembly when the print medium support reaches a home position during lowering the print medium support as represented at block  830  and actuating the plurality of driving assemblies to lower the print medium support to a printing position as represented at block  840 . 
     In some examples, the method to adjust a distance between a print medium support and a printhead of a printing system may use an adjusting system according to any of the examples herein disclosed. For example, the driving assemblies may be according to any of the examples herein disclosed. 
     A non-transitory machine-readable storage medium according to any of the examples herein disclosed may comprise instructions to perform this method. 
     At block  810 , the print medium support is lifted to a minimum predetermined distance between the print medium support and the printhead. For example, an input beam driving assembly and an output beam driving assembly may respectively lift a print medium input beam and a print medium output beam supporting the print medium support. In some examples, the print medium input beam and the print medium output beam may be lifted to an upper end according to any of the examples herein disclosed to lift the print medium support to the minimum predetermined distance. 
     After reaching the minimum predetermined distance, the print medium support may be lowered by actuating the plurality of driving assemblies as represented at block  820 . 
     At block  830 , plurality of driving assemblies is stopped when the print medium support reaches a home position during lowering the print medium support. The print medium support may thus be stopped at the home position to ensure a print medium support flatness. 
     In some examples, the method may comprise determining if the print medium support is at a home position. The home position may be determined by using a sensor according to any of the examples herein disclosed. For example, by using a reference sensor comprising an optical sensor to detect a reference line. 
     As represented at block  840  the print medium support may be lowered to a printing position. After ensuring that the print medium support is at the home position, the plurality of driving assemblies may be actuated to lower the print medium support to a printing position. 
     A sensor may provide a feedback about the position of the print medium support relative to the home position. In some examples, the method may comprise determining a distance travelled by the print medium support from the home position when the plurality of driving assemblies is lowering the print medium support from the home position to the printing position. 
     In some examples, a relative sensor according to any of the examples herein disclosed may be used for determining the distance travelled from the home position. For example, determining a distance travelled by the print medium support from the home position may comprise counting a number of sensors strips in a fixed structure detected by an optical sensor connected to the print medium support. Number of sensor strips crossed may thus be counted. Accordingly, the distance travelled may be determined. The optical sensor and the sensor strips may be according to any of the examples herein disclosed. 
     The preceding description has been presented to illustrate and describe certain examples. Different sets of examples have been described; these may be applied individually or in combination, sometimes with a synergetic effect. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any.