Patent Publication Number: US-11648772-B2

Title: Pulse waveforms for ink jet printing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/291,630, filed May 6, 2021, which is U.S. National Phase of PCT Application PCT/IB2019/056888, filed Aug. 14, 2019, which claims the benefit of U.S. Provisional Patent Application 62/767,533, filed Nov. 15, 2018. The disclosures of all these related applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to digital printing, and particularly to methods and systems for driving inkjet print heads. 
     BACKGROUND OF THE INVENTION 
     Various methods for jetting ink for presses are known in the art. For example, U.S. Patent Application Publication 2006/0164450 describes a method of driving an inkjet module having a plurality of ink jets. The method includes applying a voltage waveform to the inkjet module, the voltage waveform including a first pulse and a second pulse, activating one or more of the ink jets contemporaneously to applying the first pulse, wherein each activated ink jet ejects a fluid droplet in response to the first pulse, and activating all of the ink jets contemporaneously to applying the second pulse without ejecting a droplet. 
     As another example, U.S. Patent Application Publication 2007/0057979 describes a method and system for facilitating development of fluids having a variety of elemental compositions. A graphical user interface allows user interaction with a lab deposition system to control fluid drop ejection of fluids through multiple nozzles. Fluid drop ejection and drop formation can vary from fluid to fluid, and require adjustments to waveform parameters of a drive pulse applied to the multiple nozzles. The system implements a drop watcher camera system to capture real-time still and video images of fluid drops as they exit the multiple nozzles. The captured drop formation of the fluid drops is displayed to the user. Based on the images, the waveform parameters are adjusted and customized specific for individual fluid. In addition to adjusting the drive pulse that effects fluid drop ejection, a tickle pulse can also be adjusted and customize to prevent clogging of the nozzles. 
     U.S. Pat. No. 9,272,511 describes a method, apparatus, and system for driving a droplet ejection device with multi-pulse waveforms. In one embodiment, a method for driving a droplet ejection device having an actuator includes applying a multi-pulse waveform with a drop-firing portion having at least one drive pulse and a non-drop-firing portion to an actuator of the droplet ejection device. The non-drop-firing portion includes a jet straightening edge having a droplet straightening function and at least one cancellation edge having an energy canceling function. The drive pulse causes the droplet ejection device to eject a droplet of a fluid. 
     U.S. Pat. No. 7,988,247 describes a method for causing ink to be ejected from an ink chamber of an ink jet printer includes causing a first bolus of ink to be extruded from the ink chamber; and following lapse of a selected interval, causing a second bolus of ink to be extruded from the ink chamber. The interval is selected to be greater than the reciprocal of the fundamental resonant frequency of the chamber, and such that the first bolus remains in contact with ink in the ink chamber at the time that the second bolus is extruded. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a digital printing system including a print head and a processor. The print head is configured to jet droplets of ink. The a processor is further configured to translate a required shade of a color, to be printed at a given location on a substrate by the print head, into a sequence of pulses, the sequence including: (a) up to a predefined maximum number of driving pulses that cause the print head to jet respective droplets, and (b) a tickling pulse, which has a smaller amplitude than the driving pulses and which causes the print head to jet a droplet smaller than the droplets jetted in response to the driving pulses. The processor is additionally configured to apply the sequence of pulses to the print head. 
     In some embodiments, the processor is configured to set a same time duration for the driving pulses and for the tickling pulse. 
     In some embodiments, the processor is configured to set, for at least one of the pulses in the sequence, a time duration that matches a resonance frequency of a pressure wave in the ink inside a jetting channel of the print head. 
     In an embodiment, the processor is configured to set the time duration depending on a type of the ink. In another embodiment, the processor is configured to set an amplitude of the driving pulses to achieve a maximal speed of the jetted droplets. 
     In some embodiments, the processor is configured to apply the tickling pulse at an end of the sequence. 
     There is additionally provided, in accordance with an embodiment of the present invention, a digital printing method, including defining a required shade of a color, to be printed at a given location on a substrate by a print head that jets droplets of ink. The required shade of the color is translated into a sequence of pulses, the sequence including: (a) up to a predefined maximum number of driving pulses that cause the print head to jet respective droplets, and (b) a tickling pulse, which has a smaller amplitude than the driving pulses and which causes the print head to jet a droplet smaller than the droplets jetted in response to the driving pulses. The sequence of pulses is applied to the print head. 
     There is further provided, in accordance with an embodiment of the present invention, a manufacturing method, including, in a digital printing system that applies a sequence of pulses to a print head for jetting droplets of ink, calculating time durations, to be assigned to the pulses, so as to match a resonance frequency of a pressure wave in the ink inside a jetting channel of the print head. The digital printing system is configured to apply the calculated time durations. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic side view of a digital printing system, in accordance with an embodiment of the present invention; 
         FIG.  2    is a schematic pictorial illustration of a print bar of the digital printing system of  FIG.  1   , in accordance with an embodiment of the present invention; 
         FIG.  3    is a diagram showing a waveform applied to a print head during a jetting cycle, in accordance with embodiments of the present invention; 
         FIG.  4    is a lookup table of a four-shade-level printing scheme, in accordance with an embodiment of the present invention; and 
         FIG.  5    is a schematic graph of level 3 tickling droplet volume as a function of tickling pulse amplitude, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     In digital printing, a required shade of a color can be printed at a given location on a substrate (i.e., a printed pixel) by a print head that jets a suitable number of ink droplets of the same color. The print head jets each ink droplet from a nozzle in response to a driving pulse. Therefore, a required shade may be achieved by applying, during a jetting cycle, a suitable number of similar driving pulses to the print head. In the present context, the term “similar” means deviations of up to several percent, e.g., ±10% or ±5%. 
     During a typical printing session, some nozzles receive driving pulses that cause the nozzles to eject droplets, while other nozzles are temporarily idle. Nozzles that do not eject droplets, and the ink meniscus in them, are exposed to hot environment and may tend to dry out. When the ink starts to dry or increase its viscosity, the nozzle will not fire the first droplets until new ink arrives at the meniscus. As a result, some pixels may be missed, and when the nozzle finally jets, a resulting pixel might be distorted (i.e., have bad straightness). In extreme cases a nozzle that was idle might even clog. To prevent the above described “first drop problem” or latency problem, as well as clogging, a “tickling” pulse may be applied at the end of the jetting cycle, causing ink to flow inside the nozzle but without the nozzle jetting a droplet. 
     The duration of a jetting cycle is typically fixed and shared among all nozzles. This duration is determined by the number of ink droplets required to produce the darkest shade, and is the sum of the section durations of the driving pulse plus an identical section duration of the tickling pulse. In the present context, a “section” means a pulse and idle time intervals immediately before and/or after the pulse. Thus, a duration of a jetting cycle is fixed, regardless of whether a nozzle was idle at a certain location during a jetting cycle. 
     Embodiments of the present invention that are described hereinbelow provide methods and systems for increasing printing throughput by using the tickling section (the time duration used for the tickling pulse) in a jetting cycle to jet an ink droplet, and thereby reduce the overall duration of the jetting cycle (i.e., the time required to print the darkest shade). The disclosed technique thus uses a tickling pulse to serve two purposes at the same time: Jetting a droplet, and protecting against ink viscosity increase in the nozzle. Because of the disclosed dual role of the tickling pulses, the jetting cycle can be shortened and the overall printing throughput can be increased. 
     In some embodiments, for a given location at which a required shade is to be printed, a processor controls electrical circuitry, which in turn controls the print head, to translate the required shade into a sequence of pulses. The sequence comprises up to a predefined maximum number of driving pulses that cause the print head to jet respective droplets, and a tickling pulse that in some settings, as described below, causes the print head to jet a droplet of somewhat smaller volume than the droplets jetted in response to the driving pulses. The processor is further configured to apply the sequence of pulses to the print head. 
     In another embodiment, during or after assembly of the printing system, a professional adjusts and presets a same duration for all sections, by presetting a same delay between every two successive pulses and presetting all the pulses to the same pulse width. The pulse width and the delay values are selected so that together the duration of a driving section matches a resonance frequency of a pressure wave in the ink inside a jetting channel of the print head (i.e., matching a fluidic-structural resonance of the jetting channel of the print head) for the ink being used. As a result of pressure building up in the jetting channel by one or more driving pulses, the tickling pulse, while having smaller amplitude than the driving pulses, still causes the print head to jet a droplet, which has sufficient volume to produce a required shade. 
     In an embodiment, before, during or after assembly of the printing system, a professional adjusts and presets the amplitude of the driving pulses to achieve a maximal speed of the jetted droplets. 
     In example embodiments of the present invention, the printing system prints in a four-shade scheme, in which the system applies up to four shades (e.g., white, light gray, dark gray, and black). In these embodiments, each jetting cycle comprises two driving pulse sections followed by a tickling pulse section, in order to produce the four shades. Applying a tickling pulse capable of jetting an ink droplet at the last section of a jetting cycle, wherein using such tickling pulse, the printing system is configured to produce the darkest shade among the possible shades (e.g., a black shade), shortens the printing time by about a quarter, as described below, achieving a corresponding increase in printing throughput. 
     In an embodiment, upon receiving a tickling pulse at the end of a jetting cycle, from the electrical circuitry that controls the print heads, the print head causes ink motion in a nozzle of the print head. In an embodiment, in order for a print head to jet an ink droplet in response to tickling pulse, the tickling pulse has to be applied after applying at least one adjacent driving pulse. In general, depending on the applied sequence of driving pulses, and depending on whether any driving pulse is applied in the section immediately preceding the section in which the tickling pulse is applied, the print head may or may not jet an ink droplet in response to the tickling pulse, as described below. 
     In some embodiments, by adjusting the volume of an ink droplet jetted by a tickling pulse, the disclosed technique achieves improved printing quality over long unsupported segments of substrate, as described below. 
     By enabling jetting an ink droplet during a tickling section, the disclosed technique improves the throughput of digital printing systems, and reduce the cost of the printing hardware, and thus reduce the overall costs of printing. 
     System Description 
       FIG.  1    is a schematic side view of a digital printing system  10 , in accordance with an embodiment of the present invention. In some embodiments, system  10  comprises a rolling flexible blanket  44  that cycles through an image forming station  60 , a drying station  64 , an impression station  84  and a blanket treatment station  52 . In the context of the present invention and in the claims, the terms “blanket” and “intermediate transfer member (ITM)” are used interchangeably and refer to a flexible member comprising one or more layers used as an intermediate member configured to receive an ink image and to transfer the ink image to a target substrate, as will be described in detail below. 
     In an operative mode, image forming station  60  is configured to form a mirror ink image, also referred to herein as “an ink image” (not shown), of a digital image  42  on an upper run of a surface of blanket  44 . Subsequently the ink image is transferred to a target substrate, (e.g., a paper, a folding carton, or any suitable flexible package in a form of sheets or continuous web) located under a lower run of blanket  44 . 
     In the context of the present invention, the term “run” refers to a length or segment of blanket  44  between any two given rollers over which blanket  44  is guided. 
     In some embodiments, during installation blanket  44  may be adhered edge to edge to form a continuous blanket loop (not shown). An example of a method and a system for the installation of the seam is described in detail in U.S. Provisional Application 62/532,400, whose disclosure is incorporated herein by reference. 
     In some embodiments, image forming station  60  typically comprises multiple print bars  62 , each mounted (e.g., using a slider) on a frame (not shown) positioned at a fixed height above the surface of the upper run of blanket  44 . In some embodiments, each print bar  62  comprises a strip of print heads as wide as the printing area on blanket  44  and comprises individually controllable print nozzles. 
     In some embodiments, image forming station  60  may comprise any suitable number of bars  62 , each bar  62  may contain a printing fluid, such as an aqueous ink of a different color. The ink typically has visible colors, such as but not limited to cyan, magenta, red, green, blue, yellow, black and white. In the example of  FIG.  1   , image forming station  60  comprises seven print bars  62 , but may comprise, for example, four print bars  62  having any selected colors such as cyan, magenta, yellow and black. 
     In some embodiments, the print heads are configured to jet ink droplets of the different colors onto the surface of blanket  44  so as to form the ink image (not shown) on the surface of blanket  44 . 
     In some embodiments, different print bars  62  are spaced from one another along the movement axis of blanket  44 , represented by an arrow  94 . In this configuration, accurate spacing between bars  62 , and synchronization between directing the droplets of the ink of each bar  62  and moving blanket  44  are essential for enabling correct placement of the image pattern. 
     In the context of the present disclosure and in the claims, the terms “inter-color pattern placement,” “pattern placement accuracy,” color-to-color registration,” “C2C registration” “bar to bar registration,” and “color registration” are used interchangeably and refer to any placement accuracy of two or more colors relative to one another. 
     In some embodiments, system  10  comprises heaters, such as hot gas or air blowers  66 , which are positioned in between print bars  62 , and are configured to partially dry the ink droplets deposited on the surface of blanket  44 . This hot air flow between the print bars may assist, for example, in reducing condensation at the surface of the print heads and/or in handling satellites (e.g., residues or small droplets distributed around the main ink droplet), and/or in preventing blockage of the inkjet nozzles of the print heads, and/or in preventing the droplets of different color inks on blanket  44  from undesirably merging into one another. In some embodiments, system  10  comprises a drying station  64 , configured to blow hot air (or another gas) onto the surface of blanket  44 . In some embodiments, drying station comprises air blowers  68  or any other suitable drying apparatus. 
     In drying station  64 , the ink image formed on blanket  44  is exposed to radiation and/or to hot air in order to dry the ink more thoroughly, evaporating most or all of the liquid carrier and leaving behind only a layer of resin and coloring agent which is heated to the point of being rendered tacky ink film. 
     In some embodiments, system  10  comprises a blanket module  70  comprising a rolling ITM, such as a blanket  44 . In some embodiments, blanket module  70  comprises one or more rollers  78 , wherein at least one of rollers  78  comprises an encoder (not shown), which is configured to record the position of blanket  44 , so as to control the position of a section of blanket  44  relative to a respective print bar  62 . In some embodiments, the encoder of roller  78  typically comprises a rotary encoder configured to produce rotary-based position signals indicative of an angular displacement of the respective roller. 
     Additionally or alternatively, blanket  44  may comprise an integrated encoder (not shown) for controlling the operation of various modules of system  10 . The integrated encoder is described in detail, for example, in U.S. Provisional Application 62/689,852, whose disclosure is incorporated herein by reference. 
     In some embodiments, blanket  44  is guided over rollers  76  and  78  and a powered tensioning roller, also referred to herein as a dancer  74 . Dancer  74  is configured to control the length of slack in blanket  44  and its movement is schematically represented by a double-sided arrow. Furthermore, any stretching of blanket  44  with aging would not affect the ink image placement performance of system  10  and would merely require the taking up of more slack by tensioning dancer  74 . 
     In some embodiments, dancer  74  may be motorized. The configuration and operation of rollers  76  and  78 , and dancer  74  are described in further detail, for example, in U.S. Patent Application Publication 2017/0008272 and in the above-mentioned PCT International Publication WO 2013/132424, whose disclosures are all incorporated herein by reference. 
     In impression station  84 , blanket  44  passes between an impression cylinder  82  and a pressure cylinder  90 , which is configured to carry a compressible blanket. 
     In some embodiments, system  10  comprises a control console  12 , which is configured to control multiple modules of system  10 , such as blanket module  70 , image forming station  60  located above blanket module  70 , and a substrate transport module  80  located below blanket module  70 . 
     In some embodiments, console  12  comprises a processor  20 , typically a general-purpose computer, with suitable front end and interface circuits for interfacing with a controller  54 , via a cable  57 , and for receiving signals therefrom. In some embodiments, controller  54 , which is schematically shown as a single device, may comprise one or more electronic modules mounted on system  10  at predefined locations. At least one of the electronic modules of controller  54  may comprise an electronic device, such as control circuitry or a processor (not shown), which is configured to control various modules and stations of system  10 . In some embodiments, processor  20  and the control circuitry may be programmed in software to carry out the functions that are used by the printing system, and store data for the software in a memory  22 . The software may be downloaded to processor  20  and to the control circuitry in electronic form, over a network, for example, or it may be provided on non-transitory tangible media, such as optical, magnetic or electronic memory media. 
     In some embodiments, console  12  comprises a display  34 , which is configured to display data and images received from processor  20 , or inputs inserted by a user (not shown) using input devices  40 . In some embodiments, console  12  may have any other suitable configuration, for example, an alternative configuration of console  12  and display  34  is described in detail in U.S. Pat. No. 9,229,664, whose disclosure is incorporated herein by reference. 
     In some embodiments, processor  20  is configured to display on display  34 , a digital image  42  comprising one or more segments (not shown) of image  42  and various types of test patterns (described in detail below) stored in memory  22 . 
     In some embodiments, blanket treatment station  52 , also referred to herein as a cooling station, is configured to treat the blanket by, for example, cooling it and/or applying a treatment fluid to the outer surface of blanket  44 , and/or cleaning the outer surface of blanket  44 . At blanket treatment station  52  the temperature of blanket  44  can be reduced to a desired value before blanket  44  enters image forming station  60 . The treatment may be carried out by passing blanket  44  over one or more rollers or blades configured for applying cooling and/or cleaning and/or treatment fluid on the outer surface of the blanket. In some embodiments, processor  20  is configured to receive, e.g., from temperature sensors (not shown), signals indicative of the surface temperature of blanket  44 , so as to monitor the temperature of blanket  44  and to control the operation of blanket treatment station  52 . Examples of such treatment stations are described, for example, in PCT International Publications WO 2013/132424 and WO 2017/208152, whose disclosures are all incorporated herein by reference. 
     Additionally or alternatively, treatment fluid may be applied by jetting, prior to the ink jetting at the image forming station. 
     In the example of  FIG.  1   , station  52  is mounted between roller  78  and roller  76 , yet, station  52  may be mounted adjacent to blanket  44  at any other suitable location between impression station  84  and image forming station  60 . 
     In the example of  FIG.  1   , impression cylinder  82  impresses the ink image onto the target flexible substrate, such as an individual sheet  50 , conveyed by substrate transport module  80  from an input stack  86  to an output stack  88  via impression cylinder  82 . 
     In some embodiments, the lower run of blanket  44  selectively interacts at impression station  84  with impression cylinder  82  to impress the image pattern onto the target flexible substrate compressed between blanket  44  and impression cylinder  82  by the action of pressure of pressure cylinder  90 . In the case of a simplex printer (i.e., printing on one side of sheet  50 ) shown in  FIG.  1   , only one impression station  84  is needed. 
     In other embodiments, module  80  may comprise two impression cylinders so as to permit duplex printing. This configuration also enables conducting single sided prints at twice the speed of printing double sided prints. In addition, mixed lots of single and double-sided prints can also be printed. In alternative embodiments, a different configuration of module  80  may be used for printing on a continuous web substrate. Detailed descriptions and various configurations of duplex printing systems and of systems for printing on continuous web substrates are provided, for example, in U.S. Pat. Nos. 9,914,316 and 9,186,884, in PCT International Publication WO 2013/132424, in U.S. Patent Application Publication 2015/0054865, and in U.S. Provisional Application 62/596,926, whose disclosures are all incorporated herein by reference. 
     As briefly described above, sheets  50  or continuous web substrate (not shown) are carried by module  80  from input stack  86  and pass through the nip (not shown) located between impression cylinder  82  and pressure cylinder  90 . Within the nip, the surface of blanket  44  carrying the ink image is pressed firmly, e.g., by compressible blanket (not shown), of pressure cylinder  90  against sheet  50  (or other suitable substrate) so that the ink image is impressed onto the surface of sheet  50  and separated neatly from the surface of blanket  44 . Subsequently, sheet  50  is transported to output stack  88 . 
     In the example of  FIG.  1   , rollers  78  are positioned at the upper run of blanket  44  and are configured to maintain blanket  44  taut when passing adjacent to image forming station  60 . Furthermore, it is particularly important to control the speed of blanket  44  below image forming station  60  so as to obtain accurate jetting and deposition of the ink droplets, thereby placement of the ink image, by forming station  60 , on the surface of blanket  44 . 
     In some embodiments, impression cylinder  82  is periodically engaged to and disengaged from blanket  44  to transfer the ink images from moving blanket  44  to the target substrate passing between blanket  44  and impression cylinder  82 . In some embodiments, system  10  is configured to apply torque to blanket  44  using the aforementioned rollers and dancers, so as to maintain the upper run taut and to substantially isolate the upper run of blanket  44  from being affected by any mechanical vibrations occurred in the lower run. 
     In some embodiments, system  10  comprises an image quality control station  55 , also referred to herein as an automatic quality management (AQM) system, which serves as a closed loop inspection system integrated in system  10 . In some embodiments, station  55  may be positioned adjacent to impression cylinder  82 , as shown in  FIG.  1   , or at any other suitable location in system  10 . 
     In some embodiments, station  55  comprises a camera (not shown), which is configured to acquire one or more digital images of the aforementioned ink image printed on sheet  50 . In some embodiments, the camera may comprise any suitable image sensor, such as a Contact Image Sensor (CIS) or a Complementary metal oxide semiconductor (CMOS) image sensor, and a scanner comprising a slit having a width of about one meter or any other suitable width. 
     In some embodiments, station  55  may comprise a spectrophotometer (not shown) configured to monitor the quality of the ink printed on sheet  50 . 
     In some embodiments, the digital images acquired by station  55  are transmitted to a processor, such as processor  20  or any other processor of station  55 , which is configured to assess the quality of the respective printed images. Based on the assessment and signals received from controller  54 , processor  20  is configured to control the operation of the modules and stations of system  10 . 
     In some embodiments, station  55  is configured to inspect the quality of the printed images and test pattern so as to monitor various attributes, such as but not limited to full image registration with sheet  50 , color-to-color registration, printed geometry, image uniformity, profile and linearity of colors, and functionality of the print nozzles. In some embodiments, processor  20  is configured to automatically detect geometrical distortions or other errors in one or more of the aforementioned attributes. For example, processor  20  is configured to compare between a design version of a given digital image and a digital image of the printed version of the given image, which is acquired by the camera. 
     In other embodiments, processor  20  may apply any suitable type image processing software, e.g., to a test pattern, for detecting distortions indicative of the aforementioned errors. In some embodiments, processor  20  is configured to analyze the detected distortion in order to apply a corrective action to the malfunctioning module, and/or to feed instructions to another module or station of system  10 , so as to compensate for the detected distortion. 
     In some embodiments, by acquiring images of the testing marks printed at the bevels of sheet  50 , station  55  is configured to measure various types of distortions, such as C2C registration, image-to-substrate registration, different width between colors referred to herein as “bar to bar width delta” or as “color to color width difference”, various types of local distortions, and front-to-back registration errors (in duplex printing). In some embodiments, processor  20  is configured to: (i) sort out, e.g., to a rejection tray (not shown), sheets  50  having a distortion above a first predefined set of thresholds, (ii) initiate corrective actions for sheets  50  having a distortion above a second, lower, predefined set of thresholds, and (iii) output sheets  50  having minor distortions, e.g., below the second set of thresholds, to output stack  88 . 
     In some embodiments, processor  20  is further configured to detect, e.g., by analyzing a pattern of the printed inspection marks, additional geometric distortion such as scaling up or down, skew, or a wave distortion formed in at least one of an axis parallel to and an axis orthogonal to the movement axis of blanket  44 . 
     In some embodiments, processor  20  is configured to analyze the signals acquired by station  55  so as to monitor the nozzles of image forming station  60 . By printing a test pattern of each color of station  60 , processor  20  is configured to identify various types of defects indicative of malfunctions in the operation of the respective nozzles. 
     For example, absence of ink in a designated location in the test pattern is indicative of a missing or blocked nozzle. A shift of a printed pattern (relative to the original design) is indicative of inaccurate positioning of a respective print bar  62  or of one or more nozzles of the respective print bar. Non-uniform thickness of a printed feature of the test pattern is indicative of width differences between respective print bars  62 , referred to above as bar to bar width delta. 
     In some embodiments, processor  20  is configured to detect, based on signals received from the spectrophotometer of station  55 , deviations in the profile and linearity of the printed colors. 
     In some embodiments, processor  20  is configured to detect, based on the signals acquired by station  55 , various types of defects: (i) in the substrate (e.g., blanket  44  and/or sheet  50 ), such as a scratch, a pin hole, and a broken edge, and (ii) printing-related defects, such as irregular color spots, satellites, and splashes. 
     In some embodiments, processor  20  is configured to detect these defects by comparing between a section of the printed and a respective reference section of the original design, also referred to herein as a master. Processor  20  is further configured to classify the defects, and, based on the classification and predefined criteria, to reject sheets  50  having defects that are not within the specified predefined criteria. 
     In some embodiments, the processor of station  55  is configured to decide whether to stop the operation of system  10 , for example, in case the defect density is above a specified threshold. The processor of station  55  is further configured to initiate a corrective action in one or more of the modules and stations of system  10 , as described above. The corrective action may be carried out on-the-fly (while system  10  continues the printing process), or offline, by stopping the printing operation and fixing the problem in a respective modules and/or station of system  10 . In other embodiments, any other processor or controller of system  10  (e.g., processor  20  or controller  54 ) is configured to start a corrective action or to stop the operation of system  10  in case the defect density is above a specified threshold. 
     Additionally or alternatively, processor  20  is configured to receive, e.g., from station  55 , signals indicative of additional types of defects and problems in the printing process of system  10 . Based on these signals processor  20  is configured to automatically estimate the level of pattern placement accuracy and additional types of defects not mentioned above. In other embodiments, any other suitable method for examining the pattern printed on sheets  50  (or on any other substrate described above), can also be used, for example, using an external (e.g., offline) inspection system, or any type of measurements jig and/or scanner. In these embodiments, based on information received from the external inspection system, processor  20  is configured to initiate any suitable corrective action and/or to stop the operation of system  10 . 
     In some embodiments, the print heads are configured to jet, during the various jetting cycles, a varying number of ink droplets of a same shade onto a same location over blanket  44 , so as to form various shades of a same color (e.g., a gray level image). The ink droplets are jetted responsively to driving pulses received from image forming station  60 , as instructed by a processor, such as processor  20 . 
     In an embodiment, upon receiving a tickling pulse at the end of a jetting cycle, from electrical circuitry (not shown) that controls each print head, the print head causes ink motion in a nozzle of the print head. Depending on the values of the pulse width and the delay between driving pulses, and depending on whether or not a driving pulse is applied in the section immediately preceding the section in which the tickling pulse is applied, the print head may or may not jet an ink droplet in response to the tickling pulse, as described below. 
     In the context of the present invention and in the claims, the term “processor” refers to any processing unit, or controller, such as processor  20  or any other processor or controller in system  10 , connected to or integrated with image forming station  60 , which is configured to, for example, read a look-up table for applying waveforms, which is stored in a memory, and instruct print heads, directly or indirectly, to inkjet accordingly. Note that the control-related instructions and other computational operations described herein may be carried out by a single processor, or shared between multiple processors of one or more respective computers. 
     The configuration of system  10  is simplified and provided purely by way of example for the sake of clarifying the present invention. The components, modules and stations described in printing system  10  hereinabove and additional components and configurations are described in detail, for example, in U.S. Pat. Nos. 9,327,496 and 9,186,884, in PCT International Publications WO 2013/132438, WO 2013/132424 and WO 2017/208152, in U.S. Patent Application Publications 2015/0118503 and 2017/0008272, whose disclosures are all incorporated herein by reference. 
     The particular configurations of system  10  is shown by way of example, in order to illustrate certain problems that are addressed by embodiments of the present invention and to demonstrate the application of these embodiments in enhancing the performance of such systems. Embodiments of the present invention, however, are by no means limited to this specific sort of example systems, and the principles described herein may similarly be applied to any other sorts of printing systems. 
     Ink Jet Printing with Joint Jetting-Tickling Waveforms 
       FIG.  2    is a schematic pictorial illustration of a print bar  62  of digital printing system  10  of  FIG.  1   , in accordance with an embodiment of the present invention. As noted above, print bar  62  comprises a strip, whose width corresponds to that of the printing area on blanket  44 , of print heads  622 , and further comprises individually controllable print nozzles  624 . Print bar  62  is part of an array of print bars which may be included in image forming station  60 , as described in  FIG.  1   . 
     As seen, each print head comprises a jetting channel  626  filled with an ink  621 . In response to a pulse, a membrane in the print head (not seen) drives a pressure wave that propagates in ink  621  along jetting channel  626 . In an embodiment, to enable a tickling pulse causing jetting an ink droplet, the timings of pulse-rise and pulse-fall of the pulses are adjusted to resonantly amplify the pressure wave inside jetting channel  626  to get maximum pressure at nozzle  624  exit. The resonance is basically a fluidic (e.g., acoustic) resonance that depends primarily on speed of sound in ink  621  and on channel length  628 . 
       FIG.  3    is a diagram showing a waveform applied to a print head  622  during a jetting cycle, in accordance with embodiments of the present invention. The waveform is applied by a controlling electrical circuitry, as commanded by the processor. In some embodiments, the waveform comprises a number of (N−2) sections for driving pulses:  625 A,  625 B . . . ,  625 (N−2), and additionally, a tickling pulse section  630 , which together can produce up to N shades of a same color (e.g., N shades of gray). 
     In some embodiments, such as with the four-shade scheme described above, there are N=2 sections for driving pulses, plus a section dedicated for a tickling pulse, for achieving a total of N=4 shades. 
     As seen in  FIG.  3   , each driving pulse section  625  comprises a driving pulse  700  having an amplitude  710  and width  720  (i.e., duration  720 ) and a delay  660  between successive drive pulses. Tickling pulse section  630  comprises a tickling pulse  770  having amplitude  780  smaller than amplitude  770 , and a same width  720  and a same delay  660  relative to the last driving pulse shown (i.e., in section  625 (N−2)). In the present example, although not necessarily, a pulse width is defined as the full width at half the maximum of the pulse amplitude. 
     The smaller amplitude  780  tickling pulse  770  (i.e., smaller than the driving pulse amplitude  710 ) results in a droplet jetted by a tickling pulse being somewhat smaller than the droplets jetted in response to the driving pulses, as described in  FIG.  5   . (provided a driving pulse was applied just before tickling pulse  770  was activated). 
     A driving pulse  700  typically causes a membrane inside print head  622  to push (i.e., jet) an ink droplet through an inkjet nozzle  624  of the print head. The delay  660  and the pulse width  720  match together a resonance frequency of a pressure wave in the ink inside a jetting channel of the print for a given ink. Using the joint printing and tickling technique with the delay and the width of driving pulses preset to match the resonance of the print head results, in case of a four-shade printing cycle, in a total duration of a printing cycle that is reduced by about a quarter, as the number of sections in a jetting cycle drops from four to three. 
     The jetting cycle waveform shown in  FIG.  3    is provided by way of example, purely for the sake of clarity. Any other suitable waveforms can be used in alternative embodiments. For example, the shapes of the pulses may differ from the illustrated trapezoid shapes. 
       FIG.  4    is a lookup table  800  of a four-shade-level printing scheme, in accordance with an embodiment of the present invention. The scheme coded in lookup table  800  comprises two driving sections (denoted “1” and “2” in the figure) followed by a tickling section (denoted “3”). Table  800  is stored in memory  22  and used by the processor during a printing session. An unchecked section in table  800  results in an idle command by the processor to image forming station  60 . A section that is checked causes the processor to instruct image forming module  60  to apply the corresponding pulse at the checked section to a given print head, as described in  FIG.  3   . 
     The vertical axis of table  800  provides the four possible shade levels, in which, using printing in black and white as an example, level 0 means no shade (white), level 1 means light gray, level 2 means dark gray, and level 3 means black. 
     When a location over blanket  44  is specified as white, the processor reads the level 0 line for printing head instructions during a jetting cycle, and correspondingly the printing head applies only a tickling pulse, which does not cause jetting of an ink droplet at the location. If the location is specified as light gray, then the processor reads the level 1 line, in which a single driving pulse is applied to jet a single droplet of ink. Typically, section one is looked up for applying a light gray. Alternatively, section two can be used for this purpose. 
     If dark gray is specified at the location over blanket  44 , then the processor reads the level 2 line, in which two successive driving pulses are applied, with the second pulse jetting a droplet of ink that overlaps the first droplet injected in section one. 
     If black is specified at the location, the processor reads the level 3 line, and applies tickling pulse  770  after applying two successive driving pulses  700 , with the tickling pulse jetting a droplet of ink as described above, which overlaps the first and second droplets ejected, each responsively, to the driving pulses. 
     The description of look up table  800  of  FIG.  4   , in terms of black and white printing, is brought by way of example. In other embodiments, lookup table  800  may be implemented in the same or similar manner for color printing. Further alternatively, the use of a look-up table is not mandatory. The processor may use any other suitable data structure or format for storing the waveform definitions for the various shades. 
     A tickling pulse will cause jetting of an ink droplet only in level 3, in which the previous pulse (i.e., section 2) is active. This is because, as noted above, the previous pulse energizes (due to being in sync with a resonance frequency of a pressure wave in the ink inside a jetting channel of print head) the ink inside the nozzle. Thus, at level 0 the tickling pulse always does not cause jetting of an ink droplet. If there is no previous pulse (as the case in level 1), a tickling pulse applied at level 1 (this embodiment not reflected by table  800 ) will only agitate the meniscus without jetting. 
       FIG.  5    is a schematic graph of the volume of a tickling droplet as a function of level 3 tickling pulse amplitude, in accordance with embodiments of the present invention.  FIG.  5    shows an approximately linear dependence of the volume of the tickling droplet as a function of the amplitude of tickling pulse  770 . Data point  100  describes a tickling pulse  770  that is practically identical to a driving pulse  700 , with a resulting droplet volume similar to that of a droplet jetted by a driving pulse, when applied as a third pulse, e.g., in level 3 of  FIG.  4   . 
     Note that applying a third pulse in full amplitude in level 0, however, will result in jetting ink, which is not intended. 
     Data point  102  describes an optimized tickling pulse  770  that causes the jetting of an exact droplet volume to achieve the level 3 shade, such as black 
     Data point  104  describes a tickling pulse  770  that causes the jetting of a droplet having a minimal volume, which results in an intermediate shade, for example, darker than dark gray and paler than black in a four-shade scheme. However, in this case the level 3 ink volume (i.e., including a resulting droplet volume from pulse amplitude of data point  104 ) is not large enough to produce the maximal shade as required. 
     Any pulse amplitude below that applied in data point  104  would only cause some motion of the ink inside the nozzle, without jetting any. In an embodiment, the pulse amplitude in data point  104  is about a third of the full amplitude of a driving pulse that is represented by data point  100 . 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.