Patent Publication Number: US-9889642-B2

Title: Managing printhead nozzle conditions

Description:
BACKGROUND 
     Inkjet printing involves the release or ejection of printing fluid drops such as ink drops onto a print medium, such as paper. The ink drops bond with the paper to produce visual representations of text, images or other graphical content on the paper. In order to accurately produce the details of the printed content, nozzles in a printhead selectively release multiple ink drops as the relative positioning between the printhead and printing medium is precisely controlled. Over a period of time and use, the nozzles of the printhead may develop defects and therefore cease to operate in a desired manner. As a result, print quality may be adversely affected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples will now be described with reference to the accompanying drawings, in which: 
         FIG. 1 a    shows a block diagram of an example printer implementing an example of a nozzle condition management system; 
         FIG. 1 b    shows additional details of an example drive bubble detect test system for determining printhead nozzle conditions; 
         FIG. 2  shows an example print nozzle depicting an example process of the formation and collapse of a drive bubble; 
         FIG. 3  shows a block diagram of an example printer suitable for implementing an example nozzle condition management system to manage testing and evaluation of printhead nozzles; 
         FIG. 4  shows an example printhead that is suitable for implementing a drive bubble detect test system; 
         FIGS. 5 and 6  show flow diagrams that illustrate example methods related to managing nozzle condition tests on a printhead. 
     
    
    
     Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. 
     DETAILED DESCRIPTION 
     Systems and methods for evaluating and managing printhead nozzle conditions through drive bubble detection are described. Inkjet printing systems print image content onto a print medium, such as paper, by directing multiple drops of printing fluid, such as ink, onto the print medium. The ink is directed through multiple nozzles positioned on a printhead of the printing system as the printhead and print medium move relative to each other. For example, the printhead may move laterally with the print medium being conveyed through a conveying mechanism. Depending on the image content to be printed, the printing system determines the exact time instance and position at which the ink drops are to be released/ejected onto the print medium. In this way, the printhead releases multiple ink drops over a predefined area to produce a representation of the image content to be printed. Besides paper, other forms of print media may also be used. 
     A printhead releases/ejects ink drops through an array of nozzles provided on the printhead. The ink ejected through each nozzle comes from a corresponding ink chamber in fluid communication with the nozzle. The ink chamber is in fluid communication with an ink supply through ink delivery pathways within the printhead that enable the replenishment of ink within the chamber after each ink ejection. Each ink chamber holds the ink and periodically releases a predetermined amount to a corresponding nozzle for printing. 
     When a printhead is not printing, ink is retained within the ink chambers by capillary forces and/or back-pressures acting on the ink within the nozzle passages. Each ink chamber includes a heating element to generate heat within the chamber that causes small volumes of ink to expand and vaporize. The vaporization of ink results in the formation of a bubble within the ink chamber. The bubble, also referred to as a drive bubble, may further expand to drive or eject ink out of the chamber and through the nozzle passage. The ejected ink forms an ink drop that impacts the print medium to form an ink dot. As the ink drop is ejected, the bubble collapses and the volume of the dispensed ink drop is subsequently replenished within the chamber from an ink supply through ink delivery pathways within the printhead. 
     Ink nozzles are subjected to many such cycles of heating, drive bubble formation and collapse, and ink volume replenishments from an ink supply. Over a period of time, and depending on other operating conditions, ink nozzles within the printhead may become blocked or otherwise defective. Nozzle blockages can occur due to a variety of factors such as particulate matter within the ink that can cause the ink nozzle to get clogged. In some cases, small volumes of ink may solidify over the course of the printer&#39;s operation resulting in the clogging of the print nozzle. As a result, the formation and release of the ink drop may be adversely affected. Since the ink drop has to be formed and released at precise instances of time, any such blockages in the print nozzle are likely to have an impact on the print quality. Accordingly, in order to ensure that print quality is maintained, the condition of the print nozzle (i.e., whether it is blocked or whether it is experiencing other issues such as a deprimed chamber), is determined. 
     Various measures can be taken to help maintain nozzles in a healthy condition, such as nozzle servicing and nozzle replacement, for example. Such measures can be performed at different times, such as before printing begins, or during printing when print nozzles come to the end of a print swath, or when the media page is changing, and so on. The condition of a print nozzle can be monitored and determined through logical circuitry that can include a sensor in the nozzle chamber. The sensor can be used to detect the presence or absence of a drive bubble. For example, an ink volume present within the chamber of a print nozzle will offer less electrical impedance to a current provided by the sensor than will a drive bubble present within the chamber. When a drive bubble is present, air within the drive bubble offers a high resistance as compared to the resistance offered by the ink volume. 
     Depending on the impedance measurements and corresponding voltage variations due to the ink within the ink chamber, a determination can be made regarding whether or not a drive bubble has formed. Determining whether or not a drive bubble has formed can provide an indication about whether the print nozzle is operating in a desired manner. Furthermore, through the nozzle sensor, it may also be determined whether or not a drive bubble has formed at any specific instance or instances of time relative to the energizing of the firing resistor or heating element (i.e., relative to a fire pulse). For example, a blockage in the print nozzle can affect the formation of the drive bubble at a specific instance of time. If a drive bubble has not formed as expected at a particular instance of time, it can be determined that the nozzle is blocked and/or not working in the intended manner. Similarly, such a sensor-based mechanism can also determine whether or not a drive bubble has collapsed at a specific instance of time. Upon collapse of the drive bubble, the ink has usually been replenished, and this condition can be detected by the nozzle sensor. If it is determined that the drive bubble has not collapsed at a predetermined or expected instance of time, it can further be determined that the nozzle has become defective in some manner. 
     The printhead can incorporate circuitry that assists in implementing the functionality of the printhead. The sensor based mechanisms as described above, may operate based on signals generated by the sensors. Such signals can be communicated off the printhead circuitry, or off-chip, or off the printhead die. However, communicating such signals off the printhead to a printer (e.g., a processor or other components of the printer) to determine the condition of the print nozzle, consumes bandwidth and can introduce timing issues that might affect the accuracy of such determinations. Furthermore, processing such signals on the printhead die by prior known methods would involve complex circuitry that uses excessive die space and adds significant cost. 
     Accordingly, example drive bubble detect (DBD) test systems and methods have been developed that implement minimal circuitry on-chip (i.e., on the printhead die) to test for and store printhead nozzle conditions by detecting the presence and absence of drive bubbles within nozzle ink chambers. Determinations about nozzle conditions are performed on-chip, which reduces the demand on bandwidth for communicating condition-related information to different components of the printer, and reduces computation overhead on the printer processing unit. The minimal circuitry can be implemented using a plurality of logic-based components that reduce system complexity. 
     An example DBD test system includes a sensor within a nozzle chamber. The sensor can be an impedance sensor to determine variations in impedance of a sensed medium that changes between ink and air within the nozzle ink chamber as drive bubbles form and collapse. The impedance depends on the current passing from the sensor to ground (e.g., a ground within the print nozzle ink chamber or other locations that are in contact with the ink) through the sensed medium, and it can be compared to a threshold to determine nozzle conditions. The nozzle chamber includes a heating element, and during a printing operation the heating element causes the print nozzle to release or fire/eject ink drops onto a print medium to print desired image content. The release of an ink drop can be based on a signal, referred to as a firing pulse, received from a print processor. A fire pulse provides an indication to the print nozzle to fire or release an ink drop onto the print medium, and it results in energy being applied to the heating element to effectuate the firing of the ink drop. Energy from a fire pulse activates the heating element to generate heat, which causes a drive bubble to form within the ink chamber. As the drive bubble expands, it forces an ink drop out of the chamber and through the ink nozzle. Once the ink drop is ejected, the drive bubble collapses and the volume of ink ejected is replenished within the chamber by an ink supply reservoir in preparation for subsequent firing. 
     As the drive bubble forms and collapses within the chamber, variations in impedance can occur, and the different impedance values can be measured through the sensor positioned within the print nozzle. The varying values of impedance can be measured at specific instances of time following the end of the firing pulse (i.e., either the rising edge or the falling edge of the firing pulse). For example, impedance values can be measured at a first predetermined time instant and at a second predetermined time instant following the end of the firing pulse. The impedance values can be compared with predefined threshold values to determine whether or not the print nozzle is functioning properly or in a healthy condition. 
     For example, the first predetermined time instant may correspond to a time after the end of the firing pulse at which a drive bubble is expected to have formed. If the impedance measured at such a first predetermined time instant is high, in correspondence with a predefined threshold, it may be concluded that ink is out of the nozzle and the drive bubble has formed in an appropriate manner. However, if impedance variations occur at the first predetermined time instant (e.g., the measured impedance value increases from low to high with respect to a threshold), it may be concluded that the print nozzle is blocked. Similarly, if the measured impedance at the first predetermined time instant varies from high to low, it may be concluded that the drive bubble formed is a weak drive bubble. In addition, if the impedance measured at such a first predetermined time instant is low, which is not in correspondence with a predefined threshold, it may be concluded that no drive bubble has formed and that there may be an issue with the heating element, ink nozzle, or nozzle chamber. 
     After an ink drop is ejected from the print nozzle, the drive bubble collapses and the volume of ink expended by the print nozzle is replenished within the ink chamber through an ink supply reservoir. As a result, the sensor is brought back into contact with ink at a second predetermined time instant following the end of the fire pulse (e.g., the falling edge of the firing pulse). Thus, at the second predetermined time instant, a measured impedance should have changed from a high value (i.e., before drive bubble collapse) to a low value (i.e., after drive bubble collapse). If the measured impedance at the second predetermined time instant is at a low value that corresponds with a predefined threshold, it may be concluded that ink is back in the nozzle and the print nozzle is functioning properly. However, if the measured impedance at the second predetermined time instant is not a low value that corresponds with a predefined threshold, it may be concluded that the print nozzle is not functioning properly. In such a case, the print nozzle may be blocked or it may have a stray bubble present. 
     Measured impedance values and impedance variations associated with the print nozzle can be converted to one or more logical output signals, for example, in the form of a binary output. The logical output signals are obtained by processing the signals associated with the impedance variations through minimal logical circuitry provided on the printhead. The logical output signals are subsequently stored, registered, or latched, onto the components of the minimal circuitry to indicate the condition of the print nozzle. For example, the logical output signals represented as a combination of 0&#39;s and 1&#39;s, can be mapped to different indicative conditions of the print nozzle. The circuitry for determining the condition of the print nozzle can be implemented on the printhead using a plurality of simple logical-based components. Thus, further processing of the logical output signals off the printhead to determine a nozzle condition is unnecessary, and the use of resources to communicate and process signals indicating print nozzle conditions may be avoided. In an example, the minimal circuitry implemented on the printhead die can register the logical output signals at the first predefined time interval and the second predefined time interval. Based on the measured impedances and resulting logical output signals, the condition of the print nozzle can be recorded or stored on the printhead. The logical output signals can be a series of 0&#39;s and 1&#39;s that indicate whether the condition of the print nozzle is healthy or not (i.e., whether the nozzle is functioning properly). In some examples, logical output signals based on impedance measurements of multiple nozzles from a set of nozzles can be combined (e.g., logically OR&#39;ed) together to determine if all nozzles in the set of nozzles are functioning properly, or conversely, to determine if at least one nozzle of the set of nozzles is not functioning properly. In some examples, the logical output signals can indicate additional information such as the time elapsed between the firing pulse and the formation of a drive bubble (i.e., ink out of the nozzle), and/or the time elapsed between the firing pulse and the collapse of the drive bubble (i.e., ink back in the nozzle). 
     Accordingly, example DBD test systems on a printhead can determine and store nozzle conditions on the printhead using minimal logical circuitry to detect the presence and absence of drive bubbles within nozzle ink chambers (e.g., through impedance measurements). Furthermore, example nozzle management systems and methods on a printer are disclosed herein that provide nozzle condition management through controlling such printhead-based DBD systems. Such printer-based management systems provide for the evaluation of, and potential response to, nozzle conditions that have been determined and stored on the printhead by a printhead-based DBD test system. In some examples, DBD testing of nozzles on the printhead is fully controlled by the printer. The printer can control when DBD tests are performed, such as before, during, and after a print job is processed. For example, the printer can control DBD tests to occur in time periods between printing print swaths, or time periods between printing media pages, or time periods before or after printing the print job. The printer can set the printhead into a DBD test mode, for example, by enabling various DBD circuitry on the printhead. The printer can send a full fire pulse group to the printhead to fire specific nozzles comprising a nozzle set (e.g., firing one nozzle from each nozzle column). The printer can wait for a short period of time as the DBD circuitry stores test results (i.e., nozzle conditions) in registers (i.e., one register per nozzle column) on the printhead, and thereafter the printer can read or retrieve the DBD results from the registers over a communication bus. In some examples, the DBD circuitry can store test results in a single, cumulative register on the printhead to indicate when at least one nozzle is not functioning properly, and the printer can read a single result from the single cumulative register to determine if at least one nozzle from a nozzle set is not functioning properly. In some examples, the printer repeats the process for some or all nozzle sets. In other examples, the printer can repeatedly execute the DBD tests on a static nozzle set. In this case, the same nozzle set can be repeatedly tested and the printer can periodically read the DBD test results from printhead registers. 
     In some examples, upon evaluating nozzle conditions (i.e., DBD test results) from a nozzle set, the printer can provide varying responses. For example, in the case where an evaluation of nozzle conditions from a DBD-tested nozzle set indicates that a nozzle or multiple nozzles are not functioning properly, the printer can respond by repeating the DBD tests on the same nozzle set. This enables the printer to monitor the condition of a nozzle that has not functioned properly and to determine if repeated firing of the nozzle during the DBD tests can resolve issues within the nozzle. For example, if a nozzle has an air bubble in its chamber that causes it to not function properly during a first nozzle firing, subsequent repeated nozzle firings can often resolve this problem through elimination of the air bubble. In other cases, where an evaluation of nozzle conditions from a DBD-tested nozzle set indicates that all the nozzles in the nozzle set are functioning properly, the printer can respond by running DBD tests on nozzles within a next nozzle set. 
     The above methods and systems are further described with reference to  FIGS. 1 through 6 . It should be noted that the description and figures merely illustrate the principles of the present subject matter. It is thus understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present subject matter. Moreover, all statements herein reciting principles, aspects, and examples of the present subject matter, are intended to encompass equivalents thereof. 
       FIG. 1 a    illustrates a block diagram of an example printer  300  implementing an example of a nozzle condition management system  302 . The management system  302  manages testing and evaluation of printhead nozzles by controlling a drive bubble detect (DBD) test system  100  on the printhead  304  that determines printhead nozzle conditions through detecting the presence and absence of drive bubbles within the nozzles. The DBD system  100  is implemented within circuitry of the printhead of printer  300 . The DBD system  100  includes a print nozzle  102  coupled to a DBD module  104 . Further details of print nozzle  102  are shown in  FIG. 2 , which depicts the formation and collapse of a drive bubble within the nozzle  102  as discussed in greater detail below. Referring to  FIGS. 1 a    and  2 , a sensor  106  is provided within the ink chamber  103  of the print nozzle  102 . The sensor  106  can be implemented, for example, as an impedance sensor or a voltage sensor. In some examples, the sensor  106  can include a metal plate, such as a metal plate made of tantalum, copper, nickel, titanium, or combinations thereof. A ground element  105  may also be located anywhere within the ink chamber  103  or ink reservoir  107 . In the example of  FIG. 2 , the ground element  105  is depicted in the ink reservoir  107 . In some examples, the ground element  105  is an etched portion of a wall with a grounded, electrically conductive material that is left exposed. In other examples, the ground element  105  may be a grounded electrical pad. In the presence of liquid ink  200 , a voltage can be applied to the impedance sensor  106  and an electrical current can pass from the sensor  106  to the ground element  105 . 
     In general, the sensor  106  measures the variations in impedances that occur due to the formation or collapse of a drive bubble within the nozzle chamber at specific instants of time. Based on the measured impedances, the DBD module  104  provides output test results as logical signals, referred to as ink_out test result  108  and ink_in test result  110 . In some examples, the DBD module  104  makes the ink_out test result  108  and ink_in test result  110  available to the printer&#39;s nozzle condition management system  302  for retrieval and evaluation. For example, the DBD module  104  can place the ink_out test result  108  and ink_in test result  110  on output pads  109  of the printhead  304 , or store the results in a memory element  111  (e.g., latch, storage register) of the DBD system  100  on the printhead  304 . In one example, the sensor  106  measures an impedance associated with the nozzle chamber  103 . More specifically, the sensor  106  measures the impedance or the voltage by passing a current from the sensor  106  to ground  105  through the ink volume  200  present within the nozzle chamber  103 . Since the ink  200  is a conducting medium, the ink provides less impedance to a current than the air in a drive bubble. Once the drive bubble is formed, the impedance offered would be high. Consequently, the voltage associated with the nozzle chamber  103  would be low and high, respectively. 
     A printing process can be initiated through a firing pulse. Upon receiving the firing pulse, a heating element  202  within the print nozzle  102  can start heating the ink, which results in the formation of a drive bubble. Prior to the formation of the drive bubble, the ink will be in contact with the sensor  106  and will provide a low impedance. Once the drive bubble has formed, however, the ink ceases to be in contact with the sensor  106  and the measured impedance will be high. 
     The DBD module  104  determines the impedance at certain time instants. The timing for measuring the impedances is managed and controlled by timing circuitry  112 . The time instants are determined after a predefined time has elapsed from the occurrence of the firing pulse. In one example, the DBD module  104  measures the impedance at time instants prescribed by a first predetermined time instant and second predetermined time instant. 
     While measuring the impedance associated with the nozzle chamber, the DBD module  104  may compare the measured impedance with respect to a threshold impedance, at the first predetermined time instant. In one example, the timing circuitry  112  may activate the DBD module  104  so that the measured impedance is captured or registered at the occurrence of the first predefined time instant. The DBD module  104  may include one or multiple memory elements  111  (e.g., latches or storage registers) for storing the result of the comparison. 
     For a properly functioning print nozzle, a drive bubble will form by the first predetermined time instant. Consequently, the measured impedance associated with the print nozzle  102  (i.e., the nozzle chamber  103 ) should be high. If the DBD module  104  determines that the impedance variation has occurred by the first predetermined time instant, it may be concluded that the drive bubble either did not form properly, or was weak (e.g., collapsed prematurely). On the other hand, if the DBD module  104  determined that the impedance measured was high and no variations in the measured impedance occurred with respect to the threshold impedance, the print nozzle  102  would be considered as healthy and functioning properly. The determination of the DBD module  104  may be represented as a test result. Since the present test result corresponds to a state where the ink flows out of the print nozzle  102  (i.e., ink is pushed out by the drive bubble), the test result may be referred to as an ink_out test result  108 . 
     The DBD module  104  can also compare the measured impedance with respect to the threshold impedance at the second predetermined time instant. In one example, the timing circuitry  112  can activate the DBD module  104  so that the measured impedance is captured or registered at the occurrence of the second predefined time instant. The DBD module  104  may include a second set of memory elements  111  (e.g., latches or registers) for storing and providing the outcome of the comparison. 
     For a properly functioning print nozzle, a drive bubble will collapse after the second predetermined time instant. Consequently, the impedance measured would vary from high to low as the ink flows in from a reservoir and replenishes the ink volume within the ink chamber  103 . If the DBD module  104  determines that the impedance variation has occurred by the second predetermined time instant, it may be concluded that the drive bubble collapsed properly, and that the ink supply within the print nozzle was replenished in a timely manner. If however, the DBD module  104  determines that the variation occurs beyond the second predetermined time instant, it may be concluded that the print nozzle  102  is not functioning properly. For example, the print nozzle  102  may be blocked or a stray drive bubble may be present within the nozzle. In such a case, the DBD module  104  provides the result of such a determination as an ink_in test result  110 . 
     In order to evaluate the condition or health of the print nozzle  102 , both the ink_out test result  108  and the ink_in test result  110  can be used. For example, when both ink_out test result  108  and the ink_in test result  110  indicate that the drive bubble has formed and collapsed in a timely manner, the print nozzle  102  will be considered as healthy or as functioning properly. In another example, as discussed below, the ink_out test result  108  and the ink_in test result  110  may be stored on the printhead  304  in a register  111  of the DBD system  100  and communicated to (e.g., retrieved by, or read by) a processing unit of the printer (e.g., the printer&#39;s nozzle condition management system  302 ) for further implementing a remedial action or other response. The ink_out test result  108  and the ink_in test result  110 , in one example, may be in a binary form. 
       FIG. 1 b    illustrates additional details of an example DBD test system  100  for determining printhead nozzle conditions. The DBD system  100  as described is implemented within circuitry of a printhead installed, for example, in a printer. The system  100  includes a print nozzle  102  coupled to a DBD module  104 . The print nozzle  102  further includes a sensor  106  provided within the print nozzle  102 . In one example, the sensor  106  is a capacitive sensor configured to measure either impedance or voltage associated with the nozzle chamber  103 . The system  100  further includes timing circuitry  112 , a clock  114 , ink_out time repository  116 , ink_in time repository  118 , threshold source  120 , a firing pulse generator  122  and an ink sensing module  124 . Each of the above mentioned modules is coupled to a DBD module  104 . Although not explicitly represented, each of the modules may be further connected to each other. The DBD module  104  provides ink_out test result  108  and ink_in test result  110  based on inputs received from the modules as illustrated. 
     The operation of system  100  can be explained in conjunction with  FIG. 2 . As noted above,  FIG. 2  provides an illustration of an example print nozzle  102  depicting an example process of the formation and collapse of a drive bubble. As shown in the  FIG. 2  example, the print nozzle  102  includes a heating element  202  and the sensor  106 . Through the action of the heating element  202 , the sensor  106  may monitor the variations in the impendence measured across the print nozzle  102  due to the formation of the drive bubble  206 . 
     Initially, the print nozzle  102  prepares for ejecting an ink drop based on a fire pulse received from the firing pulse generator  106 . Prior to receiving the firing pulse, the ink  200  is retained within the print nozzle  102  due to capillary action, with the ink level  204  contained within the print nozzle  102 . Upon receiving the firing pulse, the heating element  202  initiates heating of the ink in the chamber  103  of the print nozzle  102 . As the temperature of the ink in the proximity of the heating element  202  increases, the ink vaporizes and forms a drive bubble  206 . As the heating continues, the drive bubble  206  expands and forces the ink level  204  to extend beyond the print nozzle  102  (as shown in  FIGS. 2( a )-( c ) ). 
     As previously noted, ink within the print nozzle chamber  103  offers certain electrical impedance to electrical current. Ink is typically a good conductor of electric current, and the electrical impedance offered by the ink in the nozzle chamber  103  is therefore less than for other mediums such as air. As the print nozzle  102  prepares for ejecting an ink drop, the sensor  106  may pass a finite electrical current through the ink to a ground  105  within the nozzle chamber  103 . The electrical impedance and/or the voltage associated with the print nozzle  102  can be measured through the sensor  106 . 
     The following example is presented with respect to an impedance measurement associated with the print nozzle  102 . As a drive bubble  206  forms due to the action of the heating element  202 , the ink in the proximity of the sensor  106  can lose contact with the sensor  106 . As the drive bubble  206  forms, the sensor  106  can become completely surrounded by the drive bubble  206 . At this stage, since the sensor  106  is not in contact with the ink, the impedance, and therefore the voltage measured by the sensor  106  would be correspondingly high. The impedance measured by the sensor  106  would register a mostly constant value during the time interval the sensor  106  is not in contact with the ink. As the drive bubble  206  expands further, the physical forces arising out of the capillary action are no longer able to hold the ink level  204 , and an ink drop  208  is formed and then separates from the print nozzle  102 . The separated ink drop  208  is thus ejected toward the print medium. Once the ink drop  208  is ejected, ink in the chamber  103  of print nozzle  102  is replenished by the incoming ink flow from a reservoir  107 . At this stage the heating element  202  stops heating, and as the ink is replenished, the drive bubble  206  collapses resulting in a shrinking bubble space  210  that restores ink contact with the sensor  106 , as shown in  FIG. 2( e ) . 
     The sensor  106  can measure the variations in the impedance and/or voltage that occur during the course of drive bubble  206  formation and collapse. The impedance (and voltage) associated with the print nozzle  102  will remain low at instants when ink is present and the drive bubble  206  is not present, and will be high when the drive bubble  206  is present. During the formation and collapse of the drive bubble  206 , the impedance measured by the ink sensing module  124  will vary. In some examples, the variations in impedance associated with the print nozzle  102  are measured by the ink sensing module  124  at specific time instants. The specific time instants are measured after a predefined time has elapsed following the occurrence of a firing pulse. The specific time instants can represent time instants following a firing pulse when a drive bubble (or, conversely, ink) is expected and not expected to be present within the print nozzle  102 . 
     In one example, the specific time instants may include a first predetermined time instant and a second predetermined time instant. The first predetermined time instant may correspond to a point in time when the drive bubble  206  has formed (i.e., when ink has been or is in the process of being dispensed from the print nozzle  102 ). The first predetermined time instant can be referred to as an ink_out time. Furthermore, at the second predetermined time instant, after the drive bubble  206  fully expands and the ink drop is dispensed from the print nozzle  102 , the drive bubble  206  will collapse and allow replenishment of the ink within the chamber  103 , which will restore ink contact with the sensor  106 . Because ink flows into the print nozzle  102  at this stage, the second predetermined time instant can be referred to as the ink_in time. The ink_in time and the ink_out time can be stored within ink_out time repository  116  and ink_in time repository  118 , respectively. 
     The DBD module  104  can determine the impedance at the first and second predetermined time instants. In general, for a properly functioning print nozzle  102 , the impedance associated with the nozzle will decrease over a period of time covering the two time instants, varying from a higher impedance at the first time instant (when the drive bubble is present), to a lower impedance at the second time instant (when the drive bubble has collapsed). The impedance associated with the nozzle  102  is measured after the firing pulse has been initiated. In one example, the impedance can be measured with respect to the falling edge of the firing pulse. Thus, when the falling edge of the firing pulse occurs, the ink sensing module  124  measures the impedance associated with the print nozzle  102 . In one example, when the falling edge of the firing pulse occurs, the drive bubble  206  may have formed, or may be in the process of being formed. At this stage, the ink within the print nozzle  102  is not in contact with the sensor  106 . As a result, the measured impedance would be correspondingly high. The DBD module  104  subsequently obtains the ink_out time from the ink_out time repository  116 . As mentioned previously, the ink_out time specifies the time at which the drive bubble  206  would have formed for a properly functioning print nozzle  102 . 
     Upon obtaining the ink_out time from the ink_out time repository  116 , the DBD module  104  obtains the impedance associated with the print nozzle  102  from the ink sensing module  124 . The DBD module  104  then determines and compares the impedance associated with the print nozzle  102  at the instant prescribed by the ink_out time, with a threshold impedance. Depending on whether the impedance is high, the DBD module  104  may determine whether the print nozzle  102  is functioning properly. For example, an impedance associated the print nozzle  102  that is less than the threshold impedance would indicate that the drive bubble  206  either formed late or did not form at all, which in turn would indicate that the print nozzle  102  is blocked. The ink_out time is determined with respect to the instance when the falling edge of the firing pulse occurs. In one example, the time elapsed from the instance of the falling edge of the firing pulse, may be measured through a clocked signal provided by the clock  114 . In another example, the DBD module  104  provides an output indicating the determination for the ink_out time as ink_out test result  108 . 
     As noted, the drive bubble  206  should continue to expand until an ink drop  208  is formed and ejected from the print nozzle  102 . When the ink drop  208  is ejected, the drive bubble  206  should collapse and the ink should again come in contact with the sensor  106 . As a result, the measured impedance associated with the print nozzle  102  should also drop. The DBD module  104  determines whether the expected variation in the impedance occurs, by determining if the measured impedance is lower than the threshold impedance at a second predefined time instant. In one example, the DBD module  104  determines whether the impedance variation occurring due to the collapsing of the drive bubble  206 , occurs by the time instant prescribed by the ink_in time. The ink_in time may be obtained from the ink_in time repository  118 . 
     Based on the impedance determined at the ink_in time, the DBD module  104  determines whether the print nozzle  102  is functioning properly. For example, if the impedance associated with the print nozzle  102  does not change (i.e., remains high), it may be concluded that the drive bubble  206  has persisted within the print nozzle  102  for a longer time period. This typically occurs when an ink drop takes a longer time to form, which is often the result of a blocked nozzle. Ink drops can also take a longer time to form when a stray bubble has formed within the print nozzle  102 . 
     In another example, if the DBD module  104  determines that the impedance associated with the print nozzle  102  is less than the threshold impedance at the ink_in time, it may be concluded that the print nozzle  102  is functioning properly. In one example, the DBD module  104  provides an output indicating the determination for the ink_in time as ink_in test result  110 . In one example, both the ink_out test result  108  and the ink_in test result  110  are considered for determining whether the print nozzle  102  is functioning in the proper manner. In another example, the impedance associated with the print nozzle  102  may be determined with respect to a threshold impedance, provided by threshold source  120 . 
     In yet another example, the timing circuitry  112  may be employed for measuring impedances at the ink_out time instant and the ink_in time instant. In such a case, the timing circuitry  112  may measure the time that as elapsed from the occurrence of the firing pulse based on a clocked signal from clock  114 . Once the time as prescribed by the ink_out time has been reached, the timing circuitry  112  may activate the DBD module  104  to determine a logical output based on the impedance measured at the ink_out time instant. The logical output may be determined based on the comparison between the impedance measured and a threshold impedance. 
     The logical output may be registered within the DBD module  104  as the ink_out test result  108 . In another example, the DBD module  104  may further store the ink_out test result  108  in a memory element  111  (e.g., latch, register). Similarly, the timing circuitry  112  may also monitor the time using the clocked signal from clock  114 . As the time instant prescribed by the ink_in time occurs, the timing circuitry  112  may further activate the DBD module  104  to determine another logical output and store the same. In an example, the other logical output may be stored as the ink_in test result  110 . 
       FIG. 3  illustrates a block diagram of an example printer  300  suitable for implementing an example nozzle condition management system  302  to manage testing and evaluation of printhead nozzles by controlling a DBD test system  100  on a printhead  304 . In this example, printer  300  includes an inkjet printhead assembly  306 , a fluid reservoir assembly  308 , a mounting assembly  310 , a media advance mechanism  312 , nozzle management system  302 , and a power supply  316  that provides power to the various electrical components of printer  300 . Inkjet printhead assembly  306  includes one or multiple printheads  304 , each having at least one printhead die to eject drops of printing fluid through a plurality of nozzles  102  toward a media page  318  so as to print onto the media page  318 . A media page  318  can be, for example, a precut media sheet from a media tray or a continuous media web supplied by a media roll of media from an unwinding media advance mechanism. Typically, nozzles  102  are arranged in columns or arrays such that properly sequenced ejection of ink from nozzles  102  causes characters, symbols, and/or other graphics or images to be printed upon a media page  318  as inkjet printhead assembly  306  and the media page  318  move relative to each other. 
     In some examples, fluid reservoir assembly  308  supplies printing fluids to printhead assembly  306  and can include reservoirs to store and supply different printing fluids to a printhead  304 . For example, reservoir assembly  308  can include reservoirs to supply different colored inks (e.g., cyan, magenta, yellow, and black) for different ink slots on a printhead  304 . In some examples, inkjet printhead assembly  306  and all or part of a fluid reservoir assembly  308  can be housed together in a print cartridge or pen. In some examples, individual reservoirs within reservoir assembly  308  can be removed, replaced, and/or refilled. 
     Mounting assembly  310  positions inkjet printhead assembly  306  with printhead  304  relative to media advance mechanism  312 , and media advance mechanism  312  positions media page  318  relative to inkjet printhead assembly  306 . Thus, a print zone  320  is defined adjacent to nozzles  102  in an area between inkjet printhead assembly  306  and media page  318 . In one example, printer  300  is a scanning type printer. In a scanning type inkjet printer  300 , mounting assembly  310  comprises a carriage that conveys inkjet printhead assembly  306  back and forth across the width of a print media page  318  in a manner indicated by direction arrows  322  and  324 . Thus, inkjet printhead assembly  306  moves in a generally horizontal manner that is orthogonal to the media advance direction  326 . 
     Nozzle management system  302  of printer  300  generally includes a processor (CPU)  328 , a memory  330 , firmware, and other printer electronics for communicating with and controlling inkjet printhead assembly  306 , mounting assembly  310 , and media advance mechanism  312 . In some examples, nozzle management system  302  may also include an ASIC  332  (application specific integrated circuit) and/or additional hardware components  334  to perform certain operations of the printer  300  alone or in combination with a processor  328  executing program instructions as discussed below. Thus, hardware components  334  can include physical components such as programmable logic arrays (PLAs), programmable logic controllers (PLCs), other logic and electronic circuits, and/or combinations of such physical components with programming executable by a processor. 
     Memory  330  can include both volatile (i.e., RAM) and nonvolatile (e.g., ROM, hard disk, optical disc, CD-ROM, magnetic tape, flash memory, etc.) memory components. The memory components of a memory  330  comprise non-transitory machine-readable (e.g., computer/processor-readable) media that provide for the storage of machine-readable coded program instructions, data structures, program instruction modules, and other data for printer  300 , such as module  334 . The program instructions, data structures, and modules stored in memory  330  may be part of an installation package that can be executed by processor  328  to implement various examples, such as examples discussed herein. Thus, memory  330  may be a portable medium such as a CD, DVD, or flash drive, or a memory maintained by a server from which the installation package can be downloaded and installed. In another example, the program instructions, data structures, and modules stored in memory  330  may be part of an application or applications already installed, in which case memory  330  may include integrated memory such as a hard drive. As noted, components of memory  330  comprise a non-transitory medium that does not include a propagating signal. 
     Nozzle management system  302  can receive print data  336  from a host system, such as a computer, and store the data  336  in memory  330 . Typically, data  336  comprises RIP (raster image processor) data that is in an appropriate image file format (e.g., a bitmap) suitable for printing by printer  300 . Data  336  represents, for example, a document or image file to be printed. As such, data  336  forms a print job for printer  300  that includes print job commands and/or command parameters. Using data  336 , nozzle management system  302  controls inkjet printhead assembly  306  to eject imaging fluid drops from nozzles  102  to form characters, symbols, and/or other graphics or images on media page  318 . 
     Referring now to  FIG. 4 , an example printhead  304  suitable for implementing a DBD test system  100  is illustrated. As shown in  FIG. 4 , printhead  304  includes four fluid ink slots  400 , with each slot  400  supplying fluid ink to two nozzle columns  402  on either side of the slot  400 . Therefore, in this example, printhead  304  includes eight nozzle columns  402 , and each slot  400  can provide a different ink color (e.g., cyan, magenta, yellow, black) to two of the eight nozzle columns that are adjacent to either side of the slot  400 . In some examples, a printhead  304  can have a greater or lesser number of slots and corresponding nozzle columns. Each nozzle column  402  represents a group of nozzles from which a single nozzle can be included within a nozzle set  404  for DBD testing. This is because a common result bus  405  is coupled to each nozzle in a column, which limits each nozzle set  404  to having no more than one nozzle from each nozzle column  402  for DBD testing at one time. Accordingly, there can be as many nozzle sets  404  as there are nozzles in a nozzle column  402 . In some examples, a nozzle set  404  may not include a nozzle from each nozzle column  402 . Thus, a nozzle set  404  may include one nozzle from every nozzle column  402 , or it may include one nozzle from fewer than every nozzle column  402 . In some examples, a nozzle set  404  may include one nozzle from a single nozzle column  402 . For each nozzle column  402 , there is a corresponding DBD test system  100  on printhead  304  to perform DBD tests on nozzles from within that column  402 . Each DBD test system  100  can include a memory element  111  ( FIG. 1 a   ) such as a latch or storage register to store nozzle conditions, for example, in the form of logical output (i.e., binary output) converted from impedance and/or voltage values measured during a DBD test. The logical output stored in the register of a DBD test system  100  indicates the condition of a tested print nozzle. A communication bus  406  couples each DBD test system  100  to the nozzle management system  302  on printer  300  to enable the transfer of instructions, nozzle condition information, and other data between the printhead  304  and printer  300 . In some examples, printhead  304  can include a single, cumulative register  408  on the printhead  304 . A cumulative register  408  can store a nozzle condition that indicates at least one nozzle in a nozzle set  404  is not functioning properly. Thus, any nozzle from a nozzle set  404  determined to not function properly by its associated DBD test system  100  will cause the DBD system  100  to load the cumulative register  408  with logical output indicating the tested nozzle set  404  has a non-functioning nozzle. 
     Referring now primarily to  FIGS. 3 and 4 , the nozzle management system  302  on printer  300  includes a nozzle condition management module  334  stored in memory  330 . Management module  334  comprises program instructions executable on processor  328  to cause printer  300  to retrieve, evaluate, and respond to nozzle conditions determined and stored in registers on printhead  304  by DBD test systems  100 . The DBD testing of nozzles  102  is controlled by the execution of instructions in module  334  on a printer processor  328  that cause the printer  300  to send instructions to DBD test systems  100  on printhead  304  over communication bus  406 . Instructions sent to a DBD test system  100  include, for example, instructions to enter a DBD test mode (e.g., enable DBD circuitry on the printhead), instructions to fire print nozzles in a specified nozzle set  404 , and instructions to store DBD test results (i.e., nozzle condition results) in registers on the printhead  304 . Module  334  executing on printer processor  328  further causes the printer  300  to wait for a short period of time after nozzles are fired to allow the DBD circuitry to store DBD test results (i.e., nozzle conditions) in registers (i.e., one register per nozzle column) on the printhead, and to thereafter read or retrieve the DBD results from the registers over a communication bus  406 . Further executing instructions from module  334  on printer processor  328  can cause the printer  300  to evaluate the retrieved nozzle conditions and to provide varying responses, such as by adjusting future DBD tests. For example, if a nozzle condition indicates a nozzle is not functioning properly, the printer can respond by repeating the DBD tests on the same nozzle set  404  to continue an evaluation of the nozzle and to perhaps resolve an issue within the nozzle through the repeated firing. In other examples where an evaluation of the nozzle conditions from a tested nozzle set  404  indicate the nozzles are functioning properly, the printer can respond by instructing DBD test systems  100  to test different nozzle sets  404 . 
       FIGS. 5 and 6  show flow diagrams that illustrate example methods  500  and  600 , related to managing nozzle condition tests on a printhead. Methods  500  and  600  are associated with the examples discussed above with regard to  FIGS. 1-4 , and details of the operations shown in methods  500  and  600  can be found in the related discussion of such examples. The operations of methods  500  and  600  may be embodied as programming instructions stored on a non-transitory computer/processor-readable medium, such as memory  330  of a printer  330  as shown in  FIG. 3 . In some examples, implementing the operations of methods  500  and  600  can be achieved by a processor, such as processor  328  of  FIG. 3 , reading and executing the programming instructions stored in memory  330 . In some examples, implementing the operations of methods  500  and  600  can be achieved using an ASIC  332  and/or other hardware components  334  alone or in combination with programming instructions executable by a processor. 
     Methods  500  and  600  may include more than one implementation, and different implementations of methods  500  and  600  may not employ every operation presented in the respective flow diagrams. Therefore, while the operations of methods  500  and  600  are presented in a particular order within the flow diagrams, the order of their presentation is not intended to be a limitation as to the order in which the operations may actually be implemented, or as to whether all of the operations may be implemented. For example, one implementation of method  500  might be achieved through the performance of a number of initial operations, without performing one or more subsequent operations, while another implementation of method  500  might be achieved through the performance of all of the operations. 
     Referring now to the flow diagram of  FIG. 5 , an example method  500  begins at block  502  with a printer instructing a printhead to perform impedance measurements on a plurality of nozzles in a first set of print nozzles. In some examples, the first set of nozzles comprises one nozzle from each of a plurality of nozzle groups, such as nozzle columns. As shown at blocks  504 - 510 , instructing a printhead to perform impedance measurements can include enabling DBD circuitry on the printhead ( 504 ), firing each nozzle of the plurality of nozzles ( 506 ), measuring impedance in each nozzle after the firing to determine a nozzle condition ( 508 ), and storing each nozzle condition in a result register on the printhead ( 510 ). In some examples, storing each nozzle condition can included storing each nozzle condition in a separate result register on the printhead, or storing the nozzle condition of any non-functioning nozzle in a cumulative result register to indicate that at least one nozzle in the nozzle set is not functioning properly. 
     The method  500  can continue at block  512  with retrieving from the printhead, an impedance measurement result corresponding with each nozzle, where each impedance measurement result indicates a nozzle condition of its corresponding nozzle. In some examples, retrieving an impedance measurement result comprises retrieving each impedance measurement result from a distinct printhead register on the printhead that corresponds with a nozzle column to which the nozzle belongs. In some examples, retrieving an impedance measurement result comprises retrieving a combined impedance measurement result from a single cumulative register on the printhead, where the combined impedance measurement result indicates a nozzle condition of at least one nozzle within the first set of nozzles. 
     As shown at block  514  of method  500 , each test result can be used to determine whether a corresponding nozzle is functioning properly. When each nozzle in the first set of nozzles is functioning properly, the printhead can be instructed to perform impedance measurements on a second set of nozzles, as shown at block  516 . As shown at block  518 , when a nozzle in the first set of nozzles is not functioning properly, the printhead can be instructed to perform impedance measurements on the first set of nozzles again. 
     Referring now to the flow diagram of  FIG. 6 , an example method  600  of managing nozzle conditions on a printhead begins at block  602  with a printer providing a print job to a printhead for printing. As shown at block  604 , the printhead can be instructed by the printer to perform DBD tests on a plurality of nozzles at a certain stage during printing of the print job. In different examples, the certain stage can be a printing stage selected from the group consisting of a time period between printing print swaths, a time period between printing media pages, a time period before beginning to print the print job, and a time period following printing of the print job. In some examples, the certain stage can also be during real time printing of print data where DBD testing is performed on a nozzle fired while printing actual print data. Furthermore, in some examples, performing DBD tests on a plurality of nozzles comprises performing DBD tests on a set of nozzles in which each nozzle in the set of nozzles belongs to a distinct column of nozzles. The method  600  can continue as shown at block  606  with, for each nozzle, retrieving a test result from a register on the printhead associated with a nozzle group to which the nozzle belongs. As shown at block  608 , the printhead can also be instructed to perform DBD tests on the same plurality of nozzles over again when the test results indicate that at least one of the nozzles is not functioning properly. The printhead can further be instructed to perform DBD tests on a next plurality of nozzles when the test results indicate the nozzles are functioning properly.