Method and apparatus for achieving controlled RF switching ratio to maintain thermal uniformity in the acoustic focal spot of an acoustic ink printhead

Abstract: An acoustic printhead comprises a matrix of drop ejectors configured in rows and columns, a plurality of row switches that are connected to control operation of the rows of the drop ejectors, and a plurality of column switches, connected to control operation of the columns of drop ejectors, wherein by selection of an appropriate row switch and column switch, a particular transducer of a specific drop ejector is turned on. The printhead further comprises a controller connected to the plurality of row switches and column switches, and a compensation network connected to the rows and/or columns of drop ejectors.

Description: The present invention relates to acoustic printing, and more particularly to controlling the on/off switching ratio between ejectors of an acoustic printhead. The fundamentals of acoustically ejecting droplets from an ejector device such as a printhead has been widely described, and the present assignee has obtained patents on numerous concepts related to this subject matter. In acoustic printing, an array of ejectors forming a printhead is covered by a pool of liquid. Each ejector can direct a beam of sound energy against a free surface of the liquid. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. When the radiation pressure is sufficiently high, individual droplets of liquid are ejected from the pool surface to impact upon a medium, such as paper, to complete the printing process. The ejectors may be arranged in a matrix or array of rows and columns, where the rows stretch across the width of the recording medium, and the columns of ejectors are approximately perpendicular. Ideally, each ejector when activated ejects a droplet identical in size to the droplets of all the other ejectors in the array. Thus, each ejector should operate under identical conditions. In acoustic printing, the general practice is to address individual ejectors by applying a common RF pulse to a segment of a row, and to control the current flow to each ejector using column switches. In some cases it is desirable to use one column switch for several rows in parallel in order to reduce the number of column driver chips and wire bonds, and hence cost, in the system. Unfortunately, this approach results in parasitic current paths which can cause undesired RF current to flow through ejectors that are not in an ON state. In existing systems, the switching ratio is limited and will vary with the number of ejectors that are ON in a given row. A switching ratio is defined as the RF power in an OFF ejector to the RF power in an ON ejector (i.e. P OFF /P ON ). FIGURE 1 illustrates an acoustic switching array with a desired current path for a selected row and selected column for an existing system. Switching matrix 10 is a 4-row 12a, 12b, 12c , 12d by 64 column 14a, 14b, 14zz switching matrix. Rows are connected to the matrix via switching elements 16a, 16b, 16c, 16d, and columns are connected through switching elements 18a, 18b, 18zz. At the intersection of the columns and rows are transducers 20. Current paths of matrix 10 are terminated at RF ground 21. It is to be appreciated that while the matrix of FIGURE 1 is a 4-row by 64-column matrix, the present invention may be used in other matrix designs. Matrix 10 is supplied by a power source 22 which provides its output to an RF signal matching circuit 24 . By proper switch sequencing, a desired current path for a selected row and selected column is obtained. For example, in FIGURE 1, by closing switch 16a and switch 18a , a current path is provided from the RF matching network 24 to transducer 20a via row 12a and column 14a . As the remaining rows and columns are unselected, only transducer 20a is intended to be activated to emit a droplet. Unfortunately, the interconnect paths used to implement a low cost acoustic printhead include unavoidable, undesirable current paths, as shown and discussed for example, in connection with FIGURES 2-4. FIGURE 2 is a simplified depiction of an undesired current path through an unselected transducer in the same row as a selected transducer. In this example, switches 16a and 18a are maintained in a closed position while the remaining switches are unselected. Therefore current is provided to transducer 20a . However, undesired current will also flow through transducer 20b, which is in selected row 12a but unselected column 14b. Similarly, FIGURE 3 illustrates a situation where an undesired current flows through transducer 20c, which is in selected column 14a and unselected row 12c. Column switches 18a-18zz are, for example, implemented with a component such as a PIN diode, which has a reasonably high intrinsic switching ratio, i.e., in the range of -6dB or greater. A high switching ratio of this type may insure that a particular column switch is securely turned OFF if it were the only device in the system. However, a net switching ratio of a selected column and a selected row ejector (relative to other ejectors which should be OFF) can vary between approximately -2.3dB and -6dB, depending upon the number of existing parasitic current paths through ejectors which are not selected. Turning to FIGURE 4, a more detailed discussion is provided regarding the parasitic current paths introduced in connection with FIGURES 2 and 3. The transducers are identified by the row and column numbers to which they are connected. For the case illustrated, all current paths start from the conductor of row0, 12a, and terminate at RF ground return 21 . In the following example, transducer 20b is an unselected transducer. The undesired current through unselected transducer 20b consists of three components, all of which start from row0, 12a, and proceed down through transducer 20b. The first component flows from transducer 20b , down through the top segment of columnl, 14b, up through transducer 20d, through a segment of rowl, 12b, down through transducer 20e, down through column0, 14a, and finally through the selected column0 switch, 18a , to RF ground return 21 . The path of the second component is from row0, 12a, down through transducer 20b, and the top two segments of columnl, 14b, up through transducer 20f, through a segment of row2, 12c, down through transducer 20c , down through column0, 14a, and finally through column0 switch , 18a, to RF ground return 21. The path of the third component is from row0, 12a , down through transducer 20b , and the top three segments of column1, 14b, up through transducer 20g, through a segment of row3, 12d , down through transducer 20h, down through column0, 14a, and finally through column0 switch 18a, to RF ground return 21. It is to be noted that no significant current is assumed to flow through any of the open (unselected) switches in columns 1 through 63 (14b- 14zz), and rows 1, 2 or 3 (12b-12d). Unwanted current paths, similar to those just described, also exist through other unselected transducers located on row0, 12a, and columns 2 through 63 (14b-14zz). Transducers 20e, 20c , and 20h have the largest magnitude of total unwanted current. For example, the current flowing through the unselected transducer 20e is the sum of the currents in all the other transducers in rowl, 12b. All of this unwanted current flows through the conducting path of unselected rowl, 12b . In this example, transducers 20e, 20c and 20h are on a selected column and unselected rows. The switching ratio is the poorest for this category when only one column is selected. This may also be seen in FIGURES 6 and 7. In the following description it is to be noted that FIGURES 5, 6, 9, 10, 15, 16 and 18 are block diagrams representing four categories of transducer states used in calculations of relative RF currents to determine the switching ratios for different numbers of selected columns. For example, in FIGURE 5 the block in the upper left part of the figure represents all of the transducers that are at Selected Row, Selected Column locations. Similarly, the block in the upper right part of the figure represents all of the transducers that are at Selected Row, Unselected Column locations. The block in the lower left part of the figure represents all of the transducers that are at Unselected Row, Selected column locations, and the block in the lower right part of the figure represents all of the transducers that are at Unselected Row, Unselected Column locations. Turning more particularly to FIGURES 5 and 6, set forth are similar simplified depictions of switching matrix 10 . FIGURE 5 illustrates a situation where 63 columns 26 , and one row 12a are selected, ON, and a single column 28 and remaining three rows 12b-12d are unselected, or OFF. Under this arrangement, the inventors have calculated that there is approximately 514µA flowing through each of the 63 transducers 30, which represents the transducers in selected row 12a , and 63 ON columns 26 of matrix 10 . It was also determined by this analysis that 393µA of current will flow in transducer 32, located in selected row 12a and the 64th unselected column 28 of transducers. With this information, it is found that the switching ratio between these two currents is equal to: 393 µA/514µA = 0.765 = -2.32dB. FIGURE 6 depicts an alternative arrangement where one column 34 , and one row 12a are selected, and remaining 63 columns 36 and 3 rows 12b-12d are unselected. In this situation, the selected current path for transducer 38 has a current of 504 µA, whereas an unwanted current of approximately 368µA exists through each of the unselected transducers connected to selected column 34 and unselected rows 12b-12d . This results in a switching ratio equal to: 368µA/504µA = 0.730 = - 2.73dB. The cumulative current through switch 18a is approximately 1607µA (i.e. 504 µA from the selected transducer in column 34 , row 12a and 368uA from each of the three unselected transducers on column 34 , on rows 12b-12d ). FIGURE 7 summarizes the effective switching ratios relative to selected row/unselected columns, and selected columns/unselected rows as a function of the number of ejectors in the row which are ON. Particularly, curve 40 shows that as the number of selected columns increase, for an unselected row, the relative switching ratio improves substantially, i.e. approximately to -10dB at 20 columns selected. Alternatively, curve 42 illustrates that for a selected row, as additional columns are moved to an ON state, the switching ratio is degraded substantially, i.e. from about -11 dB at one column ON, to about -2.5dB for 63 columns ON. When using aqueous inks for acoustic ink printing, the desired ejection velocity will be approximately 4 m/sec. This can be achieved using approximately 1dB of power over the ejection threshold. Given that there are ejection threshold power non-uniformities in the aqueous printhead of approximately +/- 0.5dB, and the desire to maintain some margin of safety (e.g. -0.5dB) to insure that ejectors which are unselected are truly OFF, an appropriate switching ratio may be found by the restrictions of: switching ratio (SR)< (overdrive for 4 m/sec) - (non-uniformity) - (margin to insure appropriate OFF state), which results in: SR ≤ (-1-0.5 -0.5)=-2dB. Therefore, a switching ratio of -2.5 to -3.0dB will be acceptable for printing of aqueous inks, when a -.5 to -1.0dB safety margin is added. However, and more specifically related to the present invention, phase-change inks require more power over the threshold than aqueous inks. To achieve a necessary 4m/sec ejection velocity, it has been determined that a -4dB power over the threshold will be required. For phase-change inks, it is intended to use static E-fields to reduce this power requirement, however it is still necessary to eject the droplets at approximately 2m/sec, i.e. -2dB over threshold. Non-uniformities in the phase-change printhead can be larger than in aqueous printheads (i.e. +/- 1dB), and the margin for turning the switches fully OFF will also be similar (i.e. - 0.5dB). Therefore, the switching ratio for phase-change inks will require: SR < (-2 - 1 - 0.5) = -3.5dB. Then, with a .5 to 1.0dB safety margin added, a switching ratio of -4.0 to -4.5dB is acceptable. Existing switching networks do not insure adequate switching ratio for phase-change printing when the foregoing requirements are taken into consideration. A further complication which exists for phase-change printing, is that thermal uniformity requirements are more exacting than for aqueous printing because the acoustic losses in the ink are larger, and phase-change inks change more strongly with temperature than aqueous inks. As a result, a several degrees celsius change across the printhead, or between the ON and OFF states of a given ejector can result in spatial and time-varying non-uniformities which will degrade output. For example, a 1-2C° change can result in a degradation in the drop diameter uniformity of 1-3%. It is believed the upper limit on drop diameter non-uniformity that can be tolerated for acceptable print quality is only 5%. Thus even comparatively small changes in temperature will cause printing degradation. The foregoing problem is particularly acute for low flow printheads, i.e. printheads where the ink is not quickly passed through the printhead. In these situations, the acoustic energy will raise the temperature of the focal region above the bulk of the ink. In some printheads the temperature rise has been determined to be as much as 12°C. While this temperature rise can be used to an advantage (i.e. reducing the temperature requirement for the bulk volume of the ink in the printhead), it poses a problem of non-equal thermal environments for ejectors that are ON versus those that are OFF. It has, therefore, been determined desirable to provide thermal environments for droplet ejector ON and OFF states which are essentially identical, by obtaining a specific switching ratio which balances the thermal changes associated with a hot ejected ink droplet. It is also considered beneficial to provide a controlled, specific switching ratio which is independent of the number of ejectors of any array which are ON and OFF. Thus, it would be desirable to increase the switching ratio, and provide means to control the switching ratio at a desired level, independent of the number of ejectors which are ON. Also EP704304 A shows a printhead according to the preamble of claim 1. An acoustic printhead includes a matrix of drop ejectors configured in rows and columns. Each drop ejector includes at least a transducer and a switch. When a particular drop ejector is selected, the associated transducer and switch are turned on, and the transducer functions so as to cause the particular drop ejector to eject a drop from a pool of liquid. When the particular drop ejector is not selected, the associated transducer and switch are off, and the particular drop ejector does not eject a drop from the pool of liquid. A plurality of row switches, connected to control operation of the rows of drop ejectors are provided. Provided are a plurality of column switches, connected to control operation of the columns of drop ejectors, wherein by selection of an appropriate row switch and column switch, the particular transducer of a specific drop ejector is turned on. A controller connected to the plurality of row switches and the plurality of column switches acts to control selection of the drop ejectors. Further provided is a compensation network connected to at least one of the rows of drop ejectors and columns of drop ejectors, wherein the compensation network selectively provides compensation energy to drop ejectors which are not selected, to lower undesirable current flow. The compensation network may be configured to control a switching ratio of the matrix of drop ejectors, where the switching ratio is defined as the amount of power in a drop ejector which is off compared to the amount of power in a drop ejector which is on, and wherein control of the switching ratio includes improving the switching ratio of the acoustic printhead for a four-row, 64-column drop ejector array to at least -5 dB. When a drop is ejected from the printhead, it removes thermal energy from the acoustic printhead. A switching ratio is selected to balance power differences between on/off drop ejectors, against the thermal energy which is carried away by the ejected drop. FIGURE 1 illustrates an existing switching matrix for an AIP printhead; FIGURE 2 depicts the matrix of FIGURE 1 showing a concept of current leakage in a selected row, unselected column transducer situation; FIGURE 3 shows a matrix of FIGURE 1 in an unselected row, selected column transducer configuration; FIGURE 4 sets forth a detailed illustration of undesired current paths in a switching matrix; FIGURE 5 depicts the simplified model of FIGURE 1 wherein a single row is selected and 63 columns are selected; FIGURE 6 illustrates the matrix of FIGURE 1 wherein a single row and single column are selected; FIGURE 7 depicts a graphical representation of a switching ratio dependent upon the number of selected columns; FIGURE 8 shows a detailed illustration of a switching network to improve a switching ratio. FIGURE 9 provides a simplified schematic of the switching network of FIGURE 1 including a compensation scheme to increase the switching ratio; FIGURE 10 depicts the simplified schematic of FIGURE 9 in a case where 63 ejectors are in an OFF state and a single ejector is in an ON state; FIGURE 11 is a graphical representation of the switching ratio of rows and columns of the matrix network; FIGURE 12 is a graphical representation of ink temperature in a focal region over a preselected time period; FIGURE 13 is a simplified version of an acoustic ink print head ejecting an ink droplet; FIGURE 14 is a graphical representation of focal heating which expels an ink droplet; FIGURE 15 illustrates row compensation for a 4-row, 64-column network where one column is ON, 63 columns are OFF, and one row is selected and three rows are not selected; FIGURE 16 depicts a simplified version of a dual compensation network including both row and column compensation for a 4-row, 64-column network with one column ON, 63 columns OFF, and one row is selected and three rows are not selected; FIGURE 17 illustrates a 2-row, 64-column switching network in which the present invention may be implemented; and FIGURE 18 depicts a simplified version of the 2-row, 64-column network having row compensation, where 63 columns are in an OFF state and one column is in an ON state. A general practice for controlling the emitters of an acoustic ink printer array is to address the individual ejectors by applying a common RF pulse to a segment of a row, and to control the current flow to each ejector using column switches. In existing systems, it may be preferable to use one column switch for several rows in parallel in order to reduce the number of column driver chips and wire bonds, and therefore cost, in the system matrix. Unfortunately, this approach results in parasitic current paths which can limit the effective switching ratio , which can result in switching ratios that vary with the number of ejectors in an ON state in a given row. For phase change acoustic ink printing, there is a need for switching ratios better than the typical -2 to -3dB minimum that can be achieved with ganged 4-row column switches. Therefore, the present invention describes a scheme and accompanying architectures which are able to maintain a precise switching ratio, independent of the number of ejectors ON, in order to limit thermal non-uniformities in printheads. FIGURE 8 illustrates a transducer matrix 60 , to assist in the description of the concept of providing a compensating current path to the column nodes of the transducer matrix as used in acoustic printheads Providing compensation results in a smaller amount of undesirable current flow, allowing for an improvement in the switching ratio which has been defined as the ratio of the undesired RF power in an OFF ejector to the RF power in an ejector that is in an ON state (i.e. P OFF / P ON ). The 4 row by 64 column transducer array 60 depicted in FIGURE 8 is substantially similar in part to the configuration shown in FIGURES 4 -6. In the following discussion, it is assumed column0, 14a, is selected, and all other columns are not selected. In this example, the troublesome current paths are those that flow through unselected transducers 20e, 20c and 20h, where the switching ratio is -2.73 dB, as calculated in FIGURE 6. These currents originate on selected row0, 12a, and flow through unselected transducers 20b --- 20zz, to the unselected column conductors, for columns 14b through 14zz. The unwanted currents then flow from the unselected columns, through three groups of transducers to the three unselected row conductors. The path is completed through transducers 20c, 20c and 20h and the selected column0, 14a, column conductor and column switch 18a, to a ground return 21. The switching ratio for transducers 20e, 20c and 20h can be improved to be better than -5 dB by adding a compensation current path from the unselected column conductors to ground return 21. One way to accomplish this is shown in FIGURE 8. The column switches (18a-18zz) are changed from Single-Pole-Single-Throw to Single-Pole-Double-Throw. Small value compensation capacitors (e.g. 1pF) (52a- 52zz), connect the new normally closed contact on each column switch to a column compensation bus 54. The column compensation bus 54 extracts current from each of the unselected column conductors and passes it to ground return 21, through a variable pull down capacitor 56. The formed compensation current path will carry some of the current that would, in the absence of the compensation path, flow through the transducers 20e, 20c, 20h in the Selected-Column, Unselected-Row category, thereby reducing the magnitude of the unwanted current and improving the switching ratio. For this example, the value of a pull-up capacitor 58 is very small so only a negligible current will flow through it to the column compensation bus 54. The uncompensated switching ratio for the 1 column off, 63 columns selected case is -2.32 dB as shown in FIGURE 5. When the compensation path from selected row0, through pull-up capacitor 58 to the column compensation bus 54 is added (as depicted in FIGURE 8), the compensation path is able to carry some of the unwanted current that would otherwise flow through the Selected-Row, Unselected-Column transducer such that the switching ratio is improved to -5.67 dB, as shown in FIGURE 9. FIGURE 9 depicts a simplified version of a switching matrix 60 having 4 rows and 64 columns. In this example, switching matrix 60 has 63 selected columns 62, and a single selected row 12a. This circuit also depicts a single unselected column 66 and 3 unselected rows 12b-12d. A column switch 68 is in a selected position, which corresponds to the selection of the 63 columns 62. Column switch 70 is in an unselected state. With particular attention to the concepts of the present invention, a compensation selection circuit 72 is provided which includes a first capacitor 74, which in this embodiment may be a 1 pico-farad capacitor, and switching terminals 76, 78 and 80. Terminal 78 has included therein a 32 pico-farad compensation capacitor 82, and terminal 80 has a 16 pico-farad compensation capacitor 84. In this initial representation, when a compensation selector switch 86 is connected to terminal 76, a connection is made from RF source 88 through capacitor 74 to switch 70. Similar to the discussion in connection with the switching network of FIGURE 8, this arrangement depicts an arrangement to provide a compensating current path to the column nodes of a transducer matrix used in acoustic printheads. Also, in FIGURE 8 choosing a large value capacitor, as the pull-up capacitor 58, results in operational characteristics corresponding to having compensation selector switch 86, connected to switching terminal 76. By providing compensating current paths , it is possible to improve and stabilize the effective switching ratio for a number of ejectors irrespective of those which are ON or OFF. FIGURE 9 illustrates a case where 63 ejectors are ON and one OFF, in which the switching ratio relative to the Selected Row/Unselected Column category has been increased. Specifically, transducers in the selected columns have 514µA and the transducer in the unselected column has 268µA. Therefore, the switching ratio is: 268µA / 514 µA = 0.521 = - 5.67dB. It is noted that compensation capacitor 90 is provided for connection to columns 1-63. It is to be appreciated that compensation capacitor 90 represents a network of compensation capacitors such that each column has appropriate capacitive values. Further, a column compensation bus 91, similar to column compensation bus 54 of FIGURE 8, is provided between compensation capacitor 90 and compensation selector switch 86. Capacitors 74 and 90 provide current paths from the column compensation bus 91 to individual column switches, such as switch 70. To further describe the foregoing concept, illustrated in FIGURE 10 is matrix configuration 60 of FIGURE 9 which shows an alternative view of the case dealt with in connection with FIGURE 8. In this example, a single column is selected 100 and 63 columns are unselected 102. Further, a single row is selected 12a and three rows of the matrix are unselected 12b-12d. Compensation selection network 72 is shown with compensation selector 86 connected to terminal 80, which includes compensation capacitor 84 coupled to ground. In this example, the switching ratio is: 283µA / 505µA = 0.560 = - 5.04dB. Thus, whereas the switching network 10 of FIGURE 5 (which includes 63 selected columns and one selected row) has a switching ratio of -2.32dB, the addition and use of compensation selection network 72 of FIGURE 9 is able to improve this switching ratio to - 5.67dB. Similarly, whereas the switching network of FIGURE 6 (which includes one selected column and one selected row) has a switching ratio of -2.73dB, compensation selection network 72 of FIGURE 10 improves its switching ratio to -5.04dB. The foregoing discussion illustrates the addition of a compensation selection network 72 allows for an improvement in the switching ratio for ejectors of an acoustic ink printer. It may be desirable to not only maintain the switching ratio above a certain value but also within a certain range, as the number of columns which are selected increase. In this regard, attention is drawn to FIGURE 11, which shows the switching ratio characteristics of the compensation selection network 72 for a matrix 60 as shown in the forgoing figures. Curve 110 depicts a switching ratio achievable when compensation selector switch 86 is connected to terminal 80. In this configuration, switch 70 is compensated by capacitive coupling to ground, through capacitors 74 and 84. As the number of selected columns increase, the switching ratio first improves from approximately -5dB to a best value of approximately -6dB, and then begins to degrade below a -5dB switching ratio. Curve 112 shows the characteristics of operation when compensation selector 86 is connected to terminal 78, whereby the compensation current is provided from the RF switch source 88 through coupling capacitor 82 and capacitor 74. Curve 112 shows an improvement in the switching ratio from approximately 10 columns ON to approximately 20 columns ON, and thereafter an inferior ON/OFF ratio in decibels, as additional columns are selected. Lastly, curve 114 illustrates the switching ratio characteristics when compensation selector switch 86 is connected to terminal 76 such that the RF source 88 is directly connected to the column compensation bus 91. While the switching ratio under this connection scheme is good at a lower number of selected columns, as the number of selected columns increase the switching ratio degrades. Additionally, as previously mentioned, selection of a large value capacitor as pull-up capacitor 58, in FIGURE 8, results in the same operational characteristics obtained by having compensation selector switch 86 connected to switching terminal 76 of FIGURE 9. FIGURE 11 therefore, shows the switching ratios that can be achieved with different sources driving the column compensation bus and coupling capacitor 74. It may be understood by reviewing FIGURE 11 that by having a selection of compensation options (i.e. 5 pico-farads to RF, 7 pico-farads to RF, 9 pico-farads to RF, etc.), it is possible to establish and maintain a switching ratio within a desired range, independent of the ratio of ON to OFF ejectors. It is also noted that it is necessary to take into consideration the characteristics of both transducers on the same column and transducers on the same row in order to maintain the switching ratio within a selected range over the entirety of the columns of the print head. Of course, to implement this design, the compensation current is dynamically set to the proper values as image data changes. A variety of architectures which provide improved control of the switching ratio have been developed by the inventors, and will be discussed in following sections of this application. It is, however, appreciated that in a preferred implementation of a printhead having a low flow rate, such as in a phase-change acoustic printhead, there will be a significant rise in the temperature of the acoustic focal spot, for example anywhere between 5-20°C. Particularly, it is known that in acoustic printing, a focused beam is used to achieve drop ejection. This design causes local heating substantially at the point of the ink drop which is ejected. Therefore, the drop of ink which is expelled will have a substantially raised temperature. A raised temperature at this location can be used to an advantage as a means to locally reduce the viscosity at the point of ejection, decreasing the required ejection energy, and increasing the maximum firing rate. While this temperature rise can be advantageous, the exact value of the ejection temperature will depend upon the acoustic power level being supplied to a particular transducer. As a result, ejectors which are ON (i.e. at a power level P ON ), will have a different acoustic focal temperature than those which are OFF (i.e. at a power level P OFF ). Since the focal heat spot equilibrates in temperature much more slowly than the ejection rate, this can lead to uncontrolled, data-dependent changes in the acoustic ejection process, giving rise to print quality degradation. The problem is illustrated in FIGURE 12, wherein the predicted temperature rise in the acoustic focal region is plotted as a function of time. At a firing rate of 14kH, approximately 200 drops will be ejected by the time the focal region approaches thermal equilibrium at Time (T) = 0.02 seconds 117. A presumed T = 0, 118, condition is one in which no drops have been fired for a long period of time, and where the ON/OFF switching ratio is very high. The temperature rise shown in FIGURE 12, approximately 10°C, is large enough to induce severe print non-uniformities. It was previously noted that a 1-2-degree C change in temperature can result in an increase in drop diameter non-uniformity of 1-3%. To avoid print degradation, the physical energy and heat dissipation must be proximal to that of the ink drop being expelled. As shown in FIGURE 13, an acoustic printhead 120 is designed such that acoustic energy 121, generated by well known means, is focused by a lens 122, or other focusing device, to a focal point 123, which causes a pool of liquid 124 to expel a droplet 126. The focal point 123 is located in close physical proximity to the printhead surface 128. The detail of having a large amount of energy concentrated such that the heating of the printhead occurs near the ejected drop 126, means drop 126 will carry away a significant amount of heat energy. In this design, each droplet will carry away up to 50% or more of the heat at the focal point 123. A further aspect of the present invention uses the dissipation of heat energy through expulsion of heated drop 126, in combination with the controlling of the switching ratio, to provide a precisely controlled balance in the power difference against the thermal energy which is carried away by the ejected droplet. The heat of the drop is equal to the density multiplied by the volume of the drop multiplied by the specific heat, multiplied by the firing rate of an ejector. Thus, the region of concentrated heat for which there is most concern about maintaining uniform temperature between ON and OFF, is immediately adjacent to the droplet which is carried away. Using this knowledge it is possible to select a specific switching ratio which will balance the ON and OFF states of the ejectors. The heat loss occurring due to droplet ejection is to be included, when estimating a net temperature rise on the acoustic focal region. FIGURE 14 shows two modeling results, one which includes the estimated heat temperature loss factor, and one without the inclusion of this heat temperature loss factor (this case is slightly different in detail than the values shown in FIGURE 12). Modeled curve 130 shows the focal heating region is approximately 30°C when the modeled temperature ON state does not take into consideration the thermal energy loss due to the ejected drop. Modeled curve 132 shows that the focal region temperature drops to slightly below 15°C, when the loss due to the ejection of the ejected drop is taken into consideration. Thus, the ejected drop is responsible for carrying away approximately 50% of the heat when the ejector is in an ON state. Curve 134 illustrates the temperature rise in the OFF state (i.e. when no droplet is ejected) is simply the upper ON state representation 130, reduced by the amount of the switching ratio. Plotted in FIGURE 14, is the predicted OFF state temperature for a range of switching ratios -1 to -5dB. As seen by point 136 of FIGURE 14, a switching ratio can be precisely chosen, as taught in FIGURES 9-11, such that the OFF-state temperature is equal to the ON-state temperature. Thus, for an addressed row the OFF ejector should have the exact same switching ratio relative to an ON ejector so a correct amount of power is being dissipated in the entire row. The OFF ejectors are supplied with sufficient compensation current to achieve this goal. When a next row is fired, it is desirable to have the preceding row completely OFF, with as much switching ratio as possible (i.e. not -SdB's but closer to -20dB's). FIGURE 15 depicts a 4-row, 64-column transducer switching matrix 160 which implements a row compensation network 162 according to the present invention. Row compensation is similar in function to column compensation in that adjustable compensation current paths are added around transducers that are located on unselected rows and/or columns, to modify the switching ratio as desired. The following example has one column ON 164 and 63 columns OFF 166, with row 12a selected while the remaining three rows 12b - 12d are unselected. It is to be appreciated the invention will of course work with other ON/OFF ratios. Row compensation network 162 includes a plurality of switches 168, 170, 172 and 174, which may be selectively coupled to corresponding capacitive elements 176, 178, 180 and 182. By selectively controlling operation of switches 168-174, it is possible to create a reasonably smooth profile of switching ratios by using different combinations of capacitors 176-182 to compensate unselected transducer rows. Specifically, the row compensation design will add compensation current paths to the transducers on unselected rows. The paths are to the RF source or to ground return in a manner similar to that shown for column compensation in FIGURE 8. For example, in FIGURE 15 switches 172 and 174 are closed to incorporate capacitors 180 and 182 into the switching network, and switches 168 and 170 are not selected. By this arrangement, compensation is provided to the unselected rows 12b-12d to obtain a desired switching ratio. While four capacitive elements are shown for the compensation network 162, additional capacitive elements may be provided to generate a more refined control of the switching ratio. Further, although FIGURE 15 has illustrated the concepts of the present embodiment with a single ON column and 63 columns OFF, the invention is intended to work with other ON/OFF ratios as well as with a matrix having more than 64 columns. Thus, row compensation network 160 selectively inserts capacitors 176, 178, 180 and 182 into a compensation configuration with the voltage supply 88 to thereby provide a compensation current. Turning to FIGURE 16, illustrated is a transducer switching matrix 190 implementing both row compensation network 162 and column compensation network 72. Matrix 190 has a switching configuration of one column ON 192 and 63 columns OFF 194. Similar to previous examples, row 12a is selected and rows 12b - 12d are unselected. Use of such an architecture allows for compensation to both currents in the rows of transducers and in the columns of transducers. Column compensation network 72 and row compensation network 162 will operate in a manner similar to that previously described in connection with architectures illustrating individual uses of these networks. By proper selection of compensation network components, it is possible to obtain and maintain a desired switching ratio. FIGURE 17 depicts a transducer switching network 196, comprised of 2 rows and 64 columns, although the network may employ more or less than 64 columns. As previously discussed, while using one column switch for several rows in parallel, reduces the number of column driver chips and wire bonds in a system, this approach results in increasing parasitic current paths which can limit the effective switching ratio for the RF column switches. The 2-row network 195 is implemented to improve the control of parasitic current paths. Network 195 functions under the same concepts as network 10 of FIGURES 1, 2 and 3. For example, a row 196, and column 197 are selected which allows a current to flow through transducer 198. However, similar to the discussion in connection with FIGURES 2 and 3, unwanted current paths will exist. This will result in undesirable current flow through OFF transducers, such as unselected transducer 199. Switching network 200 of FIGURE 18 is a simplified illustration of switching matrix 195 shown in FIGURE 17, further including a row compensation network 202 which conceptually operates in the same manner as row compensation network 162 of FIGURE 15. A difference between compensation network 162 and compensation network 202, is found in that compensation network is required to only provide compensation for 2 rows. It is to be further understood that 2-row transducer switching matrix 195 may also be designed with a column compensation, such as column compensation network 72 of FIGURE 10, as well as with both row and column compensation networks. Using a 2-row transducer switching matrix achieves a reduction in the number of column driver chips and wire bonds compared to a system with a single row network, while also encountering less parasitic current paths than for transducer switching networks with 3 or more rows. The 2-row transducer switching matrix, which implements row and column current path compensation, is able to achieve highly reliable switching ratio control. It is to be appreciated that while capacitors have been described in the compensation networks, other devices such as diodes, transistors, etc. may be used to control compensation current paths and magnitudes. Further, the present embodiments have focused on 4 and 2-row column matrixes, however other size matrixes may also implement the present invention.

Claims: An acoustic printhead comprising: a matrix of drop ejectors configured in rows and columns, each drop ejector including at least a transducer (20) and a switch, wherein when a particular drop ejector is selected, the associated transducer and switch are turned on, and the transducer functions so as to cause the particular drop ejector to eject a drop from a pool of liquid, and when the particular drop ejector is not selected the associated transducer and switch are off, and the particular drop ejector does not eject a drop from the pool of liquid; a plurality of row switches (16a-16d), connected to control operation of the rows of drop ejectors; a plurality of column switches (18a-18zz), connected to control operation of the columns of drop ejectors, wherein by selection of an appropriate row switch and column switch, the particular transducer (20) of a specific drop ejector is turned on ; a controller connected to the plurality of row switches and the plurality of column switches, to control selection of the drop ejectors; and characterised in a compensation network connected to at least one of the rows of drop ejectors and columns of drop ejectors, wherein the compensation network selectively provides compensation energy to drop ejectors which are not selected, to lower undesirable current flow. The invention according to claim 1 wherein the compensation network is configured to control a switching ratio of the matrix of drop ejectors, the switching ratio defined as the amount of power in a drop ejector which is off compared to the amount of power in a drop ejector which is on, and wherein control of the switching ratio includes improving the switching ratio of the acoustic printhead for a 4 row, 64 column drop ejector array to at least -5dB. The invention according to claim 1 wherein each of the ejected drops remove thermal energy from the acoustic printhead upon being ejected, and a switching ratio is selected to balance power differences between on/off drop ejectors, against the thermal energy which is carried away by the ejected drop.

Inventors: BAKER LAMAR T, BUHLER STEVEN A, EL HATEM ABDUL M, ELROD SCOTT, GUNNING WILLIAM F, HADIMIOGLU BABUR B, ROY JOY, STEARNS RICHARD, BAKER, LAMAR T., BUHLER, STEVEN A., EL HATEM, ABDUL M., ELROD, SCOTT, GUNNING, WILLIAM F., HADIMIOGLU, BABUR B., ROY, JOY, STEARNS, RICHARD
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 2000
Main classification: B41J2
IPC codes: B41J2