WO2019150085A1

WO2019150085A1 - Droplet ejection apparatus

Info

Publication number: WO2019150085A1
Application number: PCT/GB2019/050229
Authority: WO
Inventors: Mujahid-ul ISLAM
Original assignee: Xaar Technology Limited
Priority date: 2018-01-31
Filing date: 2019-01-28
Publication date: 2019-08-08
Also published as: GB2570668A; GB201801611D0

Abstract

A droplet ejection apparatus, comprising: a droplet ejection head, comprising: a plurality of fluid chambers; a plurality of nozzles, each fluid chamber communicating with at least one nozzle; a plurality of piezoelectric actuating elements, each of which is adjacent one of said fluid chambers and comprises a first and a second electrode in electrical contact with a piezoelectric member, such that drive voltage waveforms can be applied by said first and second electrodes to said piezoelectric member, so as to cause the ejection of fluid from the adjacent one of said fluid chambers, through the at least one communicating nozzle, as one or more fluid droplets; common drive waveform generating circuitry, which is configured to generate a common drive voltage waveform, which has a period T_CDW, and a baseline at a first voltage value, and which comprises a plurality of pulses during each of which the voltage decreases, the common drive waveform generating circuitry being electrically connected to the first electrodes of said actuating elements, so as to apply said common drive voltage waveform thereto; switching circuitry, comprising an array of switches, each switch being electrically connected to the second electrode of a respective one of said piezoelectric actuating elements and being configured so that, when turned on, the switch in question connects the corresponding second electrode to a common reference voltage; and a switching controller, configured to receive a portion of firing data, to select a group of switches in accordance with said portion of firing data, and to turn on each switch within said selected groupof switches for an on period T_ON during of the period T_CDW of the common drive waveform; wherein the switching controller is further configured such that substantially every on period T_ON: has a start time that is concurrent with one of said plurality of pulses of the common drive waveform; and/or has an end time that is concurrent with a different one of said plurality of pulses of the common drive waveform.

Description

DROPLET EJECTION APPARATUS

FIELD OF THE INVENTION

The present invention relates to apparatus for droplet ejection, as well as to circuitry and assemblies therefor. It may find particularly beneficial application in a printer including a printhead, such as an inkjet printhead, and circuitry therefor.

BACKGROUND TO THE INVENTION

Droplet ejection heads are now in widespread usage, whether in more traditional applications, such as inkjet printing, or in 3D printing, or other materials deposition or rapid prototyping techniques. Accordingly, the fluids may have novel chemical properties to adhere to new substrates and increase the functionality of the deposited material.

Recently, inkjet printheads have been developed that are capable of depositing ink directly onto ceramic tiles, with high reliability and throughput. This allows the patterns on the tiles to be customized to a customer’s exact specifications, as well as reducing the need for a full range of tiles to be kept in stock.

In other applications, inkjet printheads have been developed that are capable of depositing ink directly on to textiles. As with ceramics applications, this may allow the patterns on the textiles to be customized to a customer’s exact specifications, as well as reducing the need for a full range of printed textiles to be kept in stock. In still other applications, droplet ejection heads may be used to form elements such as colour filters in LCD or OLED elements displays used in flat-screen television manufacturing.

So as to be suitable for new and/or increasingly challenging deposition applications, droplet ejection heads continue to evolve and specialise. However, while a great many developments have been made, there remains room for improvements in the field of droplet ejection heads. SUMMARY OF THE INVENTION

Aspects of the invention are set out in the appended independent claims, while particular embodiments of the invention are set out in the appended dependent claims.

The following disclosure, in a first aspect, relates to a droplet ejection apparatus, comprising: a droplet ejection head; common drive waveform generating circuitry; switching circuitry; and a switching controller.

The droplet ejection head comprises: a plurality of fluid chambers; a plurality of nozzles, each fluid chamber communicating with at least one nozzle; a plurality of piezoelectric actuating elements, each of which is adjacent one of said fluid chambers and comprises a first and a second electrode in electrical contact with a piezoelectric member, such that drive voltage waveforms can be applied by said first and second electrodes to said piezoelectric member, so as to cause the ejection of fluid from the adjacent one of said fluid chambers, through the at least one communicating nozzle, as one or more fluid droplets.

The common drive waveform generating circuitry is configured to generate a common drive voltage waveform, which has a period T_CDW, and a baseline at a first voltage value, and which comprises a plurality of pulses during each of which the voltage decreases. The common drive waveform generating circuitry is electrically connected to the first electrodes of said actuating elements, so as to apply said common drive voltage waveform thereto.

The switching circuitry comprises an array of switches, each switch being electrically connected to the second electrode of a respective one of said piezoelectric actuating elements and being configured so that, when turned on, the switch in question connects the corresponding second electrode to a common reference voltage.

The switching controller is configured to receive a portion of firing data, to select a group of switches in accordance with said portion of firing data, and to turn on each switch within said selected group of switches for an on period TON during of the period T_CDW of the common drive waveform, each on period T_ON having a start time and an end time. The switching controller is further configured such that, generally, each on period T_ON: has a start time that is concurrent with one of said plurality of pulses of the common drive waveform; and/or has an end time that is concurrent with a different one of said plurality of pulses of the common drive waveform.

To meet the material needs of diverse applications, a wide variety of alternative fluids may be deposited by droplet ejection heads as described herein. For instance, a droplet ejection head may eject droplets of ink that may travel to a sheet of paper or card, or to other receiving media, such as textile or foil or shaped articles (e.g. cans, bottles etc.), to form an image, as is the case in inkjet printing applications, where the droplet ejection head may be an inkjet printhead or, more particularly, a drop-on-demand inkjet printhead. Alternatively, droplets of fluid may be used to build structures, for example electrically active fluids may be deposited onto receiving media such as a circuit board so as to enable prototyping of electrical devices.

In another example, polymer containing fluids or molten polymer may be deposited in successive layers so as to produce a prototype model of an object (as in 3D printing). In still other applications, droplet ejection heads might be adapted to deposit droplets of solution containing biological or chemical material onto a receiving medium such as a microarray.

Droplet ejection heads suitable for such alternative fluids may be generally similar in construction to printheads, with some adaptations made to handle the specific fluid in question.

Droplet ejection heads as described in the following disclosure may be drop-on-demand droplet ejection heads. In such heads, the pattern of droplets ejected varies in dependence upon the input data provided to the head.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now directed to the drawings, in which:

Figure 1A is a schematic diagram that illustrates a droplet ejection apparatus according to an illustrative embodiment;

Figure 1 B is a diagram illustrating a common drive waveform that is generated by the common drive waveform generating circuitry in the apparatus shown in Figure 1A; Figure 2 is a diagram illustrating various voltage waveforms present within the droplet ejection apparatus of Figures 1A and 1 B, as a result of the switching controller of the apparatus being suitably configured;

Figure 3 is a diagram illustrating various voltage waveforms in a droplet ejection apparatus in a situation where a switch is turned on for consecutive periods;

Figure 4 is a diagram illustrating various voltage waveforms in a droplet ejection apparatus according to a contrasting example, having a differently configured switching controller;

Figure 5 is a diagram illustrating various voltage waveforms in a droplet ejection apparatus according to a further contrasting example, having a differently configured switching controller;

Figure 6 is a diagram illustrating various voltage waveforms in a droplet ejection apparatus according to a further illustrative embodiment that allows fine adjustment of drive waveforms to be carried out; and

Figure 7 is a schematic diagram that illustrates a droplet ejection apparatus according to a further illustrative embodiment, where each switch of the switching circuitry includes a respective transistor.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is directed to Figures 1 -2, which show a droplet ejection apparatus according to a first illustrative embodiment.

Attention is firstly directed to Figure 1A, which is a block diagram that illustrates schematically the droplet ejection apparatus 1. As is shown, the droplet ejection apparatus 1 includes a droplet ejection head 100. While only one droplet ejection head 100 is shown, it should be understood that the apparatus 1 may include multiple such droplet ejection heads, for example fixed to a support in a desired arrangement.

The (or each) droplet ejection head 100 includes a number of fluid chambers and nozzles, with each fluid chamber being in fluid communication with one of the nozzles. In some embodiments, each fluid chamber will be associated with a respective nozzle, but as will be appreciated from the generality of this disclosure, this is by no means essential and thus each fluid chamber could be associated with two or potentially more nozzles. As is apparent from Figure 1A, the head 100 further includes a number of piezoelectric actuating elements 130a-130n, each of which is adjacent to one of the fluid chambers and includes a first and a second electrode 131 , 132 in electrical contact with a piezoelectric member 135. For instance, piezoelectric actuating element 130a includes first electrode 131 a, second electrode 132a, and piezoelectric member 135a. As may be appreciated, drive voltage waveforms may be applied using the first and second electrodes 131 , 132 of each actuating element 130 to the corresponding piezoelectric member 135, so as to cause the ejection of fluid from the adjacent one of the fluid chambers, through the communicating nozzle, as one or more fluid droplets.

As may be seen from Figure 1A, the droplet ejection apparatus 1 further comprises common drive waveform generating circuitry 200, which is configured to generate a common drive voltage waveform 210. Accordingly, the common drive waveform generating circuitry may, for example, include one or more amplifiers 201 , as illustrated in Figure 1A. As is apparent from Figure 1A, the common drive waveform generating circuitry 200 is electrically connected to the first electrode 131 a-131 n of each piezoelectric actuating element 130a-130n, enabling it to apply the common drive voltage waveform 210 thereto.

An example of a suitable common drive waveform 210 is illustrated in Figure 1 B. As may be seen, the common drive waveform 210 has a baseline 21 1 at a first voltage value (indicated as V_HIGH) and includes a number of pulses 215a-215c during each of which the voltage decreases. Though the example common drive waveform 210 shown in Figure 1 B includes three pulses 215a-215c, other common drive waveforms 210 may include any suitable number of pulses, such as two, four, five, six etc. Furthermore, while the pulses 215a-215c of the example common drive waveform 210 shown in Figure 1 B are simple trapezoidal pulses, other examples of common drive waveforms 210 may include pulses having more complex shapes.

As illustrated in Figure 1A, the common drive waveform generating circuitry 200 may be provided on a module (e.g. an integrated circuit) located remotely of the ejection head 100 so that heat generated by the circuitry 200 is not transferred to the ejection head 100. Moreover, where the apparatus includes multiple droplet ejection heads 100, such a module might be electrically connected to two or more heads 100 so as to supply common drive waveforms 210 thereto. As may be seen from Figure 1A, the droplet ejection apparatus 1 further includes switching circuitry 310. As is apparent from Figure 1A, the switching circuitry 310 includes an array of switches 31 1 a-311 n. As shown, each such switch 31 1 a-311 n is electrically connected to the second electrode 132a-132n of a respective one of the piezoelectric actuating elements 130a-130n. As is also apparent from Figure 1A, each switch 311 a-311 n is configured so that, when turned on (i.e. closed), it connects the corresponding second electrode 132a-132n to a common reference voltage V_REF· AS may be seen from Figure 1 B, this common reference voltage V_REF is lower than or equal to the voltage of the pulses 215a-215c of the common drive waveform 210 (and also lower than the voltage V_HIGH of the baseline 211 of the common drive waveform 210).

By contrast, each switch 311 a-31 1 n may configured such that, when turned off (i.e. opened), it isolates the corresponding second electrode 132a-132n.

For the sake of simplicity, the common reference voltage V_REF in the illustrative embodiment of Figures 1 and 2 is a constant voltage (for instance, it could be at ground, e.g. signal ground, chassis ground etc.). However, in other examples it could instead be a (time-varying) voltage waveform. Such a voltage waveform may nonetheless have a voltage lower than or equal to the pulses 215a-215c of the common drive waveform 210, at least during the portions of the common reference voltage waveform that are concurrent with the pulses of the common drive waveform.

Returning to Figure 1A, the droplet ejection apparatus 1 further includes a switching controller 350, for controlling the array of switches 311 a-311 n of the switching circuitry 310. More particularly, the switching controller 350 controls the on/off state of the switches 31 1 a-311 n based on a portion of firing data 355 that it has received. The switching controller 350 selects a group of the switches 31 1 a-31 1 n based on the portion of firing data 355 and causes each of the selected switches to be turned on for an on period T_ON- The on period T_ON for each selected switch lasts for at least one complete pulse 215a-215c within the common drive waveform 210, as is illustrated in Figure 1 B.

As a result of the selected switches being turned on, drive waveforms are applied to the actuating elements 135a-135n that correspond to the selected switches by the associated first and second electrodes 131 , 132. This leads to the ejection of fluid droplets from the corresponding fluid chambers of the head 100. In other words, a group of nozzles is fired that corresponds to the portion of firing data 355. A further selection of a group of switches may be carried out subsequently, with this further selection being based on a further portion of firing data 355. This process may be repeated multiple times, for example so as to deposit successive rows of droplets on a deposition medium, with each row of droplets corresponding to a respective portion of firing data 355. In this way, a pattern of droplets may be progressively deposited on the deposition medium.

In the particular example embodiment of a droplet ejection apparatus 1 shown in Figures 1 and 2, the switching circuitry 310 and the switching controller 350 are provided on an ejection control module 300. This may, for example, comprise one or more integrated circuit (e.g. an application-specific integrated circuit, ASIC, a field programmable gate array, FPGA, or a system on chip, SoC). As illustrated in Figure 1A, such an ejection control module 300 may form a part of the droplet ejection head 100, for example being mounted within, or upon a side surface of, the head 100. However, though this may enable the head 100, switching circuitry 310, and switching controller 350 to be installed in a straightforward manner, it is by no means essential and such an ejection control module 300 could be provided separately from the head 100.

Attention is directed next to Figure 2. With the aid of this drawing, the timing of the on period T_ON relative to the common drive waveform 210 and the resulting voltage waveforms present within the droplet ejection apparatus 1 may be better understood.

In more detail, Figure 2 further shows the voltage waveform 221 present at the first electrode 131 of an actuating element 130 that corresponds to a given one of the selected switches, as well as the voltage waveform 222 present at the second electrode 132 of the same actuating element 130. Also shown is the resulting drive waveform 220 applied between the first and second electrodes 131 , 132 to the actuating element 130.

Figure 2 further illustrates the timing of the beginning and end of the on period T_ON relative to the common drive waveform. As is apparent, in contrast to what might be expected, neither the beginning nor the end of the on period T_ON is concurrent with the baseline 21 1 of the common drive waveform 210. Rather, the beginning and end of the on period T_ON are concurrent with respective, different pulses within the common drive waveform 210. In the particular example shown, the on period T_ON begins during the first pulse 215a of the common drive waveform 210 and ends during the third and last of the pulses 215c (it being understood that these specific pulses are by no means essential). As is also apparent from Figure 2, the voltage waveform 221 present on the first electrode 131 of the actuating element 130 is the same as the common drive waveform 210.

Attention is directed next to the voltage waveform 222 present on the second electrode 132. As is shown in Figure 2, during an initial period when the switch 31 1 is turned off, the voltage on the second electrode 132 generally follows the voltage on the first electrode 131. During this period, the actuating element 130 is essentially acting as a capacitor that cannot be discharged: because the switch 31 1 is turned off, there is no route for charge to leave the second electrode 132. As the amount of charge stored by the actuating element 130 remains constant, the voltage on the second electrode 132 follows the voltage on the first electrode 131.

However, once the switch 31 1 is turned on (at the beginning of the on period TON) the second electrode 132 is connected to the common reference voltage V_REF· The common reference voltage V_REF is therefore applied to the second electrode 132 for the whole of the on period T_ON- AS noted above, in the particular example illustrated in Figures 1 and 2, the common reference voltage V_REF is a constant voltage; thus, the voltage on the second electrode 132 remains constant for the whole of the on period TON-

At the end of the on period TON, the switch 311 is turned off and the second electrode 132 is isolated and, therefore, disconnected from the common reference voltage V_REF. As during the initial off period, the actuating element 130 again acts essentially as a capacitor that cannot be discharged and thus the voltage on the second electrode 132 follows the voltage on the first electrode 131.

Attention is now directed to the resulting drive waveform 220 that is applied by the first and second electrodes 131 , 132 to the piezoelectric member 135. As may be seen from Figure 2, this waveform 220 includes two rising edges 2201 , 2203 and two falling edges 2202, 2204. Each edge will cause a change in the volume of the fluid chamber associated with the actuating element 130.

In some constructions of the droplet ejection head 100, the actuating elements 130 may be configured such that an increase in the voltage between the first and second electrodes 131 , 132 (i.e. a rising edge in Figure 2) causes an decrease in the volume of the associated fluid chamber, whereas a decrease in the voltage between the first and second electrodes 131 , 132 (i.e. a falling edge in Figure 2) causes an increase in the volume of the associated fluid chamber. In other words, the voltage applied between the first and second electrodes 131 , 132 is inversely related to the resulting change in the volume of the chamber.

With such a droplet ejection head 100, the drive waveform 220 illustrated in Figure 2 would cause an initial decrease in the volume of the chamber, as a result of rising edge 2201 , then an increase in the volume of the chamber, as a result of falling edge 2202, then a further decrease, as a result of the second rising edge 2203, then, finally, an increase in the volume of the chamber, as a result of the second falling edge 2204.

Depending on the particular circumstances, such as the magnitude of the change in voltage, the viscosity of the fluid within the chambers, the configuration (e.g. size and shape) of the nozzles and fluid chambers, a rising edge 2201 , 2203 - and the corresponding decrease in the volume of the chamber - may tend to cause the ejection of a droplet of fluid from an associated nozzle. By contrast, a falling edge - and the corresponding increase in the volume of the chamber - may tend to cause fluid to be drawn into the chamber from a fluid supply.

In some cases, in order to achieve ejection of a droplet of fluid, an expansion of the chamber (an increase in its volume), which draws fluid into the chamber, may need to immediately precede a contraction of the chamber (a decrease in its volume). In such a case (and with a head configured such that the voltage applied between the first and second electrodes 131 , 132 is inversely related to the resulting change in the volume of the chamber), ejection may not occur on the first rising edge 2201 shown in Figure 2. Rather, ejection would occur instead on the second rising edge 2203, since this is preceded by falling edge 2202.

In such a case (or otherwise), the switching controller 350 may be configured such that the beginning and end of each on period T_ON are determined so that each drive waveform 220 includes at least one instance of: a falling edge 2202, 2204 with an immediately preceding rising edge 2201 , 2203.

Where the switching controller 350 is so-configured, there may be restrictions on how closely spaced in time consecutive drive waveforms 220 may be. This is illustrated with reference to Figure 3, which shows a situation where the same switch 31 1 is part of consecutively selected groups (e.g. corresponding to successive portions of firing data). In the example illustrated in the drawing, the switch is therefore turned on for two successive on periods TON(M), TON(M+ 1 ) (though the same issues may equally result were there a short off period - less than the length of a common drive waveform pulse 215).

Prior to the first on period TON(M) , the switch is turned off, as indicated in Figure 3. As a result, the drive waveform 220(M) corresponding to the first on period TON(M) (and the first portion of firing data) is substantially the same as that shown in Figure 2 and described above, having a first rising edge 2201 , then a first falling edge 2202, then a second rising edge 2203, then a second falling edge 2204. Moreover, the effect of the first drive waveform 220(M) on droplet ejection may be assumed to be similar, for example with a droplet being ejected on the second rising edge 2203 (since this is preceded by falling edge 2202), but not on first rising edge 2201. As may be appreciated from Figure 3, the drive waveform 220(M+1 ) corresponding to the second on period TON(M+ 1 ) (and the second portion of firing data) appears generally similar to the preceding drive waveform 220(M).

However, the effect of the second drive waveform 220(M+1) on droplet ejection may differ from that of the preceding drive waveform 220(M). This is because, in the example shown in Figure 3, the first rising edge 2205 of the second drive waveform 220(M+1 ) is immediately preceded by the second falling edge 2204 of the previous drive waveform 220(M). Thus, a droplet may be ejected on the first rising edge 2205 of the second waveform 220(M+1 ), in contrast to the first rising edge 2201 of the first waveform 220(M).

Such issues may, for example, be addressed by suitable pre-processing of the firing data, for instance so as to distribute/diffuse any pattern errors that arise to nearby nozzles, or to earlier or later portions of firing data (e.g. with a dithering algorithm).

Alternatively (or in addition), such issues may be addressed by starting the second on period T_ON (M+1 ) part-way through the rising edge in the common drive waveform pulse 215c, thus reducing the amount of energy that is applied to the fluid in the chamber by the actuating element 130.

Notwithstanding the above discussion, it will be understood that, whether a contraction with an immediately preceding expansion (e.g. a rising edge 2201 , 2203 with an immediately preceding falling edge 2202, 2204) is necessary in order to achieve ejection will depend on multiple factors, such as the magnitude of the change in volume of the chamber, the viscosity of the fluid within the chambers, the configuration (e.g. size and shape) of the nozzles and fluid chambers. Hence, this is not a general restriction.

It should further be understood that, in other constructions for the droplet ejection head 100, the actuating elements 130 may be configured such that an increase in the voltage between the first and second electrodes 131 , 132 (i.e. a rising edge 2201 , 2203 in Figure 2) causes an increase in the volume of the associated fluid chamber, whereas a decrease in the voltage between the first and second electrodes 131 , 132 (i.e. a falling edge 2202, 2204 in Figure 2) causes an decrease in the volume of the associated fluid chamber. In other words, the voltage applied between the first and second electrodes 131 , 132 is directly related to the resulting change in the volume of the chamber.

It will be understood that, with such a droplet ejection head 100, the drive waveform 220 illustrated in Figure 2 would cause an initial increase in the volume of the chamber, as a result of rising edge 2201 , then an decrease in the volume of the chamber, as a result of falling edge 2202, then a further increase, as a result of the second rising edge 2203, then, finally, an decrease in the volume of the chamber, as a result of the second falling edge 2204.

With such a droplet ejection head 100 it may likewise be required that an expansion of the chamber immediately precedes a contraction of the chamber, in order to reliably achieve ejection of a droplet of fluid. In such a case, both falling edge 2202 and falling edge 2204 may lead to ejection of separate droplets of fluid (which may merge in flight, or on the receiving medium), or falling edge 2202 may cause a droplet to begin to form, with falling edge 2204 adding additional fluid (e.g. of a similar volume) and causing the breakoff of the droplet from the nozzle.

Attention is now directed to Figures 4 and 5, which illustrate contrasting examples that demonstrate the significance of the timing of the beginning and end of the on period TON-

T urning first to Figure 4, shown is a contrasting example where the end of the on period TON is concurrent with the baseline 21 1 of the common drive waveform 210. In the particular example shown, the end of the on period T_ON is prior to the third pulse 215c of the common drive waveform 210. In a similar manner to Figures 2 and 3, Figure 4 shows the voltage waveform 221 present at the first electrode 131 of an actuating element 130 corresponding to a selected switch, as well as the voltage waveform 222 present at the second electrode 132 of the same actuating element 130. Also shown is the resulting drive waveform 220 applied between the first and second electrodes 131 , 132.

As is apparent from Figure 4, the voltage waveform 221 present on the first electrode

131 of the actuating element 130 is, as before, the common drive waveform 210.

Attention is directed next to the voltage waveform 222 present on the second electrode 132. As is shown in Figure 4, during an initial period when the switch 31 1 is turned off, the voltage on the second electrode 132 generally follows the voltage on the first electrode 131 , as indicated by arrow 2221.

As before, once the switch 31 1 is turned on (at the beginning of the on period TON) the second electrode 132 is connected to the common reference voltage V_REF, with the common reference voltage V_REF therefore being applied to the second electrode 132 for the whole of the on period TON-

At the end of the on period TON, the switch 31 1 is turned off and the second electrode

132 is isolated and, therefore, disconnected from the common reference voltage V_REF. As during the initial off period, the actuating element 130 again acts essentially as a capacitor that cannot be discharged and thus the voltage on the second electrode 132 follows the voltage on the first electrode 131.

However, in contrast to the situation illustrated in Figure 2, as the voltage present on the second electrode 132 is already at the common reference voltage V_REF, when the next pulse 215c of the common drive waveform 210 arrives, the voltage on the second electrode 132 is driven to a value below the common reference voltage V_REF, in the portion of the waveform 222 indicated by arrow 2222.

Providing switching circuitry 310 that is capable of tolerating voltages lower than the common reference voltage V_REF is complex, for example requiring the provision of an additional, offsetting voltage signal. Moreover, where the switching circuitry 310 has not been so-configured, when voltages more than, for example 1 2V (or perhaps 0.6V) lower than the common reference voltage V_REF are applied, parasitic diodes within the switching circuitry 310 may be switched on, causing the switching circuitry 310 to fail. As will be apparent, such issues are avoided in the example illustrated in Figure 2, as the end of the on period T_ON is concurrent with one of the pulses of the common drive waveform 210. In particular embodiments, the switching controller 350 may be configured such that each on period T_ON ends at a time when the voltage of the common drive waveform is approximately equal to the common reference voltage V_REF, for example within 1.2V, and preferably within 0.6V, of the common reference voltage V_REF.

For similar reasons, in some embodiments, such as that illustrated in Figure 2, the switching controller 350 may be configured such that the on period T_ON ends at a time when the voltage of the common drive waveform is at or near (e.g. within 1.2V of, or more specifically within 0.6V of) the minimum voltage of the corresponding one of the common drive waveform pulses 215.

Turning next to Figure 5, shown is a further contrasting example where the beginning of the on period T_ON is concurrent with the baseline 21 1 of the common drive waveform 210. In the particular example shown, the beginning of the on period T_ON is prior to the first pulse 215a of the common drive waveform 210. As with the example of Figure 2, the end of the on period T_ON is concurrent with the third pulse 215c of the common drive waveform 210.

In a similar manner to Figures 2, 3 and 4, Figure 5 shows the voltage waveforms present at the first electrode 131 (waveform 221 ), and the second electrode 132 (waveform 222) of an actuating element 130 corresponding to a selected switch. Also shown is the resulting drive waveform 220 applied between the first and second electrodes 131 , 132.

As before, the voltage waveform 221 present on the first electrode 131 of the actuating element 130 is the common drive waveform 210.

Turning to the voltage waveform 222 present on the second electrode 132, once the switch 31 1 is turned on (at the beginning of the on period TON) the second electrode 132 is connected to the common reference voltage V_REF. In contrast to the situation depicted in Figures 2 and 4, at the point when the switch 31 1 is turned on (the beginning of the on period TON) and the second electrode 132 is connected to the common reference voltage V_REF, the second electrode 132 has the same voltage as that of the baseline 211 of the common drive waveform 210, indicated as V_HIGH in Figure 5. Thus, the connection of the second electrode 132 to the common reference voltage V_REF causes the actuating element 130 to discharge, corresponding to falling edge 2223 of waveform 222. As the amplitude of each of the pulses 215a-215c of the common drive waveform 210 may be in the region of 25-50V, and because the heat generated by such discharging scales with the square of the change in velocity, such discharging may generate significant heat within the switching circuitry 310.

As will be apparent, such effects are reduced in the example illustrated in Figure 2, as the beginning of the on period T_ON is concurrent with one of the pulses of the common drive waveform 210. In particular embodiments, the switching controller 350 may configured such that each on period T_ON begins at a time when the voltage of the common drive waveform is approximately equal to the common reference voltage V_REF. For example, it may be within 8V, and in some cases within 5V, of the common reference voltage V_REF, SO as to maintain heat generation within levels that may be managed straightforwardly.

For similar reasons, in some embodiments, such as that illustrated in Figure 2, the switching controller 350 may be configured such that the period T_ON begins at a time when the voltage of the common drive waveform is at or near (e.g. within 8V of, or more specifically within 5V of) the minimum voltage of the corresponding one of the common drive waveform pulses 215.

In some embodiments, the same on period T_ON is used for substantially all of the selected switches. In other embodiments, different switches will have different on periods T_ON- For instance, the switching controller 350 may determine a beginning and an end of the on period T_ON for each selected switch. This may allow an individually adjusted drive waveform to be applied to the actuating element 130 corresponding to each selected switch.

Such individual adjustment might, for example, be based on adjustment data relating to the characteristics of the droplets ejected by each actuating element 130, such as the volume/or the velocity of such droplets. For example, such adjustment data could ensure that substantially all actuating elements 130 eject droplets having similar characteristics.

Such individual adjustment might, in addition, or instead, be based on the firing data 355 received by the switching controller 350. For example, the switching controller 350 might be configured to determine, for each switch 31 1 within the selected group of switches, a group of the plurality of pulses 215a-215c of the common drive waveform 210, in accordance with the firing data 355, and a corresponding on period T_ON for the switch 31 1 in question, which begins prior to and ends after the thus-determined group of pulses. This may, for instance, lead to each actuating element 130 ejecting a droplet having a volume that corresponds to a portion of firing data 355 that the switching controller 350.

As noted above, an on period T_ON may begin or end at a point in time when the voltage of the common drive waveform is only approximately equal to the minimum voltage of the corresponding one of the common drive waveform pulses 215 and/or approximately equal to the common reference voltage V_REF· In some embodiments, such flexibility may allow fine adjustment of drive waveforms 220 to be carried out.

Such an embodiment is illustrated in Figure 6, which shows, for a particular selected switch 31 1 , a part of the voltage waveform present on the corresponding first electrode 131 (indicated as 221 ), and on the corresponding second electrode 132 (indicated as 222), as well as the corresponding part of the drive waveform 220 applied to the associated actuating element 130.

It may be noted that only one common drive waveform pulse 215’ is shown in Figure 6. However, this is merely for clarity and it is envisaged that the common drive waveform 210 may include a number of pulses, as before; these are simply not visible at the level of magnification of the drawing. The greater level of magnification is used to show clearly the more complex shape of the pulse 215’: as is apparent, the pulse 215’ does not have the same simple, trapezoidal shape as the common drive waveform pulses 215; rather, it has a ledge or step 2151 in its trailing/rising edge.

As is apparent from Figure 6, the switch 31 1 is initially turned on, and then is turned off for a short period of time during the pulse 215, before being turned on again. Thus, the switch has two on periods T₀N(A) and T_ON(B).

At least one of these periods may have timings as discussed further above, having a start time concurrent with one common drive waveform pulse 215 and an end time concurrent with a different common drive waveform pulse 215. Furthermore, it is envisaged that both of these periods may correspond to the same portion of firing data. Accordingly, the waveform applied across the associated actuating element 130 may be considered a single drive waveform 220, despite being concurrent with both on periods TQN(A) and TON(B). In any case, as is apparent from Figure 6, the first on period T₀N(A) ends at a point when the voltage of the common drive waveform is substantially equal to the minimum voltage of the common drive waveform pulse 215’ and to the common reference voltage V_REF. However, the second on period T₀N(B) starts at a point when the voltage of the common drive waveform is a relatively small amount greater than the minimum voltage of the common drive waveform pulse 215’ and the common reference voltage V_REF.

As is apparent from the falling edge 2225 of the waveform 222 on the second electrode 132, this leads to a small amount of charge being discharged from the second electrode 132, and therefore a small amount of heat being produced within the switching circuitry 310.

As is illustrated by dotted lines 2225’ and 2225” in Figure 6, if the start time of the second on period T₀N(B) is altered, the timing of this falling edge may be varied. The dotted lines 2225’ and 2225” show two possible timings for the falling edge. As is apparent from the respective positions for the corresponding rising edges 2206, 2206’, 2206” in the drive waveform, this variation in timing of such an edge, may enable fine adjustments to the shape of the drive voltage waveform 220 to be made. This may, for instance, allow an individually adjusted drive waveform to be applied to the actuating element 130 corresponding to each selected switch, as mentioned above.

Attention is now directed to Figure 7. It should be noted that, though Figure 1A shows the switches 31 1 a-31 1 n schematically, in some embodiments, each of the switches 31 1 a-311 n of the switching circuitry 310 may include one or more transistors. Such an embodiment is shown in Figure 7, which is a schematic diagram of a droplet ejection apparatus according to a further illustrative embodiment. As is apparent, the droplet ejection apparatus shown in Figure 7 is generally the same as that illustrated in Figure 1 , with the exception of the configuration of the switching circuitry 310’.

Specifically, the switching circuitry 310’ in the illustrative embodiment of Figure 7, is configured such that each switch 31 1 a’-311 n’ includes a single transistor. More particularly, each switch 31 1 a’-311 n’ includes a transistor whose drain terminal is electrically connected to the second electrode 132a-132n of the corresponding one of the actuating elements 130a-130n. As is apparent from Figure 7, the source terminal of each transistor is connected to the common reference voltage V_REF. As before, the switching controller 350 controls the on/off state of the switches 31 1 a’- 311 n\ based on a portion of firing data 355 that it has received. To this end, the switching controller 350 is connected to the gate terminal of each transistor.

While in the particular illustrative embodiment shown in Figure 7, each switch 31 1 a’- 31 1 n’ includes a single transistor, it should be understood that, in other embodiments, each switch 31 1 a’-311 n’ might include multiple transistors, for example connected in parallel or in series. Where each switch 31 1 a’-311 n’ includes multiple transistors connected in parallel, the switching controller 350 could, for instance, be configured so as to turn on a selected number of the group of transistors of each switch 31 1 a’-311 n’. In this way, each switch 311 a-311 n may be provided with a controllable ON resistance.

The transistors may, in some embodiments, be MOSFETs or LDMOS transistors. LDMOS transistors may be employed, as they are well-suited to handling voltages of the same order of magnitude as suitable pulse amplitudes for the common drive waveform 210, for example in the range of 25V-50V, e.g. around 40V. In addition, or instead, the transistors may be n-channel transistors (e.g. nLDMOS transistors), which may in some cases be more compact than equivalent p-channel transistors.

Referring again to Figure 7, in the particular illustrative embodiment shown, each switch 31 1 a-311 n includes an n-channel transistor whose drain terminal is electrically connected to the second electrode 132a-132n of the corresponding actuating element 130a-130n. As is apparent from Figure 7, the source terminal of each transistor is connected to the common reference voltage V_REF·

As noted above, p-channel transistors could instead be used, in which case, the connections of the drain and source terminals for each p-channel transistor may, for example, be opposite to those shown for each n-channel transistor in Figure 7.

Returning to Figure 7, as with the illustrative embodiment of Figures 1 and 2, the switching controller 350 controls the on/off state of the switches 31 1 a-311 n based on a portion of firing data 355 that it has received. To this end, the switching controller 350 is connected to the gate terminal of each transistor.

In the particular illustrative embodiment shown in Figure 7, each switch 31 1 a-31 1 n includes a single transistor. However, in other embodiments, each switch 31 1 a-31 1 n might include multiple transistors, for example connected in parallel or in series. Where each switch 31 1 a-311 n includes multiple transistors connected in parallel, the switching controller 350 could, for instance, be configured so as to turn on a selected number of the group of transistors of each switch 311 a-311 n. In this way, each switch 31 1 a-311 n may be provided with a controllable ON resistance.

The switching controller 350 may, in some embodiments, control each transistor with a relatively low voltage (e.g. around 5-10V), particularly where the common reference voltage V_REF is a relatively low voltage, for example because, in order to turn on the transistor, the voltage applied to the gate need only differ by a few volts (e.g. 3-4V) from the voltage at the source. Such a low voltage switching controller 350 may generate only a small amount of heat, and/or may be relatively simple in construction, and/or may be relatively compact.

From the generality of the foregoing description, it will be understood that the apparatus, and circuitry disclosed herein may utilise a wide range of droplet ejection heads. Solely by way of example, heads as disclosed in the Applicant’s earlier patent publications WOOO/38928, W02007/1 13554, WO2016/001679, WO2016/156792, WO2016/193749, WO2017/1 18843, and WO2017/149330 might be utilised. In several of the droplet ejection heads taught in these documents, the head includes a membrane, which bounds the fluid chambers. The application of drive voltage waveforms to the electrodes of the actuating elements causes the deformation of this membrane, thereby leading to ejection of one or more fluid droplets through the nozzles of the fluid chambers. However, the principles of this disclosure may be employed in heads having a different actuation mechanism, such as where a piezoelectric wall separating neighbouring chambers within an array is deformed, thereby leading to ejection of droplets from one or both of the neighbouring chambers.

To provide a droplet ejection apparatus as described herein, an assembly may be provided that includes a switching controller and switching circuitry as described herein. These may be configured to be electrically connectable to a suitable droplet ejection head. Such an assembly may therefore enable an existing droplet ejection head to form a part of a droplet ejection apparatus that operates according to the principles described herein. In such an assembly, the switching controller and switching circuitry may, for example, be provided on an ejection control module, as described above. Furthermore, the common drive waveform generating circuitry may, for example, be provided on a common drive waveform generating module, as also described above. More generally, though the foregoing description has presented a number of examples, it should be understood that other examples and variations are contemplated within the scope of the appended claims.

It should be noted that the foregoing description is intended to provide a number of non- limiting examples that assist the skilled reader’s understanding of the present invention and that demonstrate how the present invention may be implemented.

Claims

1. A droplet ejection apparatus, comprising:

a droplet ejection head, comprising:

a plurality of fluid chambers;

a plurality of nozzles, each fluid chamber communicating with at least one nozzle;

a plurality of piezoelectric actuating elements, each of which is adjacent one of said fluid chambers and comprises a first and a second electrode in electrical contact with a piezoelectric member, such that drive voltage waveforms can be applied by said first and second electrodes to said piezoelectric member, so as to cause the ejection of fluid from the adjacent one of said fluid chambers, through the at least one communicating nozzle, as one or more fluid droplets; common drive waveform generating circuitry, which is configured to generate a common drive voltage waveform, which has a period T_CDW, and a baseline at a first voltage value, and which comprises a plurality of pulses during each of which the voltage decreases, the common drive waveform generating circuitry being electrically connected to the first electrodes of said actuating elements, so as to apply said common drive voltage waveform thereto;

switching circuitry, comprising an array of switches, each switch being electrically connected to the second electrode of a respective one of said piezoelectric actuating elements and being configured so that, when turned on, the switch in question connects the corresponding second electrode to a common reference voltage; and

a switching controller, configured to receive a portion of firing data, to select a group of switches in accordance with said portion of firing data, and to turn on each switch within said selected group of switches for an on period T_ON during of the period TCDW of the common drive waveform;

wherein the switching controller is further configured such that substantially every on period T_ON:

has a start time that is concurrent with one of said plurality of pulses of the common drive waveform; and/or

has an end time that is concurrent with a different one of said plurality of pulses of the common drive waveform.

2. The droplet ejection apparatus of Claim 1 , wherein each of the plurality of pulses in the common drive waveform has a minimum voltage, the switching controller being configured such that, at the start time of each on period TON, the voltage of the common drive waveform is at or near the minimum voltage of the corresponding one said plurality of pulses, and preferably within 8V, more preferably within 5V, of said minimum voltage.

3. The droplet ejection apparatus of Claim 1 or Claim 2, wherein the switching controller is configured such that, at the start time of each on period TON, the voltage of the common drive waveform is approximately equal to the common reference voltage, and is preferably within 8V, and more preferably within 5V, of the common reference voltage.

4. The droplet ejection apparatus of any one of claims 1 to 3, wherein each of the plurality of pulses in the common drive waveform has a minimum voltage, the switching controller being configured such that, at the end time of each on period TON, the voltage of the common drive waveform is at or near the minimum voltage of the corresponding one said plurality of pulses, and is preferably within 1.2V, more preferably within 0.6V of said minimum voltage.

5. The droplet ejection apparatus of any preceding claim, wherein the switching controller is configured such that, at the end time of each on period TON, the voltage of the common drive waveform is approximately equal to the common reference voltage, and is preferably within 1.2V, and more preferably within 0.6V, of the common reference voltage.

6. The droplet ejection apparatus according to any preceding claim, wherein the switching controller is additionally configured to:

receive a further portion of firing data; and to

select a group of switches in accordance with said further portion of firing data, and to turn on each switch within said selected group of switches for an on period T_ON during of the period T_CDW of the common drive waveform;

wherein the on periods corresponding to the further portion of firing data do not overlap with any of the on periods corresponding to the initial portion of firing data.

7. The droplet ejection apparatus of any preceding claim, wherein the switching controller is further configured: to determine, for each switch within said selected group of switches, a group of the plurality of pulses of the common drive waveform, in accordance with said firing data, and a corresponding on period T_ON for the switch in question, which begins prior to and ends after the thus-determined group of pulses.

8. The droplet ejection apparatus of any preceding claim, wherein said actuating elements are configured such that, for each actuating element, the voltage applied between the first and second electrode is inversely related to the volume of the adjacent one of said fluid chambers.

9. The droplet ejection apparatus of any preceding claim, wherein the head further comprises at least one membrane, which bounds the fluid chambers, the application of said drive voltage waveforms to the electrodes of the actuating elements causing the deformation of said at least one membrane and thereby the ejection of one or more fluid droplets from the fluid chambers through said nozzles.

10. The droplet ejection apparatus of any preceding claim, wherein each switch comprises one or more transistors.

1 1. The droplet ejection apparatus of Claim 9, wherein each switch comprises a single transistor.

12. The droplet ejection apparatus of Claim 10, wherein each switch comprises a respective group of transistors connected in parallel, preferably wherein the switching controller is further configured to determine a number of transistors to be turned on within each group, and to therefore control the on resistance for each group of transistors.

13. The droplet ejection apparatus of any one of claims 10 to 12, wherein each of said one or more transistors is a MOSFET transistor, or an LDMOS transistor.

14. The droplet ejection apparatus of any one of claims 10 to 13, wherein each of said one or more transistors is an n-channel transistor.

15. The droplet ejection apparatus according to any preceding claim, wherein said common reference voltage is substantially constant.

16. The droplet ejection apparatus according to any preceding claim, wherein said common reference voltage is at ground.

17. The droplet ejection apparatus according to any preceding claim, wherein, for substantially the whole of each of said plurality of pulses, the common drive waveform has a higher voltage than the common reference voltage.

18. The droplet ejection apparatus according to any preceding claim, wherein said switching controller and said switching circuitry are provided on an ejection control module, the ejection head comprising said ejection control module.

19. The droplet ejection apparatus according to Claim 18, wherein said ejection control module is an integrated circuit, preferably an ASIC.

20. The droplet ejection apparatus according to any preceding claim, wherein said common drive waveform generating circuitry is provided on a module located remotely of the ejection head so that heat generated is not transferred to the ejection head.

21. An ejection control module, comprising a switching controller and switching circuitry as defined in any one of claims 1 to 17, and being electrically connectable to a droplet ejection head as defined in any one of claims 1 to 17, so as to control ejection of droplets therefrom.

22. The ejection control module according to Claim 21 , wherein the ejection control module is an integrated circuit, preferably an ASIC.

23. An assembly for a droplet ejection apparatus, the assembly comprising a switching controller, switching circuitry, and common drive waveform generating circuitry, as defined in any one of claims 1 to 17.

24. The assembly of Claim 23, wherein the switching controller, and switching circuitry are provided on an ejection control module, and said common drive waveform generating circuitry is provided on which a common drive waveform generating module; and

wherein each of said ejection control module and said common drive waveform generating module are separately electrically connectable to a droplet ejection head as defined in any one of claims 1 to 17, so as to be operable to cause ejection of droplets therefrom.