The present disclosure relates to a controller, fluid supply system and apparatus and method for printing, which may be particularly suitable for applications where there are multiple droplet ejection heads arranged at different vertical positions relative to each other. Such applications may include printing onto large surfaces, such as walls and vehicles.

BACKGROUND

Droplet ejection heads are now in widespread usage, whether in more traditional applications, such as inkjet printing, or in newer applications such as 3D printing. Droplet ejection heads have been developed that are capable of use in industrial applications, for example for printing directly onto substrates such as ceramic tiles or textiles. Such industrial printing techniques using droplet ejection heads (such as piezoelectric inkjet printheads) allow for short production runs, customization of products and even printing of bespoke designs. Droplet ejection heads continue to evolve and specialise so as to be suitable for new and/or increasingly challenging applications.

In recent years, there has been increasing interest in printing onto more complex and/or large shapes, such as three-dimensional objects, or surfaces such as walls, or onto objects such as vehicles, either to provide an overall covering, or to decorate and/or customise the surface with images and/or text and/or texture. Printing onto complex and/or large shapes and surfaces using droplet ejection heads is of interest due to the ability to print onto the surface in a targeted and controlled manner, without release of large numbers of small droplets into the atmosphere, as with conventional systems such as spray painting. This may be beneficial for environmental reasons, and also allow cost savings on ventilation systems and the like. Further, because the ink deposition is much better controlled, the requirement to mask surfaces to prevent unwanted deposition can be eliminated, saving on a time-consuming and hence expensive stage of the process.

Using a printing technique may also reduce the ink/ fluid volume requirements, and therefore the size (and possibly cost) of the system required, as the associated fluid reservoirs and supply systems may be smaller. Further, printing techniques may allow the use of multiple colours or fluid types at the same time, and the printing of complex print jobs in a limited number of passes. Printing onto large/ complex shapes and surfaces using droplet ejection techniques may, for example, require the use of industrial robots such as multi-axis machines or a gantry system or robotic arms. Multiple droplet ejection heads may be required in such arrangements, which will often be located at different height positions relative to each other. Other applications requiring a multi -head configuration whereby the droplet ejection heads are not at equal heights include Direct-to-Shape (DTS) applications (such as printing a swath exceeding 1 droplet ejection head width onto a bottle or other object), or coding and marking and packaging (for example printing labels or barcodes or other information or images on one or more of the sides and/or the top of a box simultaneously).

In most applications, some form of fluid supply system is required to deliver fluid to the droplet ejection heads. The objective of the fluid supply system may be limited to replenishing the fluid ejected by the droplet ejection head. More complex systems may require a controlled fluid flow rate through the droplet ejection head, because the fluid flow is used to, e.g., control the fluid temperature, or cool the droplet ejection head.

To ensure reliable performance of the droplet ejection head, it is desirable to maintain the fluid meniscus within the nozzles of the droplet ejection head so as to prevent fluid weeping onto a nozzle plate; in order to do this, the pressure inside the nozzle(s) of the droplet ejection head is kept below atmospheric pressure (e.g., at a negative pressure). This pressure is commonly referred to as back pressure, nozzle pressure or meniscus pressure. It is also desirable to prevent air being ingested into the droplet ejection head, which occurs when the back pressure is too low (e.g., too negative a pressure), such that the meniscus is drawn back into the nozzle(s) of the droplet ejection head. This lower limit on the back pressure may vary depending on the type of droplet ejection head and/or on the ink being used. It may readily be determined by experimentation, for example.

The meniscus pressure must therefore be kept within a window which is generally determined by:

1) the meniscus pressure at which the fluid starts to weep onto the nozzle plate, and/or

2) the meniscus pressure at which air is ingested through the nozzles.

Further, variation of the meniscus pressure within this window may be sufficient to result in undesirable droplet volume and velocity variations which may lead to observable defects in the printed image on the substrate. Therefore, for reliable and good quality droplet ejection it is often necessary to control the meniscus pressure within a smaller range and keep its variation to a minimum (for example for the Xaar 1003 printhead a range of ± 2 mbar is specified for the chosen meniscus pressure window).

Some droplet ejection heads are so-called through-flow or recirculation droplet ejection heads. This means that the fluid circulates through the droplet ejection head with a proportion of the fluid being drawn off, and ejected out, of the nozzles and the remainder exiting the droplet ejection head and being returned to the fluid supply (for example, to a reservoir or to a pump to be re-circulated).

Where there are multiple droplet ejection heads located at different height positions relative to each other, and to the fluid supply system, as shown in Fig. 6, the meniscus pressure for one droplet ejection head may not be the same as that for a droplet ejection head at a different height. A single fluid supply system can generally only maintain a static pressure (typically the desired meniscus pressure) for droplet ejection heads at a single height. Where there are droplet ejection heads at different heights, the difference in meniscus pressure (APmeniscus) between them may be large (APmeniscus=pgh, where h is the height difference between the droplet ejection heads, g is the gravitational acceleration, and p is the fluid density), resulting in observable differences in the drop s-on- substrate, and hence in the printed image. In a worst-case scenario, the result may be that, in a vertical array of droplet ejection heads, some weep fluid and some ingest air, affecting the overall print performance. A possible solution is to provide a fluid supply system per droplet ejection head, but such a solution is expensive and may require a lot of space. It is an object of the present invention to overcome such disadvantages by providing as simplified a system as possible.

SUMMARY

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

According to a first aspect of the invention, there is provided a recirculating fluid supply system for a plurality of droplet ejection heads, each having a fluid inlet and fluid outlet; the system comprising: a common supply pump for transferring fluid to each of the plurality of droplet ejection heads; a fluid reservoir fluidically connectable via said common supply pump to said plurality of droplet ejection heads so as to supply fluid, via said common supply pump, to each of said droplet ejection heads; a sensor manifold per droplet ejection head for outputting the pressure in the vicinity of the fluid inlet and in the vicinity of the fluid outlet, respectively, for each droplet ejection head; and a fluid return pump per droplet ejection head having a fluid return connection, fluidically connectable to said droplet ejection head, so as to transfer fluid away from said droplet ejection head; wherein each of said fluid return pumps is operable to control the meniscus pressure, at said respective droplet ejection head, based on the output of its respective sensor manifold.

According to a second aspect of the invention, there is provided a recirculating fluid supply system for a plurality of droplet ejection heads, each having a fluid inlet and fluid outlet; the system comprising: a common return pump for transferring fluid away from each of the plurality of droplet ejection heads; a fluid reservoir fluidically connectable via said common return pump to said plurality of droplet ejection heads, so as to receive fluid, via said common return pump, from each of said droplet ejection heads; a sensor manifold per droplet ejection head for outputting the pressure in the vicinity of the fluid inlet and in the vicinity of the fluid outlet, respectively, for each droplet ejection head; and a fluid supply pump per droplet ejection head having a fluid supply connection, fluidically connectable to said droplet ejection head, so as to transfer fluid to said droplet ejection head; wherein each of said fluid supply pumps is operable to control the meniscus pressure, at a droplet ejection head, based on the output of its respective sensor manifold.

According to a third aspect of the invention there is provided a recirculating fluid supply system for a plurality of droplet ejection heads arranged in two or more groups, each droplet ejection head having a fluid inlet and fluid outlet and each group having a group fluid inlet and group fluid outlet; the system comprising: a common supply pump for transferring fluid to each of the plurality of droplet ejection heads; a fluid reservoir fluidically connectable, via said common supply pump, to said plurality of droplet ejection heads, so as to supply fluid, via said common supply pump, to each of said droplet ejection heads; at least one sensor manifold per group for outputting the pressure in the vicinity of the group fluid inlet and in the vicinity of the group fluid outlet, or in the vicinity of the fluid inlet and in the vicinity of the fluid outlet of a chosen at least one droplet ejection head per group of said droplet ejection heads; and a fluid return pump per group having one or more fluid return connections, fluidically connectable to said group, so as to transfer fluid away from said droplet ejection heads in said group; wherein each of said fluid return pumps is operable to control the meniscus pressure at the droplet ejection heads within said group, based on the output of the respective group sensor manifold.

According to a fourth aspect of the invention, there is provided a recirculating fluid supply system for a plurality of droplet ejection heads arranged in two or more groups, each droplet ejection head having a fluid inlet and a fluid outlet and each group having a group fluid inlet and group fluid outlet; the system comprising: a common return pump for transferring fluid away from each of the plurality of droplet ejection heads; a fluid reservoir fluidically connectable, via said common return pump, to said plurality of droplet ejection heads, so as to receive fluid via said common return pump from each of said droplet ejection heads; at least one sensor manifold per group for outputting the pressure in the vicinity of the group fluid inlet and in the vicinity of the group fluid outlet, or the pressure in the vicinity of the fluid inlet and in the vicinity of the fluid outlet of a chosen at least one droplet ejection head per group of said droplet ejection heads; and a fluid supply pump per group having one or more fluid supply connections, fluidically connectable to said group, so as to transfer fluid to said droplet ejection heads in said group; wherein each of said fluid supply pumps is operable to control the meniscus pressure at the droplet ejection heads within said group based on the output of its respective sensor manifold.

According to a fifth aspect of the invention, there is provided a droplet ejection apparatus comprising one or more recirculating fluid supply systems according to any of the first, second, third or fourth aspects of the invention.

According to a sixth aspect of the invention, there is provided a method of operating a droplet ejection apparatus according to the fifth aspect of the invention, or comprising a recirculating fluid supply system according to any of the first, second, third or fourth aspects of the invention and a plurality of droplet ejection heads.

According to a seventh aspect of the invention, there is provided an electronic controller for controlling a recirculating fluid supply system according to any of the first, second, third or fourth aspects, or for controlling a droplet ejection apparatus according to the fifth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 depicts a droplet ejection apparatus comprising two droplet ejection heads arranged at different heights and a recirculating fluid supply system according to an embodiment with a common supply pump and a return pump per droplet ejection head;

Fig. 2 depicts a droplet ejection apparatus comprising two droplet ejection heads arranged at different heights, a recirculating fluid supply system according to an alternative embodiment with a common return pump and a supply pump per droplet ejection head and a droplet ejection head movement device;

Fig. 3 A depicts a droplet ejection apparatus comprising a plurality of droplet ejection heads arranged into two groups at different heights, a recirculating fluid supply system according to a yet further embodiment, which is similar to that of Fig. 1, where the recirculating fluid supply system is connected to an external reservoir via a fill path and fill pump;

Fig. 3B depicts a detail D of Fig. 3 A, indicated by dashed line D in Fig. 3 A;

Fig. 4 depicts a droplet ejection apparatus, comprising four droplet ejection heads arranged into two groups at different heights, a recirculating fluid supply system according to a yet further embodiment, similar to that of Fig. 3 A, and whereby the droplet ejection heads within a first group, sharing a first return pump, are at a height offset with respect to one-another and whereby the droplet ejection heads within a second group, sharing a second return pump, are at a height offset with respect to one-another;

Fig. 5 depicts a droplet ejection apparatus comprising four droplet ejection heads arranged into two groups at different heights, a recirculating fluid supply system according to a further embodiment, and two droplet ejection head movement devices;

Fig. 6 depicts a droplet ejection apparatus comprising two droplet ejection heads and a fluid supply system according to a known arrangement;

Fig. 7A depicts a schematic arrangement of common supply pump, droplet ejection head, and common return pump according to the known arrangement of Fig. 6;

Fig. 7B depicts a schematic arrangement similar to that of Fig. 1 with a common supply pump and a return pump per droplet ejection head;

Fig. 7C depicts a schematic arrangement similar to that of Fig. 2 with a common return pump and a supply pump per droplet ejection head; and

Fig. 7D depicts the calculated pressure at various locations through the arrangement of Fig. 7A and the embodiments of Fig. 7B and Fig. 7C.

It should be noted that the drawings are not to scale and that certain features may be shown with exaggerated sizes so that these are more clearly visible.

DETAILED DESCRIPTION

Embodiments and their various implementations will now be described with reference to the drawings. Throughout the following description, like reference numerals are used for like elements, where appropriate.

Apparatus

Considering first Fig. 1, this depicts a droplet ejection apparatus 1 comprising two droplet ejection heads 107a, 107b and a recirculating fluid supply system 10 according to an embodiment. The droplet ejection heads 107a, 107b have a fluid inlet and a fluid outlet, and are therefore recirculating or through-flow devices. The droplet ejection heads 107a, 107b may each comprise a plurality of nozzles arranged in an array, for example in one or more rows of nozzles extending in an array direction. It can be seen from Fig. 1 that the droplet ejection heads 107a, 107b are arranged at different heights to each other, such that they are vertically separated by a height h. The droplet ejection heads 107a, 107b are also oriented substantially vertically, such that the nozzles to eject droplets extend in the vertical (y- direction) - i.e., the array direction is oriented such that the nozzles are arranged at different heights in the vertical direction. The droplet ejection heads 107a, 107b are also depicted as being located directly above one another, but this is merely for convenience and is by no means essential; in other arrangements they could be offset in the x-direction.

The recirculating fluid supply system 10 comprises a fluid reservoir 101 and a common supply pump 103. The common supply pump 103 is fluidically connected to the reservoir 101 and fluidically connectable, via fluid supply connections 105a, 105b, to each of the droplet ejection heads 107a, 107b, so as to connect to and supply fluid to each of the droplet ejection heads 107a, 107b.

In the embodiment of Fig. 1, it can be seen that the fluidic connection between the common supply pump 103 and the droplet ejection heads 107a, 107b is via a fluid supply path 102. The fluid supply path 102 comprises fluid supply connections 105a, 105b that are connectable to the inlets of the respective droplet ejection heads 107a, 107b. The fluid supply path 102 further comprises a common supply path 104 that carries fluid from the reservoir 101 and then splits to form a first supply path 104a and a second supply path 104b to supply fluid to the first and second droplet ejection heads 107a, 107b respectively. Each of the supply paths 104a, 104b may comprise a plurality of sections and connectors to fluidically connect all the components located between the common supply path 104 and the droplet ejection heads 107a, 107b. For example, in the embodiment of Fig. 1, each of the supply paths 104a, 104b comprises a first part 104ai,104bi connecting the common supply path 104 to the sensor manifolds 106a, 106b. The supply paths 104a, 104b may further comprise a second part 104aii,104bii connecting the sensor manifolds 106a, 106b to the fluid supply connections 105a, 105b; or, as in Fig. 1, the fluid supply connections 105a, 105b may comprise the respective second parts 104aii, 104bii of the supply paths 104a, 104b.

The recirculating fluid supply system 10 comprises a sensor manifold 106a, 106b per respective droplet ejection head 107a, 107b. The sensor manifolds 106a, 106b are located adjacent to a respective droplet ejection head, such that they measure, and output, pressures close to the respective droplet ejection heads. The respective sensor manifolds 106a, 106b measure the inlet pressure or supply side pressure (Ps) in the vicinity of the inlet to (i.e., at the supply side of) the respective droplet ejection heads 107a, 107b, i.e. for droplet ejection head 107a, Ps _a =Pla and for droplet ejection head 107b, Psb=Plb, generally Ps=Pla,Plb. Similarly the respective sensor manifolds 106a, 106b measure the outlet pressure or return side pressure (PR) in the vicinity of the outlet from (i.e., at the return side of) the respective droplet ejection heads 107a, 107b, such that PR _a =P2a, PRb=P2b, generally PR=P2a,P2b, which may be used to determine the differential pressure PD (the pressure difference across a respective droplet ejection head 107a, 107b) and the meniscus or nozzle or back pressure PM for a respective droplet ejection head 107a, 107b using the following two equations, which calculation may be performed in the controllers 109a, 109b: i.e., for droplet ejection head 107a, for example:

PDO. Psa PRCI

_ Psa + PRO.

PMO. ~ 2

The differential pressure PD is directly related to the flow rate through the droplet ejection head, the design of the droplet ejection head (impedance), and the viscosity of the fluid. The required flow rate depends on the droplet ejection head design, the type of fluid to be ejected, the application, and the print mode, a typical flow rate value is 100 mL/min.

The inventor has determined that fundamental to the present invention is that droplet ejection performance, and hence print performance, is not as sensitive to variations in flow rate as compared to variations in the meniscus pressure. Having a common supply pump and a fluid return pump per droplet ejection head, or alternatively a common return pump and a fluid supply pump per droplet ejection head, does not allow for having the same flow rate through each droplet ejection head or group of heads, while simultaneously maintaining the same meniscus pressure for each droplet ejection head or group of heads. However, providing that the inlet pressure Ps and the outlet pressure PR of the droplet ejection heads 107a, 107b are maintained/controlled such that the meniscus pressure PM falls within the meniscus pressure window PM(WINDOW), then acceptable print performance may be obtained. The meniscus pressure window PM(WINDOW) is the pressure window in which there will be no ingestion of air or weeping of fluid from the nozzles of the droplet ejection head 107.

It is generally known that the meniscus pressure window PM(WINDOW) is predominantly determined by the nozzle size, and hence, is the same for each droplet ejection head with the same nozzle size. The meniscus pressure window PM(WINDOW) is also affected by the surface tension of the ejection fluid (e.g., the ink being used). For applications such as inkjet, the inks are typically designed to have a surface tension within a narrow range, such that the PM(WINDOW) is not significantly different for different inks. Usually, the weeping point is checked to ensure it coincides with PM = 0 mbar, and hence, that the pressure datum offset (see below) is set correctly.

The sensor manifolds may be arranged at the same height as the nozzles of their respective droplet ejection heads, or they may not. For example, the sensor manifolds may be located above or below the nozzles of their respective droplet ejection head, or they may be aligned with them. For example, in arrangements where a droplet ejection head is oriented horizontally the nozzles may all be at the same vertical height and the sensor manifold may be arranged at the same vertical height as the nozzles, or at a fixed vertical offset to them. Any height discrepancy Ah _a ,Ahb between the sensor manifolds and the nozzles can be adjusted for by setting a datum pressure offset (P offset) per sensor manifold (to correct the measured pressure values):

Poffset= pg Ah _a , b.

The datum pressure offset Poffset is typically set such that the desired meniscus pressure PM is maintained in the centre of the droplet ejection head (i.e., in the vertical centre of the array of nozzles). Where the droplet ejection heads are oriented vertically, i.e., such that the array of nozzles is oriented such that the nozzles are arranged at different heights in the vertical direction, then the datum pressure offset Poffset is typically set such that the desired meniscus pressure PM is maintained in the nozzle(s) at the vertical centre of the array of nozzles. It is to be understood that PM is a negative value, such that a higher PM means closer to zero and a lower PM means more negative. A higher PM results in a larger drop volume and a lower drop velocity. Conversely a lower PM results in a smaller drop volume and a higher drop velocity. The recirculating fluid supply system 10 additionally comprises a fluid return pump 113a, 113b per droplet ejection head 107a, 107b and fluid return connections 115a, 115b, fluidically connectable to the outlets of the droplet ejection heads 107a, 107b, so as to transfer fluid away from the heads. Each of the fluid return pumps 113a, 113b is operable to control the meniscus pressure at their respective droplet ejection head 107a, 107b based on the output of their respective sensor manifolds 106a, 106b. In the arrangement of Fig. 1, the fluid return pumps 113a, 113b are fluidically connected so as to transfer fluid away from their respective droplet ejection head 107a, 107b.

Where the droplet ejection heads 107a, 107b are arranged vertically, as shown in Fig. 1, there is a pressure gradient AP across each droplet ejection head due to the effect of gravity. A typical value of AP across a respective droplet ejection head 107a, 107b is 7 mbar for a typical Xaar printhead and a typical ink. In such a case the meniscus pressure PM may be managed to balance in the vertical centre of the droplet ejection head, as discussed above, and provided that the pressure at the highest and lowest nozzles in a respective droplet ejection head remains within the allowable range of meniscus pressures (e.g., within the meniscus pressure window PM(WINDOW), then there will be no ingestion of air or weeping of fluid from the nozzles in a respective droplet ejection head.

The fluid return path 112 is fluidically connectable between the droplet ejection heads 107a, 107b and the fluid return pumps 113a, 113b. The fluid return path 112 comprises fluid return connections 115a, 115b that are connectable to the outlets of the droplet ejection heads 107a, 107b. The fluid return path 112 comprises two return paths 114a, 114b which join to form a common return path 114. Each of the return paths 114a, 114b may comprise a plurality of sections and connectors to suitably fluidically connect all the components located between the droplet ejection heads 107a, 107b and the common return path 114. For example, in the embodiment of Fig. 1 they comprise a first part, connecting the fluid return connections 115a, 115b to the sensor manifolds 106a, 106b; a second part connecting the sensor manifolds 106a, 106b to the fluid return pumps 113a, 113b and a third part connecting the fluid return pumps 113a, 113b to the common return path 114, which, in this embodiment, returns fluid to the reservoir 101. It may be understood that the fluid supply connections 105a, 105b and the fluid return connections 115a, 115b may be simple push fit connections onto the droplet ejection heads 107a, 107b, or they may be more complex connections, e.g. a quick release self-sealing connector. In some arrangements, a valve (not shown) may be included such that the droplet ejection head may be removed without fluid spillage. Similar considerations may apply to any other connections required in the system of the invention.

To summarise, Fig. 1 depicts a recirculating fluid supply system 10 for a plurality of droplet ejection heads (in this instance two droplet ejection heads 107a, 107b, though this is by no means limiting), each having a fluid inlet and fluid outlet; the system comprising: a common supply pump 103 for transferring fluid to each of the plurality of droplet ejection heads 107a, 107b; a fluid reservoir 101 fluidically connectable, via said common supply pump 103, to said plurality of droplet ejection heads 107a, 107b, so as to supply fluid via said common supply pump 103 to each of said droplet ejection heads 107a, 107b; a sensor manifold 106a, 106b per droplet ejection head 107a, 107b for outputting the pressure in the vicinity of the fluid inlet and the fluid outlet, respectively, for each droplet ejection head; and a fluid return pump 113a, 113b per droplet ejection head 107a, 107b having a fluid return connection 115a, 115b, fluidically connectable to said droplet ejection head 107a, 107b so as to transfer fluid away from said droplet ejection head 107a, 107b via said fluid return pump 113a, 113b; wherein each of said fluid return pumps 113a, 113b is operable to control the meniscus pressure at said respective droplet ejection head 107a, 107b, based on the output of its respective sensor manifold 106a, 106b.

It may be understood that the fluid supply path 102 and the fluid return path 112, described herein with respect to the embodiment of Fig. 1, are merely a possible arrangement of the fluid paths and that any suitable arrangement of supply paths that enables the common supply pump 103 to be fluidically connected to the reservoir 101 and fluidically connectable, via fluid supply connections 105a, 105b, to the respective inlets of each of the droplet ejection heads 107a, 107b, so as to supply fluid to each of the droplet ejection heads 107a, 107b may be used. Likewise, any arrangement of return paths that enables the fluid return path 112 to be fluidically connectable, via the respective outlets of the droplet ejection heads 107a, 107b, to the fluid return connections 115a, 115b and enables the fluid return pumps 113a, 113b to transfer fluid away from the droplet ejection heads 107a, 107b, may be used.

It may further be understood that the sensor manifolds 106a, 106b may further comprise temperature sensors to measure the temperature in the vicinity of the inlet or supply side, temperature Tl, and the temperature in the vicinity of the outlet or return side, temperature T2. In the arrangement shown in Fig. 1, only the first sensor manifold 106a comprises temperature sensors Tla, T2a, but this is by no means limiting and, in other arrangements, some or all of the sensor manifolds may comprise respective temperature sensors. The temperature measurements may be used to ensure that the temperature (and hence viscosity) of the fluid, such as ink, is controlled - this will be described later.

As previously mentioned, the recirculating fluid supply system 10 of Fig. 1 is arranged in a droplet ejection apparatus 1. The droplet ejection apparatus 1 of this embodiment further comprises two droplet ejection heads 107a, 107b, where the two droplet ejection heads 107a, 107b are located at different vertical positions relative to each other, separated by a height h. The droplet ejection heads 107a, 107b are each arranged in a respective group 409a, 409b. It may be understood that, whilst the embodiment of Fig. 1 comprises two droplet ejection heads 107a, 107b, one per group 409a, 409b, this is by no means limiting and in other arrangements the groups 409a, 409b may comprise more than one droplet ejection head (e.g. 107a_l-107a_n,107b_l-107b_m respectively). Still further, the number of groups is not limited, and other arrangements may comprise more than two groups (e.g. 409a-409y).

Such a droplet ejection apparatus 1 can be used to control the pressure across the droplet ejection heads 107a, 107b so as to maintain the meniscus pressure PM within the meniscus pressure window PM(WINDOW), thereby improving the droplet ejection performance. In some operational modes, the meniscus pressure may be maintained within a chosen meniscus pressure window PM(CHOSEN WINDOW), which is a chosen subset of the meniscus pressure window PM(WINDOW), that may give improved print performance. It may be understood that the height h separating the groups 409a, 409b may be such that droplet ejection heads 107a, 107b in a given group 409a, 409b do not fall within the chosen meniscus pressure window, or alternatively within the meniscus pressure window, of a different group 409a, 409b.

It may be understood that the recirculating fluid supply system 10 according to the embodiment, that achieves the necessary control over meniscus pressure, is as simple as possible, since much of the fluid supply and optional fluid conditioning apparatus 135, as explained below, is common to the entire system and may be conveniently arranged in a common unit 11. By placing as many components as possible in the common unit 11, the minimum number of devices per droplet ejection head (or per group of droplet ejection heads) may be used, providing savings on cost, number of component parts and weight. This may be beneficial for installation into a droplet ejection head mounting and/or movement device such as a printbar, robotic arm or gantry system, and thereafter may aid the ease of use of such movement devices due to reduced weight, space savings and also reduced power consumption. Examples of devices that may also be included in the common unit 11, other than the fluid supply pump 103 and the fluid reservoir 101, include one or more fluid conditioning apparatus 135, such as heaters or coolers or filters or de-gassers.

Method of operation

In order to control the meniscus pressure at the droplet ejection heads 107a, 107b for the droplet ejection apparatus 1 of Fig. 1 comprising a recirculating fluid supply system 10 and two droplet ejection heads 107a, 107b, where the droplet ejection heads 107a, 107b are arranged in two groups, 409a, 409b respectively, each with a respective fluid return pump 113a, 113b and being at different heights h from each other, the method of operation may be as follows:

- setting the performance of the common supply pump and each return pump to an initial value;

- measuring the pressure in the vicinity of the fluid inlets (Pla,Plb) and in the vicinity of the fluid outlets (P2a,P2b) of the droplet ejection heads 107a, 107b using the respective sensor manifold 106a, 106b; and

- calculating the meniscus pressure for said group 409a, 409b using the measured inlet pressure and outlet pressure for the group; and

- altering the performance of one and/or both of the respective fluid return pump 113a, 113b for the group 409a, 409b (e.g., the fluid return pumps 113a, 113b are adjustable independently of each other) in response to the calculated meniscus pressure so as to maintain the meniscus pressure for the respective droplet ejection heads 107a, 107b, in respective groups 409a, 409b, within the meniscus pressure window.

It may be understood that altering the performance of the fluid return pumps 113a, 113b may comprise altering the voltage supplied to the respective fluid return pump 113a, 113b. In some arrangements, the method may comprise maintaining the meniscus pressure for the respective droplet ejection heads 107a, 107b within a chosen meniscus pressure window, which is narrower than the meniscus pressure window, for reasons of improved print performance.

In order to perform the above method, the fluid supply system 10 comprises controllers 109a, 109b, one per group 409a, 409b, for controlling the meniscus pressure of the respective group droplet ejection heads 107a, 107b; each group 409a, 409b having a respective sensor manifold 106a, 106b for outputting the pressure in the vicinity of the fluid inlet and in the vicinity of the fluid outlet of the droplet ejection head 107a, 107b. The controllers 109a, 109b are operable to calculate the meniscus pressure based on the output of the respective sensor manifold 106a, 106b and then to determine whether the calculated meniscus pressure lies within the meniscus pressure window (or within the chosen meniscus pressure window) for the group. Based on this determination, the controllers provide output signals to one and/or both of the fluid return pumps 113a, 113b so as to thereby adjust the respective pump’s performance, in order to control the meniscus pressure at the respective droplet ejection head 107a, 107b. One of the controllers (in this case 109a) may further be used to adjust the performance of the common supply pump 103, based on the pressure measurements from the sensor manifold 106a, so as to control the differential pressure.

Alternative Configurations

Turning now to Fig. 2, this depicts a droplet ejection apparatus 2 comprising two droplet ejection heads 107a, 107b, oriented substantially horizontally relative to the x-axis (rather than vertically, as in Fig. 1), and arranged substantially parallel to one another and so that they are at different heights, i.e., they are separated by a vertical height h. For clarity, because the droplet ejection heads 107a, 107b are oriented substantially horizontally, the array(s) of nozzles are also oriented substantially horizontally, being arranged at substantially the same vertical position for a respective droplet ejection head 107a, 107b. The droplet ejection apparatus 2 further comprises a recirculating fluid supply system 20 according to an alternative embodiment. This embodiment of the recirculating fluid supply system 20 is largely similar to that depicted in Fig. 1, differing mainly in that, instead of two fluid return pumps 113a, 113b, the recirculating fluid supply system 20 comprises two fluid supply pumps 103a, 103b and, instead of a common supply pump 103, it comprises a common return pump 113. The droplet ejection apparatus 2 further comprises a movement device 500, on which the droplet ejection heads 107a, 107b are mounted. The embodiment of Fig. 2 therefore comprises a droplet ejection apparatus 2 comprising a recirculating fluid supply system 20 for a plurality of droplet ejection heads 107a, 107b, each having a fluid inlet and fluid outlet; the system comprising: a common return pump 113 for transferring fluid away from each of the plurality of droplet ejection heads 107a, 107b; a fluid reservoir 101 fluidically connectable, via said common return pump 113, to said plurality of droplet ejection heads 107a, 107b, so as to receive fluid, via said common return pump 113, from each of said droplet ejection heads 107a, 107b; a sensor manifold 106a, 106b per droplet ejection head 107a, 107b for outputting the pressure in the vicinity of the fluid inlet and the fluid outlet, respectively, for each droplet ejection head 107a, 107b; and a fluid supply pump 103a, 103b per droplet ejection head 107a, 107b having a fluid supply connection 105a, 105b, fluidically connectable to said droplet ejection head 107a, 107b, so as to transfer fluid to said droplet ejection head 107a, 107b; wherein each of said fluid supply pumps 103a, 103b is operable to control the meniscus pressure at a droplet ejection head 107a, 107b based on the output of its respective sensor manifold 106a, 106b.

The fluid supply path 102 comprises a common supply path 104 that splits to form a first supply path 104a and a second supply path 104b. Each of the supply paths 104a, 104b may comprise a plurality of pipe sections and connectors to suitably fluidically connect all the components located between the supply path 104 and the droplet ejection heads 107a, 107b. For example, in the embodiment of Fig. 2 they comprise a first part 104ai,104bi connecting the common supply path 104 to the fluid supply pumps 103 a, 103b and a second part 104aii,104bii connecting the fluid supply pumps 103 a, 103b to the sensor manifolds 106a, 106b respectively. Fluid supply connections 105a, 105b are connectable to the inlets of the droplet ejection heads 107a, 107b so as to fluidically connect them to the sensor manifolds 106a, 106b. Fluid return connectors 115a, 115b are connectable to the outlets of the droplet ejection heads 107a, 107b and connect them to the sensor manifolds 106a, 106b, respectively. It can be seen that, in this embodiment, the two droplet ejection heads 107a, 107b are depicted as being at the same height as their respective sensor manifolds 106a, 106b. This is by no means essential, but advantageously, in such an arrangement, there is no need to account for a datum pressure offset Poffset.

The fluid return path 112 comprises two return paths 114a, 114b which comprise a first part 114ai,l 14bi connecting the sensor manifolds 106a, 106b to the common return path 114, wherein the common return path 114 connects to the common return pump 113 and hence returns fluid from the droplet ejection heads 107a, 107b to the reservoir 101. Each of the fluid supply pumps 103 a, 103b are operable to control the meniscus pressure at their respective droplet ejection head 107a, 107b, based on the output of their respective sensor manifolds 106a, 106b.

The embodiment of Fig. 2 comprises an alternative controller arrangement to that of Fig. 1 whereby there is a common controller 109 arranged in the common unit 11 and electronically connected so as to receive pressure (and temperature in the case of sensor manifold 106a) measurements from the sensor manifolds 106a, 106b. The common controller 109 may perform all necessary calculations to determine how the fluid supply pumps 103a, 103b should be operated in order to maintain the meniscus pressure window, or, more preferably, the chosen meniscus pressure window at the respective droplet ejection heads 107a, 107b. The common controller 109 may then send suitable signals to the fluid supply pumps 103a, 103b, so as to adjust their performance, e.g. by altering the voltage supplied to the fluid supply pumps 103a, 103b. The common controller 109 may further control all of the components in the fluid supply system 20, still further in the droplet ejection apparatus 2, where said components require control. For example, the common controller 109 may further control the common return pump 113 and may further control the optional fluid conditioning device 135; it may also control the movement device 500 and/or the operation of the droplet ejection heads 107a, 107b (electrical connections not shown in Fig. 2). Alternatively (not shown in Fig. 2, see Fig. 3 A and Fig. 3B) an external controller 119 may provide instructions to, and receive feedback from, the common controller 109. Such an external controller 119 may further control the droplet ejection heads 107a, 107b and/or the movement device 500.

The method of operation of the embodiment of Fig. 2 is similar to that described above with respect to the embodiment of Fig. 1, except that the method comprises altering the performance of the respective fluid supply pumps 103a, 103b for the group 409a, 409b in response to the calculated meniscus pressure for the group so as to maintain the meniscus pressure for the respective droplet ejection heads 107a, 107b in respective groups 409a, 409b within the meniscus pressure window or within the chosen meniscus pressure window.

Turning now to Fig. 3 A, this depicts a droplet ejection apparatus 3 comprising a plurality of vertically oriented droplet ejection heads 107a_l-107a_n, 107b_l-107b_m arranged into two groups 409a, 409b where the groups 409a, 409b are at different heights, vertically separated by a height h. The droplet ejection apparatus 3 further comprises a recirculating fluid supply system 30 according to a yet further embodiment, which is similar to that of Fig. 1, and an external reservoir 201, connected to the reservoir 101 of the fluid supply system 30 via a fill path 202 and fill pump 203. The droplet ejection apparatus 3 is controlled by an external controller 119. Fig, 3B depicts a detail D of Fig. 3 A, indicated by dashed line D in Fig. 3 A.

In the embodiment of Fig. 3A-Fig. 3B the recirculating fluid supply system 30 comprises a common supply pump 103 for transferring fluid to each of the two groups 409a, 409b; a fluid reservoir 101 fluidically connectable via said common supply pump 103, and the two groups 409a, 409b, so as to supply fluid, via said common supply pump 103, to each of the plurality of droplet ejection heads 107a_l-107a_n, 107b_l-107b_m in the two groups 409a, 409b. There is a respective sensor manifold 106a, 106b per group 409a, 409b and a respective fluid return pump 113a, 113b per group 409a, 409b to transfer fluid away from the respective groups 409a, 409b. The droplet ejection apparatus 3 of Fig. 3A-Fig. 3B, therefore, comprises a recirculating fluid supply system 30 for a plurality of droplet ejection heads 107a_l,107a_n,107b_l-107b_m arranged in two groups 409a, 409b, each group having a group fluid inlet 120a, 120b and group fluid outlet 125a, 125b. The recirculating fluid supply system 30 of Fig. 3A-Fig. 3B may comprise: a common supply pump 103 for transferring fluid to each of the plurality of droplet ejection heads 107a_l - 107a_n, 107b_l - 107b_m, a fluid reservoir 101 fluidically connectable via said common supply pump 103 to said plurality of droplet ejection heads 107a_l-107a_n,107b_l-107b_m so as to supply fluid, via said common supply pump 103, to each of said droplet ejection heads 107a_l , 107a_n, 107b_l - 107b_m, at least one sensor manifold 106a, 106b per group 409a, 409b, for outputting the pressure in the vicinity of the group fluid inlet and in the vicinity of the group fluid outlet for each group 409a, 409b, and a fluid return pump 113a, 113b per group 409a, 409b having one or more fluid return connections 115a_l-l 15a_n,l 15b_l-l 15b_m fluidically connectable to said group 409a, 409b so as to transfer fluid away from said droplet ejection heads 107a_l-107a_n, 107b_l-107b_m in said group 409a, 409b, where each of said fluid return pumps 113a, 113b is operable to control the meniscus pressure at the droplet ejection heads 107a_l-107a_n, 107b_l-107b_m, within said group 409a, 409b, based on the output of the respective group sensor manifold 106a, 106b.

It may be generally understood that two groups 409a, 409b is not limiting, and that the droplet ejection apparatus 3 of Fig. 3A-Fig. 3B may comprise a plurality of droplet ejection heads 107 arranged in two or more groups, 409a-y, each droplet ejection head 107 having a fluid inlet and fluid outlet and each group 409a-y having a group fluid inlet and group fluid outlet. Further, each group may have a respective sensor manifold 106a-y and a respective fluid return pump 113a-y.

It may be generally understood that, for a group 409a, 409b with a plurality of droplet ejection heads 107a_l-107a_n, 107b_l-107b_m, “in the vicinity” encompasses a point at or upstream of the group fluid inlet 120a, 120b and at or downstream of the group fluid outlet 125a, 125b. Optionally, the fluid return pump 113a, 113b may have a fluid damper 118a, 118b located adjacent to them to even out pressure fluctuations in the fluidic path. Likewise, the common supply pump 103 may have a fluid damper 108 adjacent to it in the fluidic path. Additionally, there may be an optional level sensor LI and an optional temperature sensor T3 in the reservoir 101. Further there may be an optional level sensor L2 and an optional temperature sensor T4 in the external reservoir 201 and the fill pump 203 may be controlled to supply fluid to the reservoir 101 from the external reservoir 201, as required.

It may be understood that, rather than having an arrangement similar to the embodiment of Fig. 1, the droplet ejection apparatus 3 of Fig. 3A-Fig. 3B could instead comprise a fluid supply system similar to the embodiment of Fig. 2, e.g. comprising a respective fluid supply pump 103a, 103b per group 409a, 409b and a common return pump 113 and appropriate fluid paths and connections such that each of said fluid supply pumps 103a, 103b is operable to control the meniscus pressure at the droplet ejection heads within their respective group 409a, 409b, based on the output of their respective sensor manifold 106a, 106b.

It can be seen that, in Fig. 3 A-Fig. 3B, the first group 409a is located at a vertical height h above the second group 409b and comprises n droplet ejection heads 107a_l to 107a_n located at the same height. The second group 409b comprises n droplet ejection heads 107b_l to 107b_m located at the same height (for simplicity only the first and last droplet ejection head in each group 409a, 409b, plus respective fluidic connections, are depicted in Fig. 3A-Fig. 3B). The respective fluid inlets and fluid outlets of the droplet ejection heads 107a_l-107a_n, 107b_l-107b_m in groups 409a, 409b are connectable to the recirculating fluid supply system 30 via fluid supply connections 105a_l-105a_n and 105b_l-105b_m, respectively, and fluid return connections 115a_l-l 15a_n and 115b_l-l 15b_m, respectively.

It can be seen that the fluid supply path 102 comprises a common supply path 104 that carries fluid from the reservoir 101, driven by the common supply pump 103, and then splits to form a first supply path 104a to supply fluid to the first group 409a and a second supply path 104b to supply fluid to the second group 409b. Y-connectors or any other suitable connection device may be used for the split from common supply path 104 to first and second supply paths 104a, 104b (and, likewise, for the joining of first and second return paths 114a, 114b to common return path 114).

Each of the supply paths 104a, 104b may comprise a plurality of sections and connectors to fluidically connect components located between the common supply path 104 and the droplet ejection heads 107a, 107b. For example, in the embodiment of Fig. 3A-Fig. 3B, each of the supply paths 104a, 104b comprises a first part 104ai,104bi connecting the common supply path 104 to the respective sensor manifolds 106a, 106b. The supply paths 104a, 104b may further comprise a second part 104aii, 104bii. The second part 104aii, 104bii may comprise a part that is connected to the respective sensor manifold 106a, 106b at its upstream end, thereafter the second part 104aii,104bii may comprise a group fluid inlet 120a, 120b adjacent to and upstream of the point at which the second part 104aii,104bii splits into a plurality of legs (e.g. fluid supply connections 105a_l-105a_n,105b_l-105b_m respectively) so as to be fluidically connectable to the respective fluid inlets, of the plurality of droplet ejection heads 107a_l-107a_n,107b_l-107b_m, respectively. A possible arrangement may comprise one or more Y-connectors and then separate pipework to each droplet ejection head 107a_l-107a_n,107b_l-107b_m.

The fluid return path 112 is arranged in a similar manner to the fluid supply path 102. Described in reverse order to the operational direction of fluid flow, the fluid return path comprises a common return path 114 that connects at one end to the reservoir 101 and at the other end splits into two return paths 114a, 114b, one per respective group 409a, 409b. Each return path 114a, 114b comprises a plurality of sections and connectors to fluidically connect components located between the common return path 114 and the droplet ejection heads 107a_l-107a_n and l-7b_l-107b_m. A first portion 114ai,114bi of the return paths 114a, 114b connects the common return path 114 to the respective return pumps 113a, 113b and then to the optional respective dampers 118a, 118b and subsequently to the respective sensor manifolds 106a, 106b. The return paths 114a, 114b may further comprise a second part 114aii, 114bii. The second parts 114aii, 114bii comprise a part that is connected to the respective sensor manifold 106a, 106b, thereafter the second part 114aii, 114bii comprises a group fluid outlet 125a, 125b adjacent to the point at which the second part 114aii, 114bii splits into a plurality of legs (e.g. fluid return connections 115a_l -115a_n,l 15b_l -115b_m respectively) so as to be fluidically connectable to the respective fluid outlets of the plurality of droplet ejection heads 107a_l-107a_n,107b_l-107b_m, respectively. A possible arrangement may comprise one or more Y-connectors and then separate pipework to each droplet ejection head 107a_l-107a_n,107b_l-107b_m.

For the avoidance of doubt, in operation, fluid flows from the plurality of droplet ejection heads 107a_l-107a_n,107b_l-107b_m into the respective fluid return connections 115a_l- 115a_n,115b_l-115b_m and subsequently all the legs join together to form a single fluidic path with the group fluid outlet 125a, 125b arranged adjacent to and downstream of the point at which all of the legs have been joined together, such joining being done in any suitable manner. The fluid then flows, in the return paths 114a, 114b, via the respective sensor manifolds 106a, 106b; dampers 118a, 118b (optional); return pumps 113a, 113b; and then the return paths 114a, 114b join together to form common return path 114, which returns the fluid to the reservoir 101.

It may be understood that where there are a plurality of droplet ejection heads 107a_l- 107a_n,107b_l-107b_m, the second parts 104aii, 104bii, 114aii,l 14bii of the supply and return paths 104a, 104b, 114a, 114b, respectively, may split repeatedly in a tiered or hierarchical manner (e.g. each path may split into two legs, then each leg may split into two further legs, and so on) until there are sufficient legs to connect to each of the plurality of droplet ejection heads 107a_l-107a_n,107b_l-107b_m. Each split may comprise a Y- connector. However, it may be understood that any suitable connection method may be utilised, and that the split may comprise more than two legs per tier. It may be understood that, in the case where there are a plurality of droplet ejection heads 107a_l-107a_n,107b_l-107b_m in a group 409a, 409b, preferably the fluidic resistance of the fluid paths, from the respective group fluid inlets 120a, 120b to the fluid inlet of each respective droplet ejection head, is similar or the same, such that the meniscus pressure at each droplet ejection head is similar or the same. In this way, each droplet ejection head behaves in a similar manner, reducing any variation in ejected drop volume/velocity. Likewise, preferably, the fluidic resistance of the fluid paths, from the fluid outlet of each respective droplet ejection head 107a_l-107a_n,107b_l-107b_m in a group 409a, 409b to the respective group fluid outlet 125a, 125b, is also similar or the same. It may be understood that, if the fluidic resistance of the fluid paths to and from the droplet ejection heads in a respective group 409a, 409b differs, so does the droplet ejection behaviour of the heads within the group 409a, 409b. Accordingly, the greater the requirement for high print quality, and hence consistency between heads, the greater the requirement to match fluidic resistances in the fluid supply paths and fluidic resistances in the fluid return paths, i.e., in such operating conditions it may be preferable to match the resistances of the fluid supply paths and the fluid return paths to each other. Conversely, applications requiring lower print quality may tolerate greater variation in the fluidic resistance of the respective supply and return fluid paths.

Fig. 4 depicts an embodiment which is similar to that of Fig. 3 A-Fig. 3B with a droplet ejection apparatus 4 comprising a recirculating fluid supply system 40. It differs in that the sensor manifolds 106a, 106b are located downstream of the respective group fluid inlets 120a, 120b and upstream of the respective group fluid outlets 125a, 125b (for simplicity not all of the legs of 104a, 104b, 114a, 114b are labelled). This means that they are measuring the inlet and outlet pressures Pla,Plb,P2a,P2b respectively in the vicinity of the inlet and in the vicinity of the outlet of a chosen one droplet ejection head from each respective group 409a, 409b (in this case 107a_2 and 107b_2 respectively), rather than the group inlet and group outlet pressures. It may be understood that, in the context of a chosen one droplet ejection head, “in the vicinity” may be taken to mean adjacent to the respective inlet and outlet. Provided that the fluidic resistance of the fluid paths, from the respective group fluid inlets 120a, 120b to the fluid inlet of each respective droplet ejection head, is the same, or else sufficiently similar, and that all of the droplet ejection heads in a respective group are operating within the same meniscus pressure window, or within the same chosen meniscus pressure window, then the pressures measured across one droplet ejection head and the meniscus pressure calculated from these can be applied to the other droplet ejection heads in the same group 409a, 409b.

The embodiment of Fig. 4 further differs from that of Fig. 3A-Fig. 3B in that there are two droplet ejection heads per group 409a, 409b and that the droplet ejection heads 107a_l,107a_2 in the first group 409a are within a height offset range hl with respect to one another and the droplet ejection heads 107b_l-107b_2 in the second group 409b are within a height offset range h2 with respect to one another. It may be understood that, where there are height offset ranges hl,h2, these should be such that all of the droplet ejection heads in a respective group 409a, 409b are operating within the same meniscus pressure window, or within the same chosen meniscus pressure window, such that they can share the return pump 113a or 113b for the group (or, generalising to embodiments such as that of Fig. 3A-Fig. 3B, share the supply pump 103a, 103b for the group).

It may be understood that, depending on the use to which such a droplet ejection apparatus 4 is to be put, and the tolerance on the meniscus pressure window or the chosen meniscus pressure window, the height offset hl,h2 between the droplet ejection heads 107a_l, 107a_2, 107b_l-107b_2 in the group 409a, 409b may be a design feature to enable the droplet ejection heads to address different parts of the substrate, which are at different heights, or it may be so that aligning the droplet ejection heads in the apparatus is easier as there is a greater tolerance of positional variations between heads. Such considerations may also apply more generally where the groups comprise more than two droplet ejection heads, e.g., 107a_l, 107a_n,107b_l-107b_m. It may further be understood that the above discussion regarding height offsets within a group 409a, 409b may also apply if the droplet ejection heads 107a_l-107a_n,107b_l-107b_m were mounted horizontally rather than vertically but still at a height offset with respect to one another, within a respective group 409a, 409b.

Turning now to Fig. 5, this depicts an alternative embodiment, a droplet ejection apparatus 5, which is similar to that of Fig. 3 A-Fig. 3B, except that the fluid supply system 50 has fluid supply pumps 103 a, 103b on the fluid supply path 104a, 104b, as well as fluid return pumps 113a, 113b on the fluid return path 114a, 114b. Additionally, the droplet ejection apparatus 5 comprises two movement devices 500a, 500b, one per group 409a, 409b. The two groups 409a, 409b are similar to the embodiments of Fig. 3 A-Fig. 3B and Fig. 4, except that both groups 409a, 409b comprise two droplet ejection heads 107a_l,107a_2 and 107b_l,107b_2, respectively, with associated fluid supply connections 105a_l,105a_2,105b_l,105b_2 and fluid return connections 115a_l , 115a_2,l 15b_l , 115b_2 (for simplicity only the fluid connections in group 409a are labelled). There is one supply pump 103a, 103b and one return pump 113a, 113b per group 409a, 409b, respectively.

The supply pumps 103 a, 103b are fluidically connected so as to supply fluid to their respective groups 409a, 409b and hence to the droplet ejection heads 107a_l,107a_2, 107b_l,107b_2 in each group 409a, 409b, via the fluid supply connections 105a_l,105a_2,105b_l,105b_2. Such an arrangement, with both fluid supply pumps 103a, 103b and fluid return pumps 113a, 113b, as well as a common supply pump 103, may be beneficial where the groups 409a, 409b each comprise a large number of droplet ejection heads 107a_l-107a_n, 107b_l-107b_n, or where a significant distance between the common unit 11 and the droplet ejection heads 107a_l, 107a_2, 107b_l , 107b_2 of the droplet ejection apparatus 5 is required, for example, such that a single supply pump or single return pump per group is not sufficient - due, for example, to a large number of heads, and/or the amount of fluid needing to be pumped in a particular arrangement.

It may be understood that, where there are significant distances between the common unit 11 and the other components, and/or large number of droplet ejection heads, alternative embodiments may comprise fluid supply pumps 103a, 103b and fluid return pumps 113a, 113b and a common return pump 113, such as that of the embodiment of Fig. 2. Still further embodiments with both a common supply pump 103 and a common return pump 113 may be contemplated. Alternatively, where multiple droplet ejection heads are required at each vertical position, rather than single groups at each height, comprising a large number of droplet ejection heads, an arrangement with several groups 409a_l-409a_y,409b_l- 409b_z at each vertical position may be suitable. Each such group may comprise fewer droplet ejection heads, such that a respective fluid supply pump or fluid return pump per group is sufficient.

Example

Turning now to Fig. 6, this depicts a droplet ejection apparatus 200 according to a known arrangement. The second head 107b, within the dashed box 205, will perform suitably only if the height difference h is such that both droplet ejection heads 107a, 107b fall within the same meniscus pressure window, or within the same chosen meniscus pressure window, for cases requiring high print quality. Fig. 7A depicts schematically part of a similar arrangement to that of Fig. 6 (referred to as Head 1), with a common supply pump 103 and a common return pump 113 to either side of a droplet ejection head 107, with various locations on the fluidic path labelled (1) to (6). Fig. 7B depicts a similar schematic arrangement to that of Fig. 7A, but for an embodiment similar to that of Fig. 1 with a common supply pump 103 and a fluid return pump 113a, 113b per droplet ejection head 107a, 107b (Head 2, Option 1). Similarly, Fig. 7C depicts a schematic arrangement similar to the embodiment of Fig. 2, with a common return pump 113 and a fluid supply pump 103a, 103b per droplet ejection head 107a, 107b (Head 2, Option 2). Fig. 7D depicts the pressure in mbar at the locations ( 1 )-(6) for the cases of Fig. 7A (Head 1), Fig. 7B (Head 2, option 1) and Fig. 7C (Head 2, option 2). The supply side pressure Ps, the return side pressure PR and the differential pressure PD for Head 2, Option 1 are labelled on Fig. 7D. It can be seen that the pressure variation from (1) to (6) is fairly similar for all three cases. Also labelled are a representative meniscus pressure window P M(WINDOW) and a representative chosen meniscus pressure window PM(CHOSEN WINDOW).

General considerations

It may be understood that, for any of the droplet ejection apparatus 1,2, 3, 4, 5 described herein, the groups 409 may comprise one or more droplet ejection heads 107. It may therefore be understood that the embodiments described in Fig. 1 and Fig. 2 may also be considered as comprising two groups 409a, 409b wherein each group 409a, 409b comprises one droplet ejection head 107a, 107b, respectively. A group may comprise one, two or more droplet ejection heads 107, as described herein. Still further, a group may comprise a plurality of droplet ejection heads, e.g., m or n numbers of droplet ejection heads 107a_l- 107a_n,107b_l-107b_m. It may also be understood that the groups may comprise equal (n=m) or different (n n) numbers of droplet ejection heads, depending on the application for which they are being used. Additionally, it may be understood that, whilst the embodiments depicted herein comprise two groups 409a, 409b this is by no means limiting, and any of the embodiments described and alluded to herein may comprise a plurality of groups 409a-409y arranged at different vertical heights, wherein each of said groups 409a- y may comprise the same or differing numbers of droplet ejection heads 107. It may be understood that, where there are two or more groups 409 arranged at different vertical heights, the difference in height between the groups 409 may be such that droplet ejection heads in a group 409 at one height do not fall within the same meniscus pressure window as droplet ejection heads in a group 409 located at a different height, more preferably, the droplet ejection heads in groups 409 are arranged such that they do not fall within the same chosen meniscus pressure window as those in another group 409 at a different vertical height.

Generally, embodiments described herein may comprise a recirculating fluid supply system 10,30,40,50 for a plurality of droplet ejection heads 107 arranged in two or more groups 409a-y, each group 409a-y having a group fluid inlet and group fluid outlet; the system comprising: a common supply pump 103 for transferring fluid to each of the plurality of droplet ejection heads 107; a fluid reservoir 101 fluidically connectable, via said common supply pump 103, to said plurality of droplet ejection heads 107, so as to supply fluid, via said common supply pump 103, to each of said droplet ejection heads 107; at least one sensor manifold 106a-y per group 409a-y, for outputting the pressure at the sensor manifold 106a- y; and a fluid return pump 113a-y per group 409a-y, having one or more fluid return connections, fluidically connectable to said group 409a-y so as to transfer fluid away from said droplet ejection heads 107 in said group 409a-y; wherein each of said fluid return pumps 113a-y is operable to control the meniscus pressure at the droplet ejection heads 107 within said group 409a-y, based on the output of the respective group sensor manifold 106a-y.

Alternatively, embodiments described herein may comprise a recirculating fluid supply system 20 for a plurality of droplet ejection heads 107 arranged in two or more groups 409a- y, each group 409a-y having a group fluid inlet and group fluid outlet; the system comprising: a common return pump 113 for transferring fluid away from each of the plurality of droplet ejection heads 107; a fluid reservoir 101 fluidically connectable, via said common return pump 113, to said plurality of droplet ejection heads 107, so as to supply fluid, via said common return pump 113, from each of said droplet ejection heads 107; at least one sensor manifold 106a-y per group 409a-y, for outputting the pressure at the sensor manifold 106a-y; and a fluid supply pump 103a-y per group 409a-y having one or more fluid supply connections, fluidically connectable to said group 409a-y, so as to transfer fluid to said droplet ejection heads 107 in said group 409a-y; wherein each of said fluid supply pumps 103a-y is operable to control the meniscus pressure at the droplet ejection heads 107 within said group 409a-y based on the output of the respective group sensor manifold 106a- y-

It may be understood that for a recirculating fluid supply system 10,20,30,40,50, where a group 409a-y comprises one droplet ejection head 107, then the group fluid inlet and group fluid outlet comprise the respective droplet ejection head fluid inlet and fluid outlet, and the sensor manifold 106a-y may be arranged to output the pressure in the vicinity of the head fluid inlet and the pressure in the vicinity of the head fluid outlet. It may further be understood that, where a group 409a-y comprises two or more droplet ejection heads 107, then the sensor manifold 106a-y may be arranged to output the pressure in the vicinity of the group fluid inlet and in the vicinity of the group fluid outlet, or it may be arranged to output the pressure in the vicinity of the fluid inlet and the fluid outlet of a chosen one droplet ejection head of the group of two or more droplet ejection heads.

It may further be understood, that in some applications, there may be more than one group 409 at a given vertical location, for example to provide coverage over a large area (such as printing background colour onto a large substrate). In some applications there may be a limit on the capacity of the fluid supply and/or fluid return pumps 103, 113 or, in order to use lighter and/or smaller pumps, the number of droplet ejection heads per group that can be operated by a given pump may be limited such that two or more groups 409 of droplet ejection heads 107 are required at a given vertical position, each group 409 with their own fluid supply and/or fluid return pumps 103,113, to provide fluid to all of the droplet ejection heads 107 in the group 409. However, in such an application, whilst there may be two or more groups 409 at a given vertical location, there will, nevertheless, be two or more groups at different vertical heights, as discussed above.

As described with reference to Fig. 3 A-Fig. 3B, but applicable more generally to any of the embodiments and arrangements described herein, where one or more groups comprise a plurality of droplet ejection heads n,m, all of the heads in a given group may be located at the same vertical height and, preferably, the fluidic resistance of the respective fluid paths, from the group fluid inlet 120a, 120b to each droplet ejection head in the group 409a, 409b, may be arranged to be very similar or the same. Likewise, preferably, the fluidic resistance of the fluid return paths, from each droplet ejection head in the group 409a, 409b to the respective group fluid outlet 125a, 125b, may be arranged to be very similar or the same. Also, as previously described, the droplet ejection apparatus may comprise droplet ejection heads arranged into two or more groups 409a-y and wherein at least two of said two or more groups are arranged at different heights to each other.

Still further, in any of the embodiments and arrangements described herein, where the print performance is less tightly constrained (e.g. image quality is less important, and variation may be tolerated), e.g. the application allows for meniscus pressure variation and hence drop volume/velocity variation between droplet ejection heads within a group 409, then the fluidic resistance of the fluidic paths from the group fluid inlet 120a, 120b to the respective droplet ejection heads within the group 409a, 409b may differ and/or the respective heights of the droplet ejection heads within the group 409 may vary. Likewise, in such cases, the fluidic resistance of the individual return paths from the droplet ejection heads in the group 409a, 409b to the respective group fluid outlet 125a, 125b may differ. It may be understood that in such variation tolerant cases, the differing resistance should be such that the meniscus pressure for each droplet ejection head 107 within a particular group 409 is maintained within the meniscus pressure window or within the chosen meniscus pressure window.

Further still, for any embodiment or arrangement where a group 409a, 409b comprises a plurality of droplet ejection heads 107 located at the same height, or at similar heights, and where the fluidic resistance of the fluid supply and the fluid return paths is suitably designed, such that the meniscus pressure for each of the droplet ejection heads within a group falls within the meniscus pressure window, or the chosen meniscus pressure window, then the sensor manifold 106, for the group 409a, 409b, may be connected to a chosen one of the plurality of droplet ejection heads 107 within a respective group, as shown in Fig. 4. The differential and meniscus pressure for the chosen droplet ejection head 107 can then be applied to the other droplet ejection heads in the group, with suitable adjustments being calculated for any variation in height within the group 409a, 409b, if necessary.

Still further, in arrangements where the relative vertical positions of the droplet ejection heads in the respective groups 409a, 409b are fixed, such arrangements may be operable with only one sensor manifold 106, with the height offset between the groups 409a, 409b being used to determine the meniscus pressure for the group or group(s) without a respective sensor manifold. Such a calculation may be performed by the common controller 109, or at any other suitable location. The respective fluid supply pump 103a, 103b for each group 409a, 409b would then be controlled to maintain the pressure within the respective group 409a, 409b droplet ejection heads 107 within their respective meniscus pressure window.

It may be understood that, for any of the embodiments and arrangements described herein, the meniscus pressure window is that pressure window in which the droplet ejection heads do not ingest air or weep fluid from the nozzles and that, when operating the droplet ejection apparatus 1,2, 3, 4, 5, the pressure drop between the fluid inlet and fluid outlet of each of the droplet ejection heads is maintained within this meniscus pressure window. Still further, the pressure drop may be maintained within a chosen meniscus pressure window, which is a narrower range selected from within the meniscus pressure window that gives improved print performance and hence enhanced image quality. The meniscus pressure window and the chosen meniscus pressure window may depend on the droplet ejection head or heads being used, and/or the image quality requirements of the application in which they are being used. In some applications, the meniscus pressure window and/or the chosen meniscus pressure window may vary from group to group, for example if different droplet ejection heads are being used in different groups, or if the type of printing each group is doing differs (e.g. one group may be printing background colour, where image quality is less important, whilst another group may be printing detailed images or text).

It may further be generally understood that, as described above with respect to Fig. 1, the inventor has determined that print performance is not as sensitive to variations in flow rate (and hence to the differential pressure) as it is to variations in the meniscus pressure. The differential pressure is not directly related to the drop formation process. It should be noted that for effective operation, the differential pressure (and hence, recirculation flow) should be sufficiently high such that the conditioning devices in the common unit 11 can work adequately and that the well-known advantages of throughflow for print performance, temperature control etc. are maintained. The apparatus described herein may therefore control the differential pressure for one of the droplet ejection heads, whilst controlling the meniscus pressure for all of the droplet deposition heads (as described herein). Therefore, having a common supply pump 103 and a fluid return pump 113a, 113b per droplet ejection head 107a, 107b (or group of droplet ejection heads 109a, 109b) or alternatively a common return pump 113 and a fluid supply pump 103a, 103b per droplet ejection head 107a, 107b (or group 409a, 409b), means that the flow rate through each droplet ejection head 107a, 107b, or group of droplet ejection heads 409a, 409b may differ, but that such an arrangement may maintain the meniscus pressure for each droplet ejection head 107a, 107b or group 409a, 409b within the meniscus pressure window/ chosen meniscus pressure window for the respective droplet ejection head/ group of heads. This can be done by adjusting the drive voltage of the respective fluid return pumps 113a, 113b and/or the respective fluid supply pumps 103a, 103b, whilst the voltage to the common supply 103 and/or common return pumps 113 may be maintained at a constant value.

In alternative arrangements, one or more of the sensor manifolds 106a, 106b may be integrated into the droplet ejection heads 107a, 107b, 107, 107a_l-107a_n,107b_l-107b_m. This has the advantage of eliminating the need to correct for Poffset. In such an arrangement, the fluid supply connections 105a, 105b may connect directly to the common supply path 104. Alternatively, the first and second supply paths 104a, 104b may comprise the fluid supply connections 105a, 105b. Similarly, the fluid return connections 115a, 115b may connect directly to the common return path 114, or the first and second return paths 114a, 114b may comprise the fluid return connections 115a, 115b. It may further be understood that such integrated sensor manifolds 106a, 106b may further comprise temperature sensors to measure the inlet or supply side temperature T1 and the outlet or return side temperature T2.

A control scheme for the fluid supply systems comprising a common supply pump 103 as described herein may comprise the following steps:

- set a constant value for the common supply pump 103;

- use individual proportional integral derivative loops (PID loops) to adjust all the respective return pumps 113a, 113b individually until the desired manifold pressure is obtained for all droplet ejection heads 107a, 107b; and

- release the constant drive voltage to the common supply pump 103 and use a secondary PID loop to adjust the supply pump 103 until the desired differential pressure is obtained for a single droplet ejection head 107, preferably the droplet ejection head closest to the vertical centre of a series of droplet ejection heads at a vertical offset from one another. Preferably, the initial common supply pump value is set as close as possible to the final value.

Alternatively, for the fluid supply systems comprising a common return pump 113 as described herein, the control scheme may comprise the following steps: - set a constant value of the common return pump 113, in this case the droplet ejection heads 107 must be capped (e.g., with a foil) to avoid air from being ingested, hampering the startup procedure;

- use individual PID loop to adjust all fluid supply pumps 103 a, 103b individually until the desired meniscus pressure is obtained for all droplet ejection heads 107; and

- let go of the constant drive voltage of the common return pump 113 and use a secondary PID loop to adjust the common return pump 113 until the desired differential pressure is obtained for a single droplet ejection head, preferably the droplet ejection head closest to the vertical centre of a series of droplet ejection heads at a vertical offset from one another.

The differential pressure PD depends on the viscosity of the fluid, but typically ranges between 80 mbar and 140 mbar. Preferably, a control accuracy of +/- 2 mbar may be required but an allowed control range of +/- 20 mbar may be suitable. The meniscus pressure window depends on the droplet ejection head and the nozzle size in particular. A typical meniscus pressure window is -30 mbar to 0 mbar. A typical meniscus pressure of -15 mbar lies in the centre of the meniscus pressure window PM(WINDOW). Typically, once the meniscus pressure PM is chosen, a chosen window will be allowed around that meniscus pressure, for example if the meniscus pressure is -15 mbar then the chosen meniscus pressure window PM(CHOSEN WINDOW) may be +/- 2 mbar, e.g. from -17 mbar to -13 mbar. The ink system may then be configured and controlled such that the meniscus pressure is maintained within the chosen meniscus pressure window. Accurate control of the meniscus pressure is important for print quality. During operation, the respective return pumps 113a, 113b or supply pumps 103a, 103b may require further adjustment, based on the measurements taken by the sensor manifold 106a, 106b, depending on if and how the droplet ejection heads 107 are being moved.

It may further be understood that a droplet ejection apparatus 1,2, 3, 4, 5 may comprise one or more recirculating fluid supply systems 10,20,30,40,50 as described herein, for example the droplet ejection apparatus 1,2, 3, 4, 5 may be arranged to provide more than one colour, and may therefore comprise a per colour recirculating fluid supply system 10,20,30,40,50 and associated groups of droplet ejection heads. Alternatively, a droplet ejection apparatus 1,2, 3, 4, 5 may be arranged to print more than one type of fluid, such as base coats, top coats, surface treatments, etc., each with their own recirculating fluid supply system 10,20,30,40,50 and associated groups of droplet ejection heads. Where the droplet ejection apparatus comprises two or more recirculating fluid supply systems 10,20,30,40,50, it may comprise two or more droplet ejection heads per recirculating fluid supply system. These may be the same type of recirculating fluid supply system, but this is by no means essential.

The droplet ejection apparatus 1,2, 3, 4, 5 may comprise more than one type of recirculating fluid supply system 10,20,30,40,50 as described herein, for instance when printing different fluid types or fluid viscosities, or using different print modes. An example of this might be when printing both large volumes of fluid to generate thicker layers of print on the substrate and also printing small areas of detail requiring much lower fluid volumes using a different ink. Such differing print techniques may require different types of recirculating fluid supply systems 10,20,30,40,50 with different ink volume capacities. Still further, the different types of fluid may also require different types of droplet ejection heads 107 which may, by their differences, require non-identical fluid supply systems 10,20,30,40,50.

Any of the recirculating fluid supply systems 10,20,30,40,50 described herein may further comprise optional fluid conditioning apparatus 135. The fluid conditioning apparatus 135 may comprise one or more devices, for example temperature control devices, degassing devices (to remove air initially dissolved in the fluid, e.g., ink) and filters. Where the fluid conditioning apparatus 135 comprises temperature control devices, such as heaters or coolers, the recirculating fluid supply system 10,20,30,40,50 may further comprise temperature sensors, which may be conveniently located in the sensor manifold 106a, 106b in order to measure the inlet and outlet temperature Tla,T2a in the vicinity of a droplet ejection head 107a, 107b, or in the vicinity of a group 409a, 409b of droplet ejection heads, depending on the location of the sensor manifold 106a, 106b with respect to the droplet ejection heads, as shown in the embodiments of e.g. Fig. 3A-Fig. 3B and Fig. 4. Optionally, the recirculating fluid supply systems 10,20,30,40,50 may comprise individual temperature sensors per droplet ejection head 107. Alternatively, temperature sensors may be an integral part of the droplet ejection heads 107.

The temperature sensors Tla,T2a, may be used to provide feedback in a temperature control loop. Such a temperature control loop may comprise a fluid conditioning device 135 that comprises a temperature control device such as a heater, or a cooler. A change in ink temperature would change the viscosity and therefore the differential pressure in the system. The use of a temperature control loop and a temperature control device may allow the temperature of the ink to be controlled such that the performance of one or more of the pumps does not have to be adjusted to account for temperature variations.

An example of a temperature control loop is shown in Fig. 2, for example, where the sensor manifold 106a comprises temperature measurement sensors to measure the inlet and outlet temperatures Tla,T2a in the vicinity of the droplet ejection head 107a. The sensor manifold 106a is electrically connected to a common controller 109 which is also electrically connected to the fluid conditioning device 135. The common controller 109 may use the inlet temperature Tla to control the fluid conditioning device 135, to adjust the temperature of the fluid supplied to the droplet ejection heads to remain within a desired supply temperature range, for example. A further control loop may use the average of the inlet and outlet temperatures (T2a+Tla)/2 across one or more droplet ejection heads to adjust the print duty cycle, for example to reduce the duty cycle, if the temperature rise is too great. Such a control loop may use a common controller 109, or an external controller 119 as shown in Fig. 3A-Fig. 3B.

However, it may be understood that temperature control is not an essential component of the recirculating fluid supply systems 10,20,30,40,50 described herein. For example, a recirculating fluid supply system 10,20,30,40,50 in a clean room where the temperature is well controlled, with a large enough reservoir 101 such that any heat generated by the droplet ejection heads 107a, 107b may be dissipated, may not require a temperature control loop.

For systems in locations where the temperature is not well controlled, the addition of a temperature control loop, temperature sensors and a temperature control device may be advantageous. Suitable temperature control loops may be included in any of the fluid supply systems described or contemplated herein. Temperature control may be important to maintain the viscosity of the fluid, e.g., the ink, at a chosen value or within a chosen range, as viscosity may impact the drop formation process and the pressure distribution in the fluid system. The temperature sensors may be part of the sensor manifold 106a so as to measure the temperature change across one droplet ejection head 107a, as shown in Fig. 1 and Fig. 3A-Fig. 3B, or adjacent to one group 409a, 409b, as shown in Fig. 5.

The system may comprise further temperature sensors adjacent to or in the vicinity of one of the sensor manifolds 106a, 106b. Alternatively, there may be one or more temperature sensors per group 409a, 409b, or per droplet ejection head 107. In some arrangements, temperature sensors may be located in one or more of the droplet ejection heads 107. Further, the reservoir 101 may comprise a temperature sensor to measure the reservoir temperature T3 and, where present, the external reservoir 201 may comprise a temperature sensor to measure the external reservoir temperature T4. Generally, a common heater would suffice in the applications described herein, but the system may contain individual heaters for each individual droplet ejection head 107 or for groups 409 of droplet ejection heads 107, or in the reservoir 101 and/or the external reservoir 201, where present. In some circumstances, where the required jetting temperature is below room temperature, a cooler may be required. In some circumstances a cooler may be required to dissipate heat induced by printing prior to returning the ink to the droplet ejection heads 107.

It may be understood that any of the recirculating fluid supply systems 10,20,30,40,50 described herein may comprise one or more dampers 108,118, 118a, 118b such that some or all of the pumps 103, 113, 203 present in the system, whether common pumps or group or droplet ejection head pumps, and whether supply or return pumps, may have a fluid damper 108, 118 118a, 118b, adjacent to them to even out any pressure fluctuations in the fluidic path and thereby improve print performance. It may be understood that dampers are not required in all circumstances, it depends on factors such as the required print quality, the type of pumps used, the performance of the chosen pumps and the configuration of the fluid supply system. It may further be understood that, provided pressure pulses are sufficiently damped (typically to below +/- 2 mbar), the fluid supply 103a, 103b and/or fluid return pumps 113a, 113b need not be located adjacent to the droplet ejection heads 107, which may be advantageous for weight considerations in some applications.

Any of the recirculating fluid supply systems 10,20,30,40,50 described herein may further be connectable to an external reservoir 201, which may have a much larger fluid capacity, via suitable fluidic connections 202 and a fill pump 203 as depicted in Fig. 3A-Fig. 3B. Where present, the external reservoir 201 may comprise a temperature sensor T4, it may further comprise a level sensor L2. The fill pump 203, to supply fluid to the reservoir 101 from the external reservoir 201, may be located external to the common unit 11, as shown in Fig. 3 A-Fig. 3B, or incorporated within it. Further, the system may comprise level sensors that send a signal to, for example, the external controller 119, or another suitable controller to shut-down the system if the fluid levels drop too low, i.e., below a lower limit value, that may be predefined. In some arrangements the fluid return path 112 may return fluid to the external reservoir 201 rather than the reservoir 101, as shown with a dashed line as fluid return path 112’ in Fig. 3 A-Fig. 3B.

The recirculating fluid supply systems 10,20,30,40,50 described herein may comprise an air valve as a convenient way to allow in air in order to drain the system of fluid. During normal operation, the air valve would be closed, and the reservoir 101 would be open to atmosphere so as to allow air to enter the reservoir and pressures to balance in the system as fluid in the reservoir 101 is depleted.

Instead of the fluid conditioner 135, some of the conditioning of the fluid may take place elsewhere in the recirculating fluid supply system 10,20,30,40,50, or in the droplet ejection heads 107. For example: the droplet ejection heads 107 may contain an inline filter; the reservoir 101,201 may be heated or cooled instead of having an inline heater or cooler. Still further, the reservoirs 101,201 may comprise filters to prevent lumps, contamination (e.g. dirt) or other unwanted particles entering the fluid paths. Such lumps could potentially damage a droplet ejection head 107 or cause reductions in print quality. In most of the applications described herein a single filter in the common unit 11 may suffice; in other applications individual filters per droplet ejection head 107, or per group 409 of droplet ejection heads 107 may be envisioned. As previously mentioned, the reservoirs 101,201 may comprise monitoring devices, such as temperature sensors, level sensors and the like.

Any of the recirculating fluid supply systems 10,20,30,40,50 described herein may further comprise a controller 109a, 109b, as shown in Fig. 1, to control the return and/or fluid supply pumps 113a, 113b, 103a, 103b, as applicable, based on their respective pressure measurements. Alternatively, the common unit 11 in any of the recirculating fluid supply systems 10,20,30,40,50 described herein, may comprise a controller 109, as shown in Fig. 2. Alternatively, the droplet ejection apparatus 1,2, 3, 4, 5 may comprise an external controller 119, as shown in Fig. 3 A-Fig. 3B, arranged to control the recirculating fluid supply system 10,20,30,40,50 and the groups 409a, 409b, or droplet ejection heads 107,107a, 107b, 107a_l,107a_2 as described herein. Such an external controller 119 may also control other components that the droplet ejection apparatus 1,2, 3, 4, 5 may comprise, such as droplet ejection head movement systems and the like. Any of the arrangements of controller 109,119 described herein, or any suitable alternatives, may be used with the embodiments of the invention without departing from the scope of the invention.

As previously mentioned, the droplet ejection apparatus 1,2, 3, 4, 5 described herein may comprise one or more movement devices 500 to move the droplet ejection heads. Any suitable movement device 500 may be used, of which examples are a multi-axis robot arm, a moveable print carriage or a gantry. The movement device 500 may comprise a movement mechanism, such as a movable mounting arm, to move all the droplet ejection heads 107a, 107b, or groups 409a, 409b of droplet ejection heads together, or it may comprise a separate movement mechanism for each group 409a, 409b, or for all the groups at a particular vertical location. It may be understood that the vertical locations of the groups, relative to the ground or other suitable reference point, may be fixed, or that the vertical locations of the groups may alter relative to the reference point during operation. For example, a droplet ejection apparatus 1,2, 3, 4, 5 to print a large area, such as a wall, may comprise a movement arm 500 with groups of droplet ejection heads 107 arranged on the movement arm at different heights such that a larger area can be addressed and coated or painted at one time. Alternatively, a droplet ejection apparatus 1,2, 3, 4, 5 addressing a more complex shape, for example, may comprise groups of droplet ejection heads at different heights where the groups are independently movable so as to address all of the contours of the shape.

It may further be understood that the movement devices 500 may also move the fluid supply pumps and/or fluid return pumps. Having fluid supply pumps 103 and/or fluid return pumps 113, and/or also sensor manifolds 106 (and, where present, dampers 118) that move with the droplet ejection heads 107, may be preferable in some arrangements, as doing so creates a configuration with a static frame of reference between these constituent parts, which may be more favourable for meniscus pressure control. In other embodiments, due, for example, to space or weight constraints, one or more of these components may be arranged on a nonmoving part of the droplet ejection apparatus 1,2, 3, 4, 5 or on a non-moving part of the movement device 500 and connected to the movable droplet ejection heads 107 using suitable pipework. It may be understood that, where the droplet ejection heads 107a, 107b move in tandem with their respective sensor manifolds 106a, 106b, such that the height difference between them remains the same, then the datum pressure offset Poffset will remain the same. It may be understood that the terms “horizontally” and “vertically” oriented, as used herein with reference to the droplet ejection heads, refers to the way in which the droplet ejection heads are mounted relative to the horizon, and not that they are constrained to move in a particular way or to remain in a particular orientation. Though it may be understood that in some arrangements, they may be so constrained. For example, a “vertical” orientation such as that depicted in Fig. 1 may be useful to address a substrate such as a bottle, if a single droplet ejection head would be too small to address the entire height of the bottle in one go. It may be understood that where there are a plurality of heads in a group, they may move independently of each other, but need to do so in such a manner that they remain within the same meniscus pressure window, or within the chosen meniscus pressure window, where applicable.

Still further, there is no requirement that all of the droplet ejection heads in a particular droplet ejection apparatus have the same orientation relative to each other, for example, an arrangement to apply labels and barcodes to packaging may comprise one or more horizontally oriented droplet ejection heads 107 to print onto the top of a box or parcel, whilst one or more vertically oriented droplet ejection heads 107 print onto one or more sides of the box or parcel. Still further, some apparatus may comprise two or more droplet ejection heads that are free to move relative to each other (such as on an arrangement of multi-axis robot arms) and their orientation relative to each other may be subject to change over time.

The modifications described herein, and other modifications and additions, may be made without departing from the scope of the invention. More generally, any of the modifications and additions described herein, may be combined with other modifications and additions described herein in any suitable configuration without departing from the scope of the invention.