WO2022066171A1

WO2022066171A1 - Print agent delivery systems

Info

Publication number: WO2022066171A1
Application number: PCT/US2020/052913
Authority: WO
Inventors: Cristian DIEZ QUILEZ; Miquel Boleda Busquets; Alberto Borrego Lebrato
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2020-09-25
Filing date: 2020-09-25
Publication date: 2022-03-31

Abstract

A print agent delivery system (100) comprises a printhead (140), a pump (120) to convey print agent along a delivery channel (130) to the printhead for ejection and a sensor (136) to monitor a property of print agent in the delivery channel. A controller (150) switches from operating a pump motor (122) driving the pump (120) at a first low speed condition to a second high speed condition, during a continuous printing operation, based on a signal received from the sensor (136) relating to the property and indicative of elevated demand for print agent at the printhead (140).

Description

PRINT AGENT DELIVERY SYSTEMS

BACKGROUND

[0001] Printing systems, such as inkjet printing systems, may use print agent delivery systems to deliver print agent from a print agent supply (such as an ink cartridge) to a printhead.

BRIEF DESCRIPTION OF DRAWINGS

[0002] Examples will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0003] Figure 1 is a simplified schematic of an example of a print agent delivery system;

[0004] Figure 2 is a flow diagram of an example method of operating a print agent delivery system;

[0005] Figures 3 and 4 are simplified schematics of further example print agent delivery systems;

[0006] Figure 5 is a flowchart of an example method of operating a print agent delivery system; and

[0007] Figure 6 is a simplified schematic of a tangible machine-readable medium and a processor.

DETAILED DESCRIPTION

[0008] Print agent delivery systems that receive print agent from external print agent supplies (such as an ink cartridge) may be categorised as "cold-swap" or "hot-swap" systems. In cold-swap systems, print agent may be pumped directly from the external ink supply to the printhead without any intermediate storage such as in an internal reservoir that can be charged with print agent. As such, the print agent supply is generally replaced ("swapped") when the print agent delivery system is not in use ("cold"), for example in a printing operation.

[0009] In contrast, "hot-swap" systems generally comprise some form of intermediate storage such as an internal reservoir, and may have separate pumps for pumping print agent to and from the reservoir. As such, a printing operation may continue while the external print agent supply is being swapped ("hot-swapping"). Owing to the inclusion of intermediate storage for print agent in "hot-swap” systems, such systems can operate to maintain pressure of print agent delivered to a printhead at a variable flow rate. Such systems generally utilise an internal reservoir in which the stored print agent may share an interface with a compressible gas (such as air). For example, a gas pump can be used to maintain the gas at a target pressure, thereby pressurising the print agent within the reservoir at a level sufficient for delivery to a printhead, while permitting the print agent to be discharged at a variable flow rate according the demand of the printhead. The gas pump can be operated intermittently, for example, to maintain the gas at the target pressure.

[0010] In the absence of such an intermediate reservoir, cold-swap systems are generally constrained to using pumps which act directly on the print agent. The pumps may be sized and operated to deliver a maximum flow rate for which a printhead is designed. Within the technical field such constraints may be overcome by implementing an intermediate reservoir, whether or not hot-swapping functionality is provided of the system as a whole.

[0011] Figure 1 shows an example print agent delivery system 100. Print agent delivery systems of the type disclosed herein may be integrated into printing systems that use print agents to print an image on a substrate, for example inkjet printing systems which print an image on a substrate by ejecting a print agent or a plurality of print agents. In inkjet printing systems, nozzles of a printhead are caused to eject print agent and thereby print the image.

[0012] The print agent delivery system 100 of Figure 1 is to receive print agent from an external print agent supply in the form of a print agent supply 10, e.g., a removable cartridge, or in some examples, the supply may be a re-fillable supply known in the art as CISS (Continuous Ink Supply Systems). In flow order from the print agent supply 10, the example print agent delivery system 100 of Figure 1 comprises: a print agent supply port 1 10 to receive print agent from the external print agent supply 10; a pump 120 coupled to the print agent supply port 110 by an upstream portion 132 of a delivery channel 130, and a printhead 140 coupled to the pump 120 by a downstream portion 134 of the delivery channel 130. A sensor 136 is provided to monitor a property of print agent in the delivery channel 130. The delivery channel may extend from the print agent supply port through to a nozzle of the printhead, so as to include a part of the printhead. In this example, the sensor is disposed along the downstream portion 134 of the delivery channel 130 but in other examples the sensor may be at other locations, for example: within the body of the printhead 140, in the upstream portion 132 of the delivery channel 132. The sensor 136 is coupled to a controller 150. The controller 150 is coupled to a pump motor 122 which drives the pump 120.

[0013] In this example, a recirculation loop 160 is provided to recirculate print agent discharged from the pump (e.g. from between an outlet of the pump and the delivery channel 130) to an inlet 124 of the pump. A pressure relief valve 162 is provided along the recirculation loop 160 to permit print agent to recirculate when the pressure downstream of the pump 120 is above a threshold, and to prevent print agent recirculating when the pressure downstream of the pump 120 is below the threshold. For example, the threshold may be set so that the pressure downstream of the pump is maintained at a level sufficient to convey print agent to the printhead 140, against a pressure drop along the delivery channel 130 and any pressure drop within the printhead 140.

[0014] The print agent delivery system 100 of Figure 1 may be referred to as a "coldswap" system as described above, since the print agent is conveyed directly from the external ink supply to the printhead without any intermediate storage such as in an internal reservoir that can be charged with print agent.

[0015] In the absence of an intermediate reservoir, the pump 120 may act directly on the print agent received within it. The pump 120 is driven by the pump motor 122 to pressurise the print agent and convey it along the delivery channel 130 to the printhead for ejection. The pump 120 is controlled by the controller 150 as will be described in further detail below.

[0016] In use, print agent is discharged from the pump at a pump flow rate. The printhead 140 receives print agent from the delivery channel 130 at a variable demand flow rate. The demand flow rate is variable as it is dependent on the print operation, with most print operations requiring different amounts of a print agent at different phases of the print operation. An amount of print agent corresponding to the difference between the pump flow rate and the demand flow rate is recirculated along the recirculation loop 160, provided that the pressure downstream of the pump 140 is greater than a threshold pressure at which the pressure relief valve 162 is to open.

[0017] The controller 150 is to operate the pump motor 122 based on a signal received from the sensor 136 relating to the property of print agent in the delivery channel. The expression "relating to the property" is used herein, as it will be appreciated that the signal itself may not directly represent the respective property (which may be a physical property) of the print agent. For example, the property may be a pressure of the print agent as determined by a pressure sensor, a flow rate of the print agent as determined by a flow rate sensor, or a level of print agent as determined by a level sensor associated with a reservoir in the printhead. For example, the signal may have an amplitude (e.g. in mV) which is proportional to the respective property or otherwise a function of the property of the print agent. The signal may not be directly calibrated to a linear scale of the property, but nevertheless may be a function of the property, such that the controller may process the signal to make the operation of the pump motor 122 dependent on the physical property of the print agent.

[0018] In this particular example, the controller is to switch from operating the pump motor 122 to drive the pump at a first low speed condition to a second high speed condition, based on the signal received from the sensor when the signal is indicative of elevated demand for print agent at the print head. The pump flow rate in the low speed condition may be lower than a maximum demand flow rate for which the printhead 140 is rated, or at which it may attempt to operate at in a given printing operation. Operating the print agent delivery system so that the demand flow rate exceeds the pump flow rate is unsustainable and would normally cause an "out of ink" error. The pump may be a positive displacement pump or a dynamic pump.

[0019] In this particular example, the sensor is a pressure sensor to monitor a pressure of the print agent in the downstream portion 134 of the delivery channel 130. The inventors have found that the pressure of the print agent downstream of the pump 120 tends to fall as the demand flow rate approaches and then equals the pump flow rate. A signal relating to the pressure of print agent downstream of the pump can therefore be used as a reliable indicator that the demand flow rate is approaching or exceeding the pump flow rate (i.e. indicative of elevated print agent demand), and on this basis to switch operation of the pump motor 122 to drive the pump at the second high speed condition. For example, the signal may be processed to determine whether a parameter derived from the signal relating to pressure falls outside of a predetermined range (e.g. below a threshold value) associated with the pump flow rate being sufficient to meet the demand flow rate, or may be processed to determine whether the rate of change of the parameter is indicative of the pressure of the print agent falling at a rate corresponding to an elevated print agent demand that is not being sufficiently met by the pump flow rate. The signal may not be a continuously variable signal relating to the property. For example, the signal may be a binary-type signal (i.e. having two possible signal values), and the sensor may be to issue one of the signal values when the property meets a first set of conditions, and to issue the other of the signal values when the property meets a second set of conditions. For example, the signal may issue one value when the property of pressure is at or above a threshold, and another value with the property of pressure is below the threshold.

[0020] In variant examples, other sensors monitoring other properties of print agent in the delivery channel 130 may be used. For example, a pressure sensor could be provided upstream of the pump 120. The upstream pressure may tend to fall as the demand flow rate approaches and then equals the pump flow rate. When the pump 120 is a dynamic pump, the pressure downstream of the pump 120 and upstream of the pump 120 may be related (e.g. by an operating parameter of the pump), and as such a fall in the pressure upstream of the pump may be indicative of elevated print agent demand at the printhead.

[0021] Further, in variant examples a flow rate sensor could be provided either upstream or downstream of the pump 120, and either upstream or downstream of a recirculation loop when present. It will be appreciated that in examples where a recirculation loops is present, a pump flow rate discharged from the flow rate may be greater than a demand flow rate at the printhead, with excess flow of print agent circulating through the recirculation loop. In such examples, the flow rate upstream and downstream of the recirculation loop corresponds to the demand flow rate at the printhead. Accordingly, the flow rate of print agent in the delivery channel may correspond to the demand flow rate at which print agent is received and utilised (i.e. ejected) at the printhead. Therefore, a flow rate may be directly indicative of an elevated print agent demand. For example, it may be determined that there is an elevated print agent demand when the signal corresponds to a flow rate at or above a threshold, for example a threshold corresponding to a maximum rated flow rate of the pump.

[0022] Further, in variant examples a level sensor could be provided for monitoring a property of print agent in the delivery channel. In particular, the delivery channel may be considered to extend through the printhead to a nozzle or a manifold for a plurality of nozzles. A reservoir may be disposed within the printhead to locally store a relatively small volume of print agent to be ejected, thereby forming part of the delivery channel. A level sensor may be provided within the reservoir, and the monitored property may be a level of print agent in the reservoir. For example, the level sensor may issue a signal indicating whether the level of print agent is greater or less than a threshold amount corresponding to a position of the level sensor within the reservoir. When the level is relatively low, this O is indicative of a lack of sufficient supply to meet a demand flow rate, which is indicative of an elevated demand flow rate relative to an amount of print agent available.

[0023] In the particular example being described with respect to Figure 1 , switching between the low speed condition and the high speed condition is controlled by varying a power supplied to the pump motor 122 between two fixed power levels corresponding to the two conditions. In other examples, it may be the voltage or current that is varied, or a pump with an electronic controller may be instructed to operate at a target speed (e.g. a rotational speed), or a target flow rate.

[0024] There may be a non-linear relationship between any of power, voltage, current, flow rate and target speed. With positive displacement pumps, there is generally a direct correlation between speed and flow rate. With dynamic pumps, the speed and flow rate are complex functions of the pressure conditions at the pump. Nevertheless, it will be appreciated that, with all other conditions being equal, an increase in speed correlates with an ability to deliver an increase in flow rate, provided the flow rate is not otherwise restricted. Accordingly, the expressions "low speed" and "high speed" are to be considered as relative terms (to one another) rather than being indicative of operation at predetermined speeds, with the controller varing the operation of the pump motor so as to achieve an increase in pump speed and flow rate as it switches from the first low speed condition to the second high speed condition.

[0025] By switching to the second high speed condition, the pump flow rate may rise above the demand flow rate, such that printing may continue without interruption of a continuous printing operation. For the purposes of the present disclosure, a continuous printing operation relates to printing with print agent delivered to a printhead along a delivery channel or a plurality of delivery channels associated with a respective continuously-operating pump motor. As noted herein, a pump motor may drive pumps associated with multiple delivery channels. A continuous printing operation is a printing operation in which the printhead substantially continuously ejects print agent received from any of the respective delivery channels (i.e. the delivery channels associated with the pump motor). A continuous printing operation may terminate when there is no demand for any of the print agents associated with the respective pump motor to continue printing the image, for example because a respective portion of the image is not to be printed with those print agents, or because a traversing carriage carrying the printhead is to traverse a substrate without printing before recommencing printing the image, or because a substrate is to be advanced along a feed direction without printing before recommencing printing the image. According to the disclosure, the pump motor switches from operating a pump at a first low speed condition to a second high speed condition during a continuous printing operation based on an elevated print agent demand, whereby the continuous printing operation can continue without necessitating an interruption to replenish print agent or modify operation of a pump motor.

[0026] The power consumption of the pump may be relatively lower when the controller operates the pump motor to drive the pump at the first low speed condition, relative to the second high speed condition. Accordingly, by operating at the first low speed condition until there is an elevated print agent demand that necessitates operation at the second high speed condition, power consumption may be reduced.

[0027] For components which interact with print agent, component degradation and fatigue may be a function of flow rate through the associated component (e.g. through the pump, delivery channel, sensor and recirculation loop). Accordingly, by operating at the first low speed condition until there is an elevated print agent demand, component wear may be reduced and component lifespan may be increased.

[0028] Figure 2 is a flow diagram of an example method 200 of operating a print agent delivery system, for example the print agent delivery system 100 of Figure 1 .

[0029] In block 202, a controller operates a pump motor to drive a pump to discharge print agent at a pump flow rate for conveying print agent to a printhead along a delivery channel. The printhead receives print agent conveyed by the pump at a printhead demand flow rate, and an excess flow of print agent discharged by the pump recirculates to an inlet of the pump via a recirculation loop. The excess flow corresponds to a difference between the pump flow rate and the demand flow rate.

[0030] In block 204, the controller determines a condition corresponding to elevated print agent demand beyond a threshold. For example, a signal 206 may be received from a sensor for monitoring a parameter relating to pressure downstream of the pump, and may be compared with a threshold or predetermined range as described above. In other examples, the condition may be determined in other ways. For example, a controller for a print operation may predict a demand flow rate for a print agent based on the instructions for the print operation, and may determine that the demand flow rate for print agent is to increase to an elevated level at a respective phase of the print operation. [0031] In block 208, responsive to the determination of the condition, the controller operates the pump motor to increase the pump flow rate from a baseline pump flow rate to an elevated pump flow rate, during a continuous printing operation.

[0032] Figure 3 shows a further exampie of a print agent delivery system 300. The print agent delivery system 300 differs from that described above with respect to Figure 1 in that it is to deliver a plurality of print agents. Some components of the print agent delivery system 300 may be substantially as described above with respect to Figure 1 , and as such the same reference numeral may be used where appropriate.

[0033] In flow order from an external print agent supply 10, the example print agent delivery system 300 comprises: a print agent supply port 130 to receive a plurality of separate print agents from the external print agent supply; and a respective plurality of delivery sub-assemblies 312, 314, 316, 318 for the respective print agents, each having a respective delivery channel 130 for conveying the print agent to a printhead as will be described below. [0034] Each delivery sub-assembly 312, 314, 316, 318 comprises, in flow order: a connection to receive the respective print agent from the print agent supply port 130; a pump 120 coupled to the connection by an upstream portion 132 of the delivery channel 130, and a printhead 140 coupled to the pump 120 by a downstream portion 134 of the delivery channel 130. The pump 120 may be a positive displacement pump (e.g. a piston pump or screw pump), or a dynamic pump. In this example, the system 300 is illustrated as having separate printheads 140, however it will be appreciated that there may be a common printhead, with each delivery channel 130 of the respective delivery subassemblies providing print agent into a respective port of the common printhead. In the interests of clarity in Figure 3, reference numerals are provided on the first sub-assembly 302 to the extent that components of the other sub-assemblies are identical.

[0035] In each delivery sub-assembly, a sensor 136 is provided to monitor a property of print agent in the delivery channel 130. The sensors 136 may be in accordance with any of the example descriptions above, for example they may be pressure sensors, flow rate sensors or level sensors. [0036] In this example, a recirculation loop 160 is provided to recirculate print agent discharged from the pump (e.g. from between an outlet of the pump and the delivery channel 130) to an inlet 124 of the pump. A pressure relief valve 162 is provided along the recirculation loop 160 to permit print agent to recirculate when the pressure downstream of the pump 120 is above a threshold, and to prevent print agent recirculating when the pressure downstream of the pump 120 is below the threshold. For example, the threshold may be set so that the pressure downstream of the pump is maintained at a level sufficient to convey print agent to the printhead 140, against a pressure drop along the delivery channel 130 including through the printhead 140.

[0037] In the example print agent delivery system 300 of Figure 3, there are two print motors 330, 340 each associated with a respective set of sub-assemblies. In particular, three sub-assemblies 312, 314, 316 form a first linked set of sub-assemblies as the respective pumps 120 are each driven by a first pump motor 330 which is common between them. Similarly, those three sub-assemblies 312, 314, 316 are each associated with a common first controller 350 which is coupled to the sensors 136 associated with the delivery channel 130 of each sub-assembly 312, 314, 316 within the linked set.

[0038] The second print motor 340 is associated with the fourth sub-assembly 318 alone, and is coupled to drive the pump 120 of the fourth sub-assembly 318 to convey the respective print agent along the respective delivery channel 130. A second controller 352 is coupled to the sensor 136 associated with the respective delivery channel 130, and is coupled to the second print motor 340 to control its operation.

[0039] In use, the two controllers 350, 352 independently operate to control the respective pump motors 330, 340 based on the signals received from the respective sensor or sensors 136. In this example, the second controller 352 functions substantially as described above with respect to the controller of Figure 1 , as it controls one pump motor associated with one pump, delivery channel and sensor.

[0040] In this example, the first controller 350 for the linked set of sub-assemblies 312, 314, 316 switch operation of the common pump motor 330 to drive each of the pumps 120 associated with the respective delivery channels from a first low speed condition to a second high speed condition, based on any one of the signals received from the respective sensors 136 being indicative of an elevated print agent demand. The analysis to determine whether a signal is indicative of an elevated print agent demand may be performed as described above with respect to Figure 1 . [0041] Accordingly, each of the pumps 120 associated with the linked sub-assemblies is driven io operate at the second high speed condition when the demand flow for any of the respective print agents is elevated.

[0042] By providing a common print motor for multiple print agents, but not all print agents, the equipment cost and complexity may be reduced relative to providing one print motor per print agent. By providing a print motor which is dedicated for a selected one print agent, while providing a common print motor for other print agents, overall component fatigue and energy consumption may be reduced in circumstances that the selected agent is more frequently used at elevated flow rates. For example, it may be that a dedicated print motor is provided for a particular colorant (e.g. black) that is utilised most often, whereas a common print motor is provided for other colorant channels (e.g. cyan, magenta, yellow).

[0043] In the example system 300 (and in the further example described below), two or more pumps may be provided in a common unit, for example in a common pump body coupled to multiple supply and/or delivery channels and supporting multiple pumps. A recirculation loop may be integrally provided with a pump. A pump motor or pump motors may be integrated with such a common unit. Similarly, two or more sensors may be provided by a common sensor assembly having multiple sensors for monitoring the flow in respective delivery channels. [0044] Figure 4 shows a further example of a print agent delivery system 400. Like the example of Figure 3, the system 400 is to deliver multiple print agents along a plurality of delivery channels associated with respective sub-assemblies 412, 414, 416, 418. Each of the sub-assemblies 412, 414, 416, 418 is as described above with respect to Figure 3. The system 400 of Figure 4 differs from that described above 'with respect to Figure 3 in that each one of the sub-assemblies 412, 414, 416, 418 is provided with a respective print motor 432, 434, 436, 438 to drive the respective pump 120. Further a common controller 450 is provided to receive each of the signals from the respective sensors 136 of the subassemblies, and to control each print motor 432, 434, 436, 438 individually based on the signal received from the respective sensor. It will be appreciated that in other examples there may be a plurality of individual controllers 450, for example one associated with each sub-assembly. [0045] Figure 5 is a flow diagram of a method of operating a print agent deiivery system, as implemented by a controller with respect to each individual pump motor. The method may be implemented using any of the systems described herein, for example any of the systems 100, 300, 400 described above with respect to Figures 1 , 3 and 4. [0046] In a system which is to delivery a plurality of print agents and which has a plurality of print motors for driving the pumps associated with those print agents, the method is conducted independently with respect to each print motor. As such, the following description relates to the control of a print motor associated with one pump or a plurality of linked pumps (i.e. linked by sharing a common print motor). [0047] Block 202 is conducted as described above with respect to Figure 2.

[0048] Block 204 is conducted as described above with respect to Figure 2. To the extent that the controller receives a plurality of signals from sensors associated with a plurality of print channels, the controller evaluates whether each respective signal is indicative of an elevated print agent demand as described above with respect to Figure 2. If any of the plurality of signals is indicative of an elevated print agent demand, then the controller determines an elevated print agent demand which is to be acted upon, even though the elevated print agent demand may be for one print agent of a plurality of print agents.

[0049] Block 208 is conducted as described above with respect to Figure 2. The controller may operate the pump motor at two predetermined conditions including a low speed condition corresponding to the baseline pump flow rate and a high speed condition corresponding to the elevated pump flow rate. Each condition may be defined by one of: a predetermined rotary speed of the pump motor, a predetermined voltage to the pump rotor; a predetermined current to the pump rotor; or a predetermined power to the pump rotor. [0050] In block 510, the controller determines a condition to reduce the pump flow rate from the elevated pump flow rate, and responsive to that determination controls operation of the pump motor to reduce the pump flow rate. For example, the controller may determine the condition based on a time delay since the determination of elevated print agent demand (for example, 2 seconds). [0051] The method is conducted repeatedly during a continuous printing operation.

When the determination to reduce the pump flow rate is made based on a delay, it may be that the print agent demand is still elevated. In such circumstances, the controller will once again determine the condition of elevated print agent demand at block 204, and thereby proceed to block 208 to increase the pump flow rate.

[0052] In systems incorporating a recirculation loop, it may be that pressure remains relatively constant while the pump flow rate exceeds the demand flow rate. Accordingly, it may be that a determination of reduced print agent demand is not made based on a signal from a pressure sensor. A reduced print agent demand may be determined by the controller predicting a demand flow rate based on instructions for a print operation. For example, the controller may determine that an elevated demand flow rate for a print agent is to reduce from an elevated level at a respective phase of the print operation, in another example, the controller may determine the condition responsive to a signal that a flow rate has (or all respective flow rates have) dropped below a threshold such that operation of the pump motor may be reverted to the first low speed condition.

[0053] In other examples, a reduced print agent demand may be determined by incorporation of additional sensors. For example, a sensor may be provided to monitor a flow rate through a recirculation loop. A reduced print agent demand may be determined on the basis that a flow rate through the recirculation loop has returned above a threshold.

[0054] A print agent delivery system in accordance with the examples described herein may be provided by retrofitting a print agent delivery system having a similar architecture with an updated controller which is to control the or each print agent motor as described herein. It may be that a pump motor of a system for retrofitting is constrained to operating at a single condition (e.g. a single speed, or input power, voltage or current), and so the pump motor may also be retrofitted with a dual-speed or variable-speed pump motor that may be controlled by the controller as described herein.

[0055] Examples in the present disclosure can be provided as methods, systems or machine readable instructions, such as any combination of software, hardware, firmware or the like. Figure 6 shows a tangible machine readable storage medium 602 comprising instructions 604 to be executed by a processor 606. Such machine readable instructions may be included on a computer readable storage medium (including but is not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0056] In particular, the instructions 604 may be to cause the processor to receive a signal relating to a property of print agent in a delivery channel of a print agent delivery system along which the print agent is conveyed by a pump; to determine a condition corresponding to an elevated print agent demand, based on the signal; and responsive to determining the condition, switch from operating the pump motor to drive the pump at a first low speed condition, to operating the pump motor to drive the pump at a second high speed condition. For example, the signal may relate to a pressure of print agent downstream of a pump in a print agent delivery system; and the condition may be a low pressure condition downstream of the pump determined based on the signal, as described above.

[0057] The instructions may be to cause the processor to conduct any of the methods as described herein.

[0058] The tangible machine readable medium may be providing by updating or replacing software or firmware associated with a print agent delivery system, for example software or firmware of a controller of a print agent delivery system, in order to provide a controller which is to perform any of the methods described herein.

[0059] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each flow and/or block in the flow charts and/or block diagrams, as well as combinations of the flows and/or diagrams in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0060] The machine readable instructions may, for example, be executed by a general purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine readable instructions. Thus functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors. [0061] Such machine readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0062] Such machine readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by flow(s) in the flow charts and/or block(s) in the block diagrams.

[0063] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure.

[0064] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.

[0065] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims.

[0066] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.

Claims

1 . A print agent delivery system comprising: a printhead; a pump to convey print agent along a delivery channel to the printhead for ejection; a sensor to monitor a property of print agent in the delivery channel; a controller to switch from operating a pump motor driving the pump at a first low speed condition to a second high speed condition, during a continuous printing operation, based on a signal received from the sensor relating to the property and indicative of elevated demand for print agent at the printhead.

2. A print agent delivery system according to claim 1 , wherein: the sensor is a pressure sensor and the property is a pressure of the print agent; the sensor is a flow rate sensor and the property is a flow rate of the print agent; and/or the sensor is a level sensor associated with a reservoir in the printhead, and the property is a level of print agent in the reservoir.

3. A print agent delivery system according to claim 1 , wherein the pump is to discharge print agent at a pump flow rate; wherein the printhead is to receive print agent from the pump at a demand flow rate; and wherein the print agent delivery system comprises a recirculating bypass channel to recirculate an excess flow of print agent discharged by the pump to an inlet of the pump; wherein the excess flow corresponds to a difference between the pump flow rate and the demand flow rate.

4. A print agent delivery system according to claim 3, wherein the recirculating bypass channel comprises a pressure relief valve.

5. A print agent delivery system according to claim 4, wherein the pump is a positive displacement pump.

6. A print agent deliver system according to ciaim 1 , comprising a plurality of delivery channels for conveying respective print agents to the printhead; wherein for each delivery channel, the print agent delivery system comprises: a respective pump to convey print agent along the respective delivery channel to the printhead for ejection; and a sensor to monitor a property of print agent in the delivery channel.

7. A print agent delivery system according to claim 6, wherein a linked set of the pumps are driven by a common pump motor, the linked set of the pumps comprising a plurality of pumps associated with a corresponding linked set of delivery channels and a corresponding linked set of sensors; and wherein the controller is configured to switch from operating the common pump motor at the first low speed condition to the second high speed condition, based on a signal received from a sensor of the linked set of sensors, the signal relating to the property and indicative of elevated demand for print agent at the printhead.

8. A print agent delivery system according to claim 6, comprising a plurality of pump motors to drive respective pumps, wherein each pump motor is to drive a pump associated with a delivery channel or a linked set of pumps associated with a corresponding linked set of delivery channels; wherein the controller is to separately control each pump motor to switch from operating the pump motor at the first low speed condition to the second high speed condition, based on a signal received from a sensor of the respective delivery channel or linked set of delivery channels, the signal relating to the property and indicative of elevated demand for print agent at the printhead.

9. A print agent delivery system according to claim 8, wherein the controller comprises a plurality of controllers, each controlling a respective set of one or more pump motors.

10. A method of operating a print agent delivery system, comprising: a controller operating a pump motor to drive a pump to discharge print agent at a pump flow rate for conveying print agent to a printhead along a delivery channel, the printhead receiving print agent conveyed by the pump at a printhead demand flow rate, 17 an excess flow of print agent discharged by the pump recirculating to an inlet of the pump via a recirculation loop, wherein the excess flow corresponds to a difference between the pump flow rate and the demand flow rate; the controller determining a condition corresponding to elevated print agent demand beyond a threshold; responsive to the determination, the controller operating the pump motor to increase the pump flow rate from a baseline pump flow rate to an elevated pump flow rate, during a continuous printing operation.

11. A method according to claim 10, further comprising receiving a signal from a sensor monitoring a property of print agent in the delivery channel, wherein: the sensor is a pressure sensor and the property is a pressure of the print agent; the sensor is a flow rate sensor and the property is a flow rate of the print agent; and/or the sensor is a level sensor associated with a reservoir in the printhead, and the property is a level of print agent in the reservoir; and wherein the condition corresponding to elevated print agent demand is determined based on the signal.

12. A method according to claim 10, further comprising the controller determining a condition to reduce the pump flow rate from the elevated pump flow rate, wherein the condition is determined based on: on a time delay since the determination of elevated print agent demand; or a determination of reduced print agent demand; responsive to the determination, the pump motor operating to reduce the pump flow rate from the elevated pump flow rate.

13. A method according to claim 10, wherein the controller is to operate the pump motor at two predetermined conditions including a low speed condition corresponding to the baseline pump flow rate and a high speed condition corresponding to the elevated pump flow rate; wherein each condition is defined by one of: a predetermined rotary speed of the pump motor; and 18 a predetermined voltage, current or power to the pump rotor.

14. A method according to claim 10, wherein the print agent delivery system comprises: a plurality of delivery channels for conveying respective print agents to the printhead; a plurality of pump motors to drive respective pumps, wherein each pump motor is to drive a pump associated with a delivery channel or a linked set of pumps associated with a corresponding linked set of delivery channels; wherein the method comprises: the controller determining a condition corresponding to elevated print agent demand beyond a threshold along a delivery channel associated with one of the pump motors; the controller independently varying the operation of the respective pump motor to increase the pump flow rate along the respective delivery channel.

15. A tangible machine-readable medium comprising a set of instructions which, when executed by a processor cause the processor to: receive a signal relating to a property of print agent in a delivery channel of a print agent delivery system along which the print agent is conveyed by a pump; determine a condition corresponding to an elevated print agent demand, based on the signal; responsive to determining the condition, switch from operating the pump motor to drive the pump at a first low speed condition, to operating the pump motor to drive the pump at a second high speed condition.