Printing device having a printing fluid detection system

A printing device is provided, which is configured to print a printing fluid onto a printing medium. The printing device includes a printing fluid reservoir configured to hold a volume of the printing fluid, a print head assembly configured to transfer the printing fluid to the printing medium, a conduit fluidically connecting the printing fluid reservoir to the print head assembly, and a printing fluid detector associated with the conduit. The printing fluid detector includes a first electrode and a second electrode coupled with the conduit and configured to be in contact with printing fluid in the conduit, a power supply configured to apply an alternating supply signal to the first electrode, and detector circuitry configured to detect a measured impedance value related to the presence of printing fluid within the conduit by comparing the alternating supply signal with a detected signal that has been modified from the applied signal by an impedance characteristic of the printing fluid.

BACKGROUND

Many types of printing devices, including but not limited to printers, copiers, and facsimile machines, print by transferring a printing fluid onto a printing medium. These printing devices typically include a printing fluid supply or reservoir configured to store a volume of printing fluid. The printing fluid reservoir may be located remotely from the print head assembly (“off-axis”), in which case the fluid is transferred to the print head assembly through a suitable conduit, or may be integrated with the print head assembly (“on-axis”). Where the printing fluid reservoir is located off-axis, the print head assembly may include a small reservoir that is periodically refilled from the larger off-axis reservoir.

Some printing devices may include a printing fluid detector configured to produce an out-of-fluid signal when printing fluid in the print head assembly or printing fluid reservoir drops below a predetermined level. This signal may be used to trigger the printing device to stop printing, and also to alert a user to the out-of-fluid state. The user may then replace (or replenish) the printing fluid reservoir and resume printing.

Various types of printing fluid detectors are known. Examples include, but are not limited to, optical detectors, pressure-based detectors, resistance-based detectors and capacitance-based detectors. Capacitance-based printing fluid detectors may utilize a pair of capacitor plates positioned adjacent, but external, to the printing fluid. These detectors measure changes in the capacitance of the plates with changes in printing fluid levels. However, the changes in capacitance of these systems may be too small to easily distinguish the capacitance changes from background noise. Thus, it may be difficult to accurately determine a printing fluid level, resulting in the generation of false out-of-fluid signals, and/or the failure to generate out-of-fluid signals when appropriate. Furthermore, many of these detectors may have difficulty distinguishing printing fluid from printing fluid froth, which is commonly dispensed by a printing fluid reservoir after the reservoir is substantially emptied of printing fluid.

SUMMARY

A printing device is provided, which is configured to print a printing fluid onto a printing medium. The printing device includes a printing fluid reservoir configured to hold a volume of the printing fluid, a print head assembly configured to transfer the printing fluid to the printing medium, a conduit fluidically connecting the printing fluid reservoir to the print head assembly, and a printing fluid detector associated with the conduit. The printing fluid detector includes a first electrode and a second electrode coupled with the conduit and configured to be in contact with printing fluid in the conduit, a power supply configured to apply an alternating supply signal to the first electrode, and detector circuitry configured to detect a measured impedance value related to the presence of printing fluid within the conduit by comparing the alternating supply signal with a detected signal that has been modified from the applied signal by an impedance characteristic of the printing fluid.

DETAILED DESCRIPTION

FIG. 1shows, generally at10, a block diagram of a first embodiment of a printing device according to the present invention. Printing device10may be any suitable type of printing device, including but not limited to, a printer, facsimile machine, copier, or a hybrid device that combines the functionalities of more than one of these devices. Printing device10includes a print head assembly12configured to transfer a printing fluid onto a printing medium14positioned adjacent to the print head assembly. Print head assembly12typically is configured to transfer the printing fluid onto printing medium14via a plurality of fluid ejection mechanisms16. Fluid ejection mechanisms16may be configured to eject printing fluid in any suitable manner. Examples include, but are not limited to, thermal and piezoelectric fluid ejection mechanisms.

Print head assembly12may be mounted to a mounting assembly18configured to move the print head assembly relative to printing medium14. Likewise, printing medium14may be positioned on, or may otherwise interact with, a media transport assembly20configured to move the printing medium relative to print head assembly12. Typically, mounting assembly18moves print head assembly12in a direction generally orthogonal to the direction in which media transport assembly20moves printing medium14, thus enabling printing over a wide area of printing medium14.

Printing device10also typically includes an electronic controller22configured receive data24representing a print job, and to control the ejection of printing fluid from print head assembly12, the motion of mounting assembly18, and the motion of media transport assembly20to effect printing of an image represented by data24.

Printing device10also includes a printing fluid reservoir26configured to supply printing fluid stored within the printing fluid reservoir to print head assembly12as needed. Printing fluid reservoir26is fluidically connected to print head assembly12via a conduit28configured to transport printing fluid from the printing fluid reservoir to the print head assembly. Any of print head assembly12, printing fluid reservoir26, or conduit28may include a suitable pumping mechanism (not shown) for effecting the transfer of printing fluid from the printing fluid reservoir to the print head assembly. Examples of suitable pumping devices include, but are not limited to, peristaltic pumping devices.

Printing fluid reservoir26may be configured to deliver printing fluid to print head assembly12continuously during printing, or may be configured to periodically deliver a predetermined volume of printing fluid to the print head assembly. Where printing fluid reservoir26is configured to deliver a predetermined volume of printing fluid to print head assembly12periodically, the print head assembly may include a smaller reservoir29configured to hold printing fluid transferred from printing fluid reservoir26.

Printing device10also includes a printing fluid detector30disposed along conduit28between printing fluid reservoir26and print head assembly12. Printing fluid detector30is configured to measure an impedance value associated with the printing fluid in conduit28, and to determine a characteristic of the printing fluid in the conduit based upon the measured impedance value. For example, printing fluid detector30may be configured to determine a type of printing fluid in conduit28, and/or whether any printing fluid is in the conduit.

FIG. 2shows a schematic depiction of a first exemplary embodiment of printing fluid detector32. Printing fluid detector30includes a first electrode32and a second electrode34. Each electrode has a hollow interior through which printing fluid may flow, and solid walls configured to contain the printing fluid within the hollow interior, and thus forms a portion of conduit28. First electrode32and second electrode34are each electrically conductive, and are separated by an electrically insulating conduit segment36. First electrode32and second electrode34are arranged in the conduit such that printing fluid35flowing from printing fluid reservoir26into print head assembly12first flows through one of the electrodes, then through electrically insulating conduit segment36, and then through the other electrode before reaching the print head assembly. InFIG. 2, printing fluid is depicted as flowing first through second electrode34. However, it Will be appreciated that printing fluid may also flow first through first electrode32.

Printing fluid detector30also includes power supply circuitry40configured to apply an alternating signal to the first electrode or second electrode (or, equivalently, across the first and second electrodes). A resistor42is disposed between power supply circuitry40and first electrode32, in series with first electrode32and second electrode34.

Additionally, printing fluid detector30includes detector circuitry44configured to determine a measured impedance value of the printing fluid from a comparison of the supply signal measured at einand a detected signal measured at eout. As shown inFIG. 2, einmay be measured at the power supply side of resistor42, and eoutmay be measured at the side of resistor54closer to first electrode32. The measured impedance value, either a capacitance value or a resistance value, may then be used to determine a characteristic of printing fluid42in printing fluid reservoir26, including but not limited to, a printing fluid type and an out-of-fluid condition. Furthermore, where the rate of transfer of printing fluid from printing fluid reservoir26to print head assembly12is known, a printing fluid level in printing fluid reservoir26may also be determined. These determinations are described in more detail below.

Detector circuitry44may include a memory46and a processor48for comparing the supply signal and the detected signal to determine the measured impedance value. Memory46may be configured to store instructions executable by processor48to perform the comparison of the supply signal and detected signal to determine the measured impedance value. The instructions may also be executable by processor48to compare the measured impedance value to predetermined impedance values arranged in a look-up table also stored in memory46to determine the desired characteristic of the printing fluid in conduit28.

First electrode32and second electrode34are each configured such that the electrically conductive materials that form the first and second electrodes are in direct contact with printing fluid when the printing fluid is in the conduit. In some capacitive fluid level detection systems, the capacitor plates may be positioned externally from the fluid being measured. However, as described above, the changes in capacitance due to a presence or absence of printing fluid in these systems may be too small to easily distinguish the changes from background noise.

In contrast, by placing first electrode32and second electrode34in direct contact with the printing fluid, extremely large capacitances may be formed. When two electrodes are placed in an ionic fluid, such as many printing fluids, and charged with opposite polarities, a layer of negative ions forms on the positively charged electrode, and a layer of positive ions forms on the negatively charged electrode. Furthermore, additional layers of positive and negative ions form on the innermost ion layers, forming alternating layers of oppositely charged ions extending outwardly into the printing fluid from each electrode. This charge structure is referred to as an electrical double layer (EDL), due to the double charge layer represented by the charges in the electrode and the charges in the first ion layer on the electrode surface. The EDL at each electrode acts effectively a capacitor, wherein the layer of ions acts as one plate and the electrode acts as the other plate. The effective circuit of the electrodes in the solution is shown generally at50inFIG. 3, wherein capacitor52represents the EDL at first electrode32, and capacitor54represents the EDL at second electrode44. The printing fluid will also have an associated resistance, represented by resistor55.

Due to the atomic-scale proximity of the ions to the electrode in the EDL, and to the fact that capacitance varies inversely with the distance of charge separation in a capacitor, extremely large capacitances per unit electrode surface area are generated in the EDLs associated with electrodes32and34. The capacitances may be orders of magnitude larger than those possible with electrodes located external to the printing fluid. For example, where the surface areas and separation of first electrode32and second electrode34would be expected to result in a capacitance in the femptofarad range, capacitances in the nanofarad or microfarad range are observed. These large capacitances facilitate the measurement of the impedance of the printing fluid in conduit28.

Likewise, when conduit28is drained of printing fluid, much lower capacitances are observed.FIG. 4shows the printing fluid detector ofFIG. 3drained of sufficient printing fluid such that second electrode34is not in contact with any printing fluid. In this instance, no EDL exists on the surface of second electrode34. Thus, the capacitance of electrodes32and34is lower than when both electrodes are in contact with printing fluid. The drop in capacitance may be significant and easily distinguishable from noise. Thus, this difference in capacitance may be used to detect an out-of-fluid condition within the conduit, and thus an out-of-fluid condition in printing fluid reservoir26.

First electrode32and second electrode34may each have any suitable shape and size.FIG. 5shows a first exemplary embodiment of first electrode32and second electrode34, in which each electrode takes the form of an electrically conductive cylinder, indicated at60and60′ for the first and second electrodes, respectively. Each cylinder60(and60′) includes an end62configured to fit snugly within the inner diameter of electrically insulating conduit segment36. Also, cylinder60includes a second end configured to fit within conduit segment64, which leads toward printing fluid reservoir26, and cylinder60′ includes a second end configured to fit within conduit segment66, which leads to print head assembly12. Cylinders60,60′ also include electrical connections (not shown) to detector circuitry44.

Each end62of each cylinder60,60′ may include one or more barbs68to hold the end securely within the interior of the adjacent conduit segment. Alternatively, ends62may be connected to conduit segments36,64and66via an adhesive, a clamp, and/or any other suitable attachment mechanism. Likewise, the ends of each cylinder60,60′ may be configured to fit over adjacent conduit segments such that the conduit segments fit within the interior of the cylinders.

Cylinders60,60′ may have any suitable dimensions. For example, each cylinder60,60′ may have an inner diameter of approximately 0.5-5.0 mm, and more typically approximately 1 mm. A cylinder60with an inner diameter in this range may help to ensure that printing fluid remains in contact with all inner surfaces of the cylinder while inside of the cylinder, and thus to help ensure the proper functioning of printing fluid detector32. Likewise, each cylinder60,60′ may have any suitable length. Suitable lengths include, but are not limited, to those in the range of approximately 10 mm to 30 mm. While the depicted cylinders have a generally round cross-sectional shape, it will be appreciated that the cylinders may have any other suitable cross-sectional shape, including, but not limited to, polygonal cross-sections. Thus, the term “cylindrical” as used herein is used only to describe the appearance of the depicted embodiment, and is not intended to be limiting in any sense. Likewise, it will be appreciated that the dimensional ranges stated above are merely exemplary, and that cylinders60and60′ may also have dimensions outside of these ranges.

Cylinders60,60′ may be made of any suitable electrically conductive material. Examples of suitable materials include, but are not limited to, metals such as stainless steel, platinum, gold and palladium. Alternatively, first electrode32and second electrode34may be made from an electrically conductive carbon material. Examples include, but are not limited to, activated carbon, carbon black, carbon fiber cloth, graphite, graphite powder, graphite cloth, glassy carbon, carbon aerogel, and cellulose-derived foamed carbon. To increase the conductivity of a carbon-based electrode, the carbon may be modified by oxidation. Examples of suitable techniques to oxidize the carbon include, but are not limited to, liquid-phase oxidations, gas-phase oxidations, plasma treatments, and heat treatments in inert environments.

In some embodiments, first electrode32and second electrode34may be coated with an electrically conductive coating. For example, first electrode32and second electrode34may be coated with a material having a high surface area-to-volume ratio to increase the effective surface area of the electrode. This may increase the capacitances that may be achieved with the electrode, as the electrode surface may accommodate more charge. The use of such a coating may allow smaller electrodes to be used without any sacrifice in measurement sensitivity. The use of a coating also may offer the further advantage of protecting the electrode material from corrosion by the printing fluid. Examples of suitable electrically conductive coatings include, but are not limited to, Teflon-based coatings (which may be modified with carbon), polypyrroles, polyanilines, polythiophenes, conjugated bithiazoles and bis-(thienyl) bithiazoles. Furthermore, the coating may be selectively cross linked to reduce the level and type of adsorbed printing fluid components.

FIGS. 6-7show a second exemplary embodiment of the first electrode, second electrode and electrically insulating conduit segment. Referring first toFIG. 6, the first electrode, second electrode and electrically insulating conduit segment are all integrated into a housing, indicated generally at70. Housing70includes an opening72at each end to accept the attachment of conduit28and to permit passage of a flow of printing fluid. Housing72also may include one or more electrical ports74configured to permit electrical connections to be made to first electrode32and second electrode34, which are contained within housing72. In the depicted embodiment, housing72includes two electrical ports74, one for each electrode contained within. However, it will be appreciated that housing72may include either more or fewer electrical ports.

Next,FIG. 7shows a broken-away view of housing72that shows first electrode and second electrode. Each electrode takes the form of an electrically conductive cylinder, indicated at76and76′ for the first and second electrodes, respectively. Cylinders76and76′ are each seated in a compartment configured to hold the cylinder in place within housing72. Cylinders76and76′ are separated by a separator80that holds the cylinders apart from one another at a desired spacing. Separator80thus also acts as electrically insulating conduit segment36′ in the embodiment ofFIGS. 6-7.

Housing72may have any suitable construction. For example, housing72may be formed from a molded electrically insulating material, such as a suitable plastic or ceramic, or a machined material. The inner surface of each cylinder76and76′ may be flush with the inner surface of separator78, or may be raised or lowered with respect to the inner surface of the separator. Likewise, cylinders76and76′ may be made from any suitable electrically insulating material, including but not limited to those listed above for the embodiment of FIG.5.

As mentioned above, printing fluid detector30may be used either to measure a capacitance value or a resistance value. The determination and use of capacitance measurements are described first. As is well known in the electrical arts, a capacitor may cause a phase shift in an alternating signal, in that the current through the capacitor lags the voltage across the capacitor. This effect is observed with EDL capacitance. The magnitude of the phase shift is a function of both the frequency of the signal and the capacitance of the capacitor. Thus, the phase shift of the supply signal measured at einrelative to the detected signal measured at eoutmay be used to determine the capacitance of first electrode32and second electrode34in the presence or absence of printing fluid.

FIG. 8shows, generally at90, a graph depicting the observed phase shift of a signal in an exemplary printing fluid detector as a function of the log of the frequency of the signal. The data represented in graph90was taken from a printing fluid detector full of fluid. Line92is drawn through a plurality of data points (not shown) taken over a range of frequencies from approximately 1 Hz to approximately 1 MHz. The phase shift shows a first region94between approximately 1 Hz and approximately 1 kHz in which the phase shift varies significantly as a function of the frequency of the supply signal. Referring toFIG. 9, which shows a graph100illustrating the frequency dependence of the resistive component of the total impedance of the electrodes and printing fluid at102and the capacitive portion of the total impedance at104, it can be seen that the capacitive portion dominates the total impedance at lower frequencies. Thus, the phase shift of the detected signal compared to the supply signal is expected to be greatest in this region.

Referring again toFIG. 8, the phase shift is seen to be essentially zero in a second, middle region96of graph90, between approximately 1 kHz and 100 kHz. In this region, the capacitive and inductive portions of the impedance are negligible, while the resistive portion is dominant. Finally, the phase shift increases in a third, high-frequency region98of graph90, above approximately 100 kHz. This phase shift is due to inductive effects. Thus, the capacitance of the printing fluid within conduit28may be measured most sensitively in capacitive frequency region94, between approximately 1 Hz and 1 kHz. While the phase shift is expected to be greatest at low frequencies, the use of frequencies in the range of 50-100 Hz still give large phase shifts, and also may enable the more rapid acquisition of data. Furthermore, the use of lower frequencies (<1 Hz) may result in the plating of the electrodes with metal ions present in the printing fluid, whereas the use of higher frequencies may avoid the occurrence of plating.

Because the total capacitance of first electrode32and second electrode34is a function of the amount of charge stored on each electrode, the capacitance of the electrodes drops as the fluid level (and thus the size of each EDL) drops. This drop is relatively large where one of the electrodes is not in contact with printing fluid. Thus, an absence of printing fluid in conduit28may be observed as a relatively significant change in the phase shift between the supply signal measured at einand the detected signal measured at eout.

FIG. 10shows, generally at110, a graph depicting the dependence of the phase shift112between the supply signal and the detected signal as a function of an amount of electrode surface area covered by printing fluid. Graph110shows the result of experiments performed with two electrodes in a vessel of printing fluid, but the graph may be used to extrapolate capacitances observed between a full-of-fluid condition and an out-of-fluid condition in conduit28. The full-of-fluid condition corresponds to point114, which shows a phase shift of approximately 3.0 ms, while an out-of-fluid condition corresponds approximately to point116, which shows a phase shift of approximately 0.5 ms.

The magnitude of the phase shift at these printing fluid levels has been found to be accurately reproducible. This enables a look-up table of phase shifts associated with an absence or presence of printing fluid to be constructed and stored in memory48. Thus, processor46may be programmed to match a measured phase shift value to phase shift values stored in the look-up table in memory48for both the “full of fluid” and out-of-fluid conditions, and then to determine the printing fluid level corresponding to the measured phase shift value. Processor46may then communicate this condition to printing device controller22, which may stop printing or take other suitable action in response. Alternatively, a simple threshold filter circuit may be used to detect an out-of-fluid signal without the use of a look-up table, wherein capacitances above a preselected threshold value are considered to indicate the presence of printing fluid, and capacitances below the preselected threshold value (or a separate, lower preselected value) are considered to indicate the absence of printing fluid.

As described above, printing fluid detector30may be used to detect other printing fluid characteristics besides an out-of-fluid condition. For example, printing fluid detector32may be used to detect a printing fluid type. Different ionic printing fluids typically have different metal cations ions, organometallic ions, and counterions, and also typically have different concentrations of ions, depending upon the color (and other physical characteristics) of the printing fluid.

The presence of different ions and/or different concentrations of ions may cause the electrodes to exhibit different impedance characteristics in the presence of different types of fluids.FIG. 11shows, generally at200, a bar graph representing the relative magnitudes of the phase shifts (and thus the relative magnitudes of the capacitance of the electrodes) measured for four different printing fluids at a selected frequency. Cyan is shown at202, yellow at204, magenta at206and black at208. Graph200shows that each printing fluid is distinguishable from the others across the entire frequency range by its respective measured phase shift/capacitance.

Due to the differences in the phase shifts between the different printing fluids, a printing fluid determination may be simple to implement using measured phase shift values. First, a predetermined phase shift value could be determined for each printing fluid supported by printing device10. Next, a look-up table that contains a list of the printing fluids correlated to their predetermined phase shift values may be constructed and stored in memory46. Finally, a phase shift value measured by printing fluid detector30for an unknown fluid may be compared to the phase shift values stored in the look-up table to determine which printing fluid corresponds to the measured phase shift value.

Given the wide variety of printing fluids available today, some printing fluids may exhibit phase shift values so similar that they are difficult to distinguish. To help reduce the possibility that a printing fluid is misidentified, more than one impedance characteristic may be measured for a selected printing fluid, and memory46may include a look-up table containing a list of printing fluids that are each correlated to more than one predetermined impedance characteristic. For example, printing fluid detector32may be configured to first measure a phase shift value, and then a resistance value, and then compare these values to predetermined phase shift and resistance values for selected printing fluids.

Referring briefly back toFIGS. 8 and 9, at frequencies between approximately 1 kHz and 100 kHz, the capacitive component of the total impedance is essentially zero. Thus, the resistance of the printing fluid is the major component of the total impedance between these frequencies. It has been found that the printing fluid resistance may be accurately measured at these frequencies. Furthermore, it has been found that different printing fluids exhibit different fluidic resistances. Thus, the printing fluid resistance may be used to help identify a printing fluid type. A simple bar graph showing the variation in the magnitude of the resistance between the four printing fluids ofFIG. 11is shown generally at210in FIG.12.

To implement a two-impedance-value measurement, a phase shift value may be measured at a first, lower frequency, and then a resistance value may be measured at a higher frequency. Next, processor48may look for a fluid in the look-up table in memory46that has impedance values which most closely match each of the impedance values measured for the printing fluid in the printing fluid reservoir. Alternatively, two different phase shift values may be measured at two different frequencies, and the look-up table may include two predetermined phase shift values for each fluid type. Furthermore, a phase shift and total impedance may be measured at a single frequency, and compared to corresponding values in a look-up table. It will be appreciated that these combinations of impedance values are merely exemplary, and that any other suitable combination of impedance values may be used in a printing fluid type determination.

The printing fluid resistance also may be used with any of the printing fluid detector embodiments described herein to determine an out-of-fluid condition. As described above, the fluidic resistance measurement may be made at a frequency between approximately 1 kHz and 100 kHz to reduce the capacitive component of the total impedance of the electrodes and the printing fluid. A resistance value may be determined by measuring the voltage drop at eout(of FIG.2), combined with measuring the current flowing through the circuit. A resistor (not shown) may be used in parallel with the fluidic resistance to help in the calculation and/or measurement of the voltage drop. The measured resistance value could then be compared to a look-up table containing a plurality of pre-determined printing fluid levels correlated with predetermined fluidic resistance values to determine the printing fluid level, as described above for the phase shift embodiments.

The determination of printing fluid resistance values at frequencies between 1 kHz and 100 kHz has been found to be a quick and reliable method of determining printing fluid levels, printing fluid types and out-of-fluid conditions. Furthermore, the resistance measurements have been found to be sensitive to changes in fluid types and/or the presence/absence of fluid in the electrodes. Additionally, the resistance measurements have been found to allow the resistance of printing fluid to be distinguished from residual printing fluid froth of a wide range of densities and concentrations of froth that may be left in the printing fluid reservoir after the printing fluid has been emptied.

One difficulty that may be encountered in using capacitance/phase shift and/or resistance measurements to determine an out-of-fluid condition is that, for some printing fluids, the resistance and capacitance (and therefore, the phase shift) measurements of the fluid and residual froth may be dependent to various degrees upon the temperature of the printing fluid in the printing fluid reservoir. Ordinarily, the differences in the capacitance/resistance of the printing fluid and electrodes as compared to air is sufficiently different that any minor variations in the capacitance/resistance of the fluid as a function of temperature may not effect an out-of-fluid determination. However, in some situations, the residual froth left over inside of a printing fluid reservoir after the printing fluid reservoir is substantially emptied of printing fluid may have a resistance similar to the resistance of the printing fluid.

The resistances of air, froth and printing fluid in an exemplary printing fluid detector30are shown at302,304and306, respectively, in graph300of FIG.13. Typically, it is desirable to indicate an out-of-fluid condition when only froth is present in the printing fluid reservoir. However, it can be seen that the margin between the resistance of froth at 35 degrees Celsius and the resistance of the printing fluid at 15 degrees Celsius is fairly narrow, and thus may be difficult for printing fluid detector32to distinguish.

To compensate, the following temperature calibration may be performed periodically to ensure that detector circuitry44is able to determine that a correct froth threshold is used for the actual temperature. First, the resistances of the printing fluid and froth are experimentally determined over a range of temperatures, and the determined values are recorded in a look-up table stored in memory46. Next, a series of resistance measurements are taken, and the standard deviation of the measured values is determined. It has been found that a series of resistance measurements taken from a conduit containing froth has a much higher standard deviation (on the order of 100:1) than a series of resistance measurements taken from a conduit containing printing fluid, which consistently exhibits very low standard deviations. Thus, if the standard deviation (or other suitable mathematical indication of variability) of the series of resistance measurements is above a preselected threshold, for example, approximately 5%, then the printing fluid reservoir is determined to contain froth, and no temperature recalibration is performed. On the other hand, if the standard deviation of the series of resistance measurements is below the preselected threshold, then the printing fluid reservoir is determined to contain printing fluid, and the temperature corresponding to the measured printing fluid resistance is located in the look-up table. Finally, the froth resistance corresponding to the determined temperature is set as a new out-of-fluid threshold resistance value.

The resistance value corresponding to froth may be updated at any desired frequency. For example, the value may be updated as infrequently as once an hour, or even less frequently. Likewise, the value may be updated as frequently as once every few seconds. However, the value is more typically updated every few minutes. Updating the resistance value corresponding to froth every few minutes helps to ensure that the value is updated over a shorter timeframe than typical changes in temperature, yet is not updated so often as to consume printing device resources to a detrimental extent. The measurement of the resistance value corresponding to printing fluid may be facilitated, for example, by actuating a pump to remove froth from the vicinity of the first and second electrodes, where froth is detected.

Some printing devices may include a bipolar analog power supply that may be used to produce the alternating supply signal. However, other printing devices may not utilize bipolar voltages, but instead may only have a unipolar voltage source, such as a digital clock signal. The application of such a unipolar voltage source across the electrodes may cause metal ions to plate on the electrodes, which may result in the production of gasses. These gasses may be detrimental to the properties of the printing fluid, and also may cause unwanted pressure to build within printing fluid reservoir26.

To avoid the expense of providing bipolar voltage sources in devices that would not otherwise have them, circuitry may be provided that creates a bipolar signal from a unipolar source.FIGS. 14 and 15show two exemplary circuits that may be used to produce a bipolar voltage from one or more unipolar voltage sources.

First,FIG. 14shows, generally at400, a circuit that utilizes a single unipolar alternating voltage source402to generate a bipolar signal across the first and second electrodes. Voltage source402is configured to output a digital bi-level unipolar voltage, as shown in diagram404. Capacitor408(labeled “equivalent capacitance”), and resistor410(labeled “fluid resistance”) together represent the impedance of the first electrode, second electrode and printing fluid.

Circuit400also includes a peak reading AC ammeter406positioned below a junction at which the current splits to flow through resistor412and the electrodes and printing fluid to allow the calculation of an impedance value of the printing fluid. Circuit400also includes a resistor412in parallel with the fluidic impedance, and a capacitor414located below the junction at which the currents through resister412and the fluid resistance410rejoin. The values of resistor412and capacitor are414selected such that the RC time constant of capacitor414and resistor412is larger than the frequency of voltage source402, and such that the voltage at capacitor414remains at approximately one half of the maximum output voltage of voltage source402. Thus, when voltage source402is outputting a positive voltage, the voltage at point416is more positive than the voltage at point418. On the other hand, when voltage source402is outputting 0 V, capacitor414holds point418at a more positive voltage than point416. In this manner, the first and second electrodes alternate as the most positive electrode, helping to avoid plating and gas production problems.

Next,FIG. 15shows a circuit420that utilizes two unipolar voltage sources to create a bipolar signal across the first and second electrodes. Circuit420includes a first unipolar voltage source422connected to one electrode, and a second unipolar voltage source424connected to the other electrode. The impedance of the first electrode, second electrode and printing fluid is represented by capacitor426(labeled “equivalent capacitance”) and resistor428(labeled “fluid resistance”). Circuit420may include an ammeter430to allow the current through the electrodes and printing fluid to be measured, and thus to allow a measured impedance value to be calculated.

The signals supplied by voltage sources422and424are configured to be 180 degrees out of phase, as shown in phase diagram432. Thus, whenever the signal from voltage source422is high, the signal from voltage source424is low and vice versa. This causes the polarities of the two electrodes to be reversed periodically, and thus helps to avoid plating problems and unwanted production of gases in the printing fluid reservoir.

Other electrode configurations besides cylindrical electrodes may be used in printing fluid detector30.FIGS. 16-19show two other exemplary embodiments of electrodes printing fluid detector30. First,FIG. 16shows generally at500printing fluid detector circuitry that includes a first electrode502and a second electrode504in the form of narrow, elongate electrically conductive members that protrude into the interior of conduit28. The electrically conductive materials that form first electrode502and second electrode504are configured to be in direct contact with printing fluid inside of conduit28. Thus, even though first electrode502and second electrode504may have relatively small surface areas, the electrodes display large capacitances due to the EDL effect. It will be appreciated that the circuitry described above in reference toFIGS. 2-3may be used with electrodes502and504by replacing the cylindrical electrodes in those figures with electrodes502and504.

First electrode502and second electrode504may have any suitable length. For example, first electrode502and/or second electrode504may extend either completely across or only partially across the inner diameter of conduit28. Additionally, the electrodes may have any suitable diameter, ranging from a thin, needle-like shape to a thicker shape that substantially fills the interior of conduit28. Furthermore, the electrodes may be arranged within conduit28in any suitable manner. For example, the electrodes may be arranged side-by-side across conduit28(in a direction either across the direction of fluid flow, as shown, or along the direction of fluid flow), at opposite sides of conduit28(as indicated by second electrode504′, shown in dashed lines), or in any other suitable relation. The magnitude of the EDL effect allows the electrodes to be widely separated and still sensitively measure impedance characteristics of printing fluids in conduit28.

Next,FIG. 17shows, generally at600, an exemplary embodiment of printing fluid detector circuitry that includes smaller, nub-like first602and second604electrodes. First electrode602and second electrode604as depicted have a smaller surface area than electrodes502and504of FIG.16. However, due to the EDL effect, electrodes602and604still exhibit large capacitances in the presence of printing fluid, and thus may be used to measure impedance characteristics of a printing fluid in conduit28with a high degree of sensitivity.

Electrodes602and604may each have any suitable shape and size. For example, rather than the small, nub-like electrodes depicted inFIG. 17, electrodes602and604may be recessed into the interior wall of conduit28such that the electrode protrudes less into the interior of the conduit, or is flush with the interior wall of the conduit. Likewise, electrodes602and604may be positioned in any desired relation to each other. For example, electrodes602and604may be positioned in a side-by-side manner, or one electrode (indicated at604′) may be positioned opposite the other electrode. It will be appreciated that a nub-shaped electrode, such as those depicted inFIG. 17, may be used in combination with an elongate electrode, such as those depicted inFIG. 16

FIGS. 18 and 19show other examples of suitable electrodes for detecting printing fluid in conduit28. First,FIG. 18shows, generally at700, an exemplary embodiment of printing fluid detector circuitry that includes electrodes702and704with a relatively flat, wide cross-sectional circumference. Electrodes702and704are shown as extending entirely through conduit28, but it will be appreciated that the electrodes may also extend only partially through the conduit. Also, while electrodes702and704are shown as having the long dimension of the cross-sectional circumference aligned with the direction of fluid flow through conduit28, it will be appreciated that the electrodes may have any other suitable orientation.

If desired, electrodes702and704may be integrated with, or otherwise coupled to, a casing706that forms part of conduit28. Casing706may include connectors708and708′ configured to allow the casing to be easily connected to and/or removed from adjacent segments of conduit28. This may allow electrodes702and704to be more easily serviced and/or replaced. Alternatively, electrodes702and704may be configured to be coupled with a conduit segment other than casing706.

FIG. 19shows, generally at800, yet another exemplary embodiment of printing fluid detector circuitry that includes ring-shaped electrodes802and804. Each ring-shaped electrode802and804includes an opening806through which printing fluid may flow. Electrodes802and804may be integrated with, or otherwise coupled to, a casing808configured to be easily connected to and removed from other segments of conduit28. Alternatively, electrodes802and804may be configured to be coupled with a conduit segment other than casing808.

As mentioned above, printing fluid may be transferred from printing fluid reservoir26to print head assembly12via a pumping mechanism. Where the pumping rate of the pumping mechanism and an initial level of printing fluid in printing fluid reservoir26are known, an actual fluid level of printing fluid in reservoir26may be calculated. First, when pumping is initiated, the temperature calibration described above for determining the air/froth threshold resistance value may be performed. Next, if printing fluid detector30determines that pumping fluid, as opposed to froth, is in conduit28, the length of time that the pumping mechanism transfers fluid out of printing fluid reservoir26may be monitored. Once pumping is completed (or periodically during pumping), the amount of fluid that has been transferred out of printing fluid reservoir26may be calculated by multiplying the pumping rate and the pumping time. Finally, the amount of fluid transferred may be subtracted from the initial amount of fluid to determine an amount of printing fluid remaining in printing fluid reservoir26, which may then be stored in memory46. This value may then be used as the initial printing fluid amount in a subsequent calculation of printing fluid usage.

This technique of monitoring printing fluid usage may be extended to situations in which froth is being transferred to print head assembly12instead of pure printing fluid. Printing fluid froth is typically a mixture of printing fluid and air or other gases. It has been found that the resistance of froth measured by printing fluid detector30in the 1 kHz-100 kHz frequency range varies linearly with the fluid content of the froth. Therefore, a look-up table may be constructed by measuring the resistance of froth over a range of air:printing fluid ratios for a selected printing fluid, and then stored in memory26. Then, as printing fluid or froth is transferred from printing fluid reservoir26to print head assembly12, the amount of printing fluid transferred may be determined first by measuring the resistance of the printing fluid and/or froth in printing fluid detector30, then comparing the measured resistance to the resistance values stored in the look-up table to determine the fluid:air ratio of the fluid and/or froth in the printing fluid detector, and then calculating how much fluid is transferred by multiplying the pumping time, the pumping rate, and the measured fluid:air ratio.

Although the present disclosure includes specific embodiments, specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various elements, features, functions, and/or properties disclosed herein. The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.