WO2022177582A1

WO2022177582A1 - Photoconductive element voltage determination

Info

Publication number: WO2022177582A1
Application number: PCT/US2021/019080
Authority: WO
Inventors: Asaf Shoshani; Lavi Cohen; Shimi Nakash
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2021-02-22
Filing date: 2021-02-22
Publication date: 2022-08-25

Abstract

According to an example, a method to determine a photoconductive element voltage includes engaging a developing unit with the photoconductive element. The developing unit is set at a series of voltage values. For each voltage value of the series of voltage values, a corresponding electric current flowing through the developing unit is measured such that a series of electric currents is obtained. The photoconductor element voltage is determined based on the series of voltage values and the series of electric currents.

Description

PHOTOCONDUCTIVE ELEMENT VOLTAGE DETERMINATION BACKGROUND [0001] Liquid electro-photography (LEP) printing systems form an image on a substrate by transferring a printing fluid profile associated to the image from a surface of a photoconductive element to the substrate. The printing fluid profile is generated in the photoconductive element by selectively charging or discharging the surface of the photoconductive element and selectively transferring printing fluids to the surface of the photoconductive element. In order to selectively transfer printing fluid to the photoconductive element, developing units electrically charged may be selectively engaged with the surface of the photoconductive element. BRIEF DESCRIPTION OF DRAWINGS [0002] Features of the present disclosure are illustrated by way of example and are not limited in the following figure(s), in which like numerals indicate like elements, in which: [0003] FIG.1 shows a liquid electro-photography printing system to determine a photoconductive element voltage according to an example of the present disclosure; [0004] FIG.2 shows a developing unit to determine a photoconductive element voltage, according to an example of the present disclosure; [0005] FIG.3 shows a method to determine a photoconductive element voltage, according to an example of the present disclosure; [0006] FIG.4 shows a method for determining a series of correction factors, according to an example of the present disclosure; [0007] FIG.5 shows a printing system comprising a photo imaging plate and a set of developing units, according to an example of the present disclosure; [0008] FIG.6 shows a computer-readable medium comprising instructions, according to an example of the present disclosure; [0009] FIG.7 shows a first set of line charts representing a series of voltage values and a series of electric current values, according to an example of the present disclosure; and [0010] FIG.8 shows a second set of line charts representing a series of voltage values and a series of corrected electric current values, according to an example of the present disclosure. DETAILED DESCRIPTION [0011] For simplicity and illustrative purposes, the present disclosure is described by referring mainly to examples. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be readily apparent, however, that the present disclosure may be practiced without limitation to these specific details. In other instances, some methods and structures have not been described in detail so as not to unnecessarily obscure the present disclosure. [0012] Throughout the present disclosure, the terms "a" and "an" are intended to denote at least one of a particular element. As used herein, the term "includes" means includes but not limited to, the term "including" means including but not limited to. The term "based on" means based at least in part on. [0013] Liquid electro-photography (LEP) printing systems may be used to generate images on substrates by transferring a printing fluid profile associated with the image from a surface of a photoconductive element of the printing system to the substrate. In order to generate the printing fluid profile on the surface of the photoconductive element, developing units such as binary ink developers transfer printing fluid to the surface of the photoconductive element. [0014] Liquid electro-photography (LEP) printing systems comprise charging elements such as charging rollers to electrically charge a surface of the photoconductive element. In an example, a charging roller uniformly charges a surface of a photoconductive element of an LEP printing system at a reference voltage. In addition to the charging elements, the LEP printing system comprises a discharging element such as a writing head in order to selectively discharge specific regions of the surface of the photoconductive element. Afterward, a developing unit of the LEP printing system is to develop charged printing fluid. In order to develop the printing fluid, the printing fluid is electrically charged so that the printing fluid is selectively transferred to the discharged regions of the surface of the photoconductive element. Based on the electrical charge on a region surface of the photoconductive element, the printing fluid is transferred to the surface. If an electrical charge of a region of the surface exceeds a charge value, the printing fluid is repelled from such charged regions. By subsequently engaging and disengaging other developing units of the LEP printing system, the printing fluid profile associated with the image is obtained on the surface of the photoconductive element. Then, the printing fluid profile may be transferred directly to the substrate to obtain the image, or may be transferred to additional elements of the LEP printing system such as intermediate elements. [0015] As used herein, “printing fluid” refers generally to any substance that can be applied upon a substrate by a printer during a printing operation, including but not limited to inks, electro-inks, primers and overcoat materials (such as a varnish), water, and solvents other than water. [0016] In order to effectively charge a surface of a photoconductive element, an LEP printing system may comprise a voltage sensor such as an electrometer to determine a voltage value of the photoconductive element. Such voltage sensors may be positioned at a specific position of the photoconductive element, for instance an intermediate point between the two lateral edges of the photoconductive element. Determining an accurate charging of the surface is crucial for the transfer operation because the transfer of electrically charged printing fluids is based on the voltage differences between the developing unit and the surface of the photoconductive element. Hence, by assuring an effective charge of the photoconductive element, a subsequent discharge of the photoconductive element and a subsequent transfer of the printing fluid to the surface of the photoconductive element will be performed under operative conditions. If the surface of the photoconductive element is not properly charged by the charging elements, a resulting electrical charge profile associated with the image will result in print quality issues. [0017] However, the measurement of a voltage sensor may not depict an actual voltage state of a surface of a photoconductive element. Since the voltage sensor measures a voltage at a specific width of the surface of the photoconductive element, the remaining width is disregarded. When using wider voltage sensors, an improvement of the measurement may be obtained. However, because the voltage sensors are not positioned at a contact region between the photoconductive element and the developing unit where the printing fluid transfer occurs, electric charge changes derived from factors such as charge decay are not taken into account. Charge decay may be experienced upon charging the surface of the photoconductive element. The surface of the photoconductive element may gradually start to lose the charging potential over time because the environmental conditions and other factors. [0018] In order to consider the impacts of charge decay, the measurements may be corrected based on the locations of the developing units, the voltage sensor, the charging element, or the discharging element. However, the correction of the measurements does not assure a proper calculation of the voltage of the photoconductive element at the region where the transfer of the printing fluid occurs. As a result, in some situations a calibration of the voltage used by the developing unit may not be accurately performed. [0019] Disclosed herein are examples of printing systems, methods, and computer- readable mediums comprising instructions which may be used to determine a voltage of a photoconductive element in a region where a printing fluid is transferred from a developing unit to the photoconductive element. Hence, different examples of printing systems, methods, and sets of instructions are described. [0020] According to an example, an LEP printing system comprises a photoconductive element, a series of developing units engageable with the photoconductive element, and a charging element. The LEP printing system may be used to print an image on a substrate, wherein an electrical charge profiled associated with the image has been previously generated on a surface of the photoconductive element. In order to generate the electrical charge profile, a charging element charges the surface of the photoconductive element and a discharging element selectively discharged the surface. Afterward, each developing unit may be selectively engaged with and disengaged from the surface of the photoconductive element to generate a printing fluid profile. Before performing the transfer of the printing fluid to the surface, a calibration stage may be performed. During the calibration stage, the surface of the photoconductive element is set at a voltage value by the charging element. In an example, the voltage on the surface of the photoconductive element is measured by using voltage sensors. In an example, an operative electrical value is within a range of 800 volts to 1150 volts, for instance 950 volts. [0021] However, as previously explained in the description, the usage of voltage sensors may result in measurements which differ with an actual voltage at a region where the printing fluid transfer between each of the developing unit and the photoconductive element occurs. In order to effectively measure the voltage at a transfer region, methods involving the usage of the developing units may be used. [0022] Throughout the description, the term “transfer region” will be used to refer to a region of the photoconductive element where a developing unit in an engaged position and the photoconductive element contact with each other. Since the photoconductive element may engage with multiple developing units, multiple transfer regions may be defined for the same photoconductive element. [0023] Referring now to FIG.1, an LEP printing system 100 is shown. The printing system 100 comprises a photoconductive element 110, a charging element 105, a set of developing units, and a controller 150. The set of developing units comprises a first developing unit 120, a second developing unit 130, and a third developing unit 140. As previously explained, the charging element 105 is to charge a surface of the photoconductive element 110 at a calibration voltage. In order to effectively charge the surface, the photoconductive element 110 rotates so that the charging element 105 uniformly charges the surface at the calibration voltage. Each developing unit of the set developing units is movable between an engaged position at which the developing unit is in contact with the photoconductive element 110 and a disengaged position at which the developing unit is not contacting the surface photoconductive element. In other words, each developing unit of the set of developing units is movable towards the photoconductive element 110 to the engaged position and away from the photoconductive element 110 to the disengaged position. In FIG.1, the first developing unit 120 is at an engaged position 121a, the second developing unit 130 is at a disengaged position 131b, and the third developing unit 140 is at an engaged position 141a. When the developing units are at their respective engaged position, a respective transfer region is defined at specific locations of the surface of the photoconductive element at which one of the developing units contacts with the photoconductive element 110. Consequently, in the LEP printing system of FIG.1, a first transfer region 110a is obtained when the first developing unit 120 is at the engaged position 121a, a second transfer region 110b is obtained when the second developing unit 130 is at the engaged position (not depicted in FIG.1), and a third transfer region 110c is obtained when the third developing unit 140 is at the engaged position 141a. In other examples, when having a different number of developing units, additional or fewer transfer regions may be defined. [0024] In FIG.1, controller 150 is to control each of the charging element 105, the photoconductive element 110, the first developing unit 120, the second developing unit 130, and the third developing unit 140. Referring to the charging element, controller 150 is to control the charging element 105 to electrically charge a surface of the photoconductive element 110 during a specific time frame of a printing operation. In some examples, controller 150 controls the charging element 105 so as to charge the surface of the photoconductive element 110 during a calibration operation prior to the printing operation. Referring to the photoconductive element 110, the controller 150 is to control a rotation of the photoconductive element 110. In some examples, the LEP printing system 100 may comprise optical sensors to determine an angular speed of the photoconductive element and/or an angle elapsed during the rotation. In other examples, the photoconductive element 110 may be driven by a servo-controlled motor such that an angle rotated, and an angular speed can be determined based on readings of the servo-controlled motor. Referring to the developing units, controller 150 is to selectively control the movement of the developing units between the engaged position and the disengaged position. In addition, controller 150 is to selectively control a voltage of each of the developing units 120, 130, and 140. Since the transfer at the transfer regions 110a, 110b, and 110c is caused by the difference of voltages between the developing unit and the surface of the photoconductive element 110, an aspect to consider is to precisely set a voltage difference between each of the developing units and the surface of the photoconductive element 110. [0025] In an example, controller 150 is to set a different voltage value for each developing unit. Since the locations of the first transfer region 110a, the second transfer region 110b, and the third transfer region 110c are different, if a common voltage is set in each of the developing units 120, 130, and 140, the voltage difference between the surface of the photoconductive element 110 and each of the developing units 120, 130 and 140 at the transfer regions 110a, 110b, and 110c may not be the same for each of the developing units. As previously explained, factors such as charge decay may result in an electric charge gradual loss of the charged element, i.e. the surface of the photoconductive element 110. Hence, since the locations of the developing units are different, when aiming to achieve a voltage difference at the transfer regions, different voltage values may be set for each developing unit 120, 130, and 140. In other examples, the voltage difference at the transfer region may be set based on the printing fluid. Then, when developing different types of printing fluid with the developing units, different voltages for each of the developing units may be set. In other examples, a voltage difference at each of the transfer regions 110a, 110b, and 110c between the developing unit and the photoconductive element 110 is to be the same during printing operation. Hence, in order to correct the voltage difference resulting from factors such as charge decay, a photoconductive element voltage measurement at the transfer regions can be used to calibrate the printing operation such that operative conditions for the printing operation are obtained. [0026] In order to determine a photoconductive element voltage at the transfer regions, controller 150 may set a series of voltage values in the set of developing units 120. For each voltage value of the series of voltage values, a corresponding electric current for each developing unit of the set of developing units 120 is measured. Upon determination of the electric currents, a photoconductive element voltage at each of the transfer regions may be calculated based on the series of voltage values and the corresponding electric currents of each of the developing units. [0027] According to some examples, each of the developing units is set at a different voltage with respect to each other. The controller 150 may selectively control each developing unit such that a different voltage is set. [0028] Referring now to FIG.2, a developing unit 220 of an LEP printing system 200 is shown. The LEP printing system 200 further comprises a controller 250 to selectively control each of the sub-elements of the developing unit 220. The developing unit 220 may be alternatively referred to as a binary ink developer (BID). As previously explained, the developing unit 220 serves for several functions comprising developing printing fluid, applying printing fluid to a photoconductive element of the LEP printing system 200, and removing residual printing fluid. In addition, the developing unit 220 may be used to determine a photoconductive element voltage at a transfer region where the developing unit and the photoconductive element contact when the developing units is at the engaged position. In FIG.2, the developing unit 220 comprises a developer roller 221, a first electrode 222, a squeegee 223, and a cleaner 224. The developing unit 220 further comprises a housing 225 defining a printing fluid tray 226 that collects unused printing fluid or any printing fluid that was not used for forming a printing fluid profile on the surface of the photoconductive element. During printing, printing fluid is pumped from a tank (not shown) for use in printing and collected in the printing fluid tray 226 after printing. [0029] In use during a printing operation, the first electrode 222 is held at a predetermined voltage such as, for example, a negative electrical potential to influence printing fluid movement to the developing roller 221. The negative potential may be, for instance, -1500 volts, but alternative values are possible. When the printing fluid is in a state where it is more liquid than solid, the printing fluid can migrate from the first electrode 222 to coat the developer roller 221. The developer roller 221 is held at a respective predetermined electrical potential while being rotated (clockwise in FIG.2). In some examples, the electrical potential of the developing roller 221 is less negative than the first electrode 222, for example -450 volts. Downstream the printing fluid transfer between the first electrode 222 and the developer roller 221, the squeegee 223 contacts the developing roller 221. In FIG.2, the squeegee 223 is a roller rotating in the opposite direction to the developer roller 221, i.e. a counterclockwise direction. The squeegee 223 is set at a predetermined squeegee potential. In some examples, the predetermined squeegee potential is more negative than the developer roller 221, for instance, -750 volts. The squeegee 223 is to skim the printing fluid that has been coated onto the developer roller 221 to influence its composition, in particular, its viscosity. Following skimming, printing fluid can be more solid than liquid. After skimming the printing fluid remaining on the developer roller 221 is selectively transferred to the photoconductive element. As previously explained, the photoconductive element is previously uniformly charged (for instance, by using a charging element) and selectively discharged (for instance, by using a discharging element) to obtain an electrical profile associated with the image to be printed on a substrate. In order to perform the transfer, the developing unit 220 is positioned at an engaged position. As a result, the printing fluid on the developing roller 221 of the developing unit 220 is transferred to regions of the photoconductive element where the photoconductive element has been discharged in areas intended to form a printing fluid profile associated with the image. Thereafter, the photoconductive element may contact an intermediate transfer element or the substrate. [0030] After the printing fluid transfer, printing fluid not transferred from the developing roller 221 to the photoconductive element is referred to as unused printing fluid. The unused printing fluid is to be cleaned from the surface of the developing roller 221 by the cleaner 224. In order to effectively clean the unused printing fluid, the cleaner 224 is at a predetermined potential, wherein the potential is less negative than the potential of the developing roller 221. In an example, the predetermined potential of the cleaner is -250 volts. The developing unit 220 may comprise further elements to provide better cleaning of the unused printing fluid from the developing roller 221, such as sponge rollers, wipers, scrapers, amongst others. [0031] As explained above, controller 250 is capable of selectively setting a voltage for each of the developing unit 221, the first electrode 222, the squeegee 223, and the cleaner 224. Based on the voltage difference between each of the elements and the developing unit 221, a specific operation is performed. In the same way, based on the voltage difference between the photoconductive element and the developing roller 221 at the transfer region, the transfer operation is performed. [0032] In some examples, the controller 250 may set a common voltage to each of the developing unit 221, the first electrode 222, the squeegee 223, and the cleaner 224. By setting a common voltage, an internal electrical leakage between the electrically charged elements of the developing unit 220 is prevented. By moving the developing unit 220 to an engaged position in which the developing unit 220 contacts the photoconductive element, the developing roller 221 of the developing unit 220 will contact the photoconductive element at a transfer region while having the elements at the common voltage. As a result, an electric current may flow through the transfer region because of the voltage differences between the developing unit and the photoconductive element. The controller 250 may determine the electric current in at least one of the elements of the developing unit 220 (for instance the developing roller 221), and based on the sign of the electric current, a range for a voltage value of the photoconductive element may be determined. For example, if the electric current on the elements of the developing unit 220, for instance the developing roller 221 has a positive value, the voltage value of the photoconductive element is considered to be lower than the common voltage set to each of the elements of the developing unit 220. If the electric current of one of the elements of the developing unit 220 has a negative value, the voltage value of the photoconductive element is considered to be higher than the common voltage set to each of the elements of the developing unit 220. If the electric current is null, the voltage value of the photoconductive element is considered to be equal to the common voltage set to each of the elements of the developing unit 220. Hence, by setting the elements of the developing 220 at a series voltage values and measuring the series of corresponding electric currents flowing through the developing unit 220, a voltage value of the photoconductive element may be determined at the transfer region. [0033] According to one example, as a part of a calibration operation of the LEP printing system, the controller of the LEP printing system sets the developing unit at multiple voltages values in order to determine a reference voltage of the photoconductive element. The photoconductive element may have been previously charged by a charging element of the printing system. During the calibration operation, the developing unit is used as a measuring element rather than a printing fluid transfer element. In an example, during a determination of the voltage value of the photoconductive element, the sub-elements of the developing unit perform in a similar way as during the printing operation, but without transferring printing fluid to the photoconductive element. By setting multiple voltage values in the developing unit and measuring the electric currents resulting from such values, the reference voltage of the photoconductive element can be determined based on the multiple voltage values and the electric currents resulting from such values. [0034] According to some other examples, a reference voltage determination for the photoconductive element may be performed for each developing unit of the LEP printing system. When having additional developing units (for the example three developing units in the LEP printing system of FIG.1), multiple reference voltage values of the photoconductive element may be determined. An upcoming printing operation may be calibrated based on the reference voltage values. [0035] Referring now to FIG.3, a method 300 to determine a photoconductive element voltage is shown. Method 300 may be performed, for instance, by using a developing unit. In an example, the developing unit may correspond to the developing units 120, 130, 140, and 220 previously explained in reference to FIGs.1 and 2. At block 310, method 300 comprises engaging a developing unit with the photoconductive element. Engaging the developing unit should be understood as moving the developing unit towards the photoconductive element to an engaged position. At block 320, method 300 comprises setting the developing unit at a series of voltage values. At block 330, method 300 comprises for each voltage value of the series of voltage values, measuring a corresponding electric current flowing through the developing unit such that a series of electric currents is obtained. As previously explained in reference to FIG.2, a controller of the LEP printing system can determine an electric current flowing through the developing unit (or one of its elements, for instance the developing roller). Then, at block 340, method 300 comprises determining the photoconductive element voltage based on the series of voltage values and the series of electric currents. In an example, method 300 may further comprise calibrating an upcoming printing operation based on the photoconductive element voltage. [0036] In some examples, the series of voltage values have a predetermined amount of voltage values. In some other examples, the series of voltage values may be an open series, i.e. different voltage values are set until the photoconductive element voltage is determined. In an example, the initial voltage value may be, for instance, a predetermined value based on an expected voltage value for the photoconductive element. In other examples, the initial voltage value is set based on a value determined in the last calibration operation. [0037] In some other examples, the series of voltage values may be modified based on an initial electric current obtained while setting the developing unit at an initial voltage value. In an example, setting the developing unit at the series of voltage values comprises setting the developing unit at an initial voltage value, measuring a corresponding initial electric current flowing through the developing unit, and modifying the series of voltage values base on an electrical current sign of the corresponding initial electric current. For example, if the initial electric current sign has a first sign, the series of voltage values may be modified such that each voltage value of the series of voltage values is greater than the initial voltage value. Similarly, if the initial electric current sign is a second sign opposite to the first sign, the series of voltage values may be modified such that each voltage value of the series of voltage values is lower than the initial voltage value. [0038] In other examples, determining the photoconductive element voltage based on the series of voltage values and the series of electric currents includes determining a pair of consecutive voltage values of the series of voltage values associated with corresponding electric currents having opposite signs, and determining the photoconductive element voltage based on the pair of consecutive voltage values. In an example, determining the photoconductive element voltage based on the pair of consecutive voltage values comprises interpolating the pair of consecutive voltage values associated with corresponding electric currents having opposite signs. In other examples, other determination methods to determine the “zero crossing point” may be used. [0039] In further examples, method 300 may further comprise measuring with an optical encoder an angular position of the photoconductive element. Based on the measurements, a correlation between the angular position, the series of voltages, and the series of electric currents may be performed during a time frame. The correlation may be used to determine if a surface of the photoconductive element is uniformly charged. In order to obtain an accurate correlation, the series of voltage values may be set at different angular positions of the photoconductive element. [0040] Referring now to FIG.4, a method 400 for determining a series of correction factors is shown. Method 400 may be performed, for instance, using a developing such as the developing units 120, 130, 140, and 220 previously explained in reference to FIGs.1 and 2. In an example, method 300 comprises method 400. At block 410, method 400 comprises disengaging the developing unit. Disengaging the developing unit should be understood as moving the developing unit to the disengaged position such that the developing unit is not in contact with the photoconductive element. At block 420, method 400 comprises setting the developing unit at a series of voltage values. The series of voltage values may be, for instance, the same series of voltage values set at block 320 of method 300. However, in other examples, the series of voltage values of method 300 and method 400 may be different. At block 430, method 400 comprises measuring, for each voltage value of the series of voltage values, a corresponding disengaged electric current flowing through the developing unit such that a series of disengaged electric currents is obtained. Then, at block 440, method 400 comprises determining a series of correction factors based on the series of disengaged electric currents and the series of electric currents, wherein the series of electric currents correspond to the series of electric currents measured during at the engaged position of the developing unit, i.e. during engagement of the developing unit. [0041] In some examples, the usage of method 400 provides a correction of the electric current generated by external factors to the LEP printing system, for example the presence of an electrical field generated by external devices. When the LEP printing system is located next to other devices, the presence of the devices may create an electrical field that may result in electrical currents flowing through the developing unit. In other examples, the environment where the LEP printing system may be subjected to an electric charge. The presence of electric charges may introduce noise for the determination of the photoconductive element voltage. Hence, in order to correct the noise introduced by external factors and/or elements, method 400 may be used to determine a series of correction factors based on the electric currents determined during engagement and the electric currents determined during disengagement. In other examples, block 340 of method 300 includes correcting the series of electric currents based on the series of corrections factors, wherein the corrective factors correspond to the correction factors obtained in the block 440 of method 400. [0042] Referring now to FIG.5, a printing system 500 comprising a photo imaging plate 510 and a set of developing units 520 is shown. The printing system 500 may be, for instance, an LEP printing system. In an example, the printing system 500 is used to obtain an image on a substrate. The printing system 500 further comprises a controller 530. The photo imaging plate (PIP) 510 may be alternatively referred to as a photoconductive element. The photo imaging plate 510 is to be charged by a charging element (not shown in FIG.5) so that an electrical potential is set on a surface of the photo imaging plate 510. Later, a discharging element (not shown in FIG.5) may selectively discharge the surface of the photo imaging plate 510 such that an electrical charge profile associated with the image is obtained on the surface of the photo imaging plate 510, i.e. an electric latent image is obtained. Afterward, the set of developing units 520 is to transfer developed printing fluid to the electrical charge profile so that a printing fluid profile is obtained on the surface of the photo imaging plate 510. Then, the printing fluid profile is transferred to other elements, as previously explained in other examples. [0043] The set of developing units 520 comprises at least one developing unit such as one of the developing units 120, 130, 140, and 220 previously explained in reference to FIGs.1 and 2. As previously explained, each developing unit of the set of developing units 520 is movable towards the photo imaging plate 510 to an engaged position and away from the photo imaging plate to a disengaged position. At the engaged position, the developing unit is in contact with the photo imaging plate 510 at a transfer region. At the disengaged position, the developing unit is not in contact with the photo imaging plate 510. [0044] In order to determine a voltage value of the photo imaging plate 510, controller 530 is to cause the system 500 to perform a series of actions. In an example, controller 530 cause the printing system to move each developing unit of the set of developing units 520 at the engaged position, set a series of voltage values in the set of developing units 520, for each voltage value of the series of voltage values, measure a corresponding electric current for each developing unit of the set of developing units 520, and calculate a photo imaging plate voltage for each developing unit of the set of developing units 520 based on the series of voltage values and the corresponding electric currents. In other examples, calculating a photo imaging plate voltage for each developing unit of the set of developing units based on the series of voltage values and the corresponding electric currents may further comprise for each developing unit, identifying a pair of consecutive voltages of the series of voltage values having electric currents of the corresponding electric currents with opposite signs, and for each developing unit, determining a photo imaging plate voltage based on the pair of consecutive voltages. [0045] In an example, the set of developing units 520 comprises a first developing unit, a second developing unit, and a third developing unit. By positioning the developing units at the engaged position and setting the series of voltage values, corresponding electric currents may be measured for each of the developing units, i.e. first electric currents for the first developing unit, second electric currents for the second developing unit, and third electric currents for the third developing unit. Accordingly, a first photo imaging plate voltage may be determined at a transfer region of the first developing unit, a second photo imaging a plate voltage may be determined at a transfer region of the second developing unit, and a third photo imaging plate voltage may be determined at a transfer region of the third developing unit. Based on the photo imaging plate voltages, an upcoming printing operation of the printing system 500 may be calibrated. [0046] In some examples, controller 540 is to cause the printing system 500 to determine a difference between a minimum voltage value and a maximum voltage value for the series of voltage values, and trigger a maximum voltage difference error upon the difference exceeds a maximum difference. Since the application of a wide range of voltage values may have a negative impact over the photo imaging plate 510 or the developing unit, by triggering the maximum difference error in the printing system 500, damage over the photo imaging plate is prevented. [0047] In other examples, controller 540 is to cause the printing system 500 to perform method 400, i.e. disengage each developing unit of the set of developing units 520 from the photo imaging plate 510, set the series of voltage values to the series of developing units, for each developing unit, measure a corresponding series of disengaged electric currents, and determine, for each developing unit, a series of corrective factors based on the corresponding electric currents and the corresponding series of disengaged electric currents. By determining corrective factors and subsequently applying these corrective factors to the electric currents determined during the engagement of the developing units, external factors to the printing system 500 may be corrected. [0048] As previously explained in the description, each developing unit of the set of developing units 520 may comprise a developer roller to contact the photo imaging plate 510 at a transfer region during the engaged position of the developing unit, an electrode to transfer printing fluid to the developer roller, a squeegee contacting the developer roller, and a cleaner to clean a surface of the developer roller, wherein the voltage at each of the elements can be individually set. In an example, set the series of voltage values in the set of developing units 520 comprises setting in each developing unit the series of voltage values in the developer roller, the electrode, the squeegee, and the cleaner. By setting the same voltage value in the electrically charged elements of each developing unit, electric current leakage between such elements is prevented. [0049] Referring now to FIG.6, a computer-readable medium 600 comprising instructions is shown. The instructions, when executed by a processor of a system, cause the system to perform a series of actions. Computer-readable mediums include, for example, electronic, magnetic, optical, electromagnetic, or semiconductor media. More specific examples of suitable computer-readable media include a hard drive, a random access memory (RAM), a read-only memory (ROM), memory cards and sticks, and other portable storage devices. In FIG.6, the computer-readable medium 600 comprises a first instruction 610, a second instruction 620, and a third instruction 630. The first instruction 610, when executed by the processor, causes the system to modify a voltage of a developing unit contacting a photoconductive element to a series of voltage values during a time frame. In other examples, the system may comprise multiple developing units. The second instruction 620, when executed by the processor, causes the system to measure electric currents flowing through the developing unit during the time frame. The third instruction 630, when executed by the processor, causes the system to determine a reference voltage of the photoconductive element based on the series of voltages and the electric currents during the time frame. In other examples, the computer-readable medium may comprise further instructions to determine a series of correction factors, as previously explained in method 400 in FIG.4. [0050] In an example, the computer-readable medium 600 comprises further instructions to cause the system to identify a time of occurrence within the time frame in which an electric current sign has changed, wherein determine a voltage of the photoconductive element based on the series of voltages values and the electric currents during the time frame comprises determine the reference voltage of the photoconductive based on a voltage value of the series of voltage values at the time of occurrence. [0051] In some examples, the computer-readable medium 600 comprises further instructions to cause the system to determine an upcoming operation for the system, and calibrate the upcoming operation based on the voltage of the photoconductive element. When having a system with more than one developing unit, multiple calibrations may be performed: one for each developing unit. [0052] In other examples, the computer-readable medium 600 may comprise further instructions to perform additional actions. In an example, at block 610, modify the voltage of the developing unit contacting the photoconductive element to the series of voltage value during the time frame comprises measuring an initial electric current of the developing unit when the developing unit is set at an initial voltage and modifying the initial voltage based on an electric current sign of the initial electric current. In other examples, the computer-readable medium 600 may comprise further instructions to increase or decrease the voltage value based on the electric current sign. Accordingly, modifying the initial voltage based on the electric current sign of the initial electric current may comprise increasing the initial voltage if the electric sign of the initial electric current is a first electric sign, and decreasing the initial voltage if the electric sign of the initial electric current is a second electric sign. [0053] Referring now to FIG.7, a first set of line charts 700 representing a series of voltage values 711a to 711g and a series of electric current values 763a to 763g is shown. The first set of line charts 700 comprises an upper line chart 710 and a lower line chart 760. For each of the line charts, the X-axis 702 represents time. Regarding the Y-axis, a first Y-axis 701 of the upper line chart 710 represents a voltage set in a developing unit. For lower line chart 760, a second Y-axis 703 represents an electric current flowing through the developing unit. Since the values for the X-axis 702 are common for the upper line chart 710 and the lower line chart 760, for each voltage value set in the developing unit, a corresponding electric current obtained in the developing unit is in the second Y-axis 703 of the lower chart 760. In particular, the upper line chart 710 comprises the series of voltage values 711a to 711g and the lower line chart 760 comprises the series of corresponding electric currents 763a to 763g at an engaged position of the developing unit. At the engaged position, the developing unit is capable to transfer printing fluid to a photoconductive element such as a print imaging plate. [0054] In the example of FIG.7, the developing unit is selectively set at each of the voltage values. At first, the developing unit is set at a first voltage value 711a of the series of voltage values 711a to 711g. The first voltage value 711a may be alternatively referred to as an initial voltage value. The selection of the initial voltage value may be based on previously performed calibration operations in the same printing system or a predetermined voltage value. When setting the developing unit at the first voltage value 711a, a first electric current 763a is measured flowing through the developing unit, wherein a sign of the first electric current 763a is a negative sign. Once the first electric current 763a is determined, the developing unit is set at a second volage value 711b. In some examples, the second voltage value 711b is selected based on an electrical sign of a previous electric current, i.e. the sign the first electric current 763a. In other examples, the voltage values of the series of voltage values 711a to 711g are predetermined, for example by starting from an initial voltage value and periodically adding a voltage value. Similarly, the series of voltage values 711a to 711g may be set by starting from an initial voltage value and periodically subtracting a voltage value. [0055] In the example of FIG.7, the series of voltage values 711a to 711g correspond to a series of voltages considered suitable for the determination, i.e. have been set based on the results of previous calibration operations. When setting the developing unit at the second voltage value 711b, a second electric current 763b having a negative sign is obtained. As explained above, an upcoming voltage value may be selected based on an electrical current sign obtained when setting a current voltage. Then, the developing unit is set at a third voltage value 711c that results in a third electric current 763c. Following the measurement of the third voltage value 711c, the developing unit is set at a fourth voltage value 711d. When setting the developing unit at the fourth voltage value 711d, a fourth electric current 763d is measured on the developing unit. The fourth electric current 763d has changed its sign, i.e. the electric current value is at the positive region of the lower line chart 760. Then, the developing unit is subsequently set at a fifth voltage value 711e, a sixth voltage value 711f and a seventh voltage value 711g. For the fifth voltage value 711e, a fifth electric current 763e is measured. For the sixth voltage value 711f, a sixth electric current 763f is measured. For the seventh voltage value 711g, a seventh electric current 763g is measured. In some other examples, instead of setting the developing unit at discrete voltage values of the series of voltage values 711a to 711g, the developing unit may be subjected to a continuous change of voltage, as represented by a series of continuous voltage values 712. For example, when setting the developing unit at the series of continuous voltage values 712, the series of electric currents may be measured as discrete values over time, or as a series of continuous electric values 762 over time. [0056] Once the series of voltage values 711a to 711g have been set and the series of electric current values 763a to 763g have been measured, a reference value of the photoconductive element may be determined. As previously explained in the description, a reference voltage value of a photoconductive element may be determined based on the series of voltage values 711a to 711g and the series of electric current values 763a to 763g. In the example of FIG.7, the reference voltage value of the photoconductive element is determined based on a pair of consecutive voltage values associated with electric currents having different signs. As explained above, the third electric current 763c had a negative sign and the fourth electric current 763d had a positive sign. Hence, in the example of FIG.7, the pair of consecutive voltage values having associated electric currents having different signs comprises the third voltage value 711c and the fourth voltage value 711d. Based on the third voltage value 711c and the fourth voltage value 711d, the reference voltage of the photoconductive element may be determined. In an example, the voltage is determined by monitoring a tendency of the series of electric current 763a to 763g. In other examples, other statistical methods may be used, for instance by considering the previous and subsequent voltage values to the pair of voltage values. When having available the series of continuous electric values 762, the reference voltage of the photoconductive element may correspond to an occurrence time 702a at which the series of continuous electric values 762 intersects the X-axis 702. Based on the occurrence time 702a, a corresponding voltage 713 is determined, wherein the corresponding voltage 713 corresponds to the reference voltage of the photoconductive element. [0057] According to some examples, instead of representing the series of voltage values and the series of electric currents in separate line charts, a line chart representing a correlation between the voltage values and the corresponding electric currents may be obtained. In a similar way, instead of matching a time at which the measured electric current has changed its sign, a reference voltage of the photoconductive element may be determined based on a pair of consecutive voltage values having associated electric currents with a different sign. [0058] Referring now to FIG.8, a second set of line charts 800 representing a series of voltage values 712 and a corrected electric current 862 is shown. The second set of line charts 800 comprises an upper chart 710 and a lower chart 860, wherein the upper chart 710 corresponds to the upper chart 710 previously explained in reference with FIG.7. The lower chart 860 comprises a Y-axis 703 representing electric current values and the X-axis 702 representing time. For illustrative purposes, the series of voltage values 712 is a continuous set of voltage values rather than the series of discrete voltage values 711a to 711g explained in FIG.7. [0059] In the lower chart 806, the series of electric currents 762 previously determined in FIG.7 are represented by using a dashed line. As explained above, the series of electric currents 762 are determined during the engaged position of the developing unit, i.e. a position at which printing can be transferred from the developing unit to the photoconductive element. However, as previously explained in FIG.4, a series of correction factors may be determined during a disengaged position of the developing unit. In order to determine such correction factors, a series of disengaged electric currents are determined. To determine the series of disengaged electric currents, the developing unit may be set at the series of voltages 712 while being at the disengaged position. In other examples, different voltages values may be used instead of the same voltages used during the engagement measurements. Upon measurements of disengaged electric currents, a series of correction factors may be determined based on the series of electric currents 762 at the engaged position and the series of electric currents at the disengaged position (not represented in FIG.8). Once the correction factors have been determined, a correction 861 may be applied over the series of electric currents 762. For example, in the example of FIG.8, the series of electric currents 763a to 763g represented in FIG.7 result in a series of corrected electric currents 863a to 863g after application of the correction 861. In FIG.8, after applying the correction 861 over the series of electric currents 762, a corrected electric current 862 is obtained. [0060] Once the corrected electric current 862 is determined, a reference voltage value of the photoconductive element may be determined based on the series of voltage values 712 and the corrected electric current 862. In the example of FIG.8, the corrected electric current 862 intersects the X-axis at a second time 702b. As represented in FIG.8, the second time 702b is different from the occurrence time 702a previously determined in FIG.7. Based on the corrected occurrence time 702b, a corresponding voltage 813 is determined, wherein the corresponding voltage 813 corresponds to the reference voltage of the photoconductive element when the disengaged currents are considered. [0061] It should be noted that for illustrative purposes, the voltage values and the electric currents of FIG.7 and FIG.8 have been exaggerated. As a result, a big difference is obtained between the photoconductive element voltage 713 determined in FIG.7 and the photoconductive element voltage 813 determined in FIG.8. [0062] According to some examples, when having multiple developing units available, the series of voltage values may be set for each of the developing units. Accordingly, multiple series of electric currents may be determined. In a similar way to the example of FIG.8, a series of disengaged electric currents may be subsequently obtained during the disengaged position of each of the developing units. Based on the multiple series of electric currents and their respective series of disengaged electric currents, corrected electric currents may be obtained for each developing unit. Subsequently, a reference voltage for each of the developing units may be determined based on the voltage values, the engaged electric currents, and the disengaged electric currents. [0063] What has been described and illustrated herein are examples of the disclosure along with some variations. The terms, descriptions, and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the scope of the disclosure, which is intended to be defined by the following claims (and their equivalents) in which all terms are meant in their broadest reasonable sense unless otherwise indicated.

Claims

CLAIMS What is claimed is: 1. A method to determine a photoconductive element voltage, the method comprising: engaging a developing unit with a photoconductive element; setting the developing unit at a series of voltage values; for each voltage value of the series of voltage values, measuring a corresponding electric current flowing through the developing unit such that a series of electric currents is obtained; and determining a photoconductor element voltage based on the series of voltage values and the series of electric currents.

2. The method of claim 1, wherein setting the developing unit at the series of voltage values comprises: setting the developing unit at an initial voltage value; measuring a corresponding initial electric current flowing through the developing unit; and modifying the series of voltage values based on an electric current sign of the corresponding initial electric current.

3. The method of claim 1, the method further comprising calibrating an upcoming printing operation based on the photoconductor element voltage.

4. The method of claim 1, wherein determining the photoconductor element voltage based on the series of voltage values and the series of electric currents comprises: determining a pair of consecutive voltage values of the series of voltage values associated with corresponding electric currents having opposite signs; and determining the photoconductor element voltage based on the pair of consecutive voltage values.

5. The method of claim 1, the method further comprising: disengaging the developing unit from the photoconductor element; setting the developing unit at the series of voltage values; measuring, for each voltage value of the series of voltage values, a corresponding disengaged electric current such that a series of disengaged electric currents is obtained; and determining a series of correction factors based on the series of disengaged electric currents and the series of electric currents, wherein determining the photoconductive element voltage based on the series of voltage values and the series of electric currents includes correcting the series of electric currents based on the series of corrections factors.

6. A printing system comprising: a photo imaging plate; a set of developing units, wherein each developing unit is movable towards the photo imaging plate to an engaged position and away from the photo imaging plate to a disengaged position a controller to cause the system to: move each developing unit of the set of developing units to the engaged position; set a series of voltage values in the set of developing units; for each voltage value of the series of voltages values, measure a corresponding electric current for each developing unit of the set of developing units; and calculate a photo imaging plate voltage for each developing unit of the set of developing units based on the series of voltage values and the corresponding electric currents.

7. The system of claim 6, wherein calculating a photo imaging plate voltage for each developing unit of the set of developing units based on the series of voltage values and the respective electric currents comprises: for each developing unit, identifying a pair of consecutive voltages of the series of voltage values having electric currents of the corresponding electric currents with opposite signs; and for each developing unit, determining the photo imaging plate voltage based on the pair of consecutive voltages.

8. The system of claim 6, wherein controller is to cause the system to: determine a difference between a minimum voltage value and a maximum voltage value for the series of voltage values; and trigger a maximum difference error upon the difference exceeds a maximum voltage difference.

9. The system of claim 6, wherein each developing unit comprises: a developer roller to contact the photo imaging plate at a transfer region during the engaged position of the developing unit; an electrode to transfer printing fluid to the developer roller; a squeegee contacting the developer roller; and a cleaner to clean a surface of the developer roller, wherein set the series of voltage values in the set of developing units comprises setting in each developing unit the series of voltage values in the developer roller, the electrode, the squeegee, and the cleaner.

10. The system of claim 6, wherein controller is to cause the system to: disengage each developing unit of the set of developing units from the photo imaging plate; set the series of voltage values to the series of developing units; for each developing unit, measure a corresponding series of disengaged electric currents; and determine, for each developing unit, a series of corrective factors based on the corresponding electric currents and the corresponding series of disengaged electric currents, wherein calculating the photo imaging plate voltage for each developing unit of the set of developing units based on the series of voltage values and the respective electric currents further comprises applying the respective series of correction factors to the respective electric currents.

11. A computer-readable medium comprising instructions that, when executed by a processor, cause a system to: modify a voltage of a developing unit contacting a photoconductive element to a series of voltage values during a time frame; measure electric currents flowing through the developing unit during the time frame; and determine a reference voltage of the photoconductor element based on the series of voltages values and the electric currents during the time frame.

12. The computer-readable medium of claim 11, comprising further instructions to cause the system to: identify a time of occurrence within the time frame in which an electric current sign has changed, wherein determining the reference voltage of the photoconductive element based on the series of voltages values and the electric currents during the time frame comprises determine the reference voltage of the photoconductor based on a voltage value of the series of voltage values at the time of occurrence.

13. The computer-readable medium of claim 11, comprising further instructions to cause the system to: determine an upcoming operation for the system; and calibrate the upcoming operation based on the voltage of the photoconductor element.

14. The computer-readable medium of claim 11, wherein modify the voltage of the developing unit contacting the photoconductive element to the series of voltage values during the time frame comprises: measuring an initial electric current of the developing unit when the developing unit is set at an initial voltage; and modifying the initial voltage based on an electric current sign of the initial electric current.

15. The computer-readable medium of claim 14, wherein modifying the initial voltage based on the electric current sign of the initial electric current comprises: increasing the initial voltage if the electric sign of the initial electric current is a first electric sign; and decreasing the initial voltage if the electric sign of the initial electric current is a second electric sign.