Printing device having an output level compensation function

A printing device having multiple printheads includes an acquisition device to obtain temperatures of the multiple printheads, a determination device which determines, based on the obtained temperature of one printhead, a target temperature corresponding to each other of the multiple printheads in order to maintain a predetermined output level relationship between an output level of the one printhead and an output level of each other of the multiple printheads, and an adjustment device to adjust the temperature of each other of the multiple printheads to the corresponding target temperature.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to shuttle-type image printing devices which
 print characters and images on a print medium by scanning multiple
 printheads across the print medium. In particular, the invention provides
 for improved output from shuttle-type printing devices in which multiple
 printheads are disposed at a fixed distance from each other and wherein
 each printhead scans and prints over a divided section of a print medium.
 2. Description of the Related Art
 Some conventional printing devices use full-line printheads, which are
 capable of simultaneously printing an entire line of data upon a print
 medium. Unfortunately, such printheads are quite expensive.
 In contrast, serial printing devices operate by scanning a printhead across
 a print medium. The printhead forms images upon the print medium as it is
 scanned across. These printheads are required to print only a small amount
 of data at any one time and are therefore generally less expensive than
 full-line printheads.
 However, one drawback of such serial printing is that high speed printing
 is difficult to achieve.
 Japanese Laid-Open Patent Application No. 50-81437 and U.S. Pat. No.
 4,272,771, disclose examples of methods to increase the print speed of
 serial image printing instruments. According to these documents, the left
 and right halves of each printed line are printed simultaneously. To
 achieve this feature, left and right printhead assemblies are provided,
 both of which are supported by a common carriage mechanism. Accordingly,
 print speed is approximately doubled over that of serial printing devices.
 Furthermore, these references suggest that further increases in print
 speed can be achieved by using more than two printhead assemblies or by
 printing in both the left and right scanning directions.
 Color, high quality and high speed printing have been performed using
 multiple printheads working in conjunction. While utilizing these
 configurations, certain relative output levels must be maintained among
 the multiple printheads. If these relative output levels are not
 maintained, the color, or gradient, becomes out of balance, or the print
 density becomes uneven. In general, print quality may be degraded due to
 incorrect relative output levels.
 For example, in a configuration where multiple printheads print in
 respective divided sections of a print medium, any characteristic
 differences among the printheads, the ink or the ink ribbons causes
 mismatches in print density between the divided sections.
 FIG. 1A and FIG. 1B illustrate this phenomenon. In FIG. 1A, two printheads,
 printhead 4A and printhead 4B, have printed within the section designated
 A and the section designated B, respectively. As shown, printhead 4B
 produces a more dense output than that of printhead 4A. The Figure
 illustrates the printing results for three printing duties, 25%, 50% and
 100%. The Figure shows that, for each printing duty, the difference in
 print densities between section A and section B is very noticeable at the
 border between the two sections.
 FIG. 1B illustrates similar printing results utilizing the same printheads
 while redefining section A and section B so as to add a small overlap
 between the two sections. Each printhead prints approximately half of the
 total print data in the overlapped printing area. Hence, the printing
 density of the overlapped area is greater than that of section A. However,
 the density is lower than that of section B. Therefore, in the case of
 FIG. 1B, the density differences are less noticeable than that shown in
 the above FIG. 1A, but are still obvious at both borders of the overlapped
 printing area. Accordingly, it is necessary to compensate for differences
 in print density caused by differences in output characteristics of
 utilized printheads.
 The above problem may be addressed by selecting printheads having the same
 output characteristics. This approach is not realistic for printing
 devices in which printheads (or printhead cartridges) can be replaced.
 Hence, what is needed is a method to compensate for fluctuating output
 levels of each printhead and to maintain a certain output level balance
 among multiple printheads.
 One important consideration in devising such a method is that, in
 operation, various printing signals (image data), each having various
 printing duties, are sent to the multiple printheads of a shuttle-type
 printing device. The varying duties give rise to varying driving duties of
 the printheads. The temperature of each printhead will vary accordingly.
 Furthermore, output characteristics of printing heads are dependent upon
 their respective temperatures. Hence, when the temperatures of printheads
 vary during operation, a particular output level relationship of the
 printheads becomes particularly difficult to maintain.
 SUMMARY OF THE INVENTION
 One purpose of this invention is to provide a printer driver, an image
 printing device and a method which prevent a color balance from varying
 even when the temperatures of multiple printheads vary.
 Yet another purpose of this invention is to provide a printer driver, an
 image printing device and a method which prevent a gray balance from
 varying even when the temperatures of multiple printheads vary.
 A further purpose of this invention is to provide a printer driver, an
 image printing device and a method which prevent a gradient balance from
 varying even when the temperatures of multiple printheads vary.
 In order to achieve the above purposes, this invention is characterized by
 a system to obtain a temperature of each of multiple printheads, to
 determine, based on the obtained temperature of one of the printheads, a
 target temperature corresponding to each other of the multiple printheads
 in order to maintain a predetermined relationship between an output level
 of the one printhead and an output level of each other of the multiple
 printheads to adjust the temperature of each other of the multiple
 printheads to the corresponding determined target temperature and to print
 an image using the multiple printheads.
 In one aspect, the invention is further characterized by a system to
 determine the target temperature corresponding to each other of the
 multiple printheads so that an output level of an image printed by the one
 print head is equal to an output level of an image printed by each other
 of the multiple printheads.
 In another aspect, the invention is characterized by a system wherein each
 of the multiple printheads uses a different color ink to print an image
 and the target temperature corresponding to each other of the multiple
 printheads is determined in order to maintain a predetermined color
 balance between an image printed by the one printhead and an image printed
 by each other of the multiple printheads.
 In a third aspect, the present invention is further characterized by a
 system wherein each of the multiple printheads uses a different color ink
 to print an image, one of the inks being black ink, and wherein the target
 temperature corresponding to each other of the multiple printheads is
 determined in order to maintain a predetermined gray balance between an
 image printed by the one printhead and an image printed by each other of
 the multiple printheads.
 In yet another aspect, the present invention is further characterized by a
 system wherein each of the multiple printheads uses ink of a density
 different than a density of ink used by any other of the multiple
 printheads, and wherein the target temperature corresponding to each other
 of the multiple printheads is determined in order to maintain a particular
 gradient balance between an image printed by the one printhead and an
 image printed by each of the other multiple printheads.
 This brief summary has been provided so that the nature of the invention
 may be understood quickly. A more complete understanding of the invention
 can be obtained by reference to the following detailed description of the
 preferred embodiments thereof in connection with the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The foregoing embodiments of the present invention are described in view of
 the above-mentioned Figures.
 The First Preferred Embodiment
 FIG. 2 shows printheads 4A and 4B mounted on carriage 1 at a separation of
 72 mm. Printheads 4A and 4B support, respectively, ink tanks 5A and 5B.
 Ink stored in the tanks is provided to the printheads during printing.
 This design allows for independent installation and removal of each tank
 and each printhead on the carriage 1. Alternatively, a printhead and a
 tank can be formed as one unit, and the combined unit can be installed on
 and removed from carriage 1.
 Carriage 1 is supported on guide rail 2 and can be moved freely by virtue
 of a drive mechanism, such as a drive belt. As a result, carriage 1 can be
 located anywhere within the scannable space, denoted by "358 mm" in FIG.
 2. In addition, the ink jet nozzles of each of printhead 4A and 4B can be
 located anywhere within each of scanning areas "258 mm(A)" or "258 mm(B)"
 respectively. Excluding ramp up and ramp down areas, at which carriage 1
 accelerates and decelerates, printhead 4A scans in its divided printing
 area "226 mm(A)", and printhead 4B scans in its divided printing area "226
 mm(B)".
 Caps 6A and 6B are used for the ink jet nozzles of printheads 4A and 4B,
 respectively, under a platen 3 within the scannable space of the carriage
 1. The carriage 1 rests over the platen at a home position, whereat each
 ink jet nozzle is capped with either cap 6A or 6B. Pump 7 is connected to
 cap 6B and removes ink through cap 6B. Each of heads 4A and 4B travels to
 a position facing cap 6B sometime during operation therefore ink can be
 removed from either printhead using pump 7.
 Wiper 8 is provided adjacent to cap 6B. Wiper 8 moves outward at a certain
 time into the path of one of the printheads and wipes ink jet nozzles of
 the printhead as it comes in contact with wiper 8. In addition, dummy
 ejection receptor 9 is provided on the opposite end of the scannable space
 of printhead 4A from "226 mm(B)" where cap 6A is located. Printhead 4B can
 travel to this position sometime during operation and perform a dummy
 ejection. Similarly, printhead 4A can perform a dummy ejection after it
 travels to a position facing cap 6A.
 The foregoing arrangement maximizes the printable area within the scannable
 space.
 In the above-described embodiment of a printing device, the printhead
 separation distance (72 mm), is preferably set to approximately
 one-quarter of the maximum printable area (298 mm). The printable area is
 maximized by dividing it into two scanning areas for each printhead. The
 width of the overlapped scanning area is 154 mm. These sizes are defined
 as follows. The width of A3 paper (297 mm.times.420 mm) is the width of
 the maximum printable area. The width of the overlapped scanning area
 corresponds to the width of A5 size paper (148 mm.times.210 mm).
 Therefore, the width of the maximum printable area is defined at
 approximately twice that of the overlapped scanning area.
 In this preferred embodiment, each of printheads 4A and 4B print on
 assigned printing areas, respectively, in a case where the instrument
 prints on A3 size paper. In this case, both printheads preferably eject
 the same type of ink. On the other hand, when the printing instrument
 prints on A5 size paper, which is the width of the overlapped printing
 area, one printhead may be replaced by a type of printhead which ejects
 ink with a lighter color so that ink with darker and lighter colors may be
 printed at areas of the page which can be accessed by both printhead 4A
 and printhead 4B.
 Accordingly, the printing device of the embodiment of FIG. 2 can print
 faster over A3 size print media than a printing device with one printhead
 because the work of printing over the maximum printable area is divided
 between two printheads. In addition, the size of the printing device of
 FIG. 2 is smaller than other devices having the same maximum printable
 area.
 The design of this preferred embodiment benefits single color printing,
 such as black and white. However, when using multiple color inks for color
 printing, the benefits are more pronounced.
 Regarding color printing, there are several types of printing devices which
 utilize a print medium which itself generates color. Examples of such
 devices include a device in which heating elements on a thermal printhead
 heat special thermal paper, thereby generating color, and a device in
 which optical effects create color upon photosensitive paper.
 On the other hand, various methods are used in which printheads transfer
 color ink onto print media. For example an impact printing method, ink
 ribbons contain liquid color ink which is transferred to a print medium
 when printing wires press the ribbons against the print medium. In thermal
 melt and sublimation transfer printing methods, heating elements on a
 thermal printhead heat solid ink on ink ribbon printheads and transfer the
 ink to a print medium. In an ink jet method, liquid ink is ejected onto a
 print medium.
 Of the above examples, devices in which color ink is transferred onto print
 media are used more widely due to their use of ordinary paper. Among these
 methods, ink jet printing has the advantages of low noise, lower operation
 cost, ease of miniaturization, ability to use ordinary paper, and ease of
 color printing. Hence, this method is widely used in various printing
 devices, such as printers and photocopiers.
 Ink jet printing devices include those in which the use of multiple
 printheads allows them to realize color printing, gradient printing or
 high resolution printing. For example, color printing can be performed
 using four printheads, each having a different color ink, yellow (Y),
 magenta (M), cyan (C) and black (K), or using three printheads containing
 Y, M and C. Gradient printing can be performed using a higher output level
 printhead which prints at a high density and a lower output level
 printhead which prints at a low density. High resolution printing can be
 performed using multiple printheads for each color, which are installed so
 as to provide interlaced printing.
 In this color printing embodiment, four color inks, black (Bk), cyan (C),
 magenta (M) and yellow (Y) are used. Four individually replaceable tanks,
 one for each color ink, Bk, C, M or Y, are installed on the central
 portion of carriage 1 of FIG. 2. Each printhead is equipped with a group
 of ink jet nozzles, each of which ejects, respectively, Bk, C, M or Y ink.
 The four ink tanks supply color ink to both printheads. Even though this
 embodiment is designed to supply ink from common ink tanks to each
 printhead, applications of this invention are not limited to this design.
 For example, each printhead can be equipped with an exclusive ink tank and
 each tank can thereby form a single unit with its respective printhead.
 Also, such tanks can be made removable from the printheads.
 FIG. 3 is a block diagram for a heater driver of a printhead similar to
 printheads 4A and 4B. Heaters 41-1, 41-2, . . . , 41-160 each correspond
 to a respective ink jet nozzle used for a particular color ink.
 Accordingly, each nozzle may be individually heated. Here, 16 heaters are
 used for Y (yellow) nozzles, 24 for both M (magenta) and C (cyan), 64 for
 K (black), and a total (32 for four sets) of 8 nozzles disposed between
 each of these colors. When each of heaters 41 are turned on at the same
 time, a large current flows and the load on the power supply increases. In
 addition, because voltage drops across the circuit impedance, the energy
 supplied to each of the heaters decreases. This may jeopardize normal
 printing functions.
 Thus, a concern for the ill-effect on image quality also arises. Therefore,
 in this preferred embodiment, printheads are installed at a small angle,
 and the well-known method of time-sharing driving is used for heater
 control. Under this time-sharing driving method, heaters are grouped into
 blocks, each of which contains the same number of heaters. In addition,
 the image data and print timing are adjusted block by block for ink
 ejection.
 Various ways of realizing the time-sharing driving method have been
 proposed and implemented. Any of these methods can be used. In this
 preferred embodiment, color ink jet nozzles are divided into 20 blocks.
 Each block contains 8 ink jet nozzles. These ink jet nozzles include 8 ink
 jet nozzles for mixed colors. Each block ejects ink sequentially, one
 after another, with a certain constant interval.
 The printheads are installed at an angle in order to compensate for the
 scanning speed of the printheads and the ejection time differences among
 the ink jet nozzle blocks. The angled installation of the printheads
 prevents the ejection time differences among the ink jet nozzle blocks
 from causing a straight line to be slanted.
 During printhead operation, ink is provided via shared liquid chambers
 located behind the ink paths leading to the nozzles. One liquid chamber is
 provided for each ink color. Ink is supplied from the shared liquid
 chambers through ink supply pipes to ink tanks 5A and 5B. Heater 41 and
 electrical wires are installed on the ink path leading to each ink jet
 nozzle. Heater 41 is a thermo-electrical converter which generates thermal
 energy for ink ejection. The electrical wires supply power to the heater.
 The heater and electrical wires are formed on a substrate such as a
 silicon wafer using thin film technology. A protective film is formed on
 heater 41 so that the heater does not come into direct contact with ink.
 Furthermore, the ink jet nozzle, ink path and shared liquid chamber are
 formed by stacking walls made of material such as resin and glass.
 Once heater 41 heats the ink inside a nozzle to boiling, bubbles are formed
 within the ink. The bubble formation increases pressure within the ink jet
 nozzles, and the increased pressure causes ink droplets to be ejected
 toward a print medium. An ejected ink droplet for each color weighs
 approximately 40 ng. This printing method is generally called bubble jet
 printing.
 AND gates 42-1 to 42-160 logically multiply a selection signal from a
 decoder 43, driving data from latch circuit 44 and a heat enable signal
 (Heat ENB). The selection signal is used in the time sharing process and
 the heat enable signal dictates the driving time. A shift register 45
 converts serial image data input signals into parallel signals and outputs
 the resulting driving data to the latch circuit 44. The resulting output
 signal is transmitted to respective heater 41.
 Temperature sensors 46 are provided on printheads 4A and 4B in this
 preferred embodiment. The sensors monitor the respective temperatures of
 printheads 4A and 4B. Generally, optimum driving conditions for the
 printheads are determined depending on the temperatures of printheads 4A
 and 4B. A protective mechanism is operated which is also based on the
 temperature information. Each of these provisions improve the stability of
 the printing characteristics. Furthermore, temperature control heaters 47
 are provided on printheads 4A and 4B in order to maintain printheads 4A
 and 4B at a particular temperature.
 FIG. 4A shows a system which comprises a printing device and a host
 computer which functions as a hosting instrument. In the host computer,
 various data processing is performed by OS (Operating System) 101 in
 conjunction with application software 102. In operation, image data is
 generated by application software 102 and printer driver 103 outputs the
 image data to the printing device.
 The image data is sent to printer driver 103 as multiple-level RGB data.
 After half-tone processing, the data is usually converted into binary CMYK
 data. The host computer then outputs the converted image data through a
 host computer/printing device interface or a file storage device
 interface. In the instance shown in FIG. 4A, the image data is output via
 a printing device interface.
 The printing device receives the image data under the control of controller
 software 104, checks items such as printer mode and compatibility with
 printheads 106, and transfers the image data to engine software 105.
 Engine software 105 interprets the received image data as having the print
 mode and the data structure as instructed by the controller software 104
 and generates pulses for the ink jet nozzles based on the image data. The
 pulses are sent to printheads 106. Printheads 106 use the pulses to eject
 color ink which corresponds to the pulses and to thereby print a color
 image on a print medium.
 FIG. 4B shows a block diagram of the printing device of FIG. 4A. Image data
 to be printed is transmitted into a receiving buffer in the printing
 device. In addition, data to acknowledge the correct receipt of image data
 by the printing device and data to show the operational status of the
 printing device are sent from the printing device to the host computer.
 The data in the receiving buffer is controlled under the management of CPU
 21, stored temporarily in print buffer 24, and given to printheads 4A and
 4B as print data.
 Based on the information from paper sensor 25, CPU 21 sends commands to a
 paper forwarding mechanism. The paper forwarding mechanism, such as line
 feed motor 26, controls mechanical drivers such as paper forwarding
 rollers or line feed rollers based on commands from CPU 21. CPU 21 also
 sends commands to carriage-return driving mechanism 28 based on
 information from carriage return sensor 27. Carriage return mechanism 28
 controls a carriage-driving power supply and thereby controls the
 movements of carriage 1. Purging unit 30 protects heads 4A and 4B and
 optimizes the driving conditions, using commands from CPU 21. CPU 21 sends
 such commands based on information sent by printhead sensor 29. Printhead
 sensor 29 comprises many sensors, for example, sensors such as those used
 to determine whether or not ink is present.
 Commands from CPU 21 to photosensor 31 activate LED 32. Light from LED 32
 subsequently reflected by test patterns on a print medium is then detected
 by photodiode 33. Based on the reflected light and on temperature readings
 from temperature sensors 46, CPU 21 controls the temperature of printheads
 4A and 4B by controlling temperature control heaters 47. This temperature
 control is described in greater detail below.
 In this preferred embodiment, printheads 4A and 4B print over the divided
 left and right printing areas shown in FIG. 2. Accordingly, the print data
 sent to printhead 4A is usually different from that sent to 4B. This
 difference creates a temperature difference between printhead 4A and 4B.
 Unfortunately, this temperature difference creates a difference in the
 amount of ink ejected by each printhead. One drawback of this situation is
 illustrated in FIG. 1. FIG. 5 graphically describes the phenomena of FIG.
 1 and also shows one technique which addresses the problems thereby
 resulting.
 The output levels of printheads 4A and 4B, which are indicated,
 respectively, by the solid line and the broken line in FIG. 5A, are
 originally adjusted equal by a well-known compensation method such as the
 pulse signal compensation method or the printhead temperature compensation
 method. FIG. 5B shows the case in which the duty of the printing data for
 printhead 4A, and therefore the temperature of printhead 4A, is higher
 than that of printhead 4B. In such a case, the output level of printhead
 4A increases, as shown by the thicker solid line. Accordingly, when
 printing is performed in the case shown in FIG. 5B, the contrast between
 printing density of the left and right printing sections is undesirable.
 Hence, as illustrated in FIG. 5C, it is necessary to bring the output level
 of printhead 4B up to the level of thicker broken line, which represents
 the output level of printhead 4A. FIG. 6A and 6B illustrate a method for
 doing so.
 FIG. 6A and 6B show a case where, if the temperature of printhead 4A is set
 at X and that of printhead 4B at W, the output levels of printheads 4A and
 4B are equal at O. (O.D.=Optical Density). When the duty of the printing
 data of printhead 4A is higher than that of printhead 4B, the temperature
 of printhead 4A increases by V to Y. As shown in FIG. 6A, the output level
 of printhead 4A then increases from O to Q.
 In order to keep the output levels of printheads 4A and 4B the same, the
 temperature of printhead 4B must be increased. However, a temperature
 equal to that experienced by printhead 4A will not suffice to equalize
 output levels. As illustrated in FIG. 6A, the output level of printhead 4B
 only reaches level P when the temperature of printhead 4B is increased by
 V to Z. Accordingly, as shown in FIG. 6A, the output level of printheads
 with different output level characteristics cannot be balanced simply by
 maintaining a relative temperature difference between the printheads.
 FIG. 6B shows the results of a temperature adjustment method according to
 the preferred embodiment. In particular, based on the temperature of
 printhead 4A, the temperature of printhead 4B is set so that the output
 levels of each printhead are equal. More particularly, the temperature of
 printhead 4B is set to Z' so that the output level of printhead 4B becomes
 equal to Q, which is the output level of printhead 4A at temperature Y.
 Notably, as described with respect to FIG. 6A, the temperature of
 printhead 4B must be increased by V', rather than by V.
 In this preferred embodiment, for the purpose of performing the control
 described above, a target temperature table is constructed in Control RAM
 22 or Control ROM 23. FIG. 8 shows a target temperature table of
 printheads 4A and 4B, which have the output level-temperature
 characteristics shown in FIG. 7. Specifically, for output levels of 0, P,
 Q, and R, the target temperatures of printhead 4A and 4B are AO, AP, AQ,
 AR, and BO, BP, BQ, and BR, respectively.
 A process for obtaining target temperatures according to this preferred
 embodiment is explained by the flowchart of FIG. 9.
 At step S2 of the FIG. 9 flowchart, CPU 21 obtains temperatures Ta and Tb
 of printheads 4A and 4B, respectively, from sensors 46 which are provided
 on printheads 4A and 4B. At step S3, CPU 21, using photosensor 31, LED 32,
 and photodiode 33, obtains output levels OD_A and OD_B, which correspond
 to temperatures Ta and Tb, respectively. For example, when the temperature
 Ta of printhead 4A is AQ, the output level OD_A of printhead 4A is defined
 as AQ, and when the temperature Tb of printhead 4B is BP, the output level
 OD_B of printhead 4B is defined as BP.
 At step S4, the output level OD_A of printhead 4A and the output level OD_B
 of printhead 4B are compared, and, in steps S5 and S6, the temperature of
 the printhead having a lower output level is set so that the output level
 corresponding to the set temperature increases to that of the printhead
 having a higher output level. For example, if the output level OD_A of
 printhead 4A is higher than OD_B of printhead 4B (Yes at step S4), the
 target temperature TTb of printhead 4B is set at that temperature which
 results in printhead 4B having an output level equal to OD_A. On the other
 hand, if the output level OD_A of printhead 4A is not greater than that
 OD_B of printhead 4B (No at step S4), CPU 21 determines, in step S6, the
 target temperature TTa of printhead 4A from the table in FIG. 8 which
 results in printhead 4A producing an output level equal to OD_B. Step S7
 completes the target temperature determination.
 In this preferred embodiment, heaters 47 are capable only of heating
 printheads and thereby causing output levels to increase. Hence, a
 printhead with a lower output level must be heated so that its output
 level matches that of the printhead with a higher output level. Of course,
 cooling devices, either alone or in conjunction with heaters 47, may also
 be utilized so as to adjust printhead temperatures and equalize output
 levels in accordance with the present invention.
 In the flowchart of FIG. 9, output levels corresponding to certain
 temperatures are referenced in order to determine a target temperature of
 a printhead. However, in another embodiment, the printhead temperature is
 defined without reference to printhead output levels. This method is
 explained by the flowchart of FIG. 10.
 At step S12 of FIG. 10, CPU 21 obtains the temperatures Ta and Tb of
 printheads 4A and 4B, respectively, from sensors 46 which are provided on
 printheads 4A and 4B. In step S13, tentative target temperatures BA and AB
 are obtained. The tentative target temperature BA brings the output level
 of printhead 4B to that of printhead 4A, and the tentative target
 temperature AB brings the output level of printhead 4A to that of
 printhead 4B.
 For example, if the temperature of printhead 4A is AQ and that of printhead
 4B is BP, the output level of printhead 4A is Q and that of printhead 4B
 is P. Therefore, in order to bring the output level of printhead 4B equal
 to that of printhead 4A, the temperature of printhead 4B must be adjusted
 to BQ, as defined by the table of FIG. 8. Hence, the tentative target
 temperature BA of printhead 4B is found to be BQ. Similarly, the tentative
 target temperature AB of printhead 4A is determined to be AP.
 In step S14, the tentative target temperature AB is compared with the
 temperature Ta of printhead 4A. If the tentative target temperature AB is
 greater than the temperature Ta of printhead 4A, the tentative target
 temperature AB is defined, in step S15, as the target temperature TTa for
 printhead 4A. On the other hand, in step S16, the tentative target
 temperature BA is compared with temperature Tb of printhead 4B. If
 tentative target temperature BA is greater than temperature Tb of
 printhead 4B, the tentative target temperature BA is defined, in step S17,
 as the target temperature TTb for printhead 4B. As in the process of FIG.
 9, heaters 47 can only heat printheads 4A and 4B. Hence, steps s14 to s17
 ensure that a target temperature is set only for the printhead having a
 target temperature greater than its actual temperature. Of course, in
 another embodiment, cooling devices may also be used to adjust the
 temperature of a printhead to a tentative target temperature lower than
 its actual temperature.
 If steps S14 and S16 both result in negative responses, the temperature
 sensor readings are in error. Accordingly, an error process is performed
 in step S18. The target temperature determination process terminates in
 Step S19.
 FIG. 11 shows a flowchart for constructing a target temperature table to be
 utilized by the processes of FIG. 9 and FIG. 10. In step S112, output
 levels are measured as a function of temperature for each of multiple
 printheads, and temperature-output level tables of TT1[x], TT2[x], . . . ,
 TTi[x] are built, where TTi[x] denotes the output level of the i-th
 printhead at temperature x. The measured data is illustrated in FIG. 12.
 It has been determined that, in step S112, printing data having a duty of
 less than 50% should be supplied to the printheads. In addition, it is
 preferable to choose printheads having different output levels at room
 temperature. The constructed target temperature tables are stored in
 Control ROM 23 in the printing device.
 This preferred embodiment utilizes replaceable printheads. Therefore, the
 target table should include data for all printheads which might eventually
 be installed in the printing device. However, it may be preferable to
 reduce the total amount of stored data by consolidating data corresponding
 to printheads having similar temperature-output level characteristics.
 In step S113, output levels Oda[JJ] and Odb[JJ] for installed printheads 4A
 and 4B are measured at room temperature at an appropriate time, such as
 when the printheads are installed, or when the power is turned on.
 Photosensor 31 measures these output levels on a test pattern printed
 using print data having the same duty as that used in step S112. Although
 either absolute or relative output levels can be measured, this preferred
 embodiment utilizes absolute values.
 Steps S114 and S115 estimate the characteristics of each printhead based on
 the measured output levels and, based on the estimation, choose a table to
 manage the target temperatures. In the example shown in FIG. 12,
 characteristic TT4 is chosen for printhead 4A, and characteristic TT2 for
 printhead 4B. The data in the chosen tables are stored in RAM 22 as the
 target temperature tables for both heads 4A and 4B. Accordingly, a
 customized table such as that shown in FIG. 8 is constructed.
 In a case where relative output level values are measured in step S113, the
 characteristic TT1 is assigned to the printhead having the lower output
 level and an output level characteristic TT2 to TT6 is assigned to the
 other printhead. When the room temperature measurement data does not match
 with any of TTi[JJ], the table having the most similar data, or the
 weighted average of two tables having similar data, one of which contains
 higher output levels and the other of which contains lower output levels,
 should be used.
 In the above preferred embodiment, measurement of output levels is done
 after the printheads are installed in the printing device. The output
 level of a printhead can also be measured when the printhead is
 manufactured and the measured data can be stored in the printhead during
 manufacture. The output level information or specific ID information can
 be stored in a ROM provided in a printhead or the output level information
 can be stored using a hard-wired pattern or resistors having specific
 values. After such a printhead is installed in a printing device, the
 stored data is accessed in order to determine the output level-temperature
 characteristic corresponding to the printhead.
 Alternatively, a target temperature table may be constructed by a printing
 device once a printhead is installed in the printing device. In this case,
 printhead 4A is fixed at a temperature AO while printing a test pattern
 and printhead 4B prints multiple test patterns while its temperature is
 varied. In accordance with the foregoing descriptions, the temperature BO
 at which printhead 4B produces the same output level as that of printhead
 4A at the temperature AO is determined to correspond to printhead 4A's
 temperature AO. This process is repeated at different temperatures of
 printhead 4A in order to complete the target temperature table. This
 process reduces errors caused by variations of the sensitivity of
 photosensors among printing devices.
 FIG. 13 is a flowchart describing control of printhead temperature during
 printing. In step S22, CPU 21 obtains temperatures Ta and Tb of printheads
 4A and 4B. Next, CPU 21 follows the flow of either FIG. 9 or FIG. 10 in
 order to obtain target temperatures TTa and TTb for each printhead. If, in
 step S24, temperature Ta is lower than target temperature TTa, flow
 proceeds to step S25, wherein printhead 4A is heated. On the other hand,
 if temperature Ta of printhead 4A is not lower than the target temperature
 TTa, and if, in step S26, the temperature Tb of printhead 4B is lower than
 the target temperature TTb, flow proceeds to step S27, wherein printhead
 4B is heated.
 The above steps are repeated until the temperatures of printheads 4A and 4B
 are greater than target values TTa and TTb, respectively. Thereafter, in
 step S28, actual printing starts.
 In the example of FIG. 13, printing begins once all printheads reach their
 respective target temperatures. Accordingly, the image density of each
 divided printing section is consistent.
 The flowchart of FIG. 14 sets forth another printing control procedure. The
 actions performed at each step of FIG. 14 correspond to those steps in
 FIG. 13 having identical least-significant digits. Generally, the process
 of FIG. 14 differs from that of FIG. 13 in that printing is performed as
 soon as printhead 4A or printhead 4B is heated.
 In other words, according to FIG. 14, printing begins without determining
 whether a heated printhead has reached its target temperature. Hence, the
 total printing time is less than that of the process in FIG. 13.
 On the other hand, the image density in the divided printing sections of a
 document printed using the process of FIG. 14 may not be consistent
 because in some cases, printing may start before all printheads reach
 their target temperature. However, when printheads are always controlled
 toward a target temperature as provided in FIG. 14, actual printhead
 temperatures are not likely to vary greatly from the target temperatures.
 As described above, this preferred embodiment increases the temperature of
 a printhead having a lower output level based on the temperature of a
 printhead having a higher output level in order to adjust the output level
 of the first printhead to the output level of the other printhead.
 Accordingly, the density difference between divided printing sections of a
 page is reduced even in a case where the temperatures of printheads
 fluctuate.
 In this preferred embodiment, an entire printing area is divided into two
 sections and each of two printheads print in an assigned printing section.
 The present invention, however, is not limited to this arrangement. It can
 be applied to cases in which an entire printing area is divided into three
 or more printing sections and in which three or more printheads print in
 each of the assigned printing sections.
 For example, FIG. 15 shows a target temperature table for a case in which
 an entire printing area is divided into three printing sections and in
 which three printheads print in each of the three sections. In such a
 case, based on the temperature of a printhead having the highest output
 level, the temperature of the other two printheads are adjusted so that
 both produce the same output level as the printhead having the highest
 output level.
 In the above preferred embodiment, temperature sensors 46 measure the
 temperature of the printheads. Other well-known methods may also be
 utilized to estimate the printhead temperatures. For example, printhead
 temperature variations can be estimated from the duty of the driving
 signals supplied to the printheads, and the net printhead temperatures can
 be estimated using the temperature variation estimates and a temperature
 measured within a printing device by an environment sensor. With this
 arrangement, and contrary to the above embodiment, differences in measured
 printhead temperatures will not be affected by differences among
 individual temperature sensors used to measure individual printhead
 temperatures.
 In the above preferred embodiment, control is performed by CPU 21 using
 stored process steps. Alternatively, printer driver 103 in a host computer
 can perform the control. In this case, printer driver 103 is designed to
 contain a target management table, and to receive temperature information
 of multiple printheads. For example, printer driver 103 follows the flow
 of FIG. 9 or FIG. 10 to obtain the target temperature for one of the
 printheads using the received temperature information. Printer driver 103
 then transmits the target temperature information to a printing device.
 The printing device receives the target temperature information and
 follows the flow of FIG. 13 or FIG. 14 in order to perform the temperature
 control.
 It may be advantageous to incorporate the above-mentioned temperature
 estimation method when utilizing printer driver 103 as described above
 because printer driver 103 can estimate the temperature of the printheads
 based on the printing data which the printer driver 103 itself transmits
 to the printing device. Printer driver 103 can also easily obtain the
 environment temperature from the printing device periodically.
 Furthermore, this preferred embodiment utilizes temperature control heaters
 47 in order to control printhead temperatures. Instead, cooling methods
 such as one utilizing Peltier devices can be used. In this case, the
 temperature of a printhead having a higher output level should be adjusted
 so that its output level equals that of another printhead having a lower
 output level. Both heating and cooling devices may also be used, based on
 the amount of heating or cooling necessary to equalize output levels.
 The Second Preferred Embodiment
 According to the second preferred embodiment, printhead 4B ejects lower
 density ink than that ejected by printhead 4A. Therefore, printheads 4A
 and 4B use inks having different concentration and also work together to
 print a gradient image in overlapped printing area "154 mm".
 As described above, because the printing data transmitted to printhead 4A
 will usually differ from that transmitted to printhead 4B, the respective
 temperatures of printheads 4A and 4B may also differ. Because ink
 viscosity reduces when printhead temperature increases, the amount of ink
 ejected from the hotter printhead increases and the image produced
 therefrom becomes darker. Thus, the linearity of the gradient image is not
 easily maintained.
 The thin solid lines labeled high-cyan and low-cyan in FIG. 16A show
 respective output levels for printheads 4A and 4B. The output levels of
 the two printheads are originally set to have a particular relationship by
 using a well-known output level compensation method such as pulse signal
 compensation or printhead temperature compensation. For convenience sake,
 the output levels for the high-density and low-density inks are drawn
 using equally thin lines. Both inks overlap in the printed image,
 therefore the thicker solid line indicates the net output level.
 FIG. 16B illustrates the case in which the printing data duty for printhead
 4A, the printhead having the high-density ink, is high. The output level
 of printhead 4A (shown by the thinner broken line) is greater than the
 original high-cyan output level of FIG. 16A. When printing is performed
 under the conditions of FIG. 16B, the actual combined output level (shown
 by the thicker broken line) increases non-linearly at a point in the high
 output level range. Thus, the linearity of the gradient is not maintained.
 Therefore, in order to maintain the original output level relationship with
 printhead 4A, it is necessary to increase the output level of printhead
 4B, as illustrated by the thinner broken lines in FIG. 16C. If increased
 as shown, the linearity of the gradient (the thicker solid line) can be
 re-established.
 Contrary to FIG. 16A, FIG. 16B, and FIG. 16C, FIG. 17A and FIG. 17B
 illustrate a case in which the printing data duty to printhead 4B, having
 the lower-density ink, increases with respect to the printing data duty to
 printhead 4A. Similarly to the remedy shown in FIG. 16C, FIG. 17C shows
 that, in order to maintain the linearity of the gradient, the output level
 of printhead 4A must be increased.
 Controlling the output levels shown in FIG. 16 and FIG. 17 occurs in a
 similar fashion to the control described in the first preferred
 embodiment. However, a target temperature table such as shown in FIG. 8
 cannot be used without modification because the original concentration of
 the ink is different for printheads 4A and 4B. In this second preferred
 embodiment, the output level of the high-density ink is twice as much as
 that of the low-density ink. The target temperatures must therefore be
 defined so that this original relationship is maintained. Therefore, as
 illustrated in FIG. 18, the temperature-output level characteristics are
 measured separately for printhead 4A, having high-density ink, and
 printhead 4B, having low-density ink. The target temperature table is
 therefore constructed so that a given output level of the high-density ink
 corresponds to an output level of the low-density ink having one-half the
 magnitude of the given output level. As shown in FIG. 18, the respective
 printhead temperatures at which this circumstance occurs are designated as
 corresponding to one another.
 It is also possible to utilize the table in FIG. 8 to construct the table
 of FIG. 18. In doing so, the target temperatures are obtained by comparing
 corrected output levels. The corrected printhead temperatures are obtained
 by determining each temperature at which the output level of printhead 4B
 is a desired fraction of each output level of printhead 4A.
 As described above with respect to FIGS. 13 and 14, in this preferred
 embodiment, CPU 21 increases the temperature of a printhead having a lower
 output level than that required for a particular output level
 relationship. Hence, regardless of which printhead fluctuates in
 temperature, the gradient linearity within an overlapped printing area can
 be maintained.
 Furthermore, the design of this invention is not limited to a shuttle type
 printing device. It can be applied to any serial-type or full-multi-type
 gray scale printing device.
 In addition, the various alterations explained with respect to the first
 preferred embodiment are also applicable to this second preferred
 embodiment. For example, the various method of construction of a target
 temperature table, printhead temperature estimation, and control by
 printer driver 103 can all be applied to this preferred embodiment.
 The Third Preferred Embodiment
 In the above-described first and second preferred embodiments, each
 printhead ejects inks for each of four colors. Even if the printing data
 duty is very different among the four colors, the amount of ejected ink
 does not differ greatly among the four colors because ink jet nozzles for
 all colors are formed on the same substrate and therefore the four inks
 are all at the same temperature.
 In the third preferred embodiment, an independent printhead is provided for
 each of the four colors, black, yellow, magenta and cyan. The printheads
 basically have a similar structure as the one shown in FIG. 3. However,
 each printhead contains 128 units of heater 41.
 In this example, a black image in the lower density range is printed, not
 by a black ink, but by the PCBk (Process Color Black), which utilizes the
 combination of yellow, magenta and cyan. In a higher density range, a
 black image is expressed by all four colors.
 In FIG. 19, lines A to D correspond to each of the printheads containing
 black, yellow, magenta and cyan inks, respectively. Print data transmitted
 to each of printheads A to D usually differs from that of each other
 printhead, and therefore, in some cases, the temperature of each of the
 printheads A to D differs from that of each other printhead. As described
 above, when the printhead temperature increases, the viscosity of ink
 reduces and more ink is ejected. Thus, the image density corresponding to
 the less-viscous ink increases. As a result, a desired color balance and
 gray balance may be lost.
 The solid lines in FIG. 19A represent original output levels of printheads
 A to D. The output levels are originally fixed at a certain relationship
 by a well-known output level compensation method such as pulse signal
 compensation or printhead temperature compensation. Ink ejected from each
 printhead is combined on a printed image and creates an image having the
 PCBk output level represented by the thick solid line PCBk.
 In FIG. 19B, the duty of printing data for the cyan ink printhead, or
 printhead 4B, is high and, therefore, the output level of printhead 4B
 (the thin broken line) is greater than the level needed to maintain its
 original relationship with the output from printheads A, C and D.
 Therefore, when printing is performed under the conditions of FIG. 19B,
 the resultant hue becomes different from the hue of FIG. 19A.
 Accordingly, it is necessary to increase the output levels of printheads C
 and D to the level represented by the thin broken lines C and D of FIG.
 19C, so as to maintain the original output level relationship with
 printhead 4B. As a result of doing so, a deviation in hue can be
 corrected.
 The output level for PCBk is shown in FIG. 19B and FIG. 19C as a reference
 only. Although PCBk is not linear in either Figure, it should be noted
 that this preferred embodiment is intended to maintain a particular hue or
 color balance, and not to maintain the linearity of the PCBk output level.
 This distinction is further explained by the hue diagram of FIG. 20. This
 hue diagram is created by dissecting a color space at a certain black
 output level. originally, the balance among CMY is chosen so that the
 expected black PCBk can be obtained. FIG. 19A illustrates a situation in
 which the output levels of each color satisfy the chosen balance. If one
 of the printheads, for example, the cyan-ejecting printhead, exhibits a
 rise in temperature, then its color output level increases as illustrated
 in FIG. 19B and the output shifts to a color having a hue deviation from
 the expected black, namely to a black having a strong cyan hue (cyanic
 PCBk), as shown by arrow in FIG. 20. Accordingly, this hue deviation is
 corrected by adjusting the other two colors as shown in FIG. 19C so as to
 obtain the expected black.
 The correction is illustrated by arrow b of FIG. 20.
 Once this correction is performed, the CMY level as a whole increases with
 respect to that of black. The PCBk output level therefore increases in the
 medium output level range, as illustrated in FIG. 19C. Accordingly, it may
 also be necessary to adjust the output level of printhead 4A, having the
 black ink, in order to maintain the gradient of the PCBk. This adjustment
 is the same as the compensation of the black output level for the black in
 the medium and high output levels, or the compensation for the gray scale
 printing in black color. Hence, the explanation is omitted here.
 In this preferred embodiment, the target temperature table shown in FIG. 21
 is prepared in order to define the target temperatures for printheads of
 two colors when an output level of a third printhead increases.
 The table of FIG. 21 is constructed at room temperature using the
 previously-described methods. Then, for a certain temperature of one of
 the printheads, the temperatures for the other two printheads are obtained
 so that the resultant outputs form black when mixed. The K-CMY target
 temperature tables, TTc[K], TTm[K], and TTy[K], are constructed so as to
 produce a black output level of 1/2K, or one-half of the black output
 level of the black ink-ejecting printhead after the temperature
 adjustments are completed. It is desirable to construct these tables at a
 CMY balance corresponding to 50% of the output level of the black
 printhead because such a construction gives the best compensation
 efficiency.
 The flowchart of FIG. 22 describes a method for obtaining target
 temperatures according to the third preferred embodiment of the present
 invention.
 In step S42, CPU 21 obtains the temperatures Tc, Tm and Ty for three
 printheads using temperature sensor 46 provided on each printhead. At step
 S43, CPU 21 obtains, using the table of FIG. 21, black output levels
 K[Tc], K[Tm] and K[Ty], which correspond, respectively, to the printhead
 temperatures Tc, Tm and Ty. For example, when the temperature Tc of
 printhead C is CQ, Tm of printhead M, MP, and Ty of printhead Y, YP, then
 the black output levels K[Tc], K[Tm] and K[Ty] for printheads C, M, and Y
 are Q, P, and P, respectively.
 At steps S44 to S46, the highest black output level is determined from
 among K[Tc], K[Tm] and K[Ty]. If the corresponding black output level
 K[Tc] is the highest, then, in step S47, CPU 21 uses the table to
 determine the values of the target temperatures TTm and TTy needed for
 printheads M and Y (in this example, MQ and YQ). If black output level
 K[Tm] is the highest, then, in step S49, the values of the target
 temperatures TTc and TTy for printheads C and Y are determined. The
 temperature control process following S50 is the same as that shown in
 FIG. 13 and FIG. 14. Hence, it is omitted here.
 As described above, in this preferred embodiment, the temperature of a
 printhead producing a lower black output level than that required for a
 particular output level relationship is adjusted. The temperature is
 adjusted based on the output level required for the particular output
 level relationship. Therefore, no matter which printhead fluctuates in
 temperature, a particular color balance can be maintained.
 Furthermore, the design of this embodiment is not limited to a shuttle type
 printing device. It can be applied to any serial-type or full-multi-type
 gray scale printing device.
 In addition, the various alterations explained with respect to the first
 preferred embodiment are also applicable to this preferred embodiment. In
 particular, the construction of a target temperature table, printhead
 temperature estimation, and control by printer driver 103 can all be
 incorporated into this embodiment.
 The Fourth Preferred Embodiment
 The fourth preferred embodiment includes one printhead to eject black ink
 and another to eject yellow, magenta and cyan inks separately. The
 printheads have a structure similar to the printhead of FIG. 3. However,
 the printhead to eject black ink contains 128 heaters 41, while the
 printhead to eject yellow, magenta and cyan inks contains 160 heaters 41.
 Forty-eight heaters are dedicated to each color and each of the two
 regions between the ink colors contains 8 heaters.
 In this example, a black image in a lower density range is printed, not by
 a black ink, but by PCBk (Process Color Black). In the higher density
 range, a black image is printed using the PCBk and black ink.
 In the third preferred embodiment, all four color printheads are separate
 and hence, temperature fluctuation for each ink occurs somewhat
 independently. In this preferred embodiment, one color printhead contains
 three color inks, and therefore, it is considered that the same
 temperature fluctuation occurs to each of the three colors. Therefore in
 this preferred embodiment, gray balance is more of a concern than color
 balance.
 In the following description, A denotes the printhead with black ink and B
 denotes the printhead having color ink. Because printheads 4A and 4B are
 separate entities, the output levels of black for printhead 4A and of
 color for printhead 4B fluctuate independently. Printing data transmitted
 to each of printheads 4A and 4B differs from that of the other for many
 images, and hence, in some cases, the temperature of each of the
 printheads 4A and 4B differs from that of the other. When the temperature
 of a printhead increases, the viscosity of its ink reduces and more ink is
 ejected. As a result, the gray balance cannot be maintained, as in the
 case with the third preferred embodiment.
 The solid lines in FIG. 23A represent the output levels of printheads 4A
 and 4B. The output levels of the printheads are originally set at a
 certain relationship by a well-known output level compensation method such
 as the pulse signal compensation method or the printhead temperature
 compensation method. Ink ejected from each printhead is combined on a
 printed image and, as a result, the image has the PCBk output level, which
 is represented by the thick solid line in the Figure.
 In FIG. 23B, the duty of printing data for the black ink printhead, or
 printhead 4A, is high and the output level of printhead 4A (the thinner
 broken line) is greater than the level required for the original
 relationship with printhead 4B. When printing is performed under this
 condition, the output level in the high density range is greater than it
 should be (thicker broken line) and hence, as described above, the gray
 balance cannot be maintained.
 Therefore, it is necessary to increase the output level of printhead 4B to
 the level represented by a thinner broken line in FIG. 23C, so that the
 original output level relationship with printhead 4A can be maintained. As
 a result, the proper gray balance (thick solid line) can be restored.
 Contrary to FIG. 23A to FIG. 23C, FIG. 24A to FIG. 24C show the case in
 which the duty of printing data for printhead 4B increases with respect to
 that for printhead 4A. As shown in FIG. C, the gray balance is restored by
 increasing the output level of printhead 4A.
 As in the case of the above-explained second preferred embodiment, the
 target temperatures in this preferred embodiment can be obtained by
 comparing corrected output levels. The corrected output levels are
 obtained based on a certain constant relationship required between the
 black ink output level and the PCBk output level.
 As described above with respect to FIG. 13 and FIG. 14, the temperature of
 a printhead with a lower output level than that required for a particular
 output level relationship is increased to achieve the particular output
 level relationship. Therefore, no matter which printhead fluctuates in
 temperature, the gray balance in color printing can be maintained.
 Furthermore, the design of this invention is not limited to a shuttle type
 printing device. It can be applied to any serial-type or full-type gray
 scale printing device.
 In addition, the various alterations explained in the first preferred
 embodiment are also applicable to this preferred embodiment. For example,
 the construction of a target temperature table, printhead temperature
 estimation, and control by printer driver 103 are all applicable to this
 preferred embodiment.
 In addition, this invention is not restricted to an image printing
 instrument in which binary data is printed. This invention is also
 effective for a printing device which prints multiple-level image data.
 Furthermore, the series of signal processing may all be performed by a
 printer driver.
 This invention is applicable, not only to the ink jet printing method, but
 also to the printing methods in which the print density has thermal
 characteristics, such as the above-mentioned thermal melt and sublimation
 transfer printing methods.
 The invention is also applicable to printing devices which use paper,
 cloth, leather, transparencies, metal or other types of print media.
 While the present invention is described above with respect to what is
 currently considered to be its preferred embodiments, it is to be
 understood that the invention is not limited to the disclosed embodiments.
 To the contrary, the invention is intended to cover various modifications
 and equivalent arrangements included within the spirit and scope of the
 appended claims.