Patent Publication Number: US-7712861-B2

Title: Image forming apparatus and droplet ejection correction method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus and a droplet ejection correction method, and more particularly, to an image forming apparatus for correcting droplet ejection in cases where images are formed by droplet ejection and a droplet ejection correction method. 
     2. Description of the Related Art 
     Among image forming apparatuses which form images by ejecting and depositing droplets of ink onto a medium, such as paper, apparatuses are known which read in an image formed on a medium and correct ink droplet ejection on the basis of the reading results (see, for example, Japanese Patent Application Publication Nos. 5-238012, 6-166247, 6-198866 and 7-266582). 
     Furthermore, an image forming apparatus is also known which reads in an image formed on a medium and determines ejection failure nozzles on the basis of the reading results (see, for example, Japanese Patent Application Publication No. 6-297728). 
     In a so-called single-pass image forming apparatus which forms images by moving a medium, such as paper, once only, with respect to a head having a plurality of nozzles, any variation in the droplet ejection between respective nozzles, such as variation in the direction of flight of the ejected droplets and variation in the ejected droplet volumes, directly produces image abnormalities, such as so-called “banding”, or the like. 
     In an image forming apparatus of this kind, naturally, better reliability of the head is achieved, but even in this case, if using a head having a very large number of nozzles, it is not possible to avoid droplet ejection variations which occur with a certain probability. 
     If it is sought to correct droplet ejection on the basis of image reading results, in order to avoid image deterioration due to variations in droplet ejection, then “instable variations” present a major problem. More specifically, if the variation in the droplet ejection is “relatively stable variation” which involves gradual variation over time, then if images containing relatively stable variation of this kind are read in and correction is performed directly on this basis, then an effect in maintaining high image quality is obtained; but if the variation in droplet ejection is “instable variation” which involves sudden variation over time, then even if images containing “instable variation” of this kind are read in and correction is performed directly on this basis, when the correction is actually implemented, it is possible that the direction of variation will have changed (in other words, the direction of flight of the ejected droplets or the direction of increase or decrease in the ejection droplet volume will have changed), and in cases such as this, the deterioration of the image may actually be exacerbated. In other words, there is a problem in that high-frequency noise components adversely affect the correction. 
     Well-known and commonly used methods for removing high-frequency noise components are simple averaging, such as arithmetic averaging, and filtering of the high-frequency components, but in the case of an image forming apparatus, increasing the number of measurement operations so as to achieve effective noise removal means increasing the number of prints or increasing the number of test patterns. Therefore, it is not possible to increase the number of measurement operations, readily. 
     Japanese Patent Application Publication Nos. 5-238012, 6-166247, 6-198866, 7-266582, and 6-297728 mentioned above do not make any concrete description of the problem of dealing with temporal variation in droplet ejection, and nor do they present ways of resolving this problem. 
     Japanese Patent Application Publication No. 6-198866 discloses a general averaging process between pixels, namely, a process which uses the average value of the adjacent pixels to each pixel (dot) output, (for example, the average of three pixels), as the output for that pixel, in order to reduce the effects of noise and reading error, but it does not discuss how to deal with temporal variations in droplet ejection. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of the foregoing circumstances, an object thereof being to provide an image forming apparatus and a droplet ejection correction method whereby, even if temporal variation occurs in droplet ejection when forming images by ejecting and depositing droplets, accurate correction of the droplet ejection can be implemented. 
     In order to attain the aforementioned object, the present invention is directed to an image forming apparatus, comprising: an image forming device which has a plurality of nozzles performing droplet ejection of ink to deposit droplets of the ink to form an image onto a prescribed medium; an image reading device which acquires read image data by reading in the image on the medium; a deviation amount calculation device which, for each of the nozzles, calculates a current deviation amount that is a deviation amount in the droplet ejection with respect to a prescribed central value, by finding a weighted average of an initial deviation amount and past N deviation amounts nearest to a current time calculated for the nozzle according to past read image data; and a droplet ejection correction device which corrects the droplet ejection of each of the nozzles according to the current deviation amount of the nozzle calculated by the deviation amount calculation device. 
     According to this aspect of the present invention, since the current deviation amount is calculated for each nozzle by finding a weighted average between the initial deviation amount and the past N deviation amounts nearest to the current time, and since droplet ejection is corrected for each nozzle on the basis of the current deviation amount that includes a component of the initial deviation amount in this way, then even if temporal variation occurs in the droplet ejection, the current deviation amount can still be ascertained accurately, and hence the droplet ejection can be corrected accurately. Since the current deviation amount is calculated by using the weighted value of the initial deviation amount, then it is possible to ascertain the current deviation amount accurately, and to achieve accurate correction of droplet ejection, without having to increase the number of prints or test patterns made. 
     Preferably, the deviation amount includes one of a deviation amount in a position of a dot formed by the droplet of the ink deposited on the medium, a deviation amount in a size of the dot, and a deviation amount in density of the dot. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming apparatus, comprising: an image forming device which has a plurality of nozzles performing droplet ejection of ink to deposit droplets of the ink to form an image onto a prescribed medium; an image reading device which acquires read image data by reading in the image on the medium; a deviation amount calculation device which, for each of the nozzles, calculates a deviation amount in the droplet ejection with respect to a prescribed central value according to the read image data; a droplet ejection correction device which corrects the droplet ejection of each of the nozzles according to the deviation amount of the nozzle; a storage device which stores a maximum tolerable value for a temporal variation in the deviation amount of each of the nozzles; and a control device which, for each of the nozzles, compares the temporal variation in the deviation amount with the maximum tolerable value stored in the storage device, and implements control which discards the deviation amount relating to a period where the temporal variation in the deviation amount exceeds the maximum tolerable value, in such a manner that that deviation amount is not used in correcting the droplet ejection of the nozzle. 
     According to this aspect of the present invention, since the temporal variation in the deviation amount in droplet ejection of each nozzle is compared with the maximum tolerable value stored in the storage device, and since the deviation amount in droplet ejection for any nozzle relating to a period where the temporal variation in the deviation amount in droplet ejection of the nozzle exceeds the maximum tolerable value is discarded and is not used in correcting the droplet ejection of the nozzle, then it is possible to ascertain the deviation amount in droplet ejection accurately, and hence to correct the droplet ejection accurately, without increasing the number of measurements of the deviation amount, even in cases where there is temporal variation in the droplet ejection. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming apparatus, comprising: an image forming device which has a nozzle performing droplet ejection of ink to deposit droplets of the ink to form an image onto a prescribed medium; an image reading device which acquires read image data by reading in the image on the medium; a deviation amount calculation device which calculates a deviation amount in the droplet ejection of the nozzle with respect to a prescribed central value according to the read image data; a droplet ejection correction device which corrects the droplet ejection of the nozzle according to the deviation amount; a storage device which stores a past deviation amount in the droplet ejection of the nozzle; and a control device which measures the deviation amount by using the image forming device, the image reading device and the deviation amount calculation device, before and after performing a maintenance operation to restore a state of the ink in the nozzle, judges whether or not a differential between the deviation amount measured before the maintenance operation and the deviation amount measured after the maintenance operation is within a prescribed range, and implements control in such a manner that, when the differential is within the prescribed range, then the droplet ejection of the nozzle is corrected by the droplet ejection correction device according to at least one of the deviation amount measured after the maintenance operation and the deviation amount measured before the maintenance operation, whereas when the differential is not within the prescribed range, then the droplet ejection of the nozzle is corrected by the droplet ejection correction device according to the past deviation amount stored in the storage device. 
     According to this aspect of the present invention, if the differential between the deviation amounts before and after a maintenance operation does not lie within a prescribed range, then droplet ejection correction is performed on the basis of a past deviation amount stored in the storage device, and therefore it is possible to correct droplet ejection accurately, without increasing the number of measurements of the deviation amount, even in cases where variation may arise in the deviation amount before and after a maintenance operation. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming apparatus, comprising: an image forming device which has a nozzle performing droplet ejection of ink to deposit droplets of the ink to form an image onto a prescribed medium; an image reading device which acquires read image data by reading in the image on the medium; a deviation amount calculation device which calculates a deviation amount in the droplet ejection of the nozzle with respect to a prescribed central value according to the read image data; a droplet ejection correction device which corrects the droplet ejection of the nozzle according to the deviation amount; and a control device which, in a high-precision measurement mode, measures the deviation amount by using the image forming device, the image reading device and the deviation amount calculation device at a prescribed speed, and in a high-speed measurement mode, measures the deviation amount by using the image forming device, the image reading device and the deviation amount calculation device at a higher speed than in the high-precision measurement mode, wherein the control device normally measures the deviation amount in the high-speed measurement mode and corrects the droplet ejection of the nozzle by means of the droplet ejection correction device according to the deviation amount measured in the high-speed measurement mode, whereas when the deviation amount measured in the high-speed measurement mode does not satisfy prescribed conditions, then the control device measures the deviation amount in the high-precision measurement mode and corrects the droplet ejection of the nozzle by means of the droplet ejection correction device according to the deviation amount measured in the high-precision measurement mode. 
     According to this aspect of the present invention, the deviation amount in droplet ejection is normally measured in the high-speed measurement mode and the droplet ejection is corrected on the basis of this deviation amount, whereas if this deviation amount does not satisfy prescribed conditions, then the droplet ejection is corrected on the basis of a deviation amount measured in the high-precision measurement mode, and therefore it is possible to ascertain the deviation amount in droplet ejection accurately, and hence to correct the droplet ejection accurately, without lowering the operating rate of the image forming apparatus, even in cases where there is temporal variation in the droplet ejection. 
     Preferably, the control device normally measures the deviation amount in the high-speed measurement mode and corrects the droplet ejection of the nozzle according to the deviation amount measured in the high-speed measurement mode when the deviation amount measured in the high-speed measurement mode is within a prescribed first range, whereas when the deviation amount measured in the high-speed measurement mode is not within the first range, then the control device measures the deviation amount again in the high-speed measurement mode after performing a maintenance operation for restoring a state of the ink in the nozzle, and when a differential between the deviation amount measured in the high-speed measurement mode before the maintenance operation and the deviation amount measured in the high-speed measurement mode after the maintenance operation is within a prescribed second range, then the control device also measures the deviation amount in the high-precision measurement mode and corrects the droplet ejection of the nozzle according to the deviation amount measured in the high-precision measurement mode, whereas when the differential is not within the second range, then the control device corrects the droplet ejection of the nozzle according to a past deviation amount in the droplet ejection of the nozzle stored in a prescribed storage device. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming apparatus, comprising: an image forming device which has a nozzle performing droplet ejection of ink to deposit droplets of the ink to form an image onto a prescribed medium; an image reading device which acquires read image data by reading in the image on the medium; a deviation amount calculation device which calculates a deviation amount in the droplet ejection of the nozzle with respect to a prescribed central value according to the read image data; a droplet ejection correction device which corrects the droplet ejection of the nozzle according to the deviation amount; a storage device which stores the deviation amount; and a control device which, in a high-precision measurement mode, measures the deviation amount by using the image forming device, the image reading device and the deviation amount calculation device at a prescribed speed, and in a high-speed measurement mode, measures the deviation amount by using the image forming device, the image reading device and the deviation amount calculation device at a higher speed than in the high-precision measurement mode, wherein: the control device measures the deviation amount in the high-precision measurement mode, and stores the deviation amount measured in the high-speed measurement mode in the storage device; the control device measures the deviation amount in the high-speed measurement mode, performs ejection failure detection for the nozzle according to the deviation amount measured in the high-speed measurement mode, and performs a droplet ejection correction according to a result of the ejection failure detection; and the control device also corrects the droplet ejection of the nozzle by means of the droplet ejection correction device according to the deviation amount of the nozzle previously measured in the high-precision measurement mode and stored in the storage device. 
     According to this aspect of the present invention, the deviation amount in droplet ejection is measured in the high-speed measurement mode and ejection failure detection is performed on the basis of this deviation amount, while droplet ejection is corrected on the basis of a deviation amount in droplet ejection measured previously in the high-precision measurement mode, and therefore it is possible to ascertain the deviation amount in droplet ejection accurately, and hence to correct the droplet ejection accurately, without lowering the operating rate of the image forming apparatus, even in cases where there is temporal variation in the droplet ejection. 
     In order to attain the aforementioned object, the present invention is also directed to a droplet ejection correction method, comprising the steps of: performing droplet ejection of ink from a plurality of nozzles to deposit droplets of the ink to form an image onto a prescribed medium; acquiring read image data by reading in the image on the medium; calculating, for each of the nozzles, a current deviation amount that is a deviation amount in the droplet ejection with respect to a prescribed central value, by finding a weighted average of an initial deviation amount and past N deviation amounts nearest to a current time calculated for the nozzle according to past read image data; and correcting the droplet ejection of each of the nozzles according to the current deviation amount of the nozzle calculated in the calculating step. 
     In order to attain the aforementioned object, the present invention is also directed to a droplet ejection correction method, comprising the steps of: performing droplet ejection of ink from a plurality of nozzles to deposit droplets of the ink to form an image onto a prescribed medium; acquiring read image data by reading in the image on the medium; calculating, for each of the nozzles, a deviation amount in the droplet ejection with respect to a prescribed central value according to the read image data; correcting the droplet ejection of each of the nozzles according to the deviation amount of the nozzle calculated in the calculating step; and comparing, for each of the nozzles, a temporal variation in the deviation amount with a maximum tolerable value stored in a prescribed storage device, and discarding the deviation amount relating to a period where the temporal variation in the deviation amount exceeds the maximum tolerable value, in such a manner that that deviation amount is not used in the correcting step. 
     In order to attain the aforementioned object, the present invention is also directed to a droplet ejection correction method, comprising the steps of: performing droplet ejection of ink from a nozzle to deposit droplets of the ink to form an image onto a prescribed medium; acquiring read image data by reading in the image on the medium; calculating a deviation amount in the droplet ejection of the nozzle with respect to a prescribed central value according to the read image data; performing a maintenance operation to restore a state of the ink in the nozzle; and measuring the deviation amount by the droplet ejection performing step, the read image data acquiring step and the deviation amount calculating step, before and after the maintenance operation performing step, judging whether or not a differential between the deviation amount measured before the maintenance operation performing step and the deviation amount measured after the maintenance operation performing step is within a prescribed range, and when the differential is within the prescribed range, then correcting the droplet ejection of the nozzle according to at least one of the deviation amount measured after the maintenance operation performing step and the deviation amount measured before the maintenance operation performing step, whereas when the differential is not within the prescribed range, correcting the droplet ejection of the nozzle according to a past deviation amount in the droplet ejection of the nozzle stored in a prescribed storage device. 
     In order to attain the aforementioned object, the present invention is also directed to a droplet ejection correction method, comprising the steps of: performing droplet ejection of ink from a nozzle to deposit droplets of the ink to form an image onto a prescribed medium; acquiring read image data by reading in the image on the medium; calculating a deviation amount in the droplet ejection of the nozzle with respect to a prescribed central value according to the read image data; measuring, in a high-precision measurement mode, the deviation amount by the droplet ejection performing step, the read image data acquiring step and the deviation amount calculating step, at a prescribed speed; measuring, in a high-speed measurement mode, the deviation amount by the droplet ejection performing step, the read image data acquiring step and the deviation amount calculating step, at a higher speed than in the high-precision measurement mode; and normally measuring the deviation amount in the high-speed measurement mode and correcting the droplet ejection of the nozzle according to the deviation amount measured in the high-speed measurement mode, whereas when the deviation amount measured in the high-speed measurement mode does not satisfy prescribed conditions, then measuring the deviation amount in the high-precision measurement mode and correcting the droplet ejection of the nozzle according to the deviation amount measured in the high-precision measurement mode. 
     Preferably, the droplet ejection correction method further comprises the step of performing a maintenance operation to restore a state of the ink in the nozzle, wherein the deviation amount is normally measured in the high-speed measurement mode and the droplet ejection of the nozzle is corrected according to the deviation amount measured in the high-speed measurement mode when the deviation amount measured in the high-speed measurement mode is within a prescribed first range, whereas when the deviation amount measured in the high-speed measurement mode is not within the first range, then the deviation amount is measured again in the high-speed measurement mode after the maintenance operation performing step, and when a differential between the deviation amount measured in the high-speed measurement mode before the maintenance operation performing step and the deviation amount measured in the high-speed measurement mode after the maintenance operation performing step is within a prescribed second range, then the deviation amount is also measured in the high-precision measurement mode and the droplet ejection of the nozzle is corrected according to the deviation amount measured in the high-precision measurement mode, whereas when the differential is not within the second range, then the droplet ejection of the nozzle is corrected according to a past deviation amount in the droplet ejection of the nozzle stored in a prescribed storage device. 
     In order to attain the aforementioned object, the present invention is also directed to a droplet ejection correction method, comprising the steps of: performing droplet ejection of ink from a nozzle to deposit droplets of the ink to form an image onto a prescribed medium; acquiring read image data by reading in the image on the medium; calculating a deviation amount in the droplet ejection of the nozzle with respect to a prescribed central value according to the read image data; measuring, in a high-precision measurement mode, the deviation amount by the droplet ejection performing step, the read image data acquiring step and the deviation amount calculating step, at a prescribed speed; measuring, in a high-speed measurement mode, the deviation amount by the droplet ejection performing step, the read image data acquiring step and the deviation amount calculating step, at a higher speed than in the high-precision measurement mode; measuring the deviation amount in the high-speed measurement mode, performing ejection failure detection for the nozzle according to the deviation amount measured in the high-speed measurement mode, and performing a droplet ejection correction according to a result of the ejection failure detection; and correcting the droplet ejection of the nozzle according to a deviation amount of the nozzle previously measured in the high-precision measurement mode and stored in a prescribed storage device. 
     According to the present invention, when forming images by means of droplet ejection, it is possible to correct droplet ejection accurately, even if there is temporal variation in the droplet ejection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is a block diagram showing the general composition of an image forming apparatus according to an embodiment of the present invention; 
         FIGS. 2A and 2B  are a plan view perspective diagram and a cross-sectional diagram showing the whole of a head in the image forming apparatus; 
         FIG. 3  is an illustrative diagram used to describe maintenance for restoring the state of the liquid; 
         FIG. 4  is an illustrative diagram showing a droplet ejection correction process according to a first embodiment of the present invention; 
         FIG. 5  is a flowchart showing an overview of a sequence of droplet ejection correction processing according to a second embodiment of the present invention; 
         FIG. 6  is a flowchart showing an overview of a sequence of droplet ejection correction processing according to a third embodiment of the present invention; 
         FIG. 7  is a flowchart showing an overview of a sequence of droplet ejection correction processing according to a fourth embodiment of the present invention; 
         FIG. 8  is an overall compositional diagram showing the mechanical composition of the image forming apparatus according to an embodiment of the present invention; and 
         FIG. 9  is a plan diagram showing the arrangement of the heads and the image reading unit in the image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     General Composition of Image Forming Apparatus 
       FIG. 1  is a block diagram showing the general composition of an image forming apparatus according to an embodiment of the present invention. 
     In  FIG. 1 , the image forming apparatus  10  includes: an ink droplet ejection head (hereinafter referred to as the head)  50 , a maintenance unit  102 , a communication unit  112 , a droplet deposition pattern generation unit  114 , a head drive unit  116 , an image reading unit  122 , a deviation amount calculation unit  124 , a droplet ejection correction unit  126 , a control unit  150  and a storage unit  152 . 
     This head  50  has a plurality of nozzles which eject droplets of ink. By ejecting droplets of ink from the nozzles toward the medium  16 , such as paper, an image composed of a plurality of dots formed from the deposited ink droplets is formed on the medium  16 . A concrete embodiment of the head  50  is described in detail later. 
     The maintenance unit  102  performs a maintenance operation for restoring the state of the ink in the head  50  (and in particular, the state of the ink inside the nozzles). A concrete embodiment of the maintenance unit  102  is described in detail later. 
     The communication unit  112  functions as an image data input device, which receives image data transmitted by a host  200 . It is possible to use a wired or wireless interface for the communication unit  112 . 
     On the basis of the image data, the droplet deposition pattern generation unit  114  generates droplet deposition pattern data required in order to form dots on the medium  16  by depositing ink droplets from the nozzles of the head  50  onto the medium  16 . In other words, the droplet deposition pattern generation unit  114  converts the image data into the droplet deposition pattern data. 
     Here, the image data is either data for forming an actual image desired by the user, as received from the host  200 , or test image data for measuring the deviation amount. 
     The droplet deposition pattern data is data that indicates the droplet deposition positions and the droplet ejection volumes. The data may be data of a simple format that simply indicates whether each nozzle is designated to perform droplet ejection. 
     The head drive unit  116  supplies drive signals for droplet ejection to the head  50 , on the basis of the droplet deposition pattern data. 
     The image reading unit  122  optically reads in the image (droplet deposition result) formed on the medium  16  and thereby obtains density data (read image data). The image reading unit  122  includes image sensors (imaging devices), such as CCD (charge coupled device) sensors, CMOS (complementary metal oxide semiconductor) sensors, or the like. The image sensors may be arranged in a one-dimensional configuration or a two-dimensional configuration. Furthermore, the image reading unit is not limited to a case where it is built in the image forming apparatus  10 , as shown in  FIG. 1 , and it is also possible to provide the image reading unit as a separate unit having a scanning mechanism (a so-called image scanner), outside of the image forming apparatus  10 . 
     In order to correct droplet ejection, it is possible to form a test image for determining variations in droplet ejection, such as a solid image or a fine line image, on the medium  16 . The test image data may be one sheet or a plurality of sheets of special image data held in the image forming apparatus  10 . Moreover, it is also possible to provide a special region to be read at the end section of the medium  16 , and to prepare the test image data for an image to be formed on the special region. Furthermore, it is also possible to use the actual image data received from the host  200 , as test image data. 
     The deviation amount calculation unit  124  calculates the deviation amounts from the central design values of the droplet ejection, for each nozzle in the head  50 , on the basis of the density data acquired by the image reading unit  122  (the read image data). 
     Here, the deviation amounts in droplet ejection include: the deviation amount in the position of the deposited droplet (dot position), the deviation amount in the size of the deposited droplet (size of dot) (for example, the deviation amount in the diameter of the deposited droplet (diameter of the dot) or the deviation amount in the volume of the ejected droplet), the deviation amount in the density (the density at the respective positions on the medium  16  that correspond to the central positions of the respective nozzles), and the like. In other words, the deviation amount in droplet ejection may be the deviation amount relating the position of the deposited droplet or the deviation amount relating to the volume of the deposited droplet. 
     The storage unit  152  stores information of various types. The storage unit  152  according to the present embodiment stores information relating to variations in droplet ejection, in particular, the history of the deviation amounts in droplet ejection, the tolerable ranges of the deviation amounts, the tolerable ranges of the temporal variations in the deviation amounts, and the like. 
     The droplet ejection correction unit  126  calculates droplet ejection correction data on the basis of the deviation amounts calculated by the deviation amount calculation unit  124 , and it corrects the droplet ejection of the head  50  on the basis of this droplet ejection correction data. 
     The possible of modes of droplet ejection correction are: firstly, a mode based on correcting the droplet deposition pattern data generated by the droplet deposition pattern generation unit  114  (hereinafter referred to as “droplet deposition pattern correction”), and secondly, a mode based on correcting the drive signal for droplet ejection applied to the head  50  from the head drive unit  116  (hereinafter referred to as “drive signal correction”). 
     In the droplet deposition pattern correction, first droplet deposition pattern data is generated by the droplet deposition pattern generation unit  114 , on the basis of the image data received from the host  200 , while on the other hand, droplet ejection correction data is calculated by the droplet ejection correction unit  126  on the basis of the deviation amounts in droplet ejection, and the droplet ejection correction data is supplied to the droplet deposition pattern generation unit  114 . The droplet deposition pattern generation unit  114  generates second droplet deposition pattern data required for actual driving of the head  50 , on the basis of the first droplet deposition pattern data and the droplet ejection correction data, and supplies the second droplet deposition pattern to the head drive unit  116 . In practice, the head drive unit  116  generates a drive signal on the basis of the second droplet deposition pattern data, and supplies the drive signal to the head  50 . 
     The droplet deposition pattern correction may be, for example, correction that increases the diameter of the droplets ejected from the nozzles in the vicinity of a nozzle having low density, or correction that increases the number of droplet ejecting actions performed by the nozzles in the vicinity of a nozzle having low density, and the like. 
     In the drive signal correction, droplet deposition pattern data is generated by the droplet deposition pattern generation unit  114 , on the basis of the image data received from the host  200 , and the droplet deposition pattern data is supplied to the head drive unit  116 , while on the other hand, droplet ejection correction data is calculated by the droplet ejection correction unit  126  on the basis of the deviation amounts in droplet ejection and the droplet ejection correction data is supplied to the head drive unit  116 . The head drive unit  116  generates a drive signal on the basis of the droplet deposition pattern data and the droplet ejection correction data, and supplies the drive signal to the head  50 . 
     The drive signal correction may be, for example, correction of the voltage (drive voltage) of the drive signal, correction of the waveform of the drive signal (drive waveform), and the like. 
     The control unit  150  controls the respective units of the image forming apparatus  10 , such as the maintenance unit  102 , the communication unit  112 , the droplet deposition pattern generation unit  114 , the head drive unit  116 , the image reading unit  122 , the deviation amount calculation unit  124 , the droplet ejection correction unit  126 , and the like. The specific droplet ejection correction processing, which is controlled by the control unit  150 , is described in detail later. 
     Head 
       FIG. 2A  is a plan view perspective diagram showing the basic overall structure of the head  50  in  FIG. 1 . 
     The droplet ejection head  50  shown in  FIG. 2A  is a so-called full line head, having a structure in which a plurality of nozzles  51  which eject ink droplets toward the medium  16  are arranged in a two-dimensional configuration through a length corresponding to the width Wm of the medium  16  in the direction perpendicular to the direction of conveyance of the medium  16  (the sub-scanning direction indicated by arrow S in  FIG. 2A ), in other words, in the main scanning direction indicated by arrow M in  FIG. 2A . 
     The head  50  includes a plurality of pressure chamber units  54 , each having the nozzle  51 , a pressure chamber  52  connected to the nozzle  51 , and a ink supply port  53 , the pressure chamber units  54  being arranged in two directions, namely, the main scanning direction M and an oblique direction forming a prescribed acute angle θ (where 0°&lt;θ&lt;90°) with respect to the main scanning direction M. In  FIG. 2A , in order to simplify the drawing, only a portion of the pressure chamber units  54  are depicted in the drawing. 
     More specifically, the nozzles  51  are arranged at a uniform pitch d in the direction forming the prescribed acute angle of θ with respect to the main scanning direction M, and hence the nozzle arrangement can be treated as equivalent to a configuration in which the nozzles are arranged at an interval of d×cos θ in a straight line following the main scanning direction M. 
       FIG. 2B  shows a cross-sectional diagram along line  2 B- 2 B in  FIG. 2A  of one of the aforementioned pressure chamber units  54 , which forms one of the droplet ejection elements constituting the head  50 . 
     As shown in  FIG. 2B , each pressure chamber  52  is connected to a common liquid chamber  55  through the ink supply port  53 . The common liquid chamber  55  is connected to a tank which forms a liquid supply tank (not shown), and the liquid supplied from the tank is distributed and supplied to the respective pressure chambers  52  by means of the common liquid chamber  55 . 
     A piezoelectric body  58   a  is disposed on a diaphragm  56 , which constitutes the ceiling of the pressure chamber  52 , and an individual electrode  57  is arranged on the piezoelectric body  58   a . The diaphragm  56  is earthed and also functions as a common electrode. A piezoelectric actuator  58 , which forms a device for generating a liquid ejection pressure, is constituted by the diaphragm  56 , the individual electrode  57  and the piezoelectric body  58   a.    
     When a prescribed drive voltage is applied to the individual electrode  57  of the piezoelectric actuator  58 , the piezoelectric body  58   a  deforms, thereby changing the volume of the pressure chamber  52 , and this results in a change in the pressure inside the pressure chamber  52 , which causes the liquid in the pressure chamber  52  to be ejected from the nozzle  51  as a droplet. When the volume of the pressure chamber  52  returns to normal after droplet ejection, new liquid is supplied to the pressure chamber  52  from the common liquid chamber  55  through the ink supply port  53 . 
       FIG. 2A  shows the embodiment where the nozzles  51  are arranged two-dimensionally in order to achieve a structure whereby a high-resolution image can be formed at high-speed onto the medium  16 , but the head used in the present invention is not limited in particular to a structure in which the nozzles  51  are arranged two-dimensionally, and it may also adopt a structure where the nozzles  51  are arranged one-dimensionally. Furthermore, the pressure chamber unit  54  shown in  FIG. 2B  is merely an example of a droplet ejection element constituting a part of the head, and the invention is not limited in particular to this. For example, instead of disposing the common liquid chamber  55  below the pressure chamber  52  (in other words, towards the nozzle face  510  from the pressure chamber  52 ), it is also possible to dispose the common liquid chamber  55  above the pressure chamber  52  (in other words, on the reverse side from the nozzle face  510 ). Furthermore, it is also possible to generate a liquid droplet ejection force by using heating bodies instead of piezoelectric bodies  58   a , for example. 
     Maintenance Unit 
       FIG. 3  is an illustrative diagram used to describe the maintenance unit  102  in the image forming apparatus  10  shown in  FIG. 1 . 
     The tank  60  is a base tank for supplying ink to the head  50 . A filter  62  for eliminating foreign mater and bubbles is provided at an intermediate position of a tubing  650  (ink supply channel), which connects the tank  60  with the head  50 . 
     The image forming apparatus  10  is further provided with: a cap  64  forming a device for preventing drying of the ink surface in the nozzles  51  or preventing increase in the ink viscosity in the vicinity of the ink surface during a prolonged idle period without droplet ejection; and a cleaning blade  66  forming a device for cleaning the nozzle face  510 . 
     A unit including the cap  64  and the cleaning blade  66  can be relatively moved with respect to the head  50  by a movement mechanism (not shown), and is moved from a predetermined holding position to a maintenance position below the head  50 , as and when required. 
     Furthermore, the cap  64  is relatively raised and lowered with respect to the head  50  by an elevator mechanism (not shown). The elevator mechanism raises the cap  64  to a predetermined elevated position so as to come into close contact with the head  50 , and at least the nozzle region of the nozzle face  510  is thus covered by the cap  64 . 
     The cleaning blade  66  is composed of rubber or another elastic member, and can slide on the nozzle face  510  of the head  50  by means of a cleaning blade movement mechanism (not shown). If there are ink droplets or foreign matter adhering to the nozzle face  510 , then the nozzle face  510  is wiped by causing the cleaning blade  66  to slide over the nozzle face  510 , thereby cleaning same. 
     A suction pump  67  suctions ink from the nozzles  51  of the head  50  and sends the suctioned ink to a recovery tank  68  in a state where the nozzle face  510  of the head  50  is covered by the cap  64 . 
     A suction operation of this kind is performed when ink is filled into the head  50  from the tank  60  when the tank  60  is installed in the image forming apparatus  10  (initial filling), and it is also performed when removing ink of increased viscosity after the apparatus has been out of use for a long period of time (start of use after long period of inactivity). 
     Furthermore, if bubbles infiltrate inside the nozzles  51  and the pressure chambers  52  of the head  50 , or if the increase in the viscosity of the ink inside the nozzles  51  exceeds a certain level, then it becomes impossible to eject ink droplets from the nozzles  51  by purging (dummy droplet ejection), and therefore, the cap  64  is abutted against the nozzle face  510  of the head  50 , and an operation is performed to suction out the ink containing bubbles or the ink of increased viscosity inside the pressure chambers  52  of the head  50 , by means of the suction pump  67 . 
     In the embodiment shown in  FIG. 3 , the maintenance unit  102  shown in  FIG. 1  is principally constituted by the cap  64 , the cleaning blade  66  and the suction pump  67 . 
     Below, the droplet ejection correction processing, which is controlled by the control unit  150  shown in  FIG. 1 , is described in detail, separately with respect to various different embodiments. 
     First Embodiment 
     The droplet ejection correction processing according to the first embodiment is described with reference to the illustrative diagram in  FIG. 4 . 
       FIG. 4  shows an example of temporal variation in a deviation amount in droplet ejection with respect to the central design value. Here, the vertical axis in  FIG. 4  shows the deviation amount in droplet ejection and the horizontal axis shows the elapsed time since the initial time point t 0 . 
     The initial time point t 0  is, for example, the time of the finishing checkout of the image forming apparatus  10  in the manufacturing factory, or the time of the transfer of the image forming apparatus  10  from the manufacturing factory. It is also possible that the initial time point t 0  is the time point at which the image forming apparatus  10  is installed for use. 
     It is possible that the deviation amount d 0  at the initial time point t 0  (called “initial deviation amount”) is measured by using the deviation amount measurement device built in the image forming apparatus  10 , constituted by the head  50 , the image reading unit  122  and the deviation amount calculation unit  124  in the image forming apparatus  10 , over a measurement time that is sufficiently longer than measurement made during operation of the image forming apparatus  10 . A case such as this is desirable, since it means that no special deviation amount measurement device is required, and measurement of higher reliability than measurement performed during the operation of the image forming apparatus  10  can be achieved. It is also possible that the initial deviation amount d 0  is measured by using a special measurement device that has higher reliability than the deviation amount measurement device built in the image forming apparatus  10 . Moreover, it is also possible that the initial deviation amount d 0  is indirectly determined, on the basis of measurement amounts that relate physically, or theoretically, to the deviation amount in droplet ejection, such as the dimensional values of the respective elements (parts) of the head  50 . The initial deviation amount d 0  determined by any of these methods generally has higher reliability than the deviation amount measured during operation of the image forming apparatus  10 . 
     The initial deviation amount d 0  is beforehand stored in the storage unit  152  in  FIG. 1 , before the operation of the image forming apparatus  10 . 
     During the operation of the image forming apparatus  10 , the deviation amounts shown in  FIG. 4  (d 1  to d 6 , and d P−4  to d P−1 ) are measured by using the deviation amount measurement device built in the image forming apparatus  10 , which is constituted by the head  50 , the image reading unit  122  and the deviation amount calculation unit  124  shown in  FIG. 1 . More specifically, as well as forming images on the media  16  by ejecting ink droplets onto the medium  16  from the nozzles  51  in the head  50  while moving the media  16  (e.g., paper) relatively with respect to the head  50  in  FIG. 1 , read image data is acquired by reading in the images on the media  16  by means of the image reading unit  122  in  FIG. 1 . The deviation amounts (d 1  to d 6 , and d P−4  to d P−1 ) are calculated by the deviation amount calculation unit  124  in  FIG. 1 , on the basis of this read image data. 
     These deviation amounts (d 1  to d 6 , and d P−4  to d P−1 ) are stored in the storage unit  152  in  FIG. 1  as history data (in other words, past deviation amounts). It is also possible for the past deviation amounts (for example, d 1  to d 6 ) prior to a prescribed number (N) of measurement operations to be erased successively. 
     The deviation amount d P  of the current ejected droplet is calculated by the deviation amount calculation unit  124  as follows: 
                       d   P     =           ∑   i     ⁢     (       w   i     ·     d   i       )       N     +       w   0     ·     d   0           ,           (   1   )               
where i is an index (P−N to P−1) indicating each deviation amount (d P−N  to d P−1 ) of the last N measurements nearest to the current time, w i  is a “weighting” which applies a weighting to each deviation amount (d P−N  to d P−1 ) of the last N measurements nearest to the current time, and w 0  is the “weighting” corresponding to the initial deviation amount d 0 . This weighting w 0  is in the range of 0&lt;w 0 &lt;1.
 
     As shown in Formula 1, the current deviation amount d P  is calculated by finding the weighted average of the deviation amounts (d P−N  to d P−1 ) of the past N measurements nearest to the current time obtained previously by measurement of the deviation amounts, and the initial deviation amount d 0 . 
     Desirably, the weightings w i  relating to the deviation amounts (d P−N  to d P−1 ) of the past N measurements should assume a smaller value, the older the measurement to which they relate, in other words, the greater the separation in time between that measurement and the current time. 
     Moreover, desirably, the weight w 0  relating to the initial deviation amount d 0  is changed on the basis of the magnitude of the temporal variation in the deviation amounts of the past N measurements (d P−N  to d P−1 ) (namely, the differential of d P−N  to d P−1 ). More specifically, the greater the temporal variation in the deviation amounts, the greater the weighting w 0  applied to the initial deviation amount d 0 . 
     Furthermore, if a deviation amount value is obtained during a period where the variation in the deviation amounts has exceeded a prescribed maximum tolerance value, then there is a high possibility that this obtained deviation amount value is noise, and therefore, it is desirable that this obtained deviation amount value is discarded, rather than being used in the calculation of the current deviation amount d P . More specifically, if |d K −d K−1 |&gt;d MAX , then the values d K  and d K−1  are discarded, where d MAX  is the maximum tolerable value, which is specified in advance. For each of the nozzles  51 , the current deviation amount d P  is calculated by eliminating any unnecessary past deviation amounts. By adopting this method, it is possible to improve the accuracy of the current deviation amount d P , and hence to achieve more accurate correction of droplet ejection. 
     It is possible that the weightings of the deviation amounts d P−N  to d P−1  for the past N measurements are the same weighting w A , and in this case, the current deviation amount d P  is expressed as: 
                       d   P     =         w   A     ·         ∑   i     ⁢     d   i       N       +       w   0     ·     d   0           ,           (   2   )               
where w A =1−w 0 .
 
     The current deviation amount d P  is stored in the storage unit  152  as a part of the deviation amount history. Older data may be erased successively, in order. 
     Detailed specific examples are now described with respect to the weighting of the initial deviation amount d 0  and the deviation amounts (d P−N  to d P−1 ) of the past N measurements nearest to the current time. Here, the initial deviation amount d 0  is the deviation amount determined by the measurement upon manufacture of the image forming apparatus  10  in the factory. 
     In the first example, the deviation amounts are measured a plurality of times (M times) upon manufacture of the image forming apparatus  10  in the factory, and weightings are applied using this number of measurements (i.e., M). The current deviation amount d P  in the first example is expressed as: 
                       d   P     =           d   0     ·   M     +       d   n     ·   N         M   +   N         ,           (   3   )               
where d 0  is the average value of the M deviation amounts measured upon manufacture of the image forming apparatus  10  in the factory, and d n  is the average value of the most recent N deviation amounts (d P−N  to d P−1 ) measured in the past.
 
     In the second example, after the image forming apparatus  10  is installed and before becoming in use, a running test of the image forming apparatus  10  is performed, and weightings are applied by using the deviation amounts found in the running test. The current deviation amount d P  in the second example is expressed as: 
                       d   P     =           d   0     ·     d   t_ini       +       d   n     ·     d   t_mean             d   t_ini     +     d   t_mean           ,           (   4   )               
where d t     —     ini  is the absolute value of the initial deviation amount at the start of the running test period, and d t     —     mean  is the average value of the absolute values of the deviation amounts during the whole period of the running test.
 
     It is thought that the initial deviation amount d 0  is principally governed by the mechanical accuracy of the nozzles and the drive characteristics of the actuators (which depend on the dimensional accuracy of the piezoelectric bodies, and the like), and the subsequent deviation amount d n  is principally governed by factors such as deterioration of the actuators and displacement of the installation position of the whole head. Since these factors differ, then it is logical to suppose that variation will appear in the respective deviation amounts, due to differences in these factors. 
     In the third example, the running test period in the second example is divided into a number of periods (for example, within one year from the manufacture, between one and two years, between two and three years, and the like), the values of d t     —     ini  and d t     —     mean  are measured for each period, and the current deviation amounts d P  shown in Formula 4 are calculated by using the different values of d t     —     ini  and d t     —     mean , in accordance with the operational time after the manufacture. The measure of the running time period is not limited to one based on time (for example, years), and it is also to use the number of prints as a basis for this. 
     In the third example, the weightings are altered with the passage of time. The factors that cause variation in the deviation amount include factors depending on the number of ejection actions, such as the deterioration of the piezoelectric bodies, and hence the weightings are altered in accordance with the passage of time (or the number of prints made). The weighting applied to the initial deviation amount d 0  may be decreased over time. 
     The first to third examples are described in relation to the cases where the weightings applied to the deviation amounts of the most recent past N measurements (in other words, the weightings relating to N deviation amounts) are all the same, but the present invention may also be applied to cases where the weightings applied to the deviation amounts of the past N measurements are respectively different. 
     In the above-described first embodiment, the control unit  150  shown in  FIG. 1  forms images on prescribed media  16  by means of a head  50 , and also acquires read image data by reading in the images on the media  16  by means of the image reading unit  122 , calculates the deviation amounts on the basis of the read image data, by means of the deviation amount calculation unit  124 , and stores the deviation amounts in the storage unit  152  as part of the deviation amount history. The initial deviation amount d 0  and the maximum tolerable value of the deviation amount are stored in advance in the storage unit  152 . Furthermore, the control unit  150  calculates the current deviation amount d P  for each of the nozzles  51 , by finding the weighted average of the initial deviation amount d 0  and the deviation amounts (d P−N  to d P−1 ) of the past N measurements nearest to the current time, by means of the deviation amount calculation unit  124 , and the control unit  150  calculates correction data on the basis of the current deviation amount d P  for each of the nozzles  51 . During image formation, the control unit  150  corrects droplet ejection for the respective nozzles  51  according to the thus obtained correction data. It is highly possible that the past deviation amount that shows a large temporal variation is noise, and hence such values are discarded in order that they are not used in the calculation of the current deviation amount d P . For example, if the temporal variation in the deviation amounts (d P−N  to d P−1 ) of the past N measurements nearest to the current time includes variation that is greater than a prescribed volume, then droplet ejection is corrected on the basis of the deviation amount that is determined previously as the current deviation amount. 
     Second Embodiment 
     Next, the droplet ejection correction processing according to a second embodiment of the present invention is described with reference to the flowchart in  FIG. 5 , which shows an overview of this processing. The droplet ejection correction processing shown in  FIG. 5  is performed by the control unit  150  in  FIG. 1 , in accordance with a prescribed program. 
     Firstly, the deviation amount A 1  in the droplet ejection from each nozzle  51  of the head  50  is measured by means of the head  50 , the image reading unit  122  and the deviation amount calculation unit  124  shown in  FIG. 1  (step S 202 ), and the deviation amount A 1  thus measured is stored in the storage unit  152  shown in  FIG. 1  (step S 204 ). 
     Thereupon, the maintenance unit  102  shown in  FIG. 1  performs a maintenance operation for the head  50  shown in  FIG. 1 , such as suctioning the nozzles  51 , wiping the nozzle face  510 , or the like (step S 206 ). By means of this maintenance operation, the state of the ink inside the head  50  (and in particular, the state of the ink inside the nozzles  51 ) is restored. 
     After performing the maintenance operation, the deviation amount A 2  in the droplet ejection from each nozzle  51  of the head  50  is again measured by means of the head  50 , the image reading unit  122  and the deviation amount calculation unit  124  shown in  FIG. 1  (step S 208 ), and the deviation amount A 2  thus measured is stored in the storage unit  152  shown in  FIG. 1  (step S 210 ). 
     Thereupon, the differential |A 1 −A 2 | between the deviation amount A 1  measured immediately before the maintenance operation and the deviation amount A 2  measured immediately after the maintenance operation (i.e., the temporal variation in the deviation amount) is calculated and compared with a prescribed amount (i.e., the maximum tolerable variation) beforehand stored in the storage unit  152  (step S 212 ). For example, the judgment in step S 212  is performed for each nozzle  51 . 
     Here, if the differential |A 1 −A 2 | is not greater than the prescribed amount, then the droplet ejection correction unit  126  in  FIG. 1  calculates correction data on the basis of the deviation amount A 2  (the deviation amount measured immediately after the maintenance operation), and accordingly performs droplet ejection correction (step S 214 ). 
     In general, the droplet ejection correction is not performed by performing droplet ejection again toward the medium  16  on which an image has already been formed and from which the image has been read in, but by correcting droplet ejection toward the next medium that is relatively moved to the position opposing the head  50 . More specifically, firstly, there is a mode in which the droplet deposition pattern data (which is generated by the droplet deposition pattern generation unit  114  in  FIG. 1 ) of the image that is to be formed on the next medium is corrected, and secondly, there is a mode in which the drive signals (which are generated by the head drive unit  116  in  FIG. 1 ) applied to the head  50  during droplet ejection toward the next medium are corrected, and the like. 
     It is also possible to correct droplet ejection on the basis of the average value of the deviation amount A 1  obtained by measuring immediately before the maintenance operation, and the deviation amount A 2  obtained by measuring immediately after the maintenance operation, instead of correcting droplet ejection on the basis of the deviation amount A 2 . 
     If the differential |A 1 −A 2 | is greater than the prescribed amount at step S 212 , then the deviation amount A 1  and the deviation amount A 2  are discarded in such a manner that they are not used in droplet ejection correction, in other words, any deviation amounts from a period of time in which the variation in the deviation amounts exceeds a tolerable range are discarded, and droplet ejection correction is performed by calculating correction data on the basis of the past deviation amounts stored in the storage unit  152  in  FIG. 1  (step S 216 ). In short, the droplet ejection is corrected on the basis of the deviation amounts relating to a period where the variation in the deviation amounts is within the tolerable range. 
     Factors that cause significant variation of the deviation amounts of the droplet ejection are, for instance, soiling in the vicinity of the nozzles  51 , incorporation of bubbles, and the like. It is considered that these factors are greatly affected by the maintenance operation. Therefore, the deviation amounts before and after the maintenance operation which amounts do not show great variation, are taken as the “stable” deviation amounts, and droplet ejection is corrected on the basis of these “stable” deviation amounts. 
     The control unit  150  shown in  FIG. 1  controls the measurement of the deviation amounts described above (which involves image formation by means of the head  50  in  FIG. 1 , image reading by means of the image reading unit  122  in  FIG. 1 , and calculation of the deviation amounts by means of the deviation amount calculation unit  124  in  FIG. 1 ), and the droplet ejection correction by means of the droplet ejection correction unit  126  in  FIG. 1 . 
     In the second embodiment, the control unit  150  in  FIG. 1  judges whether or not the differential between the deviation amount in droplet ejection of the nozzles  51  measured immediately before a maintenance operation and the deviation amount in droplet ejection of the nozzles  51  measured immediately after the maintenance operation is within the prescribed tolerable range, and if the differential is within the prescribed tolerable range, then the droplet ejection of the nozzles  51  is corrected by the droplet ejection correction unit  126  on the basis of the deviation amount in the droplet ejection of the nozzles  51  measured immediately after the maintenance operation and/or the deviation amount in the droplet ejection of the nozzles  51  measured immediately before the maintenance operation, whereas if the differential is not within the prescribed tolerable range, then the droplet ejection of the nozzles  51  is corrected by the droplet ejection correction unit  126  on the basis of the past deviation amounts in the droplet ejection of the nozzles  51 , as stored in the storage unit  152 . 
     Third Embodiment 
     Next, the droplet ejection correction processing according to a third embodiment of the present invention is described. 
     In the present embodiment, there are two modes for measuring the deviation amount in droplet ejection: a high-precision measurement mode and a high-speed measurement mode. The high-precision measurement mode measures the deviation amount in droplet ejection at a slower speed but with higher accuracy, than the high-speed measurement mode. On the other hand, the high-speed measurement mode measures the deviation amount in droplet ejection at a faster speed but with lower accuracy, than the high-precision measurement mode. 
     More specifically, in the high-speed measurement mode, image reading by the image reading unit  122  in  FIG. 1  is performed more quickly, and the image resolution of the read image data is lower, than in the case of the high-precision measurement mode. In the high-speed measurement mode, the speed of the medium  16  that is moved relatively with respect to the image reading unit  122  in  FIG. 1  is greater than in the high-precision measurement mode. 
     In the present embodiment, the high-speed measurement mode and the high-precision measurement mode are selectively used. Normally, the deviation amount in droplet ejection is measured for each nozzle  51  in the high-speed measurement mode, and droplet ejection is corrected for each nozzle  51  by the droplet ejection correction unit  126  in  FIG. 1  on the basis of the deviation amount thus measured; however, if the deviation amount measured in the high-speed measurement mode does not satisfy prescribed conditions (more specifically, if the deviation amount is not within a prescribed range, or if the temporal variation in the deviation amount is not within a prescribed range), then the deviation amount in droplet ejection is measured for each nozzle  51  in the high-precision measurement mode, and droplet ejection is corrected for each nozzle  51  by the droplet ejection correction  124  in  FIG. 1  on the basis of the deviation amounts thus measured in the high-precision measurement mode. 
     Below, the droplet ejection correction processing according to the third embodiment of the present invention is described with reference to the flowchart in  FIG. 6 , which shows an overview of the sequence of this processing. The processing shown in  FIG. 6  is performed by the control unit  150  in  FIG. 1 , in accordance with a prescribed program. 
     Normally, the deviation amount A 1  in the droplet ejection from each nozzle  51  of the head  50  is measured in the high-speed measurement mode, by means of the head  50 , the image reading unit  122  and the deviation amount calculation unit  124  shown in  FIG. 1  (step S 312 ), and the deviation amount A 1  thus measured is stored in the storage unit  152  shown in  FIG. 1  (step S 314 ). 
     Thereupon, the deviation amount A 1  measured in the high-speed measurement mode is compared with a prescribed amount stored previously in the storage unit  152  (namely, the maximum tolerable value of the deviation amount) (step S 316 ). 
     Here, if the deviation amount A 1  is not greater than the prescribed amount (in other words, if it is within the tolerable range), then correction data is calculated on the basis of the deviation amount A 1  by the droplet ejection correction unit  126  in  FIG. 1 , and droplet ejection is corrected accordingly (step S 318 ). 
     On the other hand, if the deviation amount A 1  is greater than the prescribed amount at step S 316 , then the maintenance unit  102  in  FIG. 1  performs a maintenance operation for the head  50  (step S 320 ). After performing the maintenance operation, the deviation amount A 2  in the droplet ejection from each nozzle  51  of the head  50  is again measured in the high-speed measurement mode (step S 322 ), and the deviation amount A 2  thus measured is stored in the storage unit  152  shown in  FIG. 1  (step S 324 ). 
     Thereupon, the differential |A 1 −A 2 | between the deviation amount A 1  measured immediately before the maintenance operation and the deviation amount A 2  measured immediately after the maintenance operation (i.e., the temporal variation in the deviation amount) is calculated and compared with a prescribed amount (i.e., the maximum tolerable variation) beforehand stored in the storage unit  152  (step S 326 ). 
     Then, if the differential |A 1 −A 2 | is not greater than the prescribed amount (i.e., if the temporal variation in the deviation amount is within the tolerable range), then the deviation amount B in droplet ejection is measured in the high-precision measurement mode (step S 328 ), and the deviation amount B thus measured is stored in the storage unit  152  in  FIG. 1  (step S 330 ). Thereupon, droplet ejection correction is performed by calculating correction data on the basis of the deviation amount B, by means of the droplet ejection correction unit  126  in  FIG. 1  (step S 332 ). 
     In general, the droplet ejection correction is not performed by performing droplet ejection again toward the medium  16  on which an image has already been formed and from which the image has been read in, but by correcting droplet ejection toward the next medium that is relatively moved to the position opposing the head  50 . More specifically, firstly, there is a mode in which the droplet deposition pattern data (which is generated by the droplet deposition pattern generation unit  114  in  FIG. 1 ) of the image that is to be formed on the next medium is corrected, and secondly, there is a mode in which the drive signals (which are generated by the head drive unit  116  in  FIG. 1 ) applied to the head  50  during droplet ejection toward the next medium are corrected, and the like. 
     If the differential |A 1 −A 2 | is greater than the prescribed amount at step S 326 , then the deviation amount A 1  and the deviation amount A 2  are discarded in such a manner that they are not used in droplet ejection correction, in other words, any deviation amounts from a period of time in which the variation in the deviation amounts exceeds a tolerable range are discarded, and droplet ejection correction is performed by calculating correction data on the basis of the past deviation amounts stored in the storage unit  152  in  FIG. 1  (step S 334 ). In short, the droplet ejection is corrected on the basis of the deviation amounts relating to a period where the variation in the deviation amounts is within the tolerable range. 
     Here, there are various modes for comparing the measured deviation amount with a prescribed amount (threshold value) that has been stored previously, in other words, modes for judging the deviation amount in step S 316 . The threshold value of the deviation amount indicates a value beyond which droplet ejection correction cannot be performed, in other words, it indicates the maximum tolerable value for correction of droplet ejection. 
     Firstly, there is a judgment mode in which it is judged whether or not the deviation amount of nozzles  51  (adjacent nozzles) which form dots that are mutually adjacent on the medium  16  is greater than the threshold value, in the case of both nozzles  51 , and if the deviation amount is greater than the threshold value for both nozzles, then a maintenance operation (step S 320 ) is performed. For example, a maintenance operation is performed when ejection has become impossible from two nozzles adjacent to each other. 
     Secondly, there is a judgment mode in which it is judged whether or not the deviation amount is greater than the threshold value, for each nozzle  51  individually, and if there is even one nozzle  51  having a deviation amount that is greater than the threshold value, then a maintenance operation (step S 320 ) is performed. For example, a maintenance operation is performed when any one nozzle is suffering ejection failure. 
     Thirdly, there is a judgment mode in which a maintenance operation (step S 320 ) is performed if the number of nozzles  51  having deviation amounts greater than the threshold value has exceeded a previously established prescribed number (a number threshold value). 
     Fourthly, there is a judgment mode in which, if the sum of the absolute values of the deviation amounts for the nozzles  51  exceeds a threshold value previously established for comparing with this sum of absolute values, then maintenance (step S 320 ) is performed. In general, the value (sum total) obtained by adding up the absolute values of the deviation amounts for all of the nozzles  51  gradually increases over time, and if there is a large number of nozzles  51 , then the temporal change in this sum is substantially similar, regardless of external factors. Therefore, it is logical to use the sum total of the absolute values of the deviation amounts, as in this fourth judgment mode. By setting the threshold value in such a manner that when this threshold value is exceeded, there still remains some margin of time before the occurrence of nozzles  51  in which droplet ejection correction is impossible, then it is possible to prevent the occurrence of uncorrectable nozzles, even if measurement is made intermittently. 
     In the case of the third judgment mode and the fourth judgment mode, it is possible to reduce the frequency of measurement of the deviation amount, in comparison with the first judgment mode or the second judgment mode. 
     As described above, in the third embodiment, the control unit  150  in  FIG. 1  normally measures the deviation amount in droplet ejection of the nozzles  51  in the high-speed measurement mode, and corrects the droplet ejection of the nozzles  51  by means of the droplet ejection correction unit  126 , on the basis of the deviation amount thus measured, but if the deviation amount does not satisfy prescribed conditions, then the control unit  150  measures the deviation amount in droplet ejection of the nozzles  51  in the high-precision measurement mode, and corrects the droplet ejection of the nozzles  51  by means of the droplet ejection correction unit  126 , on the basis of the deviation amount thus measured in the high-precision measurement mode. 
     Furthermore, for each of the nozzles  51 , the control unit  150  in  FIG. 1  compares the temporal variation in the deviation amount in droplet ejection for that nozzle  51 , with a prescribed maximum tolerable value stored in the storage unit  152 , and discards any of the deviation amounts in droplet ejection for that nozzle  51  relating to the period in which the temporal variation in the deviation amount in droplet ejection for that nozzle  51  exceeded a maximum tolerable value, in such a manner that such deviation amount values are not used in the correction of the droplet ejection of that nozzle  51 . 
     Fourth Embodiment 
     Next, the droplet ejection correction processing according to a fourth embodiment of the present invention is described. 
     In the fourth embodiment, similarly to the third embodiment, the high-precision measurement mode and the high-speed measurement mode are selectively used. These measurement modes have already been explained in the third embodiment, and therefore further description thereof is omitted here. 
     Below, the droplet ejection correction processing according to the fourth embodiment of the present invention is described with reference to the flowchart in  FIG. 7 , which shows an overview of the sequence of this processing. The processing shown in  FIG. 7  is performed by the control unit  150  in  FIG. 1 , in accordance with a prescribed program. 
     When the image forming apparatus  10  is started up, a maintenance operation is performed in the head  50  of the  FIG. 1 , by the maintenance unit  102  in  FIG. 1  (step S 402 ), and immediately thereafter the deviation amount B in droplet ejection of each nozzle  51  of the head  50  in  FIG. 1  is measured by using the head  50 , the image reading unit  122  and the deviation amount calculation unit  124  in  FIG. 1 , in the high-precision measurement mode (step S 404 ), and the deviation amount B thus measured is stored in the storage unit  152  in  FIG. 1  (step S 406 ). 
     After step S 406  in  FIG. 7 , the left-hand side loop (S 412 →S 414 →S 416 →S 418 →S 419 →S 412 ) is a sequence of droplet ejection correction processing during normal printing in which no maintenance is necessary, and the right-hand side loop (S 412 →S 414 →S 416 →S 420 →S 428 →S 430 →S 432 →S 412 ) is a sequence of droplet ejection correction processing during printing where maintenance is necessary. 
     Normally, the deviation amount A 1  in droplet ejection of each nozzle  51  of the head  50  in  FIG. 1  is measured in the high-speed measurement mode, and the nozzle suffering ejection failure is identified (ejection failure detection is performed) (step S 412 ), and the measured deviation amount A 1  and the ejection failure nozzle identification information is stored in the storage unit  152  in  FIG. 1  (step S 414 ). 
     The step of judging whether or not it is necessary to perform maintenance by comparing the deviation amount A 1  with a previously established prescribed amount (threshold value) (S 416 ) involves similar judgment processing to the judgment step (S 316 ) in the third embodiment shown in  FIG. 6 . This judgment processing has already been described in detail in respect of the third embodiment, and therefore, further explanation thereof is omitted here. 
     Here, if the deviation amount A 1  is not greater than the prescribed amount (threshold value) (in other words, if it is within the tolerable range), then droplet ejection is corrected by calculating correction data on the basis of the result of the ejection failure detection in step S 412  (the ejection failure nozzle identification information) (step S 418 ) (hereinafter, this correction is called “ejection failure correction”), and furthermore, droplet ejection is corrected by calculating correction data on the basis of the deviation amount B measured in the high-precision measurement mode and stored in the storage unit  152  (step S 419 ), (hereinafter, this correction is called “ejection correction”). 
     In the case of “ejection failure correction”, correction is performed by increasing the droplet ejection volume (or by increasing the droplet ejection rate), in respect of the nozzles that are adjacent to the ejection failure nozzle (or the nozzles that are in the peripheral region of the ejection failure nozzle). In addition, correction may also be implemented in order to stop the driving of the piezoelectric actuator  58  corresponding to the nozzle that is suffering the ejection failure, thereby preventing wasteful consumption of power. 
     In the case of “ejection correction”, if the deviation amount B corresponding to the nozzle under consideration is outside the tolerable range, then correction is performed by, for instance, increasing or decreasing the droplet ejection volume (or increasing or decreasing the droplet ejection rate) relating to the nozzle under consideration and the nozzles that are adjacent to the nozzle under consideration (or relating to the nozzle under consideration and the nozzles in the peripheral region of the nozzle under consideration). 
     During normal printing, it is expected that the nozzles  51  perform ejection at a certain operating rate or above, and therefore, a state where the deviation amount A 1  is not greater than the threshold value, in other words, a state where the viscosity of the ink in the nozzles  51  is stable and no maintenance is necessary, continues for a certain amount of time. In this state where no maintenance is necessary, it is sufficient to repeat the steps of ejection failure detection by the high-speed measurement (step S 412 ) and “ejection failure correction” (step S 418 ), and “ejection correction” (step S 419 ) based on the deviation amount B. Therefore, the loop that does not require maintenance (S 412  to S 419 ) is repeated. Consequently, printing is continued while performing the high-speed measurement only, and hence there is no reduction in productivity. 
     On the other hand, if the deviation amount A 1  is greater than the prescribed amount, then the maintenance unit  102  in  FIG. 1  performs the maintenance operation with respect to the head  50  in  FIG. 1  (step S 420 ), and immediately after this maintenance operation, the deviation amount B in droplet ejection is measured in the high-precision measurement mode, and furthermore the ejection failure nozzles are identified (ejection failure detection is performed) (step S 428 ). Thereupon, the deviation amount B thus measured, and the ejection failure identification information, are stored in the storage unit  152  in  FIG. 1  (step S 430 ). The droplet ejection correction unit  126  in  FIG. 1  then performs droplet ejection correction (ejection failure correction) based on the results of the ejection failure detection in step S 428  (on the basis of the ejection failure nozzle identification information), and droplet ejection correction (ejection correction) on the basis of the deviation amount B (step S 432 ). 
     As stated above, in the fourth embodiment, the control unit  150  in  FIG. 1  measures the deviation amounts in droplet ejection of the nozzles  51 , in high-precision measurement mode, and stores the deviation amounts thus measured in the storage unit  152 ; normally, the control unit  150  measures the deviation amounts in droplet ejection of the nozzles  51  in the high-speed measurement mode, performs ejection failure detection on the basis of these deviation amounts, and implements droplet ejection correction (ejection failure correction) by means of the droplet ejection correction unit  126 , on the basis of the results of this droplet ejection detection, while at the same time, it implements droplet ejection correction (ejection correction) for the nozzles  51  by means of the droplet ejection correction unit  126 , on the basis of the deviation amounts in droplet ejection of the nozzles  51  measured in the past in the high-precision measurement mode and stored in the storage unit  152 . 
     Embodiment of Mechanical Composition of Image Forming Apparatus 
       FIG. 8  is a general schematic drawing showing the mechanical composition of the image forming apparatus  10  according to an embodiment of the present invention. As shown in  FIG. 8 , this image forming apparatus  10  includes: a droplet ejection unit  12  having a plurality of heads  50  as shown in  FIG. 1  provided for respective ink colors, namely, heads  12 K (black ink head),  12 C (cyan ink head),  12 M (magenta ink head) and  12 Y (yellow ink head); an ink storing and loading unit  14 , which stores inks to be supplied to the respective heads  12 K,  12 C,  12 M and  12 Y; a paper supply unit  18 , which supplies a medium  16 , such as paper; a decurling unit  20 , which removes curl from the medium  16 ; a suction belt conveyance unit  22 , disposed facing the nozzle faces (droplet ejection faces) of the heads  12 K,  12 C,  12 M and  12 Y, which conveys the medium  16  while keeping the medium  16  flat; an image sensor  24  forming the image reading unit  122 , which reads in an image produced by droplet ejection from the droplet ejection unit  12 ; and a paper output unit  26 , which outputs a printed medium (printed matter) to the exterior. 
     In  FIG. 8 , a magazine for rolled paper (continuous medium) is shown as an embodiment of the paper supply unit  18 ; however, more magazines with different medium width and quality may be jointly provided. Moreover, the medium may be supplied with cassettes that contain cut papers loaded in layers and that are used jointly or in lieu of the magazine for rolled paper. 
     In the case of a configuration in which a plurality of types of media can be used, it is preferable that an information recording medium such as a bar code and a wireless tag containing information about the type of medium  16  is attached to the magazine, and by reading the information contained in the information recording medium with a predetermined reading device, the type of medium to be used is automatically determined, and ink-droplet ejection is controlled so that the ink-droplets are ejected in an appropriate manner in accordance with the type of medium. 
     The medium  16  delivered from the paper supply unit  18  retains curl due to having been loaded in the magazine. In order to remove the curl, heat is applied to the recording paper  16  in the decurling unit  20  by a heating drum  30  in the direction opposite from the curl direction in the magazine. The heating temperature at this time is preferably controlled so that the medium  16  has a curl in which the surface on which the print is to be made is slightly round outward. 
     In the case of the configuration in which roll paper is used, a cutter (first cutter)  28  is provided as shown in  FIG. 8 , and the continuous paper is cut into a desired size by the cutter  28 . The cutter  28  has a stationary blade  28 A, whose length is not less than the width of the conveyor pathway of the medium  16 , and a round blade  28 B, which moves along the stationary blade  28 A. The stationary blade  28 A is disposed on the reverse side of the printed surface of the medium  16 , and the round blade  28 B is disposed on the printed surface side of the medium  16  across the conveyor pathway. When cut papers are used, the cutter  28  is not required. 
     The decurled and cut medium  16  is delivered to the suction belt conveyance unit  22 . The suction belt conveyance unit  22  has a configuration in which an endless belt  33  is set around rollers  31  and  32  so that the portion of the endless belt  33  facing at least the nozzle face of the droplet ejection unit  12  and the sensor face of the image sensor  24  forms a horizontal plane (flat plane). 
     The belt  33  has a width that is greater than the width of the medium  16 , and a plurality of suction apertures (not shown) are formed on the belt surface. A suction chamber  34  is disposed in a position facing the sensor surface of the image sensor  24  and the nozzle face of the droplet ejection unit  12  on the interior side of the belt  33 , which is set around the rollers  31  and  32 , as shown in  FIG. 8 . The suction chamber  34  provides suction with a fan  35  to generate a negative pressure, and the recording paper  16  on the belt  33  is held by suction. 
     The belt  33  is driven in the clockwise direction in  FIG. 8  by the motive force of a motor being transmitted to at least one of the rollers  31  and  32 , which the belt  33  is set around, and the medium  16  held on the belt  33  is conveyed from left to right in  FIG. 8 . 
     Since ink adheres to the belt  33  when a marginless print job or the like is performed, a belt-cleaning unit  36  is disposed in a predetermined position (a suitable position outside the printing area) on the exterior side of the belt  33 . Although the details of the configuration of the belt-cleaning unit  36  are not shown, embodiments thereof include a configuration in which nipping with a brush roller and a water absorbent roller or the like, an air blow configuration in which clean air is blown, or a combination of these. In the case of the configuration in which the belt  33  is nipped with the cleaning rollers, it is preferable to make the line velocity of the cleaning rollers different than that of the belt  33  to improve the cleaning effect. 
     The inkjet recording apparatus  10  can comprise a roller nip conveyance mechanism, in which the recording paper  16  is pinched and conveyed with nip rollers, instead of the suction belt conveyance unit  22 . However, there is a drawback in the roller nip conveyance mechanism that the print tends to be smeared when the printing area is conveyed by the roller nip action because the nip roller makes contact with the printed surface of the medium immediately after printing. Therefore, the suction belt conveyance in which nothing comes into contact with the image surface in the printing area is preferable. 
     A heating fan  40  is disposed on the upstream side of the droplet ejection unit  12  in the medium conveyance pathway formed by the suction belt conveyance unit  22 . The heating fan  40  blows heated air onto the medium  16  to heat the medium  16  immediately before printing so that the ink deposited on the medium  16  dries more easily. 
     The droplet ejection unit  12  is a so-called “full line head” in which a line head having a length corresponding to the maximum paper width is arranged in a direction that is perpendicular to the paper feed direction (medium conveyance direction) (see  FIG. 9 ). Each of the heads  12 K,  12 C,  12 M and  12 Y is constituted by a full line head, in which a plurality of ink droplet ejection ports (nozzles) are arranged through a length that exceeds at least one side of the maximum-size medium  16  intended for use in the image forming apparatus  10 , as shown in  FIG. 9 . 
     The print heads  12 K,  12 C,  12 M and  12 Y are arranged in the order of black (K), cyan (C), magenta (M), and yellow (Y) from the upstream side, along the feed direction of the recording paper  16  (hereinafter, referred to as the sub-scanning direction). A color image can be formed on the recording paper  16  by ejecting the inks from the print heads  12 K,  12 C,  12 M and  12 Y, respectively, onto the recording paper  16  while conveying the recording paper  16 . 
     The droplet ejection unit  12 , in which the full-line heads covering the entire width of the paper are thus provided for the respective ink colors, can record an image over the entire surface of the medium  16  by performing the action of moving the medium  16  and the droplet ejection unit  12  relatively to each other in the medium conveyance direction just once (in other words, by means of a single scan in the medium conveyance direction). In this way, it is possible to achieve higher-speed printing and to improve productivity in comparison with a shuttle scanning type of head configuration, in which a head moves reciprocally in a direction that is substantially perpendicular to the medium conveyance direction. 
     Although the configuration with the KCMY four standard colors is described in the present embodiment, combinations of the ink colors and the number of colors are not limited to those. Light inks or dark inks can be added as required. For example, a configuration is possible in which heads for ejecting light-colored inks such as light cyan and light magenta are added. 
     As shown in  FIG. 8 , the ink storing and loading unit  14  has ink tanks for storing the inks of the colors corresponding to the respective print heads  12 K,  12 C,  12 M and  12 Y, and the respective tanks are connected to the print heads  12 K,  12 C,  12 M and  12 Y by means of channels (not shown). The ink storing and loading unit  14  has a warning device (for example, a display device or an alarm sound generator) for warning when the remaining amount of any ink is low, and has a mechanism for preventing loading errors among the colors. 
     The image sensor  24  reads in the droplet ejection results of the droplet ejection unit  12 , and the occurrence of nozzle blockages or other droplet ejection defects and droplet ejection variations is determined from the read image data obtained from the image sensor  24 . 
     The image sensor  24  of the present embodiment is configured with at least a line sensor having rows of photoelectric transducing elements with a width that is greater than the ink-droplet ejection width (image forming width) of the heads  12 K,  12 C,  12 M and  12 Y. This line sensor has a color separation line CCD sensor including a red (R) sensor row composed of photoelectric transducing elements (pixels) arranged in a line provided with an R filter, a green (G) sensor row with a G filter, and a blue (B) sensor row with a B filter. Instead of a line sensor, it is possible to use an area sensor composed of photoelectric transducing elements which are arranged two-dimensionally. 
     The image sensor  24  according to the present embodiment reads in an image (which may be a test pattern or an actual image) that has been formed by the heads  12 K,  12 C,  12 M and  12 Y of the respective colors, and determines the droplet ejection variations for each head. Judgment of droplet ejection variations includes determining the presence or absence of ejected droplets (dots), and measuring the droplet deposition positions (dot positions), the ejected droplet diameters (dot diameters), the density, and the like. The image sensor  24  is provided with a light source (not illustrated) which irradiates light onto the deposited dots. 
     A post-drying unit  42  is disposed following the image sensor  24 . The post-drying unit  42  is a device to dry the printed image surface, and includes a heating fan, for example. It is preferable to avoid contact with the printed surface until the ejected ink dries, and a device that blows heated air onto the printed surface is preferable. 
     In cases in which printing is performed with dye-based ink on porous paper, blocking the pores of the paper by the application of pressure prevents the ink from coming contact with ozone and other substance that cause dye molecules to break down, and has the effect of increasing the durability of the print. 
     A heating/pressurizing unit  44  is disposed following the post-drying unit  42 . The heating/pressurizing unit  44  is a device to control the glossiness of the image surface, and the image surface is pressed with a pressure roller  45  having a predetermined uneven surface shape while the image surface is heated, and the uneven shape is transferred to the image surface. 
     The printed matter generated in this manner is outputted from the paper output unit  26 . The target print (i.e., the result of printing the target image) and the test print are preferably outputted separately. In the image forming apparatus  10 , a sorting device (not shown) is provided for switching the outputting pathways in order to sort the printed matter with the target print and the printed matter with the test print, and to send them to paper output units  26 A and  26 B, respectively. When the target print and the test print are simultaneously formed in parallel on the same large medium, the test print image is cut and separated by a cutter (second cutter)  48 . The cutter  48  is disposed directly in front of the paper output unit  26 , and is used for cutting the test print portion from the target print portion when a test print has been performed in the blank portion of the target print. The structure of the cutter  48  is the same as the first cutter  28  described above, and has a stationary blade  48 A and a round blade  48 B. 
     Although not shown in  FIG. 8 , the paper output unit  26 A for the target prints is provided with a sorter for collecting prints according to print orders. The numeral  26 B signifies a test paper output unit. 
     The foregoing description related to the embodiment where the relative movement between the liquid ejection heads  50  formed with the nozzles  51  and the medium  16  is achieved by moving the medium  16  with respect to the fixed liquid ejection heads  50 , but the present invention is not limited to cases of this kind, and the present invention can also be applied to a case where the medium  16  is fixed and the liquid ejection heads  50  are moved, or to a case where both the liquid ejection heads  50  and the medium  16  are moved. 
     Furthermore, the head that uses the piezoelectric bodies to eject droplets is described as the embodiment, but the present invention is not limited in particular to this and it can also be applied to a head that uses heaters to eject droplets. 
     It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.