Patent Publication Number: US-7216949-B2

Title: Ink jet printer

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a detection of a jet failure of a nozzle in an ink jet printer. 
   2. Description of the Related Art 
   In an earlier developed ink jet printer, an image is recorded by jetting ink droplets onto a recording medium from a plurality of nozzles which use a thermal system or a piezo system actuated according to jet signals based on image signals. In the ink jet printer, fixing of ink near nozzles due to drying or increased viscosity of the ink in a case of being left for a long period of time without being used or adhering of impurities (foreign particles) or the like to the nozzles would cause clogging of the nozzles, which results in a failure of jetting ink droplets from the nozzles in spite of normally outputting ink jet signals from a drive circuit, that is, a jet failure of ink from the nozzles (hereinafter, referred to as nozzle clogging). The nozzle clogging would cause a deterioration of print quality such as generating a blank in a printed character or image, which is recognized as a white stripe, or causing difference of reproduced color in each recorded image due to a lack of ink color material. Therefore, an optical detection section as a detection section for detecting such the nozzle clogging has been disclosed. 
   For example, disclosed is an ink jetting condition detection method for detecting nozzle clogging by changing a detection timing with a photo sensor in which light emitting elements and light receiving elements are combined along a distance corresponding to the width of the head as a detection section of nozzle clogging of a carriage type ink jet printer in which ink is jetted from the head in a direction (main scanning direction) perpendicularly crossing a carrying direction of a paper (sub scanning direction) to form an image (JP-Tokukai-hei-11-188853A, hereinafter referred to as “Patent Document 1”). 
   However, applying Patent Document 1 to a line head type ink jet printer could be causative factors of cost increase due to the needs to adjust the amount of light or the diameter of beam from the light emitting elements with high accuracy because one line head has a large length and the size of the ink droplets jetted from the nozzles is small, and to move the detection section for nozzle clogging by using a positioning sensor with high accuracy. Also, the distance between the light emitting elements and the light receiving elements becomes large, which may cause misdetection due to dust or ink droplets in the form of mist. Further, since a large number of nozzles which need to be detected exist in one line head, it would raise a problem that time for detection becomes long. 
   SUMMARY OF THE INVENTION 
   The present invention is developed in view of the above described problems, and an object of the present invention is to provide an ink jet printer capable of precisely detecting nozzle clogging by utilizing impact force generated when an ink droplet jetted from a nozzle lands. 
   For solving the problems, in accordance with a first aspect of the present invention, the ink jet printer to record an image on a recording medium by jetting ink from nozzles comprises: 
   a vibration detection section to receive ink jetted from the nozzles and output a detection signal having an amplitude corresponding to a vibration generated when the ink lands on the vibration detection section; 
   a sampling section to sample an amplitude value of the detection signal by a predetermined sampling clock signal; 
   a storing section to store an amplitude value data of the detection signal sampled by the sampling section; 
   a judging section to judge a jet failure of one of the nozzles based on the amplitude value data of the detection signal stored in the storing section; and 
   a control section to control to jet the ink continuously a plurality of times with a jet drive cycle which is set by multiplying a standard drive waveform time of an ink jet signal from the nozzles by an odd number which is not less than five when detecting a jet failure of one of the nozzles. 
   In accordance with a second aspect of the present invention, the ink jet printer to record an image on a recording medium by jetting ink from nozzles comprises: 
   a vibration detection section to receive ink jetted from the nozzles and output a detection signal having an amplitude corresponding to a vibration generated when the ink lands on the vibration detection section; 
   a sampling section to sample an amplitude value of the detection signal by a predetermined sampling clock signal; 
   a storing section to store an amplitude value data of the detection signal sampled by the sampling section; 
   a judging section to judge a jet failure of one of the nozzles based on the amplitude value data of the detection signal stored in the storing section; and 
   a control section to control to jet the ink continuously n times with a predetermined jet drive cycle from the nozzles when detecting a jet failure of one of the nozzles, 
   wherein the judging section judges the jet failure of one of the nozzles based on only a control for jetting the ink from n/2 times in a control for jetting the ink continuously n times, and the n is an integer which is not less than two. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein; 
       FIG. 1  is a schematic view of an inside of an ink jet printer  1  in the first embodiment in which the present invention is applied; 
       FIG. 2A  is a view showing a state where a nozzle clogging detection part has not received an ink droplet from a head module  31   a  (initial state); 
       FIG. 2B  is a view showing a state where the nozzle clogging detection part has received an ink droplet from the head module  31   a  (operating state); 
       FIG. 3A  is a view in which a maximum projecting portion of a contact surface S 2  of the cover  61   a  contacts with a maximum projecting portion of a piezo film  62 ; 
       FIG. 3B  is a view in which the contact surface S 2  of the cover  61   a  contacts with the piezo film  62 ; 
       FIG. 4  is a control block diagram for controlling the ink jet printer  1  in the first embodiment; 
       FIG. 5  is a view showing an example of the dependence of an ink droplet speed on a cycle of a jet drive signal; 
       FIG. 6  is an example of a memory map  141  stored in a storing unit  140 ; 
       FIG. 7A  is an example of a time chart of a jet drive signal S m0 , an ink jet signal S m1  output based on the jet drive signal S m0 , a sampling start signal Ss and a piezo vibration signal Sd output based on the ink jet signal S m1 ; 
       FIG. 7B  is an example of a time chart of the sampling start signal Ss, the piezo vibration signal Sd, and a sampling clock signal Sc in a sampling period Ts shown in  FIG. 7A ; 
       FIG. 8  is a flow chart of a nozzle clogging judging operation in the first embodiment; 
       FIG. 9  is a flow chart continuing from  FIG. 8  to illustrate the nozzle clogging judging operation in the first embodiment; 
       FIG. 10  is a flow chart continuing from  FIG. 9  to illustrate the nozzle clogging judging operation in the first embodiment; 
       FIG. 11  is a control block diagram for controlling the ink jet printer in the second embodiment; 
       FIG. 12  is a flow chart of a nozzle clogging judging operation in the second embodiment; 
       FIG. 13  is a flow chart continuing from  FIG. 12  to illustrate the nozzle clogging judging operation in the second embodiment; and 
       FIG. 14  is a flow chart continuing from  FIG. 13  to illustrate the nozzle clogging judging operation in the second embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   [First Embodiment] 
   The first embodiment of the present invention will be explained below referring to the drawings. 
   The configuration will be explained first. 
     FIG. 1  shows a schematic view of the inside of an ink jet printer  1  of a line head type in the first embodiment. As shown in  FIG. 1 , the ink jet printer  1  comprises a paper feed part  10 , a carrying part  20 , a head unit part  30 , a paper discharge part  40 , a maintenance part  50  as a maintenance section, a nozzle clogging detection part  60  and the like. 
   The paper feed part  10  is provided with a paper feed tray  11  for stacking and storing a plurality of recording mediums P at the lower side of the inside of the ink jet printer  1 . A paper pick up device  12  is provided at one end portion of an upper side of the paper feed tray  11  for picking up the recording medium P on which an image is to be recorded one by one from the paper feed tray  11 . 
   The recording medium P to be applied includes various types of papers such as a plain paper, a recycled paper, a gloss paper or the like, and a cut sheet shaped recording medium made from a material such as various types of textiles, non-woven fabrics, resin, metal, glass or the like. 
   The carrying part  20  for carrying the recording medium P is provided on the upper side of the paper feed part  10 . The carrying part  20  comprises a carrying belt  21 , tension rollers  22 , a pressure roller  23 , carrying rollers  24  and a carrying path  25 . 
   The carrying belt  21  is a circular shaped belt for carrying the recording medium P in a horizontal direction while supporting it in a plane state, and is movably tensioned by the plurality of tension rollers  22 . The carrying belt  21  is provided with an opening parts  21   a , so that a nozzle clogging detection part  60  to be described later is movable and a capping module covers nozzles. An encoder film and an encoder sensor are provided at the end portion of the carrying belt  21  to make the opening part  21   a  be positioned at the lower side of the nozzles when judging nozzle clogging or performing a maintenance operation, thus the position of the opening part  21   a  can be detected based on the detection signal from the encoder sensor (not shown). 
   The pressure roller  23  is rotatably provided at a portion where the carrying belt  21  and the recording medium P start to contact with each other as a roller to put pressure onto the carrying belt  21  for carrying the recording medium P in a flat shape. 
   The carrying path  25  is a path for carrying the recording medium P which was fed from the paper feed tray  11 , and for discharging the recording medium P carried along the periphery of the carrying belt  21  to a paper discharge part  40 . The carrying rollers  24  are provided at a predetermined position of the carrying path  25  as a plurality pairs of rollers for carrying the recording medium P in a carrying direction X. 
   A head unit part  30  comprises line head type head units  31 ,  32 ,  33 ,  34  at the portion near the upper portion of the carrying belt  21 , for jetting each ink color of black (Bk), cyan (C), magenta (M) and yellow (Y) onto the recording medium P in this order along the carrying direction X, each of which comprises a plurality of nozzles (not shown) and extends along the whole width of the carrying belt  21 . Each head unit  31 ,  32 ,  33 ,  34  is disposed to make the nozzle-plates thereof face the periphery of the carrying belt  21 . 
   The recording medium P on which an image was formed by ink droplets jetted from each head unit  31 ,  32 ,  33 ,  34  is discharged from the paper discharge part  40  in order. 
   Each head unit  31 ,  32 ,  33 ,  34  extending in a direction approximately perpendicular to the carrying direction X of the recording medium P comprises a plurality of head modules as an ink jet section arranged in parallel in a longitudinal direction. Each head module extends in the longitudinal direction of each head module  31 ,  32 ,  33 ,  34 , and they are alternately arranged in parallel with each other at predetermined intervals in the carrying direction X of the recording medium P (staggered arrangement). 
   The paper discharge part  40  comprises a paper discharge tray  41  provided at the side portion of the ink jet printer  1 , and the recording medium P on which an image was formed is discharged therefrom in order. 
   The maintenance part  50  is provided at the lower side of the head unit  30  to face thereto across a portion near the lower portion of the upper surface of the carrying belt  21 . The maintenance part  50  comprises a plurality of cap units  51 ,  52 ,  53 ,  54  for covering the nozzles, a suction pump which is not shown, and a waste ink tank. 
   Each cap unit  51 ,  52 ,  53 ,  54  comprise a plurality of capping modules (not shown) each of which corresponds to each head module of each head unit  31 ,  32 ,  33 ,  34 . Each capping module is movable between a capping position for capping the nozzles of each head module corresponding thereto and a separated position where each capping module is separated from the nozzles. Coupled to each capping module is a suction pump and an air communicating valve or the like for suctioning fluid in a space which is formed after each capping module is moved to the capping position and the whole nozzles are covered by a rubber member or the like to shut off the outside air and be sealed. That is, the air and the ink inside the space are suctioned by the suction pump. The ink suctioned by the suction pump is discharged to the waste ink tank. The configurations of the suction pump, the air communicating valve, the waste ink tank and the like are same as those of the earlier technique, therefore the detailed descriptions thereof are omitted here. 
   In the first embodiment, explanation will be made to an example in which a suction operation which is a representative of a maintenance method is adopted as a method to solve nozzle clogging. However, a flashing operation may be adopted, in which electrical signals are given to the heads to jet ink droplets, and foreign materials or the like adhered to the nozzles and the nozzle-plates are flashed. 
   Further, a mechanism for performing a wiping operation to wipe unnecessary ink droplets adhered to the nozzle-plates after the suction operation or the flashing operation may be provided. 
   The nozzle clogging detection part  60  is provided at the lower portion of the head unit part  30  to face thereto across the portion near the lower portion of the upper surface of the carrying belt  21 , and is movable to the predetermined position corresponding to each head unit  31 ,  32 ,  33 ,  34 . A plurality of nozzle clogging detection parts  60  extend in the longitudinal direction of the head unit and are alternately arranged in parallel with each other at predetermined intervals in the carrying direction X of the recording medium P, to correspond to each head module. 
     FIGS. 2A and 2B  show end views of the nozzle clogging detection part  60 . 
     FIG. 2A  shows a state where the nozzle clogging detection part  60  has not received an ink droplet from the head module  31   a  (initial state), and  FIG. 2B  shows a state where the nozzle clogging detection part  60  has received an ink droplet from the head module  31   a  (operating state). 
   As shown in  FIGS. 2A and 2B , the nozzle clogging detection part  60  comprises an ink droplet receiving part  61 , a piezo film  62  as a vibration detection portion, a supporting part  63 , an adjusting part  64  and the like. 
   In the first embodiment, the explanation will be made in the case where the piezo film of a film shaped piezoelectric elements is used as a vibration detection portion, however, it is not limited thereto as long as the vibration detection portion is capable of receiving ink droplets jetted from the nozzles and outputting mechanical displacement (vibration) as electric charge (amplitude) when the ink droplets land, thus, it may be a strain gage or the like. 
   The ink droplet receiving part  61  comprises a cover  61   a  and an elastic supporting member  61   b , and transmits the impact force generated when ink droplets land to the piezo film  62 . 
   The cover  61   a  comprises an ink droplet landing surface S 1  for receiving ink droplets and a contact surface S 2  for transmitting the impact force by contacting with the piezo film  62  when ink droplets land on the ink droplet landing surface S 1 . The contact surface S 2  is provided with a projecting portion at a position to face the maximum projecting portion of the curved outer periphery of the piezo film  62  for transmitting the impact force generated when ink droplets land to the maximum projecting portion. 
   The cover  61   a  extends in the longitudinal direction of the head module corresponding thereto to be interposed between the nozzles and the piezo film  62 , so that the piezo film  62  can be protected from various ink droplets. Thus, the response property of the piezo film  62  can be protected. 
   There is an ink jet printer in which a property (viscosity) of the ink to be jetted changes depending upon the ink used. Specifically, there is a case where the ink is heated to the temperature higher than room temperature to lower the ink viscosity, and the ink droplets with high temperature are detected. In such ink jetting method, the temperature of the piezo film rises after receiving tens of ink droplets, which would cause a change to the response property of the piezo film. Thus, the cover  611   a  has a purpose to prevent such the change of the response property. When the ink to be used is electrically conductive ink, the cover  611   a  can prevent the piezo film from being damaged when the electrically conductive ink contacts the output signal terminals of the piezo film. 
   The cover  61   a  is kept in stationary state with a certain state by the elastic supporting member  61   b  provided on the bottom portion, and transmits small impact force generated when the ink droplets land onto the piezo film  62 , so that it is preferable to use workpiece materials which are light in weight such as plastic or the like and can be formed to be an arbitrary shape. 
   The elastic supporting member  61   b  supports the cover  61   a  to keep the contact surface S 2  of the cover  61   a  and the piezo film  62  in a non-contact state when the ink droplet landing surface S 1  does not receive ink droplets, and to make the contact surface S 2  of the cover  61   a  be in the contact state with the piezo film  62  when the ink droplet landing surface S 1  receives ink droplets. 
   In the first embodiment, when the ink droplet landing surface S 1  does not receive ink droplets, the contact surface S 2  and the piezo film  62  are set to be in the non-contact state, however, both of them may contact with each other, that is, the present invention is not limited to this embodiment as long as the piezo film  62  and the contact surface S 2  are in a stationary state while keeping a certain equilibrium state. 
   The piezo film  62  is curved into an approximately half cylinder ahead in the ink droplet jetting direction from the nozzles by the support part  63 , and is supported to make the maximum projecting portion of the curved outer periphery direct to an ink droplet coming direction. The piezo film  62  extends corresponding to the longitudinal direction of the head module. 
   In the first embodiment, the landing of ink droplets is detected by using piezoelectric effect of the piezo film  62 . To further improve sensitivity and directivity of the piezo film  62 , the piezo film  62  is curved into an approximately half cylinder, and ink droplets from the ink droplet coming direction land onto the maximum projecting portion of the curved outer periphery (that is, the maximum projecting portion of the piezo film  62  which is curved into an approximately half cylinder contacts with the projecting portion of the contact surface S 2  of the cover  61   a ). 
   The piezo film  62  used in the invention may be any piezoelectric element as long as the piezoelectric element is formed in a film shape, which is easy to thin even when it has a large size, with piezoelectric effect and improved productivity, having excellent flexibility, impact-resistance, chemical stability or the like in comparison with an earlier developed piezoelectric ceramic or the like, and has a better output response to impact or shape changing, wide frequency characteristic or the like. 
     FIGS. 3A and 3B  show sectional views of examples of a contact state of the contact surface S 2  of the cover  61   a  and the piezo film  62 .  FIG. 3A  shows a state that the maximum projecting portion of the contact surface S 2  of the cover  61   a  contacts with the maximum projecting portion of the piezo film  62 , and  FIG. 3B  shows a state that the contact surface S 2  of the cover  61   a  contacts with the piezo film  62 . 
   As shown in  FIG. 3A , in the case where the maximum projecting portion of the cover  61   a  contacts with the maximum projecting portion of the curved outer periphery of the piezo film  62 , when ink droplets from the nozzles land on the ink droplet landing surface S 1 , a small impact force generated by the landing of the ink droplets onto the ink droplet landing surface S 1  can be concentrated on the maximum projecting portion of the contact surface S 2 , thereby enabling the maximum projecting portion of the piezo film  62  to receive the impact force. Thus, the impact force by the ink droplets can efficiently be transmitted, so that high sensitivity and response property can be obtained. 
   As shown in  FIG. 3B , in the case where the contact surface S 2  contacts with the piezo film  62  at portions other than the maximum projecting portion, the impact force by the landing of ink droplets on the ink droplet landing surface S 1  would be dispersed, and further the dispersed impact force would be received by the curved surface of the piezo film  62 , thereby reducing the transmission efficiency of the force, which results in lowering the sensitivity and response property. 
   Thus, the position of the maximum projecting portion of the curved outer periphery of the piezo film  62  is adapted to be movable, and the nozzle clogging detection part  60  comprises the supporting part  63  and the adjusting part  64  for adapting the maximum projecting portion to the ink droplet landing position. Adaptation of the maximum projecting portion of the curved outer periphery of the piezo film  62  to the ink droplet landing position is successful in adjusting the shape of the piezo film, thereby enabling to adjust the response property of the piezo film  62 . 
   The supporting part  63  curves the piezo film  62  into approximately half cylinder to support it. The supporting part  63  comprises a fixed supporting part  63   a  and a movable supporting part  63   b . The facing surfaces of the fixed supporting part  63   a  and the movable supporting part  63   b  are provided to be perpendicular to a rotating axis A of a screw  64   a  which will be described later and in parallel with each other. 
   The piezo film  62  is fixed to the fixed supporting part  63   a  at one end side thereof in a curving direction, and the fixed supporting part  63   a  is provided not to move irrespective of the rotation of the screw  64   a.    
   The movable supporting part  63   b  supports the other end side of the piezo film  62  opposing to the one end side of the piezo film  62  in the curving direction which is fixed to the fixed supporting part  63   a . A female screw hole in which a male screw part of the screw  64   a  is screwed is formed in the movable supporting part  63   b , so that the movable supporting part  63   b  is movable to be close to or separated from the fixed supporting part  63   a  corresponding to the rotation direction of the screw  64   a.    
   The adjusting part  64  comprises the screw  64   a  and a supporting base  64   b.    
   The screw  64   a  is disposed such that the rotating axis A of the screw  64   a  is perpendicular to the ink droplet jetting direction B, and is rotatably supported by the supporting base  64   b  and a casing  69 . The screw  64   a  has the male screw part in a movable rage of the movable supporting part  63   a , which is screwed in the female screw hole of the movable supporting part  63   a.    
   For example, in  FIG. 2A , when the screw  64   a  is rotated in a right-handed screw direction, the movable supporting part  63   b  is moved to the right side, and when the screw  64   a  is rotated in a left-handed screw direction, the movable supporting part  63   b  is moved to the left side. 
   Accordingly, the other end side of the piezo film  62  can be movably supported to be close to or separated from the one end of the piezo film  62  in the curving direction, and the position of the maximum projecting portion of the curved outer periphery is adapted to the ink droplet landing position, so that the position of the maximum projecting portion of the curved outer periphery can be adjusted right to left and up and down. Therefore, the adjustment of the response property of the piezo film  62  can be realized. 
   As described above, by providing the supporting part  63  and the adjusting part  64 , the curvature of the curved piezo film  62  can be changed to adjust the position of the maximum projecting portion of the curved outer periphery, thus, the distance between the cover  61   a  and the piezo film  62 , and the initial shape of the piezo film  62  can be adjusted. Accordingly, in the initial state, appropriate adjustment of the position can be performed so as not to output a signal from the piezo film  62  by some impact (operating vibration of the machine itself, a misalignment of the setting position of the ink receiving member  60 , or the like). Appropriate adjustment of the position of the piezo film  62  to the ink droplet landing position is successful in obtaining high response property in the operating state, so that the small changes of the landing of fine ink droplets can be detected with high accuracy. 
   The rotating operation of the screw  64   a  by the adjusting part  64  may be performed manually or automatically. In the case of automatically performing the rotating operation, for example, a feed screw mechanism may be applied, in which a ball screw or the like is used as the screw  64   a  in the adjusting part  64 . That is, for example, in the case of adjusting the initial shape of the piezo film  62 , the configuration may be such that the value of rotation amounts corresponding to shape of the piezo film  62  is prestored in the nonvolatile memory which is under the control of the CPU, and the driving force from the drive source which can be controlled by the predetermined control device can be transmitted to the screw  64   a . Thus, the driving force from the drive source can be controlled based on the stored data in the nonvolatile memory. Thereby, the rotation amount can be automatically adjusted to adjust the position of the maximum projecting portion of the curved outer periphery, so that the initial shape of the piezo film  62  can be adjusted. 
     FIG. 4  shows a control block diagram for controlling the ink jet printer  1  of the first embodiment. As shown in  FIG. 4 , the control system comprises a control unit  100  and a nozzle clogging detection circuit  200 . 
   In the control unit  100 , a CPU (Central Processing Unit)  110  as a control section and a judging section, a ROM (Read Only Memory)  120 , a RAM (Random Access Memory)  130 , a storing unit  140  as a storing section, an I/O (Input/Output)  150 , a various machines control unit  160 , an I/F  170 , a drive circuit  180  and the like are connected to a system bus  190 , and the control unit  100  is connected to the nozzle clogging detection circuit  200  through the I/F  170 . 
   The nozzle clogging detection circuit  200  comprises a shape correction circuit  210 , an amplifier circuit  220 , a filter circuit  230 , a sampling clock generation unit  240  as a sampling section, a peak hold unit  250 , an A/D conversion circuit  260  or the like. 
   The CPU  110  reads out a system program, or various processing programs and data stored in the ROM  120 , expanding them in the RAM  130 , and performs a central control of operations of the whole ink jet printer  1  according to the programs expanded. That is, the CPU  110  performs a timing control of the whole system, storing and accumulation controls of data with the use of the RAM  130 , an output of print data to each head module, an input-output control of an operating portion which is not shown, an interface (I/F) to other applications, or an operation control. 
   Incidentally, due to the structural feature of the ink jet head, there is a case that ink jetting is not performed for the initial stage of the ink droplet jetting operations, and is recovered after repeating the ink droplet jetting operation a few times, so that ink droplets are jetted in the middle or last half ink droplet detection operations. 
   Therefore, in the present invention, ink jetting is performed continuously a plurality of times, and the vibration generated when an ink droplet lands is detected in each ink droplet jetting operation. Thus, even if the vibration which is generated when an ink droplet lands is not detected in the initial stage, a judgment is made that there is no nozzle clogging when the ink jetting is recovered after repeating the ink droplet jetting operation a few times, so that the detection accuracy of the nozzle clogging can be improved. 
   When the ink droplet jetting operation from the nozzles is performed continuously, the cycle of the jet drive signal S m0  needs to be set to make the speed of an ink droplet in each ink droplet jetting operation be constant. The ink jet signal S m1  for jetting ink droplets from the nozzles is an integral multiple of a standard drive waveform time AL. Thus, the cycle of the jet drive signal S m0  is set by multiplying the standard drive waveform time AL by an odd number to stabilize the ink droplet speed. 
     FIG. 5  is a view showing an example of the dependence of the ink droplet speed on the cycle of the jet drive signal. In the example shown in  FIG. 5 , when the standard drive waveform time AL is set to be 4.9 [μs], the ink droplet speed of the first ink droplet jetting operation is 5.5 [m/s]. 
   As shown in  FIG. 5 , when the cycle of the jet drive signal S m0  is set by multiplying the standard drive waveform time AL by an even number (6AL, 8AL, 10AL, 12AL and 14AL are shown in  FIG. 5 ), the ink droplet speed decreases, thereby causing the gap in the timing of an ink droplet to land with respect to the first ink droplet jetting operation. Accordingly, the cycle of the jet drive signal S m0  is preferably set by multiplying the standard drive waveform time AL by an odd number for setting the ink droplet speed to be approximately equal to the ink droplet speed of the first ink droplet jetting operation. However, when the time is three times of the standard drive waveform time AL, the nozzles are not ready to perform the second ink droplet jetting operation. Thus, the second ink droplet jetting time in the continuous ink droplet jetting operations is set by multiplying the standard drive waveform time AL by an odd number not less than five as the cycle of the jet drive signal S m0 . 
   In the example shown in  FIG. 5 , as the cycle of the jet drive signal S m0 , the shortest drive timing of the second ink droplet to have a speed close to the ink droplet speed of the first ink droplet jetting operation (5.5 [m/s]) is five times of the standard drive waveform time AL, followed by seven times and then by nine times thereof. Thus, when detecting an ink droplet by jetting two or more ink droplets continuously, it is preferable to set the cycle of the jet drive signal S m0  by multiplying the standard drive waveform time AL by seven or nine which is an odd number. Accordingly, the cycle of the jet drive signal S m0  is preferably set by multiplying the standard drive waveform time AL by an odd number not less than five, more preferably by an integral number which is one of 5, 7, 9, 11, 13 and 15. 
   Accordingly, to realize the first embodiment, the CPU  110  calculates the jet drive signal S m0  as an instruction signal to continuously jet ink droplets a plurality of times with a jet drive cycle which is set by multiplying the standard drive waveform time of the ink jet signal by an odd number not less than five, and outputs the calculated jet drive signal S m0  to the drive circuit  180 . The CPU  110  reads out an address of a detected data Sd′ as an amplitude value data showing a maximum voltage value V max  as a maximum amplitude value in each ink droplet jetting operation from a memory map  141  to be described later which is stored in the storing unit  140 , and performs the judgment of the nozzle clogging based on the address number (number of addresses) which was read out. Further, the nozzle clogging judging operation is performed by comparing the maximum voltage value V max  which is shown by the detected data Sd′ corresponding to the address which was read out and the standard voltage value V 0  as a standard value. 
   When the CPU  100  judges that nozzle clogging exists, the maintenance part  50  is controlled to drive to solve nozzle clogging. 
   The ROM  120  stores a program or a system program for driving the ink jet printer  1 , various programs corresponding to the system, data necessary for processing with the various processing programs and the like. 
   To realize the first embodiment, the ROM  120  stores the standard voltage value V 0  which is the maximum voltage value based on the detected data Sd′ when ink droplets are properly jetted onto the ink droplet landing surface S 1 , a delay time t d  to be described later and a sampling period Ts. 
   The RAM  130  is a temporally storing region for programs, input or output data, parameters read out from the ROM  120  in various processing controlled and executed by the CPU  110 . 
   The RAM  130  also temporally stores addresses read out from the storing unit  140  by the CPU  110 , a sampling number (m) (number of samplings) calculated by a sampling clock generation unit  260  to be described later and the number of ink droplet jetting operations (n) which is set by an operating portion or the like which is not shown, and comprises an ink jet counter for counting the number of ink droplet jetting operations (jet count number N). 
   To realize the first embodiment, the storing unit  140  stores the detected data Sd′ input from the A/D conversion circuit  250  through the I/F in the memory map  141  to correspond to each address. 
     FIG. 6  shows an example of the memory map  141  stored in the storing unit  140 . 
   As shown in  FIG. 6 , each address in the memory map  141  consists of an upper address. A 1  and a lower address A 2 . The upper address A 1  shows the number of ink droplet jetting operations (n) which was set, and the lower address A 2  shows the sampling number (m) in each ink droplet jetting operation, that is, a clock number (number of clocks) of sampling clock signal. 
   For example, the detected data Sd′ stored in the address “01×0001” is the voltage value (amplitude value) of the piezo vibration signal Sd as a detected signal presented in twos complement form when the clock number of the sampling clock signal is 1 in the first ink droplet jetting operation. 
   The storing unit  140  may be composed by using a part of the storing region of the RAM  130 . 
   The I/O  150  is for input and output of data between the control unit  100  and a control unit of each portion. To realize the first embodiment, the I/O  150  is connected to a shape correction control unit for controlling the shape of the piezo film  62  and a maintenance control unit for controlling the operations of the maintenance part  50 , and is also connected to control units such as an ink supply/waste fluid control unit, a waste ink control unit or the like. 
   The various machines control unit  160  is connected to a paper feed control unit for controlling various rollers and the paper pick up device  12  of the paper feed part  10 , a carrying control unit for controlling various rollers of the carrying part  20 , and a sensor control unit for driving various sensors provided in the ink jet printer  1  and the like, each of which operates based on the instructions from the CPU  110 . 
   The drive circuit  180  generates and outputs the ink jet signal S m1  for jetting ink droplets from the nozzles by driving the nozzles of each head module of each head unit  31 ,  32 ,  33 ,  34 , based on the print data and the jet drive signal S m0  from the CPU  110 . Also, the drive circuit  180  generates and outputs a sampling start signal Ss which is output after a lapse of a delay time t d  from a rise time as a generation time of the jet drive signal S m0 . The generated ink jet signal S m1  is output to the nozzles of each head module and the sampling start signal Ss is output to the sampling clock generation unit  260 . 
   The delay time td is determined as the time corresponding to the time needed for ink droplets to land based on the drive waveform condition of the ink jet signal S m1 . The drive waveform condition is determined based on the ink type to be jetted (for example, water-based ink, oil-based ink, ultraviolet curable ink, solid ink or the like), the jetting method (piezo system using piezoelectric elements, thermal system using a heater, or the like), a head configuration or the like. 
   For example, in a case of a head of piezo system, an ink droplet jetting operation is performed by the ink jet signal S m1  having two drive waveforms. In this case, a positive voltage waveform is called “ON waveform”, and a negative voltage waveform is called “OFF waveform”. A time period of the “ON waveform” (that is, the standard drive waveform time AL) is a standard for ink droplet jetting operations. 
   The delay time t d  is set to be twice of the standard drive waveform time AL of the ink jet signal S m1 . 
   The sampling start signal Ss is a signal for instructing detection of the piezo vibration signal Sd, and a time period Ts (hereinafter, referred to as a sampling time period) for detecting the piezo vibration signal Sd shows a “LOW” state. 
   For example, in a case of a head of piezo system, the time slot of the sampling time period Ts is set to be twice of the standard drive waveform time AL with the rise time of the “OFF waveform” of the ink jet signal S m1  as a center. 
   The shape correction circuit  210  adjusts the output signal from the piezo film  62  based on the instructions from the shape correction control unit to make the piezo vibration signal Sd to be output constant within the range of the preset initial value in a case that the piezo film  62  is in the initial condition. 
   The piezo vibration signal Sd as a detection signal output from the piezo film  62  is amplified and adjusted by the amplifier circuit  220 , and is subjected to filtering out noise with the filter circuit  230 . The denoised piezo vibration signal Sd is input to the peak hold unit  250 . 
   The sampling clock generation unit  240  receives the sampling start signal Ss and the sampling time period Ts input from the drive circuit  180 , and generates a sampling clock signal Sc which is a clock signal with a constant frequency to calculate the sampling number (m). The generated sampling clock signal Sc is output to the peak hold unit  250 , and the calculated sampling number (m) is output to the control unit  100 . 
   The sampling clock generation unit  240  comprises an address counter  241  for counting the clock number of the generated sampling clock signal Sc, and an address count number (M) (number of addresses counted) is output to the control unit  100 . 
   Preferably, the cycle of the sampling clock signal Sc is set with the number of sampling data, the capacity of the storing unit  140 , the data collection function and the like optimized. When the cycle is shortened and the sampling number is increased, it may increase the case to read the same data continuously, and may make the read time of the data from the storing unit  140  long, which may result in long nozzle clogging judging operations. 
   Therefore, the cycle of the sampling clock signal Sc can be calculated depending upon the time slot of the sampling time period Ts of the sampling start signal Ss. For example, in a case of a head of piezo system, when the sampling time period Ts is set to be twice of the standard drive waveform time AL, the sampling clock signal Sc with a cycle of one tenth of the standard drive waveform time AL can be calculated. 
   The peak hold unit  250  extracts the piezo vibration signal Sd input from the filter circuit  230  in the sampling time period Ts based on the sampling clock signal Sc input from the sampling clock generation unit  240  for each clock. The extracted piezo vibration signals Sd are subjected to A/D conversion by the A/D conversion circuit  260 , and are stored in the memory map  141  of the storing unit  140  through the I/F  170  as the detected signal Sd′. 
   As described above, the piezo vibration signal Sd needs to be detected only in the sampling time period Ts, so that unnecessary signal is not detected, thereby improving detection accuracy of a jet failure of the nozzles. 
     FIGS. 7A and 7B  show examples of time charts of ink droplet jetting operations from nozzles. 
     FIG. 7A  shows the jet drive signal S m0  as a signal to instruct the ink droplet jetting operations output to the drive circuit  180  from the CPU  110 , the ink jet signal S m1  output to the head of the head module  31   a  from the drive circuit  180  based on the jet drive signal S m0 , the sampling start signal Ss output to the sampling clock generation unit  240  from the drive circuit  180 , and the piezo vibration signal Sd output from the piezo film  62  based on the ink jet signal S m1 . 
     FIG. 7B  shows an example of the sampling start signal Ss, the piezo vibration signal Sd, and the sampling clock signal Sc output to the peak hold unit  250  from the sampling clock generation unit  240  in the sampling time period Ts shown in  FIG. 7A . 
   As shown in  FIG. 7A , for example, the cycle T m0  of the jet drive signal S m0  is set to be five times of the standard drive waveform time AL. When the jet drive signal S m0  is output at time t 1 , the ink jet signal S m1  is output. When the delay time td passes from the rise time t 1  of the jet drive signal S m0 , the sampling start signal Ss is output. The sampling time period Ts is set to be twice of the standard drive waveform time AL with the rise time t 2  of the “OFF waveform” of the ink jet signal S m1  as a center. Ink droplets land on the ink droplet landing surface S 1  in the sampling time period Ts, so that the piezo vibration signal Sd is output. 
   As shown in  FIG. 7B , in the sampling time period Ts, the sampling clock signal Sc is output based on the sampling start signal Ss, the piezo vibration signal Sd is extracted based on the sampling start signal Ss for each clock, and nozzle clogging judging operation is performed. 
   Next, description will be made for the nozzle clogging judging operation performed by the control unit  100 . 
     FIGS. 8 to 10  show flow charts of the nozzle clogging judging operation of the first embodiment. 
   The head unit which is subjected to the nozzle clogging judgment is set. Thereafter, the nozzle clogging detection part  60  is moved to a predetermined position below the set head unit (Step S 1 ). 
   After the nozzle clogging detection part  60  was moved to the predetermined position, cleaning of the ink droplet receiving part  61  is performed, and mechanical vibration of the nozzle clogging detection part  60  is stopped (Step S 2 ). 
   After the cleaning of the ink droplet receiving part  61 , the shape of the piezo film  62  is adjusted. Also, the output signal output from the piezo film  62  is adjusted so that the piezo vibration signal Sd to be output from the piezo film  62  is constant within the range of the preset initial value (Step S 3 ). 
   After the initial setting of the output signal from the piezo film  62 , nozzles for judging nozzle clogging are set (Step S 4 ). 
   The number of ink droplet jetting operations (n) is set by an operating portion or the like, and the sampling number (m) is calculated based on the sampling time period Ts and the sampling clock signal Sc (Step S 5 ). 
   In the storing unit  140 , addresses are set based on the number of ink droplet jetting operations (n) and the sampling number (m), and the setting of the memory map for storing detected data Sd′ is performed (Step S 6 ). 
   One is added to the jet count number N of the ink jet counter (adding one to a reference upper address), and the ink jet signal S m1  is output to the set nozzles to jet ink droplets (Step S 7 ). 
   After the lapse of the delay time t d , the sampling start signal Ss is output (the sampling start signal Ss is in a “LOW” state), and the sampling clock signal Sc starts to be output (Step S 8 ). 
   The address counter  241  counts a clock number of the sampling clock signal Sc (Step S 9 ). 
   A maximum voltage value in the piezo vibration signal Sd is detected for each clock of the sampling clock signal Sc, and is subjected to A/D conversion. Thereafter, the piezo vibration signal Sd (that is, a detected data Sd′ for each clock) is stored in an appropriate address based on the jet count number N and the address count number (M) with reference to the addresses in the memory map  141 . 
   A judgment is made whether the value of the reference lower address is equal to the sampling number (m) (that is, the address count number (M) is equal to the sampling number (m)), and the sampling start signal Ss is in the “HIGH” state (Step S 10 ). If these conditions are not satisfied (Step S 10 ; No), the operation is returned to Step S 9 . 
   When the address count number (M) is equal to the sampling number (m), and the sampling start signal Ss is in the “HIGH” state (Step S 10 ; Yes), the address counter  241  is cleared (that is, the reference lower address is set to “0”) (Step S 11 ). 
   A judgment is made whether the reference upper address is equal to the number of ink droplet jetting operations (n) (that is, the jet count number N is equal to the number of ink droplet jetting operations (n)) (Step S 12 ). When the reference upper address is not equal to the number of ink droplet jetting operations (n) (Step S 12 ; No), the operation is returned to Step S 7 . 
   When the reference upper address is equal to the number of ink droplet jetting operations (n) (Step S 12 ; Yes), the ink droplet jetting operation is finished (Step S 13 ). 
   The addresses in each of which the maximum voltage value V max  for each ink droplet jetting operation is stored are read out from the memory map  141  (Step S 14 ). 
   Preferably, there is one lower address in the addresses in each of which the maximum voltage value for each ink droplet jetting operation is stored, however, due to the relationship between the cycle of the sampling clock signal Sc and that of the piezo vibration signal Sd, there may be a case where a plurality of lower addresses storing the maximum voltage value with the same voltage consecutively exists. 
   In the embodiment, a judgment is made whether lower addresses in the addresses read out for the ink droplet jetting operations are either the same or three consecutive numbers. Hereinafter, it is defined as “judging whether the lower addresses are within ±1”. 
   When the lower addresses are within ±1 (that is, one lower address having the same maximum voltage value exists, or three or less consecutive lower addresses have the same maximum voltage value), it can be judged that the maximum voltage value exists in one lower address, or the same maximum voltage value exist in three or less consecutive lower addresses. Thus, a judgment can be made that there is no nozzle clogging. When the lower addresses are not within ±1, it can be judged that there is no address indicating the maximum voltage value, or the same maximum voltage value exists in four or more consecutive lower addresses. Thus, a judgment can be made that there was no vibration or impact by ink droplets, thereby it can be judged that nozzle clogging exists. 
   In the first embodiment, explanation is made to the case in which the lower addresses need to be within ±1, however, it may be within ±2 to 4, that is, it is preferable to set the address number with which a judgment can be made that there is no nozzle clogging based on the address number which would exists considering the relationship between the cycle of the sampling clock signal and that of the piezo vibration signal Sd. 
   A judgment is made whether the lower addresses in the addresses read out for all ink droplet jetting operations are within ±1 (Step S 15 ). 
   When not all the lower addresses in the addresses read out for all ink droplet jetting operations are not within ±1 (Step S 15 ; No), a judgment is made whether the lower addresses in the addresses read out in the last half ink droplet jetting operations are within ±1 (Step S 16 ). When not all the lower addresses in the addresses read out in the last half ink droplet jetting operations are not within ±1 (Step S 16 ; No), the ink droplet jetting operations are judged to be abnormal, therefore, a judgment is made that there is nozzle clogging (Step S 21 ). 
   When all the lower addresses in the addresses read out in all ink droplet jetting operations or in the last half ink droplet jetting operations are within ±1 (Step S 15 ; Yes, Step S 16 ; Yes), the maximum voltage values V max  written in the addresses read out in all ink droplet jetting operations and the standard voltage value V 0  stored in the ROM  120  are read out (Step S 17 ). 
   In the judgmental step (S 18 ) as a judgmental section, a judgment is made whether each maximum voltage value V max  of the ink droplet jetting operations which was read out is not less than the standard voltage value V 0 . 
   When each maximum voltage value V max  is not less than the standard voltage value V 0  (Step S 18 ; Yes), the ink droplet jetting operations are judged to be normal, therefore, a judgment is made that there is no nozzle clogging (Step S 19 ). 
   When not all the maximum voltage values V max  are not less than the standard voltage value V 0  (Step S 18 ; No), a judgment is made whether each maximum voltage value V max  in the last half of the ink droplet jetting operations is not less than the standard voltage value V 0  (Step S 20 ). 
   When each maximum voltage value V max  in the last half of the ink droplet jetting operations is not less than the standard voltage value V 0  (Step S 20 ; Yes), the ink droplet jetting operations are judged to be normal, and a judgment is made that there is no nozzle clogging (Step S 19 ). 
   When not all the maximum voltage values V max  in the last half of the ink droplet jetting operations are not less than the standard voltage value V 0  (Step S 20 ; No), the ink droplet jetting operations are judged to be abnormal, and a judgment is made that nozzle clogging exists (Step S 21 ). 
   When the judgment was made that nozzle clogging exists, the maintenance operation for solving the nozzle clogging (for example, suction operation or the like) is performed to the head module having the nozzles which need the maintenance (Step S 22 ). 
   Ink droplets are jetted from the nozzles predetermined times, reading out an address of a detected data Sd′ indicating the maximum voltage value V max  for each ink droplet jetting operation, performing a judgment of nozzle clogging based on the address number which was read out, and further comparing each maximum voltage value V max  shown by the detected data Sd′ corresponding to each address which was read out with the standard voltage value V 0 . When not all the maximum voltage values V max  are not less than the standard voltage value V 0 , a judgment is made that ink droplets are not jetted from the set nozzles (that is, nozzle clogging exists). Further, due to the structural feature of the ink jet head, there is a case that ink jetting is not performed for the initial stage of the ink droplet jetting operations, and is recovered after repeating ink droplet jetting operation a few times, so that ink droplets are jetted in the last half ink droplet detection operations to detect the maximum voltage values V max . Even in such the case, a judgment can be made that there is no nozzle clogging, thereby enabling to properly perform the maintenance operation only in the case when the maintenance operation is needed. 
   Accordingly, the landing of in droplets can be detected by using a vibration detection portion (for example, piezo film) with high accuracy, and nozzle clogging can be detected with easy structure. This results in a decrease of the cost for the apparatus. 
   [Second Embodiment] 
   The second embodiment will be explained referring to the drawings. 
   The configuration will be explained first. 
   The schematic configuration of the inside of an ink jet printer, an end surface of the nozzle clogging detection part  60 , and the cross section of a contact state of the contact surface S 2  of the cover  61   a  and the piezo film  62  in the second embodiment are the same as those in the first embodiment, therefore the explanations are omitted here and they are not shown in the drawings. 
     FIG. 11  shows a control block diagram for controlling the ink jet printer of the second embodiment. 
   In the control block diagram of the second embodiment, the component that is same as in the first embodiment will be given the same reference numeral and the detailed explanations thereof will be omitted, thus only the component which is different will be explained. 
   In the control unit  300 , a CPU  310  as a control section and a judging section, a ROM  320 , a RAM  330 , a storing unit  140  as a storing section, an I/O  150 , a various machines control unit  160 , an I/F  170 , a drive circuit  180  and the like are connected to a system bus  190 , and the control unit  300  is connected to the nozzle clogging detection circuit  200  through the I/F  170 . 
   The CPU  310  reads out a system program, or various processing programs and data stored in the ROM  320 , expanding them in the RAM  330 , and performs a central control of operations of the whole ink jet printer according to the programs expanded. That is, the CPU  310  performs a timing control of the whole system, storing and accumulation controls of data with the use of the RAM  330 , an output of print data to each head module, an input-output control of an operating portion which is not shown, an interface (I/F) to other applications, or an operation control. 
   In the ink jet printer, after removing the capping, or in a state of not performing a print recording for a while, the ink viscosity increases due to the evaporation of the moisture in ink or the like, so that it is difficult to perform the ink droplet jetting operation. Especially, in water-based pigmented ink, it is more likely to occur, and there is a case that the ink droplet jetting operation is not performed even when the jet drive signal is given. However, the ink jetting droplet operation may be recovered after repeating the ink droplet jetting operation a few times (corresponding to the flashing operation in the maintenance operation). 
   The ink jet printer causing such phenomenon may cause the same phenomenon in the ink droplet detection operation. Thus, in the present invention, in a case of performing the ink droplet detection operation after performing the ink droplet jetting operation a few times, when the ink droplet jetting operation is performed the predetermined number of times (n times), the ink droplet detection data of the first half ink droplet jetting operations is not used as the data to judge nozzle clogging, and the judgment of the nozzle clogging is performed based on the ink droplet detection data of the last half ink droplet jetting operations (from n/2 times). 
   The dependence of the ink droplet speed on the cycle of the jet drive signal S m0  is same as that in the first embodiment. That is, as the cycle of the jet drive signal S m0 , the shortest drive timing of the second ink droplet to have a speed close to the ink droplet speed of the first ink droplet jetting operation (5.5 [m/s]) is five times of the standard drive waveform time AL, followed by seven times and then by nine times thereof. Thus, when detecting an ink droplet by jetting two or more ink droplets continuously, it is preferable to set the cycle of the jet drive signal S m0  by multiplying the standard drive waveform time AL by seven or nine which is an odd number. Accordingly, the cycle of the jet drive signal S m0  is preferably set by multiplying the standard drive waveform time AL by an odd number not less than five, more preferably by an integral number which is one of 5, 7, 9, 11, 13 and 15. The detailed explanation and the drawings thereof are omitted here. 
   Accordingly, to realize the second embodiment, the CPU  310  calculates the jet drive signal S m0  as an instruction signal to continuously jet ink droplets n times with a jet drive cycle which is set by multiplying the standard drive waveform time of the ink jet signal by an odd number not less than 5, and outputs the calculated jet drive signal S m0  to the drive circuit  180 . The CPU  310  reads out an address of a detected data Sd′ as an amplitude value data showing a maximum voltage value V max  as a maximum amplitude value in each ink droplet jetting operation in the last half ink droplet jetting operations (from n/2 times) from the memory map  141  to be described later which is stored in the storing unit  140 , and performs the judgment of the nozzle clogging based on the address number (number of addresses) which was read out. Further, the nozzle clogging judging operation is performed by comparing the maximum voltage value V max  which is shown by the detected data Sd′ corresponding to the address which was read out and the standard voltage value V 0  as a standard value. 
   When the control unit  310  judges that nozzle clogging exists, the maintenance part  50  is controlled to drive to solve nozzle clogging. 
   The ROM  320  stores a program or a system program for driving the ink jet printer, various programs corresponding to the system, data necessary for processing with the various processing programs and the like. 
   To realize the second embodiment, the ROM  320  stores the standard voltage value V 0  which is the maximum voltage value based on the sampling detected signal Sd′ when ink droplets are properly jetted onto the ink droplet landing surface S 1 , a delay time t d  to be described later and a sampling period Ts. 
   The RAM  330  is a temporally storing region for programs, input or output data, parameters read out from the ROM  320  in various processing controlled and executed by the CPU  310 . 
   The RAM  330  also temporally stores addresses read out from the storing unit  140 , a sampling number (m) (number of samplings) calculated by a sampling clock generation unit  260  to be described later and the number of ink droplet jetting operations (n) which is set by an operating portion or the like which is not shown, and comprises an ink jet counter for counting the number of ink droplet jetting operations (jet count number N). 
   An example of a time chart of an ink droplet jetting operation from the nozzles is approximately the same as that in the first embodiment, thus, the drawings and the explanation thereof will be omitted here. 
   Next, description will be made for the nozzle clogging judging operation performed by the control unit  300 . 
     FIGS. 12 to 14  show flow charts of the nozzle clogging judging operation of the second embodiment. 
   Steps S 31  to S 43  are same as Steps S 1  to S 13  in the first embodiment, therefore the explanations thereof are omitted. 
   After finishing the ink droplet jetting operation (after Step S 43 ), the addresses in each of which the maximum voltage value V max  for each ink droplet jetting operation in the last half ink droplet jetting operations (from n/2 times) is stored are read out from the memory map  141  (Step S 44 ). 
   The explanation of the address number in each of which the maximum voltage value for each ink droplet jetting operation is stored is same as that in the first embodiment, thus, the explanation thereof will be omitted here. 
   A judgment is made whether lower addresses in the addresses read out for the ink droplet jetting operations in the last half ink droplet jetting operations (from n/2 times) are within ±1 (Step S 45 ). 
   When not all the lower addresses in the addresses read out for all ink droplet jetting operations in the last half ink droplet jetting operations (from n/2 times) are not within ±1 (Step S 45 ; No), the ink droplet jetting operations are judged to be abnormal, therefore, a judgment is made that there is nozzle clogging (Step S 49 ). 
   When all the lower addresses in the addresses read out for all ink droplet jetting operations in the last half ink droplet jetting operations (from n/2 times) are within ±1 (Step S 45 ; Yes), the maximum voltage values V max  written in the addresses read out in all ink droplet jetting operations in the last half ink jetting operations (from n/2 times) and the standard voltage value V 0  stored in the ROM  320  are read out (Step S 46 ). 
   In the judgmental step (S 47 ) as a judgmental section, a judgment is made whether each maximum voltage value V max  of the ink droplet jetting operations which was read out is not less than the standard voltage value V 0 . 
   When each maximum voltage value V max  is not less than the standard voltage value V 0  (Step S 47 ; Yes), the ink droplet jetting operations are judged to be normal, therefore, a judgment is made that there is no nozzle clogging (Step S 48 ). 
   When not all the maximum voltage values V max  are not less than the standard voltage value V 0  (Step S 47 ; No), the ink droplet jetting operations are judged to be abnormal, and a judgment is made that nozzle clogging exists (Step S 49 ). 
   When the judgment was made that nozzle clogging exists, the maintenance operation for solving the nozzle clogging (for example, suction operation or the like) is performed to the head module having the nozzles which need the maintenance (Step S 50 ). 
   Ink droplets are jetted from the nozzles predetermined times (n times), reading out an address of a detected data Sd′ indicating the maximum voltage value V max  for each ink droplet jetting operation in the last half ink jetting operations (from n/2 times), performing a judgment of nozzle clogging based on the address number which was read out, and further comparing each maximum voltage value V max  shown by the detected data Sd′ corresponding to each address which was read out with the standard voltage value V 0 . When not all the maximum voltage values V max  are not less than the standard voltage value V 0 , a judgment is made that ink droplets are not jetted from the set nozzles (that is, nozzle clogging exists). Further, due to the structural feature of the ink jet head, there is a case that ink jetting is not performed for the initial stage of the ink droplet jetting operations, and is recovered after repeating ink droplet jetting operation a few times, so that ink droplets are jetted in the last half ink droplet detection operations to detect the maximum voltage values V max . Even in such the case, a judgment can be made that there is no nozzle clogging, thereby enabling to improve ink droplet landing accuracy and the detection speed, and to properly perform the maintenance operation only in the case when the maintenance operation is needed. 
   Accordingly, the landing of in droplets can be detected by using a vibration detection portion (for example, piezo film) with high accuracy, and nozzle clogging can be detected with easy structure. This results in a decrease of the cost for the apparatus. 
   The entire disclosure of Japanese Patent Application Nos. Tokugan 2004-119017 which was filed on Apr. 14, 2004, and Tokugan 2004-313732 which was filed on Oct. 28, 2004, including specification, claims, drawings and summary are incorporated herein by reference in its entirety.