Patent Publication Number: US-6334658-B1

Title: Ink-jet printer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an ink-jet printer for jetting ink on a recorded medium from an ink-jet head to form an image, and in particular, relates to an ink-jet printer which can detect that an ink jetting failure has occurred in the ink-jet head. 
     2. Description of Prior Art 
     Heretofore, a conventional ink-jet printer cannot automatically detect an ink jetting failure, such that when a user finds the omission (a so-called defect) of a printed image formed on a recorded medium, the user must determine that an ink jetting failure has occurred. When an ink jetting failure occurs, a user cleans an ink-jet head with a swab, etc., to solve the ink jetting failure. An ink-jet printer provided with a maintenance device for cleaning an ink jetted path in an ink-jet head has been recently developed. For the maintenance device, a purging device in which ink in the above ink jetted path is forcedly discharged together with bubbles and dust, and the nozzle face is wiped to remove the ink and dust around the nozzle, etc., has been devised. When an ink jetting failure occurs, the above ink-jet head failure can be solved by cleaning the above ink jetted path using the above maintenance device. 
     However, as a conventional type ink-jet printer cannot detect an ink jetting failure until a user finds the omission on a printed image formed on a recorded medium, a large quantity of printing error may be caused. A plan that when an ink jetting failure may occur, the above ink jetted path is cleaned by the maintenance device at a predetermined time, has also been devised. However, in this case, maintenance is required to be frequently executed sufficiently in advance. During maintenance, the printing operation is required to be interrupted and during purging and the like, ink is consumed. Therefore, in this case, valuable ink is consumed by frequent maintenance and the time required for printing is also extended. 
     SUMMARY OF THE INVENTION 
     The invention provides an ink-jet printer for enabling precisely and automatically detecting an ink jetting failure in an ink-jet head. In particular, the invention automatically solves an ink jetting failure when the ink jetting failure occurs, by automatically supplementing the ink supply when the ink supply decreases, and reducing the load related to processing. 
     A printer according to the invention is based upon an ink-jet printer provided with an ink tank for storing ink, an ink-jet head for jetting ink in the ink tank onto a recorded medium according to a driving signal, a detector for detecting the consumed quantity of ink in the ink tank, a calculation unit for calculating the consumed quantity of ink in the ink tank according to a driving signal, and a determination unit for comparing the consumed quantity of ink calculated by the calculation unit and that detected by the detector and determining that an ink jetting failure has occurred in the ink-jet head if difference between both exceeds predetermined quantity. 
     In the invention as described above, an ink-jet head jets ink stored in an ink tank onto a recorded medium according to a driving signal. The detector detects the consumed quantity of ink in the ink tank. As an ink-jet head jets ink according to a driving signal, the consumed quantity of ink in the ink tank normally has a relation to the driving signal. The calculation unit calculates the consumed quantity of ink in the ink tank according to the driving signal. The determination unit compares the consumed quantity of ink calculated by the calculation unit and that detected by the detector and determines that an ink jetting failure has occurred in the ink-jet head if difference between both exceeds a predetermined quantity. That is, as ink is not jetted according to a driving signal when an ink jetting failure occurs, the difference between the consumed quantity of each ink calculated and detected is determined. That is, if an ink jetting failure continues for a fixed period, the counted value of the consumed quantity gradually comes off. When such a situation occurs, the determination unit determines that an ink jetting failure has occurred. 
     Therefore, according to the invention, an ink jetting failure in an ink-jet head can be precisely and automatically detected. According to the detection, a warning can be given and printing can be also halted. Therefore, even if a user fails to find the ink jetting failure, a large quantity of printing error can be satisfactorily prevented from occurring. 
     If maintenance is executed as necessary based upon the result of the determination by the determination unit according to the invention, the frequency of maintenance to be executed is appropriately set, and ink and printing time can be satisfactorily prevented from being wasted. As the time to execute maintenance according to the result of the determination, the time of maintenance may be also set and stored as the time when an ink jetting failure occurs, a total quantity of ink is jetted, a total time that an inkjet head is driven, and the like, before an ink jetting failure occurs since the last maintenance. The time of maintenance may be also set to a time immediately before the jetted quantity and the driven time occurs. In the case of the latter, the occurrence of an ink jetting failure can be prevented beforehand and the occurrence of a printing error can be more satisfactorily inhibited. 
     Further, the invention is characterized in that a maintenance unit for cleaning an ink jetted path in an ink-jet head and a maintenance controller for cleaning the ink jetted path by the maintenance unit when the above determination unit determines that an ink jetting failure occurs, are further provided. 
     According to the invention, when the determination unit determines that an ink jetting failure occurs, the maintenance controller instructs the maintenance unit to clean an ink jetted path in an ink-jet head. Therefore, according to the invention, when an ink jetting failure occurs, it can be automatically solved. Also, according to the invention, as maintenance (cleaning an ink jetted path) is executed when an ink jetting failure occurs, the frequency in which maintenance is executed can be minimized. Therefore, ink and printing time can be prevented from being wasted. 
     Further, the invention is characterized in that the above detector can detect when ink in an ink tank decreases and ink is required to be supplemented. An ink supply unit for supplying a predetermined quantity of ink in an ink tank and supply controller for instructing the above ink supply unit to supply ink when the detector detects that ink is required to be supplemented, may be further provided. When the detector detects that ink is required to be supplemented, the supply controller supplies a predetermined quantity of ink to an ink tank via the ink supply unit. Therefore, according to the invention, ink can be automatically supplemented when the ink decreases. 
     The ink supply unit according to the invention supplies a predetermined quantity of ink to an ink tank. Therefore, if the detector detects that ink is required to be supplemented, the ink supply unit supplies ink and afterward, the detector detects that ink is required to be supplemented again, the consumed quantity of ink is the above predetermined quantity. Therefore, the detector according to the invention can detect the consumed quantity of ink even if the detector only detects that ink is required to be supplemented. Therefore, in this case, the structure of the ink-jet printer can be extremely simplified and the manufacturing cost can be reduced satisfactorily. 
     Further, the invention is characterized in that the detector can detect that ink in an ink tank decreases up to a first reference value and ink in the ink tank decreases up to a second reference value different from the first reference value. Therefore, when the quantity of ink in an ink tank is between both reference values, the consumed quantity of ink can be readily detected. In this case, the consumed quantity of ink in a short period can be detected, compared with a case that the detector only detects that ink is required to be supplemented. Therefore, the consumed quantity of ink calculated by the calculation unit may also correspond to a short period. Therefore, according to the invention, the load related to calculation processing by the calculation unit can be reduced. In the invention, the supplied quantity of ink may be arbitrary. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail with reference to the following figures wherein: 
     FIG. 1 shows an ink-jet printer to which the invention is applied; 
     FIG. 2 is an exploded perspective view showing the head of the printer; 
     FIG. 3 is a top view showing the ink tank of the head; 
     FIGS.  4 (A) and  4 (B) are sectional views viewed along lines B—B and C—C in FIG. 3 showing the ink tank; 
     FIG. 5 shows the purging operation of the head; 
     FIG. 6 is a side view and a top view showing the ink feeder of the printer; 
     FIGS.  7 (A) and  7 (B) are perspective drawings showing the vicinity of the ink outlet of the ink feeder; 
     FIG. 8 shows the control board of the printer; 
     FIG. 9 is a flowchart showing processing executed by the CPU on the control board; 
     FIG. 10 shows the control board in a second embodiment; 
     FIG. 11 is a flowchart showing processing executed by the CPU on the control board; and 
     FIG. 12 is a flowchart showing processing executed by the CPU on the control board in a third embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, referring to the drawings, embodiments of the invention will be described. FIG. 1 shows an example of an ink-jet printer to which the present invention is applied. An ink-jet printer in this embodiment is a so-called hot melt ink-jet printer for melting solid ink thermically and jetting it. 
     As shown in FIG. 1, a head (an ink-jet head)  1  of an ink-jet printer in this embodiment is mounted on a carriage  3  (shown in FIG. 6) so that the carriage can be moved along a guide shaft  5 . The head  1  jets ink onto a recorded medium carried in the vicinity of the left side and moved into a printing range, and is moved to a maintenance area in the vicinity of the right end of the guide shaft  5  before or after printing or at a predetermined time when a maintenance operation described below is applied to the head. The carriage  3  is connected to a carriage motor  8  via a belt  9  and is a well-known type moved along the guide shaft  5  as the carriage motor  8  is rotated. 
     The top face of a frame  100  for supporting the head  1  via the guide shaft  5 , functions as a guide face  101  for guiding recording paper. Feed rollers  103  and  105  are arranged on the guide face  101 . Pulleys  107  and  109  are respectively fixed coaxially to each feed roller  103  and  105  and the driving force of a feed motor  111  is transmitted to the pulleys  107  and  109  via a belt  113 . Therefore, recording paper can be carried in a direction perpendicular to the guide shaft  5  along the guide face  101 . A rubber pad  115  is arranged in a position opposite to the nozzle face  36  (shown in FIG. 4) of the head  1  in the above maintenance area so that the rubber pad can be pressed or separated on/from the nozzle face  36  by a pad driving motor  117  (shown in FIG.  8 ). Further, a pump  120  for executing purging described below is arranged outside the frame  100  on the side of the maintenance area with the carriage motor  8  and the feed motor  111 . 
     Next, referring to FIGS. 2-4, the structure of the head  1  will be described. FIG. 2 is an exploded perspective view showing the head  1 , FIG. 3 is a top view showing an ink tank  10  of the head  1  and FIGS.  4 (A) and  4 (B) are sectional views viewed along lines B—B and C—C in FIG.  3 . For the structure of the head  1 , refer to Japanese published unexamined patent application No. Hei 10-146959. 
     As shown in FIG. 2, the head  1  is provided with the ink tank  10 , a front panel  30 , a melting tank  40 , a cam  50  and a control panel stage  70 . The ink tank  10  is provided with a tilted front part  15  for attaching the front panel  30 , four sets of main chambers  11  and deputy chambers  13  for color output (black, cyan, magenta and yellow) for storing hot melt ink (hereinafter, it may be called only ink), an ink tank upper cover  19  and an ink tank heater  17  attached on the rear surface of the ink tank  10  and further, as shown in FIG.  4 (B), a communicating passage  21  open downward is provided on the rear side of the bottom of each main chamber  11  and each deputy chamber  13  of the ink tank  10 . 
     The main chamber  11  is in the shape of a letter L (as shown in FIG. 2) when viewed from the top and is provided with a main chamber inlet  21 A (shown in FIG.  4 (B)) leading to the communicating passage  21 , a main chamber outlet  22 A (also shown in FIG.  4 (A)) leading to the front panel  30  and a filter  29  (shown in FIG.  4 (A)). The filter  29  is produced by pressing after fibers made of stainless steel are sintered and is changed to a shape of paper, each fiber is complicatedly bent and overlapped and is provided with a passage with a three dimensional structure (for example, Tomy firec SS (trademark) manufactured by Tomoegawa Seishijo). 
     The deputy chamber  13  is provided with a deputy chamber outlet  21 B leading to the communicating passage  21 , a deputy chamber inlet  22 B leading to the front panel  30  and a valve opening/closing lever  24  approximately in the shape of a reverse T for opening the other when either the deputy chamber outlet  21 B or the deputy chamber inlet  22 B is closed, as shown in FIGS.  2  and  4 (B). 
     The valve opening/closing lever  24  is cast in an aluminum alloy using a die and is attached with a lever pedestal  25  provided between the deputy chamber outlet  21 B and the deputy chamber inlet  22 B as a supporting point in a state in which the valve opening/closing lever can be swung, as shown in FIG.  4 (B). The valve opening/closing lever  24  is provided with pressure valves  27  and  28  and is normally kept in a state in which the pressure valve  28  seals the deputy chamber inlet  22 B by being pressed by a plate spring  26 . The pressure face of the pressure valve  27  is spherical, the edge of the deputy chamber outlet  21 B corresponding to the pressure face is tapered. The pressure face of the pressure valve  28  is flat and the edge of the deputy chamber inlet  22 B corresponding to it is circularly protruded. The pressure valves  27  and  28  are made of silicone rubber, with a Shore hardness of approximately 40° and a heat resistance temperature of approximately 200° C. 
     As shown in FIG. 2, the ink tank upper cover  19  is provided with a front panel cover  19 A fitted to the shape of the front panel  30 , a deputy chamber cover  19 B for covering the deputy chamber  13 , a long hole  19 C for exposing the upper end  24 A of the valve opening/closing lever  24 , an ink input port  19 D for supplying hot melt ink from the melting tank  40  to the deputy chamber  13 , an air chamber  20  for delivering compressed air to each main chamber  11  from the pump  120  shown in FIG. 1, a through hole  20 B open to the side of the ink tank  10  from the air chamber  20  and an air chamber cover  20 A for sealing the air chamber  20 . The air chamber  20  of the ink tank upper cover  19  is provided with a through hole  23  leading to the main chamber  11  as shown in FIG.  4 (A). 
     As shown in FIG. 2, the front panel  30  is provided with four nozzle heads  31  on the front surface and on the rear surface, an approach route  35  shown in FIGS.  4 (A) and  5  leading from each main chamber  11  to each nozzle head  31  and a return route  37  shown in FIGS.  4 (B) and  5  leading from each nozzle head  31  to each deputy chamber  13 , are formed. Further, as shown in FIGS. 2 and 4, a cover panel  30 A is attached to the rear surface of the front panel  30  so that the cover panel  30 A covers the approach route  35  and the return route  37  and a front panel heater  33  is attached to the rear surface of the cover panel  30 A. As shown in FIGS.  4 (A) and  4 (B), an approach route inlet  35 A from each main chamber  11  to each approach route  35 , an approach route outlet  35 B from each approach route  35  to each nozzle head  31 , a return route inlet  37 B from each nozzle head  31  to each return route  37  and a return route outlet  37 A from each return route  37  to each deputy chamber  13 , are respectively provided. 
     The nozzle head  31  is provided with a piezoelectric-crystal element  38  and jets ink supplied via the approach route outlet  35 B according to the change of the volume of the piezoelectric-crystal element  38 . Ink supplied to the nozzle head  31  can be circulated to the deputy chamber  13  via the return route inlet  37 B and the return route  37 . That is, as shown by an arrow in FIG. 5, ink reaches a nozzle  32  via the approach route  35 , the approach route outlet  35 B and a lower branch point  31 A and further, can be circulated to the deputy chamber  13  via an upper branch point  31 B, the return route inlet  37 B and the return route  37 . For the nozzle  32 , 128 minute jets are arranged in two rows by 64 pieces and ink is jetted onto recording paper by pressurizing ink according to the minute change of the volume of the piezoelectric-crystal element  38 . 
     The cam  50  is attached on the ink tank upper cover  19  in the right and left directions in FIG. 3 in a state in which the cam can be slid and is protruded right from the ink tank upper cover  19  in the vicinity of a contact face  50 A. The cam  50  is provided with four cam faces  50 B and a state in which the cam face  50 B does not touch the upper end  24 A of the valve opening/closing lever  24 , is normally kept by being pressed by a spring  51  provided between a projection  52  provided at the left end of the cam  50  and a projection  19 E provided to the ink tank upper cover  19 . 
     As shown in FIG. 2, the melting tank  40  is divided into four chambers for the colors of black, cyan, magenta and yellow. Each chamber is in the shape of a box the upper part of which is open so that an ink pellet  220  (shown in FIG. 7) as solid ink can be thrown into each chamber. As shown in FIG.  4 (B), a passage  47  for leading melted ink to the deputy chamber  13  is formed in the lower part of each chamber of the melting tank  40 . 
     The ink pellet  220  is supplied to the melting tank  40  from an ink feeder  200 , as shown in FIG.  6 . The melting tank  40  is provided with a heater, the ink pellet  220  is melted by the heater and supplied to the deputy chamber  13  of the ink tank  10  via the passage  47 . Further, the control panel stage  70  is provided with a control board and attached to the upper part of the head  1 . 
     In the head  1  as described above, if the piezoelectric-crystal element  38  is driven according to print data, and the like, as described above after the ink pellet  220  is kept in a melted state by driving various heaters  17 ,  33 , and the like, ink can be jetted. When the head  1  is moved to the above maintenance area, the purging can be executed as follows. 
     Purging operation for pressurizing ink in the front panel  30  and the nozzle head  31  from the side of the main chamber  11 , involves delivering bubbles and dust in ink which respectively cause a failure of jetting in the nozzle  32 , outside from the nozzle face  36 . The bubbles and dust in ink in the front panel  30  are then delivered to the deputy chamber  13  together with ink and the respective nozzle and the front panel is filled with clean ink filtered by the filter  29 . Bubbles are mixed in ink when the temperature of the head  1  lowers and ink is solidified after a power source is turned off and ink is melted after the power source is turned on again. Dust may enter from the nozzle  32 . 
     When the head  1  is moved to the maintenance area, the contact face  50 A of the cam  50  is pressed upon the frame  100 , as shown in FIG. 3 and a hollow cylindrical cap  55  provided to the frame  100  covers the through hole  20 B. The cam  50  is slid relatively left on the ink tank upper cover  19  and the cam face  50 B presses the upper end  24 A of the valve opening/closing lever  24  and the cam face  50 B presses and moves the upper end  24 A of the valve opening/closing lever  24  downward in FIG.  3 . Therefore, the valve opening/closing lever  24  is swung based upon the lever pedestal  25  as a supporting point, contact between the pressure valve  28  and the deputy chamber inlet  22 B is released, when the valve opening/closing lever is further swung, the pressure valve  27  and the deputy chamber outlet  21 B come in contact, the deputy chamber inlet  22 B is released and the deputy chamber outlet  21 B is sealed. 
     At this time, if compressed air is delivered from the pump  120  (shown in FIG. 1) via a pipe  57  connected to a hollow part of the cap  55  because the cap  55  covers the through hole  20 B, bubbles can be pushed out as follows: That is, atmospheric pressure in the main chamber  11  is increased by delivering compressed air. As the deputy chamber outlet  21 B is sealed and the deputy chamber inlet  22 B is released, bubbles and dust in the ink are filtered by the filter  29  via the main chamber  11  and afterward, ink reaches the nozzle head  31  via the main chamber outlet  22 A, the approach route inlet  35 A, the approach route  35  and the approach route outlet  35 B. Next, the above ink is divided into a path in which the ink is discharged outside (jetted) from the nozzle  32  and a path in which the ink is directed on the side of the return route inlet  37 B. If the nozzle face  36  is open, the respective flow rates are determined according to the passage resistance of the approach route  35 , the return route  37  and the nozzle  32 . Ink which flows in the path on the side of the return route inlet  37 B is carried to the deputy chamber  13  via the return route  37 , the return route outlet  37 A and the deputy chamber inlet  22 B. Hereby, ink including bubbles in the approach route  35 , the nozzle  32  and the return route  37 , is replaced with clean ink. 
     Afterward, when the head  1  is moved left and the contact face  50 A is separated from the frame  100 , the upper end  24 A of the valve opening/closing lever  24  is not pressed by the cam face  50 B. The valve opening/closing lever  24  is swung based upon the lever pedestal  25  as a supporting point by pressure by the plate spring  26 , the deputy chamber inlet  22 B is sealed and the deputy chamber outlet  21 B is released. Hereby, ink forcedly carried into the deputy chamber  13  in purging, is returned to the main chamber  11  via the communicating passage  21  and the respective levels of the main chamber  11  and the deputy chamber  13  can be equalized. 
     The head  1  in this embodiment is designed so that the passage resistance of all the nozzles is smaller than the passage resistance of the return route  37 . For a method of setting passage resistance, the higher the temperature of the ink, the more flowability is enhanced. The passage resistance of the nozzle  32  and the return route  37  can be set so that the passage resistance of the nozzle  32  is low and the passage resistance of the return route  37  is high by adjusting so that the temperature of ink in the nozzle is high and the temperature of ink in the return route  37  is low. The passage resistance may be also set based upon the cross-sectional area of a passage and the shape of a passage. Therefore, if the nozzle face  36  is open, most of ink carried to the approach route  35  is discharged outside from the nozzle  32 . If the rubber pad  115  is pressed on the nozzle face  36 , the aperture of the nozzle  32  is sealed to prevent ink from being discharged from the nozzle  32 . In this case, most of ink carried to the approach route  35  is circulated via the return route  37 . Therefore, if purging is executed twice by switching the pressure/release of the rubber pad  115 , ink including bubbles in the approach route  35 , the nozzle  32  and the return route  37 , can be effectively replaced with clean ink. 
     Next, the structure of the above mentioned ink feeder  200  and the control board of the control panel stage  70  will be described. FIG. 6 is a side view and a top view showing the structure of the ink feeder  200 . As shown in FIG. 6, the ink feeder  200  is provided with a body  221  and a transparent cover  222  provided to the body  221  so that the transparent cover  222  can be opened or closed. A housing groove part  223  for housing the ink pellet  220 , is formed in the body  221 . The housing groove parts  223  are arranged corresponding to each color of black, cyan, magenta and yellow in parallel and the ink pellet  220  of each color housed there is pressed in the direction of the ejection part  223 A by a pressing member (not shown), such as a spring. 
     As shown in FIG.  7 (A), the ejection part  223 A is open between the upper surface and the lower surface. A pellet supporting piece  235  one end of which is fixed to the bottom of the housing groove part  223 , is provided to the lower surface of the ejection part  223 A and the pellet supporting piece  235  is structured so that the ink pellet  220  moved to the ejection part  223 A is held by touching the pellet supporting piece to the lower surface of the ink pellet  220  and supporting the ink pellet  220 . The ink pellet  220  held in the ejection part  223 A is pressed downward by an ejecting mechanism  236  shown in FIG. 6, and ejected, bending the pellet supporting piece  235  as shown in FIG.  7 (B). 
     As shown in FIG. 6, the ejecting mechanism  236  is provided to each housing groove part  223  and is provided with a first turning member  237  for pressing the upper surface of the ink pellet  220  downward in turning. The first turning member  237  can be coupled to a second turning member  240  via a turning control mechanism  241  and the second turning member  240  is structured so that it is turned vertically (in a direction shown by an arrow) via a gear mechanism (not shown) by the feed motor  111  shown in FIG.  1 . 
     The turning control mechanism  241  is provided with a first coupling member  238  one end of which is coupled to the first turning member  237  so that the first turning member can be turned, a fitting member  239  provided on the side of a free end of the first coupling member  238  and a key member  242  which can be fitted to the fitting member  239 . The side of the free end of the key member  242  is formed in the shape of a key so that it is fitted to the fitting member  239  and the end of the key is designed so that the key does not come in contact with the fitting member  239  only by moving the key member  242  vertically. 
     In the meantime, a pinion member  244  is provided to the side of the fixed end of the key member  242  so that the pinion member can be rotated and the pinion member  244  supports the free end of the key member  242  so that the key member can be turned sideways. The pinion member  244  is engaged with a rack member  245 , and the pinion member  244  and the rack member  245  regulate a direction in which the key member  242  is moved to a vertical direction. The fixed end of the key member  242  is coupled to the free end of the second turning member  240  via the second coupling member  246 . 
     The above key member  242  is provided to a supply selecting member  243  so that the key member can be touched to the supply selecting member. The supply selecting member  243  is fixed to the side of the carriage  3  and when the supply selecting member comes in contact with the key member  242 , the supply selecting member moves the key member  242  sideways. Therefore, if the key member  242  does not come in contact with the supply selecting member  243  when the second turning member  240  is turned and the key member  242  is moved downward, the key member  242  and the fitting member  239  are not fitted and the first turning member  237  is not turned. In the meantime, as the key member  242  is turned, being moved sideways if the key member  242  comes in contact with the supply selecting member  243 , the key member  242  and the fitting member  239  are fitted and the first turning member  237  is also turned. 
     The supply selecting member  243  is provided corresponding to each color, however, the arrangement pitch is different from the arrangement pitch of the ejecting mechanism  236 . Therefore, if the carriage  3  is moved by controlling the carriage motor  8  and only the supply selecting member  243  corresponding to desired color comes in contact with the corresponding key member  242 , only the first turning member  237  corresponding to the desired color is turned. Hereby, only the ink pellet corresponding to the desired color can be supplied to the corresponding melting tank  40 . 
     FIG. 8 is an explanatory drawing showing the configuration of the control board of the control panel stage  70 . As shown in FIG. 8, the control board of the control panel stage  70  is structured by connecting CPU  71 , RAM  72 , ROM  73  and ASIC  74 , via a bus  75 . A head control unit  76  for driving the piezoelectric-crystal element  38  according to print data, and the like, the carriage motor  8 , the feed motor  111 , the pad driving motor  117 , a motor control unit  77  for driving various motors, such as the pump  120 , a heater control unit  78  for driving various heaters, such as the ink tank heater  17  and the front panel heater  33 , an interface  79  for inputting or outputting data to/from an external personal computer, an encoder  81  for detecting a state in which the carriage motor  8  and others are rotated, and the like, are connected to ASIC  74 . ASIC  74  comprises a well-known gate array and a drop counter  74 A for driving the piezoelectric-crystal element  38  and for counting the number of jetted ink droplets every color. 
     Liquid ink  220 L as a result of melting the ink pellet  220  is housed in the main chamber  11  and an ink sensor  300  for detecting that the ink  220 L decreases and is required to be supplemented, is arranged in the main chamber  11 . For the ink sensor  300 , a thermistor may be arranged in the ink  220 L and so that ink  220 L is detected based upon the ease with which heat is taken when the thermistor is heated or the ink  220 L may be detected based upon the state of conduction between a pair of electrodes arranged apart, to determine whether ink  220 L exists. The ink sensor  300  outputs a detection signal to the CPU  71  corresponding to whether ink  220 L decreases up to a quantity required to be supplemented (hereinafter called an ink supplement level). 
     Next, processing executed by CPU  71  of the control panel stage  70  when print data, and the like, are input via the interface  79 , will be described. FIG. 9 is a flowchart showing processing executed by the CPU  71 . CPU  71  executes the above processing for every color when print data and the like, are input. 
     When the processing is started, first, processing proceeds to S 1  (S represents a step) and the printing operation is executed, counting the number of jetted ink droplets by the drop counter  74 A. As the printing operation is a well-known process, it is not described in detail. Processing proceeds to S 3  every time the printing operation is executed and referring to a detection signal from the ink sensor  300 , it is determined whether ink  220 L decreases up to the ink supplement level. If ink does not decrease up to the ink supplement level (NO in S 3 ), the printing operation in S 1  is continued and if ink decreases (YES in S 3 ), processing proceeds to S 5 . The number of jetted ink droplets counted by the drop counter  74 A in S 1  is counted as an accumulated value since the ink pellet  220  is supplied by the ink feeder  200 . Therefore, if a power source is turned off while the process is in an executed loop in S 1  and S 3 , the number of jetted ink droplets is immediately stored before the power source is turned off in a backup RAM (not shown) and the like. Thus, when the power source is turned on again, the drop counter starts to count from the next stored value. 
     Various times, for proceeding from S 1  to S 3  is devised as follows, and suitable times can be selected according to characteristics, such as resolution required for a printer and printing speed. 
     {circle around (1)} Approximately in parallel with the printing operation, that is, during printing, the determination in S 3  is normally made. 
     {circle around (0)} When the carriage  3  is moved to the vicinity of the end of recording paper by the printing operation in S 1  (when one movement of the carriage is completed), the determination in S 3  is made. 
     {circle around (3)} When the printing operation for one page in S 1  is finished, the determination in S 3  is made. 
     {circle around (4)} When the printing operation in one paragraph across plural pages in S 1  is finished, the determination in S 3  is made. 
     However, in the cases shown in {circle around (3)} and {circle around (4)}, the position of the ink sensor  300  is required to be set so that all ink  220 L is not consumed until the next page or paragraph is finished. Therefore, even when ink  220 L decreases up to the ink supplement level, it is desirable that the quantity of ink for at least one page is secured. 
     The quantity of ink per ink droplet is normally set to an approximate fixed value (supposed the quantity of ink α). The value α also depends upon resolution, is approximately 10 to 150 pl (picoliter=10 −12  l), in the case of 300 dpi, the value is approximately 80 pl, and in the case of 600 dpi, the value is approximately 40 pl. The quantity of ink per droplet varies by approximately ±10%. If the whole one page (A4, for example) is printed at the rate of 300 dpi (80 pl/dot), ink  220 L equivalent to approximately 0.5 to 0.6 cc is consumed. As are equivalent to 10 to 20% is printed in the case of average text printing, the quantity of ink per page is approximately 0.05 to 0.12 cc per color. 
     Further, in the process, as described below, the maintenance operation in S 9  may be executed before ink is supplied in S 7 . Therefore, the position of the ink sensor  300  may also be set so that the ink  220 L (which is normally 0.1 to 0.3 cc and varies as the quantity of ink per ink droplet) required for purging, is secured even when it is determined in S 3  that ink  220 L decreases up to the ink supplement level (YES). 
     When ink  220 L decreases up to the ink supplement level and processing proceeds to S 5 , the consumed quantity of ink and the calculated quantity of ink are compared as follows: If both are equal (YES in S 5 ), one ink pellet  220  of corresponding color is supplied by the ink feeder  200  in S 7  and processing proceeds to S 1 . 
     In this embodiment, every time ink  220 L decreases up to the ink supplement level, one ink pellet  220  is supplied in S 7 . The consumed quantity of ink is equal to the supplied quantity of ink until the process proceeds to S 5  again. That is, the consumed quantity of ink is equal to the quantity of ink (fixed quantity of approximately 3.0 cc) for one ink pellet  22 . 
     The quantity of ink per droplet jetted from the nozzle face  36  is ‘α’, as described above if no ink jetting failure occurs. Therefore, the consumed quantity of ink  220 L normally remains until processing proceeds to S 5  again, at a value (the value is called calculated ink consumed quantity) obtained by multiplying the number of jetted droplets counted in S 1  by ‘α’. If the maintenance operation is executed until processing proceeds to S 5  again (including during printing and when ink is supplied), a value obtained by adding the consumed quantity of ink  220 L in purging is the calculated ink consumed quantity. 
     For a method of addition, the number of jetted droplets in purging may also be added by a value equivalent to the consumed quantity of ink. The consumed quantity of ink corresponding to the frequency of purging (stored in RAM  72 ) may be also added to the calculated ink consumed quantity calculated based upon the number of jetted droplets used by the printing operation. The quantity α of ink per droplet may vary depending upon color. For example, in the case of 300 dpi, the quantity of ink per black ink droplet may be set to 70 pl and the quantity of ink per yellow ink droplet may be set to 90 pl. In this case, the calculated ink consumed quantity used in S 5  is calculated using ‘α’ of a corresponding color. Further, the quantity of ink for one ink pellet  220  may also vary depending upon color. For example, black ink pellet frequently used may be relatively large and therefore, the main chamber  11  may be also large. In this case, the supplied quantity of ink used in S 5  is set according to the corresponding color. 
     As ink  220 L decreases up to the ink supplement level when the process proceeds to S 5 , the consumed quantity of ink is normally equal to calculated ink consumed quantity (YES in S 5 ). In this case, every time ink  220 L decreases up to the ink supplement level (YES in S 3 ), the ink pellet  220  is automatically supplied one by one in S 7  and the printing operation in S 1  according to print data and the like, can be automatically and continuously executed for a long period. 
     However, as ink is not jetted according to a driving signal (that is, the number of jetted ink droplets) from the piezoelectric-crystal element  38  when a failure of jetting ink occurs in the head  1 , the ink consumed quantity and calculated ink consumed quantity are not equal. In this case (NO in S 5 ), it is determined that an ink jetting failure has occurred in the head  1 . After the maintenance operation is executed in S 9 , the process is returned to S 7  and the ink pellet  220  is supplied. 
     In S 5 , it is not determined whether the ink consumed quantity and the calculated ink consumed quantity are strictly equal, but it is determined whether the difference between both is actually smaller than a predetermined quantity. For example, as described above, the quantity α of ink per ink droplet and the consumed quantity of ink in purging also vary and the detection precision and the characteristics of the ink sensor  300  also vary. The above predetermined quantity is set according to the dispersion of the consumed quantity of ink and the like, and the tolerance for a defect (that is, the tolerance for a jetting failure). For example, if strict detection is required, the above predetermined value is set to a value slightly larger than a value corresponding to the dispersion of the consumed quantity of ink, and the like. If high precision is required, an ink sensor  300  with a detection precision is used. Further, if the processing in S 3  is executed at the time of the above {circle around (2)} to {circle around (4)}, ink  220 L may be already smaller than the ink supplement level to some extent when an affirmative determination is made in S 3 . If the processing in S 3  is executed at the above time, the above predetermined value is set to a further larger value to catch the decreased quantity. 
     The time for proceeding from S 3  to S 5  also varies as the above {circle around (1)} to {circle around (4)}. The decrease of ink  220 L up to the ink supplement level (YES in S 3 ) occurs during the printing operation and the maintenance operation. However, if the determination process as in S 5  is executed during the printing and maintenance operations, there is a slightly questionable case. In such a case, it is desirable that the time in {circle around (2)} to {circle around (4)} is set. Proceeding from S 1  to S 3  may be also executed at the time described in {circle around (1)} and proceeding from S 3  to S 5  may be also executed at the time described in {circle around (2)} to {circle around (4)}. Such processing is realized by storing the number of jetted ink droplets when an affirmative determination is made in S 3  in RAM  72 , and the like, reading the stored value in S 5  and executing the above determination process. 
     Further, if processing from S 1  to S 3  and from S 3  to S 5  is also executed at the time described in the above {circle around (1)}, time for proceeding from S 5  to S 9 , that is, the timing of executing the maintenance operation can be devised as follows: That is, if the maintenance operation is executed at the time described in {circle around (1)}, the printing operation is halted halfway, a printing situation at that time is stored, and the maintenance operation is executed. The time may be also set as described above, however, in this case, even if the printing operation is continued again based upon the stored printing situation after the maintenance operation, a joint may be formed in a printed image. In the ink-jet printer of the type that the carriage is moved (as in this embodiment), it is conceived that the maintenance operation in S 9  is executed at the first paper feed time (for example in the above {circle around (2)}, after a negative determination in S 5 . Even in this case, the problem of a printing joint may occur. However, in the case of large paper printing and the like, there is also a benefit if it is conceived that the printing operation is halted halfway and the recording paper is discarded. 
     According to the printing operation, executing the maintenance operation in S 9  after the printing operation is completed, and halting the printing operation halfway and executing the maintenance operation, may be also switched. For example, if recording paper is of specific paper size and the printing operation is executed at a half of the paper size or more, the maintenance operation may be also set so that it is executed after the printing operation on the page is finished. In addition, even if printing operation is executed at a half or more if the recording paper is of another paper size, the maintenance operation may be also set so that the printing operation is halted halfway and the maintenance operation is executed. Further, the timing of the maintenance operation varies depending upon a method of setting the above predetermined quantity related to the determination in S 5 . For example, if the predetermined quantity according to strict detection is set, (i.e., a slightly more predetermined quantity is set), an ink jetting failure seldom occurs even if a negative determination is made in S 5 . In this case, it is desirable that the maintenance operation is executed after the printing operation on the page being printed is completed. 
     Next, various types of maintenance operations in S 9  are conceivable and for example, the following type can be adopted. That is, after purging occurs where the rubber pad  115  is pressed on the nozzle face  36  and the ink is circulated, the rubber pad  115  is separated from the nozzle face  36  and ink is discharged from the nozzle  32 , and the carriage  3  is moved, the rubber pad  115  is pressed on the nozzle face  36  and ink on the nozzle face  36  is wiped. In the ink-jet printer, when an ink jetting failure occurs (NO in S 5 ), the failure can be automatically solved by executing such a process. The maintenance operation in S 9  may be also executed after ink is supplied in S 7 . 
     As described above, if the actual ink supplied quantity (=ink consumed quantity) corresponding to the quantity of ink for one ink pellet  220  and the calculated ink consumed quantity calculated based upon print data and the like, are compared, and difference between both is a predetermined quantity or more, the ink-jet printer equivalent to this embodiment determines that an ink jetting failure occurs in the head  1 . Therefore, an ink jetting failure in the head  1  can be precisely and automatically detected. Therefore, even if a user fails to find an ink jetting failure, a large quantity of printing error can be satisfactorily prevented from occurring. In the ink-jet printer, as the maintenance operation is executed in S 9  when an ink jetting failure occurs (NO in S 5 ), the frequency with which the maintenance operation is executed can be set to the necessary minimum value. Therefore, the waste of ink and printing time can be satisfactorily prevented. 
     Further, in the ink-jet printer, it can be detected that the ink  220 L equivalent to the above ink supplied quantity is consumed by an extremely simple structure where only an ink sensor  300  for detecting that ink  220 L decreases up to the ink supplement level, is provided. Therefore, the structure of the ink-jet printer can be simplified and manufacturing costs can be satisfactorily reduced. 
     Next, FIG. 10 is a drawing showing the configuration of a control board of a control panel stage  70  in a second embodiment of the present invention. The configuration of the control board in this embodiment is different from the configuration of the control board in the above embodiment according to the following description. Therefore, the differences in the configuration will be described, the reference numbers shown in FIG. 8 will be allocated according to the configuration above, and the detailed description of similar elements described above is omitted. 
     As shown in FIG. 10, in this embodiment, an ink sensor  310  for detecting that ink  220 L in an ink tank  11  decreases up to a first reference value and an ink sensor  320  for detecting that the ink  220 L decreases up to a second reference value smaller than the above first reference value, are provided in place of the above ink sensor  300 . The ink sensors  310  and  320 , respectively, output a detection signal to the CPU  71  corresponding to whether ink  220 L decreased up to the first reference value or the second reference value. The above first reference value is set to a sufficiently smaller value than the quantity of ink obtained by melting one ink pellet  220  and the above second reference value is set to the quantity of ink approximately equal to the above ink supplement level. 
     FIG. 11 is a flowchart showing processing executed by the CPU  71  in the second embodiment of the invention. In this embodiment, the CPU  71  also executes processing in parallel for every color when print data and the like, are input. Timing for proceeding through each step is also set approximately similar to that in the above embodiment. 
     When processing is started, first, the printing operation is executed in S 11 . The printing operation is different from the printing operation in the above S 1  and is executed without counting the number of jetted ink droplets by a drop counter  74 A. Processing proceeds to S 13  every time the printing operation is executed and referring to a detection signal from the ink sensor  310 , it is determined whether ink  220 L decreases up to the first reference value. If the ink does not decrease up to the first reference value (NO in S 13 ), the printing operation in S 11  is continued and if the ink decreases (YES in S 13 ), processing proceeds to S 15 . 
     In S 15 , counting the number of jetted ink droplets using the drop counter  74 A since the number is the first reference value or smaller, the printing operation is executed as in the above S 1 . In S 17 , it is determined whether ink  220 L decreases up to the second reference value and until the ink decreases up to the second reference value, the printing operation in S 15  is continued. If the power source is turned off while the process is in a loop is executed in S 15  and S 17 , the number of jetted ink droplets is stored immediately before the power source is turned off in a backup RAM (not shown) and the like. When the power source is turned on again, counting is executed from the next stored value. 
     When ink  220 L decreases up to the second reference value (YES in S 17 ), processing proceeds to S 21  and the ink consumed quantity and the calculated ink consumed quantity, are compared. In processing in S 21 , the ink consumed quantity is the quantity of ink equivalent to the difference between the first reference value and the second reference value. The calculated ink consumed quantity is the quantity of ink corresponding to the number of jetted ink droplets counted in S 15 , that is, the quantity of ink corresponding to the number of jetted ink droplets while ink  220 L decreases from the first reference value to the second reference value. 
     If the ink consumed quantity and the calculated ink consumed quantity are equal (YES in S 21 ), it is determined that no ink jetting failure has occurred and after ink is supplied in S 23 , the printing operation in S 11  is continued. If the ink consumed quantity and the calculated ink consumed quantity are not equal (NO in S 2 ), it is determined that an ink jetting failure has occurred and after the maintenance operation is executed in S 25 , processing proceeds to S 23 . 
     In the second embodiment, as in the above embodiment, an ink jetting failure in the head  1  can also be precisely and automatically detected. Therefore, even if a user fails to find an ink jetting failure, a large quantity of printing error can be satisfactorily prevented from occurring. In the second embodiment, as the maintenance operation is also executed in S 25  when an ink jetting failure occurs (NO in S 21 ), the frequency with which the maintenance operation is executed can be set to the necessary minimum value. Therefore, the waste of ink and printing time can be extremely satisfactorily prevented. Further, in the second embodiment, the ink consumed quantity and the calculated ink consumed quantity are compared while ink  220 L decreases from the first reference value to the second reference value. Therefore, the number of jetted ink droplets counted by the drop counter  74 A corresponds to a relatively short period and the load related to counting the number of jetted ink droplets, such as the drop counter  74 A and the memory capacity of the backup RAM, can be further reduced. 
     Also, in the second embodiment, as the ink consumed quantity is compared while the ink  220 L decreases from the first reference value to the second reference value, the supplied quantity of ink in S 23  may be also arbitrary. For example, an ink pellet of different shape from the ink pellet  220  may also be supplied. If all ink pellets  220  are removed from an ink feeder  200 , processing in S 23  cannot be executed and an error occurs. At this time, if an ink pellet different in shape is inserted into a melting tank  40  manually and CPU  71  is reset after it is melted, various ink pellets can be used. In such a case, in the second embodiment, it can be also satisfactorily detected based upon ink consumed quantity while ink decreases from the first reference value to the second reference value, and the like, whether an ink jetting failure occurs. 
     Next, FIG. 12 is a flowchart showing processing executed by the CPU  71  in a third embodiment of the present invention. As the configuration of a control board of a control panel stage  70  in the third embodiment of the invention is the same as that in the above first embodiment shown in FIG. 8, the detailed description is omitted. In this embodiment, the CPU  71  also executes the parallel processing for every color when print data and others are input. Timing for proceeding to each step is also set approximately similar to that in the above embodiments. 
     As described above, as the quantity of ink per ink droplet is normally an approximately fixed value, the quantity in which ink  220 L decreases up to an ink supplement level can be converted to the number of jetted ink droplets. In the first embodiment, a quantity in which ink decreases is detected by the sensor and is based upon the detection results, and processing proceeds to the step (S 5 ) in which the ink consumed quantity and the calculated ink consumed quantity are compared. However, in the third embodiment, the number of jetted ink droplets required to supplement ink is preset, it is determined whether the number of jetted ink droplets reaches the set level and based upon the determination result, and processing proceeds to the step in which the ink consumed quantity and the calculated ink consumed quantity are compared. 
     When processing is started, first, the process proceeds to S 31  and the printing operation is executed, and the number of jetted ink droplets are counted by a drop counter  74 A. Every time the printing operation is executed, processing proceeds to S 33  and referring to a value counted by the drop counter  74 A, it is determined whether the number of jetted ink droplets reaches the set level. If the number does not reach the set level (NO in S 33 ), the printing operation in S 31  is continued and if the number reaches the set level (YES in S 33 ), processing proceeds to S 35 . 
     When the number of jetted ink droplets reaches the set level and processing proceeds to S 35 , the ink consumed quantity and the calculated ink consumed quantity are compared, as described above. If both are equal (YES in S 35 ), one ink pellet  220  of corresponding color is supplied by an ink feeder  200  in S 37  and the processing is returned to S 31 . 
     As ink  220 L normally decreases up to the ink supplement level when processing proceeds to S 35 , the calculated ink consumed quantity is equal to ink consumed quantity (YES in S 35 ). In this case, every time the number of jetted ink droplets reaches the set level, in other words, ink  220 L decreases up to the ink supplement level (YES in S 33 ), an ink pellet  220  is automatically supplied one by one in S 37 , and the printing operation in S 31  according to print data and the like, can be automatically and continuously executed for a long period. 
     However, as the jetting of ink according to a driving signal (that is, the number of jetted ink droplets) from a piezoelectric-crystal element  38  is not executed when an ink jetting failure occurs in the head  1 , the calculated ink consumed quantity and the ink consumed quantity are not equal. In this case (NO in S 35 ), after it is determined that an ink jetting failure occurs in the head  1  and the maintenance operation is executed in S 39 , processing is returned to S 37  and an ink pellet  220  is supplied. 
     The present invention is not limited to the above embodiments and various embodiments may be made. For example, in the above embodiments, when an ink jetting failure occurs (NO in S 5 , S 21  and S 35 ), the maintenance operation is executed in S 9 , S 25  and S 39 . However, the jetted quantity of ink executed by the print head unit  1 , the driving time, and the like, from the last ink jetting failure are stored and the maintenance operation may be also executed at a time immediately before the jetted quantity, driving time, and the like, when the ink jetting failure occurs. In this case, the occurrence of the ink jetting failure can be prevented beforehand and the occurrence of a printing error can be more satisfactorily inhibited. 
     The maintenance operation is not automatically executed and the occurrence of an ink jetting failure may be only informed by an alarm, and the like. That is, the structure may be provided with a controller for halting the driving (the printing operation) of an ink-jet head when the determination unit determines that an ink jetting failure has occurred and an information unit for informing a user and the like, of the occurrence of the ink jetting failure. For the information unit, a process such as sending data showing that an error has occurred in the ink-jet printer to a personal computer via an interface  79  and the like, and instructing a printer driver to display a warning on the screen, may be provided. The user can execute the maintenance operation (that is, can specify suitable processing after he/she checks a state of an ink jetting failure) by sending a command via the printer driver, to the personal computer or operating a panel switch on the ink-jet printer, according to the warning. In an ink-jet printer which is not provided with a maintenance unit, the maintenance operation can be executed by a user according to the above warning. 
     Further, a detector for continuously detecting the quantity of ink  220 L in a main chamber  11  is conceivable. For example, a pair of electrodes are provided in the vicinity of the lower part of the main chamber  11  and each variation of electrical resistance and electric capacity between both electrodes is detected (for example, the time constant of the response waveform of an applied pulse is detected), or a float may be floated in ink  220 L and the height is detected, may be used. Thus, the above control can be started from any quantity of ink, such as the quantity of ink when purging is executed. Thus, when the ink  220 L decreases by a predetermined quantity, the above stored quantity of ink and calculated ink consumed quantity, are compared. Therefore, in this case, an ink jetting failure can be more promptly detected. 
     If detection means which can continuously detect the quantity of ink is used as described above, an ink jetting failure can be determined by further various methods. For example, an ink jetting failure may also be determined by detecting the quantity of ink when the number of jetted ink droplets reaches a predetermined number, storing the quantity of ink in RAM  72 , and acquiring the difference between the value and a result of the last detection (if the maintenance operation is executed, the consumed quantity at that time is also added). Also, a timer, (may be also in the ink-jet printer or may be also a timer which can be controlled by a printer driver in a personal computer) is provided and an ink jetting failure may also be determined based upon the difference between the quantity of ink after a predetermined time (may be also timed only during printing) elapses and the quantity of ink before predetermined time elapses (last time), and the number of jetted ink droplets, since the last time. 
     Further, for the maintenance unit, well-known various structures in addition to the above one can be also applied. Particularly, if the maintenance unit which can execute the maintenance operation for every color is applied, the maintenance operation in S 9 , S 25  or S 39  can be executed only for color corresponding to an ink jetting failure. In this case, the wasted ink can be satisfactorily saved. In the meantime, if the maintenance operation is executed for all colors when an ink jetting occurs as in the above embodiments, the maintenance operation can be prevented from being frequently executed and the printing speed can be enhanced.