Abstract:
A recovery method for an ink jet printing apparatus is provided which makes it possible to use as much ink in the ink tank as possible and minimize the residual ink volume at time of a tank replacement. The recovery method for keeping an ejection performance of the print head in appropriate condition by performing a recovery operation of discharging ink from the print head, therefore, includes the steps of: detecting a residual ink volume in the ink tank; and controlling a discharge ink volume to be discharged by the recovery operation when the residual ink volume detected by the residual ink volume detection step is less than or equal to a predetermined value.

Description:
[0001]    This application is based on Patent Application No. 2001-284037 filed Sep. 18, 2001 in Japan, the content of which is incorporated hereinto by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an ink jet printing apparatus for printing an image by ejecting ink stored in an ink tank from a print head and to ink ejection performance recovery device and method for the ink jet printing apparatus. More specifically, the present invention relates to an improvement in a recovery operation performed to keep an ink ejection of the print head in an appropriate condition.  
           [0004]    2. Description of the Related Art  
           [0005]    In recent years, printers, facsimiles and copying machines using an ink jet printing system are rapidly expanding into the market.  
           [0006]    Ink jet printing apparatus in general use have a print head for ejecting ink droplets and an ink tank for supplying ink to the print head. In one type of these ink jet printing apparatus which has a print head and an ink tank formed integral as one piece, when the ink tank runs out of ink, the print head as well as the ink tank must be replaced, increasing a consumption of consumable parts and therefore a running cost.  
           [0007]    To deal with this problem, it is currently practiced to separately form a print head and an ink tank, removably assemble them together and mount them on a carriage. In this case, when the ink tank runs out of ink, only the ink tank needs to be replaced, significantly reducing the running cost when compared with the type in which they are formed integral as one piece. In the above construction having an integrally formed ink tank and print head, in further reducing the running cost it is effective to increase the capacity of the ink tank. Particularly, in a printer that prints a large number of sheets, such as a network printer in office, a problem may arise that unless a capacity of the ink tank or the number of sheets that can be printed with one ink tank is set large. a frequency at which the ink tank needs to be replaced will increase. It is therefore necessary to use a large capacity ink tank to reduce a burden of replacement work on the part of the user.  
           [0008]    As the capacity of the ink tank increases, however, the carriage holding the print head and the ink tank also increases in size and weight. As the weights of the carriage and the ink tank increase, it becomes difficult to scan the carriage at high speed, rendering a high-speed printing difficult to achieve. Of course, the high-speed printing may be realized by using a high torque motor for driving the carriage but with an increased cost of the motor.  
           [0009]    To cope with this problem, a printing apparatus has been put into practical use, in which only the print head is mounted in a carriage, with the ink tank fixed at a predetermined position on other than the carriage in the ink jet printing apparatus and connected to the print head as by a tube. In this apparatus, since what is mounted on the carriage is only a light-weight print head, the scanning of the carriage can be done at high speed without requiring a large motor. The capacity of the ink tank fixedly installed on other than the carriage can be selected appropriately by considering the running cost, the size of the apparatus and the frequency of ink tank replacement.  
           [0010]    [0010]FIG. 10 is a conceptual diagram showing a printing apparatus having a print head and an ink tank connected through a tube.  
           [0011]    Denoted  1  is a print head mounted on a carriage (not shown),  2  an ink tank,  3  a supply tube connecting the print head to the ink tank  2 , and  4  an atmosphere-communication tube connected to the ink tank  2 . When the print head  1  performs printing, ink in the ink tank  2  is supplied through the supply tube  3  to the print head  1 . The same volume of air as the ink supplied to the print head  1  is introduced into the ink tank  2 .  
           [0012]    In the construction shown in FIG. 10, the ink tank mounted in the carriage is ideally replaced with a new one when the ink in the ink tank is completely used up. Replacing the ink tank at such a timing can be said to be ideal from the standpoint of an ink use efficiency in ensuring that the ink in the ink tank is completely consumed, and also from the standpoint of an ejection function in ensuring that air is not trapped into the tube by stopping the ink use just before the ink in the tube begins to be used. When air is trapped in the tube, the air will be taken into the print head as the print head continues printing, resulting in a failure to eject ink properly. It is therefore necessary to avoid the air being trapped into the tube.  
           [0013]    Assuming that air may get trapped in the tube, it is possible to perform the recovery operation every time the ink tank is replaced In that case, there is a high possibility of unnecessary or redundant recovery operations being carried out, thus wasting the ink and time. During the recovery operation all the ink in the tube and the print head needs to be sucked out and removed, and thus a large volume of ink is consumed. When the above-described excess recovery operation is executed, a large amount of ink is wasted, increasing the running cost. Further, if the amount of ink discharged increases, a waste ink absorbing unit in the printing apparatus needs to be increased in capacity, which in turn leads to an unwanted increase in the overall size of the printing apparatus.  
           [0014]    As described above, preventing air from being rapped in the tube is desirable also to make the recovery operation unnecessary after the replacement of the tank. For this purpose, detection means for detecting the amount of ink remaining in the ink tank needs to be provided.  
           [0015]    Generally known residual ink volume detection means currently available include a discharge volume detection system that detects an ink discharge volume by counting the number of ink droplets ejected from a print head or metering an ink volume sucked out by recovery means, and a system that detects a residual ink volume by measuring a voltage change when a predetermined current is applied to a pair of electrodes installed in a tank.  
           [0016]    Although the ink in the ink tank should be used up completely as described above, because it is impossible to determine what kind of image the user will print, it is necessary to stop using the ink tank just before the ink is consumed completely to allow a certain amount of ink to remain in the tank.  
           [0017]    [0017]FIG. 11 shows a level of ink when the tank use is finished. For example, the residual ink volume detection means detects when the ink level reaches a position indicated by an arrow A in the figure, and decides that the ink tank has reached its final stage of use. At this time, the ink tank must contain a volume of ink that will allow a solid print image with a maximum print duty of 100% to be printed on a print medium of the largest size that can be used on the printing apparatus. If this volume of ink is not left in the tank, the ink in the tube may be consumed, causing air to enter into the tube.  
           [0018]    When the final stage of tank use as indicated by the arrow A is reached, the ink tank must be replaced despite the fact that the tank still contains a certain volume of ink with which a low-duty printing can be performed. This is therefore not desirable from the standpoint of an ink use efficiency.  
           [0019]    This is explained in more detail. In a printing operation that prints on, a print medium at a resolution or density of 600 pixels per inch with 30 picoliter of ink ejected to each pixel, for example, the amount of ink required to form a solid image of 100% print duty on a A3-size print medium is approximately 2 cc. Suppose that the amount of ink remaining in the tank when the residual ink level reaches the position of arrow A in FIG. 11 is 1 cc. In this condition, performing the above printing operation immediately before the ink level of arrow A is reached will cause the ink in the tube to be consumed and air to get into the tube. Thus, if a setting is made so that about 2 cc of ink remains when the ink level is at the arrow A position, i.e., at the final stage of tank use, the above problem is eliminated but the 2 cc of ink is wasted. This volume of ink wasted corresponds to 40 sheets of A4-size paper printed at a 5% duty.  
           [0020]    Next, a recovery operation will be explained. The recovery operation involves operating a pump. connected to a cap member adapted to engage the print head and generating a negative pressure in the cap to suck out ink from the print head. The amount of ink consumed by the recovery operation (discharge volume) can be set at an appropriate value according to a purpose of the recovery operation performed. For example, when several nozzles fail to eject ink or a printed image has light or dark strips or density variations, a recovery operation with a comparatively small ink consumption is executed. When air accumulated in the print head or tube is removed or when the printing apparatus has not been used for several weeks or months, a recovery operation with a comparatively large ink consumption is carried out. It is assumed here that the ink consumption by the latter recovery operation is 5 cc.  
           [0021]    The final-stage-of-use position indicated by arrow A of FIG. 11 must be set so that the ink in the tube will not be consumed whichever recovery operation is executed. That is, the residual ink volume A needs to be set to 5 cc to accommodate a worst case in which a recovery operation with the largest ink consumption may be performed immediately before the arrow A position is reached. With this setting it is possible to reliably prevent the ink in the tube from being consumed by the recovery operation. However, when the residual ink volume reaches 5 cc after the normal printing operation, it is decided that the final stage of tank use is reached and thus the ink tank must be replaced. The 5 cc of residual ink is even larger than the residual ink volume set in the preceding case that allows the A3-size medium printing.  
         SUMMARY OF THE INVENTION  
         [0022]    An object of the present invention is to provide an ink jet printing apparatus capable of using as much ink in an ink tank as possible and minimizing a residual ink volume at time of tank replacement.  
           [0023]    According to a first aspect, the present invention provides an ink jet printing apparatus for performing a printing with a print head which ejects ink supplied from an ink tank, in the form of droplets, the printing apparatus comprising: a print head for ejecting ink droplets; an ink tank for storing ink to be supplied to the print head; recovery means for keeping an ejection performance of the print head in appropriate condition by discharging ink from the print head; a residual ink volume detection means for detecting a residual ink volume in the ink tank; and a discharge ink volume control means for controlling a discharge ink volume to be discharged by the recovery means when the residual ink volume detected by the residual ink volume detection means is less than or equal to a predetermined value.  
           [0024]    In the above aspect, the ink tank may be connected to the print head through a predetermined ink supply path.  
           [0025]    Further, the discharge ink volume control means may set two or more discharge ink volumes to be discharged by the recovery means.  
           [0026]    Further, the recovery means may be able to perform a plurality of recovery operations with different discharge volumes and the control means may select one of the recovery operations according to the residual ink volume in the ink tank.  
           [0027]    Further, the recovery means may include a cap that hermetically contacts an ink nozzle surface of the print head and pressure generation means connected to the cap, and a negative pressure generated by the pressure generation means may be applied to an interior of the cap in hermetic contact with the ink nozzle surface of the print head to discharge ink from the nozzles.  
           [0028]    Further, the residual ink volume detection means may comprise metering means for measuring an ink volume flowing out of the print head and calculation means for determining the residual ink volume by subtracting the ink volume measured by the metering means from a maximum storage volume of the ink tank.  
           [0029]    Further, the residual ink volume detection means may comprise a pair of electrodes provided on a bottom of the ink tank, an annular wall enclosing at least one of the electrodes, a constant current source for applying a constant current between the electrodes through the ink and a voltage detection means for detecting a voltage between the electrodes, and detect the residual ink volume from the voltage detected by the voltage detection means.  
           [0030]    According to another aspect, the present invention provides a recovery device in an ink jet printing apparatus, wherein the ink jet printing apparatus has a print head for ejecting ink droplets and an ink tank for storing ink to be supplied to the print head, the recovery device comprising: recovery means for keeping an ejection performance of the print head in appropriate condition by discharging ink from the print head; a residual ink volume detection means for detecting a residual ink volume in the ink tank; and a discharge ink volume control means for controlling a discharge ink volume to be discharged by the recovery means when the residual ink volume detected by the residual ink volume detection means is less than or equal to a predetermined value.  
           [0031]    According to still another aspect, the present invention provides a recovery method used in an ink jet printing apparatus for keeping an ejection performance of a print head in appropriate condition by performing a recovery operation of discharging ink from the print head, wherein the ink jet printing apparatus has the print head for ejecting ink droplets and an ink tank for storing ink to be supplied to the print head, the recovery method comprising the steps of: detecting a residual ink volume in the ink tank; and controlling a discharge ink volume to be discharged by the recovery operation when the residual ink volume detected by the residual ink volume detection step is less than or equal to a predetermined value.  
           [0032]    In the invention having the construction described above, since the ink consumption by the recovery operation is limited according to the residual ink volume in the ink tank, the residual ink volume at the end of use of the ink tank can be reduced significantly, thus making an efficient use of the ink.  
           [0033]    The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.  
           [0034]    The above and other objects , effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    [0035]FIG. 1 is a perspective view showing an outline construction of an ink jet printing apparatus according to one embodiment of the present invention;  
         [0036]    [0036]FIG. 2 is a schematic diagram showing an outline construction of an ink supply system in one embodiment of the invention;  
         [0037]    [0037]FIG. 3 is a schematic diagram showing an overall configuration of an electric circuit in one embodiment of the invention;  
         [0038]    [0038]FIG. 4 is a block diagram of a main printed circuit board in one embodiment of the invention;  
         [0039]    [0039]FIG. 5 is a block diagram showing a system configuration of a host computer and a printing apparatus in one embodiment of the invention;  
         [0040]    [0040]FIG. 6 is a flow chart showing a control operation according to a first embodiment of the invention;  
         [0041]    [0041]FIG. 7 is a flow chart showing a control operation according to a second embodiment of the invention;  
         [0042]    [0042]FIG. 8 is a schematic view showing another construction of the residual ink volume detection means in one embodiment of the invention;  
         [0043]    [0043]FIG. 9 is an explanatory vertical side cross section showing a working principle of the residual ink volume detection means of FIG. 8;  
         [0044]    [0044]FIG. 10 is an explanatory schematic side view showing an ink supply system in the ink jet printing apparatus; and  
         [0045]    [0045]FIG. 11 is an explanatory schematic side view showing a volume of ink remaining in an ink tank when a final ink level set in the ink tank of the ink jet printing apparatus is reached. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0046]    Embodiments of the present invention will be described by referring to the accompanying drawings.  
         [0047]    First, a construction of an ink jet printing apparatus according to one embodiment of this invention will be explained by referring to FIG. 1 to FIG. 5.  
         [0048]    [0048]FIG. 1 is an external view showing essential portions of an ink jet printing apparatus (also referred to as a printer) that can apply the present invention This ink jet printing apparatus is of a so-called serial scan type in which an image is formed on a print medium (also referred to as print paper or simply paper) by ejecting ink from an ink jet print head according to image data as the print head is reciprocally moved in a main scan direction perpendicular to a sub-scan direction in which the print medium is fed, and by intermittently feeding the print medium in the sub-scan direction.  
         [0049]    This ink jet printing apparatus includes a carriage  2  mounting the ink jet print head (also referred to simply as a print head), a carriage motor  12  for driving the carriage  2  in the main scan direction, a flexible cable  13  for transmitting electric signals from a control unit (not shown) held in the printing apparatus body to the print heads recovery means  14  for recovering an ink ejection performance of the print head, a paper supply tray  15  for storing print mediums in a stacked state, and an optical position sensor  16  for optically reading a position of the carriage  2 .  
         [0050]    In the ink jet printing apparatus of such a construction, the print head along with the carriage  2  is serially scanned over the print medium to perform printing over a linne of a width corresponding to the number of nozzles of the print head and, during a non-printing operation, the print medium is fed a predetermined distance.  
         [0051]    Further, in an enlarged plan view of the recovery means  14  in FIG. 1, denoted  21  is suction and rest caps,  22  an ink receiver for receiving ink ejected during the ejection performance recovery operation, and  23  wiper blades for wiping a nozzle face of each head. The wiper blades  23  are moved in a direction of arrow to wipe the nozzle faces.  
         [0052]    In this embodiment, the nozzles of the print head are hermetically enclosed by the caps  21  and a suction pump (not shown) as pressure generating means connected to the caps  21  is operated to generate a negative pressure in the caps  21  and thereby suck out ink from the nozzles of the print head. This recovery means can perform three kinds of recovery operations. When several nozzles fail to eject ink or a printed image has light or dark strips or density variations, a recovery operation with a comparatively small ink consumption is performed. This recovery operation is called a recovery operation A When air accumulated in the print head or tube is removed or when the printing apparatus has not been used for several weeks or months, a recovery operation with a comparatively large ink consumption is carried out. This recovery operation is called a recovery operation B. Further, when the printing apparatus is newly installed or the print head is replaced, or when the printing apparatus has not been used for a long period of time in excess of several months, a recovery operation with the largest ink consumption is executed. This recovery operation is called a recovery operation C. Here, the recovery operation A consumes 0.5 cc of ink, the recovery operation B consumes 4 cc and the recovery operation C consumes 7 cc.  
         [0053]    [0053]FIG. 2 is a diagram showing a printing apparatus in which print head and ink tanks are connected with tubes.  
         [0054]    Ink is supplied from a main ink tank  201  through tubes  207  and a joint  208  to a sub-ink tank  202  on the carriage  2  from which it is further supplied to the print head  9  In the main ink tank  201 , denoted  201 Y,  201 M.  201 C and  201 B are ink tanks for yellow, magenta, cyan and black inks, respectively. The print head  9  is moved together with the carriage  2  in the main scan direction along the shaft  10 . Designated  203  is a buffer chamber.  
         [0055]    The ink may be directly supplied to the print head  9  from the main tank  201  installed at a fixed position in the apparatus body. However, to minimize a load on the carriage  2  for higher printing speed and lighter weight, it is effective to reduce the size of the sub-tank  202  mounted on the carriage  2 , as in this embodiment. That is, In this embodiment the sub-tank  202  of relatively small capacity is mounted on the carriage  2  to supply ink to the print head  9  from the sub-tank  202 , which in turn is supplied ink from the main tank  201  of relatively large capacity installed at a fixed position in the apparatus body. The supply joint  208  forms an ink supply path between the main tank  201  and the sub-tank  202  when the carriage  2  moves to a predetermined position such as home position. Thus, at an optimum timing according to the capacity of the sub-tank  202  and the ink consumption of the print head  9 , the ink can be supplied from the main tank  201  to the sub-tank  202 . In this embodiment an Ink volume of the main tank is set at  80  cc.  
         [0056]    Next, an electric circuit configuration according to one embodiment of this invention will be explained.  
         [0057]    [0057]FIG. 3 is a schematic diagram showing an overall configuration of an electric circuit in one embodiment. A control system in this embodiment includes mainly a carriage printed circuit board  301 , a main PCB (printed circuit board)  302 , and a power unit  303 . The power unit is connected to the main PCB  302  and supplies a variety of drive powers. The carriage PCB  301  is a printed circuit board unit mounted on the carriage  2  (FIG. 1) which functions as an interface for sending and receiving signals to and from the print head through a contact PFC  304  and which, based on pulse signals output from an encoder sensor  305  as the carriage is scanned, detects a change in a positional relation between an encoder scale  306  and the encoder sensor  305  and sends an output signal to the main PCB  302  through a flexible flat cable (CRFFC)  307 .  
         [0058]    The main PCB  302  is a printed circuit board unit that performs drive controls on various parts of the ink jet printing apparatus of this embodiment. The main PCB  302  has I/O ports for a paper end detection sensor (PE sensor)  308 , an ASF sensor  309 , a cover sensor  310 , a parallel interface (parallel I/F)  311 , a resume key  312 , an LED  313 , a power key  314  and a buzzer  315 . It also is connected with a CR motor  316 , an LF motor  317  and a PG motor  318  to control their operations and has connection interfaces with a PG sensor  319 , the CRFFC  307  and the power unit  303 .  
         [0059]    [0059]FIG. 4 is a block diagram of the main PCB in the printing apparatus of this embodiment. In the figure, designated  401  is a CPU which is connected to a ROM  402  and an ASIC (Application Specific Integrated Circuit)  403  to control the ASIC  403  according to a program stored in the ROM  402  and also detects an input signal  404  from the power key  314 , an input signal  405  from the resume key  312  and a cover detection signal  406 . The CPU also drives the buzzer  407  using a buzzer signal (BUZ), detects a residual ink volume in the ink tank, performs various logic operations and judgments of conditions, and controls the driving of the print head and the ink jet printing apparatus.  
         [0060]    Denoted  408  is a CR motor driver which, according to a CR motor control signal from the ASIC  403 , generates a CR motor drive signal to drive a CR motor  409 . Designated  410  is an LF/PG motor driver which, according to a pulse motor control signal (PM control signal) from the ASIC  403 , generates an LF motor drive signal to drive an LF motor  411  and also generates a PG motor drive signal to drive a PG motor  412 .  
         [0061]    Designated  413  is a power control circuit which, according to a power control signal from the ASIC  403 , controls power supply to various sensors having light emitting elements. A parallel I/F  414  sends a parallel I/F signal from the ASIC  403  to the parallel I/F cable connected to an external circuit. Signals from the parallel I/F cable are transferred to the ASIC  403 .  
         [0062]    This ASIC  403  is an one-chip semiconductor integrated circuit which is controlled by the CPU  401  through a control bus to output the CR motor control signal, PM control signal, power control signal, head power ON signal, motor power ON signal and others and transfer these signals to the parallel I/F  414 . Further, the ASIC  403  checks statuses of a PE detection signal from a PE sensor  415 , an ASF detection signal from an ASP sensor  416  and a PG detection signal from a PG sensor  417  and sends data representing the statuses of these signals to the CPU  401  through the control unit. The CPU  401 , based on the received data, controls the operation of an LED drive signal to turn on or off an LED  418 . Further, the ASIC  403  also has a dot count function, described later, which counts the number of ink droplets ejected from the print head  419 .  
         [0063]    [0063]FIG. 5 is a block diagram showing a concept of a system built from the ink jet printing apparatus and a host computer.  
         [0064]    Denoted  501  is a host computer which is connected to the printing apparatus  503  and mainly generates data to be used for printing operation. Reference  502  represents a printer driver. In the host computer  501 , image data supplied from an application program is sent from an image processing unit  509 , described later, in the printer driver  502  to the printing apparatus  503 . Through a two-way communication, the host computer receives from the printing apparatus  503  status information such as error data and head ejection volume information characteristic of this invention, and changes the processing method accordingly. The transfer and processing of these information will be detailed later.  
         [0065]    An ASIC  505  sends and receives data to and from the host computer  501  through an I/F unit  504  in the printing apparatus  503 . A CPU  507  sends and receives data signals and control signals to and from the ASIC  505  to control various operations of the printer  503 . The ASIC  505  sends and receives head control signals to and from the print head  506 . Further, the ASIC  505  has a dot counter (measuring means) to count the number of ink droplets ejected from the print head. The dot counter counts both the number of ink droplets ejected to form an image and the number of ink droplets ejected for a preliminary ejection to keep the ejection characteristic of the print head in appropriate condition. The CPU  507  receives the head control signals for the print head  506  through the ASIC  505  to perform a variety of controls for head driving. Further, the printing apparatus  503  has an EEPROM  508 , and the content of the EEPROM  508  is transmitted to the CPU  507  through the ASIC  505  at a predetermined timing.  
         [0066]    As a method of detecting the residual ink volume in the ink tank, this embodiment employs a dot count method using the dot counter. This method involves counting the number of ink droplets ejected for forming an image and for performing the preliminary ejection, multiplying the counted value by the ink volume (discharge volume) for each droplet to determine an ink consumption and subtracting the calculated ink consumption from the full volume of ink in the ink tank to determine a residual ink volume in the tank. At this time, an ink volume consumed by the recovery operation is also considered in addition to the number of ink droplets ejected from the print head to determine the residual ink volum.  
         [0067]    Next, a control operation in one embodiment of this invention executed by the ink jet printing apparatus of the above construction will be described.  
         [0068]    (First Embodiment)  
         [0069]    [0069]FIG. 6 is a flow chart showing a sequence of control operation performed in a first embodiment of this invention.  
         [0070]    First, when a recovery operation command is received at step  1 , the control sequence at step  2  checks whether the residual ink volume in the ink tank is less than or equal to a predetermined residual volume, which corresponds to 9% of the maximum storage volume of the ink tank. If the residual volume is found less than or equal to the predetermined residual volume (9%), the control sequence performs the specified recovery operation at step  5 .  
         [0071]    If the residual ink volume is less than or equal to the predetermined residual volume, the control sequence at step  3  checks whether the recovery operation command received is for a recovery operation C. When it is determined to be the recovery operation C, step  4  executes a recovery operation B which consumes less ink than the recovery operation C.  
         [0072]    When at step  3  the received recovery operation command is determined as not one for the recovery operation C, step  5  performs the specified recovery operation. In this case, the specified recovery operation is either a recovery operation A or B whose ink consumption is less than or equal to that of the recovery operation C, and is thus executed as specified.  
         [0073]    In this first embodiment, the full ink volume of the ink tank is 80 cc, so that the specified residual volume of 9% corresponds to 7.2 cc. This specified residual volume (7.2 cc) is larger than the ink consumption of 7 cc by the recovery operation C. When the residual Ink volume becomes lower than 7.2 cc, the execution of the recovery operation C is inhibited and changed to the execution of the recovery operation B, as described above. Thus, the use of the ink tank can be terminated with only 4 cc of ink, which corresponds to the ink consumption by the recovery operation B, remaining in the tank. This arrangement can reduce the ink residual volume when compared with a case where the kinds of recovery operations are not limited.  
         [0074]    (Second Embodiment)  
         [0075]    [0075]FIG. 7 is a flow chart showing a series of control steps in a second embodiment of this invention.  
         [0076]    First, a recovery operation command is received at step  21 , the control sequence at step  22  checks whether the residual ink volume in the ink tank is less than or equal to a predetermined residual volume, which corresponds to 9% of the maximum storage volume of the ink tank. If the residual volume is found less than or equal to the predetermined residual volume (9%), the control sequence performs the specified recovery operation at step  27 .  
         [0077]    If the residual ink volume is less than or equal to the predetermined residual volume, the control sequence at step  23  checks whether the residual ink volume is less than or equal to a second predetermined residual volume, which is 5% of the maximum storage volume of the ink tank. When it is decided that the residual ink volume is less than or equal to the second predetermined residual volume, step  24  executes the recovery operation A which consumes the least amount of ink. If at step  23  it is decided that the residual ink volume is less than or equal to the second predetermined residual volume, the control sequence proceeds to step  25  where it checks whether the specified recovery operation is the recovery operation C. If it is decided that the recovery operation C is specified, the recovery operation B instead of the recovery operation C is executed. If it is decided that the recovery operation C is not specified, the control sequence proceeds to step  24  where it executes the recovery operation A.  
         [0078]    As described above, in the second embodiment, when the residual ink volume is less than or equal to a first predetermined residual volume, the recovery operation B is executed even if the recovery operation C is specified. Further, when the residual ink volume is less than or equal to the second predetermined residual volume, the recovery operation A is executed whatever recovery operation is specified. In this second embodiment too, the first predetermined residual volume (9%) is 7.2 cc, which corresponds to the ink consumption by the recovery operation C. The second predetermined residual volume (5%) is 4 cc, which corresponds to the ink consumption by the recovery operation B.  
         [0079]    Hence, if the use of the ink tank is stopped with 0.5 cc of ink, which corresponds to the ink consumption by the recovery operation A, remaining in the tank, then the minimum residual ink volume is 0.5 cc. This arrangement can further reduce the residual ink volume when compared with a case where the kinds of recovery operations are not limited. Considering the residual ink volume required to form an image, however, it Is preferred that a greater amount of ink than 0.5 cc be left in the tank. For example, when there is a possibility of printing a 100% solid image on an A3-size print medium, the use of the ink tank must be stopped when the residual ink volume reaches about 2 cc, as explained in connection with the prior art.  
         [0080]    (Other Embodiments)  
         [0081]    Generally, there are two kinds of recovery operations, one that is automatically performed by the printing apparatus to remove air trapped in the print head and tube or to remove viscous ink near the nozzles after the printing apparatus has been left unused for a comparatively long period of time, and one that is performed at the request of the user for some purpose.  
         [0082]    Of these, the automatic recovery operation is normally preset with an execution timing and a frequency that are given predetermined allowances. Thus, when the residual ink volume in the ink tank is running low, limiting the recovery operation unconditionally as described in the preceding embodiments will rarely affect the image forming adversely. When the recovery operation is initiated by the user, however, there is a possibility that a recovery operation that may cause adverse effects on the image formation may be selected. But for the user to achieve his or her intended objectives, it is also effective to permit the user to choose a desired recovery operation. For this reason a mode may be provided which does not limit the recovery operation. In this case, however, there is a possibility of even the ink in the tube being consumed by the recovery operation initiated by the user. Thus, it is desired that the use of the ink tank be stopped somewhat earlier to avoid air entering the tube.  
         [0083]    In the above embodiments the dot count system using a dot counter was employed to detect the residual ink volume. The residual ink volume detection means of this embodiment may use other systems than the dot count system. One such example is described below.  
         [0084]    [0084]FIG. 8 shows another example of the residual ink volume detection means applied to this invention. In the figure, a supply pin  205  and an atmosphere communication pin  204  are formed of conductive metal materials and are connected with one end of conductive wires  209 A,  209 B. These conductive wires  209 A,  209 B are connected at the other end to a constant current circuit (current source)  210 . The constant current circuit  210  provides a DC current of 100 μA to produce a maximum voltage of 5 V between the pins  205  and  204  as electrodes.  
         [0085]    Thus, when there is no ink in the tank  201  or when the tank  201  is not mounted in its place, the maximum voltage of 5 V is applied between these pins  205 ,  204 . When the pins  205 ,  204  are electrically connected by the ink in the tank  201 , the voltage applied between the pins  204 ,  205  changes according to a resistance of the tank  201 . The residual ink volume detection means measures the amount of ink remaining in the tank  201  according to a change in the applied voltage.  
         [0086]    [0086]FIG. 9 shows a detection principle of the residual ink volume detection means. The ink level in the tank  201  gradually lowers as the ink is consumed, as indicated by levels L 1 , L 2 , L 3  in the figure. When the ink level is higher than an upper end of an annular wall  201   b  surrounding the atmosphere communication pin  204 , as indicated by level L 1 , the atmosphere communication pin  204  and the supply pin  205 , both functioning as electrodes, are electrically connected through the ink in the tank  201  that lies above the annular wall  201   b.    
         [0087]    When the ink level goes lower than the upper end of the annular wall  201   b , as indicated by level L 2 , the annular wall  201   b  interrupts the electrical connection between the inside ink and the outside ink, thus electrically disconnecting the pins  204 ,  205 .  
         [0088]    Therefore, when the ink level reaches the upper end of the annular wall  201   b , as shown at level L 2 , the applied voltage between the pins  204 ,  205  changes and the level L 2  can be used as a detection point P. The residual ink volume detection means having a mechanism construction as described above detects from a voltage change when the ink level reaches the level L 2 .  
         [0089]    As described above, in this embodiment any desired residual ink volume can be detected according to the height of the annular wall  201   b.    
         [0090]    The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the intention, therefore, In the apparent claims to cover all such changes and modifications as fall within the true spirit of the invention.