Patent Publication Number: US-8974019-B2

Title: Apparatus for and method of supplying liquid

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for and a method of supplying a liquid. 
     2. Description of the Background Art 
     A liquid supply apparatus for supplying a liquid through a liquid feed pipe has been hitherto known. A known example of the liquid supply apparatus includes an ink supply apparatus in which ink is supplied from an ink cartridge through a liquid feed tube to a head tank for supplying ink directly to an ink ejection head. 
     An inkjet recording apparatus including a conventional ink supply apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. 2011-189701. The inkjet recording apparatus disclosed in Japanese Patent Application Laid-Open No. 2011-189701 includes droplet ejection heads for ejecting ink, head tanks for supplying ink directly to the droplet ejection heads, ink cartridges for adding and supplying ink to the head tanks, and supply tubes for supplying ink from the ink cartridges to the head tanks (in paragraphs 0015 and 0031). 
     In such an inkjet recording apparatus, the ejection of appropriate amounts of ink droplets from the droplet ejection heads necessitates the adjustment of the pressure of ink in nozzles of the droplet ejection heads to within an appropriate range. The droplet ejection heads, on the other hand, are susceptible to the internal pressure in the head tanks (in paragraph 0003). 
     In such an ink supply apparatus for supplying ink from the ink cartridges to the head tanks, an abrupt change in ink supply volume causes an abrupt change in internal pressure in the head tanks. As a result, appropriate amounts of ink droplets cannot be ejected from the droplet ejection heads. 
     In particular, there are cases where a high-power pump is used for supply of ink in a large-scale inkjet printing machine which consumes a large amount of ink. Thus, large vibrations occur in some cases when the pump is turned on or off or when the output of the pump is changed. This results in apprehension that an abrupt change in internal pressure in the droplet ejection heads or the head tanks occurs to influence printing quality. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide an apparatus for and a method of supplying a liquid which are capable of suppressing an abrupt change in pressure in a destination to which the liquid is supplied. 
     A first aspect of the present invention is intended for a liquid supply apparatus for supplying a liquid to at least one liquid consuming apparatus. The liquid supply apparatus comprises: a reservoir for storing a liquid therein; a supply path for supplying the liquid from the reservoir to the liquid consuming apparatus; a pump for feeding the liquid from the reservoir to the supply path; and a controller for outputting a driving signal to the pump to control the supply of the liquid, the controller acquiring a target feed volume of the liquid from the pump to gradually change a feed volume of the liquid from the pump from a current feed volume to the target feed volume, based on a request signal from the liquid consuming apparatus. 
     According to the first aspect of the present invention, the liquid supply apparatus is capable of suppressing an abrupt change in pressure in the liquid consuming apparatus serving as the destination to which the liquid is supplied. 
     A second aspect of the present invention is intended for a method of supplying a liquid to a liquid consuming apparatus. The method comprising the steps of: a) providing a request signal from the liquid consuming apparatus to a controller which controls a pump for feeding a liquid stored in a reservoir to the liquid consuming apparatus; b) providing a target feed volume of the liquid from the pump, to the controller, based on the request signal; and c) outputting a driving signal from the controller to the pump, the driving signal gradually changing a feed volume of the liquid from the pump from a current feed volume to the target feed volume. 
     According to the second aspect of the present invention, the method is capable of suppressing an abrupt change in pressure in the liquid consuming apparatus which is a destination to which the liquid is supplied. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a liquid supply apparatus; 
         FIG. 2  is a block diagram showing a main control mechanism in the liquid supply apparatus shown in  FIG. 1 ; 
         FIG. 3  is a flow diagram showing a procedure for measurement of the volume of ink in a counting tank; 
         FIG. 4  is a graph showing an example of a relationship between request signals from printers and an output from a supply pump; 
         FIG. 5  is a flow diagram showing a procedure for operation of the supply pump in response to the request signals from the printers; 
         FIG. 6  is a graph showing waveforms of a driving signal during changes in output; 
         FIG. 7  shows a correspondence table between the output number of the driving signal and a duty ratio; and 
         FIG. 8  is a flow diagram showing a procedure during the output of the driving signal. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment according to the present invention will now be described with reference to the drawings. 
     1. Liquid Supply Apparatus According to One Preferred Embodiment 
     &lt;1-1. Configuration of Liquid Supply Apparatus&gt; 
       FIG. 1  is a diagram showing a configuration of a liquid supply apparatus  1  according to one preferred embodiment of the present invention.  FIG. 2  is a block diagram showing a main control mechanism in the liquid supply apparatus  1 . The liquid supply apparatus  1  is an apparatus for supplying ink from at least one ink tank  8  to at least one printer  9  that is an inkjet printing machine. The “liquid” which the liquid supply apparatus  1  according to the present preferred embodiment supplies is ink. 
     In the liquid supply apparatus  1  according to the present preferred embodiment, ink is supplied from two ink tanks  8  which are more specifically referred to as a first ink tank  81  and a second ink tank  82 . In the liquid supply apparatus  1  according to the present preferred embodiment, ink is supplied to two printers  9  which are more specifically referred to as a first printer  91  and a second printer  92 . In the present preferred embodiment, the at least one ink tank  8  serves as a “supply tank” and the at least one printer  9  serves as a “liquid consuming apparatus”. The number of ink tanks  8  may be one or greater than two. Likewise, the number of printers  9  may be one or greater than two. 
     The liquid supply apparatus  1  includes a counting tank  21 , an intermediate tank  22 , a filter  23 , a deaeration module  24 , a suction pump  31 , and a supply pump  32 . As shown in  FIG. 1 , the aforementioned components  21 ,  22 ,  23 ,  24 ,  31  and  32  are connected through pipes  41  to  43 . The counting tank  21 , the intermediate tank  22 , the deaeration module  24 , the suction pump  31  and the supply pump  32  are controlled by a controller  10 . 
     The counting tank  21  is a container which measures the volume of ink supply from the at least one ink tank  8  to the intermediate tank  22  to thereby manage the feed volume of ink. The counting tank  21  includes a low liquid level sensor  211  and a high liquid level sensor  212  both of which sense the liquid level of ink stored in the counting tank  21 . The low liquid level sensor  211  senses whether the liquid level of ink in the counting tank  21  is higher than a predetermined first vertical position or not. The first vertical position is a position higher than an ink discharge port  210  provided in the counting tank  21 . The high liquid level sensor  212  senses whether the liquid level of ink in the counting tank  21  is higher than a predetermined second vertical position or not. The second vertical position is a position higher than the first vertical position. In other words, the high liquid level sensor  212  is provided at a position higher than that of the low liquid level sensor  211 . The measurement of the volume of ink in the counting tank  21  will be described later. 
     The intermediate tank  22  is a container for temporarily storing therein ink that is a liquid. In the present preferred embodiment, the intermediate tank  22  serves as a “reservoir”. The intermediate tank  22  includes a first sensor  221  and a second sensor  222  both of which sense the liquid level of ink stored in the intermediate tank  22 . The first sensor  221  is provided at a position higher than that of an ink discharge port  220  provided in the intermediate tank  22 . The second sensor  222  is provided at a position higher than that of the first sensor  221 . Preferably, there is a greater difference in height between the first sensor  221  and the second sensor  222 . 
     The filter  23  is a filter for removing foreign matter from ink passing through the pipe  43 . 
     The deaeration module  24  is a container for removing dissolved gases from ink. The deaeration module  24  includes, for example, a vacuum pump for exhausting gases from the interior of the deaeration module  24 . The deaeration module  24  may have other structures capable of removing dissolved gases from ink. When the ink in the at least one ink tank  8  is deaerated ink, the deaeration module  24  may be dispensed with. 
     The suction pump  31  is a pump for sucking ink from the at least one ink tank  8  to supply the ink to the counting tank  21 . The suction pump  31  is interposed in the pipe  41  which connects the at least one ink tank  8  and the counting tank  21  for communication therebetween. In the present preferred embodiment, the suction pump  31  serves as a “first feeding part” for feeding ink from the at least one ink tank  8  to the counting tank  21 . 
     The supply pump  32  is a pump for sucking ink from the intermediate tank  22  to supply the ink through the deaeration module  24  to the at least one printer  9 . The supply pump  32  is interposed in the pipe  43  which connects the intermediate tank  22  and the at least one printer  9  for communication therebetween. In the present preferred embodiment, the supply pump  32  serves as a “pump” for feeding ink from the intermediate tank  22  to the pipe  43 . 
     The pipe  41  connects the at least one ink tank  8  and the counting tank  21  for communication therebetween. In the present preferred embodiment, two ink tanks, i.e. the first ink tank  81  and the second ink tank  82 , are used as the at least one ink tank  8 , as mentioned above. Accordingly, the pipe  41  includes a pipe  410  which connects to the counting tank  21  and in which the suction pump  31  is interposed, a pipe  411  which connects the pipe  410  and the first ink tank  81  to each other, and a pipe  412  which connects the pipe  410  and the second ink tank  82  to each other. Suction valves  51  and  52  that are on-off valves are provided in the pipes  411  and  412 , respectively. 
     The pipe  42  connects the counting tank  21  and the intermediate tank  22  for communication therebetween. An end of the pipe  42  which is closer to the counting tank  21  is connected to the discharge port  210  of the counting tank  21 . A discharge valve  53  that is an on-off valve is provided in the pipe  42 . In the present preferred embodiment, the discharge valve  53  serves as a “second feeding part” for feeding ink from the counting tank  21  to the intermediate tank  22 . 
     The pipe  43  connects the intermediate tank  22  and the at least one printer  9  for communication therebetween. An end of the pipe  43  which is closer to the intermediate tank  22  is the discharge port  220  of the intermediate tank  22 . In the present preferred embodiment, two printers, i.e. the first printer  91  and the second printer  92 , are used as the at least one printer  9 , as mentioned above. Accordingly, the pipe  43  includes a pipe  430  which connects to the intermediate tank  22 , a pipe  431  which connects the pipe  430  and the first printer  91  to each other, and a pipe  432  which connects the pipe  430  and the second printer  92  to each other. 
     The supply pump  32 , a supply valve  54 , the filter  23  and the deaeration module  24  are provided in the pipe  430  in the order named as seen from the intermediate tank  22 . 
     In the present preferred embodiment, the pipe  43  serves as a “supply path” for supplying ink from the intermediate tank  22  to the at least one printer  9 . Specifically, the pipe  430  and the pipe  431  serve as a supply path from the intermediate tank  22  to the first printer  91 , and the pipe  430  and the pipe  432  serve as a supply path from the intermediate tank  22  to the second printer  92 . 
     As shown in  FIGS. 1 and 2 , the first printer  91  includes a first printer controller  910 , a first head tank  911  for supplying ink directly to an ink ejection head, and a first printer valve  912  for making and breaking a connection between the first head tank  911  and the liquid supply apparatus  1 . The pipe  431  is connected for communication with the first head tank  911  through the first printer valve  912 . The first printer controller  910  controls the first printer valve  912 . 
     Likewise, the second printer  92  includes a second printer controller  920 , a second head tank  921  for supplying ink directly to an ink ejection head, and a second printer valve  922  for making and breaking a connection between the second head tank  921  and the liquid supply apparatus  1 . The pipe  432  is connected for communication with the second head tank  921  through the second printer valve  922 . The second printer controller  920  controls the second printer valve  922 . 
     As shown in  FIG. 2 , the controller  10  receives sensing signals from the low liquid level sensor  211  and the high liquid level sensor  212  of the counting tank  21  and from the first sensor  221  and the second sensor  222  of the intermediate tank  22 . The controller  10  also receives ink supply request signals from the first printer controller  910  and the second printer controller  920 . Specifically, the first printer controller  910  and the second printer controller  920  send connection making signals to the first printer valve  912  and the second printer valve  922  respectively, and at the same time send the request signals to the controller  10 . The first printer controller  910  and the second printer controller  920  send connection breaking signals to the first printer valve  912  and the second printer valve  922  respectively, and at the same time stop the request signals to the controller  10 . 
     The controller  10  is electrically connected to the suction pump  31 , the supply pump  32 , the suction valves  51  and  52 , the discharge valve  53  and the supply valve  54 . The controller  10  controls the operations of opening and closing the suction pump  31 , the supply pump  32 , the suction valves  51  and  52 , the discharge valve  53  and the supply valve  54  in accordance with user&#39;s manipulations, various input signals or previously set programs. 
     The controller  10  is formed by, for example, a computer including a computation processor such as a CPU, and a memory. As shown in  FIG. 2 , the controller  10  includes a storage area  11  for storing a correspondence table  111  to be described later therein. 
     With the aforementioned configuration, the liquid supply apparatus  1  supplies ink from the at least one ink tank  8  to the at least one printer  9 . The brief description of the operation of the liquid supply apparatus  1  is as follows. 
     First, the suction pump  31  is driven to supply ink from the at least one ink tank  8  through the pipe  41  to the counting tank  21 . The ink temporarily stored in the counting tank  21  is discharged through the pipe  42  and the discharge valve  53  to the intermediate tank  22 . The ink stored in the intermediate tank  22  is fed into the pipe  43  by the supply pump  32  in accordance with the request signal from the at least one printer  9 . The ink fed by the supply pump  32  passes through the pipe  43  and then through the supply valve  54  to the filter  23 , which in turn removes foreign matter from the ink. Then, the ink reaches the deaeration module  24 , which in turn removes dissolved gases from the ink. Finally, the ink is supplied to the at least one printer  9 . 
     The details of a configuration for suppressing an abrupt change in pressure in the at least one printer  9  which is a destination to which ink is supplied in such a liquid supply apparatus  1  will be described later. 
     &lt;1-2. Counting Tank&gt; 
     Next, the measurement of the volume of ink in the counting tank  21  will be described.  FIG. 3  is a flow diagram showing an example of a procedure for measurement of the volume of ink in the counting tank  21 . 
     In the present preferred embodiment, 18 liters of ink is stored in each ink tank  8  before use. The counting tank  21  has an ink capacity of 100 milliliters from the first vertical position to the second vertical position. The ink capacity of the counting tank  21  from the first vertical position to the second vertical position shall be referred to simply as the ink capacity of the counting tank  21  hereinafter. It should be noted that the ink capacity of each ink tank  8  and the ink capacity of the counting tank  21  are not limited to the aforementioned values. The operation of the counting tank  21  and its adjacent components is described below. 
     As mentioned above, the counting tank  21  includes the low liquid level sensor  211  and the high liquid level sensor  212  which sense the liquid level of ink stored in the counting tank  21 . 
     At the start of the operation of the liquid supply apparatus  1 , the controller  10  initially checks a count N (in Step S 101 ). The count N is treated as a variable in the computation process in the controller  10 . The count N is zero when a new ink tank  8  is to be used. When the ink tank  8  used during the preceding operation is to be continuously used without change, the count N has the same value as that obtained at the completion of the preceding operation. For convenience of description, it is assumed that the ink tank  8  to be used in this case is the first ink tank  81 . At this time, the suction pump  31  is unoperated, and the discharge valve  53  is closed. Also, the suction valve  51  which controls the communication between the first ink tank  81  to be used and the suction pump  31  is open, and the suction valve  52  which controls the communication between the second ink tank  82  not to be used and the suction pump  31  is closed. 
     Next, the controller  10  opens the discharge valve  53  to discharge the ink remaining in the counting tank  21  to the intermediate tank  22  (in Step S 102 ). Then, the controller  10  judges whether the liquid level of ink in the counting tank  21  is higher than the aforementioned first vertical position or not, based on the sensing signal sent from the low liquid level sensor  211  (in Step S 103 ). When the liquid level of ink in the counting tank  21  is higher than the first vertical position, the discharge of ink is continued, and the procedure returns to Step S 103 . 
     When the liquid level of ink in the counting tank  21  becomes lower than the first vertical position in Step S 103 , the controller  10  closes the discharge valve  53  (in Step S 104 ). This stops the discharge of ink from the counting tank  21  to the intermediate tank  22 . 
     Then, the controller  10  increases the value of the count N by one. That is, the controller  10  increments the count N (in Step S 105 ). Then, the controller  10  judges whether the count N is less than a predetermined value or not (in Step S 106 ). The predetermined value is a value determined by the ink capacity of the ink tank  8  to be used and the ink capacity of the counting tank  21 . The predetermined value is less than the quotient of the ink capacity of the ink tank  8  divided by the ink capacity of the counting tank  21 , and is preferably a greater value. In the present preferred embodiment, the predetermined value may be set to 178, for example, because the ink capacity of the ink tank  8  is 18 liters and the ink capacity of the counting tank  21  is 100 milliliters. 
     When the count N is not less than the predetermined value in Step S 106 , the procedure proceeds to Step S 111  in which the ink tank  8  is changed. In the present preferred embodiment, the suction valve  51  is closed and the suction valve  52  is opened, whereby the ink tank  8  to be used is changed from the first ink tank  81  to the second ink tank  82 . The change of the ink tank  8  may be done either manually or automatically by the controller  10 . After the controller  10  automatically changes the ink tank  8 , the count N may be reset to zero and the procedure may return to Step S 106 . 
     On the other hand, when the count N is less than the predetermined value in Step S 106 , the suction pump  31  is brought into operation (in Step S 107 ). This causes the supply of ink from the ink tank  8  to the counting tank  21 . 
     Subsequently, the controller  10  judges whether the liquid level of ink in the counting tank  21  is higher than the aforementioned second vertical position or not, based on the sensing signal sent from the high liquid level sensor  212  (in Step S 108 ). When the liquid level of ink in the counting tank  21  is not higher than the second vertical position, the supply of ink is continued, and the procedure returns to Step S 108 . 
     When it is judged in Step S 108  that the liquid level of ink in the counting tank  21  reaches the second vertical position, the suction pump  31  is stopped (in Step S 109 ). This stops the supply of ink from the ink tank  8  to the counting tank  21 . 
     Thereafter, the discharge valve  53  is opened to start the discharge of ink (in Step S 110 ). Then, the procedure returns to Step S 103 . 
     Steps S 104  to S 107  in which the suction pump  31  is brought into operation and the discharge valve  53  is closed after the liquid level of ink in the counting tank  21  becomes lower than the first vertical position shall be collectively referred to as a first process. Steps S 109  to S 110  in which the suction pump  31  is stopped and the discharge valve  53  is opened after the liquid level of ink in the counting tank  21  reaches the second vertical position shall be collectively referred to as a second process. The controller  10  repeats the first process and the second process to measure the number of repetitions by incrementing the count N in Step S 105 . Then, the controller  10  estimates the volume of ink supplied from the ink tank  8  to the intermediate tank  22 , based on the count N, thereby to estimate the volume of ink remaining in the ink tank  8  being used. As a result, before the volume of ink remaining in the ink tank  8  becomes zero, the change to another ink tank  8  is done to prevent a situation such that ink can no longer be supplied to the counting tank  21 . Also, the entry of air into the pipe  41  is suppressed. 
     While the liquid level of ink in the intermediate tank  22  is higher than the second sensor  222  in Steps S 102  and S 110 , the opening of the discharge valve  53  may be temporarily stopped. 
     &lt;1-3. Operation of Supply Pump&gt; 
     Next, the operation of the supply pump  32  will be described.  FIG. 4  is a graph showing an example of a relationship between the request signals from the two printers  9  and the feed volume of the liquid from the supply pump  32  (the feed volume of the liquid per unit time; the same shall apply hereinafter).  FIG. 5  is a flow diagram showing a procedure for operation of the supply pump  32  in response to the request signals from the two printers  9 . The abscissa of  FIG. 4  represents time which is common to upper, middle and lower parts of  FIG. 4 . The upper part of  FIG. 4  represents a request signal  91 P from the first printer  91 ; the middle part thereof represents a request signal  92 P from the second printer  92 ; and the lower part thereof represents the feed volume W of the liquid from the supply pump  32 . 
     In the present preferred embodiment, assuming that the maximum feed volume of the liquid from the supply pump  32  is 100%, the supply pump  32  supplies ink in a feed volume W of 50% to the first printer  91 , and supplies ink in a feed volume W of 50% to the second printer  92 . 
     At time T 0  in the example of  FIG. 4 , the request signal  91 P and the request signal  92 P are OFF, and the operation starts when the supply pump  32  is in a stopped state. Thereafter, the request signal  91 P is turned ON at time T 1 , and the signal  92 P is subsequently turned ON at time T 2 . Then, the request signal  91 P is turned OFF at time T 3 , and the request signal  92 P is turned OFF at time T 4 . Changes in the feed volume W of the liquid from the supply pump  32  in this operation will be described with reference to  FIG. 5 . 
     First, the request signal  91 P is turned ON at the time T 1 . In other words, the controller  10  receives the request signal  91 P (in Step S 301 ). 
     Next, the controller  10  acquires the target feed volume Wg of the liquid from the supply pump  32 , based on the received request signal  91 P (in Step S 302 ). In the present preferred embodiment, the target feed volume Wg of the liquid which is 50% is acquired because the feed volume W of the liquid to the first printer  91  is 50% of the maximum feed volume. 
     Then, the controller  10  sends a driving signal S to the supply pump  32 , based on the acquired target feed volume Wg (in Step S 303 ). Subsequently, the supply pump  32  operates, based on the received driving signal S (in Step S 304 ). Specifically, the supply pump  32  feeds the liquid in accordance with the output of the driving signal S. In response to the driving signal S, the supply pump  32  gradually changes the feed volume W from a current feed volume Wn of 0% to a target feed volume Wg of 50%. In the example of  FIG. 4 , the feed volume W gradually increases from the time T 1  at which the request signal  91 P is received, and reaches a target feed volume Wg of 50% after a lapse of time t. A procedure during the output of the driving signal S in Step S 303  will be described later. 
     Likewise, when the request signal  92 P is turned ON at the time T 2 , the controller  10  acquires a target feed volume Wg of 100%. The feed volume W of the liquid from the supply pump  32  gradually changes from 50% to 100%. Next, when the request signal  91 P is turned OFF at the time T 3 , the controller  10  acquires a target feed volume Wg of 50%. The feed volume W of the liquid from the supply pump  32  gradually changes from 100% to 50%. When the request signal  92 P is turned OFF at the time T 4 , the controller  10  acquires a target feed volume Wg of 0%. The feed volume W of the liquid from the supply pump  32  gradually changes from 50% to 0%. 
     Gradually changing the feed volume W of the liquid from the supply pump  32  in this manner suppresses an abrupt change in pressure in the printers  9  which is a destination to which ink is supplied. 
     &lt;1-4. Driving Signal&gt; 
     Next, the driving signal S sent from the controller  10  to the supply pump  32  will be described.  FIG. 6  is a graph showing waveforms of the driving signal S during changes in output.  FIG. 7  shows the correspondence table  111  between the output number of the driving signal S and a duty ratio.  FIG. 8  is a flow diagram showing a procedure in the controller  10  during the output of the driving signal S. 
     In the present preferred embodiment, the operation of the supply pump  32  is under PWM (pulse width modulation) control. Specifically, the driving signal S sent from the controller  10  to the supply pump  32  is in the form of a rectangular pulse wave whose high state voltage is constant. The pulse wave has a constant frequency and a pulse width with a variable duty ratio D. Thus, the output of the driving signal S in the present preferred embodiment is the duty ratio D of the pulse width. 
     The feed volume W of the liquid from the supply pump  32  depends on the driving signal S inputted thereto. In other words, the feed volume W of the liquid from the supply pump  32  varies in corresponding relation to the duty ratio D of the driving signal S. In the present preferred embodiment, the feed volume W of the liquid from the supply pump  32  and the duty ratio D of the driving signal S are in the following corresponding relation: D=0% for W=0%; D=50% for W=50%; and D=100% for W=100%. 
       FIG. 6  shows an example of the driving signal S in the case where the driving signal S is gradually changed from an initial output to a target output. The abscissa of  FIG. 6  represents time. The driving signal S shown in  FIG. 6  is that obtained in the case where the duty ratio D is changed from 0% to 50%, from 50% to 100%, from 100% to 50%, and from 50% to 0% in top-to-bottom order. In the example of  FIG. 6 , the duty ratio D of the driving signal S changes stepwise from the initial output to the target output in steps of 10%. 
       FIG. 7  shows the correspondence table  111  between the output number M of the driving signal and the duty ratio D. The values of the output number M, and the values of the duty ratio D, an OFF time interval and an ON time interval corresponding to the output number M are listed in the correspondence table  111 . In the present preferred embodiment, the driving signal S has a period of 10 ms, the sum of the OFF time interval and the ON time interval per period is 10 ms. A procedure using the correspondence table  111  during the output of the driving signal S will be described below with reference to  FIG. 8 . 
     At the start of driving of the liquid supply apparatus  1 , the supply pump  32  is not driven, so that the feed volume W of the liquid from the supply pump  32  is 0%. At this time, the controller  10  does not send the driving signal S, so that the duty ratio D of the driving signal S is 0%. When the liquid supply apparatus  1  is driven, the controller  10  sets a current output number Mn and a target output number Mg to zero in corresponding relation to a duty ratio D of 0% (in Step S 501 ). At this time, the output of the driving signal S is OFF. 
     Next, the controller  10  acquires the target output number Mg corresponding to a target duty ratio, based on the target feed volume Wg of the liquid from the supply pump  32  acquired in Step S 302  described above (in Step S 502 ). 
     Then, the controller  10  judges whether the current output number Mn coincides with the target output number Mg or not (in Step S 503 ). When the current output number Mn coincides with the target output number Mg, the procedure proceeds to Step S 507 . 
     When the current output number Mn does not coincide with the target output number Mg, the controller  10  judges whether the current output number Mn is less than the target output number Mg or not (in Step S 504 ). When the current output number Mn is less than the target output number Mg, the value of the current output number Mn is increased by one. That is, the current output number Mn is incremented (in Step S 505 ). Then, the procedure proceeds to Step S 507 . On the other hand, when the current output number Mn is greater than the target output number Mg, the value of the current output number Mn is decreased by one. That is, the current output number Mn is decremented (in Step S 506 ). Then, the procedure proceeds to Step S 507 . 
     In Step S 507 , the controller  10  references the correspondence table  111  stored in the storage area  11  to acquire the OFF and ON time intervals corresponding to the current output number Mn. 
     Subsequently, the controller  10  judges whether the acquired OFF time interval is zero or not (in Step S 508 ). When the acquired OFF time interval is zero, the procedure proceeds to Step S 511 . When the acquired OFF time interval is not zero, the controller  10  turns OFF the output of the driving signal S (in Step S 509 ), and waits for the acquired OFF time interval (in Step S 510 ). 
     Then, the controller  10  judges whether the acquired ON time interval is zero or not (in Step S 511 ). When the acquired ON time interval is zero, the procedure returns to Step S 502 . When the acquired ON time interval is not zero, the controller  10  turns ON the output of the driving signal S (in Step S 512 ), and waits for the acquired ON time interval (in Step S 513 ). Then, the procedure returns to Step S 502 . 
     Using the aforementioned procedure, the controller  10  changes the driving signal S in the form of a pulse wave while acquiring the duty ratio D by reference to the correspondence table  111 . Specifically, the controller  10  acquires the target duty ratio and the target output number Mg corresponding to the target feed volume Wg, based on the request signal from the at least one printer  9  to change the duty ratio D of the driving signal S stepwise from the current duty ratio to the target duty ratio. In the present preferred embodiment, the duty ratio D of the driving signal S changes stepwise in steps of 10%, as shown in  FIG. 6 . This gradually changes the feed volume W of the liquid from the supply pump  32 . As a result, an abrupt change in pressure in the at least one printer  9  which is a destination to which ink is supplied is suppressed. 
     In the aforementioned example, the duty ratio D of the driving signal S is changed for each period of a pulse. The present invention, however, is not limited to this. The duty ratio D of the driving signal S may be changed for each group of periods. This changes the feed volume W of the liquid from the supply pump  32  more slowly to further suppress an abrupt change in pressure in the at least one printer  9  which is a destination to which ink is supplied. 
     2. Modification 
     While the one preferred embodiment according to the present invention has been described hereinabove, the present invention is not limited to the aforementioned preferred embodiment. 
     In the aforementioned preferred embodiment, the required feed volume of the liquid from the supply pump  32  to the first printer  91  is 50%, and the required feed volume of the liquid from the supply pump  32  to the second printer  92  is 50%. The present invention, however, is not limited to this. When the at least one printer  9  includes a plurality of printers  9 , the required feed volumes different from each other may be set for the respective printers  9 . In this case, the maximum value of the target feed volume of the liquid from the supply pump  32  is the sum of the required feed volumes of the liquid to the respective printers  9 . Also, the target feed volume is the sum of the required feed volumes of the liquid to the respective printers  9  outputting the request signals. This achieves the supply of ink in appropriate volumes to the respective printers  9 . 
     The required feed volumes of the liquid to the printers  9  may correspond to the lengths of the supply paths to the printers  9 , respectively. For example, when the total length of the pipes  430  and  431  serving as the supply path to the first printer  91  is shorter than the total length of the pipes  430  and  432  serving as the supply path to the second printer  92 , the required feed volume of the liquid to the first printer  91  may be less than the required feed volume of the liquid to the second printer  92 . 
     The controller  10  may vary the feed volume of the liquid from the suction pump  31 , based on the request signals from the printers  9 . For example, the controller  10  may control the suction pump  31  so that the greater the number of printers  9  outputting the request signals or the target feed volume is, the greater the feed volume of the liquid from the suction pump  31  is. This allows the volume of ink in the intermediate tank  22  to be maintained at an appropriate volume. 
     In the aforementioned preferred embodiment, the duty ratio D of the driving signal S is changed in steps of 10%. The present invention, however, is not limited to this. The duty ratio D may be changed in steps of 5%, 1% or less than 1%. Similarly, the number of steps of the duty ratio D of the driving signal S for one printer  9  is five in the aforementioned preferred embodiment. The present invention, however, is not limited to this. The number of steps of the duty ratio D for one printer  9  may be less than or more than five. Also, when the at least one printer  9  includes a plurality of printers  9 , the number of steps of the duty ratio D may be set for each of the printers  9 . 
     In the aforementioned preferred embodiment, the driving signal S has a period of 10 ms, i.e. a frequency of 0.1 kHz. The present invention, however, is not limited to this. The driving signal S may have a frequency of 1 kHz, for example. An appropriate frequency may be used as the frequency of the driving signal S, as required, in accordance with the output from the liquid supply apparatus  1  and the like. 
     In the aforementioned preferred embodiment, the controller  10  uses the PWM control to control the operation of the supply pump  32 . The present invention, however, is not limited to this. The controller  10  may control the operation of the supply pump  32  by changing the driving voltage stepwise, in place of the PWM control. The PWM control is, however, preferable in being able to change the output while maintaining a rated voltage suitable for the driving of the supply pump  32 . 
     The configurations of the details of the liquid supply apparatus may differ from those shown in the figures of the present invention. The components described in the aforementioned preferred embodiment and in the various modifications may be consistently combined together, as appropriate. 
     The liquid supply apparatus according to the present invention is not limited to an ink supply apparatus for a printer, but may be, for example, an apparatus for supplying a treatment liquid to an apparatus (a liquid consuming apparatus) which applies the treatment liquid to surfaces of a semiconductor substrate and a substrate for a flat panel display device. 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.