Abstract:
A system and method for supplying replenishment fluid to an ink tank, wherein the ink tank includes a supply line to supply ink to a nozzle and a return line to receive unused ink. The method comprises the steps of periodically using a control system to recalibrate the ink pressure along the supply line to obtain an optimal print quality; and after the ink pressure has been recalibrated, using the control system to automatically adjust a supply rate of the replenishment fluid to the ink tank, wherein the supply rate of the replenishment fluid is adjusted based on the recalibrated ink pressure.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to ink replenishment systems, and more specifically to an ink replenishment system and method for a continuous flow ink jet printer using pressure feedback. 
     2. Background Art 
     Since its mass-market introduction in the mid-1970&#39;s, ink jet printing continues to be one of the most popular printing technologies. Ink jet is a non-impact dot-matrix printing technology in which droplets of ink are jetted from a small aperture directly to a specified position on a media to create an image. Continuous flow ink jet printing involves a process in which a stream of ink is broken into droplets of uniform size and spacing and passed through an electrical charge. The charged drops are deflected into a gutter for recirculation, and the uncharged drops fly directly onto the media to form an image. 
     One of the difficulties of continuous flow ink jet ink printing involves ink quality degradation caused during the recirculation process. In particular, as the ink flows from the print nozzles, water from the ink evaporates. Accordingly, the ink that is recirculated back to the ink tank tends to be more viscous. To compensate, a replenishment tank holding replenishment fluid is connected with a valve to the ink tank to help maintain a constant ink viscosity. 
     In current embodiments, the amount of replenishment fluid that flows to the ink tank is controlled using some type of manual adjustment. In one typical embodiment, the valve connecting the replenishment tank to the ink tank allows a fixed amount of replenishment fluid to enter the ink tank during periodic activations of the valve. Unfortunately, fluctuations in external conditions, such as humidity, can significantly impact the amount of required replenishment fluid. Accordingly, incorrect viscosity can occur over relatively short durations and result in print failure. 
     Thus, a need exists for an ink replenishment system that can ensure a correct amount of replenishment fluid is being added to the ink tank in a continuous flow ink jet printer. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above-mentioned problems, as well as others, by providing an ink replenishment system and method for a continuous flow ink jet printer that uses pressure feedback data to regulate the amount of replenishment fluid to be added to the ink tank. 
     In a first aspect, the invention provides a continuous flow ink jet printer, comprising: a nozzle that receives ink from a supply line and generates ink drops; an ink tank for providing ink to the supply line; an ink gutter for recirculating unused ink drops back to the ink tank; a replenishment tank that provides replenishment fluid for the ink tank; and a control system that controls the flow of replenishment fluid to the ink tank based on an ink pressure along the flow line. 
     In a second aspect, the invention provides a replenishment system for supplying replenishment fluid to a supply tank, wherein the supply tank includes a supply line to supply a source fluid to a work piece and a return line to receive unused source fluid, the system comprising: a replenishment tank that provides replenishment fluid for the supply tank, wherein the replenishment fluid adjusts a viscosity of the source fluid; a valve that activates a flow of replenishment fluid into the supply tank; and a control system that controls the activation of the valve based on a pressure in the supply line proximate the work piece. 
     In a third aspect, the invention provides a method for supplying replenishment fluid to an ink tank, wherein the ink tank includes a supply line to supply ink to a nozzle and a return line to receive unused ink, the method comprising: periodically using a control system to recalibrate the ink pressure along the supply line to obtain an optimal print quality; and after the ink pressure has been recalibrated, using the control system to automatically adjust a supply rate of the replenishment fluid to the ink tank, wherein the supply rate of the replenishment fluid is adjusted based on the recalibrated ink pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts a diagram of a continuous flow ink jet printer having a replenishment system in accordance with the present invention. 
         FIG. 2  depicts a flow diagram of a method for implementing a replenishment system in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings,  FIG. 1  depicts a diagram of a continuous flow ink jet printer  10  that utilizes a replenishment system in accordance with the present invention. Printer  10  includes an ink tank  12  for storing a source fluid, in this case, ink. The ink from the ink tank  12  is pumped to a nozzle  18  using a motor  16  via a supply line  14 . From the nozzle  18 , ink drops  19  are sprayed onto a receiving medium, e.g., paper, in a controlled manner. As noted above, a certain portion of ink drops  19  that are unused during spraying are collected in an ink gutter  20  and recirculated back to the ink tank  12 . As also noted, water contained in the ink can be evaporated as the ink flows through the nozzle  18 . Accordingly, the recirculated ink may alter the viscosity of the ink in the ink tank  12 . 
     In order to maintain an ideal viscosity, replenishment fluid is injected into the ink tank  12 . The replenishment fluid flows from a replenishment tank  22 , through a valve  24  and into the ink tank  12 . The amount of replenishment fluid added to the ink tank  12  is controlled by periodically activating the valve  24  to allow replenishment fluid to pass to the ink tank  12  for a predetermined length of time. Activation of valve  24  is automatically controlled by controller  28 , and can be activated in any manner, e.g., with a solenoid  26 . 
     Controller  28  has two functions, namely (1) to regulate the pressure in the flow line  14 , and (2) to control the replenishment rate of replenishment fluid. First, controller  28  periodically recalibrates the pressure in the supply line  14  until an optimum printing is achieved. Algorithms for calculating optimum printing based on sensed pressure are known in the art and the process for regulating pressure may be implemented in any manner. The exemplary embodiment shown in  FIG. 1  utilizes a pressure sensor  30  that senses a pressure proximate the nozzle  18 , a pressure regulator  32 , and a stepper motor  34  that steps up or down the pressure at the pressure regulator  32  based on feedback from controller  28 . Pressure is regulated in this manner periodically, e.g., when new ink is added, during printer recalibrations, etc. Thus, the pressure is changed from time to time to ensure optimum print quality is achieved. In a typical embodiment, a printer may be recalibrated every 24 hours to provide the correct pressure. However, the present invention is not limited to a specific recalibration period. 
     The second function provided by the controller  28  is to control the replenishment rate of the replenishment fluid to the ink tank based on the pressure in the supply line  14 . Specifically, once a recalibrated pressure value is determined, which will result in optimum printing, the controller  28  uses the value to calculate a new replenishment rate for the replenishment fluid, based on an algorithm  29 . Thus, the replenishment rate of the replenishment fluid is automatically changed whenever a new pressure is applied to the supply line  14 . 
     In the following exemplary embodiment, the replenishment rate is implemented by varying the time interval Ti between injections. Thus, as the value of Ti decreases, the amount of replenishment fluid that is added over a given time period increases. However, it should be understood that any method for controlling the replenishment rate based on the recalibrated pressure value may be utilized. Initially, when new ink is installed in the printer, the pressure regulator  32  is adjusted under program control until optimum printing is detected. The resultant nozzle pressure is sensed by the pressure sensor  30  and is recorded by an algorithm  29  in controller  28  as new ink pressure, Pn. The replenishment rate for the replenishment fluid when new ink is added may be set in any manner, e.g., manually via the controller  28  based on a manufacturer&#39;s recommendation, etc. A recalibration of the supply line pressure is thereafter conducted periodically, e.g., every 24 hours, and the recalibrated pressure Pr is also sensed and captured by the algorithm  29 . A replenishment rate is then calculated by the algorithm  29  based on a difference between new ink pressure Pn and the current recalibrated pressure Pr. 
     In one exemplary embodiment for implementing the algorithm  29 , two known factors are utilized, the normal evaporation rate En (ml/hr), and the volume Vp of replenishment fluid required to be added to the ink tank  12  to lower the head pressure 1 psi (ml/psi). The volume of fluid Vi (ml), injected per time interval Tn (seconds), is therefore also known. The time interval Tn, to add the nominal evaporation volume is:
 
 Tn (seconds)=( Vi/En )*3600 sec/hr
 
     After a recalibration is accomplished, the recalibrated pressure Pr is read. The difference between the recalibrated pressure, Pr and the new ink pressure Pn is calculated. This difference can be used to find the volume of fluid to be added or withheld until the next recalibration to bring the pressure back to new ink pressure. The volume of fluid delta, Vd, is: 
       Vd (ml)=( Pr−Pn )* Vp   
     The number of injections, Ni to correct the viscosity that must be injected until the next recalibration is:
 
 Ni=Vd/Vi 
 
Assuming 24 hours between recalibrations, the time period delta Td between injections is:
 
 Td (seconds)=(24 hr*3600 seconds/hr)/ Ni 
 
This is the time delta to alter the time of nominal evaporation rate, Tn. The resultant injection time interval, Ti is:
 
 Ti=Tn−Td. 
 
Thus, if the recalibrated pressure Pr is higher indicating thicker ink, the injection time interval, Ti should decrease to inject replenishment fluid at a faster rate.
 
     Referring now to  FIG. 2 , a flow diagram of an exemplary method of implementing the invention is shown. At step S 1 , new ink is added to the printer, and an initial replenishment rate is set for the ink replenishment system. At step S 2 , the ink pressure is changed using a feedback controller and stepper motor to achieve optimum print quality. Once achieved, the new ink pressure value Pn is saved at step S 3 . At step S 4 , the ink pressure is periodically recalibrated to achieve optimum print quality, and the recalibrated pressure value Pr is saved at step S 5 . Next, at step S 6 , after each recalibration, the controller is used to calculate a new replenishment rate for the ink replenishment system based on a difference Pr−Pn. Finally, at step S 7 , the new replenishment rate is used to control the ink injection into the ink tank until the next recalibration. 
     It should be appreciated that while the above description describes systems and methods for controlling the flow of replenishing fluid into an ink tank in a continuous flow ink jet printer, the invention could be applied to any continuous flow system (including ink, paint, fertilizer, liquid, gas, etc.) that utilizes a work piece (such as a nozzle) in which replenishing fluid needs to be added to a source fluid in a source tank. 
     Moreover, it is understood that the controller  28  described herein can be implemented in hardware, software, or a combination of hardware and software. It may be implemented by any type of computer system or other apparatus adapted for carrying out the methods described herein. A typical combination of hardware and software could be a general-purpose computer system with a computer program that, when loaded and executed, controls the computer system such that it carries out the methods described herein. Alternatively, a specific use computer, containing specialized hardware for carrying out one or more of the functional tasks of the invention could be utilized. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods and functions described herein, and which—when loaded in a computer system—is able to carry out these methods and functions. Computer program, software program, program, program product, or software, in the present context mean any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: (a) conversion to another language, code or notation; and/or (b) reproduction in a different material form. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teachings. Such modifications and variations that are apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.