Patent Publication Number: US-6209997-B1

Title: Impulse fluid jet apparatus with depriming protection

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part and claims the benefit of the filing date of U.S. patent application Ser. No. 08/828,758, filed Mar. 25, 1997. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a drop-on-demand or impulse fluid jets which eject a droplet of fluid such as ink in response to the energization of a transducer. 
     BACKGROUND OF THE INVENTION 
     Impulse fluid or ink jets are designed and driven so as to eject a droplet of fluid such as ink on demand from a chamber through an orifice in the chamber. Where impulse jets are utilized in many applications including industrial applications, it is important that the impulse ink jets operate reliably. Such reliability can be jeopardized where the impulse jets can be deprimed due to fluid disturbances in the supply of ink to and through the impulse jet. Such depriming can occur as a result of brief disturbances to the fluid supply as well large, longer disturbances caused by, for example, bumping the apparatus. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, an apparatus is provided for preventing depriming of an impulse jet. 
     In still further accordance with the invention, an apparatus is provided for preventing depriming of an impulse jet in response to small and/or brief disturbances in the ink or fluid supply line to the impulse ink jet or elsewhere. 
     In still further accordance with the invention, an apparatus is provided for preventing depriming of an impulse jet in response to large and/or longer disturbances in fluid supply line or elsewhere. 
     In accordance with a preferred embodiment of the invention, an impulse ink fluid apparatus comprises a transducer, and a fluid jet chamber coupled to the transducer, the chamber having an orifice through which droplets of fluid are ejected in response to the energization of the transducer. A fluid supply is coupled to the fluid jet chamber through a compliant chamber which forms at least part of a low pass filter substantially attenuating fluid disturbances having a duration substantially less than time constant of the low pass filter formed by the compliant chamber. 
     In the preferred embodiment, the time constant represented by the product of the fluidic capacitance of the compliant chamber and at least a portion of the fluidic resistance of the fluid supply substantially attenuates pressure disturbances having a duration less than 0.01 the value of the time constant. In the preferred embodiment, disturbances of less than 0.01 and preferably less than 0.05 seconds will be attenuated. 
     In the preferred embodiment, the compliant chamber comprises a compliant member for absorbing pressure waves. The compliant chamber comprises a flexible diaphragm which is nonplanar in the undisturbed state such that deformation is nonlinear with respect to changes in pressure. Preferably, pressure waves are absorbed without pressure increases in the compliant chamber. In the preferred embodiment, the compliant chamber comprises an air passage allowing ambient air pressure to flow through and reach the compliant member. 
     The compliant chamber may also include a filter permitting the flow of ink through the filter from the ink supply to the fluid jet chamber. 
     In one preferred embodiment, the apparatus further comprises at least one check valve located between the fluid jet or a manifold serving a plurality of fluid jets and the compliant chamber for preventing the reverse flow of ink from the ink jet chambers to the compliant chamber while permitting ink to flow from the compliant chamber to the ink jets. Each check valve includes passageways permitting the passage of air through the check valves. In this regard, the check valve comprises a valve seat, a valve member, a valve support comprising at least one passage permitting fluid flow between the valve seat and the valve member and through the fluid passage toward the fluid chamber. The check valve includes a valve body forming the valve seat and containing the valve member and the valve support such that the fluid passage in the valve support is located adjacent to the valve body. Preferably, the valve support comprises a plurality of passages located adjacent to the valve body. 
     In another preferred embodiment, the check valve, which acts as a rectifier to maintain positive pressure at the orifice(s), is coupled between the fluid supply and the compliant chamber. Moreover, in this embodiment the compliant chamber holds negative pressure between −0.1 and −10 in-H 2 O created by orifice jetting and the static height of the ink supply. Further, the check valve is characterized by a cracking pressure of between 0.1 and 3 in-H 2 O, whereby excessive buildup of negative pressure at the orifice during jetting is prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an impulse ink jet head incorporating the invention shown in cross-sectional form with the ink supply with the compliant chamber rotated 90° for purposes of clarity. 
     FIG. 2 is a view of the impulse ink jet head taken along section line  2 — 2  of FIG.  1 . 
     FIG. 3 is a sectional view of the compliant chamber taken along line  3 — 3  of FIG.  2 . 
     FIG. 4 is a sectional view of the compliant chamber taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is an enlarged sectional view of the check valve and the ink jet chamber shown in FIG.  1 . 
     FIG. 6 is a sectional view taken along line  6 — 6  of FIG. 5 depicting the restrictor plate forming a portion of a plurality of ink jets in the head. 
     FIG. 7 is a perspective view of a check valve support member shown in FIG.  5 . 
     FIG. 8 is an equivalent electrical circuit for the fluidic system shown in FIGS. 1-7. 
     FIG. 9 corresponds to FIG. 8 with the equivalent electrical circuit broken into sections A through C. 
     FIG. 10 is a classical first order low pass filter schematic. 
     FIG. 11 depicts an alternative embodiment of an impulse ink jet head in accordance with the present invention. The primary difference between the embodiments depicted in FIGS. 11 and 1, respectively, is in the location of the check valve  32 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an impulse fluid or ink jet print head  10  comprises a chamber plate  12  and an orifice plate  14  forming a plurality of ink jet chambers  16  coupled to a transducer  18  through a foot  20  and a diaphragm  22  shown in enlarged form in FIG.  5 . The transducer  18  is energized and de-energized by applying a voltage transverse to the longitudinal axis of the transducer  18  so as to operate the transducer in an expander mode, i.e., the transducer expands and contracts along the longitudinal axis parallel to arrows  19  to cause volumetric changes in the chamber  16  to jet droplets from orifices  15  in the plate  14 . 
     Referring to FIGS. 1 and 5, fluid in the form of ink is supplied to each chamber  16  through restricted inlets  24  formed in a restrictor plate  26  between the chamber plate  12  and a spacer plate  27  which is sandwiched up against the diaphragm  22  best shown in FIG.  5 . Such restricted inlets  24  are shown in FIG. 6 which depicts the restrictor plate  26  having enlarged openings  28  as explained in co-pending application Ser. No. 08/823,718, filed Mar., 25, 1997, titled “High Performance Impulse Ink Jet Method and Apparatus”, which is incorporated herein by reference. The foregoing structure including the size of the chambers  16  in conjunction with the resonant frequency of the transducers  18  produce a high performance ink jet. 
     Each of the restricted inlets  24  is supplied by a manifold  30  shown in FIGS. 1 and 5 as well as FIG.  2 . In accordance with one important aspect of the invention, the manifold  30  is supplied with fluid in the form of ink through two check valves  32  as shown in FIGS. 1,  2  and  5 . The check valves  32  are designed in a manner so as to prevent the reverse flow of ink from the  24  back through the fluid supply lines in the print head  10 . Any check valve design which will prevent reverse flow from chamber  24  can be employed in the present invention. No particular design is preferred, however, for the purposes of illustrating the operation of the invention, the embodiment shown in FIG. 5 is described in detail herein. The check valves  32  shown in FIGS. 1,  2  and  5  comprise a flotable valve plate  34  movable by ink flow so as to be seated against a valve seat  36  in the print head  10  when the valve is closed due to back pressure from the fluid in manifold  30  as shown in dotted lines in FIG.  5 . The valve plate  34  when open permits the flow of ink into the chamber  16  as shown in FIG.  5 . As the supply of ink in manifold  30  is depleted the pressure in the manifold drops so that the ink within supply channel  48  forces valve plate  34  against end  38 . Ink then fills manifold  30  and is ejected from the orifice  15  as was described above. Details of the support  40  can be best appreciated by reference to FIG. 7 wherein flow channels  42  leading away from the end  38  and toward the manifold  30  are provided and a flange  44  through which the channels  42  pass is also provided for engaging a seat  46  in the print head in the valve body for the diaphragm  34  as best shown in FIG.  5 . 
     Referring again to FIGS. 1 and 2, the check valves  32  are located in the print head at the end of supply channels  48 . In accordance with another important aspect of this invention, the supply channels  48  are terminated in a compliant chamber  49  shown in FIGS. 1 and 2 and best shown in FIGS. 3 and 4. The compliant chamber is provided to attenuate brief disturbances which might otherwise deprime the print head. To this end, the compliant chamber comprises a flexible membrane  50  which is capable of movement in the direction shown by the arrows  52  in an amount sufficient to absorb ink pressure disturbances in the supply line through the print head so as to prevent depriming of the head. The membrane  50  is held in place between a stainless steel member  54  and a filter assembly  56 . The stainless steel member  54  is in turn held in place between the membrane  50  and another gasket  58 . In order to provide total freedom of movement of the flexible membrane  50 , a vent hole  60  is provided in the print head juxtaposed to the membrane  50  so as to allow air to escape which is displaced by the membrane  50 . 
     The filter assembly  56  includes a filter  62 . As will be appreciated with reference to FIGS. 1-4, ink is free to flow into the compliant chamber  49  from an inlet  66  on the membrane side of the filter  62 . 
     In order for the compliant chamber to best serve its purpose of preventing depriming of the ink jet, it is preferable that the membrane displacement be non-linear with respect to changes in pressure. For this purpose, the membrane  50  is shown as concave with respect to the interior of the chamber  49  such that resistance to deformation increases concavity of the diaphragm. In other words, deformation of the diaphragm is non-linear with respect to changes in pressure within the compliant chamber. 
     As also shown in FIG. 1, the inlet  66  of the print head is supplied with ink through a flexible tube  70  leading to a reservoir  72 . The reservoir  72  as shown is including another filter  74  to assure that no agglomerations in the ink greater than the filler rating pass into the print head  10 . Also note that the level of ink  76  in the reservoir  72  is maintained below the height of the ink jet chamber  14  so as to assure that no ink pressure at the chamber  16  thus avoiding weeping of ink from the orifices  15  in the orifice plate  14 . 
     In order to better appreciate the manner in which the compliant chamber can prevent pressure disturbances from depriming ink jets, reference will now be made to FIG. 8 wherein an electrical circuit is shown which is equivalent to the fluidic circuit of the ink jet apparatus shown in FIG.  1 . In particular, the apparatus shown in FIG. 1 is depicted in equivalent electrical circuit form using capacitance, resistance and inductance and a pressure disturbance equivalent to a voltage pulse being simulated by a pulse generator. More specifically, the equivalent circuit shown in FIG. 1 comprises a capacitance  80  and a resistance  82  corresponding to the capacitance and resistance of the orifice  15  in the orifice plate  14  respectively. A resistance  84  corresponds to the resistance of the restrictor  24 . A capacitance  86  and a resistance  88  correspond to the fluidic capacitance and resistance of the manifold  30  including the feed lines  46 . A capacitance  90  corresponds to the fluidic capacitance of the compliant chamber. A resistance  92  and a resistance  94  correspond to the fluidic resistance of the feed line  70  while an inductance  96  and an inductance  98  correspond to the fluidic inductance of that feed line  72 . A capacitance  102  corresponds to the fluidic capacitance of the reservoir  72  and a voltage source  104  corresponds to the fluidic voltage or pressure generated by the reservoir  72 . Any pressure disturbance in the feed line which is of a nature which could otherwise deprime the ink jet is depicted by a signal generator  106  located between the resistances  92  and  94  and the inductances  96  and  98  of the feed line. 
     The equivalent circuit of FIG. 8 may be broken down for purposes of analysis into a series of low pass filters depicted in FIG.  9 . More specifically, a first low pass filter is provided by sub-circuit  108  comprising the fluidic capacitance  80  and the fluidic resistances  82  and  84 . A second sub-circuit  110  comprises fluidic capacitance  86  and fluidic resistance  88 . A third sub-circuit  112  comprises fluidic capacitance  90  of the compliant chamber and fluidic resistance  92  which is part of the feed line  70  resistance. 
     As may be seen be reference to FIG. 10, each of the sub-circuits  108 ,  110  and  112  effectively form a classic first order low pass filter where v i (t) is an input voltage corresponding to the disturbance in the feed line and the v o (t) is the output voltage. By proper selection of the resistance R and the capacitance C of the low pass filter, the output voltage v o (t) corresponding to the output effect of the pressure disturbance represented by the v i (t) may be severely attenuated if the duration of the disturbance is less than the time constant Υ corresponding to the product RC. In other words, disturbances which are sufficiently brief in time will be severely attenuated by the low pass filter represented by the sub-circuit  112  corresponding to the low pass filter represented by the RC combination of the compliant chamber capacitance  90  and the line resistance  92 . More specifically, disturbances having a duration shorter than 10% of Υ equal to the product of RC will be sufficiently attenuated so as not to have any material affect on the operation of the print head: i.e., will not deprime the print head or cause weeping through the orifices. In this regard, the flexible membrane is chosen so as to produce a time constant Υ of at least 0.1 second and preferably more than 0.5 seconds such that disturbances less than {fraction (1/10)} or 10% of the time constant Υ will be substantially attenuated by the low pass filter formed by the compliant chamber in conjunction with the feed line resistance. More specifically, for a time constant of 0.1 seconds, the disturbance of less than 0.01 seconds or less than {fraction (1/10)} or 10% of 0.1 seconds will have no effect on depriming or weeping. Similarly, for a time constant of at least 0.5 seconds, a disturbance of 0.05 seconds in duration will have little or no effect on depriming or weeping. It should be understood that in order for the compliant chamber to function as part of a low pass filter to serve the foregoing purpose, it is important that the compliant chamber be large enough to handle the volumetric disturbance without an undue increase in pressure. 
     As indicated previously, the check valves  32  will only prevent depriming in gross overpressure situations where they are useful in preventing back flow of ink. On the other hand, very small pressure changes which would otherwise produce depriming will not have that effect where the compliant chamber is utilized to provide the low pass filter characteristic. However, where there are very large pressure changes, the check valves do tend to pressurize the manifold section and prevent an unobjectionable massive deprime at the expense of some slightly objectionable orifice weeping. 
     It will be appreciated that the compliant chamber may take on different shapes and sizes. In particular, the flexible membrane may take on a different shape although it is preferred that the membrane provide a non-linear change in deformation with respect to changes in pressure. 
     Referring now to FIG. 11, in an alternative preferred embodiment, the check valve  32  is allowed to float between the ink tube (feed line)  70  and the compliant chamber  49 , in the “elbow” region of the filter assembly  66 . It is important for proper operation of the invention that the wall surrounding the check valve have a rough surface to create a sufficient amount of friction with the ink, to cause the ink to flow against the movable disk of the check valve rather than around the disk. This embodiment was made in the process of developing a bar code print head. It has been found that this embodiment improves upon the print head&#39;s ability to stay primed. A pressure wave, e.g., one caused by sudden movement of the reservoir  72  or tube  70  (including shock, vibration, pumping, elevation, squeezing or heating of the feed tube or ink supply), will travel past the check valve  32  and slightly pressurize the compliant chamber  49 . Subsequently, a negative part of the pressure wave will travel backward, from the face of the print head toward the compliant chamber, and seat the disk  34  of the check valve  32 . In this manner, the pressure in the compliant chamber will remain large enough to prevent negative pressure from being developed at the orifices. This sequence can result in a small amount of ink being wept out of the face of the print head (during the positive cycle of the pressure wave), but it prevents air from being pulled into the print head. An important characteristic of this alternative embodiment of the invention relates to the way in which the check valve in combination with the compliant chamber prevent air from being sucked into, and thus depriming, the print head. 
     The compliant chamber is preferably designed to hold a negative pressure of between −0.1 and −10 in-H 2 O, which is the range of pressures that are likely to be created by orifice jetting and the static height of said ink supply (i.e., the ink supply will typically be stationed slightly below the print head). In addition, to avoid jetting anomalies, the check valve is preferably designed to have a cracking pressure of between 0.1 and 3 in-H 2 O. This prevents excessive buildup of negative pressure at the orifice during jetting. 
     Although preferred embodiments of the invention have been described in detail and various modifications suggested, other such modifications will occur to those of ordinary skill in the art which will fall within the true spirit and scope of the invention as set forth in the appended claims. For example, fluids other than ink may be utilized where the fluidic jets are used, for example, as meters. In addition, alternative ink jet configurations may be utilized where different types of transducers are used including the ink itself as in a bubble jet.