Abstract:
A print head and method that are capable of detecting a plurality of performance conditions such as a dry-fire, no-fire or clogged-nozzle condition. Pressure wave sensors within a print head are disclosed that are capable of detecting pressure waves generated by the firing of an ink expulsion mechanism. The characteristics of the pressure wave generated by the firing event (e.g., magnitude and timing) are indicative of the operating condition within the head. Multiple sensor types are disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to print heads used in printers and plotters and the like and, more specifically, to detecting malfunctions within such print heads. 
     BACKGROUND OF THE INVENTION 
     Printers and plotters are known in the art and include those made by Hewlett-Packard, Canon and Epson, amongst others. In the discussion that follows, printers and plotters are referred to collectively with the term “printers”. Problems associated with current printers and print head arrangements include that the print head may run out of ink while printing, the print head nozzle may become clogged and the ink expulsion mechanism may not fire, amongst other malfunctions. Evidence of such malfunctions are usually detected when the printed document is pulled out of the printer and examined visually. At this point it is too late for appropriate correction. Some types of electronic sensing are known in the art, such as techniques for detecting when an ink expulsion mechanism has not fired. These techniques, however, are limited in scope and do not, for example, detect when a nozzle is clogged or unclogged. 
     A need thus exists to detect print head malfunction in such a manner as to eliminate or minimize corruption of a printed image. Early detection of a malfunction permits preventative steps to be taken such as print head replacement or software based compensation within the firing algorithm, etc. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a print head that can detect a malfunction therein. 
     It is another object of the present invention to provide a print head that can detect such conditions as a clogged nozzle, no fire and dry fire. 
     It is another object of the present invention to provide a print head that incorporates a pressure sensor and circuitry therefor that detects firing of an ink expulsion mechanism and determines characteristics about the firing based on the sensed signals. 
     It is also an object of the present invention to provide a print head with a piezoelectric type pressure sensor. 
     These and related objects of the present invention are achieved by use of a print head apparatus with a malfunction detector as described herein. 
     The attainment of the foregoing and related advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention taken together with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional side view of a print head in accordance with the present invention. 
         FIG. 2  is a side view of a piezoelectric acoustic wave transducer in accordance with the present invention. 
         FIG. 3  is a side view of a portion of an interdigitated pressure wave transducer in accordance with the present invention. 
         FIG. 4  is a plan view of an arrangement of piezoelectric acoustic pressure wave transducers and interdigitated piezoelectric pressure wave transducers in a print head in accordance with the present invention. 
         FIG. 5  is a graph illustrating the pressure on an expulsion mechanism surface versus time for a clogged nozzle firing and an unclogged nozzle firing. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a cross sectional side view of a print head  10  in accordance with the present invention is shown. Print head  10  includes a substrate in or on which is provided an ink expulsion mechanism  14 . Ink expulsion mechanism  14  may expel ink through thermal or mechanical excitation or through other appropriate expulsion means. In a preferred embodiment, mechanism  14  is thermally actuated and may be implemented with a resistive element as is known in the art. Ink expulsion mechanism  14  is controlled by off-die circuitry or by a combination of on-die and off-die circuitry as is known. Representative off-die coupling is indicated by signal line  15  and contact pad  16 . 
     A barrier layer  20  is formed on substrate  12  and an orifice plate  30  is formed on barrier layer  20 . The substrate, barrier layer and orifice plate define an ink well or conduit  24  that channels ink from a supply (not shown) into proximity with the expulsion mechanism. An orifice or nozzle  31  through which ink is expelled is formed in the orifice plate and positioned over ink expulsion mechanism  14 . Suitable material for barrier layer  20  and orifice plate  30  are known in the art. 
     Assuming that ink expulsion mechanism  14  is a thermally actuated device such as a resistor, an ink drop is expelled by essentially boiling a drop of ink through nozzle  31 . During formation and collapse of a boiling ink bubble, a series of acoustic pressure waves  26  (hereinafter referred to as “pressure waves”) are produced. These waves propagate through the components of the print head, including primarily the substrate and ink well. 
     In the substrate (and conventional thin film layers formed thereon), both longitudinal and shear waves are produced. Longitudinal waves can be detected by an interdigitated piezoelectric pressure wave transducer  50  or the like which is described in more detail with reference to  FIGS. 3 and 4 . In the ink well, longitudinal pressure waves are produced. These waves can be detected with a piezoelectric acoustic pressure wave transducer  40  which is described in more detail with reference to FIG.  2 . 
     For purposes of the present discussion, the term “interdigitated transducer” will be used for the interdigitated piezoelectric pressure wave transducer and the term “acoustic transducer” will be used for the piezoelectric acoustic pressure wave transducer. While both an acoustic transducer and an interdigitated transducer are described as being provided on substrate  12 , it should be recognized that they need not be provided together because either transducer is capable of sufficiently detecting pressure waves. The provision of both provides redundancy. 
     Acoustic transducer  40  and interdigitated transducer  50  are preferably coupled to processing circuit  60 . Processing circuit  60  preferably includes an amplifier, a filter and an analog to digital converter or related signal processing circuitry. Processing circuit  60  may be configured to provide the necessary processing to determine dry-fire, no-fire and clogged-fire conditions (that is, a misfire) or the sensor output signals can be delivered to off-die logic  70  for such processing. The output of processing circuit  60  is propagated over signal line  17  to contact pad  18 . 
     Referring to  FIG. 2 , a side view of an acoustic transducer in accordance with the present invention is shown.  FIG. 2  illustrates the acoustic transducer of  FIG. 1  in more detail.  FIG. 2  illustrates substrate  12  on which the following layers are formed: an insulation layer  21 , a conductive coupling layer  41 , piezoelectric material  42 , a first and a second signal conductive layer  44 , 45 , a passivation layer  47  and a surface coat layer  48 . In a preferred embodiment, these layers are made of the following or a like material: insulation layer  21  is silicon dioxide (SiO 2 ), conductive layer  41  is tantalum aluminum (TaAl), piezoelectric material  42  is aluminum nitride (AlN), first and second conductive layers or traces  44 , 45  are aluminum (Al), passivation layer  47  includes a first layer of silicon nitride (Si 3 N 4 ) and a second layer of silicon carbide (SiC), and coating layer  48  is tantalum (Ta). It should be recognized that the arrangement and composition of these layers may be altered in a manner consistent with device fabrication techniques without deviating from the present invention. It should also be recognized that other piezoelectric material such as zinc oxide (ZnO) or PZT may be used and that other types of suitable pressure sensors may be used. includes a first layer of silicon nitride (Si 3 N 4 ) and a second layer of silicon carbide (SiC), and coating  48  layer is tantalum (Ta). It should be recognized that the arrangement and composition of these layers may be altered in a manner consistent with device fabrication techniques without deviating from the present invention. It should also be recognized that other piezoelectric material such as zinc oxide (ZnO) or PZT may be used and that other types of suitable pressure sensors may be used. 
     The first and second conductive layers  44 , 45  form conductors for reading a voltage generated by piezoelectric material  42  in response to an incident pressure wave. A pressure wave traveling through the ink well compresses the thin film stack, resulting in a mechanical strain in the thin film layers. In the piezoelectric layer, this strain produces a measurable electric charge across the two conductors. 
     Referring to  FIG. 3 , a side view of a portion of an interdigitated transducer in accordance with the present invention is shown.  FIG. 3  illustrates the interdigitated transducer of FIG.  1 . The layout of this transducer and its arrangement with another interdigitated transducer are shown in FIG.  4 .  FIG. 3  illustrates substrate  12  on which are formed insulation layer  21 , piezoelectric material  52 , first and second conductors  54 , 55  (only one of which is shown), a passivation layer  57  and a coating layer  58 . The substrate, insulation layer, passivation layer and coating layer are as discussed above for acoustic transducer  40 . The piezoelectric material and conductive layers are preferably similar in composition to their counterparts in transducer  40 , however, their areal arrangement is different as shown in FIG.  4 . 
     Referring to  FIG. 4 , a plan view of an arrangement of acoustic transducers and interdigitated transducers in a print head in accordance with the present invention is shown.  FIG. 4  illustrates substrate  12 , a plurality of ink expulsion mechanisms  14 , barrier layer  20 , ink well  24 , a plurality of acoustic transducers  40  and a plurality of interdigitated transducers  50 . Orifice plate  30  would be placed over the arrangement of  FIG. 4  with nozzles aligned with the ink expulsion mechanisms  14 . It should be recognized that the transducer arrangement disclosed in  FIG. 4  is representative and provided for pedagogical purposes. The ink expulsion mechanisms, ink well and the size, number and arrangement of transducers may be modified from that of  FIG. 4  without departing from the present invention. Furthermore, it should be recognized that although the interdigitated transducers are shown in the ink well, since they detect pressure waves in the substrate they may be placed anywhere on the substrate including under the barrier layer. 
     The interdigitated transducers are preferably implemented as interdigitated conductors  54 - 55  placed over a corresponding pattern of piezoelectric material  52 . These interdigitated transducers exhibit a directional detection characteristic that is advantageous to some implementations of the present invention.  FIG. 4  illustrates two interdigitated pressure wave transducers  50  and  50 ′ that are arranged orthogonally to one another. This arrangement facilitates detection of pressure waves traveling in different directions. The acoustic transducers  40  of  FIG. 4  are essentially as described above with references to  FIGS. 1 and 2 . Each of transducers  40  and  50  are shown with their first and second conductors  44 , 45  and  54 , 55 , respectively being coupled to vias  13  (under the barrier layer) that are coupled to signal processing circuit  60  of FIG.  1 . 
     Referring to  FIG. 5 , a graph illustrating the pressure on the surface of resistor or expulsion mechanism  14  verses time for a clogged nozzle firing and an unclogged nozzle firing is shown. As alluded to above, the cavitation of the air bubble(s) at resistor or expulsion mechanism  14  during firing causes a considerable pressure spike on the surface of the resistor. This pressure spike is normally around 20 MPa (greater than 10K PSI) and occurs at approximately 13.5 μS after firing. When the nozzle associated with a particular resistor is clogged, however, the pressure spike has a different signature. Typically it is lower in magnitude by about 15-25 percent (e.g., approximately 16 Mpa) and occurs earlier (e.g., 15-20% earlier, usually approximately 11 μS). The combination of decreased magnitude and quicker response time permits differentiation of an unclogged firing from a clogged firing. The absence of a pressure wave indicates a “no-fire” event. 
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.