Abstract:
A system and method for detecting a faulty piezoelectrically actuated ink ejector includes the piezoelectric element with an input signal, and sensing a response of the piezoelectric element to the input signal. Phase relationships and frequency dependent impedances may be analyzed and used to detect faulty ink ejectors. The detection circuit may include processing in the digital domain.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to ink jet printer technology. More particularly, the invention relates to ink jet printers which employ piezoelectric elements for ejecting ink. 
     2. Background of the Related Art 
     There are currently two major technologies used in drop-on-demand ink jet printing: thermal technology and piezoelectric technology. Most currently available ink jet printers use thermal methods to eject ink droplets out of a nozzle and onto a recording medium. In these methods, the actual ejection is initiated by heating the ink adjacent to the nozzle with a thin film resistor to create a bubble which forces a drop of ink out of the nozzle. Some recently introduced ink jet printers employ piezoelectric technology to achieve the same end of ejecting ink onto the recording medium. 
     Piezoelectricity refers to the deformation of a crystalline material when subjected to an electrical potential. Instead of using heat to eject the ink, these printers employ piezoelectric deformation to reduce the volume of a small ink reservoir, thereby ejecting a droplet of ink from the reservoir. In some piezoelectric ink jet print heads, a piezoelectric element is actuated so as to exert mechanical pressure on a membrane laying against the ink channel. When a very short electrical pulse is applied to the piezoelectric element, it may expand, contract, bend, or otherwse deform. The deformation of the piezoelectric element forces the ink out of the ink channel onto the recording medium. The expansion and contraction occurs at high speed and produces high pressures inside the ink reservoir, making an ink droplet eject from the nozzle and onto the recording medium. 
     In order to enhance printing resolution, ink jet printers often use several hundred adjacent nozzles, each having a diameter of less than 50 micrometers. The use of smaller ink chambers and finer nozzles creates a commonly recurring problem in ink jet printers. The ink channels of these printers may contain non-ink material such as air bubbles. Air can be introduced if the ink channels are run completely out of ink during use, or bubbles in the ink can become trapped near the piezoelectric actuators and nozzles over time. The presence of excess air in the channel causes the ink ejection mechanism to malfunction, thereby affecting the quality and resolution of the printed material. Such degradation in print quality can seriously undermine the effective utility of ink jet printers. 
     Several attempts have been made to detect the presence of air bubbles in ink channels with varying degrees of detectability. One attempt involved activating the piezoelectric element simultaneously with a simulation capacitor, and comparing the responses to the pulse activation. This technique is described in detail in U.S. Pat. No. 4,498,088 to Kanamaya. Another technique actuates the piezoelectric element with a normal ink ejection pulse, and detects a voltage overshoot which may develop across the actuated piezoelectric element. This technique is described in U.S. Pat. No. 5,500,657 to Yauchi et al. The Kanamaya and Yauchi et al. references are hereby incorporated by reference in their entireties. 
     The Kanamaya and Yauchi techniques require fairly complex analog actuation and detection circuits. Furthermore, they attempt to detect small perturbations in relatively large actuation signals, thus increasing the chances of erroneous evaluation of an ink channel. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improvement over the prior art by simplifying the dedicated detection circuitry required for ink ejector evaluation. Advantageously, in some systems in accordance with the invention, computational hardware already present in the ink jet printer is used to perform ink ejector analysis, thereby minimizing costs associated with faulty jet detection systems. 
     In one embodiment of the invention, a fault detection circuit for a piezoelectric ink jet printer comprises a driver circuit coupled to at least one piezoelectrically actuated ink ejector for applying a test signal to the ejector and a pre-processing circuit for monitoring, processing, and digitizing a response of the ejector to the test signal. The fault detection circuit also includes digital signal processing means for receiving an output from the pre-processing circuit and for analyzing a frequency dependent impedance of the ink ejector. As the impedance may shift with the presence of air bubbles in the channel, faulty ink ejectors may be detected. 
     In another embodiment, an ink jet printer incorporating fault detection comprises a first drive circuit coupled to a plurality of ink ejectors so as to control ink ejection therefrom during normal printing operations as well as a second drive circuit periodically coupled through a resistor to a selected one of the plurality of ink ejectors. The second drive circuit is configured to apply a test signal through the resistor to the selected ink ejector. The printer also comprises a fault detection circuit having an input connected to at least one side of the resistor; wherein an electrical signal present there is detected by the fault detection circuit, and wherein characteristics of the detected electrical signal are indicative of an operational status of the selected ink ejector. It can be appreciated that in these embodiments, faulty ink ejection channels may be accurately detected using a minimum of dedicated circuitry. 
     Methods of detecting faulty ink ejectors are also provided. In one embodiment, an ink jet printer system has a plurality of ink jet channels (IJC), each IJC including a piezoelectric element. A method of detecting faulty IJCs includes driving the piezoelectric element with an input voltage signal; and sensing a phase difference between the input voltage signal and a resulting current through the piezoelectric element. In another embodiment, a method of detecting faulty IJCs comprises determining the impedance of the piezoelectric element at at least one frequency band. The above described methods take advantage of variations in a piezoelectric ink ejectors response to selected test signals, reducing the complexity of test driver and detection circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1A is a cross-section of an ink jet head primed with normal ink liquid. 
     FIG. 1B is a cross-section of the ink jet head of FIG. 1A under normal operation. 
     FIG. 2A is a cross-section of an ink jet head primed with ink liquid containing air bubbles. 
     FIG. 2B is a cross-section of the ink jet head of FIG. 2A under faulty operation. 
     FIG. 3 is a schematic diagram of one embodiment of the detection system employed in the diagnosis of faulty ink jet channels. 
     FIG. 4A is a functional block diagram of one embodiment of a fault detection circuit employed in the detection system of FIG.  3 . 
     FIG. 4B is a functional block diagram of another embodiment of a fault detection circuit employed in the detection system of FIG.  3 . 
     FIG. 4C is a functional block diagram illustrating signal processing circuitry performing both print control and fault detection. 
     FIG. 5 is a plot of signal amplitude measured across a piezoelectric element as a function of frequency. 
     FIG. 6 is a schematic diagram of another embodiment of a detection system employed in the diagnosis of faulty ink jet channels. 
     FIG. 7 is a flow chart of decisional steps employed in a fault detection system according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiments of the present invention will now be described with reference to the accompanying Figures, wherein like numerals refer to like elements throughout. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction wit a detailed description of certain specific preferred embodiments of the present invention. 
     Many different methods of fabricating drop-on-demand piezoelectric ink jet printheads have been devised. As discussed above, the general principle involves reducing the volume of an ink chamber so as to force ink out of a nozzle in the ink chamber and onto a piece of paper or other recording medium. Although the invention has application to many different types of piezoelectric ink jet methods, one example of a configuration suitable for producing such ink chamber volume reductions is illustrated in FIGS. 1A through 2B. In FIG. 1A, a cross-section of an ink jet head primed normally with liquid ink is illustrated. As shown in this Figure, the ink jet head  100  comprises an ink jet channel (IJC)  110 , a piezoelectric element  130 , and a deformable membrane  120  placed therebetween. A nozzle  140  at the tip of the ink jet head  100  is provided for ink ejection onto the media. The IJC  110  is surrounded by IJC walls  150 . Ink liquid is supplied into the IJC  110  via an ink supply tube (IST)  160 . 
     FIG. 1B is a cross-section of the ink jet head of FIG. 1A under normal operation. Upon applying a voltage pulse to the piezoelectric element  130 , the piezoelectric element  130  develops a mechanical strain and expands. This mechanical strain causes the deformable membrane  120  to convexly extend into the IJC  110 , thereby forcing the ink liquid to eject from the IJC  110  through the nozzle  140 . After expansion, the piezoelectric element  130  contracts, drawing ink up the ink supply tube  160 , replenishing the expelled ink droplet. In on alternative embodiment, the piezoelectric crystal may draw ink into the IJC  110  by first retracting, and then may elongate back to normal size to eject ink from the IJC  110 . Although FIGS. 1 and 2 specifically illustrate the first described “fire before fill” technique, both techniques are well known to those of skill in the art, and the invention is applicable to these and other ink ejection mechanisms. 
     When the IJC  110  contains air bubbles, the ejection mechanism may not function properly. FIG. 2A is a cross-section of an ink jet head filled with ink containing air bubbles. As shown in FIG. 2A, the ink jet head  200  is structurally and functionally identical to the ink jet head  100  of FIGS. 1A and 1B. However, air bubbles  210  are present in the ink liquid in the IJC  110 . The presence of air bubbles  210  in the IJC  110  may be due to several factors. Air may be introduced when replacing ink reservoirs external to the print head, or may result from continuing to print after the attached ink reservoir has run dry. Because adhesion forces between air bubbles  210  and the IJC walls  150  are often greater than flow forces associated with the flow of ink liquid through the IJC  110 , once air bubbles have been introduced into the IJC  110 , they may be difficult or impossible to remove. 
     The presence of air bubbles  210  in the IJC  110  interferes with, and often prevents, the ejection of ink through the nozzle  140 . FIG. 2B is a cross-section of the ink jet head of FIG. 2A under faulty operation. As shown in FIG. 2B, the IJC channel  110  contains one or more air bubbles  210 . A signal generator  220  is connected to the piezoelectric element  130  to provide actuation pulses to the piezoelectric element  130 . As discussed above, the piezoelectric element  130  generates a mechanical strain onto the deformable membrane  120  causing it to convexly expand. The expansion of the deformable membrane  120  increases the pressure in the IJC  110 . However, instead of ejecting ink, the increased pressure in the IJC  110  is absorbed by the air bubbles  210  causing no ink to eject from the nozzle  140 . 
     The invention provides a system and method for detecting the presence of such air bubbles  210 . A typical ink jet printer may include, for example 50-400 ink jet channels and associated nozzles. Additionally, the ink jet print head may include several spare ink jet channels. The system detects and identifies faulty ink jet channels. Once detected, the printing system may perform one or more of a variety of functions, including notifying the operator of the fault condition, running a service routine on the printhead, or replacing faulty ink jet channels with one or more spare ink jet channels. 
     FIG. 3 is a schematic diagram of one embodiment of the detection system employed in the diagnosis of faulty ink jet channels. As shown in FIG. 3, a print drive circuit (PDC)  310  is connected to a select circuit (SC)  350 . A maintenance drive circuit (MDC)  320  is connected to an impedance  330 , which may be inductive, capacitive, or resistive, and which is connected to the SC  350 . The embodiment illustrated in FIG. 3 shows a resistor as the impedance  330 . The resistance used in this embodiment may vary widely, and certain ranges may be more appropriate to certain piezoelectric printhead designs. Resistance values of 100 kohms to 200 kohms have been successful in some embodiments. It will be appreciated that a resistor is advantageous in that its impedance is not frequency dependent. The SC  350  is connected to one or more piezoelectric elements  360 , each contained in an ink jet print head. A fault detection circuit (FDC)  340  is connected to one or both sides of the resistor  330  to detect abnormalities in the operation of the piezoelectric elements  360  as will be explained further below. 
     The PDC  310  generates piezoelectric actuation signals in response to print data generated by a host computer, such as a printer server (not shown in this figure). During printing operations, the print head passes back and forth across the media, and the piezoelectric elements are selectively actuated, one or more at a time, to deliver ink droplets to the media by the signals received from the PDC  310 . During periods when the piezoelectric elements  360  are not being utilized to place ink droplets on the recording medium, such as prior to beginning a print, or even during a print at those times between passes across the media, the piezoelectric elements  360  may be individually connected through the SC  350  to the MDC  120 . The SC  350  connects the MDC  320  to a given piezoelectric element  360  in order to diagnose malfunctions or faulty ink jet channels. The SC  350  is thus configured to periodically connect the MDC  320  to individual ones of the piezoelectric elements  360 . 
     In one advantageous embodiment, the SC  350  sequentially selects piezoelectric elements  360  for testing. As mentioned above, these selections advantageously occur when the print head is not being used for printing. The test signals which the MDC  320  applies to a piezoelectric element may vary widely in their characteristics. In some embodiments, the signal is a constant amplitude and constant frequency sine wave. In other embodiments, the frequency of an applied sine wave is swept from a low initial frequency to a high ending frequency. In still other embodiments, one or more square waves or other time limited pulse shapes having a range of frequency components may be used. The amplitude, duration, and frequency applied by the MDC  320  will vary depending on the nature of the print head being tested and the desired method of fault detection incorporated into the FDC  340 . The amplitude of the maintenance drive signal is preferably below the amplitude required for droplet ejection. Furthermore, the signal preferably includes a high energy content in a frequency range around the resonant frequency of the piezoelectric element being tested. The signal will be applied for the duration required by the FDC  340  to make a determination as to the status of the piezoelectric element being tested. The FDC  340  may receive as an input both the voltage signals at points A  302  (V A ) and B  304  (V B ) with respect to a reference voltage, e.g., ground, as shown in FIG. 4A, or may utilize a measurement of only the voltage at point B  304  relative to ground, as shown in FIG.  4 B. 
     In one embodiment, illustrated in FIG. 4A, the FDC  340  evaluates the signals received from points A and B by routing them to a phase detector  410  to compare the phase of V A  to the phase of V B  to determine the presence or absence of air bubbles in the IJC  110 . The phase detector  410  may be an analog multiplier which is commonly referred to as a four-quadrant multiplier, or any other type of phase detector (analog or digital) which are well known in the art. In this embodiment, the applied signal is advantageously a continuous sine wave having a frequency at approximately the resonant frequency of the piezoelectric element/ink channel being tested. For purposes of explanation, the piezoelectric element and the chamber it is coupled to can be considered a “black box” impedance between node  304  (FIG. 3) and ground. It has been observed that if air bubbles are not present in the IJC  110 , the apparent or effective capacitance of the piezoelectric element being tested is small, and the current through the resistor  330  of FIG. 3 is in phase with the drive signal voltage at point A  302 . Hence, the phase difference between the voltage signals V A  and V B  is small or zero. On the other hand, if air bubbles are present in the IJC  110 , the response of the piezoelectric element is altered so as to increase the effective capacitive characteristics of the piezoelectric element being tested. The piezoelectric element no longer appears essentially resistive in nature, and the phase difference between the voltage signals V A  and V B  is detectable and quantifiable. 
     As illustrated in FIG. 4A, the FDC  340  may additionally comprise a low pass filter (LPF)  420  and an analog-digital converter (ADC)  430  connected to receive, filter, and digitize the output of phase detector  410 . The LPF  420  is a low pass filter which excludes high frequency components typically present in analog phase detection circuits. The ADC  430  converts the filtered phase measurement from analog to digital form for further processing. A digital signal processor (DSP)  440 , and a memory  450  are further connected to receive the digitized output from the ADC  430 . The DSP  440  is programmed to compare the phase difference measured for a given piezoelectric element with a threshold value to determine whether or not the piezoelectric element being tested is faulty. 
     Although the circuit of FIG. 4A illustrates analog phase detection and filtering, it can be appreciated that the signals at point A  302  and point B  304  could be digitized directly, and all signal processing required to perform phase analysis could be performed by the DSP  440  in the digital domain. In this case, the phase detector and filter functions would be implemented in the software running on the DSP  440 . The DSP  440  advantageously comprises a single chip processing circuit, such as are widely used throughout the electronics industry and which are commercially available from Texas Instruments, Lucent, Motorola, and others. 
     In some cases, one or more of the piezoelectric elements  360  may exhibit effective capacitive characteristics even when air bubbles are not present in the IJC  110 . Therefore, there may be a current to voltage phase offset even for a properly functioning channel. Parameters representative of this phase offset for such piezoelectric elements having a quantifiable effective capacitance at the maintenance drive signal frequency may be stored in the memory  450 . The initial phase offset values are obtained under known conditions and, particularly, when air bubbles are absent from the respective IJCs  110 . In testing a piezoelectric element, the FDC  340  compares the phase offset stored in the memory  450  which is associated with the channel being tested to determine if an increased phase offset indicative of a faulty channel is present. 
     FIG. 4B illustrates a second embodiment of the FDC  340  of FIG.  3 . In this embodiment, the FDC  340  only utilizes the voltage present at point B  304  between the resistor  330  and the piezoelectric element  360  being tested. In this embodiment, the signal level present at this node may be used as an indication of a faulty IJC  110  as the response of the piezoelectric element at selected excitation frequencies may change when air is present in the channel. In some embodiments, near the resonant frequency of the piezoelectric element/ink channel, the signal level at this node  304  may increase when air bubbles are present in the IJC  100 . Without being limited to any particular theory of operation, it is suspected that the increased signal level is due to a decrease in fluidic damping by fluid in the IJC  110 . When there is no air in the chamber, the fluid damps the response of the piezoelectric element at the resonant frequency. When there is excessive air in the chamber, the response of the piezoelectric element is not as damped, creating a hump in the frequency response curve as illustrated in FIG.  5  and described in more detail below. 
     To detect this increase, the FDC  340  includes a peak or RMS detection circuit  460  having an output connected to an analog to digital converter (ADC)  470 . As with the embodiment illustrated in FIG. 4A, the output of the ADC  470  is routed to a digital signal processor (DSP)  480  and memory  490 . The signal level present at node B  304  is thus received by the DSP  480  and compared to a threshold to determine whether or not the IJC  110  being tested is faulty. As with the embodiment of FIG. 4A, the memory may store a table of parameters indicative of signal levels associated with one or more of the piezoelectric elements of the print head when they are functioning properly. In this case, the DSP  480  may compare the received signal level with the parameter previously stored in the memory  490 . 
     As an ink jet printer typically includes a digital signal processing circuit to perform its normal printing operations, the implementation of the invention can be performed using processing capacity already present in the printer, thus minimizing costs associated with faulty jet detection. This feature is illustrated in FIG.  4 C. In this Figure, the test circuit  492  provides an input to a preprocessor circuit  494 . Example test and pre-processor circuits are illustrated and discussed with reference to FIGS. 4A and 4B, As discussed with reference to these Figures, the test circuit  492  may include a signal generator and a series impedance, and the pre-processor  494  may include filters, A/D converters, peak detectors, phase detectors, etc. 
     Referring again to FIG. 4C, the DSP  440  may be used to receive print data and to control the print drive circuit  310  during normal printing operations in addition to receiving an input from the pre-processor  494 . As mentioned above, the testing may be performed during those periods when the processing circuit  440  is not being used to process print data such as prior to beginning a print job or in between passes across the media. It will be appreciated that the fault detection methods described herein may thus be implemented via appropriate programming of the processor circuit  440  in the ink jet printer. The software implementing these methods will generally be stored in a programmable storage device in the printer, such as a ROM or EEPROM, which may be integral to or separate from the processor  440  itself. 
     As an example of the signal level differences produced by the presence of air bubbles in an IJC  110 , FIG. 5 provides plots of the measured potential at point B  304  of FIG. 3 as a function of the frequency of a continuous sine wave output from the MDC  320  for both a functioning channel and a channel containing a significant amount of air. In this Figure, the horizontal axis represents the frequency of voltage signals applied to the piezoelectric element  360  selected by the SC  350 . The y-axis represents the amplitude (A r ) in dB of the voltage signals (V B ) measured across the piezoelectric element  360 , i.e., at point B  304  (FIG. 3) relative to the applied voltage of the MDC  320 . In producing this plot, a piezoelectric element and IJC  110  of configuration similar to that illustrated in FIGS. 1 and 2 was used which had a resonant frequency of approximately 41.5 kHz. 
     The first curve  510  represents the variation in A r , for a piezoelectric element without air bubbles, as a function of frequency of V MDC . The second curve  520  represents the variation in the relative amplitude A r ′, for a piezoelectric element with air bubbles, as a function of frequency of V MDC . The two curves begin to diverge at approximately 28 kHz, with the deviation becoming most significant (between 3-4 dB) and most detectable at around the resonant frequency of approximately 41.5 kHz. Of course, different styles of piezoelectric print head will have different response curves and will be resonant at different frequencies. It will be appreciated that the embodiment shown in FIG. 5 is one illustrative example. 
     It will be appreciated by those of skill in the art that several alternative schemes may be used to detect this difference in response with and without air in the chamber. In one embodiment, the MDC  320  supplies a sine wave signal having a fixed frequency at approximately the resonant frequency of the IJC  110 . The signal level at point B.  304  is compared to a threshold expected signal level, and the IJC  110  may be detected as faulty if the signal level exceeds the threshold. Alternatively, the frequency output by the MDC  320  could be swept through a range of frequencies, and a faulty IJC  110  may be detected by detecting the region of large positive slope  530  present in the response curve  520  of an IJC  110  which contains excess air. The response at point  13  to a square wave, chirp, or other time limited waveform containing a range of frequency components may also be detected at point B, and may be used to characterize an IJC  110  as good or faulty. 
     In analogy with the embodiment described above with reference to FIG. 4A, the signal level at point B for each piezoelectric element may be recorded in the memory unit  490  during printer manufacture before regular operation. The FDC  340  may determine the presence of faulty ink jet channel by measuring the signal level at point B  304  for a piezoelectric element being tested and comparing this with the expected response measured during manufacture when the channel was known to be functioning properly. If the FDC  340  detects a deviation such as shown in FIG. 5, then the piezoelectric element being tested is considered faulty. 
     FIG. 6 is a schematic diagram of another embodiment of a detection system employed in the diagnosis of faulty ink jet channels. As shown in FIG. 6, a print drive circuit (PDC)  610  is connected to a select circuit (SC)  650 . A maintenance drive circuit (MDC)  620  is connected to the SC  650 . The SC  650  is connected to one or more piezoelectric elements  660 , each contained in an ink jet head. In the embodiment of FIG. 6, these items may be essentially identical to the ones shown and described with reference to FIG. 3 above. 
     In the embodiment of FIG. 6, however, a vibration transducer  630  is attached to the ink jet print head. Suitable vibration transducers are known in the art, and typically comprise an accelerometer which converts mechanical vibrations into an electrical signal. A fault detection circuit (FDC)  640  is connected to the vibration transducer  630  to detect abnormalities in the operation of the piezoelectric elements  660 . As described above, the SC  650  selects the PDC  610  under normal printing operation (“normal mode”) to activate printing by the ink jet nozzles. In the diagnostic mode, the SC  650  selects one piezoelectric element to be tested. The MDC  620  applies voltage signals having a predetermined amplitude, duration, and frequency which may be of insufficient intensity to eject ink from the channel, but which elicit a vibratory response in the print head. In many embodiments, it is desirable that the frequency of the test signals be substantially close to the resonant frequency of the piezoelectric element being tested. It will be appreciated, however, that a wide variety of test signals could be utilized including square pulses, frequency swept signals, etc. 
     The FDC  640  detects and measures the vibration signals generated by the piezoelectric elements  660  in response to the voltage signals driving the one piezoelectric element being tested. The energy content of the vibrations in different frequency bands may be significantly different when excess air is present in the channel being tested. Thus, in analogy with the above described electrical signal monitoring, the FDC  640  compares the vibration signals to already known, and previously recorded, vibration signals of the piezoelectric elements  660  when they are known to be functioning properly during printer manufacture. If the vibration signals show differences associated with air bubble presence or other detectable faults, then the FDC  640  determines that the operation of the channel piezoelectric element being tested is faulty. 
     FIG. 7 is a flow chart of the steps employed by a fault detection system in accordance with the present invention. As shown in FIG. 7, at step  710 , the SC  350  (FIG. 3) selects the MDC  320  to drive a particular piezoelectric element (“channel”) for testing. At step  720 , the MDC  320  drives the channel being tested with the desired signal. As noted above, the test signals preferably include a large component at or near the resonant frequency of the piezoelectric element/ink channel being tested. At step  730 , the FDC  340  monitors the response of the channel. At step  740 , the FDC  340  determines if the response by the channel being tested is satisfactory. This may advantageously be performed by comparing the measured response to an appropriate expected value which was stored when the ink jet channel being tested was known to be functioning properly. If the channel response is not satisfactory, then at step  750 , the FDC  340  records the channel as faulty in the memory unit  450 ,  490 . Next, at step  760 , the FDC determines if more channels are to be tested. If no channels remain to be tested, then the process terminates at step  780 . If it is desirable to test another channel then, at step  770 , the SC  350  selects another channel for testing, and the process loops back to step  720  to analyze an additional ink jet channel. 
     In some embodiments, the procedure illustrated in FIG. 7 is performed on all of the ink jet channels of the head prior to beginning each print job. In other embodiments, the channels are sequentially tested during print jobs as well by performing channel tests at those times when the ink jet print head is in between passes across the media being printed. 
     In view of the foregoing, it will be appreciated that the invention overcomes the long-standing need for a system and method for detecting faulty ink ejection channels without the disadvantages of inaccurate detection criteria, or obtaining measurements which may be susceptible to error. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.