Determining the operational status of the printhead

Abstract

The present application relates to determining the operating state of a printhead. A system and method for determining the operating state of a nozzle in an inkjet printhead having a piezoelectric actuator configured to eject ink through the nozzle, the system comprising: a drive circuit configured to apply a drive signal to the piezoelectric actuator during a first time period; and a sensing circuit configured to measure a current in the piezoelectric actuator as a function of time during a second time period after the first time period; wherein the system is configured to determine the operating state of the nozzle based on the time taken for the current measured during the second time period to reach a predetermined condition or the slope of the current measured as a function of time during the second time period.

Description

Technical Field

[0001] The present invention relates to inkjet printers and, more particularly, to an apparatus for determining the state of a nozzle in an inkjet piezoelectric printhead. Background Art

[0002] Inkjet printheads may use piezoelectric actuators to eject ink from nozzles that are applied to the print media.

[0003] Figure 1 A schematic diagram of an example of a piezoelectric inkjet printhead is shown. The printhead has a housing 101 that defines a nozzle chamber 102. An orifice plate 103 is located at one end of the pressure chamber, and the orifice plate 103 is located between a slit plate 104 and a channel plate 105. A vibration plate 106 is located at the other end of the pressure chamber, and a piezoelectric actuator 107 is attached to the vibration plate 106. The printhead also has a channel 108 that supplies ink to the chamber and thereby to the nozzle 109. In other printhead designs, the piezoelectric material can be attached to the orifice plate or another component of the printhead. In some designs, the pressure chamber is not required to eject ink from the nozzle.

[0004] The piezoelectric actuator 107 is connected to a driver circuit that excites the piezoelectric material in the actuator. (x) Ti (1-x) O3) is often used as a piezoelectric in an actuator.

[0005] Piezoelectrics can produce work on or extract energy from a nozzle by changing the polarization of the molecules of the piezoelectric material. In fact, by placing the piezoelectric material between electrodes, this transduction effect can convert electrical energy into mechanical energy (or vice versa). When a voltage is applied to the electrodes, an electric field is generated, which changes the polarization state of the piezoelectric material, which causes the size of the piezoelectric material to change by compressing or expanding the size of the piezoelectric material in a certain spatial direction. Conversely, when pressure is applied to the piezoelectric, the size of the piezoelectric changes. This changes the polarization of the molecules, which in turn changes the surface charge of the material. This change in surface charge can cause an electric current to flow through the electrodes of the piezoelectric or cause a voltage change between the electrodes, depending on the type of circuit to which the electrodes are connected.

[0006] Energy transfer occurs between the piezoelectric actuator and the fluid in the nozzle whenever the voltage between the electrodes of the piezoelectric actuator changes or when current flows through the electrodes. The process of energy transfer from the electrical driver to the nozzle fluid through the piezoelectric is often called nozzle actuation when the energy transfer between the driver and sensing circuits is also considered. The process of energy transfer between the fluid in the nozzle and the sensing circuit is often called nozzle sensing.

[0007] Nozzle excitation is typically performed by applying a voltage function between electrodes of the piezoelectric actuator 107. Typically, nozzle sensing is performed by measuring the current generated by the piezoelectric between its electrodes when the pressure in the nozzle chamber 102 changes. Alternative implementations of these processes are possible with the aid of circuit analysis equivalence theorems. For example, an implementation in which nozzle excitation is performed by applying a current between electrodes and nozzle sensing is performed by measuring the voltage between electrodes is also possible.

[0008] exist Figure 1 In the exemplary print head shown, during printing, a pulse voltage signal is applied to the piezoelectric actuator by the driver circuit to rapidly change the volume of the pressure chamber, and the piezoelectric actuator consumes current. The movement of the vibrating plate generates a pressure wave in the chamber, which causes ink to be ejected from the nozzle at the end or one side of the channel. Due to the pressure wave, much of the ink contained in the pressure chamber is ejected from the printer head through the channel, the channel plate, the orifice plate, and the slit plate. The ejected ink travels toward the medium to be printed in the form of droplets.

[0009] In such a printhead, the orifice of the nozzle may become partially or completely blocked. When this happens, ink drops cannot be ejected properly from the printer head. Jetting may stop altogether, or drops may be ejected erroneously, i.e., in the wrong direction. But even more common in practice is that the ink supply may form bubbles in the chamber that the pressure wave must pass through before reaching the nozzle, or accidentally draw or allow bubbles to enter the chamber that the pressure wave must pass through before reaching the nozzle. As the pressure wave passes through, the contraction of the bubble may absorb or break up the wave, causing the pressure wave to reach the nozzle to be weakened, and the ejection of ink to be weak or completely prevented.

[0010] Known techniques for determining nozzle status utilize measurements of the frequency of post-injection current oscillations in a piezoelectric actuator or the decay of oscillations observed from the current versus time profile. However, this is computationally complex and requires a relatively long period of time after the excitation of the piezoelectric has ceased to be determined.

[0011] It would be desirable to develop a faster and less computationally intensive method for detecting the status of nozzles in an inkjet printer head. Summary of the invention

[0012] According to a first aspect, a system for determining the operating state of a nozzle in an inkjet print head is provided, the inkjet print head having a piezoelectric actuator configured to cause ink to be ejected through the nozzle, the system comprising: a drive circuit configured to apply a drive signal to the piezoelectric actuator during a first time period; and a sensing circuit configured to measure a current within the piezoelectric actuator as a function of time during a second time period after the first time period; wherein the system is configured to determine the operating state of the nozzle based on the time taken for the current measured during the second time period to reach a predetermined condition.

[0013] The predetermined condition may be a threshold current value.

[0014] The predetermined condition may be a maximum current value.

[0015] The predetermined condition may be when the gradient of the measured current as a function of time is equal to zero.

[0016] The predetermined condition may be when, during said second period of time, the gradient of the measured current as a function of time is equal to zero for the first time.

[0017] The second time period may be separated from the first time period by an intermediate time period.

[0018] The time it takes for the measured current to reach the predetermined condition may be measured from the end of the intermediate time period.

[0019] The time taken for the measured current to reach the predetermined condition may be measured from the start of the second time period.

[0020] The system may also include a comparator configured to compare the measured current to a threshold current value.

[0021] The system may also include a counter configured to measure the time it takes for the measured current to reach a predetermined condition.

[0022] According to a second aspect, a system for determining the operating state of a nozzle in an inkjet print head is provided, the inkjet print head having a piezoelectric actuator configured to cause ink to be ejected through the nozzle, the system comprising: a drive circuit configured to apply a drive signal to the piezoelectric actuator during a first time period; and a sensing circuit configured to measure a current within the piezoelectric actuator as a function of time during a second time period after the first time period; wherein the system is configured to determine the operating state of the nozzle based on the slope of the measured current as a function of time during the second time period.

[0023] The system may also include a low noise amplifier.

[0024] The operational state of the nozzle may be determined by a logic processor.

[0025] The logic processor may be a microprocessor, CPLD, FPGA, digital signal processor, microcontroller, embedded PC, personal computer, server, ASIC or other programmable logic.

[0026] The operational state of the nozzle may be determined using one or more of a rule set, an algorithm, and a lookup table.

[0027] The sensing circuit may include one or more of: a current sensor resistor, a differential operational amplifier, a Hall effect current sensor, a capacitor in series with the piezoelectric actuator, and a current mirror.

[0028] The operating state may be determined as one or more of: normal firing, deviated firing, partial blockage, full blockage, and containing bubbles.

[0029] No drive signal may be applied to the actuator during the second time period.

[0030] According to a third aspect, there is provided an inkjet print head comprising: a nozzle; a piezoelectric actuator configured to cause ink to be ejected through the nozzle; and a system as described above.

[0031] According to a fourth aspect, a method for determining the operating state of a nozzle in an inkjet print head is provided, the method comprising: applying a drive signal to a piezoelectric actuator during a first time period; measuring the current within the piezoelectric actuator as a function of time during a second time period after the first time period; and determining the operating state of the nozzle based on the time taken for the current measured during the second time period to reach a predetermined condition.

[0032] According to a fifth aspect, a method for determining an operating state of a nozzle in an inkjet print head is provided, the method comprising: applying a drive signal to a piezoelectric actuator during a first time period; measuring a current within the piezoelectric actuator as a function of time during a second time period after the first time period; and determining the operating state of the nozzle based on the slope of the measured current as a function of time during the second time period. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will now be described by way of example with reference to the accompanying drawings.

[0034] In the attached picture:

[0035] Figure 1 A piezoelectric inkjet print head is shown schematically.

[0036] Figure 2An apparatus for determining the operational status of an inkjet print head is schematically shown.

[0037] Figure 3 (a) schematically shows the signal applied to the device as a function of time.

[0038] Figure 3 (b) shows a graph of the current in the piezoelectric body relative to time after the driving signal ends.

[0039] Figure 4 (a) shows an example of a current versus time graph for a non-jetting print head.

[0040] Figure 4 (b) shows an example of a current versus time graph for an ejection print head.

[0041] Figure 5 A method for determining the operational status of nozzles in an inkjet printhead is shown.

[0042] Figure 6 Another method for determining the operational status of nozzles in an inkjet printhead is shown. DETAILED DESCRIPTION

[0043] Figure 2 Schematically shows a method for determining a print head (e.g., Figure 1 Device for displaying the operating state of a piezoelectric nozzle).

[0044] A piezoelectric actuator is shown at 201 and includes a piezoelectric material disposed between a pair of electrodes. The piezoelectric actuator is configured to cause ink to be ejected through a nozzle when a drive voltage of sufficient peak amplitude is applied to the actuator. For simplicity, the remainder of the inkjet nozzle is not shown.

[0045] The device includes a driver circuit shown at 202, which is configured to apply a drive voltage to the piezoelectric actuator. As described in more detail below, during certain time periods, the driver circuit is configured to apply a drive signal to the piezoelectric actuator, which is a change in voltage. The drive signal is a voltage profile, commonly referred to as a waveform, which is applied to the piezoelectric body to produce a mechanical action on the nozzle chamber. Applying the drive signal to the piezoelectric actuator causes a change in the size of the piezoelectric material. The drive signal can be described as a sequence of voltage hold values ​​and slope (dV / dt) values ​​that transition between the hold values. It can be described as a "trapezoidal" profile. However, the drive signal can be any suitable mathematical function.

[0046] The device also includes a sensing circuit 203. The sensing circuit is configured to measure the current in the piezoelectric actuator in the inkjet print head. When the piezoelectric actuator is excited by the driver circuit, the piezoelectric body consumes current. When the piezoelectric actuator is not driven by the driver circuit, the piezoelectric actuator generates current due to the pressure drop in the chamber (which causes the movement of the piezoelectric material). The current measured by the sensing circuit may include the current generated by the piezoelectric actuator due to the movement of the piezoelectric material and / or the current present in the piezoelectric actuator due to other reasons, that is, the sensing circuit measures the current flowing in the piezoelectric actuator at a given time. The sensing circuit 203 can be implemented as a current sensor resistor and a differential operational amplifier, a Hall effect current sensor, a capacitor connected in series with the nozzle piezoelectric, a current mirror, or other common circuits for detecting current.

[0047] The device further includes a low noise amplifier 204 , a voltage comparator 205 and a logic processor 206 .

[0048] The low noise amplifier 204 is configured to amplify the current signal measured by the sensing circuit 203. The output from the low noise amplifier 204 is input to the voltage comparator 205.

[0049] The voltage comparator 205 may be implemented as a single commercially available circuit or as a combination of an analog-to-digital converter (ADC) and a microprocessor or a field programmable gate array (FPGA). The voltage comparator 205 may be implemented as an electronic circuit element or as an algorithm running on a logic processor. Figure 2 The voltage comparator 205 is shown as a discrete component in the circuit, but it can be constructed in many different ways. The logic processor can be a device such as a microprocessor, a complex programmable logic device (CPLD), an FPGA, a digital signal processor (DSP), a microcontroller, an embedded PC, a personal computer, a server, an ASIC, or other programmable logic. Figure 2 In the illustrated embodiment, the logic processor includes a timer circuit 207 (in Figure 2 200 is shown as a counter in FIG. 201 . The timer circuit can be typically implemented as a CPLD, FPGA, or microprocessor. The logic processor also includes a delay 208 that is used to avoid signals captured when the operational amplifier is stabilizing its output voltage after the nozzle excitation pulse is used for nozzle state determination.

[0050] Typically, nozzle actuation is performed by applying a voltage function between electrodes of the piezoelectric actuator 201. Typically, nozzle sensing is performed by applying a voltage function between electrodes of the piezoelectric actuator 201. Figure 1The method of measuring the current generated by the piezoelectric body between its electrodes when the pressure in 102) changes is performed. This is the preferred method used in the present invention. With the help of circuit analysis equivalence theorem, alternative implementations of these processes are possible. For example, the nozzle excitation is performed by applying a current between the electrodes and the nozzle sensing is performed by measuring the voltage between the electrodes.

[0051] Converting the current generated by a piezoelectric into a voltage has several practical advantages. Figure 1 The nozzle shown, by placing the resistor of the sensing circuit 203 in series with the piezoelectric actuator 201, the current generated by the piezoelectric body can be converted to an equivalent voltage. According to Ohm's law, there is a proportional relationship between this current and this voltage, and in this document, the terms "sensing current" and "sensing voltage" will be used as equivalent terms. In some embodiments of the present invention, electrical components or circuits that are not resistors can be used to convert the sensing current to a voltage. Particularly relevant is when a capacitor is used instead of the resistor of the sensing circuit 203. In these cases, the relationship between the sensing current and the voltage is not proportional, and an appropriate mathematical expression for the equivalent impedance of the component or circuit is used.

[0052] Figure 3 (a) schematically shows a signal provided to a device for determining the operating state of a nozzle as a function of time. Figure 3 (b) shows the Figure 3 2 and 3. Corresponding graphs of the current in the piezoelectric body as a function of time (using sampling units) measured by the sensing circuit 203 for some corresponding portions of (a).

[0053] like Figure 3 As shown in (a), during a first time period, the drive circuit 202 applies a drive signal to the piezoelectric actuator. Generally, the drive signal is indicated at 301. The drive signal includes a change in a drive voltage. The change in voltage provided by the drive signal is sufficient to excite the piezoelectric actuator. The drive signal can be in the form of a voltage pulse. The waveform of the drive signal can have a trapezoidal profile. Figure 3 (a), the drive signal includes a hold value V h Transition to hold value V e The slope value (dV / dt) of the holding value V e After returning to the holding value V through another slope h (The hold value V h The shape of the piezoelectric body is changed before returning the shape of the piezoelectric body to its previous shape. These changes in shape change the volume of the nozzle chamber, which in turn changes the pressure in the fluid in the chamber, thereby producing some work on the fluid (excitation of the nozzle), which can ultimately lead to the ejection of ink drops (if V e The voltage holding part Vh The holding voltage can be any value: zero voltage, positive voltage or negative voltage. In contrast to the excitation effect above, the change in the pressure of the fluid causes a change in the shape of the nozzle chamber and the piezoelectric element, which in turn causes some work to be done on the piezoelectric element (de-excitation of the nozzle), which in turn causes the piezoelectric body to generate current.

[0054] Therefore, the driving voltage is the voltage applied to the piezoelectric body at a given time, the driving signal is the changing voltage that causes the excitation of the piezoelectric body to determine the state, and the holding voltage ( Figure 3 V in (a) h ) is the baseline. During the voltage holding period, the nozzle state determination is performed when the pressure change in the nozzle chamber is affected by the operating state of the entire nozzle structure.

[0055] The drive signal applied to the piezoelectric actuator for nozzle state determination may have a peak amplitude sufficient to cause ink droplet ejection, or it may be insufficient to cause ink droplet ejection. The peak voltage of the drive signal (e.g., Figure 3 (a) V e ) is used to excite the piezoelectric body of the nozzle, and the state used to determine the nozzle does not have to be the same as the state used for printing. However, the state determination can also be performed after printing has stopped, and the drive signal can correspond to the drive voltage applied to the actuator during printing. The method described herein can be performed in any constant voltage segment after the piezoelectric body is excited, provided that there is enough time to perform the measurement before the piezoelectric body is excited again.

[0056] Application of the drive signal may result in ink ejection if the peak amplitude of the drive signal exceeds the ejection voltage. The ejection voltage is a threshold voltage value corresponding to a voltage sufficient to cause ejection of ink from the nozzle. Thus, the piezoelectric actuator is configured to cause ejection of ink through the nozzle when a drive voltage exceeding the ejection voltage is applied to the actuator.

[0057] The device also receives a measurement signal. Figure 3 In the example shown in (a), the measurement signal has two levels of 0 and 1, corresponding to no current in the piezoelectric body and current in the piezoelectric body being measured, respectively. During the first time period, the measurement signal is 0.

[0058] exist Figure 3 In the example of (a), after the first time period of applying the drive signal, there is an intermediate time period during which the drive voltage returns to the holding value V after the drive signal is applied. hAfterwards, the amplifier and / or parameters stabilize. In some implementations, the intermediate time period may be zero seconds. However, typically the duration of the intermediate time period is about 1 to 2 μs. Therefore, the intermediate time period after the end of the drive signal waveform actuation is a time period in which the measured current in the piezoelectric may not be reliable for state determination. It is desirable to wait for the electronics to stabilize and not take measurements until after the intermediate period is over. During the intermediate time period, the measured signal is 0.

[0059] The intermediate time period is followed by a second time period during which the operating state of the nozzle is determined. During the second time period, the measurement signal is 1, such as Figure 3 During the second time period, the driving circuit 202 does not apply a driving signal to the actuator, but instead applies a voltage hold (ie, a constant voltage) to the piezoelectric body.

[0060] Therefore, the state determination is performed during the second time period, which starts at a fixed time (corresponding to the middle time period) after the end of the waveform excitation (the end of the last slope of the drive signal).

[0061] The constant voltage applied to the piezoelectric body during the second time period may be a positive voltage, a negative voltage or a zero voltage.The voltage applied to the piezoelectric body during the second time period is generally the same as the voltage applied to the piezoelectric body before the drive signal is applied.

[0062] During the second time period, no drive signal is applied to the piezoelectric actuator. During the second time period, the voltage applied to the piezoelectric actuator is constant. During the second time period, after the drive signal is removed, the piezoelectric body generates a current as the nozzle chamber of the print head changes shape. As the maximum applied voltage corresponding to the drive signal (in Figure 3 In the example (a), V e )'s excitation of the piezoelectric actuator results in a current that is approximately six orders of magnitude higher than the current generated when the piezoelectric body is excited.

[0063] The second time period is followed by a third time period. During the third time period, there are residual pressure oscillations in the piezoelectric body. During the third time period, no drive signal is applied to the piezoelectric actuator. During the third time period, the voltage applied to the piezoelectric body is constant. The constant voltage applied to the piezoelectric body during the third time period may be a positive voltage, a negative voltage, or a zero voltage. The voltage applied to the piezoelectric body during the third time period is preferably the same as the voltage applied during the second time period. In the present invention, the current change during the third time period is not used to determine the operating state of the nozzle. During the third time period, the measurement signal may be 1, such as Figure 3 as shown in (a), or it can also be 0.

[0064] Therefore, there are four time windows in the measurement cycle: the first time period (drive signal waveform activation time, i.e., the drive signal is applied to the piezoelectric body), the middle time period (amplifier stabilization time), the second time period (slope-time to maximum measurement time), and the third time period (residual pressure oscillation time). The measurement of the current in the piezoelectric body to determine the state of the piezoelectric body is performed during the second time period (when the piezoelectric body is de-activated). The method does not use the measurement of the current in the third time period to determine the state of the nozzle (when there is residual pressure oscillation).

[0065] Further details of how the current within the piezoelectric actuator in the inkjet printhead is monitored during the second time period to determine the nozzle status will now be described.

[0066] In one embodiment, the system determines whether the measured current during the second time period is above a threshold. The threshold may be a predetermined threshold. The voltage comparator 205 may use the current measured by the sensing circuit 203 to determine whether the piezoelectric current is above or below a threshold. In this embodiment, the measurement is made over a period of time from the end of the intermediate time period (after the electronics have stabilized) until the threshold current value is reached. The time taken to reach the predetermined threshold is typically between 2 and 10 μs. The timer circuit 207 is used to measure the period of time between the end of the intermediate time period and the time when the piezoelectric current exceeds the threshold when the piezoelectric is de-energized during the second time period. The time taken to reach the current threshold may be used to determine the state of the nozzle, which will be described in more detail below.

[0067] In another embodiment, the system determines whether the current measured during the second time period has reached a local maximum. The time period from the end of the intermediate time period to the time when the local maximum is reached is measured. The voltage comparator 205 can use the current measured by the sensing circuit 203 to determine whether the piezoelectric current has reached a local maximum. The voltage comparator can be configured to detect when such a current reaches a local maximum. The system can determine that the local maximum current has been reached by determining the derivative of the current with respect to time. When the derivative of the current with respect to time is equal to zero, the local maximum is reached. When di / dt reverses (i.e., changes from positive to negative, or vice versa), the local maximum can be additionally detected. The local maximum of interest is when the first derivative is equal to zero during the second cycle or when the first di / dt reverses. The time taken to reach the local maximum from the end of the intermediate time period is typically 2 to 10 μs. The timer circuit 207 is used to measure the time period between the end of the intermediate time period and the time when the piezoelectric is de-energized during the second time period to reach the local maximum. The time taken to reach the local maximum can be used to determine the state of the nozzle, which will be described in more detail below.

[0068] In further embodiments, the system determines the slope (i.e., gradient, derivative of the current with respect to time) of the current versus time graph during the second time period. The system may determine the slope of the graph between the end of the intermediate stabilization period and the first local maximum in the current versus time graph during the second time period. Alternatively, the system may determine the slope of a smaller portion (in terms of time) of the current versus time graph during the second time period. The determined slope may be used to determine the state of the nozzle, as will be described in more detail below.

[0069] Once the time period to reach the predetermined condition or the slope of the current versus time graph is determined, the logic processor 206 can determine the state of the nozzle based on the value of the slope or the time it takes to reach the threshold or local maximum.

[0070] The operating state of the nozzle is determined using one or more of a set of rules, an algorithm, and a lookup table. For example, a measurement of the time it takes to reach a predetermined condition or the slope of a current versus time graph may be input into an algorithm that outputs a state based on the input. The measurement of the time it takes to reach a predetermined condition or the slope of a current versus time graph may be used to look up a corresponding state in a lookup table.

[0071] Figure 4 (a) and Figure 4 (b) shows a current versus time graph of a print head nozzle with different operating states. In this example, the slope of the graph is used to determine the operating state of the nozzle. Figure 4 (a) shows a current versus time graph after the end of the intermediate time period for a non-spraying nozzle, Figure 4 (b) shows a current versus time graph for the ejection nozzle. The slope of the current versus time graph during the second time period for the ejection nozzle has a greater value than that for the non-ejection nozzle.

[0072] The state of the nozzle can be described as normal jetting, deviated jetting, partially blocked, completely blocked, containing bubbles, and other states, or by a combination of any of the foregoing states. Each operating state can correspond to a different discrete value or a different range of values ​​of the time taken to reach the predetermined condition during the second time period, or a different discrete value or a different range of values ​​of the slope of the current versus time graph during the second time period.

[0073] The system may be configured to output the determined state to a user of a printer comprising an inkjet printhead. The system may send a state signal to a processor of the printer. The signal may cause the state to be transmitted to the user of the printer. For example, the state may be displayed on a display element (e.g., a user interface) of the printer, which may be an LCD screen or similar element, or the state may be transmitted to a PC or server using an appropriate communication protocol such as TCP-IP (Internet Protocol).

[0074] Figure 5 An embodiment of a method for determining an operating state of a nozzle in an inkjet printhead is summarized. At step 501, the method includes applying a drive signal to a piezoelectric actuator during a first time period. At step 502, the method includes measuring a current in the piezoelectric actuator as a function of time during a second time period after the first time period. At step 503, the method includes determining the operating state of the nozzle based on the time taken for the current measured during the second time period to reach a predetermined condition.

[0075] Figure 6 Another embodiment of a method for determining an operating state of a nozzle in an inkjet printhead is summarized. At step 601, the method includes applying a drive signal to a piezoelectric actuator during a first time period. At step 602, the method includes measuring a current in the piezoelectric actuator as a function of time during a second time period after the first time period. At step 603, the method includes determining the operating state of the nozzle based on a slope of the measured current as a function of time during the second time period.

[0076] The described method may be used between print jobs to determine nozzle status.

[0077] Alternatively, the described method can be used after a maintenance operation to check whether maintenance (e.g., cleaning of the nozzles) has been successfully completed. The method can also be performed during the print job itself, where the peak voltage of the drive signal is sufficient to cause an ink droplet to be ejected from the nozzle (i.e., the drive voltage exceeds the ejection voltage threshold).

[0078] The print head may contain a plurality of piezoelectric nozzles. The state determining means may be sequentially connected to the piezoelectric actuator of each nozzle of the print head. This may be accomplished using a switching circuit.

[0079] Using the above method, the state of the print head can be determined more quickly after the piezoelectric nozzle is energized than in prior art methods. For example, state determination using the frequency or decay rate of the current versus time profile requires a longer period of time after the drive signal stops because multiple oscillation cycles are required. The currently described method only requires the time period before the first local maximum in the current versus time curve to make a state determination.

[0080] Furthermore, measuring these properties of the current-time characteristics is much simpler than measuring the frequency or decay rate of the current in the piezoelectric as a function of time.

[0081] Thus, the operational status of the print head nozzles may be obtained more quickly and in a less computationally demanding manner.

[0082] Applicants hereby disclose separately each individual feature described herein and any combination of two or more such features, as long as these features or combinations can be implemented based on the present specification as a whole according to the common knowledge of those skilled in the art, regardless of whether these features or combinations of features solve any problems disclosed herein, and are not limited by the scope of the claims. Applicants point out that various aspects of the present invention may be composed of any such individual features or combinations of features. In view of the foregoing description, it will be apparent to those skilled in the art that various modifications may be made within the scope of the present invention.

Claims

1. A system for determining an operating state of a nozzle in an inkjet printhead, the inkjet printhead having a piezoelectric actuator configured to cause ink to be ejected through the nozzle, the system include: a drive circuit configured to apply a drive signal to the piezoelectric actuator during a first time period; as well as a sensing circuit configured to measure a current within the piezoelectric actuator as a function of time during a second time period after the first time period, wherein the drive signal remains constant during the second time period; Wherein the system is configured to determine the operating state of the nozzle based on the time taken for the current measured during the second time period to reach a predetermined condition.

2. The system according to claim 1, in, The predetermined condition is a threshold current value.

3. The system according to claim 1, in, The predetermined condition is a maximum current value.

4. The system according to claim 1 or 3, in, The predetermined condition is when the gradient of the measured current as a function of time is equal to zero.

5. The system according to claim 4, in, The predetermined condition is when, during the second time period, the gradient of the measured current as a function of time is equal to zero for the first time.

6. The system according to any one of claims 1 to 3, in, The second time period is separated from the first time period by an intermediate time period.

7. The system according to claim 6, in, The time taken for the measured current to reach the predetermined condition is measured from the end of the intermediate time period.

8. The system according to any one of claims 1 to 3, in, The time taken for the measured current to reach the predetermined condition is measured from the start of the second time period.

9. The system according to claim 2, in, The system also includes a comparator configured to compare the measured current to the threshold current value.

10. The system according to any one of claims 1 to 3, in, The system also includes a counter configured to measure the time it takes for the measured current to reach the predetermined condition.

11. A system for determining an operating state of a nozzle in an inkjet printhead, the inkjet printhead having a piezoelectric actuator configured to cause ink to be ejected through the nozzle, the system include: a drive circuit configured to apply a drive signal to the piezoelectric actuator during a first time period; as well as a sensing circuit configured to measure a current within the piezoelectric actuator as a function of time during a second time period after the first time period, wherein the drive signal remains constant during the second time period; Therein, the system is configured to determine the operating state of the nozzle based on a slope of the measured current as a function of time during the second time period.

12. The system according to claim 11, in, The system also includes a low noise amplifier.

13. The system according to claim 11 or 12, in, The operating state of the nozzle is determined by a logic processor.

14. The system according to claim 13, in, The logic processor is one of a microprocessor, a CPLD, a FPGA, a digital signal processor, a microcontroller, an embedded PC, a personal computer, a server, an ASIC.

15. The system according to claim 11 or 12, in, The operational state of the nozzle is determined using one or more of a rule set, an algorithm, and a lookup table.

16. The system according to claim 11 or 12, in, The sensing circuit includes one or more of: a current sensor resistor, a differential operational amplifier, a Hall effect current sensor, a capacitor in series with the piezoelectric actuator, and a current mirror.

17. The system according to claim 11 or 12, in, The operating state is determined to be one or more of: normal firing, deviated firing, partial blockage, full blockage, and containing bubbles.

18. The system according to claim 11 or 12, in, The drive signal is not applied to the actuator during the second time period.

19. An inkjet print head, include: nozzle; a piezoelectric actuator configured to cause ink to be ejected through the nozzle; as well as A system as claimed in any preceding claim.

20. A method for determining the operating state of a nozzle in an inkjet print head, the method include: applying a drive signal to the piezoelectric actuator during a first time period; measuring a current in the piezoelectric actuator as a function of time during a second time period after the first time period; as well as The operating state of the nozzle is determined based on the time taken for the current measured during the second time period to reach a predetermined condition, wherein the drive signal remains constant during the second time period.

21. A method for determining the operating state of a nozzle in an inkjet print head, the method include: applying a drive signal to the piezoelectric actuator during a first time period; measuring a current in the piezoelectric actuator as a function of time during a second time period after the first time period; as well as An operational state of the nozzle is determined from a slope of the measured current as a function of time during the second time period, wherein the drive signal remains constant during the second time period.

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