Patent Grant
9862187
This invention pertains to the field of inkjet printing and more particularly to temperature sensing at different locations on a drop ejector array module and providing drop volume control for the different locations.
Inkjet printing is typically done by either drop-on-demand or continuous inkjet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording medium using a drop ejector including a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the recording medium and strikes the recording medium. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image.
Motion of the recording medium relative to the printhead during drop ejection can consist of keeping the printhead stationary and advancing the recording medium past the printhead while the drops are ejected, or alternatively keeping the recording medium stationary and moving the printhead. This former architecture is appropriate if the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead drop ejector array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a medium advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the medium advance direction as the drops are ejected from the nozzles. After the carriage-mounted printhead has printed a swath of the image while traversing the print medium, the recording medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop ejector in a drop-on-demand inkjet printhead includes a pressure chamber having an ink inlet for providing ink to the pressure chamber, and a nozzle for jetting drops out of the chamber. Two side-by-side drop ejectors are shown in prior art
Other types of actuators that use resistive heaters to selectively pressurize the pressure chamber for drop ejection include thermal actuators that have a multi-layer cantilevered element that is caused to rapidly bend toward the nozzle when the resistive heater layer is pulsed. Less heating of the ink is required than for thermal inkjet, where the ink is locally vaporized to provide the ejection pressure.
Drop ejectors in drop-on-demand inkjet printheads work well within a given temperature range. Printhead temperature can vary due to variation in ambient temperature as well as to temperature rise associated with the energy dissipated on the printhead during operation. A known problem in drop-on-demand inkjet printing is the degradation in output print quality due to temperature-related changes in the volume of ink that is ejected. One reason why the size of ejected drops increases with temperature of the printhead is that ink viscosity decreases with increased temperature. In addition, for thermal inkjet printheads, the amount of ink that is vaporized by a resistive heater during a printing pulse increases with increased printhead temperature. Although a significant portion of the heat is carried off by the ejected ink drops, some of the heat remains in the printhead and results in an increased temperature. At sufficiently high temperature the drop ejection can become unreliable, resulting in missing dots in the printed image.
Various printhead temperature control and pulse waveform control systems and methods are known in the prior art for sensing inkjet printhead temperature and using sensed temperature signals to compensate for temperature fluctuations. The approach in printhead temperature control is to keep the printhead within a narrow temperature range by auxiliary heating or cooling for example. In pulse waveform control the approach is to tailor the pulses that are provided to the resistive heaters in order to compensate for temperature changes on the printhead so that the drop volume remains substantially constant. In both approaches it is important to have an accurate measurement of temperature in the vicinity of the drop ejectors.
U.S. Pat. No. 4,910,528 discloses an analog temperature sensing system where a thin film thermal sensing resistor (i.e. a thermistor) is formed on the same substrate as the drop ejectors in order to provide a temperature measurement that corresponds closely to the temperature of the drop ejector substrate. It is disclosed that preferably four leads are attached to the thermistor where two of the leads provide a current and the other two leads are used to output the voltage drop across the thermistor.
U.S. Pat. No. 4,791,435 discloses a thermal inkjet printhead temperature control system that regulates the temperature of a printhead using a temperature sensing device and a heating component. The temperature sensing device includes either a collection of transducers or a single thermistor located on the drop ejector substrate or on a printed circuit board to which the printhead is attached.
U.S. Pat. No. 5,075,690 discloses an analog temperature sensor that is formed on the drop ejector substrate and extends along the length of the array of drop ejectors. Recognizing that manufacturing variations in the thermistor can result in large inaccuracies in temperature measurement, a factory calibration is disclosed where a resistor in series with the thermistor is trimmed, for example by laser trimming, while the printhead is held at a set point temperature. Other methods of factory calibration of the thermistor on an inkjet printhead are described in U.S. Pat. No. 5,881,451 and U.S. Pat. No. 7,572,051.
The analog measurement of a temperature sensor such as a thermistor can be converted to a digital signal by an analog to digital converter for use in control circuitry as disclosed in U.S. Pat. No. 6,302,507 and in U.S. Pat. No. 6,322,189. Another alternative for providing a digital signal is to provide temperature controlled oscillator circuitry on the drop ejector substrate. Temperature controlled oscillators are described in U.S. Pat. No. 5,388,134 and typically include a thermistor as the temperature sensitive element. The number of counts recorded by a counter during a given time interval (i.e. the frequency of the oscillator signal) changes approximately linearly with temperature. Various implementations of temperature controlled oscillators on inkjet printheads are disclosed in U.S. Pat. No. 5,745,130, U.S. Pat. No. 6,037,831, U.S. Pat. No. 6,278,468, and U.S. Pat. No. 8,419,158.
In some of the prior art references listed above, such as U.S. Pat. No. 5,745,130, U.S. Pat. No. 6,302,507, U.S. Pat. No. 7,572,051 and U.S. Pat. No. 8,419,158 a plurality of small temperature sensors are located in various parts of the drop ejector substrate either for obtaining an average temperature measurement on the drop ejector substrate or for independently measuring the temperature in different locations on the drop ejector substrate. Typically for measuring temperature in different locations using temperature sensors formed on the drop ejector substrate, additional output leads are required as shown in prior art
As indicated above, drop volume tends to increase with increased temperature of the printhead and ink. It is also known that drop volume can be affected by the pulse waveform. As disclosed in U.S. Pat. No. 4,982,199, ink in the vicinity of the nozzle of a drop ejector can be pre-warmed by pulsing the resistive heater using one or more pulses that have insufficient energy to form a vapor bubble of ink prior to the firing pulse that forms the vapor bubble. By pre-warming the ink, more of the ink in the nozzle region is brought to the vaporization temperature by the firing pulse before the transfer of heat to the ink from the resistive heater is interrupted by the formation of the vapor bubble. Vaporizing more of the ink forms a larger bubble, which provides the power for ejecting a larger drop of ink. Prior art
U.S. Pat. No. 5,036,377 disclosed attaching a temperature sensor to a surface of the drop ejector substrate. The resistive heaters on the drop ejector substrate are connected to drivers that are not on the drop ejector substrate. Temperature signals from the temperature sensor are sent to a controller, and the controller enables actuation of selected resistive heaters through the drivers using packets of electrical pulses. A digital clock signal is also provided to the controller. It is disclosed that pulse widths, idle times between pulses or number of pulses per packet can be increased or decreased by one or more clock units to change the pulse waveform in order to control drop volume in response to the temperature measured by the temperature sensor according to a look-up table that provides data to the controller. U.S. Pat. No. 5,917,509 discloses one or more precursor pulses (or warming pulses) that are applied to the resistive heater for warming the ink nearby, followed by a print pulse that causes a drop of ink to be ejected.
Despite the previous advances in temperature sensing as well as temperature control and drop volume control on inkjet printheads, what is still needed are printing system designs and printing methods that provide individual temperature sensing and corresponding pulse waveform compensation for different locations on a drop ejector array substrate. In addition, it is desirable to provide drop ejector arrays having a small number of input/output connections, while still providing drop volume control for drop ejectors in different locations on a drop ejector array substrate that can be at different temperatures.
According to an aspect of the present invention, an inkjet printing system includes at least one drop ejector array module. Each drop ejector array module includes a substrate having an array of drop ejectors disposed on the substrate. Each drop ejector includes a nozzle, an ink inlet, a pressure chamber in fluidic communication with the nozzle and the ink inlet, and an actuator configured to selectively pressurize the pressure chamber for ejecting ink through the nozzle. A primary temperature sensor is disposed on the drop ejector array substrate in a first location near to a first set of drop ejectors. At least one secondary temperature sensor disposed on the substrate in a second location near to a second set of drop ejectors. Temperature comparison circuitry is disposed on the substrate, and is configured to compare signals from the primary temperature sensor and the at least one secondary temperature sensor. Pulse modification circuitry disposed on the substrate is electrically connected to the temperature comparison circuitry and is configured to modify an input pulse waveform. A temperature output pad on the drop ejector array module is connected to the primary temperature sensor. A pulse waveform input pad on the drop ejector array module is connected to the pulse modification circuitry. The inkjet printing system also includes a controller that is electrically connected to the primary temperature sensor via the temperature output pad and to the pulse modification circuitry via the pulse waveform input pad. In some embodiments the inkjet printing system further includes a reference temperature sensor that is separate from the at least one drop ejector array module. In such embodiments the controller is electrically connected to the reference temperature sensor and to the temperature output pad of the at least one drop ejector array module, and is configured to calibrate the primary temperature sensor on the at least one drop ejector array module.
According to another aspect of the present invention, a method is provided for controlling actuation of drop ejectors disposed at different locations on a drop ejector array module having a primary temperature sensor in a first location near to a first set of drop ejectors and a secondary temperature sensor in a second location near to a second set of drop ejectors. The method includes performing a first temperature measurement with the primary temperature sensor and performing a second temperature measurement with the secondary temperature sensor. A temperature difference between the first temperature measurement and the second temperature measurement is determined using temperature comparison circuitry disposed on the drop ejector array module. A controller receives the first temperature measurement, determines electrical pulse waveforms corresponding to the first temperature measurement, and sends the electrical pulse waveforms corresponding to the first temperature measurement to the drop ejector array module. The electrical pulse waveforms are used to provide first actuation pulse waveforms to the first set of drop ejectors corresponding to the first temperature measurement. Pulse modification circuitry disposed on the drop ejector array module is used to modify the first actuation pulse waveforms based on the temperature difference measured by the comparison circuitry to provide second actuation pulse waveforms to the second set of drop ejectors.
This invention has the advantage that drop volume control is provided for drop ejectors in different locations on a drop ejector array substrate that can be at different temperatures. It has the additional advantage that only a small number of input/output pads are required. A further advantage is that the temperature sensor can be calibrated within the printing system.
It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures.
The invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense.
Printhead die 110 includes at least one drop ejector array 120 including a plurality of drop ejectors 125 formed on a top surface 112 of a substrate 111 that can be made of silicon or other appropriate material. In the example shown in
Maintenance station 70 keeps the drop ejectors 125 of printhead die 110 on printhead 50 in proper condition for reliable printing. Maintenance can include operations such as wiping the top surface 112 of printhead die 110 in order to remove excess ink, or applying suction to the drop ejector array 120 in order to prime the nozzles. Maintenance operations can also include spitting, i.e. the firing of non-printing ink drops into a reservoir in order to provide fresh ink to the pressure chambers and the nozzles, especially if the drop ejectors have not been fired recently. Volatile components of the ink can evaporate through the nozzle over a period of time and the resulting increased viscosity can make jetting unreliable.
Temperature comparison circuitry 160 makes it possible to sense the temperature in different regions of drop ejector array module 110 without requiring an output pad for each temperature sensor. Temperature output pad 131 is connected to the primary temperature sensor 156 by lead 181, but no output pads are provided for secondary temperature sensors 157 and 158. Instead, primary temperature sensor 156, secondary temperature sensor 157 and secondary temperature sensor 158 are connected to temperature comparison circuitry 160 by connections 183, 184 and 185 respectively. Although single lines are shown for representing leads from the group 130 of input/output pads and for other connections within drop ejector array module 110, it is to be understood herein that each single line can represent more than one electrical trace. Temperature comparison circuitry 160 is used to determine a temperature difference between a first temperature measurement made using the primary temperature sensor 156 and a second (or third) temperature measurement made using a secondary temperature sensor 157 (or 158). Differences in temperature along drop ejector array 120 can occur due to uneven printing usage by first set 126, second set 127 and third set 128 of drop ejectors 125. For example, if the image data has recently required relatively heavy printing usage by second set 127 and lighter printing usage by first set 126 and third set 128, then the temperature measured by secondary temperature sensor 157 can be higher than the temperature measured by either primary temperature sensor 156 or secondary temperature sensor 158. Rather than controlling pulse waveforms for drop volume control based on an average temperature on the drop ejector array module 110, or on the temperature signal provided at temperature output pad 131 (corresponding to the temperature measured by primary temperature sensor 156), pulse waveform control can be provided independently for the various regions of drop ejector array 120.
For embodiments where primary temperature sensor 156, secondary temperature sensor 157 and secondary temperature sensor 158 are all thermistors, temperature comparison circuitry 160 can operate by comparing a signal corresponding to the resistance of the thermistor of primary temperature sensor 156 with signals corresponding to the resistances of the thermistors of secondary temperature sensors 157 and 158 respectively. Alternatively, signals representing voltage drops across the respective thermistors can be compared. For embodiments where primary temperature sensor 156, secondary temperature sensor 157 and secondary temperature sensor 158 are all temperature controlled oscillators, a frequency of a signal from primary temperature sensor 156 is compared to a frequency of a signal from secondary temperature sensor 157 and a frequency of a signal from secondary temperature sensor 158. Temperature comparison circuitry 160 makes comparisons on an ongoing basis so that temperature differences between the primary temperature sensor 156 and the secondary temperature sensors 157 and 158 are updated continually during the printing process.
In response to the temperature that is measured by primary temperature sensor 156 and sent to controller 14 via temperature output pad 131 and printhead output line 52 (
Pulse waveforms 201-206 of
In the examples shown in
In previous implementations of drop volume control using pulse waveforms to compensate for the tendency of drop volume to increase with temperature, a temperature measurement representing a group of drop ejectors 125 would be sent periodically to the controller 14. In response to the most recent temperature measurement the controller would send pulse waveforms to be used by the entire group of drop ejectors 125. For example, if a drop ejector array module 110 had a single temperature sensor, the temperature measured by that temperature sensor 110 would be used by the controller to determine the pulse waveform to be used for all of the drop ejectors 120 on the drop ejector array module. For a printhead 50 having a plurality of drop ejector array modules 110, a single temperature measurement would be used to characterize each drop ejector array module 110 and the controller 14 would send pulse waveforms which could differ for the different drop ejector array modules 110.
In embodiments of the invention it is recognized that temperature can vary across the drop ejector array module 110 so that it can be advantageous to use different pulse waveforms for first set 126, second set 127 and third set 128 (
However, if the temperatures measured by secondary temperature sensors 157 or 158 are sufficiently different from the temperature measured by primary temperature sensor 156, the pulse modification circuitry 170 on drop ejector array module 110 can be used for modifying the pulse waveforms as appropriate for drop ejectors 125 in different regions of the drop ejector array module 110. An example of how the pulse modification circuitry 170 operates can be understood with reference to
When it is said herein that pulse modification circuitry 170 modifies first actuation pulse waveforms it is meant that the modified actuation pulse waveforms have a different shape than the first actuation pulse waveforms. It is not meant to imply that pulse modification operations are restricted to the sequence of providing the first actuation pulse waveforms from the electrical pulse waveforms that controller 14 sends to pulse waveform input pad 132 and then performing modification operations. The language is also meant to include optionally directly modifying the electrical pulse waveforms that controller 14 sends to pulse waveform input pad 132.
The substrate 111 (
In another embodiment the pulse modification circuitry 170 is configured or directed to modify the first actuation pulse waveforms based on the temperature difference measured by the temperature comparison circuitry 160 and to provide the modified actuation pulse waveforms to the first set 126 of drop ejectors 125. In other words, the shape of the actuation pulse waveforms used to pulse the resistive heaters of the first set 126 of drop ejectors can be different from the electrical pulse waveforms sent by controller 14 to pulse waveform input pad 132 in response to the temperature measurement made by primary temperature sensor 156. For example, if primary temperature sensor 156 measures a temperature T4 and both secondary temperature sensor 157 and secondary temperature sensor 158 measure a temperature T2, the controller 14 will send electrical pulse waveform 204 (
In the examples described above with reference to
As indicated in the prior art, temperature sensors that are fabricated on drop ejector array modules typically need to be calibrated in order to provide an accurate temperature measurement. This is important for printheads having a single drop ejector array module, but even more important for printheads having a plurality of drop ejector array modules.
Controller 14 is configured to calibrate the primary temperature sensor 156 (
Optionally the primary temperature sensor 156 for each drop ejector array module 110 can be calibrated in the factory as described in the prior art references cited above. However, such a calibration would need to be stored in memory on the pagewidth printhead 105, so that if a particular pagewidth printhead 105 needed to be replaced, the controller 14 would have access to the calibration data for the new printhead. In addition, if the characteristics of the temperature sensors drift over time the factory calibration can lose accuracy.
European Patent No. 0 622 209 discloses a carriage printer where a thermistor mounted on the carriage is used to calibrate a printhead substrate temperature sensor that is fabricated on the substrate of the drop ejector array module. It is disclosed that the printhead temperature sensor can be calibrated once at power-on or continuously. A drawback of the disclosed calibration method is that it does not ensure that the drop ejector array module is in a state of thermal equilibrium with the reference temperature sensor. For example, many inkjet printers have maintenance routines that dissipate energy on the drop ejector array module during long periods of printer inactivity. As long as the printer is plugged into an active electrical outlet, maintenance operations such as firing non-printing drops of ink from the nozzles into a reservoir are routinely done on a periodic basis, even if the printer is turned off or is in a sleep mode. When the user starts a print job and turns the power on or exits the sleep mode, he has no information about when maintenance spitting of ink drops has last occurred.
Because a printing system is in a less predictable environment than a factory environment, it must be established that the drop ejector array module 110 is in a state of thermal equilibrium with the reference temperature sensor 150. In one embodiment successive electrical signals from the primary temperature sensor 156 are sent to the controller 14. A signal sent from the primary temperature sensor 156 at an initial time is compared with a later signal that is sent after a predetermined delay time. The predetermined delay time can be between one second and one minute for example. The controller 14 determines whether the successive electrical signals differ from each other by less than a predetermined threshold value that is stored in printer memory. If the successive electrical signals differ by less than the predetermined threshold value, the controller 14 determines that the temperature of the drop ejector array module 110 is not appreciably changing as a function of time. Since other parts of the printing system tend to change temperature only very slowly (for example as the ambient temperature of the room changes), in some embodiments establishing that the temperature of the drop ejector array module 110 is not changing appreciably is sufficient for the controller 14 to determine that the drop ejector array module 110 is in a state of equilibrium with the reference temperature sensor 150. In other embodiments, the controller 14 similarly compares successive signals from the reference temperature sensor 150 to verify that its temperature measurement is also not changing appreciably as a function of time.
In another embodiment the controller 14 determines that the drop ejector array module 110 is in a state of thermal equilibrium with the reference temperature sensor 150 by monitoring how long it has been since any drop ejector 125 on the drop ejector array module 110 has been fired, whether for printing or for maintenance operations. A clock 11 on controller 14 for example is used to track time. In this embodiment a firing incidence time that corresponds to a most recent firing of any drop ejector 125 on drop ejector array module 110 is stored in memory, for example in memory 19 on controller 14. Controller 14 measures a time interval between a current time and the firing incidence time. Controller 14 compares the measured time interval to a predetermined threshold time interval that is stored in memory 19. If the time interval between the current time and the firing incidence time is greater than the predetermined threshold time interval, then the controller determines that the drop ejector array module 110 is in a state of thermal equilibrium with the reference temperature sensor 150.
Once the controller 14 has determined that the drop ejector array module 110 is in a state of thermal equilibrium with the reference temperature sensor 150 the calibration process can proceed. At a first time the controller 14 receives a first electrical signal from the primary temperature sensor 156. Substantially simultaneously at the first time, for example within one second and preferably within 0.1 second, controller 14 receives a corresponding first reference temperature reading from the reference temperature sensor 150 and associates the first reference temperature reading with the first electrical signal. Controller 14 calculates a temperature calibration coefficient using the first temperature reading and the first electrical signal, and stores the temperature calibration coefficient in memory such as memory 19.
In the embodiments described above with reference to
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
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