The present application is related to U.S. patent application Ser. No. 11/061,120, filed on Feb. 18, 2005 and entitled “METHODS AND APPARATUS FOR PRECISION CONTROL OF PRINT HEAD ASSEMBLIES” which is hereby incorporated by reference herein in its entirety.
The present application is also related to U.S. patent application Ser. No. 11/238,637, filed on Sep. 29, 2005 and entitled “METHODS AND APPARATUS FOR A HIGH RESOLUTION INKJET FIRE PULSE GENERATOR” which is hereby incorporated by reference herein in its entirety.
The present invention relates generally to systems for manufacturing color filters for flat panel displays, and is more particularly concerned with apparatus and methods for testing and maintaining such systems.
The flat panel display industry has been attempting to employ inkjet printing to manufacture display devices, in particular, color filters. When inkjet printing techniques are applied in high throughput manufacturing, it is beneficial to maximize system reliability while minimizing system down time by rapid troubleshooting. System failures can arise in one or more printing channels due to clogging, electronics malfunction and variation of printhead parameters. In the case of electronics malfunction and variation of printhead parameters, it is cumbersome to manually examine signals to isolate the location and nature of a specific failure. Accordingly, apparatus and methods are needed to efficiently acquire data, test reliability and troubleshoot failures in inkjet printer systems.
In some aspects, the present invention provides a method for reliability testing an inkjet printing system including a print head having a capacitance and printer control electronics adapted to transmit a firing voltage signal to activate the print head. The method includes pre-calibrating a relationship between a capacitance of the print head and a measured voltage value of the firing voltage signal; measuring an actual firing voltage signal; determining the value of the print head capacitance by interpolation based on the measured firing voltage signal and the pre-calibrated relationship between the print head capacitance and measured voltage; and calculating a voltage at the print head based on the determined print head capacitance. Operability of the print head is then ascertainable based on the values of the print head capacitance and calculated print head voltage.
In some other aspects, the present invention provides a method that includes measuring a capacitance of printer control electronics (Cpce) and a test capacitance of a known value (Cknown) once per channel; measuring a per-channel voltage (Vno load) at a measurement apparatus used to measure Cpce and Cknown without a capacitive load; measuring a per-channel voltage (Vknown load) at the measurement apparatus used to measure Cpce and Cknown with a known capacitive load coupled to a testing interface; measuring a per-channel voltage (Vunknown load) at the measurement apparatus used to measure Cpce and Cknown with an unknown capacitive load coupled to the testing interface; calculating a data acquisition capacitance (CDAQ) based on the measured voltages; calculating a print head capacitance (Chead) based on a slew-rate ratio; and reconstructing a fire pulse voltage signal for each print head channel based on the ratio of Vknown load to Vunknown load, CDAQ, Vno load, Cpce and Cknown.
In yet other aspects, the present invention provides a system for reliability testing an inkjet printing system. The system for reliability testing includes a testing interface, a print head coupled to the testing interface, printer control electronics coupled to the testing interface and coupled to the print head via the testing interface, the printer control electronics adapted to transmit a firing voltage signal through the testing interface to the print head, and a measurement apparatus coupled to the testing interface. The testing interface includes an input path for receiving the firing voltage signal from the printer control electronics, the input path splitting into a first path coupled to the print head and a second path coupled to the measurement apparatus.
In still yet other aspects, the present invention provides an apparatus for testing an inkjet printing system. The apparatus includes a test interface adapted to be coupled to a print head and to a print control circuit, wherein the print control circuit is coupled to the print head via the test interface and is adapted to transmit a firing signal through the test interface to the print head; and a measurement circuit coupled to the test interface. The test interface includes an input path for receiving the firing signal from the print control circuit, the input path being split into a first path coupled to the print head and a second path coupled to the measurement circuit.
Other features and aspects of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings.
In an inkjet printer, an inkjet printer control system operates one or more inkjet print heads to dispense ink (or other fluid) onto a substrate. The inkjet print heads typically include multiple separately-controllable nozzles which each dispense drops upon being activated. The control path for each nozzle comprises a channel along which voltage signals may be propagated for nozzle activation. For example, some print heads include piezoelectric transducers (PZTs) coupled to each nozzle that expand and contract to release a drop of ink through an opening in response to a voltage pulse. When a channel is functioning properly, the amplitude of a voltage pulse measured across the channel is well defined; this allows the proper functioning of the channel to be tested through measurement of the channel voltage.
According to some embodiments, the present invention provides a system and method for determining whether each channel of a print head is functioning properly based on a fire pulse voltage measured across each respective channel. In some embodiments, a measurement apparatus acquires voltage data from the multiple print head channels via a testing interface that modifies the voltage signal to match requirements of a measurement apparatus. Since the measurement apparatus as well as the testing interface introduce their own capacitance to the printing system to which it is applied (the combined capacitance of the measurement apparatus, the printer control electronics, and testing interface is termed the ‘data acquisition capacitance’ (CDAQ)), the voltages that are recorded by the measurement apparatus reflect contributions from its own capacitance in addition to the capacitance of the print head channels. For this reason, the recorded voltages do not reflect accurate measurements of the channel voltages. To obtain accurate channel voltages, each contribution to the total capacitance (hereinafter ‘Ctot’), that is, the sum of the capacitance of the data acquisition system (CDAQ), and the capacitance of the print head channel (hereinafter ‘Chead’), is isolated and determined separately.
In some embodiments of the present invention, the channel capacitance Chead is determined based on the measured voltage (hereinafter ‘V2’). However, the relationship between Chead and V2 is usually not linear. Thus, in some embodiments, the present invention provides a method of calibrating numerous per channel capacitance Chead values with measured voltage values V2 such that unknown channel capacitances can be interpolated from calibrated values. The per channel voltage V1 may then be ascertained based on the measured voltage V2, and the separately determined capacitances Chead and Ctot.
System Overview
Turning to
Printer control electronics 130 are coupled to the print head 110 through the testing interface 120. The printer control electronics 130 includes logic, communication, and memory devices configured to control the operation of the print head 110. The print control electronics 130 may be implemented using one or more field programmable gate arrays (FPGA) or other similar devices. In some embodiments, discrete components may alternatively or additionally be used. In particular, the printer control electronics 130 may include one or more drivers that may each include logic to transmit control signals (e.g., fire pulse signals) to one or more print heads e.g., print head 110. Each driver of the printer control electronics 130 is adapted to transmit signals on multiple channels so that each actuator corresponding to each nozzle of the print head can be individually and independently actuated. For example, if the print head 110 comprises a 128-channel device, then the driver is adapted to address control signals to each of the 128 channels by separate connections, a multiplexing arrangement or any other electronic addressing mechanism.
The print control electronics 130 may be coupled to a power supply (not shown) so as to be able to generate relatively high voltage firing pulses to trigger the nozzles of the print head 110 to “jet” ink. In some embodiments, the power supply may be a high voltage negative power supply adapted to generate signals having amplitudes of approximately 140 volts or more. Other voltages may be used. The print control electronics 130 may send firing pulse voltage signals with specific amplitudes and durations so as to cause the nozzles of the print head 110 to dispense fluid drops of specific drop sizes as described, for example, in previously incorporated U.S. patent application Ser. No. 11/061,120. The print control electronics 130 may additionally be coupled to a host computer 150 for receiving data or instructions for generating the firing pulses.
As shown in
An output path 209 taps node 211 between resistors 202, 204 and also taps node 213 between capacitors 205, 207 such that the resistors act as a voltage divider to lower the firing voltage signal to match input requirements of the measurement apparatus employed. Example values for resistors 202, 204 are 10 Mega-ohms and 200 Kilo-ohms, respectively, which provides a large amount of attenuation. Other resistor values can also be used. In operation, when a firing voltage signal is transmitted, a large proportion of current from the signal flows through the capacitors 205, 207 which then become charged. The capacitors 205, 207 then act as voltage sources with respect to resistors 202, 204 and aid in reconstructing the firing voltage waveform. Example values for capacitors 205, 207 are 10 picofarads (10 pF) and 500 picofarads (500 pF), respectively.
The output path 209 leads from nodes 211, 213 to a buffer 215 which may comprise an operational amplifier or similar device having high input impedance and low output impedance to improve measurement accuracy. The output from the buffer 215 represents the output of the compensator circuit 1221.
Referring again to
The output path 127 from the selector circuit 128 leads to a measurement apparatus 140 which is adapted to accurately measure the voltage signal supplied to it along path 129. An example of a suitable commercially available measurement apparatus that may be used in the context of the present invention is the PXI-6239 Multifunction Data Acquisition (DAQ) device manufactured by National Instruments Inc. of Austin, Tex. The measurement apparatus 140 may additionally comprise or be coupled to a computer or control electronics that allow user control of the measurement apparatus. The measurement apparatus may be coupled via a TTL or other type of connection to a switching device 129 situated within (as shown) or coupled the testing interface 120. By activating the switching device 129, the testing interface 120 can be disconnected via the measurement apparatus when it is desired to stop testing operations.
As can be seen from
dV=(1/C)*I*dt (1)
From this equation, it can be seen that the slew rate of a signal is inversely proportional to capacitance along the signal path. Taking V1 to be the voltage drop across a print head channel without a contribution from the data acquisition path and V2 to be the voltage drop across a the print head channel including the contribution from the data acquisition path, the ratio of V1 to V2 can be expressed according to equation (2) as:
V1/V2=Ctot/Chead=a1/a2 (2),
where a1, a2 represent respective slew rates of voltage signals V1, V2.
As indicated, the measured voltage pulse shown in curve 306 varies considerably from curve 302 which it is meant to reproduce. If the print head capacitance Chead were known beforehand, it would be a trivial matter to reconstruct V1 from V2 as measured (i.e., from equation (2)); however, the print head capacitance varies from one channel to another within a print head, and between different heads, resulting in a nonlinear relationship between V2 and Chead. Owing to this nonlinear relationship, Chead typically cannot be determined by linear scaling.
Overall Reliability Testing Method
Referring to the flowchart of
Exemplary Calibration Method
In step 605 of the calibration process, a per-channel voltage reading is taken at the measurement apparatus in the state depicted in
Vno-load=I·t/M(CDAQ) (3),
where M is a resistive attenuation factor equal to the ratio of R1 to R2 in the voltage compensator circuit.
In step 607, a per-channel voltage reading is taken at the measurement apparatus in the state depicted in
Vknown-load=I·t/M(CDAQ+Cknown) (4).
In step 609, a per-channel voltage reading is taken at the measurement apparatus in the state depicted in
Vunknown-load=I·t/M(CDAQ+Cunknown) (5).
In step 611, the total data acquisition capacitance CDAQ is calculated using equations (3) and (4) above as follows:
CDAQ=[(Vno-load/t)/(Vknown-load/t)−1]*Cknown (6),
where CDAQ=Ctest+Cpce.
In step 613, the print head capacitance Chead (or Cunknown) is calculated from the slew-rate ratios of Vknown-load to Vunknown-load as follows:
(Vknown-load/t)/(Vunknown-load/t)=(CDAQ+Chead)/(CDAQ+Cknown) (7).
Since all variables other than Chead are known or have been ascertained, Chead can be determined numerically. This process can then be repeated over numerous channels to derive a calibrated relationship between Chead and the measured value of unknown-load (V2).
It is again noted that the print head voltage signal can be reconstructed once Chead is known. For example, when a print head channel is connected directly to the printer control electronics without the testing interface, the voltage signal Vhead can be expressed as:
Vhead=I·t/(Chead+Cpce) (8)
Having calculated CDAQ and Chead, the actual fire pulse voltage signal for each print head channel (Vhead) taking into account the effects of the testing interface can be reconstructed in step 615 using equation (8) as follows:
Vhead/Vunknown-load=M*(Ctot/(Chead+Cpce)) (9),
where Ctot=CDAQ+Chead.
Since all of the variables in equation (9) other than Vhead are known or have been ascertained, Vhead can be determined numerically.
The foregoing description discloses only particular embodiments of the invention; modifications of the above disclosed methods and apparatus which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art.
Accordingly, while the present invention has been disclosed in connection with specific embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.
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