THERMAL INKJET PEN TEMPERATURE CONTROL

Abstract: Thermal inkjet printhead temperature control is provided in a temperature control system responsive to printhead temperature, which in the presence of printhead overheating selectively causes the printhead to stand idle, or, if multiple nozzles are available, shifts to a nozzle which is not overheated, which in the event a nozzle is unused for some time and the dye transport agent may have evaporated leaving a viscous plug in the nozzle employs warm up pulsing and/or nozzle spitting to clear the nozzles, and which when the temperature of the nozzle is below acceptable printing temperatures, nozzle pulsing for warm up and/or nozzle spitting to clear the nozzles, all such decisions and actions being provided in advance of beginning a printing operation.

Description: This invention relates to thermal inkjet types of printers for producing printed text and/or graphics and more particularly to arrangements for controlling the uniformity of the ink drops in such printers by providing a control of the temperature of the printhead or pen. The appearance of printed text or graphics produced by thermal inkjet print heads varies if the viscosity of the ink changes. Viscosity is affected by the printhead temperature which in turn varies with the use profile of the printhead and the temperature environment in which the printer operates. One prior art approach taken in dealing with this problem has been to provide a spittoon into which ink drops are ejected prior to commencing printing. The purpose of this is twofold. First such ink drop ejection tends to clear viscous plugs from the nozzle of the thermal inkjet printhead and second, this preliminary use of the printhead provides a warm up interval, hopefully to achieve a printhead temperature at or near a desired temperature for printing purposes. Another prior art effort in dealing with this problem has been to provide a multi-grade ink in which the change in viscosity over a limited range of printhead operacting temperatures would not result in significant degradation of print quality. While such prior art developments have provided improvements in the quality of printed text, improvements in thermal inkjet printhead operation are known from DE-A-3546138 and US-A-4544931. The present invention aims to improve over this known prior art by improving the control of thermal inkjet printhead temperature. According to the present invention there is provided a temperature control system for a thermal inkjet printer, having a printer carriage drive, a printer carriage movable by said printer carriage drive across a printing zone between sweep limit positions and movable to and from a rest position in response to print commands, and having a thermal inkjet printhead mounted on said carriage, comprising: means including print drive circuits coupled to said thermal inkjet printhead for producing electrical pulses for firing inkdrops from said thermal inkjet printhead in said printing zone and for stopping said electrical pulses outside of said printing zone; temperature sensor means for sensing the temperature of said thermal inkjet printhead; and means responsive to said temperature sensor means for controlling said means including print drive circuits to reduce the data throughput by idling the printer carriage outside of the printing zone above a predetermined sensed temperature and thereby to maintain the temperature of said thermal inkjet printhead substantially at a predetermined temperature. Normal nozzle substrate temperatures for satisfactory printhead operation are about 40°C. Variations of about ± 5°C can be tolerated. Many things influence the temperature of the nozzle, these include: the ambient temperature of the environment, the amount of use a particular nozzle gets, the location of a nozzle on the nozzle substrate, i.e., near an edge or toward the center of the nozzle substrate. In addition, certail dyes (and dye transport agents) are more sensitive to temperature than others. The magenta nozzles may be more sensitive to low temperatures than the black nozzles, for instance. Therefore, the determination of temperature at or near each individual nozzle in a nozzle substrate is necessary to optimize printhead performance and hence to maximize print quality. The printhead temperature is determined by several means. One is by placing temperature sensing transducers on the substrate for each nozzle. Alternatively, a thermistor is placed on the printed circuit board to which the printhead is attached. This assembly is mounted on the printer carriage. Using the output of the thermistor a close estimate of the printhead temperature is achieved. Thermal models of the pens or printheads are provided and these are used in conjunction with printhead temperature sensors to provide the information useful in controlling the printhead or temperature. Profiles of the use of each nozzle are developed. These profiles when compared with a thermal model provide information useful in controlling head temperature. Temperature compensation and control is provided for both low printhead temperature and high printhead temperature. At low temperatures low energy pulses are sent to a nozzle to heat it. These pulses are below the threshold which would cause a drop of ink to be fired. The number of pulses used in this warm up process is based on the nozzle's temperature, the location of the nozzle in the substrate, the dye (color) in the nozzle, and the use profile of the nozzle. Another warming method which is employed is to fire some drops of ink from the nozzle into a spittoon which is located near the writing area but in a position outside of the writing area. The number of drops fired into this spittoon are based upon the temperature which is sensed, the nozzle location on the substrate, the color of the ink in the nozzle and the use profile of the nozzle. At high temperature, the use profile and the temperature sensors are monitored to see if a particular nozzle exceeds its operable range. If this is the case, printing is stopped until the temperature drops or in the alternative where more than one nozzle is on the substrate another nozzle is used. If a nozzle is unused for some time, the dye transport agent can evaporate, leaving a viscous plug in the nozzle. This evaporation is both temperature and time dependent. The nozzle use and temperature profiles are used in this situation to indicate when a nozzle needs to be cleared by firing ink drops into the spittoon. Low energy pulses which are below the level needed to fire ink drops are also used to warm and thin the viscous plugs depending upon the temperature and nozzle use profiles. Pulsing may be used independently of spitting or may be used prior to spitting to facilitate clearing the nozzles. The invention will be further understood by reference to the following specification when considered in conjunction with the accompanying drawings, in which: Figure 1 is a block diagram of an improved thermal inkjet printer control system, including provisions for controlling the temperature of the printhead, in accordance with the principles of this invention; Figure 2 is a block diagram illustrating details of the printhead temperature control system of this invention; and Figure 3 is a flow chart illustrating the decision making process in the different functional modes of operation. Figure 1 is a block diagram of a thermal inkjet printhead temperature control system embodying the principles of this invention. Print data may be supplied from an instrumentality such as a computer (not shown). Such print data is applied as input via a bus 1 to a microprocessor 2. In response to this input, as well as other inputs, yet to be described, the microprocessor produces control signals which are coupled by a bus 3 to a control circuit 4 which has multiple functions. The control circuit 4 produces a pulse width modulated signal which is coupled by a circuit 5 to a pulse width modulation amplifier 6 supplied with power from a power source 7. The amplifier 6 transforms and amplifies the input signal thereto to produce a drive voltage coupled by a circuit 8 to a motor 9. The motor 9 in this application is a DC motor functioning as a print carriage drive motor. The motor 9 drives a mechanism 10, such as a pulley and belt system, connected to a print carriage 11, to move the carriage in its axis. An encoder 12, comprising an encoder body 12a and an encoder scale 13 responds to carriage motion. The encoder scale 13 is secured at its ends to the printer chassis (not shown) in a position spanning and paralleling the carriage axis. The encoder body 12a which is mounted on the printer carriage, includes an optical scale detector therewithin which scans the tape as the carriage moves in its axis. Scale count signals, as well as signals indicative of start-of-print or end-of-print, from print limit bands B, carriage sweep limit signals from sweep limit bands A, etc., which are produced by the encoder, are coupled as feedback via circuit 14 to the control circuit 4. Control circuit 4, using the encoder signals, produces ink drop firing rate signals, coupled via a bus 15 to the microprocessor for controlling ink drop firing, produces scale count signals coupled to the microprocessor via a bus 16 for motor control, produces print limit signals from print bands B of the scale, and produces carriage sweep limit signals from the sweep limit bands A of the scale, respectively. The microprocessor compares the desired carriage position, which it generates in response to its input 1 with the carriage position derived from encoder feedback while scanning the scale divisions, and then computes the required control for the motor. This is an incremental process and is repeated in one embodiment of this invention at 200 times per second. This computation of motor control voltage provides the basis for control of print carriage speed between the print limit bands B within which printing takes place. The encoder which is shown, is a single channel incremental position encoder. This encoder functions as the feedback element in the control system. Its description here is believed to be sufficient for an understanding of this invention. In one practical embodiment of this invention, there are 90 scale divisions per 2.54 cm (per inch) on the encoder scale. The control circuit 4 doubles this to provide 180 pulse conts per 2.54 cm (per inch) required by the print heads for print drop firing. Control circuit 4 also quadruples the scale division pulse counts to provide 360 pulse counts per 2.54 cm (per inch) of scale required by the motor control. When leaving the printing zone, the carriage is decelerated in the space between the print limit bands B and the sweep limit bands A. During printing, the carriage is stopped and reversed in the sweep limit bands A, and then accelerated to print speed between the sweep limit band A and the print limit band B. At the print limit band B, stat-of-print is initiated resulting in the production of the print drop firing signals coupled by the bus 15 to the microprocessor. A printhead assembly 20 comprising a printhead 21 and print drive circuit 22 is mounted on the print carriage and moves with the print carriage in the axis. The printhead 21 is of the thermal inkjet type. It may be a single color or a multi-color printhead. A nozzle array is provided for each color of ink in the printhead. Thermal excitation for each nozzle in each nozzle array is used to fire the ink drops. This thermal excitation in the form of voltage pulses is provided by the print drive circuit 22. Such arrangements are well known. The print drive circuits 22 conventionally comprise a printed circuit board to which the printhead is connected, forming the printhead assembly 20. The microprocessor produces print data signals for controlling the firing of the printhead nozzles. The print data signals provide information for pulse formation, for nozzle firing, for printing text and/or graphics and for maintaining uniformity of ink drops by controlling printhead temperature. In accomplishing this, the print data output microprocessor is coupled via a bus 23 to a logic array circuit 24. The logic array circuit comprises a pulse generator and a pulse counter with provisions for pulse width control. The logic array circuit produces pulses coupled to the print drive circuits for selectively, and individually firing the nozzles of the print heads in a sequence to produce the text and/or graphics of the print data 1 as the printer carriage moves through the print zone between the print limit bands B on the scale. Temperature compensation is provided in part by measuring the temperature of the printhead. This may be done by providing a nozzle substrate having temperature sensitivity, or by placing a temperature sensor TS on the nozzle substrate, or by locating a temperature sensor TS such as a thermistor on the carriage printed circuit board or on the printhead. Such temperature sensors are used to provide the input needed to estimate the printhead temperature, which, in turn, can be used to control the printhead temperature, using inexpensive electronics. As indicated in Figure 1 the output of the temperature sensor TS is connected to the microprocessor 2. The print drive circuits are supplied with power by a power supply 26. The output of the temperature sensor TS is also coupled as a control input to the power supply 26 and is used to regulate print pulse energy inversely proportionally to printhead or nozzle temperature. Thus, temperature sensing at the printhead is used directly to control the power supply so that the pulse energy which is applied for firing the ink drops results in uniformity of the ink drops. In the microprocessor, the indication of printhead temperature is employed in a decision making process to determine the temperature condition of the nozzles, i.e., whether the nozzles are cold or whether the nozzles are overheating and is used with processor based information as to the location of the nozzles on the substrate the color of the ink in a particular printhead and the use profile of that printhead, for providing input to the logic array circuit 24 for producing print pulses for firing the nozzles of that particular printhead, to maintain uniformity in the ink drops which are fired. The organizational concept of that aspect of this temperature control system is illustrated in Figure 2. In Figure 2 the microprocessor is shown in dot-dash outline. For the purpose of this description, it comprises a data processing section 2a and a read only memory section 2b. The data processing section uses the print data instructions on bus 1 to provide input by a bus 1a to a pulse generator 24a in the logic array circuit 24 for printing text. Print pulse timing in this respect is determined by the microprocessor using the print drop firing signals on the bus 15 at an input of the data processing section. Thus text is printed by the printhead 21 as the print carriage sweeps in its axis between the print limit bands B on the scale. The output of the pulse generator 24 is coupled to the printhead drive circuits 22 through a print pulse counter 24b forming part of the logic array circuit 24. The output of the print pulse counter is coupled back to the data processing section 2a of the microprocessor where it is used to compute the print drop pulse rate of the printhead. This print drop pulse rate is used by the data processor in accessing use profiles in its read only memory section, for providing pulse generating input to the pulse generator so that, for example, in a multi-printhead printer another printhead may be selected for printing. In the alternative, for example, in a single printhead arrangement, excessive temperature alone or rising temperature with a high use profile may be processed by the data processing section of the microprocessor to produce a control to reduce data throughput to prevent the rise in temperature. This concept is tied in with the dwell time between the lines of print data. It is feasible because the printhead temperature time constant is long in comparison with the carriage sweep time in the axis. Thus the microprocessor produces motor control of a character to provide a predetermined dwell time of the carriage in either of the sweep limit bands A on the scale. These dwell intervals may take place at the end of each carriage sweep or at the end of selected carriage sweeps to control the printhead temperature as required. Where multiple nozzle arrays are provided on a single substrate, the location of the nozzle array on the substrate has a bearing on its temperature. Similarly ink color is a factor in temperature control because some colors are more sensitive to low temperatures than others. When the printhead is not in use, it resides in a park or rest position in a limit of carriage movement in which the carriage is removed entirely of the carriage print sweep range. This position is determined by a park band C on the scale, as seen in Figure 1. When not in use, head temperatures may be below those which are acceptable for printing. The printhead assembly 20 is shown in park position in dot-dash outline in Figure 1. Adjacent the printhead, in a position toward the adjacent sweep limit band A on the scale, is a spittoon 27, also shown in dot-dash outline. In this circumstance, when a print demand is made, the data processor section of the microprocessor may determine that a viscous plug exists in the printhead nozzle. Thus, when the command is issued for the carriage to move out of park position to perform a printing operation, the microprocessor provides an instruction to the pulse generator 24a to produce print drop firing pulses timed to expel print drops into the spittoon as the carriage moves out of the park position for a printing operation. This operation clears any plugs which may exist in the nozzles and additionally provides a degree of warm up depending upon the number of print pulses that have been applied in firing ink drops into the spittoon. In other circumstances, if the printhead exists in a low temperature situation unacceptable for printing and the use profile is such that no viscous ink plugs exist in the nozzle, warm up pulses for the printhead may be selected. Warm up pulse instructions from the microprocessor, initiated by the data processing section accessing the warm up pulse data of the read only memory section, provides instructions to the pulse width control section of the pulse generator 24a to produce warm up pulses. These are time limited voltage bursts which heat but are too short to expel ink from the printhead. The flow chart of Figure 3 characterizes these functions of the temperature control system. If there is overheating the decision is to stand idle as in dwell time in the sweep limit bands A of the carriage, or in a multi-nozzle single color head assembly, to shift nozzles. In the event of a viscous plug, warming pulses and/or spitting of the nozzles may be employed. In the event the nozzles are cold, nozzle pulsing for warming and/or spitting may be employed. These decisions and actions always precede a following printing operation. This printhead temperature control for maintaining uniformity and quality of print or graphics is applicable in all thermal inkjet systems.

Claims: 1. A temperature control system for a thermal inkjet printer, having a printer carriage drive (9,10), a printer carriage (11) movable by said printer carriage drive (9,10) across a printing zone between sweep limit positions (A,B,C) and movable to and from a rest position in response to print commands, and having a thermal inkjet printhead (21) mounted on said carriage (11), comprising: means (20) including print drive circuits (22) coupled to said thermal inkjet printhead (21) for producing electrical pulses for firing inkdrops from said thermal inkjet printhead (21) in said printing zone and for stopping said electrical pulses outside of said printing zone; temperature sensor means (TS) for sensing the temperature of said thermal inkjet printhead (21); and means (2) responsive to said temperature sensor means (TS) for controlling said means including print drive circuits (22) to reduce the data throughput by idling the printer carriage (11) outside of the printing zone above a predetermined sensed temperature and thereby to maintain the temperature of said thermal inkjet printhead (21) substantially at a predetermined temperature. 2. A system according to claim 1, wherein the means responsive to said temperature sensor means (TS) comprises: means for controlling said printer carriage drive (9,10) to stop said printer carriage (11) in a sweep limit position and to dwell therein to permit said thermal inkjet printhead (21) to cool when the temperature thereof is above said predetermined temperature. 3. A system according to claim 1, wherein the means responsive to said temperature sensor means (TS) comprises: means responsive to a print command as said printer carriage (11) is moved from said rest position by said printer carriage drive (9,10) for causing said means including print drive circuits (22) to apply electric pulses to said thermal inkjet printhead (21) for firing ink drops when the temperature of said thermal inkjet printhead (21) is below said predetermined temperature. 4. A system according to claim 1, wherein the means (20) including print drive circuits (22) coupled to said thermal inkjet printhead (21) comprises: a power supply (26) coupled to said print drive circuits (22) and controlled by said means (2) responsive to said temperature sensor means (TS) so that power supplied to said print drive circuits (22) is reduced for temperatures of said thermal inkjet printhead (21) above said predetermined temperature and increased for temperatures of said thermal inkjet printhead (21) below said predetermined temperature. 5. A system according to claim 1, comprising: means (24) for counting said electrical pulses; means (2) for computing a pulse rate from said electrical pulses; means for determining a quantity representing the intensity of use, i.e., the use profile of said thermal inkjet printhead (21) from said pulse rate; and means responsive to said quantity for additionally controlling said means (20) including print drive circuits (22). 6. A system according to claim 1, wherein: said means (2) responsive to said temperature sensor means (TS) controls said means (20) including print drive circuits (22) to produce electrical pulses of limited duration to warm said thermal inkjet printhead (21) but not to fire inkdrops.

Inventors: MOSTAFA, HATEM E., SMITH, JAMES C., WALSH, WILLIAM J.
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1988
Main classification: B41J2
IPC codes: B41J2