Printing apparatus

Abstract: A print timing signal is generated which has 1/n the previous cycle of the encoder output. The print processing based on this print timing signal allows the actual print processing to extend into the next encoder cycle when the encoder cycle varies due to carriage speed variations and other causes, and corrects the print start trigger of the next cycle and the print timing cycle on the basis of a time difference (ΔT) by which the actual print processing extends into the next encoder cycle. When performing printing with high resolution in particular, this printing apparatus generates precise print timings, thereby improving the print position accuracy of the print head and performing high quality printing.

Description: The present invention relates to a printing apparatus and more particularly to a print timing control of a print head in the printing apparatus. A variety of techniques for controlling the print timing have been known and one such example is a technology that generates a print timing by using an encoder. The encoder used by such a printing apparatus is generally known to correspond to the print resolution. Another conventionally known example uses an encoder with a lower resolution than the print resolution of the printing apparatus, detects leading and trailing edges of two pulse signals AS and BS 90 degrees out of phase with each other and outputs a quadruple-cycle signal to generate a print timing signal with a quadruple resolution. Still another example uses an encoder with a lower resolution than the print resolution, as in the above example, measures an interval between pulses output from the encoder, and divides the pulse interval one cycle before into n equal parts to generate a print timing signal with a resolution n times the original encoder resolution. In the case where an encoder matching the print resolution is used, however, printing at high resolution requires an encoder that has a correspondingly high resolution, making the apparatus expensive. Further, in the case where an encoder with a lower resolution than the print resolution is used and where the leading and trailing edges of two pulse signals AS, BS 90 degrees out of phase are detected to produce a quadruple-cycle signal and thereby generate a print timing signal with a quadruple resolution, when an optical sensor is used in the encoder, interval errors occur which include errors due to sensor characteristic of a light receiving device or the like, phase shifts between A- and B-phase signals, and errors induced by a circuit that transforms an output from the light receiving device into a rectangular pulse signal. The interval errors may cause the print timing to deviate relatively greatly from a desired position. As to a digital servo apparatus, on the other hand, Japanese Patent Laid-Open No. 7-210249 for example discloses a technique that corrects the quadruple interval errors to correct the print position represented by the quadruple signal. In the case of a printing apparatus, however, what is required is not the detection of an accurate print position corresponding to the signal but the generation of a precise print timing for a predetermined print position, and therefore what is disclosed in the official gazette cannot correct the deviation of the print position. Further, in the case where an encoder with a lower resolution than the print resolution is used and where an interval between adjacent pulse signals is measured and the pulse signal interval one cycle before is divided into n equal parts to generate a print timing signal with a resolution n times the original encoder resolution, a time difference may occur among intervals between two adjacent pulse signals. When such a time difference occurs, a pulse signal interval in the previous cycle may extend and, to that extent, a pulse signal interval in the current cycle shorten, giving rise to a problem that a time required for data transfer and for print processing associated with head driving may become insufficient. In any of the conventional techniques described above, sudden noise or the like can cause variations in the timing and cycle of the encoder and may result in a failure or error of the print processing. US-A-4436439 describes a small printer having a timing mechanism in the form of a tacho-generator built into the print head moving motor. The printer has a printing position control unit in which the timing signal provided by the timing mechanism is shaped and is then applied to a timing signal period 1/N division circuit that generates n pulses for each timing signal producing an n timing signal. The n timing signal is applied to a coincidence circuit and to a timing data storage for timing calculation circuits. A reset signal resets the timing data storage or timing calculations circuits in accordance with a signal from a position detecting mechanism so that a decision signal is applied to the coincidence circuit. The decision signal determines whether the present end timing signal should be provided as a printing position signal to operate the print head. JP-A-9-71008 describes a print head control system including a pulse signal output unit for generating a pulse signal having periods including adjacent high and low levels in a number proportional to movement of the print carriage. A period calculation unit detects each rising and falling edge of the pulse signal and calculates therefrom the duration of the most recent period directly prior to a detected one of the rising and falling edges. A print period calculation unit uniformly divides each received duration by a predetermined number corresponding to the print resolution of the printer to determine a print period for each received duration. The period duration is successively updated at both the rising edge and the falling edge of the pulse signal. Each time the period duration is updated the print period for determining timing of printing dots is newly determined by dividing the updated print duration by the predetermined number appropriate for this print resolution. According to the present invention, there is provided a printing apparatus for printing on a print medium using a print head, the apparatus comprising: a head moving means for moving the print head; a head position information detection means for outputting a cyclic signal according to the amount of movement of the print head driven by the head moving means; a phase interval detection means for detecting the time interval between predetermined reference phases of the cyclic signal output from the head position information detection means; a division means for dividing each time interval detected by the phase interval detection means into n equal parts; a generation means for generating n print timing signals, according to the time interval divided by the division means, by taking as references the predetermined reference phases associated with the time interval measurement by the phase interval detection means; characterised by: a time difference detection means for detecting the time difference between a print end time of a previous cycle and the predetermined reference phase of a current cycle on the basis of the cyclic signal output by the head position information detection means and the print timing signals generated by the generation means; and a correction means for correcting the print timing signals of a current cycle generated by the generation means by an amount corresponding to the detected time difference when the print end time of the previous cycle lags the predetermined reference phase of the current cycle. An embodiment of the present invention provides a printing apparatus which, when performing high resolution printing, can generate a precise print timing to improve the print position accuracy of a print head and thereby perform high quality printing. An embodiment of the present invention provides a printing apparatus capable of performing printing that may prevent errors associated with print positions due to disturbances such as noise from being produced. In one embodiment, the correction means is arranged to delay a start timing of the current cycle print timing signal generated by the print timing generation means by an amount corresponding to the detected time difference and to correct a cycle of the current cycle print timing signal when the print end time of the previous cycle lags the predetermined reference phase of the current cycle. Further when the detected cycle is above or below a predetermined value, this is taken as being abnormal. Thus, the print timing can be generated in a cycle that falls within at least a predetermined range. As a result, it is possible to generate a print timing with virtually high resolution at low cost and perform high quality printing with high resolution. At the same time, the print processing time can be set sufficiently large to enable efficient high-speed printing. It is also possible to perform stable printing without error even when there are variations in the encoder outputs due to disturbances such as noise. The above and other aspects, effects, features and advantages of the present invention will become more apparent from the following description of an embodiment thereof taken in conjunction with the accompanying drawings. Figure 1 is a schematic top view showing an outline structure of an ink jet printing apparatus according to an embodiment of the present invention. Figure 2 is a block diagram showing how a print timing is generated according an embodiment of the invention. Figure 3 is a timing chart showing the generation of a print timing signal according to the embodiment of the invention. An embodiment of the present invention and example will be described by referring to the accompanying drawings. Embodiment An embodiment of the inkjet printing apparatus according to the present invention will be described by referring to Figures 1, 2 and 3. Figure 1 is a top view showing the outline construction of the ink jet printing apparatus. Figure 2 is a circuit block diagram showing the generation of a print timing based on the output of the encoder, and Figure 3 is a timing chart showing the generation of a print timing signal based on a phase output from the encoder. In Figure 1, reference number 1 represents an ink jet unit that comprises a print head and an ink cartridge. Among various types of ink jet system, this system employs the print head that utilizes thermal energy to form a bubble to eject an ink droplet. The print head and the cartridge are both removably mounted on a carriage 5. The carriage 5 slidably engages a guide shaft 6 and can be driven by a drive mechanism not shown along the guide shaft 6 in the direction of an arrow in the figure. Thus, the print head can scan over a print medium 2 such as paper and, during the scan, ejects ink onto the print medium 2 to perform printing. Denoted 3 is a linear scale provided with slits at constant intervals and which extends in the direction of movement of the carriage 5. Mounted on the carriage 5 is an encoder 4 having a pair of light emitting portion and a light receiving portion. As the carriage 5 moves, the encoder 4 outputs a signal according to the position of a slit on the linear scale 3. The linear encoder comprising the linear scale 3 and the optical encoder 4 is known, and the optical encoder 4 has two stationary slits disposed at an angle of 90 degrees to each other and facing the slits of the linear scale 3 to generate encoder outputs of A- and B-phase signals 90 degrees out of phase with each other. Each of the two stationary slits is provided with a light receiving portion, and these two light receiving portions receive light from the light emitting portion that has passed through the slit of the linear scale 3. With this construction, the linear encoder can detect the position of the moving print head 1 and output a signal as a position information of the print head. A print head drive control drives the print head according to the position information of the print head to eject ink at a specified position as the print head scans over the print medium 2. The linear scale 3 is formed with slits that correspond to a relatively low resolving power which is 1/n the print resolution. As the demand for a higher printed image quality increases, there is a growing demand on the linear encoder itself for a higher resolution. Forming the slits and optical encoder with a high resolving power may increase cost as well as noise and error components, as described earlier, which in turn requires a filter circuit for eliminating the noise and error components and thus results in a cost increase. In this embodiment, however, the slits of the linear scale 3 are formed at a relatively low resolving power which is 1/n the print resolution, thus allowing the linear encoder to be constructed inexpensively. Figure 3 shows the A-phase and B-phase outputs of the light receiving portions of the optical encoder 4. The power of received light of the light receiving portions becomes maximum when a window of the slit of the linear scale 3 coincides with a window of the stationary slit of the optical encoder 4 and minimum when these windows are shifted 180 degrees out of phase. The power of the received light changes almost linearly between the maximum and the minimum. As a result, the continuous waveforms of power of the received light as the carriage 5 moves are actually triangular waveforms. The output signals of phase A and phase B as shown in Figure 3 are obtained by converting the triangular waves into pulse signals with a predetermined average level taken as a reference. These pulse signals are 90 degrees out of phase as shown in the figure. The leading and trailing edges of the A- and B-phase signals are detected to generate a timing signal, whose cycle is four times that of the original signal, i.e., four times the original resolving power. As described above, however, this also increases errors that are associated with characteristics of the light receiving sensor of the optical encoder 4, phase shifts of the A-and B-phase signals, and a signal processing circuit that converts an analog signal output from the light receiving portion into a rectangular waveform signal. Therefore, using the quadruple-cycle signal as is to generate a print timing signal cannot produce an accurate print timing. Further, even if an arrangement for correction is made to produce an accurate print position corresponding to the quadruple-cycle signal as disclosed in the above-described official gazette, because the correction provided by this method does not eliminate effects of the above-described errors, the corrected print position represented by the quadruple-cycle signal deviates from the intended print position when the errors occur. Thus, the signal produced by this method also cannot be used as the print timing representing the precise print position. This embodiment thus generates a print timing by a circuit configuration shown in Figure 2. In Figure 2, a cycle counter 201 measures a previous cycle between reference phases of an output signal of the encoder 4 and outputs the measured cycle to a cycle correction circuit 202. An n-multiplication circuit 203 multiplies the corrected cycle output from the cycle correction circuit 202 by 1/n and sends a multiplied result to a print timing generation circuit 204. A print processing circuit 205 drives the print head 1 according to a print timing signal from the print timing generation circuit 204 to perform printing. When the printing for n cycles has finished, the print processing circuit 205 outputs a print end signal to a time difference counter 206. The time difference counter 206 measures a time difference between a reference phase of the encoder 4 for the current printing and the print end signal. When, according to the measured result produced by the time difference counter 206, the print end signal is found lagging the reference phase of the encoder 4 for the current printing, a trigger correction circuit 207 makes correction to delay the start of the print timing signal (trigger) by the time difference. When the print end signal is found leading the reference phase of the encoder 4 for the current printing, the trigger correction circuit 207 does not perform correction. Similarly, when the print end signal is found lagging the current printing reference phase of the encoder 4 according to the measured result produced by the time difference counter 206, the cycle correction circuit 202 makes correction to shorten the cycle by the time difference. When the print end signal occurs before the current printing reference phase of the encoder 4, the cycle correction circuit 202 does not perform correction. The print timing generation circuit 204 generates the print timing signal for n cycles according to the cycle output from the n-multiplication circuit 203 and the trigger output from the trigger correction circuit 207. Although we have shown an example case that uses the trigger correction circuit 207 and the cycle correction circuit 202 to correct the print trigger and the print cycle, it is possible to produce the print trigger at a point in time when the print end signal is generated or at a point in time when the encoder's reference phase rises, whichever is later. If the next reference signal occurs before the printing ends, the cycle counter may be decremented until the print ends. Figure 3 is a waveform diagram showing example signals generated by the circuit blocks shown in Figure 2. This example represents a case where printing is performed at a cycle, equal to 1/8 the encoder cycle, which is produced by an 8-multiplication circuit. The reference phase of the encoder output for generating the print timing is a rising edge of the A-phase signal at any print cycle. Because the encoder output B-phase signal or the trailing edge is not used and the same phase is used as a reference at all times, a precise timing can be generated even when inexpensive linear scale and encoder are used. Then, if the print end signal associated with the print timing signal generated by the previous encoder cycle ((1) in the figure) lags the reference phase for the current printing, a correction is made to delay the start of the current printing trigger signal by the time difference ΔT ((2) in the figure), thus allowing the printing to continue without error. Further, because the length of the print cycle is shortened by the time difference ΔT ((3) in the figure), it is possible to eliminate the problem that the time difference accumulates shifting the print positions successively as would occur when the print timing is simply delayed by the time difference. This assures printing with fewer errors. Further, if the actual print processing associated with the previous print cycle moves into the next print cycle, the time length by which the print processing gets into the next print cycle is taken as the time difference ΔT and the corresponding correction as described above is performed. This correction prevents the time difference from getting accumulated and shifting the print position greatly. When viewed in another way, this allows the print processing to get into the next print processing, making it possible to set the print processing time at a sufficiently large value regardless of cycle variations, which in turn assures stable and high-speed printing. As shown in Figure 3, the similar control is made of the print timing on the backward or return printing pass. On the backward printing pass, the trailing edge of the A-phase signal is used as a reference phase. This allows the print timing to be generated at the same phase with respect to the linear scale position as the forward printing pass. In an ink jet printing system like this embodiment, there is a certain space between the print head and the print medium. Hence, the time it takes for an ink droplet to fly and reach the print medium may change depending on whether the printing is on the forward pass or on the backward pass, which may in turn cause deviations in ink droplet landing positions. Hence, a forward/backward pass timing correction is performed to correct the landing deviations. While the foregoing describes example cases of ink jet printing apparatus, the present invention can also be applied to other printing apparatus, such as those of thermal type and thermal ink transfer type. Further, although the above concerns the use of an optical linear encoder, the present invention can also be applied to other types of detection systems, such as those of rotary type and magnetic type. (Others) The present invention achieves distinct effects when applied to a recording head or a recording apparatus which has means for generating thermal energy such as electrothermal transducers or laser light, and which causes changes in ink by the thermal energy so as to eject ink. This is because such a system can achieve a high density and high resolution recording. A typical structure and operational principle thereof is disclosed in U.S. patent Nos. 4,723,129 and 4,740,796, and it is preferable to use this basic principle to implement such a system. Although this system can be applied to either on-demand type or continuous type ink jet recording systems, it is particularly suitable for the on-demand type apparatus. This is because the on-demand type apparatus has electrothermal transducers, each disposed on a sheet or liquid passage that retains liquid (ink), and operates as follows: first, one or more drive signals are applied to the electrothermal transducers to cause thermal energy corresponding to recording information; second, the thermal energy induces sudden temperature rise that exceeds the nucleate boiling so as to cause the film boiling on heating portions of the recording head; and third, bubbles are grown in the liquid (ink) corresponding to the drive signals. By using the growth and collapse of the bubbles, the ink is expelled from at least one of the ink ejection orifices of the head to form one or more ink drops. The drive signal in the form of a pulse is preferable because the growth and collapse of the bubbles can be achieved instantaneously and suitably by this form of drive signal. As a drive signal in the form of a pulse, those described in U.S. patent Nos. 4,463,359 and 4,345,262 are preferable. In addition, it is preferable that the rate of temperature rise of the heating portions described in U.S. patent No. 4,313,124 be adopted to achieve better recording. U.S. patent Nos. 4,558,333 and 4,459,600 disclose the following structure of a recording head, which is incorporated to the present invention: this structure includes heating portions disposed on bent portions in addition to a combination of the ejection orifices, liquid passages and the electrothermal transducers disclosed in the above patents. Moreover, the present invention can be applied to structures disclosed in Japanese Patent Application Laying-open Nos. 59-123670 (1984) and 59-138461 (1984) in order to achieve similar effects. The former discloses a structure in which a slit common to all the electrothermal transducers is used as ejection orifices of the electrothermal transducers, and the latter discloses a structure in which openings for absorbing pressure waves caused by thermal energy are formed corresponding to the ejection orifices. Thus, irrespective of the type of the recording head, the present invention can achieve recording positively and effectively. In addition, the present invention can be applied to various serial type recording heads: a recording head fixed to the main assembly of a recording apparatus; a conveniently replaceable chip type recording head which, when loaded on the main assembly of a recording apparatus, is electrically connected to the main assembly, and is supplied with ink therefrom; and a cartridge type recording head integrally including an ink reservoir. It is further preferable to add a recovery system, or a preliminary auxiliary system for a recording head as a constituent of the recording apparatus because they serve to make the effect of the present invention more reliable. Examples of the recovery system are a capping means and a cleaning means for the recording head, and a pressure or suction means for the recording head. Examples of the preliminary auxiliary system are a preliminary heating means utilizing electrothermal transducers or a combination of other heater elements and the electrothermal transducers, and a means for carrying out preliminary ejection of ink independently of the ejection for recording. These systems are effective for reliable recording. The number and type of recording heads to be mounted on a recording apparatus can be also changed. For example, only one recording head corresponding to a single color ink, or a plurality of recording heads corresponding to a plurality of inks different in color or concentration can be used. In other words, the present invention can be effectively applied to an apparatus having at least one of the monochromatic, multi-color and full-color modes. Here, the monochromatic mode performs recording by using only one major color such as black. The multi-color mode carries out recording by using different color inks, and the full-color mode performs recording by color mixing. Furthermore, although the use of liquid ink is described above, inks that are liquid when the recording signal is applied can be used: for example, inks can be employed that solidify at a temperature lower than the room temperature and are softened or liquefied in the room temperature. This is because in the ink jet system, the ink is generally temperature adjusted in a range of the temperature 30° - 70° so that the viscosity of the ink is maintained at such a value that the ink can be ejected reliably. In addition, the present invention can be applied to such apparatus where the ink is liquefied just before the ejection by the thermal energy as follows so that the ink is expelled from the orifices in the liquid state, and then begins to solidify on hitting the recording medium, thereby preventing the ink evaporation: the ink is transformed from solid to liquid state by positively utilizing the thermal energy which would otherwise cause the temperature rise; or the ink, which is dry when left in air, is liquefied in response to the thermal energy of the recording signal. In such cases, the ink may be retained in recesses or through holes formed in a porous sheet as liquid or solid substances so that the ink faces to the electrothermal transducers as described in Japanese Patent Application Laying-open Nos. 54-56847 (1979) or 60-71260 (1985). The present invention is most effective when it uses the film boiling phenomenon to expel the ink. Furthermore, the ink jet recording apparatus of the present invention can be employed not only as an image output terminal of an information processing device such as a computer, but also as an output device of a copying machine including a reader, and as an output device of a facsimile apparatus having a transmission and receiving function. The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention claimed in the appended claims.

Claims: A printing apparatus for printing on a print medium using a print head (1), the apparatus comprising: a head moving means (5) for moving the print head; a head position information detection means (4) for outputting a cyclic signal according to the amount of movement of the print head driven by the head moving means; a phase interval detection means (201) for detecting the time interval between predetermined reference phases of the cyclic signal output from the head position information detection means; a division means (203) for dividing each time interval detected by the phase interval detection means into n equal parts; a generation means (204) for generating n print timing signals, according to the time interval divided by the division means, by taking as references the predetermined reference phases associated with the time interval measurement by the phase interval detection means; characterised by : a time difference detection means (206) for detecting the time difference between a print end time of a previous cycle and the predetermined reference phase of a current cycle on the basis of the cyclic signal output by the head position information detection means (4) and the print timing signals generated by the generation mean (204); and a correction means (202,207) for correcting the print timing signals of a current cycle generated by the generation means by an amount corresponding to the detected time difference when the print end time of the previous cycle lags the predetermined reference phase of the current cycle. A printing apparatus as claimed in claim 1, wherein the correction means (202,207) is arranged to delay a start timing of the current cycle print timing signal generated by the generation means by an amount corresponding to the detected time difference and to correct the cycle of the current cycle print timing signal when the print end time of the previous cycle lags the predetermined reference phase of the current cycle. A printing apparatus as claimed in claims 1 or 2, wherein the generation means (204;504;704) is arranged such that n is two or more. A printing apparatus as claimed in any one of the preceding claims, comprising as the print head a print head arranged to print by ejecting ink. A printing apparatus as claimed in claim 4, wherein the print head is arranged to use thermal energy to generate a bubble to cause ink ejection.

Inventors: UCHIKATA YOSHIO, UCHIKATA, YOSHIO, UCHIKATA, YOSHIO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1999
Main classification: B41J19
IPC codes: B41J19, B41J2