An ink jet recording method and recording apparatus

Abstract: An ink jet recording method comprises a pixel-recording step for forming each pixel by fewer droplets than the number of droplets to be required for obtaining a gradation of an image. Therefore all of the pixels required for the image is recorded without removing any pixel from them .

Description: The present invention relates to an ink jet recording apparatus, and an ink jet recording method for recording information such as characters, images, and pictures on a recording medium. Also, the present invention relates to an information-processing system such as a copying machine, a facsimile machine, a printer, a word processor, and a personal computer, using such apparatus as its output device. Heretofore, various kinds of recording apparatus for performing an image formation on a recording medium, for example a sheet of recording paper, a piece of fabric, and a sheet of plastic material such as one used for overhead projector (generally called as a OHP sheet in Japan) have been proposed as in the form of mounting a recording head in the type of wire dot matrix recording, thermal recording, thermal transfer recording, ink jet recording, or the like. Among the conventional recording methods, the ink jet method has been known as one of the non-impact methods and mainly grouped into two different types from the points of how to prepare ink droplets and how to generate energies for ejecting them. That is, one is a continuous type further grouped into a charge particle control type and a spray type; and another is on-demand type further grouped into a piezo type, a spark type, and a bubble jet type. In the case of the continuous type, a plurality of ink droplets are ejected continuously but only a required part of them is charged so as to be adhered on the recording medium while the others are of no use. In the case of the on-demand type, on the other hand, ink droplets are ejected when they are required for the printing. Therefore, the ink can be used without causing any useless droplets and thus the internal parts of the ink jet apparatus does not stained, According to the on-demand type, furthermore, a response frequency of the recording means is lower than that of the continuous one because the on-demand type is able to start and stop the ink ejection. Therefore, the on-demand type realized a high speed printing by increasing the number of the orifices to be formed on the recording head. Accordingly, many ink jet recording apparatuses being commercially available are provided as the on-demand type and thus they have been used as output devices of data-processing systems because of their high-density and high-speed recording behaviors. For example, the ink jet recording apparatuses have been provided as printers as output terminals of copying machines, facsimile machines, printing machines, word processors, work stations, or the like, or provided as handy- or portable-printers, personal computers, host computers, optical disc- or video-equipment, or the like. In general, furthermore, each ink jet recording apparatus can be constructed so as to meet the demand (for mechanical apply, business or private use, or the like) of each system described above, respectively. The ink jet recording apparatus generally comprises: a carriage for carrying a recording means (i.e., a recording head) and an ink tank; a transfer means for transferring a recording medium (e.g., a sheet of recording paper); and a control means for controlling the drive of these means. During the recording period, the ink jet recording head moves along the direction (main-scanning direction) perpendicular to the direction (sub-scanning direction) of transmitting the recording medium for ejecting ink droplets from a plurality of ejection orifices. During the non-recording period, on the other hand, the recording medium is shifted at a distance corresponding to the recording width of the recording medium. Therefore, the ink jet recording method is responsible for ejecting ink droplets on the recording medium in accordance with the recording signals and has several advantages over the others in that it provides more excellent image qualities and performs the high-speed printing with a low-running cost. By using the recording head comprising a plurality of ink-ejecting nozzles being arranged in the sub-scanning direction, an image having a width corresponding to the number of the nozzles can be recorded by one scanning movement of the recording head in accordance with the ink jet method, resulting that the high-speed printing is attained. In the case of the ink jet recording apparatus for performing a color printing, furthermore, a colored image can be obtained by using droplets to be ejected from different recording heads corresponding different colors, respectively, and placing one droplet upon another. In general, a recording apparatus capable of forming a full color image comprises with recording heads and ink tanks which are corresponding to the number of color inks to be required. For example, the number of recording heads is three or four when three primary colors, i.e., yellow (Y), magenta (M), and red(R), or with black (B) are used, respectively. In recent years, the above kind of recording apparatus capable of forming a full color image with a recording head unit adapted to eject three or four kinds of ink has been put in practical use. Besides, the ink jet recording apparatus easily records an image on a larger sized recording medium such as a sheet of A1 sized paper or the like. That is, a recording system for recording A1 sized color images, connecting with a device for reading an image to reproduce an original, such as a plotter (i.e., a printer used as an output device of the CAD system or the like), has been commercially available. Furthermore, there is a growing demand of applying the ink jet recording method in various fields. In the case of a presentation, for example, the ink jet recording method is applied in the process of recording an image on an OHP film to be projected for the presentation on a conference, a lecture, or the like. To meet the above demands, the ink jet recording apparatus has been developed and commercially manufactured to perform an excellent image formation without depending on a kind of the recording medium in spite that it is selected from the media having different properties of absorbing ink. Accordingly, there is a sharp rise in demand of the ink jet recording method not only in the field of data-processing but also extensively in the field of using ink-supporting medium (e.g., a sheet of paper, an OHP sheet, or a sheet of cloth) to be given ink or the like, such as textile and clothing industries, as an outstanding recording means. In addition, an image with an excellent qualities have been desired in every fields. We are now explaining how to form a gradation (hereinafter, also referred as a gray level or a gray scale) of image by the ink jet recording apparatus described above. For the gradation, several methods have been proposed, for example an area factor method in which the entire gray scale of an image is represented by one picture element of the plurality of picture elements consisting of only one of two levels of the scale in accordance with a predetermined threshold; a liquid droplet modification method (JP-A-59-207265 and JP-A-57-160654) in which the size of each droplet to be attached on the recording medium is changed by modifying the volume of liquid droplet; and a multi-droplet method in which a plurality of liquid droplets are successively ejected onto a same point to form one dot with a gradation thereof determined by the number of the droplets. Among these methods, however, the area factor method brings with it a decline in the image-resolution and also the liquid droplet modulation method cannot take a range of the gradation within the practical use, for example it is difficult to modify the volume of liquid droplet. In the case of the multi-droplet method, on the other hand, it is possible to perform an image formation with a high-resolution and a high gradation by selectively using an ink jet recording head capable of ejecting smaller sized droplets compared with that of the other methods. Next we are going to explain the image formation in accordance with the multi-droplet method, briefly. As already described above, each pixel of the image is formed by placing a plurality of droplets one on top of the other or on top of one another. In this case, a pixel formed by only one droplet is also used for the image formation. After placing the liquid droplet on the recording medium, the liquid droplet requires over several hundred seconds of the time to sink into the medium. As a result, a semicircular pattern of the permeation can be observed as a cross sectional view after placing the liquid droplet on the recording medium and thus a larger sized semicircular pattern is obtained by placing a subsequent droplet upon a prior droplet. Consequently, each size of the pixel can be individually determined by selecting the number of the droplets to be placed on the recording medium. It is expected that the recording speed is decreased when the number of the droplets to be piled up is increased for obtaining a more extended extend range of the image's gradation. However, this problem can be solved by combining the several methods, such as the liquid droplet modification method described above and another method described in Japanese Patent Application Laying-open No. 63-502261 in which a plurality of droplets respectively having different volumes is ejected from nozzles of the ink jet head and piled on the recording medium within an extremely short time. The recording process in accordance with the multi-droplet method will be explained in the following description. Figure 1 is a perspective illustration for explaining the construction of a recording head and its peripheral structure in the conventional ink jet recording apparatus. The recording head 1 is mounted on a carriage 4 slidably engaged on guide shafts 5A and 5B. These shafts 5A and 5B serve to guide a shuttle movement of the carriage 4 in the direction of an arrow A a main-scanning direction) in Fig. 1 with the aid of a driving means (not shown). Consequently, the recording head 1 is able to record an input image data on a recording medium 2 by its reciprocating motion in company with the carriage 4. The recording head 1 has a plurality of orifices for ejecting ink droplets. In this example, thirty-two orifices are formed and arranged in the direction of transporting the recording medium 2, defined as a sub-scanning direction B perpendicular to the above direction A. In the arrangement, furthermore, the orifices are in close formation with the density of 400 dpi (dot per inch) and the distance of 63.5 µm between them. Each orifice communicates with a corresponding ink path where a heater element is installed for generating a thermal energy to be used for ejecting an ink droplet. That is, the heater element causes heat by receiving an electric pulse in accordance with the driving data. Then the heat causes a film-boiling phenomena in the ink to generate a bubble responsible for pushing a part of ink out from the orifice. In general, a driving frequency of the heater element (i.e., an ejection frequency) is 10 kHz. Furthermore, a size of each ink droplet can be determined so as to express a desired concentration with an optical density (OD) of 1.4 when three ink droplets are placed on the recording medium to form a pixel covering an area of 63.5 micrometers square. In this case, the image can be formed by four different types of pixel. That is, the number of droplets in one pixel takes a value selected from 0, 1, 2, and 3, corresponding to four levels of the gradation, respectively. Consequently, a substantial frequency for forming a pixel takes a value of 10/3 kHz as a result of using the recording head with the ejection frequency of 10 kHz for ejecting three ink droplets at the maximum to form one pixel. As described above, the carriage 4 having the ink jet head 1 mounted thereon is guided by the guide shafts 5A and 5B to shift its position. The carriage's movement is performed with the aid of a timing belt having a part fixed with the carriage 4. The timing belt is being pulled by a pair of pulleys to be driven by a rotation of a motor (not shown). An ink tube 6 is connected with the recording head 1 to supply ink from an ink tank (not shown), while a flexible cable 7 is connected with the recording head 1 having a head-driving circuit (a head driver) that receives a driving signal and a control signal according to recording data from a host system or an internal control unit through the cable. It is noted that these constituents 6 and 7 are made of flexible materials so as to follow in the movement of the carriage 4. A platen roller 3 is in the type of a rolling cylinder having a longitudinal axis parallel to the guide shafts 5A and 5B. The platen roller 3 is driven by a sheet-feeder motor (not shown) to feed a sheet of recording paper 2 provided as the recording medium and is responsible for positioning and holding a recording surface thereof. Accompanying with the movement of the carriage, subsequently, the recording head 1 ejects ink droplets onto the recording surface facing with the orifices. Figure 2 is a block diagram for explaining a control mechanism of the ink jet recording apparatus shown in Figure 1. A main controller 112 comprises a central processing unit (CPU) and the like to store image data (8 bit and 256 level gradation for each pixel) from a host computer 111 into a frame memory 113. With a predetermined timing, the main controller 112 transfers the image data corresponding to each pixel from the frame memory 113 to a drive data RAM 155 through a halftoning unit 114, resulting that the frame memory 113 stores the image data. In the halftoning unit 114, the image data corresponding to each pixel is converted into four-level (quaternary) data (2 bit for each pixel, 0-3 levels of the gray scale). The halftoning unit realizes an algorithm known as a area factor method or an error-diffusion method by means of an electric circuit. According to a control signal from the main controller 112, the driver controller 116 reads drive data stored in the drive data RAM 115 with respect to a series of numbers used for identifying orifices of the recording head 1. Then the read data is supplied into a head driver 117 to control its drive timing. According to the above construction, the main controller 112 controls the events of: an ink ejection by the recording head 1 through the driver controller 116; a revolution of the carriage motor 121 through the carriage motor driver 119; and a revolution of the sheet feeding motor 122 through 122 the sheet feeding driver 120, to successively record characters, pictures, drawings, or the like on the recording paper 2 in according with the image data. The above description refers to a multi-level ink jet recording apparatus in which each pixel is formed by 0, 1, 2 or 3 ink droplets dependent upon the gradation level represented by the image data to be recorded. Ink jet recording apparatus are also known wherein each image pixel to be printed is formed by a single ink droplet. In such known bi-level ink jet recording apparatus, it has been proposed to provide a high speed printing mode, known as a draft mode (or an ink saving mode), for use when the user wishes to verify a printed image or for other purposes, that is the draft mode may be selected when the user requires the output quickly because the draft mode performs higher speed printing than the normal mode, or when the purpose for which the user requires the printed output is not important, or when the user wants to save ink. Figure 3 is a schematic illustration for explaining image recording in the draft mode of a conventional bi-level ink jet recording apparatus as disclosed e.g. in US-A-4 216 480. In the figure, reference numerals from #1 to #32 indicate orifices of the recording head 1, respectively, while reference numeral 500 indicates an area of the recording medium. The recording head 1 moves in the direction of an arrow A'. As shown in the figure, the draft mode is responsible for thinning down the overwhelming number of pixels in the image to carry out the high speed printing. In the figure, each pixel is formed by, at most, one droplet, that is each pixel is printed in accordance with binarised or bi-level image data and so is formed either by ejecting an ink droplet or not. The following description is for explaining why the printing speed is improved by thinning down the overwhelming number of pixels in the binarised image. In Figure 3, the image 500 consists of a plurality of rows (L1 - L32) each made up of a series of pixels placed next to each other so as to form a checkered pattern. In the figure, an open square represents a pixel where an ink droplet cannot be placed; and a crosshatch square represents a pixel where an ink droplet can be placed when the image data is provided . During the recording procedure, the recording head ejects an ink droplet onto a checkered pattern's point (i.e., a pixel indicated by the crosshatch square in the figure) on the recording medium when the drive data is provided. However, the recording head does not eject any ink droplet onto adjacent checkered pattern's point (i.e., a pixel indicated by the open square in the figure) on the recording medium in spite of being received the drive data. In the case of an orifice #1 of the recording head 1, for example, an ink droplet can be ejected onto a pixel #501 when the drive data is provided while it cannot be ejected when the drive data is not provided. On the other hand, a pixel #502 adjacent to the pixel #502 cannot receive any ink droplet regardless of receiving the drive data or not. Just as in the case of the pixels #501 and #502, furthermore, an ink droplet can be ejected onto a pixel #503 when the drive data is provided while it cannot be ejected when the drive data is not provided. On the other hand, a pixel #504 adjacent to the pixel #503 cannot receive any ink droplet regardless of receiving the drive data or not. Consequently, the orifice #501 ejects ink droplets on alternate pixels in the row L1 to form a series of pixels as a pattern of checker, regardless of receiving the drive data or not. Also, the other orifices performs the same recording as that of the orifice #501, so that they eject ink droplets on alternate pixels in their row to each form a series of pixels as a pattern of checker, regardless of receiving their drive data or not. By the way, the recording head has a heater's drive frequency (i.e., an ejection frequency) of 10 kHz. Therefore, each orifice may be 10 kHz in ejection frequency for two pixels when the draft mode corresponding to a binary image is performed. That is, the recording head 1 performs a recording movement two times as faster as that of the standard, so that a speed of printing is improved. However, if the pixels are thinning out to form a checker pattern as shown in Figure 3, pixels to be recorded is a half of the all, so that it is difficult to make out characters or the like in the obtained image. As a matter of fact, the obtained image can be only used for roughly observing a layout of the paper and thus it comes to nothing or wastes the time of printing. In the case of the multi-droplet method, in addition, a substantial increase of the printing speed cannot be attained because the recording head ejects a plurality of ink droplets on each pixel in spite of thinning out pixels to form a checkered pattern as shown in Figure 3. Therefore, it is difficult to raise the ejection frequency of each nozzle, so that the printing speed cannot be increased. EP-A-0124190 describes a thermal ink jet printer wherein multi-level images are formed by causing ink droplets to coalesce in flight. An object of the present invention is to provide an ink jet recording method enabling to perform a high-speed printing and to provide a high-quality print, an ink jet recording apparatus using such method, and an image-processing system using such apparatus as an output device thereof. In a first aspect of the present invention, there is provided an ink jet recording method as set out in claim 1. In a second aspect of the present invention, there is provided an ink jet recording apparatus as set out in claim 10. An embodiment of the present invention provides an ink jet recording method using a multi-droplet procedure to perform a high-quality draft mode recording with a predetermined gradation, an ink jet recording apparatus using such method, and an image-processing system using such apparatus as an output device thereof. A pixel may be formed by one fewer ink droplets than the number of ink droplets to be required for obtaining the gradation in the second mode. All of the plurality of pixels in the image may be recorded on the recording medium in the second mode. A predetermined part of the pixels in the image may be recorded on the recording medium in the second mode. The second mode may select one of: a first second mode for forming a pixel using one fewer ink droplets than the number of the ink droplets to be required for obtaining the gradation; and a second second mode for forming a pixel using two fewer ink droplets than the number of the ink droplets to be required for obtaining the gradation. The second mode may select from: a first second mode for forming a pixel using one fewer ink droplets than the number of the ink droplets to be required for obtaining the gradation; a second second mode for forming a pixel using two fewer ink droplets than the number of the ink droplets to be required for obtaining the gradation; and a third second mode for recording a predetermined part of the pixels in the image during the course of performing the first second mode or second second mode in the pixel-recording step. A speed of shifting of the recording medium relative to the recording head may be determined in accordance with the number of the fewer ink droplets. The recording means may be an ink jet recording head that employs an electrothermal transducer for ejecting ink droplets by generating a film boiling in ink by thermal energy, as means for generating energy for ejecting ink droplets. The recording means may eject ink droplets of different inks corresponding to different colors, respectively. The allocating step may allocate ink droplets for each pixel in the draft mode, in which the number of the ink droplets is fewer than the maximum number N, and the image-forming step forms the multilevel image by each pixel constituted by the ink droplets numbered by the allocating step. The allocating step may allocate ink droplets for each pixel according to the second mode in which the number of the ink droplets is one fewer than the maximum number N. The allocating step may allocate ink droplets for each pixel according to the second mode, in which the number of the ink droplets is two fewer than the maximum number N where N is greater than three. Where N is greater than three the allocating step may allocate ink droplets for each pixel according to one second mode selected from: a first second mode for allocating ink droplets, in which the number of the ink droplets is one fewer than the maximum number N; and a second second mode for allocating ink droplets, in which the number of the ink droplets is two fewer than the maximum number N. The image-forming step may perform a scanning movement of the recording head in the second mode at a higher speed compared with a scanning movement of the recording head in the first mode. The image-forming step may perform a scanning movement of the recording head at a speed corresponding to the maximum number of ink droplets to be allocated for each pixel. An ink jet recording apparatus embodying the invention may further comprise: means for allocating the number of ink droplets for allocating the number of ink droplets for each pixel in accordance with the inputted recording data including the gray level data, in which the maximum number of ink droplets to be allocated is M (<N), wherein the image forming means is arranged to form the multilevel image by forming each pixel by ejecting ink droplets from the recording head, in which the number of the ink droplets is being allocated by the allocating means. M may be equal to N-1. M may be equal to N-2 where N is greater than three. M may be equal to 1. In an embodiment, the ink jet recording apparatus may further comprise: a mask means for masking the ink droplets allocated for each pixel to be formed by the image forming means. In an embodiment, the recording apparatus may comprise scanning means for scanning the recording head relative to the recording medium during a recording operation. A scanning rate of the scanning means may be consistent with a predetermined speed in accordance with the maximum number of ink droplets which is allocated for each pixel to be formed by the image forming means. The recording head may be removably mounted. An ink jet recording apparatus may further comprise a carriage on which the recording head is mounted. An ink jet recording apparatus may further comprise a transfer means for moving the recording medium relative to the recording head. The recording apparatus may be applied to a copying machine. The recording apparatus may be applied to a facsimile machine. The recording apparatus may be applied to a terminal of a computer. The above and other aspects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. The present invention is illustrated in the following drawings in which: Fig. 1 is a perspective view which schematically shows the structure of a prior art ink jet recording apparatus; Fig. 2 is a block diagram which shows a control mechanism for the ink jet recording apparatus shown in Fig. 1; Fig. 3 is a schematic illustration for explaining an image recording in accordance with a prior art ink jet recording method; Fig 4. is a block diagram which shows a preferable embodiment of a control mechanism to be applied in an ink jet recording apparatus in accordance with the present invention; Fig 5. is a block diagram which shows another embodiment of a control mechanism to be applied in an ink jet recording apparatus in accordance with the present invention; Fig 6. is a block diagram which shows an embodiment of a sequence of limiting a printing mode of an ink jet recording method in accordance with the present invention; Fig 7. is a block diagram which shows an embodiment of image data to be used in an ink jet recording method in accordance with the present invention (normal mode); Fig 8. is a block diagram which shows an embodiment of image data to be used in an ink jet recording method in accordance with the present invention (draft modes #1 and #3); and Fig 9. is a block diagram which shows an embodiment of image data to be used in an ink jet recording method in accordance with the present invention (draft mode #2). The present invention will now be described in detail hereinafter with reference to the accompanying drawings, i.e., Fig. 4 to Fig. 9 which illustrate preferred embodiments thereof. In the figures, same reference numbers denote same constituents. Figure 4 is a block diagram for explaining a sequence of determining a print mode in accordance with an ink jet recording method of the present invention. According to the present invention, an image is formed by using the fewer number of droplets, preferably one fewer droplets, compared with the number of droplets to be required for obtaining a predetermined level of a gray scale (hereinafter also referred as a gradation). In this case, however, all of the smallest image-forming units (i.e., pixels) is printed on the recording medium. More specifically, for example, a main controller of an ink jet recording apparatus receives a print command 401 from a host computer 111 connected with an ink jet recording apparatus. The main controller judges whether the print command 401 is of a normal mode or one of draft modes #1, #2, and #3 which are functionally related with one of halftoning units 403, 406, 409, and 412. Then the main controller initializes the conditions of a drive controller and a motor driver respectively, through one of the halftoning units, according with the selected mode. The driver controller and the motor driver are responsible for controlling a recording movement of the recording means. The details of each initial condition mentioned above is as follows. That is, each halftoning unit is initialized so as to perform a bi-level (binary), 3-level (ternary), or 4-level (quaternary) halftoning; the driver controller is initialized so as to print one pixel by one droplet; and the carriage motor driver is initialized for determining a velocity of the carriage (CR) in accordance with contents of the draft mode. By using the draft mode, accordingly, all of the pixels in the image can be printed on the recording medium by means of the ink jet recording apparatus of the present invention. In Figure 4, there are several modes which can be selected as the draft mode. That is, a normal mode (normal), a first mode (draft #1), a second mode (draft #2), and a third mode (draft #3). Examples of the image data corresponding to these modes are represented in Figures 5 to 7, respectively. That is, Figure 5 is a 4-level image data corresponding to the normal mode, Figure 6 is a bi-level image data corresponding to the first or third mode, and Figure 7 is a 3-level image data corresponding to the second mode. As shown in the figures, each mode determines the type of the halftoning and an individual CR's velocity. That is, (i) when the normal mode 402 is selected, the 4-level halftoning (the maximum number of ink droplets to be ejected for each pixel is three) is performed at the CR's speed of 211.7 mm/second (404); (ii) when the first draft mode 405 is selected, the bi-level halftoning (the maximum number of ink droplet to be required for each pixel is one) is performed at the CR's speed of 635 mm/second (407), which is three times as high as that of the normal mode; (iii) when the second mode is selected, the 3-level halftoning (the maximum number of ink droplets to be ejected for each pixel is two) is performed at the CR's speed of 317.5 mm/second (410), which is one-and-a-half times as high as that of the normal mode; and (iv) when the third draft mode is selected, the bi-level halftoning is performed at CR speed of 1270 mm/second, which is six times as high as that of the normal mode. In the case of the third draft mode, furthermore, a mask pattern creation is also performed. Comparing with the conventional method, therefore, the image of characters and the like obtained by the method of the present invention can be more clearly discriminated. The image can be printed more speedily and a quality of the print is improved, compared with that of the conventional one. It is noted that an extremely high speed printing can be attained by more decreasing the number of the droplets to be placed on the recording medium and the user save the total ink volume. The following embodiments 1 and 2 are more detailed descriptions with respect to preferred embodiments of the ink jet recording apparatus that performs the above modes. <Embodiment 1> Figure 8 is a block diagram for explaining a control mechanism of the ink jet recording apparatus that performs a multi-droplet process in accordance with the present invention. A main controller 112 comprises a central processing unit (CPU) and the like to house image data (8 bit and 256-levels of gray scale for one pixel) from a host computer 111 into a frame memory 113. With a predetermined timing, the main controller 112 transfers the image data corresponding to each pixel from the frame memory 113 to a drive data RAM 155 through a halftoning unit 114, resulting that the image data is stored in the frame memory 113. In the halftoning unit 114, the image data (256 levels) corresponding to each pixel is converted into the data selected from 4-level data (0-3 levels for each pixel), 3-level data (0-2 levels for each pixel), and bi-level data (0-1 levels for each pixel). That is, one mode is selected from the group of the normal mode, the first draft mode #1, and the second draft mode #2, which are shown in Figure 4. In this embodiment, the maximum number of ink droplets to be ejected for each pixel is three. The halftoning unit is provided as a unit for realizing an algorithm known as a area factor method or an error-difusion method by means of an electric circuit. According to a control signal from the main controller 112, the driver controller 116 reads drive data stored in the drive data RAM 115 with respect to a series of numerals used for identifying orifices of the recording head 1. Then the read data is supplied into a head driver 117 to control its drive timing. According to the above construction, the main controller 112 controls the events of: (i) an ink ejection by the recording head 1 through the driver controller 116; (ii) a revolution of the carriage motor 121 through the carriage motor driver 119; and (iii) a revolution of the sheet feeding motor 122 through 122 the sheet feeding driver 120, to successively print characters, pictures, drawings, or the like on the recording medium (e.g., a sheet of recording paper 2 as shown in Figure 1) in according with the image data. In the case that for example an instruction of printing the image with the high-speed printing mode (i.e., the draft mode) is provided from the host computer 111 to the main controller 112, the following events are occurred. As shown in Figure 4, the high-speed printing mode is the first or second draft mode #1 or #2. Within the two, the first mode #1 performs the highest speed printing, so that the data corresponding to the first draft mode #1 is used in the present embodiment. Depending on the first draft mode #1, the main controller 112 sends instructions to the components to specify their initial conditions. That is, (i) an instruction of performing a bi-level halftoning (0-1 level gradation for each pixel) is provided to the halftoning unit 114; (ii) an instruction of performing a printing movement (one droplet for each pixel) is provided to the driver controller 116; and (iii) an instruction of moving the carriage at a three-times higher printing speed compared with that of the normal one to the carriage motor driver 119 (i.e., a printing rate of about 635 mm/second which is three times higher than the normal speed of about 211.7 mm/second because of the normal condition by which three droplets are provided in 400 dpi with a frequency of 10 kHz per each droplet). As shown in Figure 8, these instructions for the initial conditions are provided from the main controller 112 through a control line 123. A printing process and a data processing process in the condition of using the above first draft mode #1 will be now described. At first, the host computer 111 sends an instruction of using the first draft mode #1 for the print to initialize the main controller 112. After the initialization, the main controller 112 sends a signal of requiring print data to the host computer 111. The print data may be image data (individual data corresponding to each pixel) and/or code data such as characters. It is possible to use the print data as a command of ending the print. In this embodiment, the print data is provided as image data that consists of individual data corresponding to each pixel. That is, the main computer 111 transfers 8 bit (256-level gradation) data per pixel in order along an image raster direction to the main controller 112. The frame memory 113 stores each pixel data in a raster unit for a while. Then the main controller 112 transfers the data stored in the frame memory 113 to the halftoning unit 114 every each pixel. In this unit 114, the bi-level halftoning is performed in accordance with the parameters in the initial set-up and then 1 bit data per each pixel is transferred into the drive data RAM 115. When the print data corresponding to the number of the nozzles of the recording head (in this embodiment, 32 nozzles) is stored in the drive data RAM 115, the main controller 112 concludes that a printing movement is being ready to start and then it sends print instructions into each devices. According to the print instructions, the carriage motor driver 119 is initialized so as to shift the CR's position at a rate of 635 mm/second. On the other hand, the driver controller 116 is initialized so as to perform the steps of: reading 1 bit data for one pixel from the drive data RAM; transferring 1 bit data for one pixel into the recording head; and providing one heat pulse for one pixel to the orifice. In the present embodiment, that is, the carriage moves at a speed as three times as high as that of the normal one, on the other hand the image is formed by ejecting one droplet for printing each pixel that originally requires three droplets. Consequently, the printed image is of a lower concentration compared with that of the normal printing but no defects because of comprising all pixels required for the entire image. In the case of printing characters, particularly, it is possible to read the result because all of the pixels constituting the entire character's structure is provided . In accordance with the present invention, as described above, the printing process comprises the steps of ejecting the ink droplets corresponding to all of the pixels to be required for the image printing, so that the output image can be sufficiently allowable for the users in spite of being obtainable at a printing rate about three times higher than that of the normal one. In the present embodiment, the halftoning unit 114 sets up the drive data against the bi-level data. However, it is also possible to picture the print data such as fonts against the drive data RAM 115. In the present embodiment, furthermore, the recording head is conditioned so as to eject one droplet for one pixel but not limited to because it can be satisfied when the number of such droplets is three or less. Therefore, it is also possible to use two or three droplets for one pixel. For embodiment, the second draft mode #2 for ejecting two ink droplets at the maximum can be used under the condition of using the 3-level halftoning in the halftoning unit 114 with the CR's velocity of 317.5 mm/second. Accordingly, the printing speed can be determined by the number of droplets to be ejected for each pixel. <Embodiment 2> Figure 9 is a block diagram for explaining a control mechanism of the ink jet recording apparatus that performs the multi-drop process in accordance with the present invention. In Figure 9, the same reference numbers as that of Figure 8 of the first embodiment denote same constituents. In this embodiment, the normal mode and the third draft mode #3 shown in Figure 4 are employed. In Figure 9, reference numeral 211 denotes a mask pattern logic multiplication circuit consisting of a mask pattern unit for masking drive data and a logic circuit unit for outputting a result of logic multiplication for each of the drive data and the mask patterns at the period of transferring data from the drive data RAM 115 to the recording head through the driver controller 116. Usually, the drive data by-passes the mask pattern logical multiplication circuit 211. In the case that an instruction of printing the image with the high-speed printing mode (i.e., draft mode) is provided from the host computer 111 to the main controller 112, the following events are occurred. The high-speed printing mode is the draft mode that performs the highest speed printing. The main controller 112 sends instructions to the devices to specify their initial conditions. That is, (i) an instruction of performing a bi-level halftoning processing (0-1 level for one pixel) is provided to the halftoning unit 114; (ii) an instruction of performing a printing movement (one droplet for one pixel) is provided to the driver controller 116; (iii) an instruction of moving the carriage at a six-times higher printing speed compared with that of the normal one to the carriage motor driver 119 (i.e., a printing rate of about 1,270 mm/second which is six times as high as the normal speed of about 211.7 mm/second because of the normal condition in which three droplets are provided in 400 dpi with a frequency of 10 kHz for each droplet); and (iv) an instruction of masking the drive data to the mask pattern unit and the logic circuit unit 211. In this embodiment, the mask pattern is created as a cross stripes pattern (i.e., a checkered pattern) of pixels as explained in the related art. That is, the initial condition is determined so that each orifice ejects ink droplets alternately against each pixel in a longitudinal row (i.e., 32 pixels) to be printed by the recording head. The initial condition can be determined by the main controller 112 through the control line 123. A printing process and a data processing process in the condition of using the above draft mode will be described. Firstly, the host computer 111 sends an instruction of using the draft mode in the recording process to the main controller 112. After the initialization, the main controller 112 sends a signal of requesting print data to the host computer 111. The print data may be image data (individual data corresponding to each pixel) and/or code data such as characters. It is also possible to use the print data as a command of ending the print. In this embodiment, the print data is provided as image data that consists of individual data corresponding to each pixel. That is, the main computer 111 transfer 8 bit (256-level gray scale) data for each pixel in order along an image raster direction to the main controller 112. The frame memory 113 stores each pixel data in a raster group for a while. Then the main controller 112 transfers the data stored in the frame memory 113 to the halftoning unit 114 every each pixel. In this unit 114, the bi-level halftoning is performed in accordance with the parameters in the initial set-up and then 1 bit data for each pixel is transferred into the drive data RAM 115. When the print data corresponding to the number of the nozzles of the recording head (in this embodiment, 32 nozzles) is stored in the drive data RAM 115, the main controller 112 concludes that a printing movement is being ready to start and then it sends print instructions into each devices. According to the print instructions, the carriage motor driver 119 is initialized to shift its position at a rate of 1,270 mm/second, while the driver controller 116 is initialized so as to perform the steps of: reading 1 bit data for one pixel from the drive data RAM; transferring 1 bit data for one pixel into the recording head; providing one heat pulse for one pixel to the orifice; and masking the drive data to be transferred to the recording head by the mask pattern unit and logic circuit unit 211 during the period of transferring each data. In the present embodiment, that is, the image is formed by ejecting one droplet for printing each pixel that originally requires three droplets. In the present embodiment, consequently, only one droplet is provided for one pixel in spite of essentially requiring three droplets and also each pixel at each cross point does not receive any droplet in spite of existing the drive data thereof or not. That is, an ejection frequency of each orifice in the recording head is 10 kHz for two pixels in which one pixel is formed by one droplet, resulting that the carriage can be moved at a rate of 1,270 mm/second. In this case, the printed image is of a lower concentration compared with that of the standard printing. Therefore, it may be difficult to print and recognize the image in detail. However, it must be satisfied by the users because this kind of the printing is mainly performed for the purpose of confirming an arrangement of printed matter on a page or the overall design thereof. As described above, the present invention attains a high speed printing about six times as high as that of the conventional one and confirms an entire arrangement of the printed image or its overall design. In the present embodiment, the halftoning unit 114 sets up the drive data against the bi-level data. However, it is also possible that the CPU or the like pictures the print data such as fonts to the drive data RAM 115. In the above embodiments 1 and 2, the ink jet recording apparatus comprising a single recording head has been described for a simple and plain illustration of the present invention. Therefore, the color printing can be performed by easily modifying the construction in the above embodiments without departing from the invention. In this case, a plurality of recording heads corresponding to colors can be applied. In the above embodiments, furthermore, the halftoning is performed so as to fit to the selected draft mode. Especially in the case of printing one droplet for one pixel, however, it is also possible to transfer each pixel's data to the drive data RAM by the same way as that of the general procedure. In this case, it is better to check the existence of the pixel's data during the period of transferring each pixel's data to the drive data RAM. When it is being presented, the data is transferred one time for one pixel. The present invention achieves distinct effect 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. 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. 123670/1984 and 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. The present invention can be also applied to a so-called full-line type recording head whose length equals the maximum length across a recording medium. Such a recording head may consists of a plurality of recording heads combined together, or one integrally arranged recording head. 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. As embodiments 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. As embodiments 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 embodiment, 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 above-described embodiments use liquid ink, inks that are liquid when the recording signal is applied can be used: for embodiment, 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 30°C - 70°C 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 the electrothermal transducers as described in Japanese Patent Application Laying-open Nos. 56847/1979 or 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 various 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 scope of the invention claimed.

Claims: An ink jet recording method for recording on a recording medium a multi-level image composed of a plurality of pixels each formed by at least one ink droplet ejected from an ink jet recording head, the method comprising the steps of: allocating the number of ink droplets to be ejected to form each pixel of the image on the recording medium on the basis of gradation recording data such that the maximum number of ink droplets is N, where N is integer greater than one; and forming each pixel of the image on the recording medium by causing the recording head to eject ink droplets during a recording operation of the recording head, characterised by the forming step having a first mode and a second mode in which the recording speed is greater than that of the first mode and by carrying out the forming step in the first mode by causing the recording head to form each pixel by ejecting the corresponding allocated number of ink droplets and by carrying out the forming step in the second mode by causing the recording head to form each pixel by ejecting ink droplets from the recording head such that the maximum number of ink droplets ejected is less than N. A method as claimed in claim 1, wherein said allocating step allocates ink droplets for each pixel in said second mode, in which the number of said ink droplets is fewer than said maximum number N, and said image-forming step forms said multi-level image such that each pixel is constituted by the corresponding number of ink droplets allocated by said allocating step. A method as claimed in claim 1, wherein said allocating step allocates ink droplets for each pixel in said second mode such that the number of said ink droplets is one fewer than said maximum number N. A method as claimed in claim 1, where N is greater than three and wherein said allocating step allocates ink droplets for each pixel in said second mode such that the number of said ink droplets is two fewer than said maximum number N. A method as claimed in claim 1, where N is greater than three and wherein said allocating step allocates ink droplets for each pixel in one second mode selected from: a first second mode for allocating ink droplets, in which the number of said ink droplets is one fewer than said maximum number N; and a second second mode for allocating ink droplets, in which the number of said ink droplets is two fewer than said maximum number N. A method as claimed in any one of the preceding claims, wherein, in a recording operation, said image-forming step performs a scanning movement of said recording head in said second mode at a higher speed than a scanning movement of said recording head in said first mode. A method as claimed in claim 6, wherein said image-forming step performs a scanning movement of said recording head at a speed corresponding to the maximum number of ink droplets to be allocated for each pixel. A method as claimed in any one of claims 1 to 7, wherein said recording head ejects ink by using thermal energy. A method as claimed in any one of claims 1 to 8, wherein said recording head ejects a plurality of inks. An ink jet recording apparatus for recording on a recording medium a multi-level image composed of a plurality of pixels each formed by at least one ink droplet ejected from an ink jet recording head (118), the apparatus comprising: allocating means (112, 115) for allocating the number of ink droplets to be ejected to form each pixel of the image on the recording medium on the basis of gradation recording data such that the maximum number of ink droplets is N, where N is an integer greater than one; and image-forming means (112 to 117) for causing the recording head (118) to carry out a recording operation during which each pixel of the image is recorded on the recording medium by ejection of ink droplets by the recording head, characterised in that the image-forming means (112 to 117) is operable in a first mode and in a second mode in which the recording speed is greater than that of the first mode such that, in the first mode, said image-forming means is arranged to form an image by causing the recording head (118) to form each pixel by ejecting the corresponding allocated number of ink droplets onto the recording medium and, in the second mode, the image-forming means is arranged to form an image by causing the recording head to form each pixel by ejecting ink droplets from the recording head such that the maximum number of ink droplets ejected is less than N. An apparatus as claimed in claim 10, further comprising: further allocating means for allocating a number of ink droplets for each pixel in accordance with said gradation level recording data such that the maximum number of allocated ink droplets is M, where M<N, said image-forming means being arranged to form said multi-level image by forming each pixel by ejecting the corresponding number of ink droplets allocated by said further allocating in the second mode. An apparatus as claimed in claim 11, wherein M is equal to N-1. An apparatus as claimed in claim 11, wherein M is equal to N-2. An apparatus as claimed in claim 11, 12 or 13, wherein M is equal to 1. An apparatus as claimed in any one of claims 10 to 14, further comprising mask means for masking said ink droplets allocated for each pixel to be formed by said image forming means. An apparatus as claimed in any one of claims 10 to 15, further comprising scanning means for scanning the recording head relative to the recording medium during a recording operation. An apparatus as claimed in claim 16, wherein the scanning speed of said scanning means is consistent with a predetermined speed determined in accordance with the maximum number of ink droplets allocated for each pixel to be formed by said image forming means. An apparatus as claimed in any one of claims 10 to 17, further comprising a recording head arranged to eject ink by using thermal energy. An apparatus as claimed in any one of claims 10 to 17, further comprising a recording head for ejecting a plurality of inks. An apparatus as claimed in claim 10, further comprising a recording head arranged to be removably mounted to the mounting means of the apparatus. An apparatus as claimed in claim 20, wherein the mounting means comprises a carriage. An apparatus as claimed in any one of claims 10 to 21, further comprising transfer means for shifting the recording medium relative to said recording head. A copying machine comprising an ink jet recording apparatus as claimed in any one of claims 10 to 22. A facsimile machine comprising an ink jet recording apparatus as claimed in any one of claims 10 to 22. A computer terminal comprising an ink jet recording apparatus as claimed in any one of claims 10 to 22.

Inventors: HYOTANI HIROYUKI, TAKAGI SHINJI, TANAKA HIDEKI, HYOTANI, HIROYUKI, TAKAGI, SHINJI, TANAKA, HIDEKI, HYOTANI, HIROYUKI, C/O CANON K.K., TAKAGI, SHINJI, C/O CANON K.K., TANAKA, HIDEKI, C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: H04N1
IPC codes: H04N1, B41J2, B41J3, B41M5