JP2000280555A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase print speed by measuring the moving speed of a carriage and generating a head drive timing at a print head drive section based on the output from an encoder sensor and employing an encoder having at least two resolutions. SOLUTION: An encoder for measuring the moving speed of a carriage 1 in an ink jet printer comprises an encoder film 11 arranged with a plurality of slits in the moving direction of an ink jet head, and a sensor 12 for detecting the slits optically. Based on the output from the encoder sensor 12, a CPU measures the moving speed of the carriage 1 and generates a head drive timing at a print head drive section. The encode being used has a high resolution in low speed region, e.g. initial acceleration region, and a low resolution in high speed region. According to the arrangement, fluctuation of scanning can be prevented during acceleration and deceleration.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming an image using ejected ink.

[0002]

2. Description of the Related Art In a general ink-jet printer, a carriage including a print head having a plurality of nozzles is opposed to a paper during printing, and scanning is performed from one end of the paper to the other end. The ink is ejected (hereinafter referred to as main scanning). An image is formed on paper by repeating this main scanning and paper feeding (hereinafter referred to as sub-scanning). Here, in order to control the timing of ink ejection, generally, the carriage is accelerated to a predetermined scanning speed until the carriage reaches a printing start position on the paper, and the ink is ejected by scanning at a constant speed in the printing range. The printing is performed, and after passing the printing end position, deceleration is stopped and the next scanning is started.

[0003]

It is desired to increase the printing speed in an ink jet printer. In order to improve the printing speed in an ink jet printer, high-speed scanning, printing in an acceleration / deceleration region, and bidirectional printing (printing in both directions of main scanning reciprocation) are desirable. The timing at which ink is ejected during printing may be obtained by measuring a timer, or may be obtained from a detection signal from an encoder sensor provided with an encoder. In the case of using an encoder, generally, the resolution of the encoder is closely related to the corresponding scanning speed, and the higher the resolution, the lower the corresponding scanning speed. Generally, in order to increase the scanning speed, an encoder having a low resolution is used, and an encoder output signal is decoded to obtain a head drive timing. Therefore, this method can be used in a region where the speed is stable, but has low reliability in a region where the speed changes, and the ink landing position varies. When measuring the carriage moving speed using an encoder, when scanning at a high speed, a low resolution encoder must be used, and the scanning fluctuation is large, especially in the acceleration / deceleration region where the speed changes, especially immediately after acceleration after switching the scanning direction. In a region, it is difficult to obtain the ink ejection timing using a low-resolution encoder, and the ink ejection timing cannot be practically used. The same applies to the case where the ink ejection timing is obtained by using timer counting. JP-A-7-25103 describes printing in an acceleration / deceleration area. In this printing method, the moving speed and acceleration of the carriage are calculated by the output of the encoder sensor and the timer, and the head drive timing is determined according to the calculation result, thereby enabling printing even when the carriage is accelerated or decelerated. However, this printing method also assumes that an encoder having a lower resolution than the image pitch is used in the entire scanning area. Also, when using an encoder with a resolution lower than the image density, the decoding process for multiplying the output signal from the encoder sensor
High-resolution ink ejection timing can be obtained. here,
A method is also conceivable in which the timer sets the time width of the waveform of the ink ejection timing signal output during decoding to a value calculated from the speed at the time of acceleration / deceleration, and sequentially changes the timer value according to the speed at that time. Can be However, when there is a large amount of scanning blur of the carriage or the like, there is a large error between the actual ink ejection timing and the ink ejection timing obtained from the timer, and the dot positions of the image may vary, which may significantly reduce image quality. Was.

[0004] It is an object of the present invention to provide an ink jet type image forming apparatus having a higher printing speed.

[0005]

An image forming apparatus according to the present invention holds an ink jet head, and has a carriage reciprocating in parallel with a recording surface of a recording sheet and a plurality of slits arranged in the moving direction of the ink jet head. An encoder film, a sensor for detecting a slit of the encoder film, a carriage moving speed measuring unit for measuring a moving speed of the carriage from an output of the encoder sensor, a print head driving unit for discharging ink from the print head, and an encoder. A trigger signal generation section for generating a head drive timing of the print head drive section from an output of the sensor is provided, and the encoder has at least two or more resolutions. For example, printing timing can be set using encoders having different resolutions in the constant speed region and the acceleration / deceleration region of the carriage movement. For example, the encoder includes a portion having a resolution equal to or higher than the resolution of an image to be printed. Therefore, in a low-speed area such as the initial stage of acceleration, an output signal of a high-resolution encoder (that is, a resolution equal to or higher than the image resolution) is decoded to obtain an ink ejection timing. The output signal of the encoder having a resolution lower than the image resolution) is decoded to obtain the ink ejection timing. This makes it possible to suppress variations in the ink landing position even in an acceleration / deceleration region where the carriage scanning blur is large. Further, for example, the image forming apparatus includes a plurality of encoders having different resolutions.
Also, for example, the encoder includes an encoder film having a plurality of pitches.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In the drawings, the same reference symbols indicate the same or equivalent ones. FIG. 1 is a perspective view of an inkjet printer incorporating an inkjet head according to an embodiment of the present invention. 2 and 3 are views of an example of the periphery of the carriage unit of the ink jet printer viewed from the top and front, respectively. An ink jet printer prints on a recording sheet such as paper or an OHP sheet, and is composed of an ink jet head scanning system and a recording sheet feeding system.

In the ink jet head scanning system, the carriage 1 is provided with nozzles for four color inks (black, yellow, magenta, and cyan), and inks of the colors corresponding to the respective nozzles are also mounted. ing. Nozzles of each color are provided below the carriage 1, and the recording sheet (paper) 2 faces the nozzles with a gap of about 1 mm. The scan shaft 4 and the guide shaft 5 are provided to reciprocate the carriage 1 in parallel with the recording surface of the paper 3. The scan shaft 4 is held by the side plates at both ends and penetrates the carriage 1. The carriage 1 is connected to a timing belt 7 at a connection portion 6 (see FIG. 2).
Is mounted between a driving pulley 9 attached to a pulse motor (stepping motor) 8 and a driven pulley 10. The pulse motor 8 reciprocates the carriage 1 along the scan shaft 4, and the timing belt 7 and the driven pulley 10 change the rotation of the pulse motor 8 into a reciprocating motion of the carriage 1. Although an encoder for measuring the moving speed of the carriage 1 is not shown in FIG. 1 for simplification, as shown in FIGS. 2 and 3, a plurality of slits are formed in the moving direction of the inkjet head. And a sensor 12 for optically detecting the slits. Note that the encoder may use an encoder scale or the like.

On the other hand, in a recording sheet feeding system, a recording sheet
The (paper) 2 is placed on a platen 13 also serving as a guide plate for guiding along the transport path, and the paper pressing plate 14 presses the recording sheet 2 between the platen 13 and prevents the recording sheet 2 from floating.
The paper 2 is fed from a paper feeding unit (not shown) via a paper feeding roller (not shown) in the direction of arrow B, and is sent to a recording unit where the carriage 1 and the platen 13 face each other. It stops when the leading end of the printing area on the paper 2 is set on the nozzle-facing surface below the carriage 1. The recording sheet 2 on which the image has been recorded is discharged by a discharge roller 15 disposed downstream of the recording sheet in the transport direction and a paper discharge pressing roller 16 in contact with the discharge roller 15 at a constant pressure.

At the time of image formation, when paper 2 is fed,
Normally, the paper 2 is moved in the sub-scanning direction at a pixel pitch every main scanning once or twice (one reciprocation) of the carriage 2. Ink droplets ejected from the ink jet head below the carriage 1 land on the paper 2, and an image is formed on the paper 2. The carriage 1 is scanned by a pulse motor 8, a driven pulley 10, and a timing belt 7 in the horizontal direction (main scanning direction), that is, in parallel with the paper 2, and the nozzles under the carriage 2 record an image for one scan. I do. Each time printing for one main scan is completed, the paper 2 is fed in the vertical direction (sub-scanning direction) by a sub-scanning motor (not shown), and printing for the next main scan is performed. This is repeated to print the entire print area. When the printing is completed, the paper 2 is discharged.

FIG. 4 is a block diagram of a control system. CP
U30 is connected to host computer 32 via interface 31. The CPU 30 includes a ROM 33
The printing operation is controlled based on a program stored in the RAM 34 and print data from the host computer 32 stored in the RAM 34. The operator can set various modes on the operation panel 35 and displays the status of the printer. The sensor 36 detects the presence or absence and the position of the paper. CPU
Reference numeral 30 controls the rotation of the main scanning motor 8 and the sub-scanning motor 39 by motor drivers 37 and 38. The encoder includes an encoder film 11 and a sensor 12 that optically detects a slit provided in the encoder film 11. The encoder film 11 has a plurality of slits arranged in the main scanning direction, and the encoder sensor 12 outputs a signal corresponding to a period of the slit. The output of the encoder sensor 12 is the
The CPU 30 can recognize the moving speed of the carriage 1. The output of the encoder sensor 12 is used as a trigger signal generation unit 41 for generating head drive timing.
CPU 30 corresponding to the carriage moving speed
Is multiplied as necessary by the instruction from. CPU 30
Drives the head under the carriage 1 via the head driver 42 in synchronization with the trigger signal to eject ink. The scanning speed of the carriage 1 is controlled by the driving pulse frequency of the motor 8, but may be controlled by using a servomotor.

Before describing the acceleration / deceleration control of the present invention, a conventional acceleration / deceleration control will be described for comparison.
FIG. 5 shows an encoder film 11 for obtaining ink ejection timing in a conventional inkjet printer.
And the relationship between the main scanning time of the carriage 2 and printing. Encoder film 1 with lower resolution than image density
Numeral 1 is provided corresponding to the printing area, and the slit provided in the encoder film 11 is read by the encoder sensor 12 to obtain the timing for discharging ink from the nozzles. Here, during the period (a) during which the carriage 1 is being accelerated, printing is not performed, printing is performed in the constant speed area (b), and deceleration (c) is performed after printing for one main scan is completed. In this way, printing is performed only in the low-speed area (b).

On the other hand, the encoder used in the present invention has a high resolution in a low speed region such as an initial stage of acceleration, and has a low resolution in a high speed region. FIG. 6 shows an encoder film 11 having two resolutions and speed control using the same. In order to print even in the low-speed area, this encoder film has a length including not only the low-speed area b of the main scanning but also the acceleration / deceleration areas a and c. Further, although slits are provided in the encoder film 11, the periods thereof are different between the central part and the end part of the main scanning. At both ends, the period is short and has high resolution equal to or higher than the image density, and at the center, the period is long and has low resolution equal to or lower than the image density.
The printing area includes not only the center but also both ends.

When this encoder is used, even if printing is performed during the acceleration / deceleration period in order to improve the printing speed, variations in the printing position can be suppressed. In an area where scanning is performed at a low speed such as immediately after acceleration (acceleration area a and deceleration area c), the encoder resolution is equal to or higher than the image density. can get. On the other hand, in a region (center portion) b where scanning is performed at a high speed, an ink jet timing is obtained by decoding an encoder output having a resolution lower than the image density as in the related art. Thus, highly accurate ink ejection timing can be obtained even in the acceleration / deceleration region. In more detail, FIG.
As shown in, the encoder further supports intermediate resolution between the low-speed region and the high-speed region, that is, in the region where the speed gradually increases during acceleration and the high-resolution encoder cannot handle the speed. Let me. Therefore, in this area, an encoder having a resolution lower than the image density but higher than the encoder resolution in the high-speed scanning area is used.

FIG. 7 shows a known encoder sensor output decoding process required to obtain ink ejection timing from an encoder having a resolution lower than the image density. The cycle of the output signal from the encoder sensor is the same as the cycle of the slit provided on the encoder film. However, if a waveform whose time width is half (set in advance by a timer or the like) is output in synchronization with the falling and rising edges of this signal, the signal decoded twice (that is, the resolution is twice that of the original) Signal). Similarly, if a waveform whose time width is further halved (set in advance by a timer) is output in synchronization with the fall and rise of the signal decoded by the double, the signal is decoded by the quadruple as shown in C. (That is, a signal whose resolution is four times that of the original). If the same is repeated, a signal with higher resolution can be obtained. In this manner, ink ejection timing can be obtained using an encoder having a resolution lower than the image density.

As shown in FIG. 6, the encoder film may have a different slit resolution between the center and the end. Further, a plurality of encoders having different slit resolutions may be used. In this case, a plurality of encoder films are installed in parallel. In controlling the carriage speed, a plurality of resolutions are switched based on the measured carriage movement speed so that scanning blur does not occur.

In the method described in Japanese Patent Application Laid-Open No. H10-287014, the detection interval in the non-print area of the image (the scale of the linear encoder) is made larger than the detection interval in the area where the image is printed. The printing is controlled based on the detection signal from the detecting means. The detection of the outside of the printing area is for setting a timing for maintenance of the print head. The detection interval of the encoder is not different in the print area as in the present invention.

[0017]

By using encoders having at least two or more resolutions, printing timing is obtained by using encoders having different resolutions in the constant speed region and the acceleration / deceleration region of the carriage movement. As a result, since there is no scanning blur even during acceleration / deceleration, printing can be performed without lowering the image quality. In addition, since printing can be performed even at low speed, the printing speed is improved. Further, the size of the device can be reduced, and the cost can be reduced. Further, the printing speed can be improved without increasing the performance of the motor.

[Brief description of the drawings]

FIG. 1 is a perspective view of an inkjet printer incorporating an inkjet head according to an embodiment of the present invention.

FIG. 2 is a top view around a carriage unit of the inkjet recording apparatus.

FIG. 3 is a front view around a carriage unit of the inkjet recording apparatus.

FIG. 4 is a block diagram of a control system.

FIG. 5 is a diagram showing the relationship between an encoder film and its carriage scanning and printing in a conventional ink jet printer.

FIG. 6 shows an ink jet printer according to the present invention.
Diagram showing the relationship between the encoder film and its carriage scanning and printing

FIG. 7 is a diagram showing an encoder sensor output decoding process required to obtain ink ejection timing from an encoder having a resolution lower than the image density.

[Explanation of symbols]

1 carriage, 2 recording sheet, 11 encoder film, 12 encoder sensor.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yasuhiro Yamato 2-3-13 Azuchicho, Chuo-ku, Osaka-shi, Osaka Inside Osaka International Building Minolta Co., Ltd. (72) Inventor Hideo Hotomi Azuchi-cho, Chuo-ku, Osaka-shi, Osaka 2-3-1-3 Osaka International Building Minolta Co., Ltd. F-term (reference) 2C056 EA01 EB11 EB35 EC11 EC37 2C480 CA01 CA33 CB31 CB35 EA02 EA05 EC04

Claims (4)

[Claims] 1. A carriage which holds an ink jet head and is reciprocated in parallel with a recording surface of a recording sheet, an encoder film in which a plurality of slits are arranged in a moving direction of the ink jet head, and a sensor which detects the slits. An encoder, a carriage moving speed measuring unit that measures a moving speed of the carriage from an output of the encoder sensor, a print head driving unit that discharges ink from the print head, and a head driving timing of the print head driving unit based on an output of the encoder sensor. An image forming apparatus comprising: a trigger signal generating unit configured to generate a signal; and the encoder has at least two or more resolutions. 2. The image forming apparatus according to claim 1, wherein the encoder includes a portion having a resolution equal to or higher than a resolution of an image to be printed. 3. The image forming apparatus according to claim 1, further comprising a plurality of encoders having different resolutions. 4. The image forming apparatus according to claim 1, wherein the encoder includes an encoder scale having a plurality of pitches.