Image Forming Apparatus, and Method Thereof

1. Field of the Invention

The present invention relates to an image forming apparatus and method for forming an image in accordance with, e.g., an input image signal or a document to be read.

2. Prior Art

Printer apparatuses using various image forming means have been put into practical use. Of these means, an ink-jet scheme, a thermal transfer scheme, and the like are widely used, ranging from personal printers to office printers, because they allow a reduction in size and noise at a relatively low cost.

In image forming apparatuses using these printing schemes, image formation is generally performed while a print medium such as a paper sheet is moved relative to an internal printing section. In a serial scan printer using the ink-jet scheme or the like, a printing medium is intermittently fed step by step a predetermined amount at a time, and an image is formed the predetermined amount at a time. A printing medium is generally conveyed by a convey roller, a drive unit, e.g., a pulse motor, for driving the convey roller, and a drive transmission means such as pulleys, gears, and belt, for transmitting driving force.

In the printing scheme for forming an image while intermittently moving a printing medium relative to the printing section in the apparatus, the printing medium must be accurately conveyed line by line a predetermined amount at a time. If the printing medium is fed by an amount smaller than the predetermined amount, image overlapping occurs to form black stripes. In contrast to this, if the medium is fed by an amount exceeding the predetermined amount, image discontinuity occurs to form white stripes. In addition, such a lack or excess of convey amount also causes color misregistration, resulting in a considerable deterioration in image quality.

The main causes that make it difficult to accurately convey a printing medium line by line a desired convey amount at a time are the decentering of a printing medium convey roller and of a drive transmission means such as pulleys and gears, fluctuations in the speed of a drive source, the gear cutting precision of, e.g., belts, pulleys, and gear, and the like. Of these causes, the decentering of the roller for conveying a printing medium or of pulleys has the greatest influence.

In order to solve this problem, a conventional printer apparatus is set such that a printing medium is conveyed by a convey amount corresponding to one line when each decentered element such as a roller or a pulley is rotated one time or a plurality (integer) of times, thereby reducing variations in convey amount corresponding to one line owing to decentering.

Consider, for example, the convey roller. If, for example, a printing medium is to be conveyed by an amount corresponding to one line per rotation, the actual convey amount deviates from a predetermined amount unless the roller is processed to have an accurate diameter. In a general printer apparatus, the print width of the print head is about several mm to a few cm. In this case, the diameter of the convey roller is about several mm, which is very small as compared with its length, resulting in deflection, warpage, or the like.

Under the circumstances, therefore, a printer apparatus is reluctantly set such that a printing medium is conveyed by a convey amount corresponding to an integer multiple of a convey amount corresponding to one line when the convey roller is rotated once, thereby canceling variations in convey amount due to the decentering of the roller with a convey operation corresponding to several lines. In this method, however, the convey amount varies for each line to some degree. That is, the method is not very effective. In addition, from the viewpoint of the overall system, it is difficult to completely cancel variations in convey amount corresponding to one line owing to the influence of each element because, the relative positional relationship between the drive transmission means (belts and pulleys) and the convey roller varies depending on a printing medium used. Furthermore, it takes much labor and cost to process a recording convey roller, pulleys, and the like with high precision, and hence the cost is inevitably increased.

The present invention has been made to solve the above-described problem in the prior art, and has as its object to provide an image forming apparatus which can always form a high-definition image at a low cost without using a complicated mechanism.

It is another object of the present invention to provide an image forming apparatus for forming an image on a printing medium line by line by a serial scheme, comprising:

convey means for conveying the printing medium during an image forming operation;

setting means for setting a predetermined relationship between a state of the convey means and a position of the printing medium;

storage means for storing drive amount data of the convey means in units of the lines;

drive means for driving the convey means in accordance with the drive amount data stored in the storage means, on the basis of the relationship set by the setting means; and

print means for printing an image on the printing medium line by line.

Preferably, the convey means is a roller, and conveyance of a predetermined amount can always be performed even if the roller is decentered, thereby forming a high-definition image.

The printing medium is divided into pages, and conveyance of a predetermined amount can always be performed for each line on each page.

The setting means sets a predetermined relationship between the printing medium, the roller, a pulley, and a belt, and can restrict not only the decentering of the roller but also a convey amount error caused by the pulley and the belt.

A predetermined image pattern is printed and read, and a convey amount error for each line can be calculated and corrected.

It is still another object of the present invention to provide an image forming apparatus in which at least one roller is pressed against a printing medium, and an image is formed line by line by a printing head while the printing medium is conveyed by rotation of the roller, comprising:

drive means for driving the roller;

means for detecting that the roller is located at a predetermined angular position;

storage means for storing drive amount data of the drive means line by line to convey a printing medium having a predetermined length;

means for controlling rotation of the roller through the drive means on the basis of the drive amount data stored in the storage means; and

means for controlling rotation of the roller to set the roller at the predetermined angular position before image formation on the printing medium is started.

It is still another object of the present invention to provide a method of controlling a serial printer for printing an image line by line, comprising the steps of:

setting a roller for conveying a printing medium at a predetermined angular position;

detecting that the printing medium is located at a predetermined position;

printing an image line by line; and

conveying the printing medium by rotating the roller by a predetermined amount stored in a memory every time one-line recording is performed.

With the above-described arrangement, the image forming apparatus of the present invention can always form a high-definition image at a low cost without using a complicated mechanism.

Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a perspective view showing an image forming section of an image forming apparatus according to an embodiment of the present invention;

FIG. 2 is a sectional view showing a main part of the image forming apparatus of the embodiment;

FIG. 3 is a chart showing rollers for governing the convey amount of a printing medium for each line in the image forming apparatus of the embodiment;

FIG. 4 is a view showing the dimensions of the image forming section of the image forming apparatus of the embodiment;

FIGS. 5A to 5C are charts showing the number of drive pulses, as an initial value, for a motor and the corresponding convey amount of a printing medium for each line in the image forming apparatus of the embodiment;

FIGS. 6A to 6C are charts showing the number of drive pulses for a motor and the corresponding convey amount of a printing medium for each line during an actual operation of the image forming apparatus of the embodiment;

FIG. 7 is a block diagram showing a motor control mechanism of the printer of the embodiment;

FIG. 8 is a flow chart showing a sequence of printing an image on one page;

FIG. 9 is a perspective view showing an image forming section of an image forming apparatus according to the second embodiment of the present invention;

FIG. 10 is a perspective view showing an image forming section of an image forming apparatus according to the third embodiment of the present invention;

FIG. 11 is a block diagram showing a motor control mechanism of the printer of the third embodiment;

FIG. 12 is a schematic view showing a document reading section of an image forming apparatus according to the fourth embodiment of the present invention;

FIG. 13 is a flow chart showing a sequence of convey amount adjustment according to the fourth embodiment; and

FIG. 14 is a graph showing convey amounts recorded when the convey amount of a printing medium for each line is inaccurate.

(First Embodiment)

FIG. 1 is a perspective view showing a printing section of an ink-jet printer to which the present invention is applied. FIG. 2 is a sectional view showing a main part of the apparatus. This printer prints images in units of pages.

<Arrangement of the Apparatus>

Referring to FIG. 1, a convey roller 1 and a driven roller 2 for pressing a printing medium against the roller 1 are disposed on the upstream side (the lower right side in FIG. 1) of a printing section. Similarly, a discharge roller 3 and a driven roller 4 for pressing the printing medium against the roller 3 are disposed on the downstream side (the upper left side in FIG. 1) of the printing section.

Each of pulleys 5a and 5b is mounted on one end of a corresponding one of the convey and discharge rollers 1 and 3. The convey and discharge rollers 1 and 3 are respectively driven by pulse motors 6a and 6b through motor pulleys 7a and 7b and timing belts 8a and 8b meshed with the respective pulleys.

Each of the pulse motors 6a and 6b used in this embodiment is a high-resolution five-phase stepping motor having a basic step angle of 0.36.degree. and half-step-driven (0.18.degree. per pulse) by a motor driver (not shown). Note that in FIG. 1, the ratio of the number of gear teeth of each of the pulleys 5a and 5b respectively mounted on the convey and discharge rollers 1 and 3 to that of a corresponding one of the motor pulleys 7a and 7b is set to be 3:1.

Flags 9a and 9b are respectively attached to the pulleys 5a and 5b mounted on the respective drive roller ends. These flags 9a and 9b respectively interrupt photointerrupters 10a and 10b arranged on the apparatus main body side to detect predetermined rotational displacements of the pulleys 5a and 5b.

A platen 11 serves to support a printing medium at the printing section and has a large number of small holes in a printing medium passing surface to prevent the printing medium from floating during an image printing operation. The platen 11 is connected to a suction blower 12.

A print head 13 is a head of an ink-jet scheme. In the embodiment, in order to form a full-color image, four color print heads, i.e., cyan, magenta, yellow, and black print heads 13C, 13M, 13Y, and 13Bk are disposed side by side in the scanning direction of the print head 13. Each head has 256 nozzles arranged in a line, from which ink particles are discharged to print an image having a width of 16.25 mm. Note that print heads 13C, 13M, 13Y, and 13Bk will be referred to as the print head 13 as a whole.

In the apparatus of the embodiment, the print width of one line is set to be 1/3 the circumference of the convey roller 1 and the discharge roller 3. Since the print width corresponds to one rotation of the motor pulleys 7a and 7b, the process precision of the motor pulleys 7a and 7b has no influence on the print precision.

The print head 13 scans on guide rails 15a and 15b mounted on a carriage 14 in a direction perpendicular to the convey direction of a printing medium. The carriage 14 is driven by a pulse motor (not shown) through a drive belt 16.

Referring to FIG. 2, a CPU 17 controls a driving operation of each motor. A memory 18 stores the number of drive pulses input to each motor in an image forming operation for each line.

In the embodiment, the ratio of the pressure of the driven roller 2 against the convey roller 1 to the pressure of the driven roller 4 against the discharge roller 3 is set to be about 4:1. In addition, it takes a convey operation corresponding to 10 lines for the trailing end of a printing medium passes through the convey roller 1 and the driven roller 2 after the leading end of the printing medium is caught therebetween, and a convey operation corresponding to two more lines is required to separate the trailing end from the discharge roller 3.

For this reason, as shown in FIG. 3, the convey amount of a printing medium is governed by the convey roller 1 in the interval between the instant at which the printing medium is caught by only the convey roller 1 and the instant immediately before a convey operation of separating the trailing end of the printing medium from the convey roller 1 (first to ninth convey operations). During a convey operation (10th convey operation) of separating the trailing end of the printing medium from the convey roller 1, the convey amount of the printing medium is influenced by both the convey roller 1 and the discharge roller 3. During convey operations (11th and 12th convey operations) in which the printing medium is caught by only the discharge roller 3, the convey amount of the printing medium is governed by the discharge roller 3. Note that the above-mentioned convey operation counts are based on the assumption that an A4-size printing medium is conveyed transversely (the same will apply hereinafter).

FIG. 4 shows the dimensions of the cross-section of a main part of an image forming section of the apparatus of the embodiment.

As shown in FIG. 4, the distance between the axes of the convey and discharge rollers 1 and 3 is set to be four times larger than a print width.times.(16.256 mm). Both the distances between the centers of the convey and discharge rollers 1 and 3 and the center of the print head 13 are 2.times..

In this apparatus, therefore, when a printing medium is conveyed after image formation corresponding to the second line is completed, the leading end of the printing medium reaches the discharge roller 3. When a convey operation is performed once more after the trailing end of the printing medium is separated from the convey roller 1, image formation corresponding to the last line is performed.

<Image Forming Operation>

An operation to be performed when an image is formed on a printing medium by the apparatus of the embodiment will be described next. FIG. 8 is a flow chart showing an image forming operation to be performed under the control of the CPU 17 in the arrangement shown in FIGS. 1, 2, and 7.

Before an image forming operation is started, the pulse motors 6a and 6b are driven to rotate the convey and discharge rollers 1 and 3 in step S801. The convey and discharge rollers 1 and 3 are rotated counterclockwise until the flags 9a and 9b on the pulleys 5a and 5b mounted on the end portions of the respective rollers interrupt light beams from the photointerrupters 10a and 10b. In step S802, the points of time at which light beams from the photointerrupters 10a and 10b are respectively interrupted by the flags 9a and 9b are detected. In step S803, the pulse motors 6a and 6b for driving the convey and discharge rollers 1 and 3 are stopped, excited, and held in the excited state under the control of the CPU 17.

Even if light beams from the photointerrupters 10a and 10b have already been interrupted by the flags 9a and 9b at the start of the above-mentioned operation, the same operation as described above is performed.

In step S804, feeding of a paper sheet is started. Referring to FIG. 2, the leading end of the printing medium fed from a cassette 19 by a paper feed unit 20 is detected by a timing sensor 33 in step S805. As the sensor 33, a photointerrupter is used. In step S806, the motor 6a is driven. At this point of time, the convey roller 1 starts to rotate and holds the printing medium, thus conveying the printing medium to an image formation start position for the first line.

Since the above-described operation is performed at the start of image formation for each printing medium, the relationship between an image formation position on a printing medium and the rotational displacement of the convey roller 1 is kept constant.

When the leading end of the printing medium is held by the convey roller 1 and reaches the image formation start position for the first line, the carriage 14 having the print head 13 mounted thereon scans on the guide rails 15a and 15b to form a first-line image on the printing medium in step S809.

In step S810, the CPU 17 sends a control signal to a motor driver (not shown) to drive each pulse motor in accordance with information on a first-line drive pulse count stored in the memory 18 in advance. The convey roller 1 makes 1/3 rotation through the timing belts 8a and 8b to convey the printing medium by an amount equal to the print width of one line. When the above-described operation is repeated, and printing for the second line is completed, the printing medium is conveyed by the convey roller 1. In the subsequent convey operations for the printing medium, the discharge roller 3 is also driven. The leading end of the printing medium then reaches the discharge roller 3 and held by the discharge roller 3. That is, the relationship between an image formation position on a printing medium and the rotational displacement of the discharge roller 3 is also kept constant.

Note that steps S808 and S812 in FIG. 8 are added to repeat the sequence and are set for the sake of convenience. Although a paper feed operation is started in step S804, this operation may be performed before or after step S801.

Subsequently, the above-described operation is repeated to perform image formation line by line. In order to prevent floating and creasing of a printing medium on the platen 11, the number of drive pulses supplied to the discharge roller 3 is set to be slightly larger than that of drive pulses supplied to the convey roller 1 (larger in convey amount by 2% in this embodiment). The amounts of rotation of the convey and discharge rollers 1 and 3 for each line are controlled by the pulse motors 6a and 6b for driving the respective rollers, which motors are driven by the motor drivers controlled by control signals supplied from the CPU 17 on the basis of drive pulse information stored in the memory 18 in advance.

When the trailing end of the printing medium is separated from the convey roller 1 after the above-described operation is repeated, the convey amount of the discharge roller 3, which is set to be slightly larger than that of the convey roller 1, is set to be equal to that of the convey roller 1. Subsequently, the discharge roller 3 makes about 1/3 rotation for each line to convey the printing medium until image formation is completed up to the trailing end of the printing medium. Upon completion of image formation, the printing medium is discharged outside the apparatus by a discharge roller pair 21 in FIG. 2.

<Adjustment of Convey Amount>

The above-described operation is an operation to be performed when the apparatus of the embodiment actually forms an image. A sequence of determining the number of drive pulses supplied to each pulse motor for each line will be described below.

When assembly of the apparatus of the embodiment is completed, an image is actually output according to the sequence described above in, for example, the last step in the manufacturing process. For example, an image pattern to be output is a monochrome image and is preferably a lattice-like continuous pattern to measure the convey amount of a printing medium. Data of this print pattern is stored in a ROM beforehand in the apparatus.

As the number of drive pulses, as an initial value, input to each of the motors for driving the convey and discharge rollers 1 and 3, a pulse count (2,000 pulses) corresponding to one rotation of each of the motors is stored in the memory 18. As described above, in the apparatus of the embodiment, when the respective motor drive shafts are rotated once, the convey and discharge rollers 1 and 3 make 1/3 rotation. At this time, the convey amount of a printing medium becomes equal to the print width of the print head according to calculation.

The discharge roller 3 is driven in accordance with the convey amount for each line as an initial value, which is larger than the convey amount of the convey roller 1 by 2%, while the trailing end of a printing medium is held by the convey roller 1. After the trailing end of the printing medium is separated from the convey roller 1, the discharge roller 3 is driven in accordance with the convey amount equal to the print width of the print head according to calculation.

In this state, a lattice pattern is printed on the entire printing medium region, and the convey amount of the printing medium for each line is measured by using a reader or the like on the basis of the pitch of the lattice pattern.

In practice, as shown in FIGS. 5A through 5C, the measured convey amount for each line varies due to the influence of swing of the convey and discharge rollers 1 and 3 or the pulleys 5a and 5b mounted on the respective roller ends, the influence made when the leading end of the printing medium is brought into contact with the discharge roller 3, and the influence made when the trailing end of the printing medium is separated from the convey roller 1. FIGS. 5A and 5B are tables showing the numbers of drive pulses supplied, as initial values, to the pulse motors 6a and 6b for the first line to the 12th line, and the differences between the desired convey amounts and the measured convey amounts, recorded when the motors are driven according to the respective pulse counts. FIG. 5C is a graph showing the differences.

When image formation is performed in accordance with the above-described sequence, the rotational displacements of the convey and discharge rollers 1 and 3 relative to the apparatus are constant at the start of image formation on a printing medium. For this reason, the influence of swing of the convey and discharge rollers 1 and 3 and the influences made when a printing medium comes into contact with the rollers or separates therefrom are the same for each printing medium. In addition, the swing of the motor pulleys 7a and 7b mounted on the pulse motors has no influence on the convey amount of a printing medium because the circumference of each pulley is equal to the print width, as described above.

The two broken curves in FIG. 5C indicate the upper and lower limit data of the feed amount of a printing medium for each line in a case wherein measurement is performed with respect to 10 samples in the above-described state. For the above-described reason, variations in the convey amounts of all printing media for all lines are very small. Therefore, the convey amount of a printing medium for each line can be measured with high precision by performing measurement with respect to only one print sample.

As described above, in the apparatus of the embodiment, the motors 6a and 6b have a basic step angle of 0.18.degree., and the convey and discharge rollers 1 and 3 have a circumference three times larger than the print width and are driven by the motors at a reduction ratio of 1/3. Therefore, one pulse input to the pulse motors 6a and 6b is equivalent to 8.128 .mu.m in terms of the convey amount of a printing medium.

In order to obtain a desired convey amount of a printing medium, the number of drive pulses as an initial value for each line is increased/decreased on the basis of the convey amount of the printing medium for each line, which amount is measured in accordance with the above-described sequence, and the resultant value is stored, as a drive pulse count for each line, in the memory 18.

For example, such a drive pulse count for each motor can be stored in the memory 18 by the following method. A program for writing data in the memory 18 is prepared in advance. A keyboard or the like is connected to an input terminal connected to the CPU 17. Input data is written at a desired address of the memory 18 through the CPU 17.

According to the embodiment, the convey amount of a printing medium is governed by the convey roller 1 until a convey operation immediately before a convey operation of separating the trailing end of the printing medium from the convey roller 1, and is governed by both the convey and discharge rollers 1 and 3 in a convey operation of separating the trailing end of the printing medium from the convey roller 1. In the subsequent convey operations, the convey amount of the printing medium is governed by only the discharge roller 3.

Therefore, in order to increase/decrease the convey amount of a printing medium for the first to ninth lines, the number of drive pulses input to the pulse motor 6a is increased/decreased. Similarly, the numbers of drive pulses input to the pulse motors 6a and 6b are increased/decreased to increase/decrease the convey amount of the printing medium for the 10th line, and the number of drive pulses input to the pulse motor 6b for driving the discharge roller 3 is increased/decreased to increase/decrease the convey amount of the printing medium for 11th and subsequent lines.

FIGS. 6A to 6C show drive pulse counts set for the convey and discharge rollers 1 and 3 to realize a constant convey amount, and corresponding convey amounts of a printing medium.

The two broken lines in FIG. 6C indicate the upper and lower limits of the convey amount of a printing medium for each line in a case wherein 10 printing media are conveyed in the above-described state. As shown in FIG. 6C, the convey amount of a printing medium for each line throughout the entire region of the printing medium deviates from a target value by 10 .mu.m or less.

That is, the apparatus of the embodiment, in which the numbers of drive pulses input to the pulse motors 6a and 6b for each line are stored in the memory 18 in accordance with the above-described sequence, can realize accurate conveyance of a printing medium when a consumer uses the apparatus.

As described above, according to the embodiment, a printing medium can be intermittently conveyed at intervals each equal to the print width, from the leading end to the trailing end, with high precision. Therefore, high-definition images can be output onto the entire region of a printing medium while minimizing the marginal spaces on the leading and trailing ends of an image.

FIG. 14 shows the convey amount of a printing medium for each line in a case wherein each printing medium is conveyed in the printing apparatus without restoring the convey and discharge rollers 1 and 3 to predetermined displacements.

That is, the convey and discharge rollers 1 and 3 are driven by the drive pulses shown in FIGS. 5A and 5B, and the rotational displacement of each roller or the like for each printing medium at the start of image formation is not constant.

Note that each of the convey and discharge rollers 1 and 3 is decentered by 10.mu., and each of the pulleys inserted in the respective roller ends with pressure is decentered by 10.mu..

As described above, even if components processed with very high precision are used, variation in the convey amounts of printing media for the same line reaches .+-.20.mu., as shown in FIG. 14.

This variation occurs because the influence of decentering of each component described above varies for each convey operation.

Referring to FIG. 14, the mean value of the convey amounts of printing media considerably decreases near the 10th line. This is because the number of rollers which govern the conveyance of a printing medium is decreased from two to one, and the conveyance resistance based on suction at the platen portion changes. The mean value reaches about 50.mu. at worst.

In comparison with this data, it is clear that the printing apparatus of the embodiment can convey a printing medium with high accuracy.

In addition, in the apparatus of the embodiment, since the conveyance precision of a printing medium does not depend on the process precision of pulleys, a convey roller, and the like mounted on motors or roller ends, the manufacturing cost of these components can be greatly reduced.

(Second Embodiment)

The apparatus of the first embodiment is designed such that the rotational displacements of the convey roller and the pulse motor output shaft at the start of image formation are set to be the same for each printing medium.

In the second embodiment to be described next, the relative displacements of transmission means such as pulleys and belts as well as a convey roller and a pulse motor output shaft at the start of image formation are set to be the same for each printing medium.

FIG. 9 shows part of a printer according to the second embodiment of the present invention. The same reference numerals in FIG. 9 denote the same parts having the same functions as in FIG. 1 showing the printer of the first embodiment, and a description thereof will be omitted.

The arrangement of the apparatus and the operation principle in image formation are almost the same as those in the first embodiment. In the second embodiment, a convey roller 1, a discharge roller 3, and pulse motors 6a and 6b are restored to the initial states before a printing operation for each printing medium, and belts 8a and 8b as drive transmission means are also restored to the initial states. For this reason, means for detecting the positions of the belts are arranged. Only the drive section of the convey roller 1 will be described below because the drive sections of the convey roller 1 and the discharge roller 3 are identical.

The apparatus is set such that the ratio of the number of gear teeth of a pulley 7a to that of a roller pulley 5a is an integer, and the ratio of the number of gear teeth of the roller pulley 5a to that of the belt 8a is an integer. A slit-like notched portion is formed in the belt 8a at a position outside the range in which the belt 8a is meshed with the pulleys 5a and 7a. The pulse motor 6a keeps rotating after image formation is completed and the printing medium is discharged outside the apparatus. When the notched portion of the belt 8a reaches the position of a photointerrupter 10c arranged on the main body side, interruption of a light beam from the photointerrupter 10c is canceled. With this operation, a drive pulse stop signal is supplied from a CPU to a motor driver (not shown), thus stopping the driving operation. With this operation, the drive section is restored to an initial state before image formation.

As described above, since the overall drive section can be restored to the initial state by only arranging the means for detecting the positions of the belts, repeatability of the convey amount for each printing operation is improved, thus increasing the conveyance precision of a printing medium in combination of correction of motor drive pulse counts in the first embodiment.

If the ratio of the number of gear teeth of a belt to that of a pulley cannot be set to be an integer because of a problem associated with the arrangement of the apparatus or the like, the flags 9a and 9b and the photointerrupters 10a and 10b in the first embodiment are arranged together with the belt position detecting means described above so as to obtain the same effect as described above.

(Third Embodiment)

The third embodiment designed to achieve the same object as that of the second embodiment will be described below.

FIG. 10 shows the arrangement of an apparatus of the third embodiment. The same reference numerals in FIG. 10 denote the same parts as in FIG. 1, and a description thereof will be omitted. The principle of driving of convey and discharge rollers is the same as that in the first embodiment.

A method of keeping the relative displacement of a convey roller, a transmission belt, and a pulse motor output shaft constant for the sake of image formation on the next printing medium will be described below.

The apparatus of the third embodiment includes a counter for counting drive pulses input to a pulse motor, as shown in FIG. 11. The counter increments the count value when a drive pulse for the rotation direction in an image formation operation is input to a pulse motor 6a, and decrements the count value when a drive pulse for a direction opposite to the rotation direction is input. The count value of the counter before an image forming operation is set to be 0, and the pulse count is incremented by the counter in accordance with the number of drive pulses input during an image forming operation. At the same time, the resultant pulse count is stored in a memory.

After the image forming operation, the pulse motor is driven in the direction opposite to the direction in a convey operation of the printing medium until the drive pulse count stored in the memory becomes 0.

In this embodiment, a drive pulse count measured by the counter is always stored in the memory, and the contents of the memory are retained even if the power switch is turned off. When the power switch is turned off, a voltage applied to a solenoid 22a is simultaneously interrupted. As a result, a leaf spring 23a operates to press a pulley 7a so as to restrict the rotational displacement of the pulse motor 6a. In addition, a paper sheet jammed in the apparatus can be removed by releasing a driven roller 2, which biases a convey roller, 1 without displacing the convey roller 1, a transmission belt 8a, the pulse motor 6a, and the like.

When the power switch is turned on, the pulse motor 6a is driven until the drive pulse count is set to be 0 (the origin of the pulse motor) by referring to the latest drive pulse count stored in the memory.

By applying the same method to the discharge roller side, image formation on the next printing medium can be performed while keeping the relative displacement of the convey roller 1, the discharge roller 3, the timing belts 8a and 8b, and the output shafts of the pulse motors 6a and 6b in the same state as that at the start of the previous image forming operation, even if the numbers of gear teeth of a roller pulley 5a mounted on an end of the convey roller 1, a roller pulley 5b inserted in an end of the discharge roller 3 with a pressure, the timing belts 8a and 8b for driving the respective pulleys, and the motor pulleys 7a and 7b mounted on the pulse motors are not kept in the relationship of an integer multiple.

(Fourth Embodiment)

The fourth embodiment of the present invention will be described next. In the first embodiment, the actual image forming operation and the necessity of determining the number of drive pulses input to each pulse motor for each line have been described. In the fourth embodiment, one example of a sequence of determining each drive pulse count will be described. An image forming operation is the same as that in the first embodiment.

This embodiment is applied to an image printing apparatus having a document reader, e.g., a copying machine or a facsimile apparatus, or an image printing apparatus connected to a document reader through a personal computer.

A copy printing operation of a copying machine using an ink-jet scheme according to the fourth embodiment will be described first. FIG. 12 shows a document reader of the copying machine according to the fourth embodiment of the present invention. FIG. 13 is a flow chart showing a sequence of adjusting the convey amount for each line.

A method of reading a document by using the document reader will be described below. Referring to FIG. 12, a CCD sensor 25, an optical system (not shown), an illumination system (not shown), a printed board (not shown), and the like are mounted on a reader carriage 24. The reader carriage 24 is driven by a motor 27 as a drive source through a drive belt 26 to scan on two rails 28a and 29b. With one scanning operation, a document is read by a pixel width equal to or slightly larger than that of a print head 13. Subsequently, a rail base 29 is driven by a motor 31 through a drive transmission wire 30 to move along rails 32a and 32b in a direction perpendicular to the scanning direction of the reader carriage 24 by an amount corresponding to a print width, thus reading the next line. In the image forming section, printing is performed on a printing medium line by line in accordance with the read information. By repeating this operation, copy printing of the document is performed. Since the details of the image forming method in the image forming section is the same as that in the first embodiment, a description thereof will be omitted.

A sequence of determining the number of drive pulses input to each pulse motor for each line will be described next with reference to FIG. 13.

Similar to the first embodiment, for example, a lattice-like continuous pattern of a monochrome image, which is stored in a ROM in the apparatus to measure the convey amount of a printing medium, is printed on a printing medium. The printing medium having this pattern printed thereon is set on a document table (not shown) of the document reader such that a printing direction on the printing medium is perpendicular to the document reading direction. Subsequently, in step S132, the reader carriage 24 is caused to scan once on a central portion of the printing medium to read the lattice-like pattern in accordance with a sequence of convey amount measurement stored in the ROM. Since the scanning direction of the reader carriage 24 is perpendicular to the scanning direction of the print head 13, the reader carriage 24 can scan from the first print line to the last line with one stroke. In this case, the scanning speed is preferably set to be lower than that in a normal read operation to read the pattern with precision as high as possible. In step S133, the read information is analyzed in an image processing section to obtain a convey amount for each line. That is, variations in the density of the read pattern are checked, and the distance between the adjacent lines are quantized on the basis of the density variations. The convey amount for each line is obtained from these values and the line width.

The differences between the measured convey amounts and the desired convey amounts are converted into drive pulse counts for each pulse motor, and a correction value for the drive pulse count for each line is determined in step S134. In step S135, the respective correction values are stored as a table in a nonvolatile memory. In a subsequent printing operation, a CPU 17 supplies a control signal to each motor driver on the basis of the information of this table, thereby controlling drive pulses input to pulse motors 6a and 6b for driving the respective rollers.

The above-described operation is performed once at the time of shipment or installation of the apparatus. Subsequently, when a component of the convey section, e.g., a convey roller, needs to be replaced for some reason, the above-described operation is performed again after the replacement. With this operation, the drive pulse count table in the memory is updated, and a printing medium can be conveyed again in a desired convey amount. In addition, a change in quality of each roller over time, e.g., wear, can be properly handled.

The present invention can be effectively applied to a printing apparatus of an ink-jet scheme, of the ink-jet printing schemes, in which scattering droplets are formed by using thermal energy to perform a printing operation.

With regard to typical arrangement and principle, for example, a scheme using the basic principle disclosed in U.S. Pat. Nos. 4,723,129 and 4,740,796 is preferable. This scheme can be applied to both a so-called on-demand type apparatus and a continuous type apparatus. The present invention is especially effective for an on-demand type apparatus for the following reason. When one drive signal for providing a rapid increase in temperature exceeding a nucleus boiling temperature in accordance with print information is supplied to a thermoelectric conversion member arranged on a sheet holding a liquid (ink) or in a liquid path, thermal energy is generated in the thermoelectric conversion member to cause film boiling at a heat effect surface of a print head. As a result, a bubble can be formed in the liquid (ink) in one-to-one correspondence with the drive signal. The liquid (ink) is discharged through a discharge opening upon growth and reduction of a bubble, thereby forming at least one droplet. If this drive signal is formed as a pulse-like signal, growth/reduction of a bubble can be quickly and properly performed, which allows discharging of the liquid (ink) with excellent response characteristics. Therefore, it is preferable that the drive signal be formed as a pulse-like signal.

As this pulse-like drive signal, a signal such as the one disclosed in U.S. Pat. Nos. 4,463,359 and 4,345,262 is suitable. Note that excellent printing can be performed by using the conditions described in U.S. Pat. No. 4,313,124 associated with the rate of temperature rise at the above-mentioned heat effect surface.

As an arrangement for a print head, in addition to the arrangement described above, in which the discharge opening, the liquid path, and the thermoelectric conversion member are combined (a straight liquid path or a right angle liquid path), the arrangement disclosed in U.S. Pat. Nos. 4,558,333 and 4,459,600 in which a heat effect surface is arranged at a bent region may be employed.

In addition, the arrangement disclosed Japanese Patent Laid-Open No. 59-123670 in which a slit common to a plurality of thermoelectric conversion member is used as the discharge portion of a thermoelectric conversion member or the arrangement disclosed in Japanese Patent Laid-Open No. 59-138461 in which an opening for absorbing pressure waves of thermal energy is arranged in correspondence with a discharge portion may be employed.

As a full-line type print head having a length corresponding to the width of a maximum-size printing medium on which the printing apparatus can print an image, either an apparatus having a plurality of print heads combined to satisfy the length requirement, as described above, or an apparatus having a single integral print head may be used.

Furthermore, an exchangeable chip type print head which can be electrically connected to the apparatus main body and allows supply of an ink from the apparatus main body, or a cartridge type print head having an ink tank integrally formed thereon may be used.

A recovery means, a preliminary auxiliary means, and the like are preferably added to a print head arranged as a component of the printing apparatus of the present invention in order to further stabilize the effects of the present invention. For example, a capping means for the print head, a cleaning means, a pressurizing or suction means, a preliminary heating means constituted by a thermoelectric conversion means or another type of heating element or a combination thereof, and a means for performing a preliminary discharge mode different from a discharge mode in a printing operation are effective for a stable printing operation.

In addition to the print mode using only a main color such as black, the printing apparatus may include at least one of color modes, i.e., a multi-color mode using different colors and a full-color mode using color mixtures.

In the above-described embodiments of the present invention, an ink is handled as a liquid. However, an ink which is solidified at room temperature or less and is softened or liquified at room temperature may be used, or any type of ink which is liquified when the current print signal is applied may be used because in the above-described ink-jet scheme, temperature control is generally performed by performing temperature adjustment of an ink itself within the range of 30.degree. C. to 70.degree. C. to set the viscosity of the ink within a stable discharge range.

Furthermore, a rise in temperature due to thermal energy may be positively prevented by using the thermal energy as energy for a state change from a solid to a liquid, or an ink which is solidified when it is left to stand may be used to prevent evaporation of the ink. In any case, an ink which is liquified to be discharged as a liquid ink upon application of thermal energy in accordance with a print signal, or an ink which is liquified only by thermal energy, e.g., an ink which starts to solidify when it reaches a printing medium, may be used in the present invention. In such a case, as disclosed in Japanese Patent Laid-Open Nos. 54-56847 and 60-71260, an ink may be held as a liquid or a solid in a porous sheet recess or through hole to oppose a thermoelectric conversion member. In the present invention, execution of the above-described film boiling scheme is most effective for each ink described above.

Moreover, the printing apparatus according to the present invention may take various forms. For example, the apparatus may be integrally or separately arranged as an image output terminal of an information processing apparatus such as a word processor or a computer, as described above, or may take the form of a copying machine combined with a reader or the like. Alternatively, the apparatus may take the form of a facsimile apparatus having a transmission/reception function.

The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention, the following claims are made.