Printing apparatus and print timing control method

There is provided a printing apparatus that can perform print timing control while moving a print head with high precision even when variation in moving speed of the print head is relatively large. More specifically, a prediction speed is obtained based on a detected moving speed of the print head and a control target speed at the time of detection, and then a print timing of the print head is controlled based on the prediction speed. Accordingly, a correction speed of the print timing is considered to be the obtained prediction speed. This makes it possible to minimize a landing error even when a speed variation is large.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printing apparatus and a print timing control method, and more specifically to print timing control in printing while moving a print head.

2. Description of the Related Art

As one mode for controlling a print timing in a printing apparatus which performs printing while moving a print head, there is provided a printing apparatus that detects information about a moving speed of the print head by an encoder or the like and controls a print timing based on the information. The printing apparatus which performs printing not only in constant-speed control but also in acceleration/deceleration control performs print timing control according to a variable speed so that a desirable print position is obtained even when the speed of the print head is variable. Furthermore, the moving speed of the print head may change for some reason even in the constant-speed control, and thus the printing apparatus performs the print timing control in response to the speed variation.

Japanese Patent Laid-Open No. 2007-118425 discloses estimating for each detection cycle of an encoder a moving speed of a cycle by using a moving speed obtained in a previous cycle and determining a print timing of the cycle based on the estimation. More specifically, an estimated speed of a cycle is set based on a result obtained by subtracting a difference between the detected speed of the last cycle but one and the detected speed of the last cycle from the detected speed of the last cycle, and a print timing is controlled based on the estimated speed.

However, as disclosed in Japanese Patent Laid-Open No. 2007-118425, in a mode that the moving speed of the print head in a certain cycle is obtained based on the detected speed of the previous cycle, if the moving speed of the print head changes relatively larger, the difference between an estimated speed and an actual speed of the print head is greater. As a result, there is a problem that if the variation in moving speed of the print head is large, the difference between a target print position and an actual print position is large.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a printing apparatus and a print timing control method that can perform print timing control while moving a print head with high precision even when variation in moving speed of the print head is relatively large.

In a first aspect of the present invention, there is provided a printing apparatus for performing printing by moving a print head relative to a print medium, the printing apparatus comprising: a detection unit configured to detect a moving speed of the print head; a driving control unit configured to control a driving mechanism for driving the print head based on the detected moving speed of the print head and a target speed so that the print head moves at a moving speed that is made closer to the target speed; and a timing control unit configured to obtain a prediction speed based on the detected moving speed and the target speed and control a print timing of the print head based on the prediction speed.

In a second aspect of the present invention, there is provided a print timing control method for performing printing by moving a print head relative to a print medium, the print timing control method comprising: a detection step for detecting a moving speed of the print head; a driving control step for controlling a driving mechanism for driving the print head based on the detected moving speed of the print head and a target speed so that the print head moves at a moving speed that is made closer to the target speed; and a timing control step for obtaining a prediction speed based on the detected moving speed and the target speed and control a print timing of the print head based on the prediction speed.

The above structure makes it possible to perform the print timing control while moving the print head with high precision even when variation in moving speed of the print head is relatively large.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

FIG. 1is a block diagram showing the structure of print timing control of a printing apparatus according to one embodiment of the present invention. A printing apparatus101according to the present embodiment is a serial-type printing apparatus for printing by moving a print head to scan a print medium such as a print sheet. A print head is an ink-jet print head for printing on a print medium by ejecting ink droplets from an ejection port. The printing apparatus101has a motor102that serves as a driving mechanism for moving a carriage having the print head mounted thereon. A motor speed control part104controls driving of the motor102based on detection information inputted from an encoder103for detecting a moving speed of the carriage, and as will described later with reference toFIG. 3and others, provides various control parameters for a print timing control part105. A printing part106having the print head causes the print head to eject ink at a timing specified by the print timing control part105during scanning of the print head to perform printing. Incidentally, the motor speed control part104and the print timing control part105shown inFIG. 1are achieved by a control structure such as a CPU in the printing apparatus, and the printing part106has a mechanism for the print head and its movement.

FIG. 11is a perspective view of the printing apparatus according to the first embodiment of the present invention. InFIG. 11, a print medium1101is supported by print medium feeding rollers1102provided in a print area, ribs on a platen1110, and spurs1111, and is conveyed by the feeding rollers1102that are driven by a sheet feeding motor1103in a sub-scanning direction as shown by an arrow α. In front of the feeding rollers1102, there is provided a shaft1104in parallel with the feeding rollers1102. A carriage1105is movably guided by the shaft1104and reciprocally moves in a main scanning direction shown by an arrow β via a belt1107based on an output of a carriage motor1106.

On the carriage1105serving as a print head moving unit, there are mounted print heads1108and tanks1109for storing print ink.

The moving speed of the carriage motor1106is measured by an encoder1112.

The carriage motor1106and the encoder1112correspond to the motor102and the encoder103ofFIG. 1, respectively.

FIG. 2is a graph showing exemplary driving control for the motor102performed by the motor speed control part104and shows the relationship between an elapsed time and a moving speed of the print head (carriage) by the motor102. The motor speed control part104performs servo control at a given cycle to control the speed of the motor102.FIG. 2shows control at the acceleration of carriage movement (print head scanning) and shows that the motor102is controlled by the motor speed control part104at every servo cycle that is a constant cycle. InFIG. 2, timings at which the motor speed control part104controls the motor102are set as201(t0),202(t1),203(t2), and204(t3). The motor speed control part104monitors that the moving speed of the print head is v0based on an encoder signal received from the encoder103at the timing201. Then, as will be described later, the motor speed control part104controls driving of the motor102on the basis of two parameters: a current speed v0of the print head as monitored by the motor speed control part104and a control target speed v1of the print head at the timing201. In the same manner, the motor speed control part104controls the motor102on the basis of a target speed v2, a target speed v3, and a target speed v4, at the timing202, the timing203, and the timing204, respectively. It should be noted that the change in moving speed of the motor102and the change in moving speed of the print head from a certain control timing to the next control timing vary depending on the characteristics and the structure of the printing apparatus.

FIG. 3is a block diagram showing a detailed structure of the print timing control part105. The print timing control part105is configured by using an elapsed time counter301, a prediction speed calculation part302, a look-up table303, a latch trigger generation part304, and a landing position correction part305.

The print timing control part105receives a motor control timing signal from the motor speed control part104. The print timing control part105also receives a current speed parameter and a target speed parameter which are used for the control of the motor102by the motor speed control part104from the motor speed control part104, for each motor control timing. The elapsed time counter301of the print timing control part105counts the time from when the motor control timing signal is received, and passes the result to the prediction speed calculation part302as an elapsed time parameter. The prediction speed calculation part302receives a current speed parameter and a target speed parameter from the motor speed control part104and an elapsed time parameter from the elapsed time counter301. With reference to the look-up table303based on the parameters, the prediction speed calculation part302calculates a prediction speed and outputs the result as prediction speed information to the latch trigger generation part304and the landing position correction part305. Based on the prediction speed information, the latch trigger generation part304outputs a latch trigger to the landing position correction part305so as to control cycles at which the printing apparatus101performs printing. The landing position correction part305delays a latch trigger timing generated by the latch trigger generation part304based on the prediction speed information so as to generate and output a print trigger, that is, a timing at which the printing apparatus101performs printing, to the printing part106.

FIG. 4is a graph used for explaining the content of the look-up table303. The look-up table303is used for obtaining a prediction speed based on three parameters: an elapsed time parameter, a target speed parameter, and a current speed parameter. More specifically, the table defines a prediction speed for each combination of a current speed, an elapsed time, and a target speed. Accordingly, among the combinations, a pair of a current speed detected at a motor control timing for each servo cycle and a target speed with respect to an elapsed time is determined, and a prediction speed corresponding to the determined pair is obtained.FIG. 12shows an exemplary look-up table for obtaining a prediction speed. A prediction speed is obtained based on three parameters: a current speed obtained at a timing of a servo cycle, a target speed, and a time elapsed from the timing of a servo cycle. InFIG. 12, parameter values corresponding to the current speed, the target speed, the elapsed time, and the prediction speed are stored in a horizontal row of the table.FIG. 12shows that the prediction speed parameter is 13 in a case where the current speed parameter is 10, the target speed parameter is 20, and the elapsed time parameter is 1. The prediction speed parameter is 15 in a case where the current speed parameter is 10, the target speed parameter is 20, and the elapsed time parameter is 4.

Incidentally, the look-up table is created in advance on the basis of the characteristics of the motor102and the structure of the printing apparatus101. The look-up table may be set for each type of apparatus or for each apparatus in view of errors of the motor102and the printing apparatus101. Further, an ink level parameter may also be used in a case where weight variations due to the ink level affect the characteristics.

Referring back toFIG. 3, the latch trigger generation part304controls latch trigger cycles so that a space between ink dots formed on a print medium is constant based on the prediction speed. As shown inFIG. 5, the latch trigger generation part304makes the latch trigger cycle smaller as the prediction speed in one of the servo cycles increases, whereas the latch trigger generation part304makes the latch trigger cycle larger as the prediction speed in one of the servo cycles decreases.

FIG. 6is a diagram illustrating landing correction processing performed by the landing correction part305based on the prediction speed thus obtained.

InFIG. 6, ink from a print head601is ejected at an ejection speed602. In a case where a moving speed of the print head is set as a speed603, the ejected ink moves according to a resultant speed of the speed602and the speed603. As a result, the ink passes through a passage604and lands in a position605. In a case where a reference speed of the print head is set as a speed606, the ink moves according to a resultant speed of the speed602and the speed606while the print head moves at a reference speed, and the ink passes through a passage607and lands in a target landing position608. In other words, an error of the landing position occurs since an expected landing position is the position608, whereas an actual landing position is the position605. In the present embodiment therefore, landing is delayed by a time610so as to correct the error of the landing position caused by the moving speed of the print head. As a result, the ejected ink passes through a passage609and lands in the position608. In this manner, the landing position correction part305obtains the delay time610based on the inputted prediction speed which is set as the speed603. Then, by delaying the inputted latch trigger timing by the obtained time, the landing position correction part305generates a print trigger. Incidentally, the “reference speed” of the print head is a speed of the print head in scanning determined based on, for example, driving frequencies and a print resolution of the print head.

FIGS. 7A and 7Bshow the relationship between an actual speed (measured speed) and a prediction speed (landing position correction speed) according to the prior art, andFIGS. 7C and 7Dshow the relationship between an actual speed (measured speed) and a prediction speed (landing position correction speed) according to an embodiment of the present invention.

FIGS. 7A and 7Bshow landing correction according to the prior art, which are represented by prior art landing correction A and prior art landing correction B, respectively.FIG. 7Cshows landing correction according to the present embodiment. As shown inFIGS. 7A and 7B, the prior art landing correction A uses a measured speed of a last cycle as a landing position correction speed. Meanwhile, as a landing position correction speed, the prior art landing correction B uses a speed at which a difference between the measured speed of the last cycle but one and the measured speed of the last cycle is equal to a difference between the landing position correction speed and the measured speed of the last cycle. In a case where a measured speed at a time t is set as vm(t) and a landing position correction speed at a time t is set as vc(t), the landing position correction speed obtained by the prior art landing correction A is represented by vc(tn)=vm(tn−1). Meanwhile, the landing position correction speed obtained by the prior art landing correction B is represented by vc(tn)=vm(tn−1)+(vm(tn−1)−vm(tn−2)).

On the other hand, according to the present embodiment, in a case where a prediction speed at a time t to be inputted to the landing position correction part305is set as vp(t), the landing position correction speed is represented by vc(tn)=vp(tn). More specifically, as described with reference toFIG. 6, the prediction speed is equal to the landing position correction speed. Accordingly, as shown inFIGS. 7A and 7B, according to the prior art landing correction, when a change in speed is large, as at time t1, t2, or t3, as the difference between the actual speed and the prediction speed increases, the land error increases. On the other hand, as shown inFIG. 7C, according to the landing correction of the present embodiment, the landing error can be reduced by setting the table as described with reference toFIG. 4.

Second Embodiment

FIG. 8is a block diagram showing the structure of print timing control of a printing apparatus according to a second embodiment of the present invention. The difference between the present embodiment and the first embodiment is that a signal from an encoder103is directly sent to a print timing control part802in the second embodiment.

FIG. 9is a graph showing exemplary driving control of a motor102performed by a motor speed control part104according to the present embodiment, and shows the relationship between an elapsed time at acceleration and a moving speed of a print head. The timings at which the motor speed control part104controls the motor102are set as901(t0),902(t1),903(t2), and904(t3). The motor speed control part104monitors that the moving speed of the print head is v0based on an encoder signal received from the encoder103at the timing901. Then, the motor speed control part104controls the motor102on the basis of two parameters: a current speed v0of the print head as monitored and a target speed v4of the print head at the timing901. In the same manner, the motor speed control part104controls the motor102on the basis of the target speed v4at the motor speed control timings902,903, and904.

FIG. 10is a block diagram showing a detailed structure of the print timing control part802shown inFIG. 8. The print timing control part802is made up of an elapsed time counter301, a prediction speed calculation part302, a look-up table303, an encoder interval counter1001, a latch trigger generation part1002, and a landing position correction part1003. The encoder interval counter1001calculates a speed in an encoder section based on the information received from the encoder103, and outputs the result to the latch trigger generation part1002. The latch trigger generation part1002generates a latch trigger for each encoder section based on the encoder information received from the encoder103. The landing position correction part1003predicts a speed based on the received prediction speed information and encoder speed information, performs landing position correction, and outputs a print trigger to a printing part106.

FIG. 7Dshows the relationship between an actual speed (measured speed) and a prediction speed (landing position correction speed) by the print timing control according to the present embodiment. In the present embodiment, a speed obtained by adding a difference between a prediction speed and a measured speed inputted at the previous timing to the landing position correction part1003to a prediction speed at a time t is used as a landing position correction speed. More specifically, the landing position correction speed of the present embodiment can be represented by vc(t)=vp(tn)+vm(tn−1)−vp(tn−1). As a result of the correction, the error of the prediction speed when the speed variation is large can be reduced, and the landing error can be reduced.

FIG. 13is a flowchart of the print timing control shown inFIG. 7D. At a servo control timing, that is, a control timing of the motor by the motor speed control part104(S1301), a print timing control part105obtains a current speed and a target speed from the motor speed control part104(S1302). Then, at a print control timing, that is, a timing in a cycle shorter than that of the servo control timing (S1303), the print timing control part105obtains speed information at the time of the last print control (S1304). The speed information at the time of the last print control is calculated such that the encoder103measures the travel distance of a carriage1105between the last print control timing and the current print control timing, and the result is divided by the last control cycle. The difference between the last speed information obtained in Step1304and the prediction speed used at the last print control timing is calculated, and a speed error is obtained (S1305). Further, with reference to the look-up table303and based on an elapsed time from the servo control timing of a current print control timing obtained in Step1306and the current speed parameter and the target speed parameter obtained in Step1302, a prediction speed is obtained (S1307). Then, the speed error information obtained in Step1305is reflected on the prediction speed obtained in Step1307, so that the landing position correction speed is calculated (S1308). The print timing control part105generates a print timing based on the landing position correction speed calculated in Step1308. At the print timing generated by the print timing control part (S1309), the printing part106performs printing processing (S1310).

Other Embodiment

The above-described embodiments relate to the serial-type printing apparatus. However, the application of the present invention is not limited to these embodiments. The present invention is also applicable to a full line-type printing apparatus in which a print medium is conveyed with respect to a fixed print head. In this case, the moving speed of the print head is a relative moving speed that is a speed of the print medium conveyed with respect to the print head.

This application claims the benefit of Japanese Patent Application No. 2013-127786, filed Jun. 18, 2013, which is hereby incorporated by reference herein in its entirety.