JP2013146860A - Printing apparatus, ink amount specifying method, and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To specify an amount of ink that causes detection results of a sensor to become unstable.SOLUTION: A printing apparatus includes a head for ejecting ink, a sensor and a controller. The controller includes: forming a gradation pattern to vary an amount of ink for each unit area from a first ink amount to a second ink amount; moving the sensor along a belt-like gradation pattern; making the sensor detect a density of the gradation pattern; making the sensor acquire a correspondence between a position of the sensor and the density; and specifying a third ink amount being the third ink amount more than the first ink amount and less than the second ink amount, and unstabilizing detection results of the sensor on the basis of the correspondence.

Description

  The present invention relates to a printing apparatus, an ink amount specifying method, and a program.

  Conventionally, in a printing apparatus that prints an image on a medium (paper, cloth, etc.), a test pattern is printed on the medium, and density correction processing (so-called color calibration) is performed according to the result of reading the test pattern.

  For example, Patent Document 1 describes that a large number of patches having different gradation values are printed on a test pattern, and the gradation value of an image to be printed is corrected according to the read density of each patch.

JP 2008-302521 A

When performing the density correction process, a large number of patches having different gradation values are printed. However, in a patch having a high density, the amount of ink per unit area is large, so that the medium that has absorbed the ink may be deformed. If the medium is deformed, the detection result of the sensor for detecting the density tends to become unstable, and if such a detection result is used, there is a possibility that appropriate density correction processing cannot be performed.
Accordingly, an object of the present invention is to specify an ink amount at which the detection result of the sensor becomes unstable.

The main invention for achieving the above object is:
A head for ejecting ink;
A sensor,
A controller,
A printing apparatus comprising:
The controller is
Forming a gradation pattern in which the ink amount per unit area changes from the first ink amount to the second ink amount;
Move the sensor along the gradation pattern, let the sensor detect the density of the gradation pattern, get the correspondence between the position of the sensor and the density,
Based on the correspondence relationship, the third ink amount that is larger than the first ink amount and smaller than the second ink amount, and the detection result of the sensor is unstable is specified.
It is a printing device.

  Other features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

1 is a block diagram of the overall configuration of a printing apparatus 1. FIG. FIG. 2A is a schematic diagram of the overall configuration of the printing apparatus 1, and FIG. 2B is a cross-sectional view of the overall configuration of the printing apparatus 1. FIG. 4 is an explanatory diagram showing the arrangement of nozzles on the lower surface of a head 41. 4A and 4B are explanatory diagrams of operations at the time of printing and detection of the density correction processing of the reference example. It is explanatory drawing of correction | amendment of a certain gradation value X. FIG. 6A and 6B are explanatory diagrams of a state in which the medium is deformed so as to wave, and FIGS. 6C and 6D are explanatory diagrams of light reception by the optical sensor 54. FIG. FIG. 7A is an explanatory diagram of a comparative example, and FIG. 7B is an explanatory diagram of the density detection result of FIG. 7A. It is a flowchart of unstable ink amount specification processing. FIG. 9A is an external view of a strip-shaped gradation pattern for specifying the amount of unstable ink, FIG. 9B is an explanatory diagram of the configuration of the strip-shaped gradation pattern, and FIG. 9C is a diagram when detecting the density of the gradation pattern. FIG. 9D is an explanatory diagram when an intermediate gradation value pattern is detected, and FIG. 9E is another explanatory diagram when an intermediate gradation value pattern is detected. It is a graph of the detected value of the detected density | concentration. It is a figure which shows the inclination of a moving average when the number of object points of a moving average is set to "10". It is a figure which shows the inclination of a moving average when the number of object points of a moving average is set to "30". It is a figure which shows the inclination of a moving average when the number of object points of a moving average is set to "50". It is a figure which shows the inclination of a moving average when the number of object points of a moving average is set to "70". It is a graph which shows the amount of unstable ink. It is explanatory drawing at the time of performing linear approximation about the detected value of the area | region of an unstable ink amount. It is explanatory drawing at the time of performing polynomial approximation about the detected value of the area | region of an unstable ink amount.

At least the following matters will become clear from the description of the present specification and the accompanying drawings. That is,
A head for ejecting ink;
A sensor,
A controller,
A printing apparatus comprising:
The controller is
Forming a gradation pattern in which the ink amount per unit area changes from the first ink amount to the second ink amount;
Move the sensor along the gradation pattern, let the sensor detect the density of the gradation pattern, get the correspondence between the position of the sensor and the density,
Based on the correspondence relationship, the third ink amount that is larger than the first ink amount and smaller than the second ink amount, and the detection result of the sensor is unstable is specified.
It is a printing device.
In this way, it is possible to specify the ink amount at which the detection result of the sensor becomes unstable. When density correction is performed using the density acquired by the sensor, various processes can be applied in an ink amount region where the detection result of the sensor becomes unstable.

In the printing apparatus, after the third ink amount is specified, when forming a correction pattern according to the gradation value for the density correction process,
Forming the correction pattern having a gradation value in which the ink amount per unit area does not exceed the third ink amount;
The correction pattern having a gradation value in which the ink amount per unit area exceeds the third ink amount is not formed.
By doing so, it is possible not to perform density correction using the correction pattern for an ink amount in which the detection result of the sensor becomes unstable. Then, it is possible to perform density correction that excludes measurement values with low reliability.

In the printing apparatus, after the third ink amount is specified, when the sensor detects a correction pattern corresponding to a gradation value for density correction processing,
Detecting the correction pattern having a gradation value in which the ink amount per unit area does not exceed the third ink amount using the sensor;
The correction pattern having a gradation value in which the ink amount per unit area exceeds the third ink amount may not be detected by the sensor.
By doing so, it is possible not to perform density correction using the correction pattern for an ink amount in which the detection result of the sensor becomes unstable. Then, it is possible to perform density correction that excludes measurement values with low reliability.

In the printing apparatus, after the third ink amount is specified, a correction pattern corresponding to a gradation value is detected by the sensor for density correction processing, and the level is determined based on a detection result of the sensor. When calculating the correction value for correcting the key value,
The correction value is calculated by a predetermined calculation method based on the detection result of the correction pattern having a gradation value in which the ink amount per unit area does not exceed the third ink amount,
The correction value is calculated by a calculation method different from the predetermined calculation method based on the detection result of the correction pattern having a gradation value in which the ink amount per unit area exceeds the third ink amount. Also good.
In this way, a correction value is obtained using a predetermined calculation method for an ink amount with a stable sensor detection result, while a calculation method different from the predetermined calculation method for an ink amount with an unstable sensor detection result. The correction value can be obtained with An appropriate correction value can be obtained for any ink amount.

Further, in the printing apparatus, the third ink amount is specified by forming the gradation pattern using black ink,
The amount of ink for which the detection result of the sensor becomes unstable in other colors of ink may be the third ink amount specified using the black ink.
By doing so, the ink whose detection result of the sensor becomes unstable is determined for the third ink amount specified by the black ink, even for the ink of other colors whose ink detection result becomes unstable. Can be applied as a quantity.

In addition, at least the following matters will become clear from the description of the present specification and the accompanying drawings. That is,
Forming a gradation pattern in which the ink amount per unit area changes from the first ink amount to the second ink amount;
Moving the sensor along the gradation pattern, detecting the density of the gradation pattern with the sensor, and obtaining the correspondence between the position of the sensor and the density;
Identifying the third ink amount that is greater than the first ink amount and less than the second ink amount based on the correspondence relationship, and for which the detection result of the sensor is unstable. An ink amount specifying method characterized by comprising:
In this way, it is possible to specify the ink amount at which the detection result of the sensor becomes unstable. When density correction is performed using the density acquired by the sensor, various processes can be applied in an ink amount region where the detection result of the sensor becomes unstable.

In addition, at least the following matters will become clear from the description of the present specification and the accompanying drawings. That is,
In a printing control apparatus that controls a printing apparatus that includes a head that ejects ink and a sensor that detects density,
A function of forming a gradation pattern in which the ink amount per unit area changes from the first ink amount to the second ink amount;
A function of moving a sensor along the gradation pattern, causing the sensor to detect the density of the gradation pattern, and acquiring a correspondence between the position of the sensor and the density;
A function of specifying the third ink amount that is greater than the first ink amount and less than the second ink amount, and on which the detection result of the sensor is unstable, based on the correspondence relationship. Is a program characterized by realizing the above.
In this way, it is possible to specify the ink amount at which the detection result of the sensor becomes unstable. When density correction is performed using the density acquired by the sensor, various processes can be applied in an ink amount region where the detection result of the sensor becomes unstable.

=== Explanation of basic contents ===
Before describing the color calibration of the present embodiment, the basic contents (apparatus configuration, normal color calibration, etc.) will be described.

<Overall configuration>
FIG. 1 is a block diagram of the overall configuration of the printing apparatus 1. FIG. 2A is a schematic diagram of the overall configuration of the printing apparatus 1. FIG. 2B is a cross-sectional view of the overall configuration of the printing apparatus 1. Hereinafter, a basic configuration of the printing apparatus will be described.

  The printing apparatus 1 includes a transport unit 20, a carriage unit 30, a head unit 40, a detector group 50, and a controller 60. The printing apparatus 1 that has received print data from the computer 110 (printing control apparatus), which is an external apparatus, controls each unit (conveyance unit 20, carriage unit 30, and head unit 40) by the controller 60. The situation in the printing apparatus 1 is monitored by a detector group 50, and the detector group 50 outputs a detection result to the controller 60.

  The transport unit 20 is for transporting a medium (for example, paper S) in a first direction (hereinafter referred to as a transport direction). The transport unit 20 includes a paper feed roller 21, a transport motor 22 (also referred to as a PF motor), a transport roller 23, a platen 24, and a paper discharge roller 25. The paper feed roller 21 is a roller for feeding the paper S inserted into the paper insertion slot into the printing apparatus. The transport roller 23 is a roller that transports the paper S fed by the paper feed roller 21 to a printable area, and is driven by the transport motor 22. The platen 24 supports the paper S being printed. The paper discharge roller 25 is a roller for discharging the paper S to the outside of the printing apparatus, and is provided on the downstream side in the transport direction with respect to the printable area.

  The carriage unit 30 is for moving (also referred to as “scanning”) the head in a second direction (hereinafter referred to as a moving direction). The carriage unit 30 includes a carriage 31 and a carriage motor 32 (also referred to as a CR motor). The carriage 31 can reciprocate in the moving direction and is driven by a carriage motor 32. Further, the carriage 31 detachably holds an ink cartridge that stores ink.

  The head unit 40 is for ejecting ink onto paper. The head unit 40 includes a head 41 having a plurality of nozzles. Since the head 41 is provided on the carriage 31, when the carriage 31 moves in the movement direction, the head 41 also moves in the movement direction. Then, a dot line (raster line) along the moving direction is formed on the paper S by intermittently ejecting ink while the head 41 is moving in the moving direction.

  The detector group 50 includes a linear encoder 51, a rotary encoder 52, a paper detection sensor 53, an optical sensor 54, and the like. The linear encoder 51 detects the position of the carriage 31 in the moving direction. The rotary encoder 52 detects the rotation amount of the transport roller 23. The paper detection sensor 53 detects the position of the leading edge of the paper S being fed. The optical sensor 54 detects a pattern printed on the paper S by a light source and a light receiving sensor attached to the carriage 31.

  The controller 60 is a control unit (control unit) for controlling the printing apparatus. The controller 60 includes an interface unit 61, a CPU 62, a memory 63, and a unit control circuit 64. The interface unit 61 transmits and receives data between the computer 110 that is an external device and the printing apparatus 1. The CPU 62 is an arithmetic processing unit for controlling the entire printing apparatus. The memory 63 is for securing an area for storing a program of the CPU 62, a work area, and the like, and includes storage elements such as a RAM and an EEPROM. The CPU 62 controls each unit via the unit control circuit 64 in accordance with a program stored in the memory 63.

  FIG. 3 is an explanatory diagram showing the arrangement of nozzles on the lower surface of the head 41. On the lower surface of the head 41, a black ink nozzle group K, a cyan ink nozzle group C, a magenta ink nozzle group M, and a yellow ink nozzle group Y are formed. Each nozzle group includes a plurality of nozzles (here, n = 180) which are ejection openings for ejecting ink of each color.

  The plurality of nozzles of each nozzle group are aligned at a constant interval (nozzle pitch: k · D) along the transport direction. Here, D is the minimum dot pitch in the carrying direction (that is, the interval at the highest resolution of dots formed on the paper S). K is an integer of 1 or more. For example, when the nozzle pitch is 180 dpi (1/180 inch) and the dot pitch in the transport direction is 720 dpi (1/720 inch), k = 4.

  The nozzles of each nozzle group are assigned a smaller number as the nozzles on the downstream side (# 1 to # 180). That is, the nozzle # 1 is located downstream of the nozzle # 180 in the transport direction.

  The position of the optical sensor 54 in the transport direction is downstream in the transport direction with respect to the nozzle # 1 located on the most downstream side.

<Density correction processing (color calibration)>
A printer driver (program) for controlling the printing apparatus 1 is installed in the computer 110 (see FIG. 1). Then, the computer 110 causes the printing apparatus 1 to execute density correction processing described below according to a density correction program incorporated in the printer driver.

  4A and 4B are explanatory diagrams of operations at the time of printing and detection of the density correction processing of the reference example. Here, the black density correction process will be described.

  As shown in FIG. 4A, the printing apparatus 1 that has received an instruction from the computer 110 ejects black ink from the black ink nozzle group K of the head 41 to form a plurality of (here, nine) square correction patterns ( A so-called patch pattern) is formed on the medium. Each correction pattern is formed by applying a different amount of ink per unit area according to a specific gradation value. However, an ideal ink amount (ink amount per unit area) is not applied to each correction pattern due to the influence of manufacturing errors and the like. For example, when the ink droplets ejected from the head 41 are small, the amount of ink per unit area is reduced, so that a light correction pattern is formed. That is, at this stage, the ideal density (the target density of the specific gradation value) and the actual density are different.

  Next, as illustrated in FIG. 4B, the printing apparatus 1 detects the density of each correction pattern using the optical sensor 54, and transmits the output value of the optical sensor 54 to the computer 110. The computer 110 normalizes the output value of the optical sensor 54 received from the printing apparatus 1 as follows.

P (X1) = {Praw (X1) − Praw (255)} / {Praw (0) − Praw (255)}
Praw (X1) in the equation is an output value (raw data) of the optical sensor 54 when a correction pattern having the specific gradation value X1 is detected.
Praw (255) is an output value of the optical sensor when a correction pattern with a specific gradation value of 255 (so-called solid pattern) is detected.
Praw (0) is an output value of the optical sensor of the correction pattern having a specific gradation value of 0.
P (X1) is a normalized detection value.

  In this embodiment, the optical sensor outputs a low value when the density is high (dark). A high value is output when the density is low (bright).

  Here, normalization is performed so that the detection value of the pattern with the lightest gradation value 0 is “1” and the detection value of the pattern with the darkest gradation value 255 is “0”. For this reason, P (X1) is a value between 0 and 1. However, normalization may be performed based on another expression (for example, P (X1) = Praw (X1) / Praw (255)). Further, normalization may be performed based on other gradation values instead of using the gradation value 0 or the gradation value 255 as a reference.

  FIG. 5 is an explanatory diagram of correction of a certain gradation value X. The horizontal axis of the graph indicates the gradation value on the image data. The vertical axis of the graph indicates the density of the image printed on the medium. T (X) in the figure indicates a graph of the target density with respect to the gradation value X. Black circles in the figure indicate detection values (normalized detection values) of nine correction patterns. P (X) in the figure shows a graph of actual density obtained from the detected value of the correction pattern. P (X) is obtained, for example, by linearly interpolating between two black circles.

  In the case as shown in the figure, if an image is printed as it is according to the gradation value without correcting the gradation value, it is printed relatively lightly. For example, when an image is printed according to the gradation value A, it is printed with a density P (A) that is lighter than the target density T (A). Therefore, for example, when printing an image having a gradation value A, the gradation value A is corrected to the gradation value A ′ (= A + α) by the correction value α, and the image is displayed according to the corrected gradation value A ′. If printing is performed, an image having an initial target density T (A) can be obtained. As described above, the computer 110 calculates the correction value for each gradation value based on the detection value of the correction pattern. The calculated correction value is stored in the storage device of the computer 110.

  When the image is printed after obtaining the correction value, the computer 110 corrects the gradation value (for example, gradation value A) of each pixel constituting the image with the correction value (for example, correction value α), and after the correction. The image is printed based on the tone value (for example, tone value A ′). Thereby, the density of the image can be corrected.

  In the above description, the computer 110 performs a process for calculating the correction value and a process for correcting the gradation value of the image based on the correction value. However, the printing apparatus may perform a process of calculating a correction value and a process of correcting the gradation value of the image based on the correction value.

<Causes of unstable optical sensor detection>
In a region where the amount of ink per unit area is large, the output value (detection result) of the optical sensor 54 may become unstable. One of the causes is a phenomenon (cockling) in which the medium is deformed so as to wave.

6A and 6B are explanatory diagrams showing a state in which the medium is deformed so as to wave. FIG. 6A is a top view of the medium. 6B is a cross-sectional view taken along the line AA in FIG. 6A.
As shown in FIG. 6A, a belt-like filling pattern is printed on the medium. The filled pattern is formed by applying ink so that the amount of ink per unit area increases. When the medium absorbs ink and expands and contracts, the medium is deformed so as to wave as shown in FIG. 6B.

  6C and 6D are explanatory diagrams of light reception by the optical sensor 54. The optical sensor 54 irradiates the medium with light from the light source, receives the light reflected by the medium with the light receiving sensor, and outputs a signal corresponding to the amount of received light. As shown in FIG. 6C, if the distance between the optical sensor 54 and the medium is close, the optical path is shortened and the intensity of reflected light increases. Conversely, if the distance between the optical sensor 54 and the medium is increased, the optical path is It becomes longer and the intensity of reflected light decreases. As a result, when the medium is deformed so as to wave as shown in FIGS. 6B and 6C, the distance between the optical sensor 54 and the medium fluctuates, so the output value of the optical sensor 54 is considered to be unstable.

  Further, the output value of the optical sensor 54 becomes unstable not only because the distance between the optical sensor 54 and the medium fluctuates, but also due to local inclination of the medium as described below. Conceivable.

  Since the optical sensor 54 is intended to detect the density of the pattern, a light source and a light receiving sensor are arranged so as to detect diffuse reflected light instead of regular reflected light from the medium (left side in FIG. 6D). reference). For example, the light source is disposed so as to irradiate light from an angle of 45 degrees with respect to the surface of the medium, and the light receiving sensor is disposed above the light irradiation spot. However, when the medium has a local inclination, for example, when the medium is deformed so as to wave, the light receiving sensor may receive specularly reflected light (see the right side of FIG. 6D). In such a case, since the amount of light received by the light receiving sensor becomes high, the output value of the optical sensor 54 becomes unstable. In particular, when the optical sensor 54 detects a pattern with a high density (a pattern with a large amount of ink per unit area), the amount of diffused light is small, and therefore the change in the amount of light received when regular reflected light is received. Becomes abrupt and the output value of the optical sensor 54 tends to become unstable.

  As described above, in an area where the amount of ink per unit area is large, the output value of the optical sensor 54 becomes unstable due to a synergistic effect.

  If the amount of ink per unit area exceeds a certain value, it is considered that the output value of the optical sensor 54 becomes unstable. Therefore, in the following description, the ink amount (ink amount per unit area) in which the output value of the optical sensor 54 becomes unstable is referred to as “unstable ink amount”.

=== Specifying Method of Unstable Ink Amount ===
Next, a method for specifying the unstable ink amount will be described. First, a comparative example will be described, and then this embodiment will be described.

<Comparative example>
FIG. 7A is an explanatory diagram of a comparative example. In the comparative example, first, the printing apparatus 1 forms a plurality of belt-like patterns having a predetermined gradation value. The reason why the pattern is formed in a strip shape is that it is necessary to widen a region where the amount of ink is applied in order to deform the medium, and if the pattern is small, the medium does not deform.

  Here, four patterns of tone values (32, 96, 160, 224) are formed on the medium. As already described, a pattern having a larger amount of ink per unit area is deformed so that the medium is waved. Here, it is considered that the belt-like pattern having the gradation value 224 is most deformed.

  After forming the belt-like pattern, the printing apparatus 1 detects the density of each belt-like pattern using the optical sensor 54. FIG. 7B is an explanatory diagram of the density detection result of FIG. 7A.

  Originally, since the belt-like pattern has a constant density, the output value of the optical sensor 54 should be a constant value regardless of the position. However, in practice, the output of the optical sensor 54 changes depending on the position as a result of the medium absorbing and contracting the ink and deforming so that the medium undulates. In particular, in a pattern with a large amount of ink per unit area, the change in the output value of the optical sensor 54 becomes large.

  Therefore, in the comparative example, the amount of change in the output value of the optical sensor 54 when the belt-like pattern of each gradation value is detected is obtained. For example, as the amount of change in the output value of the optical sensor 54, the difference between the maximum value and the minimum value of the output value of the optical sensor 54 is obtained. In the comparative example, if the change amount of the output value of the optical sensor 54 is larger than a predetermined value, the ink amount per unit area corresponding to the strip pattern is set as the unstable ink amount.

  In the above comparative example, since it is necessary to form a plurality of belt-like patterns, the amount of ink consumption increases. Further, in FIG. 7A, since the unstable ink amount is specified in four stages, the accuracy of specifying the unstable ink amount becomes rough. If it is attempted to increase the accuracy of specifying the unstable ink amount in the comparative example, it is necessary to form a large number of belt-like patterns on the medium, and thus the amount of ink consumption becomes extremely large.

<This embodiment>
FIG. 8 is a flowchart of the unstable ink amount specifying process.
In the unstable ink amount specifying process, a belt-like gradation pattern is formed (S102).

  FIG. 9A is an external view of a belt-like gradation pattern for specifying an unstable ink amount. FIG. 9B is an explanatory diagram of a configuration of a belt-like gradation pattern. The numbers in parentheses in the figure indicate the black tone values.

  The belt-like gradation pattern is formed by arranging 256 rectangular patterns with gradation values 0 to 255 in the moving direction so that the gradation values continuously change. Thereby, a band-like gradation pattern is formed in which the ink amount per unit area gradually changes from the minimum amount to the maximum amount.

  Two rectangular patterns (patterns with gradation values 0 and 255) located at both ends are square. These two square patterns are larger in both the width in the moving direction and the width in the transport direction than the diameter (spot diameter) of the detection region of the optical sensor 54.

  A large number of rectangular patterns (patterns having gradation values of 1 to 254, which are intermediate gradation values) at the center have a width in the transport direction larger than the spot diameter of the optical sensor 54, but the width in the movement direction is that of the optical sensor 54. Smaller than the spot diameter. For this reason, a rectangular pattern corresponding to many gradation values can be formed in a narrow print region.

Next, the density of the belt-like gradation pattern is detected (S104).
FIG. 9C is an explanatory diagram of the operation when detecting the density of the gradation pattern. FIG. 9D is an explanatory diagram when detecting a pattern of intermediate gradation values.

  In the present embodiment, when the optical sensor 54 detects the pattern of the intermediate gradation value A, the detection area of the optical sensor (shown by a broken line in FIG. 9D) protrudes from the pattern. However, as shown in FIG. 9D, a gradation value A-1 pattern is formed adjacent to the left side of the intermediate gradation value A pattern, and a gradation value A + 1 pattern is adjacent to the right side. Is formed. That is, a light pattern is formed adjacent to the left side, and a dark pattern is formed adjacent to the right side. For this reason, even if the detection area of the optical sensor protrudes from the pattern of the intermediate gradation value A, the optical sensor 54 can output a value substantially corresponding to the density of the pattern of the intermediate gradation value A.

  FIG. 9E is another explanatory diagram when detecting a pattern of intermediate gradation values. Here, the center position of the detection area of the optical sensor 54 is located at the boundary between the pattern of the intermediate gradation value A and the pattern of the intermediate gradation value A + 1. At this position, the amount of ink per unit area in the detection area of the optical sensor 54 is larger than the amount of ink per unit area of the pattern of the intermediate gradation value A, and per unit area of the pattern of the intermediate gradation value A + 1. Less than the amount of ink. That is, the ink amount per unit area in the detection region of the optical sensor 54 at this position is an ink amount corresponding to the intermediate gradation value A + 0.5.

  As described above, in this embodiment, the density with respect to the gradation value is detected more finely than 256 levels. In other words, in the present embodiment, the optical sensor 54 detects the density of the gradation pattern at an interval shorter than the width of the rectangular pattern in the moving direction.

  FIG. 10 is a graph of the detected value of the detected density. In FIG. 10, the horizontal axis is the ink amount, and the vertical axis is the detection value. The amount of ink on the horizontal axis is shown corresponding to 256 gradations. The detected value on the vertical axis indicates a lower value as the concentration is higher. Referring to the figure, the detected value has high-order fluctuations.

Next, the gradient of the moving average of the detection values of the optical sensor 54 is obtained (S106).
FIG. 11 is a diagram illustrating the gradient of the moving average when the number of target points of the moving average is “10”. FIG. 12 is a diagram illustrating the gradient of the moving average when the number of target points of the moving average is “30”. FIG. 13 is a diagram illustrating the gradient of the moving average when the number of target points of the moving average is “50”. FIG. 14 is a diagram illustrating the gradient of the moving average when the number of target points of the moving average is “70”. In these drawings, the horizontal axis indicates the gradation value associated with the ink amount. The vertical axis represents the slope of the detected value.

  As the number of moving average points increases from “10” to “70”, the gradient of the moving average gradually becomes smoother. In the case where the number of points is “10”, the fluctuation of the inclination is still remarkable. On the other hand, when the number of points exceeds “30”, the fluctuation of the inclination is settled. Therefore, it is desirable to employ a point having 30 or more points, but in the present embodiment, description will be made with reference to a graph (FIG. 14) when the number of points with the most stable fluctuation is “70”. . An appropriate number of moving average points can be adopted according to the number of detected values sampled.

  Next, an ink amount (gradation value) at which the change of the moving average line is not simply reduced is specified (S108). The specified gradation value is set as the unstable ink amount. Note that since the detection value (output value) of the optical sensor is unstable even at a gradation value higher than the specified gradation value, a gradation value equal to or higher than the specified gradation value is also an unstable ink amount.

  Referring to FIG. 14, the ink amount is “77 and the inclination is“ 0 ”. In this embodiment, when the slope first becomes “0”, it is assumed that the change of the moving average line is not simply reduced, and the ink amount thereafter is set as an unstable ink amount.

  In the gradation value higher than the specified gradation value, the inclination value is often a negative value, but the value fluctuates gently. The negative value is often because the ink amount is gradually increased and the density is increased. On the other hand, there is a gradual fluctuation in such a tendency, as described above, after the ink amount per unit area exceeds a certain value, the output value (detected value) of the optical sensor 54 This is because it became unstable.

  FIG. 15 is a graph showing the unstable ink amount. As described above, the unstable ink amount has a corresponding gradation value of “77” or more. In FIG. 15, the detection value of the gradation value corresponding to the unstable ink amount is shown in black, and the detection value of the gradation value that is not the unstable ink amount is shown in gray.

  In this way, when the specification of the gradation value corresponding to the unstable ink amount is completed, the unstable ink amount specifying process is ended. As described above, when the unstable ink amount can be specified, the following processing can be performed using the specified unstable ink amount.

<Process after specifying ink amount (1)>
As described above, the detection value of the optical sensor 54 is unstable at the gradation value corresponding to the unstable ink amount. For this reason, if density correction processing is performed based on these detection values, density correction may not be performed appropriately.

  Therefore, in the first method as the process after specifying the ink amount, when forming the density correction pattern (patch pattern (FIG. 4A)), the gradation value corresponding to the unstable ink amount with low reliability, that is, It is possible to prevent the pattern from being formed for the portion of the ink amount corresponding to the gradation value of 77 to 255. In this case, specifically, patches with gradation values 0 to 64 are formed in the correction pattern of FIG. 4A, but patches with gradation values 96 to 255 are not formed. Then, in the range of the gradation value 96 to the gradation value 255 having low reliability, the density correction process can be omitted. By doing so, the amount of ink to be used can be reduced.

<Process after specifying ink amount (2)>
As a second method as a process after the ink amount is specified, a density correction pattern (patch pattern (FIG. 4A)) is formed, while a gradation value pattern corresponding to an unstable ink amount with low reliability is formed. The optical sensor 54 may not read. In this case, specifically, in the correction pattern of FIG. 4A, patches with gradation values 0 to 64 are read, but patches with gradation values 96 to 255 are not read. Then, in the range of the gradation value from 96 to gradation value 255, it is possible not to perform density correction processing.

<Process after specifying ink amount (3)>
As a third method as the process after the ink amount is specified, the correction value calculation method in the density correction process can be changed. Specifically, the density correction process can be performed using the gradation pattern shown in FIG. 9A without using the density correction pattern shown in FIG. 4A. Further, for the detection value corresponding to the gradation value of the unstable ink amount, a pseudo detection value is obtained based on the actual detection value, and the correction value calculation method is changed by adopting these detection values. Processing can be employed.

  It can be considered that the density correction processing using the gradation pattern shown in FIG. 9A is a finer gradation value step of the correction pattern patch shown in FIG. 4A. Therefore, when the gradation pattern shown in FIG. 9A is read, the detection value P (X) subdivided in FIG. 5 can be obtained.

  FIG. 16 is an explanatory diagram in a case where linear approximation is performed on the detection value of the gradation value corresponding to the unstable ink amount. In FIG. 16, the detection value of the optical sensor 54 is shown in gray, and the approximate straight line is shown in black. The approximate curve is shown as a straight line having a negative slope when the gradation value is 77 to 255. As described above, for the detection value corresponding to the gradation value of the unstable ink amount, a value obtained by an approximate line can be used as an alternative to the actual detection value.

  Such an approximate curve can be obtained by applying the most square method to the detected value of the unstable ink amount. By doing so, it is possible to perform appropriate density correction processing based on a pseudo detection value that eliminates fluctuations in the detection value due to the amount of unstable ink.

  Note that the method of obtaining the approximate straight line is not limited to using the least square method. For example, linear approximation may be performed using the slope of the detected value immediately before the unstable ink amount is reached (here, the gradation value is “76”). For example, a straight line that extends from the detection value at the gradation value 76 immediately before the unstable ink amount to the gradient of the detection value at the gradation value 76 may be used as the approximate line.

  In this way, the unstable detection value is not used, but on the other hand, density correction is appropriately performed using a pseudo detection value obtained from an approximate straight line that is obtained based on the actual detection value and does not vary. It can be performed.

  FIG. 17 is an explanatory diagram when the polynomial approximation is performed on the detected value of the unstable ink amount. Although the approximate straight line is used in FIG. 16 described above, the present invention is not limited to this, and a polynomial approximate curve can also be used. Also at this time, a polynomial approximate curve of an arbitrary order can be obtained by the least square method.

<Process after specifying ink amount (4)>
In general, the dynamic range of black ink detection values is wider than that of yellow ink detection values. This is because yellow ink has a smaller difference in brightness due to gradation values than black ink. For this reason, in yellow ink, since the width of the detection value is small, it may be difficult to specify the gradation value that is the amount of unstable ink.

  Therefore, as a fourth method as a process after specifying the ink amount, the gradation value that becomes the unstable ink amount is specified using the black ink, and the gradation value specified by the black ink is also applied to the yellow ink. It is good.

  That is, for ink that is difficult to specify the gradation value that is the unstable ink amount, the gradation value that is specified by using the ink that easily specifies the gradation value that is the unstable ink amount is reflected in the inks of other colors. You can also.

=== Others ===
The above embodiment mainly describes a printing apparatus provided with an optical sensor, but includes disclosure of a printing method, a color calibration method, a test pattern printing method, a test pattern, and the like. Needless to say.

  The above-described embodiments are for facilitating understanding of the present invention, and are not intended to limit the present invention. The present invention can be changed and improved without departing from the gist thereof, and it is needless to say that the present invention includes equivalents thereof.

<About printing devices>
In the printing apparatus 1 described above, the head 41 is movable in the movement direction, and a process of forming dots by intermittently ejecting ink while the head is moving in the movement direction, and the conveyance unit 20 in the conveyance direction. It was a type (so-called serial type printing apparatus) that performs printing by alternately repeating the conveying process. However, the configuration of the printing apparatus is not limited to this. For example, the head may be a fixed type (so-called line type printing apparatus) that performs printing by intermittently discharging ink to form dots on the medium while the medium is being transported.

1 printing device,
20 transport unit, 21 paper feed roller, 22 transport motor (PF motor),
23 transport roller, 24 platen, 25 discharge roller,
30 Carriage unit, 31 Carriage,
32 Carriage motor (CR motor),
40 head units, 41 heads,
50 detector groups, 51 linear encoder, 52 rotary encoder,
53 Paper detection sensor, 54 Optical sensor,
60 controller, 61 interface unit, 62 CPU,
63 memory, 64 unit control circuit

Claims (7)

A head for ejecting ink;
A sensor,
A controller,
A printing apparatus comprising:
The controller is
Forming a gradation pattern in which the ink amount per unit area changes from the first ink amount to the second ink amount;
Move the sensor along the gradation pattern, let the sensor detect the density of the gradation pattern, get the correspondence between the position of the sensor and the density,
Based on the correspondence relationship, the third ink amount that is larger than the first ink amount and smaller than the second ink amount, and the detection result of the sensor is unstable is specified.
Printing device. The printing apparatus according to claim 1,
When the correction pattern corresponding to the gradation value is formed for the density correction process after the third ink amount is specified,
Forming the correction pattern having a gradation value in which the ink amount per unit area does not exceed the third ink amount;
The printing apparatus is characterized in that the correction pattern having a gradation value in which the ink amount per unit area exceeds the third ink amount is not formed. The printing apparatus according to claim 1,
After the third ink amount is specified, when the sensor detects a correction pattern corresponding to a gradation value for density correction processing,
Detecting the correction pattern having a gradation value in which the ink amount per unit area does not exceed the third ink amount using the sensor;
The printing apparatus according to claim 1, wherein the correction pattern having a gradation value in which the ink amount per unit area exceeds the third ink amount is not detected by the sensor. The printing apparatus according to claim 1,
After the third ink amount is specified, a correction pattern corresponding to a gradation value is detected by the sensor for density correction processing, and the gradation value is corrected based on the detection result of the sensor. When calculating the correction value,
The correction value is calculated by a predetermined calculation method based on the detection result of the correction pattern having a gradation value in which the ink amount per unit area does not exceed the third ink amount,
The correction value is calculated by a calculation method different from the predetermined calculation method based on the detection result of the correction pattern having a gradation value in which the ink amount per unit area exceeds the third ink amount. Characteristic printing device. The printing apparatus according to claim 1,
Identifying the third ink amount by forming the gradation pattern using black ink;
A printing apparatus characterized in that an ink amount in which the detection result of the sensor becomes unstable in other color inks is set as the third ink amount specified by using the black ink. Forming a gradation pattern in which the ink amount per unit area changes from the first ink amount to the second ink amount;
Moving the sensor along the gradation pattern, detecting the density of the gradation pattern with the sensor, and obtaining the correspondence between the position of the sensor and the density;
Identifying the third ink amount that is greater than the first ink amount and less than the second ink amount based on the correspondence relationship, and for which the detection result of the sensor is unstable. An ink amount specifying method comprising: In a printing control apparatus that controls a printing apparatus that includes a head that ejects ink and a sensor that detects density,
A function of forming a gradation pattern in which the ink amount per unit area changes from the first ink amount to the second ink amount;
A function of moving a sensor along the gradation pattern, causing the sensor to detect the density of the gradation pattern, and acquiring a correspondence between the position of the sensor and the density;
A function of specifying the third ink amount that is greater than the first ink amount and less than the second ink amount, and on which the detection result of the sensor is unstable, based on the correspondence relationship. The program characterized by realizing.