JP2010050910A - Image reader, image forming apparatus, method for reading image, and computer program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve accuracy in setting offset levels without increasing a resolution of a DAC and achieve a stable black level. <P>SOLUTION: The image reader includes: a CCD 9; a sample hold circuit 25 for sample-holding an analog image signal from the CCD 9; an ADC 24 for digitizing the analog signal after sample holding; a means for setting a black level clamping period 34, which detects a black level of digital image data; an arithmetic circuit 31 for offset adjustment values, which detects a difference between the detected black level and a preset black level target value; and a DAC 32 for performing feedback processing on an offset adjustment value so that an offset level of the digital image data approaches a fixed level, by comparing the difference with a reference value. The DAC 32 has at least two setting accuracies of an offset adjustment value, and adjusts a read value of a white reference plate to become a preset white level target value by switching the setting accuracy of the offset adjustment value. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image reading apparatus that converts an original image signal into a digital signal and performs analog signal processing, an image forming apparatus such as a copying machine, a facsimile, or a digital multifunction peripheral equipped with the image reading apparatus, the image reading apparatus or the image forming apparatus. The present invention relates to an image reading method executed by an apparatus and a computer program for executing the image reading method by a computer, and more particularly to a technique characterized by black level offset adjustment at the time of image reading.

  The image reader illuminates the document surface with illumination light, guides the reflected light from the document to a photoelectric conversion element such as a CCD, converts it into an analog electrical signal, and converts the analog electrical signal into a digital electrical signal. Used as read data. When scanning the document and reading the image data in this way, correction data is obtained prior to reading the document.

  Document reading is performed by two methods called a sheet-through method and a flatbed method. The sheet-through method is a method in which the original is moved and read while the reading optical system is stopped, and the flat bed method is a method in which the reading optical system is moved and read while the original is stopped. In any case, the reference white plate is read to generate shading correction data, which is stored in the memory. Then, normalization is performed while reading the image data of the document with the shading correction data. By processing in this way, the unevenness of the light amount distribution and the output fluctuation in the image reading apparatus are corrected, and the reading accuracy of the original image is increased.

  At the time of shading correction, processing for detecting an offset of the image data level and subtracting it from the image data is performed. In general, the output signal of the CCD has an offset of about 3 to 6V. Therefore, in order to cancel this offset component, as shown in FIG. 16, the output signal S1 of the CCD is AC-coupled to remove the DC component, and then the DC level is set so that the signal level S2 is suitable for the analog signal processing unit in the subsequent stage. Biased. This DC bias is applied for a certain period of the image signal area (excluding reset noise) of the OPB portion of the CCD output S1, and this DC bias voltage becomes a reference voltage for sample-holding the image signal.

  Further, a fixed offset is added to the analog image signal data after the sample hold so that the AD-converted image data becomes output data having a fixed offset after the sample hold. This addition of offset is performed in order to accurately detect a black level including a noise component and to digitally subtract it during shading correction.

  The period during which the black level is detected is a region where light called OPB (Optical Black) is masked, and the above-described clamping period is also this period. FIG. 17 is a timing chart showing the timing among the line clamp signal, sampling signal (SHD), effective clamp period (OPBCLP), and CCD output signal after AC coupling at this time. As can be seen from FIG. 17, the effective clamp period is a stable area of the black level signal, and this area is in the OPB section (or the empty transfer section).

  However, when the offset of the CCD 9 varies, the offset level of the output image data after AD conversion varies. Therefore, as shown in the block diagram of FIG. 18, the difference between the offset level of the digitally converted image data after ADC and the target offset level is detected, and the image signal after sample hold is canceled so as to cancel the difference. A feedback loop is provided that varies the amount of analog offset added to the data by the DAC. That is, in FIG. 18, the output of the CCD 9 is clamped by the input clamp circuit 26 and sampled and held by the sample and hold circuit 25. The voltage sampled and held by the amplifying unit 30 is amplified and converted into a digital signal by the ADC unit 24. Then, during the effective clamp period (black level clamp period), the difference between the offset level of the image data after ADC digitally converted by the offset adjustment value calculation circuit 31 and the target offset level is detected, and the DAC unit 32 determines the difference. A loop of adding the analog signal to the input signal input clamped by the adder circuit 33 and re-sampling the sample is repeated so that the analog signal is converted into a corresponding analog signal and the difference is canceled. By this feedback loop, the influence on the image data due to the offset fluctuation is reduced. The effective clamp period is set by the black level clamp period setting means 34.

  For example, the inventions disclosed in Patent Documents 1 to 3 are also known as techniques related to such a technique.

  Among these, Patent Document 1 discloses an analog image obtained by converting reflected light from a document into an electric signal by a photoelectric conversion element in order to perform appropriate offset adjustment at high speed with respect to offset fluctuation of the photoelectric conversion element of the image reading unit. Sample hold means for sampling and holding the signal, amplification means for amplifying the analog image signal after sample hold, an AD converter for digitizing the amplified analog image signal and outputting digital image data, and the digital image data Difference detection means for detecting a difference between a black level detection value and a preset black level target value, and an adjustment coefficient selection for selecting one of a plurality of adjustment coefficients according to a result of comparing the difference with a reference value Means and an offset adjustment value based on the selected adjustment coefficient so that the offset level of the digital image data approaches a certain level. Offset adjusting apparatus characterized by comprising a feedback means for fed back processing is disclosed.

  Further, in Patent Document 2, in order to appropriately correct the black level of an analog image signal, the reflected light of the light emitted from the light source to the original is converted into an electric signal by a photoelectric conversion element and read. Analog signal processing means for performing analog signal processing after line clamping and matching to a predetermined clamp potential after AC coupling of image signals, and an image signal subjected to analog signal processing by the analog signal processing means to a predetermined reference voltage A digital conversion means for converting into a digital signal by comparing with a black offset level of an analog image signal generated by a line clamp operation in the analog signal processing means and a preset reference There is disclosed an invention characterized by comprising a deviation amount detecting means for detecting a deviation amount from a clamp voltage.

  Further, Patent Document 3 discloses a photoelectric conversion element that reads an image of a document line by line and outputs an analog image signal, an A / D converter that converts the analog image signal into a digital image signal, and the photoelectric conversion element. An offset level detection circuit for obtaining an average value of the output level of the digital image signal corresponding to the optical black pixel range, and an offset setting unit for providing an offset to the analog image signal before the conversion based on the average value of the output level; The first comparison means for comparing the average value of the output level with a predetermined target value and detecting the difference between the average value of the output level and the predetermined target value, and the comparison result of the first comparison means Obtained by an adjusting means for adjusting the number of detection lines for calculating an average value in the offset level detection circuit based on the first comparison means. A second comparing means for comparing the difference with a first predetermined value set in advance; and a second smaller than the first predetermined value set for the difference obtained by the first comparing means. And a third comparison means for comparing with a predetermined value, wherein the adjustment means has a number of detection lines for calculating an average value in the offset level detection circuit as the difference obtained by the first comparison means is smaller. And the adjustment by the adjusting means is repeated until the second comparing means determines that the difference obtained by the first comparing means is less than the first predetermined value, and the first comparing means The number of detection lines for calculating the average value in the offset level detection circuit is set to the maximum value that can be set when it is determined that the difference obtained in step (b) is less than the first predetermined value. The comparison means of The image reading is characterized in that the difference obtained by the first comparison means is compared with the second predetermined value after the number of detection lines for calculating the average value in the offset level detection circuit is maximized. An apparatus is disclosed.

In addition, Patent Document 4 discloses that the accuracy of black offset correction is improved without increasing the number of conversion bits of the A / D converter during digital conversion or increasing the circuit scale, and the gradation reproducibility of the dark portion. In order to make the image quality excellent, an offset adjustment unit that adds an offset voltage to the analog amount of the image signal read by the photoelectric conversion unit, and an image signal that has been added the offset voltage by the offset adjustment unit An analog amount image signal read by the photoelectric conversion means based on the output signal of the A / D conversion means for converting into a signal and the A / D conversion means obtained at the time of reading a black reference image as a black reference in advance. On the other hand, a first adjusting unit that adds an offset voltage by the offset adjusting unit to obtain a predetermined black offset amount with respect to the output signal of the A / D converting unit; An image reading apparatus and a second adjustment means for fine adjustment of the offset adjustment means at the time of original image reading based on the newly black offset amount read disclosed by adjusting means.
JP 2007-158663 A JP 2006-217205 A Japanese Patent No. 3954258 JP-A-11-55511

  By the way, when the offset level adjustment range is ± 250 mV and the DAC resolution is 10 bits (bits), ideally the DAC set value (10 bits) and the DAC output level (mV) are as shown in FIG. It becomes a linear relationship. However, in practice, it may not be linear as shown in FIG. 20, for example, when the DAC setting value is 512, the offset level (DAC output level) is +10 mV, and the DAC setting value is 513. When the original value of +10 mV is 0 mV, the difference between the offset level of the image data after ADC and the target offset level is detected and fed back so as to cancel the difference. When the set value is increased from 512 to 513 in an attempt to increase, the offset level decreases from 10 mV to 0 mV.

  FIG. 21 is a plot of the average value of OPB when the black level target value of the black offset level is 40 in the sub-scanning direction. The non-linear portion as shown in FIG. 20 is near the offset adjustment level. If there is a line, the black level suddenly deviates from the average level. Such problems cannot be dealt with by the inventions described in Patent Documents 1 to 4.

  Therefore, the problem to be solved by the present invention is to increase the setting accuracy of the offset level without increasing the resolution of the DAC so as to achieve a stable black level.

  In order to solve the above problem, the first means includes a photoelectric conversion element that photoelectrically converts optically read image information, a sample hold means that samples and holds an analog image signal from the photoelectric conversion element, and a sample hold An AD converter that digitizes a later analog signal and outputs digital image data; a detection unit that detects a black level of the digital image data; and a difference between the detected black level and a preset black level target value. A difference detecting unit for detecting, a feedback unit for performing a feedback process on an offset adjustment value according to a result of comparing the difference and a reference value so that an offset level of the digital image data approaches a certain level, and setting of the offset adjustment value Has at least two types of accuracy and cuts the setting accuracy of the offset adjustment value Switching means for obtaining, characterized the white level adjustment means for adjusting so that reading of the white reference plate is preset white level target value, the image reading apparatus having a.

  The second means is characterized in that, in the first means, the setting precision of the at least two kinds of offset adjustment values is two kinds of high precision and low precision.

  The third means is characterized in that, in the second means, the switching means switches the setting accuracy of the initial offset adjustment value to the low accuracy and switches the setting accuracy to a high accuracy during the white level adjustment. To do.

  A fourth means is characterized in that, in the second means, the switching means switches the setting accuracy to a high accuracy before the start of image reading, and switches the setting accuracy to a low accuracy after the end of image reading.

  A fifth means is characterized in that, in the first means, the offset adjustment value is set by a DA converter that converts a digital signal into an analog signal.

  A sixth means is characterized in that, in the fifth means, the DA converter has a resolution of N bits, and a circuit having excellent linearity in the lower M bits of the N bits.

  The seventh means includes amplification means for amplifying the analog signal sampled and held by the sample hold means in the first to sixth means, and the AD converter digitizes the amplified analog signal. It is characterized by that.

  The eighth means is characterized in that, in the seventh means, the white level adjusting means adjusts the gain of the amplifying means and adjusts the read value of the white reference plate to the white level target value.

  A ninth means is characterized in that the image forming apparatus includes the image reading apparatus according to any one of the first to eighth means.

  A tenth means includes a photoelectric conversion element that photoelectrically converts image information, and in an image reading method that reads an image by converting the image information into optical information, a step of sample-holding an analog image signal from the photoelectric conversion element A step of digitizing the analog signal after sample hold and outputting digital image data, a step of detecting a black level of the digital image data, and a difference between the detected black level and a preset black level target value A step of detecting the offset adjustment value according to a result of comparing the difference and the reference value so that the offset level of the digital image data approaches a certain level, and setting accuracy of the offset adjustment value. A step of having at least two types and switching the setting accuracy of the offset adjustment value, and a white reference Wherein the readings is provided with a step of adjusting to be preset white level target value.

  The eleventh means includes a photoelectric conversion element that photoelectrically converts image information, and a computer program for executing control for reading the image by converting the image information into optical information, and the analog from the photoelectric conversion element A procedure for sample-holding the image signal, a procedure for digitizing the analog signal after the sample-hold and outputting digital image data, a procedure for detecting the black level of the digital image data, and the detected black level are preset. A procedure for detecting a difference from a black level target value, a procedure for performing feedback processing on an offset adjustment value according to a result of comparing the difference and a reference value so that the offset level of the digital image data approaches a certain level, At least two types of offset adjustment value setting accuracy and the offset A step of switching a setting accuracy of bets adjustment values, characterized in that the reading of the white reference plate is provided with, and procedures adjusted to be predetermined white level target value.

  In the embodiment described later, the photoelectric conversion element is the CCD 9, the sample hold means is the sample hold circuit 25, the AD converter is the ADC 24, the detection means for detecting the black level is the period setting means 34 and the offset adjustment value calculation circuit. 31, the difference detection circuit is set to the offset adjustment value calculation circuit 31, the feedback circuit is set to reference numeral 29, the switching means is set to the register setting for the AFE 22, the white level adjusting means is set to the feedback circuit 29 including the DAC 32 and PGA 30, and the DA converter is Each corresponds to the DAC 32.

  According to the present invention, the difference between the detected black level and a preset black level target value is detected, and the offset adjustment value corresponding to the result of comparing the difference and the reference value is constant in the offset level of the digital image data. Since the feedback processing is performed so as to approach the level, the setting accuracy of the offset level can be increased and the black level can be stabilized without increasing the DAC resolution.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of an image reading apparatus according to an embodiment of the present invention. In the figure, an image reading device is a device such as a scanner device in a digital copying machine that reads a document image with a CCD and converts the image signal into a digital signal for processing. As shown in FIG. 1, an image reading apparatus 200 includes a scanner body 12, a contact glass 1 on which an original 13 is placed, a halogen lamp 2 for original exposure, and a first reflecting mirror. 3, a second carriage 7 on which the second reflecting mirror 4 and the third reflecting mirror 5 are mounted, and a CCD linear image sensor 9 (hereinafter abbreviated as CCD) that photoelectrically converts incident light. And a lens unit 8 for forming an image of the reading light incident on the CCD 9 from the third mirror 5 and a white reference plate provided on the upper portion of the scanner body 12 for correcting various distortions caused by the reading optical system and the like. 11.

  The CCD 9 is mounted on the sensor board unit 10, and predetermined processing is performed on a signal photoelectrically converted by the CCD 9 on the sensor board unit 10. The sensor board unit 10 on which the first carriage 6, the second carriage 7, the lens unit 8, and the CCD 9 are mounted is installed in the scanner body 12. During document scanning, the first carriage 6 and the second carriage 7 are moved in the sub-scanning direction A at a speed ratio of 2: 1 along a rail (not shown) by a stepping motor (not shown).

  FIG. 2 is a block diagram showing the configuration of the sensor board unit 10. In the figure, the output from the CCD 9 is input to an AFE (Analog Front End) 22 through an AC coupling unit 21. The AFE 22 includes an analog signal processing circuit unit 23 and an A / D conversion unit 24, and the analog signal circuit unit 23 includes a sample hold unit (SH) 25 and a clamp circuit unit 26. In this configuration, analog image data photoelectrically converted by the CCD 9 is input to the AFE 22. In the AFE 22, analog image data is subjected to various image processing such as sample hold (SH) processing and black level correction, and then output to the A / D conversion (circuit) unit 24. The analog image data is converted into 12-bit digital image data, and the converted digital image data is output to the digital processing unit or data transfer unit in the next stage. The CCD drive timing signal and the timing signal to the AFE 22 are generated by the timing control IC 28. Since the CCD 9 has a large capacitive load, the CCD 9 is not normally driven directly by the timing control IC 28 due to problems such as heat generation, and the CCD 9 is driven via the driver 20.

  FIG. 3 is a diagram showing details of the AC coupling unit 21 and the clamp circuit unit 26. In the present embodiment, as described above, the capacitor 21a is connected in series (AC coupling) between the CCD 9 and the AFE 22, and this charging / discharging is used to reduce the influence of an excessive voltage on the AFE 22 input. With this configuration, charging / discharging is performed so as to compensate for the DC level (CCD input offset) shifted by the AC component (change) of the signal, and a so-called clamping operation that follows an arbitrary potential is performed after a certain period of time. Is called. That is, when an excessive voltage (AC component) is input to the AFE 22, the influence can be reduced due to the effect of following an arbitrary potential by charging / discharging the capacitor 21a (returned to the clamp potential from the excessive voltage state).

  The normal clamping operation is performed not on the effective pixel region R1 of the CCD 9 but on the dark output part (OPB) R2, the idle transfer part R3, etc. as shown in the explanatory diagram of the clamped state in FIG. It is performed only for an arbitrary period in one line. This is generally called a line clamp. In the present embodiment, this is performed over the entire line including the effective pixel region. This is referred to herein as a solid clamp. Although it has been described that the clamping is performed over the entire area of one line, strictly speaking, the clamping operation cannot be performed during the period in which the line synchronization signal (the reference signal of the timing control IC 28) is output.

  Thereby, the time of the clamp operation can be effectively increased, and the influence of the excessive voltage can be further reduced. In addition, the clamping operation can usually be switched by setting the register of the AFE 22 from the outside. By solid-clamping this setting as a hardware default of the AFE 22, solid-clamping can be easily realized without requiring any special setting or processing when the system power is turned on.

  When the image reading apparatus is instructed to perform a scanning operation, the lamp is turned on and the document reading operation is started. Prior to the document reading operation, black level offset data to be subtracted from the image data during shading correction is acquired. As this offset data, the data of the OPB portion (optical shading area) R2 of the CCD 9 shown in FIGS. 4 and 12 is generally used.

  In the case of the so-called Book scan operation in which the flat-level scan is performed after the black level offset data has been acquired, the first carriage 6 moves and scans the reference white plate 11 and the document 13, and the shading data and the image data are read. Is called. FIG. 13 is a timing chart showing the output timing of the line synchronization signal (XLSINC), the shading gate signal (SHGT) and the frame gate signal (FGATE) at this time. White level data used for shading correction is generated based on the read data of the reference white plate 11 acquired in the reference white plate reading region R4 in FIG. Then, shading correction of the document reading region R5 is executed using the white level data and the black offset data.

In shading correction,
Dsh (n) = (Dorg (n) −B) / (Dw (n) −B) × 255
Dsh (n): n-th pixel shading corrected data
Dorg (n): nth pixel document data
Dw (n): nth pixel reference white plate reading data
B: Calculation such as black level (OPB portion) read data is performed.

  The black level offset data is acquired before the start of the scanning operation. If the offset level fluctuates during reading of the original, an image defect occurs. For this reason, the response to the offset fluctuation during reading of the document is detected by detecting the difference between the output data of the black level (OPB portion R2 or empty transfer portion R3) and the output target data every scan of the CCD 9 line. Feedback is provided for each line to the offset adjusting means (DAC) with a certain responsiveness so that the black level output data becomes a certain level.

In such an offset tracking method, in order not to perform inadvertent tracking due to noise, it is essential to set an appropriate response (emphasizing stability). As an adjustment amount calculation method, an average value (D (n)) of image data in the BLKSAMPLE period is calculated, a difference from the target level (BLACKTARGET) is detected, and the difference is multiplied by a certain coefficient and fed back. The adjustment amount is DAC (n + 1) = DAC (n) + (BLACKTARGET−D (n)) × 1 / A
However, DAC (n + 1): amount of next line feedback to DAC
DAC (n): Feedback amount to DAC on current line
BLACKTARGET: Black level target level
D (n): Current line black level reading level
1 / A: Correction coefficient
n: A method of calculating by line number is common. The calculation of the feedback adjustment amount is executed by the offset adjustment value calculation circuit 31.

  FIG. 5 is a block diagram showing the black level adjustment feedback circuit 29 in the present embodiment. Since the circuit itself is not different from the conventional example described with reference to FIG. 18, the description of the configuration of each part is omitted. The DAC 32 will be described as an example of 10 bits, and the ADC will be described as an example of 12 bits.

  The DAC 32 in the feedback loop has two modes. One mode is a normal mode that uses the entire 10-bit region (resolution is N bits = 10 bits), and the other mode is a circuit with excellent output linearity. This is a lower bit mode (lower M bit = lower 8 bits) using a lower 8 bit area having a configuration. Switching between normal / lower bits is possible by register settings for the AFE 22. In the lower bit mode, as shown in FIGS. 6 and 7, when changing from the normal mode to the lower bit mode, the immediately preceding DAC setting value is automatically adjusted to be near the center of the lower 8 bits.

  The two modes are properly used in the normal mode (Fig. 6) so that adjustment is possible even when the offset level of the clamp is large during standby for a long time, AGC (white bell adjustment) requiring accuracy, and image reading Since the offset level does not fluctuate greatly because it is a short time and is sufficiently adjustable in the lower bit mode, the lower bit mode (FIG. 7) is used.

  FIG. 8 is a flowchart showing a processing procedure of black level adjustment after the power is turned on.

  In FIG. 8, when the power is turned on (STEP 01), the connection confirmation of the sensor board 10 is performed (STEP 02), and the register initial setting of the timing control IC 28 and the AFE 22 is performed (STEP 03). Thereafter, the lamp 2 is turned on (STEP 04), and the carriage 6 is moved to the center of the white reference plate 11 (STEP 05). After the motor stops, AGC (white level adjustment) is performed (STEP06). Next, a black level confirmation step (STEP07) is executed, the lamp 2 is turned OFF (STEP08), the carriage 6 is moved to HP (home position) (STEP09), and a standby state is entered (STEP10).

  FIG. 9 is a flowchart illustrating a processing procedure from the standby of the image reading apparatus 200 to the execution of scanning. That is, when the scan starts from the standby state (STEP 10) of the first carriage 6 (STEP 11-YES), the first and second carriages 6 and 7 are scanned and the scan is executed (STEP 12). Then, when the scan is completed, a standby state is entered (step S13).

  FIG. 10 is a flowchart showing a processing procedure in a subroutine of AGC processing (white level adjustment) in STEP 06. When the AGC process is started (STEP 21), the DAC 32 is first set to the lower bit mode by register setting for the AFE 22 (STEP 22). Similarly, AGC is automatically started by register setting for the AFE 22. The gain of the amplifier 30 is adjusted so as to reach a preset white target level (STEP 23). After waiting for the AGC processing time (STEP 24), the peak detection value in the main scanning direction stored in the register is read (STEP 25). It is determined whether or not the read peak detection value is within the tolerance range (STEP 26). If it is within the tolerance range, the DAC 32 is returned to the normal mode (STEP 27). If it is outside the tolerance range, the same process is performed twice (STEP 28), and if it is NG for the third time, an error process is performed as an AGC process error (STEP 29).

  Note that the process of returning the DAC 32 to the normal mode may be performed during standby (STEP 10), and may be performed, for example, after the next black level confirmation (STEP 07).

  FIG. 11 is a flowchart showing the operation procedure of the scanning operation of the image reading apparatus.

  In the figure, when the scan operation is started (STEP 40), the DAC 32 is first set to the lower bit mode by register setting for the AFE 22 (STEP 41) as in the AGC process. Thereafter, the lamp 2 is turned on (STEP 42), and the carriage 6 is moved in the direction of arrow A (forward) in FIG. 1 (STEP 43). Next, as described above, the image of the white reference plate is read to generate shading data (STEP 44). Subsequently, image reading is performed. The signal FGATE indicating that the image is being read is asserted (STEPs 45 and 46). After reading the image, the lamp is turned off (STEP 47), and the DAC 32 is returned to the normal mode (STEP 48). Thereafter, the carriage 6 is returned (STEP 49), moved to the home position, and the scanning operation is terminated (STEP 50).

  Thus, during scanning, by setting the DAC 32 to the lower bit mode in which the setting accuracy of the offset level is high, the black level changes almost as intended as shown in FIG. This can be clearly seen by comparing with the black level of FIG. 21 in the conventional example.

  Each processing is executed while a CPU of a control circuit (not shown) reads a program stored in the ROM and does not use a RAM (not shown) as a work area. For the processing, an ASIC can be used instead of the CPU.

  FIG. 15 is a schematic configuration diagram of the entire system showing an example of an image forming apparatus to which the present invention is applied. In FIG. 1, the image forming apparatus includes a main body 100, an image reading apparatus 200 installed on the upper portion of the image forming apparatus main body 100, and an automatic document feeder (hereinafter referred to as “ADF”) mounted thereon. 300, a large-capacity paper feeding device 400 disposed on the right side in FIG. 1 of the image forming apparatus main body 100, and a sheet post-processing device 500 disposed on the left side in FIG. Has been. The image forming apparatus main body 100 includes an image writing unit 110, an image forming unit 120, a fixing unit 130, a duplex conveying unit 140, a sheet feeding unit 150, a vertical conveying unit 160, and a manual feeding unit 170.

  The image writing unit 110 modulates the LD, which is a light source, based on the image information of the original read by the image reading device 200, and performs laser writing on the surface of the photosensitive drum 121 by a scanning optical system such as a polygon mirror and an fθ lens. Is. The image forming unit 120 includes a photosensitive drum 121, and known electrophotographic imaging elements such as a developing unit 122, a transfer unit 123, a cleaning unit 124, and a charge eliminating unit provided along the outer periphery of the photosensitive drum 121. Consists of. The fixing unit 120 fixes the image transferred by the transfer unit 123 onto the transfer paper.

  The double-sided conveyance unit 140 is provided on the downstream side of the fixing unit 120 in the transfer sheet conveyance direction, and includes a first switching claw 141 for switching the transfer sheet conveyance direction to the sheet post-processing apparatus 500 side or the double-side conveyance unit 140 side, , The reverse conveying path 142 guided by the switching claw 141, the image forming side conveying path 143 that conveys the transfer paper reversed by the reverse conveying path 142 to the transfer unit 123 side again, and the reverse transfer paper after the sheet post-processing device 500. A second switching claw 145 is disposed at a branch portion between the image forming side conveying path 143 and the post processing side conveying path 144.

  The sheet feeding unit 150 includes four sheet feeding stages, and the transfer sheets stored in the sheet feeding stages selected by the pickup roller and the sheet feeding roller are drawn out and guided to the vertical conveyance unit 160. The vertical conveyance unit 160 conveys the transfer paper fed from each paper feed stage to the registration roller 161 just before the upstream of the transfer unit 123 in the paper conveyance direction, and the registration roller 161 uses the visible image on the photosensitive drum 121. The transfer paper is fed into the transfer unit 123 in time with the leading edge. The manual feed unit 170 includes an openable and closable manual feed tray 171. If necessary, the manual feed tray 171 is opened to supply transfer paper manually. Also in this case, the transfer timing of the transfer paper is taken by the registration roller 161 and is conveyed.

  The large-capacity paper feeding device 400 supplies a large amount of transfer sheets of the same size, and the bottom plate 402 rises as the transfer sheets are consumed, so that the pickup roller 401 can always pick up the sheets. ing. The transfer paper fed from the pickup roller 401 is transported from the vertical transport unit 160 to the nip of the registration roller 161.

  The sheet post-processing apparatus 500 performs predetermined processing such as punching, alignment, stapling, and sorting. In this embodiment, the punch 501, the staple tray (alignment) 502, the stapler 503, and the shift tray 504 are used for the above functions. I have. That is, the transfer paper carried into the paper post-processing device 500 from the image forming apparatus main body 100 is punched one by one with the punch 501 when punching, and thereafter, if there is no particular processing, When sorting, stacking, and sorting to the proof tray 505, the sheets are discharged to the shift tray 504, respectively. In this embodiment, the sorting is performed by the reciprocating movement of the shift tray 504 by a predetermined amount in a direction orthogonal to the sheet conveyance direction. In addition, sorting can be performed by moving the paper in a direction orthogonal to the paper transport direction on the paper transport path.

  In the case of alignment, the transfer paper that has been punched or not punched is guided to the lower transport path 506, and the staple tray 504 is aligned in the direction perpendicular to the paper transport direction by the rear end fence. In the jogger fence, alignment in the direction parallel to the paper transport direction is performed. Here, when binding is performed, a predetermined position of the aligned sheet bundle, for example, a predetermined position such as a corner portion and two central positions is bound by the stapler 503 and discharged to the shift tray 504 by the discharge belt.

  In this embodiment, a pre-stack conveyance path 507 is provided in the lower conveyance path 506, and a plurality of sheets are stacked at the time of conveyance to avoid interruption of the image forming operation on the image forming apparatus main body 100 side during post-processing. Be able to.

  The image reading apparatus 200 is guided onto the contact glass 210 by the ADF 300, optically scans the stopped document, and reads the image formed by the imaging lens through the first to third mirrors, such as a CCD or CMOS. Read by the photoelectric conversion element. The read image data is subjected to predetermined image processing by an image processing circuit (not shown) and temporarily stored in a storage device. Then, it is read from the storage device by the image writing unit 110 at the time of image formation, modulated according to the image data, and optical writing is performed.

  The ADF 300 has a double-sided reading function, and is attached to the contact glass 210 installation surface of the image reading apparatus 200 so as to be freely opened and closed.

  Further, the black level adjustment of the image reading apparatus 200 in FIG. 15 is performed as described above.

As described above, according to the present embodiment,
1) Since the setting accuracy of the offset adjustment value can be switched, white level adjustment (AGC) that requires stable black level output and image reading can be performed, and offset fluctuations in the input clamp during standby can also be handled. be able to.
2) Since there are two types of offset adjustment value setting accuracy, high accuracy and low accuracy, the offset adjustment range is wide but the low accuracy mode and the offset adjustment range is narrow but the setting accuracy is high. A stable black level can be obtained when necessary.
3) Since the setting accuracy of the offset adjustment value at the initial time is set to a low accuracy and the setting accuracy is switched to a high accuracy during the white level adjustment, a stable black level can be obtained during the white level adjustment.
4) Since the setting accuracy is switched to a high accuracy before the image reading is started and the setting accuracy is switched to a low accuracy after the image reading is completed, a stable black level can be obtained during the image reading.
5) Since the offset adjustment value is set by the DAC 32 that converts a digital signal into an analog signal, the setting accuracy can be easily changed.
6) DAC32 has N-bit resolution (10 bits in the embodiment), and has a circuit with excellent linearity in the lower M bits (8 bits) of N bits, providing a mode in which setting accuracy can be switched can do.
There are effects such as.

  In addition, this invention is not limited to this embodiment, It cannot be overemphasized that all the technical matters contained in the technical idea described in the claim are object.

1 is a diagram illustrating a schematic configuration of an image reading apparatus according to an embodiment of the present invention. It is a block diagram which shows the structure of a sensor board unit. It is a figure which shows the detail of AC coupling | bond part and a clamp circuit part. It is explanatory drawing which shows the relationship between the line clamp in 1 line, a solid clamp, idle transfer, OPB, and an effective pixel clamp state. It is a block diagram which shows the feedback circuit of the black level adjustment in this embodiment. It is a figure which shows the relationship between the DAC setting value and DAC output level at the time of normal mode. The relationship between the DAC setting value and the DAC output level for explaining that the previous DAC setting value is automatically adjusted to be near the center of the lower 8 bits when changing from the normal mode to the lower bit mode is shown. FIG. It is a flowchart which shows the process sequence of the black level adjustment after power supply ON. 6 is a flowchart showing a processing procedure for executing scan from standby of the image reading apparatus 200. It is a flowchart which shows the process sequence in the subroutine of AGC process (white level adjustment) of STEP06. It is a flowchart which shows the operation | movement procedure of the scanning operation | movement of an image reading apparatus. 4 is a timing chart showing the relationship among a line synchronization signal, an input clamp synchronization signal, a black level correction period signal, a black level correction data capturing period, and a CCD output. It is a timing chart which shows the output timing of a line synchronous signal, a shading gate signal, and a frame gate signal in the case of reading by a flat bed system. It is the figure which plotted the average value of OPB when the black level target value of the black offset level in this embodiment is 40 in the sub-scanning direction. 1 is a schematic configuration diagram of an entire system showing an example of an image forming apparatus to which the present invention is applied. It is a figure for demonstrating operation | movement of the conventional clamp circuit. It is a timing chart which shows the timing of the conventional clamp operation | movement. It is a block diagram which shows the structure of the conventional offset adjustment means. It is a figure which shows the characteristic in case the adjustment range of an offset level and the resolution of DAC are ideal. It is a figure which shows the characteristic of the adjustment range of an actual offset level, and the resolution of DAC. It is the figure which plotted the average value of OPB in the case of the black level target value of the black offset level in a prior art example being 40 in the subscanning direction.

Explanation of symbols

9 CCD
11 White reference plate 12 Scanner body 21 AC coupling 22 AFE
23 Analog signal processing circuit section 24 ADC
25 Sample hold circuit 26 Input clamp circuit 29 Feedback circuit 30 PGA
31 Offset adjustment value calculation circuit 32 DAC
33 Adder circuit 34 Black level clamp period setting means 100 Image forming apparatus main body 200 Image reading apparatus

Claims (11)

A photoelectric conversion element for photoelectrically converting optically read image information;
Sample-and-hold means for sample-holding an analog image signal from the photoelectric conversion element;
An AD converter that digitizes an analog signal after sample-holding and outputs digital image data;
Detecting means for detecting a black level of the digital image data;
Difference detection means for detecting a difference between the detected black level and a preset black level target value;
Feedback means for performing a feedback process so that an offset level of the digital image data approaches a constant level, according to a result of comparing the difference and a reference value;
Switching means having at least two types of setting accuracy of the offset adjustment value and switching the setting accuracy of the offset adjustment value;
White level adjusting means for adjusting the read value of the white reference plate to a preset white level target value;
An image reading apparatus comprising: The image reading apparatus according to claim 1,
An image reading apparatus characterized in that the setting accuracy of the at least two types of offset adjustment values is two types of high accuracy and low accuracy. The image reading apparatus according to claim 2.
An image reading apparatus characterized in that the switching means sets the initial offset adjustment value setting accuracy to the low accuracy and switches the setting accuracy to a high accuracy during the white level adjustment. The image reading apparatus according to claim 2.
The switching unit switches the setting accuracy to a high accuracy before the start of image reading, and switches the setting accuracy to a low accuracy after the image reading ends. The image reading apparatus according to claim 1,
The image reading apparatus, wherein the offset adjustment value is set by a DA converter that converts a digital signal into an analog signal. The image reading apparatus according to claim 5.
The D / A converter has an N-bit resolution, and has a circuit with excellent linearity in the lower M bits of N bits. The image reading apparatus according to any one of claims 1 to 6,
Amplifying means for amplifying the analog signal after being sample-held by the sample-hold means,
The A / D converter digitizes an amplified analog signal. The image reading apparatus according to claim 7.
The image reading apparatus according to claim 1, wherein the white level adjusting means adjusts the gain of the amplifying means to adjust the read value of the white reference plate to the white level target value.   An image forming apparatus comprising the image reading apparatus according to claim 1. In an image reading method that includes a photoelectric conversion element that photoelectrically converts image information, reads the image by converting the image information into optical information,
Sample-holding an analog image signal from the photoelectric conversion element;
A process of digitizing an analog signal after sample-holding and outputting digital image data;
Detecting a black level of the digital image data;
Detecting a difference between the detected black level and a preset black level target value;
A step of performing a feedback process so that an offset level of the digital image data approaches a certain level as an offset adjustment value according to a result of comparing the difference and a reference value;
Having at least two types of setting accuracy of the offset adjustment value and switching the setting accuracy of the offset adjustment value;
Adjusting the read value of the white reference plate to a preset white level target value;
An image reading method comprising: In a computer program for performing control by a computer to have photoelectric conversion elements that photoelectrically convert image information, convert image information into optical information, and read an image,
Sample-holding an analog image signal from the photoelectric conversion element;
A procedure for digitizing an analog signal after sample hold and outputting digital image data,
Detecting a black level of the digital image data;
A procedure for detecting a difference between the detected black level and a preset black level target value;
A procedure for performing a feedback process on an offset adjustment value according to a result of comparing the difference and a reference value so that an offset level of the digital image data approaches a certain level;
A procedure having at least two types of setting accuracy of the offset adjustment value and switching the setting accuracy of the offset adjustment value;
A procedure for adjusting the reading value of the white reference plate to a preset white level target value;
A computer program comprising:

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