Abstract:
In a density control apparatus capable of coping with a change in light sensitivity, an image formation device forms a toner image on a photosensitive body on the basis of an image signal, a sensor measures density of the toner image formed on the photosensitive body, a correction device causes the sensor to measure density of a portion where the toner image is not formed on the photosensitive body and corrects sensitivity of the sensor on the basis of the measured result, a setting device sets a level shifting quantity of an output signal from the sensor after the sensitivity of the sensor is corrected by the correction device, and a determination device causes to form plural images of different densities and determines an image formation condition on the basis of the result of measuring the density of the images and the shifting quantity set by the setting device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the control of image density in an image formation apparatus. 
     2. Related Background Art 
     As a conventional example, the structure of a multi-color image formation apparatus as well as its operation will be described with reference to FIG.  1 . 
     In FIG. 1, latent images formed every each of colors on an image support body (photosensitive drum)  100  by an optical unit  101  are developed and visualized using each of color toners of Y (yellow), M (magenta), C (cyan) and K (black) respectively supplied from color developing units of Dy, Dm, Dc and Dk. Then the developed and visualized images are plural times transferred on an external surface of a transfer belt  102  so as to form a multi-color image. In this condition, the toner is transferred to a surface of the transfer belt  102  by applying high voltage on the transfer belt  102 . Then a recording sheet (paper)  105  fed from a sheet feed unit  103  or a sheet feed toner  104  is conveyed through a sheet conveying path and the multi-color image is re-transferred from the transfer belt  102 . 
     Thereafter, the recording sheet  105  is conveyed by a conveying roller  106  and is fixed by a fixing unit  107  to be discharged to a discharge tray  108  or a discharge unit  109 . The each of color developing units, which has a rotation spindle at its both edges, capable of being rotated around the spindle is held in a developing mechanical unit  110  and is rotated to be selected. Numeral  111  denotes a cleaning unit for cleaning the toner on the transfer belt  102 . Numeral  112  denotes a discharged toner collection unit for collecting discharged toners from the image support body  100 . Numeral  113  denotes a density sensor for measuring density of a toner image formed on the photosensitive body  100 . 
     In the above-described structure, as shown in FIG. 2, a beam is radiated from a light emitting element  1  structured within the density sensor  113 , which is disposed in the vertical direction to a surface of the image support body  100 , to a toner image  20  (called as patch hereinafter) formed on the image support body  100 . Then a reflected light is detected by a light reception element  2  to realize such a structure as stabilizing image density by correcting a difference between the detected result and a predetermined detection level as a change quantity corresponding to a developing bias. 
     An example of the content concerning a density control will be described. The density control can be categorized into a developing bias control for obtaining a developing bias value treated as a maximum value of toner density and a halftone density control for controlling halftone density by varying image data upon fixing the developing bias value defined by the developing bias control. Hereinafter, the developing bias control will be described. 
     FIG. 3 is a block diagram of the structure concerning the developing bias control. In FIG. 3, numeral  3  denotes an A/D converter which converts an analog detection signal transferred from the density sensor (light reception element)  113  into a digital signal. Numeral  4  denotes a data comparison means which judges whether or not an output value of the density sensor for a surface ground of the image support body reaches a predetermined value. Numeral  5  denotes an LED light quantity setting unit which varies light quantity so as to secure an output range in case of measuring several kinds of patches. 
     Numeral  6  denotes a data storage means which interpolates detected data. Numeral  7  denotes a density calculation unit which changes a sensor output value of the measured patch in terms of a density value. Numeral  8  denotes a density/developing bias comparison unit which determines the density value changed by the density calculation unit  7  and the developing bias value corresponding to the density value. 
     Numeral  9  denotes a developing bias control unit which gives a command to a high-voltage output unit  10  so as to output the determined developing bias value. The high-voltage output unit  10  applies an output designated from the developing bias control unit  9  to the developing unit. Numeral  50  denotes a CPU within a DC controller (not shown) provided in an image formation apparatus main body. Depending on the structure, each of the blocks  3 ,  4 ,  5 ,  6 ,  7 ,  8  and  9  provided in the CPU  50  may be provided in the DC controller (not shown) or the density sensor  113 . 
     FIG. 4 is an operational flow chart of the developing bias control. In FIG. 4, the ground of the image support body surface is measured in a step S 101 . A flow advances to a step S 102 , where if a read value of the ground of the image support body surface is lower than a predetermined density value, the flow advances to a step S 104 . If the read value of the ground of the image support body surface is higher than the predetermined density value because of dirt or an inferior change in time of the density sensor  113 , a density sensor output is corrected in a step S 103 . 
     The density sensor output is corrected by each block of the A/D converter  3 , data comparison means  4 , the LED light quantity setting unit  5  and the data storage means  6  shown in FIG.  3 . In the step S 104 , density of the ground surface on a position, where the patch on the image support body is to be printed, is measured. Then density of the patch is measured in a step S 105 . In this case, the density of the patch for the ground can be measured as contrast by measuring the density of the ground in the step S 104 . The flow advances to a step S 107  after converting the read value of the density sensor  113  into the density value in a step S 106 . A method for determining an optimum developing bias value performed in the step S 107  will be described hereinafter. 
     FIG. 5 shows examples of measured patches. A patch  20 - a  is in the lowest density and a patch  20 - e  is in the highest density. The density is schematically expressed by hatching lines. The number of patches is not limited to these examples but may be varied depending on a diameter of the image support body or time spend in controlling the density. FIG. 6 shows the relationship between the density value being the measured result of patches  20 - a ,  20 - b ,  20 - c  and  20 - d  shown in FIG. 5 by the density sensor  113  and the developing bias value when the patches are formed. 
     In FIG. 6, in a case where a target density value want to be obtained among the measured density of five patches exits on somewhere between two points, the optimum developing bias value for the target density can be obtained by performing a linear interpolation for the two points. 
     FIG. 7 indicates a case that all patches which are measured can not reach the target density. In this case, the linear interpolation is performed for the two patches of which density is closer to the target density so as to estimate the optimum developing bias value for the target density. 
     FIG. 8 indicates a case that all patches which are measured can not reach the target density and the density value reaches peak between the patches  20 - b  and  20 - d . In this case, the developing bias value of the patch of which density is closest to the target density (patch reaches a peak of the density) is treated as the optimum developing bias value. 
     The halftone density control is performed after determining the optimum developing bias indicating a maximum density by the developing bias control. Also, in case of the halftone density control, a plurality of patches are printed on the image support body to measure the patches density by the density sensor  113  similar to the case of the developing bias control. The relationship between image data of the halftone density control patch and the density value is shown in FIG.  9 . On an image data/density characteristic curve shown in FIG. 9, since a raise of the density is remarkable in the vicinity of center position of the image data, a halftone correction curve is obtained by calculation so as to perform such a process as correcting the characteristic curve in linear as shown in FIG.  10 . Consequently, reproductive precision of halftone density, which is largely changed in color reproductivity of an image, can be improved. 
     However, conventionally, in measuring of the patch density when toner image density is controlled, there occurs such an inconvenient situation as an output voltage when a beam is radiated to the image support body (assumed as reference voltage V 0 ) becomes more sensitive for the light due to a material characteristic of the image support body of which surface is scraped off because of a long period use of a color image formation apparatus. Also, there occurs such an inconvenient situation as resulted in deteriorating light sensitivity because of dirt of the density sensor due to dirt of toners in the image formation apparatus. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a density control apparatus and method thereof for eliminating the above-described drawback. 
     Another object of the present invention is to provide the density control apparatus and method thereof for enabling to cope with a change in light sensitivity. 
     Still another object of the present invention is to provide the density control apparatus and method thereof for controlling density adapting to an inferior change in time of an apparatus by correcting a measured value of the density and light quantity when a patch is measured. 
     Still another object of the present invention is to provide the density control apparatus and method thereof for precisely controlling the density by effectively using a dynamic range of an A/D converter. 
     Other objects of the present invention will become apparent from the following description based on the attached drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an image formation apparatus; 
     FIG. 2 is a structural view showing a conventional toner patch measurement; 
     FIG. 3 is a block diagram for explaining a conventional developing bias control; 
     FIG. 4 is a flow chart showing contents of the conventional developing bias control; 
     FIG. 5 is an explanation view showing examples of patches measured by a developing bias control; 
     FIG. 6 is an explanation view for explaining a process to obtain an optimum developing bias value; 
     FIG. 7 is an explanation view for explaining the process to obtain the optimum developing bias value; 
     FIG. 8 is an explanation view for explaining the process to obtain the optimum developing bias value; 
     FIG. 9 is an explanation view showing an image data/density characteristic curve in a halftone density control; 
     FIG. 10 is an explanation view for explaining a process to obtain a halftone correction curve; 
     FIG. 11 is a block diagram showing circuit structure in a first embodiment of the present invention; 
     FIG. 12 is a flow chart showing process contents in the first embodiment of the present invention; 
     FIG. 13 is an explanation view for explaining an operation in the first embodiment of the present invention; 
     FIG. 14 is a block diagram showing a circuit structure in a second embodiment of the present invention; 
     FIG. 15 is a flow chart showing process contents in the second embodiment of the present invention; 
     FIG. 16 is an explanation view for explaining an operation in the second embodiment of the present invention; 
     FIG. 17 is a flow chart showing process contents in a third embodiment of the present invention; 
     FIG. 18 is an explanation view for explaining an operation in the third embodiment of the present invention; and 
     FIG. 19 is an explanation view for explaining the operation in the third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the attached drawings. 
     First Embodiment 
     FIG. 11 shows a structure of a circuit in the first embodiment of the present invention. In FIG. 11, the same numerals are given to the same portions as those shown in FIG. 3 of a conventional example and a detailed description will be omitted. In FIG. 11, numeral  11  denotes an initial setting means, which transmits a command of an initial light quantity setting value to an LED light quantity setting unit  5 . Numeral  12  denotes an output shift level setting means, which shifts voltage for an output voltage from a density sensor (light reception element)  113  depending on a obtained result in a data comparison means  4 . Numeral  50  denotes a CPU within a DC controller (not shown) provided in a color image formation apparatus main body. Depending on the structure, each of blocks  3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9 ,  10  and  11  provided in the CPU  50  may be provided in the DC controller or the density sensor  113 . 
     FIG. 12 is a flow chart for explaining an operation in the first embodiment of the present invention. In FIG. 12, when a sensitivity correction mode is started, at first, a reference light quantity P 0  used in measuring surface ground density of an image support body is set from the LED light quantity setting unit  5  by the command from the initial setting means  11  in a step S 201 . In a step S 202 , the ground density of the image support body is measured using the reference light quantity P 0 . A flow advances to a step S 203 , where the output voltage of the density sensor  113  being the measured result is transmitted to the CPU  50  in the step S 202  to be compared whether or not the output voltage is within an acceptable error range for a reference voltage value by the data comparison means  4  after passing through an A/D converter  3 . 
     In a case where the compared result is in N.G. state, the flow advances to a step S 204 . In the step S 204 , it is judged whether or not data transmitted from the density sensor  113  is within a defined output voltage range. Herein, the defined output voltage range indicates the output voltage range of the density sensor  113  and an input voltage range of the CPU  50 . In a case where the data is not within the defined output voltage range, the flow advances to a step S 205 , where the output voltage is shifted by the output shift level setting means  12 . This process is repeated until the data is included within the defined output voltage range. When the data is included within the defined output voltage range, the flow advances to a step S 206 . 
     In the step S 206 , v 1 /v 0  (=α) is calculated as a correction value using a predetermined reference voltage (V 0 ) of the image support body and an actually measured output voltage data (V 1 ). The flow advances to a step S 207 , where the LED light quantity setting unit is set to correct a light emission quantity to (P 0 /α) by introducing the correction value obtained in the step S 206 . The flow returns to the step S 203 , where the sensitivity correction mode is terminated if the data is within the acceptable error range for the reference value, as a result of comparing the data. 
     The above-described process is described with reference to a graphic chart shown in FIG.  13 . In FIG. 13, a previously estimated voltage value as an output, when a beam is radiated on a surface of the image support body  100  by the density sensor  113  and a reflected light is measured, is defined as v 0 . That is, the output voltage for the ground density is defined as v 0 . When the v 0  is assumed as a reference value, an output voltage characteristic curve estimated as the output voltage value corresponding to density of a toner image formed on the image support body is defined as (A). An output voltage value corresponding to the surface ground density of the image support body in case of changing sensitivity due to an use of the image support body is defined as v 1 . An output voltage characteristic curve corresponding to the density of the toner image to be formed is defined as (B). 
     As described in the flow chart in FIG. 12, the output voltage v 1  of the surface in case of changing the sensitivity of the image support body sometimes exceeds an upper limit value of the output range. On the curve (B), in a case where the v 1  exceeds the upper limit value of the output range or a voltage value of the output voltage characteristic curve is seemed to exceed the upper limit value of the output range, the output voltage is shifted below such that the characteristic curve is to be included in the output range. In this case, an output voltage corresponding to the surface ground density of the image support body is defined as v 2  and an output voltage characteristic curve corresponding to the density of the toner image is defined as (C). In case of comparing the curve (A) with the curve (C), since the sensitivity characteristic of the image support body has been changed due to a long period use of the image support body, the characteristic of the curve (A) does not coincide with that of the curve (C). For this reason, the v 0  and v 2 , which are the output voltages corresponding to the surface ground density of the image support body, are compared each other to execute a process for coinciding two points by varying light emission quantity of an LED  1 . Therefore, a change in the sensitivity of the image support body can be corrected. 
     In the above-described embodiment, a case of changing the sensitivity of the image support body is described. However, also in case of decreasing the light emission quantity because of dirt of the density sensor due to dust such as toners, a correction can be similarly performed by shifting the output voltage and varying the light emission quantity. 
     Second Embodiment 
     FIG. 14 shows a circuit structure in the second embodiment of the present invention. The same numerals are given to the same portions as those shown in FIG. 3 of a conventional example and FIG. 11 of the first embodiment and a detailed description will be omitted. In FIG. 14, numeral  13  denotes a toner image bias setting unit for developing a latent image formed on an image support body. 
     FIG. 15 is a flow chart for explaining an operation in the second embodiment of the present invention. The same numerals are given to the same process steps as those shown in FIG. 12 of the first embodiment and a detailed description will be omitted. 
     In FIG. 15, as a compared result, in a case where data of an output voltage value corresponding to surface ground density of the image support body is within an acceptable error range in a step S 203 , a flow advances to a step S 301 . In the step S 301 , a toner image is printed on the image support body to measure density by a density sensor  113 . In the present embodiment, the number of toner images of different density to be printed on the image support body is assumed as three. However, any number of toner images may be available if the number is equal to or larger than one. 
     In the present embodiment, a first toner image is defined as x, a second toner image is defined as y and a third toner image is defined as z. The flow advances to a step S 302 . If an obtained result of measuring density of the toner image a is within an output voltage range, the flow advances to a step S 303 . In a case where the obtained result of measuring the density is not within the output voltage range, the flow advances to a step S 305 , where an output shift level is set to shift the level. This process is repeated until the measured result is to be included within the output voltage range. 
     In this case, the toner image to be printed once more on the image support body may be only the toner image x. In the step S 303 , similar to the case in the step S 302 , if an obtained result of measuring density of the toner image y is within the output voltage range, the flow advances to a step S 304 . In a case where the obtained result of measuring the density is not within the output voltage range, the flow advances to the step S 305 , where the output shift level is set to shift the level. This process is repeated until the measured result is to be included within the output voltage range. 
     In this case, the toner image to be printed once more on the image support body may be only the toner image y. In the step S 304 , similar to the case in the step S 302 , if an obtained result of measuring density of the toner image z is within the output voltage range, a measuring error should not be occurred when the density of the toner image is controlled, thereby outputting data. In a case where the obtained result of measuring the density is not within the output voltage range, the flow advances to the step S 305 , where the output shift level is set to shift the level. This process is repeated until the measured result is to be included within the output voltage range. 
     In this case, the toner image to be printed once more on the image support body may be only the toner image z. 
     The above-described process will be described with reference to a graphic chart shown in FIG.  16 . In FIG. 16, as a result of measuring the density, obtained output voltage values of the three toner images of different density are assumed as x 0 , y 0  and z 0  respectively. These output voltage values are to be existed on a reference ideal curve (A) after terminating a sensitivity correction by measuring the surface density of the image support body  100  by the density sensor. Output voltage values of the density sensor when the toner images are actually printed on the image support body are assumed as x 1 , y 1  and z 1  respectively and a curve obtained by connecting each of the three points is assumed as (B). In a case where an output of actually measured toner image density exceeds an upper limit value of output range, the output shift level is set to shift the level such that a density output value is to be included within the output range. At this time, output voltage values are assumed as x 2 , y 2  and z 2  respectively and a sensitivity characteristic curve of the density sensor is assumed as (C). At this time, if the output voltage value y 2  or z 2  is not within the output range, the output shift level has to be further decreased. 
     In a case where a characteristic of the density sensor is deteriorated because of dirt of the density sensor due to dust such as toners, the output shift level has to be increased. 
     For the toner images to be printed on the image support body in the above-described description, any toner of yellow, magenta, cyan or black may be used. 
     A calculating formula indicating sensitivity characteristic curves of the density sensor shown in FIG. 13 can be obtained by a statistical method (e.g., least square method) on the basis of the output values of density of the three toner images. Upon obtaining this calculating formula by a calculating function of a CPU, since density other than the above-described density of the three images, for example, an output value of the density sensor for a maximum density (called as expectation value or prediction value) can be obtained, it is judged whether or not the output value for the maximum density is included within a tolerance. When a denied judgment is obtained, the above-described light quantity and the output shift level may be variably set. As to a minimum density, of course, the same process may be executed. Therefore, the sensitivity characteristic curves of the density sensor can be included within the tolerance. 
     It is preferable that the sensitivity correction of the density sensor described in the above-described first and second embodiments may be performed just before setting of an image formation condition such as a toner density control or the like described in the conventional example. 
     Third Embodiment 
     A circuit structure in the third embodiment is identical with that in the first embodiment shown in FIG.  11 . 
     FIG. 17 is a flow chart for explaining an operation in the third embodiment of the present invention. In FIG. 17, when a sensitivity correction mode is started in a step S 401 , at first, a reference light quantity (P 0 ) used in measuring surface ground density of an image support body  100  is set by an LED light quantity setting unit  5  according to a command from an initial setting means  11  in a step S 402 . In a step S 403 , the image support body  100  is radiated with the reference light quantity P 0  to measure the density using a reflected light. At this time, a reference voltage value of the surface ground density of the image support body  100  is assumed as V 0  and a measured value is assumed as V. A flow advances to a step S 404 . An output voltage obtained by measuring the surface ground density of the image support body  100  is transmitted to a CPU  50  in the step S 403  and is converted into digital data by an A/D converter  3 , thereafter, the data is compared if it is within an acceptable error range for the reference voltage value by a data comparison means  4 . Reference symbols a and b shown in the step S 404  respectively denote a lower limit value and an upper limit value of an error tolerance so as to shift to a density control mode. 
     If the compared result is in NO state, the flow advances to a step S 405 , where it is judged whether or not the data transmitted to the data comparison means  4  is within a defined output voltage range. Herein, the defined output voltage range means such a range as enabling to correct sensitivity within an output voltage range of a density sensor  113 . Reference symbols c and d respectively denote a lower limit value and an upper limit value of this range. In a case where the data is within the defined output voltage range, the flow advances to a step S 406  to set a light emission quantity P 1  used in correcting the sensitivity of the image support body. For example, when the measured value is V for the reference voltage value V 0 , a correction light quantity P 1 =P 0 /α (α=V/V 0 ) is to be set. Thereafter, the flow returns to the step S 403  to repeat the consecutive process until the transmitted data is included with the defined output voltage range. 
     In the step S 405 , in a case where the data is not within the defined output voltage range, the flow advances to a step S 407  to judge whether or not the measured value V is lower than a lower limit value of a sensitivity correctable range (V&lt;c). In a case where the measured value is lower than the lower limit value, the process is terminated as an error. If the measured value V is not lower than the lower limit value c, since the measured value is to be upper than the upper limit value d, subsequently, in a step S 409 , light quantity of an LED is corrected to judge whether or not the number of measuring times of measuring the density reaches three. If the number of measuring times reaches three, the process is terminated as an error. If the number of measuring times does not reach three, reference symbol n, which represents the measuring times, is counted up by one in a step S 410  and the flow advances to a step S 411 . It should be noted that a value of n is initialized when the sensitivity correction mode is to be started. In case of advancing to the step s 411 , it is expected that the measured value V exceeds the output range of the density sensor. In this case a correction light quantity P 2  is set. For example, when an upper limit value of the output range of the density sensor is assumed as A, the correction light quantity P 2 =P 0 /β (β=A/V 0 ) is to be set. After setting the correction light quantity P 2 , the flow returns to the step S 403 , where the density is measured to perform a comparison. 
     In the step S 404 , if the measured value V is included within the error tolerance, the flow advances to a step S 412 . From this step S 412 , the density control mode is started. At first, in a step S 413 , it is shifted to a color toner image measuring mode. The flow advances to a step S 414 , where a shift level (|V 0 −Vs|) equivalent to the quantity shown in FIG. 18 is set for the output voltage of the density sensor. The shift level, which is a fixed value previously determined according to individual dispersion of the image support body, can be optionally varied. Therefore, since an output signal from the sensor becomes to be indicated by such a form as shown in FIG. 19, a dynamic range of the A/D converter  3  can be effectively used. Reference symbol Vs denotes a ground output value of the image support body defined by setting the shift level of the density sensor. After setting the shift level, a plurality of color toner images of different density are formed and the density thereof is measured in a step S 415 . When it is terminated to measure the density of the color toner images, the flow advances to a step S 416  to release the previously set shift level. The flow advances to a step S 417 , where a plurality of black toner images of different density are formed and the density thereof is measured. 
     According to the above order, when the sensitivity of the density sensor is corrected and the density of the toner image is measured, the process is shifted to a developing bias control. 
     In the above-described embodiments, a case of changing the sensitivity of the image support body is described. However, also in case of decreasing the light emission quantity because of dirt of the density sensor due to dust such as toners, a correction can be similarly performed by varying the light emission quantity and the output voltage shift quantity. 
     It is preferable to perform the sensitivity correction of the density sensor in the above-described embodiment just before setting an image formation condition such as a toner density control or the like described in the conventional example. 
     The present invention is not limited to the above-described embodiments, but, various modifications can be effected within the scope of the appended claims.