Electrophotographic image forming apparatus including means for correcting density drift

Claim 2

An image forming apparatus comprising: means for generating gradation data for a test pattern containing a high density portion and a low density portion; means for supplying image information of the gradation data generated by said generating means to an image carrier, said image carrier having a contrast potential depending on the high density portion, and a background potential depending on the low density portion; means for sensing the image information supplied from said supplying means to the image carrier so as to provide a high density value corresponding to the high density portion and a low density value corresponding to the low density portion; first detecting means for detecting a first difference between a high density reference value and the high density value provided by said sensing means; second detecting means for detecting a second difference between a low density reference value and the low density value provided by said sensing means; first converting means for converting the first and second differences, respectively, detected by said first and second detecting means into a first variable based on a first predetermined relationship among the first difference, second difference and the first variable; second converting means for converting the first and second differences, respectively, detected by said first and second detecting means, into a second variable based on a second predetermined relationship among the first difference, second difference and the second variable; first modifying means for modifying the contrast potential of said image carrier using the first variable obtained by said first converting means; and second modifying means for modifying the background potential of said image carrier using the second variable obtained by said second converting means.

Claim 3

An image forming apparatus according to claim 1, further comprising: means for applying a grid bias voltage to said image carrier; and means for applying a developing bias voltage to said image carrier having contrast potential and background potential, each said grid base voltage and developing bias voltage corresponding to a combination of the contrast potential and the background potential.

Claim 4

An image forming apparatus according to claim 2, further comprising: first determining means for determining the grid bias voltage based on the contrast potential, modified by said first modifying means, and the background potential, modified by said second modifying means.

Claim 5

An image forming apparatus according to claim 3, further comprising: second determining means for determining the developing bias voltage based on the background potential and the grid bias voltage determined by said first determining means.

Claim 6

An image forming apparatus comprising: means for generating gradation data for a test pattern containing a high density portion and a low density portion; means for supplying image information of the gradation data generated by said generating means to an image carrier to which a toner is attached; means for sensing the image information supplied from said supplying means to the image carrier so as to provide a high density value corresponding to the high density portion and a low density value corresponding to the low density portion; first detecting means for detecting a first difference between a high density reference value and the high density value provided by said sensing means; second detecting means for detecting a second difference between a low density reference value and the low density value provided by said sensing means; first comprising means for comparing an absolute value of the first difference, detected by said first detecting means with a first comparison reference, so as to provide a first comparison result if the absolute value of the first difference is larger than the first comparison reference; second comparing means for comparing an absolute value of the second difference detected by said second detecting means with a second comparison reference, so as to provide a second comparison result if the absolute value of the second difference is larger than the second comparison reference; means for changing the high density value provided by said sensing means, so as to decrease the absolute value of the first difference during which the first comparison result is provided by said first comparing means; and means for changing the low density value provided by said sensing means, so as to decrease the absolute value of the second difference during when the second comparison result is provided by said second comparing means.

Claim 7

An image forming apparatus comprising: means for generating gradation data for a test pattern containing a high density portion and a low density portion; means for supplying image information of the gradation data generated by said generating means to an image carrier to which a toner is attached; means for sensing the image information supplied from said supplying means to the image carrier so as to provide a high density value corresponding to the high density portion and a low density value corresponding to the low density portion; first detecting means for detecting a first difference between a high density reference value and the high density value provided by said sensing means; second detecting means for detecting a second difference between a low density reference value and the low density value provided by said sensing means; first comprising means for comparing an absolute value of the first difference, detected by said first detecting means with a first comparison reference, so as to provide a first comparison result if the absolute value of the first difference is larger than the first comparison reference; second comparing means for comparing an absolute value of the second difference detected by said second detecting means with a second comparison reference, so as to provide a second comparison result if the absolute value of the second difference is larger than the second comparison reference; means for changing the high density value provided by said sensing means, so as to decrease the absolute value of the first difference during which the first comparison result is provided by said first comparing means; and means for changing the low density value provided by said sensing means, so as to decrease the absolute value of the second difference during when the second comparison result is provided by said second comparing means.

Claim 8

An image forming apparatus comprising: means for supplying an image carrier with image information containing gradation data having a high density portion and a low density portion; means for applying a grid bias voltage and a developing bias voltage to said image carrier having a contrast potential depending on the high density portion and a background potential depending on the low density portion,, each said grid bias voltage and a developing bias voltage corresponding to a combination of the contrast potential and the background potential; means for sensing the image information supplied from said supplying means to the image carrier so as to provide a high density value corresponding to the high density portion and a low density value corresponding to the low density portion; means for detecting the contrast potential an the background potential in accordance with the high and low density values provided by said sensing means; first determining means for determining the grid bias voltage based on the contrast potential and the background potential; and second determining means for determining the developing bias voltage based on the background potential and the grid bias voltage determined by said first determining means.

Claim 9

An image forming apparatus comprising: means for supplying an image carrier with image information containing gradation data having a high density portion and a low density portion; means for applying a grid bias voltage and a developing bias voltage to said image carrier having a contrast potential depending on the high density portion and a background potential depending on the low density portion,, each said grid bias voltage and a developing bias voltage corresponding to a combination of the contrast potential and the background potential; means for sensing the image information supplied from said supplying means to the image carrier so as to provide a high density value corresponding to the high density portion and a low density value corresponding to the low density portion; means for detecting the contrast potential an the background potential in accordance with the high and low density values provided by said sensing means; first determining means for determining the grid bias voltage based on the contrast potential and the background potential; and second determining means for determining the developing bias voltage based on the background potential and the grid bias voltage determined by said first determining means.

Claim 10

An image forming apparatus comprising: means for supplying an image carrier with image information containing gradation data having a high density portion and a low density portion; means for applying a grid bias voltage and a developing bias voltage to said image carrier having a contrast potential depending on the high density portion and a background potential depending on the low density portion, each said grid bias voltage and developing bias voltage corresponding to a combination of the contrast potential and the background potential; means for sensing the image information supplied from said supplying means to the image carrier so as to provide a high density value corresponding to the high density portion and a low density value corresponding the low density portion; first detecting means for detecting a first difference between a high density reference value and the high density value provided by said sensing means; second detecting means for detecting a second difference between a low density reference value and the low density value provided by said sensing means; first converting means for converting the first and second differences, respectively, detected by said first and second detecting means into a first variable based on a first predetermined relationship among the first difference, second difference and the first variable; second converting means for converting the first and second differences, respectively, detected by said first and second detecting means, into a second variable based on a second predetermined relationship among the first difference, second difference and the second variable; first modifying means for modifying the contrast potential of said image carrier using the first variable obtained by said first converting means; second modifying means for modifying the background potential of said image carrier using the second variable obtained by said second converting means; first determining means for determining the grid bias voltage based on the contrast potential, modified by said first modifying means, and the background potential, modified by said second modifying means; and second determining means for determining the developing bias voltage based on the background potential and the grid bias voltage determined by said first determining means.

Claim 11

An image forming apparatus comprising: means for supplying an image carrier with image information containing gradation data having a high density portion and a low density portion; means for applying a grid bias voltage and a developing bias voltage to said image carrier having a contrast potential depending on the high density portion and a background potential depending on the low density portion, each said grid bias voltage and developing bias voltage corresponding to a combination of the contrast potential and the background potential; means for sensing the image information supplied from said supplying means to the image carrier so as to provide a high density value corresponding to the high density portion and a low density value corresponding the low density portion; first detecting means for detecting a first difference between a high density reference value and the high density value provided by said sensing means; second detecting means for detecting a second difference between a low density reference value and the low density value provided by said sensing means; first converting means for converting the first and second differences, respectively, detected by said first and second detecting means into a first variable based on a first predetermined relationship among the first difference, second difference and the first variable; second converting means for converting the first and second differences, respectively, detected by said first and second detecting means, into a second variable based on a second predetermined relationship among the first difference, second difference and the second variable; first modifying means for modifying the contrast potential of said image carrier using the first variable obtained by said first converting means; second modifying means for modifying the background potential of said image carrier using the second variable obtained by said second converting means; first determining means for determining the grid bias voltage based on the contrast potential, modified by said first modifying means, and the background potential, modified by said second modifying means; and second determining means for determining the developing bias voltage based on the background potential and the grid bias voltage determined by said first determining means.