Sheet conveying device having function of correcting skew of sheet

1. A sheet conveying device comprising:

first and second detection sensors that are disposed in a direction crosswise to a conveying direction for conveying an index tab sheet having an index tab, for detecting the index tab sheet;

an acquiring unit configured to acquire a size of the index tab;

first and second conveying units that are disposed in a direction crosswise to the conveying direction, for conveying the index tab sheet;

a timer configured to measure a leading edge detection time from when one of the first or second detection sensor detects a leading edge of the index tab sheet to thereby output a first signal to when the other of the first or second detection sensor detects the leading edge of the index tab sheet to thereby output a second signal; and

a skew correction unit configured to control respective conveying speeds of the first and second conveying units independently of each other so that a skew corresponding to the leading edge detection time is corrected,

wherein, in a case where the index tab sheet is conveyed, when the leading edge detection time measured by the timer is not smaller than a predetermined threshold value since one of the first or second detection sensor detects the index tab of the index tab sheet, the skew correction unit determines a skew correction amount based on the measured leading edge detection time and size of the index tab acquired by the acquiring unit, and

wherein when the leading edge detection time measured by the timer is smaller than the predetermined threshold value since both the first and second detection sensors do not detect the index tab, the skew correction unit determines the skew correction amount based on the measured leading edge detection time, regardless of the size of the index tab.

2. The sheet conveying device according to claim 1 , wherein the index tab is a protruding portion of the index tab sheet, which protrudes in the conveying direction.

3. The sheet conveying device according to claim 1 , wherein:

the first and second detection sensors are disposed at respective locations upstream of the first and second conveying units in the conveying direction,

the timer further measures a trailing edge detection time from when one of the first or second detection sensor detects a trailing edge of the index tab sheet to when the other of the first or second detection sensor detects the trailing edge of the index tab sheet, and

the skew correction unit further controls the conveying speeds of the first and second conveying units so that a skew corresponding to the trailing edge detection time is corrected.

4. A sheet conveying device comprising:

first and second detection sensors that are disposed in a direction crosswise to a conveying direction for conveying an index tab sheet having an index tab, for detecting the index tab sheet, the first and second detection sensors being disposed at respective locations upstream of the first and second conveying units in the conveying direction;

first and second conveying units that are disposed in a direction crosswise to the conveying direction, for conveying the index tab sheet;

a timer configured to measure:

a leading edge detection time from when one of the first or second detection sensor detects a leading edge of the index tab sheet to thereby output a first signal to when the other of the first or second detection sensor detects the leading edge of the index tab sheet to thereby output a second signal; and

a trailing edge detection time from when one of the first or second detection sensor detects a trailing edge of the index tab sheet to when the other of the first or second detection sensor detects the trailing edge of the index tab sheet; and

a skew correction unit configured to control:

respective conveying speeds of the first and second conveying units independently of each other so that a skew corresponding to the leading edge detection time is corrected; and

the conveying speeds of the first and second conveying units so that a skew corresponding to the trailing edge detection time is corrected,

wherein when the time measured by the timer is not smaller than a first threshold value since one of the first or second detection sensor detects the index tab of the index tab sheet, the skew correction unit reduces a skew correction amount by a predetermined amount for correcting the skew corresponding to the leading edge detection time measured based on the first and second signals output from the first and second detection sensors, and

wherein when the trailing edge detection time is not smaller than a second threshold value and is smaller than a third threshold value, the skew correction unit corrects the skew corresponding to the trailing edge detection time.

5. The sheet conveying device according to claim 4 , including a notification unit configured to notify a skew correction error when the trailing edge detection time is not smaller than the third threshold value.

6. The sheet conveying device according to claim 1 , further including an image reading unit configured to read an image on the sheet of which the skew has been corrected by the skew correction unit.

7. The sheet conveying device according to claim 1 , further including an image forming unit configured to form an image on the sheet of which the skew has been corrected by the skew correction unit.

8. The sheet conveying device according to claim 1 , wherein the acquiring unit comprises:

an input unit for manually inputting the size of the index tab; and

a storage unit for storing the size input by the input unit.