1. A document conveying device comprising: a document conveying unit configured to feed a document; a controller configured to control a document feeding operation performed by the document conveying unit; a first document detector located on a feeding path along which a document is fed by the document conveying unit, the first document detector configured to detect a leading edge of the document; a second document detector located at a position which is a first distance d 1 downstream from the first document detector in the document feeding direction and different from that of the first document detector in a direction orthogonal to the document feeding direction, the document feeding direction being a direction in which the document conveying unit feeds the document, the second document detector configured to detect the leading edge of the document; a pulse count detector configured to detect the number of pulses of a pulse signal that the controller uses to drive-control the document conveying unit; a determining unit configured to determine whether the number of pulses detected by the pulse count detector during the period from the detection of a document leading edge by one of the first and second document detectors to the detection of the document leading edge by the other of the first and second document detectors exceeds a predetermined value; and the predetermined value includes a first value and a second value, the first value is the sum of the number of the first pulses p 1 for causing the controller to drive-control the document conveying unit to move the document by the first distance d 1 in the document feeding direction and the number of the second pulses p 2 for moving the document by the second distance d 2 which is greater than or equal to the first distance d 1 in the document feeding direction, and the second value is a value given by subtracting the number of first pulses p 1 from the number of the second pulses p 2 , wherein when the determining unit determines that the detection of the document leading edge by the first document detector precedes the detection of the document leading edge by the second document detector, the determining unit uses the first value as the predetermined value to determine whether the number of pulses exceeds the predetermined value, when the determining unit determines that the detection of the document leading edge by the second document detector precedes the detection of the document leading edge by the first document detector, the determining unit uses the second value as the predetermined value to determine whether the number of pulses exceeds the predetermined value, when the determining unit determines that the number of pulses exceeds the predetermined value, the controller stops the document feeding operation.
1. A document conveying device comprising: a document conveying unit configured to feed a document; a controller configured to control a document feeding operation performed by the document conveying unit; a first document detector located on a feeding path along which a document is fed by the document conveying unit, the first document detector configured to detect a leading edge of the document; a second document detector located at a position which is a first distance d 1 downstream from the first document detector in the document feeding direction and different from that of the first document detector in a direction orthogonal to the document feeding direction, the document feeding direction being a direction in which the document conveying unit feeds the document, the second document detector configured to detect the leading edge of the document; a pulse count detector configured to detect the number of pulses of a pulse signal that the controller uses to drive-control the document conveying unit; a determining unit configured to determine whether the number of pulses detected by the pulse count detector during the period from the detection of a document leading edge by one of the first and second document detectors to the detection of the document leading edge by the other of the first and second document detectors exceeds a predetermined value; and the predetermined value includes a first value and a second value, the first value is the sum of the number of the first pulses p 1 for causing the controller to drive-control the document conveying unit to move the document by the first distance d 1 in the document feeding direction and the number of the second pulses p 2 for moving the document by the second distance d 2 which is greater than or equal to the first distance d 1 in the document feeding direction, and the second value is a value given by subtracting the number of first pulses p 1 from the number of the second pulses p 2 , wherein when the determining unit determines that the detection of the document leading edge by the first document detector precedes the detection of the document leading edge by the second document detector, the determining unit uses the first value as the predetermined value to determine whether the number of pulses exceeds the predetermined value, when the determining unit determines that the detection of the document leading edge by the second document detector precedes the detection of the document leading edge by the first document detector, the determining unit uses the second value as the predetermined value to determine whether the number of pulses exceeds the predetermined value, when the determining unit determines that the number of pulses exceeds the predetermined value, the controller stops the document feeding operation. 2. The document conveying device according to claim 1 , comprising a conveying roller pair having rotation axes extending in the direction orthogonal to the document feeding direction, the conveying roller pair being a pair of conveying rollers with which a leading edge of a document that has passed through the first and second document detectors is brought into contact by the document conveying unit, the conveying roller pair configured to feed the document whose leading edge has been brought into contact therewith while holding the document in the nip between the conveying rollers.