Patent ID: 7172195

Claim:
An image reading apparatus comprising: a processing platen having photoelectric converting means for reading an image on a sheet, a sheet feeding stacker for feeding the sheet to the processing platen, a discharge stacker for storing the sheet discharged from the processing platen, a transport guide for guiding the sheet from the sheet feeding stacker to the processing platen, sheet separating means for sequentially separating and feeding the sheet on the sheet feeding stacker, register means for temporarily holding the sheet transported from the separating means, and an ultrasonic wave sensor arranged between the sheet separating means and the processing platen for detecting the sheet or a double feed of the sheet, said ultrasonic wave sensor having a wave sending element for emitting an ultrasonic wave with a predetermined frequency and a wave receiving element for receiving the ultrasonic waves from the wave sending element, said wave sending element being arranged at a lower position in a direction of gravity relative to the sheet transport guide, said wave receiving element being arranged at an upper position opposite to the lower position, said wave sending element having a wave sending surface inclined with a first predetermined angle relative to a horizontal direction, a piezoelectric diaphragm, and an oscillation circuit for exciting the piezoelectric diaphragm with a specific frequency, said oscillation circuit selectively exciting the piezoelectric diaphragm with at least two different amplitudes, wherein said wave sending element further includes a casing with said wave sending surface directly facing the wave receiving element, said piezoelectric diaphragm being fixed to the casing at a side opposite to the wave sending surface to oscillate the casing, and said oscillation circuit supplies power to the piezoelectric diaphragm greater than that required to detect the sheet when a sheet detection is not performed so that dust on the wave sending surface is removed.