Image reading apparatus and its driving method

A driving method for driving an image reading apparatus including a sensor array having a detecting surface on which a detecting object is placed and a drive controller which reads an image pattern of the detecting object placed on the detecting surface, the method comprising:

• applying a signal voltage having a first signal waveform that varies periodically to a counter electrode which is provided on an upper portion of the sensor array such that the counter electrode is provided at a lower side of a first detection electrode which comprises the detection surface at an upper side thereof with an interlayer insulating film provided between the counter electrode and the first detection electrode, to excite a second signal waveform to the first detection electrode;
• detecting a third signal waveform, which is excited to a second detection electrode upon contact of the detecting object with both the first detection electrode and the second detection electrode, the second detection electrode comprising a conductive case member that surrounds the sensor array and being electrically insulated and spaced apart from the first detection electrode, and the detecting being performed by a detecting circuit connected to the second detection electrode;
• detecting whether the detecting object has been brought into contact with the detecting surface and determining whether the detecting object that has been brought into contact with the detecting surface is a specific detecting object based on a voltage level value of the third signal waveform detected by the detecting circuit;
• and
• starting reading of the image pattern by the drive controller when it is determined that the detecting object has been brought into contact with the detecting surface and that the detecting object is the specific detecting object;
• wherein detecting whether the detecting object has been brought into contact with the detecting surface and determining whether the detecting object is the specific detecting object comprises:
• comparing the third signal waveform with a threshold voltage that is preset based on a capacitance component and a resistance component of the specific detecting object, and determining whether the threshold voltage is included within a range of a voltage amplitude of the third signal waveform;
• and
• detecting whether the detecting object has been brought into contact with the detecting surface and determining whether the detecting object is the specific detecting object when it is determined that the threshold voltage is included within the range of the voltage amplitude of the third signal waveform, and
• wherein the threshold voltage is set either to a voltage that is higher than an upper limit value of the third signal waveform excited to the second detection electrode in a state in which the detecting object does not come into contact with the detecting surface, or to a voltage that is lower than a lower limit value of the third signal waveform excited to the second detection electrode in a state in which the detecting object does not come into contact with the detecting surface.