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A Photodiode Array Sensor with Digital Autolevel Setting for Detecting a Small Moving Mark in a Wide Area

In order to detect a small moving mark such as a thin edge mark of a rotating disc, a highly sensitive photodiode array sensor is developed. The sensor is composed of an infrared light-emitting diode array and a receiving diode array. As the output signal of the sensor is very small due to slight changes of the light, the detecting slice level of the output signal must be very stable. For this purpose, the slice level is automatically set between the peak and bottom of the output signal by using the digital-to-analogue converter (DAC). Furthermore, the emitted light is chopped and modulated with a high frequency by switching the current of the emitting diode on/off and the output signal of the receiving diode is demodulated for the purpose of preventing the influence of outside light interference. The developed sensor has no focusing lens system and does not demand precise positioning of the sensor. These are great merits compared with conventional sensors.