Sheet feeder that detects multi-feed of sheets and image forming apparatus

摘要

A sheet feeder which is capable of accurately detecting multi-feed of sheets even when the distance between sensors is changed. The sheet feeder includes a sheet container for holding a stack of sheets. Ultrasonic sensors for transmitting and receiving ultrasonic waves, respectively, are provided on a conveying path. The ultrasonic sensor as a transmitter transmits ultrasonic waves to a sheet being fed and conveyed. A reception circuit detects a level of ultrasonic waves received by the ultrasonic sensor as a receiver. The level of ultrasonic waves detected after lapse of a predetermined time from transmission of ultrasonic waves is compared with a predetermined threshold value to determine multi-feed of sheets. A distance between the ultrasonic sensors is detected, and at least one of the predetermined time period and the predetermined threshold value is corrected based on the detected distance between the sensors.

主权项

1. A sheet feeder comprising:
a container unit configured to contain a plurality of sheets; a feeding unit configured to feed sheets from said container unit; a transmitter configured to transmit ultrasonic waves to a sheet being fed by said feeding unit; a receiver configured to receive ultrasonic waves transmitted by said transmitter; a first detection unit configured to detect a level of ultrasonic waves received by said receiver; a determination unit configured to determine whether or not sheets are being multi-fed, by comparing the level of ultrasonic waves detected by said first detection unit after lapse of a predetermined time period from transmission of the ultrasonic waves by said transmitter with a predetermined threshold value; a second detection unit configured to detect a distance between said transmitter and said receiver; and a changing unit configured to change at least one of the predetermined time period and the predetermined threshold value based on the distance detected by said second detection unit.