In a recirculating document handler for a copier for recirculating document sheets from a stack thereof in a document tray of the document handler to be copied and to be restacked therein after copying, with a document feeder for feeding the documents from the stack which is automatically controlled...http://www.google.com.au/patents/US4589645?utm_source=gb-gplus-sharePatent US4589645 - Document set separator and stack height sensor
