Control device and method for "on the fly" printing machines

A method for controlling an on the fly franking machine wherein documents (20) fed into said machine at a feed rate are conveyed in series at a transport velocity along a conveyor path one behind the other and spaced apart from one another, each of said documents (20) being conveyed past a rotating print head, said print head being rotated at a circumferential surface velocity (V) said method comprising the steps of:

• providing a print head and a conveyor, said conveyor having a transport velocity, and said print head having a first angular portion (2) of its circumferential surface in contact with each document as the document is conveyed past said print head during a portion of a rotational cycle of the print head defining a printing phase, and said print head having a complementary angular portion (1) of its circumferential surface not in contact with said document as the document is conveyed past said print head during the remainder of the rotational cycle of the print head defining a compensation phase
• holding the surface velocity of said print head equal to said transport velocity during the printing phase of a first document,
• using a continuous learning process to vary said surface velocity of said print head by rotating said print head at a catch-up velocity to correctly position said first angular portion of the print head relative to a second document following said first document without stopping the rotation of said print head during said compensation phase, and
• using said continuous learning process to vary said transport velocity to hold said catch-up velocity as close as possible to said transport velocity during said compensation phase while preventing the overlapping of new consecutive documents as they are fed in series into said machine.