A combine harvester, comprising: a threshing assembly comprising one or more threshing rotors mounted in the harvester; a grain pan configured to: receive a mixture of grains and chaff from the threshing assembly; and undergo a reciprocating motion in a longitudinal direction of the harvester; one or more sieves configured to: receive a layer of grains and chaff from the grain pan ; and undergo a reciprocating motion in the longitudinal direction of the harvester; and a drive mechanism configured to drive the longitudinal reciprocating motion of the grain pan , and further capable of superimposing on said longitudinal reciprocating motion a lateral reciprocating motion component; said combine harvester further comprising a control unit configured to receive input signals representative of one or more parameters related to the operation of the combine harvester, said control unit being further configured to: calculate a lateral reciprocating motion component of the grain pan , as a function of said input signals; and send a control signal to said drive mechanism, commanding the drive mechanism to superimpose the calculated lateral reciprocating motion component on said longitudinal reciprocating motion of the grain pan , characterized in that the combine harvester further comprises a plurality of sensors electrically coupled to said control unit and configured to measure a thickness of a grain portion of a stratified grain/chaff layer advancing on the grain pan towards an exit edge of the grain pan, said plurality of sensors being located in the vicinity of the exit edge and distributed at locations across the width of the grain pan , and wherein the control unit comprises an algorithm configured to calculate a lateral reciprocating motion component of the grain pan that minimizes the difference between the thickness of said grain portion of the grain/chaff layer measured by said plurality of sensors . a threshing assembly comprising one or more threshing rotors mounted in the harvester; a grain pan configured to: receive a mixture of grains and chaff from the threshing assembly; and undergo a reciprocating motion in a longitudinal direction of the harvester; receive a mixture of grains and chaff from the threshing assembly; and undergo a reciprocating motion in a longitudinal direction of the harvester; one or more sieves configured to: receive a layer of grains and chaff from the grain pan ; and undergo a reciprocating motion in the longitudinal direction of the harvester; and receive a layer of grains and chaff from the grain pan ; and undergo a reciprocating motion in the longitudinal direction of the harvester; a drive mechanism configured to drive the longitudinal reciprocating motion of the grain pan , and further capable of superimposing on said longitudinal reciprocating motion a lateral reciprocating motion component; calculate a lateral reciprocating motion component of the grain pan , as a function of said input signals; and send a control signal to said drive mechanism, commanding the drive mechanism to superimpose the calculated lateral reciprocating motion component on said longitudinal reciprocating motion of the grain pan , <EOS>