Substrate bending stiffness measurement method and system

Apparatus for real time measuring of print substrate sheet bending stiffness and thereby estimated basis weight of different basis weight print substrate sheets for sheet separating control comprising,

• a sheet corrugator having a plurality of substantially spaced apart extending ribs of different extensions providing a variable sheet engaging profile,
• and a vacuum system for applying a vacuum between a stack of print substrate sheets of unknown basis weight and said corrugator,
• said vacuum being sufficient to raise the topmost said print substrate sheet in said stack up against said corrugator and to deform said print substrate sheet against said spaced apart extending ribs of different extensions,
• forming with said vacuum a variable deformed profile of said print substrate sheet against said spaced apart extending ribs of different extensions which deformed profile varies with said sheet bending stiffness of said print substrate sheet
• a sensing system for sensing said variable deformed profile of said print substrate sheet against said spaced apart extending ribs of different extensions to provide electrical signals corresponding to the basis weight of said print substrate sheet, and
• a lookup table of stored information for comparing said related electrical signals therewith to provide a variable sheet separating control signal for separating said print substrate sheets from said stack thereof with variable sheet separating forces related to their estimated basis weight.