A low cost, fully automatic, thickness detection system, readily integrated into an existing paper path sheet transport, in which a regular and known small displacement signal is generated, compared, combined and/or superimposed on the signals from the small displacement of a sheet transport element with differences in sheet thickness. Even sub-millimeter sheet thicknesses can be measured of sheets moving through a regular sheet feeding nip with nip idler roller displacement using a low-cost sensor, where that sensor is additionally repetitively actuated by known small movements to provide calibration signals, such as those provided by plural small radius differences of a rotating idler roller shaft area which will enable existing idler and idler shaft run-out movement error noise signals to be overcome.