Abstract
Sensors used in sheet manufacturing environments can be sensitive to the gap between the scanner heads. Nuclear gauges measure both the sheet product and gas in the column between the scanner heads. The gap distance can affect the measurement in a linear or non-linear fashion depending on the sensing principle High frequency vibrations can cause the sensor devices therein to move. Techniques for controlling the sensor gap between operative surfaces on dual scanner heads during measurement operation employs a closed-loop control where piezoelectric actuators are used to maintain a constant gap throughout the scan by closing the error signal on the gap measurement sensor. A system for measuring a property of a continuous sheet includes dual scanner heads housing radiation emitters and receivers. Gap measurement signals energize the actuators that adjust the position of one or both sensor devices to maintain the gap at a desired distance.