Dynamic Optical Displacement Calibration across High Speed Packaging Board Lines
Inline optical displacement calibration requires dynamic differential sensing and spatial encoder synchronization to eliminate web flutter and thermal drift errors.
Invar Reference Rod
Metrology Standard
A physical calibration bar made of a nickel-iron alloy provides a stable baseline for measuring thickness on a moving web. This component is known as an invar reference rod and is used in continuous thickness gauges because of its low coefficient of thermal expansion. In paperboard mill environments, where temperatures fluctuate due to drying steam, this stability prevents measurement drift.
Thermal Stability
Structural shifts in standard steel or aluminum parts caused by mill temperature swings can distort thickness readings. By using an invar reference rod as the datum point, the distance sensor maintains its zero point regardless of local heat variations. The alloy maintains its dimensions over a wide temperature range, ensuring that the sensor detects actual board thickness changes rather than sensor-frame expansion.
This stability is critical for keeping paperboard caliper within the tight tolerances required by automated printing lines.
Structural Integration
The rod is housed within a protected enclosure to shield it from physical impacts while allowing it to react to the ambient temperature of the sensor frame. Its length dictates the absolute reference distance between the top and bottom sensor heads. Because the alloy exhibits almost no change in length, the calibration remains accurate without frequent manual resets.