Measuring unsteady axial velocity of fibres and threads

To measure – in fact even sense – axial motions of fibres and threads is difficult. Especially synthetic fibres may lack any distinguishable features on their surface, so that they seem to be motionless and optical sensing fails. The motion may be transferred by friction to a mechanical component (e.g., a pulley) the speed of which is measured, but this fails if the fibre motion is unsteady (periodic—a common situation in textile machinery). The unique solution by the described fluidic sensor was developed to measure velocity of the weft in shuttle-less looms. In the sensor, the weft passes axially through colliding air jets, accelerating one of them while the opposite jet is slowed down. Output signal is the pressure difference in collectors surrounding the stagnation point of the collision (and converted by a differential pressure sensor to the electric output). The design successfully tested is described in detail. Surprisingly, later performed computations discovered that the sensor actually operates in a manner not in complete agreement with the original idea upon which its design was based.