Optically based on-line fiber monitoring system with drift compensation

Abstract

Denier and other optically transmissivity-dependent parameters of yarn are monitored on-line and in real-time with a preferably microprocessor-controlled high frequency modulated-demodulated optically-based system. An LED is spaced-apart from an array of preferably identical photodetector ('PD') diodes that will track one another, with the yarn spaced between one of the PDs. The LED outputs preferably collimated light in response to a periodic LED drive signal, and yarn characteristics will affect light received by at least one PD in the array. An output signal from a first PD is fedback to the LED driver to continuously compensate the system against dust or other factors affect LED transmission and PD light reception. An output signal from a second PD is coupled to a fraction of the drive signal so as to enhance dynamic range. This signal is amplified and AC-coupled to a sample and hold that samples during a stable active portion of the LED drive signal to demodulate the relevant signal. The sampled and held magnitude is digitized and coupled to a CPU to provide measurement data as to denier and other characteristics of the yarn. The CPU controls operation of the system and can communicate bilaterally with a host computer. A third PD is used to self-calibrated the system.