Abstract:

Objective In order to fulfill the online detection of the moisture regain of the sized yarn in the sizing process, to improve the sizing quality, to reduce the carbon emission caused by sizing, and to solve the problem that the use of a contact type resistive measuring instrument may damage the sizing film, a measurement system for sizing moisture regain was developed.
Method According to the electric field model in the arc capacitor and the measurement principle of capacitive method, the model relating the capacitance difference and moisture regain was established. The sized yarn passed through the arc capacitor, and the LCR (inductance, capacitance, resistance) bridge tester captured the capacitance signal and transmitted it to the computer. The moisture regain of the sized yarn was calculated according to the model. The influences of the structure of the arc capacitor, the test parameters of the LCR bridge tester, the test environment, and the solid content of the slurry on the capacitance readings were discussed.
Results The experimental results show that the parameters of voltage, acquisition frequency, and internal resistance of the LCR bridge tester have little effect on capacitance value, while it is greatly affected by the testing frequency (Fig.3). At the frequency of 4 000 Hz, the capacitor was found to have the most significant response to water content (Fig.4), and hence 4 000 Hz was selected as the testing frequency of LCR bridge tester. It was revealed that as the curvature radius of the arc capacitor’s electrode plates increases, the capacitance value decreases, and that when the length of the electrode plate increases, the capacitance value increases. With the wrap angle of the electrode plate was increased, the capacitance value also increased (Fig.5). The capacitance simulated by the finite element technology were consistent with the measured results. The differences between the measured and the simulated values are caused by additional capacitance and environmental interference. Temperature and humidity were found to have a complex impact on the capacitance value (Fig.6), indicating that the system should be deployed under constant temperature and humidity conditions. The moisture regain of sized yarn demonstrated a significant effect on the capacitance, while the influence of fineness and size solid content were not obvious (Fig.7), where it can be seen that moisture regain is nonlinearly related to the capacitance difference. When moisture regain was increased, condensed water on and inside the yarn increased and a strong polarization reaction occurred at the frequency of 4 000 Hz with the capacitance value increasing. An exponential regression model of capacitance difference-sizing moisture regain is established. On the sizing machine, the capacitive sizing moisture regain online detection system developed by the research was adopted to measure the sizing moisture regain. Compared with the results of the oven method, the average error sizing moisture regain measurement was less than 5% (Tab.1).
Conclusion The capacitance value is greatly affected by the test frequency of the LCR bridge tester. The structure of the arc capacitor also influences the test capacitance value. The ambient temperature and humidity have complex effects on capacitance values. The influences of sized yarn moisture regain on capacitance are significant, while fineness and size solid content are not so obvious. The core components of a capacitive sizing moisture regain measurement system designed include an arc-capacitor sensor, an LCR bridge tester and a computer. The arc capacitor has a small and simple structure with low cost. It can measure sized yarns in a non-contact manner. Therefore, this system can be used in engineering practical situations.

Key words: sized yarn, sizing, moisture regain, arc capacitor sensor, online detection