| The Modern Plastics extruder of the Material Science and Engineering Department is used to extrude high density polyethylene (Alathon 7040). Diameter sensor characterization is done due to the fiber positioning sensitivity of the sensor (Zimmer) and observed variations in the sensor signal, this reveals that there are two fiber dimeter variations, one is a periodic variation with a 4.1 second period with an amplitude of about {dollar}pm{dollar}5 {dollar}mu{dollar}m for typical 80 {dollar}mu{dollar}m diameter fiber which is called a draw resonance like variation in the research, the other is a sample to sample variation at sampling rate of 100 samples/sec. Detailed step test transfer function models are developed where the input variables are take-up speed, screw speed, gear pump speed, and die temperature set point. The output variables are die temperature, die pressure, end of the barrel temperature and pressure. A total of 18 transfer functions are obtained. The relationships between known periodic input variations and the output variations are analyzed by the power spectrum analysis. Due to the dominance of the draw resonance like variation, the main control objective is to eliminate this variation. Classical control methods, proportional-integral (PI), integral (I), and the Smith predictor dead time compensator with PI and the model predictive control method called dynamic matrix control (DMC) are applied to control fiber diameter. The results show that all control methods can handle set point tracking but achieve only a limited reduction in amplitude of the fiber diameter variation. Suggestions are made for future work to address this interesting phenomenon. |
