Control Approach for Mitigating Off-Axis Position Variation in Vapor-Phase Axial Deposition

VAD (vapor-phase axial deposition) is a widely used glass soot fabrication process for the creation of high purity optical glass fiber. It is critical for low signal loss and manufacturing productivity that the core and clad geometry remain constant. Variation (off-axis) of the deposition torch position relative to the deposited core soot tip can cause an appreciable change in the deposition rate and the resulting glass soot cylinder core and clad diameters. This paper presents deposition torch position control schemes to effectively eliminate the soot growth rate variation caused by the mechanical, off-axis (horizontal) positioning errors. Two control approaches are compared for their effectiveness on the resulting geometry of the deposited glass soot cylinder. A direct approach, clad diameter control, requires the addition of a non-contact sensor to feedback the measured clad diameter. The second approach, pull speed control, uses the change in the axial growth rate of the core soot as feedback. Using a previously determined empirical model of the soot deposition process, a suitable system model was implemented to simulate the VAD deposition and the resulting soot cylinder diameters. The system response to mechanical positioning errors was evaluated using the two control approaches as well as open-loop. Uniformity of soot clad diameter was used as an evaluation metric for comparison. Simulation results indicated significant diameter regulation improvement obtained by both control methods. However, since diameter measurement lagged the pull speed change, pull speed control was more effective when comparing maximum, minimum, and range of core and clad soot diameters over the deposited soot length. The control approach presented here might be extended to other situations such as machining.