Cascaded Control for Regulating Soot Geometry in Vapor-Phase Axial Deposition

The development of a cascaded feedback control strategy for a vapor-phase axial deposition (VAD) process is investigated in this paper. VAD is a widely used process in the creation of high purity glass for optical fiber. In previous work a soot tip surface temperature controller was developed for the VAD process to reduce the effects of core soot temperature variation on deposition, leading to a more stable process. However, this approach did not address the need to regulate and link the deposition rates of the core and clad torches. To maintain a constant distance between the core and clad deposition surfaces, it is desired to have the core soot and clad soot depositing at the same linear speed to provide a more uniform product. This paper presents the design and development of a cascaded controller strategy and process model to couple and regulate the surface temperature and deposition rates of core and clad soot. Simulations for the process and control scheme demonstrate a potential improvement in the uniformity of the core and clad soot geometry over the soot product length.