Capstan Speed Control in the Optical Fiber Drawing Process: A Case Study for Mechatronics

Industrial case studies of successful implementations of combined mechanical and closed-loop control design provide students with meaningful examples of classroom theory. Such a case study on the development of a capstan drive with feedback control for use in optical fiber production is presented in this paper. Optical fiber is manufactured by the draw process, heating and pulling high purity glass cylinders to diameters smaller than human hair. Many process and product parameters are controlled during drawing of the fiber. Of critical concern is producing a constant diameter for the glass fiber and its light-guide core. Uniformity of fiber glass diameter within the necessary tolerances creates a product capable of high bandwidth optical data transmission as well as costeffective production. The optical fiber draw capstan design has direct impact on the resulting fiber quality. A systems approach to the design of mechanical and control aspects is demonstrated through mechanical/electrical parametric evaluations and modeling as well as in the simulation of the capstan drive. Disturbances in the draw process arise from several sources including the starting glass diameter variation and the draw tension control which affects the glass temperature and viscosity. Simulations reveal the achievable fiber diameter tolerance, with the completed design and control scheme in the presence of disturbances. The fiber drawing process description, process model, and capstan design are presented as a case study suitable for undergraduate or graduate courses in system dynamics, control or mechatronics. The problem discussed is of a multi-disciplinary nature, typical of many manufacturing processes problems. The case study highlights the use of mechanical and electrical modeling, system identification, and control tools as means of product/process improvement.