Capstan design and control for drawing optical fiber: A case study in mechatronics design

This paper presents a case study on the design of a draw capstan drive with feedback control for use in optical fiber manufacturing. Optical fiber is manufactured by the draw process, which involves heating and pulling high purity glass cylinders to diameters of 125 micron. Of critical concern is producing a constant diameter for the glass fiber and its lightguide core. The diameter of the optical fiber must remain constant to create a product capable of transmitting highbandwidth optical data. The optical fiber draw capstan design has a significant impact on the resulting fiber quality. As the draw speed is used to control the fiber diameter, the ability of the draw capstan to follow velocity commands directly affects the resulting fiber diameter. In this case study a systems approach is used for the design of the mechanical and control aspects through parametric evaluations and modeling, as well as simulation studies of the capstan drive. Disturbances in the draw process arise from sources such as the variation in the diameter of the input glass cylinder and the draw tension control, affecting the glass temperature and viscosity. Simulation studies demonstrate that speed regulation, to manufacture optical fiber within allowable diameter tolerances, is achievable in the presence of representative disturbances.The capstan model and design along with the fiber-drawing process model presented in this case study are suitable for undergraduate and graduate courses in system dynamics, control, and mechatronics. As is typical of many problems in manufacturing processes, the problem discussed is multi-disciplinary. The study highlights the use of mechanical and electrical modeling, system identification, and control design as necessary parts of product and process improvement.