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 fiberdrawing 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 multidisciplinary. The study highlights the use of mechanical and electrical modeling, system identification, and control design as necessary parts of product and process improvement. NOMENCLATURE B rotational damping d fiber diameter I motor current J polar mass moment of inertia Ki integral gain Kp proportional gain Kt motor torque constant M mass Q volumetric flow rate r outside radius of capstan pulley t time τ thickness of capstan pulley rim v belt speed ( r × ω ) ω motor rotational speed ω& ) (s motor rotational acceleration Laplace transform of ) (t ω Ω INTRODUCTION Industrial case studies of successful implementations of combined mechanical and closed-loop control designs provide students with meaningful, real-life examples that demonstrate classroom theory. The case study of this paper – the design and control of an optical fiber draw capstan – offers modeling and design challenges that make it highly suitable for teaching undergraduate and graduate students in courses such as system dynamics, controls, and mechatronics. The study exposes 1 Copyright © 2007 by ASME students to system models, practical mechanical design, motor selection, and closed-loop control system design. It highlights how design and performance are linked and can be improved through an integrated, systems-level understanding predicated on engineering fundamentals and tools from multiple technical disciplines. In industry and too often in academia, the boundaries and barriers between engineering disciplines can be significant. An objective of this case study is to broaden the perspective, and help reduce boundaries – artificial or real – by encouraging a blended problem-solving approach that draws upon several areas of technical knowledge and competence. The design problem here is solved with knowledge and tools from mechanical, electrical, and control engineering. Drawing upon multiple backgrounds is not a unique approach to solving design problems. There are many similar types of design challenges in industrial applications, and they can be viewed as mechatronic designs. Other real-life examples of mechatronic designs include auto-focus cameras, CD players, smart toasters, and high-tech toys. In general, machines and processes that rely on sensors, actuators, mechanisms, instrumentation, controllers and micro-processors of various types, sizes, and attributes can be called mechatronic systems. It could be said that large-scale systems, such as industrial plants or vehicles, with many control loops and interconnections under computer control are at one end of the mechatronic spectrum and relatively simpler devices such as magnetic bearings are at the other extreme [1, 2, 3]. DRAW PROCESS BACKGROUND Optical fiber is a light guide providing high-speed data transmission at the terabit (10) per second level [4]. Moving light through the glass fiber core relies upon the principle of total internal reflection, where the inside glass core has a higher refractive index than the cladding glass around the core, as shown in Figure 1. CLADDING (125 μm) CORE (9 μm) Figure 1. Typical glass components of optical fiber Optical fiber is manufactured by the draw process. This process is the only cost-effective means of creating optical fiber from ultra-high purity glass cylinders with the required refractive index profile [4, 5, 6]. An important manufacturing quality concern is maintaining a constant core diameter for efficient light transmission. Core geometry variations lead to dispersion of light and loss of signal strength. Figure 2 illustrates the draw manufacturing process for optical fiber. A high purity glass cylinder with a prescribed optical index profile, known as a ‘preform,’ is heated in a specially designed furnace to the point where the glass flows under low pulling tension. The draw capstan pulls the fiber from the bottom of the glass preform in the furnace; the glass preform feed drive above the furnace maintains material flow equilibrium through the furnace. The fiber is then cooled, coated with protective polymers, cured under ultraviolet lights, and wound onto spools. Figure 2. Draw manufacturing process for optical fiber The capstan controls the diameter of the fiber by adjusting the speed at which the fiber is drawn. Measurement of the input glass diameter occurs just below the furnace. (Cooling occurs rapidly in the thin fiber, so thermal expansion of the fiber relative to room temperature is inconsequential.) The capstan relies on the draw speed to control the diameter. Based on the diameter error about a set point, the capstan’s speed is increased when the fiber is too thick and the speed is decreased when the fiber is too thin. (The temperature of the furnace affects the pulling tension placed in the fiber by the capstan. While the pulling tension of the fiber is important in the draw process, it is not discussed in this paper.) The basic design of the fiber draw capstan is a flexible belt partially wound over a flat pulley that moves/pulls a continuous optical fiber all the way from the heated preform, as shown in Figure 3. Tests have validated a relationship between the fiber diameter and the line speed about an operating point. The basis of the relationship is constant volumetric flow rate of glass. 2 Copyright © 2007 by ASME Variations in fiber diameter arise quite often from process disturbances, such as non-uniformity of the preform diameter, mismatches of the preform feed volumetric flow relative to that of the draw, and drifts in the furnace/glass temperature. Thus, in addition to the mechanical design and motor selection, a speed controller must be incorporated into the capstan design to prevent impermissible variations in the fiber diameter from process disturbances. Figure 3. Optical fiber drawn by capstan DESIGN SPECIFICATIONS AND PARAMETERS The target dimensions of the optical fiber glass are an outside diameter of 125 μm and an inside core diameter of 9 μm. Product specifications call for outside diameter tolerances of ±1 μm. To achieve this specification, the permissible diameter error must be targeted to a lower dimension. Typically, the permissible diameter design deviation is ± 0.1 μm. The capstan mechanical, electrical, and control designs synergistically impact the ability to achieve this fiber diameter tolerance. With the permissible deviations specified, the next step is to understand how the selected parameters of the capstan design affect the resulting product. While many decisions must be made in any design, there are usually a limited number of critical parameters that have significant influence on operational performance. These parameters include: • The diameter and tolerances of the capstan pulley • Inertia limits • Belts, contact length, and bearings • Torques and speeds of the motors • Control gains • Maximum current and power limits of the amplifier In this mechatronic system, both the mechanical design and the control design influence the overall system performance. The primary mechanical design decision is the diameter of the capstan pulley. The required line speed and machining tolerances are the basis of the design. When no disturbances are present, these determine the maximum possible variations in diameter. The typical capstan drive is shown in Figure 4. Belt tension, damping, and bearing loads are present. The tension in the belt is significantly higher than the small tension in the fiber required to pull it (typically 50 to 100 grams tension). Dynamic effects of the capstan belt and bearings can be treated as viscous damping and additional inertia. Although the belts are not the focus of this case study, they are an important part of the design. The belt material, contact length, and belt tension must be selected appropriately to achieve non-slip belt movement with respect to the capstan pulley. The belt/capstan friction and prevention of fiber coating damage are other key considerations.