Robust control of nonlinear tape transport systems with and without tension sensors

Web-winding systems, such as tape drives, are often modeled as linear and time-invariant, but at least two nonlinearities are common in these systems. First, the reel radii and moments of inertia change as web media spools from one reel to an other Second, friction can draw a thin layer of air between the layers of web media wrapped on the take-up reel, making the system's spring and damping characteristics nonlinear by allowing a greater length of media to vibrate freely. In addition to these nonlinearities, there is often uncertainty in the motor parameters. In the first part of this paper, feedback linearization ideas motivate state feedback and changes of variables that transform the system into decoupled and intuitively meaningful tension and velocity loops. For the case where tension measurements are available, Lyapunov redesign techniques are then used to develop control laws that are robust with respect to these nonlinearities and uncertainties. The second part of this paper then develops an observer-based controller for the case where no tension measurements art, available. Performance is established analytically for both the measurement-based and observer-based schemes. Simulations illustrate this performance.