Back Story [About This Issue]

For several decades, one of the most popular themes in control research has been the desire to use feedback to suppress vibrations. Within an idealized linear range of motion, structures without rigid-body modes are open-loop stable and thus need not be actively -stabilized. This property holds regardless of the components that make up the structure, such as beams, plates, or membranes. But these systems are extremely challenging to control for several reasons. First, structures typically have lightly damped eigenvalues and thus can be inadvertently destabilized. Next, the frequencies of vibration, damping levels, and mode shapes are typically uncertain despite the elegance of textbook models. And third, the density of modes typically increases at high frequency, where density refers to the number poles per frequency range. All of these issues apply to active noise (that is, acoustic) control as well since air vibrates in compression as a kind of three-dimensional structure. Active vibration control has thus served as a motivation for developing new control techniques as well as a testbed for determining the -limitations of methods developed for other applications. To get a feel for some of the challenges of active vibration control, see the article by Fuller and Flotow in the December 1995 issue of this magazine.