Vibration Control of Active Structures, an Introduction 3 Rd Edition

1 Introduction 1.1 Active versus passive Consider a precision structure subjected to varying thermal conditions; unless carefully designed, it will distort as a result of the thermal gradients. One way to prevent this is to build the structure from a thermally stable composite material; this is the passive approach. An alternative way is to use a set of actuators and sensors connected by a feedback loop; such a structure is active. In this case, we exploit the main virtue of feedback, which is to reduce the sensitivity of the output to parameter variations and to attenuate the effect of disturbances within the bandwidth of the control system. Depending on the circumstances, active structures may be cheaper or lighter than passive structures of comparable performances; or they may offer performances that no passive structure could offer, as in the following example. Until a few years ago, the general belief was that atmospheric turbulence would constitute an important limitation to the resolution of earth based telescopes; this was one of the main reasons for developing the Hubble space telescope. Nowadays, it is possible to correct in real time the disturbances produced by atmospheric turbulence on the optical wave front coming from celestial objects; this allows us to improve the ultimate resolution of the telescope by one order of magnitude, to the limit imposed by diffraction. The correction is achieved by a deformable mirror coupled to a set of actuators (Fig.1.1). A wave front sensor detects the phase difference in the turbulent wave front and the control computer supplies the shape of the deformable mirror which is required to correct this error. Adaptive optics has become a standard feature in ground-based astronomy.