Active Damping of Structures with Guy Cables

Abstract

HE current design of the space station is largely based on trusses, but it is quite likely that the future large space structures (LSS) will use large trusses connected by tension cables, to increase their global stiffness, in a way similar to that used to stiffen the airplanes in the early days of aeronautics. This concept of tension trussstructure has already beenused for large mesh antennas (a 10-m deployable mesh antenna was e own by the Russians in 1979 ); it has the advantages of being deployable and easily recone gurable by changing the static tension in the cables and of lending itself naturally to shape control. 1;2 The active damping of LSS has long been recognized as a major issueforvariousreasonssuchastheinteractionofthee exiblemodes with the attitude control system, the pointing requirements of various instruments mounted on the station, or simply preservation of the microgravity environment. The active damping of truss structures has received considerable attention for the past 10 years or so, and effective solutions have been proposed 3;4 ; active struts including piezoelectric actuators have been developed, and control laws with guaranteed stability have been tested successfully. 5 The active damping of cable structures is more dife cult because cables and strings behave in a nonlinear manner and are prone to parametricexcitationwhenthefrequencyofthesupportingstructure is close to twice the natural frequency of the cable. Chen 6 showed thatthevibrationofastringcanbecontrolledby apositiveuseofthe parametric excitation resulting from the longitudinal motion of the support at afrequencyequal to twice the frequency of the transverse vibration of the string (stiffness control ). The damping of cable structures has also become a major issue in civil engineering because the ever-increasing span of the cable-stayed bridges 7 makes them more sensitive to wind and trafe c induced vibrations as well as to e utter instability. The distinctive feature here is the presence of some sag in the vertical plane, resulting from the gravity loads (typical value of the sag to length ratio is 0.5%). The active damping of cable-stayed bridges with an active