Abstract: Along with spacecraft growing complex, flexible space structures such as large solar panels and large deployable antenna are often applied. To achieve ideal attitude performance and antenna pointing accuracy, vibration control receives much attention. It is worth noticing that flexible space structures often possess close modes at low frequency, which potentially poses problems for the vibration control system. Characteristics and impacts of close modes must be taken into consideration in object modelling, actuators/sensors placement and controller design. In this paper, the reason for modal instability of close modes is first deduced, followed by the proof of model uncertainty caused by this modal instability. Then, based on the singular values of controllability Grammian, the particularity on the controllability of close modes is analyzed. Moreover, the spillover disturbance is demonstrated to show the impact of close modes further. Based on these characteristics, suggestions for vibration control of structures with close modes are proposed, which offers important reference value to onorbit vibration control of flexible space structures.

Key words: close modes, modal instability, low controllability, spillover