Dynamics of tendon vibration control system for flexible beam.

Abstract

Large Space Structures (LSS) such as space stations are expected to have the dynamic characteristics of high flexibility and low. inherent damping owing to their large dimensions and lightweight construction materials. Therefore, the needs for active control of vibrating LSS arise and the concept of actively controlled large space structures has received a great deal of recent attention. A tendon control system is proposed by the authors for the vibration control of a cantilever beam-like flexible space structure. It consists of a torque generator, placed at the root of a structure and linked with a couple of moment arms through tensile wires (tendons), thereby applying vibration control torques to the structure at the arm position. This paper discusses the dynamics of the tendon control system for the ground test beam hung in a vertical direction to investigate the feasibility of this control method. First, the tendon actuator is modeled as an added one-degree-of-freedom vibrating system and the equation of motion is derived for the flexible beam considering the effect of gravitational load and axial force generated by the tension of the tendons. Then, the mathematical model of the whole system is formulated, combining both sub-systems. Finally, a discretized dynamical model is constructed through FEM. By using this model, an open-loop transfer function from the applied moment at the upper link to the rotation angle of the moment arm is investigated. It is shown that only one open-loop zero vanishes at the frequency point determined by the natural frequency of the tendon actuator system and that its phase lags by as much as 360 degrees. Further, this fact is verified experimentally. Finally it is concluded that the dynamics of the tendon actuator system plays an important role in the design of the tendon control system for the flexible beam.