Semi-Active Control by a Cantilever-type Dynamic Absorber

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The vibration reduction by a cantilever-type dynamic absorber whose natural frequency can be controlled was theoretically analyzed. Then, an auto-tuning system to keep the antiresonance was made. The control algorithm has two stages, rough tuning and precise tuning. In the rough tuning, the exciting frequency is detected and the natural frequency of the arm length of absorber is tuned due to the data base stored in computer. In the precise tuning, the arm length of absorber is controlled such that the phase lag of the vibration between the absorber and the main body to be 90 decree.This system was applied to a beam and a rigid body which was supported by four sprines. In the case of rigid body, the absorber must be set such that the direction of the arm is normal to the nodal line. Then, both the natural frequency and the direction of the arm must be controlled. In the experiment, the vibration was reduced by about 15 decibel.When the exciting frequency shifts slowly and the absorber can track it without delay, the system keeps the antiresonance. However, when the tracking has a delay, the system goes apart from the antiresonance and the vibration increases. By increasing the damping, the increase of vibration can be avoided. This system was also applied to a duct noise.