A semi-active vibration control system can be used for control of the overall vibration level of a structure where the excitation is dominated by a single frequency. An application of such a system would be to control the overall vibration response of a helicopter as the dominant excitation occurs at the blade passing frequency. The advantage of such a semi-active vibration control system is that it provides a high vibration reduction for a small mass penalty. For a 5000kg all up weight helicopter, a semi-active vibration control system could consist of 10 actuators each with a moving mass of 2kg. Such a system would reduce the overall vibration levels to less than 10% and thus provide a vibration reduction that is comparable with that of an fully active vibration control system.

A semi-active vibration control system consists of a set of inherently passive devices namely tuned mass dampers that are positioned in their optimal locations. The ‘semi’ in semi-active refers to that each device is equipped with a tuning device that locks it to the excitation frequency. Fig. 1 shows a schematic drawing of a tuned mass damper (TMD). Basically, a TMD is a mass and spring device where the natural frequency is tuned to match the frequency that is sought controlled.

Fig. 2 shows the dynamic stiffness of a TMD. The larger the dynamic stiffness is, the larger is the force that the TMD can provide and the higher the vibration reduction. The drawback of a small loss factor, η, is that the TMD is only effective over a narrow frequency band. As the excitation frequency might change slightly during operation it is necessary to make each device tuneable. The arrow through the spring in Fig. 1 indicates that the device has variable stiffness, that makes it tuneable so that it can be locked to the excitation frequency.

Positioning of the TMDs is critical for high vibration reductions. If the TMDs are positioned using a trial and error approach it is normally not possible to reduce the overall vibration reductions by more than 50%. A trial and error approach is inefficient, as the number of possible configurations easily becomes very large. For example for a semi-active vibration control system with 10 actuators and 90 possible positions, the number of possible configurations is 5.72 10¹². To determine the optimal configuration a discrete search algorithm is employed. Fig. 3 shows the progress of the search algorithm for the optimal TMD positions. When the 'search control parameter' is 30 the positions are randomly generated. As the 'search control parameter' is reduced, the algorithm converges towards the optimal positions for the TMD resulting in an overall vibration level of less than 10%.

Fig. 4 shows the vibration levels in the sensor positions with and without the 10 TMD each having a moving mass of 1 kg. The structure is an aircraft frame with a total mass of 3000 kg.

Fig. 5 shows the influence of a similar set of 10 TMDs (2 kg each) on the spatially averaged response. The TMDs causes a dip in the overall response that coincides with the excitation frequency. At the other frequencies, the influence of the TMDs is negligible as their total mass is small compared to the total mass (3000 kg) of the airframe structure.

Fig. 6 shows an example of the optimal TMD positions for a helicopter structure. The main excitation forces and moments are generated by the blades and transmitted via the hub. The majority of the TMDs are seen to be located near the hub.