Vibration control using a tunable vibration neutralizer

The harmonic vibration of a machine at one position can be controlled using a vibration neutralizer whose natural frequency is the same as the excitation frequency. However, a conventional vibration neutralizer is only effective over a narrow frequency band and is unable to track changes in excitation frequency. A way of overcoming this problem is to use a tunable vibration neutralizer which, due its variable stiffness capability, can be tuned to operate at different frequencies. With this system, however, there is an inevitable delay between a change in the excitation frequency and the re-tuning of the neutralizer. This delay is dependent upon the control system, and the way it affects the vibration of the machine depends upon certain design parameters. This paper discusses the behaviour of a machine fitted with a tunable vibration neutralizer during the adaptation process when the system is in a transient state. It is shown that if a neutralizer cannot respond quickly enough to a frequency transient, its contribution can be to increase the vibration level of the machine. A theoretical study is presented which shows that to avoid such an increase in machine vibration a maximum control time delay should be imposed. This delay is dependent upon the rate of change of excitation frequency and the ratio of the mass of the neutralizer to that of the machine. Experiments are conducted to validate the theoretical predictions using a tunable neutralizer which consists of a mass supported by an air mount, whose natural frequency can be adjusted by varying the pressure in the mount.