Robust Multi Degree of Freedom Hybrid Vibration Absorption

A robust vibration absorber reduces vibrations of a system over a range of frequencies. Passive vibration absorption is achieved with a tuned mass and spring (known as a dynamic vibration absorber (DVA)). This is a simple, low cost method of reducing vibrations at a single frequency; however, in order to achieve robust absorption a heavy mass is required. Active vibration control is achieved with an actuator capable of applying a force to the system at any range of frequencies, but this method is expensive. Combining active and passive absorbers allows for robust absorption with minimum passive absorber mass. Various methods of combined absorption have already been proposed for systems with a single resonant frequency. This paper investigates hybrid robust vibration control of a primary system with frequency components which are the first three harmonics of the excitation of the system. The hybrid absorber consists of a three mass-spring system. Passive absorption is achieved with three masses and joining springs, the active absorption is modelled as springs in parallel with the passive springs but with variable stiffness. Variations in frequency harmonics are proportional; absorption is maintained at a range of frequencies by altering the active spring stiffnesses.