COMPARISON OF VIBRATION AMPLITUDE SUPPRESSION VS. DYNAMIC BEARING LOAD SUPPRESSION IN ACTIVE VIBRATION CONTROL OF ROTATING MACHINERY

Excessive vibration in rotating machinery is a problem encountered in many different fields, causing such difficulities as fatigue of machinery components and failure of supporting bearings. Passive techniques, though sometimes limited in their capabilities, have been used in the past to attenuate vibrations. Recently active techniques have been developed to provide vibration control performance beyond that provided by their passive counterparts. Most often, the focus of active control methods has been to suppress rotating machinery displacements. In cases where vibrations result in bearing failures, displacement suppression may not be the best choice of control approaches (it can, in fact, increase dynamic bearing loads which would be even more harmful to bearings). This paper presents two optimal control methods for attenuating steady-state vibrations in rotating machinery. One method minimizes shaft displacements while the other minimizes dynamic bearing reaction forces. The two methods are applied to a model of a typical rotating machinery system and their results are compared. It is found that displacement minimization can increase bearing loads, while bearingload minimization, on the other hand, decreases bearing loads without significant change in shaft displacements. Unwanted vibrations have historically been a major problem facing users of rotating machinery. The primary sources of these vibrations are unbalanced machinery or misaligned shafts, both of which are steady-state in nature. Vibration problems manifest * Assistant Professor, Associate Member AIAA, ASME. ** Associate Professor, Member ASME. themselves in two ways; either in producing excessive displacements of the rotating shaft in machinery, or in producing excessive dynamic reaction forces at supporting bearings. Excessive displacements can bring about severe damage if large enough to cause machinery to exceed clearances, or fatigue of the shaft or other components. The most common problem, however, is bearing failure brought about by excessive dynamic bearing loads. Bearings which are designed for their running speed under static load conditions may see marked increases in loads as rotating unbalance and shaft misalignment problems arise. These dynamic loads cause increased wear on the bearings, which ultimately result in more maintenance and down-time for machinery . Classical, passive vibration compensation techniques include adding damping or altering the system's natural frequencies to avoid an excitation harmonic. For best performance these techniques require constant monitoring of the system so that adjustments can be made as changes occur in the system and disturbance dynamics. In recent years, attention has been focused on developing active techniques which can not only provide better vibration control than passive methods, but which can continuously adjust to changing disturbances in the system. By far, the majority of the work done to date in active vibration control of rotating machinery has focused on suppressing the amplitude of shaft or rotor displa~ements'~~. This is a relatively straight-forward idea, since displacement or acceleration measurements are easily accessible. In the past, bearing loads have not been measurable in typical rotating systems, so direct minimization of bearing loads has not been given much attention. This paper presents an active control method whereby dynamic bearing loads are suppressed instead of shaft displacement amplitudes. It will be shown that Copyright