Rotating machines include motors, generators, turbines, pumps, engines, and the like or a combination of these. Inherent with any machine that has rotating elements is the dynamic property that at specific speeds the machine tends to vibrate badly. This property has nothing to do with the "balance" of the rotating elements but rather stems from the combination of masses, that make up the machine, and the coupling of these masses, by other elements, which in themselves are described by the term "stiffnesses".
The undesirable machine vibrations are related to specific speeds, which are called "critical speeds". Many different means to reduce the vibration amplitudes at the critical speeds are described in the literature. These include different spring and damping devices whereby the energy associated with these critical speeds is redirected into some other motion or converted into another form of energy, specifically, heat.
The so-called dynamic vibration absorber takes the vibratory motions of the main components of the rotating machine and converts them into the motion of some other, less critical, component. The motion of this absorber is designed to function at a particular critical speed and thereby control the vibratory nature of the machine at one speed.
Many machines employ both a dynamic vibration absorber and some type of energy damper (which converts motion into heat). All of these devices are "passive" as a mode of operation. This means that once the device is designed and installed on the machine, no significant changes can be attained, so far as the capability to change the amount of energy absorbed is concerned. This limitation is severe for machines which may have changes in the dynamic properties over a period of time. So machines that suffer changes from erosion of the moving components (rotating or translating), or conversely the build-up of mass, generally run worse the longer they are in operation.
The alternative to passive devices, for the control of vibrations, is an "active" device. This implies a device that somehow senses the vibration level or other parameters and changes, is "active", in its response to the vibration. This is to imply some association, actively, with the speed of the machine and/or vibratory levels of some specific parts of the moving elements.
Active vibration level control systems are rather limited in number. One approach is through magnetic support systems which have been applied primarily to so called "rigid body" devices. Rigid body means that the main moving element behaves primarily as a stiff body with very little relative motion of one point on the body with respect to every other point on the body.
Long turbines, motors, pumps, et cetera, that do not behave as rigid bodies are generally more difficult to design so as to avoid having the undesirable dynamic motions when they are running. These "flexible rotor" machines are of special importance to the concepts embodied in this invention. By proper control, the vibrations inherent with the many critical speeds of flexible rotor machines can be reduced by this invention.