Design and prototyping of an actuator for vibration control

As space structures become larger and lighter, there is an increasing problem with low frequency vibrations. Such vibrations are difficult to remove using passive means, but developments in structural vibration control have made it possible to characterize and remove these vibrations actively However, such approaches typically rely on high order, adaptive central systems with many inputs and outputs. A challenge arises in that these controllers are dependent on the dynamics of the structure they seek to isolate. To assist in the development of vibration isolation systems, we have developed an actuator which seeks to improve performance using only internal position measurements. As such, the actuator can act as a variable-stiffness member and can improve vibrational performance of the structure. In addition to providing good isolation itself, such an actuator also has the potential to enhance the performance of systems based on more sophisticated controllers. The actuator consists of two linear motors connected in series via a common housing. Inexpensive optical sensors provide measurements of the position of each armature relative to the housing. These sensors allow compensation at low frequencies for flexure stiffness and magnetic damping as a function of internal position. Using this approach, the dynamics of the structures connected to the actuator do not affect the stability of the controller. The two-stage nature of the actuator provides not only a measure of redundancy, but additional functionality and performance. Since the two sides of the actuator operate in series, the overall vibration isolation may be enhanced. Alternatively one side can provide positioning while the other provides isolation, or one side could provide isolation while the other acts as an active tuned-mass-damper to suppress a particular pure tone disturbance. In this paper we present the design and analytical model of the actuator. Additionally, we discuss the construction, characterization, and initial tests of a prototype.