Adaptive Control for a Microgravity Vibration Isolation System

Most active vibration isolation systems that try to a provide quiescent acceleration environment for space-science experiments have utilized linear design methods. In this paper, we address adaptive control augmentation of an existing classical controller that combines a high-gain acceleration inner-loop feedback together with a low-gain position outer-loop feedback to regulate the platform about its center position. The control design considers both parametric and dynamic uncertainties because the isolation system must accommodate a variety of payloads having different inertial and dynamic characteristics. An important aspect of the design is the accelerometer bias. Two neural networks are incorporated to adaptively compensate for the uncertainties within the acceleration and the position loop. A novel feature in the design is that highband pass and low pass filters are applied to the error signal used to adapt the weights in the neural network and the adaptive signals, so that the adaptive processes operate over targeted ranges of frequency. This prevents the inner and outer loop adaptive processes from interfering with each other. Simulations show that adaptive augmentation improves the performance of the existing acceleration controller and at the same time reduces the maximal position deviation and thus also improves the position controller. Introduction The low-acceleration environment on the International Space Station (ISS) will enable microgravity science experiments that are practically impossible on the surface of the Earth. However, a variety of vibro-acoustic disturbances on the ISS are present and can degrade the performance of many microgravity experiments. In fact, the acceleration environment on the ISS is expected to exceed the requirements of many acceleration sensitive experiments as shown in Figure 1(a). By comparing the requirement with the expected ISS acceleration in Figure 1(a), an isolation performance specification can be derived as in Figure 1(b). The isolation system must attenuate the ambient ISS acceleration by one order of magnitude at 0.1 Hz, which for a second order system implies maximum break-frequency of 0.01 Hz. That is, while the isolated system can transmit the quasi-steady accelerations of the vehicle below 0.01 Hz to the isolated assembly, it must attenuate all disturbances above 0.01 Hz. This performance specification requires the implementation of an active vibration isolation system because passive isolation systems, in general, are not able to provide sufficient attenuation of low vibration frequency disturbances. ∗Postdoctoral Fellow, [email protected], AIAA member †Professor, [email protected], AIAA Fellow ‡Professor, [email protected], Senior AIAA member §Branch Head, [email protected], AIAA Associate Fellow