Steve L. Hadden

Miniature vibration isolation system for space applications. Phase II

In recent years, there has been a significant interest in, and move towards using highly sensitive, precision payloads on space vehicles. In order to perform tasks such as communicating at extremely high data rates between satellites using laser cross-links, or searching for new planets in distant solar systems using sparse aperture optical elements, a satellite bus and its payload must remain relatively motionless. The ability to hold a precision payload steady is complicated by disturbances from reaction wheels, control moment gyroscopes, solar array drives, stepper motors, and other devices. Because every satellite is essentially unique in its construction, isolating or damping unwanted vibrations usually requires a robust system over a wide bandwidth. The disadvantage of these systems is that they typically are not retrofittable and not tunable to changes in payload size or inertias. During the Phase I MVIS program, funded by AFRL and DARPA, a hybrid piezoelectric/D-strut isolator was built and tested to prove its viability for retroffitable insertion into sensitive payload attachments. A second phase of the program, which is jointly funded between AFRL and Honeywell, was started in November of 2002 to build a hexapod and the supporting interface electronics and do a flight demonstration of the technology. The MVIS-II program is a systems-level demonstration of the application of advanced smart materials and structures technology that will enable programmable and retrofittable vibration control of spacecraft precision payloads. This paper describes the simulations, overall test plan and product development status of the overall MVIS-II program as it approaches flight.

Heavy load vibration isolation system for airborne payloads

A high-performance vibration isolation system has been developed to isolate large-sensitive payloads from aircraft disturbances. The isolation system senses and adjusts for low frequency aircraft maneuvers and changes in the aircrafts flight angle of attack. Additionally, the isolation system passively filters higher frequency disturbances from aircraft to payload. Six pneumatic struts configured as a hexapod or Stewart Platform make up the primary portion of the isolation system and accomplish vibration isolation and payload support. Each isolator strut is a unique Patented design that takes advantage of gas (the ultimate smart material), because it has a capacity for large energy storage and it possesses a near linear viscosity over a broad temperature range. Any gas that exhibits a somewhat perfect-gas characteristic can be used inside the strut with similar performance results. For our application, gaseous nitrogen (GN2) was used. The pneumatic strut has shown an ideal isolator roll-off quality that is tunable for a variety of payloads and linear over a large dynamic range. Tunability stems from a dual chamber design that allows air-spring-rate changes while maintaining constant support of the load. The strut performance trait combined with the deterministic nature of the hexapod affords predictability and controllability. The system design enables a soft floating support of large payloads with accurate knowledge of their orientation with respect to the aircraft. Another distinctive feature of the isolation system design is a servo-controlled leveling system that senses a set point from an integrally mounted LVDT and fills or exhausts gas, as necessary, maintaining strut position during the rigors of flight. A combination of Commercial Off The Shelf (COTS) and control cards with custom plumbing provides the leveling function. All tolled, the isolation system has functioned flawlessly in service, and has raised the bar for vibration isolator performance. In this paper the isolation system design will be detailed, and its performance measurements will be presented.

Structural control of a flexible satellite bus for improved jitter performance

The increasing demand for global communications and limitations on RF communications bandwidth has driven several constellations to baseline laser cross-links between the satellites within their constellations. The use of laser communications over a long distance dictates the need for accurate pointing and jitter suppression in order to maintain signal continuity. Vibrations upon a satellite bus or orbit come from several sources including: momentum systems, flexible appendages, motors and cryocoolers. Attenuation of these vibrations requires a combination of disturbance reduction, disturbance isolation, payload isolation, input command shaping, appendage damping and passive/active bus structural control. This paper addresses these techniques in a systems approach to satellite structural control. Experimental results from a representative flexible satellite truss structure using a series of integral D-Strut structural dampers is presented. The passive damping system is used to reduce resonant amplification of disturbances on precision optical equipment jitter. The use of different combinations of longitudinal, transverse and diagonal dampers is discussed to achieve specific modal damping. In addition, the design of the integral truss dampers is discussed along with their application to satellite bus construction.

Design and fabrication of a full-scale actively controlled satellite appendage simulator unit

Modern satellites require the ability to slew and settle quickly in order to acquire or transmit data efficiently. Solar arrays and communication antennas cause low frequency disturbances to the satellite bus during these maneuvers causing undesirable induced vibration of the payload. The ability to develop and experimentally demonstrate attitude control laws which compensate for these flexible body disturbances is of prime importance to modern day satellite manufacturers. Honeywell has designed and fabricated an actively controlled Appendage Simulator Unit (ASU) which can physically induce the modal characteristics of satellite appendages on to a ground based satellite test bed installed on an air bearing. The ASU consists of two orthogonal fulcrum beams weighting over 800 pounds each utilizing two electrodynamic shakers to induce active torques onto the bus. The ASU is programmed with the state space characteristics of the desired appendage and responds in real time to the bus motion to generate realistic disturbances back onto the satellite. Two LVDTs are used on each fulcrum beam to close the loop and insure the system responds in real time the same way a real solar array would on-orbit. Each axis is independently programmable in order to simulate various orientations or modal contributions from an appendage. The design process for the ASU involved the optimization of sensors, actuators, control authority, weight, power and functionality. The smart structure system design process and experimental results are described in detail.