Douglas J. Phillips

Robust decentralized control laws for the ACES structure

Control system design for the Active Control Technique Evaluation for Spacecraft (ACES) structure at NASA Marshall Space Flight Center is discussed. The primary objective of this experiment is to design controllers that provide substantial reduction of the line-of-sight pointing errors. Satisfaction of this objective requires the controllers to attenuate beam vibration significantly. The primary method chosen for control design is the optimal projection approach for uncertain systems (OPUS). The OPUS design process allows the simultaneous tradeoff of five fundamental issues in control design: actuator sizing, sensor accuracy, controller order, robustness and system performance. A brief description of the basic ACES configuration is given. The development of the models used for control design and control design for eight system loops that were selected by analysis of test data collected from the structure are discussed. Experimental results showing that very significant performance improvement is achieved when all eight feedback loops are closed are presented.<<ETX>>

High performance, accelerometer-based control of the Mini-MAST structure

Many large space system concepts will require active vibration control to satisfy critical performance requirements such as line of sight pointing accuracy and constraints on rms surface roughness. In order for these concepts to become operational, it is imperative that the benefits of active vibration control be shown to be practical in ground based experiments. The results of an experiment shows the successful application of the Maximum Entropy/Optimal Projection control design methodology to active vibration control for a flexible structure. The testbed is the Mini-Mast structure at NASA-Langley and has features dynamically traceable to future space systems. To maximize traceability to real flight systems, the controllers were designed and implemented using sensors (four accelerometers and one rate gyro) that are actually mounted to the structure. Ground mounted displacement sensors that could greatly ease the control design task were available but were used only for performance evaluation. The use of the accelerometers increased the potential of destabilizing the system due to spillover effects and motivated the use of precompensation strategy to achieve sufficient compensator roll-off.

Vibration isolation system using plural signals for control

An active isolation device and method reduces the transmission of vibrations from interconnected elements. The device maintains a high stiffness between interconnected elements while actively reducing the transmission of relative movements, such as vibration. In one embodiment, the device uses accelerometers to measure the vibrations experienced by each of the interconnected elements and selectively operates an actuator between the two elements to maintain the position of one of the elements. In another embodiment, plural of the devices are used cooperatively to reduce the vibrations that would otherwise be transmitted from one element to a platform, permitting simultaneous control of multiple degrees of freedom of movement.

Experimental demonstration of active vibration control for flexible structures

Active vibration control of flexible structures for future space missions is addressed. Three experiments that successfully demonstrate control of flexible structures are described. The first is a pendulum experiment. The structure is a 5 m compound pendulum and was designed as an end-to-end test bed for a linear proof mass actuator and its supporting electronics. Experimental results are shown for a maximum-entropy/optimal-projection controller designed to achieve 5% damping in the first two pendulum modes. The second experiment was based upon the Harris Multi-Hex prototype experiment apparatus. This is a large optical reflector structure comprising a seven-panel array and supporting truss which typifies a number of generic characteristics of large space systems. The third experiment involved control design and implementation for the ACES structure at NASA Marshall Space Flight Center. The authors conclude with some remarks on the lessons learned from conducting these experiments.<<ETX>>

The Multi-Hex Prototype Experiment

The authors describe the Multi-Hex Prototype Experiment (MHPE), which was developed to experimentally investigate the technologies associated with the active control of flexible structures. The MHPE has as its main component a structure that was designed to emulate the generic properties of large space structures, such as high-frequency RF or optical antennas and solar concentrators. The authors describe the primary features of the MHPE and present some experimental results which illustrate the efficacy of system identification and active control. Experimentation has provided validation of system identification using the eigensystem realization algorithm and has also shown the ability of active control to provide significant vibration attenuation.<<ETX>>

Active control experiments for large-optics vibration alleviation

Many future space missions involving flexible structures for large optics may require active vibration control to satisfy mission objectives. Thus, it is important for active control of flexible structures to be practically demonstrated in ground based experiments. These experiments can validate (or invalidate) existing theories and technology and provide directions for future research. This paper discusses three experiments conducted by Harris which successfully demonstrate control of flexible structures. The paper concludes with some remarks on the lessons learned from conducting these experiments.